Regulation
of Exocytosis by Complexin
Ok-Ho
Shin
Ok-Ho
Shin, Department of Neuroscience and Cell Biology, University
of Texas Medical Branch, 301 University Blvd, Galveston, TX 77555, USA
Correspondence to: Ok-Ho Shin, Department of Neuroscience and Cell
Biology, University of Texas Medical Branch, 301 University Blvd, Galveston, TX
77555, USA
Email: okshin@utmb.edu
Telephone: + 1-409-772-1188
Fax:
+1-409-762-9382
Received: February 26,
2015
Revised: April 1, 2015
Accepted: April 6, 2015
Published online: June 6, 2015
ABSTRACT
Complexin (Cpx),
which is expressed mainly in the nervous system, binds efficiently to the SNARE
complex that is composed of either Syntaxin 1 or Syntaxin 3. Both Syntaxin 1
and Syntaxin 3 are involved in mediating synchronous neurotransmitter release
in the nervous system. Cpx stabilizes the SNARE complex upon binding, and
potentiates the efficacy of synchronous exocytotic process. Cpx is consisted of
N-terminal, accessory -helix, central -helix, and C-terminal domains. Each
domain of Cpx has a distinct function that is differentially involved in the
regulation of priming, clamping, and activating in the exocytotic process.
These functions of Cpx domains coordinately potentiate the efficacy of
synchronous exocytotic process.
© 2015 ACT. All
rights reserved.
Key words: Complexin; Exocytosis; SNARE Complex; Syntaxin;
Transmitter release
Shin OH. Regulation of
Exocytosis by Complexin. Journal of
Biochemistry and Molecular Biology Research 2015; 1(2): 25-29 Available from: URL:
http://www.ghrnet.org/index.php/jbmbr/article/view/1087
Abbreviations
AMPA: -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid;
Cpx: Complexin
(also known as Synaphin);
GST: glutathione
S-transferase;
IGF-1:
insulin-like growth factor-1;
KD: knock-down;
KO: knock-out;
LTP: long-term
potentiation;
SNAP-23 or -25:
synaptosomal-associated protein 23 or 25;
SNARE: soluble
N-ethylmaleimide-sensitive factor attachment protein receptor;
SNARE complex: a
ternary complex composed of Syntaxin, Synaptobrevin, and either SNAP-23 or
SNAP-25;
Syb2:
Synaptobrevin 2 (also known as vesicle-associated membrane protein 2, VAMP2);
Syt1:
Synaptotagmin 1;
Syt10:
Synaptotagmin 10;
WT: wild-type.
INTRODUCTION
Ca2+-triggered exocytosis is an essential cellular process
that mediates transmitter release to the extracellular space for intercellular
communications particularly in neurons[1-3]. The same or similar
processes are used for intracellular trafficking to deliver membrane patches or
proteins to the plasma membrane[4,5]. Stimulations of neurons cause
an increase in cytosolic Ca2+ concentrations and trigger fusions of
synaptic vesicles into the plasma membrane to release transmitters to the
extracellular space[1-3]. Synaptotagmin 1 (Syt1), a vesicular Ca2+
binding C2 domain protein, regulates Ca2+-triggered neurotransmitter
release by interacting with membranes and the SNARE complex in a Ca2+-dependent
manner[6,7]. Synaptobrevin 2 (Syb2; also known as vesicle-associated
membrane protein 2, VAMP2), a vesicular SNARE protein, forms a ternary SNARE
complex with Syntaxin and SNAP-25 that are located in the target plasma
membrane[8]. The interaction between Syt1 and the membrane is
essential for the Ca2+-binding to the C2 domains of Syt1 in
physiological Ca2+ concentrations[7,9,10]. Meanwhile, the
Ca2+-dependent affinities of interactions between Syt1 and SNARE
complexes are correlated with the efficacies of the corresponding exocytosis[6].
Complexin (Cpx; also known as Synaphin) binds to the assembled SNARE complex,
stabilizes the SNARE complex upon binding, and potentiates the efficacy of
Syt1-regulated neurotransmitter release[11-14].
Cpx is
expressed mainly in the nervous system
Cpx is initially identified as a cytosolic protein that interacts with
the SNARE complex[12,13]. Cpx binds to the assembled SNARE complex
composed of Syntaxin 1, Syb2, and SNAP-25 with a high affinity[11-13].
The structure of Cpx-bound SNARE complex revealed that Cpx interacts directly
with all components of the SNARE complex[11]. Consistent with this
observation, Cpx does not bind to partially assembled SNARE complexes[13,15].
Cpx family has four isoforms (Cpx1-4) that are expressed mainly in the nervous
system such as the brain, spinal cord, and retina (Figure 1A)[13,16].
However, trace amounts of Cpx1-3 isoforms are also expressed in the liver,
pancreas, kidney, spleen, skeletal muscle, and lung[13,16]. Cpx1 and
Cpx2 are soluble cytosolic proteins[12,13]. Meanwhile, the
C-terminals of Cpx3 and Cpx4 are farnesylated, and this modification is
required for the targeting of these Cpx isoforms to the presynaptic plasma
membrane[16].
Cpx binds
efficiently to the assembled SNARE complexes that are composed of either
Syntaxin 1 or Syntaxin 3
Eukaryotic cells express at least four different Syntaxin isoforms
(Syntaxin 1, 2, 3, and 4) that are involved in mediating various types of
exocytotic processes[18]. Systematic analyses of interactions
between Cpx and SNARE complexes by employing both glutathione S-transferase
(GST)-pulldowns and a yeast four-hybrid interaction assay showed that Cpx binds
efficiently to the SNARE complexes that are composed of either Syntaxin 1 or
Syntaxin 3[17-20]. Cpx does not bind efficiently to the SNARE
complexes that are composed of either Syntaxin 2 or Syntaxin 4[18,20].
Consistent with these findings, the Syntaxin 1 residues that interact directly
with Cpx are conserved in Syntaxin 3, but not in Syntaxin 2 and Syntaxin 4
(Figure 1B).
SNARE
complexes that are involved in mediating synchronous release are probably
associated with Cpx
Stimulated neurons release neurotransmitters in two distinct modes,
synchronous and asynchronous (Figure 2A). Synchronous release causes rapid
actions, while asynchronous release triggers long-lasting actions of the target
cells[7,21]. Both modes of release are required to maintain the
functions of biological systems. In addition, neurons are also capable of
releasing neurotransmitters without evoked stimulations but still mainly in a
Ca2+-dependent manner, which is called spontaneous release (Figure
2A)[7,21]. Four Cpx isoforms (Cpx1-4) are expressed mainly in the
nervous system such as the brain, spinal cord, and retina, where
neurotransmitters are released in a synchronous manner (Figure 2A)[13,16].
In particular, membrane-bound Cpx3 and Cpx4 are expressed in retinal ribbon
synapses where strong synchronous neurotransmitter release is required[17].
Cpx binds efficiently to the SNARE complexes that are composed of either
Syntaxin 1 or Syntaxin 3[17-20]. These Syntaxin isoforms are
expressed at presynaptic nerve terminals where neurotransmitters are released
in a synchronous manner[6,17]. Neurons also express other Syntaxin
isoforms such as Syntaxin 2 and Syntaxin 4, which are not involved in mediating
synchronous neurotransmitter release[22,23]. Therefore, the
differential binding of Cpx to SNARE complexes is probably required for the
regulation of subsequent interaction between the SNARE complex and responsible
Ca2+-sensors. Cpx-deficient neurons, in the presence of Syt1,
display a significant reduction of the efficacy of Syt1-regulated synchronous
release[14]. However, these neurons still maintain the mode of
synchronous release[14]. Meanwhile, Syt1-deficient neurons, in the
presence of Cpx, change the mode of release to asynchronous from synchronous
release[24]. These observations suggest that Syt1 is a key regulator
for synchronous neurotransmitter release and that Cpx potentiates the
Syt1-regulated synchronous release (Figure 2B).
Cpx is
involved in regulating various types of
exocytotic processes
Cpx-deficient neurons consistently display a significant reduction of
the efficacy of synchronous neurotransmitter release in many different release
systems (Table 1). Cpx is also involved in regulating large dense-core vesicle
exocytosis[25-27] and rapid delivery of AMPA receptors to the plasma
membrane[3]. Cpx-deficient chromaffin cells show a significant
decrease in both evoked norepinephrine release and the pool size of releasable
vesicles[25,26]. In olfactory neurons, Cpx-deficiency is associated
with an impairment of Synaptotagmin 10 (Syt10)-regulated insulin-like growth
factor-1 (IGF-1) release[27]. These findings suggest that Cpx is
involved in regulating various types of exocytotic processes in the nervous
system. Trace amounts of Cpx1-3 isoforms are also expressed in many
non-neuronal tissues such as the liver, pancreas, kidney, spleen, skeletal muscle,
and lung[13,16]. Therefore, Cpx is also likely involved in
regulating various synchronous exocytotic processes occurring in these
non-neuronal cells.
Cpx
domains have distinct functions that coordinately potentiate the efficacy
of synchronous release
Cpx-deficient cortical neurons and neuromuscular
junctions show an increase in spontaneous release and a decrease in the pool
size of releasable vesicles, in addition to a significant reduction of the
efficacy of synchronous release (Table 1). These observations suggest that Cpx
is involved in regulating at least three different steps of the exocytotic
process: priming of vesicles, clamping of spontaneous vesicle fusions, and
activation of Ca2+-triggered vesicle fusions (Figure 3A). However,
Cpx-deficient hippocampal neurons do not show any increase in spontaneous
release or decrease in the pool size of releasable vesicles[14,28].
These differences among different neurotransmitter release systems are probably
caused by the differential expression of other Ca2+-sensors or
regulatory proteins. To resolve this discrepancy, isolated/purified vesicle
fusion assays, such as in vitro flipped SNARE cell fusion assay and
single vesicle-vesicle microscopy imaging, are developed and employed[29-31].
The investigation of Cpx functions by employing these methods supports the
multiple roles of Cpx in the regulation of priming, clamping, and activating
during the Ca2+-triggered vesicle fusion process. Cpx is consisted
of N-terminal, accessory -helix, central -helix, and C-terminal domains (Figure 3B). And each domain of Cpx has
a distinct function that is differentially involved in the regulation of
priming, clamping, and activating during Ca2+-triggered vesicle
fusions. These distinct functions of Cpx domains coordinately contribute to
potentiate the efficacy of synchronous release (Figure 3B)[3,25,26,29-37].
Conclusion
Cpx binds efficiently to the assembled SNARE complex that is composed
of either Syntaxin 1 or Syntaxin 3, stabilizes the SNARE complex upon binding,
and potentiates synchronous neurotransmitter release[11,14,18,20].
In stimulated neurons, Syt1 interacts with membranes and the Cpx-bound SNARE
complex in a Ca2+-dependent manner to mediate synchronous
neurotransmitter release[6]. The efficacies of Syt1-regulated
synchronous release are correlated with the strengths of interactions between
Syt1 and SNARE complexes[6]. Therefore, it is reasonable to assume
that Cpx most likely facilitates the interaction between synchronous Ca2+-sensors
and SNARE complexes, at a minimum, by selecting, tagging, and stabilizing the
assembled SNARE complex to potentiate the efficacy of synchronous release.
CONFLICT OF INTERESTS
The Author has no conflicts of interest to declare.
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Peer reviewer: Zui Pan, PhD, Associate
Professor, Davis Heart and Lung Research Institute, Comprehensive Cancer
Center, The Ohio State University Wexner Medical Center, 460 West 12th Ave. BRT. Rm398, Columbus, OH 43210, USA.
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