S100A10‐Mediated Translocation of Annexin‐A2 to SNARE Proteins in Adrenergic Chromaffin Cells Undergoing Exocytosis

In neuroendocrine cells, annexin‐A2 is implicated as a promoter of monosialotetrahexosylganglioside (GM1)‐containing lipid microdomains that are required for calcium‐regulated exocytosis. As soluble N‐ethylmaleimide‐sensitive factor attachment protein receptors (SNAREs) require a specific lipid environment to mediate granule docking and fusion, we investigated whether annexin‐A2‐induced lipid microdomains might be linked to the SNAREs present at the plasma membrane. Stimulation of adrenergic chromaffin cells induces the translocation of cytosolic annexin‐A2 to the plasma membrane, where it colocalizes with SNAP‐25 and S100A10. Cross‐linking experiments performed in stimulated chromaffin cells indicate that annexin‐A2 directly interacts with S100A10 to form a tetramer at the plasma membrane. Here, we demonstrate that S100A10 can interact with vesicle‐associated membrane protein 2 (VAMP2) and show that VAMP2 is present at the plasma membrane in resting adrenergic chromaffin cells. Tetanus toxin that cleaves VAMP2 solubilizes S100A10 from the plasma membrane and inhibits the translocation of annexin‐A2 to the plasma membrane. Immunogold labelling of plasma membrane sheets combined with spatial point pattern analysis confirmed that S100A10 is present in VAMP2 microdomains at the plasma membrane and that annexin‐A2 is observed close to S100A10 and to syntaxin in stimulated chromaffin cells. In addition, these results showed that the formation of phosphatidylinositol (4,5)‐bisphosphate (PIP₂) microdomains colocalized with S100A10 in the vicinity of docked granules, suggesting a functional interplay between annexin‐A2‐mediated lipid microdomains and SNAREs during exocytosis.     

Annexin A2–dependent actin bundling promotes secretory granule docking to the plasma membrane and exocytosis

Annexin A2, a calcium-, actin-, and lipid-binding protein involved in exocytosis, mediates the formation of lipid microdomains required for the structural and spatial organization of fusion sites at the plasma membrane. To understand how annexin A2 promotes this membrane remodeling, the involvement of cortical actin filaments in lipid domain organization was investigated. 3D electron tomography showed that cortical actin bundled by annexin A2 connected docked secretory granules to the plasma membrane and contributed to the formation of GM1-enriched lipid microdomains at the exocytotic sites in chromaffin cells. When an annexin A2 mutant with impaired actin filament–bundling activity was expressed, the formation of plasma membrane lipid microdomains and the number of exocytotic events were decreased and the fusion kinetics were slower, whereas the pharmacological activation of the intrinsic actin-bundling activity of endogenous annexin A2 had the opposite effects. Thus, annexin A2–induced actin bundling is apparently essential for generating active exocytotic sites.     

Annexin 2 "secretion" accompanying exocytosis of chromaffin cells: possible mechanisms of annexin release

Annexin 2 is a Ca(2+)-dependent phospholipid-binding protein that is involved in secretion. Despite the fact that this protein does not have signals for its secretion, many reports have shown its presence in the extracellular milieu. Here we demonstrate that, upon stimulation of exocytosis in chromaffin cells, a fraction of annexin 2 is secreted into the culture medium. This release of annexin 2 is specific, correlated with catecholamine secretion, and independent of cell death. To explain the liberation of cytosolic annexin 2 into the medium, we propose and bring evidence for a mechanism of multiporic membrane disruption during membrane fusion. Prior, in cross-linking experiments, annexin 2 forms aggregates of high molecular weight, revealing its capacity to form networks. Second, immunoelectron microscopy studies of fused chromaffin granules revealed the presence of annexin 2 and membrane proteins inside the fused vesicles, as would be predicted by the multiporic hypotheses. These data suggest that annexin 2 "secretion" in chromaffin cells is the consequence of membrane disruption during exocytosis. The role of annexin 2 in exocytosis is also discussed.     

The participation of annexin II (calpactin I) in calcium-evoked exocytosis requires protein kinase C

Permeabilized adrenal chromaffin cells secrete catecholamines by exocytosis in response to micromolar calcium concentrations. Recently, we have demonstrated that chromaffin cells permeabilized with digitonin progressively lose their capacity to secrete due to the release of certain cytosolic proteins essential for exocytosis (Sarafian T., D. Aunis, and M. F. Bader. 1987. J. Biol. Chem. 34:16671-16676). Here we show that one of the released proteins is calpactin I, a calcium-dependent phospholipid-binding protein known to promote in vitro aggregation of chromaffin granules at physiological micromolar calcium levels. The addition of calpactin I into digitonin- or streptolysin-O-permeabilized chromaffin cells with reduced secretory capacity as a result of the leakage of cytosolic proteins partially restores the calcium-dependent secretory activity. This effect is specific of calpactin I since other annexins (p32, p37, p67) do not stimulate secretion at similar or higher concentrations. Calpactin I requires the presence of Mg-ATP, suggesting that a phosphorylating step may regulate the activity of calpactin. Calpactin is unable to restore the secretory activity in cells which have completely lost their cytosolic protein kinase C or in cells having their protein kinase C inhibited by sphingosine or downregulated by long-term incubation with TPA. In contrast, calpactin I prephosphorylated in vitro by purified protein kinase C is able to reconstitute secretion in cells depleted of their protein kinase C activity. This stimulatory effect is also observed with thiophosphorylated calpactin I which is resistant to cellular phosphatases or with phosphorylated calpactin I introduced into cells in the presence of microcystin, a phosphatase inhibitor. These results suggest that calpactin I is involved in the exocytotic machinery by a mechanism which requires phosphorylation by protein kinase C.     

Phosphorylation cycling of Annexin A2 Tyr23 is critical for calcium-regulated exocytosis in neuroendocrine cells

Annexin A2 (AnxA2) is a calcium and lipid binding protein involved in neuroendocrine secretion. We have previously demonstrated that AnxA2 participates in the formation and/or stabilization of lipid microdomains required for structural and spatial organization of the exocytotic machinery in chromaffin cells. However, the regulation of AnxA2 is not fully understood. Numerous phosphorylation sites have been identified in the amino-terminal domain of AnxA2. Phosphorylation of Ser25 and Tyr23 are well established and confirmed to be functionally significant. In particular, phosphorylation of Tyr23 by the tyrosine kinase pp60Src reduces the binding of AnxA2 to both actin filaments and the plasma membrane, two major actors of exocytosis, thus, we examined whether AnxA2 was phosphorylated on Tyr23 during exocytosis. Using immunolabelling and a biochemical approach, we found that nicotine stimulation triggered the phosphorylation of Anx A2 on Tyr23. The expression of two AnxA2 mutants carrying phosphorylation deficient (Y23A) or phosphomimetic (Y23E) mutations reduced the number exocytotic sites. Furthermore, expression of AnxA2-Y23A inhibited the formation of lipid microdomains, whereas the expression of AnxA2-Y23E altered actin filaments associated with docked granules. These results suggest that phosphorylation/dephosphorylation switch at Tyr23 in AnxA2 is critical for calcium-regulated exocytosis in neuroendocrine cells. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.     

Regulation of neuroendocrine exocytosis by the ARF6 GTPase-activating protein GIT1

Neuroendocrine cells release hormones and neuropeptides by exocytosis, a highly regulated process in which secretory granules fuse with the plasma membrane to release their contents in response to a calcium trigger. Using chromaffin and PC12 cells, we have recently described that the granule-associated GTPase ARF6 plays a crucial role in exocytosis by activating phospholipase D1 at the plasma membrane and, presumably, promoting the fusion reaction between the two membrane bilayers. ARF6 is activated by the nucleotide exchange factor ARNO following docking of granules to the plasma membrane. We show here that GIT1, a GTPase-activating protein stimulating GTP hydrolysis on ARF6, is the second molecular partner that turns over the GDP/GTP cycle of ARF6 during cell stimulation. Western blot and immunofluorescence experiments indicated that GIT1 is cytosolic in resting cells but is recruited to the plasma membrane in stimulated cells, where it co-localizes with ARF6 at the granule docking sites. Over-expression of wild-type GIT1 inhibits growth hormone secretion from PC12 cells; this inhibitory effect was not observed in cells expressing a GIT1 mutant impaired in its ARF-GTPase-activating protein (GAP) activity or in cells expressing other ARF6-GAPs. Conversely reduction of GIT1 by RNA interference increased the exocytotic activity. Using a real time assay for individual chromaffin cells, we found that microinjection of GIT1 strongly reduced the number of exocytotic events. These results provide the first evidence that GIT1 plays a function in calcium-regulated exocytosis in neuroendocrine cells. We propose that GIT1 represents part of the pathway that inactivates ARF6-dependent reactions and thereby negatively regulates and/or terminates exocytotic release.     

A role for soluble NSF attachment proteins (SNAPs) in regulated exocytosis in adrenal chromaffin cells.

Digitonin-permeabilized chromaffin cells secrete catecholamines by exocytosis in response to micromolar Ca2+ concentrations, but lose the ability to secrete in response to Ca2+ as the cells lose soluble proteins through the plasma membrane pores. Such secretory run-down can be retarded by cytosolic fractions, thus providing an assay for proteins potentially involved in the exocytotic process. We have used this assay to investigate the role of N-ethylmaleimide-sensitive fusion protein (NSF) and soluble NSF attachment proteins (SNAPs) in regulated exocytosis. Recombinant alpha- and gamma-SNAP stimulated Ca(2+)-dependent exocytosis, although recombinant NSF was ineffective, despite the fact that NSF and alpha-SNAP leak from the permeabilized cells with similar time courses. However, around one third of cellular NSF was found to be present in a non-cytosolic form and so it is possible that this is sufficient for exocytosis and that exogenous SNAPs stimulate the exocytotic mechanism by acting on the leakage-insensitive NSF. The stimulatory effect of alpha-SNAP displayed a biphasic dose-response curve and was maximal at 20 micrograms/ml. The effect of alpha-SNAP was Ca(2+)- and MgATP-dependent and was inhibited by N-ethylmaleimide and botulinum A neurotoxin, indicating a bona fide action on the exocytotic mechanism. Furthermore, Ca2+ concentrations which trigger catecholamine secretion acted to prevent the leakage of NSF and alpha-SNAP from permeabilized cells. These findings provide functional evidence for a role of SNAPs in regulated exocytosis in chromaffin cells.     

Syntillas release Ca2+ at a site different from the microdomain where exocytosis occurs in mouse chromaffin cells

Spontaneous, short-lived, focal cytosolic Ca2+ transients were found for the first time and characterized in freshly dissociated chromaffin cells from mouse. Produced by release of Ca2+ from intracellular stores and mediated by type 2 and perhaps type 3 ryanodine receptors (RyRs), these transients are quantitatively similar in magnitude and duration to Ca2+ syntillas in terminals of hypothalamic neurons, suggesting that Ca2+ syntillas are found in a variety of excitable, exocytotic cells. However, unlike hypothalamic nerve terminals, chromaffin cells do not display syntilla activation by depolarization of the plasma membrane, nor do they have type 1 RyRs. It is widely thought that focal Ca2+ transients cause "spontaneous" exocytosis, although there is no direct evidence for this view. Hence, we monitored catecholamine release amperometrically while simultaneously imaging Ca2+ syntillas, the first such simultaneous measurements. Syntillas failed to produce exocytotic events; and, conversely, spontaneous exocytotic events were not preceded by syntillas. Therefore, we suggest that a spontaneous syntilla, at least in chromaffin cells, releases Ca2+ into a cytosolic microdomain distinct from the microdomains containing docked, primed vesicles. Ryanodine (100 microM) reduced the frequency of Ca2+ syntillas by an order of magnitude but did not alter the frequency of spontaneous amperometric events, suggesting that syntillas are not involved in steps preparatory to spontaneous exocytosis. Surprisingly, ryanodine also increased the total charge of individual amperometric events by 27%, indicating that intracellular Ca2+ stores can regulate quantal size.     

Annexin II in exocytosis: catecholamine secretion requires the translocation of p36 to the subplasmalemmal region in chromaffin cells

Annexin II is a Ca(2+)-dependent membrane-binding protein present in a wide variety of cells and tissues. Within cells, annexin II is found either as a 36-kD monomer (p36) or as a heterotetrameric complex (p90) coupled with the S-100-related protein, p11. Annexin II has been suggested to be involved in exocytosis as it can restore the secretory responsiveness of permeabilized chromaffin cells. By quantitative confocal immunofluorescence, immunoreplica analysis and immunoprecipitation, we show here the translocation of p36 from the cytosol to a subplasmalemmal Triton X-100 insoluble fraction in chromaffin cells following nicotinic stimulation. A synthetic peptide corresponding to the NH2-terminal domain of p36 which contains the phosphorylation sites was microinjected into individual chromaffin cells and catecholamine secretion was monitored by amperometry. This peptide blocked completely the nicotine-induced recruitment of p36 to the cell periphery and strongly inhibited exocytosis evoked by either nicotine or high K+. The light chain of annexin II, p11, was selectively expressed by adrenergic chromaffin cells, and was only present in the subplasmalemmal Triton X-100 insoluble protein fraction of both resting and stimulated cells. p11 can modify the Ca(2+)- and/or the phospholipid-binding properties of p36. We found that loss Ca2+ was required to stimulate the translocation of p36 and to trigger exocytosis in adrenergic chromaffin cells. Our findings suggest that the translocation of p36 to the subplasmalemmal region is an essential event in regulated exocytosis and support the idea that the presence of p11 in adrenergic cells may confer a higher Ca2+ affinity to the exocytotic pathway in these cells.     

Intersectin-1L nucleotide exchange factor regulates secretory granule exocytosis by activating Cdc42

Rho GTPases are key regulators of the actin cytoskeleton in membrane trafficking events. We previously reported that Cdc42 facilitates exocytosis in neuroendocrine cells by stimulating actin assembly at docking sites for secretory granules. These findings raise the question of the mechanism activating Cdc42 in exocytosis. The neuronal guanine nucleotide exchange factor, intersectin-1L, which specifically activates Cdc42 and is at an interface between membrane trafficking and actin dynamics, appears as an ideal candidate to fulfill this function. Using PC12 and chromaffin cells, we now show the presence of intersectin-1 at exocytotic sites. Moreover, through an RNA interference strategy coupled with expression of various constructs encoding the guanine nucleotide exchange domain, we demonstrate that intersectin-1L is an essential component of the exocytotic machinery. Silencing of intersectin-1 prevents secretagogue-induced activation of Cdc42 revealing intersectin-1L as the factor integrating Cdc42 activation to the exocytotic pathway. Our results extend the current role of intersectin-1L in endocytosis to a function in exocytosis and support the idea that intersectin-1L is an adaptor that coordinates exo-endocytotic membrane trafficking in secretory cells.     

Dynamin-2 Regulates Fusion Pore Expansion and Quantal Release through a Mechanism that Involves Actin Dynamics in Neuroendocrine Chromaffin Cells

Over the past years, dynamin has been implicated in tuning the amount and nature of transmitter released during exocytosis. However, the mechanism involved remains poorly understood. Here, using bovine adrenal chromaffin cells, we investigated whether this mechanism rely on dynamin’s ability to remodel actin cytoskeleton. According to this idea, inhibition of dynamin GTPase activity suppressed the calcium-dependent de novo cortical actin and altered the cortical actin network. Similarly, expression of a small interfering RNA directed against dynamin-2, an isoform highly expressed in chromaffin cells, changed the cortical actin network pattern. Disruption of dynamin-2 function, as well as the pharmacological inhibition of actin polymerization with cytochalasine-D, slowed down fusion pore expansion and increased the quantal size of individual exocytotic events. The effects of cytochalasine-D and dynamin-2 disruption were not additive indicating that dynamin-2 and F-actin regulate the late steps of exocytosis by a common mechanism. Together our data support a model in which dynamin-2 directs actin polymerization at the exocytosis site where both, in concert, adjust the hormone quantal release to efficiently respond to physiological demands.     

The Rho Guanine Nucleotide Exchange Factors Intersectin 1L and β-Pix Control Calcium-Regulated Exocytosis in Neuroendocrine PC12 Cells

GTPases of the Rho family are molecular switches that play an important role in a wide range of membrane-trafficking processes including neurotransmission and hormone release. We have previously demonstrated that RhoA and Cdc42 regulate calcium-dependent exocytosis in chromaffin cells by controlling actin dynamics, whereas Rac1 regulates lipid organisation. These findings raised the question of the upstream mechanism activating these GTPases during exocytosis. The guanine nucleotide exchange factors (GEFs) that catalyse the exchange of GDP for GTP are crucial elements regulating Rho signalling. Using an RNA interference approach, we have recently demonstrated that the GEFs Intersectin-1L and β-Pix, play essential roles in neuroendocrine exocytosis by controlling the activity of Cdc42 and Rac1, respectively. This review summarizes these results and discusses the functional importance of Rho GEFs in the exocytotic machinery in neuroendocrine cells.     

Evidence that the ability to respond to a calcium stimulus in exocytosis is determined by the secretory granule membrane: Comparison of exocytosis of injected bovine chromaffin granule membranes and endogenous cortical granules inXenopus laevis oocytes

Summary 1. To understand better the mechanisms which govern the sensitivity of secretory vesicles to a calcium stimulus, we compared the abilities of injected chromaffin granule membranes and of endogenous cortical granules to undergo exocytosis inXenopus laevis oocytes and eggs in response to cytosolic Ca²⁺. Exocytosis of chromaffin granule membranes was detected by the appearance of dopamine--hydroxylase of the chromaffin granule membrane in the oocyte or egg plasma membrane. Cortical granule exocytosis was detected by release of cortical granule lectin, a soluble constituent of cortical granules, from individual cells.2. Injected chromaffin granule membranes undergo exocytosis equally well in frog oocytes and eggs in response to a rise in cytosolic Ca²⁺ induced by incubation with ionomycin.3. Elevated Ca²⁺ triggered cortical granule exocytosis in eggs but not in oocytes.4. Injected chromaffin granule membranes do not contribute factors to the oocyte that allow calcium-dependent exocytosis of the endogenous cortical granules.5. Protein kinase C activation by phorbol esters stimulates cortical granule exocytosis in bothXenopus laevis oocytes andX. laevis eggs (Bement, W. M., and Capco, D. G.,J. Cell Biol. 108, 885–892, 1989). Activation of protein kinase C by phorbol ester also stimulated chromaffin granule membrane exocytosis in oocytes, indicating that although cortical granules and chromaffin granule membranes differ in calcium responsiveness, PKC activation is an effective secretory stimulus for both.6. These results suggest that structural or biochemical characteristics of the chromaffin granule membrane result in its ability to respond to a Ca²⁺ stimulus. In the oocytes, cortical granule components necessary for Ca²⁺-dependent exocytosis may be missing, nonfunctional, or unable to couple to the Ca²⁺ stimulus and downstream events.     

Exocytosis of single chromaffin granules in cell-free inside-out membrane patches

In chromaffin cells, exocytosis of single granules and properties of the fusion pore--the first connection between vesicular lumen and extracellular space --can be studied by cell-attached patch amperometry, which couples patch-clamp capacitance measurements with simultaneous amperometric recordings of transmitter release. Here we have studied exocytosis of single chromaffin granules and endocytosis of single vesicles in cell-free inside-out membrane patches by patch capacitance measurements and patch amperometry. We excised patches from chromaffin cells by using methods developed for studying properties of single ion channels. With low calcium concentrations in the pipette and bath, the patches showed no spontaneous exocytosis, but exocytosis could be induced in some patches by applying calcium to the cytoplasmic side of the patch. Exocytosis was also stimulated by calcium entry through the patch membrane. Initial conductances of the fusion pore were undistinguishable in cell-attached and excised patch recordings, but the subsequent pore expansion was slower in excised patches. The properties of exocytotic fusion pores in chromaffin cells are very similar to those observed in mast cells and granulocytes. Excised patches provide a tool with which to study the mechanisms of fusion pore formation and endocytosis in vitro.     

Phospholipase D1 production of phosphatidic acid at the plasma membrane promotes exocytosis of large dense-core granules at a late stage

Substantial efforts have recently been made to demonstrate the importance of lipids and lipid-modifying enzymes in various membrane trafficking processes, including calcium-regulated exocytosis of hormones and neurotransmitters. Among bioactive lipids, phosphatidic acid (PA) is an attractive candidate to promote membrane fusion through its ability to change membrane topology. To date, however, the biosynthetic pathway, the dynamic location, and actual function of PA in secretory cells remain unknown. Using a short interference RNA strategy on chromaffin and PC12 cells, we demonstrate here that phospholipase D1 is activated in secretagogue-stimulated cells and that it produces PA at the plasma membrane at the secretory granule docking sites. We show that phospholipase D1 activation and PA production represent key events in the exocytotic progression. Membrane capacitance measurements indicate that reduction of endogenous PA impairs the formation of fusion-competent granules. Finally, we show that the PLD1 short interference RNA-mediated inhibition of exocytosis can be rescued by exogenous provision of a lipid that favors the transition of opposed bi-layer membranes to hemifused membranes having the outer leaflets fused. Our findings demonstrate that PA synthesis is required during exocytosis to facilitate a late event in the granule fusion pathway. We propose that the underlying mechanism is related to the ability of PA to alter membrane curvature and promote hemi-fusion.     

Protein kinase C and guanosine triphosphate combine to potentiate calcium-dependent membrane fusion driven by annexin 7

Exocytotic secretion is promoted by the concerted action of calcium, guanine nucleotide, and protein kinase C. We now show that the calcium-dependent membrane fusion activity of annexin 7 in vitro is further potentiated by the combined addition of guanine nucleotide and protein kinase C. The observed increment involves the simultaneous activation of annexin 7 by these two effectors. Guanosine triphosphate (GTP) and its non-hydrolyzable analogues optimally enhance the phosphorylation of annexin 7 by protein kinase C in vitro. Reciprocally, phosphorylation by protein kinase C significantly potentiates the binding and hydrolysis of GTP by annexin 7. Only protein kinase C-dependent phosphorylation has a significant positive effect on annexin 7 GTPase, although other protein kinases, including cAMP-dependent protein kinase, cGMP-dependent protein kinase, and pp60(c-)(src), have been shown to label the protein with high efficiency. In vivo, the ratio of bound GDP/GTP and phosphorylation of annexin 7 change in direct proportion to the extent of catecholamine release from chromaffin cells in response to stimulation by carbachol, or to inhibition by various protein kinase C inhibitors. These results thus lead us to hypothesize that annexin 7 may serve as a common site of action for calcium, guanine nucleotide, and protein kinase C in the exocytotic membrane fusion process in chromaffin cells.     

Light-induced exocytosis in cell development and differentiation

Calcium-dependent exocytosis of fluorescently labeled single secretory vesicles in PC12 cells and primary embryonic telencephalon cells can be triggered by illumination with visible light and imaged by TIRF or epifluorescence microscopy. Opsin 3 was identified by quantitative PCR expression analysis as the putative light receptor molecule for light-induced exocytosis. In primary chicken telencephalon cells, light-induced exocytosis is restricted to a specific period during embryonic development, and involves fusion of rather large vesicles. Strictly calcium-dependent exocytosis starts after a delay of a few seconds of illumination and lasts for up to 2 min. We analyzed the frequency, time course and spatial distribution of exocytotic events. Exocytosis in PC12 cells and telencephalon cells occurs at the periphery or the interface between dividing cells, and the duration of single secretion events varies considerably. Our observation strongly supports the idea that light induced exocytosis is most likely a mechanism for building plasma membrane during differentiation, development and proliferation rather than for calcium-dependent neurotransmitter release.     

Requirement for metalloendoprotease in exocytosis: evidence in mast cells and adrenal chromaffin cells

Exocytosis is initiated by the receptor-mediated influx of calcium that results in fusion of the secretory vesicle with the plasma membrane. We examined the possibility that calcium-dependent exocytosis in mast cells and adrenal chromaffin cells requires metalloendoprotease activity. Metalloendoprotease inhibitors and dipeptide substrates block exocytosis in these cells with the same specificity and dose dependency as that with which they interact with metalloendoproteases. Metalloendoprotease activity is identified in these cells with fluorogenic synthetic substrates, which also blocked exocytosis. Metalloendoprotease activity is highest in the plasma membrane of chromaffin cells. The metalloendoprotease appears to be required in exocytosis at a step dependent on or after calcium entry, since exocytosis initiated by direct calcium introduction in both mast cells and chromaffin cells is blocked by metalloendoprotease inhibitors.     

Vesicular Ca(2+) -induced secretion promoted by intracellular pH-gradient disruption

The actions of the protonophore CCCP on intracellular Ca2+ regulation and exocytosis in chromaffin cells have been examined. Simultaneous fura-2 imaging and amperometry reveal that exposure to CCCP not only perturbs mitochondrial function but that it also alters vesicular storage of Ca2+ and catecholamines. By disrupting the pH gradient of the secretory vesicle membrane, the protonophore allows both Ca(2+) and catecholamine to leak into the cytosol. Unlike the high cytosolic Ca2+ concentrations resulting from mitochondrial membrane disruption, Ca2+ leakage from secretory vesicles may initiate exocytotic release. In conjunction with previous studies, this work reveals that catalytic and self-sustained vesicular Ca(2+) -induced exocytosis occurs with extended exposure to weak acid or base protonophores.     

Annexin II tetramer: structure and function

The annexins are a family of proteins that bind acidic phospholipids in the presence of Ca²⁺. The interaction of these proteins with biological membranes has led to the suggestion that these proteins may play a role in membrane trafficking events such as exocytosis, endocytosis and cell-cell adhesion. One member of the annexin family, annexin II, has been shown to exist as a monomer, heterodimer or heterotetramer. The ability of annexin II tetramer to bridge secretory granules to plasma membrane has suggested that this protein may play a role in Ca²⁺-dependent exocytosis. Annexin II tetramer has also been demonstrated on the extracellular face of some metastatic cells where it mediates the binding of certain metastatic cells to normal cells. Annexin II tetramer is a major cellular substrate of protein kinase C and pp60^src. Phosphorylation of annexin II tetramer is a negative modulator of protein function.Supported by a grant from the Medical Research Council of Canada