Non-polarized distribution of synaptotagmin IV in neurons: evidence that synaptotagmin IV is not a synaptic vesicle protein

Mechanisms Controlling the Expression and Secretion of BDNF

Brain-derived nerve factor (BDNF), through TrkB receptor activation, is an important modulator for many different physiological and pathological functions in the nervous system. Among them, BDNF plays a crucial role in the development and correct maintenance of brain circuits and synaptic plasticity as well as in neurodegenerative diseases. The proper functioning of the central nervous system depends on the available BDNF concentrations, which are tightly regulated at transcriptional and translational levels but also by its regulated secretion. In this review we summarize the new advances regarding the molecular players involved in BDNF release. In addition, we will address how changes of their levels or function in these proteins have a great impact in those functions modulated by BDNF under physiological and pathological conditions.

The Synaptotagmins: Calcium Sensors for Vesicular Trafficking

The synaptotagmin family of vesicle proteins is believed to mediate calcium-dependent regulation of membrane trafficking. Detailed biochemical and in vivo studies of the most characterized isoform, synaptotagmin 1 (syt 1), have provided compelling evidence that it functions as a calcium sensor for fast neurotransmitter release at synapses. However, the function of the remaining isoforms is unclear, and multiple roles have been hypothesized for several of these. Recent evidence in Drosophila has given insight into the function of some of the remaining synaptotagmin family members. Of the five evolutionarily conserved isoforms in Drosophila, only two, syt 1 and syt 4, localize to most, if not all, synapses. The former is localized to presynaptic terminals, whereas the latter is predominantly postsynaptic. This suggests an intriguing possibility that syt 4 may mediate a postsynaptic vesicle trafficking pathway, providing a molecular basis for an evolutionarily conserved bidirectional vesicular trafficking communication system at synapses.

Up-regulation of Synaptotagmin IV within amyloid plaque-associated dystrophic neurons in Tg2576 mouse model of Alzheimer's disease

AIM

To investigate the involvement of the vesicular membrane trafficking regulator Synaptotagmin IV (Syt IV) in Alzheimer's disease pathogenesis and to define the cell types containing increased levels of Syt IV in the β-amyloid plaque vicinity.

METHODS

Syt IV protein levels in wild type (WT) and Tg2576 mice cortex were determined by Western blot analysis and immunohistochemistry. Co-localization studies using double immunofluorescence staining for Syt IV and markers for astrocytes (glial fibrillary acidic protein), microglia (major histocompatibility complex class II), neurons (neuronal specific nuclear protein), and neurites (neurofilaments) were performed in WT and Tg2576 mouse cerebral cortex.

RESULTS

Western blot analysis showed higher Syt IV levels in Tg2576 mice cortex than in WT cortex. Syt IV was found only in neurons. In plaque vicinity, Syt IV was up-regulated in dystrophic neurons. The Syt IV signal was not up-regulated in the neurons of Tg2576 mice cortex without plaques (resembling the pre-symptomatic conditions).

CONCLUSIONS

Syt IV up-regulation within dystrophic neurons probably reflects disrupted vesicular transport or/and impaired protein degradation occurring in Alzheimer's disease and is probably a consequence but not the cause of neuronal degeneration. Hence, Syt IV up-regulation and/or its accumulation in dystrophic neurons may have adverse effects on the survival of the affected neuron.

Distinct subsets of Syt-IV/BDNF vesicles are sorted to axons versus dendrites and recruited to synapses by activity

BDNF plays a critical role in the regulation of synaptic strength and is essential for long-term potentiation, a phenomenon that underlies learning and memory. However, whether BDNF acts in a diffuse manner or is targeted to specific neuronal subcompartments or synaptic sites to affect circuit function remains unknown. Here, using photoactivation of BDNF or syt-IV (a regulator of exocytosis present on BDNF-containing vesicles) in transfected rat hippocampal neurons, we discovered that distinct subsets of BDNF vesicles are targeted to axons versus dendrites and are not shared between these compartments. Moreover, syt-IV- and BDNF-harboring vesicles are recruited to both presynaptic and postsynaptic sites in response to increased neuronal activity. Finally, using syt-IV knockout mouse neurons, we found that syt-IV is necessary for both presynaptic and postsynaptic scaling of synaptic strength in response to changes in network activity. These findings demonstrate that BDNF-containing vesicles can be targeted to specific sites in neurons and suggest that syt-IV-regulated BDNF secretion is subject to spatial control to regulate synaptic function in a site-specific manner.

Small GTPase Rab17 regulates dendritic morphogenesis and postsynaptic development of hippocampal neurons

Neurons are compartmentalized into two morphologically, molecularly, and functionally distinct domains: axons and dendrites, and precise targeting and localization of proteins within these domains are critical for proper neuronal functions. It has been reported that several members of the Rab family small GTPases that are key mediators of membrane trafficking, regulate axon-specific trafficking events, but little has been elucidated regarding the molecular mechanisms that underlie dendrite-specific membrane trafficking. Here we show that Rab17 regulates dendritic morphogenesis and postsynaptic development in mouse hippocampal neurons. Rab17 is localized at dendritic growth cones, shafts, filopodia, and mature spines, but it is mostly absent in axons. We also found that Rab17 mediates dendrite growth and branching and that it does not regulate axon growth or branching. Moreover, shRNA-mediated knockdown of Rab17 expression resulted in a dramatically reduced number of dendritic spines, probably because of impaired filopodia formation. These findings have revealed the first molecular link between membrane trafficking and dendritogenesis.

Transcriptional responses of cultured rat sympathetic neurons during BMP-7-induced dendritic growth

BACKGROUND

Dendrites are the primary site of synapse formation in the vertebrate nervous system; however, relatively little is known about the molecular mechanisms that regulate the initial formation of primary dendrites. Embryonic rat sympathetic neurons cultured under defined conditions extend a single functional axon, but fail to form dendrites. Addition of bone morphogenetic proteins (BMPs) triggers these neurons to extend multiple dendrites without altering axonal growth or cell survival. We used this culture system to examine differential gene expression patterns in naïve vs. BMP-treated sympathetic neurons in order to identify candidate genes involved in regulation of primary dendritogenesis.

METHODOLOGY/PRINCIPAL FINDINGS

To determine the critical transcriptional window during BMP-induced dendritic growth, morphometric analysis of microtubule-associated protein (MAP-2)-immunopositive processes was used to quantify dendritic growth in cultures exposed to the transcription inhibitor actinomycin-D added at varying times after addition of BMP-7. BMP-7-induced dendritic growth was blocked when transcription was inhibited within the first 24 hr after adding exogenous BMP-7. Thus, total RNA was isolated from sympathetic neurons exposed to three different experimental conditions: (1) no BMP-7 treatment; (2) treatment with BMP-7 for 6 hr; and (3) treatment with BMP-7 for 24 hr. Affymetrix oligonucleotide microarrays were used to identify differential gene expression under these three culture conditions. BMP-7 significantly regulated 56 unique genes at 6 hr and 185 unique genes at 24 hr. Bioinformatic analyses implicate both established and novel genes and signaling pathways in primary dendritogenesis.

CONCLUSIONS/SIGNIFICANCE

This study provides a unique dataset that will be useful in generating testable hypotheses regarding transcriptional control of the initial stages of dendritic growth. Since BMPs selectively promote dendritic growth in central neurons as well, these findings may be generally applicable to dendritic growth in other neuronal cell types.

Differential distribution of synaptotagmin-1, -4, -7, and -9 in rat adrenal chromaffin cells

Neurons and certain kinds of endocrine cells, such as adrenal chromaffin cells, have large dense-core vesicles (LDCVs) and synaptic vesicles or synaptic-like microvesicles (SLMVs). These secretory vesicles exhibit differences in Ca(2+) sensitivity and contain diverse signaling substances. The present work was undertaken to identify the synaptotagmin (Syt) isoforms present in secretory vesicles. Fractionation analysis of lysates of the bovine adrenal medulla and immunocytochemistry in rat chromaffin cells indicated that Syt 1 was localized in LDCVs and SLMVs, whereas Syt 7 was the predominant isoform present in LDCVs. In contrast to PC12 cells and the pancreatic β cell line INS-1, Syt 9 was not immunodetected in LDCVs in rat chromaffin cells. Double-staining revealed that Syt 9-like immunoreactivity was nearly identical with fluorescent thapsigargin binding, suggesting the presence of Syt 9 in the endoplasmic reticulum (ER).The exogenous expression of Syt 1-GFP in INS-1 cells, which had a negligible level of endogenous Syt 1, resulted in an increase in the amount of Syt 9 in the ER, suggesting that Syt 9 competes with Syt 1 for trafficking from the ER to the Golgi complex. We conclude that LDCVs mainly contain Syt 7, whereas SLMVs contain Syt 1, but not Syt 7, in rat and bovine chromaffin cells.

Rat and Drosophila synaptotagmin 4 have opposite effects during SNARE-catalyzed membrane fusion

Synaptotagmins (Syt) are a large family of proteins that regulate membrane traffic in neurons and other cell types. One isoform that has received considerable attention is SYT4, with apparently contradictory reports concerning the function of this isoform in fruit flies and mice. SYT4 was reported to function as a negative regulator of neurotrophin secretion in mouse neurons and as a positive regulator of secretion of a yet to be identified growth factor from muscle cells in flies. Here, we have directly compared the biochemical and functional properties of rat and fly SYT4. We report that rat SYT4 inhibited SNARE-catalyzed membrane fusion in both the absence and presence of Ca²⁺. In marked contrast, fly SYT4 stimulated SNARE-mediated membrane fusion in response to Ca²⁺. Analysis of chimeric molecules, isolated C2 domains, and point mutants revealed that the C2B domain of the fly protein senses Ca²⁺ and is sufficient to stimulate fusion. Rat SYT4 was able to stimulate fusion in response to Ca²⁺ when the conserved Asp-to-Ser Ca²⁺ ligand substitution in its C2A domain was reversed. In summary, rat SYT4 serves as an inhibitory isoform, whereas fly SYT4 is a bona fide Ca²⁺ sensor capable of coupling Ca²⁺ to membrane fusion.

Synaptotagmins in neurodegeneration

Synaptotagmins (Syts) are transmembrane proteins involved in the regulation of membrane trafficking. Here, we summarize literature data that provide growing evidence that several Syts are involved in the pathophysiological mechanisms of temporal lobe epilepsy and Parkinson's disease, as well as few reports related to brain ischemia and Alzheimer's disease (AD). We also report new data from our laboratories, showing changes of the expression of several Syts in Tg2576 mouse model of AD that may be related to neuroinflammation surrounding the beta-amyloid plaques. Furthermore, we demonstrate N-methyl-D-aspartate receptor-mediated upregulation of Syt 4 mRNA in a model of excitotoxic striatal lesion induced by unilateral striatal injection of quinolinic acid, associating the upregulation of Syt 4 with mechanisms of excitotoxicity. We propose that pharmacological manipulation of Syt expression in animal models of neurodegeneration should be further explored, as it may help to clarify the role of individual Syt isoforms in the regulation of membrane trafficking in neurodegeneration.

Synaptotagmin-IV modulates synaptic function and long-term potentiation by regulating BDNF release

Synaptotagmin-IV (syt-IV) is a membrane trafficking protein that influences learning and memory, but its localization and role in synaptic function remain unclear. Here we discovered that syt-IV localizes to BDNF-containing vesicles in hippocampal neurons. Syt-IV/BDNF-harboring vesicles undergo exocytosis in both axons and dendrites, and syt-IV inhibits BDNF release at both sites. Knockout of syt-IV increases, and over-expression decreases, the rate of FM dye destaining from presynaptic terminals indirectly via changes in post-synaptic release of BDNF. Hence, post-synaptic syt-IV regulates the trans-synaptic action of BDNF to control presynaptic vesicle dynamics. Furthermore, selective loss of presynaptic syt-IV increased spontaneous quantal release, while loss of post-synaptic syt-IV increased quantal amplitude. Finally, syt-IV knockout mice exhibit enhanced LTP, which depends entirely on disinhibition of BDNF release. Thus, regulation of BDNF secretion by syt-IV emerges as a mechanism to maintain synaptic strength within a useful range during long-term potentiation.

JNK phosphorylates synaptotagmin-4 and enhances Ca2+-evoked release

Ca2+ influx induced by membrane depolarization triggers the exocytosis of secretory vesicles in various cell types such as endocrine cells and neurons. Peptidyl growth factors enhance Ca2+-evoked release, an effect that may underlie important adaptive responses such as the long-term potentiation of synaptic transmission induced by growth factors. Here, we show that activation of the c-Jun N-terminal kinase (JNK) plays an essential role in nerve growth factor (NGF) enhancement of Ca2+-evoked release in PC12 neuroendocrine cells. Moreover, JNK associated with phosphorylated synaptotagmin-4 (Syt 4), a key mediator of NGF enhancement of Ca2+-evoked release in this system. NGF treatment led to phosphorylation of endogenous Syt 4 at Ser135 and translocation of Syt 4 from immature to mature secretory vesicles in a JNK-dependent manner. Furthermore, mutation of Ser135 abrogated enhancement of Ca2+-evoked release by Syt 4. These results provide a molecular basis for the effect of growth factors on Ca2+-mediated secretion.

Singing, but not seizure, induces synaptotagmin IV in zebra finch song circuit nuclei

Synaptotagmins are a family of proteins that function in membrane fusion events, including synaptic vesicle exocytosis. Within this family, synaptotagmin IV (Syt IV) is unique in being a depolarization-induced immediate early gene (IEG). Experimental perturbation of Syt IV modulates neurotransmitter release in mice, flies, and PC12 cells, and modulates learning in mice. Despite these features, induction of Syt IV expression by a natural behavior has not been previously reported. We used the zebra finch, a songbird species, to investigate Syt IV because song is a naturally learned behavior whose neuroanatomical basis is largely identified. We observed that, similar to rodents, Syt IV is inducible in songbirds. This induction was selective and depended on the nature of neuronal depolarization. Generalized seizures caused by the GABA(A) receptor antagonist, metrazole, induced the IEG, ZENK, in zebra finch brain. However, these same seizures failed to induce Syt IV in song control areas. In contrast, when nontreated birds sang, three song control areas showed striking Syt IV induction. Further, this induction appeared sensitive to the social context in which song was sung. Together, these data suggest that neural activity during singing can drive Syt IV expression within song circuitry whereas generalized seizure activity fails to do so even though song control areas are depolarized. Our findings indicate that, within this neural circuit for a procedurally learned sensorimotor behavior, Syt IV is selective and requires precisely patterned neural activity and/or neuromodulation associated with singing.

Synaptotagmin IV is necessary for the maturation of secretory granules in PC12 cells

In neuroendocrine PC12 cells, immature secretory granules (ISGs) mature through homotypic fusion and membrane remodeling. We present evidence that the ISG-localized synaptotagmin IV (Syt IV) is involved in ISG maturation. Using an in vitro homotypic fusion assay, we show that the cytoplasmic domain (CD) of Syt IV, but not of Syt I, VII, or IX, inhibits ISG homotypic fusion. Moreover, Syt IV CD binds specifically to ISGs and not to mature secretory granules (MSGs), and Syt IV binds to syntaxin 6, a SNARE protein that is involved in ISG maturation. ISG homotypic fusion was inhibited in vivo by small interfering RNA–mediated depletion of Syt IV. Furthermore, the Syt IV CD, as well as Syt IV depletion, reduces secretogranin II (SgII) processing by prohormone convertase 2 (PC2). PC2 is found mostly in the proform, suggesting that activation of PC2 is also inhibited. Granule formation, and the sorting of SgII and PC2 from the trans-Golgi network into ISGs and MSGs, however, is not affected. We conclude that Syt IV is an essential component for secretory granule maturation.

Synaptotagmin IV does not alter excitatory fast synaptic transmission or fusion pore kinetics in mammalian CNS neurons

Synaptotagmin IV (Syt IV) is a brain-specific isoform of the synaptotagmin family, the levels of which are strongly elevated after seizure activity. The dominant hypothesis of Syt IV function states that Syt IV upregulation is a neuroprotective mechanism for reducing neurotransmitter release. To test this hypothesis in mammalian CNS synapses, Syt IV was overexpressed in cultured mouse hippocampal neurons, and acute effects on fast excitatory neurotransmission were assessed. We found neurotransmission unaltered with respect to basal release probability, Ca2+ dependence of release, short-term plasticity, and fusion pore kinetics. In contrast, expression of a mutant Syt I with diminished Ca2+ affinity (R233Q) reduced release probability and altered the Ca2+ dependence of release, thus demonstrating the sensitivity of the system to changes in neurotransmission resulting from changes to the Ca2+ sensor. Together, these data refute the dominant model that Syt IV functions as an inhibitor of neurotransmitter release in mammalian neurons.

Altered hippocampal short-term plasticity and associative memory in synaptotagmin IV (-/-) mice

Synaptotagmin IV (Syt IV) is an activity-inducible, secretory vesicle protein that is thought to function as an inhibitor of neurotransmitter release (Littleton et al. Nature 400:757-760, 1999). To test this hypothesis in neurons of the mammalian CNS, we measured field excitatory postsynaptic potentials (fEPSPs) in hippocampal slice preparations from Syt IV (-/-) mice. At Schaffer collateral synapses, the basal properties of neurotransmission are largely normal. However, two forms of short-term plasticity, paired-pulse facilitation (PPF) and post-tetanic potentiation (PTP), are significantly enhanced in area CA1 of Syt IV (-/-) slices. Similarly, the early stages of long-term potentiation (LTP) are also enhanced at these synapses. Consistent with the low levels of Syt IV observed in dentate granule cells, the mossy fiber synapses in Syt IV (-/-) slices display largely normal PPF and LTP. In addition, we find that Syt IV (-/-) mice have deficits in the associative passive avoidance memory paradigm, but are normal in the novel object recognition paradigm. The synaptic architecture and connectivity of Syt IV (-/-) brains is indistinguishable from wild-type mice as indicated by immunohistochemical analysis. These results suggest Syt IV is a presynaptic negative regulator of short-term plasticity in area CA1 of the hippocampus and is required for some, but not all, forms of hippocampus-dependent memory.

RNA interference-mediated silencing of synaptotagmin IX, but not synaptotagmin I, inhibits dense-core vesicle exocytosis in PC12 cells

Although PC12 cells express three synaptotagmin isoforms (Syts I, IV and IX), all of which have been proposed to regulate dense-core vesicle exocytosis, it remains unknown which of the Sytisoforms acts as the major Ca2+ sensor for dense-core vesicle exocytosis. In the present study, it has been shown by immunoaffinity purification and immunocytochemistry that Syts I and IX, but not Syt IV, are present on the same secretory vesicles in PC12 cells. Silencing of Syt IX with specific small interfering RNA significantly reduced high KCl-dependent neuropeptide Y secretion from PC12 cells, whereas silencing of Syt I with specific small interfering RNA had no significant effect. The results indicate that Syts I and IX are not functionally equivalent and that Syt IX, and not Syt I, is indispensable for the regulation of Ca2+-dependent dense-core vesicle exocytosis in PC12 cells.

Synaptotagmins are trafficked to distinct subcellular domains including the postsynaptic compartment

The synaptotagmin family has been implicated in calcium-dependent neurotransmitter release, although Synaptotagmin 1 is the only isoform demonstrated to control synaptic vesicle fusion. Here, we report the characterization of the six remaining synaptotagmin isoforms encoded in the Drosophila genome, including homologues of mammalian Synaptotagmins 4, 7, 12, and 14. Like Synaptotagmin 1, Synaptotagmin 4 is ubiquitously present at synapses, but localizes to the postsynaptic compartment. The remaining isoforms were not found at synapses (Synaptotagmin 7), expressed at very low levels (Synaptotagmins 12 and 14), or in subsets of putative neurosecretory cells (Synaptotagmins α and β). Consistent with their distinct localizations, overexpression of Synaptotagmin 4 or 7 cannot functionally substitute for the loss of Synaptotagmin 1 in synaptic transmission. Our results indicate that synaptotagmins are differentially distributed to unique subcellular compartments. In addition, the identification of a postsynaptic synaptotagmin suggests calcium-dependent membrane-trafficking functions on both sides of the synapse.

Effect of acute and chronic treatment with methamphetamine on mRNA expression of synaptotagmin IV and 25 KDa-synaptic-associated protein in the rat brain

The effects of acute and chronic administration of methamphetamine (METH) on mRNA levels of synaptotagmin IV (SytIV) and an isoform of synaptic-associated protein of 25 KDa (SNAP25a) have been investigated in rat brain using in situ hybridization. Pretreatment with 0.5 mg/kg dopamine D1 receptor antagonist (SCH23390), but not 0.5 mg/kg N-methyl-D-aspartate (NMDA) receptor antagonist (MK-801), significantly attenuated the increased SytIV mRNA levels induced by acute METH administration in the striatum and the nucleus accumbens. Pretreatment with 0.5 mg/kg SCH23390, but not 0.5 mg/kg MK-801, significantly attenuated the increased SNAP25a mRNA levels induced by acute METH administration in the striatum and the dentate gyrus of the hippocampus. In the chronic treatment experiment, the SytIV mRNA levels of the group that received chronic treatment with METH followed by a METH challenge showed an increase similar to that seen after acute METH administration. In addition, those in the striatum, nucleus accumbens, and dentate gyrus were significantly higher than those of the group that received chronic treatment with saline followed by a METH challenge. The SNAP25a mRNA levels of the group that received chronic treatment with METH followed by a saline challenge were significantly higher than those of the group that received chronic treatment with saline followed by a saline challenge in the striatum and nucleus accumbens. The results of the present study suggest that SytIV may play an important role in the synaptic plasticity underlying METH-induced neuroadaptive changes including behavioral sensitization.

Discrete gene sets depend on POU domain transcription factor Brn3b/Brn-3.2/POU4f2 for their expression in the mouse embryonic retina

Brn3b/Brn-3.2/POU4f2 is a POU domain transcription factor that is essential for retinal ganglion cell (RGC) differentiation, axonal outgrowth and survival. Our goal was to establish a link between Brn3b and the downstream events leading to RGC differentiation. We sought to determine both the number and types of genes that depend on Brn3b for their expression. RNA probes from wild-type and Brn3b(-/-) E14.5, E16.5 and E18.5 mouse retinas were hybridized to a microarray containing 18,816 retina-expressed cDNAs. At E14.5, we identified 87 genes whose expression was significantly altered in the absence of Brn3b and verified the results by real-time PCR and in situ hybridization. These genes fell into discrete sets that encoded transcription factors, proteins associated with neuron integrity and function, and secreted signaling molecules. We found that Brn3b influenced gene expression in non RGCs of the retina by controlling the expression of secreted signaling molecules such as sonic hedgehog and myostatin/Gdf8. At later developmental stages, additional alterations in gene expression were secondary consequences of aberrant RGC differentiation caused by the absence of Brn3b. Our results demonstrate that a small but crucial fraction of the RGC transcriptome is dependent on Brn3b. The Brn3b-dependent gene sets therefore provide a unique molecular signature for the developing retina.

Nerve growth factor-dependent sorting of synaptotagmin IV protein to mature dense-core vesicles that undergo calcium-dependent exocytosis in PC12 cells

Synaptotagmin IV (Syt IV) is a fourth member of the Syt family and has been shown to regulate some forms of memory and learning by analysis of Syt IV null mutant mice (Ferguson, G. D., Anagnostaras, S. G., Silva, A. J., and Herschman, H. R. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 5598-5603). However, the involvement of Syt IV protein in vesicular trafficking and even its localization in secretory vesicles are still matters of controversy. Here we present several lines of evidence showing that the Syt IV protein in PC12 cells is normally localized in the Golgi or immature vesicles at the cell periphery and is sorted to fusion-competent mature dense-core vesicles in response to short nerve growth factor (NGF) stimulation. (i) In undifferentiated PC12 cells, Syt IV protein is mainly localized in the Golgi and small amounts are also present at the cell periphery, but according to the results of an immunocytochemical analysis, they do not colocalize with conventional secretory vesicle markers (Syt I, Syt IX, Rab3A, Rab27A, vesicle-associated membrane protein 2, and synaptophysin) at all. By contrast, limited colocalization of Syt IV protein with dense-core vesicle markers is found in the distal parts of the neurites of NGF-differentiated PC12 cells. (ii) Immunoelectron microscopy with highly specific anti-Syt IV antibody revealed that the Syt IV protein in undifferentiated PC12 cells is mainly present on the Golgi membranes and immature secretory vesicles, whereas after NGF stimulation Syt IV protein is also present on the mature dense-core vesicles. (iii) An N-terminal antibody-uptake experiment indicated that Syt IV-containing vesicles in the neurites of NGF-differentiated PC12 cells undergo Ca(2+)-dependent exocytosis, whereas no uptake of the anti-Syt IV-N antibody was observed in undifferentiated PC12 cells. Our results suggest that Syt IV is a stimulus (e.g. NGF)-dependent regulator for exocytosis of dense-core vesicles.