Rab3A effector domain peptides induce insulin exocytosis via a specific interaction with a cytosolic protein doublet

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.

Inhibition of Monoacylglycerol Lipase Activity Decreases Glucose-Stimulated Insulin Secretion in INS-1 (832/13) Cells and Rat Islets

Lipid signals derived from lipolysis and membrane phospholipids play an important role in glucose-stimulated insulin secretion (GSIS), though the exact secondary signals remain unclear. Previous reports have documented a stimulatory role of exogenously added mono-acyl-glycerol (MAG) on insulin secretion from cultured β-cells and islets. In this report we have determined effects of increasing intracellular MAG in the β-cell by inhibiting mono-acyl-glycerol lipase (MGL) activity, which catalyzes the final step in triacylglycerol breakdown, namely the hydrolysis of MAG to glycerol and free fatty acid (FA). To determine the role of MGL in GSIS, we used three different pharmacological agents (JZL184, MJN110 and URB602). All three inhibited GSIS and depolarization-induced insulin secretion in INS-1 (832/13). JZL184 significantly inhibited both GSIS and depolarization-induced insulin secretion in rat islets. JZL184 significantly decreased lipolysis and increased both mono- and diacyglycerol species in INS-1 cells. Analysis of the kinetics of GSIS showed that inhibition was greater during the sustained phase of secretion. A similar pattern was observed in the response of Ca²⁺ to glucose and depolarization but to a lesser degree suggesting that altered Ca²⁺ handling alone could not explain the reduction in insulin secretion. In addition, a significant reduction in long chain-CoA (LC-CoA) was observed in INS-1 cells at both basal and stimulatory glucose following inhibition of MGL. Our data implicate an important role for MGL in insulin secretion.

Design and characterization of a photo-activatable hedgehog probe that mimics the natural lipidated form

We have generated a photoactivatable form of sonic hedgehog protein by modifying the N-terminal cysteine with the heterobifunctional photocrosslinker 4-maleimidobenzophenone (Bzm). The Bzm modification on ShhN imparted a significant increase in activity as assessed in the C3H10T1/2 functional assay with potency comparable to that of the endogenous dual-lipidated form of ShhN (ShhNp). Reversed-phase HPLC analysis indicated that the increase in activity compared to unmodified ShhN may be due in part to the hydrophobic nature of the benzophenone group. In contrast to the fully processed ShhNp, Bzm-ShhN is monomeric as assessed by analytical SEC and does not require detergent to be soluble. Further, we demonstrated that the Bzm-ShhN was able to crosslink in vitro in the presence of a known binding partner, heparin. We suggest that Bzm-ShhN can serve as a relatively facile and preferred source of ShhNp for in vitro assays and as a probe to identify novel Hh protein interactions.

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.

A cryptic deletion in 5q31.2 provides further evidence for a minimally deleted region in myelodysplastic syndromes

Recurrent deletions of 5q in myeloid malignancies encompass two separate regions: deletion of 5q33, which is associated with the 5q− syndrome and haploinsufficiency of RPS14, and deletion of a more proximal locus at 5q31. We present a case with a cryptic 1.3 Mb deletion in 5q31.2 identified by array comparative genomic hybridization that places the proximal boundary of the deletion proximal and close to the candidate EGR1 gene. The patient was diagnosed initially with a myelodysplastic syndrome, with a del(20)(q11.2q13.3) as the sole abnormality identified by karyotyping. The patient progressed to acute myeloid leukemia with no change to the G-banded karyotype. The 1.3 Mb deletion on the long arm of one chromosome 5 was confirmed to have been present both at presentation with myelodysplastic syndrome and at transformation. This is an interesting case because there are few array studies identifying cryptic 5q deletions, and the study of these small deletions helps to refine the common deleted region. This case, together with previously published studies, suggests that the proximal boundary of the common deleted region may lie within the KDM3B gene.

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.

Hyperinsulinism in Infancy: From Basic Science to Clinical Disease

Dunne, Mark J., Karen E. Cosgrove, Ruth M. Shepherd, Albert Aynsley-Green, and Keith J. Lindley. Hyperinsulinism in Infancy: From Basic Science to Clinical Disease. Physiol Rev 84: 239–275, 2004; 10.1152/physrev.00022.2003.—Ion channelopathies have now been described in many well-characterized cell types including neurons, myocytes, epithelial cells, and endocrine cells. However, in only a few cases has the relationship between altered ion channel function, cell biology, and clinical disease been defined. Hyperinsulinism in infancy (HI) is a rare, potentially lethal condition of the newborn and early childhood. The causes of HI are varied and numerous, but in almost all cases they share a common target protein, the ATP-sensitive K+channel. From gene defects in ion channel subunits to defects in β-cell metabolism and anaplerosis, this review describes the relationship between pathogenesis and clinical medicine. Until recently, HI was generally considered an orphan disease, but as parallel defects in ion channels, enzymes, and metabolic pathways also give rise to diabetes and impaired insulin release, the HI paradigm has wider implications for more common disorders of the endocrine pancreas and the molecular physiology of ion transport.

Intracellular Delivery of Bioactive Peptides to RBL-2H3 Cells Induces β-Hexosaminidase Secretion and Phospholipase D Activation

This investigation compared the secretory efficacies of a series of peptides delivered to the cytoplasm of RBL-2H3 mast cells. Mimetic peptides, designed to target intracellular proteins that regulate cell signalling and membrane fusion, were synthesised as transportan 10 (TP10) chimeras for efficient plasma membrane translocation. Exocytosis of beta-hexosaminidase, a secretory lysosomal marker, indicated that peptides presenting sequences derived from protein kinase C (PKC; C1 H-CRRLSVEIWDWDL-NH(2)) and the CB(1) cannabinoid receptor (C3 H-RSKDLRHAFRSMFPSCE-NH(2)) induced beta-hexosaminidase secretion. Other peptide cargoes, including a Rab3A-derived sequence and a homologue of C3, were inactive in similar assays. Translocated C1 also activated phospholipase D (PLD), an enzyme intimately involved in the regulated secretory response of RBL-2H3 cells, but C1-induced secretion was not dependent upon phosphatidate synthesis. Neither down-regulation of Ca(2+)-sensitive isoforms of PKC nor the application of a selective PKC inhibitor attenuated the secretory efficacy of C1. These observations indicate that the molecular target of C1 is a protein involved in the regulated secretory pathway that is upstream of PLD but is not a PKC isoform. This study also confirmed that TP10 is a relatively inert cell-penetrating vector and is, therefore, widely suitable for studies in cells that are sensitive to peptidyl secretagogues.

A link between Cdc42 and syntaxin is involved in mastoparan-stimulated insulin release

Mastoparan, a hormone receptor-mimetic peptide isolated from wasp venom, stimulates insulin release from pancreatic beta-cells in a Ca(2+)-independent but GTP-dependent manner. In this report, the role of the Rho family GTP-binding protein Cdc42, in the mastoparan stimulus-secretion pathway, was examined. Overexpression of wild-type Cdc42 in beta HC-9 cells, an insulin-secreting mouse-derived cell line, resulted in a 2-fold increase in mastoparan-stimulated insulin release over vector-transfected beta HC-9 cells. This effect was not seen with secretagogues such as glucose that stimulate secretion via Ca(2+)-dependent pathways. GDP/GTP exchange assay data and studies with pertussis (PTX) toxin suggest that mastoparan may work directly to activate Cdc42 and not via PTX-sensitive heterotrimeric GTP-binding proteins. Using bacterial glutathione S-transferase-Cdc42 fusion proteins and co-immunoprecipitation and transient transfection studies, Cdc42 was shown to be an upstream regulator of the exocytotic protein, syntaxin. These results suggest that the GTP-dependent signal underlying the mastoparan effect acts at a "distal site" in stimulus-secretion coupling on one of the SNARE proteins essential for exocytosis. In vitro binding assays, using purified Cdc42 and syntaxin proteins, show that Cdc42 mediates the GTP signal through an indirect association with syntaxin. The H3 domain at the C-terminus of syntaxin, which participates in the formation of the ternary SNARE complex with the core proteins, SNAP-25 and synaptobrevin, is also required for the association with Cdc42. Thus, these studies indicate that Cdc42 could be a putative GTP-binding protein thought to be involved in the mastoparan-stimulated GTP-dependent pathway of insulin release.

Ca2+-dependent dephosphorylation of kinesin heavy chain on beta-granules in pancreatic beta-cells. Implications for regulated beta-granule transport and insulin exocytosis

The specific biochemical steps required for glucose-regulated insulin exocytosis from beta-cells are not well defined. Elevation of glucose leads to increases in cytosolic [Ca2+]i and biphasic release of insulin from both a readily releasable and a storage pool of beta-granules. The effect of elevated [Ca2+]i on phosphorylation of isolated beta-granule membrane proteins was evaluated, and the phosphorylation of four proteins was found to be altered by [Ca2+]i. One (a 18/20-kDa doublet) was a Ca2+-dependent increase in phosphorylation, and, surprisingly, three others (138, 42, and 36 kDa) were Ca2+-dependent dephosphorylations. The 138-kDa beta-granule phosphoprotein was found to be kinesin heavy chain (KHC). At low levels of [Ca2+]i KHC was phosphorylated by casein kinase 2, but KHC was rapidly dephosphorylated by protein phosphatase 2B beta (PP2Bbeta) as [Ca2+]i increased. Inhibitors of PP2B specifically reduced the second, microtubule-dependent, phase of insulin secretion, suggesting that dephosphorylation of KHC was required for transport of beta-granules from the storage pool to replenish the readily releasable pool of beta-granules. This is distinct from synaptic vesicle exocytosis, because neurotransmitter release from synaptosomes did not require a Ca2+-dependent KHC dephosphorylation. These results suggest a novel mechanism for regulating KHC function and beta-granule transport in beta-cells that is mediated by casein kinase 2 and PP2B. They also implicate a novel regulatory role for PP2B/calcineurin in the control of insulin secretion downstream of a rise in [Ca2+]i.

Imaging secretory vesicles by fluorescent protein insertion in propeptide rather than mature secreted peptide

We combined confocal and live-cell imaging with a novel molecular strategy aimed at revealing mechanisms underlying glucose-regulated insulin vesicle secretion. The 'Ins-C-GFP' reporter monitors secretory peptide targeting, trafficking, and exocytosis without directly tagging the mature secreted peptide. We trapped a green fluorescent protein (GFP) reporter in equimolar quantity within the secretory vesicle by fusing it within the C peptide of proinsulin which only after nascent vesicle sealing and acidification is cleaved from the mature secreted A and B chains of insulin. Ins-C-GFP expression in mouse islets without fail exhibited punctate distribution of green fluorescence by confocal microscopy. Ins-C-GFP colocalized GFP with insulin at vesicle dense cores by immuno-electron microscopy. Glucose stimulation decreased vesicle fluorescence coordinately with enhanced secretion from islets of C-GFP detected by anti-GFP Western blots, and of insulin detected by anti-insulin radioimmunoassay. An insulin secretagogue with a red fluorescent label, glibenclamide BODIPY TR, was applied to islets expressing Ins-C-GFP. The stimulus response was imaged as a rise in red secretagogue leading to marked loss in green granules. Since neuropeptides as well as peptide hormones are processed from propeptides after sealing of secretory granules, vesicle trapping likely is widely applicable for studies on targeting, trafficking, and regulated release of secretory peptides.

Triggering and augmentation mechanisms, granule pools, and biphasic insulin secretion

The insulin secretory response by pancreatic beta-cells to an acute "square wave" stimulation by glucose is characterized by a first phase that occurs promptly after exposure to glucose, followed by a decrease to a nadir, and a prolonged second phase. The first phase of release is due to the ATP-sensitive K(+) (K(ATP)) channel-dependent (triggering) pathway that increases [Ca(2+)](i) and has been thought to discharge the granules from a "readily releasable pool." It follows that the second phase entails the preparation of granules for release, perhaps including translocation and priming for fusion competency before exocytosis. The pathways responsible for the second phase include the K(ATP) channel-dependent pathway because of the need for elevated [Ca(2+)](i) and additional signals from K(ATP) channel-independent pathways. The mechanisms underlying these additional signals are unknown. Current hypotheses include increased cytosolic long-chain acyl-CoA, the pyruvate-malate shuttle, glutamate export from mitochondria, and an increased ATP/ADP ratio. In mouse islets, the beta-cell contains some 13,000 granules, of which approximately 100 are in a "readily releasable" pool. Rates of granule release are slow, e.g., one every 3 s, even at the peak of the first phase of glucose-stimulated release. As both phases of glucose-stimulated insulin secretion can be enhanced by agents such as glucagon-like peptide 1, which increases cyclic AMP levels and protein kinase A activity, or acetylcholine, which increases diacylglycerol levels and protein kinase C activity, a single "readily releasable pool" hypothesis is an inadequate explanation for insulin secretion. Multiple pools available for rapid release or rapid conversion of granules to a rapidly releasable state are required.

A low-affinity Ca2+-dependent association of calmodulin with the Rab3A effector domain inversely correlates with insulin exocytosis

The stimulus-response coupling pathway for glucose-regulated insulin secretion has implicated a rise in cytosolic [Ca2+]i as a key factor to induce insulin exocytosis. However, it is unclear how elevated [Ca2+]i communicates with the pancreatic beta-cell's exocytotic apparatus. As Rab3A is a model protein involved in regulated exocytosis, we have focused on its role in regulating insulin exocytosis. By using a photoactivatable cross-linking synthetic peptide that mimics the effector domain of Rab3A and microsequence analysis, we found calmodulin to be a major Rab3A target effector protein in pancreatic beta-cells. Coimmunoprecipitation analysis from pancreatic islets confirmed a Rab3A-calmodulin interaction in vivo, and that it inversely correlated with insulin exocytosis. Calmodulin affected neither GTPase nor guanine nucleotide exchange activity of Rab3A. The calmodulin-Rab3A interaction was pH- and Ca2+-dependent, and it was preferential for GTP-bound Rab3A. However, Rab3A affinity for calmodulin was relatively low (Kd = 18-22 micromol/l at 10(-5) mol/l [Ca2+]) and competed by other calmodulin-binding proteins that had higher affinity (e.g., Ca2+/calmodulin-dependent protein kinase-2 [CaMK-2] [Kd = 300-400 nmol/l at 10(-5) mol/l [Ca2+]]). Moreover, the Ca2+ dependence of the calmodulin-Rab3A interaction (K0.5 = 15-18 micromol/l [Ca2+], maximal at 100 micromol/l [Ca2+]) was significantly lower compared with that of the calmodulin-CaMK-2 association (K0.5 = 40 micromol/l [Ca2+], maximal at 1 mmol/l [Ca2+]). The data suggested that a transient Rab3A-calmodulin interaction might represent a means of directing calmodulin to the cytoplasmic face of a beta-granule, where it can be subsequently transferred for activation of other beta-granule-associated calmodulin-binding proteins as local [Ca2+]i rises to promote insulin exocytosis.

Metabolic control of beta-cell function

Glucose-induced insulin secretion is pulsatile. Glucose metabolism generates oscillations in the ATP/ADP ratio which lead to opening and closing of ATP-sensitive K(+)-channels producing subsequent oscillations in membrane potential, cytoplasmic calcium and insulin release. Metabolic signals derived from glucose can also stimulate insulin release independent of their effects on ATP-sensitive K(+)-channels. The ATP/ADP ratio may mediate both ATP-sensitive K(+)-channel-dependent and -independent pathways of secretion. Glucose metabolism also results in an increase in long-chain acyl-CoA, which is proposed to act as an effector molecule in the beta -cell. Long-chain acyl-CoA has a variety of effects in the beta -cell that may effect insulin secretion including opening ATP-sensitive K(+)-channels, activating endoplasmic reticulum Ca(2+)-ATPases and stimulating classical protein kinase C activity. In addition to stimulating insulin release, nutrients also effect gene expression, protein synthesis and beta -cell proliferation. Gene expression is effected by nutrient induction of a variety of immediate early response genes. Glucose stimulates proinsulin biosynthesis both at the translational and transcriptional level. beta -cell proliferation, as a result of insulin-like growth factor and growth hormone mitogenic pathways, is also glucose dependent. Thus, many beta -cell functions in addition to secretion are controlled by nutrient metabolism.

Beta-granule transport and exocytosis

Regulated beta -granule exocytosis is critical for the ability of the beta -cell to finely control body glucose homeostasis. This is now understood to be a multistage process whereby beta -granules are transported from biosynthetic/storage sites in the cell cytoplasm and targeted to specific regions of the plasma membrane. Exocytosis is achieved when these granules are triggered to fuse with the membrane by an elevated cytosolic Ca(2+). Dramatic advances have been made recently in our understanding of the protein-protein interactions and regulatory signals that govern intracellular transport and fusion. Although best understood for exocytosis from neurons and neuroendocrine cells, similar processes are thought to be conserved in the beta -cell.

The role of long-chain fatty acyl-CoA esters in beta-cell signal transduction

Glucose-induced insulin secretion is associated with inhibition of free fatty acid (FFA) oxidation, increased esterification and complex lipid formation by pancreatic beta-cells. Abundant evidence favors a role for cytosolic long-chain acyl-CoA (LC-CoA), including the rapid rise in malonyl CoA, the inhibitory effect of hydroxycitrate or acetyl CoA carboxylase knockout, both of which prevent malonyl CoA formation, and the stimulatory effect of exogenous FFA. On the other hand, some evidence opposes the concept, including the fall in total LC-CoA levels in response to glucose, the stimulatory effect of LC-CoA on K(ATP) channels and the lack of inhibition of glucose-stimulated secretion either by overexpression of malonyl CoA decarboxylase, which markedly lowers malonyl CoA levels, or by triacsin C, which blocks FFA conversion to LC-CoA. Alternative explanations for these data are presented. A revised model of nutrient-stimulated secretion involving two arms of signal transduction that occur simultaneously is proposed. One arm depends on modulation of the K(ATP) channel evoked by changes in the ATP/ADP ratio. The other arm depends upon anaplerotic input into the tricarboxylic acid cycle, generation of excess citrate, and increases in cytosolic malonyl-CoA. Input from this arm is increased LC-CoA. Signaling through both arms would be required for normal secretion. LC-CoA esters and products formed from them are potent regulators of enzymes and channels. It is hypothesized that their elevations directly modulate the activity of enzymes, genes and various beta-cell functions or modify the acylation state of key proteins involved in regulation of ion channels and exocytosis.

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

Insulin-secreting cells express four GTPases of the Rab3 family. After separation of extracts of INS-1 cells on a sucrose density gradient, the bulk of the A, B, and C isoforms was recovered in the fractions enriched in insulin-containing secretory granules. Rab3D was also mainly associated with secretory granules, but a fraction of this isoform was localized on lighter organelles. Analyses by confocal microscopy of immunostained HIT-T15 cells transfected with epitope-tagged constructs confirmed the distribution of the Rab3 isoforms. Transfection of HIT-T15 cells with GTPase-deficient mutants of the Rab3 isoforms decreased nutrient-induced insulin release to different degrees (D>B>A>>C), while overexpression of Rab3 wild types had minor or no effects. Expression of the same Rab3 mutants in PC12 cells provoked an inhibition of K+-stimulated secretion of dense core vesicles, indicating that, in beta-cells and neuroendocrine cells, the four Rab3 isoforms play a similar role in exocytosis. A Rab3A/C chimera in which the carboxyterminal domain of A was replaced with the corresponding region of C inhibited insulin secretion as Rab3A. In contrast, a Rab3C/A chimera containing the amino-terminal domain of C was less potent and reduced exocytosis as Rab3C. This suggests that the degree of inhibition obtained after transfection of the Rab3 isoforms is determined by differences in the variable amino-terminal region.

Molecular mechanisms and regulation of insulin exocytosis as a paradigm of endocrine secretion

Secretion of the peptide hormone insulin from pancreatic beta cells constitutes an important step in the regulation of body homeostasis. Insulin is stored in large dense core vesicles and released by exocytosis, a multistage process involving transport of vesicles to the plasma membrane, their docking, priming and finally their fusion with the plasma membrane. Some of the protein components necessary for this process have been identified in beta cells. The export of potent and potentially harmful substances has to be tightly controlled. The secretory response in pancreatic beta cells requires the concerted action of nutrients together with enteric hormones and neurotransmitters acting on G-protein coupled receptors. It is well established that glucose and other metabolizable nutrients depolarize the beta-cell membrane and the ensuing Ca2+ influx through voltage-dependent channels constitutes a main stimulus for insulin exocytosis. Theoretical considerations and recent observations suggest in addition an organizing role for the Ca2+ channel similar to neurotransmission. A second regulatory control on exocytosis is exerted by monomeric and heterotrimeric G-proteins. The monomeric GTPase Rab3A controls insulin secretion through cycling between a guanosine triphosphate liganded vesicle-bound form and a guanosine diphosphate liganded, cytosolic form. The effect of neurohormones is transduced by the heterotrimeric GTPases. Whereas pertussis-toxin sensitive alpha-subunits exert direct inhibition at the level of exocytosis, the Gbeta gamma-subunits are required for stimulation. It is possible that these GTPases exert immediate regulation, while protein kinases and phosphatases may modulate long-term adaptation at the exocytotic machinery itself. The molecular nature of their activators and effectors still await identification. Insights into the progression of the exocytotic vesicle from docking to fusion and how these processes are precisely regulated by proteins and second messengers may provide the basis for new therapeutic principles.

rab3 mediates cortical granule exocytosis in the sea urchin egg

Egg activation at fertilization in the sea urchin results in the exocytosis of approximately 15,000 cortical granules that are docked at the plasma membrane. Previously, we reported that several integral membrane proteins modeled in the SNARE hypothesis, synaptotagmin, VAMP, and syntaxin, in addition to a small GTPase of the ras superfamily, rab3, were present on cortical granules (Conner, S., Leaf, D., and Wessel, G., Mol. Reprod. Dev. 48, 1-13, 1997). Here we report that rab3 is associated with cortical granules throughout oogenesis, during cortical granule translocation, and while docked at the egg plasma membrane. Following cortical granule exocytosis, however, rab3 reassociates with a different population of vesicles, at least some of which are of endocytic origin. Because of its selective association with cortical granules in eggs and oocytes, we hypothesize that rab3 functions in cortical granule exocytosis. To test this hypothesis, we used a strategy of interfering with rab3 function by peptide competition with its effector domain, a conserved region within specific rab types. We first identified the effector domain sequence in Lytechinus variegatus eggs and find the sequence 94% identical to the effector domain of rab3 in Stronglocentrotus purpuratus. Then, with synthetic peptides to different regions of the rab3 protein, we find that cortical granule exocytosis is inhibited in eggs injected with effector domain peptides, but not with peptides from the hypervariable region or with a scrambled effector peptide. Additionally, effector-peptide-injected eggs injected with IP3 are blocked in their ability to exocytose cortical granules, suggesting that the inhibition is directly on the membrane fusion event and not the result of interference with the signal transduction mechanism leading to calcium release. We interpret these results to mean that rab3 functions in the regulation of cortical granule exocytosis following vesicle docking.

Exocytotic stimulation promotes association of the ADP-ribosylation factor with PC12 cell membranes

ADP-ribosylation factors (ARFs) are a family of small molecular, monomeric GTP-binding (G) proteins, initially identified by their ability to enhance cholera toxin (CTX) ADP-ribosyltransferase activity. ARFs have been implicated in protein transport and vesicle and endosome fusion. Although several reports show that synthetic peptides of the N-terminus of ARF inhibited Ca(2+)-dependent exocytosis in permeabilized adrenal chromaffin cells, the role of ARFs in exocytosis has not been established. In this study, we investigated the translocation of ARFs to the membrane fraction from the cytosol fraction in PC12 cells after exocytotic stimulation by measuring the immunoreactivity of ARFs (with anti-ARF anti-serum and with anti-ARF3 antibodies) and enzymatic ARF activity, which enhances the CTX effect. Both the immunoreactivity and the enzymatic activity of ARF in the membrane fraction increased about twofold, significantly, after exocytotic stimulation with ATP and KCl. The translocation of ARF and noradrenaline release was observed in the presence of extracellular CaCl2, but not in the absence of CaCl2. The ARF translocated to the membrane fraction after stimulation in intact cells seemed to be an inactive, perhaps is the GDP form, because ARF did not activate CTX in the absence of guanosine 5'-O-(thiotriphosphate) (GTP gamma S). As previously reported, ARF in the active, GTP gamma S-bound state bound to the membrane fractions. Thus ARF may have been active during translocation and inactivated later. The immunoreactivity of Gs alpha, one of the trimeric G proteins, was not changed before or after stimulation. These findings suggest that ARFs translocate to membranes from the cytosolic fraction after exocytotic stimulation in PC12 cells, and raise the possibility that ARFs regulate exocytosis.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

Differential expression pattern of Rab-GDI isoforms during the parotid gland secretion cycle

Rab GDP dissociation inhibitor (GDI) plays an important role in regulating the GDP/GTP cycle of small GTP binding proteins of the Rab family. It also regulates their association to membranes. The small family of Rab-GDI consists of several closely related isoforms, the functional differences between which are still unknown. Here we show that multiple GDI isoforms are expressed in rat parotid gland and that the individual GDI isoforms have a characteristic expression both at the RNA and at the protein level, during the parotid secretory cycle. GDIalpha, the major isoform in brain, is expressed throughout the secretory process and is equally distributed between cytoplasmic and membranous fractions. In contrast, an isoform related to, but different from GDIbeta is found predominantly in the cytoplasmic fraction and its expression is detected only after beta-adrenergic stimulation of the gland, at the end of the secretion phase, when exocytosis is already completed. The induction of such a GDI isoform at the beginning of the recovery stage correlates with the expression pattern of Rab1 and Rab5, but not Rab2 and Rab4. Our results suggest different functional roles for multiple GDI isoforms along the secretion and recovery phases in rat parotid gland.

aex-3 encodes a novel regulator of presynaptic activity in C. elegans

C. elegans aex-3 mutations cause pleiotropic behavioral defects that are suggestive of reduced synaptic transmission. aex-3 mutations also show strong genetic interactions with mutations in unc-31 and unc-64, two other genes implicated in synaptic transmission. Physiological and pharmacological studies indicate that aex-3 defects are presynaptic. In aex-3 mutants, the synaptic vesicle-associated RAB-3 protein aberrantly accumulates in neuronal cell bodies and is reduced in synapse-rich axons. This localization defect is specific to RAB-3, since other synaptic proteins are localized normally in aex-3 mutants. aex-3 encodes a 1409 amino acid protein with strong homology to DENN, a human protein of unknown function. In C. elegans, aex-3 is expressed in all or nearly all neurons. These results suggest that AEX-3 is a novel regulator of presynaptic activity that interacts with RAB-3 to regulate synaptic vesicle release.

Mechanisms of inhibition of insulin release

Several agonists including norepinephrine, somatostatin, galanin, and prostaglandins inhibit insulin release. The inhibition is sensitive to pertussis toxin, indicating the involvement of heterotrimeric Gi and/or Go proteins. Receptors for the different agonists have different selectivity for these G proteins. After G protein activation, the alpha- and beta gamma-subunits dissociate and interact with multiple targets to inhibit release. These include 1) the ATP-sensitive K+ channel and perhaps other K+ channels, 2) L-type voltage-dependent Ca2+ channels, 3) adenylyl cyclase, and 4) a "distal" site late in stimulus-secretion coupling. The latter effect, which may be exerted close to the final stage of exocytosis, is the most powerful of the individual inhibitory mechanisms. G protein action on the target molecules is determined by the individual G proteins activated and their specificity for the targets. The L-type Ca2+ channel is inhibited by G(o)-1. Adenylyl cyclase is inhibited by Gi-2 and Gi-3. The distal inhibition can be exerted by Gi-1, Gi-2, Gi-3, and G(o)-2. Thus there is both selectivity and promiscuity in G protein action in the beta-cell. These characteristics allow an inhibitory ligand to be effective at multiple targets and to act differentially from other inhibitory ligands.

Glucose-induced tyrosine phosphorylation of p125 in beta cells and pancreatic islets. A novel proximal signal in insulin secretion

In this study, we demonstrate that stimulation of beta cells with carbachol and glucose causes increased tyrosine phosphorylation of a 125-kDa protein concurrently with increased insulin secretion. The effect was observed in two different insulin-secreting cell lines and in rat pancreatic islets. Tyrosine phosphorylation was largely calcium independent and occurred within 2 min after stimulation of beta cells with glucose and the muscarinic agonist carbachol. In islets, the effect of glucose was greatly diminished by the addition of mannoheptulose, a seven-carbon sugar that inhibits glucokinase, suggesting that glucose metabolism is required for tyrosine phosphorylation of the protein to occur. Neither insulin nor insulin-like growth factor I significantly increased tyrosine phosphorylation of the 125-kDa protein, suggesting that it was not an autocrine effect. Depolarization of beta cells with glyburide or 50 m potassium dramatically increased insulin secretion but had no significant effect on tyrosine phosphorylation. Addition of phorbol ester caused a less than 2-fold increase in tyrosine phosphorylation, whereas the calcium ionophore A23187 had no effect. Among the various fuel secretagogues tested, only -glucose stimulated tyrosine phosphorylation, both alone and in combination with carbachol. Finally, the tyrosine kinase inhibitor AG879 inhibited both tyrosine phosphorylation and insulin secretion in a dose-dependent manner. Taken together, these data demonstrate the presence of a novel signaling pathway in glucose-induced insulin secretion: tyrosine phosphorylation of beta cell p125, which is a proximal step in insulin secretion. Our current working hypothesis is that glucose stimulation of beta cell p125 tyrosine phosphorylation is an essential step for insulin secretion.

Expression, localization and functional role of small GTPases of the Rab3 family in insulin-secreting cells

We examined the presence of small molecular mass GTP-binding proteins of the Rab3 family in different insulin-secreting cells. Rab3B and Rab3C were identified by western blotting in rat and in human pancreatic islets, in two rat insulin-secreting cell lines, RINm5F and INS-1, as well as in the hamster cell line HIT-T15. In contrast, Rab3A was detected in rat pancreatic islets as well as in the two insulin-secreting rat cell lines but not in human pancreatic islets and was only barely discernible in HIT-T15 cells. These findings were confirmed by two-dimensional gel electrophoresis followed by GTP-overlay of homogenates of pancreatic islets and of the purified protein. Northern blotting analysis revealed that Rab3D is expressed in the same insulin-secreting cells as Rab3A. Separation of the cells of the rat islets by fluorescence-activated cell sorting demonstrated that Rab3A was exclusively expressed in beta-cells. Rab3A was found to be associated with insulin-containing secretory granules both by immunofluorescence, immunoelectron microscopy and after sucrose density gradient. Overexpression in HIT-T15 cells of a Rab3A mutant deficient in GTP hydrolysis inhibited insulin secretion stimulated by a mixture of nutrients and bombesin. Insulin release triggered by these secretagogues was also slightly decreased by the overexpression of wild-type Rab3A but not by the overexpression of wild-type Rab5A and of a Rab5A mutant deficient in GTP hydrolysis. Finally, we studied the expression in insulin-secreting cells of rabphilin-3A, a putative effector protein that associates with the GTP-bound form of Rab3A. This Rab3A effector was not detectable in any of the cells investigated in the present study. Taken together these results indicate an involvement of Rab3A in the control of insulin release in rat and hamster. In human beta-cells, a different Rab3 isoform but with homologous function may replace Rab3A.

rab 3-peptide stimulates exocytosis of the ram sperm acrosome via interaction with cyclic AMP and phospholipase A2 metabolites

Acrosomal exocytosis triggered with A23187/Ca2+ was enhanced by rab3AL, a synthetic peptide corresponding to the effector domain of the small GTP-binding protein rab3. Exocytosis was further enhanced when spermatozoa were also exposed to dibutyryl-cAMP, but was prevented when H-89, a protein kinase A (PKA) inhibitor, was included. The action of rab3AL was not on, or upstream of, phospholipase A2 (PLA2). Inhibition of exocytosis by the PLA2 inhibitor aristolochic acid was overcome by rab3AL when it was included together with lysophosphatidylcholine; this effect was prevented by H-89. These results suggest a functional coupling between rab3 protein, metabolites generated by PLA2, and cAMP-activated PKA, in the final steps leading to membrane fusion during acrosomal exocytosis.

Insulin-dependent translocation of the small GTP-binding protein rab3C in cardiac muscle: studies on insulin-resistant Zucker rats

The failure of insulin-regulated recruitment of the GLUT4 glucose transporter in cardiac muscle of obese Zucker rats is associated with alterations of the subcellular distribution of the small-molecular-mass GTP-binding protein rab4A. Here, we show by subcellular fractionation and Western blotting a translocation of the small-molecular-mass GTP-binding protein rab3C from microsomal membranes to plasma membranes in lean control rats following in vivo insulin stimulation. However, in cardiac muscle of obese animals no significant effect of the hormone on the subcellular distribution of rab3C was observed. In GLUT4-enriched membrane vesicles, obtained from cardiac microsomes of the obese group as well as of lean controls, rab3C was not detectable. It is suggested that the altered behaviour of rab3C may contribute to an impaired trafficking of GLUT4 in the insulin-resistant state.

Prenylcysteine analogs mimicking the C-terminus of GTP-binding proteins stimulate exocytosis from permeabilized HIT-T15 cells: comparison with the effect of Rab3AL peptide

Most guanine nucleotide binding proteins (G-proteins) possess an S-prenylated C-terminal cysteine whose carboxyl group can be reversibly methylated. The prenylcysteine analog N-acetyl-S-geranylgeranyl-cysteine (AGGC) (50 microM), a competitive inhibitor of prenylcysteine methyl transferases, introduced into streptolysin-O permeabilized HIT-T15 cells doubled the rate of basal (0.1 microM Ca2+) and of stimulated (10 microM Ca2+ or 100 microM GTP gamma S) insulin secretion in a reversible and ATP-dependent manner. N-acetyl-S-farnesylcysteine (AFC) was less potent while N-acetyl-S-geranyl-cysteine was inactive. Prenylcysteine action on exocytosis did not involve inhibition of G-protein methylation, since (1) the methyl ester derivative of AFC, an inefficient inhibitor of methyltransferases in HIT-T15 cell fractions, was as potent as AGGC in stimulating exocytosis; (2) S-adenosyl-homocysteine, a general inhibitor of methylation reactions, did not alter basal or GTP gamma S-triggered secretion while inhibiting Ca(2+)-induced insulin release. The binding of G-proteins to Rab/GDP-dissociation inhibitor, Rab3A/GTPase activating protein or rabphilin-3A was not affected by the prenylcysteine analogs. AGGC or AFC had the same effect on insulin release as a synthetic peptide mimicking the amino acid residues 52-67 of the G-protein Rab3A (Rab3AL). Moreover, the action on secretion of the combination of Rab3AL and prenylcysteines was not additive. We propose that the prenylcysteines and the Rab3AL peptide influence exocytosis by affecting the association of Rab3A with different proteins of the exocytotic machinery of insulin-secreting cells.

The heterotrimeric G-protein Gi is localized to the insulin secretory granules of beta-cells and is involved in insulin exocytosis

Mastoparan, a tetradecapeptide found in wasp venom that stimulates G-proteins, increases insulin secretion from beta-cells. In this study, we have examined the role of heterotrimeric G-proteins in mastoparan-induced insulin secretion from the insulin-secreting beta-cell line beta-TC3. Mastoparan stimulated insulin secretion in a dose-dependent manner from digitonin-permeabilized beta-TC3 cells. Active mastoparan analogues mastoparan 7, mastoparan 8, and mastoparan X also stimulated secretion. Mastoparan 17, an inactive analogue of mastoparan, did not increase insulin secretion from permeabilized beta-TC3 cells. Mastoparan-induced insulin secretion from permeabilized beta-TC3 cells was inhibited by pretreatment of the cells with pertussis toxin, suggesting that mastoparan-induced insulin secretion is mediated through a pertussis toxin-sensitive G-protein present distally in exocytosis. Enriched insulin secretory granules (ISG) were prepared by sucrose/nycodenz ultracentrifugation. Western immunoblotting performed on beta-TC3 homogenate and ISG demonstrated that G alpha i was dramatically enriched in ISG. Levels of G alpha o and G alpha q were comparable in homogenate and ISG. Mastoparan stimulated ISG GTPase activity in a pertussis toxin-sensitive manner. Mastoparan 7 and mastoparan 8 also stimulated GTPase activity in the ISG, while the inactive analogue mastoparan 17 had no effect. Selective localization of G alpha i to ISG was confirmed with electron microscopic immunocytochemistry in beta-TC3 cells and beta-cells from rat pancreas. In contrast to G alpha o and G alpha q, G alpha was clearly localized to the ISG. Together, these data suggest that mastoparan may act through the heterotrimeric G-protein G alpha i located in the ISG of beta-cells to stimulate insulin secretion.