Silence of synaptotagmin I in INS-1 cells inhibits fast exocytosis and fast endocytosis

Rab26 restricts insulin secretion via sequestering Synaptotagmin-1

Rab26 is known to regulate multiple membrane trafficking events, but its role in insulin secretion in pancreatic β cells remains unclear despite it was first identified in the pancreas. In this study, we generated Rab26-/- mice through CRISPR/Cas9 technique. Surprisingly, insulin levels in the blood of the Rab26-/- mice do not decrease upon glucose stimulation but conversely increase. Deficiency of Rab26 promotes insulin secretion, which was independently verified by Rab26 knockdown in pancreatic insulinoma cells. Conversely, overexpression of Rab26 suppresses insulin secretion in both insulinoma cell lines and isolated mouse islets. Islets overexpressing Rab26, upon transplantation, also failed to restore glucose homeostasis in type 1 diabetic mice. Immunofluorescence microscopy revealed that overexpression of Rab26 results in clustering of insulin granules. GST-pulldown experiments reveal that Rab26 interacts with synaptotagmin-1 (Syt1) through directly binding to its C2A domain, which interfering with the interaction between Syt1 and SNAP25, and consequently inhibiting the exocytosis of newcomer insulin granules revealed by TIRF microscopy. Our results suggest that Rab26 serves as a negative regulator of insulin secretion, via suppressing insulin granule fusion with plasma membrane through sequestering Syt1.

The type 2 diabetes gene product STARD10 is a phosphoinositide-binding protein that controls insulin secretory granule biogenesis

OBJECTIVE

Risk alleles for type 2 diabetes at the STARD10 locus are associated with lowered STARD10 expression in the β-cell, impaired glucose-induced insulin secretion, and decreased circulating proinsulin:insulin ratios. Although likely to serve as a mediator of intracellular lipid transfer, the identity of the transported lipids and thus the pathways through which STARD10 regulates β-cell function are not understood. The aim of this study was to identify the lipids transported and affected by STARD10 in the β-cell and the role of the protein in controlling proinsulin processing and insulin granule biogenesis and maturation.

METHODS

We used isolated islets from mice deleted selectively in the β-cell for Stard10 (βStard10KO) and performed electron microscopy, pulse-chase, RNA sequencing, and lipidomic analyses. Proteomic analysis of STARD10 binding partners was executed in the INS1 (832/13) cell line. X-ray crystallography followed by molecular docking and lipid overlay assay was performed on purified STARD10 protein.

RESULTS

βStard10KO islets had a sharply altered dense core granule appearance, with a dramatic increase in the number of "rod-like" dense cores. Correspondingly, basal secretion of proinsulin was increased versus wild-type islets. The solution of the crystal structure of STARD10 to 2.3 Å resolution revealed a binding pocket capable of accommodating polyphosphoinositides, and STARD10 was shown to bind to inositides phosphorylated at the 3' position. Lipidomic analysis of βStard10KO islets demonstrated changes in phosphatidylinositol levels, and the inositol lipid kinase PIP4K2C was identified as a STARD10 binding partner. Also consistent with roles for STARD10 in phosphoinositide signalling, the phosphoinositide-binding proteins Pirt and Synaptotagmin 1 were amongst the differentially expressed genes in βStard10KO islets.

CONCLUSION

Our data indicate that STARD10 binds to, and may transport, phosphatidylinositides, influencing membrane lipid composition, insulin granule biosynthesis, and insulin processing.

Fusion pore in exocytosis: More than an exit gate? A β-cell perspective

Secretory vesicle exocytosis is a fundamental biological event and the process by which hormones (like insulin) are released into the blood. Considerable progress has been made in understanding this precisely orchestrated sequence of events from secretory vesicle docked at the cell membrane, hemifusion, to the opening of a membrane fusion pore. The exact biophysical and physiological regulation of these events implies a close interaction between membrane proteins and lipids in a confined space and constrained geometry to ensure appropriate delivery of cargo. We consider some of the still open questions such as the nature of the initiation of the fusion pore, the structure and the role of the Soluble N-ethylmaleimide-sensitive-factor Attachment protein REceptor (SNARE) transmembrane domains and their influence on the dynamics and regulation of exocytosis. We discuss how the membrane composition and protein-lipid interactions influence the likelihood of the nascent fusion pore forming. We relate these factors to the hypothesis that fusion pore expansion could be affected in type-2 diabetes via changes in disease-related gene transcription and alterations in the circulating lipid profile. Detailed characterisation of the dynamics of the fusion pore in vitro will contribute to understanding the larger issue of insulin secretory defects in diabetes.

Silence of Synaptotagmin VII inhibits release of dense core vesicles in PC12 cells

Synaptotagmin VII (Syt VII), which has a higher Ca(2+) affinity and slower disassembly kinetics with lipid than Syt I and Syt IX, was regarded as being uninvolved in synaptic vesicle (SV) exocytosis but instead possibly as a calcium sensor for the slower kinetic phase of dense core vesicles (DCVs) release. By using high temporal resolution capacitance and amperometry measurements, it was demonstrated that the knockdown of endogenous Syt VII attenuated the fusion of DCV with the plasma membrane, reduced the amplitude of the exocytotic burst of the Ca(2+)-triggered DCV release without affecting the slope of the sustained component, and blocked the fusion pore expansion. This suggests that Syt VII is the Ca(2+) sensor of DCV fusion machinery and is an essential factor for the establishment and maintenance of the pool size of releasable DCVs in PC12 cells.

Synaptotagmin-7 is a principal Ca2+ sensor for Ca2+ -induced glucagon exocytosis in pancreas

Hormones such as glucagon are secreted by Ca(2+)-induced exocytosis of large dense-core vesicles, but the mechanisms involved have only been partially elucidated. Studies of pancreatic beta-cells secreting insulin revealed that synaptotagmin-7 alone is not sufficient to mediate Ca(2+)-dependent insulin granule exocytosis, and studies of chromaffin cells secreting neuropeptides and catecholamines showed that synaptotagmin-1 and -7 collaborate as Ca(2+) sensors for exocytosis, and that both are equally involved. As no other peptide secretion was analysed, it remains unclear whether synaptotagmins generally act as Ca(2+) sensors in large dense-core vesicle exocytosis in endocrine cells, and if so, whether synaptotagmin-7 always functions with a partner in that role. In particular, far less is known about the mechanisms underlying Ca(2+)-triggered glucagon release from alpha-cells than insulin secretion from beta-cells, even though insulin and glucagon together regulate blood glucose levels. To address these issues, we analysed the role of synaptotagmins in Ca(2+)-triggered glucagon exocytosis. Surprisingly, we find that deletion of a single synaptotagmin isoform, synaptotagmin-7, nearly abolished Ca(2+)-triggered glucagon secretion. Moreover, single-cell capacitance measurements confirmed that pancreatic alpha-cells lacking synaptotagmin-7 exhibited little Ca(2+)-induced exocytosis, whereas all other physiological and morphological parameters of the alpha-cells were normal. Our data thus identify synaptotagmin-7 as a principal Ca(2+) sensor for glucagon secretion, and support the notion that synaptotagmins perform a universal but selective function as individually acting Ca(2+) sensors in neurotransmitter, neuropeptide, and hormone secretion.

Synaptotagmins bind calcium to release insulin

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

Synaptotagmin-1 utilizes membrane bending and SNARE binding to drive fusion pore expansion

In regulated vesicle exocytosis, SNARE protein complexes drive membrane fusion to connect the vesicle lumen with the extracellular space. The triggering of fusion pore formation by Ca(2+) is mediated by specific isoforms of synaptotagmin (Syt), which employ both SNARE complex and membrane binding. Ca(2+) also promotes fusion pore expansion and Syts have been implicated in this process but the mechanisms involved are unclear. We determined the role of Ca(2+)-dependent Syt-effector interactions in fusion pore expansion by expressing Syt-1 mutants selectively altered in Ca(2+)-dependent SNARE binding or in Ca(2+)-dependent membrane insertion in PC12 cells that lack vesicle Syts. The release of different-sized fluorescent peptide-EGFP vesicle cargo or the vesicle capture of different-sized external fluorescent probes was used to assess the extent of fusion pore dilation. We found that PC12 cells expressing partial loss-of-function Syt-1 mutants impaired in Ca(2+)-dependent SNARE binding exhibited reduced fusion pore opening probabilities and reduced fusion pore expansion. Cells with gain-of-function Syt-1 mutants for Ca(2+)-dependent membrane insertion exhibited normal fusion pore opening probabilities but the fusion pores dilated extensively. The results indicate that Syt-1 uses both Ca(2+)-dependent membrane insertion and SNARE binding to drive fusion pore expansion.

Central corticosteroid actions: Search for gene targets

Although many of the physiological effects of corticosteroid stress hormones on neuronal function are well recognised, the underlying genomic mechanisms are only starting to be elucidated. Linking physiology and genomics has proven to be a complicated task, despite the emergence of large-scale gene expression profiling technology in the last decade. This is in part due to the complexity of glucocorticoid-signaling, in part due to the complexity of the brain itself. The presence of a binary receptor system for glucocorticoid hormones in limbic brain structures, the coexistence of membrane and intracellular receptors and the highly contextual action of glucocorticoids contribute to this complexity. In addition, the anatomical complexity, extensive cellular heterogeneity of brain and the modest changes in gene expression (mostly in the range of 10-30%) hamper detection of responsive genes, in particular of low abundant transcripts, such as many neurotransmitter receptors and growth factors. Nonetheless, ongoing research into central targets of glucocorticoids has identified many different functional gene classes that underlie the diverse effects of glucocorticoids on brain function. These functional classes include genes involved in energy metabolism, signal transduction, neuronal structure, vesicle dynamics, neurotransmitter catabolism, cell adhesion, genes encoding neurotrophic factors and their receptors and genes involved in regulating glucocorticoid-signalling. The aim of this review is to give an overview of the current status of the field on identification of central corticosteroid targets, discuss the opportunities and pitfalls and highlight new developments in understanding central corticosteroid action.

Distinct roles of the C2A and the C2B domain of the vesicular Ca2+ sensor synaptotagmin 9 in endocrine beta-cells

Synaptotagmins form a family of calcium-sensor proteins implicated in exocytosis, and these vesicular transmembrane proteins are endowed with two cytosolic calcium-binding C2 domains, C2A and C2B. Whereas the isoforms syt1 and syt2 have been studied in detail, less is known about syt9, the calcium sensor involved in endocrine secretion such as insulin release from large dense core vesicles in pancreatic beta-cells. Using cell-based assays to closely mimic physiological conditions, we observed SNARE (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor)-independent translocation of syt9C2AB to the plasma membrane at calcium levels corresponding to endocrine exocytosis, followed by internalization to endosomes. The use of point mutants and truncations revealed that initial translocation required only the C2A domain, whereas the C2B domain ensured partial pre-binding of syt9C2AB to the membrane and post-stimulatory localization to endosomes. In contrast with the known properties of neuronal and neuroendocrine syt1 or syt2, the C2B domain of syt9 did not undergo calcium-dependent membrane binding despite a high degree of structural homology as observed through molecular modelling. The present study demonstrates distinct intracellular properties of syt9 with different roles for each C2 domain in endocrine cells.

The Ins and Outs of Secretion from Pancreatic β-Cells: Control of Single-Vesicle Exo- and Endocytosis

Exocytosis of insulin-containing secretory vesicles in pancreatic β-cells is crucial to maintenance of plasma glucose levels. They fuse with the plasma membrane in a regulated manner to release their contents and are subsequently recaptured either intact or through conventional clathrin-mediated endocytosis. Here, we discuss these mechanisms in β-cells at the single-vesicle level.

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

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

'Fractional recovery' analysis of a presynaptic synaptotagmin 1-anchored endocytic protein complex

Background

The integral synaptic vesicle protein and putative calcium sensor, synaptotagmin 1 (STG), has also been implicated in synaptic vesicle (SV) recovery. However, proteins with which STG interacts during SV endocytosis remain poorly understood. We have isolated an STG-associated endocytic complex (SAE) from presynaptic nerve terminals and have used a novel fractional recovery (FR) assay based on electrostatic dissociation to identify SAE components and map the complex structure. The location of SAE in the presynaptic terminal was determined by high-resolution quantitative immunocytochemistry at the chick ciliary ganglion giant calyx-type synapse.

Methodology/Principle Findings

The first step in FR analysis was to immunoprecipitate (IP) the complex with an antibody against one protein component (the IP-protein). The immobilized complex was then exposed to a high salt (1150 mM) stress-test that caused shedding of co-immunoprecipitated proteins (co-IP-proteins). A Fractional Recovery ratio (FR: recovery after high salt/recovery with control salt as assayed by Western blot) was calculated for each co-IP-protein. These FR values reflect complex structure since an easily dissociated protein, with a low FR value, cannot be intermediary between the IP-protein and a salt-resistant protein. The structure of the complex was mapped and a blueprint generated with a pair of FR analyses generated using two different IP-proteins. The blueprint of SAE contains an AP180/X/STG/stonin 2/intersectin/epsin core (X is unknown and epsin is hypothesized), and an AP2 adaptor, H-/L-clathrin coat and dynamin scission protein perimeter. Quantitative immunocytochemistry (ICA/ICQ method) at an isolated calyx-type presynaptic terminal indicates that this complex is associated with STG at the presynaptic transmitter release face but not with STG on intracellular synaptic vesicles.

Conclusions/Significance

We hypothesize that the SAE serves as a recognition site and also as a seed complex for clathrin-mediated synaptic vesicle recovery. The combination of FR analysis with quantitative immunocytochemistry provides a novel and effective strategy for the identification and characterization of biologically-relevant multi-molecular complexes.