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Rahman MM, Pathak A, Schueler KL, Alsharif H, Michl A, Alexander J, Kim JA, Bhatnagar S. Genetic ablation of synaptotagmin-9 alters tomosyn-1 function to increase insulin secretion from pancreatic β-cells improving glucose clearance. FASEB J 2023; 37:e23075. [PMID: 37432648 PMCID: PMC10348599 DOI: 10.1096/fj.202300291rr] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/18/2023] [Revised: 06/19/2023] [Accepted: 06/22/2023] [Indexed: 07/12/2023]
Abstract
Stimulus-coupled insulin secretion from the pancreatic islet β-cells involves the fusion of insulin granules to the plasma membrane (PM) via SNARE complex formation-a cellular process key for maintaining whole-body glucose homeostasis. Less is known about the role of endogenous inhibitors of SNARE complexes in insulin secretion. We show that an insulin granule protein synaptotagmin-9 (Syt9) deletion in mice increased glucose clearance and plasma insulin levels without affecting insulin action compared to the control mice. Upon glucose stimulation, increased biphasic and static insulin secretion were observed from ex vivo islets due to Syt9 loss. Syt9 colocalizes and binds with tomosyn-1 and the PM syntaxin-1A (Stx1A); Stx1A is required for forming SNARE complexes. Syt9 knockdown reduced tomosyn-1 protein abundance via proteasomal degradation and binding of tomosyn-1 to Stx1A. Furthermore, Stx1A-SNARE complex formation was increased, implicating Syt9-tomosyn-1-Stx1A complex is inhibitory in insulin secretion. Rescuing tomosyn-1 blocked the Syt9-knockdown-mediated increases in insulin secretion. This shows that the inhibitory effects of Syt9 on insulin secretion are mediated by tomosyn-1. We report a molecular mechanism by which β-cells modulate their secretory capacity rendering insulin granules nonfusogenic by forming the Syt9-tomosyn-1-Stx1A complex. Altogether, Syt9 loss in β-cells decreases tomosyn-1 protein abundance, increasing the formation of Stx1A-SNARE complexes, insulin secretion, and glucose clearance. These outcomes differ from the previously published work that identified Syt9 has either a positive or no effect of Syt9 on insulin secretion. Future work using β-cell-specific deletion of Syt9 mice is key for establishing the role of Syt9 in insulin secretion.
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Affiliation(s)
- Md Mostafizur Rahman
- Heersink School of Medicine, Division of Endocrinology, Diabetes, & Metabolism, Comprehensive Diabetes Center, University of Alabama, Birmingham, AL, 35294
| | - Asmita Pathak
- Heersink School of Medicine, Division of Endocrinology, Diabetes, & Metabolism, Comprehensive Diabetes Center, University of Alabama, Birmingham, AL, 35294
| | | | - Haifa Alsharif
- Heersink School of Medicine, Division of Endocrinology, Diabetes, & Metabolism, Comprehensive Diabetes Center, University of Alabama, Birmingham, AL, 35294
| | - Ava Michl
- Heersink School of Medicine, Division of Endocrinology, Diabetes, & Metabolism, Comprehensive Diabetes Center, University of Alabama, Birmingham, AL, 35294
| | - Justin Alexander
- Heersink School of Medicine, Division of Endocrinology, Diabetes, & Metabolism, Comprehensive Diabetes Center, University of Alabama, Birmingham, AL, 35294
| | - Jeong-A Kim
- Heersink School of Medicine, Division of Endocrinology, Diabetes, & Metabolism, Comprehensive Diabetes Center, University of Alabama, Birmingham, AL, 35294
| | - Sushant Bhatnagar
- Heersink School of Medicine, Division of Endocrinology, Diabetes, & Metabolism, Comprehensive Diabetes Center, University of Alabama, Birmingham, AL, 35294
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Abstract
We discovered a novel alternatively spliced form of synaptotagmin I (Syt I). This splicing event is conserved over evolution and, in Aplysia, results in a two amino acid insert in the juxtamembrane domain of Syt I (Syt IVQ). Both Syt I and Syt IVQ are localized to synaptic vesicles; however, we also observed punctae that contained one or the other spliced products. Both Syt I and Syt IVQ are phosphorylated at the adjacent PKC site. Overexpression of Syt IVQ, but not of Syt I, in Aplysia neurons blocked the ability of serotonin to reverse synaptic depression. This effect is upstream of PKC activation, because neither Syt IVQ nor Syt I blocked the effects of phorbol esters on reversing synaptic depression or the effects of serotonin on facilitating nondepressed synapses. Our results demonstrate a physiological role for splicing in the juxtamembrane domain of Syt I.
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Abstract
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.
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Affiliation(s)
- B Marquèze
- INSERM U464, Institut Fédératif Jean-Roche, Université de la Méditerranée, Faculté de Médecine, Boulevard Pierre-Dramard, 13916 cedex 20, Marseille, France.
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Krasnov PA, Enikolopov G. Targeting of synaptotagmin to neurite terminals in neuronally differentiated PC12 cells. J Cell Sci 2000; 113 ( Pt 8):1389-404. [PMID: 10725222 DOI: 10.1242/jcs.113.8.1389] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023] Open
Abstract
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.
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Affiliation(s)
- P A Krasnov
- Cold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724, USA
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Abstract
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.
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Affiliation(s)
- M Craxton
- Medical Research Council, Laboratory of Molecular Biology, Hills Road, Cambridge, UK.
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Cooper PR, Nowak NJ, Higgins MJ, Church DM, Shows TB. Transcript mapping of the human chromosome 11q12-q13.1 gene-rich region identifies several newly described conserved genes. Genomics 1998; 49:419-29. [PMID: 9615227 DOI: 10.1006/geno.1998.5291] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Despite the localization of several human diseases to 11q13, the majority of the genes responsible for these disorders have not yet been cloned. Exon amplification and EST mapping were performed using clones derived from an approximately 1.65-Mb P1 artificial chromosome contig encompassing the region that reportedly harbors the gene mutated in the dominantly inherited eye disorder, Best disease. Fifty-eight exons isolated from the region were sequenced, resulting in 41.3% showing weak or no similarity to database sequences. Four exons had exact matches with human ESTs and 2 exons were highly similar to mouse ESTs. The sequence of 1 of these human ESTs was highly similar to that of the rat Rabin3 and mouse Pat-12 genes, which potentially encode Ras-like GTPase binding proteins. Three exon sequences were similar to those of the inner centromere proteins of Gallus gallus and Xenopus laevis, which are mitotic phosphoproteins, and 1 exon sequence had similarity to the epidermal growth factor-like repeat from several proteins. High-resolution mapping of 34 ESTs binned to the 11q12-q13 region by the Human Transcript Mapping Project identified 5 present in the PAC contig, with 1 of these ESTs identifying a human homologue of the rat synaptotagmin VII gene. Database searches identified two overlapping cDNA clones representing almost the entire open reading frame of this human gene and a sequenced cosmid indicating its partial genomic structure. Further database analyses identified another sequenced cosmid from this region that contained both exon-trap and mapped EST sequences. PowerBLAST and GRAIL analysis of this cosmid sequence identified matches with several other ESTs, the previously described FEN1 gene, and a novel evolutionarily conserved gene. These experiments identify candidate genes for disorders that map to this region and indicate that this is a gene-rich region of the human genome.
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Affiliation(s)
- P R Cooper
- Department of Human Genetics, Roswell Park Cancer Institute, Buffalo, New York 14263, USA
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