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HIV-1 Tat Protein Increases Microglial Outward K
+
Current and Resultant Neurotoxic Activity
Jianuo Liu
1
*, Peng Xu
1
, Cory Collins
1
, Han Liu
1
, Jingdong Zhang
1
, James P. Keblesh
1
, Huangui Xiong
1,2
*
1Neurophysiology Laboratory, Department of Pharmacology and Experimental Neuroscience, University of Nebraska Medical Center, Omaha, Nebraska, United States of
America, 2Department of Pathology and Microbiology, University of Nebraska Medical Center, Omaha, Nebraska, United States of America
Abstract
Microglia plays a crucial role in the pathogenesis of HIV-1-associated neurocognitive disorders. Increasing evidence
indicates the voltage-gated potassium (K
v
) channels are involved in the regulation of microglia function, prompting us to
hypothesize K
v
channels may also be involved in microglia-mediated neurotoxic activity in HIV-1-infected brain. To test this
hypothesis, we investigated the involvement of K
v
channels in the response of microglia to HIV-1 Tat protein. Treatment of
rat microglia with HIV-1 Tat protein (200 ng/ml) resulted in pro-inflammatory microglial activation, as indicated by increases
in TNF-a, IL-1b, reactive oxygen species, and nitric oxide, which were accompanied by enhanced outward K
+
current and
K
v
1.3 channel expression. Suppression of microglial K
v
1.3 channel activity, either with K
v
1.3 channel blockers Margatoxin, 5-
(4-Phenoxybutoxy)psoralen, or broad-spectrum K
+
channel blocker 4-Aminopyridine, or by knockdown of K
v
1.3 expression
via transfection of microglia with K
v
1.3 siRNA, was found to abrogate the neurotoxic activity of microglia resulting from HIV-
1 Tat exposure. Furthermore, HIV-1 Tat-induced neuronal apoptosis was attenuated with the application of supernatant
collected from K
+
channel blocker-treated microglia. Lastly, the intracellular signaling pathways associated with K
v
1.3 were
investigated and enhancement of microglial K
v
1.3 was found to correspond with an increase in Erk1/2 mitogen-activated
protein kinase activation. These data suggest targeting microglial K
v
1.3 channels may be a potential new avenue of therapy
for inflammation-mediated neurological disorders.
Citation: Liu J, Xu P, Collins C, Liu H, Zhang J, et al. (2013) HIV-1 Tat Protein Increases Microglial Outward K
+
Current and Resultant Neurotoxic Activity. PLoS
ONE 8(5): e64904. doi:10.1371/journal.pone.0064904
Editor: Michelle L. Block, Virginia Commonwealth University, United States of America
Received March 21, 2013; Accepted April 19, 2013; Published May 30, 2013
Copyright: ß2013 Liu et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work was supported by a research grant from National Institues of Health (NIH): R01NS077873 to HX. The funders had no role in study design, data
collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: jnliu@unmc.edu (JL); hxiong@unmc.edu (HX)
Introduction
Individuals infected with human immunodeficiency virus type 1
(HIV-1) often suffer from neurocognitive impairments which are
referred to as HIV-1-associated neurocognitive disorders (HAND)
[1,2]. The severity of HAND varies, ranging from asymptomatic
neurocognitive impairment to its severest form: HIV-1-associated
dementia [2]. Despite the widespread use of potent antiretroviral
therapy (ART), the incidence of HAND has not been fully
prevented and its prevalence remains high ranging from 39% to
52% in varied settings [3,4,5]. Although the persistence of HAND is
multifactorial, the paucity of effective therapeutic modalities in the
control of brain macrophage and microglia activation and resultant
production of neurotoxins, a striking pathological feature in HIV-1-
infected brain, plays an important role as pathogenesis and severity
of HAND is highly correlated with activated brain macrophages
and microglia but not the presence and amount of virus in the brain
[6,7]. It is well known that the activated microglia secrete a number
of neurotoxins including, but not limited to, pro-inflammatory
cytokines, and excitatory amino acids, reactive oxygen species
(ROS), nitric oxygen (NO), which can result in neuronal injury and
consequent neurocognitive impairments [8,9,10]. As such, studies
on elucidation of the mechanisms by which HIV-1 triggers
microglial neurotoxicity and identification of specific target(s) to
control microglia activation are imperative.
Voltage-gated potassium (K
v
) channels have recently gained
much attention as the potential targets for therapy of neurological
disorders [11,12]. Electrophysiological studies of microglia in
culture and tissue slices have demonstrated that microglia express
several types of K
v
channels including inward rectifier K
ir
2.1 and
outward rectifiers K
v
1.5 and K
v
1.3. Exposure to a variety of
activating stimuli produces a characteristic pattern of up-regula-
tion of K
v
1.3 [13,14,15,16]. Whereas the expression of K
ir
2.1
channels are often found in resting microglia [17,18], the
expression of K
v
1.5 and K
v
1.3, especially the latter, appear to
be associated with microglia activation and neurotoxin production
[15,19,20,21]. Indeed, studies have shown that activation of
microglia results in neuronal injury through a process requiring
K
v
1.3 activity in microglia. Studies have also shown that blocking
microglia K
v
1.3 or decrease of K
v
1.3 expression inhibits microg-
lia-induced neurotoxicity [22,23]. We hypothesize that HIV-1
brain infection triggers microglia neurotoxic activity by increasing
K
v
1.3 activity, resulting in microglia activation and consequent
neuronal injury. To test this hypothesis, we studied involvement of
K
v
1.3 in HIV-1 Tat protein-induced microglia activation and
resultant neurotoxic activity in primary microglia culture prepared
from Sprague-Dawley rats. Our results demonstrated that HIV-1
Tat increases microglia production of neurotoxins and resultant
neurotoxicity through enhancements of K
v
1.3 protein expression
and outward K
+
currents, which can be blocked by pretreatment
PLOS ONE | www.plosone.org 1 May 2013 | Volume 8 | Issue 5 | e64904
of microglia with specific K
v
channel blockers Margatoxin (MgTx)
or 5-(4-Phenoxybutoxy)psoralen (PAP), or by transfection of
microglia with K
v
1.3 siRNA, suggesting an involvement of
K
v
1.3 in microglia-mediated neurotoxic activity. The enhance-
ments of K
v
1.3 channel activity and microglia neurotoxicity
resulting from HIV-1 Tat protein exposure are dependent on the
Erk1/2 MAPK signal pathway. Here we present evidence for the
reduction of neurotoxic secretions from microglia and associated
neuronal injury by modulation of K
+
channel activity as a
potential new treatment approach deserving further investigation.
Materials and Methods
Animals
Sprague-Dawley rats were purchased from Charles River
Laboratories (Wilmington, MA) and maintained under ethical
guidelines for care of laboratory animals at the University of
Nebraska Medical Center. All animal-use procedures were
reviewed and approved by the Institutional Animal Care and
Use Committee (IACUC) of University of Nebraska Medical
Center (IACUC #00-062-07).
Primary microglia and neuron cultures
Microglia were derived from the cerebral cortices of 0–1 day old
neonatal Sprague Dawley rats as described previously [24].
Cortical tissues were dissected in cold Hanks’ Balanced Salt
Solution (HBSS: Madiatech, Inc. Manassas, VA) and digested in a
solution consisting of 0.25% trypsin and 200 Kunitz DNase
(Sigma, St. Louis. MO) at 37uC for 30 min. Tissues were then
suspended in cold HBSS and filtered using 100 mm and 40 mm
cellular strainers (BD Bioscience, Durham, NC). Isolated cells
(30610
6
) were plated into T75 cm
2
flasks in a high-glucose
Dulbecco’s modified Eagle’s medium (DMEM) supplemented with
10% fetal bovine serum (FBS), 2 mM L-glutamine, 1% penicillin/
streptomycin, and 1 mg/ml macrophage colony-stimulating factor
(Life Technologies, Grand Island, NY). After 10 day’s culture,
flasks were shaken gently to detach cells, which were plated based
on experimental requirements in either 35 mm
2
culture dishes
(2.5610
6
cells/dish), 60 mm
2
culture dishes (7.5610
6
cells/dish),
12-well plates (1610
6
/well), or 96-well plates (0.4610
6
/well) and
incubated at 37uC. After 30 min, suspended glial cells were
removed by aspiration of culture supernatant and fresh culture
media was applied. The resulting cultures were stained with OX-
42 antibody (Serotec, Oxford, UK), a microglial CR3/CD11b
receptor marker, and determined to consist of 98–100% microglia.
Primary cortical neurons were prepared from 18-day old
Sprague Dawley embryonic rats (Charles River Laboratories).
Dissected cortices were digested with 0.25% trypsin and DNase
(200 Kunitz) in 37uC for 15 min, then filtered through 100 and
40 mm pore cellular strainers. Isolates were seeded in pre-coated
poly-D-lysine plates at a density of 0.05610
6
cells/well in 96-well
plates, 0.15610
6
cells/well in 24-well plates, or 1.0610
6
cells/well
in 6 well-plates. Neuronal cultures were maintained at 37uC for 10
days in neurobasal medium (Gibco by Life Technologies)
supplemented with 2% B27, 1% penicillin/streptomycin and
0.5 mM L-glutamine (Invitrogen by Life Technologies). The
purity of neuronal cells was determined to be .90% by staining
with microtubule-associated protein-2 antibody (MAP-2: 1:1000,
Chemicon International, Inc. Temecula, California).
Electrophysiology
Whole-cell outward K
+
currents were recorded from primary
rat microglia cultures at room temperature. Microglia were
perfused with artificial cerebrospinal fluid (ACSF) contained (in
mM) NaCl 150, KCl 4.5, CaCl
2
2, MgCl
2
1, HEPES 5, and
glucose 11. The ACSF was continuously oxygenated with 95% O
2
and 5% CO
2
with a pH of 7.4 and an osmolarity of 310 mOsm.
Patch-clamp electrodes were made from borosilicate glass
capillaries (WPI, Sarasota, FL) with a resistance of 4–6 Vwhen
filled with pipette solution contained (in mM) KCl 150, MgCl
2
1,
CaCl
2
1, EGTA 11, and HEPES 10; adjusted to a pH of 7.3 with
KOH. Voltage-dependent currents were evoked by voltage steps
(600 ms in duration) with the first step from the holding potential
of 270 mV to 2170 mV and then stepped to +50 mV with a
20 mV increments [13]. The seal resistance was 1–10 GV.
Junction potentials were corrected and the cell capacitance was
compensated (,70%) in most cells. Current signals were amplified
with an Axopatch 200B amplifier (Molecular Devices, Sunnyvale,
CA). The current traces were displayed and recorded on a Dell
computer using a pClamp 10.1 data acquisition/analysis system.
K
+
current density (pA/pF) was calculated by dividing the peak
current amplitude generated at a given voltage step by the cell
capacitance.
Measurement of reactive oxygen species (ROS) and Nitric
oxide (NO) production
Intracellular ROS were measured by fluorometric assay using
29,79-dichlorofluorescein diacetate (DCFH-DA, Sigma, St. Louis,
MO), a well-established compound for detecting and quantifying
intracellular ROS production. Microglia were first treated for
30 minutes with a K
V
1.3 channel blocker, either 5 nM MgTx,
10 nM PAP, or 1 mM 4-AP (all purchased from Sigma-Aldrich
Co, LLC, St. Louis, MO), followed by treatment with either
200 ng/ml of HIV-1 Tat
1-72
protein (Tat) or heat inactivated-
Tat
1-72
(HI Tat) (purchased from University of Kentucky). After
24 hr, microglia were exposed to 20 mM DCFH-DA for 30 min.
Cells were then washed twice in PBS and the fluorescence
immediately measured in a plate reader at an excitation
wavelength of 485 nm and an emission wavelength of 520 nm.
NO production was estimated by measuring the concentration
of nitrite using the Griess Reagent System according to the
manufacturer instruction (Promega, Madison, WI). 50 ml aliquots
of supernatant were collected from cultures of pre-treated
microglia, mixed with equal volume of Sulfanilamide Solution
for 10 min, combined with 50 ml of NED solution, and incubated
for 30 min at room temperature. The optical density was then
measured at 520 nm and 540 nm using an ELISA plate reader.
All experiments were repeated at least three times.
Cytokine assay
Cytokines IL-1band TNF-ain pre-treated microglia superna-
tants were quantified using specific enzyme-linked immunosorbent
assay (ELISA) kits (R&D Systems) in accordance with manufac-
turer protocol.
TUNEL staining and MTT assay
Neuronal apoptosis was evaluated using a Fluorescein In Situ
Cell Death Detection Kit (Roche Applied Science, Indianapolis,
IN). In brief, neurons growing on poly-D-lysine-coated coverslips
(0.15610
6
cells/well in a 24-well plate) were exposed to
supernatants collected from pre-treated microglia at 1:5 dilution
for 24 hr. Neurons were then fixed with 4% paraformaldehyde
(PFA) and permeabilized with 0.1% Triton X-100. Neurons were
subsequently incubated in the TUNEL reaction mixture for 1 hr
at 37uC and then mounted using ProLong Gold antifade reagent
with 49,69-diamidino-2-phenylindol (DAPI) counterstain (Molecu-
lar Probes, Eugene, OR). Cells were visualized using the 406oil-
HIV-1 Tat Enhances Microglial K+Channel Activity
PLOS ONE | www.plosone.org 2 May 2013 | Volume 8 | Issue 5 | e64904
immersion objective of a Zeiss LSM 510 META NLO microscope
(Zeiss MicroImaging, Inc., Thornwood, NY). The percentage of
apoptotic neurons was determined based on TUNEL-positive cells
normalized to DAPI-stained nuclei.
Cell viability was assessed by MTT assay. Pre-treated neurons
were exposed to fresh neurobasal medium containing 500 mg/ml
3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide
(MTT) for 3 hr. The MTT solution was then replaced with
300 ml of dimethyl sphingosine (DMSO: Sigma-Aldrich) for cell
lysis and the optical density (OD) was measured at 560 nm.
Immunocytochemistry
Microglia were seeded on coverslips at a density of 1.0610
6
/
well in 12-well plates, treated for 30 min with 5 nM MgTx,
10 nM PAP, or 1 mM 4-AP, and incubated with Tat (200 ng/ml).
After 24 hr, cells were washed, fixed with 4% PFA for 30 min, and
incubated with 10% normal goat serum blocking solution for
30 min. Primary antibodies (Ab) anti-CD11b Ab (CD11b; 1:500;
abcam, Cambridge, MA) and anti-K
V
1.3 (KCNA3 1:100,
Alomone Lab Ltd, Jerusalem Israel) were then applied to
coverslips for 3 hr at RT. Cells were subsequently incubated for
1 hr with Alexa Fluor 488 and Alexa Fluor 594-conjugated
secondary Abs (1:1000, Molecular Probes, Invitrogen by Life
Technologies). After washing, cells were mounted using ProLong
Gold antifade reagent with DAPI counterstain (Molecular Probes).
Images were obtained using the 406oil-immersion objective of a
Zeiss LSM 510 META NLO microscope. A minimum of 5 images
were taken from each slide.
Immunohistochemistry
Brain hippocampal tissues were dissected out from 20–30 d old
Sprague Dawley rats (Charles River Laboratories), cut into slices
at 400 mM in thickness, and placed on a 100 mm pore cellular
strainer in a 6-well plate. The hippocampal slices were then
treated for 30 min with MgTx (5 nM), PAP (10 nM), or 4-AP
(1 mM) and subsequently incubated with 200 ng/ml of Tat. After
24 hr, hippocampal slices were fixed in 4% PFA for another 24 hr,
immersed in 30% sucrose for 48 hr, embedded in optimal cutting
temperature (OCT) media, and cryosectioned to a thickness of
10 mm. Hippocampal sections were then immunostained with
either anti-Iba1 Ab (1:500, WAKO Chemicals USA, Inc.
Richmond, VA), K
V
1.3 Ab (1:200, Santa Cruz biotechnology,
Inc, CA), or TUNEL stain and visualized using the 406oil-
immersion objective of a Zeiss LSM 510 META NLO microscope.
A minimum of 5 images were taken from each slides.
Western blot analysis
Membrane proteins were prepared using a Membrane Protein
Extraction Kit (BioVision, Mountain View, CA, USA) according
to manufacturer instruction, while total proteins were isolated
using a RIPA buffer (Bio-Rad, Hercules, CA). Well volumes of
20 mg for membrane proteins and 30 mg for total proteins were
separated by electrophoresis using 4–15% Mini-PROTEAN TGX
precast gel and transferred to nitrocellulose polyvinylidene
difluoride (PVDF) membranes. PVDF membranes were then
blocked with 5% dry milk in Tris-Buffered Saline (TBS) (all
products from Bio-Rad Laboratories, Hercules, CA) and probed
overnight at 4uC with either rabbit polyclonal K
V
1.3 (1:100;
Alomone Lab, Israel), phospho-p44/42 MAPK (pERK1/2), total
p44/42 MAPK (ERK1/2) (1:1000; Cell Signaling Technology,
Danvers, MA), or anti-mouse b-actin monoclonal antibody
(1:10,000, Sigma-Aldrich) primary Abs. Membranes were next
washed (4610 min) in TBS with 0.2% Tween (TBS-T) and
incubated for 1 hr at RT with either horseradish peroxidase
(HRP)-conjugated anti-rabbit or anti-mouse secondary antibody
(1:10,000, Jackson ImmunoResearch Laboratories, West Grove,
PA). Labeled proteins were visualized by Pierce ECL Western
Blotting Substrate (Thermo Scientific, Rockford, IL). Band
densities of p-pErk1/2 were normalized to total ERK1/2 in each
sample.
Reverse transcription (RT)-PCR
Total RNA was isolated from microglia using TRIzol Reagent
(Invitrogen, Carlsbad, CA), purified by RNeasy Mini Kit
(QIAGEN, Inc., Valencia, CA), and reverse transcribed at 65uC
for 50 min according to SuperScript III reverse transcriptase
(Invitrogen) protocol. PCR amplification with Platinum PCR
SuperMix (Invitrogen) then included 2 min incubation at 94uC
followed by 30 cycles consisting of a 30 s denaturing phase at
94uC, a 30 s annealing phase at 55uC, a 1 min extension phase at
72uC, and a final extension phase of 10 min at 72uC. Densitom-
etry analysis of DNA products was performed using Northern
Eclipse 6.0 software (Bio-Rad) and results normalized to b-actin
internal controls. PCR primers used were as follows: forward
K
v
1.3 primer was GTA CTT CGA CCC GCT CCG CAA TGA;
reverse K
v
1.3 primer was GGG CAA GCA AAG AAT CGC
ACC AG; forward b-Actin primer was GTG GGG CGC CCC
AGG CAC CA; reverse b-Actin primer was CTT CCT TAA
TGT CAC GCA CGA TTT C.
siRNA transfection
Pre-designed ON-TARGETplus SMARTpool siRNA against
rat KCNA3 (K
v
1.3, NM-019270) mRNA was purchased from
Dharmacon, Inc. (Chicago, IL). Microglia plated to 2610
6
cells/
well in 6-well plates were transfected with 100 mlof2mM siRNA
for 48 or 72 hr in the presence of Dharma FECT Transfection
Reagent (Dharmacon, Inc) according to the manufacturer
instruction. A non-specific ON-TARGETplus GAPD Control
Pool siRNA (rat) (Dharmacon, Inc) was also similarly transfected
at the same concentration as the control. Transfected microglia
were then incubated for 24 hr with or without Tat (200 ng/ml),
after which the supernatant was collected (for conditioned media)
and the cells were harvested (for preparation of RNA and protein).
Statistical Analysis
Experimental data are expressed as mean6S.D. unless other-
wise indicated. Statistical analyses were performed by Student t
tests. A minimum pvalue of 0.05 was estimated as the significance
level for all tests.
Results
HIV-1 Tat exposure induces K
v
1.3 currents in microglia
HIV-1 pathogenesis involves the release of soluble viral proteins
such as gp120, Tat, and Nef. In previous studies, we demonstrated
HIV-1 gp120 IIIB enhanced whole-cell outward K
+
current in
cultured rat microglia through K
v
1.3 channels [24,25]. Here we
propose HIV-1 Tat protein may alter microglia channel profiles in
a similar manner. To test our hypothesis, we first examined the
effect of HIV-1 Tat protein on the electrophysiological properties
of microglia. Although sera levels of HIV-1 Tat have been
reported to range from 1–40 ng/ml in HIV-1 positive individuals
[26,27], localized concentrations are reasoned to be higher and
nM concentrations are commonly used in vitro to elicit the effects of
Tat exposure [28,29]. In our study, purified rat microglia were
pretreated with HIV-1 Tat protein at 20–1000 ng/ml for 24 hr
before recording. Electrophysiological recordings were performed
using a conventional whole-cell recording under voltage clamp
HIV-1 Tat Enhances Microglial K+Channel Activity
PLOS ONE | www.plosone.org 3 May 2013 | Volume 8 | Issue 5 | e64904
configuration. The average inward K
+
current (I
in
) and outward
K
+
current (I
out
) densities (pA/pF) were calculated by dividing the
K
+
current amplitude by the membrane capacitance. At hyper-
polarizing potentials, both untreated and Tat treated microglia
displayed an I
in
(Fig. 1A). The I
in
density in untreated microglia
(28.9663.76 pA/pF; n = 32) was only minimally affected by
exposure to either 20 ng/ml Tat (28.1463.61 pA/pF; n = 25),
200 ng/ml Tat (215.567.36 pA/pF; n = 27), or 1000 ng/ml Tat
(212.967.21 pA/pF; n = 24). With depolarizing pulses however,
Tat treated microglia responded with a substantial I
out
(Fig. 1A). In
fact, the I
out
density in microglia pretreated with 20 ng/ml Tat
(22.6967.46 pA/pF; n = 25) was over fourfold greater than in
untreated microglia (5.0962.84 pA/pF; n = 32). Furthermore, the
I
out
densities in microglia exposed to 200 ng/ml Tat
(30.7616.29 pA/pF; n = 27) and 1000 ng/ml Tat
(31.86614.69 pA/pF; n = 24) demonstrate this effect to be dose-
dependent (Fig. 1C). To confirm these observations were due
specifically to Tat protein function, we disrupted its tertiary
structure with heat (75uC for 5 hr) prior to incubation with
microglia. Similarly to untreated cells, microglia treated with
200 ng/ml heat-inactivated Tat protein (HI Tat, n = 9) exhibited
hyperpolarization-evoked I
in
currents and lacked significant I
out
current in response to depolarizing pulses (Fig. 1B). Next, to
determine whether the Tat-induced I
out
currents were conducted
via K
v
1.3 channels, Tat-treated microglia were perfused with
ACSF contained specific K
v
1.3 blockers PAP (10 nM), MgTx
(5 nM), or a broad spectrum K
v
channel blocker 4-AP (1 mM),
and the I
out
was significantly reduced by 52.1614.72% (n = 8),
87.2667.79% (n = 8) or 89.8163.09% (n = 7) (Fig. 1D & 1E).
Taken together, these findings strongly suggest HIV-1 Tat
exposure induces outward K
+
currents in microglia through
K
v
1.3 channels.
HIV-1 Tat upregulates K
V
1.3 expression in rat microglia
K
v
channel activity can be altered by numerous factors,
including by membrane potential, redox potential, transcription,
translation, posttranslational modification, or via direct interaction
with organic molecules or peptides. To better determine the
mechanism through which Tat induces K
v
1.3currents in rat
microglia, K
v
1.3 mRNA and protein levels were ascertained by
RT-PCR and western blot. RT-PCR performed after 24 hr
incubation of rat microglia with 200 ng/ml Tat protein showed
marked elevation in K
v
1.3 mRNA expression (Fig. 2A), with K
v
1.3
mRNA density in Tat-treated cells (1.3260.10) measuring 1.8
times greater than in untreated microglia (0.746023). As a
negative control, the K
v
1.3 mRNA density was measured in
microglia treated with 200 ng/ml heat-inactivated Tat protein
and found to be essentially unchanged (0.8160.13) (Fig. 2A).
Consistent with these effects, treatment of microglia with 200 ng/
ml Tat for 24 hr led to a nearly threefold increase in K
v
1.3 protein
levels (Fig. 2B), which was further confirmed and visualized by
immunocytochemical labeling (Fig. 2C). These findings clearly
indicate Tat protein exposure upregulates the expression of K
v
1.3
channels in rat microglia.
Involvement of K
V
1.3 in Tat-induced microglia-mediated
neurotoxicity
Having established Tat protein exposure increases K
v
1.3
expression and current density in microglia, we next sought to
determine if this change in channel profile contributes to the
neurotoxicity of HIV-1 Tat-activated microglia. Microglial super-
natants were first collected after 24 hr treatment with either HIV-
1 Tat protein (at doses of 0, 20, 200, and 1000 ng/ml) or heat-
inactivated Tat (200 ng/ml). Rat cortical neurons growing on
poly-D-lysine-coated coverslips in 24-well plates were then
subjected to these supernatants (1:5 dilution) for an additional
24 hr and neuronal viability was assessed by MTT assay. As
shown in Figure 3C, neuronal cell viability was found to be
essentially unaffected at doses of 0 ng/ml (100%) and 20 ng/ml
(97.0269.38%), however was progressively and significantly
reduced as the microglial supernatant Tat treatment dose was
increased to 200 ng/ml (70.1963.33%; p,.01) and 1000 ng/ml
(58.4564.65%; p,.01). Neuronal viability was further unaffected
by incubation with supernatant collected from microglia treated
with heat-inactivated Tat protein (HI Tat, 98.464.48%), demon-
strating the dose-dependent reductions in viability to be specific to
functional Tat protein.
Next, the role of K
v
channels in Tat-activated microglial
neurotoxicity was investigated using a similar experimental design
and examining the effect of K
V
channel blocker pretreatment on
neuronal health. Microglia were first pretreated with either 5 nM
MgTx, 10 nM PAP, or 1 mM 4-AP for 30 min. Based on the
capacity to substantially reduce neuronal viability, a dose of
200 ng/ml Tat protein was applied and cultures incubated for
24 hr. Microglia supernatants were then collected and added to
cultured rat cortical neurons at a dilution ratio of 1:5. After 24 hr,
neuronal viability and neuronal apoptosis were assessed by MTT
assay and TUNEL staining, respectively. As noted previously, cell
viability was shown by MTT assay to be decreased in neuronal
cultures exposed to Tat-treated microglial supernatant
(70.1963.33%), an effect which was significantly attenuated
(p,.01) by the pretreatment of microglia with MgTx
(85.4661.00%), PAP (82.9560.54%), or 4-AP (92.8364.66%)
(Fig. 3C). The complementary study of neuronal apoptosis
revealed similar findings, with the percentage of apoptotic neurons
greatly increasing (p,.001) with application of Tat protein
(30.6864.3%) compared to control (5.661.3%) (Fig. 3A & 3B).
Again, this result was largely reversed (p,.01) when microglial
cultures were treated with MgTx (9.5662.78%), PAP
(11.7262.42%), or 4-AP (7.2963.84%) prior to the application
of Tat protein (Fig. 3A & 3B). The recovery of neuronal viability
and attenuation of neuronal apoptosis by K
V
channel blockade,
including the use of K
v
1.3 specific inhibitors, suggests that K
v
1.3
channel activity greatly impacts Tat-induced microglia-mediated
neurotoxicity.
K
V
1.3 channel blockade decreases neurotoxic secretions
by Tat-activated microglia
The production and release of bioactive molecules by activated
microglia is believed to be the principal pathway in HAND
associated neuropathology. To better clarify the functional role of
K
v
1.3 channels in this process, we next examined the capacity for
Tat exposure to induce the secretion of proinflammatory cytokines
such as TNF-aand IL-1b, in the presence and absence of K
v
channel blockers. For this experiment, purified microglia were first
pre-treated for 30 min with a K
v
channel blocker, either MgTx
(5 nM), PAP (10 nM), or 4-AP (1 mM), and then incubated with
200 ng/ml Tat protein for 24 hr. Subsequent cytokine assays
revealed marked increases (p,.001) in levels of TNF-a
(2.2860.07 ng/ml) and IL-1b(4.1060.68 ng/ml) in the superna-
tants of Tat-treated microglia compared to the nearly undetectable
levels in untreated controls (Fig. 4A & 4B). Further, this Tat-
induced production of cytokines was significantly inhibited (p,.01)
in cultures pretreated with K
v
channel blockers MgTx, PAP, or 4-
AP. In addition to measuring cytokines, we used a similar
experimental design to measure other neurotoxic microglial
products including NO and ROS. The level of NO in
supernatants from Tat-treated microglia was (23.6560.60 nM)
HIV-1 Tat Enhances Microglial K+Channel Activity
PLOS ONE | www.plosone.org 4 May 2013 | Volume 8 | Issue 5 | e64904
compared to (0.4260.16 nM) in untreated matched controls
(Fig. 4C). This production was significantly limited (p,.01) by
blockade of K
V
channels using either MgTx (15.8160.56 nM),
PAP (11.0063.20 nM), or 4-AP (16.9661.60 nM). Similarly,
ROS production in Tat-stimulated microglia was found to be
590.646160.72% of the level in untreated control (Fig. 4D). Pre-
treatment with MgTx, PAP, or 4-AP prior to the addition of Tat
significantly reduced the levels of ROS to 175.76662.37%,
169.91684.58%, and 252.70689.4% of control, respectively.
Collectively, these results strongly support a critical role for K
v
1.3
channels in the production and secretion of neurotoxins by Tat-
activated microglia.
Neurotoxic activity of Tat-stimulated microglia is
mitigated by knockdown of K
V
1.3 gene
Having demonstrated HIV-1 Tat upregulates K
v
1.3 expression
in rat microglia and that K
v
1.3 channels are involved in Tat-
induced microglia-mediated neurotoxicity, complementary exper-
iments were next performed to address whether gene silencing by
knockdown of the K
v
1.3 gene (KCNA3) with siRNA would
attenuate these effects. First, microglia were transfected with
K
v
1.3-siRNA or nonspecific GAPD control siRNA (control
siRNA) for 48 hr or 72 hr, depending on whether mRNA or
protein expression was to be measured, and incubated with or
without 200 ng/ml Tat for an additional 24 hr. RT-PCR and
western blot were then used to examine K
v
1.3 mRNA expression
and K
v
1.3 protein levels, respectively. As expected, the upregula-
tion of K
v
1.3 mRNA expression in Tat-stimulated microglia was
efficiently inhibited by transfection with K
v
1.3 siRNA as
compared to those transfected with control siRNA (Fig. 5A).
Paralleling these results, Tat-enhanced K
v
1.3 protein expression
was found to be significantly decreased with K
v
1.3 siRNA
transfection (Fig. 5B).
To examine the effect of K
v
1.3 channel knockdown on the
neurotoxic properties of Tat-exposed microglia, we again
employed measures of neuronal viability and apoptosis. For this
experiment, microglia were transfected with K
v
1.3-siRNA for
72 hr and then incubated with 200 ng/ml Tat protein an
additional 24 hr. Microglial supernatants were next collected
and applied at 1:5 dilution to rat cortical neurons. Neurons were
then incubated for an additional 24 hr before being assessed by
MTT assay and TUNEL staining. As demonstrated by MTT
assay, the decline in neuronal viability due to Tat-exposed
microglial supernatants (66.8762.55%) was improved by pre-
transfection with K
v
1.3-siRNA (84.7063.93%) (Fig. 5C). Similar-
Figure 1. HIV-1 Tat protein enhances outward K
+
currents (
I
out
) in rat microglia. A: Representative whole-cell membrane currents recorded
from microglia treated with or without Tat at varied concentrations (0, 20, 200, 1000 ng/ml). B: Whole-cell membrane current recordings of microglia
treated with heat-inactivated Tat (200 ng/ml). C: Dose-response curve of the Tat-induced effect on I
out
. Current densities in response to different
concentrations of Tat is calculated (mean 6S.E.M) from 32, 25, 27 and 24 different cells in control, 20 ng/ml, 200 ng/ml, 1000 ng/ml groups. D:
Pharmacology of Tat-induced I
out
. Tat induced I
out
were evoked by voltage from the holding potential 270 mV to +50 mV for 600 ms are shown
before and during superfusion with extracellular solution containing 10 nM PAP, 5 nM MgTX, 1 mM 4-AP. E: Blockade of Tat(200 ng/ml)
enhancement of I
out
by specific Kv1.3 blockers PAP (n =8) and MgTx (n= 8) or by a broad spectrum K
+
channel blocker 4-AP (n = 7). #p,0.05 vs Tat;
### p,0.001 vs Tat.
doi:10.1371/journal.pone.0064904.g001
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ly, TUNEL staining revealed the percentage of apoptotic neurons
was decreased from 31.4063.37% to 11.863.03% with K
v
1.3
gene knockdown (Fig. 5D & 5E). The capacity for K
v
1.3 gene
knockdown to mitigate the neuronal damage caused by Tat-
activated microglia indicates the upregulation of K
v
1.3 mRNA
and protein is a key component in the mechanism of this
neurotoxicity.
ERK1/2 MAPK signaling pathway involvement in Tat-
induced microglial neurotoxicity
Thus far we have shown Tat-induced upregulation of K
v
1.3
channels and currents to be critical to the activation of microglia
and subsequent damage to neurons. To better clarify the
mechanisms underlying these observations, we next turned our
attention to the extracellular signal-related kinases (ERK1/2)
MAPK signaling pathway, which has been implicated elsewhere in
chronic neurodegenerative disease and may mediate the channel
profile alterations associated with Tat exposure [30,31,32]. For
this experiment, microglia were exposed to 200 ng/ml Tat and
proteins were harvested at various time points for assessment by
western blot. Our results demonstrate ERK1/2 phosphorylation
(pERK1/2) was enhanced by Tat exposure beginning at 30 min
and peaking at 5 hr after stimulation (Fig. 6A). To determine if
ERK1/2 activation mediates the Tat-induced increases in K
v
1.3
channel expression and subsequent neurotoxicity, U0126 was used
to inhibit MEK1 and MEK2, the MAPK kinases responsible for
phosphorylation of ERK1/2 MAPK. Reduction of ERK1/2
MAPK activity using U0126 (10 mM) was found to markedly
reduce both the expression levels of microglial K
v
1.3 (Fig. 6C) and
the neurotoxicity of supernatants collected from Tat-exposed
microglia (Fig. 6D). These results provide evidence for the
involvement of the ERK MAPK pathway in Tat-induced K
v
1.3
expression and consequent microglia neurotoxic activity. Lastly
and perhaps surprisingly, application of MgTx (5 nM), PAP
(10 nM), or 4-AP (1 mM) to microglia after 30 min of Tat
exposure was also found to significantly inhibit the Tat-enhanced
phosphorylation of ERK1/2 (Fig. 6B).
K
V
1.3 channel involvement in ex vivo HIV-1 Tat-induced
microglia-mediated neurotoxicity
Thus far we have demonstrated Tat protein exposure results in
the upregulation of K
v
1.3 expression and the production of
neurotoxic substances in cultured microglia. To better approxi-
mate in vivo conditions, we next used a rat hippocampal slice
culture to evaluate ex vivo the alterations in microglial K
v
1.3
expression and neuronal apoptosis resulting from HIV-1 Tat
application. Hippocampal slices were first dissected from rats aged
20–30 days, cultured in NeuroBasal medium, and incubated for
24 hr with 200 ng/ml Tat protein. Brain slices were then double
stained with the microglia marker Iba1 and K
v
1.3 antibodies. In
Tat protein treated slices, K
v
1.3 expression was found to be
enhanced and co-localized with Iba1 stained microglia (Fig. 7A).
In addition, TUNEL staining revealed Tat-induced neuronal
apoptosis could be attenuated with 30 min K
V
channel antagonist
pre-treatment, either MgTx (5 nM), PAP (10 nM), or 4-AP
(1 mM) (Fig. 7B). These results are fully consistent with our in
vitro studies and corroborate the involvement of K
v
1.3 channels in
the neuronal damage caused by Tat-activated microglia.
Discussion
Microglia are functionally related to cells of the monocyte/
macrophage lineage and play an important role as resident
immunocompetent phagocytic cells in the HAND pathogenesis. A
prominent pathological feature in HIV-1-infected brain is
microglia activation and the activated microglia exert neurotoxic
effects in the brain by releasing a variety of potentially neurotoxic
substances. In addition to their production of neurotoxins,
microglia express a large number of chemokine receptors that
are involved in cell migration and serve as co-receptors for HIV-1
infection. Indeed, microglia are the predominant resident CNS
cell type productively infected by HIV-1 [8,9]. Due to poor
penetration of antiretroviral drugs through the blood-brain barrier
(BBB), resident microglia (and brain macrophages) constitute a
cellular reservoir of HIV-1 in the brain and a source of potential
neurotoxic substances [33,34,35]. Thus, suppression of microglia
production of neurotoxins is critical to the control of HAND onset
and progression. In the present study, we demonstrated HIV-1
Tat-induced microglia activation and associated neurotoxicity
requires K
v
1.3 channel activity. Exposure of microglia to HIV-1
Tat protein was found to enhance both K
v
1.3 currents and
Figure 2. Tat upregulates microglia K
v
1.3 channel expression.
A: A representative RT-PCR gel (left) and its corresponding densitom-
etry bar graph (right) showing enhanced levels of K
V
1.3 mRNA in
microglia treated with Tat (200 ng/ml), but not heat-inactivated Tat (HI
Tat, 200 ng/ml). B: The levels of K
v
1.3 protein was also elevated by Tat
as detected by Western blot (left) and its densitometry bar graph (right).
Data were obtained from three independent experiments. C: Tat-
activated microglia were immunostained for expression of K
v
1.3 (red),
CD11b (green) and Dapi nuclei. Images were visualized by fluorescent
confocal microscopy at 6400 original magnification. Scale bars are
equal to 50 mm. * p,0.05, ** p,0.01 vs Ctrl.
doi:10.1371/journal.pone.0064904.g002
HIV-1 Tat Enhances Microglial K+Channel Activity
PLOS ONE | www.plosone.org 6 May 2013 | Volume 8 | Issue 5 | e64904
production of neurotoxins including IL-1b, TNF-a, NO and ROS,
ultimately leading to neuronal damage. Suppression of K
v
1.3
channels, either by K
v
1.3 channel antagonists or via gene
knockdown, significantly inhibited Tat-induced microglia-mediat-
ed neurotoxicity. These findings indicate K
v
1.3 channel modula-
tion has the potential to mitigate microglia-associated neurotoxic
activity.
Microglia express a defined pattern of K
v
channels including
inward rectifier K
ir
2.1 and outward rectifiers K
v
1.5 and K
v
1.3
[19,36]. Exposure to a variety of stimuli produces a characteristic
pattern of up-regulation of K
v
1.3 and activation of microglia
results in neuronal injury via a process requiring K
v
1.3 activity
[13,14,15,16]. In a previous study we found HIV-1 gp120
exposure activates microglia, in conjunction with enhanced
K
v
1.3 expression and outward K
+
currents, leading to neuronal
apoptosis [24,25]. The gp120-induced microglial neurotoxicity
was significantly attenuated via suppression of K
v
1.3 expression or
blockade of K
v
1.3 current. Similarly, our present study revealed
exposure to HIV-1 Tat increases microglial K
v
1.3 channel
expression and outward K
+
current in association with activation,
neurotoxin secretion, and neuronal apoptosis, which were
successfully inhibited by either siRNA knockdown of the K
v
1.3
gene (Fig. 4) or specific K
v
1.3 blockers MgTx and PAP (Fig. 1,
Fig. 2, and Fig. 7). We confirmed these results in an ex vivo study
using hippocampal slice culture, in which HIV-1 Tat-induced
microglia-mediated neuronal apoptosis was attenuated by pre-
treatment with K
v
1.3 antagonists MgTx or PAP, or a broad
spectrum Kv channel blocker, 4-AP. Collectively, these findings
reveal the integral role of K
v
1.3 channels in regulating microglia
activation and establish a new approach for controlling microglia
mediated neurotoxic activity.
Although reported in several studies to be associated with
diseases including B cell lymphoma [37], breast cancer [38,39]
and Alzheimer’s disease [40], research on the importance of
enhanced microglial K
v
1.3 channel activity in HIV-1 related
cognitive impairment has thus far been sparse. The evidence for
the pivotal role of microglia in HAND pathogenesis is abundant
however, as microglia are well known to mediate HIV entry into
the brain, serve as a reservoir for productive and latent HIV-1
infection, and function as a source of neurotoxic substances
[8,19,41,42,43]. After infection with HIV-1, microglia undergo
dramatic phenotypic, immunological, and functional changes to
produce the cytokines, chemokines, superoxides, and viral proteins
that result in neuronal injury. In order to determine the role of
microglial K
v
1.3 in this process, we examined the neurotoxic
secretions of HIV-1 Tat-treated microglia in relation to K
v
1.3
channel activity. We found blockade of K
v
1.3 channels using
either specific K
v
1.3 antagonists, MgTx and PAP, or a broad
spectrum K
V
blocker, 4-AP, was sufficient to inhibit microglial
production of IL-1b, TNF-a, NO, and ROS. These results are
consistent with our previous findings [24] and suggest enhanced
microglial K
v
1.3 channel activity is required for the HIV-1 Tat-
induced secretion of neurotoxins by microglia.
HIV-1 Tat exposure has been shown to lead to microglia/
macrophage activation, neurotoxin secretion, and subsequent
neuronal damage [22,44,45,46] in a process mediated through
microglial signal transduction pathways such as ERK1/2, PI3K,
and p38 MAPK [47,48]. Tat protein has also been shown capable
of increasing an outward-rectifying K
+
current in rat microglia
through regulation of transcription factor NF-kB [49]. However,
while K
v
1.3 channel activity has here been demonstrated to be
necessary for Tat-induced microglial neurotoxicity, the underlying
Figure 3. K
v
channel antagonists attenuate Tat-induced microglia neurotoxicity. Neurons were exposed to conditioned media recovered
from microglia pre-treated with MgTx (5 nM), PAP (10 nM), or 4-AP (1 mM) for 30 min followed by Tat at varied concentrations (0, 20, 200, 1000 ng/
ml). After 24 hr treatment, TUNEL staining and MTT were performed. A: TUNEL positive neurons were visualized by confocal microscopy at 6400
original magnification. Scale bar equals 50 mm. Note that Tat-treated conditioned media, but not heat inactivated Tat (HI Tat)-treated conditioned
media, induced neuronal apoptosis and that the Tat-induced neuronal apoptosis was blocked by MgTx, PAP or 4-AP. B: Quantitative exhibition of
apoptotic neurons determined by ratio of the number of TUNEL-positive cells to the total number of Dapi-positive cells under different experimental
conditions as indicated. C: MTT assay showed increased viabilities in MgTx, PAP, and 4-AP-treated groups, respectively. Data were from three
independent experiments. ** p,0.01, *** p,0.001 vs Ctrl;
##
p,0.01,
###
p,0.001 vs conditioned media treated with Tat alone.
doi:10.1371/journal.pone.0064904.g003
HIV-1 Tat Enhances Microglial K+Channel Activity
PLOS ONE | www.plosone.org 7 May 2013 | Volume 8 | Issue 5 | e64904
connection remains to be elucidated. Although the breadth of
mechanisms for modulating K
v
channels are numerous, including
regulation of gene expression, post-translational modification,
direct interactions with organic molecules and peptides, and
responsiveness to membrane potential, to name a few, we chose as
an appropriate starting place those signaling pathways which can
convert an extracellular signal such as HIV-1 Tat protein into a
functional cellular response. In particular, we focused on ERK1/2,
which constitute one of the MAPK pathways that commonly
transduce microenvironmental conditions in microglia and have
been implicated in chronic neurodegenerative diseases [30,31,32].
Depending on the cell type, the stimulus, and the duration of cell
activation, a variety of biological responses including cell
proliferation, differentiation, migration, and apoptosis have been
correlated with ERK activation [30,31,32]. In the present study,
we explored whether ERK1/2 activation was involved in the
enhancement of microglial K
v
1.3 expression and neuronal
apoptosis resulting from exposure to HIV-1 Tat. We found
ERK1/2 phosphorylation increased in HIV-1 Tat-treated mi-
croglia in a time-dependent manner (Fig. 6A), but could be
prevented by pre-treatment with U1026, an inhibitor of the
upstream kinase responsible for regulating ERK1/2 activity.
Furthermore, pre-treatment with U1026 ameliorated Tat-induced
microglial K
v
1.3 expression and associated neurotoxicity (Fig. 6C,
Fig. 6D), indicating this process is dependent on activation of the
ERK1/2 MAPK pathway. Lastly, our experiments revealed the
novel finding that HIV-1 Tat-induced ERK1/2 phosphorylation
could be inhibited with K
v
channel antagonists, MgTx, PAP, or 4-
AP. It appears that while Tat-induced microglial K
v
1.3 expression
is dependent on ERK1/2 MAPK, this same pathway is also
responsive to K
v
currents. Given that K
v
currents set the
membrane potential and thus influence Ca
2+
influx, the latter
effect may be mediated through Ca
2+
-dependent intracellular
processes or pathways. While more investigation is necessary, this
reciprocal regulation may allow for intervention in the crucial
transition between functional immune activation and reactive
microgliosis.
In this study, we provided evidence demonstrating K
v
1.3
channels to be an integral component of HIV-1 Tat-induced
Figure 5. K
v
1.3 siRNA abrogates neurotoxic activity of Tat-
activated microglia. Microglia were transfected with siRNA targeting
K
v
1.3 (K
v
1.3-siRNA) or nonspecific GAPD control siRNA (Ctrl-siRNAS) for
48 or 72 hr, followed by an additional 24 hr exposure to Tat (200 ng/
ml). Cells were then harvested for detections of K
v
1.3 mRNA (48 hr post-
transfection/24 hr Tat treatment) and K
v
1.3 proteins (72 hr post-
transfection/24 hr Tat treatment). Supernatants were subjected to
neuronal culture. Neuronal apoptosis and viability assay were
determined using TUNEL staining and MTT assay. A: Representative
gels show RT-PCR products for K
v
1.3 mRNA and internal control b-actin
and bar graph reflects the density of each band after normalization of
its b-actin. B: Western blots show K
v
1.3 protein and internal control b-
actin protein expression of microglia, and bar graph shows densito-
metric quantification of each band. C: Collected supernatants were
subjected to primary neuronal culture at a dilution of 1:5 for 24 hr and
neuronal viability was evaluated by MTT assay. An increased viability
was observed in neurons treated with supernatants recovered from
microglia transfected with K
v
1.3-siRNA, but not transfected with Ctrl-
siRNA. D: Transfection of microglia with K
v
1.3-siRNA significantly
reduced neuronal apoptosis. In contrast, transfection of microglia with
Ctrl-siRNA exhibited no significant protective effect. E: Apoptotic
neurons were visualized by fluorescence microscopy at 6400 original
magnification. Scale bar equals 100 mm. * p,0.05, *** p,0.001 vs Ctrl-
siRNA;
###
p,0.001 vs Ctrl (blank).
doi:10.1371/journal.pone.0064904.g005
Figure 4. K
v
channel blockers inhibited Tat-activated microglia
secretion of neurotoxins. Microglia were treated with MgTx (5 nM),
PAP (10 nM), or 4-AP (1 mM) for 30 min before addition of Tat at
200 ng/ml. After 24 hr incubation, the supernatants were harvested for
detection of IL-1b(A), TNF-a(B), NO (C) and the cells were used for
analysis of ROS production (D). Data presented were from three
independent experiments. *** p,0.001) vs Ctrl;
##
p,0.01 or
###
p,0.001 illustrates Tat alone vs pre-Tat plus MgTx, PAP or 4-AP.
doi:10.1371/journal.pone.0064904.g004
HIV-1 Tat Enhances Microglial K+Channel Activity
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microglia-mediated neurotoxicity and a potential site of regula-
tion. This promising data opens the future possibility of using
K
v
1.3 channel inhibitors as a novel strategy to combat HAND and
other neurodegenerative disorders in which the pathophysiological
process involves microglia-mediated immune and inflammatory
responses. In a previous in vivo study, we demonstrated the
administration of K
v
channel antagonist 4-AP could ameliorate
HIV-1-induced encephalitis and cognitive disorder, improving
spatial learning and memory in a severe combined immunodefi-
cient (SCID) mouse model of HIV-1 encephalitis (HIVE) [50].
Nevertheless, as K
v
channel blockers selectively target immune
cells including macrophage, microglia, and lymphocytes, the
therapeutic benefit of this approach must be carefully considered
for potential risks to the immune system. Of these cells, the most
abundant K
v
1.3 channel expression is reported to be found the
human effector memory T cells (CD4
+
CCR7
2
CD45RA
2
), which
regulate Th1 cell inflammatory responses [36,51,52]. The use of
small molecule K
V
blockers, such as verapamil, dilitazem, and
nifedipine, has been shown to reduce IL-12 secretion and inhibit T
cell proliferation [53]. Notably, these immunomodulatory effects
have been found to depend heavily on the level of K
v
1.3 channel
expression, which changes dramatically as T cells differentiate
from naı
¨ve to memory states or transition from resting to
activation [54,55,56]. Given that the immune response functions
of microglia, macrophage, Helper T cells, and B cells remain
basically intact, the potential side effects of pharmacologically
blocking K
v
1.3 channels could be minimal [52,57]. In fact, the
potential side effects of using K
V
channel blockers to treat a wide
range of autoimmune conditions, including those involving
effector memory T cells, delayed type hypersensitivity, type 1
diabetes, rheumatoid arthritis, multiple sclerosis, and inflammato-
ry bone resorption, have been investigated without revealing any
generalized immunesuppression [58,59,60,61]. Recently, the
safety of this approach was given further credence when the US
Food and Drug Administration approved the K
V
channel blocker
dalfampridine (Ampyra) as a treatment multiple sclerosis (http://
www.fad.gov/NewsEvents/Newsroom/PressAnouncements/
ucm198463.htm).
In summary, the present study serves to establish the integral
role of K
v
1.3 channel activity in HIV-1 Tat-induced microglia-
mediated neurotoxicity. The identification of K
v
1.3 channels as a
point of intervention in this process may open new avenues for
therapeutic modalities. Given its feasibility and safety, it may now
be advantageous to consider studying a K
v
1.3 channel-based
therapeutic approach in the treatment of HAND and other
neurodegenerative disorders characterized by microglia-mediated
neuroinflammation.
Acknowledgments
The Authors thank Ms. Julie Ditter, Ms. Robin Taylor, Ms. Johna Belling
and Ms. Sandra Wiese for their excellent administrative support.
Author Contributions
Conceived and designed the experiments: JL HX. Performed the
experiments: JL PX CC HL JZ. Analyzed the data: JL PX CC HX.
Wrote the paper: JL JK HX.
Figure 7. K
v
channel blockers ameliorated Tat-induced microg-
lia neurotoxicity in rat hippocampus slices. Rat hippocampus
slices were pretreated with MgTx (5 nM), PAP (10 nM), or 4-AP (1 mM)
for 30 min before addition of Tat (200 ng/ml). Immunohistochemistry
or TUNEL staining was performed 24 hr later. A: Tat increased levels of
K
v
1.3 expression that were co-localized with microglia (Iba1 staining) in
rat hippocampus slices. Rat hippocampus slices were stained with
mouse anti-Iba1 Ab (1:1000, red), whereas K
v
1.3 was stained with goat
polyclonal antibody (1:200, green). Images were visualized by confocal
microscopy. B: TUNEL staining showed that Tat produced neuronal
apoptosis in rat hippocampus slices that was attenuated by MgTx, PAP,
or 4-AP. Numerical numbers in each image panel represents the
average apoptotic cells (M 6SD, n= 3 slices, 5 random visual fields
were counted in each slice) in experimental conditions as indicated.
doi:10.1371/journal.pone.0064904.g007
Figure 6. Involvement of ERK1/2 MAPK pathway in Tat-
mediated upregulation of K
V
1.3 expression. A: Western blot
analysis for ERK1/2 MAPK. Microglia were exposed to 200 ng/ml of Tat
and harvested at indicated times. Protein expression was analyzed by
immunoblot using antibodies against ERK1/2 phosphorylation (pERK1/
2) and total ERK1/2 (ERK1/2) MAPK. Tat up-regulated pERK1/2 MAPK in a
time window from 30 min to 5 hr. B:K
v
channel antagonists inhibit
ERK1/2 MAPK phosphorylation. Microglia were treated with MgTx
(5 nM), PAP (10 nM), or 4-AP (1 mM) for 30 min followed by Tat at
200 ng/ml for additional 5 hr. Gel blots reveal a reduction of Tat-
induced ERK1/2 phosphorylation in microglia treated with MgTx, PAP,
or 4-AP, indicating a link between K
v
1.3 channel activation and ERK1/2
MAPK signal pathway. C: Western blot results showing that the
blockade of Tat enhancement of K
v
1.3 expression in microglia was
blocked by U0126, an inhibitor for MEK1 and MEK 2, further
demonstrating the link between ERK1/2 MAPK and Tat-induced
increase of K
v
1.3 expression. D: TUNEL staining exhibited a significant
increase of neuronal apoptosis induced by the supernatants collected
from Tat-treated microglia and its blockade by U0126, a MEK1 and
MEK2 inhibitor. Data were from three independent experiments. ***
p,0.001 vs Ctrl;
##
p,0.001 vs Tat-treated alone.
doi:10.1371/journal.pone.0064904.g006
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