Subcellular distribution and function of Rab3A, B, C, and D isoforms in insulin-secreting cells

Integrative structural modelling and visualisation of a cellular organelle

Models of insulin secretory vesicles from pancreatic beta cells have been created using the cellPACK suite of tools to research, curate, construct and visualise the current state of knowledge. The model integrates experimental information from proteomics, structural biology, cryoelectron microscopy and X-ray tomography, and is used to generate models of mature and immature vesicles. A new method was developed to generate a confidence score that reconciles inconsistencies between three available proteomes using expert annotations of cellular localisation. The models are used to simulate soft X-ray tomograms, allowing quantification of features that are observed in experimental tomograms, and in turn, allowing interpretation of X-ray tomograms at the molecular level.

SNARE Complex-Associated Proteins in the Lateral Amygdala of Macaca mulatta Following Long-Term Ethanol Drinking

BACKGROUND

Recent work with long-term ethanol (EtOH) self-administration in nonhuman primate models has revealed a complex array of behavioral and physiological effects that closely mimic human alcohol abuse. Detailed neurophysiological analysis in these models suggests a myriad of pre- and postsynaptic neurobiological effects that may contribute to the behavioral manifestations of long-term EtOH drinking. The molecular mechanisms regulating presynaptic effects of this chronic EtOH exposure are largely unknown. To this end, we analyzed the effects of long-term EtOH self-administration on the levels of presynaptic SNARE complex proteins in Macaca mulatta basolateral amygdala, a brain region known to regulate both aversive and reward-seeking behaviors.

METHODS

Basolateral amygdala samples from control and EtOH-drinking male and female monkeys were processed. Total basolateral amygdala protein was analyzed by Western blotting using antibodies directed against both core SNARE and SNARE-associated proteins. We also performed correlational analyses between protein expression levels and a number of EtOH drinking parameters, including lifetime grams of EtOH consumed, preference, and blood alcohol concentration.

RESULTS

Significant interactions or main effects of sex/drinking were seen for a number of SNARE core and SNARE-associated proteins. Across the range of EtOH-drinking phenotypes, SNAP25 and Munc13-1 proteins levels were significantly different between males and females, and Munc13-2 levels were significantly lower in animals with a history of EtOH drinking. A separate analysis of very heavy-drinking individuals revealed significant decreases in Rab3c (females) and complexin 2 (males).

CONCLUSIONS

Protein expression analysis of basolateral amygdala total protein from controls and animals following long-term EtOH self-administration suggests a number of alterations in core SNARE or SNARE-associated components that could dramatically alter presynaptic function. A number of proteins or multiprotein components were also correlated with EtOH drinking behavior, which suggest a potentially heritable role for presynaptic SNARE proteins.

Pancreatic β-Cell Electrical Activity and Insulin Secretion: Of Mice and Men

The pancreatic β-cell plays a key role in glucose homeostasis by secreting insulin, the only hormone capable of lowering the blood glucose concentration. Impaired insulin secretion results in the chronic hyperglycemia that characterizes type 2 diabetes (T2DM), which currently afflicts >450 million people worldwide. The healthy β-cell acts as a glucose sensor matching its output to the circulating glucose concentration. It does so via metabolically induced changes in electrical activity, which culminate in an increase in the cytoplasmic Ca2+ concentration and initiation of Ca2+-dependent exocytosis of insulin-containing secretory granules. Here, we review recent advances in our understanding of the β-cell transcriptome, electrical activity, and insulin exocytosis. We highlight salient differences between mouse and human β-cells, provide models of how the different ion channels contribute to their electrical activity and insulin secretion, and conclude by discussing how these processes become perturbed in T2DM.

Role of G-proteins in islet function in health and diabetes

Glucose-stimulated insulin secretion (GSIS) involves interplay between metabolic and cationic events. Seminal contributions from multiple laboratories affirm essential roles for small G-proteins (Rac1, Cdc42, Arf6, Rab27A) in GSIS. Activation of these signalling proteins promotes cytoskeletal remodeling, transport and docking of insulin granules on the plasma membrane for exocytotic secretion of insulin. Evidence in rodent and human islets suggests key roles for lipidation (farnesylation and geranylgeranylation) of these G-proteins for their targeting to appropriate cellular compartments for optimal regulation of effectors leading to GSIS. Interestingly, however, inhibition of prenylation appears to cause mislocalization of non-prenylated, but (paradoxically) activated G-proteins, in "inappropriate" compartments leading to activation of stress kinases and onset of mitochondrial defects, loss in GSIS and apoptosis of the islet β-cell. This review highlights our current understanding of roles of G-proteins and their post-translational lipidation (prenylation) signalling networks in islet function in normal health, metabolic stress (glucolipotoxicity and ER stress) and diabetes. Critical knowledge gaps that need to be addressed for the development of therapeutics to halt defects in these signalling steps in β-cells in models of impaired insulin secretion and diabetes are also highlighted and discussed.

Endogenous beta-cell CART regulates insulin secretion and transcription of beta-cell genes

Impaired beta-cell function is key to the development of type 2 diabetes. Cocaine- and amphetamine-regulated transcript (CART) is an islet peptide with insulinotropic and glucagonostatic properties. Here we studied the role of endogenous CART in beta-cell function. CART silencing in INS-1 (832/13) beta-cells reduced insulin secretion and production, ATP levels and beta-cell exocytosis. This was substantiated by reduced expression of several exocytosis genes, as well as reduced expression of genes important for insulin secretion and processing. In addition, CART silencing reduced the expression of a network of transcription factors essential for beta-cell function. Moreover, in RNAseq data from human islet donors, CARTPT expression levels correlated with insulin, exocytosis genes and key beta-cell transcription factors. Thus, endogenous beta-cell CART regulates insulin expression and secretion in INS-1 (832/13) cells, via actions on the exocytotic machinery and a network of beta-cell transcription factors. We conclude that CART is important for maintaining the beta-cell phenotype.

Key proteins involved in insulin vesicle exocytosis and secretion

In vivo insulin secretion is predominantly affected by blood glucose concentration, blood concentration of amino acids, gastrointestinal hormones and free nerve functional status, in addition to other factors. Insulin is one of the most important hormones in the body, and its secretion is precisely controlled by nutrients, neurotransmitters and hormones. The insulin exocytosis process is similar to the neurotransmitter release mechanism. There are various types of proteins and lipids that participate in the insulin secretory vesicle fusion process, such as soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein, Ras-related proteins and vacuolar-type H⁺-ATPase (V-ATPase). Notably, the SNARE protein is the molecular basis of exocytotic activity. In the current review, the role of the vesicle membrane proteins (synaptobrevins, vesicle associated membrane proteins and target membrane proteins) and auxiliary proteins (Rab proteins and Munc-18 proteins) in vesicle fusion activity were summarized. A summary of these key proteins involved in insulin granule secretion will facilitate understanding of the pathogenesis of diabetes.

Calmodulin interacts with Rab3D and modulates osteoclastic bone resorption

Calmodulin is a highly versatile protein that regulates intracellular calcium homeostasis and is involved in a variety of cellular functions including cardiac excitability, synaptic plasticity and signaling transduction. During osteoclastic bone resorption, calmodulin has been reported to concentrate at the ruffled border membrane of osteoclasts where it is thought to modulate bone resorption activity in response to calcium. Here we report an interaction between calmodulin and Rab3D, a small exocytic GTPase and established regulator osteoclastic bone resorption. Using yeast two-hybrid screening together with a series of protein-protein interaction studies, we show that calmodulin interacts with Rab3D in a calcium dependent manner. Consistently, expression of a calcium insensitive form of calmodulin (i.e. CaM1234) perturbs calmodulin-Rab3D interaction as monitored by bioluminescence resonance energy transfer (BRET) assays. In osteoclasts, calmodulin and Rab3D are constitutively co-expressed during RANKL-induced osteoclast differentiation, co-occupy plasma membrane fractions by differential gradient sedimentation assay and colocalise in the ruffled border as revealed by confocal microscopy. Further, functional blockade of calmodulin-Rab3D interaction by calmidazolium chloride coincides with an attenuation of osteoclastic bone resorption. Our data imply that calmodulin- Rab3D interaction is required for efficient bone resorption by osteoclasts in vitro.

Rab3A, a possible marker of cortical granules, participates in cortical granule exocytosis in mouse eggs

Fusion of cortical granules with the oocyte plasma membrane is the most significant event to prevent polyspermy. This particular exocytosis, also known as cortical reaction, is regulated by calcium and its molecular mechanism is still not known. Rab3A, a member of the small GTP-binding protein superfamily, has been implicated in calcium-dependent exocytosis and is not yet clear whether Rab3A participates in cortical granules exocytosis. Here, we examine the involvement of Rab3A in the physiology of cortical granules, particularly, in their distribution during oocyte maturation and activation, and their participation in membrane fusion during cortical granule exocytosis. Immunofluorescence and Western blot analysis showed that Rab3A and cortical granules have a similar migration pattern during oocyte maturation, and that Rab3A is no longer detected after cortical granule exocytosis. These results suggested that Rab3A might be a marker of cortical granules. Overexpression of EGFP-Rab3A colocalized with cortical granules with a Pearson correlation coefficient of +0.967, indicating that Rab3A and cortical granules have almost a perfect colocalization in the egg cortical region. Using a functional assay, we demonstrated that microinjection of recombinant, prenylated and active GST-Rab3A triggered cortical granule exocytosis, indicating that Rab3A has an active role in this secretory pathway. To confirm this active role, we inhibited the function of endogenous Rab3A by microinjecting a polyclonal antibody raised against Rab3A prior to parthenogenetic activation. Our results showed that Rab3A antibody microinjection abolished cortical granule exocytosis in parthenogenetically activated oocytes. Altogether, our findings confirm that Rab3A might function as a marker of cortical granules and participates in cortical granule exocytosis in mouse eggs.

Increased Expression of the Diabetes Gene SOX4 Reduces Insulin Secretion by Impaired Fusion Pore Expansion

The transcription factor Sox4 has been proposed to underlie the increased type 2 diabetes risk linked to an intronic single nucleotide polymorphism in CDKAL1 In a mouse model expressing a mutant form of Sox4, glucose-induced insulin secretion is reduced by 40% despite normal intracellular Ca(2+) signaling and depolarization-evoked exocytosis. This paradox is explained by a fourfold increase in kiss-and-run exocytosis (as determined by single-granule exocytosis measurements) in which the fusion pore connecting the granule lumen to the exterior expands to a diameter of only 2 nm, which does not allow the exit of insulin. Microarray analysis indicated that this correlated with an increased expression of the exocytosis-regulating protein Stxbp6. In a large collection of human islet preparations (n = 63), STXBP6 expression and glucose-induced insulin secretion correlated positively and negatively with SOX4 expression, respectively. Overexpression of SOX4 in the human insulin-secreting cell EndoC-βH2 interfered with granule emptying and inhibited hormone release, the latter effect reversed by silencing STXBP6 These data suggest that increased SOX4 expression inhibits insulin secretion and increased diabetes risk by the upregulation of STXBP6 and an increase in kiss-and-run exocytosis at the expense of full fusion. We propose that pharmacological interventions promoting fusion pore expansion may be effective in diabetes therapy.

RAB-6.1 and RAB-6.2 Promote Retrograde Transport in C. elegans

Retrograde transport is a critical mechanism for recycling certain membrane cargo. Following endocytosis from the plasma membrane, retrograde cargo is moved from early endosomes to Golgi followed by transport (recycling) back to the plasma membrane. The complete molecular and cellular mechanisms of retrograde transport remain unclear. The small GTPase RAB-6.2 mediates the retrograde recycling of the AMPA-type glutamate receptor (AMPAR) subunit GLR-1 in C. elegans neurons. Here we show that RAB-6.2 and a close paralog, RAB-6.1, together regulate retrograde transport in both neurons and non-neuronal tissue. Mutants for rab-6.1 or rab-6.2 fail to recycle GLR-1 receptors, resulting in GLR-1 turnover and behavioral defects indicative of diminished GLR-1 function. Loss of both rab-6.1 and rab-6.2 results in an additive effect on GLR-1 retrograde recycling, indicating that these two C. elegans Rab6 isoforms have overlapping functions. MIG-14 (Wntless) protein, which undergoes retrograde recycling, undergoes a similar degradation in intestinal epithelia in both rab-6.1 and rab-6.2 mutants, suggesting a broader role for these proteins in retrograde transport. Surprisingly, MIG-14 is localized to separate, spatially segregated endosomal compartments in rab-6.1 mutants compared to rab-6.2 mutants. Our results indicate that RAB-6.1 and RAB-6.2 have partially redundant functions in overall retrograde transport, but also have their own unique cellular- and subcellular functions.

An efficient two-step subcellular fractionation method for the enrichment of insulin granules from INS-1 cells

Insulin is one of the key regulators for blood glucose homeostasis. More than 99% of insulin is secreted from the pancreatic β-cells. Within each β-cell, insulin is packaged and processed in insulin secretary granules (ISGs) before its exocytosis. Insulin secretion is a complicated but well-organized dynamic process that includes the budding of immature ISGs (iISGs) from the trans-Golgi network, iISG maturation, and mature ISG (mISG) fusion with plasma membrane. However, the molecular mechanisms involved in this process are largely unknown. It is therefore crucial to separate and enrich iISGs and mISGs before determining their distinct characteristics and protein contents. Here, we developed an efficient two-step subcellular fractionation method for the enrichment of iISGs and mISGs from INS-1 cells: OptiPrep gradient purification followed by Percoll solution purification. We demonstrated that by using this method, iISGs and mISGs can be successfully distinguished and enriched. This method can be easily adapted to investigate SGs in other cells or tissues, thereby providing a useful tool for elucidating the mechanisms of granule maturation and secretion.

GTP-bound Rab3A exhibits consecutive positive and negative roles during human sperm dense-core granule exocytosis

Exocytosis of mammalian sperm dense-core secretory granule relies on the same fusion molecules as all other secretory cells; one such molecule is the small GTPase Rab3A. Here, we report an in-depth biochemical characterization of the role of Rab3A in secretion by scrutinizing the exocytotic response of streptolysin O-permeabilized human sperm to the acute application of a number of Rab3A-containing constructs and correlating the findings with those gathered with the endogenous protein. Full length, geranylgeranylated, and active Rab3A elicited human sperm exocytosis per se. With Rab3A/Rab22A chimeric proteins, we demonstrated that the carboxy-terminal domain of the Rab3A molecule was necessary and sufficient to promote exocytosis, whereas its amino-terminus prevented calcium-triggered secretion. Interestingly, full length Rab3A halted secretion when added after the docking of the acrosome to the plasma membrane. This effect depended on the inability of Rab3A to hydrolyze GTP. We combined modified immunofluorescence and acrosomal staining protocols to detect membrane fusion and the activation status of endogenous Rab3 simultaneously in individual cells, and found that GTP hydrolysis on endogenous Rab3 was mandatory for fusion pores to open. Our findings contribute to establishing that Rab3 modulates regulated exocytosis differently depending on the nucleotide bound and the exocytosis stage under study.

Distinct actions of Rab3 and Rab27 GTPases on late stages of exocytosis of insulin

Rab GTPases associated with insulin-containing secretory granules (SGs) are key in targeting, docking and assembly of molecular complexes governing pancreatic β-cell exocytosis. Four Rab3 isoforms along with Rab27A are associated with insulin granules, yet elucidation of the distinct roles of these Rab families on exocytosis remains unclear. To define specific actions of these Rab families we employ Rab3GAP and/or EPI64A GTPase-activating protein overexpression in β-cells from wild-type or Ashen mice to selectively transit the entire Rab3 family or Rab27A to a GDP-bound state. Ashen mice carry a spontaneous mutation that eliminates Rab27A expression. Using membrane capacitance measurements we find that GTP/GDP nucleotide cycling of Rab27A is essential for generation of the functionally defined immediately releasable pool (IRP) and central to regulating the size of the readily releasable pool (RRP). By comparison, nucleotide cycling of Rab3 GTPases, but not of Rab27A, is essential for a kinetically rapid filling of the RRP with SGs. Aside from these distinct functions, Rab3 and Rab27A GTPases demonstrate considerable functional overlap in building the readily releasable granule pool. Hence, while Rab3 and Rab27A cooperate to generate release-ready SGs in β-cells, they also direct unique kinetic and functional properties of the exocytotic pathway.

Rab3D is critical for secretory granule maturation in PC12 cells

Neuropeptide- and hormone-containing secretory granules (SGs) are synthesized at the trans-Golgi network (TGN) as immature secretory granules (ISGs) and complete their maturation in the F-actin-rich cell cortex. This maturation process is characterized by acidification-dependent processing of cargo proteins, condensation of the SG matrix and removal of membrane and proteins not destined to mature secretory granules (MSGs). Here we addressed a potential role of Rab3 isoforms in these maturation steps by expressing their nucleotide-binding deficient mutants in PC12 cells. Our data show that the presence of Rab3D(N135I) decreases the restriction of maturing SGs to the F-actin-rich cell cortex, blocks the removal of the endoprotease furin from SGs and impedes the processing of the luminal SG protein secretogranin II. This strongly suggests that Rab3D is implicated in the subcellular localization and maturation of ISGs.

Rab-geranylgeranyl transferase regulates glucose-stimulated insulin secretion from pancreatic β cells

A growing body of evidence implicates essential roles for small molecular weight G-proteins (e.g., Cdc42, Rac1, Arf6 and Rab3A and Rab27A) in islet β-cell function including glucose-stimulated insulin secretion (GSIS). One of the known mechanisms for optimal activation of small G-proteins involves post-translational prenylation, which is mediated by farnesyltransferase (FTase) and geranylgeranyl transferases (GGTases I and II). The FTase catalyzes incorporation of a 15-carbon farnesyl group while the GGTase mediates incorporation of a 20-carbon geranylgeranyl group into the C-terminal cysteines of G-proteins. The FTase, GGTase I and GGTase II prenylate Ras, Cdc42/Rac1, and Rab G-proteins, respectively. While considerable evidence exists on FTase/GGTase I-mediated regulation of GSIS, very little is known about GGTase II (also referred to as Rab GGTase; RGGT) and its regulatory proteins in the cascade of events leading to GSIS. Herein, we provide the first immunological evidence to suggest expression of α- and β-subunits of RGGT in clonal INS 832/13 β-cells, normal rat islets and human islets. Furthermore, Rab escort protein1 (REP1), which has been shown to be critical for prenylation of Rab G-proteins, is also expressed in these cells. Furthermore, evidence is presented to suggest that siRNA-mediated knockdown of α- or β-subunits of RGGT and REP1 markedly attenuates GSIS in INS 832/13 cells. These findings provide the first evidence in support of key roles for RGGT and its regulatory proteins in GSIS.

Comprehensive phosphoproteome analysis of INS-1 pancreatic β-cells using various digestion strategies coupled with liquid chromatography-tandem mass spectrometry

Type 2 diabetes results from aberrant regulation of the phosphorylation cascade in beta-cells. Phosphorylation in pancreatic beta-cells has not been examined extensively, except with regard to subcellular phosphoproteomes using mitochondria. Thus, robust, comprehensive analytical strategies are needed to characterize the many phosphorylated proteins that exist, because of their low abundance, the low stoichiometry of phosphorylation, and the dynamic regulation of phosphoproteins. In this study, we attempted to generate data on a large-scale phosphoproteome from the INS-1 rat pancreatic beta-cell line using linear ion trap MS/MS. To profile the phosphoproteome in-depth, we used comprehensive phosphoproteomic strategies, including detergent-based protein extraction (SDS and SDC), differential sample preparation (in-gel, in-solution digestion, and FASP), TiO2 enrichment, and MS replicate analyses (MS2-only and multiple-stage activation). All spectra were processed and validated by stringent multiple filtering using target and decoy databases. We identified 2467 distinct phosphorylation sites on 1419 phosphoproteins using 4 mg of INS-1 cell lysate in 24 LC-MS/MS runs, of which 683 (27.7%) were considered novel phosphorylation sites that have not been characterized in human, mouse, or rat homologues. Our informatics data constitute a rich bioinformatics resource for investigating the function of reversible phosphorylation in pancreatic beta-cells. In particular, novel phosphorylation sites on proteins that mediate the pathology of type 2 diabetes, such as Pdx-1, Nkx.2, and Srebf1, will be valuable targets in ongoing phosphoproteomics studies.

Neurexin-1α contributes to insulin-containing secretory granule docking

Neurexins are a family of transmembrane, synaptic adhesion molecules. In neurons, neurexins bind to both sub-plasma membrane and synaptic vesicle-associated constituents of the secretory machinery, play a key role in the organization and stabilization of the presynaptic active zone, and help mediate docking of synaptic vesicles. We have previously shown that neurexins, like many other protein constituents of the neurotransmitter exocytotic machinery, are expressed in pancreatic β cells. We hypothesized that the role of neurexins in β cells parallels their role in neurons, with β-cell neurexins helping to mediate insulin granule docking and secretion. Here we demonstrate that β cells express a more restricted pattern of neurexin transcripts than neurons, with a clear predominance of neurexin-1α expressed in isolated islets. Using INS-1E β cells, we found that neurexin-1α interacts with membrane-bound components of the secretory granule-docking machinery and with the granule-associated protein granuphilin. Decreased expression of neurexin-1α, like decreased expression of granuphilin, reduces granule docking at the β-cell membrane and improves insulin secretion. Perifusion of neurexin-1α KO mouse islets revealed a significant increase in second-phase insulin secretion with a trend toward increased first-phase secretion. Upon glucose stimulation, neurexin-1α protein levels decrease. This glucose-induced down-regulation may enhance glucose-stimulated insulin secretion. We conclude that neurexin-1α is a component of the β-cell secretory machinery and contributes to secretory granule docking, most likely through interactions with granuphilin. Neurexin-1α is the only transmembrane component of the docking machinery identified thus far. Our findings provide new insights into the mechanisms of insulin granule docking and exocytosis.

Role for inducible cAMP early repressor in promoting pancreatic beta cell dysfunction evoked by oxidative stress in human and rat islets

AIMS/HYPOTHESIS

Pro-atherogenic and pro-oxidant, oxidised LDL trigger adverse effects on pancreatic beta cells, possibly contributing to diabetes progression. Because oxidised LDL diminish the expression of genes regulated by the inducible cAMP early repressor (ICER), we investigated the involvement of this transcription factor and of oxidative stress in beta cell failure elicited by oxidised LDL.

METHODS

Isolated human and rat islets, and insulin-secreting cells were cultured with human native or oxidised LDL or with hydrogen peroxide. The expression of genes was determined by quantitative real-time PCR and western blotting. Insulin secretion was monitored by EIA kit. Cell apoptosis was determined by scoring cells displaying pycnotic nuclei.

RESULTS

Exposure of beta cell lines and islets to oxidised LDL, but not to native LDL raised the abundance of ICER. Induction of this repressor by the modified LDL compromised the expression of important beta cell genes, including insulin and anti-apoptotic islet brain 1, as well as of genes coding for key components of the secretory machinery. This led to hampering of insulin production and secretion, and of cell survival. Silencing of this transcription factor by RNA interference restored the expression of its target genes and alleviated beta cell dysfunction and death triggered by oxidised LDL. Induction of ICER was stimulated by oxidative stress, whereas antioxidant treatment with N-acetylcysteine or HDL prevented the rise of ICER elicited by oxidised LDL and restored beta cell functions.

CONCLUSIONS/INTERPRETATION

Induction of ICER links oxidative stress to beta cell failure caused by oxidised LDL and can be effectively abrogated by antioxidant treatment.

Rab3-GAP controls the progression of synaptic homeostasis at a late stage of vesicle release

Homeostatic signaling systems stabilize neural function through the modulation of neurotransmitter receptor abundance, ion channel density, and presynaptic neurotransmitter release. Molecular mechanisms that drive these changes are being unveiled. In theory, molecular mechanisms may also exist to oppose the induction or expression of homeostatic plasticity, but these mechanisms have yet to be explored. In an ongoing electrophysiology-based genetic screen, we have tested 162 new mutations for genes involved in homeostatic signaling at the Drosophila NMJ. This screen identified a mutation in the rab3-GAP gene. We show that Rab3-GAP is necessary for the induction and expression of synaptic homeostasis. We then provide evidence that Rab3-GAP relieves an opposing influence on homeostasis that is catalyzed by Rab3 and which is independent of any change in NMJ anatomy. These data define roles for Rab3-GAP and Rab3 in synaptic homeostasis and uncover a mechanism, acting at a late stage of vesicle release, that opposes the progression of homeostatic plasticity.

Transport of receptors, receptor signaling complexes and ion channels via neuropeptide-secretory vesicles

Stimulus-induced exocytosis of large dense-core vesicles (LDCVs) leads to discharge of neuropeptides and fusion of LDCV membranes with the plasma membrane. However, the contribution of LDCVs to the properties of the neuronal membrane remains largely unclear. The present study found that LDCVs were associated with multiple receptors, channels and signaling molecules, suggesting that neuronal sensitivity is modulated by an LDCV-mediated mechanism. Liquid chromatography-mass spectrometry combined with immunoblotting of subcellular fractions identified 298 proteins in LDCV membranes purified from the dorsal spinal cord, including G-protein-coupled receptors, G-proteins and other signaling molecules, ion channels and trafficking-related proteins. Morphological assays showed that δ-opioid receptor 1 (DOR1), β2 adrenergic receptor (AR), G(αi2), voltage-gated calcium channel α2δ1 subunit and P2X purinoceptor 2 were localized in substance P (SP)-positive LDCVs in small-diameter dorsal root ganglion neurons, whereas β1 AR, Wnt receptor frizzled 8 and dishevelled 1 were present in SP-negative LDCVs. Furthermore, DOR1/G(αi2)/G(β1γ5)/phospholipase C β2 complexes were associated with LDCVs. Blockade of the DOR1/G(αi2) interaction largely abolished the LDCV localization of G(αi2) and impaired stimulation-induced surface expression of G(αi2). Thus, LDCVs serve as carriers of receptors, ion channels and preassembled receptor signaling complexes, enabling a rapid, activity-dependent modulation of neuronal sensitivity.

Tctex-1, a novel interaction partner of Rab3D, is required for osteoclastic bone resorption

Vesicular transport along microtubules must be strictly regulated to sustain the unique structural and functional polarization of bone-resorbing osteoclasts. However, the molecular mechanisms bridging these vesicle-microtubule interactions remain largely obscure. Rab3D, a member of the Rab3 subfamily (Rab3A/B/C/D) of small exocytotic GTPases, represents a core component of the osteoclastic vesicle transport machinery. Here, we identify a new Rab3D-interacting partner, Tctex-1, a light chain of the cytoplasmic dynein microtubule motor complex, by a yeast two-hybrid screen. We demonstrate that Tctex-1 binds specifically to Rab3D in a GTP-dependent manner and co-occupies Rab3D-bearing vesicles in bone-resorbing osteoclasts. Furthermore, we provide evidence that Tctex-1 and Rab3D intimately associate with the dynein motor complex and microtubules in osteoclasts. Finally, targeted disruption of Tctex-1 by RNA interference significantly impairs bone resorption capacity and mislocalizes Rab3D vesicles in osteoclasts, attesting to the notion that components of the Rab3D-trafficking pathway contribute to the maintenance of osteoclastic resorptive function.

Roles of myosin Va and Rab3D in membrane remodeling of immature secretory granules

Neuroendocrine secretory granules (SGs) are formed at the trans-Golgi network (TGN) as immature intermediates. In PC12 cells, these immature SGs (ISGs) are transported within seconds to the cell cortex, where they move along actin filaments and complete maturation. This maturation process comprises acidification-dependent processing of cargo proteins, condensation of the SG matrix, and removal of membrane and proteins not destined to mature SGs (MSGs) into ISG-derived vesicles (IDVs). We investigated the roles of myosin Va and Rab3 isoforms in the maturation of ISGs in neuroendocrine PC12 cells. The expression of dominant-negative mutants of myosin Va or Rab3D blocked the removal of the endoprotease furin from ISGs. Furthermore, expression of mutant Rab3D, but not of mutant myosin Va, impaired cargo processing of SGs. In conclusion, our data suggest an implication of myosin Va and Rab3D in the maturation of SGs where they participate in overlapping but not identical tasks.

The pseudokinase tribbles homolog 3 interacts with ATF4 to negatively regulate insulin exocytosis in human and mouse beta cells

Insufficient insulin secretion and reduced pancreatic beta cell mass are hallmarks of type 2 diabetes (T2DM). Here, we confirm that a previously identified polymorphism (rs2295490/Q84R) in exon 2 of the pseudokinase-encoding gene tribbles 3 (TRB3) is associated with an increased risk for T2DM in 2 populations of people of mixed European descent. Carriers of the 84R allele had substantially reduced plasma levels of C-peptide, the product of proinsulin processing to insulin, suggesting a role for TRB3 in beta cell function. Overexpression of TRB3 84R in mouse beta cells, human islet cells, and the murine beta cell line MIN6 revealed reduced insulin exocytosis, associated with a marked reduction in docked insulin granules visualized by electron microscopy. Conversely, knockdown of TRB3 in MIN6 cells restored insulin secretion and expression of exocytosis genes. Further analysis in MIN6 cells demonstrated that TRB3 interacted with the transcription factor ATF4 and that this complex acted as a competitive inhibitor of cAMP response element-binding (CREB) transcription factor in the regulation of key exocytosis genes. In addition, the 84R TRB3 variant exhibited greater protein stability than wild-type TRB3 and increased binding affinity to Akt. Mice overexpressing TRB3 84R in beta cells displayed decreased beta cell mass, associated with reduced proliferation and enhanced apoptosis rates. These data link a missense polymorphism in human TRB3 to impaired insulin exocytosis and thus increased risk for T2DM.

Small G proteins in islet beta-cell function

Glucose-stimulated insulin secretion from the islet beta-cell involves a sequence of metabolic events and an interplay between a wide range of signaling pathways leading to the generation of second messengers (e.g., cyclic nucleotides, adenine and guanine nucleotides, soluble lipid messengers) and mobilization of calcium ions. Consequent to the generation of necessary signals, the insulin-laden secretory granules are transported from distal sites to the plasma membrane for fusion and release of their cargo into the circulation. The secretory granule transport underlies precise changes in cytoskeletal architecture involving a well-coordinated cross-talk between various signaling proteins, including small molecular mass GTP-binding proteins (G proteins) and their respective effector proteins. The purpose of this article is to provide an overview of current understanding of the identity of small G proteins (e.g., Cdc42, Rac1, and ARF-6) and their corresponding regulatory factors (e.g., GDP/GTP-exchange factors, GDP-dissociation inhibitors) in the pancreatic beta-cell. Plausible mechanisms underlying regulation of these signaling proteins by insulin secretagogues are also discussed. In addition to their positive modulatory roles, certain small G proteins also contribute to the metabolic dysfunction and demise of the islet beta-cell seen in in vitro and in vivo models of impaired insulin secretion and diabetes. Emerging evidence also suggests significant insulin secretory abnormalities in small G protein knockout animals, further emphasizing vital roles for these proteins in normal health and function of the islet beta-cell. Potential significance of these experimental observations from multiple laboratories and possible avenues for future research in this area of islet research are highlighted.

Impaired insulin secretion in transgenic mice over-expressing calpastatin in pancreatic β-cells

Calpains are a family of calcium-activated proteases involved in a number of cellular functions including cell death, proliferation and exocytosis. The finding that variation in the calpain-10 gene increases type 2 diabetes risk in some populations has increased interest in determining the potential role of calpains in pancreatic β-cell function. In the present study, transgenic mice (Cast (RIP)) expressing an endogenous calpain inhibitor, calpastatin, in pancreatic β-cells were used to dissect the role of the calpain system in the regulation insulin secretion in vivo and in vitro. Glucose concentrations after the administration of intraperitoneal glucose were significantly increased in Cast (RIP) mice compared with wildtype littermate controls. This was associated with a reduction in glucose-stimulated insulin secretion in vivo. Using pancreas perfusion, static islet incubation and islet perifusion, it was demonstrated that Cast (RIP) islets hypersecreted insulin at low glucose, but exhibited significantly impaired insulin responses to high glucose. Examination of insulin release and calcium signals from isolated islets indicated that distal components of the insulin exocytotic pathway were abnormal in Cast (RIP) mice. Cast (RIP) islets had modestly reduced expression of Rab3a and other critical components in the late steps of insulin exocytosis. These studies provide the first evidence that blocking endogenous calpain activity partially impairs insulin release in vivo and in vitro by targeting distal components of the insulin exocytotic machinery.

Rab11 and its effector Rip11 participate in regulation of insulin granule exocytosis

Rab GTPases and their effectors play important roles in membrane trafficking between cellular compartments in eukaryotic cells. In the present study, we examined the roles of Rab11B and its effectors in insulin secretion in pancreatic beta-cells. In the mouse insulin-secreting cell line MIN6, Rab11 was co-localized with insulin-containing granules, and over-expression of the GTP- or the GDP-bound form of Rab11B significantly inhibited regulated secretion, indicating involvement of Rab11B in regulated insulin secretion. To determine the downstream signal of Rab11-mediated insulin secretion, we examined the effects of various Rab11-interacting proteins on insulin secretion, and found that Rip11 is involved in cAMP-potentiated insulin secretion but not in glucose-induced insulin secretion. Analyses by immunocytochemistry and subcellular fractionation revealed Rip11 to be co-localized with insulin granules. The inhibitory effect of the Rip11 mutant was not altered in MIN6 cells lacking Epac2, which mediates protein kinase A (PKA)-independent potentiation of insulin secretion, compared with wild-type MIN6 cells. In addition, Rip11 was found to be phosphorylated by PKA in MIN6 cells. The present study shows that both Rab11 and its effector Rip11 participate in insulin granule exocytosis and that Rip11, as a substrate of PKA, regulates the potentiation of exocytosis by cAMP in pancreatic beta-cells.

The GTPase Rab3b/3c-positive recycling vesicles are involved in cross-presentation in dendritic cells

Antigen cross-presentation in dendritic cells is a complex intracellular membrane transport process, but the underlying molecular mechanisms remain to be thoroughly investigated. In this study, we examined the effect of siRNA-mediated knockdown of 57 Rab GTPases, the key regulators of membrane trafficking, on antigen cross-presentation. Twelve Rab GTPases were identified to be associated with antigen cross-presentation, and Rab3b/3c was indicated to be colocalized with MHC class I molecules at perinuclear tubular structure. Tracing with fluorescence protein-tagged beta(2)-microglobulin demonstrated that the MHC class I molecules were internalized from the plasma membrane to Rab3b/3c-positive compartments, which were also colocalized with the internalized transferrin. Moreover, depletion of Rab3b/3c strongly reduced the fast phase recycling rate of transferrin receptors. Furthermore, the Rab3b/3c-positive compartments were colocalized with a fraction of Rab27a at a juxtaposition of phagosomes. Together, these data demonstrate that Rab3b/3c-positive recycling vesicles are involved in and may constitute one of the recycling compartments in exogenous antigen cross-presentation.

Maturation of secretory granules

Exocrine, endocrine, and neuroendocrine cells store hormones and neuropeptides in secretory granules (SGs), which undergo regulated exocytosis in response to an appropriate stimulus. These cargo proteins are sorted at the trans-Golgi network into forming immature secretory granules (ISGs). ISGs undergo maturation while they are transported to and within the F-actin-rich cortex. This process includes homotypic fusion of ISGs, acidification of their lumen, processing, and aggregation of cargo proteins as well as removal of excess membrane and missorted cargo. The resulting mature secretory granules (MSGs) are stored in the F-actin-rich cell cortex, perhaps as segregated pools exhibiting specific responses to stimuli for regulated exocytosis. During the last decade our understanding of the maturation of ISGs advanced substantially. The use of biochemical approaches led to the identification of membrane molecules mechanistically involved in this process. Furthermore, live cell imaging in combination with fluorescently tagged marker proteins of SGs provided insights into the dynamics of maturing ISGs, and the functional implications of cytoskeletal elements and motor proteins.

Kinetics of Rab27a-dependent actions on vesicle docking and priming in pancreatic beta-cells

The small GTPase Rab27a, along with the isoforms of Rab3, is present on insulin secretory granules and has been implicated in regulation of Ca(2+)-triggered exocytosis. We have used membrane capacitance measurements to define the role of Rab27a in regulating the size and refilling of distinct pools of insulin granules by comparison of evoked secretory responses from Rab27a-null ashen and strain-matched wild-type control pancreatic beta-cells. We find that ashen beta-cells display a kinetic defect in refilling of readily releasable and immediately releasable vesicle pools (RRP and IRP, respectively) in response to depolarization-evoked Ca(2+) influx. The deficit in IRP refilling was not observed in the presence of stimulatory glucose concentrations (16.7 mm), though incomplete refilling of the RRP persisted. Comparatively, beta-cells from Rab3a(-/-) mice exhibited complete refilling of the IRP and RRP, demonstrating that Rab27a and Rab3a exert distinct roles in the insulin granule secretory pathway. Further, depletion of the RRP in ashen beta-cells was twofold faster than that of control beta-cells. These deficits in refilling and exocytotic rate in ashen beta-cells were absent when cAMP-regulatory pathways were activated. Elevated cAMP increased the RRP pool size, and complete refilling of the RRP occurred in ashen beta-cells; responses were comparable to wild-type controls. These effects of cAMP were largely eliminated by Rp-cAMP inhibition of PKA, indicating that PKA acts on vesicle priming downstream or via pathways independent of Rab27a. In summary, Rab27a exerts dual roles in glucose-mediated insulin granule exocytosis, facilitating refilling of releasable granule pools while also limiting the rate of release from these pools.

Dual effect of cell-cell contact disruption on cytosolic calcium and insulin secretion

Cell-to-cell interactions play an important role in insulin secretion. Compared with intact islets, dispersed pancreatic beta-cells show increased basal and decreased glucose-stimulated insulin secretion. In this study, we used mouse MIN6B1 cells to investigate the mechanisms that control insulin secretion when cells are in contact with each other or not. RNAi-mediated silencing of the adhesion molecule E-cadherin in confluent cells reduced glucose-stimulated secretion to the levels observed in isolated cells but had no impact on basal secretion. Dispersed cells presented high cytosolic Ca(2+) activity, depolymerized cytoskeleton and ERK1/2 activation in low glucose conditions. Both the increased basal secretion and the spontaneous Ca(2+) activity were corrected by transient removal of Ca(2+) or prolonged incubation of cells in low glucose, a procedure that restored the ability of dispersed cells to respond to glucose (11-fold stimulation). In conclusion, we show that dispersed pancreatic beta-cells can respond robustly to glucose once their elevated basal secretion has been corrected. The increased basal insulin secretion of dispersed cells is due to spontaneous Ca(2+) transients that activate downstream Ca(2+) effectors, whereas engagement of cell adhesion molecules including E-cadherin contributes to the greater secretory response to glucose seen in cells with normal intercellular contacts.

MyRIP anchors protein kinase A to the exocyst complex

The movement of signal transduction enzymes in and out of multi-protein complexes coordinates the spatial and temporal resolution of cellular events. Anchoring and scaffolding proteins are key to this process because they sequester protein kinases and phosphatases with a subset of their preferred substrates. The protein kinase A-anchoring family of proteins (AKAPs), which target the cAMP-dependent protein kinase (PKA) and other enzymes to defined subcellular microenvironments, represent a well studied group of these signal-organizing molecules. In this report we demonstrate that the Rab27a GTPase effector protein MyRIP is a member of the AKAP family. The zebrafish homolog of MyRIP (Ze-AKAP2) was initially detected in a two-hybrid screen for AKAPs. A combination of biochemical, cell-based, and immunofluorescence approaches demonstrate that the mouse MyRIP ortholog targets the type II PKA holoenzyme via an atypical mechanism to a specific perinuclear region of insulin-secreting cells. Similar approaches show that MyRIP interacts with the Sec6 and Sec8 components of the exocyst complex, an evolutionarily conserved protein unit that controls protein trafficking and exocytosis. These data indicate that MyRIP functions as a scaffolding protein that links PKA to components of the exocytosis machinery.

BMP4-BMPR1A signaling in beta cells is required for and augments glucose-stimulated insulin secretion

Impaired glucose-stimulated insulin secretion (GSIS) and perturbed proinsulin processing are hallmarks of beta cell dysfunction in type 2 diabetes. Signals that can preserve and/or enhance beta cell function are therefore of great therapeutic interest. Here we show that bone morphogenetic protein 4 (Bmp4) and its high-affinity receptor, Bmpr1a, are expressed in beta cells. Mice with attenuated BMPR1A signaling in beta cells show decreased expression of key genes involved in insulin gene expression, proinsulin processing, glucose sensing, secretion stimulus coupling, incretin signaling, and insulin exocytosis and develop diabetes due to impaired insulin secretion. We also show that transgenic expression of Bmp4 in beta cells enhances GSIS and glucose clearance and that systemic administration of BMP4 protein to adult mice significantly stimulates GSIS and ameliorates glucose tolerance in a mouse model of glucose intolerance. Thus, BMP4-BMPR1A signaling in beta cells plays a key role in GSIS.

Proteomics analysis of insulin secretory granules

Insulin secretory granules (ISGs) are cytoplasmic organelles of pancreatic beta-cells. They are responsible for the storage and secretion of insulin. To date, only about 30 different proteins have been clearly described to be associated with these organelles. However, data from two-dimensional gel electrophoresis analyses suggested that almost 150 different polypeptides might be present within ISGs. The elucidation of the identity and function of the ISG proteins by proteomics strategies would be of considerable help to further understand some of the underlying mechanisms implicated in ISG biogenesis and trafficking. Furthermore it should give the bases to the comprehension of impaired insulin secretion observed during diabetes. A proteomics analysis of an enriched insulin granule fraction from the rat insulin-secreting cell line INS-1E was performed. The efficacy of the fractionation procedure was assessed by Western blot and electron microscopy. Proteins of the ISG fraction were separated by SDS-PAGE, excised from consecutive gel slices, and tryptically digested. Peptides were analyzed by nano-LC-ESI-MS/MS. This strategy identified 130 different proteins that were classified into four structural groups including intravesicular proteins, membrane proteins, novel proteins, and other proteins. Confocal microscopy analysis demonstrated the association of Rab37 and VAMP8 with ISGs in INS-1E cells. In conclusion, the present study identified 130 proteins from which 110 are new proteins associated with ISGs. The elucidation of their role will further help in the understanding of the mechanisms governing impaired insulin secretion during diabetes.

Structural and functional changes in human insulin induced by methylglyoxal

Elevated methylglyoxal (MG) levels have been reported in insulin-resistance syndrome. The present study investigated whether MG, a highly reactive metabolite of glucose, induced structural and functional changes of insulin. Incubation of human insulin with MG in vitro yielded MG-insulin adducts, as evidenced by additional peaks observed on mass spectrometric (MS) analysis of the incubates. Tandem MS analysis of insulin B-chain adducts confirmed attachment of MG at an arginine residue. [3H]-2-deoxyglucose uptake by 3T3-L1 adipocytes was significantly and concentration-dependently decreased after the treatment with MG-insulin adducts, in comparison with the effect of native insulin at the same concentrations. A significant decrease of glucose uptake induced by MG-insulin adducts was also observed in L8 skeletal muscle cells. MG alone had no effect on glucose uptake or the transcriptional expression of insulin receptor. Unlike native insulin, MG-insulin adducts did not inhibit insulin release from pancreatic beta-cells. The degradation of MG-insulin through liver cells was also decreased. In conclusion, MG modifies insulin by attaching to internal arginine residue in beta-chain of insulin. The formation of this MG-insulin adduct decreases insulin-mediated glucose uptake, impairs autocrine control of insulin secretion, and decreases insulin clearance. These structural and functional abnormalities of insulin molecule may contribute to the pathogenesis of insulin resistance.

Regulation of pancreatic beta-cell insulin secretion by actin cytoskeleton remodelling: role of gelsolin and cooperation with the MAPK signalling pathway

We have previously isolated two MIN6 beta-cell sublines, B1, highly responsive to glucose-stimulated insulin secretion, and C3, markedly refractory (Lilla, V., Webb, G., Rickenbach, K., Maturana, A., Steiner, D. F., Halban, P. A. and Irminger, J. C. (2003) Endocrinology 144, 1368-1379). We now demonstrate that C3 cells have substantially increased amounts of F-actin stress fibres whereas B1 cells have shorter cortical F-actin. Consistent with these data, B1 cells display glucose-dependent actin remodelling whereas, in C3 cells, F-actin is refractory to this secretagogue. Furthermore, F-actin depolymerisation with latrunculin B restores glucose-stimulated insulin secretion in C3 cells. In parallel, glucose-stimulated ERK1/2 activation is greater in B1 than in C3 cells, and is potentiated in both sublines following F-actin depolymerisation. Glucose-activated phosphoERK1/2 accumulates at actin filament tips adjacent to the plasma membrane, indicating that these are the main sites of action for this kinase during insulin secretion. In addition, B1 cell expression of the calcium-dependent F-actin severing protein gelsolin is >100-fold higher than that of C3 cells. Knock-down of gelsolin reduced glucose-stimulated insulin secretion, whereas gelsolin over-expression potentiated secretion from B1 cells. Gelsolin localised along depolymerised actin fibres after glucose stimulation. Taken together, these data demonstrate that F-actin reorganization prior to insulin secretion requires gelsolin and plays a role in the glucose-dependent MAPK signal transduction that regulates beta-cell insulin secretion.

SV2A and SV2C are not vesicular Ca2+ transporters but control glucose-evoked granule recruitment

Synaptic vesicle protein 2 (SV2) is expressed in neuroendocrine cells as three homologous isoforms, SV2A, SV2B and SV2C. Ca2+-dependent function in exocytosis has been attributed to SV2A and SV2B, without elucidation of the mechanism. The role of SV2C has not yet been addressed. Here we characterize the three SV2 isoforms and define their involvement in regulated insulin secretion. SV2A and SV2C are associated with insulin-containing granules and synaptic-like-microvesicles (SLM) in INS-1E insulinoma and primary beta-cells, whereas SV2B is only present on SLM. Neither overexpression nor isoform-specific silencing of SV2A or SV2C by RNA interference modifies depolarization-triggered cytosolic [Ca2+] rises or secretory granule [Ca2+], measured with a VAMP-2 aequorin chimera. This strongly argues against any Ca2+ transport function of SV2. Moreover, up- or downregulation of these isoforms has no influence on K+-induced insulin release suggesting that SV2 does not affect the Ca2+-dependent step(s) of exocytosis. By contrast, glucose-elicited secretion is inhibited during the sustained rather than the early phase, placing the action of SV2 on the recruitment of granules from the reserve pool to the plasma membrane. This conclusion is reinforced by capacitance measurements in glucose-stimulated SV2C-deficient cells. Like capacitance, evoked and basal hormone release are attenuated more by silencing of SV2C compared with SV2A. This indicates only partial redundancy and highlights a key role for SV2C in the secretory process.

ICER induced by hyperglycemia represses the expression of genes essential for insulin exocytosis

The GTPases Rab3a and Rab27a and their effectors Granuphilin/Slp4 and Noc2 are essential regulators of neuroendocrine secretion. Chronic exposure of pancreatic beta-cells to supraphysiological glucose levels decreased selectively the expression of these proteins. This glucotoxic effect was mimicked by cAMP-raising agents and blocked by PKA inhibitors. We demonstrate that the transcriptional repressor ICER, which is induced in a PKA-dependent manner by chronic hyperglycemia and cAMP-raising agents, is responsible for the decline of the four genes. ICER overexpression diminished the level of Granuphilin, Noc2, Rab3a and Rab27a by binding to cAMP responsive elements located in the promoters of these genes and inhibited exocytosis of beta-cells in response to secretagogues. Moreover, the loss in the expression of the genes of the secretory machinery caused by glucose and cAMP-raising agents was prevented by an antisense construct that reduces ICER levels. We propose that induction of inappropriate ICER levels lead to defects in the secretory process of pancreatic beta-cells possibly contributing, in conjunction with other known deleterious effects of hyperglycemia, to defective insulin release in type 2 diabetes.

A unique set of SH3-SH3 interactions controls IB1 homodimerization

Islet-brain 1 (IB1 or JIP-1) is a scaffold protein that interacts with components of the c-Jun N-terminal kinase (JNK) signal-transduction pathway. IB1 is expressed at high levels in neurons and in pancreatic beta-cells, where it controls expression of several insulin-secretory components and secretion. IB1 has been shown to homodimerize, but neither the molecular mechanisms nor the function of dimerization have yet been characterized. Here, we show that IB1 homodimerizes through a novel and unique set of Src homology 3 (SH3)-SH3 interactions. X-ray crystallography studies show that the dimer interface covers a region usually engaged in PxxP-mediated ligand recognition, even though the IB1 SH3 domain lacks this motif. The highly stable IB1 homodimer can be significantly destabilized in vitro by three individual point mutations directed against key residues involved in dimerization. Each mutation reduces IB1-dependent basal JNK activity in 293T cells. Impaired dimerization also results in a reduction in glucose transporter type 2 expression and in glucose-dependent insulin secretion in pancreatic beta-cells. Taken together, these results indicate that IB1 homodimerization through its SH3 domain has pleiotropic effects including regulation of the insulin secretion process.

PKA-dependent and PKA-independent pathways for cAMP-regulated exocytosis

Stimulus-secretion coupling is an essential process in secretory cells in which regulated exocytosis occurs, including neuronal, neuroendocrine, endocrine, and exocrine cells. While an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) is the principal signal, other intracellular signals also are important in regulated exocytosis. In particular, the cAMP signaling system is well known to regulate and modulate exocytosis in a variety of secretory cells. Until recently, it was generally thought that the effects of cAMP in regulated exocytosis are mediated by activation of cAMP-dependent protein kinase (PKA), a major cAMP target, followed by phosphorylation of the relevant proteins. Although the involvement of PKA-independent mechanisms has been suggested in cAMP-regulated exocytosis by pharmacological approaches, the molecular mechanisms are unknown. Newly discovered cAMP-GEF/Epac, which belongs to the cAMP-binding protein family, exhibits guanine nucleotide exchange factor activities and exerts diverse effects on cellular functions including hormone/transmitter secretion, cell adhesion, and intracellular Ca(2+) mobilization. cAMP-GEF/Epac mediates the PKA-independent effects on cAMP-regulated exocytosis. Thus cAMP regulates and modulates exocytosis by coordinating both PKA-dependent and PKA-independent mechanisms. Localization of cAMP within intracellular compartments (cAMP compartmentation or compartmentalization) may be a key mechanism underlying the distinct effects of cAMP in different domains of the cell.

Rabphilin Localizes with the Cell Actin Cytoskeleton and Stimulates Association of Granules with F-actin Cross-linked by α-Actinin

In endocrine cell, granules accumulate within an F-actin-rich region below the plasma membrane. The mechanisms involved in this process are largely unknown. Rabphilin is a cytosolic protein that is expressed in neurons and neuroendocrine cells and binds with high affinity to members of the Rab3 family of GTPases localized to synaptic vesicles and dense core granules. Rabphilin also interacts with alpha-actinin, a protein that cross-links F-actin into bundles and networks and associates with the granule membrane. Here we asked whether rabphilin, in addition to its granule localization, also interacts with the cell actin cytoskeleton. Immunofluorescence and immunoelectron microscopy show that rabphilin localizes to the sub-plasmalemmal actin cytoskeleton both in neuroendocrine and unspecialized cells. By using purified components, it is found that association of rabphilin with F-actin is dependent on added alpha-actinin. In an in vitro assay, granules, unlike endosomes or mitochondria, associate with F-actin cross-linked by alpha-actinin. Rabphilin is shown to stimulate this process. Rabphilin enhances by approximately 8-fold the granule ability to localize within regions of elevated concentration of cross-linked F-actin. These results suggest that rabphilin, by interacting with alpha-actinin, organizes the cell cytoskeleton to facilitate granule localization within F-actin-rich regions.

Rab3D regulates a novel vesicular trafficking pathway that is required for osteoclastic bone resorption

Rab3 proteins are a subfamily of GTPases, known to mediate membrane transport in eukaryotic cells and play a role in exocytosis. Our data indicate that Rab3D is the major Rab3 species expressed in osteoclasts. To investigate the role of Rab3D in osteoclast physiology we examined the skeletal architecture of Rab3D-deficient mice and found an osteosclerotic phenotype. Although basal osteoclast number in null animals is normal the total eroded surface is significantly reduced, suggesting that the resorptive defect is due to attenuated osteoclast activity. Consistent with this hypothesis, ultrastructural analysis reveals that Rab3D(-/-) osteoclasts exhibit irregular ruffled borders. Furthermore, while overexpression of wild-type, constitutively active, or prenylation-deficient Rab3D has no significant effects, overexpression of GTP-binding-deficient Rab3D impairs bone resorption in vitro. Finally, subcellular localization studies reveal that, unlike wild-type or constitutively active Rab3D, which associate with a nonendosomal/lysosomal subset of post-trans-Golgi network (TGN) vesicles, inactive Rab3D localizes to the TGN and inhibits biogenesis of Rab3D-bearing vesicles. Collectively, our data suggest that Rab3D modulates a post-TGN trafficking step that is required for osteoclastic bone resorption.

Suppression of Pdx-1 perturbs proinsulin processing, insulin secretion and GLP-1 signalling in INS-1 cells

AIMS/HYPOTHESIS

Mutations in genes encoding HNF-4alpha, HNF-1alpha and IPF-1/Pdx-1 are associated with, respectively, MODY subtypes-1, -3 and -4. Impaired glucose-stimulated insulin secretion is the common primary defect of these monogenic forms of diabetes. A regulatory circuit between these three transcription factors has also been suggested. We aimed to explore how Pdx-1 regulates beta cell function and gene expression patterns.

METHODS

We studied two previously established INS-1 stable cell lines permitting inducible expression of, respectively, Pdx-1 and its dominant-negative mutant. We used HPLC for insulin processing, adenovirally encoded aequorin for cytosolic [Ca2+], and transient transfection of human growth hormone or patch-clamp capacitance recordings to monitor exocytosis.

RESULTS

Induction of DN-Pdx-1 resulted in defective glucose-stimulated and K+-depolarisation-induced insulin secretion in INS-1 cells, while overexpression of Pdx-1 had no effect. We found that DN-Pdx-1 caused down-regulation of fibroblast growth factor receptor 1 (FGFR1), and consequently prohormone convertases (PC-1/3 and -2). As a result, DN-Pdx-1 severely impaired proinsulin processing. In addition, induction of Pdx-1 suppressed the expression of glucagon-like peptide 1 receptor (GLP-1R), which resulted in marked reduction of both basal and GLP-1 agonist exendin-4-stimulated cellular cAMP levels. Induction of DN-Pdx-1 did not affect glucokinase activity, glycolysis, mitochondrial metabolism or ATP generation. The K+-induced cytosolic [Ca2+] rise and Ca2+-evoked exocytosis (membrane capacitance) were not abrogated.

CONCLUSIONS/INTERPRETATION

The severely impaired proinsulin processing combined with decreased GLP-1R expression and cellular cAMP content, rather than metabolic defects or altered exocytosis, may contribute to the beta cell dysfunction induced by Pdx-1 deficiency.

Nuclear factor-{kappa}B activity in {beta}-cells is required for glucose-stimulated insulin secretion

Glucose-stimulated insulin secretion (GSIS) in pancreatic beta-cells depends on coordinated glucose uptake, oxidative metabolism, and Ca(2+)-triggered insulin exocytosis. Impaired GSIS is a hallmark of type 2 diabetes. However, at present we know very little about the molecular mechanisms that induce and maintain the expression of genes required for GSIS in beta-cells. The transcription factor nuclear factor-kappaB (NF-kappaB) is activated by an increase in intracellular Ca(2+) in beta-cells. Here, we show that attenuation of NF-kappaB activation in beta-cells generates mice with impaired GSIS, and that the beta-cells show perturbed expression of genes required for glucose uptake, oxidative metabolism, and insulin exocytosis. Thus, NF-kappaB appears to be part of a positive regulatory circuit that maintains GSIS in pancreatic beta-cells.

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

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

Complexin I regulates glucose-induced secretion in pancreatic β-cells

The neuronal-specific protein complexin I (CPX I) plays an important role in controlling the Ca2+-dependent neurotransmitter release. Since insulin exocytosis and neurotransmitter release rely on similar molecular mechanisms and that pancreatic β-cells and neuronal cells share the expression of many restricted genes, we investigated the potential role of CPX I in insulin-secreting cells. We found that pancreatic islets and several insulin-secreting cell lines express high levels of CPX I. The β-cell expression of CPX I is mediated by the presence of a neuron restrictive silencer element located within the regulatory region of the gene. This element bound the transcriptional repressor REST, which is found in most cell types with the exception of mature neuronal cells and β-cells. Overexpression of CPX I or silencing of the CPX I gene (Cplx1) by RNA interference led to strong impairment in β-cell secretion in response to nutrients such as glucose, leucine and KCl. This effect was detected both in the early and the sustained secretory phases but was much more pronounced in the early phase. We conclude that CPX I plays a critical role in β-cells in the control of the stimulated-exocytosis of insulin.

Neuronal traits are required for glucose-induced insulin secretion

The transcriptional repressor RE1 silencer transcription factor (REST) is an important factor that restricts some neuronal traits to neurons. Since these traits are also present in pancreatic beta-cells, we evaluated their role by generating a model of insulin-secreting cells that express REST. The presence of REST led to a decrease in expression of its known target genes, whereas insulin expression and its cellular content were conserved. As a consequence of REST expression, the capacity to secrete insulin in response to mitochondrial fuels, a particularity of mature beta-cells, was impaired. These data provide evidence that REST target genes are required for an appropriate glucose-induced insulin secretion.

Identification and characterization of Noc2 as a potential Rab3B effector protein in epithelial cells

The Rab3 family small G proteins (Rab3A-D) are involved in the regulated secretory pathway of brain and secretory tissues. Among Rab3-interacting proteins, Rabphilin-3, Rim, and Noc2, all of which contain a conserved Rab3-binding domain (RBD3), are generally recognized Rab3 effector proteins in neurons and secretory cells. Although Rab3B was also detected in epithelial cells, its function remained unknown. We isolated cDNA sequences from human epithelial Caco2-cell mRNA by degenerate RT-PCR based on the conserved amino acid sequence of RBD3. Multiple cDNA clones were identified as encoding Noc2. Northern blot analysis revealed that Noc2 mRNA was expressed not only in secretory tissues but also in epithelial tissues and cell lines. A pull-down assay demonstrated that Noc2 bound to Rab3B in a GTP-dependent manner. When Noc2 was co-expressed with the GTP-bound form of Rab3B, it was recruited from the cytosol to perinuclear membranes. Furthermore, overexpression of Noc2 inhibited the cell-surface transport of basolateral vesicular stomatitis virus glycoprotein. These results suggest that Noc2 functions as a potential Rab3B effector protein in epithelial cells.

Rab3A negatively regulates activity-dependent modulation of exocytosis in bovine adrenal chromaffin cells

Members of the Rab family of monomeric GTPases have been implicated in vesicle trafficking, and Rab3A, located on synaptic vesicles in neurones and secretory vesicles in neuroendocrine cells, is likely to be involved in vesicle fusion leading to neurotransmitter release. A hydrolysis-deficient mutant of Rab3A, Rab3AQ81L, has been shown to potently inhibit hormone release. Here we show that the inhibition of hormone release by Rab3AQ81L is activity-dependent. Bovine adrenal chromaffin cells were induced to express Rab3AQ81L and green fluorescent protein by adenoviral gene transfer of a bicistronic construct. Fluorescent cells were stimulated with single depolarizations and trains of depolarizing pulses in whole cell perforated patch clamp recordings, and exocytosis was detected with cell capacitance measurements and carbon fibre amperometry. When single depolarizations were used to evoke exocytosis, cells expressing Rab3AQ81L showed a 50% reduction in response amplitude. When trains of brief depolarizations (10 or 40 ms) were used to evoke exocytosis, responses rapidly declined to zero in cells expressing Rab3AQ81L. Wild-type Rab3A had effects similar to Rab3AQ81L, causing significant inhibition of exocytosis only during repetitive stimulation. Expression of Rab5A did not alter exocytosis evoked by single depolarizations or repetitive stimulation. Applying a long duration depolarization in the middle of a stimulus train revealed that exocytotic efficacy (capacitance increase per amount of calcium influx) was not decreased in Rab3AQ81L-expressing cells. Instead, the activity-dependent increase in exocytotic efficacy observed in control cells did not occur in Rab3AQ81L-expressing cells. Our results suggest that Rab3A in the GTP bound conformation prevents activity-dependent facilitation.

Rab3D redistribution and function in rat parotid acini

Rab3D is a low molecular weight GTP-binding protein believed to be involved with regulated secretion in many cell types. In parotid, Rab3D is localized to secretory granule membranes or present in the cytosol as a complex with Rab escort protein. In the present study, we examined the redistribution of membrane-associated Rab3D during secretion in permeabilized parotid acini. When permeabilized acini were stimulated with calcium and cAMP, amylase release increased greater than twofold over basal. Quantitative immunoblotting of subcellular fractions revealed that Rab3D did not dissociate from parotid membranes during secretion. Immunohistochemical staining demonstrated that Rab3D co-localizes with amylase containing granules that are found in the apical pole of the cell. Upon stimulation with calcium and cAMP, Rab3D and amylase immunostaining of granules appeared to be more dispersed. However, Rab3D immunostaining was not observed on the plasma membrane and appeared to reside in the apical cytoplasm. To examine the role of Rab3D in amylase release, cytosolic extracts containing myc-tagged Rab3D and Rab3DQ81L, a GTP-binding mutant, were prepared and incubated with streptolysin O-permeabilized acini. Rab3D, but not Rab3DQ81L, bound to parotid membranes suggesting that Rab3D-binding to parotid membranes is guanine nucleotide-dependent. Moreover, wild-type and mutant Rab3D inhibited agonist-induced amylase release from permeabilized parotid acini. These observations indicate that in parotid acini, Rab3D does not dissociate from parotid membranes or redistribute to the plasma membrane during secretion, and may play an inhibitory role in regulated secretion. The fact that both wild-type Rab3D and the GTP-binding mutant inhibit amylase release suggests that binding of Rab3D to the membrane is not essential for secretory inhibition.