CAPS drives trans-SNARE complex formation and membrane fusion through syntaxin interactions

Systematizing and cloning of genes involved in the cerebellar cortex circuit development

The cerebellar cortical circuit of mammals develops via a series of magnificent cellular events in the postnatal stage of development to accomplish the formation of functional circuit architectures. The contribution of genetic factors is thought to be crucial to cerebellar development. Therefore, it is essential to analyze the underlying transcriptome during development to understand the genetic blueprint of the cerebellar cortical circuit. In this review, we introduce the profiling of large numbers of spatiotemporal gene expression data obtained by developmental time-series microarray analyses and in situ hybridization cellular mRNA mapping, and the creation of a neuroinformatics database called the Cerebellar Development Transcriptome Database. Using this database, we have identified thousands of genes that are classified into various functional categories and are expressed coincidently with related cellular developmental stages. We have also suggested the molecular mechanisms of cerebellar development by functional characterization of several identified genes (Cupidin, p130Cas, very-KIND, CAPS2) responsible for distinct cellular events of developing cerebellar granule cells. Taken together, the gene expression profiling during the cerebellar development demonstrates that the development of cerebellar cortical circuit is attributed to the complex but orchestrated transcriptome.

Calcium-dependent activator protein for secretion 2 (CAPS2) promotes BDNF secretion and is critical for the development of GABAergic interneuron network

Calcium-dependent activator protein for secretion 2 (CAPS2) is a dense-core vesicle-associated protein that is involved in the secretion of BDNF. BDNF has a pivotal role in neuronal survival and development, including the development of inhibitory neurons and their circuits. However, how CAPS2 affects BDNF secretion and its biological significance in inhibitory neurons are largely unknown. Here we reveal the role of CAPS2 in the regulated secretion of BDNF and show the effect of CAPS2 on the development of hippocampal GABAergic systems. We show that CAPS2 is colocalized with BDNF, both synaptically and extrasynaptically in axons of hippocampal neurons. Overexpression of exogenous CAPS2 in hippocampal neurons of CAPS2-KO mice enhanced depolarization-induced BDNF exocytosis events in terms of kinetics, frequency, and amplitude. We also show that in the CAPS2-KO hippocampus, BDNF secretion is reduced, and GABAergic systems are impaired, including a decreased number of GABAergic neurons and their synapses, a decreased number of synaptic vesicles in inhibitory synapses, and a reduced frequency and amplitude of miniature inhibitory postsynaptic currents. Conversely, excitatory neurons in the CAPS2-KO hippocampus were largely unaffected with respect to field excitatory postsynaptic potentials, miniature excitatory postsynaptic currents, and synapse number and morphology. Moreover, CAPS2-KO mice exhibited several GABA system-associated deficits, including reduced late-phase long-term potentiation at CA3-CA1 synapses, decreased hippocampal theta oscillation frequency, and increased anxiety-like behavior. Collectively, these results suggest that CAPS2 promotes activity-dependent BDNF secretion during the postnatal period that is critical for the development of hippocampal GABAergic networks.

Single secretory granules of live cells recruit syntaxin-1 and synaptosomal associated protein 25 (SNAP-25) in large copy numbers

Before secretory vesicles undergo exocytosis, they must recruit the proteins syntaxin-1 and synaptosomal associated protein 25 (SNAP-25) in the plasma membrane. GFP-labeled versions of both proteins cluster at sites where secretory granules have docked. Single-particle tracking shows that minority populations of both molecules are strongly hindered in their mobility, consistent with their confinement in nanodomains. We measured the fluorescence of granule-associated clusters, the fluorescence of single molecules, and the numbers of unlabeled syntaxin-1 and SNAP-25 molecules per cell. There was a more than 10-fold excess of SNAP-25 over syntaxin-1. Fifty to seventy copies each of syntaxin-1 and SNAP-25 molecules were associated with a single docked granule, many more than have been reported to be required for fusion.

Syntaxin clusters assemble reversibly at sites of secretory granules in live cells

Syntaxin resides in the plasma membrane, where it helps to catalyze membrane fusion during exocytosis. The protein also forms clusters in cell-free and granule-free plasma-membrane sheets. We imaged the interaction between syntaxin and single secretory granules by two-color total internal reflection microscopy in PC12 cells. Syntaxin-GFP assembled in clusters at sites where single granules had docked at the plasma membrane. Clusters were intermittently present at granule sites, as syntaxin molecules assembled and disassembled in a coordinated fashion. Recruitment to granules required the N-terminal domain of syntaxin, but not the entry of syntaxin into SNARE complexes. Clusters facilitated exocytosis and disassembled once exocytosis was complete. Syntaxin cluster formation defines an intermediate step in exocytosis.

Lipid dynamics in exocytosis

Regulated exocytosis of neurotransmitter- and hormone-containing vesicles underpins neuronal and hormonal communication and relies on a well-orchestrated series of molecular interactions. This in part involves the upstream formation of a complex of SNAREs and associated proteins leading to the eventual fusion of the vesicle membrane with the plasma membrane, a process that enables content release. Although the role of lipids in exocytosis is intuitive, it has long been overlooked at least compared to the extensive work on SNAREs. Here, we will present the latest advances in this rapidly developing field revealing that lipids actually play an active role in exocytosis by focusing on cholesterol, 3'-phosphorylated phosphoinositides and phosphatidic acid.

The t-SNARE complex: a close up

The SNARE proteins, syntaxin, SNAP-25, and synaptobrevin have long been known to provide the driving force for vesicle fusion in the process of regulated exocytosis. Of particular interest is the initial interaction between SNAP-25 and syntaxin to form the t-SNARE heterodimer, an acceptor for subsequent synaptobrevin engagement. In vitro studies have revealed at least two different dynamic conformations of t-SNARE heterodimer defined by the degree of association of the C-terminal SNARE motif of SNAP-25 with syntaxin. At the plasma membrane, these proteins are organized into dense clusters of 50-60 nm in diameter. More recently, the t-SNARE interaction within these clusters was investigated in live cells at the molecular level, estimating each cluster to contain 35-70 t-SNARE molecules. This work reported the presence of both partially and fully zippered t-SNARE complex at the plasma membrane in agreement with the earlier in vitro findings. It also revealed a spatial segregation into distinct clusters containing predominantly one conformation apparently patterned by the surrounding lipid environment. The reason for this dynamic t-SNARE complex in exocytosis is uncertain; however, it does take us one step closer to understand the complex sequence of events leading to vesicle fusion, emphasizing the role of both membrane proteins and lipids.

Two distinct secretory vesicle-priming steps in adrenal chromaffin cells

The calcium-dependent activator proteins for secretion, CAPS1 and CAPS2, facilitate syntaxin opening during synaptic vesicle priming.

Priming of large dense-core vesicles (LDCVs) is a Ca²⁺-dependent step by which LDCVs enter a release-ready pool, involving the formation of the soluble N-ethyl-maleimide sensitive fusion protein attachment protein (SNAP) receptor complex consisting of syntaxin, SNAP-25, and synaptobrevin. Using mice lacking both isoforms of the calcium-dependent activator protein for secretion (CAPS), we show that LDCV priming in adrenal chromaffin cells entails two distinct steps. CAPS is required for priming of the readily releasable LDCV pool and sustained secretion in the continued presence of high Ca²⁺ concentrations. Either CAPS1 or CAPS2 can rescue secretion in cells lacking both CAPS isoforms. Furthermore, the deficit in the readily releasable LDCV pool resulting from CAPS deletion is reversed by a constitutively open form of syntaxin but not by Munc13-1, a priming protein that facilitates the conversion of syntaxin to the open conformation. Our data indicate that CAPS functions downstream of Munc13s but also interacts functionally with Munc13s in the LDCV-priming process.

Novel interactions of CAPS (Ca2+-dependent activator protein for secretion) with the three neuronal SNARE proteins required for vesicle fusion

CAPS (aka CADPS) is required for optimal vesicle exocytosis in neurons and endocrine cells where it functions to prime the exocytic machinery for Ca(2+)-triggered fusion. Fusion is mediated by trans complexes of the SNARE proteins VAMP-2, syntaxin-1, and SNAP-25 that bridge vesicle and plasma membrane. CAPS promotes SNARE complex formation on liposomes, but the SNARE binding properties of CAPS are unknown. The current work revealed that CAPS exhibits high affinity binding to syntaxin-1 and SNAP-25 and moderate affinity binding to VAMP-2. CAPS binding is specific for a subset of exocytic SNARE protein isoforms and requires membrane integration of the SNARE proteins. SNARE protein binding by CAPS is novel and mediated by interactions with the SNARE motifs in the three proteins. The C-terminal site for CAPS binding on syntaxin-1 does not overlap the Munc18-1 binding site and both proteins can co-reside on membrane-integrated syntaxin-1. As expected for a C-terminal binding site on syntaxin-1, CAPS stimulates SNARE-dependent liposome fusion with N-terminal truncated syntaxin-1 but exhibits impaired activity with C-terminal syntaxin-1 mutants. Overall the results suggest that SNARE complex formation promoted by CAPS may be mediated by direct interactions of CAPS with each of the three SNARE proteins required for vesicle exocytosis.

SNARE bundle and syntaxin N-peptide constitute a minimal complement for Munc18-1 activation of membrane fusion

Sec1/Munc18 (SM) proteins activate intracellular membrane fusion through binding to cognate SNAP receptor (SNARE) complexes. The synaptic target membrane SNARE syntaxin 1 contains a highly conserved H(abc) domain, which connects an N-peptide motif to the SNARE core domain and is thought to participate in the binding of Munc18-1 (the neuronal SM protein) to the SNARE complex. Unexpectedly, we found that mutation or complete removal of the H(abc) domain had no effect on Munc18-1 stimulation of fusion. The central cavity region of Munc18-1 is required to stimulate fusion but not through its binding to the syntaxin H(abc) domain. SNAP-25, another synaptic SNARE subunit, contains a flexible linker and exhibits an atypical conjoined Q(bc) configuration. We found that neither the linker nor the Q(bc) configuration is necessary for Munc18-1 promotion of fusion. As a result, Munc18-1 activates a SNARE complex with the typical configuration, in which each of the SNARE core domains is individually rooted in the membrane bilayer. Thus, the SNARE four-helix bundle and syntaxin N-peptide constitute a minimal complement for Munc18-1 activation of fusion.

UNC-31/CAPS docks and primes dense core vesicles in C. elegans neurons

UNC-31 or its mammalian homologue, Ca(2+)-dependent activator protein for secretion (CAPS), is indispensable for exocytosis of dense core vesicle (DCV) and synaptic vesicle (SV). From N- to the C-terminus, UNC-31 contains putative functional domains, including dynactin 1 binding domain (DBD), C2, PH, (M)UNC-13 homology domain (MHD) and DCV binding domain (DCVBD), the last four we examined in this study. We employed UNC-31 null mutant C. elegans worms to examine whether UNC-31 functions could be rescued by ectopic expression of full length UNC-31 vs each of these four domain-deleted mutants. Full length UNC-31 cDNA rescued the phenotypes of C. elegans null mutants in response to Ca(2+)-elevation in ALA neurons. Surprisingly, MHD deletion also rescued UNC-31 exocytotic function in part because the relatively high Ca(2+) level (pre-flash Ca(2+) was 450 nM) used in the capacitance study could bypass the MHD defect. Nonetheless, the three other domain-truncation cDNAs had almost no rescue on Ca(2+) evoked secretion. Importantly, this genetic null mutant rescue strategy enabled physiological studies at levels of whole organism to single cells, such as locomotion assay, pharmacological study of neurotransmission at neuromuscular junction, in vivo neuropeptide release measurement and analysis of vesicular docking. Our results suggest that each of these UNC-31 domains support distinct sequential molecular actions of UNC-31 in vesicular exocytosis, including steps in vesicle tethering and docking that bridge vesicle with plasma membrane, and subsequently priming vesicle by initiating the formation of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) core complex.

HOPS initiates vacuole docking by tethering membranes before trans-SNARE complex assembly

Vacuole homotypic fusion has been reconstituted with all purified components: vacuolar lipids, four soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins, Sec17p, Sec18p, the Rab Ypt7p, and the hexameric homotypic fusion and vacuole protein sorting complex (HOPS). HOPS is a Rab-effector with direct affinity for SNAREs (presumably via its Sec1-Munc18 homologous subunit Vps33p) and for certain vacuolar lipids. Each of these pure vacuolar proteins was required for optimal proteoliposome clustering, raising the question of which was most directly involved. We now present model subreactions of clustering and fusion that reveal that HOPS is the direct agent of tethering. The Rab and vacuole lipids contribute to tethering by supporting the membrane association of HOPS. HOPS indirectly facilitates trans-SNARE complex formation by tethering membranes, because the synthetic liposome tethering factor polyethylene glycol can also stimulate trans-SNARE complex formation and fusion. SNAREs further stabilize the associations of HOPS-tethered membranes. HOPS then protects newly formed trans-SNARE complexes from disassembly by Sec17p/Sec18p.

Synaptobrevin N-terminally bound to syntaxin-SNAP-25 defines the primed vesicle state in regulated exocytosis

Time-resolved measurements of exocytosis identify a domain of the SNARE complex required to keep vesicles readily releasable.

Rapid neurotransmitter release depends on the ability to arrest the SNAP receptor (SNARE)–dependent exocytosis pathway at an intermediate “cocked” state, from which fusion can be triggered by Ca²⁺. It is not clear whether this state includes assembly of synaptobrevin (the vesicle membrane SNARE) to the syntaxin–SNAP-25 (target membrane SNAREs) acceptor complex or whether the reaction is arrested upstream of that step. In this study, by a combination of in vitro biophysical measurements and time-resolved exocytosis measurements in adrenal chromaffin cells, we find that mutations of the N-terminal interaction layers of the SNARE bundle inhibit assembly in vitro and vesicle priming in vivo without detectable changes in triggering speed or fusion pore properties. In contrast, mutations in the last C-terminal layer decrease triggering speed and fusion pore duration. Between the two domains, we identify a region exquisitely sensitive to mutation, possibly constituting a switch. Our data are consistent with a model in which the N terminus of the SNARE complex assembles during vesicle priming, followed by Ca²⁺-triggered C-terminal assembly and membrane fusion.