A direct interaction between Cdc42 and vesicle-associated membrane protein 2 regulates SNARE-dependent insulin exocytosis

SNARE proteins: Core engines of membrane fusion in cancer

Vesicles are loaded with a variety of cargoes, including membrane proteins, secreted proteins, signaling molecules, and various enzymes, etc. Not surprisingly, vesicle transport is essential for proper cellular life activities including growth, division, movement and cellular communication. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) mediate membrane fusion of vesicles with their target compartments that is fundamental for cargo delivery. Recent studies have shown that multiple SNARE family members are aberrantly expressed in human cancers and actively contribute to malignant proliferation, invasion, metastasis, immune evasion and treatment resistance. Here, the localization and function of SNARE proteins in eukaryotic cells are firstly mapped. Then we summarize the expression and regulation of SNAREs in cancer, and describe their contribution to cancer progression and mechanisms, and finally we propose engineering botulinum toxin as a strategy to target SNAREs for cancer treatment.

BCAS2 Participates in Insulin Synthesis and Secretion via mRNA Alternative Splicing in Mice

Insulin secreted by pancreatic β cells is essential for maintaining blood glucose levels. Diabetes is caused primarily by a loss of β cells or impairment of β-cell function. A previous whole-transcriptome analysis of islets from a type 2 diabetes group and a control group showed that a splicing disorder occurred in approximately 25% of splicing events. Breast carcinoma amplified sequence 2 (BCAS2) is a spliceosome component whose function in islet β cells is unclear. Here, we report that knockdown of Bcas2 decreased glucose- and KCl-stimulated insulin secretion in the NIT-1 cell line. Pancreas weight, glucose tolerance, and insulin sensitivity were measured in normal chow-fed Bcas2 f/f-βKO mice, and β-cell mass and islet size were analyzed by immunohistochemistry. Glucose intolerance developed in Bcas2 f/f-βKO mice, but there were no significant differences in pancreas weight, insulin sensitivity, β-cell mass, or islet size. Furthermore, observation of glucose-stimulated insulin secretion and insulin secretion granules in normal chow-fed mice revealed that the insulin level in serum and the number of insulin secretion granules were decreased in Bcas2 f/f-βKO mice. These differences were related to abnormal splicing of Syt7 and Tcf7l2 pre-mRNA. Taken together, these results demonstrate that BCAS2 is involved in alternative splicing during insulin synthesis and secretion.

A metabolic role for CD47 in pancreatic β cell insulin secretion and islet transplant outcomes

Diabetes is a global public health burden and is characterized clinically by relative or absolute insulin deficiency. Therapeutic agents that stimulate insulin secretion and improve insulin sensitivity are in high demand as treatment options. CD47 is a cell surface glycoprotein implicated in multiple cellular functions including recognition of self, angiogenesis, and nitric oxide signaling; however, its role in the regulation of insulin secretion remains unknown. Here, we demonstrate that CD47 receptor signaling inhibits insulin release from human as well as mouse pancreatic β cells and that it can be pharmacologically exploited to boost insulin secretion in both models. CD47 depletion stimulated insulin granule exocytosis via activation of the Rho GTPase Cdc42 in β cells and improved glucose clearance and insulin sensitivity in vivo. CD47 blockade enhanced syngeneic islet transplantation efficiency and expedited the return to euglycemia in streptozotocin-induced diabetic mice. Further, anti-CD47 antibody treatment delayed the onset of diabetes in nonobese diabetic (NOD) mice and protected them from overt diabetes. Our findings identify CD47 as a regulator of insulin secretion, and its manipulation in β cells offers a therapeutic opportunity for diabetes and islet transplantation by correcting insulin deficiency.

The function of VAMP2 in mediating membrane fusion: An overview

Vesicle-associated membrane protein 2 (VAMP2, also known as synaptobrevin-2), encoded by VAMP2 in humans, is a key component of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex. VAMP2 combined with syntaxin-1A (SYX-1A) and synaptosome-associated protein 25 (SNAP-25) produces a force that induces the formation of fusion pores, thereby mediating the fusion of synaptic vesicles and the release of neurotransmitters. VAMP2 is largely unstructured in the absence of interaction partners. Upon interaction with other SNAREs, the structure of VAMP2 stabilizes, resulting in the formation of four structural domains. In this review, we highlight the current knowledge of the roles of the VAMP2 domains and the interaction between VAMP2 and various fusion-related proteins in the presynaptic cytoplasm during the fusion process. Our summary will contribute to a better understanding of the roles of the VAMP2 protein in membrane fusion.

β1-Integrin-A Key Player in Controlling Pancreatic Beta-Cell Insulin Secretion via Interplay With SNARE Proteins

Shortcomings in cell-based therapies for patients with diabetes have been revealed to be, in part, a result of an improper extracellular matrix (ECM) environment. In vivo, pancreatic islets are emersed in a diverse ECM that provides physical support and is crucial for healthy function. β1-Integrin receptors have been determined to be responsible for modulation of beneficial interactions with ECM proteins influencing beta-cell development, proliferation, maturation, and function. β1-Integrin signaling has been demonstrated to augment insulin secretion by impacting the actin cytoskeleton via activation of focal adhesion kinase and downstream signaling pathways. In other secretory cells, evidence of a bidirectional relationship between integrins and exocytotic machinery has been demonstrated, and, thus, this relationship could be present in pancreatic beta cells. In this review, we will discuss the role of ECM-β1-integrin interplay with exocytotic proteins in controlling pancreatic beta-cell insulin secretion through their dynamic and unique signaling pathway.

Membrane Fusion and SNAREs: Interaction with Ras Proteins

The superfamily of Ras proteins comprises different molecules belonging to the GTPase family. They normally cycle between an active state bound to GTP which activates effectors while the protein is membrane-associated, and an inactive GDP-bound state. They regulate the intracellular trafficking and other cellular processes. The family of Rab proteins includes several members and they have been found, among other Ras proteins, to be fundamental for important biological processes, such as endocytosis and exocytosis. SNARE proteins control the fusion of vesicles by forming quaternary complexes which are divided into two small groups on the two different compartments. Generally, the association of three SNARE proteins on the donor compartment with the one on the target compartment determines the formation of the SNARE complex, the opening of the fusion pore and the formation of one single bigger vesicle. Interestingly, novel interactions between other molecules involved in intracellular trafficking, endosomal fusion and maturation have recently been found, such as the interaction between invariant chain and the Qb SNARE vti1b, and more functional connections between Rab proteins and SNAREs are supposed to be fundamental for the regulation of membrane fusion.

DOC2b Enhances β-Cell Function via a Novel Tyrosine Phosphorylation-Dependent Mechanism

Double C2 domain Β (DOC2b) protein is required for glucose-stimulated insulin secretion (GSIS) in β-cells, the underlying mechanism of which remains unresolved. Our biochemical analysis using primary human islets and human and rodent clonal β-cells revealed that DOC2b is tyrosine phosphorylated within 2 min of glucose stimulation, and Src family kinase member YES is required for this process. Biochemical and functional analysis using DOC2bY301 mutants revealed the requirement of Y301 phosphorylation for the interaction of DOC2b with YES kinase and increased content of VAMP2, a protein on insulin secretory granules, at the plasma membrane (PM), concomitant with DOC2b-mediated enhancement of GSIS in β-cells. Coimmunoprecipitation studies demonstrated an increased association of DOC2b with ERM family proteins in β-cells following glucose stimulation or pervanadate treatment. Y301 phosphorylation-competent DOC2b was required to increase ERM protein activation, and ERM protein knockdown impaired DOC2b-mediated boosting of GSIS, suggesting that tyrosine-phosphorylated DOC2b regulates GSIS via ERM-mediated granule localization to the PM. Taken together, these results demonstrate the glucose-induced posttranslational modification of DOC2b in β-cells, pinpointing the kinase, site of action, and downstream signaling events and revealing a regulatory role of YES kinase at various steps in GSIS. This work will enhance the development of novel therapeutic strategies to restore glucose homeostasis in diabetes.

Targeting Small GTPases and Their Prenylation in Diabetes Mellitus

A fundamental role of pancreatic β-cells to maintain proper blood glucose level is controlled by the Ras superfamily of small GTPases that undergo post-translational modifications, including prenylation. This covalent attachment with either a farnesyl or a geranylgeranyl group controls their localization, activity, and protein–protein interactions. Small GTPases are critical in maintaining glucose homeostasis acting in the pancreas and metabolically active tissues such as skeletal muscles, liver, or adipocytes. Hyperglycemia-induced upregulation of small GTPases suggests that inhibition of these pathways deserves to be considered as a potential therapeutic approach in treating T2D. This Perspective presents how inhibition of various points in the mevalonate pathway might affect protein prenylation and functioning of diabetes-affected tissues and contribute to chronic inflammation involved in diabetes mellitus (T2D) development. We also demonstrate the currently available molecular tools to decipher the mechanisms linking the mevalonate pathway’s enzymes and GTPases with diabetes.

MicroRNA Sequences Modulated by Beta Cell Lipid Metabolism: Implications for Type 2 Diabetes Mellitus

Alterations in lipid metabolism within beta cells and islets contributes to dysfunction and apoptosis of beta cells, leading to loss of insulin secretion and the onset of type 2 diabetes. Over the last decade, there has been an explosion of interest in understanding the landscape of gene expression which influences beta cell function, including the importance of small non-coding microRNA sequences in this context. This review sought to identify the microRNA sequences regulated by metabolic challenges in beta cells and islets, their targets, highlight their function and assess their possible relevance as biomarkers of disease progression in diabetic individuals. Predictive analysis was used to explore networks of genes targeted by these microRNA sequences, which may offer new therapeutic strategies to protect beta cell function and delay the onset of type 2 diabetes.

Emerging Roles of Small GTPases in Islet β-Cell Function

Several small guanosine triphosphatases (GTPases) from the Ras protein superfamily regulate glucose-stimulated insulin secretion in the pancreatic islet β-cell. The Rho family GTPases Cdc42 and Rac1 are primarily involved in relaying key signals in several cellular functions, including vesicle trafficking, plasma membrane homeostasis, and cytoskeletal dynamics. They orchestrate specific changes at each spatiotemporal region within the β-cell by coordinating with signal transducers, guanine nucleotide exchange factors (GEFs), GTPase-activating factors (GAPs), and their effectors. The Arf family of small GTPases is involved in vesicular trafficking (exocytosis and endocytosis) and actin cytoskeletal dynamics. Rab-GTPases regulate pre-exocytotic and late endocytic membrane trafficking events in β-cells. Several additional functions for small GTPases include regulating transcription factor activity and mitochondrial dynamics. Importantly, defects in several of these GTPases have been found associated with type 2 diabetes (T2D) etiology. The purpose of this review is to systematically denote the identities and molecular mechanistic steps in the glucose-stimulated insulin secretion pathway that leads to the normal release of insulin. We will also note newly identified defects in these GTPases and their corresponding regulatory factors (e.g., GDP dissociation inhibitors (GDIs), GEFs, and GAPs) in the pancreatic β-cells, which contribute to the dysregulation of metabolism and the development of T2D.

Targeting SNARE-Mediated Vesicle Transport to Block Invadopodium-Based Cancer Cell Invasion

During metastasis, cancer cells can invade extracellular matrix (ECM) through a process mediated by matrix-degrading protrusions of the plasma membrane, termed invadopodia. Formation of invadopodia correlates with cells’ invasive and metastatic potential, and thus presents a potential target for therapeutic approaches to target metastatic progression. Invadopodia formation is dependent on the recruitment of proteins involved in intracellular signaling, actin cytoskeleton remodeling, and proteolytic matrix modification. The latter includes matrix degrading enzymes such as MT1-MMP, MMP2, and MMP9. These essential invadopodium-associated enzymes are required for localized matrix degradation, and their localization at invadopodia is central to invadopodium-based cancer cell invasion. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) facilitate intracellular vesicle traffic, including that involved in the transport of invadopodium-associated proteins, and in so doing promote modification of ECM and modulation of signaling pathways involved in the movement of cancer cells. Specific SNARE complexes have been found to support invadopodia formation, and these complexes are, in turn, regulated by associated proteins that interact specifically with SNAREs. Targeting SNARE regulatory proteins thus provides a possible approach to disrupt SNARE-dependent delivery of invadopodial proteins, including MT1-MMP, to sites of ECM modification. Here, we review recent studies of SNARE regulators that hold potential as targets for the development of anti-metastatic therapies for patients burdened with invadopodia-forming cancer types.

RhoG deficiency abrogates cytotoxicity of human lymphocytes and causes hemophagocytic lymphohistiocytosis

Exocytosis of cytotoxic granules (CG) by lymphocytes is required for the elimination of infected and malignant cells. Impairments in this process underly a group of diseases with dramatic hyperferritinemic inflammation termed hemophagocytic lymphohistiocytosis (HLH). Although genetic and functional studies of HLH have identified proteins controlling distinct steps of CG exocytosis, the molecular mechanisms that spatiotemporally coordinate CG release remain partially elusive. We studied a patient exhibiting characteristic clinical features of HLH associated with markedly impaired cytotoxic T lymphocyte (CTL) and natural killer (NK) cell exocytosis functions, who beared biallelic deleterious mutations in the gene encoding the small GTPase RhoG. Experimental ablation of RHOG in a model cell line and primary CTLs from healthy individuals uncovered a hitherto unappreciated role of RhoG in retaining CGs in the vicinity of the plasma membrane (PM), a fundamental prerequisite for CG exocytotic release. We discovered that RhoG engages in a protein-protein interaction with Munc13-4, an exocytosis protein essential for CG fusion with the PM. We show that this interaction is critical for docking of Munc13-4+ CGs to the PM and subsequent membrane fusion and release of CG content. Thus, our study illuminates RhoG as a novel essential regulator of human lymphocyte cytotoxicity and provides the molecular pathomechanism behind the identified here and previously unreported genetically determined form of HLH.

Conventional and Unconventional Mechanisms by which Exocytosis Proteins Oversee β-cell Function and Protection

Type 2 diabetes (T2D) is one of the prominent causes of morbidity and mortality in the United States and beyond, reaching global pandemic proportions. One hallmark of T2D is dysfunctional glucose-stimulated insulin secretion from the pancreatic β-cell. Insulin is secreted via the recruitment of insulin secretory granules to the plasma membrane, where the soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and SNARE regulators work together to dock the secretory granules and release insulin into the circulation. SNARE proteins and their regulators include the Syntaxins, SNAPs, Sec1/Munc18, VAMPs, and double C2-domain proteins. Recent studies using genomics, proteomics, and biochemical approaches have linked deficiencies of exocytosis proteins with the onset and progression of T2D. Promising results are also emerging wherein restoration or enhancement of certain exocytosis proteins to β-cells improves whole-body glucose homeostasis, enhances β-cell function, and surprisingly, protection of β-cell mass. Intriguingly, overexpression and knockout studies have revealed novel functions of certain exocytosis proteins, like Syntaxin 4, suggesting that exocytosis proteins can impact a variety of pathways, including inflammatory signaling and aging. In this review, we present the conventional and unconventional functions of β-cell exocytosis proteins in normal physiology and T2D and describe how these insights might improve clinical care for T2D.

Specific deletion of CDC42 in pancreatic β cells attenuates glucose-induced insulin expression and secretion in mice

Insulin is a key hormone for maintaining glucose homeostasis in organisms. In general, deficiency of insulin synthesis and secretion results in type I diabetes, whereas insulin resistance leads to type 2 diabetes. Cell division cycle 42 (CDC42), a member of Rho GTPases family, has been shown as an essential regulator in the second phase of glucose-induced insulin secretion in pancreatic islets β cells in vitro. However, the effect of CDC42 on insulin expression has not been explored. Here we reported that the glucose-induced insulin expression and secretion were significantly inhibited in mice lacking CDC42 gene in pancreatic β cells (Rip-CDC42cKO) in vivo and in vitro. Deletion of CDC42 gene in pancreatic β cells did not affect survival or reproduction in mice. However, the Rip-CDC42cKO mice showed the systemic glucose intolerance and the decrease of glucose-induced insulin secretion without apparent alterations of peripheral tissues insulin sensitivity and the morphology of islets. Furthermore, we demonstrated that deletion of CDC42 gene in pancreatic β cells significantly attenuated the insulin expression through inhibiting the ERK1/2-NeuroD1 signaling pathway. Taken together, our study presents novel evidence that CDC42 is an important modulator in glucose-induced insulin expression as well as insulin secretion in pancreatic β cells.

Robo2 regulates synaptic oxytocin content by affecting actin dynamics

The regulation of neuropeptide level at the site of release is essential for proper neurophysiological functions. We focused on a prominent neuropeptide, oxytocin (OXT) in the zebrafish as an in vivo model to visualize and quantify OXT content at the resolution of a single synapse. We found that OXT-loaded synapses were enriched with polymerized actin. Perturbation of actin filaments by either cytochalasin-D or conditional Cofilin expression resulted in decreased synaptic OXT levels. Genetic loss of robo2 or slit3 displayed decreased synaptic OXT content and robo2 mutants displayed reduced mobility of the actin probe Lifeact-EGFP in OXT synapses. Using a novel transgenic reporter allowing real-time monitoring of OXT-loaded vesicles, we show that robo2 mutants display slower rate of vesicles accumulation. OXT-specific expression of dominant-negative Cdc42, which is a key regulator of actin dynamics and a downstream effector of Robo2, led to a dose-dependent increase in OXT content in WT, and a dampened effect in robo2 mutants. Our results link Slit3-Robo2-Cdc42, which controls local actin dynamics, with the maintenance of synaptic neuropeptide levels.

Epigenetic alteration of Rho guanine nucleotide exchange Factor 11 (ARHGEF11) in cord blood samples in macrosomia exposed to intrauterine hyperglycemia

Background: Macrosomia at birth is associated with maternal hyperglycemia and leads to subsequent susceptibility to obesity, abnormal glucose metabolism, hypertension, and dyslipidemia in offspring. Epigenetic reprogramming has been reported to be involved in the development of human diseases caused by suboptimal environmental or nutritional factors. The study was aiming to explore epigenetic mechanism influences on macrosomic infants exposed to intrauterine hyperglycemia. We performed a genome-wide analysis of DNA methylation in cord blood from macrosomic infants born to women with gestational diabetes in order to identify genes related to fetal growth or early adipose tissue development.Methods: To analyze the epigenetic patterns in umbilical cord blood in gestational diabetes mellitus (GDM), we collected umbilical cord blood from women with GDM (mean pregestational BMI of 24.4 kg/m² and mean neonatal birth weight of 4366 g) and normal glucose-tolerant women (mean pregestational BMI of 19.8 kg/m² and mean neonatal birth weight of 3166 g). Differentially methylated genes in the GDM group were identified using the Infinium HumanMethylation450 BeadChip array.Results: A total of 1251 genes were differentially methylated compared to the controls (p < .01). The methylation microarray data showed that two specific CpG sites (cg12604331 and cg08480098) in the gene body of ARHGEF11 were significantly hypomethylated in the cord blood in macrosomic infants. Altered DNA methylation levels of ARHGEF11 were negatively correlated with glucose levels and neonatal birth weight.Conclusions: Exposure to adverse intrauterine environments can alter fetal development, such as by affecting the nutritional status of the fetus. Such exposure can also result in significant epigenetic modifications, including DNA methylation, which could serve as a potential marker for nutrition and metabolic conditions at the neonatal stage or even in the adult.

Increased expression levels of Syntaxin 1A and Synaptobrevin 2/Vesicle-Associated Membrane Protein-2 are associated with the progression of bladder cancer

Gene expression is tightly regulated in time and space through a multitude of factors consisting of signaling molecules. Soluble N-ethylmaleimide-sensitive-factor attachment protein receptors (SNARE) are membrane proteins responsible for the intercellular trafficking of signals through endocytosis and exocytosis of vesicles. Altered expression of SNARE proteins in cellular communication is the major hallmark of cancer phenotypes as indicated in recent studies. SNAREs play an important role in maintaining cell growth and epithelial membrane permeability of the bladder and are not only involved in cancer progression but also metastatic cell invasion through SNARE-mediated trafficking. Synaptobrevin2/Vesicle associated membrane protein-2 (v-SNARE) and Syntaxin (t-SNARE) form a vesicular docking complex during endocytosis. Some earlier studies have shown a critical role of SNARE in colon, lungs, and breast cancer progression and metastasis. In this study, we analyzed the relative expression of the STX1A and VAMP2 (SYB2) for their possible association in the progression and metastasis of bladder cancer. The profiling of the genes showed a significant increase in STX1A and VAMP2 expression (p < 0.001) in high-grade tumor cells compared to normal and low-grade tumors. These findings suggest that elevated expression of STX1A and VAMP2 might have caused the abnormal progression and invasion of cancer cells leading to the transformation of cells into high-grade tumor in bladder cancer.

An in vivo screen for neuronal genes involved in obesity identifies Diacylglycerol kinase as a regulator of insulin secretion

Objective

Obesity is a complex disorder involving many genetic and environmental factors that are required to maintain energy homeostasis. While studies in human populations have led to significant progress in the generation of an obesity gene map and broadened our understanding of the genetic basis of common obesity, there is still a large portion of heritability and etiology that remains unknown. Here, we have used the genetically tractable fruit fly, Drosophila melanogaster, to identify genes/pathways that function in the nervous system to regulate energy balance.

Methods

We performed an in vivo RNAi screen in Drosophila neurons and assayed for obese or lean phenotypes by measuring changes in levels of stored fats (in the form of triacylglycerides or TAG). Three rounds of screening were performed to verify the reproducibility and specificity of the adiposity phenotypes. Genes that produced >25% increase in TAG (206 in total) underwent a second round of screening to verify their effect on TAG levels by retesting the same RNAi line to validate the phenotype. All remaining hits were screened a third time by testing the TAG levels of additional RNAi lines against the genes of interest to rule out any off-target effects.

Results

We identified 24 genes including 20 genes that have not been previously associated with energy homeostasis. One identified hit, Diacylglycerol kinase (Dgk), has mammalian homologues that have been implicated in genome-wide association studies for metabolic defects. Downregulation of neuronal Dgk levels increases TAG and carbohydrate levels and these phenotypes can be recapitulated by reducing Dgk levels specifically within the insulin-producing cells that secrete Drosophila insulin-like peptides (dILPs). Conversely, overexpression of kinase-dead Dgk, but not wild-type, decreased circulating dILP2 and dILP5 levels resulting in lower insulin signalling activity. Despite having higher circulating dILP levels, Dgk RNAi flies have decreased pathway activity suggesting that they are insulin-resistant.

Conclusion

Altogether, we have identified several genes that act within the CNS to regulate energy homeostasis. One of these, Dgk, acts within the insulin-producing cells to regulate the secretion of dILPs and energy homeostasis in Drosophila.

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RNAi screen in neurons identifies 24 regulators of energy homeostasis.

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One of the hits, Dgk, affects lipid and carbohydrate homeostasis.

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Dgk acts within the IPCs to regulate dILP secretion and insulin signalling activity.

Excess cholesterol inhibits glucose-stimulated fusion pore dynamics in insulin exocytosis

Type 2 diabetes is caused by defects in both insulin sensitivity and insulin secretion. Glucose triggers insulin secretion by causing exocytosis of insulin granules from pancreatic β-cells. High circulating cholesterol levels and a diminished capacity of serum to remove cholesterol from β-cells are observed in diabetic individuals. Both of these effects can lead to cholesterol accumulation in β-cells and contribute to β-cell dysfunction. However, the molecular mechanisms by which cholesterol accumulation impairs β-cell function remain largely unknown. Here, we used total internal reflection fluorescence microscopy to address, at the single-granule level, the role of cholesterol in regulating fusion pore dynamics during insulin exocytosis. We focused particularly on the effects of cholesterol overload, which is relevant to type 2 diabetes. We show that excess cholesterol reduced the number of glucose-stimulated fusion events, and modulated the proportion of full fusion and kiss-and-run fusion events. Analysis of single exocytic events revealed distinct fusion kinetics, with more clustered and compound exocytosis observed in cholesterol-overloaded β-cells. We provide evidence for the involvement of the GTPase dynamin, which is regulated in part by cholesterol-induced phosphatidylinositol 4,5-bisphosphate enrichment in the plasma membrane, in the switch between full fusion and kiss-and-run fusion. Characterization of insulin exocytosis offers insights into the role that elevated cholesterol may play in the development of type 2 diabetes.

Syntaxin 2 Acts as Inhibitory SNARE for Insulin Granule Exocytosis

Of the four syntaxins specialized for exocytosis, syntaxin (Syn)-2 is the least understood. In this study, we used Syn-2/epimorphin knockout mice to examine the role of Syn-2 in insulin secretory granule (SG) exocytosis. Unexpectedly, Syn-2 knockout mice exhibited paradoxical superior glucose homeostasis resulting from an enhanced insulin secretion. This was confirmed in vitro by pancreatic islet perifusion showing an amplified biphasic glucose-stimulated insulin secretion arising from an increase in size of the readily releasable pool of insulin SGs and enhanced SG pool refilling. The increase in insulin exocytosis was attributed mainly to an enhanced recruitment of the larger pool of newcomer SGs that undergoes no residence time on plasma membrane before fusion and, to a lesser extent, also the predocked SGs. Consistently, Syn-2 depletion resulted in a stimulation-induced increase in abundance of exocytotic complexes we previously demonstrated as mediating the fusion of newcomer SGs (Syn-3/VAMP8/SNAP25/Munc18b) and predocked SGs (Syn-1A/VAMP2/SNAP25/Muncn18a). This work is the first to show in mammals that Syn-2 could function as an inhibitory SNARE protein that, when relieved, could promote exocytosis in pancreatic islet β-cells. Thus, Syn-2 may serve as a potential target to treat diabetes.

Long-Term Potentiation: From CaMKII to AMPA Receptor Trafficking

For more than 20 years, we have known that Ca(2+)/calmodulin-dependent protein kinase (CaMKII) activation is both necessary and sufficient for the induction of long-term potentiation (LTP). During this time, tremendous effort has been spent in attempting to understand how CaMKII activation gives rise to this phenomenon. Despite such efforts, there is much to be learned about the molecular mechanisms involved in LTP induction downstream of CaMKII activation. In this review, we highlight recent developments that have shaped our current thinking about the molecular mechanisms underlying LTP and discuss important questions that remain in the field.

Intracellular periodontal pathogen exploits recycling pathway to exit from infected cells

Although human gingival epithelium prevents intrusions by periodontal bacteria, Porphyromonas gingivalis, the most well-known periodontal pathogen, is able to invade gingival epithelial cells and pass through the epithelial barrier into deeper tissues. We previously reported that intracellular P. gingivalis exits from gingival epithelial cells via a recycling pathway. However, the underlying molecular process remains unknown. In the present study, we found that the pathogen localized in early endosomes recruits VAMP2 and Rab4A. VAMP2 was found to be specifically localized in early endosomes, although its localization remained unclear in mammalian cells. A single transmembrane domain of VAMP2 was found to be necessary and sufficient for localizing in early endosomes containing P. gingivalis in gingival epithelial cells. VAMP2 forms a complex with EXOC2/Sec5 and EXOC3/Sec6, whereas Rab4A mediates dissociation of the EXOC complex followed by recruitment of RUFY1/Rabip4, Rab4A effector, and Rab14. Depletion of VAMP2 or Rab4A resulted in accumulation of bacteria in early endosomes and disturbed bacterial exit from infected cells. It is suggested that these novel dynamics allow P. gingivalis to exploit fast recycling pathways promoting further bacterial penetration of gingival tissues.

Interplay between Rab27a effectors in pancreatic β-cells

The small GTPase Rab27a is a member of the Rab family that is involved in membrane trafficking in various kinds of cells. Rab27a has GTP- and GDP-bound forms, and their interconversion regulates intracellular signaling pathways. Typically, only a GTP-bound GTPase binds its specific effectors with the resulting downstream signals controlling specific cellular functions. We previously identified novel Rab27a-interacting proteins. Surprisingly, some of these proteins interacted with GDP-bound Rab27a. The present study reviews recent progress in our understanding of the roles of Rab27a and its effectors in the secretory process. In pancreatic β-cells, GTP-bound Rab27a regulates insulin secretion at the pre-exocytotic stages via its GTP-specific effectors such as Exophilin8/Slac2-c/MyRIP and Slp4/Granuphilin. Glucose stimulation causes insulin exocytosis. Glucose stimulation also converts Rab27a from its GTP- to its GDP-bound form. GDP-bound Rab27a interacts with GDP-specific effectors and controls endocytosis of the secretory membrane. Thus, Rab27a cycling between GTP- and GDP-bound forms synchronizes with the recycling of secretory membrane to re-use the membrane and keep the β-cell volume constant.

Distinct pathways of cholesterol biosynthesis impact on insulin secretion

Results from previous investigations have indicated that glucose-stimulated insulin secretion (GSIS) is affected by changes in cholesterol and its intermediates, but the precise link between secretion and cholesterol has not been thoroughly investigated. In this study, we show the contribution of both protein isoprenylation and cholesterol-dependent plasma membrane structural integrity to insulin secretion in INS-1E cells and mouse islets. Acute (2 h) inhibition of hydroxyl-methylglutaryl-CoA reductase by simvastatin (SIM) resulted in inhibition of GSIS without reduction in total cellular cholesterol content. This effect was prevented by cell loading with the isoprenyl molecule geranylgeranyl pyrophosphate. Chronic (24 h) inhibition of cholesterol biosynthesis resulted in inhibition of GSIS with a significant reduction in total cellular cholesterol content, which was also observed after the inhibition of cholesterol biosynthesis downstream of isoprenoid formation. Electron paramagnetic resonance analyses of INS-1E cells showed that the SIM-induced reduction in cholesterol increased plasma membrane fluidity. Thus, the blockade of cholesterol biosynthesis resulted in the reduction of availability of isoprenoids, followed by a reduction in the total cholesterol content associated with an increase in plasma membrane fluidity. Herein, we show the different contributions of cholesterol biosynthesis to GSIS, and propose that isoprenoid molecules and cholesterol-dependent signaling are dual regulators of proper β-cell function.

Involvement of RhoA/ROCK in insulin secretion of pancreatic β-cells in 3D culture

Cell-cell contacts and interactions between pancreatic β-cells and/or other cell populations within islets are essential for cell survival, insulin secretion, and functional synchronization. Three-dimensional (3D) culture systems supply the ideal microenvironment for islet-like cluster formation and functional maintenance. However, the underlying mechanisms remain unclear. In this study, mouse insulinoma 6 (MIN6) cells were cultured in a rotating 3D culture system to form islet-like aggregates. Glucose-stimulated insulin secretion (GSIS) and the RhoA/ROCK pathway were investigated. In the 3D-cultured MIN6 cells, more endocrine-specific genes were up-regulated, and GSIS was increased to a greater extent than in cells grown in monolayers. RhoA/ROCK inactivation led to F-actin remodeling in the MIN6 cell aggregates and greater insulin exocytosis. The gap junction protein, connexin 36 (Cx36), was up-regulated in MIN6 cell aggregates and RhoA/ROCK-inactivated monolayer cells. GSIS dramatically decreased when Cx36 was knocked down by short interfering RNA and could not be reversed by RhoA/ROCK inactivation. Thus, the RhoA/ROCK signaling pathway is involved in insulin release through the up-regulation of Cx36 expression in 3D-cultured MIN6 cells.

Population distribution analyses reveal a hierarchy of molecular players underlying parallel endocytic pathways

Single-cell-resolved measurements reveal heterogeneous distributions of clathrin-dependent (CD) and -independent (CLIC/GEEC: CG) endocytic activity in Drosophila cell populations. dsRNA-mediated knockdown of core versus peripheral endocytic machinery induces strong changes in the mean, or subtle changes in the shapes of these distributions, respectively. By quantifying these subtle shape changes for 27 single-cell features which report on endocytic activity and cell morphology, we organize 1072 Drosophila genes into a tree-like hierarchy. We find that tree nodes contain gene sets enriched in functional classes and protein complexes, providing a portrait of core and peripheral control of CD and CG endocytosis. For 470 genes we obtain additional features from separate assays and classify them into early- or late-acting genes of the endocytic pathways. Detailed analyses of specific genes at intermediate levels of the tree suggest that Vacuolar ATPase and lysosomal genes involved in vacuolar biogenesis play an evolutionarily conserved role in CG endocytosis.

Rho GTPases, phosphoinositides, and actin: a tripartite framework for efficient vesicular trafficking

Rho GTPases are well known regulators of the actin cytoskeleton that act by binding and activating actin nucleators. They are therefore involved in many actin-based processes, including cell migration, cell polarity, and membrane trafficking. With the identification of phosphoinositide kinases and phosphatases as potential binding partners or effectors, Rho GTPases also appear to participate in the regulation of phosphoinositide metabolism. Since both actin dynamics and phosphoinositide turnover affect the efficiency and the fidelity of vesicle transport between cell compartments, Rho GTPases have emerged as critical players in membrane trafficking. Rho GTPase activity, actin remodeling, and phosphoinositide metabolism need to be coordinated in both space and time to ensure the progression of vesicles along membrane trafficking pathways. Although most molecular pathways are still unclear, in this review, we will highlight recent advances made in our understanding of how Rho-dependent signaling pathways organize actin dynamics and phosphoinositides and how phosphoinositides potentially provide negative feedback to Rho GTPases during endocytosis, exocytosis and membrane exchange between intracellular compartments.

WNT5A induces release of exosomes containing pro-angiogenic and immunosuppressive factors from malignant melanoma cells

Background

Wnt proteins are important for developmental processes and certain diseases. WNT5A is a non-canonical Wnt protein that previously has been shown to play a role in the progression of malignant melanoma. High expression of WNT5A in melanoma tumors correlates to formation of distant metastasis and poor prognosis. This has partly been described by the findings that WNT5A expression in melanoma cell lines increases migration and invasion.

Methods

Malignant melanoma cell lines were treated with rWNT5A or WNT5A siRNA, and mRNA versus protein levels of soluble mediators were measured using RT-PCR, cytokine bead array and ELISA. The induced signaling pathways were analyzed using inhibitors, Rho-GTPase pull down assays and western blot. Ultracentrifugation and electron microscopy was used to analyze microvesicles. Gene expression microarray data obtained from primary malignant melanomas was used to verify our data.

Results

We show that WNT5A signaling induces a Ca²⁺-dependent release of exosomes containing the immunomodulatory and pro-angiogenic proteins IL-6, VEGF and MMP2 in melanoma cells. The process was independent of the transcriptional machinery and depletion of WNT5A reduced the levels of the exosome-derived proteins. The WNT5A induced exosomal secretion was neither affected by Tetanus toxin nor Brefeldin A, but was blocked by the calcium chelator Bapta, inhibited by a dominant negative version of the small Rho-GTPase Cdc42 and was accompanied by cytoskeletal reorganization. Co-cultures of melanoma/endothelial cells showed that depletion of WNT5A in melanoma cells decreased endothelial cell branching, while stimulation of endothelial cells with isolated rWNT5A-induced melanoma exosomes increased endothelial cell branching in vitro. Finally, gene expression data analysis of primary malignant melanomas revealed a correlation between WNT5A expression and the angiogenesis marker ESAM.

Conclusions

These data indicate that WNT5A has a broader function on tumor progression and metastatic spread than previously known; by inducing exosome-release of immunomodulatory and pro-angiogenic factors that enhance the immunosuppressive and angiogenic capacity of the tumors thus rendering them more aggressive and more prone to metastasize.

Activated Cdc42-bound IQGAP1 determines the cellular endocytic site

Recruitment of specific molecules to a specific membrane site is essential for communication between specialized membranous organelles. In the present study, we identified IQGAP1 as a novel GDP-bound-Rab27a-interacting protein. We found that IQGAP1 interacts with GDP-bound Rab27a when it forms a complex with GTP-bound Cdc42. We also found that IQGAP1 regulates the endocytosis of insulin secretory membranes. Silencing of IQGAP1 inhibits both endocytosis and the glucose-induced redistribution of endocytic machinery, including Rab27a and its binding protein coronin 3. These processes can also be inhibited by disruption of the trimeric complex with dominant negative IQGAP1 and Cdc42. These results indicate that activation of Cdc42 in response to the insulin secretagogue glucose recruits endocytic machinery to IQGAP1 at the cell periphery and regulates endocytosis at this membrane site.

Signaling mechanisms of glucose-induced F-actin remodeling in pancreatic islet β cells

The maintenance of whole-body glucose homeostasis is critical for survival, and is controlled by the coordination of multiple organs and endocrine systems. Pancreatic islet β cells secrete insulin in response to nutrient stimuli, and insulin then travels through the circulation promoting glucose uptake into insulin-responsive tissues such as liver, skeletal muscle and adipose. Many of the genes identified in human genome-wide association studies of diabetic individuals are directly associated with β cell survival and function, giving credence to the idea that β-cell dysfunction is central to the development of type 2 diabetes. As such, investigations into the mechanisms by which β cells sense glucose and secrete insulin in a regulated manner are a major focus of current diabetes research. In particular, recent discoveries of the detailed role and requirements for reorganization/remodeling of filamentous actin (F-actin) in the regulation of insulin release from the β cell have appeared at the forefront of islet function research, having lapsed in prior years due to technical limitations. Recent advances in live-cell imaging and specialized reagents have revealed localized F-actin remodeling to be a requisite for the normal biphasic pattern of nutrient-stimulated insulin secretion. This review will provide an historical look at the emergent focus on the role of the actin cytoskeleton and its regulation of insulin secretion, leading up to the cutting-edge research in progress in the field today.

A p21-activated kinase (PAK1) signaling cascade coordinately regulates F-actin remodeling and insulin granule exocytosis in pancreatic β cells

Human islet studies implicate an important signaling role for the Cdc42 effector protein p21-activated kinase (PAK1) in the sustained/second-phase of insulin secretion. Because human islets from type 2 diabetic donors lack ∼80% of normal PAK1 protein levels, the mechanistic requirement for PAK1 signaling in islet function was interrogated. Similar to MIN6 β cells, human islets elicited glucose-stimulated PAK1 activation that was sensitive to the PAK1 inhibitor, IPA3. Given that sustained insulin secretion has been correlated with glucose-induced filamentous actin (F-actin) remodeling, we tested the hypothesis that a Cdc42-activated PAK1 signaling cascade is required to elicit F-actin remodeling to mobilize granules to the cell surface. Live-cell imaging captured the glucose-induced cortical F-actin remodeling in MIN6 β cells; IPA3-mediated inhibition of PAK1 abolished this remodeling. IPA3 also ablated glucose-stimulated insulin granule accumulation at the plasma membrane, consistent with its role in sustained/second-phase insulin release. Both IPA3 and a selective inhibitor of the Cdc42 GTPase, ML-141, blunted the glucose-stimulated activation of Raf-1, suggesting Raf-1 to be downstream of Cdc42→PAK1. IPA3 also inhibited MEK1/2 activation, implicating the MEK1/2→ERK1/2 cascade to occur downstream of PAK1. Importantly, PD0325901, a new selective inhibitor of MEK1/2→ERK1/2 activation, impaired F-actin remodeling and the sustained/amplification pathway of insulin release. Taken together, these data suggest that glucose-mediated activation of Cdc42 leads to activation of PAK1 and prompts activation of its downstream targets Raf-1, MEK1/2 and ERK1/2 to elicit F-actin remodeling and recruitment of insulin granules to the plasma membrane to support the sustained phase of insulin release.

The association between CDC42 and caveolin-1 is involved in the regulation of capacitation and acrosome reaction of guinea pig and mouse sperm

In the mammalian sperm, the acrosome reaction (AR) is considered to be a regulated secretion that is an essential requirement for physiological fertilization. The AR is the all-or-nothing secretion system that allows for multiple membrane fusion events. It is a Ca2+-regulated exocytosis reaction that has also been shown to be regulated by several signaling pathways. CDC42 has a central role in the regulated exocytosis through the activation of SNARE proteins and actin polymerization. Furthermore, the lipid raft protein caveolin-1 (CAV1) functions as a scaffold and guanine nucleotide dissociation inhibitor protein for CDC42, which is inactivated when associated with CAV1. CDC42 and other RHO proteins have been shown to localize in the acrosome region of mammalian sperm; however, their relationship with the AR is unknown. Here, we present the first evidence that CDC42 and CAV1 could be involved in the regulation of capacitation and the AR. Our findings show that CDC42 is activated early during capacitation, reaching an activation maximum after 20 min of capacitation. Spontaneous and progesterone-induced ARs were inhibited when sperm were capacitated in presence of secramine A, a specific CDC42 inhibitor. CAV1 and CDC42 were co-immunoprecipitated from the membranes of noncapacitated sperm; this association was reduced in capacitated sperm, and our data suggest that the phosphorylation (Tyr14) of CAV1 by c-Src is involved in such reductions. We suggest that CDC42 activation is favored by the disruption of the CAV1–CDC42 interaction, allowing for its participation in the regulation of capacitation and the AR.

Munc18-1 regulates first-phase insulin release by promoting granule docking to multiple syntaxin isoforms

Attenuated levels of the Sec1/Munc18 (SM) protein Munc18-1 in human islet β-cells is coincident with type 2 diabetes, although how Munc18-1 facilitates insulin secretion remains enigmatic. Herein, using conventional Munc18-1(+/-) and β-cell specific Munc18-1(-/-) knock-out mice, we establish that Munc18-1 is required for the first phase of insulin secretion. Conversely, human islets expressing elevated levels of Munc18-1 elicited significant potentiation of only first-phase insulin release. Insulin secretory changes positively correlated with insulin granule number at the plasma membrane: Munc18-1-deficient cells lacked 35% of the normal component of pre-docked insulin secretory granules, whereas cells with elevated levels of Munc18-1 exhibited a ∼20% increase in pre-docked granule number. Pre-docked syntaxin 1-based SNARE complexes bound by Munc18-1 were detected in β-cell lysates but, surprisingly, were reduced by elevation of Munc18-1 levels. Paradoxically, elevated Munc18-1 levels coincided with increased binding of syntaxin 4 to VAMP2 at the plasma membrane. Accordingly, syntaxin 4 was a requisite for Munc18-1 potentiation of insulin release. Munc18c, the cognate SM isoform for syntaxin 4, failed to bind SNARE complexes. Given that Munc18-1 does not pair with syntaxin 4, these data suggest a novel indirect role for Munc18-1 in facilitating syntaxin 4-mediated granule pre-docking to support first-phase insulin exocytosis.

Synapses of amphids defective (SAD-A) kinase promotes glucose-stimulated insulin secretion through activation of p21-activated kinase (PAK1) in pancreatic β-Cells

The p21-activated kinase-1 (PAK1) is implicated in regulation of insulin exocytosis as an effector of Rho GTPases. PAK1 is activated by the onset of glucose-stimulated insulin secretion (GSIS) through phosphorylation of Thr-423, a major activation site by Cdc42 and Rac1. However, the kinase(s) that phosphorylates PAK1 at Thr-423 in islet β-cells remains elusive. The present studies identified SAD-A (synapses of amphids defective), a member of AMP-activated protein kinase-related kinases exclusively expressed in brain and pancreas, as a key regulator of GSIS through activation of PAK1. We show that SAD-A directly binds to PAK1 through its kinase domain. The interaction is mediated by the p21-binding domain (PBD) of PAK1 and requires both kinases in an active conformation. The binding leads to direct phosphorylation of PAK1 at Thr-423 by SAD-A, triggering the onset of GSIS from islet β-cells. Consequently, ablation of PAK1 kinase activity or depletion of PAK1 expression completely abolishes the potentiating effect of SAD-A on GSIS. Consistent with its role in regulating GSIS, overexpression of SAD-A in MIN6 islet β-cells significantly stimulated cytoskeletal remodeling, which is required for insulin exocytosis. Together, the present studies identified a critical role of SAD-A in the activation of PAK1 during the onset of insulin exocytosis.

The small GTPase Cdc42 modulates the number of exocytosis-competent dense-core vesicles in PC12 cells

Although the small GTPase Rho family Cdc42 has been shown to facilitate exocytosis through increasing the amount of hormones released, the precise mechanisms regulating the quantity of hormones released on exocytosis are not well understood. Here we show by live cell imaging analysis under TIRF microscope and immunocytochemical analysis under confocal microscope that Cdc42 modulated the number of fusion events and the number of dense-core vesicles produced in the cells. Overexpression of a wild-type or constitutively-active form of Cdc42 strongly facilitated high-KCl-induced exocytosis from the newly recruited plasma membrane vesicles in PC12 cells. By contrast, a dominant-negative form of Cdc42 inhibited exocytosis from both the newly recruited and previously docked plasma membrane vesicles. The number of intracellular dense-core vesicles was increased by the overexpression of both a wild-type and constitutively-active form of Cdc42. Consistently, activation of Cdc42 by overexpression of Tuba, a Golgi-associated guanine nucleotide exchange factor for Cdc42 increased the number of intracellular dense-core vesicles, whereas inhibition of Cdc42 by overexpression of the Cdc42/Rac interactive binding domain of neuronal Wiskott-Aldrich syndrome protein decreased the number of them. These findings suggest that Cdc42 facilitates exocytosis by modulating both the number of exocytosis-competent dense-core vesicles and the production of dense-core vesicles in PC12 cells.

Factors affecting the quantification of biomolecular interactions by fluorescence cross-correlation spectroscopy

Fluorescence cross-correlation spectroscopy (FCCS) is used to determine interactions and dissociation constants (K(d)s) of biomolecules. The determination of a K(d) depends on the accurate measurement of the auto- and cross-correlation function (ACF and CCF) amplitudes. In the case of complete binding, the ratio of the CCF/ACF amplitudes is expected to be 1. However, measurements performed on tandem fluorescent proteins (FPs), in which two different FPs are linked, yield CCF/ACF amplitude ratios of ~0.5 or less for different FCCS schemes. We use single wavelength FCCS and pulsed interleaved excitation FCCS to measure various tandem FPs constituted of different red and green FPs and determine the causes for this suboptimal ratio. The main causes for the reduced CCF/ACF amplitude ratio are differences in observation volumes for the different labels, the existence of dark FPs due to maturation problems, photobleaching, and to a lesser extent Förster (or fluorescence) resonance energy transfer between the labels. We deduce the fraction of nonfluorescent proteins for EGFP, mRFP, and mCherry as well as the differences in observation volumes. We use this information to correct FCCS measurements of the interaction of Cdc42, a small Rho-GTPase, with its effector IQGAP1 in live cell measurements to obtain a label-independent value for the K(d).

New insights into the role of the cortical cytoskeleton in exocytosis from neuroendocrine cells

The cortical cytoskeleton is a dense network of filamentous actin (F-actin) that participates in the events associated with secretion from neuroendocrine cells. This filamentous web traps secretory vesicles, acting as a retention system that blocks the access of vesicles to secretory sites during the resting state, and it mediates their active directional transport during stimulation. The changes in the cortical cytoskeleton that drive this functional transformation have been well documented, particularly in cultured chromaffin cells. At the biochemical level, alterations in F-actin are governed by the activity of molecular motors like myosins II and V and by other calcium-dependent proteins that influence the polymerization and cross-linking of F-actin structures. In addition to modulating vesicle transport, the F-actin cortical network and its associated motor proteins also influence the late phases of the secretory process, including membrane fusion and the release of active substances through the exocytotic fusion pore. Here, we discuss the potential interactions between the F-actin cortical web and proteins such as SNAREs during secretion. We also discuss the role of the cytoskeleton in organizing the molecular elements required to sustain regulated exocytosis, forming a molecular structure that foments the efficient release of neurotransmitters and hormones.

Lipid rafts, caveolae and GPI-linked proteins

Lipid rafts and caveolae are specialized membrane microdomains enriched in sphingolipids and cholesterol. They function in a variety of cellular processes including but not limited to endocytosis, transcytosis, signal transduction and receptor recycling. Here, we outline the similarities and differences between lipid rafts and caveolae as well as discuss important components and functions of each.

Cool-1/βPIX functions as a guanine nucleotide exchange factor in the cycling of Cdc42 to regulate insulin secretion

Second-phase insulin release requires the sustained mobilization of insulin granules from internal storage pools to the cell surface for fusion with the plasma membrane. However, the detailed mechanisms underlying this process remain largely unknown. GTP-loading of the small GTPase Cdc42 is the first glucose-specific activation step in the process, although how glucose triggers Cdc42 activation is entirely unknown. In a directed candidate screen for guanine nucleotide exchange factors (GEFs), which directly activate small GTPases, Cool-1/βPix was identified in pancreatic islet beta cells. In support of its role as the beta cell Cdc42 GEF, βPix coimmunoprecipitated with Cdc42 in human islets and MIN6 beta cells in a glucose-dependent manner, peaking just prior to Cdc42 activation. Furthermore, RNAi-mediated βPix reduction by 50% corresponded to full ablation of glucose-induced Cdc42 activation and significant attenuation of basal and glucose-stimulated insulin secretion. Of the two Cdc42 guanine nucleotide dissociation inhibitor (GDI) proteins identified in beta cells, βPix competed selectively with caveolin-1 (Cav-1) but not RhoGDI in coimmunoprecipitation and GST-Cdc42-GDP interaction assays. However, a phospho-deficient Cav-1-Y14F mutant failed to compete with βPix; Cav-1(Tyr14) is an established phosphorylation site for Src kinase. Taken together, these data support a new model, wherein glucose stimulates Cav-1 and induces its dissociation from Cdc42, possibly via Src kinase activation to phosphorylate Cav-1(Tyr14), to promote Cdc42-βPix binding and Cdc42 activation, and to trigger downstream signaling and ultimately sustain insulin release.

T-cadherin (Cdh13) in association with pancreatic β-cell granules contributes to second phase insulin secretion

Glucose homeostasis depends on adequate control of insulin secretion. We report the association of the cell-adhesion and adiponectin (APN)-binding glycoprotein T-cadherin (Cdh13) with insulin granules in mouse and human β-cells. Immunohistochemistry and electron microscopy of islets in situ and targeting of RFP-tagged T-cadherin to GFP-labeled insulin granules in isolated β-cells demonstrate this unusual location. Analyses of T-cadherin-deficient (Tcad-KO) mice show normal islet architecture and insulin content. However, T-cadherin is required for sufficient insulin release in vitro and in vivo. Primary islets from Tcad-KO mice were defective in glucose-induced but not KCl-mediated insulin secretion. In vivo, second phase insulin release in T-cad-KO mice during a hyperglycemic clamp was impaired while acute first phase release was unaffected. Tcad-KO mice showed progressive glucose intolerance by 5 mo of age without concomitant changes in peripheral insulin sensitivity. Our analyses detected no association of APN with T-cadherin on β-cell granules although colocalization was observed on the pancreatic vasculature. These data identify T-cadherin as a novel component of insulin granules and suggest that T-cadherin contributes to the regulation of insulin secretion independently of direct interactions with APN.

SNARE-mediated rapid lysosome fusion in membrane raft clustering and dysfunction of bovine coronary arterial endothelium

The present study attempted to evaluate whether soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) mediate lysosome fusion in response to death receptor activation and contribute to membrane raft (MR) clustering and consequent endothelial dysfunction in coronary arterial endothelial cells. By immunohistochemical analysis, vesicle-associated membrane proteins 2 (VAMP-2, vesicle-SNAREs) were found to be abundantly expressed in the endothelium of bovine coronary arteries. Direct lysosome fusion monitoring by N-(3-triethylammoniumpropyl)-4-[4-(dibutylamino)styryl]pyridinium dibromide (FM1-43) quenching demonstrated that the inhibition of VAMP-2 with tetanus toxin or specific small interfering ribonucleic acid (siRNA) almost completely blocked lysosome fusion to plasma membrane induced by Fas ligand (FasL), a well-known MR clustering stimulator. The involvement of SNAREs was further confirmed by an increased interaction of VAMP-2 with a target-SNARE protein syntaxin-4 after FasL stimulation in coimmunoprecipitation analysis. Also, the inhibition of VAMP-2 with tetanus toxin or VAMP-2 siRNA abolished FasL-induced MR clustering, its colocalization with a NADPH oxidase unit gp91(phox), and increased superoxide production. Finally, FasL-induced impairment of endothelium-dependent vasodilation was reversed by the treatment of bovine coronary arteries with tetanus toxin or VAMP-2 siRNA. VAMP-2 is critical to lysosome fusion in MR clustering, and this VAMP-2-mediated lysosome-MR signalosomes contribute to redox regulation of coronary endothelial function.

A distinct trans-Golgi network subcompartment for sorting of synaptic and granule proteins in neurons and neuroendocrine cells

Golgi-to-plasma-membrane trafficking of synaptic-like microvesicle (SLMV) proteins, vesicular acetylcholine transporter (VAChT) and synaptophysin (SYN), and a large dense-core vesicle (LDCV) protein, chromogranin A (CgA), was investigated in undifferentiated neuroendocrine PC12 cells. Live cell imaging and 20°C block-release experiments showed that VAChT-GFP, SYN-GFP and CgA-RFP specifically and transiently cohabitated in a distinct sorting compartment during cold block and then separated into synaptic protein transport vesicles (SPTVs) and LDCVs, after release from temperature block. We found that in this trans-Golgi subcompartment there was colocalization of SPTV and LDCV proteins, most significantly with VAMP4 and Golgin97, and to some degree with TGN46, but not at all with TGN38. Moreover, some SNAP25 and VAMP2, two subunits of the exocytic machinery, were also recruited onto this compartment. Thus, in neuroendocrine cells, synaptic vesicle and LDCV proteins converge briefly in a distinct trans-Golgi network subcompartment before sorting into SPTVs and LDCVs, ultimately for delivery to the plasma membrane. This specialized sorting compartment from which SPTVs and LDCVs bud might facilitate the acquisition of common exocytic machinery needed on the membranes of these vesicles.

How important are Rho GTPases in neurosecretion?

Rho GTPases are small GTP binding proteins belonging to the Ras superfamily which act as molecular switches that regulate many cellular function including cell morphology, cell to cell interaction, cell migration and adhesion. In neuronal cells, Rho GTPases have been proposed to regulate neuronal development and synaptic plasticity. However, the role of Rho GTPases in neurosecretion is poorly documented. In this review, we discuss data that highlight the importance of Rho GTPases and their regulators into the control of neurotransmitter and hormone release in neurons and neuroendocrine cells, respectively.

Rho GTPase Cdc42 is a direct interacting partner of Adenomatous Polyposis Coli protein and can alter its cellular localization

Adenomatous Polyposis Coli (APC) is a tumor suppressor gene product involved in colon cancer. APC is a large multidomain molecule of 2843 amino acid residues and connects cell-cell adhesion, the F-actin/microtubule cytoskeleton and the nucleus. Here we show that Cdc42 interacts directly with the first three armadillo repeats of APC by yeast two-hybrid screens. We confirm the Cdc42-APC interaction using pulldown assays in vitro and FRET assays in vivo. Interestingly, Cdc42 interacts with APC at leading edge sites where F-actin is enriched. In contrast, Cdc42 interacts with the truncated mutant APC^1–1638 in cellular puncta associated with the golgi-lysozome pathway in transfected CHO cells. In HCT116 and SW480 cells, Cdc42 induces the relocalization of endogenous APC and the mutant APC^1–1338 to the plasma membrane and cellular puncta, respectively. Taken together, these data indicate that the Cdc42-APC interaction induces localization of both APC and mutant APC and may thus play a direct role in the functions of these proteins.

R1467H Variants of Rho Guanine Nucleotide Exchange Factor 11 (ARHGEF11) are Associated with Type 2 Diabetes Mellitus in Koreans

Background

The human Rho guanine nucleotide exchange factor 11 (ARHGEF11) functions as an activator of Rho GTPases and is thought to influence insulin signaling. The R1467H variant of ARHGEF11 has been reported to be associated with susceptibility to type 2 diabetes mellitus (T2DM) in Western populations.

Methods

We investigated the effects of the R1467H variant on susceptibility to T2DM as well as related traits in a Korean population. We genotyped the R1467H (rs945508) of ARHGEF11 in 689 unrelated T2DM patients and 249 non-diabetic individuals and compared the clinical and biochemical characteristics according to different alleles.

Results

The H allele was significantly more frequent in T2DM cases than in controls (P = 0.037, 17.1% and 13.1%; respectively). H homozygocity was associated with a higher risk of T2DM compared to those with R/R or R/H genotype (odds ratio, 5.24; 95% confidence interval, 1.06 to 25.83; P = 0.042). The fasting plasma glucose, HbA1c, fasting insulin, HOMA2-IR and HOMA2-%β levels did not differ significantly between different genotypes.

Conclusion

Our study replicated associations of the ARHGEF11 polymorphism with increased risk of T2DM in a Korean population and thus supports previous data implicating a potential role of ARHGEF11 in the etiology of T2DM. Further studies revealing the underlying mechanism for this association are needed.

Rho GTPases and exocytosis: what are the molecular links?

Delivery of proteins or lipids to the plasma membrane or into the extracellular space occurs through exocytosis, a process that requires tethering, docking, priming and fusion of vesicles, as well as F-actin rearrangements in response to specific extracellular cues. GTPases of the Rho family have been implicated as important regulators of exocytosis, but how Rho proteins control this process is an open question. In this review, we focus on molecular connections that drive Rho-dependent exocytosis in polarized and regulated exocytosis. Specifically, we present data showing that Rho proteins interaction with the exocyst complex and IQGAP mediates polarized exocytosis, whereas interaction with actin-binding proteins like N-WASP mediates regulated exocytosis.

Glucose-induced ERM protein activation and translocation regulates insulin secretion

A key step in regulating insulin secretion is insulin granule trafficking to the plasma membrane. Using live-cell time-lapse confocal microscopy, we observed a dynamic association of insulin granules with filamentous actin and PIP2-enriched structures. We found that the scaffolding protein family ERM, comprising ezrin, radixin, and moesin, are expressed in β-cells and target both F-actin and PIP2. Furthermore, ERM proteins are activated via phosphorylation in a glucose- and calcium-dependent manner. This activation leads to a translocation of the ERM proteins to sites on the cell periphery enriched in insulin granules, the exocyst complex docking protein Exo70, and lipid rafts. ERM scaffolding proteins also participate in insulin granule trafficking and docking to the plasma membrane. Overexpression of a truncated dominant-negative ezrin construct that lacks the ERM F-actin binding domain leads to a reduction in insulin granules near the plasma membrane and impaired secretion. Conversely, overexpression of a constitutively active ezrin results in more granules near the cell periphery and an enhancement of insulin secretion. Diabetic mouse islets contain less active ERM, suggestive of a novel mechanism whereby impairment of insulin granule trafficking to the membrane through a complex containing F-actin, PIP2, Exo70, and ERM proteins contributes to defective insulin secretion.

Cholesterol biosynthesis pathway intermediates and inhibitors regulate glucose-stimulated insulin secretion and secretory granule formation in pancreatic beta-cells

Cholesterol is reportedly abundant in the endocrine secretory granule (SG) membrane. In this study, we examined the involvement of cholesterol biosynthesis intermediates and inhibitors in insulin secretion and SG formation mechanisms. There are two routes for the supply of cholesterol to the cells: one via de novo biosynthesis and the other via low-density lipoprotein receptor-mediated endocytosis. We found that insulin secretion and content are diminished by β-hydroxy-β-methylglutaryl-coenzyme A inhibitor lovastatin but not by lipoprotein depletion from the culture medium in MIN6 β-cells. Cholesterol biosynthesis intermediates mevalonate, squalene, and geranylgeranyl pyrophosphate enhanced glucose-stimulated insulin secretion, and the former two increased insulin content. The glucose-stimulated insulin secretion-enhancing effect of geranylgeranyl pyrophosphate was also confirmed in perifusion with rat islets. Morphologically, mevalonate and squalene increased the population of SGs without affecting their size. In contrast, lovastatin increased the SG size with reduction of insulin-accumulating dense cores, leading to a decrease in insulin content. Furthermore, insulin was secreted in a constitutive manner, indicating disruption of regulated insulin secretion. Because secretogranin III, a cholesterol-binding SG-residential granin-family protein, coincides with SG localization based on the cholesterol composition, secretogranin III may be associated with insulin-accumulating mechanisms. Although the SG membrane exhibits a high cholesterol composition, we could not find detergent-resistant membrane regions using a lipid raft-residential protein flotillin and a fluorescent cholesterol-Si-pyrene probe as markers on a sucrose-density gradient fractionation. We suggest that the high cholesterol composition of SG membrane with 40-50 mol% is crucial for insulin secretion and SG formation functions.