Role of the Doc2 alpha-Munc13-1 interaction in the neurotransmitter release process

Vesicle-associated proteins and transmitter release from sympathetic ganglionic boutons

A method is reported for introducing peptides derived from SNARE proteins that control exocytosis of vesicles at boutons formed by sympathetic ganglion cells in tissue culture. These peptides were coupled to the DNA binding domain of the Drosophila transcription factor antennapedia, called penetratin. This facilitated the passage of peptides across the bouton membrane. FM1-43 was used to monitor the exocytosis of transmitter from depolarized boutons after their exposure to the penetratin-peptide sequences IETRHNEIIKLETSIRELHD of syntaxin and KGFLSSLFGGSSK of alpha-SNAP, both of which blocked secretion, whereas the peptide sequences SELDDRA-DALQAGASQFETSAAKLKRK of synaptobrevin did not. This report introduces a readily applicable method for determining the effect of different peptide sequences of vesicle-associated proteins on secretion at vertebrate boutons and presents an account of the effects of a selection of such peptides on exocytosis.

Synaptotagmin-like protein 1-3: a novel family of C-terminal-type tandem C2 proteins

Synaptotagmins (Syt), rabphilin-3A, and Doc2 belong to a family of carboxyl terminal type (C-type) tandem C2 proteins and are thought to be involved in vesicular trafficking. We have cloned and characterized a novel family of C-type tandem C2 proteins, designated Slp1-3 (synaptotagmin-like protein 1-3). The Slp1-3 C2 domains show high homology to granuphilin-a C2 domains, but the amino-terminal domain of Slp1-3 does not contain any known protein motifs or a transmembrane domain. A subcellular fractionation study indicated that Slp1-3 proteins are peripheral membrane proteins. Phospholipid binding experiments indicated that Slp3 is a Ca(2+)-dependent isoform, but Slp1 and Slp2 are Ca(2+)-independent isoforms, because only the Slp3 C2A domain showed Ca(2+)-dependent phospholipid binding activity. The C-terminus of Slp1-3 also bound neurexin Ialpha in vitro, in the same manner as Syt family proteins, which may be important for the membrane association of Slp1-3. In addition, Slp family proteins are differentially distributed in different mouse tissues and at different developmental stages.

Characterization of KIAA1427 protein as an atypical synaptotagmin (Syt XIII)

Synaptotagmin (Syt) belongs to a family of type-I membrane proteins and is a protein that consists of a short extracellular N-terminus, a single transmembrane domain, two C2 domains and a short C-terminus. Here, we cloned and characterized a mouse orthologue of human KIAA1427 protein as an atypical Syt (named Syt XIII). Subcellular fractionation and antibody-uptake experiments indicate that Syt XIII is indeed a type-I membrane protein, but, unlike other Syt isoforms, lacks an N-terminal extracellular domain. Syt XIII C2 domains show relatively little similarity to Syt I (less than 35% identity at the amino acid level), and lack key amino acids responsible for Ca2+ binding. Because of these substitutions, the Syt XIII C2 domains did not show Ca2+-dependent phospholipid-binding activity, and Syt XIII is thus classified as a Ca2+ -independent isoform. By contrast, the Syt XIII C-terminal domain is highly homologous with other Syt isoforms and can function as a common receptor for neurexin Ialpha in vitro. Since Syt XIII is expressed in various tissues outside the brain, Syt XIII may be involved in constitutive vesicle transport.

Is double C2 protein (DOC2) expressed in bovine adrenal medulla? A commercial anti-DOC2 monoclonal antibody recognizes a major bovine mitochondrial antigen

We have examined the expression in bovine adrenal medulla of double C2 protein (DOC2), a vesicular protein which associates with intracellular phospholipid and Ca(2+) and is implicated in the modulation of regulated exocytosis. Extensive reverse transcription-PCR, Northern blot analyses and in vitro translation reactions have been combined with immunological studies to provide data to suggest that neither DOC2alpha nor DOC2beta is expressed at detectable levels in bovine adrenal chromaffin cells, and that a widely used monoclonal antibody directed against DOC2 also recognizes mitochondrial complex III core protein 2.

Doc2gamma, a third isoform of double C2 protein, lacking calcium-dependent phospholipid binding activity

The Doc2 (double C2) family consists of two isoforms (Doc2alpha and Doc2beta) characterized by an N-terminal Munc13-1 interacting domain (Mid) and two C2 domains that interact with Ca(2+) and phospholipid at the C-terminus. This Ca(2+)-binding property is thought to be important to the regulation of neurotransmitter release. In this paper, we report a third isoform of mouse Doc2, named Doc2gamma. Doc2gamma also contains a putative Mid domain and two C2 domains, and it is 45.6 and 43.2% identical to mouse Doc2alpha and Doc2beta, respectively, at the amino acid level. In contrast to the other Doc2 isoforms, the C2 domains of Doc2gamma impair Ca(2+)-dependent phospholipid binding activity. The highest expression of Doc2gamma mRNA was found in the heart, but occurs ubiquitously, the same as Doc2beta. These findings indicate that Doc2gamma may also function as an effector for Munc13-1 and that it may be involved in the regulation of vesicular trafficking.

Munc13-1 acts as a priming factor for large dense-core vesicles in bovine chromaffin cells

In chromaffin cells the number of large dense-core vesicles (LDCVs) which can be released by brief, intense stimuli represents only a small fraction of the 'morphologically docked' vesicles at the plasma membrane. Recently, it was shown that Munc13-1 is essential for a post-docking step of synaptic vesicle fusion. To investigate the role of Munc13-1 in LDCV exocytosis, we overexpressed Munc13-1 in chromaffin cells and stimulated secretion by flash photolysis of caged calcium. Both components of the exocytotic burst, which represent the fusion of release-competent vesicles, were increased by a factor of three. The sustained component, which represents vesicle maturation and subsequent fusion, was increased by the same factor. The response to a second flash, however, was greatly reduced, indicating a depletion of release-competent vesicles. Since there was no apparent change in the number of docked vesicles, we conclude that Munc13-1 acts as a priming factor by accelerating the rate constant of vesicle transfer from a pool of docked, but unprimed vesicles to a pool of release-competent, primed vesicles.

Regulation of transmitter release by Unc-13 and its homologues

Neurotransmitters are released by Ca(2+)-triggered exocytotic fusion of synaptic vesicles. Before fusion, vesicles dock at a specialised presynaptic plasma membrane region, the active zone, where they are primed to a fusion competent state. The nature of this priming reaction has long been enigmatic. Recent evidence demonstrates that priming is an essential and rate-limiting step in secretion from neurons and neuroendocrine cells. Members of the Unc-13 protein family, which are highly conserved during evolution and act as novel targets of the diacylglycerol second-messenger pathway, have been identified to play an essential role in this process.

Double C2 protein. A review

Concerted effort has led to the identification of dozens of synaptic proteins and has thereby opened the door for the characterisation of the molecular mechanisms underlying regulated exocytosis. Calcium is known to play a number of roles in regulated exocytosis, acting as the trigger for fast synaptic transmission and also acting at some of the steps preceding vesicle fusion. Investigators have therefore focussed considerable attention on possible calcium sensors. What many of the candidate proteins have in common is a C2 domain, one of the four conserved domains originally described in protein kinase C. Such domains have been shown to bind calcium and phospholipid in a large number of intracellular proteins. Synaptotagmin, a C2-domain protein, is a very strong candidate for the protein involved in triggering fast calcium-dependent vesicle fusion. Recent attention has also concerned the other calcium sensors, which may play roles in the 'priming' or transport of vesicles. This review concerns one of these tentative calcium-binding proteins, double C2 or DOC2. DOC2 was originally isolated from nervous tissue but subsequently has been found to be more widely expressed. DOC2 is a vesicular protein that may be involved in the early stages of preparing vesicles for exocytosis.

Protein-protein interactions in neurotransmitter release

The arrival of a nerve impulse at a nerve terminal leads to the opening of voltage-gated Ca(2+) channels and a rapid influx of Ca(2+). The increase in Ca(2+) concentration at the active zone from the basal level of 100-200 mM triggers the fusion of docked synaptic vesicles, resulting in neurotransmitter release. A large number of proteins have been identified at nerve terminals and a cascade of protein-protein interactions has been suggested to be involved in the cycling of synaptic vesicle states. Functional studies in last half decade on synaptic-terminal proteins, including Ca(2+) channels, have revealed that the SNARE core complex, consisting of synaptobrevin VAMP, a synaptic vesicle-associated protein, syntaxin and SNAP-25, synaptic membrane-associated proteins, acts as the membrane fusion machinery and that proteins interacting with the SNARE complex play essential roles in synaptic vesicle exocytosis by regulating assembly and disassembly of the SNARE complex.

Doc2alpha is an activity-dependent modulator of excitatory synaptic transmission

Doc2alpha is a synaptic vesicle-associated Ca2 + -binding protein. To study the role of Doc2alpha in synaptic transmission and modulation, we generated homozygous null Doc2alpha mutant mice. In the CA1 region of hippocampal slices in the mutant mice, excitatory synaptic responses evoked with prolonged 5 Hz stimulation showed a significantly larger frequency facilitation followed by a steeper depression than those in wild-type mice, whereas there was no difference in synaptic transmission at lower frequencies or in paired-pulse facilitation. These results suggest that Doc2alpha regulates synaptic transmission when high Ca2 + concentrations in the presynaptic terminal are sustained. Furthermore, the mutant mice showed impairment in long-term potentiation and passive avoidance task. Thus, Doc2alpha may regulate transmitter release during repetitive synaptic activation, thereby contributing to memory formation.

Drosophila UNC-13 is essential for synaptic transmission

The UNC-13 protein family has been suggested to be critical for synaptic vesicle dynamics based on its interactions with Syntaxin, Munc-18 and Doc 2alpha. We cloned the Drosophila homolog (Dunc-13) and characterized its function using a combination of electrophysiology and ultrastructural analyses. Dunc-13 contained a C1 lipid-binding motif and two C2 calcium-binding domains, and its expression was restricted to neurons. Elimination of dunc-13 expression abolished synaptic transmission, an effect comparable only to removal of the core complex proteins Syntaxin and Synaptobrevin. Transmitter release remained impaired under elevated calcium influx or application of hyperosmotic saline. Ultrastructurally, mutant terminals accumulated docked vesicles at presynaptic release sites. We conclude that Dunc-13 is essential for a stage of neurotransmission following vesicle docking and before fusion.

Transient, phorbol ester-induced DOC2-Munc13 interactions in vivo

Munc13-1 and DOC2 have been implicated in the regulation of exocytosis. Here we demonstrate in vivo that these two proteins undergo a transient phorbol ester-mediated and protein kinase C-independent interaction, resulting in the translocation of DOC2 from a vesicular localization to the plasma membrane. The translocation of DOC2 is dependent upon the DOC2 Munc interacting domain that binds specifically to Munc13-1, whereas the association of DOC2 with intracellular membranes is dependent on its C2 domains. This is the first direct in vivo demonstration of a protein-protein interaction between two presynaptic proteins and may represent a molecular basis for phorbol ester-dependent enhancement of exocytosis.

Presynaptic mechanism for phorbol ester-induced synaptic potentiation

Phorbol ester facilitates transmitter release at a variety of synapses, and the phorbol ester-induced synaptic potentiation (PESP) is a model for presynaptic facilitation. To address the mechanism underlying PESP, we have made paired whole-cell recordings from the giant presynaptic terminal, the calyx of Held, and its postsynaptic target in the medial nucleus of the trapezoid body in rat brainstem slices. Phorbol ester potentiated EPSCs without affecting either presynaptic calcium currents or potassium currents. Protein kinase C inhibitors applied from outside or injected directly into the presynaptic terminal attenuated the PESP. Furthermore, presynaptic loading of a synthetic peptide with the sequence of the N-terminal domain of Doc2alpha interacting with Munc13-1 (Mid peptide) significantly attenuated PESP, whereas mutated Mid peptide had no effect. We conclude that the target of the presynaptic facilitatory effect of phorbol ester resides downstream of calcium influx and may involve both protein kinase C and Doc2alpha - Munc13-1 interaction.

Presynaptic mechanism for phorbol ester-induced synaptic potentiation

Phorbol ester facilitates transmitter release at a variety of synapses, and the phorbol ester-induced synaptic potentiation (PESP) is a model for presynaptic facilitation. To address the mechanism underlying PESP, we have made paired whole-cell recordings from the giant presynaptic terminal, the calyx of Held, and its postsynaptic target in the medial nucleus of the trapezoid body in rat brainstem slices. Phorbol ester potentiated EPSCs without affecting either presynaptic calcium currents or potassium currents. Protein kinase C inhibitors applied from outside or injected directly into the presynaptic terminal attenuated the PESP. Furthermore, presynaptic loading of a synthetic peptide with the sequence of the N-terminal domain of Doc2alpha interacting with Munc13-1 (Mid peptide) significantly attenuated PESP, whereas mutated Mid peptide had no effect. We conclude that the target of the presynaptic facilitatory effect of phorbol ester resides downstream of calcium influx and may involve both protein kinase C and Doc2alpha - Munc13-1 interaction.

Direct interaction between the ARF-specific guanine nucleotide exchange factor msec7-1 and presynaptic Munc13-1

Msec7-1, a mammalian homologue of yeast sec7p, is a specific GDP/GTP exchange factor for small G-proteins of the ARF family. Overexpression of msec7-1 in Xenopus neuromuscular junctions leads to an increase in synaptic transmitter release that is most likely caused by an increase in the pool of readily releasable vesicles. However, the molecular mechanisms by which msec7-1 is targeted to presynaptic compartments and enhances neurotransmitter release are not known. In the present study, we demonstrate that msec7-1 interacts directly with Munc13-1, a phorbol ester-dependent enhancer of neurotransmitter release that is specifically localized to presynaptic transmitter release zones. Given that Munc13-1 and msec7-1 participate in very similar presynaptic processes and because Munc13-1 is specifically targeted to presynaptic active zones, we suggest that the msec7-1/Munc13-1 interaction serves to colocalize the two proteins at the active zone, a subcellular compartment with extremely high membrane turnover.

Interaction of Doc2 with tctex-1, a light chain of cytoplasmic dynein. Implication in dynein-dependent vesicle transport

Doc2 has one Munc13-interacting domain at the N-terminal region and two C2-like domains interacting with Ca2+ and phospholipid at the C-terminal region. Doc2 consists of two isoforms, Doc2alpha and -beta. Doc2alpha is specifically expressed in neuronal cells and implicated in Ca2+-dependent neurotransmitter release, whereas Doc2beta is ubiquitously expressed and its function is unknown. We show here that both Doc2alpha and -beta interact with rat tctex-1, a light chain of cytoplasmic dynein, in both cell-free and intact cell systems. Overexpression of the N-terminal fragment of Doc2 containing the tctex-1-interacting domain induces changes in the intracellular localization of cation-independent mannose 6-phosphate receptor and its ligand, cathepsin D, which are transported from trans-Golgi network to late endosomes. Overexpression of the C-terminal fragment containing two C2-like domains shows the similar effect, but to a lesser extent, whereas overexpression of full-length Doc2 or the C-terminal fragment of rabphilin3 containing two C2-like domains does not show this effect. Because dynein is a minus-end-directed microtubule-based motor protein, these results suggest that Doc2, especially Doc2beta, plays a role in dynein-dependent intracellular vesicle transport.