Synaptotagmin I is essential for Ca(2+)-independent release of neurotransmitter induced by alpha-latrotoxin

Disorders of synaptic vesicle fusion machinery

The revolution in genetic technology has ushered in a new age for our understanding of the underlying causes of neurodevelopmental, neuromuscular and neurodegenerative disorders, revealing that the presynaptic machinery governing synaptic vesicle fusion is compromised in many of these neurological disorders. This builds upon decades of research showing that disturbance to neurotransmitter release via toxins can cause acute neurological dysfunction. In this review, we focus on disorders of synaptic vesicle fusion caused either by toxic insult to the presynapse or alterations to genes encoding the key proteins that control and regulate fusion: the SNARE proteins (synaptobrevin, syntaxin-1 and SNAP-25), Munc18, Munc13, synaptotagmin, complexin, CSPα, α-synuclein, PRRT2 and tomosyn. We discuss the roles of these proteins and the cellular and molecular mechanisms underpinning neurological deficits in these disorders.

V-1, a catecholamine biosynthesis regulatory protein, positively controls catecholamine secretion in PC12D cells

Stably transfected PC12D cell lines overexpressing a catecholamine biosynthesis regulatory protein, V-1, were used to examine the functional role of V-1 in catecholamine secretion. High K(+)-induced dopamine secretion in V-1 overexpressing clones was shown to be markedly potentiated compared with control clones carried with a vector alone. As assayed intracellular calcium concentration ([Ca(2+)](i)) using fura-PE3, V-1 overexpression was observed to enhance high K(+)-elicited [Ca(2+)](i) elevation. Electron microscopic analysis revealed an increase in dense-cored vesicle formation by V-1 overexpression. These results suggest that the enhancement of high K(+)-induced dopamine secretion by V-1 overexpression results from the potentiation of high K(+)-induced [Ca(2+)](i) elevation and the increase in the number of dense-cored vesicles.

Modulation of spontaneous quantal release of neurotransmitters in the hippocampus

Presynaptic action potentials trigger the exocytosis of neurotransmitters. However, even in the absence of depolarisation-dependent Ca2+ entry nearby release sites, spontaneous vesicular release still occurs. Even though this happens at low rate, such spontaneous release may play a trophic role in maintaining the shape of dendritic structures. Like evoked responses, action potential-independent release is subject to modulation. This review describes some of the regulatory factors that rapidly and presynaptically regulate the ongoing Ca2+-independent release of neurotransmitters in the hippocampus. For instance, the electrical activity of the nerve ending, neurotransmitters, hypertonic solutions, neurotoxins, polycations, neurotrophic factors, immunoglobulins, cyclothiazide and psychotropic drugs can all modify the rate of spontaneous release. This can be achieved through various mechanisms that can be Ca2+-dependent or Ca2+-independent, protein kinase-dependent or independent. Since action potential-independent release contributes to the maintenance of dendritic structures, neuromodulators are likely to influence the density and/or length of dendritic spines, which in turn may modulate information processing in the central nervous system (CNS).

Establishment of variant PC12 subclones deficient in stimulation-secretion coupling

Clonal rat pheochromocytoma (PC12) cells have been widely used to study the molecular mechanism of exocytosis. We have isolated variant PC12 subclones with deficiencies in stimulation-secretion coupling, by a single cell recloning, and investigated the defects. PC12-1G2 hardly released dopamine following high-K(+)-induced depolarization, but normal release was evoked by the Ca(2+)-ionophore, ionomycin. Fura-2 fluorometry indicated that a nicardipine-sensitive component of Ca(2+) influx was missing, suggesting that PC12-1G2 has defects in L-type Ca(2+) channel function. PC12-2B3 was not responsive to high-K(+)-induced depolarization and ionomycin, and voltage-dependent Ca(2+) entry was identical to that of the normal clone. Electron microscopy revealed that the number of vesicles adjacent or directly attached to the plasma membrane was decreased in PC12-2B3. The expression of presynaptic proteins was analyzed by immunoblotting using a panel of antibodies. Syntaxin 1, VAMP-2, SNAP-25, Munc18, Rab3C and Sec-6 were decreased compared to the control clone and that of synaptophysin was extremely low. PC12-D60 synthesized and released dopamine normally, but had almost lost its catecholamine-uptake activity. These results show that multiple PC12 cells variants are spontaneously generated, and that recloning can select PC12 subclones useful for the study of the molecular mechanisms of neurotransmitter release.

Role of the conserved WHXL motif in the C terminus of synaptotagmin in synaptic vesicle docking

Synaptotagmin (Syt) I, an abundant synaptic vesicle protein, consists of one transmembrane region, two C2 domains, and a short C terminus. This protein is essential for both synaptic vesicle exocytosis and endocytosis via its C2 domains. Although the short C terminus is highly conserved among the Syt family and across species, little is known about the exact role of the conserved C terminus of Syt I. In this paper, we report a function of the Syt I C terminus in synaptic vesicle docking at the active zones. Presynaptic injection of a peptide corresponding to the C-terminal 21 amino acids of Syt I (named Syt-C) into the squid giant synapse blocked synaptic transmission without affecting the presynaptic action potential or the presynaptic Ca(2+) currents. The same procedure repeated with a mutant C-terminal peptide (Syt-CM) had no effect on synaptic transmission. Repetitive presynaptic stimulation with Syt-C produced a rapid decrease in the amplitude of the postsynaptic potentials as the synaptic block progressed, indicating that the peptide interferes with the docking step rather than the fusion step of synaptic vesicles. Electron microscopy of the synapses injected with the Syt-C peptide showed a marked decrease in the number of docked synaptic vesicles at the active zones, as compared with controls. These results indicate that Syt I is a multifunctional protein that is involved in at least three steps of synaptic vesicle cycle: docking, fusion, and reuptake of synaptic vesicles.

Alpha-latrotoxin stimulates inward current, rise in cytosolic calcium concentration, and exocytosis in at pituitary gonadotropes

Alpha-latrotoxin (LTX) from the black widow spider venom, stimulates neurotransmitter release from neuronal cells via Ca2+ -dependent as well as Ca2+ -independent mechanisms. In some peptide-secreting endocrine cells, however, LTX stimulates hormone release mainly via a Ca2+ -independent mechanism. Here we investigated the action of LTX in rat pituitary gonadotropes that secrete the peptide, LH. Using the patch-clamp technique in conjunction with the fluorescent Ca2+ indicator (indo-1) to simultaneously measure the cytosolic Ca2+ concentration ([Ca2+]i) and ionic current, we showed that LTX elicited bursts of inward current that were accompanied by [Ca2+]i elevations. In the presence of a physiological concentration of extracellular Ca2+, the unitary conductance of the LTX-induced current was about 300 pS, and only about 6.4% of the current was carried by Ca2+. The LTX-induced current was occasionally followed by intracellular Ca2+ release. At [Ca2+]i of 1 microM or more, exocytosis (detected by membrane capacitance measurement) was consistently triggered, and it was frequently followed by endocytosis. Thus, LTX triggers Ca2+ -dependent exocytosis in gonadotropes via extracellular Ca2+ entry as well as intracellular Ca2+ release. In approximately 25% of the cells, LTX could also trigger a slow exocytosis in the absence of [Ca2+]i elevation. Therefore, LTX has both Ca2+ -dependent and Ca2+ -independent actions in gonadotropes.

Ca2+-independent insulin exocytosis induced by alpha-latrotoxin requires latrophilin, a G protein-coupled receptor

alpha-Latrotoxin (alpha-LTX) induces exocytosis of small synaptic vesicles (SSVs) in neuronal cells both by a calcium-independent mechanism and by opening cation-permeable pores. Since the basic molecular events regulating exocytosis in neurons and endocrine cells may be similar, we have used the exocytosis of insulin-containing large dense core vesicles (LDCVs) as a model system. In primary pancreatic beta-cells and in the derived cell lines INS-1 and MIN6, alpha-LTX increased insulin release in the absence of extracellular calcium, but the insulin-secreting cell lines HIT-T15 and RINm5F were unresponsive. alpha-LTX did not alter membrane potential or cytosolic calcium, and its stimulatory effect on exocytosis was still observed in pre-permeabilized INS-1 cells kept at 0.1 microM Ca2+. Consequently, pore formation or ion fluxes induced by alpha-LTX could be excluded. The Ca2+-independent alpha-LTX-binding protein, latrophilin, is a novel member of the secretin family of G protein-coupled receptors (GPCR). Sensitivity to alpha-LTX correlated with expression of latrophilin, but not with synaptotagmin I or neurexin Ialpha expression. Moreover, transient expression of latrophilin in HIT-T15 cells conferred alpha-LTX-induced exocytosis. Our results indicate that direct stimulation of exocytosis by a GPCR mediates the Ca2+-independent effects of alpha-LTX in the absence of altered ion fluxes. Therefore, direct regulation by receptor-activated heterotrimeric G proteins constitutes an important feature of the endocrine exocytosis of insulin-containing LDCVs and may also apply to SSV exocytosis in neurons.

alpha-Latrotoxin stimulates exocytosis by the interaction with a neuronal G-protein-coupled receptor

alpha-Latrotoxin is a potent stimulator of neurosecretion. Its action requires extracellular binding to high affinity presynaptic receptors. Neurexin I alpha was previously described as a high affinity alpha-latrotoxin receptor that binds the toxin only in the presence of calcium ions. Therefore, the interaction of alpha-latrotoxin with neurexin I alpha cannot explain how alpha-latrotoxin stimulates neurotransmitter release in the absence of calcium. We describe molecular cloning and functional expression of the calcium-independent receptor of alpha-latrotoxin (CIRL), which is a second high affinity alpha-latrotoxin receptor that may be the major mediator of alpha-latrotoxin's effects. CIRL appears to be a novel orphan G-protein-coupled receptor, a member of the secretin receptor family. In contrast with other known serpentine receptors, CIRL has two subunits of the 120 and 85 kDa that are the result of endogenous proteolytic cleavage of a precursor polypeptide. CIRL is found in brain where it is enriched in the striatum and cortex. Expression of CIRL in chromaffin cells increases the sensitivity of the cells to the effects of alpha-latrotoxin, demonstrating that this protein is functional in coupling to secretion. Syntaxin, a component of the fusion complex, copurifies with CIRL on an alpha-latrotoxin affinity column and forms stable complexes with this receptor in vitro. Interaction of CIRL with a specific presynaptic neurotoxin and with a component of the docking-fusion machinery suggests its role in regulation of neurosecretion.

Acetylcholine release from rat brain cortical slices evoked by the fraction P4 of the venom of the spider Phoneutria nigriventer has Ca2+ and temperature independent components

Fractionation of the venom of the spider Phoneutria nigriventer revealed that it was a mixture of several neurotoxic peptides. The peptides so far characterized either inhibited or induced neurotransmitter release. These effects were mediated by Ca2+ channels or increasing Na+ permeability through voltage sensitive Na(+)-channels, respectively. The pooled toxic components (fraction P4) showed stimulatory effects on acetylcholine release from brain cortical slices. In addition, a component of the observed effects resembling that of alpha-latrotoxin was identified, which was characterized by the ability to provoke release of acetylcholine (ACh) at low temperature and in a manner independent of extracellular Ca2+ and of voltage sensitive Na(+)-channels.

Single-cell measurements of quantal secretion induced by alpha-latrotoxin from rat adrenal chromaffin cells: dependence on extracellular Ca2+

alpha-Latrotoxin (alpha-LT), from black widow spider venom, is a potent enhancer of the spontaneous quantal release of neurotransmitter from a variety of nerve terminals and clonal neurosecretory cells. Using electrochemical amperometry and estimation of membrane impedance by phase detection, we present evidence that alpha-LT induces exocytosis of catecholamines from rat adrenal chromaffin cells beginning as rapidly as 30 s after close application of the toxin. This release is largely dependent on adequate levels of extracellular Ca2+ ([Ca2+]o). Lowering [Ca2+]o from 2 mM to </= 10-20 microM reduces the alpha-LT-induced rise in membrane capacitance by at least sixfold, on average, and nearly abolishes alpha-LT-induced quantal amperometric events, while still permitting insertion of non-selective cation channels. Based on these experiments, we argue that the rapid onset of alpha-LT action in promoting massive quantal release from chromaffin cells is primarily due to an increase in the Ca2+ permeability of the plasma membrane through non-selective cation channels.

Probing the structure-function relationship of alpha-latrotoxin-formed channels with antibodies and pronase

The major toxic component of black widow spider (Latrodectus mactans tredecimguttatus) venom, alpha-latrotoxin, is known to form ionic channels in different membranes. In order to probe the extramembrane domains of alpha-latrotoxin molecule, alpha-latrotoxin channels in planar lipid membrane were treated with antibodies to latrotoxin or with pronase added to different sides of the membrane. It was found that antibody addition to the same side as the toxin (cis) decreased channel conductance only at positive potentials across the membrane. In contrast, trans side addition of antibodies changed the channel conductance at both positive and negative potentials: at positive potential conductance first slightly increased then decreased by more then 50%; at negative potential it decreased much more quickly, to only about 20% of the initial value. No dependence on membrane potential was found for pronase treatment of incorporated channels. For both cis and trans application of pronase, channel selectivity for Ca2+, Mg2+, Ba2+ and K+, Na+, Li+ ions did not change significantly but Cd2+ block was decreased. Trans pronase treatment also resulted in some rectification of I/V curves and an increase in channel conductance. We interpret these findings as evidence that alpha-latrotoxin channel has protruding parts on both sides of the membrane and that its conformation in the membrane depends on membrane potential.

Phosphorylation of 25-kDa synaptosome-associated protein. Possible involvement in protein kinase C-mediated regulation of neurotransmitter release

Protein kinase C-mediated phosphorylation of a 25-kDa synaptosome-associated protein (SNAP-25) was examined in living PC12 cells. Phorbol 12-myristate 13-acetate treatment enhanced high potassium-induced [3H]-norepinephrine release, and a 28-kDa protein recognized by an anti-SNAP-25 antibody was phosphorylated on Ser residues. The molecular size of the phosphorylated band decreased slightly following treatment with Clostridium botulinum type A neurotoxin, whereas the band disappeared after treatment with botulinum type E neurotoxin, indicating that the 28-kDa protein was SNAP-25. A phosphorylation is likely to occur at Ser187, as this is the only Ser residue located between the cleavage sites of botulinum type A and E neurotoxins. SNAP-25 of PC12 cells was phosphorylated by purified protein kinase C in vitro, and the amount of syntaxin co-immunoprecipitated with SNAP-25 was decreased by phosphorylation. These results suggest that the phosphorylation of SNAP-25 may be involved in protein kinase C-mediated regulation of catecholamine release from PC12 cells.

Synaptotagmin I- and II-deficient PC12 cells exhibit calcium-independent, depolarization-induced neurotransmitter release from synaptic-like microvesicles

Synaptotagmin I- and II-deficient PC12 cells (Shoji-Kasai et al. [1]) were used to compare the role of this protein in the calcium-dependent exocytosis of secretory granules and synaptic-like microvesicles (SLMVs). While neither catecholamine nor protein secretion from secretory granules were altered, the depolarization-induced acetylcholine release from SLMVs was no longer calcium-dependent. We propose that within the exocytotic process of SLMVs, there exist two depolarization-induced steps. One is calcium-dependent and no longer present in synaptotagmin I- and II-deficient cells. The other is induced by depolarization, does not require calcium, and suffices to trigger neurotransmitter release from SLMVs in synaptotagmin I- and II-deficient PC12 cells.