Distinct roles of C2A and C2B domains of synaptotagmin in the regulation of exocytosis in adrenal chromaffin cells

Dissociation of inositol polyphosphates from the C2B domain of synaptotagmin facilitates spontaneous release of catecholamines in adrenal chromaffin cells. A suggestive evidence of a fusion clamp by synaptotagmin

Synaptotagmins (Syts) serve as a Ca²+ sensor in the release of neurotransmitters and hormones. Inositol polyphosphates (InsPPs) such as Inositol 1,3,4,5,6-pentakisphosphate (InsP₅) and inositol hexakisphosphate (InsP₆) bind to Ca²+-binding C2B domain of Syt I and II, and inhibit transmitter release. We have shown that the inhibition by InsPPs is reversed by Ca²+ in adrenal chromaffin cells, while a rapid accumulation of endogenous InsP₅ and InsP₆ upon depolarizing stimuli have been reported in these and some other cells. Such a rapid accumulation of InsPPs, if not all, might reflect their dissociation from C2B domain of Syt. To elucidate the functional relevance, we studied the effects of antibodies against C2A and C2B domains (anti-C2A Ab, anti-C2B Ab) on the accumulation of InsPPs induced by Ca²+ in digitonin-permeabilized adrenal chromaffin cells. Anti-C2B Ab by itself caused an accumulation of InsPPs in the permeabilizing medium, and increased spontaneous release of catecholamines (CA). Anti-C2A Ab abolished Ca²+-induced increase of InsPPs in cytosolic component, and inhibited Ca²+-evoked release of CA with little effect on the spontaneous release. Microinjection of InsP₆ but not inositol hexakissulfate into intact chromaffin cells inhibited both spontaneous and nicotine-evoked exocytotic events. These results suggest that endogenous InsPPs bound to the C2B domain clamp spontaneous fusion of the docked or primed vesicles at resting level of intracellular Ca²+, and binding of Ca²+ to the C2A or/and C2B domain facilitate fusion dissociating InsPPs from Syt in adrenal chromaffin cells. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Calcium Signaling and Exocytosis in Adrenal Chromaffin Cells

At a given cytosolic domain of a chromaffin cell, the rate and amplitude of the Ca2+concentration ([Ca2+]c) depends on at least four efficient regulatory systems: 1) plasmalemmal calcium channels, 2) endoplasmic reticulum, 3) mitochondria, and 4) chromaffin vesicles. Different mammalian species express different levels of the L, N, P/Q, and R subtypes of high-voltage-activated calcium channels; in bovine and humans, P/Q channels predominate, whereas in felines and murine species, L-type channels predominate. The calcium channels in chromaffin cells are regulated by G proteins coupled to purinergic and opiate receptors, as well as by voltage and the local changes of [Ca2+]c. Chromaffin cells have been particularly useful in studying calcium channel current autoregulation by materials coreleased with catecholamines, such as ATP and opiates. Depending on the preparation (cultured cells, adrenal slices) and the stimulation pattern (action potentials, depolarizing pulses, high K+, acetylcholine), the role of each calcium channel in controlling catecholamine release can change drastically. Targeted aequorin and confocal microscopy shows that Ca2+entry through calcium channels can refill the endoplasmic reticulum (ER) to nearly millimolar concentrations, and causes the release of Ca2+(CICR). Depending on its degree of filling, the ER may act as a sink or source of Ca2+that modulates catecholamine release. Targeted aequorins with different Ca2+affinities show that mitochondria undergo surprisingly rapid millimolar Ca2+transients, upon stimulation of chromaffin cells with ACh, high K+, or caffeine. Physiological stimuli generate [Ca2+]cmicrodomains in which the local subplasmalemmal [Ca2+]crises abruptly from 0.1 to ∼50 μM, triggering CICR, mitochondrial Ca2+uptake, and exocytosis at nearby secretory active sites. The fact that protonophores abolish mitochondrial Ca2+uptake, and increase catecholamine release three- to fivefold, support the earlier observation. This increase is probably due to acceleration of vesicle transport from a reserve pool to a ready-release vesicle pool; this transport might be controlled by Ca2+redistribution to the cytoskeleton, through CICR, and/or mitochondrial Ca2+release. We propose that chromaffin cells have developed functional triads that are formed by calcium channels, the ER, and the mitochondria and locally control the [Ca2+]cthat regulate the early and late steps of exocytosis.

External Ca2+ dependency of synaptic transmission in drosophila synaptotagmin I mutants

To resolve some of differences in reports on the function of Synaptotagmin I (Syt I), we re-examined synaptic transmission at the neuromuscular junction of Drosophila embryos that have mutations in the Syt I gene (syt I). Two major questions addressed were which Ca2+ binding domain, C2A or C2B, sense Ca2+ and is Syt I a negative regulator of spontaneous vesicle fusion. Synaptic currents were induced by nerve stimulation or by high K+ treatment in external solutions containing various Ca2+ concentrations. In a null allele, syt I(AD4), synchronous synaptic currents were rarely observed but not abolished. The quantal content was about 1/60 of control but increased linearly with [Ca2+](e) with a slope of 0.95 (N) in the double logarithmic plot, in contrast to 3.01 in control. The slope of 1.06 in an allele, syt I(AD1), which lacks the second Ca2+ binding domain, C2B, was not different from in syt I(AD4). In another allele, syt I(AD3), in which one amino acid in C2B is mutated, synchronous synaptic transmission was also impaired and N was 1.54, which is significantly smaller than in control. In high K+ saline, the [Ca2+](e) dependency of vesicle release in syt I(AD4) was lower than in controls, whereas that in syt I(AD3) was even lower than in syt I(AD4), suggesting that syt I(AD3) is inhibiting vesicle fusion. These findings led us to conclude that C2B, not C2A, senses Ca2+, and Syt I is a negative regulator of vesicle fusion.

Adenovirus-mediated silencing of Synaptotagmin 9 inhibits Ca2+-dependent insulin secretion in islets

Synaptotagmins (Syts) are involved in Ca(2+)-dependent insulin release. However, which Syt isoform is functional in primary beta-cells remains unknown. We demonstrate by electron microscopy of pancreatic islets, the association of Syt 9 with insulin granules. Silencing of Syt 9 by RNA interference adenovirus in islet cells had no effect on the expression of Syt 5, Syt 7 and Syt 3 isoforms. The latter was localized at the plasma membrane of pancreatic polypeptide cells. Insulin release in response to glucose or tolbutamide was strongly inhibited in Syt 9 deficient islets, whereas exocytosis potentiated by raising cAMP levels, was unaltered. Thus, Syt 9 may act as Ca(2+) sensor for beta-cell secretion.

Evidence for the association of synaptotagmin with glutathione S-transferases: implications for a novel function in human breast cancer

OBJECTIVE

To analyze the pattern of changes in GSTs in cancerous and adjacent non-cancerous tissues obtained from breast cancer patients undergoing surgery.

DESIGN AND METHODS

Cytosolic GST purification, assay of GST, protein expression levels, and GST-synaptotagmin association were analyzed using standard biochemical techniques like GSH-affinity purification, spectrophotometry, SDS-PAGE, Western blots, and matrix-assisted laser desorption and ionization-time of flight (MALDI-TOF).

RESULTS

GST activity in cancerous tissues (0.26 U/mg protein) was significantly higher (P < 0.05) as compared to those from adjacent non-cancerous tissues (0.14 U/mg protein) of breast cancer patients. Further analysis of GST subunits on SDS-PAGE and Western blots using class-specific GST antibodies revealed significant elevation in GST-pi levels in cancer tissues with no appreciable changes in GST-alpha and GST-mu. Along with the elevation of GST-pi levels, high molecular weight proteins (approximately 70 kDa) cross reacting with GST antibodies were detected only in surgically resected tumor biopsies but not in the non-cancerous tissues adjacent to the tumor. Based on MALDI-TOF analysis, the high molecular weight band was identified as synaptotagmin V bound to GST-M1 with 47% sequence coverage after processing on an MS-FIT search engine.

CONCLUSIONS

Our results suggest a novel putative functional role for the GST-synaptotagmin complex in human breast cancers. As this association of GST M1-synaptotagmin was not seen in adjacent non-cancerous tissues, this can be used as a marker for breast cancers.

Synaptotagmin I synchronizes transmitter release in mouse hippocampal neurons

We have asked whether loss of the Ca2+ sensor protein synaptotagmin I influences the total amount of neurotransmitter released after a presynaptic action potential. Hippocampal neurons from synaptotagmin I knock-out mice had a greatly reduced fast synchronous component of glutamate release, as reported previously. However, the amount of glutamate released during the slow asynchronous component increased in these knock-out neurons. As a result of these changes in the kinetics of release, there was no significant difference between wild-type and knock-out neurons in the total amount of transmitter released within 400 msec after a presynaptic stimulus. Fluorescence imaging experiments demonstrated that wild-type and knock-out neurons take up and release similar amounts of FM dye after depolarization, indicating normal amounts of synaptic vesicle trafficking in the knock-out neurons. These results indicate that synaptotagmin I knock-out neurons are fully capable of releasing neurotransmitter, with the increased slow component of release serving to compensate for loss of the fast component. Thus, synaptotagmin I synchronizes the rapid release of neurotransmitters after Ca2+ entry into presynaptic terminals and also appears to suppress the slower, asynchronous form of transmitter release.

Synaptotagmin VII Is Targeted to Dense-core Vesicles and Regulates Their Ca2+-dependent Exocytosis in PC12 Cells

It has recently been proposed that synaptotagmin (Syt) VII functions as a plasma membrane Ca2+ sensor for dense-core vesicle exocytosis in PC12 cells based on the results of transient overexpression studies using green fluorescent protein (GFP)-tagged Syt VII; however, the precise subcellular localization of Syt VII is still a matter of controversy (plasma membrane versus secretory granules). In this study we established a PC12 cell line "stably expressing" the Syt VII-GFP molecule and demonstrated by immunocytochemical and immunoelectron microscopic analyses that the Syt VII-GFP protein is localized on dense-core vesicles as well as in other intracellular membranous structures, such as the trans-Golgi network and lysosomes. Syt VII-GFP forms a complex with endogenous Syts I and IX, but not with Syt IV, and it colocalize well with Syts I and IX in the cellular processes (where dense-core vesicles are accumulated) in the PC12 cell line. We further demonstrated by an N-terminal antibody-uptake experiment that Syt VII-GFP-containing dense-core vesicles undergo Ca2+ -dependent exocytosis, the same as endogenous Syt IX-containing vesicles. Moreover, silencing of Syt VII-GFP with specific small interfering RNA dramatically reduced high KCl-dependent neuropeptide Y secretion from the stable PC12 cell line (approximately 60% of the control cells), whereas the same small interfering RNA had little effect on neuropeptide Y secretion from the wild-type PC12 cells (approximately 85-90% of the control cells), indicating that the level of endogenous expression of Syt VII molecules must be low. Our results indicate that the targeting of Syt VII-GFP molecules to specific membrane compartment(s) is affected by the transfection method (transient expression versus stable expression) and suggested that Syt VII molecule on dense-core vesicles functions as a vesicular Ca2+ sensor for exocytosis in endocrine cells.

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.

Regulation of Casein Kinase-2 (CK2) Activity by Inositol Phosphates

Casein kinase 2 (CK2) was one of the first protein kinases to be discovered and has been suggested to be responsible for as much as one-fifth of the eukaryotic phosphoproteome. Despite being responsible for the phosphorylation of a vast array of proteins central to numerous dynamic cellular processes, the activity of CK2 appears to be unregulated. In the current study, we identified a protein kinase activity in rat liver supernatant that is up-regulated by inositol 1,3,4,5-tetrakisphosphate (IP4) and inositol hexakisphosphate (IP6). The substrate for the inositol phosphate-regulated protein kinase was identified as a phosphatidylcholine transfer protein-like protein. Using the phosphorylation of this substrate in an assay, we purified the inositol phosphate-regulated protein kinase and determined it to be CK2. Bacterially expressed recombinant CK2, however, showed very high basal activity and was only modestly activated by IP6 and not regulated by IP. We found that an endogenous component present in rat liver supernatant was able to inhibit both recombinant and liver-purified CK2 basal activity. Under these conditions, recombinant CK2 catalytic activity could be increased substantially by IP4, inositol 1,3,4,5,6-pentakisphosphate (IP5), and IP6. We concluded that, contrary to the previously held view, CK2 can exist in a state of low constitutive activity allowing for its regulation by inositol phosphates. The ability of the higher inositol phosphates to directly stimulate CK2 catalytic activity provides the first evidence that these signaling molecules can operate via a direct control of protein phosphorylation.

Regulated exocytosis of an H+/myo-inositol symporter at synapses and growth cones

Phosphoinositides, synthesized from myo-inositol, play a critical role in the development of growth cones and in synaptic activity. As neurons cannot synthesize inositol, they take it up from the extracellular milieu. Here, we demonstrate that, in brain and PC12 cells, the recently identified H(+)/myo-inositol symporter HMIT is present in intracellular vesicles that are distinct from synaptic and dense-core vesicles. We further show that HMIT can be triggered to appear on the cell surface following cell depolarization, activation of protein kinase C or increased intracellular calcium concentrations. HMIT cell surface expression takes place preferentially in regions of nerve growth and at varicosities and leads to increased myo-inositol uptake. The symporter is then endocytosed in a dynamin-dependent manner and becomes available for a subsequent cycle of stimulated exocytosis. HMIT is thus expressed in a vesicular compartment involved in activity-dependent regulation of myo-inositol uptake in neurons. This may be essential for sustained signaling and vesicular traffic activities in growth cones and at synapses.

Secretory granule exocytosis

Regulated exocytosis of secretory granules or dense-core granules has been examined in many well-characterized cell types including neurons, neuroendocrine, endocrine, exocrine, and hemopoietic cells and also in other less well-studied cell types. Secretory granule exocytosis occurs through mechanisms with many aspects in common with synaptic vesicle exocytosis and most likely uses the same basic protein components. Despite the widespread expression and conservation of a core exocytotic machinery, many variations occur in the control of secretory granule exocytosis that are related to the specialized physiological role of particular cell types. In this review we describe the wide range of cell types in which regulated secretory granule exocytosis occurs and assess the evidence for the expression of the conserved fusion machinery in these cells. The signals that trigger and regulate exocytosis are reviewed. Aspects of the control of exocytosis that are specific for secretory granules compared with synaptic vesicles or for particular cell types are described and compared to define the range of accessory control mechanisms that exert their effects on the core exocytotic machinery.

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.

The C2A domain of synaptotagmin-like protein 3 (Slp3) is an atypical calcium-dependent phospholipid-binding machine: comparison with the C2A domain of synaptotagmin I

The synaptotagmin-like protein (Slp) family consists of an N-terminal Rab27-binding domain and C-terminal tandem C2 motifs, and although it has been suggested to regulate Rab27-dependent membrane trafficking, such as Ca2+-regulated granule exocytosis in T-lymphocytes [Kuroda, Fukuda, Ariga and Mikoshiba (2002) J. Biol. Chem. 277, 9212-9218], little is known about the Ca2+-binding property of the Slp family. In this study, I demonstrated that the C2A domain of Slp3 exhibits Ca(2+)-dependent phospholipid-binding activity similar to that of the C2A domain of synaptotagmin I (Syt I) with regard to phospholipid selectivity, bivalent cation selectivity and effect of ionic strength. This finding was surprising because the C2A domains of other C-terminal-type (C-type) tandem C2 proteins require five conserved acidic residues in the putative Ca2+-binding loops 1 and 3 on the top of the beta-sandwich structure for their Ca2+-/phospholipid-binding activity, whereas the C2A domain of Slp3 contains only one conserved acidic residue in the putative Ca2+-binding loop 1. Site-directed mutagenesis and chimaeric analysis of the C2A domains of Syt I and Slp3 showed that Glu-336 and Glu-337 in the putative Ca2+-binding loop 1 and polybasic sequence (Lys-359, Lys-360 and Lys-361) in the beta-4 strand of the C2 structure are crucial for Ca2+-dependent phospholipid-binding activity of the Slp3 C2A domain, whereas the similar polybasic sequence in the C2A domain of Syt I is dispensable for Ca2+-dependent phospholipid-binding activity. These results indicate that the C2A domain of Slp3 is an atypical Ca2+-/phospholipid-binding machine, compared with other C-type tandem C2 proteins.

Vesicle-associated membrane protein-2/synaptobrevin binding to synaptotagmin I promotes O-glycosylation of synaptotagmin I

Synaptotagmin I (Syt I), an evolutionarily conserved integral membrane protein of synaptic vesicles, is now known to regulate Ca2+-dependent neurotransmitter release. Syt I protein should undergo several post-translational modifications before maturation and subsequent functioning on synaptic vesicles (e.g. N-glycosylation and fatty acylation in vertebrate Syt I), because the apparent molecular weight of Syt I on synaptic vesicles (mature form, 65,000) was much higher than the calculated molecular weight (47,400) predicted from the cDNA sequences both in vertebrates and invertebrates. Common post-translational modification(s) of Syt I conserved across phylogeny, however, have never been elucidated. In the present study, I discovered that dithreonine residues (Thr-15 and Thr-16) at the intravesicular domain of mouse Syt I are post-translationally modified by a complex form of O-linked sugar (i.e. the addition of sialic acids) in PC12 cells and that the O-glycosylation of Syt I in COS-7 cells depends on the coexpression of vesicle-associated membrane protein-2 (VAMP-2)/synaptobrevin. I also showed that a transmembrane domain of Syt I directly interacts with isolated VAMP-2, but not VAMP-2, in the heterotrimeric SNARE (SNAP receptor) complex (vesicle SNARE, VAMP-2, and two target SNAREs, syntaxin IA and SNAP-25). Since di-Thr or di-Ser residues are often found at the intravesicular domain of invertebrate Syt I, and VAMP-dependent O-glycosylation was also observed in squid Syt expressed in COS-7 cells, I propose that VAMP-dependent O-glycosylation of Syt I is a common modification during evolution and may have important role(s) in synaptic vesicle trafficking.

Inositol and higher inositol phosphates in neural tissues: homeostasis, metabolism and functional significance

Inositol phospholipids and inositol phosphates mediate well-established functions in signal transduction and in Ca2+ homeostasis in the CNS and non-neural tissues. More recently, there has been renewed interest in other roles that both myo-inositol and its highly phosphorylated forms may play in neural function. We review evidence that myo-inositol serves as a clinically relevant osmolyte in the CNS, and that its hexakisphosphate and pyrophosphorylated derivatives may play roles in such diverse cellular functions as DNA repair, nuclear RNA export and synaptic membrane trafficking.

Synaptotagmin I-DeltaC2B. A novel synaptotagmin isoform with a single C2 domain in the bovine adrenal medulla

Synaptotagmin I is a 65 kDa type 1 membrane glycoprotein found in secretory organelles that plays a key role in regulated exocytosis. We have characterised two forms (long and short) of synaptotagmin I that are present in the bovine adrenal medulla. The long form is a type I integral membrane protein which has two cytoplasmic C2 domains and corresponds to the previously characterised full-length synaptotagmin I isoform. The short-form synaptotagmin I-DeltaC2B has the same structure in the lumenal and transmembrane sequences, but synaptotagmin I-DeltaC2B is truncated such that it only has a single cytoplasmic C2 domain. Analysis of synaptotagmin I-DeltaC2B expression indicates that synaptotagmin I-DeltaC2B is preferentially expressed in the bovine adrenal medulla. However, it is absent from the dense core chromaffin granules. Furthermore, when expressed in the rat pheochromocytoma cell line PC12 bovine synaptotagmin I-DeltaC2B is largely absent from dense core granules and synaptic-like microvesicles. Instead, indirect immunofluorescence microscopy reveals the intracellular location of synaptotagmin I-DeltaC2B to be the plasma membrane.

Synaptotagmin IX regulates Ca2+-dependent secretion in PC12 cells

Synaptotagmin (Syt) I-deficient phaeochromocytoma (PC12) cell lines show normal Ca(2+)-dependent norepinephrine (NE) release (Shoji-Kasai, Y., Yoshida, A., Sato, K., Hoshino, T., Ogura, A., Kondo, S., Fujimoto, Y., Kuwahara, R., Kato, R., and Takahashi, M. (1992) Science 256, 1821-1823). To identify an alternative Ca(2+) sensor, we searched for other Syt isoforms in Syt I-deficient PC12 cells and identified Syt IX, an isoform closely related to Syt I, as an abundantly expressed dense-core vesicle protein. Here we show that Syt IX is required for the Ca(2+)-dependent release of NE from PC12 cells. Antibodies directed against the C2A domain of either Syt IX or Syt I inhibited Ca(2+)-dependent NE release in permeable PC12 cells indicating that both Syt proteins function in dense-core vesicle exocytosis. Our results support the idea that Syt family proteins that co-reside on secretory vesicles may function cooperatively and redundantly as potential Ca(2+) sensors for exocytosis.

Formation of crystalloid endoplasmic reticulum induced by expression of synaptotagmin lacking the conserved WHXL motif in the C terminus. Structural importance of the WHXL motif in the C2B domain

Synaptotagmin (Syt) is a family of type I membrane proteins that consists of a single transmembrane domain, a spacer domain, two Ca(2+)-binding C2 domains, and a short C terminus. We recently showed that deletion of the short C terminus (17 amino acids) of Syt IV prevented the Golgi localization of Syt IV proteins in PC12 cells and induced granular structures of various sizes in the cell body by an unknown mechanism (Fukuda, M., Ibata, K., and Mikoshiba, K. (2001) J. Neurochem. 77, 730-740). In this study we showed by electron microscopy that these structures are crystalloid endoplasmic reticulum (ER), analyzed the mechanism of its induction, and demonstrated that: (a) mutation or deletion of the evolutionarily conserved WHXL motif in the C terminus of the synaptotagmin family (Syt DeltaC) destabilizes the C2B domain structure (i.e. causes misfolding of the protein), probably by disrupting the formation of stable anti-parallel beta-sheets between the beta-1 and beta-8 strands of the C2B domain; (b) the resulting malfolded proteins accumulate in the ER rather than being transported to other membrane structures (e.g. the Golgi apparatus), with the malfolded proteins also inducing the expression of BiP (immunoglobulin binding protein), one of the ER stress proteins; and (c) the ERs in which the Syt DeltaC proteins have accumulated associate with each other as a result of oligomerization capacity of the synaptotagmin family, because the Syt IDeltaC mutant, which lacks oligomerization activity, cannot induce crystalloid ER. Our findings indicate that the conserved WHXL motif is important not only for protein interaction site but for proper folding of the C2B domain.

Direct interaction of the Rab3 effector RIM with Ca2+ channels, SNAP-25, and synaptotagmin

To define the role of the Rab3-interacting molecule RIM in exocytosis we searched for additional binding partners of the protein. We found that the two C(2) domains of RIM display properties analogous to those of the C(2)B domain of synaptotagmin-I. Thus, RIM-C(2)A and RIM-C(2)B bind in a Ca(2+)-independent manner to alpha1B, the pore-forming subunit of N-type Ca(2+) channels (EC(50) = approximately 20 nm). They also weakly interact with the alpha1C but not the alpha1D subunit of L-type Ca(2+) channels. In addition, the C(2) domains of RIM associate with SNAP-25 and synaptotagmin-I. The binding affinities for these two proteins are 203 and 24 nm, respectively, for RIM-C(2)A and 224 and 16 nm for RIM-C(2)B. The interactions of the C(2) domains of RIM with SNAP-25 and synaptotagmin-I are modulated by Ca(2+). Thus, in the presence of Ca(2+) (EC(50) = approximately 75 microm) the interaction with synaptotagmin-I is increased, whereas SNAP-25 binding is reduced. Synaptotagmin-I binding is abolished by mutations in two positively charged amino acids in the C(2) domains of RIM and by the addition of inositol polyphosphates. We propose that the Rab3 effector RIM is a scaffold protein that participates through its multiple binding partners in the docking and fusion of secretory vesicles at the release sites.

Identification of a mammalian H(+)-myo-inositol symporter expressed predominantly in the brain

Inositol and its phosphorylated derivatives play a major role in brain function, either as osmolytes, second messengers or regulators of vesicle endo- and exocytosis. Here we describe the identification and functional characterization of a novel H(+)-myo- inositol co-transporter, HMIT, expressed predominantly in the brain. HMIT cDNA encodes a 618 amino acid polypeptide with 12 predicted transmembrane domains. Functional expression of HMIT in Xenopus oocytes showed that transport activity was specific for myo-inositol and related stereoisomers with a Michaelis-Menten constant of approximately 100 microM, and that transport activity was strongly stimulated by decreasing pH. Electrophysiological measurements revealed that transport was electrogenic with a maximal transport activity reached at pH 5.0. In rat brain membrane preparations, HMIT appeared as a 75-90 kDa protein that could be converted to a 67 kDa band upon enzymatic deglycosylation. Immunofluorescence microscopy analysis showed HMIT expression in glial cells and some neurons. These data provide the first characterization of a mammalian H(+)-coupled myo- inositol transporter. Predominant central expression of HMIT suggests that it has a key role in the control of myo-inositol brain metabolism.

Synaptotagmin VI participates in the acrosome reaction of human spermatozoa

Acrosomal exocytosis is a calcium-dependent secretion event causing the release of the acrosomal contents and the loss of the membranes surrounding the acrosome. The synaptotagmins are a family of calcium-binding proteins that participate in the exocytosis of synaptic vesicles. The ubiquitous synaptotagmin VI isoform was found in human sperm cells by Western blot analysis. Immunocytochemistry at the optical and electron microscopy levels localized the protein to the outer acrosomal membrane. Calcium-triggered acrosomal exocytosis in permeabilized sperm cells was abrogated by a specific anti-synaptotagmin VI antibody, indicating that the protein is required for the process. Moreover, a recombinant fusion protein between glutathione S-transferase and the two calcium and phospholipid binding domains of synaptotagmin VI completely inhibited calcium-triggered exocytosis. Interestingly, phorbol ester-dependent in vitro phosphorylation of this recombinant protein abolished its inhibitory effect. We previously showed that, in permeabilized spermatozoa, addition of active Rab3A triggers acrosomal exocytosis at very low calcium concentration. Rab3A-promoted exocytosis was inhibited by the cytosolic domain of synaptotagmin VI and by the anti-synaptotagmin VI antibody, indicating that synaptotagmin is also necessary for Rab-mediated acrosomal content release. In conclusion, the results strongly indicate that synaptotagmin VI is a key component of the secretory machinery involved in acrosomal exocytosis.

A unique spacer domain of synaptotagmin IV is essential for Golgi localization

Synaptotagmin (Syt) family members consist of six separate domains: a short amino terminus, a single transmembrane domain, a spacer domain, a C2A domain, a C2B domain and a short carboxyl (C) terminus. Despite sharing the same domain structures, several synaptotagmin isoforms show distinct subcellular localization. Syt IV is mainly localized at the Golgi, while Syt I, a possible Ca(2+)-sensor for secretory vesicles, is localized at dense-core vesicles and synaptic-like microvesicles in PC12 cells. In this study, we sought to identify the region responsible for the Golgi localization of Syt IV by immunocytochemical and biochemical analyses as a means of defining the distinct subcellular localization of the synaptotagmin family. We found that the unique C-terminus of the spacer domain (amino acid residues 73-144) between the transmembrane domain and the C2A domain is essential for the Golgi localization of Syt IV. In addition, the short C-terminus is probably involved in proper folding of the protein, especially the C2B domain. Without the C-terminus, Syt IVdeltaC proteins are not targeted to the Golgi and seem to colocalize with an endoplasmic reticulum (ER) marker (i.e. induce crystalloid ER-like structures). On the basis of these results, we propose that the divergent spacer domain among synaptotagmin isoforms may contain certain signals that determine the final destination of each isoform.

Expression and localisation of synaptotagmin isoforms in endocrine (β)-cells: their function in insulin exocytosis

Exocytosis of insulin containing Large Dense Core Vesicles (LDCVs) from pancreatic (β)-cells and derived cell lines is mainly controlled by Ca(2+). Several lines of evidence have demonstrated a role of the Ca(2+)- and phospholipid-binding protein synaptotagmin (syt) in this event. Synaptotagmins form a large protein family with distinct affinities for Ca(2+) determined by their two C(2) domains (C(2)A/B). Except for the well-characterized isoforms I and II, their role is still unclear. We have used here insulin-secreting cells as a model system for LDCV exocytosis to gain insight into the function of synaptotagmins. Immunocytochemical analysis revealed that of the candidate Ca(2+) sensors in LDCV exocytosis, syt III was not expressed in primary (β)-cells, whereas syt IV was only found adjacent to the TGN. However, syt V-VIII isoforms were expressed at different levels in various insulin-secreting cells and in pancreatic islet preparations. In streptolysin-O permeabilized primary (β)-cells the introduction of recombinant peptides (100 nM) corresponding to the C(2) domains of syt V, VII and VIII, but not of syt III, IV or VI, inhibited Ca(2+)-evoked insulin exocytosis by 30% without altering GTP*S-induced release. Our observations demonstrate that syt III and IV are not involved in the exocytosis of LDCVs from primary (β)-cells whereas V, VII and VIII may mediate Ca(2+)-regulation of 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.

Assessing the omnipotence of inositol hexakisphosphate

This review assesses the authenticity of inositol hexakisphosphate (InsP(6)) being a wide-ranging regulator of many important cellular functions. Against a background in which the possible importance of localized InsP(6) metabolism is discussed, there is the facile explanation that InsP(6) is merely an "inactive" precursor for the diphosphorylated inositol phosphates. Indeed, many of the proposed cellular functions of InsP(6) cannot sustain a challenge from the implementation of a rigorous set of criteria, which are designed to avoid experimental artefacts.

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.

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.

Plasmalemmal repair of severed neurites of PC12 cells requires Ca(2+) and synaptotagmin

Ca(2+) and synaptotagmin (a Ca(2+)-binding protein that regulates axolemmal fusion of synaptic vesicles) play essential roles in the repair of axolemmal damage in invertebrate giant axons. We now report that neurites of a rat pheochromocytoma (PC12) cell line transected and maintained in a serum medium form a dye barrier (exclude an external hydrophilic fluorescent dye) and survive for 24-hr posttransection (based on morphology and retention of another hydrophilic dye internally loaded at 6-hr posttransection). Some (25%) transected neurites that form a dye barrier regrow. Most (83%) neurites transected in a saline solution containing divalent cations (PBS(++)) also exclude entry of an externally placed hydrophilic fluorescent dye at 15-min posttransection. In contrast, only 14 or 17% of neurites maintained in a divalent cation-free solution (PBS(=)) or in PBS(=) + Mg(2+), respectively, form a dye barrier. Neurites that do not form a dye barrier do not survive for 24 hr. When PC12 neurites are loaded with an antibody to squid synaptotagmin, most (81%) antibody-loaded neurites do not form a dye barrier, whereas most (>/=81%) neurites loaded with heat-inactivated antibody or preimmune IgG do form a barrier. These data show that: 1) transected neurites of PC12 cells have mechanism(s) for plasmalemmal repair (dye barrier formation and survival); 2) Ca(2+) is necessary for dye barrier formation, which occurs minutes after transection and is necessary for survival and regrowth; and 3) synaptotagmin is an essential mediator of barrier formation. The similarity in the requirements for plasmalemmal repair in this mammalian cell preparation with those reported previously for invertebrate axons suggests that mechanisms necessary for plasmalemmal repair have been conserved phylogenetically.

Drosophila AD3 mutation of synaptotagmin impairs calcium-dependent self-oligomerization activity

Genetic analysis of a Drosophila synaptotagmin (Syt) I mutant (AD3) has revealed that Tyr-334 within the C2B domain is essential for efficient Ca(2+)-dependent neurotransmitter release. However, little is known as to why a missense mutation (Tyr-334-Asn) disrupts the function of the C2B domain at the molecular level. Here, we present evidence that a Tyr-312 to Asn substitution in mouse Syt II, which corresponds to the Drosophila AD3 mutation, completely impairs Ca(2+)-dependent self-oligomerization activity mediated by the C2B domain but allows partial interaction with wild-type proteins in a Ca(2+)-dependent manner. This observation is consistent with the fact that the AD3 allele is homozygous lethal but complements another mutant phenotype. We also showed that the Ca(2+)-dependent C2B self-oligomerization is inhibited by inositol 1,3,4, 5-tetrakisphosphate, a potent inhibitor of neurotransmitter release. All of these findings strongly support the idea that self-oligomerization of Syt I or II is essential for neurotransmitter release in vivo.

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.

Axolemmal repair requires proteins that mediate synaptic vesicle fusion

A damaged cell membrane is repaired by a seal that restricts entry or exit of molecules and ions to that of the level passing through an undamaged membrane. Seal formation requires elevation of intracellular Ca(2+) and, very likely, protein-mediated fusion of membranes. Ca(2+) also regulates the interaction between synaptotagmin (Syt) and syntaxin (Syx), which is thought to mediate fusion of synaptic vesicles with the axolemma, allowing transmitter release at synapses. To determine whether synaptic proteins have a role in sealing axolemmal damage, we injected squid and crayfish giant axons with an antibody that inhibits squid Syt from binding Ca(2+), or with another antibody that inhibits the Ca(2+)-dependent interaction of squid Syx with the Ca(2+)-binding domain of Syt. Axons injected with antibody to Syt did not seal, as assessed at axonal cut ends by the exclusion of extracellular hydrophilic fluorescent dye using confocal microscopy, and by the decay of extracellular injury current compared to levels measured in uninjured axons using a vibrating probe technique. In contrast, axons injected with either denatured antibody to Syt or preimmune IgG did seal. Similarly, axons injected with antibody to Syx did not seal, but did seal when injected with either denatured antibody to Syx or preimmune IgG. These results indicate an essential involvement of Syt and Syx in the repair (sealing) of severed axons. We suggest that vesicles, which accumulate and interact at the injury site, re-establish axolemmal continuity by Ca(2+)-induced fusions mediated by proteins such as those involved in neurotransmitter release.

The use of permeabilized cells to assay protein phosphorylation and catecholamine release

A number of approaches can be used to determine the protein kinases and protein phosphatases acting on particular phosphoproteins in vivo. Cell permeabilization represents one such approach. In this overview we discuss the different permeabilization procedures used in bovine adrenal chromaffin cells and in particular the use of digitonin. The effect of various factors on the extent of digitonin-permeabilization, protein phosphorylation and catecholamine release are also discussed. The factors include the permeabilization medium, the ions such as calcium, and the second messengers, such as cAMP, IP3, cADPR and calmodulin. The effect of specific peptide inhibitors of protein kinases on tyrosine hydroxylase phosphorylation is illustrated. Advantages and disadvantages of cell permeabilization procedures are discussed throughout the text.

Rapid regulated dense-core vesicle exocytosis requires the CAPS protein

Although many proteins essential for regulated neurotransmitter and peptide hormone secretion have been identified, little is understood about their precise roles at specific stages of the multistep pathway of exocytosis. To study the function of CAPS (Ca(2+)-dependent activator protein for secretion), a protein required for Ca(2+)-dependent exocytosis of dense-core vesicles, secretory responses in single rat melanotrophs were monitored by patch-clamp membrane capacitance measurements. Flash photolysis of caged Ca(2+) elicited biphasic capacitance increases consisting of rapid and slow components with distinct Ca(2+) dependencies. A threshold of approximately 10 microM Ca(2+) was required to trigger the slow component, while the rapid capacitance increase was recorded already at a intracellular Ca(2+) activity < 10 microM. Both kinetic membrane capacitance components were abolished by botulinum neurotoxin B or E treatment, suggesting involvement of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor)-dependent vesicle fusion. The rapid but not the slow component was inhibited by CAPS antibody. These results were further clarified by immunocytochemical studies that revealed that CAPS was present on only a subset of dense-core vesicles. Overall, the results indicate that dense-core vesicle exocytosis in melanotrophs occurs by two parallel pathways. The faster pathway exhibits high sensitivity to Ca(2+) and requires the presence of CAPS, which appears to act at a late stage in the secretory pathway.

Genomic structures of synaptotagmin II protein: comparison of exon-intron organization of the synaptotagmin gene family

Synaptotagmin (Syt) I (or II) is thought to be a Ca(2+) sensor for neurotransmitter release in the rostral (or caudal) region of the mammalian brain. In this study, we first report the genomic structures and the transcription initiation site of the mouse syt II gene. Syt II protein coding regions consist of 8 exons with a total size of about 7 kbp. Exon 2 encodes the transmembrane domain, and exons 3-5 and 6-8 encode the C2A and C2B domains, respectively, although the precise intron positions within the two C2 domains are different. The syt II gene structures are quite similar to the mouse syt VIII and human syt IX genes, but not Caenorhabditis elegans syt I, human syt VII, or the mouse syt III genes, indicating that the exon-intron patterns of the functional domain of synaptotagmins (especially the C2A domain) have not been as well conserved during evolution as among isoforms. The only conserved feature among the synaptotagmin gene family seems to have a single exon that encodes the whole transmembrane domain.

Synaptotagmin VII regulates Ca(2+)-dependent exocytosis of lysosomes in fibroblasts

Synaptotagmins (Syts) are transmembrane proteins with two Ca(2+)-binding C(2) domains in their cytosolic region. Syt I, the most widely studied isoform, has been proposed to function as a Ca(2+) sensor in synaptic vesicle exocytosis. Several of the twelve known Syts are expressed primarily in brain, while a few are ubiquitous (Sudhof, T.C., and J. Rizo. 1996. Neuron. 17: 379-388; Butz, S., R. Fernandez-Chacon, F. Schmitz, R. Jahn, and T.C. Sudhof. 1999. J. Biol. Chem. 274:18290-18296). The ubiquitously expressed Syt VII binds syntaxin at free Ca(2+) concentrations ([Ca(2+)]) below 10 microM, whereas other isoforms require 200-500 microM [Ca(2+)] or show no Ca(2+)-dependent syntaxin binding (Li, C., B. Ullrich, Z. Zhang, R.G.W. Anderson, N. Brose, and T.C. Sudhof. 1995. Nature. 375:594-599). We investigated the involvement of Syt VII in the exocytosis of lysosomes, which is triggered in several cell types at 1-5 microM [Ca(2+)] (Rodríguez, A., P. Webster, J. Ortego, and N.W. Andrews. 1997. J. Cell Biol. 137:93-104). Here, we show that Syt VII is localized on dense lysosomes in normal rat kidney (NRK) fibroblasts, and that GFP-tagged Syt VII is targeted to lysosomes after transfection. Recombinant fragments containing the C(2)A domain of Syt VII inhibit Ca(2+)-triggered secretion of beta-hexosaminidase and surface translocation of Lgp120, whereas the C(2)A domain of the neuronal- specific isoform, Syt I, has no effect. Antibodies against the Syt VII C(2)A domain are also inhibitory in both assays, indicating that Syt VII plays a key role in the regulation of Ca(2+)-dependent lysosome exocytosis.

Alternative splicing of synaptotagmins involving transmembrane exon skipping

The synaptotagmin gene family currently includes 12 members. Analysis of the three known genomic synaptotagmin sequences reveals conserved exon-intron patterns which delineate the synaptotagmin structural domains. We used expressed sequence tag, reverse transcription PCR and RNAse protection assay analysis of synaptotagmin messenger RNAs to demonstrate the occurrence of alternative splicing events involving a number of exons. Exon-skipped messages where transmembrane sequences have been removed or altered were found to be abundantly expressed by synaptotagmins 1, 4, 6 and 7. Although the expression of most synaptotagmins predominates in neural tissue, we find that by contrast, synaptotagmin 6 is more abundant in thymus.

A novel alternatively spliced variant of synaptotagmin VI lacking a transmembrane domain. Implications for distinct functions of the two isoforms

Synaptotagmins are a family of membrane proteins that are characterized by a single transmembrane region and tandem C2 domains and that are likely to regulate constitutive and/or regulated vesicle traffic. We have shown that a subclass of synaptotagmins (III, V, VI, and X) forms homo- and heterodimers through an evolutionarily conserved cysteine motif at their N termini (Fukuda, M., Kanno, E., and Mikoshiba, K. (1999) J. Biol. Chem. 274, 31421-31427). In this study, we identified a novel alternatively spliced variant of synaptotagmin (Syt) VI that lacks the N-terminal 85 amino acids including the transmembrane region (thus designated as Syt VIDeltaTM). Because it lacks the cysteine motif responsible for self-dimerization, Syt VIDeltaTM could not associate with Syt VI even in the presence of Ca(2+). Despite lacking the transmembrane region, Syt VIDeltaTM can associate with the plasma membrane through the C-terminal 29 amino acids. In adult mouse brain, two closely comigrating bands at M(r) approximately 50,000, which closely corresponded to the molecular weight of recombinant Syt VIDeltaTM, were detected by anti-Syt VI antibody. These immunoreactive bands were found in both soluble and membrane fractions of mouse brain, indicating that they are membrane-associated proteins (Syt VIDeltaTM), but not transmembrane proteins (Syt VI). Expression of Syt VI and Syt VIDeltaTM in PC12 or COS-7 cells indicated that the two molecules have a distinct subcellular distribution: Syt VIDeltaTM is present in the cytosol or is associated with the plasma membrane or internal membrane structures, whereas Syt VI is localized to the endoplasmic reticulum and/or Golgi-like perinuclear compartment. These results suggest that Syt VI and Syt VIDeltaTM may play distinct roles in vesicular trafficking.

Conserved N-terminal cysteine motif is essential for homo- and heterodimer formation of synaptotagmins III, V, VI, and X

The synaptotagmins now constitute a large family of membrane proteins characterized by one transmembrane region and two C2 domains. Dimerization of synaptotagmin (Syt) I, a putative low affinity Ca(2+) sensor for neurotransmitter release, is thought to be important for expression of function during exocytosis of synaptic vesicles. However, little is known about the self-dimerization properties of other isoforms. In this study, we demonstrate that a subclass of synaptotagmins (III, V, VI, and X) (Ibata, K., Fukuda, M., and Mikoshiba, K. (1998) J. Biol. Chem. 273, 12267-12273) forms beta-mercaptoethanol-sensitive homodimers and identify three evolutionarily conserved cysteine residues at the N terminus (N-terminal cysteine motif, at amino acids 10, 21, and 33 of mouse Syt III) that are not conserved in other isoforms. Site-directed mutagenesis of these cysteine residues and co-immunoprecipitation experiments clearly indicate that the first cysteine residue is essential for the stable homodimer formation of Syt III, V, or VI, and heterodimer formation between Syts III, V, VI, and X. We also show that native Syt III from mouse brain forms a beta-mercaptoethanol-sensitive homodimer. Our results suggest that the cysteine-based heterodimerization between Syt III and Syt V, VI, or X, which have different biochemical properties, may modulate the proposed function of Syt III as a putative high affinity Ca(2+) sensor for neurotransmitter release.

Physiological aspects of exocytosis in chromaffin cells of the adrenal medulla

The adrenal medulla is composed principally of groups of adrenergic and noradrenergic chromaffin cells, with minor populations of small intensely fluorescent cells and ganglionic neurones. Different molecular stimuli evoke distinct secretory events in the gland, involving the release of either adrenaline or noradrenaline together with various neuroactive peptides. The nature of the secretory response can be controlled at a central level or regulated locally within the gland. Specific innervation patterns to the different types of chromaffin cell have been implicated in central regulatory mechanisms, while several explanations for regulating secretion locally have been proposed. The differential distribution of various types of receptors between cell phenotypes, such as muscarinic or nicotinic acetylcholine receptors, histamine receptors, angiotensin receptors and different classes of opiate receptors between the two principal chromaffin cell populations could be involved in local control. In addition exocytosis parameters could be modulated differently in adrenergic and noradrenergic cells by phenotype-specific mechanisms, possibly involving molecules like Growth Associated Protein-43, Synaptosomal Associated Protein-25 isoforms or the p11 annexin subunit. The distribution of the various types of calcium channels is also known to vary between chromaffin cell subtypes. This short review examines possible ways in which specific innervation patterns in the adrenal gland could be programmed and discusses exocytosis mechanisms that could differ between chromaffin cell phenotypes. Data reviewed here suggest that the adrenal medulla should no longer be viewed as a homogeneous entity but as consisting of an ensemble of individual cell subpopulations each with a distinct secretory response that could in part reflect its local history.

Functional and biochemical analysis of the C2 domains of synaptotagmin IV

Synaptotagmins (Syts) are a family of vesicle proteins that have been implicated in both regulated neurosecretion and general membrane trafficking. Calcium-dependent interactions mediated through their C2 domains are proposed to contribute to the mechanism by which Syts trigger calcium-dependent neurotransmitter release. Syt IV is a novel member of the Syt family that is induced by cell depolarization and has a rapid rate of synthesis and a short half-life. Moreover, the C2A domain of Syt IV does not bind calcium. We have examined the biochemical and functional properties of the C2 domains of Syt IV. Consistent with its non-calcium binding properties, the C2A domain of Syt IV binds syntaxin isoforms in a calcium-independent manner. In neuroendocrine pheochromocytoma (PC12) cells, Syt IV colocalizes with Syt I in the tips of the neurites. Microinjection of the C2A domain reveals that calcium-independent interactions mediated through this domain of Syt IV inhibit calcium-mediated neurotransmitter release from PC12 cells. Conversely, the C2B domain of Syt IV contains calcium binding properties, which permit homo-oligomerization as well as hetero-oligomerization with Syt I. Our observation that different combinatorial interactions exist between Syt and syntaxin isoforms, coupled with the calcium stimulated hetero-oligomerization of Syt isoforms, suggests that the secretory machinery contains a vast repertoire of biochemical properties for sensing calcium and regulating neurotransmitter release accordingly.

Functional involvement of synaptotagmin I/II C2A domain in neurite outgrowth of chick dorsal root ganglion neuron

Synaptotagmin I or II (Syt I/II) is involved in Ca2+-regulated exocytosis of secretory vesicles, probably serving as a Ca2+-sensor via its C2A domain. Synaptotagmin is also known to be expressed in neuronal growth cone vesicles, but its functional involvement in neurite outgrowth remains largely unknown. In this study, we examined the function of Syt I/II in neurite outgrowth in cultured chick dorsal root ganglion neurons using an anti-synaptotagmin I and II C2A domain (anti-STI/II-C2A) antibody that inhibits Ca2+-regulated exocytosis. Immunoblots confirmed the high specificity of the anti-STI/II-C2A antibody and showed the expression of synaptotagmin I or II in chick dorsal root ganglion neurons. Immunocytochemistry revealed that synaptotagmin I or II is enriched at the growth cone region of chick dorsal root ganglion neurons, in both lamellipodia and filopodia. Whole or Fab-fragment of the anti-STI/II-C2A antibody loaded into dorsal root ganglion neurons by trituration significantly inhibited neurite outgrowth, whereas preimmune immunoglobulin G had no effect. These results showed that the C2A domain of synaptotagmin I or II plays a crucial role in neurite outgrowth.

Role of synaptotagmin, a Ca2+ and inositol polyphosphate binding protein, in neurotransmitter release and neurite outgrowth

Synaptotagmin I (or II), a possible Ca(2+)-sensor of synaptic vesicles, has two functionally distinct C2 domains: the C2A domain binds Ca2+ and the C2B domain binds inositol high polyphosphates (IP4, IP5, and IP6). Ca(2+)-regulated exocytosis of secretory vesicles is proposed to be activated by Ca2+ binding to the C2A domain and inhibited by inositol polyphosphate binding to the C2B domain. Synaptotagmins now constitute a large family and are thought to be involved in both regulated and constitutive vesicular trafficking. They are classified from their distribution as neuronal (synaptotagmin I-V, X, and XI) and the ubiquitous type (synaptotagmin VI-IX). Among them, synaptotagmins III, V, VI and X are deficient in IP4 binding activity due to the amino acid substitutions in the C-terminal region of the C2B domain, suggesting that these isoforms can work for vesicular trafficking even in the presence of inositol high polyphosphates. Synaptotagmin I is also known to be present in neuronal growth cone vesicles. Antibody against the C2A domain (anti-C2A) that inhibits Ca(2+)-regulated exocytosis also blocked neurite outgrowth of the chick dorsal root ganglion (DRG) neuron, suggesting that Ca(2+)-dependent synaptotagmin activation is also crucial for neurite outgrowth.

Phosphoinositide lipids as signaling molecules: common themes for signal transduction, cytoskeletal regulation, and membrane trafficking

Signaling roles for phosphoinositides that involve their regulated hydrolysis to generate second messengers have been well characterized. Recent work has revealed additional signaling roles for phosphoinositides that do not involve their hydrolysis. PtdIns 3-P, PtdIns 3,4,5-P3, and PtdIns 4,5-P2 function as site-specific signals on membranes that recruit and/or activate proteins for the assembly of spatially localized functional complexes. A large number of phosphoinositide-binding proteins have been identified as the potential effectors for phosphoinositide signals. Common themes of localized signal generation and the spatially localized recruitment of effector proteins appear to underlie mechanisms employed in signal transduction, cytoskeletal, and membrane trafficking events.

Molecular mechanisms and regulation of insulin exocytosis as a paradigm of endocrine secretion

Secretion of the peptide hormone insulin from pancreatic beta cells constitutes an important step in the regulation of body homeostasis. Insulin is stored in large dense core vesicles and released by exocytosis, a multistage process involving transport of vesicles to the plasma membrane, their docking, priming and finally their fusion with the plasma membrane. Some of the protein components necessary for this process have been identified in beta cells. The export of potent and potentially harmful substances has to be tightly controlled. The secretory response in pancreatic beta cells requires the concerted action of nutrients together with enteric hormones and neurotransmitters acting on G-protein coupled receptors. It is well established that glucose and other metabolizable nutrients depolarize the beta-cell membrane and the ensuing Ca2+ influx through voltage-dependent channels constitutes a main stimulus for insulin exocytosis. Theoretical considerations and recent observations suggest in addition an organizing role for the Ca2+ channel similar to neurotransmission. A second regulatory control on exocytosis is exerted by monomeric and heterotrimeric G-proteins. The monomeric GTPase Rab3A controls insulin secretion through cycling between a guanosine triphosphate liganded vesicle-bound form and a guanosine diphosphate liganded, cytosolic form. The effect of neurohormones is transduced by the heterotrimeric GTPases. Whereas pertussis-toxin sensitive alpha-subunits exert direct inhibition at the level of exocytosis, the Gbeta gamma-subunits are required for stimulation. It is possible that these GTPases exert immediate regulation, while protein kinases and phosphatases may modulate long-term adaptation at the exocytotic machinery itself. The molecular nature of their activators and effectors still await identification. Insights into the progression of the exocytotic vesicle from docking to fusion and how these processes are precisely regulated by proteins and second messengers may provide the basis for new therapeutic principles.

Protein kinase C controls the priming step of regulated exocytosis in adrenal chromaffin cells

1. To investigate the mechanism whereby protein kinase C enhances secretory function in adrenal chromaffin cells, we examined the effects of 12-O-tetradecanoylphorbor-13-acetate (TPA) on Ca(2+)-induced catecholamine release from digitonin-permeabilized cells, resolving the release into a MgATP-dependent priming step and a MgATP-independent Ca(2+)-triggered step. Treatment with TPA selectively potentiated the priming activity of MgATP, with little increase in the MgATP-independent release. The potentiation by TPA of the MgATP-dependent priming was blocked by [Ser25]protein kinase C(19-31), a specific substrate of protein kinase C. Gö 6976, an inhibitor selective for protein kinase C alpha and beta isoforms, also blocked the potentiation by TPA. These results suggest that activation of protein kinase C, probably the alpha isoform, potentiates the MgATP-dependent priming step. 2. The antibody raised against GAP-43, a known substrate of protein kinase C, also potentiated the MgATP-dependent priming. The effect of TPA and that of the anti-GAP-43 antibody were not additive. Calmodulin, which binds to GAP-43 and inhibits its phosphorylation by protein kinase C, abolished the effect of TPA. Thus, the present results suggest that protein kinase C potentiates MgATP-dependent priming, at least in part, through phosphorylation of GAP-43.

Functional analysis of the C2A domain of synaptotagmin 1: implications for calcium-regulated secretion

Synaptotagmin 1 is proposed to function as a low affinity calcium sensor for calcium-triggered exocytosis from neural and neuroendocrine cells. Because of the calcium-binding properties of the C2A domain of synaptotagmin 1, calcium-dependent interactions through this domain may modulate neurotransmitter release. We addressed this question by using alanine-scanning mutagenesis to generate a series of mutations within the C2A domain of synaptotagmin 1. The effects of these mutations on synaptotagmin 1 C2A function were analyzed for (1) calcium-dependent phospholipid binding, (2) calcium-dependent binding to syntaxin 1A, a plasma membrane protein critical for vesicle docking or fusion, and (3) calcium-regulated secretion after microinjection into neuroendocrine pheochromocytoma (PC12) cells. Our analyses reveal that a polylysine motif at residues 189-192 confers an inhibitory effect on secretion by recombinant synaptotagmin C2A fragments. The synaptotagmin 1 C2A polylysine motif functions independently of calcium-mediated interactions with phospholipids and syntaxin 1A. Furthermore, alpha-latrotoxin reverses the inhibitory effect of injected recombinant C2A fragments, suggesting that they perturb the cellular calcium-sensing machinery by interfering with synaptotagmin 1 activity in vivo. Our results indicate that novel calcium-independent interactions mediated through the C2A polylysine motif of synaptotagmin 1 function to modulate neurotransmitter release.

Inositol 1,3,4,5-tetrakisphosphate binding activities of neuronal and non-neuronal synaptotagmins. Identification of conserved amino acid substitutions that abolish inositol 1,3,4,5-tetrakisphosphate binding to synaptotagmins III, V, and X

Synaptotagmins I and II are essential for Ca2+-regulated exocytosis of synaptic vesicles from neurons, probably serving as Ca2+ sensors. This Ca2+-sensing function is thought to be disrupted by binding of an inositol 1,3,4,5-tetrakisphosphate (IP4) to the C2B domain of synaptotagmin I or II (Fukuda, M., Moreira, J. E., Lewis, F. M. T., Sugimori, M., Niinobe, M., Mikoshiba, K., and Llinás, R. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 10708-10712). Recently, several synaptotagmin isoforms, expressed outside the nervous system, have been identified in rats and proposed to be involved in constitutive vesicle traffic. To test whether the inositol high polyphosphates also regulate constitutive vesicle traffic by binding to the non-neuronal synaptotagmins, we examined the IP4 binding properties of the recombinant C2 domains of both neuronal (III, V, X, and XI) and non-neuronal (VI-VIII and IX) synaptotagmins. The C2B domains of synaptotagmins VII-IX and XI had strong IP4 binding activity, but the C2B domain of synaptotagmin VI showed very weak IP4 binding activity. In contrast, there was no significant IP4 binding activity of the C2B domains of synaptotagmins III, V, and X or any of the C2A domains. A phylogenetic tree of the C2 domains of 11 isoforms revealed that synaptotagmins III, V, VI, and X (IP4-insensitive or very weak IP4-binding isoforms) belong to the same branch. Based on the sequence comparison between the IP4-sensitive and -insensitive isoforms, we performed site-directed mutagenesis of synaptotagmin III and identified several amino acid substitutions that abolish IP4 binding activity. Our data suggest that the inositol high polyphosphates might also regulate constitutive vesicle traffic via binding to the IP4-sensitive non-neuronal synaptotagmins.