Synaptotagmin V: a novel synaptotagmin isoform expressed in rat brain

Expression and distribution of synaptotagmin family members in the zebrafish retina

Synaptotagmins belong to a large family of proteins. Although various synaptotagmins have been implicated as Ca²⁺ sensors for vesicle replenishment and release at conventional synapses, their roles at retinal ribbon synapses remain incompletely understood. Zebrafish is a widely used experimental model for retinal research. We therefore investigated the homology between human, rat, mouse, and zebrafish synaptotagmins 1-10 using a bioinformatics approach. We also characterized the expression and distribution of various synaptotagmin (syt) genes in the zebrafish retina using RT-PCR, qPCR, and in situhybridization, focusing on the family members whose products likely underlie Ca²⁺ -dependent exocytosis in the central nervous system (synaptotagmins 1, 2, 5, and 7). Most zebrafish synaptotagmins are well conserved and can be grouped in the same classes as mammalian synaptotagmins, based on crucial amino acid residues needed for coordinating Ca²⁺ binding and determining phospholipid binding affinity. The only exception is synaptotagmin 1b, which lacks 34 amino acid residues in the C2B domain and is therefore unlikely to bind Ca²⁺ there. Additionally, the products of zebrafish syt5a and syt5b genes share identity with mammalian class 1 and 5 synaptotagmins. Zebrafish syt1, syt2, syt5, and syt7 paralogues are found in the zebrafish brain, eye, and retina, excepting syt1b, which is only present in the brain. The complementary expression pattern of the remaining paralogues in the retina suggests that syt1a and syt5a may underlie synchronous release and syt7a and syt7b may mediate asynchronous release or other Ca²⁺ -dependent processes in different retinal neurons.

Evaluation of the Therapeutic Effect of Lycoramine on Alzheimer's Disease in Mouse Model

BACKGROUND

Alzheimer's disease is one of the leading health problems characterized by the accumulation of Aβ and hyperphosphorylated tau that account for the senile plaque formations causing extensive cognitive decline. Many of the clinical diagnoses of Alzheimer's disease are made in the late stages, when the pathological changes have already progressed.

OBJECTIVE

The objective of this study is to evaluate the promising therapeutic effects of a natural compound, lycoramine, which has been shown to have therapeutic potential in several studies and to understand its mechanism of action on the molecular level via differential protein expression analyses.

METHODS

Lycoramine and galantamine, an FDA approved drug used in the treatment of mild to moderate AD, were administered to 12 month-old 5xFAD mice. Effects of the compounds were investigated by Morris water maze, immunohistochemistry and label- free differential protein expression analyses.

RESULTS

Here we demonstrated the reversal of cognitive decline via behavioral testing and the clearance of Aβ plaques. Proteomics analysis provided in-depth information on the statistically significant protein perturbations in the cortex, hippocampus and cerebellum sections to hypothesize the possible clearance mechanisms of the plaque formation and the molecular mechanism of the reversal of cognitive decline in a transgenic mouse model. Bioinformatics analyses showed altered molecular pathways that can be linked with the reversal of cognitive decline observed after lycoramine administration but not with galantamine.

CONCLUSION

Lycoramine shows therapeutic potential to halt and reverse cognitive decline at the late stages of disease progression, and holds great promise for the treatment of Alzheimer's disease.

Synaptotagmin 5 regulates Ca2+-dependent Weibel-Palade body exocytosis in human endothelial cells

Elevations of intracellular free Ca²⁺ concentration ([Ca²⁺]_i) are a potent trigger for Weibel-Palade body (WPB) exocytosis and secretion of von Willebrand factor (VWF) from endothelial cells; however, the identity of WPB-associated Ca²⁺-sensors involved in transducing acute increases in [Ca²⁺]_i into granule exocytosis remains unknown. Here, we show that synaptotagmin 5 (SYT5) is expressed in human umbilical vein endothelial cells (HUVECs) and is recruited to WPBs to regulate Ca²⁺-driven WPB exocytosis. Western blot analysis of HUVECs identified SYT5 protein, and exogenously expressed SYT5-mEGFP localised almost exclusively to WPBs. shRNA-mediated knockdown of endogenous SYT5 (shSYT5) reduced the rate and extent of histamine-evoked WPB exocytosis and reduced secretion of the WPB cargo VWF-propeptide (VWFpp). The shSYT5-mediated reduction in histamine-evoked WPB exocytosis was prevented by expression of shRNA-resistant SYT5-mCherry. Overexpression of SYT5-EGFP increased the rate and extent of histamine-evoked WPB exocytosis, and increased secretion of VWFpp. Expression of a Ca²⁺-binding defective SYT5 mutant (SYT5-Asp197Ser-EGFP) mimicked depletion of endogenous SYT5. We identify SYT5 as a WPB-associated Ca²⁺ sensor regulating Ca²⁺-dependent secretion of stored mediators from vascular endothelial cells.

Genome-wide analysis and expression profiles of NTMC2 family genes in Oryza sativa

N-terminal-TM-C2 domain proteins (NTMC2), which share domain architecture and sequence similarity to synaptotagmins (Syts) in mammals and FAM62 (extended Syts) in metazoans, form a small gene family in plants. Previous studies showed that the Arabidopsis thaliana NTMC2 type 1.1 protein (NTMC2T1.1, named AtSyt1) possesses calcium- and membrane-binding activities that allow it to function in a plasma membrane repair pathway induced by stress. However, we lack understanding of the diverse biological roles of plant NTMC2 family genes. In this study, a total of 13 OsNTMC2 genes was identified through a comprehensive bioinformatics analysis of the rice (Oryza sativa L.) genome and classified into six OsNTMC2 groups (OsNTMC2T1 to OsNTMC2T6) based on phylogeny and motif constitution. OsNTMC2T1 to OsNTMC2T3 have two calcium-binding domains (C2A and C2B), but OsNTMC2T4 to OsNTMC2T6 have single C2 domain. The expression profiles of OsNTMC2 genes were analysed at different stages of vegetative and reproductive development. This analysis revealed that at least one OsNTMC2 gene was abundantly expressed at each stage of development. These results should facilitate research on this gene family and provide new insights elucidating their functions in higher plants.

Analysis of gene expression in two large schizophrenia cohorts identifies multiple changes associated with nerve terminal function

Schizophrenia is a severe psychiatric disorder with a world-wide prevalence of 1%. The pathophysiology of the illness is not understood, but is thought to have a strong genetic component with some environmental influences on aetiology. To gain further insight into disease mechanism, we used microarray technology to determine the expression of over 30 000 mRNA transcripts in post-mortem tissue from a brain region associated with the pathophysiology of the disease (Brodmann area 10: anterior prefrontal cortex) in 28 schizophrenic and 23 control patients. We then compared our study (Charing Cross Hospital prospective collection) with that of an independent prefrontal cortex dataset from the Harvard Brain Bank. We report the first direct comparison between two independent studies. A total of 51 gene expression changes have been identified that are common between the schizophrenia cohorts, and 49 show the same direction of disease-associated regulation. In particular, changes were observed in gene sets associated with synaptic vesicle recycling, transmitter release and cytoskeletal dynamics. This strongly suggests multiple, small but synergistic changes in gene expression that affect nerve terminal function.

Synaptotagmins bind calcium to release insulin

Plasma insulin levels are determined mainly by the rate of exocytosis of the insulin-containing large dense core vesicles (LDCVs) of pancreatic islet beta-cells. This process involves the recruitment of LDCVs to the plasma membrane, where they are docked by the assembly of multiprotein SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complexes. However, fusion of the two membranes will proceed only in the presence of Ca(2+) ions, implicating a Ca(2+) sensor protein. The synaptotagmin gene family, comprising 15 members, was proposed to act as such Ca(2+) sensor in regulated exocytosis in neurons and neuroendocrine and endocrine cells. Herein, we review the physiological function of the various synaptotagmins with reference to their impact on insulin exocytosis. Cumulating evidence emphasizes the crucial role of synaptotagmin VII and IX as mediators of glucose-induced insulin secretion.

Synaptotagmin-1, -2, and -9: Ca2+ Sensors for Fast Release that Specify Distinct Presynaptic Properties in Subsets of Neurons

Synaptotagmin-1 and -2 are known Ca(2+) sensors for fast synchronous neurotransmitter release, but the potential Ca(2+)-sensor functions of other synaptotagmins in release remain uncharacterized. We now show that besides synaptotagmin-1 and -2, only synaptotagmin-9 (also called synaptotagmin-5) mediates fast Ca(2+) triggering of release. Release induced by the three different synaptotagmin Ca(2+) sensors exhibits distinct kinetics and apparent Ca(2+) sensitivities, suggesting that the synaptotagmin isoform expressed by a neuron determines the release properties of its synapses. Conditional knockout mice producing GFP-tagged synaptotagmin-9 revealed that synaptotagmin-9 is primarily expressed in the limbic system and striatum. Acute deletion of synaptotagmin-9 in striatal neurons severely impaired fast synchronous release without changing the size of the readily-releasable vesicle pool. These data show that in mammalian brain, only synaptotagmin-1, -2, and -9 function as Ca(2+) sensors for fast release, and that these synaptotagmins are differentially expressed to confer distinct release properties onto synapses formed by defined subsets of neurons.

Synaptotagmins I and IX function redundantly in regulated exocytosis but not endocytosis in PC12 cells

Synaptotagmin I is considered to be a Ca2+ sensor for fast vesicle exocytosis. Because Ca2+-dependent vesicle exocytosis persists in synaptotagmin I mutants, there must be additional Ca2+ sensors. Multiple synaptotagmin isoforms co-reside on vesicles, which suggests that other isoforms complement synaptotagmin I function. We found that full downregulation of synaptotagmins I and IX, which co-reside on vesicles in PC12 cells, completely abolished Ca2+-dependent vesicle exocytosis. By contrast, Ca2+-dependent exocytosis persisted in cells expressing only synaptotagmin I or only synaptotagmin IX, which indicated a redundancy in function for these isoforms. Although either isoform was sufficient to confer Ca2+ regulation on vesicle exocytosis, synaptotagmins I and IX conferred faster and slower release rates, respectively, indicating that individual isoforms impart distinct kinetic properties to vesicle exocytosis. The downregulation of synaptotagmin I but not synaptotagmin IX impaired compensatory vesicle endocytosis, which revealed a lack of isoform redundancy and functional specialization of synaptotagmin I for endocytic retrieval.

Insulin vesicle release: walk, kiss, pause ... then run

The mechanisms by which insulin-containing dense core secretory vesicles approach and finally fuse with the plasma membrane are of considerable current interest: defects in these processes may be one of the contributing factors to Type 2 diabetes. In this review, we discuss the molecular mechanisms involved in vesicle trafficking within the pancreatic beta-cell and the mechanisms whereby these may be regulated. We then go on to describe recent evidence that suggests that vesicle fusion at the plasma membrane is a partly reversible process ("kiss and run" or "cavity recapture"). We propose that vesicles may participate in a exo-endocytotic cycle in which a proportion of those that have already undergone an interaction with the plasma membrane may exchange exocytotic machinery with maturing vesicles.

Distinct developmental expression of synaptotagmin I and IX in the mouse brain

Synaptotagmin IX has been postulated to function as a major Ca2+ sensor for dense-core vesicle exocytosis in neuroendocrine cells. In this study, we investigated the subcellular localization and developmental expression profile of synaptotagmin IX in the mouse brain and found that it is mainly present in the dense-core vesicle fraction, which is devoid of synaptotagmin I and synaptophysin. We also found that the synaptotagmin IX expression level is constant throughout the postnatal development of the mouse brain, whereas the synaptotagmins I, II, III, VI, and XII are upregulated in parallel with synaptogenesis. These findings suggest that synaptotagmin IX regulates the transport of certain vesicles in the brain other than synaptic vesicles.

Synaptotagmin VII is targeted to secretory organelles in PC12 cells, where it functions as a high-affinity calcium sensor

Synaptotagmin (syt) I is thought to act as a Ca2+ sensor that regulates neuronal exocytosis. Fifteen additional isoforms of syt have been identified, but their functions are less well understood. Here, we used PC12 cells to test the idea that different isoforms of syt impart cells with distinct metal (i.e., Ca2+, Ba2+, and Sr2+) requirements for secretion. These cells express syt's I and IX (syt IX sometimes referred to as syt V), which have low apparent metal affinities, at much higher levels than syt VII, which we show has a relatively high apparent affinity for metals. We found that syt I and VII partially colocalize on large dense core vesicles and that upregulation of syt VII produces a concomitant increase in the divalent cation sensitivity of catecholamine release from PC12 cells. Furthermore, RNA interference-mediated knockdown of endogenous syt VII reduced the metal sensitivity of release. These data support the hypothesis that the complement of syt's expressed by a cell, in conjunction with their metal affinity, determines the divalent cation sensitivity of exocytosis.

Synaptotagmin isoforms couple distinct ranges of Ca2+, Ba2+, and Sr2+ concentration to SNARE-mediated membrane fusion

Ca2+-triggered exocytosis of synaptic vesicles is controlled by the Ca2+-binding protein synaptotagmin (syt) I. Fifteen additional isoforms of syt have been identified. Here, we compared the abilities of three syt isoforms (I, VII, and IX) to regulate soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-mediated membrane fusion in vitro in response to divalent cations. We found that different isoforms of syt couple distinct ranges of Ca2+, Ba2+, and Sr2+ to membrane fusion; syt VII was approximately 400-fold more sensitive to Ca2+ than was syt I. Omission of phosphatidylserine (PS) from both populations of liposomes completely abrogated the ability of all three isoforms of syt to stimulate fusion. Mutations that selectively inhibit syt.target-SNARE (t-SNARE) interactions reduced syt stimulation of fusion. Using Sr2+ and Ba2+, we found that binding of syt to PS and t-SNAREs can be dissociated from activation of fusion, uncovering posteffector-binding functions for syt. Our data demonstrate that different syt isoforms are specialized to sense different ranges of divalent cations and that PS is an essential effector of Ca2+.syt action.

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 gene content of the sequenced genomes

Background

Synaptotagmins exist as a large gene family in mammals. There is much interest in the function of certain family members which act crucially in the regulated synaptic vesicle exocytosis required for efficient neurotransmission. Knowledge of the functions of other family members is relatively poor and the presence of Synaptotagmin genes in plants indicates a role for the family as a whole which is wider than neurotransmission. Identification of the Synaptotagmin genes within completely sequenced genomes can provide the entire Synaptotagmin gene complement of each sequenced organism. Defining the detailed structures of all the Synaptotagmin genes and their encoded products can provide a useful resource for functional studies and a deeper understanding of the evolution of the gene family. The current rapid increase in the number of sequenced genomes from different branches of the tree of life, together with the public deposition of evolutionarily diverse transcript sequences make such studies worthwhile.

Results

I have compiled a detailed list of the Synaptotagmin genes of Caenorhabditis, Anopheles, Drosophila, Ciona, Danio, Fugu, Mus, Homo, Arabidopsis and Oryza by examining genomic and transcript sequences from public sequence databases together with some transcript sequences obtained by cDNA library screening and RT-PCR. I have compared all of the genes and investigated the relationship between plant Synaptotagmins and their non-Synaptotagmin counterparts.

Conclusions

I have identified and compared 98 Synaptotagmin genes from 10 sequenced genomes. Detailed comparison of transcript sequences reveals abundant and complex variation in Synaptotagmin gene expression and indicates the presence of Synaptotagmin genes in all animals and land plants. Amino acid sequence comparisons indicate patterns of conservation and diversity in function. Phylogenetic analysis shows the origin of Synaptotagmins in multicellular eukaryotes and their great diversification in animals. Synaptotagmins occur in land plants and animals in combinations of 4–16 in different species. The detailed delineation of the Synaptotagmin genes presented here, will allow easier identification of Synaptotagmins in future. Since the functional roles of many of these genes are unknown, this gene collection provides a useful resource for future studies.

Synaptotagmin V and IX isoforms control Ca2+ -dependent insulin exocytosis

Synaptotagmin (Syt) is involved in Ca2+ -regulated secretion and has been suggested to serve as a general Ca2+ sensor on the membrane of secretory vesicles in neuronal cells. Insulin exocytosis from the pancreatic beta-cell is an example of a Ca2+ -dependent secretory process. Previous studies have yielded conflicting results as to which Syt isoform is present on the secretory granules in the native beta-cell. Here we show by western blotting and RT-PCR analysis, the presence of both Syt V and Syt IX in rat pancreatic islets and in the clonal beta-cell line INS-1E. The subcellular distribution of the two Syt isoforms was assessed by confocal microscopy and by sedimentation in a continuous sucrose density gradient in INS-1E cells. These experiments show that both proteins colocalize with insulin-containing secretory granules but are absent from synaptic-like microvesicles. Further immunofluorescence studies performed in primary pancreatic endocrine cells revealed that Syt V is present in glucagon-secreting alpha-cells, whereas Syt IX is associated with insulin granules in beta-cells. Transient overexpression of Syt V and Syt IX did not alter exocytosis in INS-1E cells. Finally, reduction of the expression of both Syt isoforms by RNA interference did not change basal secretion. Remarkably, hormone release in response to glucose was selectively and strongly reduced, indicating that Syt V and Syt IX are directly involved in the Ca2+ -dependent stimulation of exocytosis.

Synaptotagmin IX, a possible linker between the perinuclear endocytic recycling compartment and the microtubules

The pericentriolar endocytic recycling compartment (ERC) is involved in receptor and lipid recycling as well as in the delivery of internalized cargo from early endosomes to the trans Golgi network (TGN). We show that synaptotagmin (Syt) IX, a member of the Syt family of proteins, localizes to the ERC and is required for export from the ERC to the cell surface. We demonstrate that rat basophilic leukemia (RBL-2H3) mast cells endogenously express Syt IX mRNA and protein. Localization studies employing fractionation on linear sucrose gradients combined with confocal microscopy by indirect immunofluorescence or stable expression of a Syt IX-green fluorescent fusion protein demonstrate that Syt IX colocalizes with internalized transferrin (Tfn) and with Rab 11 at the perinuclear ERC. Syt IX also colocalizes with tubulin at the microtubules organizing center (MTOC) and remains associated with tubulin clusters formed in taxol-treated cells. Moreover, Syt IX coimmunoprecipitates with tubulin from intact RBL cells, and chimeric fusion proteins comprising either the C2A or the C2B domain of Syt IX are able to pull down tubulin from RBL cell lysates. To study the functional role of Syt IX, we have stably transfected RBL cells with Syt IX sense or antisense cDNA and monitored the routes of Tfn internalization and recycling in cells that overexpress (RBL-Syt IX+) or display substantially reduced (<90%) levels of Syt IX (RBL-Syt IX-). In these cells, Tfn binding and internalization into early endosomes and the ERC are unaltered. However, recycling from the ERC to the cell surface is significantly slowed down in the RBL-Syt IX- cells. These results therefore indicate that Syt IX is involved in regulating transport from the ERC to the cell surface, and suggest that it may play a role in linking vesicles that exit the ERC with the microtubules network.

Synaptotagmin I increases the probability of vesicle fusion at low [Ca2+] in pituitary cells

Synaptotagmin I (Syt I), a low-affinity Ca(2+)-binding protein, is thought to serve as the Ca(2+) sensor in the release of neurotransmitter. However, functional studies on the calyx of Held synapse revealed that the rapid release of neurotransmitter requires only approximately micromolar [Ca(2+)], suggesting that Syt I may play a more complex role in determining the high-affinity Ca(2+) dependence of exocytosis. Here we tested this hypothesis by studying pituitary cells, which possess high- and low-affinity Ca(2+)-dependent exocytic pathways and express Syt I. Using patch-clamp capacitance measurements to monitor secretion and the acute antisense deletion of Syt I from differentiated cells, we have shown that the rapid and the most Ca(2+)-sensitive pathway of exocytosis in rat melanotrophs requires Syt I. Furthermore, stimulation of the Ca(2+)-dependent exocytosis by cytosol dialysis with solutions containing 1 microM [Ca(2+)] was completely abolished in the absence of Syt I. Similar results were obtained by the preinjection of antibodies against the CAPS (Ca(2+)-dependent activator protein for secretion) protein. These results indicate that synaptotagmin I and CAPS proteins increase the probability of vesicle fusion at low cytosolic [Ca(2+)].

Cytochemical and molecular biological aspects of the pituitary and pituitary adenomas--cell differentiation and transcription factors

The anterior pituitary is composed of several cell types, each responsible for the production of specific hormones. Each hormone secreting cells is defined by the activation of its respective hormone genes in a temporally and spatially regulated manner. Recent development in cytochemistry and molecular biology have provided various aspects of human pituitary adenomas, i.e., functional differentiation and classification. The molecular factors that determine hormone production have now been identified as transcription factors. Many novel transcription factors that play a role in anterior pituitary development are implicated. In this review, we focus on the transcriptional factors roles on functional differentiation of the pituitary cells and adenomas and the contribution of cytochemistry and recent development in molecular biological techniques.

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.

Genomic analysis of synaptotagmin genes

I used TBLASTn to probe DNA sequence databases with a consensus peptide sequence corresponding to the most highly conserved region of the rodent synaptotagmin (Syt) gene family, which is within the C2B domain. I found human homologues for all known rodent genes, and found six further human genomic loci which encode potential family members. I found eight potential family members in Caenorhabditis elegans, six in Drosophila melanogaster, and four in Arabidopsis thaliana. The C. elegans Syt1 homologue uniquely encodes two alternative C2B exons, one or the other of which is expressed at a time. Comparison of the genomic structures of the Syt genes makes clear the different phylogenies of the different subgroups. Knowledge of the genomic structures will aid the systematic investigation of alternative splicing in Syt genes.

Synaptotagmin VII as a plasma membrane Ca(2+) sensor in exocytosis

Synaptotagmins I and II are Ca(2+) binding proteins of synaptic vesicles essential for fast Ca(2+)-triggered neurotransmitter release. However, central synapses and neuroendocrine cells lacking these synaptotagmins still exhibit Ca(2+)-evoked exocytosis. We now propose that synaptotagmin VII functions as a plasma membrane Ca(2+) sensor in synaptic exocytosis complementary to vesicular synaptotagmins. We show that alternatively spliced forms of synaptotagmin VII are expressed in a developmentally regulated pattern in brain and are concentrated in presynaptic active zones of central synapses. In neuroendocrine PC12 cells, the C(2)A and C(2)B domains of synaptotagmin VII are potent inhibitors of Ca(2+)-dependent exocytosis, but only when they bind Ca(2+). Our data suggest that in synaptic vesicle exocytosis, distinct synaptotagmins function as independent Ca(2+) sensors on the two fusion partners, the plasma membrane (synaptotagmin VII) versus synaptic vesicles (synaptotagmins I and II).

Synaptotagmin 13: structure and expression of a novel synaptotagmin

Synaptotagmins represent a family of putative vesicular trafficking proteins. With synaptotagmin 13, we have now identified a novel synaptotagmin, making this one of the largest families of trafficking proteins. Similar to synaptotagmins 3, 4, 6, 7, 9, and 11, synaptotagmin 13 is expressed at highest levels in brain but is also detectable at lower levels in non-neuronal tissues. Synaptotagmin 13 is composed of the canonical domains of synaptotagmins that include an N-terminal transmembrane region and two C-terminal cytoplasmic C2-domains (C2A- and C2B-domain) and a connecting sequence between the transmembrane region and the C2-domains. Different from most other synaptotagmins, however, synaptotagmin 13 does not have an N-terminal sequence preceding the transmembrane region, and features an unusually long connecting sequence that is proline-rich. Furthermore, the C2-domains of synaptotagmin are degenerate and lack almost all of the residues involved in Ca2+ binding, suggesting that synaptotagmin 13 is not a Ca2+-binding protein unlike most other synaptotagmins. Our data demonstrate that synaptotagmins represent a larger and more complex gene family than previously envisioned.

Partial transcriptome of the developing pituitary gland

The anterior lobe of the pituitary gland is composed of five hormone-producing cell types and develops from Rathke's pouch, an invagination of oral ectoderm. In mice, rapid cell proliferation occurs in the pouch from embryonic day 12.5 (e12.5) to e14.5, preceding the appearance of most hormone transcripts. Cell-type-specific commitment probably occurs prior to e14.5, but cell differentiation can be demonstrated only by detection of hormone transcripts. Although several transcription factors critical for pouch expansion are known, few of their target genes have been identified. To identify putative transcription factor target genes and cell-type-specific markers, we used differential display PCR analysis of RNA prepared from e12.5 and e14.5 Rathke's pouches. We present an expression profile of the developing pituitary gland including 83 transcripts, 40% of which are novel. The tissue distribution, cell specificity, and developmental regulation were determined for a subset of the transcripts.

Synaptotagmins in membrane traffic: which vesicles do the tagmins tag?

The aim of this review is to give a broad picture of what is actually known about the synaptotagmin family. Synaptotagmin I is an abundant synaptic vesicle and secretory granule protein in neurons and endocrine cells which plays a key role in Ca(2+)-induced exocytosis. It belongs to the large family of C2 domain-proteins as it contains two internal repeats that have homology to the C2 domain of protein kinase C. Eleven synaptotagmin genes have been described in rat and mouse. Except for synaptotagmin I, and by analogy synaptotagmin II, the functions of these proteins are unknown. In this review we focus on data obtained on the various isoforms without exhaustively discussing the role of synaptotagmin I in neurotransmission. Numerous in vitro interactions of synaptotagmin I with key components of the exocytosis-endocytosis machinery have been reported. Additional data concerning the other synaptotagmins are now becoming available and are reviewed here. Only interactions which have been described for several synaptotagmins, are mentioned. It is unlikely that a single isoform displays all of these potential interactions in vivo and probably the subcellular distribution of the protein will favor some of them and preclude others. Therefore, to discuss the putative role of the various synaptotagmins we have examined in detail published data concerning their localization.

Targeting of synaptotagmin to neurite terminals in neuronally differentiated PC12 cells

We have investigated structural elements that determine the accumulation of synaptotagmin, a major synaptic vesicle protein, in neurite terminals of neuronally differentiated neuroendocrine pheochromocytoma PC12 cells. We performed extensive deletion and point mutagenesis of rat synaptotagmin II, expressed mutant proteins in PC12 cells differentiated by nerve growth factor (NGF) and monitored their intracellular distribution by immunofluorescence. We found a structural element located at the carboxy-terminal domain of the synaptotagmin molecule, which is necessary for its accumulation at the terminal. Using alanine-scanning mutagenesis, we have identified two amino acids in this element, tryptophan W405 and leucine L408, that are critical for correct targeting of synaptotagmin II to neurite terminals. Changing either one of them to alanine prevents the accumulation of the protein at the terminals. These amino acids are evolutionarily conserved throughout the entire synaptotagmin family and also among synaptotagmin-related proteins, suggesting that different synaptotagmins may have similar mechanisms of targeting to neuronal cell terminals.

Contributions of immunohistochemistry and in situ hybridization to the functional analysis of pituitary adenomas

Immunohistochemistry (IHC) and recently in situ hybridization (ISH) have elucidated various aspects of human pituitary adenomas, i.e., functional differentiation and classification, transcription factors and mechanism of hormone production, regulation of hormone secretion, and processing of prohormones. Recently, the use of tyramide (catalyzed signal amplification; TSA or CSA) and RT-PCR has been effective for detection of trivial amount of proteins (peptides) and mRNA, respectively. Immunomolecular histochemistry is expected to further clarify the function and biology of human pituitary adenomas.

Specificity of Ca2+-dependent protein interactions mediated by the C2A domains of synaptotagmins

Synaptotagmins represent a family of neuronal proteins thought to function in membrane traffic. The best characterized synaptotagmin, synaptotagmin I, is essential for fast Ca2+-dependent synaptic vesicle exocytosis, indicating a role in the Ca2+ triggering of membrane fusion. Synaptotagmins contain two C2 domains, the C2A and C2B domains, which bind Ca2+ and may mediate their functions by binding to specific targets. For synaptotagmin I, several putative targets have been identified, including the SNARE proteins syntaxin and SNAP-25. However, it is unclear which of the many binding proteins are physiologically relevant. Furthermore, more than 10 highly homologous synaptotagmins are expressed in brain, but it is unknown if they execute similar binding reactions. To address these questions, we have performed a systematic, unbiased study of proteins which bind to the C2A domains of synaptotagmins I-VII. Although the various C2A domains exhibit similar binding activities for phospholipids and syntaxin, we found that they differ greatly in their protein binding patterns. Surprisingly, none of the previously characterized binding proteins for synaptotagmin I are among the major interacting proteins identified. Instead, several proteins that were not known to interact with synaptotagmin I were bound tightly and stoichiometrically, most prominently the NSF homologue VCP, which is thought to be involved in membrane fusion, and an unknown protein of 40 kDa. Point mutations in the Ca2+ binding loops of the C2A domain revealed that the interactions of these proteins with synaptotagmin I were highly specific. Furthermore, a synaptotagmin I/VCP complex could be immunoprecipitated from brain homogenates in a Ca2+-dependent manner, and GST-VCP fusion proteins efficiently captured synaptotagmin I from brain. However, when we investigated the tissue distribution of VCP, we found that, different from synaptic proteins, VCP was not enriched in brain and exhibited no developmental increase paralleling synaptogenesis. Moreover, binding of VCP, which is an ATPase, to synaptotagmin I was inhibited by both ATP and ADP, indicating that the native, nucleotide-occupied state of VCP does not bind to synaptotagmin. Together our findings suggest that the C2A-domains of different synaptotagmins, despite their homology, exhibit a high degree of specificity in their protein interactions. This is direct evidence for diverse roles of the various synaptotagmins in brain, consistent with their differential subcellular localizations. Furthermore, our results indicate that traditional approaches, such as affinity chromatography and immunoprecipitations, are useful tools to evaluate the overall spectrum of binding activity for a protein but are not sufficient to estimate physiological relevance.

Synaptotagmin IV is present at the Golgi and distal parts of neurites

Synaptotagmin IV (SytIV) is an immediate early gene induced by membrane depolarization in PC12 cells and in rat brain. However, little is known about the function of SytIV or the functional relationship between SytIV and SytI, because SytIV has yet to be localized. Here we show that SytIV was localized at the Golgi and distal part of neurites in nerve growth factor-differentiated PC12 cells and cultured hippocampal neurons by immunocytochemistry using an isoform-specific antibody (anti-SytIV). These SytIV signals were not colocalized well with SytI signals. Upon membrane depolarization, SytIV signals were increased at both the Golgi and distal part of neurites within several hours in both types of cells. We further show that the increase of SytIV protein levels results from protein kinase A-dependent gene up-regulation. In hippocampal neurons, SytIV was developmentally regulated. These results suggest that SytIV may play a role at the Golgi and tips of neurites during development and synaptic plasticity.

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.

The subcellular localizations of atypical synaptotagmins III and VI. Synaptotagmin III is enriched in synapses and synaptic plasma membranes but not in synaptic vesicles

Multiple synaptotagmins are expressed in brain, but only synaptotagmins I and II have known functions in fast, synchronous Ca2+-triggered neurotransmitter release. Synaptotagmin III was proposed to regulate other aspects of synaptic vesicle exocytosis, particularly its slow component. Such a function predicts that synaptotagmin III should be an obligatory synaptic vesicle protein, as would also be anticipated from its high homology to synaptotagmins I and II. To test this hypothesis, we studied the distribution, developmental expression, and localization of synaptotagmin III and its closest homolog, synaptotagmin VI. We find that synaptotagmins III and VI are present in all brain regions in heterogeneous distributions and that their levels increase during development in parallel with synaptogenesis. Furthermore, we show by immunocytochemistry that synaptotagmin III is concentrated in synapses, as expected. Surprisingly, however, we observed that synaptotagmin III is highly enriched in synaptic plasma membranes but not in synaptic vesicles. Synaptotagmin VI was also found to be relatively excluded from synaptic vesicles. Our data suggest that synaptotagmins III and VI perform roles in neurons that are not linked to synaptic vesicle exocytosis but to other Ca2+-related nerve terminal events, indicating that the functions of synaptotagmins are more diverse than originally thought.

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.

SNARE complex at the ribbon synapses of cochlear hair cells: analysis of synaptic vesicle- and synaptic membrane-associated proteins

Neurotransmitters are released via exocytosis of synaptic vesicles involving a fusion complex consisting of a set of highly conserved proteins, which form a multiprotein complex resulting in the docking of synaptic vesicles at the site of release. There are three major differences between cochlear hair cell synapses and CNS synapses: (i) hair cells have a specialized structure, the synaptic ribbon, to which synaptic vesicles are attached; (ii) hair cells can maintain high and sustained release of neurotransmitter; and (iii) hair cells lack synaptophysin and synapsin. These differences suggest that an unconventional mechanism of neurotransmitter release may be involved at ribbon synapses. In this study we used different and complementary approaches to determine whether or not ribbon-containing hair cells of the cochlea express any component of the core fusion complex found in conventional synapses. Syntaxin 1, the synaptic membrane synaptosome-associated protein (SNAP)-25 and vesicle-associated membrane protein (VAMP or synaptobrevin) were found to be present in the organ of Corti of both rat and guinea-pig, as shown by reverse transcription polymerase chain reaction and Western blotting. In situ hybridization and immunocytochemistry showed mRNA and protein expression, respectively, in both inner and outer hair cells. Synaptotagmins I and II, generally considered to play major roles in neurotransmitter release at central synapses, were not detected in the organ of Corti.

A sequential view of neurotransmitter release

Only a few years ago it was thought that a single Ca2+-dependent membrane binding protein might control regulated exocytosis, but it is now clear that the coordinated actions of a large number of proteins and lipids are required for the precise targeting, docking and fusion of vesicles to the plasma membrane. Thinking was focused in 1993 by the SNARE (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor) hypothesis, which proposed that certain synaptic vesicle membrane proteins combined specifically with particular proteins in the synaptic membrane active zone to form a complex that interacted with synaptoplasmic proteins, ATP and calcium ions to fuse the vesicles with the presynaptic membrane. Much research that has followed has verified the basic predictions of the SNARE hypothesis. However, recent research indicates that SNARE proteins are more widely distributed in secretory systems and that the sequence in which the proteins function may not occur as was originally proposed. That has recently produced a period of deconstruction and reinterpretation of the SNARE hypothesis. Our present state of knowledge is briefly summarized in this review.

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.

Acute administration of cocaine, but not amphetamine, increases the level of synaptotagmin IV mRNA in the dorsal striatum of rat

Synaptotagmin IV (Syt IV) is an inducible member of a multi-gene family of synaptic vesicle proteins that participate in Ca2+-dependent and Ca2+-independent interactions during membrane trafficking. We have examined the pattern of expression of Syt IV mRNA following the administration of cocaine and amphetamine. A single acute dose of cocaine, but not amphetamine, resulted in a transient increase, as determined by in situ hybridization, in the steady-state level of Syt IV mRNA in the dorsal striatum of rats 1 h after the administration of the drug. No change in the hybridization pattern of the Syt IV-specific probe to other regions of the rat brain were observed following cocaine or amphetamine administration at the time points examined (1, 3, 6, 12 and 24 h). The pattern of synaptotagmin I-(Syt I) specific hybridization remained constant, relative to controls, for both the cocaine- and amphetamine-treated animals. Northern hybridization analysis of mRNA isolated from striatal tissue using oligonucleotide probes specific to Syt I and Syt IV demonstrated that the probes hybridized exclusively to transcripts of the sizes previously reported for these two synaptotagmins and confirmed that the relative level of Syt IV to Syt I mRNA increased following the administration of cocaine but not amphetamine. These results indicate that these drugs have different effects on altering the levels of Syt IV mRNA. This work, in conjunction with earlier work that demonstrated that cocaine and amphetamine have different effects on the expression of immediate early genes such as c-Fos, supports the hypothesis that these psychotropic agents evoke different patterns of gene expression which may lead to alteration in synaptic efficacy.

Aberrant intracellular targeting and cell cycle-dependent phosphorylation of emerin contribute to the Emery-Dreifuss muscular dystrophy phenotype

The product of the X-linked Emery-Dreifuss muscular dystrophy gene is a protein called emerin, which is localized to the nuclear membrane. We have expressed full-length recombinant human emerin in an in vitro coupled reticulocyte system; it has a molecular mass of 34 kDa, inserts into microsomes in a type II orientation, and does not exhibit any N-linked glycosylation or cleavage event. Affinity-purified human emerin antiserum cross-reacts with the in vitro-expressed emerin and with a 34 kDa band present in a wide range of human tissue samples. Expression and subcellular distribution of emerin were studied in lymphoblastoid cell lines established from four patients with Emery-Dreifuss muscular dystrophy containing different mutations in the emerin gene. Emerin protein was detected in two of these patients by immunoblotting. In striking contrast to wild-type emerin, which was localized to the nuclear fraction and was insoluble in non-ionic detergents and high salt, emerin from these two patients exhibited a more random subcellular localization and increased solubility. On the basis of the mutations present in these patients, it would appear that emerin possesses two non-overlapping nuclear envelope targeting sequences. We have also demonstrated that emerin can occur in four different phosphorylated forms, three of which appear to be associated with the cell cycle. The mutant forms of emerin taken from the two patients exhibited aberrant cell cycle-dependent phosphorylated forms. This data suggests that for emerin to function normally it must be correctly localized, retained at the nuclear membrane and phosphorylated by cell cycle-mediated events.

The first C2 domain of synaptotagmin is required for exocytosis of insulin from pancreatic beta-cells: action of synaptotagmin at low micromolar calcium

The Ca2+- and phospholipid-binding protein synaptotagmin is involved in neuroexocytosis. Its precise role and Ca2+-affinity in vivo are unclear. We investigated its putative function in insulin secretion which is maximally stimulated by 10 microM cytosolic free Ca2+. The well-characterized synaptotagmin isoforms I and II are present in pancreatic beta-cell lines RINm5F, INS-1 and HIT-T15 as shown by Northern and Western blots. Subcellular fractionation and confocal microscopy revealed their presence mainly on insulin-containing secretory granules whereas only minor amounts were found on synaptic vesicle-like microvesicles. Antibodies or Fab-fragments directed against the Ca2+-dependent phospholipid binding site of the first C2 domain of synaptotagmin I or II inhibited Ca2+-stimulated, but not GTPgammaS-induced exocytosis from streptolysin-O-permeabilized INS-1 and HIT-T15 cells. Transient expression of wild-type synaptotagmin II did not alter exocytosis in HIT-T15 cells. However, mutations in the Ca2+-dependent phospholipid binding site of the first C2 domain (Delta180-183, D231S) again inhibited only Ca2+-, but not GTPgammaS-evoked exocytosis. In contrast, mutations in the IP4-binding sites of the second C2 domain (Delta325-341; K327,328, 332Q) did not alter exocytosis. Synaptotagmin II mutated in both C2 domains (Delta180-183/K327,328,332Q) induced greater inhibition than mutant Delta180-183, suggesting a discrete requirement for the second C2 domain. Thus, synaptotagmin isoforms regulate exocytotic events occurring at low micromolar Ca2+.

Direct interaction of the calcium sensor protein synaptotagmin I with a cytoplasmic domain of the alpha1A subunit of the P/Q-type calcium channel

Synaptotagmins are synaptic vesicle proteins containing two calcium-binding C2 domains which are involved in coupling calcium influx through voltage-gated channels to vesicle fusion and exocytosis of neurotransmitters. The interaction of synaptotagmins with native P/Q-type calcium channels was studied in solubilized synaptosomes from rat cerebellum. Antibodies against synaptotagmins I and II, but not IV co-immunoprecipitated [125I]omega-conotoxin MVIIC-labelled calcium channels. Direct interactions were studied between in vitro-translated [35S]synaptotagmin I and fusion proteins containing cytoplasmic loops of the alpha1A subunit (BI isoform). Gel overlay revealed the association of synaptotagmin I with a single region (residues 780-969) located in the intracellular loop connecting homologous domains II and III. Saturable calcium-independent binding occurred with equilibrium dissociation constants of 70 nM and 340 nM at 4 degrees C and pH 7.4, and association was blocked by addition of excess recombinant synaptotagmin I. Direct synaptotagmin binding to the pore-forming subunit of the P/Q-type channel may optimally locate the calcium-binding sites that initiate exocytosis within a zone of voltage-gated calcium entry.

The BASP1 family of myristoylated proteins abundant in axonal termini. Primary structure analysis and physico-chemical properties

Proteins BASP1 and GAP-43/B-50, which are abundant in nerve endings, show a number of similar physico-chemical properties. Nevertheless, they belong to different protein families. In this work, complete amino acid sequences of bovine BASP1 and human BASP1 were established. They proved to be very similar to the sequences of rat brain protein NAP-22 and chicken brain protein CAP-23. Relatively to human BASP1 its bovine, rat and chicken analogues show 80%, 70% and 45% sequence identity respectively, confirming their membership of a definite protein family (BASP1 family). All members of BASP1 family contain several 'good' PEST sequences characteristic for short-living proteins. Conservation of PEST sequences in BASP1 of different species points to their significance for BASP1 functions. In contrast to GAP-43/B-50 showing high immunological cross-reactivity between the proteins belonging to different species of mammals, immunological properties of BASP1 are species specific. BASP1 shows both high hydrophilicity and some properties characteristic for hydrophobic proteins. These properties are caused by N-terminal myristoylation of BASP1 molecules. Unlike GAP-43/B-50, BASP1 is present in high amounts also in some non-nervous tissues: testis, kidney and lymphoid organs (spleen, thymus). So far examined characteristics, including myristoylation, peptide maps and detected by isoelectrofocusing microheterogeneity, proved to be the same for BASP1 samples isolated from both brain and non-nervous tissues. Therefore, in spite of different physiological consequences, biochemical functions of BASP1 must also be similar in different tissues.

The evolutionary pressure to inactivate. A subclass of synaptotagmins with an amino acid substitution that abolishes Ca2+ binding

Synaptotagmin I is a Ca2+-binding protein of synaptic vesicles that serves as a Ca2+ sensor for neurotransmitter release and was the first member found of a large family of trafficking proteins. We have now identified a novel synaptotagmin, synaptotagmin XI, that is highly expressed in brain and at lower levels in other tissues. Like other synaptotagmins, synaptotagmin XI has a single transmembrane region and two cytoplasmic C2-domains but is most closely related to synaptotagmin IV with which it forms a new subclass of synaptotagmins. The first C2-domain of synaptotagmin I (the C2A-domain) binds phospholipids as a function of Ca2+ and contains a Ca2+-binding site, the C2-motif, that binds at least two Ca2+ ions via five aspartate residues and is conserved in most C2-domains (Shao, X., Davletov, B., Sutton, B., Südhof, T. C., Rizo, J. R. (1996) Science 273, 248-253). In the C2A-domains of synaptotagmins IV and XI, however, one of the five Ca2+-binding aspartates in the C2-motif is substituted for a serine, suggesting that these C2-domains do not bind Ca2+. To test this, we produced recombinant C2A-domains from synaptotagmins IV and XI with either wild type serine or mutant aspartate in the C2-motif. Circular dichroism showed that Ca2+ stabilizes both mutant but not wild type C2-domains against temperature-induced denaturation, indicating that the mutations restore Ca2+-binding to the wild type C2-domains. Furthermore, wild type C2A-domains of synaptotagmins IV and XI exhibited no Ca2+-dependent phospholipid binding, whereas mutant C2A-domains bound phospholipids as a function of Ca2+ similarly to wild type synaptotagmin I. These experiments suggest that a class of synaptotagmins was selected during evolution in which the Ca2+-binding site of the C2A-domain was inactivated by a single point mutation. Thus, synaptotagmins must have Ca2+-independent functions as well as Ca2+-dependent functions that are selectively maintained in distinct members of this gene family.

Human synaptotagmin V (SYT5): sequence, genomic structure, and chromosomal location

We have determined the sequence, genomic structure, and chromosomal location of the human synaptotagmin V (SYTV) gene. The human SYTV gene encodes a 386-amino-acid product which is 91% identical to rat Syt V. The human SYTV open reading frame is interrupted by seven introns which can be alternatively spliced. Human SYTV was found to lie very close to SYTIII on chromosome 19q13.4 by PCR analysis of somatic cell hybrid DNA and by DNA hybridization to arrayed cosmids of the chromosome 19 metric physical map. This provides the first report of linked synaptotagmin genes.

Regulation by bivalent cations of phospholipid binding to the C2A domain of synaptotagmin III

Synaptotagmins are Ca2+-and phospholipid-binding proteins of synaptic vesicles that might function as Ca2+ receptors for neurotransmitter release via their first C2 (C2A) domain. Here we describe the effect of Mg2+ on phospholipid binding to the C2A domains of multiple synaptotagmins (II-VI), and demonstrate that only synaptotagmin III can bind negatively charged phospholipids [phosphatidylserine (PS) and phosphatidylinositol] in a Mg2+-dependent manner. The Mg2+-dependent interaction with PS was found to have an EC50 of approx. 30 microM Mg2+, which is comparable to that of Sr2+ and Ba2+ (EC50 values of approx. 10 microM). This binding property of the C2A domain is specific to synaptotagmin III, because none of the C2A domains of other proteins, such as rabphilin 3A, Doc2alpha, Doc2beta or Gap1(m), showed phospholipid binding activity in the presence of 1 mM Mg2+. Our results suggest that synaptotagmin III is involved in presynaptic functions different from those of synaptotagmins I and II.

A novel seizure-induced synaptotagmin gene identified by differential display

Systemic administration of kainic acid, a cyclic analogue of glutamate, produces many of the clinical features of human temporal lobe epilepsy and status epilepticus in rats, including the induction of motor convulsions and the degeneration of neurons in the hippocampus and piriform cortex. Differential display PCR was used to identify mRNAs that are differentially expressed between degenerating and nondegenerating tissues in the brain after kainic acid-induced seizure activity. A novel cDNA fragment expressed in the degenerating hippocampus and piriform cortex, but not in the nondegenerating parietal cortex, was identified, cloned, and sequenced. This novel cDNA fragment identified a new member of the synaptotagmin gene family that is rapidly and transiently induced in response to seizure activity. Differential expression of this synaptotagmin gene, syt X, was confirmed by Northern blot analysis and in situ hybridization. This novel, inducible synaptotagmin gene may provide a direct link between seizure-induced neuronal gene expression and subsequent modulation of synaptic structure and function.

Developmental regulation of synaptotagmin I, II, III, and IV mRNAs in the rat CNS

Synaptotagmin I is an abundant synaptic vesicle protein that has an essential function in mediating Ca2+-triggered neurotransmitter release. We have analyzed the distribution of four neural synaptotagmin isoforms during postnatal development of the rat CNS by in situ hybridization. Synaptotagmin I, II, III, and IV genes have distinct patterns of spatiotemporal expression except in cerebellum granule cells, where the four transcripts were detected during the formation of parallel fiber/Purkinje cell synapses. Throughout development synaptotagmin I mRNAs were widely expressed in brain, whereas synaptotagmin II transcripts were predominant in spinal cord. At all stages synaptotagmin III mRNAs were expressed uniformly in most neurons examined, although at a low level. Synaptotagmin I, II, and III gene expressions mainly increased during development and persisted in adulthood, mirroring neuronal differentiation. Conversely, synaptotagmin IV transcripts were predominant during perinatal development in a heterogeneous population of neurons and subsequently were expressed uniformly at a low level. Intense labeling was observed in the hippocampal CA3 field and in the subiculum, but not in the CA1 field, of the newborn rat. In cerebral cortex, lamina-specific labeling was detected with a high expression in cell layer V. Only a small number of Purkinje cell clusters were labeled in the flocculus and paraflocculus of the cerebellum. Heterogeneous sets of neurons expressing synaptotagmin IV gene also were observed in spinal cord. We thus speculate that synaptotagmin IV may a play a role in the development of the mammalian nervous system.

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

Synaptotagmin that contains two repeats of C2 regulatory domains is considered to be involved in neurotransmitter release. To reveal the roles of synaptotagmin in the regulation of exocytosis, we examined the effects of antibodies against C2A and C2B domains on Ca2+-evoked catecholamine (CA) release from digitonin-permeabilized adrenal chromaffin cells, resolving the Ca2+-evoked release into ATP-dependent priming and ATP-independent Ca2+-triggered steps. Anti-C2A antibody clearly reduced the ATP-independent release, suggesting that the C2A domain directly facilitate or promote Ca2+-triggered step, vesicular fusion. In contrast, anti-C2B antibody did not affect Ca2+-evoked release by itself, but significantly increased the spontaneous Ca2+-independent release. In addition, inositol high-polyphosphate series (IHPS) that bind the C2B domain inhibited both the ATP-independent Ca2+-evoked release and the spontaneous release in a dose-dependent manner. The inhibition by IHPS was totally reversed by anti-C2B antibody and significantly reversed by high concentration of Ca2+. These results suggest that IHPS binding to C2B domain arrests membrane fusion by presumably preventing interaction of synaptotagmin with phospholipids or with proteins of plasma membrane. Thus, IHPS binding to the C2B domain might keep the docked or primed vesicles away from spontaneous fusion at resting level of intracellular Ca2+. Binding of the increased intracellular Ca2+ to the C2A domain may facilitate or trigger the vesicular fusion by releasing this suppression by IHPS.