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Mukherjee M, Mukherjee C, Ghosh V, Jain A, Sadhukhan S, Dagar S, Sahu BS. Endoplasmic reticulum stress impedes regulated secretion by governing key exocytotic and granulogenic molecular switches. J Cell Sci 2024; 137:jcs261257. [PMID: 38348894 DOI: 10.1242/jcs.261257] [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: 04/19/2023] [Accepted: 02/05/2024] [Indexed: 03/20/2024] Open
Abstract
Dense core vesicles (DCVs) and synaptic vesicles are specialised secretory vesicles in neurons and neuroendocrine cells, and abnormal release of their cargo is associated with various pathophysiologies. Endoplasmic reticulum (ER) stress and inter-organellar communication are also associated with disease biology. To investigate the functional status of regulated exocytosis arising from the crosstalk of a stressed ER and DCVs, ER stress was modelled in PC12 neuroendocrine cells using thapsigargin. DCV exocytosis was severely compromised in ER-stressed PC12 cells and was reversed to varying magnitudes by ER stress attenuators. Experiments with tunicamycin, an independent ER stressor, yielded similar results. Concurrently, ER stress also caused impaired DCV exocytosis in insulin-secreting INS-1 cells. Molecular analysis revealed blunted SNAP25 expression, potentially attributed to augmented levels of ATF4, an inhibitor of CREB that binds to the CREB-binding site. The effects of loss of function of ATF4 in ER-stressed cells substantiated this attribution. Our studies revealed severe defects in DCV exocytosis in ER-stressed cells for the first time, mediated by reduced levels of key exocytotic and granulogenic switches regulated via the eIF2α (EIF2A)-ATF4 axis.
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Affiliation(s)
- Mohima Mukherjee
- National Brain Research Centre, Manesar, Gurgaon, Haryana 122052, India
| | | | - Vinayak Ghosh
- National Brain Research Centre, Manesar, Gurgaon, Haryana 122052, India
| | - Aamna Jain
- National Brain Research Centre, Manesar, Gurgaon, Haryana 122052, India
| | - Souren Sadhukhan
- National Brain Research Centre, Manesar, Gurgaon, Haryana 122052, India
| | - Sushma Dagar
- National Brain Research Centre, Manesar, Gurgaon, Haryana 122052, India
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Function of Drosophila Synaptotagmins in membrane trafficking at synapses. Cell Mol Life Sci 2021; 78:4335-4364. [PMID: 33619613 PMCID: PMC8164606 DOI: 10.1007/s00018-021-03788-9] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/10/2020] [Revised: 01/29/2021] [Accepted: 02/09/2021] [Indexed: 12/13/2022]
Abstract
The Synaptotagmin (SYT) family of proteins play key roles in regulating membrane trafficking at neuronal synapses. Using both Ca2+-dependent and Ca2+-independent interactions, several SYT isoforms participate in synchronous and asynchronous fusion of synaptic vesicles (SVs) while preventing spontaneous release that occurs in the absence of stimulation. Changes in the function or abundance of the SYT1 and SYT7 isoforms alter the number and route by which SVs fuse at nerve terminals. Several SYT family members also regulate trafficking of other subcellular organelles at synapses, including dense core vesicles (DCV), exosomes, and postsynaptic vesicles. Although SYTs are linked to trafficking of multiple classes of synaptic membrane compartments, how and when they interact with lipids, the SNARE machinery and other release effectors are still being elucidated. Given mutations in the SYT family cause disorders in both the central and peripheral nervous system in humans, ongoing efforts are defining how these proteins regulate vesicle trafficking within distinct neuronal compartments. Here, we review the Drosophila SYT family and examine their role in synaptic communication. Studies in this invertebrate model have revealed key similarities and several differences with the predicted activity of their mammalian counterparts. In addition, we highlight the remaining areas of uncertainty in the field and describe outstanding questions on how the SYT family regulates membrane trafficking at nerve terminals.
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Wang JT, Yu ZY, Tao YH, Liu YC, Wang YM, Guo QL, Xue JZ, Wen XH, Zhang Q, Xu XD, He CF, Xue WJ, Guo JC, Zhou HG. A novel palmitic acid hydroxy stearic acid (5-PAHSA) plays a neuroprotective role by inhibiting phosphorylation of the m-TOR-ULK1 pathway and regulating autophagy. CNS Neurosci Ther 2021; 27:484-496. [PMID: 33459523 PMCID: PMC7941174 DOI: 10.1111/cns.13573] [Citation(s) in RCA: 10] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/24/2020] [Revised: 12/07/2020] [Accepted: 12/11/2020] [Indexed: 12/13/2022] Open
Abstract
Aims Type 2 diabetes mellitus (T2DM) can lead to brain dysfunction and a series of neurological complications. Previous research demonstrated that a novel palmitic acid (5‐PAHSA) exerts effect on glucose tolerance and chronic inflammation. Autophagy was important in diabetic‐related neurodegeneration. The aim of the present study was to investigate whether 5‐PAHSA has specific therapeutic effects on neurological dysfunction in diabetics, particularly with regard to autophagy. Methods 5‐PAHSA was successfully synthesized according to a previously described protocol. We then carried out a series of in vitro and in vivo experiments using PC12 cells under diabetic conditions, and DB/DB mice, respectively. PC12 cells were treated with 5‐PAHSA for 24 h, while mice were administered with 5‐PAHSA for 30 days. At the end of each experiment, we analyzed glucolipid metabolism, autophagy, apoptosis, oxidative stress, cognition, and a range of inflammatory factors. Results Although there was no significant improvement in glucose metabolism in mice administered with 5‐PAHSA, ox‐LDL decreased significantly following the administration of 5‐PAHSA in serum of DB/DB mice (p < 0.0001). We also found that the phosphorylation of m‐TOR and ULK‐1 was suppressed in both PC12 cells and DB/DB mice following the administration of 5‐PAHSA (p < 0.05 and p < 0.01), although increased levels of autophagy were only observed in vitro (p < 0.05). Following the administration of 5‐PAHSA, the concentration of ROS decreased in PC12 cells and the levels of CRP increased in high‐dose group of 5‐PAHSA (p < 0.01). There were no significant changes in terms of apoptosis, other inflammatory factors, or cognition in DB/DB mice following the administration of 5‐PAHSA. Conclusion We found that 5‐PAHSA can enhance autophagy in PC12 cells under diabetic conditions. Our data demonstrated that 5‐PAHSA inhibits phosphorylation of the m‐TOR‐ULK1 pathway and suppressed oxidative stress in PC12 cells, and exerted influence on lipid metabolism in DB/DB mice.
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Affiliation(s)
- Jian-Tao Wang
- Department of Geriatric Neurology of Huashan Hospital, National Clinical Research Center for Aging and Medicine, Fudan University, Shanghai, China
| | - Zhong-Yu Yu
- Department of Geriatric Neurology of Huashan Hospital, National Clinical Research Center for Aging and Medicine, Fudan University, Shanghai, China
| | - Ying-Hong Tao
- Department of Medical Examination Center, Huashan Hospital, Fudan University, Shanghai, China
| | - Ying-Chao Liu
- Department of Neurosurgery, Provincial Hospital Affiliated to Shandong University, Jinan, China
| | - Yan-Mei Wang
- Department of Geriatric Neurology of Huashan Hospital, National Clinical Research Center for Aging and Medicine, Fudan University, Shanghai, China
| | - Qi-Lin Guo
- State Key Laboratory of Medical Neurobiology and MOE Frontiers Center for Brain Science, Institutes of Brain Science, Department of Translational Neuroscience, Jing'an District Centre Hospital of Shanghai, Fudan University, Shanghai, China
| | - Jian-Zhong Xue
- Department of Neurology, Fifth Clinical Medical College of Yangzhou University, Changshu Second People's Hospital of Jiangsu Province, Changshu, China
| | - Xiao-Hong Wen
- State Key Laboratory of Medical Neurobiology and MOE Frontiers Center for Brain Science, Institutes of Brain Science, Department of Translational Neuroscience, Jing'an District Centre Hospital of Shanghai, Fudan University, Shanghai, China
| | - Qian Zhang
- State Key Laboratory of Medical Neurobiology and MOE Frontiers Center for Brain Science, Institutes of Brain Science, Department of Translational Neuroscience, Jing'an District Centre Hospital of Shanghai, Fudan University, Shanghai, China
| | - Xiao-Die Xu
- State Key Laboratory of Medical Neurobiology and MOE Frontiers Center for Brain Science, Institutes of Brain Science, Department of Translational Neuroscience, Jing'an District Centre Hospital of Shanghai, Fudan University, Shanghai, China
| | - Cheng-Feng He
- State Key Laboratory of Medical Neurobiology and MOE Frontiers Center for Brain Science, Institutes of Brain Science, Department of Translational Neuroscience, Jing'an District Centre Hospital of Shanghai, Fudan University, Shanghai, China
| | - Wen-Jiao Xue
- State Key Laboratory of Medical Neurobiology and MOE Frontiers Center for Brain Science, Institutes of Brain Science, Department of Translational Neuroscience, Jing'an District Centre Hospital of Shanghai, Fudan University, Shanghai, China
| | - Jing-Chun Guo
- State Key Laboratory of Medical Neurobiology and MOE Frontiers Center for Brain Science, Institutes of Brain Science, Department of Translational Neuroscience, Jing'an District Centre Hospital of Shanghai, Fudan University, Shanghai, China
| | - Hou-Guang Zhou
- Department of Geriatric Neurology of Huashan Hospital, National Clinical Research Center for Aging and Medicine, Fudan University, Shanghai, China
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Guan Z, Quiñones-Frías MC, Akbergenova Y, Littleton JT. Drosophila Synaptotagmin 7 negatively regulates synaptic vesicle release and replenishment in a dosage-dependent manner. eLife 2020; 9:e55443. [PMID: 32343229 PMCID: PMC7224696 DOI: 10.7554/elife.55443] [Citation(s) in RCA: 17] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/24/2020] [Accepted: 04/28/2020] [Indexed: 01/03/2023] Open
Abstract
Synchronous neurotransmitter release is triggered by Ca2+ binding to the synaptic vesicle protein Synaptotagmin 1, while asynchronous fusion and short-term facilitation is hypothesized to be mediated by plasma membrane-localized Synaptotagmin 7 (SYT7). We generated mutations in Drosophila Syt7 to determine if it plays a conserved role as the Ca2+ sensor for these processes. Electrophysiology and quantal imaging revealed evoked release was elevated 2-fold. Syt7 mutants also had a larger pool of readily-releasable vesicles, faster recovery following stimulation, and intact facilitation. Syt1/Syt7 double mutants displayed more release than Syt1 mutants alone, indicating SYT7 does not mediate the residual asynchronous release remaining in the absence of SYT1. SYT7 localizes to an internal membrane tubular network within the peri-active zone, but does not enrich at active zones. These findings indicate the two Ca2+ sensor model of SYT1 and SYT7 mediating all phases of neurotransmitter release and facilitation is not applicable at Drosophila synapses.
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Affiliation(s)
- Zhuo Guan
- The Picower Institute for Learning and Memory, Department of Biology and Department of Brain and Cognitive Sciences, Massachusetts Institute of TechnologyCambridgeUnited States
| | - Monica C Quiñones-Frías
- The Picower Institute for Learning and Memory, Department of Biology and Department of Brain and Cognitive Sciences, Massachusetts Institute of TechnologyCambridgeUnited States
| | - Yulia Akbergenova
- The Picower Institute for Learning and Memory, Department of Biology and Department of Brain and Cognitive Sciences, Massachusetts Institute of TechnologyCambridgeUnited States
| | - J Troy Littleton
- The Picower Institute for Learning and Memory, Department of Biology and Department of Brain and Cognitive Sciences, Massachusetts Institute of TechnologyCambridgeUnited States
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Rozov A, Bolshakov AP, Valiullina-Rakhmatullina F. The Ever-Growing Puzzle of Asynchronous Release. Front Cell Neurosci 2019; 13:28. [PMID: 30809127 PMCID: PMC6379310 DOI: 10.3389/fncel.2019.00028] [Citation(s) in RCA: 16] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2018] [Accepted: 01/22/2019] [Indexed: 11/13/2022] Open
Abstract
Invasion of an action potential (AP) to presynaptic terminals triggers calcium dependent vesicle fusion in a relatively short time window, about a millisecond, after the onset of the AP. This allows fast and precise information transfer from neuron to neuron by means of synaptic transmission and phasic mediator release. However, at some synapses a single AP or a short burst of APs can generate delayed or asynchronous synaptic release lasting for tens or hundreds of milliseconds. Understanding the mechanisms underlying asynchronous release (AR) is important, since AR can better recruit extrasynaptic metabotropic receptors and maintain a high level of neurotransmitter in the extracellular space for a substantially longer period of time after presynaptic activity. Over the last decade substantial work has been done to identify the presynaptic calcium sensor that may be involved in AR. Several models have been suggested which may explain the long lasting presynaptic calcium elevation a prerequisite for prolonged delayed release. However, the presynaptic mechanisms underlying asynchronous vesicle release are still not well understood. In this review article, we provide an overview of the current state of knowledge on the molecular components involved in delayed vesicle fusion and in the maintenance of sufficient calcium concentration to trigger AR. In addition, we discuss possible alternative models that may explain intraterminal calcium dynamics underlying AR.
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Affiliation(s)
- Andrei Rozov
- Laboratory of Neurobiology, Institute of Fundamental Medicine and Biology, Kazan Federal University, Kazan, Russia.,Department of Physiology and Pathophysiology, University of Heidelberg, Heidelberg, Germany
| | - Alexey P Bolshakov
- Institute of Higher Nervous Activity and Neurophysiology, Russian Academy of Sciences (RAS), Moscow, Russia.,Laboratory of Electrophysiology, Pirogov Russian National Research Medical University, Moscow, Russia
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Synaptic loss in schizophrenia: a meta-analysis and systematic review of synaptic protein and mRNA measures. Mol Psychiatry 2019; 24:549-561. [PMID: 29511299 PMCID: PMC6004314 DOI: 10.1038/s41380-018-0041-5] [Citation(s) in RCA: 145] [Impact Index Per Article: 29.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 11/01/2017] [Revised: 01/05/2018] [Accepted: 01/31/2018] [Indexed: 02/06/2023]
Abstract
Although synaptic loss is thought to be core to the pathophysiology of schizophrenia, the nature, consistency and magnitude of synaptic protein and mRNA changes has not been systematically appraised. Our objective was thus to systematically review and meta-analyse findings. The entire PubMed database was searched for studies from inception date to the 1st of July 2017. We selected case-control postmortem studies in schizophrenia quantifying synaptic protein or mRNA levels in brain tissue. The difference in protein and mRNA levels between cases and controls was extracted and meta-analysis conducted. Among the results, we found a significant reduction in synaptophysin in schizophrenia in the hippocampus (effect size: -0.65, p < 0.01), frontal (effect size: -0.36, p = 0.04), and cingulate cortices (effect size: -0.54, p = 0.02), but no significant changes for synaptophysin in occipital and temporal cortices, and no changes for SNAP-25, PSD-95, VAMP, and syntaxin in frontal cortex. There were insufficient studies for meta-analysis of complexins, synapsins, rab3A and synaptotagmin and mRNA measures. Findings are summarised for these, which generally show reductions in SNAP-25, PSD-95, synapsin and rab3A protein levels in the hippocampus but inconsistency in other regions. Our findings of moderate-large reductions in synaptophysin in hippocampus and frontal cortical regions, and a tendency for reductions in other pre- and postsynaptic proteins in the hippocampus are consistent with models that implicate synaptic loss in schizophrenia. However, they also identify potential differences between regions and proteins, suggesting synaptic loss is not uniform in nature or extent.
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Exceptionally tight membrane-binding may explain the key role of the synaptotagmin-7 C 2A domain in asynchronous neurotransmitter release. Proc Natl Acad Sci U S A 2017; 114:E8518-E8527. [PMID: 28923929 DOI: 10.1073/pnas.1710708114] [Citation(s) in RCA: 32] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Synaptotagmins (Syts) act as Ca2+ sensors in neurotransmitter release by virtue of Ca2+-binding to their two C2 domains, but their mechanisms of action remain unclear. Puzzlingly, Ca2+-binding to the C2B domain appears to dominate Syt1 function in synchronous release, whereas Ca2+-binding to the C2A domain mediates Syt7 function in asynchronous release. Here we show that crystal structures of the Syt7 C2A domain and C2AB region, and analyses of intrinsic Ca2+-binding to the Syt7 C2 domains using isothermal titration calorimetry, did not reveal major differences that could explain functional differentiation between Syt7 and Syt1. However, using liposome titrations under Ca2+ saturating conditions, we show that the Syt7 C2A domain has a very high membrane affinity and dominates phospholipid binding to Syt7 in the presence or absence of l-α-phosphatidylinositol 4,5-diphosphate (PIP2). For Syt1, the two Ca2+-saturated C2 domains have similar affinities for membranes lacking PIP2, but the C2B domain dominates binding to PIP2-containing membranes. Mutagenesis revealed that the dramatic differences in membrane affinity between the Syt1 and Syt7 C2A domains arise in part from apparently conservative residue substitutions, showing how striking biochemical and functional differences can result from the cumulative effects of subtle residue substitutions. Viewed together, our results suggest that membrane affinity may be a key determinant of the functions of Syt C2 domains in neurotransmitter release.
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Pinheiro PS, Houy S, Sørensen JB. C2-domain containing calcium sensors in neuroendocrine secretion. J Neurochem 2016; 139:943-958. [DOI: 10.1111/jnc.13865] [Citation(s) in RCA: 44] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/29/2016] [Revised: 09/17/2016] [Accepted: 10/05/2016] [Indexed: 12/11/2022]
Affiliation(s)
- Paulo S. Pinheiro
- Center for Neuroscience and Cell Biology; University of Coimbra; Coimbra Portugal
| | - Sébastien Houy
- Department of Neuroscience and Pharmacology; Faculty of Health and Medical Sciences; University of Copenhagen; Copenhagen Denmark
| | - Jakob B. Sørensen
- Department of Neuroscience and Pharmacology; Faculty of Health and Medical Sciences; University of Copenhagen; Copenhagen Denmark
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Bacaj T, Wu D, Burré J, Malenka RC, Liu X, Südhof TC. Synaptotagmin-1 and -7 Are Redundantly Essential for Maintaining the Capacity of the Readily-Releasable Pool of Synaptic Vesicles. PLoS Biol 2015; 13:e1002267. [PMID: 26437117 PMCID: PMC4593569 DOI: 10.1371/journal.pbio.1002267] [Citation(s) in RCA: 63] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/21/2015] [Accepted: 08/27/2015] [Indexed: 12/29/2022] Open
Abstract
In forebrain neurons, Ca(2+) triggers exocytosis of readily releasable vesicles by binding to synaptotagmin-1 and -7, thereby inducing fast and slow vesicle exocytosis, respectively. Loss-of-function of synaptotagmin-1 or -7 selectively impairs the fast and slow phase of release, respectively, but does not change the size of the readily-releasable pool (RRP) of vesicles as measured by stimulation of release with hypertonic sucrose, or alter the rate of vesicle priming into the RRP. Here we show, however, that simultaneous loss-of-function of both synaptotagmin-1 and -7 dramatically decreased the capacity of the RRP, again without altering the rate of vesicle priming into the RRP. Either synaptotagmin-1 or -7 was sufficient to rescue the RRP size in neurons lacking both synaptotagmin-1 and -7. Although maintenance of RRP size was Ca(2+)-independent, mutations in Ca(2+)-binding sequences of synaptotagmin-1 or synaptotagmin-7--which are contained in flexible top-loop sequences of their C2 domains--blocked the ability of these synaptotagmins to maintain the RRP size. Both synaptotagmins bound to SNARE complexes; SNARE complex binding was reduced by the top-loop mutations that impaired RRP maintenance. Thus, synaptotagmin-1 and -7 perform redundant functions in maintaining the capacity of the RRP in addition to nonredundant functions in the Ca(2+) triggering of different phases of release.
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Affiliation(s)
- Taulant Bacaj
- Department of Molecular and Cellular Physiology and Howard Hughes Medical Institute, Stanford University, Stanford, California, United States of America
| | - Dick Wu
- Department of Molecular and Cellular Physiology and Howard Hughes Medical Institute, Stanford University, Stanford, California, United States of America
- Nancy Pritzker Laboratory, Department of Psychiatry and Behavioral Sciences, Stanford University Medical School, Stanford, California, United States of America
| | - Jacqueline Burré
- Department of Molecular and Cellular Physiology and Howard Hughes Medical Institute, Stanford University, Stanford, California, United States of America
| | - Robert C. Malenka
- Nancy Pritzker Laboratory, Department of Psychiatry and Behavioral Sciences, Stanford University Medical School, Stanford, California, United States of America
| | - Xinran Liu
- Department of Cell Biology, Yale University School of Medicine, New Haven, Connecticut, United States of America
| | - Thomas C. Südhof
- Department of Molecular and Cellular Physiology and Howard Hughes Medical Institute, Stanford University, Stanford, California, United States of America
- * E-mail:
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Gautam V, D'Avanzo C, Berezovska O, Tanzi RE, Kovacs DM. Synaptotagmins interact with APP and promote Aβ generation. Mol Neurodegener 2015. [PMID: 26202512 PMCID: PMC4511450 DOI: 10.1186/s13024-015-0028-5] [Citation(s) in RCA: 32] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022] Open
Abstract
Background Accumulation of the β-amyloid peptide (Aβ) is a major pathological hallmark of Alzheimer’s disease (AD). Recent studies have shown that synaptic Aβ toxicity may directly impair synaptic function. However, proteins regulating Aβ generation at the synapse have not been characterized. Here, we sought to identify synaptic proteins that interact with the extracellular domain of APP and regulate Aβ generation. Results Affinity purification-coupled mass spectrometry identified members of the Synaptotagmin (Syt) family as novel interacting proteins with the APP ectodomain in mouse brains. Syt-1, −2 and −9 interacted with APP in cells and in mouse brains in vivo. Using a GST pull-down approach, we have further demonstrated that the Syt interaction site lies in the 108 amino acids linker region between the E1 and KPI domains of APP. Stable overexpression of Syt-1 or Syt-9 with APP in CHO and rat pheochromocytoma cells (PC12) significantly increased APP-CTF and sAPP levels, with a 2 to 3 fold increase in secreted Aβ levels in PC12 cells. Moreover, using a stable knockdown approach to reduce the expression of endogenous Syt-1 in PC12 cells, we have observed a ~ 50 % reduction in secreted Aβ generation. APP processing also decreased in these cells, shown by lower CTF levels. Lentiviral-mediated knock down of endogenous Syt-1 in mouse primary neurons also led to a significant reduction in both Aβ40 and Aβ42 generation. As secreted sAPPβ levels were significantly reduced in PC12 cells lacking Syt-1 expression, our results suggest that Syt-1 regulates Aβ generation by modulating BACE1-mediated cleavage of APP. Conclusion Altogether, our data identify the synaptic vesicle proteins Syt-1 and 9 as novel APP-interacting proteins that promote Aβ generation and thus may play an important role in the pathogenesis of AD. Electronic supplementary material The online version of this article (doi:10.1186/s13024-015-0028-5) contains supplementary material, which is available to authorized users.
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Affiliation(s)
- Vivek Gautam
- Genetics and Aging Research Unit, MassGeneral Institute for Neurodegenerative Disease, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA, 02129, USA.
| | - Carla D'Avanzo
- Genetics and Aging Research Unit, MassGeneral Institute for Neurodegenerative Disease, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA, 02129, USA.
| | - Oksana Berezovska
- MassGeneral Institute for Neurodegenerative Disease, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA, 02129, USA.
| | - Rudolph E Tanzi
- Genetics and Aging Research Unit, MassGeneral Institute for Neurodegenerative Disease, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA, 02129, USA.
| | - Dora M Kovacs
- Genetics and Aging Research Unit, MassGeneral Institute for Neurodegenerative Disease, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA, 02129, USA.
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Molecular origins of synaptotagmin 1 activities on vesicle docking and fusion pore opening. Sci Rep 2015; 5:9267. [PMID: 25791821 PMCID: PMC4366854 DOI: 10.1038/srep09267] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/24/2014] [Accepted: 02/26/2015] [Indexed: 11/25/2022] Open
Abstract
Synaptotagmin 1 (Syt1), a major Ca2+ sensor in neuroexocytosis, utilizes SNARE- and membrane-binding to regulate vesicle fusion, a required process for neurotransmitter release at the synapse. However, the mechanism by which Syt1 orchestrates SNARE- and membrane- binding to control individual vesicle fusion steps is still unclear. In this study, we used a number of single vesicle assays that can differentiate intermediates of neuroexocytosis, to focus on Syt1 mutants that might impair Syt1-SNARE/PIP2 interaction, Ca2+-binding, or membrane penetration. Our results show that, although putative Syt1-SNARE/PIP2 coupling through the polybasic region of the C2B domain is critical for vesicle docking, its disruption does not affect content release. In contrast, Ca2+-binding and membrane-penetration mutants significantly reduce content release. Our results thus delineate multiple functions of Syt1 along the pathway of Ca2+-triggered exocytosis in unprecedented detail.
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Abstract
Synaptic vesicles release their vesicular contents to the extracellular space by Ca(2+)-triggered exocytosis. The Ca(2+)-triggered exocytotic process is regulated by synaptotagmin (Syt), a vesicular Ca(2+)-binding C2 domain protein. Synaptotagmin 1 (Syt1), the most studied major isoform among 16 Syt isoforms, mediates Ca(2+)-triggered synaptic vesicle exocytosis by interacting with the target membranes and SNARE/complexin complex. In synapses of the central nervous system, synaptobrevin 2, a major vesicular SNARE protein, forms a ternary SNARE complex with the plasma membrane SNARE proteins, syntaxin 1 and SNAP25. The affinities of Ca(2+)-dependent interactions between Syt1 and its targets (i.e., SNARE complexes and membranes) are well correlated with the efficacies of the corresponding exocytotic processes. Therefore, different SNARE protein isoforms and membrane lipids, which interact with Syt1 with various affinities, are capable of regulating the efficacy of Syt1-mediated exocytosis. Otoferlin, another type of vesicular C2 domain protein that binds to the membrane in a Ca(2+)-dependent manner, is also involved in the Ca(2+)-triggered synaptic vesicle exocytosis in auditory hair cells. However, the functions of otoferlin in the exocytotic process are not well understood. In addition, at least five different types of synaptic vesicle proteins such as synaptic vesicle protein 2, cysteine string protein α, rab3, synapsin, and a group of proteins containing four transmembrane regions, which includes synaptophysin, synaptogyrin, and secretory carrier membrane protein, are involved in modulating the exocytotic process by regulating the formation and trafficking of synaptic vesicles.
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Affiliation(s)
- Ok-Ho Shin
- Department of Neuroscience and Cell Biology, University of Texas Medical Branch, Galveston, Texas
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Ramirez DMO, Kavalali ET. The role of non-canonical SNAREs in synaptic vesicle recycling. CELLULAR LOGISTICS 2014; 2:20-27. [PMID: 22645707 PMCID: PMC3355972 DOI: 10.4161/cl.20114] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Indexed: 01/23/2023]
Abstract
An increasing number of studies suggest that distinct pools of synaptic vesicles drive specific forms of neurotransmission. Interspersed with these functional studies are analyses of the synaptic vesicle proteome which have consistently detected the presence of so-called “non-canonical” SNAREs that typically function in fusion and trafficking of other subcellular structures within the neuron. The recent identification of certain non-canonical vesicular SNAREs driving spontaneous (e.g., VAMP7 and vti1a) or evoked asynchronous (e.g., VAMP4) release integrates and corroborates existing data from functional and proteomic studies and implies that at least some complement of non-canonical SNAREs resident on synaptic vesicles function in neurotransmission. Here, we discuss the specific roles in neurotransmission of proteins homologous to each member of the classical neuronal SNARE complex consisting of synaptobrevin2, syntaxin-1 and SNAP-25.
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de Groot MW, Westerink RH. Chemically-induced oxidative stress increases the vulnerability of PC12 cells to rotenone-induced toxicity. Neurotoxicology 2014; 43:102-109. [DOI: 10.1016/j.neuro.2014.02.008] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/07/2013] [Revised: 02/20/2014] [Accepted: 02/28/2014] [Indexed: 10/25/2022]
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Deák F. Neuronal vesicular trafficking and release in age-related cognitive impairment. J Gerontol A Biol Sci Med Sci 2014; 69:1325-30. [PMID: 24809352 DOI: 10.1093/gerona/glu061] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
Aging is a common major risk factor for many neurological disorders resulting in cognitive impairment and neurodegeneration including Parkinson's and Alzheimer's diseases. Novel results from the fields of molecular neuroscience and aging research provide evidence for a link between decline of various cognitive, executive functions and changes in neuronal mechanisms of intracellular trafficking and regulated vesicle release processes in the aging nervous system. In this Perspective, we review these recent findings and formulate a hypothesis on how cargo delivery to the synapses and the release of neurotrophic factors may be involved in maintaining learning and memory capabilities during healthy aging and present examples on how defects of those disrupt normal cognition. We provide an overview of emerging new concepts and approaches that will significantly advance our understanding of the aging brain and pathophysiology of dementia. This knowledge will be instrumental in defining drug targets and designing novel therapeutic strategies.
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Affiliation(s)
- Ferenc Deák
- Reynolds Oklahoma Center on Aging, Department of Geriatric Medicine, University of Oklahoma Health Sciences Center.
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16
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Bacaj T, Wu D, Yang X, Morishita W, Zhou P, Xu W, Malenka RC, Südhof TC. Synaptotagmin-1 and synaptotagmin-7 trigger synchronous and asynchronous phases of neurotransmitter release. Neuron 2014; 80:947-59. [PMID: 24267651 DOI: 10.1016/j.neuron.2013.10.026] [Citation(s) in RCA: 208] [Impact Index Per Article: 20.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 10/02/2013] [Indexed: 01/04/2023]
Abstract
In forebrain neurons, knockout of synaptotagmin-1 blocks fast Ca(2+)-triggered synchronous neurotransmitter release but enables manifestation of slow Ca(2+)-triggered asynchronous release. Here, we show using single-cell PCR that individual hippocampal neurons abundantly coexpress two Ca(2+)-binding synaptotagmin isoforms, synaptotagmin-1 and synaptotagmin-7. In synaptotagmin-1-deficient synapses of excitatory and inhibitory neurons, loss of function of synaptotagmin-7 suppressed asynchronous release. This phenotype was rescued by wild-type but not mutant synaptotagmin-7 lacking functional Ca(2+)-binding sites. Even in synaptotagmin-1-containing neurons, synaptotagmin-7 ablation partly impaired asynchronous release induced by extended high-frequency stimulus trains. Synaptotagmins bind Ca(2+) via two C2 domains, the C2A and C2B domains. Surprisingly, synaptotagmin-7 function selectively required its C2A domain Ca(2+)-binding sites, whereas synaptotagmin-1 function required its C2B domain Ca(2+)-binding sites. Our data show that nearly all Ca(2+)-triggered release at a synapse is due to synaptotagmins, with synaptotagmin-7 mediating a slower form of Ca(2+)-triggered release that is normally occluded by faster synaptotagmin-1-induced release but becomes manifest upon synaptotagmin-1 deletion.
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Affiliation(s)
- Taulant Bacaj
- Department of Molecular and Cellular Physiology and Howard Hughes Medical Institute, Stanford University Medical School, Stanford, CA 94305, USA
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17
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Abstract
During an action potential, Ca(2+) entering a presynaptic terminal triggers synaptic vesicle exocytosis and neurotransmitter release in less than a millisecond. How does Ca(2+) stimulate release so rapidly and precisely? Work over the last decades revealed that Ca(2+) binding to synaptotagmin triggers release by stimulating synaptotagmin binding to a core fusion machinery composed of SNARE and SM proteins that mediates membrane fusion during exocytosis. Complexin adaptor proteins assist synaptotagmin by activating and clamping this core fusion machinery. Synaptic vesicles containing synaptotagmin are positioned at the active zone, the site of vesicle fusion, by a protein complex containing RIM proteins. RIM proteins activate docking and priming of synaptic vesicles and simultaneously recruit Ca(2+) channels to active zones, thereby connecting in a single complex primed synaptic vesicles to Ca(2+) channels. This architecture allows direct flow of Ca(2+) ions from Ca(2+) channels to synaptotagmin, which then triggers fusion, thus mediating tight millisecond coupling of an action potential to neurotransmitter release.
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Affiliation(s)
- Thomas C Südhof
- Department of Molecular and Cellular Physiology, and Howard Hughes Medical Institute, Lorry Lokey SIM1 Building, 265 Campus Drive, Stanford University School of Medicine, Stanford, CA 94305, USA.
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18
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Malsam J, Parisotto D, Bharat TAM, Scheutzow A, Krause JM, Briggs JAG, Söllner TH. Complexin arrests a pool of docked vesicles for fast Ca2+-dependent release. EMBO J 2012; 31:3270-81. [PMID: 22705946 DOI: 10.1038/emboj.2012.164] [Citation(s) in RCA: 74] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/05/2011] [Accepted: 05/15/2012] [Indexed: 12/16/2022] Open
Abstract
Regulated exocytosis requires that the assembly of the basic membrane fusion machinery is temporarily arrested. Synchronized membrane fusion is then caused by a specific trigger--a local rise of the Ca(2+) concentration. Using reconstituted giant unilamellar vesicles (GUVs), we have analysed the role of complexin and membrane-anchored synaptotagmin 1 in arresting and synchronizing fusion by lipid-mixing and cryo-electron microscopy. We find that they mediate the formation and consumption of docked small unilamellar vesicles (SUVs) via the following sequence of events: Synaptotagmin 1 mediates v-SNARE-SUV docking to t-SNARE-GUVs in a Ca(2+)-independent manner. Complexin blocks vesicle consumption, causing accumulation of docked vesicles. Together with synaptotagmin 1, complexin synchronizes and stimulates rapid fusion of accumulated docked vesicles in response to physiological Ca(2+) concentrations. Thus, the reconstituted assay resolves both the stimulatory and inhibitory function of complexin and mimics key aspects of synaptic vesicle fusion.
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Affiliation(s)
- Jörg Malsam
- Heidelberg University Biochemistry Center, Heidelberg, Germany
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Tobin V, Schwab Y, Lelos N, Onaka T, Pittman QJ, Ludwig M. Expression of exocytosis proteins in rat supraoptic nucleus neurones. J Neuroendocrinol 2012; 24:629-41. [PMID: 21988098 PMCID: PMC3569506 DOI: 10.1111/j.1365-2826.2011.02237.x] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
In magnocellular neurones of the supraoptic nucleus (SON), the neuropeptides vasopressin and oxytocin are synthesised and packaged into large dense-cored vesicles (LDCVs). These vesicles undergo regulated exocytosis from nerve terminals in the posterior pituitary gland and from somata/dendrites in the SON. Regulated exocytosis of LDCVs is considered to involve the soluble N-ethylmaleimide sensitive fusion protein attachment protein receptor (SNARE) complex [comprising vesicle associated membrane protein 2 (VAMP-2), syntaxin-1 and soluble N-ethylmaleimide attachment protein-25 (SNAP-25)] and regulatory proteins [such as synaptotagmin-1, munc-18 and Ca(2+) -dependent activator protein for secretion (CAPS-1)]. Using fluorescent immunocytochemistry and confocal microscopy, in both oxytocin and vasopressin neurones, we observed VAMP-2, SNAP-25 and syntaxin-1-immunoreactivity in axon terminals. The somata and dendrites contained syntaxin-1 and other regulatory exocytosis proteins, including munc-18 and CAPS-1. However, the distribution of VAMP-2 and synaptotagmin-1 in the SON was limited to putative pre-synaptic contacts because they co-localised with synaptophysin (synaptic vesicle marker) and had no co-localisation with either oxytocin or vasopressin. SNAP-25 immunoreactivity in the SON was limited to glial cell processes and was not detected in oxytocin or vasopressin somata/dendrites. The present results indicate differences in the expression and localisation of exocytosis proteins between the axon terminals and somata/dendritic compartment. The absence of VAMP-2 and SNAP-25 immunoreactivity from the somata/dendrites suggests that there might be different SNARE protein isoforms expressed in these compartments. Alternatively, exocytosis of LDCVs from somata/dendrites may use a different mechanism from that described by the SNARE complex theory.
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Affiliation(s)
- V. Tobin
- Centre for Integrative Physiology, University of Edinburgh, Edinburgh, UK
| | - Y. Schwab
- Hotchkiss Brain Institute, Department of Physiology and Pharmacology, University of Calgary, Calgary, Canada
| | - N. Lelos
- Centre for Integrative Physiology, University of Edinburgh, Edinburgh, UK
| | - T. Onaka
- Department of Physiology, Jichi Medical University, Tochigi, Japan
| | - Q. J. Pittman
- Hotchkiss Brain Institute, Department of Physiology and Pharmacology, University of Calgary, Calgary, Canada
| | - M. Ludwig
- Centre for Integrative Physiology, University of Edinburgh, Edinburgh, UK
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20
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Abstract
In chromaffin cells, Ca(2+) binding to synaptotagmin-1 and -7 triggers exocytosis by promoting fusion pore opening and fusion pore expansion. Synaptotagmins contain two C2 domains that both bind Ca(2+) and contribute to exocytosis; however, it remains unknown whether the C2 domains act similarly or differentially to promote opening and expansion of fusion pores. Here, we use patch amperometry measurements in WT and synaptotagmin-7-mutant chromaffin cells to analyze the role of Ca(2+) binding to the two synaptotagmin-7 C2 domains in exocytosis. We show that, surprisingly, Ca(2+) binding to the C2A domain suffices to trigger fusion pore opening but that the resulting fusion pores are unstable and collapse, causing a dramatic increase in kiss-and-run fusion events. Thus, synaptotagmin-7 controls fusion pore dynamics during exocytosis via a push-and-pull mechanism in which Ca(2+) binding to both C2 domains promotes fusion pore opening, but the C2B domain is selectively essential for continuous expansion of an otherwise unstable fusion pore.
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21
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Therrien C, Di Fulvio S, Pickles S, Sinnreich M. Characterization of lipid binding specificities of dysferlin C2 domains reveals novel interactions with phosphoinositides. Biochemistry 2010; 48:2377-84. [PMID: 19253956 DOI: 10.1021/bi802242r] [Citation(s) in RCA: 73] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Dysferlin is a type II transmembrane protein implicated in Ca(2+)-dependent sarcolemmal membrane repair. Dysferlin has seven C2 domains, which are lipid and protein binding modules. In this study, we sought to characterize the lipid binding specificity of dysferlin's seven C2 domains. Dysferlin's C2A domain was able to bind to phosphatidylserine (PS), phosphatidylinositol 4-phosphate [PtdIns(4)P], and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] in a Ca(2+)-dependent fashion. The remainder of the C2 domains exhibited weaker and Ca(2+)-independent binding to PS and no significant binding to phosphoinositides.
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Affiliation(s)
- Christian Therrien
- Neuromuscular Research Group, Montreal Neurological Institute and Hospital, McGill University, Montreal, Canada
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22
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Munc13 C2B domain is an activity-dependent Ca2+ regulator of synaptic exocytosis. Nat Struct Mol Biol 2010; 17:280-8. [PMID: 20154707 PMCID: PMC2916016 DOI: 10.1038/nsmb.1758] [Citation(s) in RCA: 180] [Impact Index Per Article: 12.9] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/23/2009] [Accepted: 12/04/2009] [Indexed: 11/08/2022]
Abstract
Munc13 is a multidomain protein present in presynaptic active zones that mediates the priming and plasticity of synaptic vesicle exocytosis, but the mechanisms involved remain unclear. Here we use biophysical, biochemical and electrophysiological approaches to show that the central C(2)B domain of Munc13 functions as a Ca(2+) regulator of short-term synaptic plasticity. The crystal structure of the C(2)B domain revealed an unusual Ca(2+)-binding site with an amphipathic alpha-helix. This configuration confers onto the C(2)B domain unique Ca(2+)-dependent phospholipid-binding properties that favor phosphatidylinositolphosphates. A mutation that inactivated Ca(2+)-dependent phospholipid binding to the C(2)B domain did not alter neurotransmitter release evoked by isolated action potentials, but it did depress release evoked by action-potential trains. In contrast, a mutation that increased Ca(2+)-dependent phosphatidylinositolbisphosphate binding to the C(2)B domain enhanced release evoked by isolated action potentials and by action-potential trains. Our data suggest that, during repeated action potentials, Ca(2+) and phosphatidylinositolphosphate binding to the Munc13 C(2)B domain potentiate synaptic vesicle exocytosis, thereby offsetting synaptic depression induced by vesicle depletion.
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23
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Ca2+-dependent release of Munc18-1 from presynaptic mGluRs in short-term facilitation. Proc Natl Acad Sci U S A 2009; 106:18385-9. [PMID: 19822743 DOI: 10.1073/pnas.0910088106] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Short-term synaptic facilitation plays an important role in information processing in the central nervous system. Although the crucial requirement of presynaptic Ca(2+) in the expression of this plasticity has been known for decades, the molecular mechanisms underlying the plasticity remain controversial. Here, we show that presynaptic metabotropic glutamate receptors (mGluRs) bind and release Munc18-1 (also known as rbSec1/nSec1), an essential protein for synaptic transmission, in a Ca(2+)-dependent manner, whose actions decrease and increase synaptic vesicle release, respectively. We found that mGluR4 bound Munc18-1 with an EC(50) for Ca(2+) of 168 nM, close to the resting Ca(2+) concentration, and that the interaction was disrupted by Ca(2+)-activated calmodulin (CaM) at higher concentrations of Ca(2+). Consistently, the Munc18-1-interacting domain of mGluR4 suppressed both dense-core vesicle secretion from permeabilized PC12 cells and synaptic transmission in neuronal cells. Furthermore, this domain was sufficient to induce paired-pulse facilitation. Obviously, the role of mGluR4 in these processes was independent of its classical function of activation by glutamate. On the basis of these experimental data, we propose the following model: When neurons are not active, Munc18-1 is sequestered by mGluR4, and therefore the basal synaptic transmission is kept low. After the action potential, the increase in the Ca(2+) level activates CaM, which in turn liberates Munc18-1 from mGluR4, causing short-term synaptic facilitation. Our findings unite and provide a new insight into receptor signaling and vesicular transport, which are pivotal activities involved in a variety of cellular processes.
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24
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Calcium-sensing beyond neurotransmitters: functions of synaptotagmins in neuroendocrine and endocrine secretion. Biosci Rep 2009; 29:245-59. [PMID: 19500075 DOI: 10.1042/bsr20090031] [Citation(s) in RCA: 81] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
Abstract
Neurotransmitters, neuropeptides and hormones are released through the regulated exocytosis of SVs (synaptic vesicles) and LDCVs (large dense-core vesicles), a process that is controlled by calcium. Synaptotagmins are a family of type 1 membrane proteins that share a common domain structure. Most synaptotagmins are located in brain and endocrine cells, and some of these synaptotagmins bind to phospholipids and calcium at levels that trigger regulated exocytosis of SVs and LDCVs. This led to the proposed synaptotagmin-calcium-sensor paradigm, that is, members of the synaptotagmin family function as calcium sensors for the regulated exocytosis of neurotransmitters, neuropeptides and hormones. Here, we provide an overview of the synaptotagmin family, and review the recent mouse genetic studies aimed at understanding the functions of synaptotagmins in neurotransmission and endocrine-hormone secretion. Also, we discuss potential roles of synaptotagmins in non-traditional endocrine systems.
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25
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Insulin granule biogenesis, trafficking and exocytosis. VITAMINS AND HORMONES 2009; 80:473-506. [PMID: 19251047 DOI: 10.1016/s0083-6729(08)00616-x] [Citation(s) in RCA: 142] [Impact Index Per Article: 9.5] [Reference Citation Analysis] [Abstract] [Subscribe] [Scholar Register] [Indexed: 01/18/2023]
Abstract
It is becoming increasingly apparent that beta cell dysfunction resulting in abnormal insulin secretion is the essential element in the progression of patients from a state of impaired glucose tolerance to frank type 2 diabetes (Del Prato, 2003; Del Prato and Tiengo, 2001). Although extensive studies have examined the molecular, cellular and physiologic mechanisms of insulin granule biogenesis, sorting, and exocytosis the precise mechanisms controlling these processes and their dysregulation in the developed of diabetes remains an area of important investigation. We now know that insulin biogenesis initiates with the synthesis of preproinsulin in rough endoplastic reticulum and conversion of preproinsulin to proinsulin. Proinsulin begins to be packaged in the Trans-Golgi Network and is sorting into immature secretory granules. These immature granules become acidic via ATP-dependent proton pump and proinsulin undergoes proteolytic cleavage resulting the formation of insulin and C-peptide. During the granule maturation process, insulin is crystallized with zinc and calcium in the form of dense-core granules and unwanted cargo and membrane proteins undergo selective retrograde trafficking to either the constitutive trafficking pathway for secretion or to degradative pathways. The newly formed mature dense-core insulin granules populate two different intracellular pools, the readily releasable pools (RRP) and the reserved pool. These two distinct populations are thought to be responsible for the biphasic nature of insulin release in which the RRP granules are associated with the plasma membrane and undergo an acute calcium-dependent release accounting for first phase insulin secretion. In contrast, second phase insulin secretion requires the trafficking of the reserved granule pool to the plasma membrane. The initial trigger for insulin granule fusion with the plasma membrane is a rise in intracellular calcium and in the case of glucose stimulation results from increased production of ATP, closure of the ATP-sensitive potassium channel and cellular depolarization. In turn, this opens voltage-dependent calcium channels allowing increased influx of extracellular calcium. Calcium is thought to bind to members of the fusion regulatory proteins synaptogamin that functionally repressors the fusion inhibitory protein complexin. Both complexin and synaptogamin interact as well as several other regulatory proteins interact with the core fusion machinery composed of the Q- or t-SNARE proteins syntaxin 1 and SNAP25 in the plasma membrane that assembles with the R- or v-SNARE protein VAMP2 in insulin granules. In this chapter we will review the current progress of insulin granule biogenesis, sorting, trafficking, exocytosis and signaling pathways that comprise the molecular basis of glucose-dependent insulin secretion.
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26
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Gaffaney JD, Dunning FM, Wang Z, Hui E, Chapman ER. Synaptotagmin C2B domain regulates Ca2+-triggered fusion in vitro: critical residues revealed by scanning alanine mutagenesis. J Biol Chem 2008; 283:31763-75. [PMID: 18784080 PMCID: PMC2581593 DOI: 10.1074/jbc.m803355200] [Citation(s) in RCA: 58] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
Synaptotagmin (syt) 1 is localized to synaptic vesicles, binds
Ca2+, and regulates neuronal exocytosis. Syt 1 harbors two
Ca2+-binding motifs referred to as C2A and C2B. In this study we
examine the function of the isolated C2 domains of Syt 1 using a
reconstituted, SNARE (soluble N-ethylmaleimide-sensitive factor
attachment receptor)-mediated, fusion assay. We report that inclusion of
phosphatidylethanolamine into reconstituted SNARE vesicles enabled isolated
C2B, but not C2A, to regulate Ca2+-triggered fusion. The isolated
C2B domain had a 6-fold lower EC for Ca2+ 50-activated fusion than
the intact cytosolic domain of Syt 1 (C2AB). Phosphatidylethanolamine
increased both the rate and efficiency of C2AB- and C2B-regulated fusion
without affecting their abilities to bind membrane-embedded syntaxin-SNAP-25
(t-SNARE) complexes. At equimolar concentrations, the isolated C2A domain was
an effective inhibitor of C2B-, but not C2AB-regulated fusion; hence, C2A has
markedly different effects in the fusion assay depending on whether it is
tethered to C2B. Finally, scanning alanine mutagenesis of C2AB revealed four
distinct groups of mutations within the C2B domain that play roles in the
regulation of SNARE-mediated fusion. Surprisingly, substitution of Arg-398
with alanine, which lies on the opposite end of C2B from the
Ca2+/membrane-binding loops, decreases C2AB t-SNARE binding and
Ca2+-triggered fusion in vitro without affecting
Ca2+-triggered interactions with phosphatidylserine or vesicle
aggregation. In addition, some mutations uncouple the clamping and stimulatory
functions of syt 1, suggesting that these two activities are mediated by
distinct structural determinants in C2B.
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Affiliation(s)
- Jon D Gaffaney
- Department of Physiology, University of Wisconsin, Madison, Wisconsin 53706, USA
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27
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Abstract
Neurotransmitter release at synapses involves a highly specialized form of membrane fusion that is triggered by Ca(2+) ions and is optimized for speed. These observations were established decades ago, but only recently have the molecular mechanisms that underlie this process begun to come into view. Here, we summarize findings obtained from genetically modified neurons and neuroendocrine cells, as well as from reconstituted systems, which are beginning to reveal the molecular mechanism by which Ca(2+)-acting on the synaptic vesicle (SV) protein synaptotagmin I (syt)-triggers rapid exocytosis. This work sheds light not only on presynaptic aspects of synaptic transmission, but also on the fundamental problem of membrane fusion, which has remained a puzzle that has yet to be solved in any biological system.
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Affiliation(s)
- Edwin R Chapman
- Howard Hughes Medical Institute and Department of Physiology, University of Wisconsin, Madison, WI 53706, USA.
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Bhalla A, Chicka MC, Chapman ER. Analysis of the synaptotagmin family during reconstituted membrane fusion. Uncovering a class of inhibitory isoforms. J Biol Chem 2008; 283:21799-807. [PMID: 18508778 PMCID: PMC2490792 DOI: 10.1074/jbc.m709628200] [Citation(s) in RCA: 53] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Ca(2+)-triggered exocytosis in neurons and neuroendocrine cells is regulated by the Ca(2+)-binding protein synaptotagmin (syt) I. Sixteen additional isoforms of syt have been identified, but little is known concerning their biochemical or functional properties. Here, we assessed the abilities of fourteen syt isoforms to directly regulate SNARE (soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptor)-catalyzed membrane fusion. One group of isoforms stimulated neuronal SNARE-mediated fusion in response to Ca(2+), while another set inhibited SNARE catalyzed fusion in both the absence and presence of Ca(2+). Biochemical analysis revealed a strong correlation between the ability of syt isoforms to bind 1,2-dioleoyl phosphatidylserine (PS) and t-SNAREs in a Ca(2+)-promoted manner with their abilities to enhance fusion, further establishing PS and SNAREs as critical effectors for syt action. The ability of syt I to efficiently stimulate fusion was specific for certain SNARE pairs, suggesting that syts might contribute to the specificity of intracellular membrane fusion reactions. Finally, a subset of inhibitory syts down-regulated the ability of syt I to activate fusion, demonstrating that syt isoforms can modulate the function of each other.
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Affiliation(s)
- Akhil Bhalla
- Howard Hughes Medical Institute, and Department of Physiology, University of Wisconsin, 1300 University Avenue, Madison, WI 53706, USA
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29
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Rajalingam D, Graziani I, Prudovsky I, Yu C, Kumar TKS. Relevance of partially structured states in the non-classical secretion of acidic fibroblast growth factor. Biochemistry 2007; 46:9225-38. [PMID: 17636870 PMCID: PMC3656169 DOI: 10.1021/bi7002586] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Acidic fibroblast growth factor (aFGF) is a signal peptide-less protein that is secreted into the extracellular compartment as part of a multiprotein release complex, consisting of aFGF, S100A13 (a calcium binding protein), and a 40 kDa (p40) form of synaptotagmin (Syt1), a protein that participates in the docking of a variety of secretory vesicles. p40 Syt1, and specifically its C2A domain, is believed to play a major role in the non-classical secretion of the aFGF release complex mediated by the interaction of aFGF and p40 Syt1with the phospholipids of the cell membrane inner leaflet. In the present study, we investigate the structural characteristics of aFGF and the C2A domain of p40 Syt1 under acidic conditions, using a variety of biophysical techniques including multidimensional NMR spectroscopy. Urea-induced equilibrium unfolding (at pH 3.4) of both aFGF and the C2A domain are non-cooperative and proceed with the accumulation of stable intermediate states. 1-Anilino-8-napthalene sulfonate (ANS) binding and size-exclusion chromatography results suggest that both aFGF and the C2A domain exist as partially structured states under acidic conditions (pH 3.4). Limited trypsin digestion analysis and 1H-15N chemical shift perturbation data reveal that the flexibility of certain portions of the protein backbone is increased in the partially structured state(s) of aFGF. The residues that are perturbed in the partially structured state(s) in aFGF are mostly located at the N- and C-terminal ends of the protein. In marked contrast, most of the interactions stabilizing the native secondary structure are preserved in the partially structured state of the C2A domain. Isothermal titration calorimetry data indicate that the binding affinity between aFGF and the C2A domain is significantly enhanced at pH 3.4. In addition, both aFGF and the C2A domain exhibit much higher lipid binding affinity in their partially structured states. The translocation of the multiprotein FGF release complex across the membrane appears to be facilitated by the formation of partially structured states of aFGF and the C2A domain of p40 Syt1.
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Affiliation(s)
| | - Irene Graziani
- Department of Chemistry and Biochemistry, Center for Molecular Medicine, Maine Medical Center Research Institute, Scarborough, Maine 04074
| | - Igor Prudovsky
- Department of Chemistry and Biochemistry, Center for Molecular Medicine, Maine Medical Center Research Institute, Scarborough, Maine 04074
| | - Chin Yu
- Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701
- Department of Chemistry, National Tsing Hua University, Hsinchu 30043, Taiwan
- To whom correspondence should be addressed. Phone: 479-575-5646. Fax: 479-575-4049. (T.K.S.K.). Phone: 886-35-711082. Fax: 886-35-721524. cyu@ mx.nthu.edu.tw (C.Y.)
| | - Thallapuranam Krishnaswamy S. Kumar
- Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701
- To whom correspondence should be addressed. Phone: 479-575-5646. Fax: 479-575-4049. (T.K.S.K.). Phone: 886-35-711082. Fax: 886-35-721524. cyu@ mx.nthu.edu.tw (C.Y.)
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Tsuboi T, Fukuda M. Synaptotagmin VII modulates the kinetics of dense-core vesicle exocytosis in PC12 cells. Genes Cells 2007; 12:511-9. [PMID: 17397398 DOI: 10.1111/j.1365-2443.2007.01070.x] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
In our previous study, we showed that PC12 cell lines stably expressing synaptotagmin (Syt) VII have greater ability to release hormones Ca(2+)-dependently than the original PC12 cells. However, the precise molecular mechanism of the enhancement of hormone secretion by Syt VII has never been elucidated. In this study, we established a PC12 cell line that stably expresses Syt VII-green fluorescent protein (Syt VII-GFP) or its Ca(2+)-binding-site-deficient mutant (D172N/D303N substitutions; Syt VII-DN-GFP), and examined the effect of Syt VII-GFP expression on the kinetics of dense-core vesicle exocytosis by total internal reflection fluorescence (TIRF) microscopy. Both Syt VII-GFP and Syt VII-DN-GFP co-localized well with dense-core vesicle markers, monomeric red fluorescent protein (mRFP)-tagged neuropeptide Y (NPY-mRFP) and cyan fluorescent protein (CFP)-tagged tissue plasminogen activator (tPA-CFP). Expression of Syt VII-GFP enhanced the number of dense-core vesicle exocytotic events, whereas expression of Syt VII-DN-GFP or knockdown of Syt VII-GFP with specific small interfering RNA (siRNA) attenuated the number of exocytotic events. Monitoring individual tPA-CFP release events revealed that "full release" events are increased in Syt VII-GFP-expressing cells, but not in Syt VII-DN-GFP-expressing or Syt VII-silenced cells. Our data indicate that Syt VII modulates the kinetics of Ca(2+)-dependent dense-core vesicle exocytosis in neuroendocrine PC12 cells, possibly by modulating fusion pore opening.
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Affiliation(s)
- Takashi Tsuboi
- Fukuda Initiative Research Unit, RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
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Min SW, Chang WP, Südhof TC. E-Syts, a family of membranous Ca2+-sensor proteins with multiple C2 domains. Proc Natl Acad Sci U S A 2007; 104:3823-8. [PMID: 17360437 PMCID: PMC1820668 DOI: 10.1073/pnas.0611725104] [Citation(s) in RCA: 146] [Impact Index Per Article: 8.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
C(2) domains are autonomously folded protein modules that generally act as Ca(2+)- and phospholipid-binding domains and/or as protein-protein interaction domains. We now report the primary structures and biochemical properties of a family of evolutionarily conserved mammalian proteins, referred to as E-Syts, for extended synaptotagmin-like proteins. E-Syts contain an N-terminal transmembrane region, a central juxtamembranous domain that is conserved from yeast to human, and five (E-Syt1) or three (E-Syt2 and E-Syt3) C-terminal C(2) domains. Only the first E-Syt C(2) domain, the C(2)A domain, includes the complete sequence motif that is required for Ca(2+) binding in C(2) domains. Recombinant protein fragments of E-Syt2 that include the first C(2) domain are capable of Ca(2+)-dependent phospholipid binding at micromolar concentrations of free Ca(2+), suggesting that E-Syts bind Ca(2+) through their first C(2) domain in a phospholipid complex. E-Syts are ubiquitously expressed, but enriched in brain. Expression of myc-tagged E-Syt proteins in transfected cells demonstrated localization to intracellular membranes for E-Syt1 and to plasma membranes for E-Syt2 and E-Syt3. Structure/function studies showed that the plasma-membrane localization of E-Syt2 and E-Syt3 was directed by their C-terminal C(2)C domains. This result reveals an unexpected mechanism by which the C(2)C domains of E-Syt2 and E-Syt3 functions as a targeting motif that localizes these proteins into the plasma membrane independent of their transmembrane region. Viewed together, our findings suggest that E-Syts function as Ca(2+)-regulated intrinsic membrane proteins with multiple C(2) domains, expanding the repertoire of such proteins to a fourth class beyond synaptotagmins, ferlins, and MCTPs (multiple C(2) domain and transmembrane region proteins).
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Affiliation(s)
| | | | - Thomas C. Südhof
- Departments of *Neuroscience and
- Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, TX 75390-9111
- To whom correspondence should be addressed. E-mail:
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Lynch KL, Martin TFJ. Synaptotagmins I and IX function redundantly in regulated exocytosis but not endocytosis in PC12 cells. J Cell Sci 2007; 120:617-27. [PMID: 17264148 DOI: 10.1242/jcs.03375] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
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.
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Affiliation(s)
- Kara L Lynch
- Department of Biochemistry, 433 Babcock Drive, University of Wisconsin, Madison, WI 53706, USA
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Abstract
Neuronal communication is tightly regulated in time and space. Following neuronal activation, an electrical signal triggers neurotransmitter (NT) release at the active zone. The process starts by the signal reaching the synapse followed by a fusion of the synaptic vesicle (SV) and diffusion of the released NT in the synaptic cleft. The NT then binds to the appropriate receptor and induces a membrane potential change at the target cell membrane. The entire process is controlled by a fairly small set of synaptic proteins, collectively called SYCONs. The biochemical features of SYCONs underlie the properties of NT release. SYCONs are characterized by their ability to detect and respond to changes in environmental signals. For example, consider synaptotagmin I (Syt1), a prototype of a protein family with over 20 gene and variants in mammals. Syt1 is a specific example of a multi-sensor device with a large repertoire of discrete states. Several of these states are stimulated by a local concentration of signaling molecules such as Ca2+. The ability of this protein to sense signaling molecules and to adopt multiple biochemical states is shared by other SYCONs such as the synapsins (Syns). Specific biochemical states of Syns determine the accessibility of SV for NT release. Each of these states is defined by a specific alternative spliced variant with a unique profile of phosphorylation modified sites. The plasticity of the synapse is a direct reflection of SYCON's multiple biochemical states. State transitions occurs in a wide range of time scales, and therefore these molecules need to cope with events that last milliseconds (i.e., exocytosis in fast responding synapses) and with events that can carry on for many minutes (i.e., organization of SV pools). We suggest that SYCONs are optimized throughout evolution as multi-sensor devices. A full repertoire of the switches leading to alternation of protein states and a detailed characterization of protein-protein network within the synapse is critical for the development of a dynamic model of synaptic transmission.
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Affiliation(s)
- Guy Brachya
- Dept of Biological Chemistry, The Hebrew University of Jerusalem, 91904, Israel
| | - Chava Yanay
- Dept of Biological Chemistry, The Hebrew University of Jerusalem, 91904, Israel
| | - Michal Linial
- Dept of Biological Chemistry, The Hebrew University of Jerusalem, 91904, Israel
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Xiong X, Zhou KM, Wu ZX, Xu T. Silence of synaptotagmin I in INS-1 cells inhibits fast exocytosis and fast endocytosis. Biochem Biophys Res Commun 2006; 347:76-82. [PMID: 16808897 DOI: 10.1016/j.bbrc.2006.06.045] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/03/2006] [Accepted: 06/08/2006] [Indexed: 10/24/2022]
Abstract
Synaptotagmin I (Syt I) is a Ca(2+) sensor for triggering fast synchronized release of neurotransmitters. However, controversy remains whether Syt I is also obligatory for the exocytosis and endocytosis of larger dense core vesicles (LDCVs) in endocrine cells. In this study, we used a short hairpin RNA (shRNA) to silence the expression of Syt I and investigated the roles of Syt I on exocytosis and endocytosis in INS-1 cells. Our results demonstrated that expression of Syt I is remarkably reduced by the Syt I gene targeting shRNA. Using high-time resolution capacitance measurement, we found that the silence of Syt I decreased the calcium sensitivity of fusion of insulin granules and therefore reduced the exocytotic burst triggered by step-like [Ca(2+)](i) elevation. In addition, the occurrence frequency and amplitude of fast endocytosis were remarkably reduced in the silenced cells. We conclude that Syt I not only participates in the Ca(2+)-sensing of LDCV fusion with plasmalemma, but also plays a crucial role in fast endocytosis in INS-1 cells.
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Affiliation(s)
- Xiong Xiong
- Institute of Biophysics and Biochemistry, HuaZhong University of Science and Technology, Wuhan, PR China
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35
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Deák F, Shin OH, Tang J, Hanson P, Ubach J, Jahn R, Rizo J, Kavalali ET, Südhof TC. Rabphilin regulates SNARE-dependent re-priming of synaptic vesicles for fusion. EMBO J 2006; 25:2856-66. [PMID: 16763567 PMCID: PMC1500841 DOI: 10.1038/sj.emboj.7601165] [Citation(s) in RCA: 84] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/20/2005] [Accepted: 05/03/2006] [Indexed: 11/08/2022] Open
Abstract
Synaptic vesicle fusion is catalyzed by assembly of synaptic SNARE complexes, and is regulated by the synaptic vesicle GTP-binding protein Rab3 that binds to RIM and to rabphilin. RIM is a known physiological regulator of fusion, but the role of rabphilin remains obscure. We now show that rabphilin regulates recovery of synaptic vesicles from use-dependent depression, probably by a direct interaction with the SNARE protein SNAP-25. Deletion of rabphilin dramatically accelerates recovery of depressed synaptic responses; this phenotype is rescued by viral expression of wild-type rabphilin, but not of mutant rabphilin lacking the second rabphilin C2 domain that binds to SNAP-25. Moreover, deletion of rabphilin also increases the size of synaptic responses in synapses lacking the vesicular SNARE protein synaptobrevin in which synaptic responses are severely depressed. Our data suggest that binding of rabphilin to SNAP-25 regulates exocytosis of synaptic vesicles after the readily releasable pool has either been physiologically exhausted by use-dependent depression, or has been artificially depleted by deletion of synaptobrevin.
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Affiliation(s)
- Ferenc Deák
- Center for Basic Neuroscience, University of Texas Southwestern Medical Center, Dallas, TX, USA
- Howard Hughes Medical Institute, UT Southwestern Medical Center, Dallas, TX, USA
| | - Ok-Ho Shin
- Center for Basic Neuroscience, University of Texas Southwestern Medical Center, Dallas, TX, USA
| | - Jiong Tang
- Center for Basic Neuroscience, University of Texas Southwestern Medical Center, Dallas, TX, USA
| | - Phyllis Hanson
- Department of Pharmacology and Howard Hughes Medical Institute, Yale University School of Medicine, New Haven, CT, USA
| | - Josep Ubach
- Departments of Biochemistry and Pharmacology, UT Southwestern Medical Center, Dallas, TX, USA
| | - Reinhard Jahn
- Department of Pharmacology and Howard Hughes Medical Institute, Yale University School of Medicine, New Haven, CT, USA
| | - Josep Rizo
- Departments of Biochemistry and Pharmacology, UT Southwestern Medical Center, Dallas, TX, USA
| | - Ege T Kavalali
- Center for Basic Neuroscience, University of Texas Southwestern Medical Center, Dallas, TX, USA
- Department of Physiology, University of Texas Southwestern Medical Center, Dallas, TX, USA
| | - Thomas C Südhof
- Center for Basic Neuroscience, University of Texas Southwestern Medical Center, Dallas, TX, USA
- Howard Hughes Medical Institute, UT Southwestern Medical Center, Dallas, TX, USA
- Department of Molecular Genetics, UT Southwestern Medical Center, Dallas, TX, USA
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Pang ZP, Sun J, Rizo J, Maximov A, Südhof TC. Genetic analysis of synaptotagmin 2 in spontaneous and Ca2+-triggered neurotransmitter release. EMBO J 2006; 25:2039-50. [PMID: 16642042 PMCID: PMC1462977 DOI: 10.1038/sj.emboj.7601103] [Citation(s) in RCA: 132] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/09/2006] [Accepted: 03/28/2006] [Indexed: 11/09/2022] Open
Abstract
Synaptotagmin 2 resembles synaptotagmin 1, the Ca2+ sensor for fast neurotransmitter release in forebrain synapses, but little is known about synaptotagmin 2 function. Here, we describe a severely ataxic mouse strain that harbors a single, destabilizing amino-acid substitution (I377N) in synaptotagmin 2. In Calyx of Held synapses, this mutation causes a delay and a decrease in Ca2+-induced but not in hypertonic sucrose-induced release, suggesting that synaptotagmin 2 mediates Ca2+ triggering of evoked release in brainstem synapses. Unexpectedly, we additionally observed in synaptotagmin 2 mutant synapses a dramatic increase in spontaneous release. Synaptotagmin 1-deficient excitatory and inhibitory cortical synapses also displayed a large increase in spontaneous release, demonstrating that this effect was shared among synaptotagmins 1 and 2. Our data suggest that synaptotagmin 1 and 2 perform equivalent functions in the Ca2+ triggering of action potential-induced release and in the restriction of spontaneous release, consistent with a general role of synaptotagmins in controlling 'release slots' for synaptic vesicles at the active zone.
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Affiliation(s)
- Zhiping P Pang
- Departments of Molecular Genetics, Pharmacology, and Biochemistry, Center for Basic Neuroscience, Howard Hughes Medical Institute, UT Southwestern Medical Center, Dallas, TX, USA
| | - Jianyuan Sun
- Departments of Molecular Genetics, Pharmacology, and Biochemistry, Center for Basic Neuroscience, Howard Hughes Medical Institute, UT Southwestern Medical Center, Dallas, TX, USA
| | - Josep Rizo
- Departments of Molecular Genetics, Pharmacology, and Biochemistry, Center for Basic Neuroscience, Howard Hughes Medical Institute, UT Southwestern Medical Center, Dallas, TX, USA
| | - Anton Maximov
- Departments of Molecular Genetics, Pharmacology, and Biochemistry, Center for Basic Neuroscience, Howard Hughes Medical Institute, UT Southwestern Medical Center, Dallas, TX, USA
| | - Thomas C Südhof
- Departments of Molecular Genetics, Pharmacology, and Biochemistry, Center for Basic Neuroscience, Howard Hughes Medical Institute, UT Southwestern Medical Center, Dallas, TX, USA
- Department of Molecular Genetics, Center for Basic Neuroscience, Howard Hughes Medical Institute, UT Southwestern Medical Center, 6000 Harry Hines Blvd., Dallas, TX 75390-9111 USA. Tel.: +1 214 648 1876; Fax: +1 214 648 1879; E-mail:
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Li L, Shin OH, Rhee JS, Araç D, Rah JC, Rizo J, Südhof T, Rosenmund C. Phosphatidylinositol phosphates as co-activators of Ca2+ binding to C2 domains of synaptotagmin 1. J Biol Chem 2006; 281:15845-52. [PMID: 16595652 DOI: 10.1074/jbc.m600888200] [Citation(s) in RCA: 94] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Ca2+-dependent phospholipid binding to the C2A and C2B domains of synaptotagmin 1 is thought to trigger fast neurotransmitter release, but only Ca2+ binding to the C2B domain is essential for release. To investigate the underlying mechanism, we have compared the role of basic residues in Ca2+/phospholipid binding and in release. Mutations in a polybasic sequence on the side of the C2B domain beta-sandwich or in a basic residue in a top Ca2+-binding loop of the C2A domain (R233) cause comparable decreases in the apparent Ca2+ affinity of synaptotagmin 1 and the Ca2+ sensitivity of release, whereas mutation of the residue homologous to Arg233 in the C2B domain (Lys366) has no effect. Phosphatidylinositol polyphosphates co-activate Ca2+-dependent and -independent phospholipid binding to synaptotagmin 1, but the effects of these mutations on release only correlate with their effects on the Ca2+-dependent component. These results reveal clear distinctions in the Ca2+-dependent phospholipid binding modes of the synaptotagmin 1 C2 domains that may underlie their functional asymmetry and suggest that phosphatidylinositol polyphosphates may serve as physiological modulators of Ca2+ affinity of synaptotagmin 1 in vivo.
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Affiliation(s)
- LiYi Li
- Department of Neuroscience, Baylor College of Medicine, Houston, Texas 77030, USA
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38
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Araç D, Chen X, Khant HA, Ubach J, Ludtke SJ, Kikkawa M, Johnson AE, Chiu W, Südhof TC, Rizo J. Close membrane-membrane proximity induced by Ca2+-dependent multivalent binding of synaptotagmin-1 to phospholipids. Nat Struct Mol Biol 2006; 13:209-17. [PMID: 16491093 DOI: 10.1038/nsmb1056] [Citation(s) in RCA: 212] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2005] [Accepted: 01/03/2006] [Indexed: 11/09/2022]
Abstract
Synaptotagmin acts as a Ca(2+) sensor in neurotransmitter release through its two C(2) domains. Ca(2+)-dependent phospholipid binding is key for synaptotagmin function, but it is unclear how this activity cooperates with the SNARE complex involved in release or why Ca(2+) binding to the C(2)B domain is more crucial for release than Ca(2+) binding to the C(2)A domain. Here we show that Ca(2+) induces high-affinity simultaneous binding of synaptotagmin to two membranes, bringing them into close proximity. The synaptotagmin C(2)B domain is sufficient for this ability, which arises from the abundance of basic residues around its surface. We propose a model wherein synaptotagmin cooperates with the SNAREs in bringing the synaptic vesicle and plasma membranes together and accelerates membrane fusion through the highly positive electrostatic potential of its C(2)B domain.
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Affiliation(s)
- Demet Araç
- Department of Biochemistry, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, Texas 75390, USA
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39
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Rhee JS, Li LY, Shin OH, Rah JC, Rizo J, Südhof TC, Rosenmund C. Augmenting neurotransmitter release by enhancing the apparent Ca2+ affinity of synaptotagmin 1. Proc Natl Acad Sci U S A 2005; 102:18664-9. [PMID: 16352718 PMCID: PMC1311909 DOI: 10.1073/pnas.0509153102] [Citation(s) in RCA: 127] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Synaptotagmin 1 likely acts as a Ca2+ sensor in neurotransmitter release by Ca2+-binding to its two C2 domains. This notion was strongly supported by the observation that a mutation in the C2A domain causes parallel decreases in the apparent Ca2+ affinity of synaptotagmin 1 and in the Ca2+ sensitivity of release. However, this study was based on a single loss-of-function mutation. We now show that tryptophan substitutions in the synaptotagmin 1 C2 domains act as gain-of-function mutations to increase the apparent Ca2+ affinity of synaptotagmin 1. The same substitutions, when introduced into synaptotagmin 1 expressed in neurons, enhance the Ca2+ sensitivity of release. Mutations in the two C2 domains lead to comparable and additive effects in release. Our results thus show that the apparent Ca2+ sensitivity of release is dictated by the apparent Ca2+ affinity of synaptotagmin 1 in both directions, and that Ca2+ binding to both C2 domains contributes to Ca2+ triggering of release.
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Affiliation(s)
- J-S Rhee
- Department of Neuroscience, Baylor College of Medicine, One Baylor Plaza, Houston, TX 7703, USA
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40
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Crivellato E, Nico B, Ribatti D. Ultrastructural evidence of piecemeal degranulation in large dense-core vesicles of brain neurons. ACTA ACUST UNITED AC 2005; 210:25-34. [PMID: 16044317 DOI: 10.1007/s00429-005-0002-z] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 06/03/2005] [Indexed: 10/25/2022]
Abstract
Large dense-core vesicles (LDCV) are a group of neuronal secretory organelles with different size and characteristically condensed morphology. LDCV release their specific cargo by regulated exocytosis, either in the form of "full fusion" or "kiss-and-run" exocytosis. In this paper, we provide ultrastructural evidence indicative of a slow and particulate mode of secretion from LDCV, called piecemeal degranulation (PMD). A number of LDCV in the nerve boutons of mouse brain presented marked increase in their size accompanied by reduction and also disappearance of content material. Residual secretory constituents in altered LDCV displayed eroded marginated patterns, leading to eccentric "haloed" morphologies. Remarkably, altered LDCV never appeared to be fused with each other or with the nerve plasma membrane. Very small vesicles, empty or apparently loaded with the same material making-up the LDCV content, could be seen near or attached to LDCV and the plasma membrane. First described in basophils, mast cells and eosinophils, PMD has recently been recognized in various neuro-endocrine cells, like adrenal chromaffin cells and endocrine cells of the gastro-intestinal epithelia. Here we suggest that PMD may be a hitherto unrecognized pathway of neuron secretion. It would represent the morphological correlate of a long-lasting and low-level process of neuro-transmitter release. It extends the patterns of neuron secretion and possibly opens new perspectives in understanding neuron plasticity.
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Affiliation(s)
- E Crivellato
- Department of Medical and Morphological Researches, Anatomy Section, University of Udine Medical School, P.le Kolbe, n. 3, 33100, Udine, Italy.
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41
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Gong LW, Di Paolo G, Diaz E, Cestra G, Diaz ME, Lindau M, De Camilli P, Toomre D. Phosphatidylinositol phosphate kinase type I gamma regulates dynamics of large dense-core vesicle fusion. Proc Natl Acad Sci U S A 2005; 102:5204-9. [PMID: 15793002 PMCID: PMC555604 DOI: 10.1073/pnas.0501412102] [Citation(s) in RCA: 80] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Phosphatidylinositol-4,5-bisphosphate was proposed to be an important regulator of large dense-core vesicle exocytosis from neuroendocrine tissues. Here, we have examined the kinetics of secretion in chromaffin cells from mice lacking phosphatidylinositol phosphate kinase type I gamma, the major neuronal phosphatidylinositol-4-phosphate 5-kinase. Absence of this enzyme caused a reduction of the readily releasable vesicle pool and its refilling rate, with a small increase in morphologically docked vesicles, indicating a defect in vesicle priming. Furthermore, amperometry revealed a delay in fusion pore expansion. These results provide direct genetic evidence for a key role of phosphatidylinositol-4,5-bisphosphate synthesis in the regulation of large dense-core vesicle fusion dynamics.
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Affiliation(s)
- Liang-Wei Gong
- Department of Cell Biology, Yale University School of Medicine, New Haven, CT 06510, USA
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42
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Wang L, Li G, Sugita S. A Central Kinase Domain of Type I Phosphatidylinositol Phosphate Kinases Is Sufficient to Prime Exocytosis. J Biol Chem 2005; 280:16522-7. [PMID: 15728183 DOI: 10.1074/jbc.m413263200] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Exocytosis, a critical process for neuronal communication and hormonal regulation, involves several distinct steps including MgATP-dependent priming (which involves the synthesis of phosphatidylinositol 4,5-bisphosphate). Type I phosphatidylinositol phosphate kinases (PIPKIs) were purified biochemically as a priming factor. PIPKI consists of three domains: the N-terminal region, the central kinase domain, and the C-terminal region. Three isoforms (alpha, beta, and gamma) of PIPKI have been identified, and each is alternatively spliced at the C-terminal region. In the present study, we conducted a structure/function analysis of PIPKIs in the priming of exocytosis, and we found that recombinant PIPKIalpha and PIPKIgamma had priming activity. However, an unexpected finding of these results was that PIPKIbeta did not prime exocytosis. The N- or C-terminal region of PIPKIalpha and PIPKIgamma was not required for priming, which indicates that the central kinase domain is sufficient for this process. Alternative splicing in each isoform did not affect the isoform specificity in priming. Priming activity by isoforms is strongly correlated with their phosphatidylinositol phosphate kinase activity because PIPKIalpha and PIPKIgamma had higher kinase activity than PIPKIbeta. These results suggest that PIPKIalpha and PIPKIgamma are the critical priming factors for exocytosis; it also suggests that the levels of phosphatidylinositol phosphate kinase activity in producing phosphatidylinositol 4,5-bisphosphate specify the function of PIPKI isoforms in priming.
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Affiliation(s)
- Li Wang
- Division of Cellular and Molecular Biology, Toronto Western Research Institute, University Health Network and Department of Physiology, University of Toronto, 399 Bathurst Street, Toronto, Ontario M5T 2S8, Canada
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43
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Atiya-Nasagi Y, Cohen H, Medalia O, Fukudan M, Sagi-Eisenberg R. O-glycosylation is essential for intracellular targeting of synaptotagmins I and II in non-neuronal specialized secretory cells. J Cell Sci 2005; 118:1363-72. [PMID: 15755799 DOI: 10.1242/jcs.01710] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
We have examined the trafficking of synaptotagmin (Syt) I and II in the mast cell line rat basophilic leukemia (RBL-2H3). We demonstrate that both Syt I and Syt II travel through the plasma membrane and require endocytosis to reach their final intracellular localization. However, N- or C-terminal tagging of Syt II, but not of Syt I, prevents its internalization, trapping the tagged protein at the plasma membrane. Furthermore, a chimeric protein comprising a tagged luminal domain of Syt II fused with the remaining domains of Syt I also localizes to the plasma membrane, whereas a chimera consisting of tagged luminal domain of Syt I fused with Syt II colocalizes with Syt I on secretory granules. We also show that endocytosis of both Syt I and Syt II is strictly dependent on O-glycosylation processing, whereby O-glycosylation mutants of either protein fail to internalize and remain at the plasma membrane. Our results indicate that the luminal domains of Syt I and Syt II govern their internalization capacity from the plasma membrane and identify O-glycosylation as playing a crucial role in Syt trafficking in non-neuronal secretory cells.
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Affiliation(s)
- Yafit Atiya-Nasagi
- Department of Cell and Developmental Biology, Sackler School of Medicine, Tel Aviv University, Tel Aviv 69978, Israel
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44
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Fukuda M, Kanno E, Satoh M, Saegusa C, Yamamoto A. Synaptotagmin VII Is Targeted to Dense-core Vesicles and Regulates Their Ca2+-dependent Exocytosis in PC12 Cells. J Biol Chem 2004; 279:52677-84. [PMID: 15456748 DOI: 10.1074/jbc.m409241200] [Citation(s) in RCA: 72] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
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.
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Affiliation(s)
- Mitsunori Fukuda
- Fukuda Initiative Research Unit, RIKEN (the Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
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45
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Han W, Rhee JS, Maximov A, Lin W, Hammer RE, Rosenmund C, Südhof TC. C-terminal ECFP fusion impairs synaptotagmin 1 function: crowding out synaptotagmin 1. J Biol Chem 2004; 280:5089-100. [PMID: 15561725 DOI: 10.1074/jbc.m408757200] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
To allow the monitoring of synaptotagmin 1 trafficking in vivo, we generated transgenic mice expressing a synaptotagmin 1-enhanced cyan fluorescent protein (ECFP) fusion protein under control of the Thy1 promoter. Transgenic synaptotagmin 1-ECFP is expressed throughout the brain where it localizes to synapses and marks synapses in vivo. However, when we crossed transgenic synaptotagmin 1-ECFP mice with synaptotagmin 1 knock-out mice, we detected no rescue of survival or function. Furthermore, viral overexpression of synaptotagmin 1-ECFP in synaptotagmin 1-deficient neurons failed to restore normal Ca2+-triggered release, whereas overexpression of wild type synaptotagmin 1 did so efficiently. To determine whether synaptotagmin 1-ECFP is non-functional because the ECFP-fusion interferes with its biochemical activities, we measured Ca2+-independent binding of synaptotagmin 1-ECFP to SNARE complexes, and Ca2+-dependent binding of synaptotagmin 1-ECFP to phospholipids and to itself. Although the apparent Ca2+ affinity of synaptotagmin 1-ECFP was decreased compared with wild type synaptotagmin 1, we observed no major changes in Ca2+-dependent or -independent activities, indicating that the non-functionality of the synaptotagmin 1-ECFP fusion protein was not because of inactivation of its biochemical properties. These data suggest that synaptotagmin 1-ECFP is suitable for monitoring synaptic vesicle traffic in vivo because the synaptotagmin 1-ECFP marks synaptic vesicles without participating in exocytosis. In addition, the data demonstrate that synaptotagmin 1 function requires a free C terminus, possibly because of spatial constraints at the release sites.
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Affiliation(s)
- Weiping Han
- Center for Basic Neuroscience, the Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, Texas 75390-9111, USA
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46
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Abstract
Several members of the synaptotagmin (syt) family of vesicle proteins have been proposed to act as Ca2+ sensors on synaptic vesicles. The mechanism by which calcium activates this class of proteins has been the subject of controversy, yet relatively few detailed biophysical studies have been reported on how isoforms other than syt I respond to divalent metal ions. Here, we report a series of studies on the response of syt II to a wide range of metal ions. Analytical ultracentrifugation studies demonstrate that Ca2+ induces protein dimerization upon exposure to 5 mM Ca2+. Whereas Ba2+, Mg2+, or Sr2+ do not potentiate self-association as strongly as Ca2+, Pb2+ triggers self-association of syt II at concentrations as low as 10 microM. Partial proteolysis studies suggest that the various divalent metals cause different changes in the conformation of the protein. The high calcium concentrations required for self-association of syt II suggest that the oligomerized state of this protein is not a critical intermediate in vesicle fusion; however, low-affinity calcium sites on syt II may play a critical role in buffering calcium at the presynaptic active zone. In addition, the high propensity of lead to oligomerize syt II offers a possible molecular explanation for how lead interferes with calcium-evoked neurotransmitter release.
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Affiliation(s)
- Ricardo A García
- Department of Biochemistry, Molecular Biology, and Cell Biology, Northwestern University, Evanston, Illinois 60208, USA
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47
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Fukuda M. RNA interference-mediated silencing of synaptotagmin IX, but not synaptotagmin I, inhibits dense-core vesicle exocytosis in PC12 cells. Biochem J 2004; 380:875-9. [PMID: 15015935 PMCID: PMC1224215 DOI: 10.1042/bj20040096] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/16/2004] [Revised: 03/08/2004] [Accepted: 03/11/2004] [Indexed: 11/17/2022]
Abstract
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.
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Affiliation(s)
- Mitsunori Fukuda
- Fukuda Initiative Research Unit, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
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Shin OH, Han W, Wang Y, Südhof TC. Evolutionarily conserved multiple C2 domain proteins with two transmembrane regions (MCTPs) and unusual Ca2+ binding properties. J Biol Chem 2004; 280:1641-51. [PMID: 15528213 DOI: 10.1074/jbc.m407305200] [Citation(s) in RCA: 73] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
C2 domains are primarily found in signal transduction proteins such as protein kinase C, which generally contain a single C2 domain, and in membrane trafficking proteins such as synaptotagmins, which generally contain multiple C2 domains. In both classes of proteins, C2 domains usually regulate the respective protein's function by forming Ca(2+)-dependent or Ca(2+)-independent phospholipid complexes. We now describe MCTPs (multiple C2 domain and transmembrane region proteins), a novel family of evolutionarily conserved C2 domain proteins with unusual Ca(2+)-dependent properties. MCTPs are composed of a variable N-terminal sequence, three C2 domains, two transmembrane regions, and a short C-terminal sequence. The invertebrate organisms Caenorhabditis elegans and Drosophila melanogaster express a single MCTP gene, whereas vertebrates express two MCTP genes (MCTP1 and MCTP2) whose primary transcripts are extensively alternatively spliced. Most of the MCTP sequences, in particular the C2 domains, are highly conserved. All MCTP C2 domains except for the second C2 domain of MCTP2 include a perfect Ca2+/phospholipid-binding consensus sequence. To determine whether the C2 domains of MCTPs actually function as Ca2+/phospholipid-binding modules, we analyzed their Ca2+ and phospholipid binding properties. Surprisingly, we found that none of the three MCTP1 C2 domains interacted with negatively charged or neutral phospholipids in the presence or absence of Ca2+. However, Ca2+ titrations monitored via intrinsic tryptophan fluorescence revealed that all three C2 domains bound Ca2+ in the absence of phospholipids with a high apparent affinity (EC50 of approximately 1.3-2.3 microM). Our data thus reveal that MCTPs are evolutionarily conserved C2 domain proteins that are unusual in that the C2 domains are anchored in the membrane by two closely spaced transmembrane regions and represent Ca(2+)-binding but not phospholipid-binding modules.
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Affiliation(s)
- Ok-Ho Shin
- Center for Basic Neuroscience, Department of Molecular Genetics and Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, Texas 75390, USA
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Marshall C, Hitman GA, Partridge CJ, Clark A, Ma H, Shearer TR, Turner MD. Evidence that an isoform of calpain-10 is a regulator of exocytosis in pancreatic beta-cells. Mol Endocrinol 2004; 19:213-24. [PMID: 15471947 DOI: 10.1210/me.2004-0064] [Citation(s) in RCA: 92] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
Abstract
Calpain-10 (CAPN10) is the first type 2 diabetes susceptibility gene to be identified through a genome scan, with polymorphisms being associated with altered CAPN10 expression. Functional data have been hitherto elusive, but we report here a corresponding increase between CAPN10 expression level and regulated insulin secretion. Pancreatic beta-cell secretory granule exocytosis is mediated by the soluble N-ethylmaleimide-sensitive fusion protein attachment receptor protein complex of synaptosomal-associated protein of 25 kDa (SNAP-25), syntaxin 1, and vesicle-associated membrane protein 2. We report, for the first time, direct binding of a calpain-10 isoform with members of this complex. Furthermore, SNAP-25 undergoes a Ca2+-dependent partial proteolysis during exocytosis, with calpain protease inhibitor similarly suppressing both insulin secretion and SNAP-25 proteolysis. Based upon these findings, we postulate that an isoform of calpain-10 is a Ca2+-sensor that functions to trigger exocytosis in pancreatic beta-cells.
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Affiliation(s)
- Catriona Marshall
- Centre for Diabetes and Metabolic Medicine, Institute of Cell and Molecular Science, Barts and The London, Queen Mary's School of Medicine and Dentistry, University of London, Whitechapel, London E1 1BB, United Kingdom
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Meldolesi J, Chieregatti E, Luisa Malosio M. Requirements for the identification of dense-core granules. Trends Cell Biol 2004; 14:13-9. [PMID: 14729176 DOI: 10.1016/j.tcb.2003.11.006] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
Dense-core granules (DCGs), cytoplasmic organelles competent for regulated exocytosis, show considerable heterogeneity depending upon the specificity of their expressing cells--primarily neurons and neurosecretory cells. DCGs have been mainly identified by detecting their cargo molecules, often members of the granin family, and using conventional electron microscopy and immunocytochemistry. However, by a critical analysis of the various stages of DCG "life" within neurosecretory cells, we have highlighted several specific molecular and functional properties that are common to all these organelles. We propose that these properties be considered as strict requirements for the identification of DCGs.
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Affiliation(s)
- Jacopo Meldolesi
- Vita-Salute San Raffaele University and Scientific Institute San Raffaele, Department of Neuroscience and Immunology, Centre of Excellence in Physiopathology of Cell Differentiation, via Olgettina 58, 20132 Milan, Italy.
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