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biomolecules
Review
Exocytosis in Astrocytes
Aleksandra Mielnicka and Piotr Michaluk *
Citation: Mielnicka, A.; Michaluk, P.
Exocytosis in Astrocytes. Biomolecules
2021,11, 1367. https://doi.org/
10.3390/biom11091367
Academic Editor: Agnieszka Jurga
Received: 16 August 2021
Accepted: 14 September 2021
Published: 16 September 2021
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BRAINCITY, Laboratory of Neurobiology, The Nencki Institute of Experimental Biology, PAS,
02-093 Warsaw, Poland; a.mielnicka@nencki.edu.pl
*Correspondence: p.michaluk@nencki.edu.pl; Tel.: +48-22-5892-382
Abstract:
Until recently, astrocytes were thought to be a part of a simple “brain glue” providing
only a supporting role for neurons. However, the discoveries of the last two decades have proven
astrocytes to be dynamic partners participating in brain metabolism and actively influencing com-
munication between neurons. The means of astrocyte-neuron communication are diverse, although
regulated exocytosis has received the most attention but also caused the most debate. Similar to
most of eukaryotic cells, astrocytes have a complex range of vesicular organelles which can undergo
exocytosis as well as intricate molecular mechanisms that regulate this process. In this review, we
focus on the components needed for regulated exocytosis to occur and summarise the knowledge
about experimental evidence showing its presence in astrocytes.
Keywords: SNARE; secretion; vesicles; transmitter; lysosome; gliotransmission
1. Introduction
Astrocytes, together with oligodendrocytes and microglia, form three main categories
of glial cells in the central nervous system. They are found throughout the brain and
occupy around half of the brain’s volume. Astrocytes have a bushy morphology with
few main processes, starting from a relatively small cell body and extending many fine
processes that invade extracellular space. It has been estimated that an astrocyte can
contact over 100,000 synapses in rats and almost two million in the human cortex [
1
].
Even though astrocytes tend to occupy distinct and non-overlapping domains, their fine
processes are in contact with one another via gap junctions. They create, therefore, a large
network of interconnected cells which can easily exchange ions and small molecules of up
to 1–1.2 kDa [2].
For a long time, neuroscientists considered glia as merely supportive cells in the brain
that control ion and water homeostasis, produce and remove neurotransmitters, induce
synaptogenesis, provide trophic factors for neurons and maintain the blood–brain barrier.
Their view as “passive” cells was reinforced by observations that astrocytes expressed
potassium and sodium channels and could exhibit evoked inward currents, but they did
not “fire” or propagate action potentials [
3
]. Therefore, it can be safely said that the entire
field was revolutionised by the discovery of glutamate-induced Ca
2+
waves in cultured
astrocytes. This raised the possibility that astrocytes may contribute to modulating neu-
ronal activity [
4
]. This observation was later confirmed many times in different
in vitro
and
in vivo
models [
5
–
9
]. Further demonstrations that intracellular Ca
2+
levels in astrocytes
can cause a release of glutamate and a subsequent Ca
2+
increase in neurons [
10
,
11
] led to
the concept of gliotransmission i.e., the release of transmitters from astrocytes and other
glial cells to neurons. This, in turn, helped to coin the term “tripartite synapse”, where
astrocyte processes form an integral part of a functional brain connection in addition to
pre- and post-synaptic compartments [
12
]. The classical understanding of this model as-
sumes that through spillover neurotransmitters and other factors released by neurons, bind
to high-affinity astrocytic G protein-coupled receptors (GPCRs), triggering inositol-1,4,5-
trisphosphate (IP
3
) production and Ca
2+
release from the endoplasmic reticulum (ER) [
13
].
Biomolecules 2021,11, 1367. https://doi.org/10.3390/biom11091367 https://www.mdpi.com/journal/biomolecules
Biomolecules 2021,11, 1367 2 of 19
Increases in astrocytic Ca
2+
levels lead to a release of gliotransmitters, including glutamate,
ATP, D-serine and
γ
-aminobutyric acid (GABA) [
14
]. The release of gliotransmitters has a
wide range of effects on neurons, such as stimulating N-methyl-D-aspartate receptor (NM-
DAR), synchronising neuronal spiking [
15
], as well as regulating synaptic vesicle release
probability, synaptic plasticity and even behaviour [
7
,
14
,
16
–
18
]. Despite this description
of different modes of transmitter release from astrocytes, regulated exocytosis became the
most studied but also criticised (for a review, see [7,19,20]).
Exocytosis is a process in which the cargo of a secretory vesicle is released across
the cell membrane. This universal process, which is common to all eukaryotic cells, can
be generally divided into two types: unregulated and regulated exocytosis. Unregulated
(constitutive) exocytosis typically involves the formation of membranous secretory vesicles
within the cell, in which the cargo is packaged and then continually released through the
cell membrane. In regulated exocytosis, the secretory vesicle with its cargo is stored until
a signal triggers the process of secretion [
21
–
23
]. The secretory vesicle has to go through
several stages before secretion actually occurs. The first stage, docking, is the tethering and
linking of the vesicle to the plasma membrane. Docking is followed by priming, during
which the attached vesicle’s membrane is brought in proximity to the release site at the
plasma membrane. Finally, a trigger (most commonly an influx of Ca
2+
) leads to fusion,
during which the vesicle and the plasma membrane combine with each other. After fusion,
the secretory vesicle can either be recycled by the closing of the fusion pore, a model termed
“kiss-and-run”, or it can collapse and fully integrate with the plasma membrane [
21
,
24
].
Regardless of the mechanism of exocytosis, all cells require vesicular organelles with a
cargo and the molecular machinery to conduct exocytosis.
Similar to all eukaryotic cells, astrocytes contain different types of vesicular organelle
which cargo can be released to the environment. Generally, secretory vesicles form a
complex network originating from the ER or the Golgi apparatus, as well as from endo-
somes. Astrocytes are not different, and they contain several secretory organelles, including
synaptic-like microvesicles (SLMV) [
25
–
28
], dense-core vesicles (DCV) [
29
–
32
], secretory
lysosomes (SL) [
33
–
35
] and extracellular vesicles, which can be further divided into ex-
osomes and ectosomes [
36
,
37
]. Out of these, the SLMVs, DCVs and SLs have been well
described in the literature as undergoing exocytosis in astrocytes (see Table 1). In this
review, we focus on these three types of vesicular organelle, and the molecular machinery
regulating exocytosis and providing its spatial organisation, as well as on the Ca
2+
sensors
which can trigger membrane fusion.
Table 1. Secretory vesicles undergoing exocytosis in astrocytes.
Secretory Organelle Diameter Cargo Associated Proteins
Protein Name Gene Name
Synaptic-Like
Microvesicles (SLMVs) 30–100 nm Glutamate
d-serine
VGluT1
VGlut2
VGlut3
VAMP2 (synaptobrevin 2)
VAMP3 (cellubrevin)
Rab3a
V-ATPase
Slc17a7
Slc17a6
Slc17a8
Vamp2
Vamp3
Rab3a
Atp6v0, Atp6v1 1
Dense-Core Vesicles
(DCV) 100–600 nm
ANP
ATP
BDNF
Secretogranin II
Secretogranin III
Chromogranin
NPY
VAMP2 (synaptobrevin 2)
VAMP3 (cellubrevin)
VNuT
Vamp2
Vamp3
Slc17a9
Biomolecules 2021,11, 1367 3 of 19
Table 1. Cont.
Secretory Organelle Diameter Cargo Associated Proteins
Protein Name Gene Name
Secretory Lysosome
(SL) 300–500 nm
ATP
Cathepsin B
Cathepsin D
Proteolytic enzymes
VAMP7 (TI-VAMP)
Rab7
CD63
LAMP1
Sialin
VNuT
Vamp7
Rab7a
Cd63
Lamp1
Slc17a5
Slc17a9
1
V-ATPase is a large complex consisting of 13 subunits coded by many more genes; however, all the gene names start with Atp6v0 or with
Atp6v1 for the VOand V1domains, respectively.
1.1. Synaptic-like Microvesicles (SLMVs)
Astroglial SLMVs are small (30–100 nm in dimeter) electron-lucent vesicles, similar to
neuronal synaptic vesicles. They were first identified in the brain tissue in the molecular
layer of the dentate gyrus using electron microscopy [
25
]. They are much less numerous
than synaptic vesicles in neurons; however, they form small groups of 2–15 in the astro-
cytic cytoplasm and were shown to be present in the vicinity of NMDARs at asymmetric
synapses [
25
,
38
,
39
]. Further studies have also found astrocytic SLMVs close to extrasy-
naptic NMDARs containing the GluN2B subunit [
26
]. Despite being grouped, SLMVs in
astrocytes are not concentrated by structurally organised active zones, as it is the case in
neurons [
26
]; however, the endoplasmic reticulum (ER) seems to be in proximity to these
clusters [
25
,
38
]. This localisation is in line with the canonical tripartite synapse model,
where local increases in Ca
2+
concentration released from the ER can trigger a release of
gliotransmitters. It should be noted, however, that ER is usually not present in perisynaptic
processes [
40
]. Additionally, it was shown that SLMVs containing vesicle-associated mem-
brane protein 2 (VAMP2) are highly mobile in astrocytes, and that an increase in cytosolic
Ca
2+
causes vesicle docking [
29
]. Similarly, Ca
2+
-regulated mobility of vesicular glutamate
transporter 1 (VGluT1)-positive vesicles have been found also in other studies [41].
SLMVs contain mainly small signalling molecules: glutamate and D-serine [
37
]. Fill-
ing vesicles with a transmitter requires active transport fuelled by an electrochemical
gradient generated by the vesicular ATP-dependent H
+
pump (V-ATPase), a large complex
composed of two domains, V
O
(transmembrane, responsible for H+ translocation) and V
1
(responsible for ATP hydrolysis) [
23
]. Expression of V-ATPase has been shown in cultured
astrocytes by fractionation and immunoblotting [
27
,
42
], immunofluorescence [
43
] and func-
tionally, by usage of its blocker—bafilomycin [
44
–
46
]. The transport of a neurotransmitter is
mediated by vesicular transporters. There are three vesicular glutamate transporters (VGluT1-
3) and they all have been identified in cultured astrocytes [
25
,
27
,
29
,
33
–
35
,
45
,
47
–
49
]. Moreover,
expression of VGluT1 and -3 has been demonstrated in acute and fixed tissue by a number
of techniques, including confocal and electron microscopy, and RT-PCR [
25
,
39
,
46
,
50
,
51
].
Electron microscopy studies have shown that VGluT1-3 colocalises with SLMVs in the hip-
pocampus [
25
,
39
,
51
] and, importantly, in the case of VGluT1 and -3, immunogold labelling
was not present in knockout animals of those proteins [
39
,
51
]. Additionally, Bezzi and
co-workers [
25
], by means of immunogold staining, have shown coloclization of VGluT1
and -2 with VAMP3. Notably, other studies have not confirmed the expression of VGluTs
in astrocytes by means of RNA-seq [
52
,
53
], gene chip microarrays [
54
], or by confocal
microscopy [
55
]. It has to be noted, however, that high-throughput methods such as RNA
sequencing or gene chip microarrays might not be sensitive enough to detect low protein
expression, and that because reported exocytosis in astrocytes is slow and glutamate is also
a metabolite for astrocytes, they might not need high expression of VGluTs [56,57].
So far, no D-serine vesicular transporter has been identified in astrocytes in situ;
however, it was shown to be present in immunopurified astrocytic vesicles in one study [
27
].
The identified transporter was proposed to be the D-serine/chloride co-transporter, which
uses the H
+
gradient created by V-ATPase concentrated D-serine in the vesicles [
27
]. It is
Biomolecules 2021,11, 1367 4 of 19
still not clear whether D-serine is loaded together with glutamate to the same SLMVs. This
process of vesicular synergy is present in neurons where glutamate is co-released with
dopamine, serotonin or acetylcholine [
58
]. VGluTs and D-serine are located in astrocytic
VAMP2- and VAMP3-positive vesicles, which raises the possibility that these may be the
same vesicles [
25
,
45
,
49
,
59
,
60
]. Additionally, in immunopurified SLMVs, both transmitters
can be present within the same vesicle, and D-serine application modulates the uptake of
glutamate to immunopurified SLMVs and vice versa [
27
]. This suggests that these two
amino acids may be released from the same vesicles. Notably, D-serine had no effect on
glutamate uptake into immunopurified synaptic vesicles [
27
]. Contrary to this evidence,
in fixed tissue, it was shown that D-serine and glutamate are stored in distinct vesicles
within the same astrocyte [38]. Notably, it has also been recently shown that the source of
D-serine is mainly neuronal, as serine racemase (SR)—the enzyme which converts L-serine
into D-serine—is expressed almost entirely in the neurons [
61
,
62
]. This, however, does
not exclude the possibility of D-serine transport from neurons to astrocytes and secondary
astroglial release of D-serine [
63
]. Additionally, traumatic brain injury causes a population
of reactive astrocytes to express SR, so astroglial D-serine may be important in pathological
conditions [64].
1.2. Dense-Core Vesicles (DCV)
Dense-core vesicles (DCVs) have been well studied in a variety of tissues, where
they are responsible for the storage and secretion of biogenic amines, peptides and neu-
rotrophins, including catecholamines released from adrenal chromaffin cells or insulin
released from DCVs in pancreatic
β
cells [
21
]. In astrocytes, DCVs are larger than SLMVs,
being 100–600 nm [
30
–
32
] in diameter; however, it has been reported that atrial natri-
uretic peptide (ANP)-containing vesicles can be 50 nm in diameter [
65
]. As their name
suggests, DCVs have an electron-dense core, although it is not as dense as that in neuroen-
docrine cells [
36
,
65
]. DCVs are not very abundant in astrocytes, accounting for roughly
2% of the vesicles [
29
]. Despite their low numbers, they have been shown in culture to
contain an array of molecules such as secretogranin II [
30
,
31
,
66
,
67
] and III [
68
,
69
], chromo-
granins [
32
], ANP [
66
,
70
,
71
], neuropeptide Y (NPY) [
31
,
72
], brain-derived neurotrophic
factor (BDNF) [
73
,
74
] and ATP [
75
,
76
]. To our knowledge, only secretogranins consisting
of DCVs have been shown to be present in situ in human brain tissue [
32
]. Interestingly,
the same study showed the existence of inositol-1,4,5-triphosphate (IP
3
) receptors (IP
3
Rs)
in DCVs, suggesting that they can serve as IP
3
-sensitive intracellular Ca
2+
reservoirs. As
they are relatively small, they could be directed to many distinct processes of the astrocyte,
including perisynaptic processes [
32
], and provide an additional source of Ca
2+
for regu-
lated exocytosis. Similar to SMLVs, DCVs have also been found to be highly mobile, and
an increase in cytosolic Ca2+ levels seems to decrease this mobility [77,78].
Of all molecules present in DCVs, ATP has attracted the most attention, as it is a
potent transmitter influencing glial and neuronal signalling, as well as behaviour [
79
,
80
].
Its localisation in DCVs in cultured astrocytes was demonstrated on a few occasions [
74
–
76
]
but, to our knowledge, not in the tissue. ATP is transported into vesicles by the vesicular
nucleotide transporter (VNuT) [
81
], which is also present in microglia [
82
]. VNuT is present
in cultured astrocytes [
47
,
83
–
85
] as well as in freshly isolated astrocytes [
86
]. Even though
there seems to be a lack of evidence showing VNuT in the astrocytes in tissue, an astrocyte-
selective VNuT knockout was shown to be important for fluoxetine-induced antidepressive
behaviour [
87
]. This provides proof, albeit indirect, that ATP is transported into the vesicles
in astrocytes in vivo.
1.3. Secretory Lysosomes (SL)
Secretory lysosomes have diameters ranging from 300 to 500 nm [
34
,
88
] and have
been identified multiple times in cultured astrocytes or in freshly isolated astrocytes,
where they store and release ATP in a Ca
2+
-dependent manner [
34
,
35
,
89
]. Moreover,
lysosomes can coexist with SLMVs in the same astrocyte, and they fuse with the plasma
Biomolecules 2021,11, 1367 5 of 19
membrane in a Ca
2+
-regulated manner, although small vesicles are exocytosed more
efficiently than lysosomes [
33
]. Secretory lysosomes contain ATP, cathepsin B and D and
other proteolytic enzymes [
34
,
37
,
90
,
91
]. Secretory lysosomes in cultured astrocytes can be
labelled by dextrans [
92
,
93
], various FM dyes and MANT-ATP—a fluorescent analogue
used in studies of ATP stores [
34
,
94
]—as well as quinacrine, which emits a green fluorescent
signal in the presence of intracellular ATP [
91
]. One study has found, however, that FM
dyes do not enter astrocytes on the endocytic pathway but rather via the store-operated
calcium channel and do not stain lysosomes [
95
]. In this study, the authors did not use
any lysosomal/endosome markers except for fluorescently labelled dextran. Astroglial
secretory lysosomes lack VGluTs, VAMP2 and VAMP3 [
33
,
34
]; however, they express
lysosomal-specific markers including cathepsin D, lysosomal-associated membrane protein
1 (LAMP1) [
34
,
59
], sialin, CD63/LAMP3 [
35
], monomeric ras-related protein Rab7 [
96
] and
VAMP7 [
35
]. Interestingly, VAMP7 is insensitive to cleavage by the tetanus neurotoxin
(TeNT) and contributes to TeNT-independent exocytotic release of ATP, hence its alternative
name (TI-VAMP) [
33
,
91
]. Downregulation of VAMP7 expression inhibits the fusion of ATP-
storing vesicles and decreases ATP-mediated calcium wave propagation [
91
], which is an
important form of long-range communication in the astrocytic network. Similar to DCV,
secretory lysosomes in the astrocytes express VNuT [
83
], which transports ATP from the
cytoplasm into the vesicles.
2. The Exocytosis Machinery in Astrocytes
Secretory vesicle docking, priming and fusion with a plasma membrane are generally
mediated by SNARE proteins (soluble N-ethylmaleimide-sensitive fusion protein attach-
ment protein receptors) and SM proteins (Sec1/Munc18-like proteins) that undergo a cycle
of association and dissociation during the fusion reaction. The mechanism of regulated
SNARE complex assembly is conserved in many different cell types, including neuronal,
exocrine, haematopoetic and endocrine cells (Figure 1; for reviews, see [
21
,
23
,
97
–
99
]). The
SNARE complex consists of two target membrane proteins (t-SNARE), namely syntaxin
and SNAP-25 (or SNAP-23), and one vesicle-associated (v-SNARE) protein, VAMP2. Syn-
taxins are ~35 kDa proteins containing a carboxy-terminal transmembrane domain and
an amino terminus oriented toward the cytoplasm. The other t-SNARE, SNAP-25 or -23
(25 and 23 kDa respectively), is attached to the plasma membrane via four palmitoylated
cysteine residues. The v-SNARE VAMP2 is a 18 kDa protein with a vesicle lumen-oriented
carboxyl terminus and an amino terminus oriented toward the cytoplasm. During the
vesicle docking process, the SM protein Munc18-1 initially binds to the closed conformation
of syntaxin1. When the closed conformation of syntaxin1 “opens” during priming and
SNARE complexes starts forming, Munc18-1 remains attached to syntaxin1 in the assem-
bling SNARE complex but switches its binding mode to an interaction with the SNARE
complex. The SNARE complex is extremely stable, where one v-SNARE binds with two t-
SNARE proteins in a 1:1:1 ratio. The SNARE complex forms initially a “trans” configuration
(SNARE proteins are on opposite membranes) with amino- to carboxy-terminal zippering
of proteins, which brings the vesicle closer to the plasma membrane. Assembly of the
full trans-SNARE complex, together with the action of the SM protein, opens the fusion
pore. After fusion pore opening, the membrane of the vesicle and the plasma membrane
completely merge, and trans-SNARE complexes are converted into cis-SNARE complexes,
where the SNARE complexes are on a single membrane. Next, the cis-SNARE complex pro-
teins are bound by N-ethylmaleimide sensitive factor (NSF) and soluble NSF-attachment
proteins (SNAPs, no relation to SNAP-25 and its homologs) to catalyse SNARE complex
dissociation into monomers. This allows for endocytosis of the v-SNARE and recycling of
the individual t-SNAREs back to their respective plasma membrane compartments.
Biomolecules 2021,11, 1367 6 of 19
Biomolecules 2021, 11, x FOR PEER REVIEW 6 of 19
ment proteins (SNAPs, no relation to SNAP-25 and its homologs) to catalyse SNARE com-
plex dissociation into monomers. This allows for endocytosis of the v-SNARE and recy-
cling of the individual t-SNAREs back to their respective plasma membrane compart-
ments.
Figure 1. Schematic SNARE/SM cycle. SNARE and SM proteins undergo a cycle of assembly and disassembly. At the
beginning of docking, syntaxin1 is present in a “closed” conformation in which its Habc domain (purple) blocks its SNARE
motif (dark blue rectangle). In this position, Munc18-1 binds monomeric syntaxin1. For the SNARE complex to assemble,
syntaxin1 has to ‘‘open’’. During this conformational change, the SNARE complex assembly and Munc18-1 change their
binding to syntaxin1 by binding to assemble the trans-SNARE complexes via interacting with the syntaxin1 N-peptide.
Once the SNARE complexes have partly assembled, complexin binds to further tighten secretory vesicle priming. The
‘‘superprimed’’ SNARE/SM protein complexes are then ready for the Ca
2+
-trigger. Ca
2+
binds to synaptotagmin, which
causes an interaction between synaptotagmin and SNAREs and phospholipids of the plasma membrane. After fusion pore
opening, the vesicular membrane and plasma membrane merge, resulting in a change from trans- to cis-SNARE complexes.
The association of NSF/SNAP ATPases disassembles SNARE complexes to free SNAREs, and the vesicle is recycled, can
be refilled with neurotransmitters, and reused for another release (modified from [98]).
For over two decades, a large number of studies identified components of the SNARE
complex and SM proteins in the astrocytes; however, the issue of regulated exocytosis still
remains a matter of debate. Out of the v-SNAREs, the expression of VAMP2 and VAMP3
has been shown multiple times in vitro [29,45,100–102] and in vivo [43,46,52–54,103].
The functionality of VAMP2/VAMP3 has been confirmed by multiple studies, espe-
cially by the use of tetanus neurotoxin (TeNT) and botulinum neurotoxin (BoNT), which
cleave SNARE proteins. In particular, application of TeNT to cultured astrocytes attenu-
ated the exocytotic release of glutamate [25,45,104–106]. Moreover, the enzymatically ac-
tive light chain of TeNT was applied through a patch pipette to astrocytes in acute slices,
where it abolished the release of
D
-serine [16] or glutamate [26,107], which affected neigh-
bouring neurons. Importantly, Haydon’s laboratory has developed a transgenic mouse
Figure 1.
Schematic SNARE/SM cycle. SNARE and SM proteins undergo a cycle of assembly and disassembly. At the
beginning of docking, syntaxin1 is present in a “closed” conformation in which its Habc domain (purple) blocks its SNARE
motif (dark blue rectangle). In this position, Munc18-1 binds monomeric syntaxin1. For the SNARE complex to assemble,
syntaxin1 has to “open”. During this conformational change, the SNARE complex assembly and Munc18-1 change their
binding to syntaxin1 by binding to assemble the trans-SNARE complexes via interacting with the syntaxin1 N-peptide.
Once the SNARE complexes have partly assembled, complexin binds to further tighten secretory vesicle priming. The
“superprimed” SNARE/SM protein complexes are then ready for the Ca
2+
-trigger. Ca
2+
binds to synaptotagmin, which
causes an interaction between synaptotagmin and SNAREs and phospholipids of the plasma membrane. After fusion pore
opening, the vesicular membrane and plasma membrane merge, resulting in a change from trans- to cis-SNARE complexes.
The association of NSF/SNAP ATPases disassembles SNARE complexes to free SNAREs, and the vesicle is recycled, can be
refilled with neurotransmitters, and reused for another release (modified from [98]).
For over two decades, a large number of studies identified components of the SNARE
complex and SM proteins in the astrocytes; however, the issue of regulated exocytosis still
remains a matter of debate. Out of the v-SNAREs, the expression of VAMP2 and VAMP3
has been shown multiple times in vitro [29,45,100–102] and in vivo [43,46,52–54,103].
The functionality of VAMP2/VAMP3 has been confirmed by multiple studies, espe-
cially by the use of tetanus neurotoxin (TeNT) and botulinum neurotoxin (BoNT), which
cleave SNARE proteins. In particular, application of TeNT to cultured astrocytes atten-
uated the exocytotic release of glutamate [
25
,
45
,
104
–
106
]. Moreover, the enzymatically
active light chain of TeNT was applied through a patch pipette to astrocytes in acute slices,
where it abolished the release of D-serine [
16
] or glutamate [
26
,
107
], which affected neigh-
bouring neurons. Importantly, Haydon’s laboratory has developed a transgenic mouse
which overexpress a dominant negative SNARE (cytosolic tail of VAMP2, dnSNARE) in
astrocytes [
17
]. This method had been previously shown to decrease astrocytic gliotrans-
mission
in vitro
[
46
] and was later explained in detail as stabilizing the fusion pore in a
narrow, release-unproductive state, thus effectively decreasing exocytosis [
73
]. Experiments
Biomolecules 2021,11, 1367 7 of 19
with dnSNARE transgenic mice performed not only by Haydon’s group showed reduced
SNARE-dependent gliotransmission (mostly of adenosine and ATP), which influences the
behaviour, synaptic transmission and maturation of neurons [
17
,
86
,
108
–
113
]. Although
studies using dnSNARE mice were convincing and showed the transgenic expression
specifically in astrocytes, a study by Fujita and co-workers [
114
] found that dnSNARE
can be also expressed in the neurons of transgenic mice, thus questioning the previous
findings. This has led to a heated debate (see [
115
,
116
]). The discussion has pointed to
technical matters in studying gliotransmission, particularly the use of the GFAP promoter
as a glial-specific promoter. Importantly, another study using astrocyte-specific expression
of the BoNT serotype B light chain (BoNT/B) in a Cre/Lox system (named “iBot mice”)
showed decreased VAMP-dependent glutamate release from the astrocytes and impaired
glial volume regulation [
117
]. Later, the same model (iBot mice) was used in parallel with
dnSNARE mice to show that blockade of gliotransmitter release from astrocytes influences
adult-born neurons, reduces their glutamatergic synaptic input as well as dendritic spine
density, and leads to their lower functional integration and hence survival [
110
]. This
result validated the relevance of the dnSNARE mouse model, as well as the importance of
astrocytic exocytosis.
Other v-SNARE proteins have also been identified in astrocytes. In particular, VAMP7
was shown in astrocytes
in vitro
as a v-SNARE associated with lysosomes [
35
,
59
,
91
], as was
VAMP8 [
35
,
59
]. Additionally, VAMP4 and VAMP8 can be found to be actively expressed
in vivo
in the cortex, hippocampus and striatal astrocytes, based on RNA sequencing [
53
].
For t-SNARE proteins, astrocytes were shown to express syntaxin1; however, the
results of various studies have been mixed. Syntaxin1 has been demonstrated in cultured
astrocytes [
45
,
66
], albeit at a lower level than in isolated synaptosomal membranes [
118
].
Additionally, one group failed to find syntaxin1 in cultured astrocytes [
119
], while an-
other was able to identify it in astrocytes in situ [
120
]. Syntaxin4, however, seems to
have a much higher expression level in astrocytes than syntaxin1, and it was shown to
be expressed in culture [
66
], in situ [
121
] and ex vivo from freshly isolated astrocytes,
based on RNA sequencing [
53
]. With regard to the second t-SNARE, it seems that as-
trocytes express SNAP-23 rather than SNAP-25. The latter failed to be found even in
cultured
astrocytes [45,66,118,119,122,123]
. SNAP-23 was, however, identified in astro-
cytes
in vitro
[
27
,
29
,
43
,
45
,
46
,
48
,
66
,
122
,
123
] and in fixed tissue
in vivo
[
43
,
120
], although
one recent study has found its expression to be insignificant, based on RNA sequencing,
while showing SNAP-25 expression [
53
]. Therefore, the expression profile of SNARE
proteins in astrocytes seems to follow other non-neuronal cells, where the SNARE com-
plex consists mostly of SNAP-23, syntaxin 4 and VAMP2 (synaptobrevin 2) or VAMP3
(cellubrevin) [124–126].
As mentioned above, the SNARE complex requires SM proteins for its formation, in
particular Munc-18 is a prominent and well-studied example. It was shown to be significant
for vesicle docking in chromaffin cells where Munc18-1 knockout reduces fraction of
docked vesicles from 30% to 5% [
21
,
127
]. Moreover, Munc18-1 binding to assembling
SNARE complexes is essential for synaptic vesicle fusion, and in every SNARE-dependent
fusion reaction studied, an SM protein participated and was essential for that fusion
reaction [
98
]. Munc18-1 and Munc18-3 have been found in cultured and freshly isolated
astrocytes [
46
,
66
,
128
], while RNA sequencing from freshly isolated astrocytes has shown
that they express Munc18-3 rather than Munc18-1, which is expressed in the neurons [
52
].
A similar result has recently been found in another study using RNA sequencing, where
Munc18-1 expression was found in astrocytes at a low level; however, Munc18-3 and
Munc18-4 were enriched in the astrocytes [53].
Other components of the exocytosis machinery in astrocytes have not been intensively
studied. Notable exceptions are the Rab proteins—small GTPases that are localised on
synaptic vesicles and are important for their docking. Cultured astrocytes were shown to
express Rab3 [
100
,
101
,
129
,
130
], which is usually associated with SMLVs or DCVs. A recent
study in cultured astrocytes has shown that disruption of Rab3 function by mutated Hunt-
Biomolecules 2021,11, 1367 8 of 19
ingtin blocks the docking of DCVs and ultimately decreases BDNF and ATP release [
74
].
Expression of Rab7, which is associated with late endosomes/lysosomes, was also reported
in astrocytes
in vitro
[
29
,
35
,
91
,
131
]. Additionally, Rab10 and Rab35 have also been shown
to be associated with lysosomes in cultured astrocytes [
131
]. Thanks to access to a database
(http://astrocyternaseq.org/ accessed on 1 January 2021) of sequenced astrocytic RNAs
from different brain structures [
53
], we were able to confirm the expression of Rab3a, Rab7
and Rab35 in freshly isolated astrocytes. All these proteins are expressed at a significantly
different level from the expression threshold set as fragments per kilobase per million
(FPKM) >10 (see [53]).
3. Spatial Organisation of Exocytosis
Ultrafast neurotransmitter release, which occurs in neurons in response to an action
potential, can only be achieved by bringing Ca
2+
channels to docked and primed synaptic
vesicles at the active zone. Most of the active zones in vertebrates are specialised disc-
like structures at the plasma membrane with a 0.2–0.5
µ
m diameter. Active zones are
surrounded by a perisynaptic zone containing transsynaptic cell adhesion molecules and
receptors regulating neurotransmitter release, which is the site of synaptic vesicle endocyto-
sis [
132
]. The core of the active zone is formed mainly by the members of six evolutionarily
conserved proteins: Rab3-interacting molecules (RIM), RIM-binding proteins (RIM-BP),
Munc13 (no relation to Munc18),
α
-liprin and ELKS protein, as well as piccolo/bassoon.
RIM, RIM-BP and Munc13, are multidomain proteins, whereas
α
-liprin and ELKS exhibit
a simpler structure. Active zone proteins form a single large protein complex that docks
and primes synaptic vesicles, recruits Ca
2+
channels to the docked and primed vesicles,
tethers the vesicles and Ca
2+
channels to synaptic cell adhesion molecules, and mediates
synaptic plasticity. Astrocytes do not have an active zone that could be observed under an
electron microscope; however, the fact that there have been observations of small groups of
SMLVs in astrocytes (see above) [
25
,
38
,
39
] suggest the possibility of its existence. To our
best knowledge, components of the active zone have not been intensively studied, except
for Munc13, which is a priming factor catalysing the conformational switch of syntaxin1
from closed to open, promoting SNARE complex assembly. Mungenast [
133
] found the
expression of Munc13-1 in cultured astrocytes by RT-PCR, immunostaining and Western
blotting, and showed that siRNA downregulation of Munc13-1 inhibits astrocytic ATP
release, induced by diacylglycerol (DAG). In another study based on RNA sequencing
from freshly isolated astrocytes, Munc13-2 and -3 were enriched in the astrocytes, while
Munc13-1 was expressed mainly by neurons [
52
,
134
]. Similar results can be found in RNA
sequencing data from freshly isolated astrocytes; however, only Munc13-3 has shown a
significant level of expression in these cells in the striatum [
53
]. Results showing the ex-
pression and function of Munc13 in regulated exocytosis in the astrocytes are very limited,
and the involvement of other active zone components in vesicular release could provide
more insights into explaining the process. It can be theorised, however, that the spatial
organisation of exocytosis in astrocytes might be completely different from that present
at a presynaptic site in neurons. For example, a recent report by Buscemi and co-workers
showed that mGlu5 interacts with Homer1b/c in astrocytes and that this interaction is
crucial for the their glutamate release [
135
]. The interaction between Homer and mGluR5
in astrocytes has been previously suggested but not shown directly [
136
]. Homer1 is,
however, a neuronal postsynaptic marker (for a review, see [
137
]) and its role in astrocytes
is quite novel. Nevertheless, in line with these observations, neurons may also have “non-
canonical” exocytosis machinery, which has been reported, for example, in dendrites [
138
].
Interestingly, the composition of the SNARE complex, which was proposed to regulate
exocytosis from neuronal dendrites, is very similar to that proposed in astrocytes and
consists of SNAP-23, syntaxin 4 and VAMP2 or VAMP3 [138].
Biomolecules 2021,11, 1367 9 of 19
4. The Ca2+ Sensor for Regulated Exocytosis in Astrocytes
Under physiological circumstances, primed vesicles are stimulated to exocytosis by
Ca
2+
; therefore, to trigger the final stage of the fusion reaction, a Ca
2+
sensor is required
at the site of exocytosis. Since the discovery of synaptotagmin1 (Syt1) [
139
], it has been
proposed to be the Ca
2+
sensor for regulated exocytosis. Syts are evolutionarily conserved
proteins containing a short amino-terminal sequence directed toward the vesicle lumen,
followed by a transmembrane region, a central linker sequence of variable length, and
two carboxy-terminal C2 domains that bind Ca
2+
. [
98
,
140
]. C2 domains were initially
defined in protein-kinase C isozymes and were later shown to constitute autonomously
folding Ca2+/phospholipid-binding domains. In addition, C2 domains constitute protein
interaction domains and, in the case of Syt1, bind to syntaxin-1 and to SNARE complexes.
There are 16 Syts expressed in the brain and eight of them bind Ca
2+
: Syt1, Syt2, Syt3,
Syt5, Syt6, Syt7, Syt9 and Syt10 [
140
]. Out of those, Syt1, Syt2 and Syt9 are responsible
for triggering fast fusion of the synaptic vesicles; however, astrocytes do not seem to
express any of them [
52
,
53
,
123
,
141
,
142
]. Strikingly, for many other forms of regulated,
Ca
2+
-triggered exocytosis, Syt1, Syt2 and Syt9 are involved in chromaffin cells, neuropep-
tide secretion in neurons or mast cell degranulation. However, Syt7 has also been shown
to be involved in triggering exocytosis in chromaffin cells [
143
], pancreatic insulin- and
glucagon-secreting cells [
144
,
145
], or exocytosis of lysosomes in fibroblasts [
146
]. In neu-
rons, Syt7 was also shown to be important for Ca
2+
-triggered asynchronous release [
147
].
For astrocytes, however, results showing Syt7 expression are quite ambiguous. Studies
in cultured astrocytes did not detect Syt7 [
142
,
148
] or even showed that overexpression
of Syt7 inhibited lysosomal exocytosis [
148
]. However, Mittelsteadt and colleagues [
141
]
showed, using single cell RT-PCR, that patch-clamped hippocampal astrocytes in the CA1
stratum radiatum express Syt7 (in seven out of nine cells). However, cultured astrocytes
express Syt4 and Syt11 [
29
,
65
,
123
,
142
,
148
–
150
]. Syt4 expression was also found in freshly
isolated astrocytes by RT-PCR [
142
]; similarly, Syt11 was found by RNA sequencing [
52
,
53
]
and RT-PCR [
141
]. Zhang and co-workers also confirmed the expression of Syt4 in situ
by confocal and electron microscopy. Even though Syt4 and Syt11 do not bind Ca
2+
[
151
],
Zhang and co-workers have shown that it regulates astrocytic Ca
2+
-dependent glutamate
release in cultured astrocytes [
142
]. Similarly, Syt11 was shown to regulate Ca
2+
-dependent
lysosome exocytosis in injured astrocytes [
148
]. It should be mentioned that there is a
whole set of studies showing that astrocytes do not release neurotransmitters in response
to Ca
2+
elevation, arguing against the existence of Ca
2+
-dependent exocytosis in astrocytes
(for in depth reviews, see [
7
,
19
,
20
,
115
,
152
]). Clearly, the involvement of synaptotagmins
in regulated exocytosis in astrocytes needs further studies and clarification, especially in
more intact preparations like brain slices or in vivo.
Synaptotagmins, however, do not act alone in triggering fusion but require complexin
as a cofactor [
140
]. Complexin was discovered by virtue of its tight binding to SNARE
complexes. It functions as a priming factor for SNARE complexes, as well as an activator
of these complexes, preparing them for subsequent synaptotagmin action, and also as a
clamp of spontaneous release, preventing unregulated fusion [
98
]. Complexin’s expression
in astrocytes has been studied mainly
in vitro
, where cultured astrocytes have been shown
to express complexin 2, while complexin 1 was expressed in the neurons [
46
,
153
,
154
].
Recently, complexin 2 expression was confirmed by RNA sequencing data [
53
], while
complexin 1 was also expressed at a significant level, albeit much lower than the general
tissue level, and only in cortical astrocytes.
It should also be noted that Ca
2+
is not the only signal that can trigger vesicle secretion,
as many cells show GTP-dependent DCV secretion. Non-hydrolysable GTP was shown to
trigger secretion in a Ca
2+
-independent manner in chromaffin cells [
155
], mast cells [
156
]
and pancreatic
β
cells [
157
]. Even though GTP-dependent exocytosis differs from Ca
2+
-
dependent exocytosis in terms of the signal sensor for triggering, the final fusion steps are
still dependent on the SNARE proteins [
21
]. The major sensors for GTP in GTP-dependent
exocytosis are considered to be Ral proteins (RalA and RalB) and GTPases [
158
]. Ral
Biomolecules 2021,11, 1367 10 of 19
proteins interact with the exocyst protein complex, which is an octameric protein complex
containing, among other proteins, Sec5, which is bound by Ral in a GTP-dependent
manner [
21
,
159
]. It is believed that the exocyst complex tethers secretory vesicles to
the plasma membrane, where fusion occurs in a SNARE-dependent manner [
159
]. For
example, live-cell imaging of Sec8 (another member of the exocyst complex) showed that it
is transported to a cellular membrane where it remains for seconds until fusion occurs [
160
],
which is in line with the slow kinetics of astrocytic release after stimulus [
161
]. Additionally,
members of the exocyst complex can interact with Rab proteins or v-SNARE [
162
], directly
with plasma membrane via phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) [
163
], and
with a Sec1/Munc18 (SM) protein family member, Sec1 [
164
]. Interestingly, when we
searched for expression of RalA, RalB and Sec5 (encoded in mice by Ral1a, Ral1b and Exoc2,
respectively) in a database of sequenced astrocytic RNAs (http://astrocyternaseq.org/
accessed on 1 January 2021), we found that all these proteins are expressed at a significantly
different level from the expression threshold set as fragments per kilobase per million
(FPKM) >10 [
53
]. However, to our knowledge, so far, there are no experimental data
exploring the exocyst complex in astrocytes, as it has mostly been studied in yeast. Given
the lack of Ca
2+
-binding synaptotagmins in astrocytes and their slow exocytosis [
37
,
161
],
GTP-dependent exocytosis could be an interesting alternative hypothesis.
5. Exocytosis in Pathological Conditions
In nearly all brain pathologies, such as traumatic brain injury, stroke, ischemia, infec-
tious disease, neuroinflammatory and neurodegenerative disease, epilepsy, brain tumours,
schizophrenia, migraine or depression, there is clearly a presence of “reactive astroglio-
sis” [
165
,
166
]. This term is a result of a consensus statement [
167
] recently proposed to
describe the process in which, in response to pathology, astrocytes engage in molecularly
defined programs involving changes in transcriptional regulation, biochemical, morpho-
logical, metabolic and physiological remodelling, which result in a loss of or increase in
homeostatic functions and/or the gain of completely new function(s).
Accumulating evidence suggests that in pathological conditions, microglia are acti-
vated first and, through the release of ATP and inflammatory mediators (mainly interleukin-
1
α
(IL-1
α
), tumour necrosis factor (TNF) and complement component 1, subcomponent q
(C1q)), subsequently trigger astrocytic activation [
168
,
169
]. Activated astrocytes, in turn,
increase their production and secretion of chemotactic cytokine stromal cell-derived factor-
1
α
(SDF-1
α
) [
170
,
171
], the proinflammatory cytokine TNF and the inflammatory mediator
prostaglandin E2 (PGE2) [
105
,
172
], IL-1
α
, IL-6, interferon-
γ
(IFN-
γ
) [
173
] and many oth-
ers [
174
]. As mentioned above, some groups have argued that astrocytes are capable of
Ca
2+
-regulated exocytosis only after activation by microglia (see, for example, [
115
,
175
]).
In support of this view, a previous report by Pascual and co-workers [
176
] showed that
activation of the microglia by lipopolysaccharide (LPS) induced a rapid (within minutes)
and transient (several minutes long) increase in the frequency of excitatory synaptic cur-
rents in acute hippocampal slices. The proposed mechanism involved the activation of
metabotropic P2Y1 receptors (P2Y1Rs) on astrocytes by the release of ATP from the mi-
croglia, thus triggering glutamate release from astrocytes and finally modulating synaptic
mGluRs. Similarly, it was shown that astrocytes from TNF-deficient or TNF type 1 receptor
(TNFR1)-deficient mice displayed altered P2Y1R-dependent Ca
2+
signalling and decreased
glutamate release [
44
,
177
]. Interestingly, a recent study [
178
] has shown that the presence
of microglia downregulates the expression of VAMP2 in astrocytes, which slows down the
vesicular release by astrocytes. This favours the longer release of transmitters in opposition
to a rapid release and exhaustion of the vesicular pool in the absence of microglial factors.
Additionally, the authors showed that the gliotransmission triggered by the P2Y1 agonist
is impaired in slices from transgenic mice devoid of microglia [178].
SDF-1
α
, TNF or PGE2 by themselves are also sufficient to induce glutamate release
from astrocytes, albeit still in a Ca
2+
-dependent manner. They act through activation of their
respective receptors—CXCR4 and-, TNFR (both G
i/o
–associated GPCRs)—or prostaglandin
Biomolecules 2021,11, 1367 11 of 19
E (EP) receptors (G
i
/G
s
–associated GPCRs) [
104
,
105
,
175
,
179
–
181
]. The Ca
2+
-dependence
of the exocytosis of glutamate after activation of these receptors is based on an observation
that the response is blocked by intracellular Ca
2+
chelators or inhibitors of exocytosis [
105
].
In physiological conditions, astrocytes maintain constant levels of blood and brain
PCO
2
/pH and they respond to physiological decreases in pH with vigorous elevations
in intracellular Ca
2+
and regulate the exocytosis of ATP [
182
]. However, in pathological
conditions of hypoxia, they can react to a decrease in PO2 by Ca
2+
signalling, accompa-
nied by increases in mitochondrial reactive oxygen species (ROS) production and ATP
exocytosis [
183
,
184
]. In the context of hypoxia, a recent study by Byts and co-workers [
185
]
has shown that transmembrane prolyl 4-hydroxylase (P4H-TM), which is located in the
ER and has a Ca
2+
-sensing EF-domain, controls ATP-induced Ca
2+
signalling and glio-
transmission. This effect is mediated by hypoxia-inducible factor 1 (HIF1), which is a
heterodimeric transcription factor. In normoxia, prolyl 4-hydroxylase (P4H) hydroxy-
lates two prolyl residues located on HIF’s
α
subunit. This hydroxylation leads to von
Hippel–Lindau (VHL)-targeted degradation of HIFa, which suppresses the transcription
of hypoxia-responsive genes. However, in hypoxia, P4H is inactive, which leads to stabil-
isation, accumulation and activation of HIF and induction of hypoxia-responsive genes.
One study [
185
] showed that P4H-TM knockout animals had a changed expression of
several genes related to Ca
2+
signalling and vesicular transport/docking pathways, thus
influencing receptor-operated calcium entry (ROCE) and store-operated calcium entry
(SOCE), as well as calcium re-uptake by mitochondria. Moreover, in an
in vitro
model
of cerebral ischemia, oxygen–glucose deprivation (OGD), there was a biphasic increase
in Ca
2+
signalling in astrocytes as well as neurons and a subsequent accumulation of
pro-inflammatory factors, such as IL-1b and TNF
α
, leading to hyperexcitation of the neu-
rons and their death after reoxygenation [
186
]. Interestingly, pretreatment of cell cultures
with the selective
α
2-adrenergic receptor agonists guanfacine and UK-14,304 showed a
neuroprotective effect through Ca2+-regulated exocytosis of ATP [186].
Reactive astrocytes have been also shown to release complement system proteins [
187
].
The complement system represents one of the most basic immune cascades. The comple-
ment proteins C3a, C1q and C5 are present in the brain, where they regulate neurogenesis,
neuronal survival and synaptic elimination [
188
,
189
]. It has been shown that NF-
κ
B sig-
nalling promotes the secretion of C3a. In Alzheimer’s disease, exposure to amyloid
β
strongly activates astroglial NF-
κ
B, which increases the astroglial C3a release that, in turn,
contributes to neurodegeneration [187].
Finally, the activation of astrocytes is often associated with their morphological and
biochemical remodelling. The reactivity is manifested by an increased expression of
intermediate filaments (most notably GFAP and vimentin). It has been suggested that such
an upregulation of intermediate filaments allows faster and therefore more efficient delivery
of major histocompatibility complex (MHC) Class II molecules to the cell surface. Exposure
of astrocytes to INF-γinduced MHC Class II expression in late endosomes/lysosomes [92,96].
6. Conclusions
Astrocytes are receiving increasing attention as their complex role in the central
nervous system becomes more prominent and accepted. In this review, we presented
only a fraction of the studies which not only show that astrocytes have various types of
secretory vesicles but also that they express a complex molecular machinery associated
with those vesicles which is sufficient for their regulated exocytosis. For other excellent
in-depth reviews, please see [
36
,
134
,
161
]. Even though some of the components of this
release machinery, such as SNARE or synaptotagmins, have been intensively studied for
over two decades, others have remained mostly undescribed. These include potential
scaffolding proteins or components of the active zone, which can direct exocytosis to
the most crucial places for astrocyte–neuron communication, but also alternative to Ca
2+
mechanisms of triggering exocytosis. Moreover, much attention needs to be given to
designing new experiments to bring them to the more physiology-relevant environment
Biomolecules 2021,11, 1367 12 of 19
of neural tissue. New techniques of astrocyte culturing also give hope for creating new
study models which will resemble astrocytes in the brain. Regardless of the discussion on
Ca
2+
sources for triggering exocytosis and gaps in our knowledge about the elements of
the exocytotic molecular machinery, astrocytes have proven to be an important component
of neuronal networks.
Author Contributions:
Conceptualisation, P.M.; investigation, P.M. and A.M.; writing—original draft
preparation, P.M., A.M.; writing—review and editing, P.M.; funding acquisition, P.M. All authors
have read and agreed to the published version of the manuscript.
Funding:
This research was funded by National Science Centre, Poland, grant number 2017/26/D/
NZ3/01017.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments:
We would like to thank Katarzyna Kuter and Agnieszka Jurga for giving us the
opportunity to participate in the Special Issue: “Astroglia in Physiology, Pathology and Therapy”,
and Leszek Kaczmarek for all the support.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Oberheim, N.A.; Takano, T.; Han, X.; He, W.; Lin, J.H.C.; Wang, F.; Xu, Q.; Wyatt, J.D.; Pilcher, W.; Ojemann, J.; et al. Uniquely
Hominid Features of Adult Human Astrocytes. J. Neurosci. 2009,29, 3276–3287. [CrossRef] [PubMed]
2.
Giaume, C.; Naus, C.C. Connexins, gap junctions, and glia. Wiley Interdiscip. Rev. Membr. Transp. Signal.
2013
,2, 133–142.
[CrossRef]
3.
Edallérac, G.; Echever, O.; Erouach, N. How do astrocytes shape synaptic transmission? Insights from electrophysiology. Front.
Cell. Neurosci. 2013,7, 159. [CrossRef]
4.
Cornell-Bell, A.H.; Finkbeiner, S.M.; Cooper, M.S.; Smith, S.J. Glutamate induces calcium waves in cultured astrocytes: Long-range
glial signaling. Science 1990,247, 470–473. [CrossRef]
5.
Agulhon, C.; Petravicz, J.; McMullen, A.B.; Sweger, E.J.; Minton, S.K.; Taves, S.; Casper, K.B.; Fiacco, T.A.; McCarthy, K.D. What Is
the Role of Astrocyte Calcium in Neurophysiology? Neuron 2008,59, 932–946. [CrossRef]
6.
Araque, A.; Carmignoto, G.; Haydon, P.G.; Oliet, S.H.; Robitaille, R.; Volterra, A. Gliotransmitters Travel in Time and Space.
Neuron 2014,81, 728–739. [CrossRef]
7. Bazargani, N.; Attwell, D. Astrocyte calcium signaling: The third wave. Nat. Neurosci. 2016,19, 182–189. [CrossRef]
8.
Bekar, L.K.; He, W.; Nedergaard, M. Locus Coeruleus
α
-Adrenergic–Mediated Activation of Cortical Astrocytes In Vivo. Cereb.
Cortex 2008,18, 2789–2795. [CrossRef]
9.
Hirase, H.; Qian, L.; Barthó, P.; Buzsaki, G. Calcium Dynamics of Cortical Astrocytic Networks In Vivo. PLoS Biol.
2004
,2, e96.
[CrossRef]
10.
Nedergaard, M. Direct signaling from astrocytes to neurons in cultures of mammalian brain cells. Science
1994
,263, 1768–1771.
[CrossRef]
11.
Parpura, V.; Basarsky, T.A.; Liu, F.; Jeftinija, K.; Jeftinija, S.; Haydon, P.G. Glutamate-mediated astrocyte–neuron signalling. Nat.
Cell Biol. 1994,369, 744–747. [CrossRef]
12.
Perea, G.; Navarrete, M.; Araque, A. Tripartite synapses: Astrocytes process and control synaptic information. Trends Neurosci.
2009,32, 421–431. [CrossRef]
13.
Volterra, A.; Liaudet, N.; Savtchouk, I. Astrocyte Ca
2+
signalling: An unexpected complexity. Nat. Rev. Neurosci.
2014
,15, 327–335.
[CrossRef]
14.
Hamilton-Whitaker, N.; Attwell, D. Do astrocytes really exocytose neurotransmitters? Nat. Rev. Neurosci.
2010
,11, 227–238.
[CrossRef] [PubMed]
15.
Angulo, M.C.; Kozlov, A.S.; Charpak, S.; Audinat, E. Glutamate Released from Glial Cells Synchronizes Neuronal Activity in the
Hippocampus. J. Neurosci. 2004,24, 6920–6927. [CrossRef]
16.
Henneberger, C.; Papouin, T.; Oliet, S.H.R.; Rusakov, D.A. Long-term potentiation depends on release of d-serine from astrocytes.
Nature 2010,463, 232–236. [CrossRef] [PubMed]
17.
Pascual, O.; Casper, K.B.; Kubera, C.; Zhang, J.; Revilla-Sanchez, R.; Sul, J.-Y.; Takano, H.; Moss, S.J.; McCarthy, K.; Haydon, P.G.
Astrocytic Purinergic Signaling Coordinates Synaptic Networks. Science 2005,310, 113–116. [CrossRef] [PubMed]
18.
Serrano, A.; Haddjeri, N.; Lacaille, J.-C.; Robitaille, R. GABAergic Network Activation of Glial Cells Underlies Hippocampal
Heterosynaptic Depression. J. Neurosci. 2006,26, 5370–5382. [CrossRef]
Biomolecules 2021,11, 1367 13 of 19
19.
Fiacco, T.A.; McCarthy, K.D. Multiple Lines of Evidence Indicate That Gliotransmission Does Not Occur under Physiological
Conditions. J. Neurosci. 2018,38, 3–13. [CrossRef]
20. Savtchouk, I.; Volterra, A. Gliotransmission: Beyond Black-and-White. J. Neurosci. 2018,38, 14–25. [CrossRef] [PubMed]
21. Sugita, S. Mechanisms of exocytosis. Acta Physiol. 2007,192, 185–193. [CrossRef]
22. Thorn, P.; Zorec, R.; Rettig, J.; Keating, D.J. Exocytosis in non-neuronal cells. J. Neurochem. 2016,137, 849–859. [CrossRef]
23. Südhof, T.C.; Rizo, J. Synaptic Vesicle Exocytosis. Cold Spring Harb. Perspect. Biol. 2011,3, a005637. [CrossRef]
24.
Chanaday, N.L.; Cousin, M.A.; Milosevic, I.; Watanabe, S.; Morgan, J.R. The Synaptic Vesicle Cycle Revisited: New Insights into
the Modes and Mechanisms. J. Neurosci. 2019,39, 8209–8216. [CrossRef] [PubMed]
25.
Bezzi, P.; Gundersen, V.; Galbete, J.L.; Seifert, G.; Steinhäuser, C.; Pilati, E.; Volterra, A. Astrocytes contain avesicular compartment
that is competent for regulated exocytosis of glutamate. Nat. Neurosci. 2004,7, 613–620. [CrossRef]
26.
Jourdain, P.; Bergersen, L.H.; Bhaukaurally, K.; Bezzi, P.; Santello, M.; Domercq, M.; Matute, C.; Tonello, F.; Gundersen, V.; Volterra,
A. Glutamate exocytosis from astrocytes controls synaptic strength. Nat. Neurosci. 2007,10, 331–339. [CrossRef]
27.
Martineau, M.; Shi, T.; Puyal, J.; Knolhoff, A.; Dulong, J.; Gasnier, B.; Klingauf, J.; Sweedler, J.V.; Jahn, R.; Mothet, J.-P. Storage
and uptake of d-serine into astrocytic synaptic-like vesicles specify gliotransmission. J. Neurosci.
2013
,33, 3413–3423. [CrossRef]
[PubMed]
28.
Bergersen, L.; Gundersen, V. Morphological evidence for vesicular glutamate release from astrocytes. Neuroscience
2009
,158,
260–265. [CrossRef] [PubMed]
29.
Crippa, D.; Schenk, U.; Francolini, M.; Rosa, P.; Verderio, C.; Zonta, M.; Pozzan, T.; Matteoli, M.; Carmignoto, G. Synaptobrevin2-
expressing vesicles in rat astrocytes: Insights into molecular characterization, dynamics and exocytosis. J. Physiol.
2006
,570,
567–582. [CrossRef]
30.
Calegari, F.; Coco, S.; Taverna, E.; Bassetti, M.; Verderio, C.; Corradi, N.; Matteoli, M.; Rosa, P. A Regulated Secretory Pathway in
Cultured Hippocampal Astrocytes. J. Biol. Chem. 1999,274, 22539–22547. [CrossRef]
31.
Prada, I.; Marchaland, J.; Podini, P.; Magrassi, L.; D’Alessandro, R.; Bezzi, P.; Meldolesi, J. REST/NRSF governs the expression of
dense-core vesicle gliosecretion in astrocytes. J. Cell Biol. 2011,193, 537–549. [CrossRef]
32.
Hur, Y.S.; Kim, K.D.; Paek, S.H.; Yoo, S.H. Evidence for the Existence of Secretory Granule (Dense-Core Vesicle)-Based Inositol
1,4,5-Trisphosphate-Dependent Ca2+ Signaling System in Astrocytes. PLoS ONE 2010,5, e11973. [CrossRef] [PubMed]
33.
Liu, T.; Sun, L.; Xiong, Y.; Shang, S.; Guo, N.; Teng, S.; Wang, Y.; Liu, B.; Wang, C.; Wang, L.; et al. Calcium Triggers Exocytosis
from Two Types of Organelles in a Single Astrocyte. J. Neurosci. 2011,31, 10593–10601. [CrossRef]
34.
Zhang, Z.; Chen, G.; Zhou, W.; Song, A.; Xu, T.; Luo, Q.; Wang, W.; Gu, X.-S.; Duan, S. Regulated ATP release from astrocytes
through lysosome exocytosis. Nat. Cell Biol. 2007,9, 945–953. [CrossRef]
35.
Li, D.; Ropert, N.; Koulakoff, A.; Giaume, C.; Oheim, M. Lysosomes Are the Major Vesicular Compartment Undergoing
Ca2+-Regulated Exocytosis from Cortical Astrocytes. J. Neurosci. 2008,28, 7648–7658. [CrossRef] [PubMed]
36.
Vardjan, N.; Parpura, V.; Verkhratsky, A.; Zorec, R. Gliocrine System: Astroglia as Secretory Cells of the CNS. In Advances in
Experimental Medicine and Biology; Springer Science and Business Media LLC: Berlin, Germany, 2019; Volume 1175, pp. 93–115.
37.
Verkhratsky, A.; Matteoli, M.; Parpura, V.; Mothet, J.; Zorec, R. Astrocytes as secretory cells of the central nervous system:
Idiosyncrasies of vesicular secretion. EMBO J. 2016,35, 239–257. [CrossRef] [PubMed]
38.
Bergersen, L.; Morland, C.; Ormel, L.; Rinholm, J.; Larsson, M.; Wold, J.; Røe, Å.T.; Stranna, A.; Santello, M.; Bouvier, D.; et al.
Immunogold Detection of L-glutamate and D-serine in Small Synaptic-Like Microvesicles in Adult Hippocampal Astrocytes.
Cereb. Cortex 2012,22, 1690–1697. [CrossRef]
39.
Ormel, L.; Stensrud, M.J.; Bergersen, L.H.; Gundersen, V. VGLUT1 is localized in astrocytic processes in several brain regions.
Glia 2012,60, 229–238. [CrossRef] [PubMed]
40.
Patrushev, I.; Gavrilov, N.; Turlapov, V.; Semyanov, A. Subcellular location of astrocytic calcium stores favors extrasynaptic
neuron–astrocyte communication. Cell Calcium 2013,54, 343–349. [CrossRef]
41.
Stenovec, M.; Kreft, M.; Grilc, S.; Potokar, M.; Kreft, M.E.; Pangršiˇc, T.; Zorec, R. Ca
2+
-dependent mobility of vesicles capturing
anti-VGLUT1 antibodies. Exp. Cell Res. 2007,313, 3809–3818. [CrossRef]
42.
Hiasa, M.; Miyaji, T.; Haruna, Y.; Takeuchi, T.; Harada, Y.; Moriyama, S.; Yamamoto, A.; Omote, H.; Moriyama, Y. Identification of
a mammalian vesicular polyamine transporter. Sci. Rep. 2015,4, 6836. [CrossRef] [PubMed]
43.
Wilhelm, A.; Volknandt, W.; Langer, D.; Nolte, C.; Kettenmann, H.; Zimmermann, H. Localization of SNARE proteins and
secretory organelle proteins in astrocytes in vitro and in situ. Neurosci. Res. 2004,48, 249–257. [CrossRef]
44.
Domercq, M.; Brambilla, L.; Pilati, E.; Marchaland, J.; Volterra, A.; Bezzi, P. P2Y1 Receptor-evoked Glutamate Exocytosis from
Astrocytes. J. Biol. Chem. 2006,281, 30684–30696. [CrossRef]
45.
Montana, V.; Ni, Y.; Sunjara, V.; Hua, X.; Parpura, V. Vesicular Glutamate Transporter-Dependent Glutamate Release from
Astrocytes. J. Neurosci. 2004,24, 2633–2642. [CrossRef]
46.
Zhang, Q.; Pangršiˇc, T.; Kreft, M.; Kržan, M.; Li, N.; Sul, J.-Y.; Halassa, M.; Van Bockstaele, E.; Zorec, R.; Haydon, P.G. Fusion-
related Release of Glutamate from Astrocytes. J. Biol. Chem. 2004,279, 12724–12733. [CrossRef] [PubMed]
47.
Kasymov, V.; Larina, O.; Castaldo, C.; Marina, N.; Patrushev, M.; Kasparov, S.; Gourine, A.V. Differential Sensitivity of Brainstem
versus Cortical Astrocytes to Changes in pH Reveals Functional Regional Specialization of Astroglia. J. Neurosci.
2013
,33, 435–441.
[CrossRef]
Biomolecules 2021,11, 1367 14 of 19
48.
Höltje, M.; Hofmann, F.; Lux, R.; Veh, R.W.; Just, I.; Ahnert-Hilger, G. Glutamate Uptake and Release by Astrocytes Are Enhanced
by Clostridium botulinum C3 Protein. J. Biol. Chem. 2008,283, 9289–9299. [CrossRef] [PubMed]
49.
Bowser, D.; Khakh, B.S. Two forms of single-vesicle astrocyte exocytosis imaged with total internal reflection fluorescence
microscopy. Proc. Natl. Acad. Sci. USA 2007,104, 4212–4217. [CrossRef] [PubMed]
50.
Fremeau, R.T.; Burman, J.; Qureshi, T.; Tran, C.H.; Proctor, J.; Johnson, J.; Zhang, H.; Sulzer, D.; Copenhagen, D.R.; Storm-Mathisen,
J.; et al. The identification of vesicular glutamate transporter 3 suggests novel modes of signaling by glutamate. Proc. Natl. Acad.
Sci. USA 2002,99, 14488–14493. [CrossRef] [PubMed]
51.
Ormel, L.; Stensrud, M.J.; Chaudhry, F.A.; Gundersen, V. A distinct set of synaptic-like microvesicles in atroglial cells contain
VGLUT3. Glia 2012,60, 1289–1300. [CrossRef] [PubMed]
52.
Zhang, Y.; Chen, K.; Sloan, S.A.; Bennett, M.L.; Scholze, A.R.; O’Keeffe, S.; Phatnani, H.P.; Guarnieri, P.; Caneda, C.; Ruderisch, N.;
et al. An RNA-Sequencing Transcriptome and Splicing Database of Glia, Neurons, and Vascular Cells of the Cerebral Cortex. J.
Neurosci. 2014,34, 11929–11947. [CrossRef] [PubMed]
53.
Chai, H.; Diaz-Castro, B.; Shigetomi, E.; Monte, E.; Octeau, J.C.; Yu, X.; Cohn, W.; Rajendran, P.S.; Vondriska, T.M.; Whitelegge,
J.P.; et al. Neural Circuit-Specialized Astrocytes: Transcriptomic, Proteomic, Morphological, and Functional Evidence. Neuron
2017,95, 531–549.e9. [CrossRef] [PubMed]
54.
Cahoy, J.D.; Emery, B.; Kaushal, A.; Foo, L.C.; Zamanian, J.; Christopherson, K.S.; Xing, Y.; Lubischer, J.; Krieg, P.A.; Krupenko,
S.A.; et al. A Transcriptome Database for Astrocytes, Neurons, and Oligodendrocytes: A New Resource for Understanding Brain
Development and Function. J. Neurosci. 2008,28, 264–278. [CrossRef]
55.
Li, D.; Hérault, K.; Silm, K.; Evrard, A.; Wojcik, S.; Oheim, M.; Herzog, E.; Ropert, N. Lack of Evidence for Vesicular Glutamate
Transporter Expression in Mouse Astrocytes. J. Neurosci. 2013,33, 4434–4455. [CrossRef]
56.
Guˇcek, A.; Vardjan, N.; Zorec, R. Exocytosis in Astrocytes: Transmitter Release and Membrane Signal Regulation. Neurochem. Res.
2012,37, 2351–2363. [CrossRef]
57.
Vardjan, N.; Kreft, M.; Zorec, R. Regulated Exocytosis in Astrocytes is as Slow as the Metabolic Availability of Gliotransmitters:
Focus on Glutamate and ATP. Adv. Neurobiol. 2014,11, 81–101.
58.
El Mestikawy, S.; Mackenzie, Å.; Fortin, G.M.; Descarries, L.; Trudeau, L.-E. From glutamate co-release to vesicular synergy:
Vesicular glutamate transporters. Nat. Rev. Neurosci. 2011,12, 204–216. [CrossRef] [PubMed]
59.
Martineau, M.; Galli, T.; Baux, G.; Mothet, J.-P. Confocal imaging and tracking of the exocytotic routes for d-serine-mediated
gliotransmission. Glia 2008,56, 1271–1284. [CrossRef] [PubMed]
60.
Mothet, J.-P.; Pollegioni, L.; Ouanounou, G.; Martineau, M.; Fossier, P.; Baux, G. Glutamate receptor activation triggers a calcium-
dependent and SNARE protein-dependent release of the gliotransmitter d-serine. Proc. Natl. Acad. Sci. USA
2005
,102, 5606–5611.
[CrossRef] [PubMed]
61.
Wolosker, H.; Balu, D.T.; Coyle, J.T. The Rise and Fall of the d -Serine-Mediated Gliotransmission Hypothesis. Trends Neurosci.
2016,39, 712–721. [CrossRef] [PubMed]
62.
Papouin, T.; Henneberger, C.; Rusakov, D.A.; Oliet, S.H. Astroglial versus Neuronal d-Serine: Fact Checking. Trends Neurosci.
2017,40, 517–520. [CrossRef]
63.
Neame, S.; Safory, H.; Radzishevsky, I.; Touitou, A.; Marchesani, F.; Marchetti, M.; Kellner, S.; Berlin, S.; Foltyn, V.N.; Engelender,
S.; et al. The NMDA receptor activation by d-serine and glycine is controlled by an astrocytic Phgdh-dependent serine shuttle.
Proc. Natl. Acad. Sci. USA 2019,116, 20736–20742. [CrossRef] [PubMed]
64.
Perez, E.J.; Tapanes, S.A.; Loris, Z.B.; Balu, D.; Sick, T.J.; Coyle, J.T.; Liebl, D.J. Enhanced astrocytic d-serine underlies synaptic
damage after traumatic brain injury. J. Clin. Investig. 2017,127, 3114–3125. [CrossRef] [PubMed]
65.
Potokar, M.; Stenovec, M.; Kreft, M.; Kreft, M.E.; Zorec, R. Stimulation inhibits the mobility of recycling peptidergic vesicles in
astrocytes. Glia 2007,56, 135–144. [CrossRef] [PubMed]
66.
Paco, S.; Margelí, M.A.; Olkkonen, V.M.; Imai, A.; Blasi, J.; Aguado, F.; Fischer-Colbrie, R. Regulation of exocytotic protein
expression and Ca2+-dependent peptide secretion in astrocytes. J. Neurochem. 2009,110, 143–156. [CrossRef]
67.
Fischer-Colbrie, R.; Kirchmair, R.; Schobert, A.; Olenik, C.; Meyer, D.K.; Winkler, H. Secretogranin II Is Synthesized and Secreted
in Astrocyte Cultures. J. Neurochem. 1993,60, 2312–2314. [CrossRef] [PubMed]
68.
Paco, S.; Pozas, E.; Aguado, F. Secretogranin III Is an Astrocyte Granin That Is Overexpressed in Reactive Glia. Cereb. Cortex
2009
,
20, 1386–1397. [CrossRef]
69.
Zhan, X.; Wen, G.; Jiang, E.; Li, F.; Wu, X.; Pang, H. Secretogranin III upregulation is involved in parkinsonian toxin-mediated
astroglia activation. J. Toxicol. Sci. 2020,45, 271–280. [CrossRef]
70.
Kreft, M.; Stenovec, M.; Rupnik, M.S.; Grilc, S.; Kržan, M.; Potokar, M.; Pangršiˇc, T.; Haydon, P.G.; Zorec, R. Properties of
Ca2+-dependent exocytosis in cultured astrocytes. Glia 2004,46, 437–445. [CrossRef]
71.
Chatterjee, S.; Sikdar, S.K. Corticosterone treatment results in enhanced release of peptidergic vesicles in astrocytes via cytoskeletal
rearrangements. Glia 2013,61, 2050–2062. [CrossRef]
72.
Ramamoorthy, P.; Whim, M.D. Trafficking and Fusion of Neuropeptide Y-Containing Dense-Core Granules in Astrocytes. J.
Neurosci. 2008,28, 13815–13827. [CrossRef] [PubMed]
73.
Guˇcek, A.; Jorgaˇcevski, J.; Singh, P.; Geisler, C.; Lisjak, M.; Vardjan, N.; Kreft, M.; Egner, A.; Zorec, R. Dominant negative SNARE
peptides stabilize the fusion pore in a narrow, release-unproductive state. Cell. Mol. Life Sci.
2016
,73, 3719–3731. [CrossRef]
[PubMed]
Biomolecules 2021,11, 1367 15 of 19
74.
Hong, Y.; Zhao, T.; Li, X.-J.; Li, S. Mutant Huntingtin Impairs BDNF Release from Astrocytes by Disrupting Conversion of
Rab3a-GTP into Rab3a-GDP. J. Neurosci. 2016,36, 8790–8801. [CrossRef] [PubMed]
75.
Coco, S.; Calegari, F.; Pravettoni, E.; Pozzi, D.; Taverna, E.; Rosa, P.; Matteoli, M.; Verderio, C. Storage and Release of ATP from
Astrocytes in Culture. J. Biol. Chem. 2003,278, 1354–1362. [CrossRef]
76.
Pangršiˇc, T.; Potokar, M.; Stenovec, M.; Kreft, M.; Fabbretti, E.; Nistri, A.; Pryazhnikov, E.; Khiroug, L.; Giniatullin, R.; Zorec, R.
Exocytotic Release of ATP from Cultured Astrocytes. J. Biol. Chem. 2007,282, 28749–28758. [CrossRef]
77.
Potokar, M.; Kreft, M.; Pangršiˇc, T.; Zorec, R. Vesicle mobility studied in cultured astrocytes. Biochem. Biophys. Res. Commun.
2005
,
329, 678–683. [CrossRef]
78.
Potokar, M.; Vardjan, N.; Stenovec, M.; Gabrijel, M.; Trkov, S.; Jorgaˇcevski, J.; Kreft, M.; Zorec, R. Astrocytic Vesicle Mobility in
Health and Disease. Int. J. Mol. Sci. 2013,14, 11238–11258. [CrossRef]
79.
Bowser, D.; Khakh, B.S. Vesicular ATP Is the Predominant Cause of Intercellular Calcium Waves in Astrocytes. J. Gen. Physiol.
2007,129, 485–491. [CrossRef] [PubMed]
80.
Hines, D.J.; Haydon, P.G. Astrocytic adenosine: From synapses to psychiatric disorders. Philos. Trans. R. Soc. B Biol. Sci.
2014
,369,
20130594. [CrossRef]
81.
Sawada, K.; Echigo, N.; Juge, N.; Miyaji, T.; Otsuka, M.; Omote, H.; Yamamoto, A.; Moriyama, Y. Identification of a vesicular
nucleotide transporter. Proc. Natl. Acad. Sci. USA 2008,105, 5683–5686. [CrossRef]
82.
Imura, Y.; Morizawa, Y.; Komatsu, R.; Shibata, K.; Shinozaki, Y.; Kasai, H.; Moriishi, K.; Moriyama, Y.; Koizumi, S. Microglia
release ATP by exocytosis. Glia 2013,61, 1320–1330. [CrossRef]
83.
Oya, M.; Kitaguchi, T.; Yanagihara, Y.; Numano, R.; Kakeyama, M.; Ikematsu, K.; Tsuboi, T. Vesicular nucleotide transporter is
involved in ATP storage of secretory lysosomes in astrocytes. Biochem. Biophys. Res. Commun. 2013,438, 145–151. [CrossRef]
84.
Angelova, P.R.; Iversen, K.Z.; Teschemacher, A.G.; Kasparov, S.; Gourine, A.V.; Abramov, A.Y. Signal transduction in astrocytes:
Localization and release of inorganic polyphosphate. Glia 2018,66, 2126–2136. [CrossRef]
85.
Beckel, J.M.; Gómez, N.M.; Lu, W.; Campagno, K.; Nabet, B.; AlBalawi, F.; Lim, J.C.; Boesze-Battaglia, K.; Mitchell, C.H.
Stimulation of TLR3 triggers release of lysosomal ATP in astrocytes and epithelial cells that requires TRPML1 channels. Sci. Rep.
2018,8, 1–14. [CrossRef]
86.
Lalo, U.; Palygin, O.; Rasooli-Nejad, S.; Andrew, J.; Haydon, P.G.; Pankratov, Y. Exocytosis of ATP From Astrocytes Modulates
Phasic and Tonic Inhibition in the Neocortex. PLoS Biol. 2014,12, e1001747. [CrossRef] [PubMed]
87.
Kinoshita, M.; Hirayama, Y.; Fujishita, K.; Shibata, K.; Shinozaki, Y.; Shigetomi, E.; Takeda, A.; Le, H.P.N.; Hayashi, H.; Hiasa, M.;
et al. Anti-Depressant Fluoxetine Reveals its Therapeutic Effect Via Astrocytes. EBioMedicine 2018,32, 72–83. [CrossRef]
88.
Chen, X.; Wang, L.; Zhou, Y.; Zheng, L.-H.; Zhou, Z. “Kiss-and-Run” Glutamate Secretion in Cultured and Freshly Isolated Rat
Hippocampal Astrocytes. J. Neurosci. 2005,25, 9236–9243. [CrossRef]
89.
Jaiswal, J.K.; Fix, M.; Takano, T.; Nedergaard, M.; Simon, S.M. Resolving vesicle fusion from lysis to monitor calcium-triggered
lysosomal exocytosis in astrocytes. Proc. Natl. Acad. Sci. USA 2007,104, 14151–14156. [CrossRef] [PubMed]
90.
Li, L.; Lundkvist, A.; Andersson, D.; Wilhelmsson, U.; Nagai, N.; Pardo, A.; Nodin, C.; Ståhlberg, A.; Aprico, K.; Larsson, K.; et al.
Protective Role of Reactive Astrocytes in Brain Ischemia. Br. J. Pharmacol. 2007,28, 468–481. [CrossRef]
91.
Verderio, C.; Cagnoli, C.; Bergami, M.; Francolini, M.; Schenk, U.; Colombo, A.; Riganti, L.; Frassoni, C.; Zuccaro, E.; Danglot, L.;
et al. TI-VAMP/VAMP7 is the SNARE of secretory lysosomes contributing to ATP secretion from astrocytes. Biol. Cell
2012
,104,
213–228. [CrossRef]
92.
Vardjan, N.; Gabrijel, M.; Potokar, M.; Švajger, U.; Kreft, M.; Jeras, M.; de Pablo, Y.; Faiz, M.; Pekny, M.; Zorec, R. IFN-
γ
-induced
increase in the mobility of MHC class II compartments in astrocytes depends on intermediate filaments. J. Neuroinflam.
2012
,9,
144. [CrossRef] [PubMed]
93.
Jaiswal, J.; Andrews, N.; Simon, S.M. Membrane proximal lysosomes are the major vesicles responsible for calcium-dependent
exocytosis in nonsecretory cells. J. Cell Biol. 2002,159, 625–635. [CrossRef]
94.
Soerensen, C.; Novak, I. Visualization of ATP Release in Pancreatic Acini in Response to Cholinergic Stimulus. J. Biol. Chem.
2001
,
276, 32925–32932. [CrossRef]
95.
Li, D.; Herault, K.; Oheim, M.; Ropert, N. FM dyes enter via a store-operated calcium channel and modify calcium signaling of
cultured astrocytes. Proc. Natl. Acad. Sci. USA 2009,106, 21960–21965. [CrossRef] [PubMed]
96.
Boži´c, M.; Verkhratsky, A.; Zorec, R.; Stenovec, M. Exocytosis of large-diameter lysosomes mediates interferon
γ
-induced
relocation of MHC class II molecules toward the surface of astrocytes. Cell. Mol. Life Sci.
2019
,77, 3245–3264. [CrossRef] [PubMed]
97.
Söllner, T.; Bennett, M.K.; Whiteheart, S.; Scheller, R.H.; Rothman, J.E. A protein assembly-disassembly pathway
in vitro
that may
correspond to sequential steps of synaptic vesicle docking, activation, and fusion. Cell 1993,75, 409–418. [CrossRef]
98.
Südhof, T.C. Neurotransmitter release: The last millisecond in the life of a synaptic vesicle. Neuron
2013
,80, 675–690. [CrossRef]
[PubMed]
99.
Aslamy, A.; Thurmond, D.C. Exocytosis proteins as novel targets for diabetes prevention and/or remediation? Am. J. Physiol.
Integr. Comp. Physiol. 2017,312, R739–R752. [CrossRef]
100.
Anlauf, E.; Derouiche, A. Astrocytic exocytosis vesicles and glutamate: A high-resolution immunofluorescence study. Glia
2004
,
49, 96–106. [CrossRef]
101.
Maienschein, V.; Marxen, M.; Volknandt, W.; Zimmermann, H. A plethora of presynaptic proteins associated with ATP-storing
organelles in cultured astrocytes. Glia 1999,26, 233–244. [CrossRef]
Biomolecules 2021,11, 1367 16 of 19
102.
Singh, P.; Jorgaˇcevski, J.; Kreft, M.; Grubiši´c, V.; Stout, R.; Potokar, M.; Parpura, V.; Zorec, R. Single-vesicle architecture of
synaptobrevin2 in astrocytes. Nat. Commun. 2014,5, 1–12. [CrossRef] [PubMed]
103.
Kim, J.-H.; Kim, J.-H.; Cho, Y.-E.; Baek, M.-C.; Jung, J.-Y.; Lee, M.-G.; Jang, I.-S.; Lee, H.-W.; Suk, K. Chronic Sleep Deprivation-
Induced Proteome Changes in Astrocytes of the Rat Hypothalamus. J. Proteome Res. 2014,13, 4047–4061. [CrossRef] [PubMed]
104.
Bezzi, P.; Carmignoto, P.; Pasti, L.; Vesce, S.; Rossi, D.M.; Rizzini, B.L.; Pozzan, T.; Volterra, A. Prostaglandins stimulate
calcium-dependent glutamate release in astrocytes. Nat. Cell Biol. 1998,391, 281–285. [CrossRef]
105.
Bezzi, P.; Domercq, M.; Brambilla, L.; Galli, R.; Schols, D.; De Clercq, E.; Vescovi, A.; Bagetta, G.; Kollias, G.; Meldolesi, J.; et al.
CXCR4-activated astrocyte glutamate release via TNF
α
: Amplification by microglia triggers neurotoxicity. Nat. Neurosci.
2001
,4,
702–710. [CrossRef]
106.
Hua, X.; Malarkey, E.B.; Sunjara, V.; Rosenwald, S.E.; Li, W.-H.; Parpura, V. Ca
2+
-dependent glutamate release involves two
classes of endoplasmic reticulum Ca2+ stores in astrocytes. J. Neurosci. Res. 2004,76, 86–97. [CrossRef]
107.
Perea, G.; Araque, A. Astrocytes Potentiate Transmitter Release at Single Hippocampal Synapses. Science
2007
,317, 1083–1086.
[CrossRef]
108. Halassa, M.M.; Florian, C.; Fellin, T.; Munoz, J.R.; Lee, S.-Y.; Abel, T.; Haydon, P.G.; Frank, M.G. Astrocytic Modulation of Sleep
Homeostasis and Cognitive Consequences of Sleep Loss. Neuron 2009,61, 213–219. [CrossRef]
109. Hines, D.; Haydon, P.G. Inhibition of a SNARE-sensitive pathway in astrocytes attenuates damage following stroke. J. Neurosci.
2013,33, 4234–4240. [CrossRef] [PubMed]
110.
Sultan, S.; Li, L.; Moss, J.; Petrelli, F.; Cassé, F.; Gebara, E.; Lopatar, J.; Pfrieger, F.; Bezzi, P.; Bischofberger, J.; et al. Synaptic
Integration of Adult-Born Hippocampal Neurons Is Locally Controlled by Astrocytes. Neuron
2015
,88, 957–972. [CrossRef]
[PubMed]
111.
Sardinha, V.M.; Guerra-Gomes, S.; Caetano, I.; Tavares, G.; Martins, M.; Reis, J.S.; Correia, J.S.; Teixeira-Castro, A.; Pinto, L.;
Sousa, N.; et al. Astrocytic signaling supports hippocampal-prefrontal theta synchronization and cognitive function. Glia
2017
,
65, 1944–1960. [CrossRef] [PubMed]
112.
Nadjar, A.; Blutstein, T.; Aubert, A.; Laye, S.; Haydon, P.G. Astrocyte-derived adenosine modulates increased sleep pressure
during inflammatory response. Glia 2013,61, 724–731. [CrossRef] [PubMed]
113.
Turner, J.R.; Ecke, L.E.; Briand, L.A.; Haydon, P.G.; Blendy, J.A.; Haydon, P. Cocaine-related behaviors in mice with deficient
gliotransmission. Psychopharmacology 2013,226, 167–176. [CrossRef] [PubMed]
114.
Fujita, T.; Chen, M.J.; Li, B.; Smith, N.; Peng, W.; Sun, W.; Toner, M.J.; Kress, B.T.; Wang, L.; Benraiss, A.; et al. Neuronal transgene
expression in dominant-negative SNARE mice. J. Neurosci. 2014,34, 16594–16604. [CrossRef]
115.
Sloan, S.A.; Barres, B.A. Looks Can Be Deceiving: Reconsidering the Evidence for Gliotransmission. Neuron
2014
,84, 1112–1115.
[CrossRef]
116. Petrelli, F.; Bezzi, P. Novel insights into gliotransmitters. Curr. Opin. Pharmacol. 2016,26, 138–145. [CrossRef]
117.
´
Sl˛ezak, M.; Grosche, A.; Niemiec, A.; Tanimoto, N.; Pannicke, T.; Münch, T.; Crocker, B.; Isope, P.; Härtig, W.; Beck, S.C.; et al.
Relevance of Exocytotic Glutamate Release from Retinal Glia. Neuron 2012,74, 504–516. [CrossRef]
118.
Parpura, V.; Fang, Y.; Basarsky, T.; Jahn, R.; Haydon, P.G. Expression of synaptobrevin II, cellubrevin and syntaxin but not
SNAP-25 in cultured astrocytes. FEBS Lett. 1995,377, 489–492. [CrossRef]
119.
Jeftinija, S.D.; Jeftinija, K.V.; Stefanovi´c, G. Cultured astrocytes express proteins involved in vesicular glutamate release. Brain Res.
1997,750, 41–47. [CrossRef]
120.
Schubert, V.; Bouvier, D.; Volterra, A. SNARE protein expression in synaptic terminals and astrocytes in the adult hippocampus:
A comparative analysis. Glia 2011,59, 1472–1488. [CrossRef]
121.
Tao-Cheng, J.-H.; Pham, A.; Yang, Y.; Winters, C.; Gallant, P.; Reese, T. Syntaxin 4 is concentrated on plasma membrane of
astrocytes. Neuroscience 2015,286, 264–271. [CrossRef] [PubMed]
122.
Hepp, R.; Perraut, M.; Chasserot-Golaz, S.; Galli, T.; Aunis, D.; Langley, K.; Grant, N.J. Cultured glial cells express the SNAP-25
analogue SNAP-23. Glia 1999,27, 181–187. [CrossRef]
123.
Malarkey, E.B.; Parpura, V. Temporal characteristics of vesicular fusion in astrocytes: Examination of synaptobrevin 2-laden
vesicles at single vesicle resolution. J. Physiol. 2011,589, 4271–4300. [CrossRef]
124.
Smithers, N.P.; Hodgkinson, C.P.; Cuttle, M.; Sale, G.J. Insulin-triggered repositioning of munc18c on syntaxin-4 in GLUT4
signalling. Biochem. J. 2008,410, 255–260. [CrossRef]
125.
Predescu, S.; Predescu, D.N.; Shimizu, K.; Klein, I.K.; Malik, A.B. Cholesterol-dependent Syntaxin-4 and SNAP-23 Clustering
Regulates Caveolar Fusion with the Endothelial Plasma Membrane. J. Biol. Chem. 2005,280, 37130–37138. [CrossRef] [PubMed]
126.
Brandie, F.M.; Aran, V.; Verma, A.; McNew, J.A.; Bryant, N.J.; Gould, G. Negative Regulation of Syntaxin4/SNAP-23/VAMP2-
Mediated Membrane Fusion by Munc18c In Vitro. PLoS ONE 2008,3, e4074. [CrossRef] [PubMed]
127.
Voets, T.; Toonen, R.F.; Brian, E.C.; de Wit, H.; Moser, T.; Rettig, J.; Südhof, T.C.; Neher, E.; Verhage, M. Munc18-1 Promotes Large
Dense-Core Vesicle Docking. Neuron 2001,31, 581–592. [CrossRef]
128.
Oh, E.; Kalwat, M.; Kim, M.-J.; Verhage, M.; Thurmond, D.C. Munc18-1 Regulates First-phase Insulin Release by Promoting
Granule Docking to Multiple Syntaxin Isoforms. J. Biol. Chem. 2012,287, 25821–25833. [CrossRef]
129.
Motoike, T.; Sano, K.; Nakamura, H.; Takai, Y. Expression of smg p25A/rab 3A guanine nucleotide dissociation inhibitor (GDI) in
neurons and glial cells from rat brain. Neurosci. Lett. 1993,156, 87–90. [CrossRef]
Biomolecules 2021,11, 1367 17 of 19
130.
Madison, D.; Krüger, W.; Kim, T.; Pfeiffer, S. Differential expression of rab3 isoforms in oligodendrocytes and astrocytes. J.
Neurosci. Res. 1996,45, 258–268. [CrossRef]
131.
Bonet-Ponce, L.; Beilina, A.; Williamson, C.D.; Lindberg, E.; Kluss, J.H.; Saez-Atienzar, S.; Landeck, N.; Kumaran, R.; Mamais, A.;
Bleck, C.K.E.; et al. LRRK2 mediates tubulation and vesicle sorting from lysosomes. Sci. Adv. 2020,6, eabb2454. [CrossRef]
132. Südhof, T.C. The Presynaptic Active Zone. Neuron 2012,75, 11–25. [CrossRef]
133. Mungenast, A.E. Diacylglycerol Signaling Underlies Astrocytic ATP Release. Neural Plast. 2011,2011. [CrossRef] [PubMed]
134.
Bohmbach, K.; Schwarz, M.K.; Schoch, S.; Henneberger, C. The structural and functional evidence for vesicular release from
astrocytes in situ. Brain Res. Bull. 2018,136, 65–75. [CrossRef]
135.
Buscemi, L.; Ginet, V.; Lopatar, J.; Montana, V.; Pucci, L.; Spagnuolo, P.; Zehnder, T.; Grubiši´c, V.; Truttman, A.; Sala, C.;
et al. Homer1 Scaffold Proteins Govern Ca2+ Dynamics in Normal and Reactive Astrocytes. Cereb. Cortex
2017
,27, 2365–2384.
[CrossRef]
136.
Paquet, M.; Ribeiro, F.M.; Guadagno, J.; Esseltine, J.L.; Ferguson, S.S.; Cregan, S.P. Role of metabotropic glutamate receptor 5
signaling and homer in oxygen glucose deprivation-mediated astrocyte apoptosis. Mol. Brain 2013,6, 9. [CrossRef] [PubMed]
137. Foa, L.; Gasperini, R. Developmental roles for Homer: More than just a pretty scaffold. J. Neurochem. 2009,108. [CrossRef]
138. Kennedy, M.J.; Ehlers, M.D. Mechanisms and Function of Dendritic Exocytosis. Neuron 2011,69, 856–875. [CrossRef]
139.
Perin, M.S.; Fried, V.A.; Mignery, G.A.; Jahn, R.; Südhof, T.C. Phospholipid binding by a synaptic vesicle protein homologous to
the regulatory region of protein kinase C. Nat. Cell Biol. 1990,345, 260–263. [CrossRef] [PubMed]
140. Südhof, T.C. Calcium Control of Neurotransmitter Release. Cold Spring Harb. Perspect. Biol. 2011,4, a011353. [CrossRef]
141.
Mittelsteadt, T.; Seifert, G.; Alvárez-Barón, E.; Steinhäuser, C.; Becker, A.J.; Schoch, S. Differential mRNA expression patterns of
the synaptotagmin gene family in the rodent brain. J. Comp. Neurol. 2009,512, 514–528. [CrossRef]
142.
Zhang, Q.; Fukuda, M.; Van Bockstaele, E.; Pascual, O.; Haydon, P.G. Synaptotagmin IV regulates glial glutamate release. Proc.
Natl. Acad. Sci. USA 2004,101, 9441–9446. [CrossRef]
143.
Schonn, J.-S.; Maximov, A.; Lao, Y.; Sudhof, T.C.; Sorensen, J.B. Synaptotagmin-1 and -7 are functionally overlapping Ca
2+
sensors
for exocytosis in adrenal chromaffin cells. Proc. Natl. Acad. Sci. USA 2008,105, 3998–4003. [CrossRef]
144.
Gustavsson, N.; Lao, Y.; Maximov, A.; Chuang, J.-C.; Kostromina, E.; Repa, J.; Li, C.; Radda, G.K.; Südhof, T.C.; Han, W. Impaired
insulin secretion and glucose intolerance in synaptotagmin-7 null mutant mice. Proc. Natl. Acad. Sci. USA
2008
,105, 3992–3997.
[CrossRef] [PubMed]
145.
Gustavsson, N.; Wei, S.-H.; Hoang, D.N.; Lao, Y.; Zhang, Q.; Radda, G.K.; Rorsman, P.; Südhof, T.C.; Han, W. Synaptotagmin-7 is
a principal Ca2+sensor for Ca2+-induced glucagon exocytosis in pancreas. J. Physiol. 2009,587, 1169–1178. [CrossRef]
146.
Martinez, I.; Chakrabarti, S.; Hellevik, T.; Morehead, J.; Fowler, K.; Andrews, N.W. Synaptotagmin VII Regulates Ca
2+
-Dependent
Exocytosis of Lysosomes in Fibroblasts. J. Cell Biol. 2000,148, 1141–1150. [CrossRef] [PubMed]
147.
Luo, F.; Bacaj, T.; Südhof, T.C. Synaptotagmin-7 Is Essential for Ca
2+
-Triggered Delayed Asynchronous Release But Not for
Ca2+-Dependent Vesicle Priming in Retinal Ribbon Synapses. J. Neurosci. 2015,35, 11024–11033. [CrossRef]
148.
Sreetama, S.C.; Takano, T.; Nedergaard, M.; Simon, S.; Jaiswal, J.K. Injured astrocytes are repaired by Synaptotagmin XI-regulated
lysosome exocytosis. Cell Death Differ. 2015,23, 596–607. [CrossRef] [PubMed]
149.
Oh, S.-J.; Han, K.-S.; Park, H.; Woo, D.H.; Kim, H.Y.; Traynelis, S.F.; Lee, C.J. Protease activated receptor 1-induced glutamate
release in cultured astrocytes is mediated by Bestrophin-1 channel but not by vesicular exocytosis. Mol. Brain
2012
,5, 38.
[CrossRef]
150.
Stenovec, M.; Lasiˇc, E.; Boži´c, M.; Bobnar, S.T.; Stout, R.F.; Grubiši´c, V.; Parpura, V.; Zorec, R. Ketamine Inhibits ATP-Evoked
Exocytotic Release of Brain-Derived Neurotrophic Factor from Vesicles in Cultured Rat Astrocytes. Mol. Neurobiol.
2016
,53,
6882–6896. [CrossRef]
151.
Dai, H.; Shin, O.-H.; Machius, M.; Tomchick, D.; Südhof, T.C.; Rizo, J. Structural basis for the evolutionary inactivation of Ca
2+
binding to synaptotagmin 4. Nat. Struct. Mol. Biol. 2004,11, 844–849. [CrossRef]
152. Rusakov, D. Disentangling calcium-driven astrocyte physiology. Nat. Rev. Neurosci. 2015,16, 226–233. [CrossRef] [PubMed]
153.
Hazell, A.S.; Wang, N. Identification of complexin II in astrocytes: A possible regulator of glutamate release in these cells. Biochem.
Biophys. Res. Commun. 2011,404, 228–232. [CrossRef]
154.
Wang, Z.; Wei, X.; Liu, K.; Zhang, X.; Yang, F.; Zhang, H.; He, Y.; Zhu, T.; Li, F.; Shi, W.; et al. NOX2 deficiency ameliorates cerebral
injury through reduction of complexin II-mediated glutamate excitotoxicity in experimental stroke. Free. Radic. Biol. Med.
2013
,
65, 942–951. [CrossRef] [PubMed]
155.
Burgoyne, R.D.; Handel, S.E. Activation of exocytosis by GTP analogues in adrenal chromaffin cells revealed by patch-clamp
capacitance measurement. FEBS Lett. 1994,344, 139–142. [CrossRef]
156.
Fernandez, J.M.; Neher, E.; Gomperts, B.D. Capacitance measurements reveal stepwise fusion events in degranulating mast cells.
Nat. Cell Biol. 1984,312, 453–455. [CrossRef]
157.
Regazzi, R.; Li, G.; Ullrich, S.; Jaggi, C.; Wollheim, C.B. Different requirements for protein kinase C activation and Ca
2+
-
independent insulin secretion in response to guanine nucleotides. J. Biol. Chem. 1989,264, 9939–9944. [CrossRef]
158.
Li, G.; Han, L.; Chou, T.-C.; Fujita, Y.; Arunachalam, L.; Xu, A.; Wong, A.; Chiew, S.-K.; Wan, Q.; Wang, L.; et al. RalA and RalB
Function as the Critical GTP Sensors for GTP-Dependent Exocytosis. J. Neurosci. 2007,27, 190–202. [CrossRef]
159. Wu, B.; Guo, W. The Exocyst at a Glance. J. Cell Sci. 2015,128, 2957–2964. [CrossRef]
Biomolecules 2021,11, 1367 18 of 19
160.
Rivera-Molina, F.; Toomre, D. Live-cell imaging of exocyst links its spatiotemporal dynamics to various stages of vesicle fusion. J.
Cell Biol. 2013,201, 673–680. [CrossRef]
161.
Zorec, R.; Verkhratsky, A.; Rodríguez, J.; Parpura, V. Astrocytic vesicles and gliotransmitters: Slowness of vesicular release and
synaptobrevin2-laden vesicle nanoarchitecture. Neuroscience 2016,323, 67–75. [CrossRef]
162.
Lepore, D.; Martínez-Núñez, L.; Munson, M. Exposing the Elusive Exocyst Structure. Trends Biochem. Sci.
2018
,43, 714–725.
[CrossRef] [PubMed]
163.
Liu, J.; Zuo, X.; Yue, P.; Guo, W. Phosphatidylinositol 4,5-Bisphosphate Mediates the Targeting of the Exocyst to the Plasma
Membrane for Exocytosis in Mammalian Cells. Mol. Biol. Cell 2007,18, 4483–4492. [CrossRef]
164.
Morgera, F.; Sallah, M.R.; Dubuke, M.L.; Gandhi, P.; Brewer, D.N.; Carr, C.M.; Munson, M. Regulation of exocytosis by the exocyst
subunit Sec6 and the SM protein Sec1. Mol. Biol. Cell 2012,23, 337–346. [CrossRef] [PubMed]
165.
Moulson, A.J.; Squair, J.W.; Franklin, R.J.M.; Tetzlaff, W.; Assinck, P. Diversity of Reactive Astrogliosis in CNS Pathology:
Heterogeneity or Plasticity? Front. Cell. Neurosci. 2021. [CrossRef] [PubMed]
166.
Nosi, D.; Lana, D.; Giovannini, M.; Delfino, G.; Zecchi-Orlandini, S. Neuroinflammation: Integrated Nervous Tissue Response
through Intercellular Interactions at the “Whole System” Scale. Cells 2021,10, 1195. [CrossRef]
167.
Escartin, C.; Galea, E.; Lakatos, A.; O’Callaghan, J.P.; Petzold, G.C.; Serrano-Pozo, A.; Steinhäuser, C.; Volterra, A.; Carmignoto, G.;
Agarwal, A.; et al. Reactive astrocyte nomenclature, definitions, and future directions. Nat. Neurosci.
2021
,24, 312–325. [CrossRef]
168.
Liddelow, S.A.; Guttenplan, K.A.; Clarke, L.E.; Bennett, F.C.; Bohlen, C.J.; Schirmer, L.; Bennett, M.L.; Münch, A.E.; Chung, W.S.;
Petersong, T.C.; et al. Neurotoxic reactive astrocytes are induced by activated microglia. Nature
2017
,541, 481–487. [CrossRef]
[PubMed]
169.
Liddelow, S.A.; Barres, B.A. Reactive Astrocytes: Production, Function, and Therapeutic Potential. Immunity
2017
,46, 957–967.
[CrossRef]
170.
Peng, H.; Erdmann, N.; Whitney, N.; Dou, H.; Gorantla, S.; Gendelman, H.E.; Ghorpade, A.; Zheng, J. HIV-1-infected and/or
immune activated macrophages regulate astrocyte SDF-1 production through IL-1β.Glia 2006,54, 619–629. [CrossRef]
171.
Zheng, J.C.; Huang, Y.; Tang, K.; Cui, M.; Niemann, D.; Lopez, A.; Morgello, S.; Chen, S. HIV-1-infected and/or immune-activated
macrophages regulate astrocyte CXCL8 production through IL-1
β
and TNF-
α
: Involvement of mitogen-activated protein kinases
and protein kinase R. J. Neuroimmunol. 2008,200, 100–110. [CrossRef]
172.
Álvarez, S.; Blanco, A.; Fresno, M.; Muñoz-Fernández, M. Ángeles Nuclear factor-
κ
B activation regulates cyclooxygenase-2
induction in human astrocytes in response to CXCL12: Role in neuronal toxicity. J. Neurochem. 2010,113, 772–783. [CrossRef]
173.
Lau, L.T.; Yu, A.C.-H. Astrocytes Produce and Release Interleukin-1, Interleukin-6, Tumor Necrosis Factor Alpha and Interferon-
Gamma Following Traumatic and Metabolic Injury. J. Neurotrauma 2001,18, 351–359. [CrossRef] [PubMed]
174.
Verhoog, Q.P.; Holtman, L.; Aronica, E.; Van Vliet, E.A. Astrocytes as Guardians of Neuronal Excitability: Mechanisms Underlying
Epileptogenesis. Front. Neurol. 2020,11, 11. [CrossRef] [PubMed]
175.
Agulhon, C.; Sun, M.-Y.; Murphy, T.; Myers, T.; Lauderdale, K.; Fiacco, T.A. Calcium Signaling and Gliotransmission in Normal
vs. Reactive Astrocytes. Front. Pharmacol. 2012,3, 139. [CrossRef]
176.
Pascual, O.; Ben Achour, S.; Rostaing, P.; Triller, A.; Bessis, A. Microglia activation triggers astrocyte-mediated modulation of
excitatory neurotransmission. Proc. Natl. Acad. Sci. USA 2012,109, E197–E205. [CrossRef]
177.
Santello, M.; Bezzi, P.; Volterra, A. TNF
α
Controls Glutamatergic Gliotransmission in the Hippocampal Dentate Gyrus. Neuron
2011,69, 988–1001. [CrossRef] [PubMed]
178.
Takata-Tsuji, F.; Chounlamountri, N.; Do, L.; Philippot, C.; Ducassou, J.N.; Couté, Y.; Ben Achour, S.; Honnorat, J.; Place, C.;
Pascual, O. Microglia modulate gliotransmission through the regulation of VAMP2 proteins in astrocytes. Glia
2021
,69, 61–72.
[CrossRef]
179.
Calì, C.; Marchaland, J.; Regazzi, R.; Bezzi, P. SDF 1-alpha (CXCL12) triggers glutamate exocytosis from astrocytes on a millisecond
time scale: Imaging analysis at the single-vesicle level with TIRF microscopy. J. Neuroimmunol. 2008,198, 82–91. [CrossRef]
180.
Canedo, T.; Portugal, C.C.; Socodato, R.; Almeida, T.O.; Terceiro, A.F.; Bravo, J.; Silva, A.I.; Magalhães, J.D.; Guerra-Gomes,
S.; Oliveira, J.F.; et al. Astrocyte-derived TNF and glutamate critically modulate microglia activation by methamphetamine.
Neuropsychopharmacology 2021. [CrossRef]
181.
Habbas, S.; Santello, M.; Becker, D.; Stubbe, H.; Zappia, G.; Liaudet, N.; Klaus, F.; Kollias, G.; Fontana, A.; Pryce, C.R.; et al.
Neuroinflammatory TNFαImpairs Memory via Astrocyte Signaling. Cell 2015,163, 1730–1741. [CrossRef]
182.
Gourine, A.V.; Kasymov, V.; Marina, N.; Tang, F.; Figueiredo, M.F.; Lane, S.; Teschemacher, A.G.; Spyer, K.M.; Deisseroth,
K.; Kasparov, S. Astrocytes Control Breathing Through pH-Dependent Release of ATP. Science
2010
,329, 571–575. [CrossRef]
[PubMed]
183.
Angelova, P.R.; Kasymov, V.; Christie, I.; Sheikhbahaei, S.; Turovsky, E.; Marina, N.; Korsak, A.; Zwicker, J.D.; Teschemacher, A.G.;
Ackland, G.L.; et al. Functional Oxygen Sensitivity of Astrocytes. J. Neurosci. 2015,35, 10460–10473. [CrossRef] [PubMed]
184.
Marina, N.; Turovsky, E.; Christie, I.N.; Hosford, P.; Hadjihambi, A.; Korsak, A.; Ang, R.; Mastitskaya, S.; Sheikhbahaei, S.;
Theparambil, S.M.; et al. Brain metabolic sensing and metabolic signaling at the level of an astrocyte. Glia
2018
,66, 1185–1199.
[CrossRef] [PubMed]
185.
Byts, N.; Sharma, S.; Laurila, J.; Paudel, P.; Miinalainen, I.; Ronkainen, V.-P.; Hinttala, R.; Törnquist, K.; Koivunen, P.; Myllyharju,
J. Transmembrane Prolyl 4-Hydroxylase is a Novel Regulator of Calcium Signaling in Astrocytes. Eneuro
2021
,8, 1–23. [CrossRef]
[PubMed]
Biomolecules 2021,11, 1367 19 of 19
186.
Gaidin, S.G.; Turovskaya, M.V.; Mal’Tseva, V.N.; Zinchenko, V.P.; Blinova, E.; Turovsky, E.A. A Complex Neuroprotective Effect of
Alpha-2-Adrenergic Receptor Agonists in a Model of Cerebral Ischemia–Reoxygenation In Vitro. Biochem. (Moscow) Suppl. Ser. A
Membr. Cell Biol. 2019,13, 319–333. [CrossRef]
187.
Lian, H.; Yang, L.; Cole, A.; Sun, L.; Chiang, A.C.-A.; Fowler, S.W.; Shim, D.J.; Rodriguez-Rivera, J.; Taglialatela, G.; Jankowsky,
J.L.; et al. NF
κ
B-Activated Astroglial Release of Complement C3 Compromises Neuronal Morphology and Function Associated
with Alzheimer’s Disease. Neuron 2015,85, 101–115. [CrossRef]
188.
Stephan, A.H.; Barres, B.A.; Stevens, B. The Complement System: An Unexpected Role in Synaptic Pruning During Development
and Disease. Annu. Rev. Neurosci. 2012,35, 369–389. [CrossRef] [PubMed]
189.
Stevens, B.; Allen, N.J.; Vazquez, L.E.; Howell, G.R.; Christopherson, K.S.; Nouri, N.; Micheva, K.; Mehalow, A.; Huberman, A.D.;
Stafford, B.; et al. The Classical Complement Cascade Mediates CNS Synapse Elimination. Cell
2007
,131, 1164–1178. [CrossRef]
