Rodent Models and Contemporary Molecular Techniques: Notable Feats yet Incomplete Explanations of Parkinson’s Disease Pathogenesis

Abstract

Rodent models and molecular tools, mainly omics and RNA interference, have been rigorously used to decode the intangible etiology and pathogenesis of Parkinson's disease (PD). Although convention of contemporary molecular techniques and multiple rodent models paved imperative leads in deciphering the role of putative causative factors and sequential events leading to PD, complete and clear-cut mechanisms of pathogenesis are still hard to pin down. The current article reviews the implications and pros and cons of rodent models and molecular tools in understanding the molecular and cellular bases of PD pathogenesis based on the existing literature. Probable rationales for short of comprehensive leads and future possibilities in spite of the extensive applications of molecular tools and rodent models have also been discussed.

Full text

Rodent Models and Contemporary Molecular Techniques:
Notable Feats yet Incomplete Explanations of Parkinson’s
Disease Pathogenesis
Sharawan Yadav &Anubhuti Dixit &Sonal Agrawal &
Ashish Singh &Garima Srivastava &Anand Kumar
Singh &Pramod Kumar Srivastava &Om Prakash &
Mahendra Pratap Singh
Received: 14 May 2012 /Accepted: 13 June 2012
#Springer Science+Business Media, LLC 2012
Abstract Rodent models and molecular tools, mainly
omics and RNA interference, have been rigorously used to
decode the intangible etiology and pathogenesis of Parkin-
son’s disease (PD). Although convention of contemporary
molecular techniques and multiple rodent models paved
imperative leads in deciphering the role of putative causa-
tive factors and sequential events leading to PD, complete
and clear-cut mechanisms of pathogenesis are still hard to
pin down. The current article reviews the implications and
pros and cons of rodent models and molecular tools in
understanding the molecular and cellular bases of PD path-
ogenesis based on the existing literature. Probable rationales
for short of comprehensive leads and future possibilities in
spite of the extensive applications of molecular tools and
rodent models have also been discussed.
Keywords Parkinson’s disease .Rodent models .
Genomics .Transcriptomics .Proteomics .RNA interference
Introduction
James Parkinson offered the first landmark portrayal on
shaking palsy; however, the name Parkinson’sdisease
(PD) was given by Jean-Martin Charcot [1,2]. PD is
recognized as the most common progressive, baffling, and
devastating neurodegenerative disorder in the elderly after
the Alzheimer disease [3,4]. This movement disorder is
distinguished by the selective degeneration of the nigros-
triatal dopaminergic neurons, accumulation of cytoplasmic
protein aggregates and onset of phenotypic features, such as
resting tremor, rigidity and bradykinesia, etc., leading to loss
of control over the movement [3–6]. The degeneration of
selective neurons is accountable for the decreased dopamine
level in the striatum that ultimately results in the clinical
manifestations [6]. Although an early diagnosis is dreadfully
difficult, physical and clinical examinations and sympto-
matic features are used to diagnose the patients after consid-
erable dopaminergic neurodegeneration and manifestation of
noticeable complications [7]. Moreover, the comprehensive
explanations of pathogenesis and permanent cure are not yet
established, and therapeutic and surgical procedures offer
provisional aids [7,8].
One of the most commonly accepted notions for the onset
of symptomatic features of PD is the resultant interplay of
the environmental factors, increased age, and genetic sus-
ceptibility of an individual [3,5,9]. Administration of 6-
hydroxydopamine (6-OHDA), 1-methyl-4-phenyl-1,2,3,6-
tetrahydropyridine (MPTP), reserpine, methamphetamine,
rotenone, maneb, zinc, manganese, paraquat, and cyper-
methrin in rodents develop many symptomatic features
mimicking PD [9–14]. These chemicals either alone or in
combination inhibit mitochondrial function resulting in
depleted energy metabolism, free radical generation, and
neuroinflammation leading to programmed cell death and
selective neurodegeneration [15–19]. Some environmentally
relevant chemicals directly cross the blood–brain barrier
(BBB) and enter the brain owing to their lipophilic nature,
such as MPTP and rotenone. Others, those are hydrophilic in
Authors Sharawan Yadav and Anubhuti Dixit contributed equally to
this work.
S. Yadav :A. Dixit :S. Agrawal :A. Singh :G. Srivastava :
A. K. Singh :M. P. Singh (*)
CSIR-Indian Institute of Toxicology Research (CSIR-IITR),
M.G. Marg, Post Box 80, Lucknow -226 001, Uttar Pradesh, India
e-mail: singhmahendrapratap@rediffmail.com
P. K. Srivastava :O. Prakash
Banaras Hindu University,
Varanasi -221 005, Uttar Pradesh, India
Mol Neurobiol
DOI 10.1007/s12035-012-8291-8

nature, require either specific transporters, such as paraquat
that requires a neutral amino acid transporter or needs to be
imported directly in the target tissue, such as 6-OHDA [9,14].
Environmental factors could contribute notably, if the genetic
factors clutch them appropriately, as every synthetic heroin
(which contains MPTP) user does not develop PD-like symp-
toms. Pesticides and other environmental chemicals, when
enter the body and subsequently in the brain, get converted
by the phase I and II xenobiotic metabolizing enzymes either to
more or less toxic intermediary metabolites. The bioconversion
is directly proportional to the catalytic activity, protein expres-
sion, and DNA sequence ofthe coding and non-coding regions
of a gene, which encode the enzyme [20]. Twin analysis,
omics, and RNA interference (RNAi) further supported the
genetic bases in a few cases and deciphered autosomal pattern
of inheritance of many PARK genes [5,21–23]. Omics and
RNAi also deduced many indefinable aspects, which include
the identification of molecular fingerprints and molecular
explanations of PD pathogenesis [7,22]. Although amalgama-
tion of modern techniques and multiple rodent models were
expected to offer novel clues to disease pathogenesis and
molecular biomarkers, clinical and phenotypic symptoms are
still used as the gold standard to diagnose PD [7]. Despite
noteworthy and extensive endeavors made by rodent models
and molecular tools to pinpoint the biochemical, clinical,
pathological, epidemiological, and molecular bases of disease
pathogenesis, the complete molecular machinery of PD patho-
genesis is still mysterious [21,22,24].
Salient characteristics, achievements, and limitations of
various rodent models and/or molecular tools employed for
understanding PD pathogenesis have been comprehensively
reviewed elsewhere [3–5,10,21,22,24–27]. This article
updates the contributions made by imperative rodent mod-
els, omics, and RNAi approaches altogether based on the
information available in the literature along with the reasons
why such sincere efforts could not yet embrace the desired
success. Attempts are also made to portray likely explan-
ations for lack of comprehensive translation of the informa-
tion generated from rodents to humans.
Contributory Factors of PD
Since most of the rodent models are based on the alleged
causative factors, it is worthwhile to discuss about the undeni-
able and suspected contributory factors of sporadic PD.
Undoubtedly, aging is the main perpetrator, as it has been
found to increase the incidences of PD in humans. It is
estimated that approximately 1 % of the population of 50–
60 years of age may develop PD, and the incidences may go
up further in elderly individuals [5]. The effect of age is
reflected even in the experimental rodent models, as the aged
rats have been found to be more susceptible to a chemical that
induces the nigrostriatal dopaminergic neurodegeneration
[13]. Environmental factors that include pesticides exposure,
rural living, well water drinking, and head trauma could be
other contributory factors, as evidenced from epidemiological
studies and/or rodent experimentations [3–5].
Appearance of the disease warning signs in an early age
has been limited; however, onset of the disease in the
individuals below 50 years of age gave a notion that genetic
factors could play a critical role in PD pathogenesis. Now,
the familial PD is well recognized by the twin analyses and
case studies [5]. Several genes have been mapped and are
suspected to manipulate PD pathogenesis in the genetically
susceptible persons owing to point mutations and inappro-
priate epigenetic regulation [28–30]. Despite the fact that
the majority of the patients do not possess familial history,
absence of cardinal differences between the sporadic and
familial forms and the lack of symptoms in a few patients
carrying defective genes, point out the influence of herit-
able factors on the age of onset, particularly when the
appropriate environmental conditions are available [5,24,
30,31].
Rodent Models
Many chemicals, which include pesticides and metals, turn
out to be inseparable parts of the environment, and their
involvement in PD pathogenesis could be incredible. A few
pesticides and metals may lead to PD in humans and also
reproduce PD-like symptoms in the exposed experimental
rodents [5,25,32,33]. Salient characteristics and pros and
cons of a few extensively used rodent models are described
below.
6-OHDA
Even after several decades of history of its use for develop-
ing PD-like features in rats, 6-OHDA is still widely used for
the same purpose [21]. The severity of the symptoms pro-
duced by 6-OHDA depends on the site of its administration
in the substantia nigra and the extent of the lesions [32]. The
6-OHDA model provided new insights to understand PD
pathogenesis and validated pre-existing information gath-
ered from sporadic cases and also from other rodent models.
6-OHDA offered multifaceted confirmations of the degen-
eration of cell bodies of dopaminergic neurons in the sub-
stantia nigra and fibers in the striatum [34], the fundamental
features of sporadic PD in humans. Mitochondrial dysfunc-
tion has been widely accepted as the priming event leading
to oxidative stress and thereby the nigrostriatal dopaminer-
gic neurodegeneration. 6-OHDA mimics sporadic PD in the
sense that it inhibits mitochondrial electron transport chain
complex I (nicotinamide adenine dinucleotide-ubiquinone
Mol Neurobiol

reductase), generates free radicals, and induces programmed
cell death [4].
6-OHDA induces neurodegeneration by multiple means.
Oxidative stress is critical in PD pathogenesis whether it is
generated by the mitochondrial complex I inhibition or owing
to other means. Although growth arrest and DNA damage-
induced gene 153 is also reported to act as a neuronal cell
death mediator, a null mutation of the gene results in reduction
of apoptosis in the 6-OHDA model [35]. Similarly, free rad-
icals produced owing to 6-OHDA exposure causes several
changes in dopaminergic neurons that mediate neurodegener-
ation. Fos expression also mediates some of the abnormal
sensory circuits of neurodegeneration in 6-OHDA-treated rats
[36]. Proteases, such as caspases, are known to regulate apop-
totic machinery and induce toxicity in dopaminergic neurons.
Caspase-3 mediated proteolytic cleavage and activation of
protein kinase C-δis reported to be decisive in dopaminergic
neurodegeneration and cleavage resistant form of the protein
is found to protect against apoptosis in 6-OHDA-treated
rodents [37]. 6-OHDA augments unfolded protein stress,
causes cytochrome-c release and upregulates pro-apoptotic
proteins, which include caspases and p53 [38]. 6-OHDA
downregulates tumorous imaginal discs 1 protein, which ham-
pers functional and structural compensation and exacerbates
neurodegeneration [39]. Moreover, the 6-OHDA model high-
lighted the fact that auto-oxidation of dopamine into free
radicals may also lead to PD-like features [32].
6-OHDA neither induces the formation of the Lewy
bodies nor produces the similar degree of phenotypic symp-
toms in all experimental rodents [24,32]. Owing to inability
of 6-OHDA to cross the BBB and directly enter the brain
[4], the nigrostriatal administration is required, which is tricky
and cumbersome. Due to lack of its environmental occurrence
and direct human exposure, and inconvenient delivery in brain
the 6-OHDA model could not be considered very ideal for
extrapolation of the results to humans and is now becoming
less popular as compared with its popularity in the past [24].
MPTP
MPTP itself is not as toxic as its intermediary metabolite, 1-
methyl-4-phenylpyridinium cation (MPP
+
), which is highly
reactive and severely neurotoxic [4,16,25,40]. Monoamine
oxidase B (MAO-B) of the astrocytes synthesizes MPP
+
from
MPTP [4,25,41]. The organic cation transporter (Oct)-3
present in the astrocytes (glial cells) and dopamine transporter
(DAT) localized on neurons regulate the entry of MPP
+
into
dopaminergic neurons (Fig. 1)[42]. The MPTP rodent model
supported the notion that the mitochondrion is an epicenter of
PD pathogenesis [40]. Although it is not yet clear whether
mitochondrial depolarization or free radical generation after
the mitochondrial complex I inhibition is the most critical event,
the MPTP model elucidated the contributions made by the
nicotinamide adenine dinucleotide phosphate (NADPH) oxi-
dase, microglial cells, neuronal inflammation, and secondary
signaling molecules, including p38 mitogen activated protein
kinase, c-Jun N-terminal kinase (JNK) and tumor suppressor
protein 53, in PD pathogenesis [10,25,43]. MPTP triggers the
activation of JNK, which is regulated by GST Pi protein via
protein–protein interactions [44]. Although the role of free
radicals and microglial activation in the MPTP model is estab-
lished, a recent study has shown that MPTP-induced dopami-
nergic neurodegeneration primarily depends on the free radical
outburst and activation of NADPH oxidase in dopaminergic
neurons,and the activationof microglial NADPHoxidase takes
place at later stage [45]. Furthermore, theMPTP model showed
that phosphorylated Akt/protein kinase B depletion hinders
with the normal functioning of cell survival [46].
PINK 1 and DJ-1, two known neuroprotective molecules,
modulate the dopaminergic sensitivity towards MPTP-
induced toxicity [24]. This assumption gave a conduit of
how genetic factors play important roles even in toxin-
induced disease pathogenesis. It is validated by a recent
study in which loss of PARK 7 (DJ-1), a cellular target, is
found to be associated with the modulation of MPTP-
induced PD phenotype. In this study, a cell permeable Tat-
DJ-1 protein is reported to protect dopaminergic neurode-
generation by reducing the effects produced by MPTP-
mediated oxidative stress [47]. MPTP model elucidated the
inputs of peroxisome proliferator-activated receptor γin
dopaminergic neuroprotection and treatment outcomes
[48]. Moreover, the MPTP model deciphered the roles of
heat shock proteins (HSPs), such as HSP1b, in the regula-
tion of neurodegeneration, as knocking down the HSP1b
gene increases the vulnerability of dopaminergic neurons
[49]. Neurotrophic factors and apolipoprotein could encoun-
ter PD pathogenesis and help in the repairing of dopaminer-
gic neurons, as their expressions are increased in MPTP-
induced rodent models [50,51]. The MPTP model also
improved our awareness about the contribution of environ-
mental factors, the mechanisms of PD pathogenesis and
therapeutic strategy to encounter the disease.
MPTP develops an acute rodent model; low doses and
long-term exposures may help in reproducing chronic PD
phenotype [52]. Despite species to species variation,
absence of slow and progressive degeneration and distinct
Lewy body formation, the MPTP model is widely used to
understand pathogenesis and to appraise the efficacy of anti-
PD chemical entities [25,40,41,52].
Paraquat and Maneb
Initially, 1,1′-dimethyl-4,4′-bipyridinium dichloride (para-
quat), which has a structural similarity to MPP
+
and produ-
ces neurotoxicity, was assessed for neurodegenerative
potential in the experimental rodents [53,54]. Paraquat
Mol Neurobiol

enhances alpha (α)-synuclein-induced disruption of mem-
brane integrity and increases the conductance, as a result of
increased oxidative stress [55]. Later on, a fungicide, man-
ganese ethylene bis-dithiocarbamate (maneb) was also
tested for its potential in experimental animals either alone
or with paraquat [15]. Structural and functional anomalies in
the endoplasmic reticulum (ER) and mitochondrion are
found to be associated with PD pathogenesis. Paraquat
slows down mitochondrial complex I activity and augments
the microglial activation and free radical production through
NADPH oxidase, while maneb reduces mitochondrial com-
plex III activity and both of them together or individually
may lead to oxidative stress, DNA damage, defective energy
metabolism, and cellular apoptosis (Fig. 1)[4,10,15,25,
56]. Roles of ER and mitochondrion in PD pathogenesis are
also validated in the paraquat alone induced dopaminergic
neurodegeneration model in a study. ER stress and mito-
chondrial dysfunction were found to trigger caspase-12
activation, hydrogen peroxide release, and PARK 13
(HtrA2/Omi) activation; however, minocycline, a microglial
activation inhibitor, encountered such alterations [57].
Toxicant responsive genes, such as cytochrome P450
(CYP/Cyp) 2D6, play critical roles in PD pathogenesis and
treatment outcomes. The expression of Cyp2d22, a human
CYP2D6 ortholog in the mice, is increased in maneb and/or
paraquat-induced PD phenotypes, showing the role of detox-
ification machinery in the disease pathogenesis [20]. Toxicity
of pesticide-induced PD depends on the metabolic conversion
of pesticides in to too high or too low toxic species. The
conversion of paraquat dication to paraquat cation and sub-
sequent transport to dopaminergic neurons mainly depend on
Oct-3 and DAT [58]. Maneb and paraquat induce apoptosis
through Bak-dependent pathway when administered alone;
however, on combined systemic exposure, they follow the
Bax-dependent apoptotic pathway [59]. Although it is known
that males are often at the higher risk for PD as compared with
females, supportive evidence from an animal experimentation
is provided in a recent study in which the effect of paraquat
was monitored. Paraquat administration was found to increase
the brain-derived neurotrophic factor (BDNF) expression in
the hippocampus of female rats, which was responsible for
reduced susceptibility of the females towards the nigrostriatal
NADH-
ubiquinone
oxidoreductase
Succina te -
ubiquinone
oxidoreductase
Ubiquinol -
cytochrome c
redu cta s e
cytochrome
coxidase
6-OHDA
PARAQUAT MANEB
DOPAMINERGIC NEURON
MPTP
MITOCHONDRIA
BBB
MPP+
MAO-B
F1Fo-ATP
syntha se
ATP
MAIN PROS
AND CONS
Specific to
injected site
Direct
administration
to the target site
is required, as it
can not cross the
BBB
MAIN PROS AND
CONS
Mimics most of the
features of sporadic PD
but can not induce
Lewy body f ormation
An acute mod el,
however, alteration in
dose and treatment
schedule can make this
suitable as chron ic
MAIN PROS AND
CONS
Induces slow and
progressive
degeneration in the
mice
Reported toxic in
rats, if admin ister ed
for longer duration
Lipoph ilic
DAT
MPP+
6-OHDA
6-OHDA
CYPERMETHRIN
Hydrophilic
Lipophilic
Neutral amino
acid transporter
PQ
+2
METHAMPHETAMINE
ROS
Lipophilic
PQ
+
Astroc yte
ROTENONE
MAIN PROS
AND CONS
Forms Lewy
bodies
Non specific
neurodegenerat
ion
Lipophilic
Rotenone
Maneb
MAIN PROS AND
CONS
Causes slow and
progressive
dopaminergic
neurodegeneration in
rats
Mechanism is not yet
completely known
MAIN PROS AND
CONS
Degeneration of the
striatal nerve terminal s
only
Not suitable for
mechani st ic
understanding
OCT-3
Lipophilic
DA
Syna ptic
vesicle
Extracellular DA
DAT
Metham phetamine
autooxidation
PQ
+
Microglial
activation
Cypermethrin
Hydrophilic
MPTP
-
-
-
-
-
Needs direct target-
specific a dministration
Microglia
OCT-3
Fig. 1 Routes of entry of the major PD-inducing chemicals in the
brain, their enzymatic/non-enzymatic conversion into active radicals in
glial cells, subsequent entry into dopaminergic neurons, and the inhib-
itory effects produced by them at the level of mitochondrial complexes
or dopamine auto-oxidation [4,10,24,25,42,58,68]. 6-OHDA,
MAO-B, Oct-3, DAT, PQ
2+
,PQ
+
, MPTP, MPP
+
, NADH, DA, ATP,
BBB, and ROS abbreviate 6-hydroxydopamine, monoamine transporter
B, organic cation transporter-3, dopamine transporter, paraquat dication,
paraquat cation, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, 1-
methyl-4-phenylpyridinium cation, reduced nicotinamide adenine
dinucleotide, dopamine, adenosine triphosphate, blood–brain barrier,
and reactive oxygen species, respectively
Mol Neurobiol

dopaminergic neurodegeneration as compared with males, as
decreased BDNF expression was noted in males [60].
Despite mild to severe toxicity of paraquat and/or maneb
administrations in rodents after prolonged exposure [9,61],
the use of their combination in rodents is widely accepted
and offers two main advantages—inevitable human expo-
sure that makes them environmentally relevant and ability of
progressive and slow neurodegeneration [61]. Epidemiolog-
ical studies have also shown the direct relevance of maneb
and paraquat co-exposure to humans [25,62].
Rotenone
Like other animal models, the rotenone model also exhibits
mitochondrial complex I inhibition and induction of oxida-
tive stress and apoptosis [4]. The release of cytochrome c is
found to be independent of caspase activation in rotenone-
induced dopaminergic neurodegeneration. This phenome-
non exists even in the presence of cytoplasmic cytochrome
c release after mitochondrial complex I inhibition [63].
Matrix metalloproteinase-3 (MMP-3) is also implicated in
dopaminergic neurodegeneration induced by rotenone,
which is regulated by multiple mechanisms. MMP-3
requires activation from proMMP-3 by an intracellular ser-
ine protease. Under stress, HtrA2/Omi, a mitochondrial
serine protease, translocates into the cytosol, causes MMP-
3 activation, and triggers apoptosis in dopaminergic cells
[64].
Although rotenone readily enters the brain, inhibits mito-
chondrial complex I, generates free radicals, exhibits Lewy
bodies formation and other anatomical and phenotypic
symptoms of PD (Fig. 1)[65], it is not as popular as MPTP
or 6-OHDA, owing to its non-specificity, high mortality
rate, and severity of the lesions [9,10,41].
Other Pesticides
Epidemiological studies showed the contribution of a few
other pesticides in increased incidences of PD [5,33,66,
67]. Several classes of pesticides, which include, pyrethroids,
dithiocarbamates, organochlorines, and organophosphates
have been reported to induce PD-like symptoms in the exper-
imental rodents [68,69]. As these pesticides are commonly
used globally, therefore, could be of major health concern.
Dieldrin and cypermethrin induce neurodegeneration in the
adult experimental animals after prolonged exposure [13,69].
Postnatal pre-exposure of cypermethrin is also found to
enhance the susceptibility of the animals, when re-exposed
upon adulthood [13]. Despite the fact that little is known about
the mechanism of cypermethrin-induced neurodegeneration,
the neuronal loss is specific to dopaminergic neurons of the
nigrostriatal pathway [68]. Moreover, its effect on microglial
activation is known, but prolonged opening of ion channels or
mitochondrial dysfunction could possibly be the most impor-
tant events involved therein [19,68].
Methamphetamine
Methamphetamine produces dopamine depletion leading to
temporary or permanent disturbance in the dopaminergic
system after chronic or intermittent exposure and has estab-
lished the role of growth factors, particularly glial derived
neurotrophic factor [10,70]. Methamphetamine selectively
reduces phasic, but not tonic, dopaminergic signaling in the
striatum [71]. Methamphetamine-induced response is found
to be age dependent in the primates owing to age-related
changes in the neurotrophic capacity of the striatal dopa-
mine system [72]; however, in rodents, such information has
not yet been reported. As methamphetamine depletes dop-
amine level in the striatum, the effects of such drugs have
also been studied in the exposed humans to assess the actual
risk. Drug users are found to have a high risk for developing
PD as compared with unexposed individuals [73].
Methamphetamine-induced neurotoxicity also involves the
striatal vasoconstriction leading to hypoxia and dopamine
reduction in the exposed individuals [74].
In general, the methamphetamine model lacks many fun-
damental features of PD and is considered as a dopamine
depletion model rather than a true PD model.
Metals
Apart from chemicals, several metals, including iron, lead,
zinc, and manganese have been found to be associated with
PD pathogenesis, as abnormal metal exposures or accumu-
lations have been associated with the increased incidences
of PD in humans or PD-like pathology in experimental
animals [12,75,76]. “Metal accumulation may lead to
PD”came into existence, when the postmortem brain of
PD patients was seen to possess high level of metals [75].
Epidemiological studies revealed that zinc, lead, and iron
are increased in the substantia nigra of PD patients, whereas
the copper level is decreased. Like pesticides, metals induce
free radical biosynthesis leading to the degeneration of
dopaminergic neurons of the nigrostriatal pathway [77].
Metals catalyze the formation of free radicals either by
Fenton’s reaction or directly by reacting with macromole-
cules, which lead to the generation of fatty acid radicals and
4-hydroxynonenal causing DNA damage and apoptosis.
Additionally, the metals, such as cadmium, arsenic, and lead
bind to sulphydryl group of proteins and lead to depletion of
glutathione [76]. Zinc, another metal, also induces oxidative
stress via the activation of NADPH oxidase and depletion of
glutathione, which in turn activate the apoptotic machinery
leading to dopaminergic neurodegeneration similar to para-
quat [11,78]. Although metal-based rodent models have not
Mol Neurobiol

yet been fully understood, the contributions of metals in
increasing oxidative stress and reducing antioxidant
defense system cannot be straightforwardly ignored
[11,76].
Like other chemical-induced PD models, metal models
lack many fundamental features of sporadic disease. How-
ever, more exposure time is required to develop a few
cardinal features of the disease in rodents like that of spora-
dic PD in humans.
Some Other Toxins
In addition to the above-mentioned toxins, many chemicals
contribute to PD pathogenesis as reported in epidemiolog-
ical investigations and/or from animal experimentations.
Majority of such chemicals inhibit the mitochondrial com-
plex I or elicit neurotoxicity by multiple mechanisms
[79–81]. Trichloroethylene, a complex I inhibitor, is
reported to cause PD-like features in humans upon exposure
and also elicit motor impairment in experimental animals.
Its oral exposure to experimental animals for 6 weeks selec-
tively inhibits complex I and leads to the nigrostriatal dop-
aminergic neurodegeneration [79]. Annonacin, another
mitochondrial complex I inhibitor, causes the nigrostriatal
dopaminergic neurodegeneration by impairing the energy
metabolism [81]. Several bacterial neurotoxins, from which
humans and aquatic animals are exposed, could also elicit
PD-like features. β-Methylamino-L-alanine, a cyanobacte-
rial neurotoxin, elicits a few features quite similar to PD
[80]. Isoquinoline derivatives, such as tetrahydroisoquino-
line, elicit PD-like features in the animals possibly because
of its structural similarity with MPTP [82,83]. Similarly,
haloperidol, a dopamine D2 receptor blocker; epoxomixin, a
proteasome inhibitor; and 3,4-dihydroxyphenylacetalde-
hyde, a dopamine metabolite, also cause some or the other
characteristic feature of PD [84–86]. Such agents although
not yet very widely studied, could be tested across multiple
studies either alone or in combination with established and
widely studied toxins for developing better rodent models to
understand PD pathogenesis and to assess the efficacy of
neuroprotective agents.
Genetic Models
The role of genetic factors becomes vital when the mutated
gene carrier is exposed to pesticides and heavy metals. The
genetic theory of PD came into existence from the studies
related to close relatives and twins who were diagnosed with
an early onset of the disease [5]. The inheritance of genes,
critical for an early onset of the disease, follows either
autosomal dominant or autosomal recessive pattern [87].
Early onset PD-related genes are mapped in the specific
chromosomal locations, together designated as the PARK
loci [88,89]. α-Synuclein protein is encoded by PARK 1
and 4, Parkin by PARK 2, ubiquitin carboxyl terminal
hydrolase (UCH) L-1 by PARK 5, PTEN-induced kinase 1
(PINK1) by PARK 6, DJ-1 by PARK 7, leucine-rich repeat
kinase 2 (LRRK2) by PARK 8, ATP13A2 by PARK 9 and
Omi/HtrA2 by PARK 13 genes [88,89]. Moreover, many
other genes of elusive nature or function have also been
mapped as PARK 3, PARK 10, PARK 11, and PARK 12
[87–89]. Apart from PARK genes, synphilin 1, paired like
homeodomain transcription factor 3 (Pitx 3) and nuclear
receptor-related factor 1 genes, etc., have also been mapped
at various chromosomal locations and could possibly influ-
ence the genetics of the disease [28,29].
Genetic models proved to be quite useful in under-
standing familial PD pathogenesis. Owing to absence of
most of the critical phenotypic symptoms altogether and
the roles of all the genes involved in PD make little
sense of genetic models in understanding the sporadic
PD.
Dual Models
Several rodent models of the genetically linked PD have
been developed by knocking down the critical gene(s)
responsible for familial forms of the disease [65,90,91].
By and large, the genetic models provided evidence-based
proofs of the molecular mechanisms elucidated by the
toxins-induced rodent models [24,65,90]. Recently, dual/
fusion/combinational/two-hit rodent models have been
generated to test the hypothesis that the environmental
factors mainly act on the genetically susceptible individuals,
as apt environmental factors and suitable genetic makeup
together could be decisive for an early onset of PD [92].
Dual rodent models are mainly based on the principle of the
combination of two contributory factors for creation of a
new animal category to study the multifactorial etiology and
to reiterate the major cardinal disease symptoms [24,92,
93]. Generation of such model is an animated tool to com-
prehend PD in the conditions, which maximally imitate
idiopathic PD [92,94]. For example, lipopolysaccharide
(LPS) induces several neuroinflammatory molecules that
include nitric oxide, cytokines, and interferons, while α-
synuclein dysfunction is associated with its abnormal
accumulation, but low-dose of LPS and dysfunctional α-
synuclein together may cause all the above-mentioned
events and significantly mimic idiopathic PD features [24,
94].
Since dual models could be chronic and progressive in
nature as far as dopaminergic neurodegeneration is con-
cerned, they prove to be quite useful to investigate the
mechanistic and therapeutic aspects of idiopathic PD [24,
94]. Fusion models need to be developed across laboratories
all over the world employing major causative gene(s) and
Mol Neurobiol

toxin(s) for wider acceptance and real implication in under-
standing sporadic PD.
Advantages and Limitations of Rodent Models
Over Other Models
Despite the fact that nonhuman primate models proved to be
very useful in understanding a few decisive aspects of PD
pathogenesis, rodents have many advantages over primates
owing to several reasons. Convenient availability, housing,
maintenance, handling, and plausible use of a large number
of experimental animals per set for generation of much
reliable data, make rodents the preferred choice over pri-
mates [95]. Additionally, it is possible to test multiple toxins
alone as well as in combination to assess their neurodegen-
erative potential in the limited time. Although nonhuman
primates are close to humans, generation to generation stud-
ies to assess the role of a particular gene [96], or the effect of
toxins could be easily performed in rodents within the short
span as compared with nonhuman primates. Nonetheless,
the short life span of rodents does not always reflect an
advantage, as the sporadic disease in humans generally
appear very late and after long-term exposure to environ-
mental factors. Owing to such advantages and disadvan-
tages, many suspected toxins have been widely tested to
check neurodegenerative potential in rodents, but limited
studies are available with primates [32,97]. Although lower
animal models that include, Drosophila, zebra fish, nemat-
odes have been continuously used to reveal the familial PD
and genetic bases of onset and progression of the disease
[98], successful application of such models for more than
90 % of the cases of the disease, which are sporadic in
nature, is difficult. Furthermore, the genetic makeup of
lower animals does not share a noticeable resemblance with
humans, if a comparison is made with rodents or primates
[99], leading to a possibility that data obtained from the
toxicological response to a PD toxin or pharmacological
response of anti-PD compounds could vary significantly
and data extrapolation in humans would become more
difficult.
No Model Is Absolutely Close to Ideal to Understand PD
Pathogenesis
None of the rodent models developed so far can be said to
be perfectly ideal since an ideal animal model should be an
indicator of the multifactorial etiology and reproduce slow,
progressive, and exposure-dependent onset of the pheno-
typic, behavioral, biochemical, and anatomical impairments
along with the secondary changes associated with sporadic
PD [24]. The major achievements of rodent models have
been the confirmation of the contributory roles of pesticides
and heavy metals in PD pathogenesis. Rodent models also
validated the contributory roles of environmental factors,
which were reported from the epidemiological studies [5,
11,25]. Rodent models have shown that susceptibility to
environmental neurotoxins is age dependent, as in sporadic
PD. The postnatal exposure enhances the vulnerability upon
adulthood in the rats [13] also validates the theory that age is
the most important contributory factor. Although rodent
models gave a validation of the major contributory factors
reported through epidemiological or clinical investigations,
rodent models fail to offer the role of newer contributory
factors or all the culprits of PD [100,101].
Chemically induced rodent models established the role of
mitochondrial dysfunction, defective ubiquitin proteasomal
pathway and energy metabolism, apoptosis, neuroinflamma-
tion, microglial activation, dopamine autoxidation and oxi-
dative stress in the pathogenesis of PD [4,10,101]. Many
chemicals used to mimic PD symptoms in rodents lead to α-
synuclein aggregation (but lack of defined Lewy body for-
mation, except in rotenone) and subsequent cell death owing
to their ability to inhibit proteasomal and mitochondrial
functions. The results obtained from rodent models substan-
tially mimic the results obtained from the clinical samples
and postmortem brain of PD patients [101]. Owing to such
similarities, rodent models are also used to test the thera-
peutic efficacy of several drugs and natural products and
also to design new modes of therapy for the disease [52].
Despite all possible efforts, none of the current models
completely mimics sporadic PD [100,102]. The difference
in the life span from humans could also contribute to the
absence of cardinal pathogenic features of sporadic PD,
such as Lewy body formation in rodents. Furthermore, lack
of a neuromelanin and distinct regulatory pattern of tyrosine
hydroxylase (TH) makes rodent models less reliable [101,
102]. Although immunohistochemical and biochemical
observations exhibited the similar pattern of results, the
degree of differences is still enormous not only between
sporadic and chemically induced PD but also among various
rodent models [100]. A few of them develop PD symptoms
after acute exposure contrary to the chronic progression of
sporadic and familial forms of PD in the humans [68,100,
102]. A few others though chronic in nature, are either not
yet established unequivocally or reported from specific lab-
oratories or do not produce the cardinal features of PD
(Fig. 1)[9]. For example, rotenone is nonspecific, MPTP
mainly leads to rapid neurodegeneration, and paraquat may
lead to death of many experimental animals at the concen-
tration and time of exposure, which induce PD phenotype in
mice. The development of defined Lewy body, one of the
basic hallmarks of sporadic PD, is reported only in a
rotenone-induced rodent model [25,32]. Moreover,
reserpine and amphetamine do not produce significant
Mol Neurobiol

degeneration of dopaminergic neurons and pesticides, and
metals possess one or the other drawbacks detailed above
[52,65,100]. Dual rodent models are relevant to PD patho-
genesis and could be more appropriate for assessing the
efficacy of therapeutic agents [92]; however, developing
such models in experimental rodents is difficult, cumber-
some, and cost-consuming.
While it seems quite difficult to translate findings of
rodent models in humans, as most of rodent models possess
different chromosomal localization of a few identified
causative genes with significant degree of differences in
behavior, environment and anatomy of the brain and
responses towards a toxin as compared with humans, it is
expected that after the advancement of newer tools and
development of an ideal animal model, it would become
easier [100–102]. Most of the neuroprotective agents, which
succeeded in preclinical investigations in rodents, could not
be successfully translated into clinical interventions. For
proper translation of the mechanistic observations and
extrapolation of rodent data to humans, it is essential to
develop and validate newer rodent models that could help
to overcome the disparity and drawbacks in connection with
the current animal models.
Omics and RNAi in Understanding PD Pathology
Omics tools, such as genomics, proteomics and transcrip-
tomics, are large-scale technologies used to generate a pleth-
ora of information based on genetic variations and global
expression profiles of genes and proteins and to identify the
differentially expressed transcripts/proteins. Omics-driven
information provided clues to identify the individuals at
high risk, develop molecular fingerprints for diagnosis as
well as for discrimination of various stages of PD patho-
genesis (Fig. 2)[3,26,103,104].
Single Nucleotide Polymorphisms
Genome-wide association studies of several single nucleotide
polymorphisms (SNPs) with PD-linked genes are reported
[105]. For example, SNPs in various forms of synuclein (SNC-
A, SNC-B, and SNC-G), Parkin, UCHL1, PINK1, DJ1,
LRRK2,ATP13A2, and Omi/HtrA2 genes and their association
with PD are recently established [105–107]. Polymorphism in
mitochondrial DNA-encoded complex I gene validated the role
of mitochondria in PD pathogenesis [108]. As most of such
studies are performed in humans rather than rodents, such
GENOMICS: Facts
Identificat ion of SNPs
in PARK genes -
SNCA, UCHL1, DJ-1
and LRR K2, etc.
Evidence to genetic
basis of diseases
Inconsistency in
results owing t o small
sample size and
environmental and life
style factors
PROTEOMICS:Facts
Establishes the role of pathogeni c
modifications of proteins in PD related genes ,
proteasomal proteins and peroxiredoxins
Provid es many prot ein-prot ein in teract ions ,
post-translational modifications and
diff erential protein expression pat terns
Lack of proper tool t o indentify membrane
bound and high and low abundant proteins
Reproducibility of experiments is a tricky
issue
TRANSCRIPTOMICS:Facts
Establishes the roles of ILs,
cyclins an d G STs
Val id at e s rol e s of mu lt i p l e
pathways in PD
Needs quantitat ive tools for
valid atio n
Presen ce of f alse po sitive a nd
negative transcripts
Short life of most of the
transcripts
RNAi: Facts
Explores early events and
validates the roles of
suspected genes
Helps in designing
therapeutic strategy by
targeting desired RNA
Off-targeting and
undesired immune
responses are the major
pitfalls
GENOMICS: Facts
Ident ification of SNPs of
mitochondrial genes
suspected to part icipate in
PD pathogenesis
Provides evidence that
mitochondrion is the
epicenter of PD
Interpretation of the
results with absolute
cert ainty is dif ficu lt due to
sample size
TRANSCRIPTOMICS: Facts
Establishes the roles of
mitochondrial genes in PD
pathogenesis
Investigates mitochondrial
gene network and their
interactions
Lacks fully reprod ucible
patterns across various studies
owing to systemic and global
variations
PROTEOMICS: Facts
Esta blishes the ro les of mito chond rial prot eins, suc h
as NADH-ubiquinone oxidoreductase and DJ-1
mainly in the stria tum
Investigates mitoch ondrial protein expression
patterns and their interactions
Yields specific but not absolutely reproducible
patterns because of multiple reasons leading to
biological variations, mainly in the substantia nigra
Extremely diff icult to isolate pure mitochond rial
fraction
RNAi: Facts
Validate s the
roles of PINK
1and Parkin
Substitutes gene
knockouts
Difficult to
target the brain
NEURON
RNA
PROTEIN
Exogenous
siRNA
miRNA
Mitochondria
Nucleus
RNA
DNA
mRNA
PROTEIN
mtDNA
Golgi bod y
Endoplasmic
reticulum
Fig. 2 Contribution of genomics, transcriptomics, proteomics, and RNAi in understanding PD pathogenesis at cellular and organellar levels and
their pros and cons in understanding PD pathogenesis [3,5,21,22,26,27,106–108,116,137]
Mol Neurobiol

investigations are not being discussed in detail. In summary,
conflicting reports are available in literature even in the same
population. The results of a few investigations revealed the
association of SNPs of the selected genes with PD; however,
others have shown lack of such associations [105,107].
Transcriptomics
The differential transcription profiling has been used to
simultaneously assess the role of many transcripts in PD
pathogenesis [109]. Several groups of investigators working
in this area have generated enormous information employ-
ing various rodent models. The main biological pathways
involved in PD pathogenesis, which could get consensus
among the studies, are neurotransmission, dopamine metab-
olism, biodegradation and transportation, oxidative stress,
mitochondrial function, energy metabolism, neuroinflam-
mation, protein accumulation and degradation, and apopto-
sis (Table 1)[110–115]. Several studies concentrated on the
role of transcripts derived from nuclear origin; however,
studies are also available in which the role of the mitochon-
drial system is assessed. A study based on mitochondrial
transcripts profiling has shown specific susceptibility of the
striatum for oxidative phosphorylation deficiency [116].
Transcriptional profiling of brain and blood showed an
alteration in SNC-A as a critical pathogenic feature [117].
Microarray has been extensively used to assess the changes
in gene expression patterns in most of toxins-induced rodent
models, except a few models that were recently reported
[118–122]. Microarray data established the roles of many
genes that are associated with familial and toxins-induced
PD. For example, Parkin and α-synuclein are reported to
play critical roles in the regulation of ubiquitin proteasomal
pathway and subsequently in neurodegeneration and neuro-
protection [110]. Although many studies have also been
conducted using human sample, which can provide direct
information about the alteration in the gene expression at the
later stages of PD [109,110], such information is not dis-
cussed in detail as the main purpose of the article is to
discuss rodent models. Transcriptomics can even be done
with the human samples as well, since the mRNA of the
postmortem brain can be stable up to 36 h after death [123].
Proteomics
Classical proteomics and modern quantitative proteomics
in combination with bioinformatics are used to analyze
the expression levels and posttranslational changes in
the proteins of the tissues and biological fluids [124,
125]. Nigrostriatal proteomics starting from the postmor-
tem brain up to animal models not only transformed the
understanding of the genetic basis of the disease but
also provided information to develop therapeutic
strategies to encounter PD by identifying specific targets
[126]. Posttranslational modification-based molecular finger-
prints could be identified and protein localization and trans-
location can be recognized by employing proteomic tools
[127–129]. Furthermore, proteomics provided the clues to
PD pathogenesis by generating novel information or by vali-
dating the traditional assumptions and established the roles of
mitochondrial dysfunction, oxidative stress, kinase and
autophagy modulators, and disturbances in protein aggrega-
tion and degradation [130–134]. Proteomic approaches,
applied to various toxins-induced rodent models, demonstra-
ted that the alteration in the proteins related to mitochondrial
complex I, neuronal cytoskeleton, and ubiquitin proteasome
system (UPS) was common among them (Table 2)[19,114,
135–137].
Cellular or quantitative proteomicsof blood and cerebrospi-
nal fluid (CSF) is being used to identify, if any suitable bio-
marker exists for diagnosis of the disease [138]. Blood is
consideredto be the best sample because it is not onlyobtained
using a less invasive tool but also is the most dynamic tissue of
the body [139]. CSF is also an attractive and ideal body fluid to
search for PD biomarkers [140,141]. By proteomics-based
approaches, neuromelanin, mortalin, DJ-1, haptoglobin deriv-
atives, truncated globin, aggregated serum amyloid P compo-
nent, and ion channel proteins have been identified as
differentially regulated proteins in the blood or CSF of PD
patients [21,135,140,142–145]. Many proteins that include,
α-synuclein and UCH L-1, are dysregulated in patients and are
expected to help in understanding and solving a few mysteries
of PD pathogenesis [146,147]. Proteomic approaches con-
firmed that α-synuclein undergoes posttranslational modifica-
tion, a triggering event for neurotoxicity [146].
RNAi
Silencing of autosomal dominant genes, which play critical
roles in PD pathogenesis could act as novel therapeutic
targets for treating PD. RNAi, used to silence the selected
gene expression, is projected as a substitute for the gene
knockout approach to study the complex molecular and
biochemical interactions within the pathways known to
involve in PD pathogenesis [148]. As mutation in the genes
belonging to PARK loci, oxidative stress, dysfunctional
xenobiotic metabolizing machinery, inflammation, autoph-
agy, and apoptosis [4,22,88] are associated with PD path-
ogenesis, therefore, direct or indirect modulators of such
genes could be astonishing targets of RNAi-based studies
[149,150]. Small interfering RNAs (siRNAs) and micro
RNAs (miRNAs) have made substantial input towards under-
standing the early events implicated in the functioning of
dopaminergic neurons. The siRNA knockdown studies eluci-
dated that the expression of homeoprotein LIM homeobox
transcription factor 1 α(Lmx1 α) is the prerequisite for
Mol Neurobiol

neuronal progenitors to determine a dopaminergic fate [22,
151]. Similarly, the mutual interaction between miRNA-133b
and Pitx 3 is reported to regulate the differentiation and
survival of the midbrain dopaminergic neurons [152]. The
tiny non-coding RNAs carry out localized control of gene
expression in the neurons and any anomaly in it may lead to
PD [153]. Tiny non-coding RNAs play a fundamental role in
neurodegenerative diseases in rodents, as evidenced by the
complete deficiency of a particular miRNA expression during
neuronal loss [154]. Downregulations of miRNA-34b and
miRNA-34c in PD brains hypothesize the involvement of
these tiny non-coding molecules in mitochondrial dysfunction
[155]. Furthermore, a few miRNAs are also identified to
regulate the synthesis of neurotransmitter substance P by the
tachykinin (TA C 1 )gene[156]. Small non-coding RNAs val-
idated the existing knowledge regarding various genes, such
Table 1 Details of some transcriptomics studies conducted employing rodent models and list of genes and pathways involved in PD pathogenesis
Serial
no.
Toxin
(rodent
model)
Tissue Affected
pathways
Differential gene expression level References
1. 6-OHDA
(rat)
Striatum Neurotrophic
factors and
neurotransmitter
release
Increases neurotensin, neuromedin U receptor, Finkel–
Biskis–Jinkins murine osteosarcoma viral oncogene
cellular homolog, cyclooxygenase-2, follistatin, neu-
romedin U, platelet-derived growth factor-D, orphan
nuclear receptor-1 and TAC 2 expressions
[115]
Reduces TAC 1 expression
2. MPTP (rat) Striatum Cell growth,
differentiation,
regeneration
and survival
Upregulates cerebellin 1 precursor protein, galanin,
nerve growth factor receptor, and signal transducer and
activator of transcription 4 expressions
[118]
Downregulates ciliary neurotrophic factor expression
3. MPTP
(mouse)
Nigro-
striatum
Mitochondrial
dysfunction,
oxidative stress,
and apoptosis
Augments cathepsin D and UCH-14 expressions [114]
Attenuates ATP synthase protein 8, glutathione S-
transferase (GST) mu-5 (Gstm5), and NADH–ubiqui-
none oxidoreductase expressions
4. MPTP
(mouse)
Striatum Inflammatory responses,
cytokine and mammalian
target of rapamycin-signaling
pathways, activation of astro-
cytes and cellular stress
Increases heme oxygenase 1 metallothionein 2,
uncoupling protein 2, growth arrest and DNA-damage-
inducible-beta, nuclear factor of kappa light polypep-
tide gene enhancer inhibitor, FBJ murine osteosarcoma
viral oncogene homolog B, DNA-damage-inducible
transcript 4, CD9 antigen and heparin-binding epider-
mal growth factor-like growth factor expressions
[119]
Reduces retinoid X receptor gamma and paired box gene
8 expressions
5. Maneb and
paraquat
(mouse)
Striatum Electron transport,
lipid metabolism,
cell cycle, oxido-reductase
activity, ubiquitin
proteosomal system,
and apoptosis
Upregulates ubiquitin C and programmed cell death 10
expressions
[111]
Downregulates cytochrome c oxidase subunit VI a
polypeptide 1, diazepam binding inhibitor, growth
arrest specific 5, superoxide dismutase 1, and clusterin
expressions
6. MPTP
(mouse)
Substantia
nigra
Apoptosis, proteasomal system,
and energy metabolism
Increases lactate dehydrogenase 2 B chain, fibroblast
growth factor 1, cytochrome P450 family 4 subfamily
V polypeptide 3, RAS-like estrogen-regulated growth-
inhibitor, and protein tyrosine phosphatase receptor
type Z polypeptide 1 expressions
[120]
Reduces lipoprotein lipase, cadherin 2, G protein-
coupled receptor 83, neuropilin, cysteine-rich motor
neuron 1 and growth hormone receptor expressions
7. MPTP
(mouse)
Nigro-
striatum
Inflammation, oxidative stress,
glutamate toxicity, cell cycle,
and cell death processes
Increases interleukin (IL)-1b, IL-10, nuclear factor kappa
B (NF-kB) p65, N-methyl D-aspartate (NMDA)
adenosine A
2A
receptor (A
2A
-R), cyclin B2, tumor
necrosis factor (TNF) αand parkin expressions
[121]
8. MPTP
(mouse)
Nigro-
striatum
Cell cycle, oxidative stress,
inflammatory processes,
glutamate signaling, and
neuronal differentiation
Increases Bax membrane isoform α, IL-2 receptor
gamma, TNF-βand IL-1βexpressions
[122]
Reduces G2/M-specific cyclin B2, proliferation-
associated protein 1, cytochrome P450 1A1, GST- A,
inhibitor-kB αsubunit and NF-kB p65 expressions
Mol Neurobiol

as α-synuclein, TH, Parkin, PINK1 and LRRK2, which are
implicated in PD pathogenesis and assisted to gain newer
insights [149,157–159]. Thus, siRNA-mediated depletion of
the major players involved in PD pathology and progression
may help in designing a new therapeutic strategy for PD.
Additionally, miRNA mimetic or expression vector could be
used to restore or overexpress miRNAs of interest [22,160,
161].
Omics and RNAi in PD: Noteworthy Validation
but Incomplete Applications
Genomic (SNP-based) studies have a number of limitations,
including sample size, life style factors, data interpretation,
and varying levels of environmental exposure. Indeed,
genomics offered an initiative to assess the association of
SNPs in a larger scale at a rapid rate employing a huge
sample size and predicted the possible association of many
genes in PD pathogenesis (Fig. 2)[162]. The major disad-
vantage is variation among studies even within the same
population.
Evidences about the dysregulation of genes, associated
with the neurotransmission, synaptic function, oxidative
stress, mitochondrial function and energy production, pro-
tein misfolding and aggregation, UPS dysfunction, autoph-
agy and apoptosis, etc., are known, however, the microarray
data yielded inconsistent results across multiple studies
[110–113]. Despite validation with secondary quantitative
tools, transcriptomics of the nigrostriatal tissues could not
help in projecting the role of infrequently expressed genes in
PD pathogenesis [111–113]. One of the major reasons could
be the loss of a substantial number of cell bodies of the
dopaminergic neurons in the substantia nigra and related
Table 2 Details of a few proteomics studies conducted employing various rodent models and list of proteins and pathways involved in PD
pathogenesis
Serial
no.
Toxin
(rodent model)
Used tissue Affected pathways Differential protein expression level References
1. 6-OHDA (rat) Substantia nigra Mitochondrial function Increases prohibitin and complex I 30-kDa subunit
expressions
[135]
2. 6-OHDA (rat) Striatum Energy metabolism,
calcium homeostasis,
antioxidation, and
cytoskeletal
Reduces calreticulin and calmodulin expressions [136]
Increases peroxiredoxin 2, mitochondrial complex
I and III expressions
3. MPTP (mouse) Mitochondrial
fraction of the
striatum
UPS Attenuates 19S proteasome
ATPase Rpt6 expression
[132]
Augments α-synuclein expression
4. MPTP (mouse) Striatum,
cortex,
cerebellum
Dopamine signaling,
mitochondrial function,
UPS, calcium signaling
Upregulates ubiquitin-specific protease expression [133]
Downregulates cytochrome c oxidase subunit Vic,
vacuolar ATP synthase subunit F, and TH
expressions
5. MPTP (mouse) Mitochondrial
fraction of the
substantia
nigra
Mitochondrial function
and oxidative stress
signaling
Increases DJ-1 expression [137]
Reduces complex I expression
6. MPTP and
methampheta-
mine (mouse)
Striatum Mitochondrial function,
oxidative stress,
signaling, and UPS
Upregulates cytochrome c1, calpain-2,
and ubiquitin ligases expressions
[114]
Downregulates glutathione peroxidase-4
and GST-M5, F- and V-type ATPase
and proteasome subunit protein expressions
7. Maneb and
paraquat (mouse)
Striatum Neurotransmitter release
and glycolysis
Decreases complexin I, α-enolase,
and glia maturation factor-βexpressions
[166]
8. Cypermethrin
(rat)
Striatum and
substantia
nigra
Mitochondrial function,
neurotransmitter release,
and cytoskeletal
assembly
Reduces stathmin, nicotinamide adenine
dinucleotide isocitrate dehydrogenase
α-subunit, prohibitin, ubiquitin conjugating
enzyme, nicotinamide adenine dinucleotide
dehydrogenase 24 k chain precursor, heat shock
protein (Hsp)-70, and synaptosomal associated
protein-25 expressions
[19]
Increases phosphoethanolamine binding protein-1,
mitogen-activated protein kinase activated
kinase-5, Hsp-60, and α-internexin intermediate
filament expressions
Mol Neurobiol

fibers in the striatum, which are used for data generation and
subsequent analysis. Many genes expressed at low levels
and in a particular cell type make the implication of global
study of the gene expression pattern questionable [163].
Therefore, the gene expression patterns of the striatum or
substantia nigra, the ideal tissues used for microarray assess-
ment of rodents with PD phenotype, need an error-free
approach. The disagreement could also be due to the use
of multiple arrays (mitochondrial genes specific, apoptotic
genes specific, cytoskeletal genes specific, etc.), varying
labeling procedures (direct or specific indirect methods),
variable hybridization patterns (time of hybridization, types,
and compositions of hybridization and washing buffers),
and difference in the number and type of replicates (with
or without dye swapping/use of single dye or two different
dyes for controls and experimental sets) [26]. Therefore,
microarray data generated and analyzed from the rodents
across multiple studies would offer invaluable insights, if all
such variables are kept unchanged. Furthermore, the RNA
sample of the striatum or substantia nigra represents RNAs
of multiple neuronal origins (such as dopaminergic, seroto-
ninergic, glutaminergic, cholinergic, etc.); therefore, the
representation of the RNAs of the dopaminergic neurons
are compromised, which makes it complicated to distinguish
[26]. Employing a microarray technique, several novel tran-
scripts involved in PD pathogenesis, and treatment out-
comes are identified [109]. But the transcripts getting
upregulated or downregulated do not necessarily mimic
the level of protein expressions, which are the actual regu-
lators of neurodegeneration, as many genes are transcribed
but not translated [27,114]. Similarly, microarray profiling
of blood offered important clues to assess the role of specific
genes in PD pathogenesis [164]; however, in blood
microarray patterns, several regulating factors could be
compromised. Overall, the identification of causative and
modulatory genes improved our understanding of the under-
lying etiology and mechanisms of pathogenesis, prevention
and cure are perhaps at the same stage as those were before
the application of these tools in PD research. A microarray
technique is not more than a semiquantitative tool even after
considering all the available corrective measures and vali-
dation of data with real-time PCR or other secondary tools is
mandatory [27,111]. Despite significant contribution of
microarray tool in assessing the putative mechanism of the
disease pathogenesis, the technology still fails to provide
accurate and complete mechanism of sporadic PD. Overall,
genomic approaches generated huge amounts of data, the
main drawbacks remain to be elusive etiology and patho-
genesis, as little success could be achieved in translating the
information.
Several groups of investigators have identified a few
differentially expressed proteins, which could not be tested
across multiple laboratories due to lack of expensive tools
and requirement of expert technical skills [124]. As
observed in the clinical investigation [165], reproducibility
across multiple studies could have been a major concern
even in the rodents due to lesser sample size and varying
conditions of animals storage and maintenance as well
underlying experimental procedures [9,21]. One of the most
important reasons of failure is the lack of standard exper-
imental procedures for the collection, storage and process-
ing of samples, and lack of common strategies across
various studies for the removal of the highly abundant
proteins [21,142,166]. Proteomics has not yet delivered
an expected breakthrough in PD diagnosis and identification
of potential and efficacious counteractive measures [21].
The major reasons have been the huge discrepancy in the
translation of rodent proteome data to human and most
importantly difference in the expression patterns of the
identified proteins from one rodent stock to another [102,
167]. The difficulty is more critical in rodents where the
variation could be observed from a rodent to another one as
well as between the inbred and outbred stocks of the same
rodent. Owing to the BBB, many proteins are not able to
enter the blood stream [168] and the proteins identified to
date are the outcomes of pathogenesis rather than the trig-
gering response and therefore cannot serve the purpose of a
genuine biomarker [21]. In summary, proteomic approaches
identified hundreds and hundreds of proteins that are differ-
entially expressed but have not yet satisfactorily met the
goal of developing biomarkers or molecular fingerprints
suitable for real application in clinics.
RNAi elucidates mainly the role of already suspected
genes/proteins in PD and mainly acts as a validation tool
[22]. RNAi could be very successful for monogenic domi-
nant genetic disorders, as only one gene is responsible for
the overall consequence of that disease. But for PD, which
has a multifactorial etiology and elusive pathogenesis, only
combinational approaches, such as silencing of multiple
targets, could possibly help in developing therapy. Similarly,
the drugs developed from the synthetic tiny non-coding
RNA molecules need to be tested for their toxicity, toler-
ability, and undesired off-target effects before they are used
for clinical interventions [22]. Tiny non-coding RNA-
mediated interference elicits immune responses and produ-
ces pro-inflammatory cytokines and interferons that may
affect the disease progression [169–171], which tip off a
caution for its use in treating PD. This technique may get
desirable success beyond laboratory only when the unde-
sired immune responses could be defeated. RNAi tools are
still in their infancy and need to be continuously exploited to
gain certain novel information regarding elusive aspects of
PD pathophysiology.
The exact translation of omics and RNAi-generated data
employing rodent models is not possible because of many
primary reasons. The total number of genes and transcripts,
Mol Neurobiol

translated proteins, and posttranslational modifications
varies from rodent to rodent and from rodent to human
[27,102]. Although these sophisticated tools gave the role
of multiple genes/proteins/transcripts and validate the role
of the most important ones (Fig. 2), lack of consensus
among the results obtained from many studies and their
subsequent translation without fail in humans are limited
owing to lack of an ideal rodent model [102,167]. Many
proteins do not perform the similar functions across multiple
species/genus [102]. Apart from such limitations, there are
many additional hurdles for the application of sophisticated
tools in understanding the disease pathogenesis. Such stud-
ies need to be performed in the multiple rodent models and
postmortem human brains at extensive scale across laborato-
ries, globally. But the drawbacks of the recent molecular tools
are lack of cheap consumables, need of high technical exper-
tise, and requirement of highly specialized and costly equip-
ments [7]. Many more questions are still unresolved even with
the availability of these sophisticated techniques as the rela-
tionship between protein aggregation and the molecular events
leading to neurodegeneration has not yet been clarified [131].
Future Possibilities
Despite extensive efforts, rodent models and molecular tools
could not identify the realistic biomarkers and still face
barricading to pick up the fingerprints for an early diagnosis
and unbiased assessment of treatment outcomes. Although
lower animal models are beneficial to understand the role of
genetic factors in PD, development of only an appropriate
rodent or primate model could help in understanding the
pathogenesis by employing modern molecular tools and
RNAi. Success of a rodent or nonhuman primate model is
purely dependent on the availability of epidemiological
information across multiple populations, followed by appro-
priate statistical analyses. If everything goes all right, an
ideal rodent or nonhuman primate model can be developed.
Once all genuine causative factors would be identified, an
authentic dual model may be developed to understand the
precise pathology of sporadic PD. A combinational
approach of employing an ideal model with multifaceted
omics and RNAi could be expected to offer the pragmatic
mechanistic understanding of PD pathogenesis and success-
fully be translated to sporadic PD. Theoretically, the dual or
interactive model approach would offer the best prospect to
understand sporadic PD pathogenesis; however, practical
conquest lies in the development of a suitable combination
approach.
Acknowledgments We sincerely thank the Council of Scientific and
Industrial Research (CSIR), New Delhi, India/University Grants Com-
mission, New Delhi, India for providing research fellowships to
Sharawan Yadav, Sonal Agrawal, Garima Srivastava, Anand Kumar
Singh, and Anubhuti Dixit. The CSIR-IITR communication number of
the article is 3012.
Conflicts of Interest None declared.
References
1. Parkinson J (2002) An essay on the shaking palsy 1817. J Neuro-
psychiatry Clin Neurosci 14:223–236
2. Goetz CG (1986) Charcot on Parkinson’s disease. Mov Disord
1:27–32
3. Singh MP, Patel S, Dikshit M, Gupta YK (2006) Contribution of
genomics and proteomics in understanding the role of modifying
factors in Parkinson’s disease. Indian J Biochem Biophys 43:69–81
4. Miller RL, James-Kracke M, Sun GY, Sun AY (2009) Oxidative
and inflammatory pathways in Parkinson’s disease. Neurochem
Res 34:55–65
5. Wirdefeldt K, Adami HO, Cole P, Trichopoulos D, Mandel J
(2011) Epidemiology and etiology of Parkinson’sdisease:a
review of the evidence. Eur J Epidemiol 26:S1–S58
6. Klockgether T (2004) Parkinson’s disease: clinical aspects. Cell
Tissue Res 318:115–120
7. Morgan JC, Mehta SH, Sethi KD (2010) Biomarkers in Parkin-
son’s disease. Curr Neurol Neurosci Rep 10:423–430
8. Ebin J (1951) Surgical treatment of Parkinsonism: indications and
results. Bull N Y Acad Med 27:653–678
9. Cicchetti F, Drouin-Ouellet J, Gross RE (2009) Environmental
toxins and Parkinson’s disease: what have we learned from
pesticide-induced animal models? Trends Pharmacol Sci
30:475–483
10. Singhal NK, Srivastava G, Agrawal S, Jain SK, Singh MP (2012)
Melatonin as a neuroprotective agent in the rodent models of
Parkinson’s disease: is it all set to irrefutable clinical translation?
Mol Neurobiol 45:186–199
11. Kumar A, Ahmad I, Shukla S, Singh BK, Patel DK, Pandey HP,
Singh C (2010) Effect of zinc and paraquat co-exposure on
neurodegeneration: modulation of oxidative stress and expression
of metallothioneins, toxicant responsive and transporter genes in
rats. Free Radic Res 44:950–965
12. Khalid M, Aoun RA, Mathews TA (2011) Altered striatal dopa-
mine release following a sub-acute exposure to manganese. J
Neurosci Methods 202:182–191
13. Singh AK, Tiwari MN, Upadhyay G, Patel DK, Singh D, Prakash
O, Singh MP (2012) Long-term exposure to cypermethrin indu-
ces nigrostriatal dopaminergic neurodegeneration in adult rats:
postnatal exposure enhances the susceptibility during adulthood.
Neurobiol Aging 33:404–415
14. Betarbet R, Sherer TB, Greenamyre JT (2002) Animal models of
Parkinson’s disease. Bioessays 24:308–318
15. Thiruchelvam M, Brockel BJ, Richfield EK, Baggs RB, Cory-
Slechta DA (2000) Potentiated and preferential effects of com-
bined paraquat and maneb on nigrostriatal dopamine systems:
environmental risk factors for Parkinson’s disease? Brain Res
873:225–234
16. Langston JW, Ballard P, Tetrud JW, Irwin I (1983) Chronic
parkinsonism in humans due to a product of meperidine-analog
synthesis. Science 219:979–980
17. Patel S, Singh V, Kumar A, Gupta YK, Singh MP (2006) Status of
antioxidant defense system and expression of toxicant responsive
genesin striatum of maneband paraquat-induced Parkinson’sdisease
phenotype in mouse: mechanism of neurodegeneration. Brain Res
108:9–18
Mol Neurobiol

18. Gorell JM, Johnson CC, Rybicki BA, Peterson EL, Kortsha GX,
Brown GG, Richardson RJ (1999) Occupational exposure to
manganese, copper, lead, iron, mercury and zinc and the risk of
Parkinson’s disease. Neurotoxicology 20:239–247
19. Singh AK, Tiwari MN, Dixit A, Upadhyay G, Patel DK, Singh D,
Prakash O, Singh MP (2011) Nigrostriatal proteomics of
cypermethrin-induced dopaminergic neurodegeneration: micro-
glial activation dependent and independent regulations. Toxicol
Sci 122:526–538
20. Srivastava G, Dixit A, Yadav S, Patel DK, Prakash O, Singh MP
(2012) Resveratrol potentiates cytochrome P450 2d22-mediated
neuroprotection in maneb- and paraquat-induced parkinsonism in
the mouse. Free Radic Biol Med 52:1294–1306
21. Srivastava G, Singh K, Tiwari MN, Singh MP (2010) Proteomics
in Parkinson’s disease: current trends, translational snags and
future possibilities. Expert Rev Proteomics 7:127–139
22. Srivastava G, Dixit A, Prakash O, Singh MP (2011) Tiny non-
coding RNAs in Parkinson’s disease: Implications, expectations
and hypes. Neurochem Int 59:759–769
23. Klein C, Schlossmacher MG (2007) Parkinson disease, 10 years
after its genetic revolution: multiple clues to a complex disorder.
Neurology 69:2093–2104
24. Gao HM, Hong JS (2011) Gene–environment interactions: key to
unravelingthemysteryofParkinson’sdisease.ProgNeurobiol94:1–19
25. Uversky VN (2004) Neurotoxicant-induced animal models of
Parkinson’s disease: understanding the role of rotenone, maneb
and paraquat in neurodegeneration. Cell Tissue Res 318:225–241
26. Ginsberg SD, Mirnics K (2006) Functional genomic methodolo-
gies. Prog Brain Res 158:15–40
27. Shilling PD, Kelsoe JR (2002) Functional genomics approaches
to understanding brain disorders. Pharmacogenomics 3:31–45
28. Vargas RH, Ornelas LF, González IL, Escovar JR, Zurita M,
Reynaud E (2011) Synphilin suppresses α-synuclein neurotoxicity
in a Parkinson’sdiseaseDrosophila model. Genesis 49:392–402
29. Ramsden DB, Parsons RB, Ho SL, Waring RH (2001) The aetiol-
ogy of idiopathic Parkinson’s disease. Mol Pathol 54:369–380
30. Migliore L, Coppede F (2009) Environmental-induced oxidative
stress in neurodegenerative disorders and aging. Mutat Res
674:73–84
31. Wood-Kaczmar A, Gandhi S, Wood NW (2006) Understanding
the molecular causes of Parkinson’s disease. Trends Mol Med
12:521–528
32. Bove J, Prou D, Perier C, Przedborski S (2005) Toxin-induced
models of Parkinson’s disease. NeuroRx 2:484–494
33. Ascherio A, Chen H, Weisskopf MG, O’Reilly E, McCullough ML,
Calle EE, Schwarzschild MA, Thun MJ (2006) Pesticide exposure
and risk for Parkinson’sdisease. Ann Neurol 60:197–203
34. He Y, Appel S, Le W (2001) Minocycline inhibits microglial
activation and protects nigral cells after 6-hydroxydopamine
injection into mouse striatum. Brain Res 909:187–193
35. Silva RM, Ries V, Oo TF, Yarygina O,Jackson-Lewis V, Ryu EJ, Lu
PD, Marciniak SJ, Ron D, Przedborski S, Kholodilov N, Greene
LA, Burke RE (2005) CHOP/GADD153 is a mediator of apoptotic
death in substantia nigra dopamine neurons in an in vivo neurotoxin
model of parkinsonism. J Neurochem 95:974–986
36. Reyes S, Mitrofanis J (2008) Patterns of FOS expression in the
spinal cord and periaqueductal grey matter of 6OHDA-lesioned
rats. Int J Neurosci 118:1053–1079
37. Latchoumycandane C, Anantharam V, Jin H, Kanthasamy A,
Kanthasamy A (2011) Dopaminergic neurotoxicant (2011) 6-
OHDA induces oxidative damage through proteolytic activation
of PKCδin cell culture and animal models of Parkinson’s disease.
Toxicol Appl Pharmacol 256:314–323
38. Bernstein AI, Garrison SP, Zambetti GP, O’Malley KL (2011) 6-
OHDA generated ROS induces DNA damage and p53- and
PUMA-dependent cell death. Mol Neurodegener 6:2
39. Proft J, Faraji J, Robbins JC, Zucchi FC, Zhao X, Metz GA,
Braun JE (2011) Identification of bilateral changes in TID1
expression in the 6-OHDA rat model of Parkinson’s disease.
PLoS One 6:e26045
40. Przedborski S, Vila M (2003) The 1-methyl-4-phenyl-1,2,3,6-
tetrahydropyridine mouse model: a tool to explore the patho-
genesis of Parkinson’s disease. Ann N Y Acad Sci 991:189–198
41. Ross CA, Smith WW (2007) Gene-environment interactions in
Parkinson’s disease. Parkinsonism Relat Disord 13:S309–S315
42. Cui M, Aras R, Christian WV, Rappold PM, Hatwar M, Panza J,
Jackson-Lewis V, Javitch JA, Ballatori N, Przedborski S, Tieu K
(2009) The organic cation transporter-3 is a pivotal modulator of
neurodegeneration in the nigrostriatal dopaminergic pathway.
Proc Natl Acad Sci U S A 106:8043–8048
43. Karunakaran S, Saeed U, Mishra M, Valli RK, Joshi SD, Meka
DP, Seth P, Ravindranath V (2008) Selective activation of p38
mitogen-activated protein kinase in dopaminergic neurons of
substantia nigra leads to nuclear translocation of p53 in 1-
methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated mice. J Neu-
rosci 28:12500–12509
44. Castro-Caldas M, Carvalho AN, Rodrigues E, Henderson C, Wolf
CR, Gama MJ (2012) Glutathione S-transferase pi mediates MPTP-
induced c-Jun N-terminal kinase activation in the nigrostriatal path-
way. Mol Neurobiol. doi:10.1007/s12035-012-8266-9
45. Zawada WM, Banninger GP, Thornton J, Marriott B, Cantu D,
Rachubinski AL, Das M, Griffin WS, Jones SM (2011) Generation
of reactive oxygen species in 1-methyl-4-phenylpyridinium (MPP
+) treated dopaminergic neurons occurs as an NADPH oxidase-
dependent two-wave cascade. J Neuroinflammation 8:129
46. Durgadoss L, Nidadavolu P, Valli RK, Saeed U, Mishra M, Seth
P, Ravindranath V (2012) Redox modification of Akt mediated
by the dopaminergic neurotoxin MPTP, in mouse midbrain, leads
to down-regulation of pAkt. FASEB J 26:1473–1483
47. Jeong HJ, Kim DW, Woo SJ, Kim HR, Kim SM, Jo HS, Park M,
Kim DS, Kwon OS, Hwang IK, Han KH, Park J, Eum WS, Choi
SY (2012) Transduced Tat-DJ-1 protein protects against oxida-
tive stress-induced SH-SY5Y cell death and Parkinson disease in
a mouse model. Mol Cells. doi:10.1007/s10059-012-2255-8
48. Martin HL, Mounsey RB, Mustafa S, Sathe K, Teismann P
(2012) Pharmacological manipulation of peroxisome
proliferator-activated receptor γ(PPARγ) reveals a role for
anti-oxidant protection in a model of Parkinson’s disease. Exp
Neurol 235:528–538
49. Park HK, Cho AR, Lee SC, Ban JY (2012) MPTP-induced model
of Parkinson’s disease in heat shock protein 70.1 knockout mice.
Mol Med Report 5:1465–1468
50. Domenger D, Dea D, Theroux L, Moquin L, Gratton A, Poirier J
(2012) The MPTP neurotoxic lesion model of Parkinson’s disease
activates the apolipoprotein E cascade in the mouse brain. Exp
Neurol 233:513–522
51. Airavaara M, Harvey BK, Voutilainen MH, Shen H, Chou J,
Lindholm P, Lindahl M, Tuominen RK, Saarma M, Wang Y,
Hoffer B (2011) CDNF protects the nigrostriatal dopamine sys-
tem and promotes recovery after MPTP treatment in mice. Cell
Transplant. doi:10.3727/096368911X600948
52. Bezard E, Przedborski S (2011) A tale on animal models of
Parkinson’s disease. Mov Disord 26:993–1002
53. LoPachin RM, Gavin T (2008) Response to “Paraquat: the red
herring of Parkinson’s disease research”. Toxicol Sci 103:219–221
54. Shimizu K, Ohtaki K, Matsubara K, Aoyama K, Uezono T, Saito
O, Suno M, Ogawa K, Hayase N, Kimura K, Shiono H (2001)
Carrier-mediated processes in blood–brain barrier penetration and
neural uptake of paraquat. Brain Res 906:135–142
55. Feng LR, Maguire-Zeiss KA (2011) Dopamine and paraquat
enhance α-synuclein-induced alterations in membrane conduc-
tance. Neurotox Res 20:387–401
Mol Neurobiol

56. Miller RL, Sun GY, Sun AY (2007) Cytotoxicity of paraquat in
microglial cells: involvement of PKCdelta- and ERK1/2-dependent
NADPH oxidase. Brain Res 1167:129–139
57. Huang CL, Lee YC, Yang YC, Kuo TY, Huang NK (2012)
Minocycline prevents paraquat-induced cell death through
attenuating endoplasmic reticulum stress and mitochondrial dys-
function. Toxicol Lett 209:203–210
58. Rappold PM, Cui M, Chesser AS, Tibbett J, Grima JC, Duan L,
SenN,JavitchJA,TieuK(2011) Paraquat neurotoxicity is
mediated by the dopamine transporter and organic cation
transporter-3. Proc Natl Acad Sci U S A 108:20766–20771
59. Fei Q, Ethell DW (2008) Maneb potentiates paraquat neurotox-
icity by inducing key Bcl-2 family members. J Neurochem
105:2091–2097
60. Litteljohn D, Nelson E, Bethune C, Hayley S (2011) The effects
of paraquat on regional brain neurotransmitter activity, hippo-
campal BDNF and behavioural function in female mice. Neurosci
Lett 502:186–191
61. Kachroo A, Irizarry MC, Schwarzschild MA (2010) Caffeine
protects against combined paraquat and maneb-induced dopami-
nergic neuron degeneration. Exp Neurol 223:657–661
62. Wang A, Costello S, Cockburn M, Zhang X, Bronstein J, Ritz B
(2011) Parkinson’s disease risk from ambient exposure to pesti-
cides. Eur J Epidemiol 26:547–555
63. Kang H, Han BS, Kim SJ, Oh YJ (2012) Mechanisms to prevent
caspase activation in rotenone-induced dopaminergic neurode-
generation: role of ATP depletion and procaspase-9 degradation.
Apoptosis 17:449–462
64. Shin EJ, Kim EM, Lee JA, Rhim H, Hwang O (2012) Matrix
metalloproteinase-3 is activated by HtrA2/Omi in dopaminergic
cells: relevance to Parkinson’s disease. Neurochem Int 60:249–256
65. Meredith GE, Sonsalla PK, Chesselet MF (2008) Animal models of
Parkinson’s disease progression. Acta Neuropathol 115:385–398
66. Ritz BR, Manthripragada AD, Costello S, Lincoln SJ, Farrer MJ,
Cockburn M, Bronstein J (2009) Dopamine transporter genetic
variants and pesticides in Parkinson’s disease. Environ Health Per-
spect 117:964–969
67. Moretto A, Colosio C (2011) Biochemical and toxicological
evidence of neurological effects of pesticides: the example of
Parkinson’s disease. Neurotoxicology 32:383–391
68. Singh AK, Tiwari MN, Prakash O, Singh MP (2012) A current
review of cypermethrin-induced neurotoxicity and nigrostriatal
dopaminergic neurodegeneration. Curr Neuropharmacol 10:64–71
69. Sharma H, Zhang P, Barber DS, Liu B (2010) Organochlorine
pesticides dieldrin and lindane induce cooperative toxicity in
dopaminergic neurons: role of oxidative stress. Neurotoxicology
31:215–222
70. Cass W (1996) GDNF selectively protects dopamine neurons
over serotonin neurons against the neurotoxic effects of metham-
phetamine. J Neurosci 16:8132–8139
71. Howard CD, Keefe KA, Garris PA, Daberkow DP (2011) Meth-
amphetamine neurotoxicity decreases phasic, but not tonic, dopa-
minergic signaling in the rat striatum. J Neurochem 118:668–676
72. Morrow BA, Roth RH, Redmond DE, Elsworth JD (2011) Impact
of methamphetamine on dopamine neurons in primates is
dependent on age: implications for development of Parkinson’s
disease. Neuroscience 189:277–285
73. Callaghan RC, Cunningham JK, Sykes J, Kish SJ (2012)
IncreasedriskofParkinson’s disease in individuals hospital-
ized with conditions related to the use of methamphetamine
or other amphetamine-type drugs. Drug Alcohol Depend
120:35–40
74. Kousik SM, Graves SM, Napier TC, Zhao C, Carvey PM
(2011) Methamphetamine-induced vascular changes lead to
striatal hypoxia and dopamine reduction. Neuroreport
22:923–928
75. Brar S, Henderson D, Schenck J, Zimmerman EA (2009) Iron
accumulation in the substantia nigra of patients with Alzheimer
disease and parkinsonism. Arch Neurol 66:371–374
76. Jomova K, Valko M (2011) Advances in metal-induced oxidative
stress and human disease. Toxicology 283:65–87
77. Kienzl E, Puchinger L, Jellinger K, Linert W, Stachelberger H,
Jameson RF (1995) The role of transition metals in the patho-
genesis of Parkinson’s disease. J Neurol Sci 134:69–78
78. Kumar A, Singh BK, Ahmad I, Shukla S, Patel DK, Srivastava G,
Kumar V, Pandey HP, Singh C (2012) Involvement of NADPH
oxidase and glutathione in zinc-induced dopaminergic neurode-
generation in rats: similarity with paraquat neurotoxicity. Brain
Res 1438:48–64
79. Gash DM, Rutland K, Hudson NL, Sullivan PG, Bing G, Cass
WA, Pandya JD, Liu M, Choi DY, Hunter RL, Gerhardt GA,
Smith CD, Slevin JT, Prince TS (2008) Trichloroethylene: par-
kinsonism and complex 1 mitochondrial neurotoxicity. Ann Neu-
rol 63:184–192
80. Holtcamp W (2012) The emerging science of BMAA: do cyano-
bacteria contribute to neurodegenerative disease? Environ Health
Perspect 120:A110–A116
81. Lannuzel A, Michel PP, Höglinger GU, Champy P, Jousset A,
Medja F, Lombès A, Darios F, Gleye C, Laurens A, Hocquemiller
R, Hirsch EC, Ruberg M (2003) The mitochondrial complex I
inhibitor annonacin is toxic to mesencephalic dopaminergic neu-
rons by impairment of energy metabolism. Neuroscience
121:287–296
82. Storch A, Ott S, Hwang YI, Ortmann R, Hein A, Frenzel S,
Matsubara K, Ohta S, Wolf HU, Schwarz J (2002) Selective
dopaminergic neurotoxicity of isoquinoline derivatives related
to Parkinson’s disease: studies using heterologous expression
systems of the dopamine transporter. Biochem Pharmacol
63:909–920
83. Ohta S, Tachikawa O, Makino Y, Tasaki Y, Hirobe M (1990)
Metabolism and brain accumulation of tetrahydroisoquinoline
(TIQ) a possible parkinsonism inducing substance, in an animal
model of a poor debrisoquine metabolizer. Life Sci 46:599–605
84. Mohajjel Nayebi AA, Sheidaei H (2010) Buspirone improves
haloperidol-induced Parkinson disease in mice through 5-HT
(1A) recaptors. Daru 18:41–45
85. McNaught KS, Perl DP, Brownell AL, Olanow CW (2004) Sys-
temic exposure to proteasome inhibitors causes a progressive
model of Parkinson’s disease. Ann Neurol 56:149–162
86. Panneton WM, Kumar VB, Gan Q, Burke WJ, Galvin JE (2010)
The neurotoxicity of DOPAL: behavioral and stereological evi-
dence for its role in Parkinson disease pathogenesis. PLoS One 5:
e15251
87. Gasser T (2007) Update on the genetics of Parkinson’s disease.
Mov Disord 17:S343–S350
88. Bekris LM, Mata IF, Zabetian CP (2010) The genetics of Parkin-
son disease. J Geriatr Psychiatry Neurol 23:228–242
89. Fujioka S, Wszolek ZK (2012) Update on genetics of parkinson-
ism. Neurodegener Dis 10:257–260
90. Terzioglu M, Galter D (2008) Parkinson’s disease: genetic versus
toxin-induced rodent models. FEBS J 275:1384–1391
91. Hisahara S, Shimohama S (2010) Toxin-induced and genetic
animal models of Parkinson’s disease. Parkinsons Dis
2011:951709
92. Manning-Bog AB, Langston JW (2007) Model fusion, the next
phase in developing animal models for Parkinson’sdisease. Neu-
rotox Res 11:219–240
93. Boger HA, Granholm AC, McGinty JF, Middaugh LD (2010) A
dual-hit animal model for age-related Parkinsonism. Prog Neuro-
biol 90:217–229
94. Gao HM, Zhang F, Zhou H, Kam W, Wilson B, Hong JS (2011)
Neuroinflammation and α-synuclein dysfunction potentiate each
Mol Neurobiol

other, drivingchronic progression of neurodegeneration in a
mouse model of Parkinson’s disease. Environ Health Perspect
119:807–814
95. Jakowec MW, Petzinger GM (2004) 1-methyl-4-phenyl-1,2,3,6-
tetrahydropyridine-lesioned model of Parkinson’s disease, with
emphasis on mice and nonhuman primates. Comp Med 54:497–513
96. Zhu XR, Maskri L, Herold C, Bader V, Stichel CC, Güntürkün O,
Lübbert H (2007) Non-motor behavioural impairments in parkin-
deficient mice. Eur J Neurosci 26:1902–1911
97. Emborg ME (2007) Nonhuman primate models of Parkinson’s
disease. ILAR J 48:339–355
98. Duty S, Jenner P (2011) Animal models of Parkinson’s disease: a
source of novel treatments and clues to the cause of the disease.
Br J Pharmacol 164:1357–1391
99. Pienaar IS, Götz J, Feany MB (2010) Parkinson’sdisease:
insights from non-traditional model organisms. Prog Neurobiol
92:558–571
100. Antony PM, Diederich NJ, Balling R (2011) Parkinson’s disease
mouse models in translational research. Mamm Genome 22:401–
419
101. Chesselet MF, Richter F (2011) Modelling of Parkinson’s disease
in mice. Lancet Neurol 10:1108–1118
102. Potashkin JA, Blume SR, Runkle NK (2010) Limitations of
animal models of Parkinson’s disease. Parkinsons Dis
2011:658083
103. Mellick GD, Silburn PA, Sutherland GT, Siebert GA (2010)
Exploiting the potential of molecular profiling in Parkinson’s
disease: current practice and future probabilities. Expert Rev
Mol Diagn 10:1035–1050
104. Miller RM, Federoff HJ (2006) Microarrays in Parkinson’s dis-
ease: a systematic approach. NeuroRx 3:319–326
105. Chang XL, Mao XY, Li HH, Zhang JH, Li NN, Burgunder JM,
Peng R, Tan EK (2011) Association of GWAS loci with PD in
China. Am J Med Genet B Neuropsychiatr Genet 156B:334–339
106. Biernacka JM, Armasu SM, Cunningham JM, Eric Ahlskog J,
Chung SJ, Maraganore DM (2011) Do interactions between
SNCA, MAPT, and LRRK2 genes contribute to Parkinson’s
disease susceptibility? Parkinsonism Relat Disord 17:730–736
107. Chung SJ, Armasu SM, Biernacka JM, Lesnick TG, Rider DN,
Lincoln SJ, Ortolaza AI, Farrer MJ, Cunningham JM, Rocca WA,
Maraganore DM (2011) Common variants in PARK loci and
related genes and Parkinson’s disease. Mov Disord 26:280–288
108. Autere J, Moilanen JS, Finnila S, Soininen H, Mannermaa A,
Hartikainen P, Hallikainen M, Majamaa K (2004) Mitochondrial
DNA polymorphisms as risk factors for Parkinson’s disease and
Parkinson’s disease dementia. Hum Genet 115:29–35
109. Simunovic F, Yi M, Wang Y, Macey L, Brown LT, Krichevsky
AM, Andersen SL, Stephens RM, Benes FM, Sonntag KC (2009)
Gene expression profiling of substantia nigra dopamine neurons:
further insights into Parkinson’s disease pathology. Brain
132:1795–1809
110. Grunblatt E, Mandel S, Jacob-Hirsch J, Zeligson S, Amariglo N,
Rechavi G, Li J, Ravid R, Roggendorf W, Riederer P, Youdim
MB (2004) Gene expression profiling of parkinsonian substantia
nigra pars compacta alterations in ubiquitin-proteasome, heat
shock protein, iron and oxidative stress regulated proteins, cell
adhesion/cellular matrix and vesicle trafficking genes. J Neural
Transm 111:1543–1573
111. Patel S, Singh K, SinghS, Singh MP (2008) Gene expression profiles
of mouse striatum in control and maneb+ paraquat-induced Parkin-
son’s disease phenotype: validation of differentially expressedenergy
metabolizing transcripts. Mol Biotechnol 40:59–68
112. Duke DC, Moran LB, Kalaitzakis ME, Deprez M, Dexter DT,
Pearce RK, Graeber MB (2006) Transcriptome analysis reveals
link between proteasomal and mitochondrial pathways in Parkin-
son’s disease. Neurogenetics 7:139–148
113. Miller RM, Kiser GL, Kaysser-Kranich TM (2006) Robust dys-
regulation of gene expression in substantia nigra and striatum in
Parkinson’s disease. Neurobiol Dis 21:305–313
114. Chin MH, Qian WJ, Wang H, Petyuk VA, Bloom JS, Sforza DM,
Laćan G, Liu D, Khan AH, Cantor RM, Bigelow DJ, Melega WP,
Camp DG 2nd, Smith RD, Smith DJ (2008) Mitochondrial dys-
function, oxidative stress, and apoptosis revealed by proteomic
and transcriptomic analyses of the striata in two mouse models of
Parkinson’s disease. J Proteome Res 7:666–677
115. Cadet JL, Brannock C, Krasnova IN, Ladenheim B, McCoy MT,
Chou J, Lehrmann E, Wood WH, Becker KG, Wang Y (2010)
Methamphetamine-induced dopamine-independent alterations in
striatal gene expression in the 6-hydroxydopamine hemiparkin-
sonian rats. PLoS One 5:e15643
116. Pickrell AM, Fukui H, Wang X, Pinto M, Moraes CT (2011) The
striatum is highly susceptible to mitochondrial oxidative phos-
phorylation dysfunctions. J Neurosci 631:9895–9904
117. Grünblatt E (2012) Parkinson’s disease: molecular risk factors.
Parkinsonism Relat Disord 18(Suppl 1):S45–S48
118. Wernicke C, Hellmann J, Zieba B, Kuter K, Ossowska K, Frenzel
M, Dencher NA, Rommelspacher H (2010) 9-Methyl-beta-carbo-
line has restorative effects in an animal model of Parkinson’s
disease. Pharmacol Rep 62:35–53
119. Pattarini R, Rong Y, Qu C, Morgan JI (2008) Distinct mecha-
nisms of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyrimidine resist-
ance revealed by transcriptome mapping in mouse striatum.
Neuroscience 155:1174–1194
120. Chung CY, Seo H, Sonntag KC, Brooks A, Lin L, Isacson O
(2005) Cell type-specific gene expression of midbrain dopami-
nergic neurons reveals molecules involved in their vulnerability
and protection. Hum Mol Genet 14:1709–1725
121. Mandel S, Grunblatt E, Maor G, Youdim MB (2002) Early and
late gene changes in MPTP mice model of Parkinson’s disease
employing cDNA microarray. Neurochem Res 27:1231–1243
122. Grunblatt E, Mandel S, Maor G, Youdim MB (2001) Gene
expression analysis in N-methyl-4-phenyl-1, 2, 3, 6-
tetrahydropyridine mice model of Parkinson’s disease using
cDNA microarray: effect of R-apomorphine. J Neurochem
78:1–12
123. Tomita H, Vawter MP, Walsh DM, Evans SJ, Choudary PV, Li J,
Overman KM, Atz ME, Myers RM, Jones EG, Watson SJ, Akil
H, Bunney WE Jr (2004) Effect of agonal and postmortem factors
on gene expression profile: quality control in microarray analyses
of postmortem human brain. Biol Psychiatry 55:346–352
124. Mischak H, Apweiler R, Banks RE, Conaway M, Coon J,
Dominiczak A, Ehrich JH, Fliser D, Girolami M, Hermjakob H,
Hochstrasser D, Jankowski J, Julian BA, Kolch W, Massy ZA,
Neusuess C, Novak J, Peter K, Rossing K, Schanstra J, Semmes
OJ, Theodorescu D, Thongboonkerd V, Weissinger EM, Van Eyk
JE, Yamamoto T (2007) Clinical proteomics: a need to define the
field and to begin to set adequate standards. Proteomics Clin
Appl 1:148–156
125. Zhang J, Keene CD, Pan C, Montine KS, Montine TJ (2008)
Proteomics of human neurodegenerative diseases. J Neuropathol
Exp Neurol 67:923–932
126. Pienaar IS, Daniels WM, Gotz J (2008) Neuroproteomics as a
promising tool in Parkinson’s disease research. J Neural Transm
115:1413–1430
127. Butler GS, Overall CM (2009) Proteomic identification of multi-
tasking proteins in unexpected locations complicates drug target-
ing. Nat Rev Drug Discov 8:935–948
128. Farley AR, Link AJ (2009) Identification and quantification of
protein posttranslational modifications. Methods Enzymol
463:725–763
129. Hwang H, Zhang J, Chung KA, Leverenz JB, Zabetian CP,
Peskind ER, Jankovic J, Su Z, Hancock AM, Pan C, Montine
Mol Neurobiol

TJ, Pan S, Nutt J, Albin R, Gearing M, Beyer RP, Shi M, Zhang J
(2010) Glycoproteomics in neurodegenerative diseases. Mass
Spectrom Rev 29:79–125
130. Zhang J, Goodlett DR (2004) Proteomic approach to studying
Parkinson’s disease. Mol Neurobiol 29:271–288
131. Robinson PA (2010) Understanding the molecular basis of Par-
kinson’s disease, identification of biomarkers and routes to ther-
apy. Expert Rev Proteomics 7:565–578
132. Liu B, Shi Q, Ma S, Feng N, Li J, Wang L, Wang X (2008)
Striatal 19S Rpt6 deficit is related to alpha-synuclein accumula-
tion in MPTP-treated mice. Biochem Biophys Res Commun
376:277–282
133. Zhang X, Zhou JY, Chin MH, Schepmoes AA, Petyuk VA, Weitz
KK, Petritis BO, Monroe ME, Camp DG, Wood SA, Melega WP,
Bigelow DJ, Smith DJ, Qian WJ, Smith RD (2010) Region-
specific protein abundance changes in the brain of MPTP-
induced Parkinson’s disease mouse model. J Proteome Res
9:1496–1509
134. Jin J, Hulette C, Wang Y (2006) Proteomic identification of a
stress protein, mortalin/mthsp70/GRP75: relevance to Parkinson
disease. Mol Cell Proteomics 5:1193–1204
135. Park B, Yang J, Yun N, Choe KM, Jin BK, Oh YJ (2010)
Proteomic analysis of expression and protein interactions in a 6-
hydroxydopamine-induced rat brain lesion model. Neurochem Int
57:16–32
136. Lessner G, Schmitt O, Haas SJ, Mikkat S, Kreutzer M, Wree A,
Glocker MO (2010) Differential proteome of the striatum from
hemiparkinsonian rats displays vivid structural remodeling pro-
cesses. J Proteome Res 9:4671–4687
137. Jin J, Meredith GE, Chen L, Zhou Y, Xu J, Shie FS, Lockhart P,
Zhang J (2005) Quantitative proteomic analysis of mitochondrial
proteins: relevance to Lewy body formation and Parkinson’s
disease. Brain Res Mol Brain Res 134:119–138
138. Shi M, Caudle WM, Zhang J (2009) Biomarker discovery in
neurodegenerative diseases: a proteomic approach. Neurobiol
Dis 35:157–164
139. Goldknopf IL (2008) Blood-based proteomics for personalized
medicine: examples from neurodegenerative disease. Expert Rev
Proteomics 5:1–8
140. Sinha A, Srivastava N, Singh S, Singh AK, Bhushan S, Shukla R,
Singh MP (2009) Identification of differentially displayed pro-
teins in cerebrospinal fluid of Parkinson’s disease patients: a
proteomic approach. Clin Chim Acta 400:14–20
141. van Dijk KD, Teunissen CE, Drukarch B, Jimenez CR,
Groenewegen HJ, Berendse HW, van de Berg WD (2010) Diag-
nostic cerebrospinal fluid biomarkers for Parkinson’s disease: a
pathogenetically based approach. Neurobiol Dis 39:229–241
142. Tribl F, Marcus K, Bringmann G, Meyer HE, Gerlach M, Riederer P
(2006) Proteomics of the human brain: sub-proteomes might hold
the key to handle brain complexity. J Neural Transm 113:1041–
1054
143. Sinha A, Patel S, Singh MP, Shukla R (2007) Blood proteome
profiling in case controls and Parkinson’s disease patients in
Indian population. Clin Chim Acta 380:232–234
144. Chen HM, Lin CY, Wang V (2011) Amyloid P component as a
plasma marker for Parkinson’s disease identified by a proteomic
approach. Clin Biochem 44:377–385
145. Waragai M, Wei J, Fujita M, Nakai M, Ho GJ, Masliah E, Akatsu
H, Yamada T, Hashimoto M (2006) Increased level of DJ-1 in the
cerebrospinal fluids of sporadic Parkinson’s disease. Biochem
Biophys Res Commun 345:967–972
146. Muntané G, Dalfó E, Martinez A, Ferrer I (2008) Phosphoryla-
tion of tau and alpha-synuclein in synaptic-enriched fractions of
the frontal cortex in Alzheimer’s disease, and in Parkinson’s
disease and related alpha-synucleinopathies. Neuroscience
152:913–923
147. Choi J, Levey AI, Weintraub ST, Rees HD, Gearing M, Chin LS,
Li L (2004) Oxidative modifications and down-regulation of
ubiquitin carboxyl-terminal hydrolase L1 associated with idio-
pathic Parkinson’sandAlzheimer’s diseases. J Biol Chem
279:13256–13264
148. Salahpour A, Medvedev IO, Beaulieu JM, Gainetdinov RR,
Caron MG (2007) Local knockdown of genes in the brain using
small interfering RNA: a phenotypic comparison with knockout
animals. Biol Psychiatry 61:65–69
149. Ko HS, Bailey R, Smith WW, Liu Z, Shin JH, Lee YI, Zhang YJ,
Jiang H, Ross CA, Moore DJ, Patterson C, Petrucelli L, Dawson
TM, Dawson VL (2009) CHIP regulates leucine-rich repeat
kinase-2 ubiquitination, degradation, and toxicity. Proc Natl Acad
Sci U S A 106:2897–2902
150. Ko HS, Lee Y, Shin JH, Karuppagounder SS, Gadad BS, Koleske
AJ, Pletnikova O, Troncoso JC, Dawson VL, Dawson TM (2010)
Phosphorylation by the c-Abl protein tyrosine kinase inhibits
parkin’s ubiquitination and protective function. Proc Natl Acad
Sci U S A 107:16691–16696
151. Cai J, Donaldson A, Yang M, German MS, Enikolopov G,
Iacovitti L (2009) The role of Lmx1a in the differentiation of
human embryonic stem cells into midbrain dopamine neurons in
culture and after transplantation into a Parkinson’s disease model.
Stem Cells 27:220–229
152. Kim J, Inoue K, Ishii J, Vanti WB, Voronov SV, Murchison E,
Hannon G, Abeliovich A (2007) A MicroRNA feedback circuit
in midbrain dopamine neurons. Science 317:1220–1224
153. Nelson PT, Wang WX, Rajeev BW (2008) MicroRNAs (miR-
NAs) in neurodegenerative diseases. Brain Pathol 18:130–138
154. Hebert SS, De Strooper B (2009) Alterations of the microRNA
network cause neurodegenerative disease. Trends Neurosci
32:199–206
155. Miñones-Moyano E, Porta S, Escaramís G, Rabionet R, Iraola S,
Kagerbauer B, Espinosa-Parrilla Y, Ferrer I, Estivill X, Martí E
(2011) MicroRNA profiling of Parkinson’s disease brains identi-
fies early downregulation of miR-34b/c which modulate mito-
chondrial function. Hum Mol Genet 20:3067–3078
156. Greco SJ, Rameshwar P (2007) MicroRNAs regulate synthesis of
the neurotransmitter substance P in human mesenchymal stem
cell-derived neuronal cells. Proc Natl Acad Sci U S A
104:15484–15489
157. Ziviani E, Tao RN, Whitworth AJ (2010) Drosophila parkin
requires PINK1 for mitochondrial translocation and ubiquitinates
mitofusin. Proc Natl Acad Sci USA 107:5018–5023
158. Geisler S, Holmstrom KM, Skuja D, Fiesel FC, Rothfuss OC,
Kahle PJ, Springer W (2010) PINK1/Parkin-mediated mitophagy
is dependent on VDAC1 and p62/SQSTM1. Nat Cell Biol
12:119–131
159. Scherzer CR, Grass JA, Liao Z, Pepivani I, Zheng B, Eklund AC,
Ney PA, Ng J, McGoldrick M, Mollenhauer B, Bresnick EH,
Schlossmacher MG (2008) GATA transcription factors directly
regulate the Parkinson’s disease-linked gene alpha-synuclein.
Proc Natl Acad Sci U S A 105:10907–10912
160. Doxakis E (2010) Post-transcriptional regulation of alpha-
synuclein expression by mir-7 and mir-153. J Biol Chem
285:12726–12734
161. Wang G, van der Walt JM, Mayhew G, Li YJ, Zuchner S, Scott
WK, Martin ER, Vance JM (2008) Variation in the miRNA-433
binding site of FGF20 confers risk for Parkinson disease by
overexpression of alpha-synuclein. Am J Hum Genet 82:283–289
162. Toda T, Momose Y, Murata M, Tamiya G, Yamamoto M,
Hattori N, Inoko H (2003) Toward identification of suscep-
tibility genes for sporadic Parkinson’s disease. J Neurol 250:
III40–III43
163. Galvin JE (2004) Neurodegenerative diseases: pathology and the
advantage of single-cell profiling. Neurochem Res 29:1041–1051
Mol Neurobiol

164. Scherzer CR, Eklund AC, Morse LJ, Liao Z, Locascio JJ, Fefer D,
Schwarzschild MA, Schlossmacher MG, Hauser MA, Vance JM,
Sudarsky LR, Standaert DG, Growdon JH, Jensen RV, Gullans SR
(2007) Molecular markers of early Parkinson’s disease based on
gene expression in blood. Proc Natl Acad Sci U S A 104:955–960
165. Sinha A, Singh C, Parmar D, Singh MP (2007) Proteomics in
clinical interventions: achievements and limitations in biomarker
development. Life Sci 2080:1345–1354
166. Patel S, Sinha A, Singh MP (2007) Identification of differentially
expressed proteins in striatum of maneb- and paraquat-induced
Parkinson’s disease phenotype in mouse. Neurotoxicol Teratol
29:578–585
167. Gerlach M, Riederer P (1996) Animal models of Parkinson’s
disease: an empirical comparison with the phenomenology of
the disease in man. J Neural Transm 103:987–1041
168. Laterra J, Keep R, Betz LA, Goldstein GW (1999) Blood–brain
barrier. In: Siegel GJ, Agranoff BW, Albers RW et al (eds) Basic
neurochemistry: molecular, cellular and medical aspects, 6th edn.
Lippincott-Raven, Philadelphia
169. Robbins M, Judge A, Liang L, McClintock K, Yaworski E,
MacLachlan I (2007) 2′-O-methyl-modified RNAs act as TLR7
antagonists. Mol Ther 15:1663–1669
170. Ma Z, Li J, He F, Wilson A, Pitt B, Li S (2005) Cationic lipids
enhance siRNA-mediated interferon response in mice. Biochem
Biophys Res Commun 330:755–759
171. Hornung V, Guenthner-Biller M, Bourquin C, Ablasser A, Schlee
M, Uematsu S, Noronha A, Manoharan M, Akira S, de Fougerolles
A, Endres S, Hartmann G (2005) Sequence-specific potent induc-
tion of IFN-alpha by short interfering RNA in plasmacytoid den-
dritic cells through TLR7. Nat Med 11:263–270
Mol Neurobiol