A redistribution of actin and myosin IIA accompanies Ca(2+)-dependent insulin secretion

Novel roles of mTORC2 in regulation of insulin secretion by actin filament remodeling

Mammalian target of rapamycin (mTOR) kinase is an essential hub where nutrients and growth factors converge to control cellular metabolism. mTOR interacts with different accessory proteins to form complexes 1 and 2 (mTORC), and each complex has different intracellular targets. Although mTORC1's role in β-cells has been extensively studied, less is known about mTORC2's function in β-cells. Here, we show that mice with constitutive and inducible β-cell-specific deletion of RICTOR (βRicKO and iβRicKO mice, respectively) are glucose intolerant due to impaired insulin secretion when glucose is injected intraperitoneally. Decreased insulin secretion in βRicKO islets was caused by abnormal actin polymerization. Interestingly, when glucose was administered orally, no difference in glucose homeostasis and insulin secretion were observed, suggesting that incretins are counteracting the mTORC2 deficiency. Mechanistically, glucagon-like peptide-1 (GLP-1), but not gastric inhibitory polypeptide (GIP), rescued insulin secretion in vivo and in vitro by improving actin polymerization in βRicKO islets. In conclusion, mTORC2 regulates glucose-stimulated insulin secretion by promoting actin filament remodeling.NEW & NOTEWORTHY The current studies uncover a novel mechanism linking mTORC2 signaling to glucose-stimulated insulin secretion by modulation of the actin filaments. This work also underscores the important role of GLP-1 in rescuing defects in insulin secretion by modulating actin polymerization and suggests that this effect is independent of mTORC2 signaling.

Organization and dynamics of the cortical complexes controlling insulin secretion in β-cells

Insulin secretion in pancreatic β-cells is regulated by cortical complexes that are enriched at the sites of adhesion to extracellular matrix facing the vasculature. Many components of these complexes, including bassoon, RIM, ELKS and liprins, are shared with neuronal synapses. Here, we show that insulin secretion sites also contain the non-neuronal proteins LL5β (also known as PHLDB2) and KANK1, which, in migrating cells, organize exocytotic machinery in the vicinity of integrin-based adhesions. Depletion of LL5β or focal adhesion disassembly triggered by myosin II inhibition perturbed the clustering of secretory complexes and attenuated the first wave of insulin release. Although previous analyses in vitro and in neurons have suggested that secretory machinery might assemble through liquid–liquid phase separation, analysis of endogenously labeled ELKS in pancreatic islets indicated that its dynamics is inconsistent with such a scenario. Instead, fluorescence recovery after photobleaching and single-molecule imaging showed that ELKS turnover is driven by binding and unbinding to low-mobility scaffolds. Both the scaffold movements and ELKS exchange were stimulated by glucose treatment. Our findings help to explain how integrin-based adhesions control spatial organization of glucose-stimulated insulin release.

Summary: Characterization of the composition of cortical complexes controlling insulin secretion, showing that their dynamics is inconsistent with assembly through liquid–liquid phase separation.

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.

The RhoGAP Stard13 controls insulin secretion through F-actin remodeling

Objective

Actin cytoskeleton remodeling is necessary for glucose-stimulated insulin secretion in pancreatic β-cells. A mechanistic understanding of actin dynamics in the islet is paramount to a better comprehension of β-cell dysfunction in diabetes. Here, we investigate the Rho GTPase regulator Stard13 and its role in F-actin cytoskeleton organization and islet function in adult mice.

Methods

We used Lifeact-EGFP transgenic animals to visualize actin cytoskeleton organization and dynamics in vivo in the mouse islets. Furthermore, we applied this model to study actin cytoskeleton and insulin secretion in mutant mice deleted for Stard13 selectively in pancreatic cells. We isolated transgenic islets for 3D-imaging and perifusion studies to measure insulin secretion dynamics. In parallel, we performed histological and morphometric analyses of the pancreas and used in vivo approaches to study glucose metabolism in the mouse.

Results

In this study, we provide the first genetic evidence that Stard13 regulates insulin secretion in response to glucose. Postnatally, Stard13 expression became restricted to the mouse pancreatic islets. We showed that Stard13 deletion results in a marked increase in actin polymerization in islet cells, which is accompanied by severe reduction of insulin secretion in perifusion experiments. Consistently, Stard13-deleted mice displayed impaired glucose tolerance and reduced glucose-stimulated insulin secretion.

Conclusions

Taken together, our results suggest a previously unappreciated role for the RhoGAP protein Stard13 in the interplay between actin cytoskeletal remodeling and insulin secretion.

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Lifeact-EGFP mice allow in vivo labeling of the actin cytoskeleton in islets.

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The RhoGAP Stard13 regulates actin cytoskeleton organization in mouse islets.

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Stard13 deficiency hampers glucose-induced insulin secretion by mouse β-cells.

Stigmasterol prevents glucolipotoxicity induced defects in glucose-stimulated insulin secretion

Type 2 diabetes results from defects in both insulin sensitivity and insulin secretion. Elevated cholesterol content within pancreatic β-cells has been shown to reduce β-cell function and increase β-cell apoptosis. Hyperglycemia and dyslipidemia contribute to glucolipotoxicity that leads to type 2 diabetes. Here we examined the capacity of glucolipotoxicity to induce free cholesterol accumulation in human pancreatic islets and the INS-1 insulinoma cell line. Glucolipotoxicity treatment increased free cholesterol in β-cells, which was accompanied by increased reactive oxygen species (ROS) production and decreased insulin secretion. Addition of AAPH, a free radical generator, was able to increase filipin staining indicating a link between ROS production and increased cholesterol in β-cells. We also showed the ability of stigmasterol, a common food-derived phytosterol with anti-atherosclerotic potential, to prevent the increase in both free cholesterol and ROS levels induced by glucolipotoxicity in INS-1 cells. Stigmasterol addition also inhibited early apoptosis, increased total insulin, promoted actin reorganization, and improved insulin secretion in cells exposed to glucolipotoxicity. Overall, these data indicate cholesterol accumulation as an underlying mechanism for glucolipotoxicity-induced defects in insulin secretion and stigmasterol treatment as a potential strategy to protect β-cell function during diabetes progression.

The skeleton in the closet: actin cytoskeletal remodeling in β-cell function

Over the last few decades, biomedical research has considered not only the function of single cells but also the importance of the physical environment within a whole tissue, including cell-cell and cell-extracellular matrix interactions. Cytoskeleton organization and focal adhesions are crucial sensors for cells that enable them to rapidly communicate with the physical extracellular environment in response to extracellular stimuli, ensuring proper function and adaptation. The involvement of the microtubular-microfilamentous cytoskeleton in secretion mechanisms was proposed almost 50 years ago, since when the evolution of ever more sensitive and sophisticated methods in microscopy and in cell and molecular biology have led us to become aware of the importance of cytoskeleton remodeling for cell shape regulation and its crucial link with signaling pathways leading to β-cell function. Emerging evidence suggests that dysfunction of cytoskeletal components or extracellular matrix modification influences a number of disorders through potential actin cytoskeleton disruption that could be involved in the initiation of multiple cellular functions. Perturbation of β-cell actin cytoskeleton remodeling could arise secondarily to islet inflammation and fibrosis, possibly accounting in part for impaired β-cell function in type 2 diabetes. This review focuses on the role of actin remodeling in insulin secretion mechanisms and its close relationship with focal adhesions and myosin II.

Cytoskeletal dependence of insulin granule movement dynamics in INS-1 beta-cells in response to glucose

For pancreatic β-cells to secrete insulin in response to elevated blood glucose, insulin granules retained within the subplasmalemmal space must be transported to sites of secretion on the plasma membrane. Using a combination of super-resolution STORM imaging and live cell TIRF microscopy we investigate how the organization and dynamics of the actin and microtubule cytoskeletons in INS-1 β-cells contribute to this process. GFP-labeled insulin granules display 3 different modes of motion (stationary, diffusive-like, and directed). Diffusive-like motion dominates in basal, low glucose conditions. Upon glucose stimulation no gross rearrangement of the actin cytoskeleton is observed but there are increases in the 1) rate of microtubule polymerization; 2) rate of diffusive-like motion; and 3) proportion of granules undergoing microtubule-based directed motion. By pharmacologically perturbing the actin and microtubule cytoskeletons, we determine that microtubule-dependent granule transport occurs within the subplasmalemmal space and that the actin cytoskeleton limits this transport in basal conditions, when insulin secretion needs to be inhibited.

Regulation of myosin IIA and filamentous actin during insulin-stimulated glucose uptake in 3T3-L1 adipocytes

Insulin stimulated glucose uptake requires the colocalization of myosin IIA (MyoIIA) and the insulin-responsive glucose transporter 4 (GLUT4) at the plasma membrane for proper GLUT4 fusion. MyoIIA facilitates filamentous actin (F-actin) reorganization in various cell types. In adipocytes F-actin reorganization is required for insulin-stimulated glucose uptake. What is not known is whether MyoIIA interacts with F-actin to regulate insulin-induced GLUT4 fusion at the plasma membrane. To elucidate the relationship between MyoIIA and F-actin, we examined the colocalization of MyoIIA and F-actin at the plasma membrane upon insulin stimulation as well as the regulation of this interaction. Our findings demonstrated that MyoIIA and F-actin colocalized at the site of GLUT4 fusion with the plasma membrane upon insulin stimulation. Furthermore, inhibition of MyoII with blebbistatin impaired F-actin localization at the plasma membrane. Next we examined the regulatory role of calcium in MyoIIA-F-actin colocalization. Reduced calcium or calmodulin levels decreased colocalization of MyoIIA and F-actin at the plasma membrane. While calcium alone can translocate MyoIIA it did not stimulate F-actin accumulation at the plasma membrane. Taken together, we established that while MyoIIA activity is required for F-actin localization at the plasma membrane, it alone is insufficient to localize F-actin to the plasma membrane.

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.

Non-muscle myosin IIA is involved in focal adhesion and actin remodelling controlling glucose-stimulated insulin secretion

AIMS/HYPOTHESIS

Actin and focal adhesion (FA) remodelling are essential for glucose-stimulated insulin secretion (GSIS). Non-muscle myosin II (NM II) isoforms have been implicated in such remodelling in other cell types, and myosin light chain kinase (MLCK) and Rho-associated coiled-coil-containing kinase (ROCK) are upstream regulators of NM II, which is known to be involved in GSIS. The aim of this work was to elucidate the implication and regulation of NM IIA and IIB in beta cell actin and FA remodelling, granule trafficking and GSIS.

METHODS

Inhibitors of MLCK, ROCK and NM II were used to study NM II activity, and knockdown of NM IIA and IIB to determine isoform specificity, using sorted primary rat beta cells. Insulin was measured by radioimmunoassay. Protein phosphorylation and subcellular distribution were determined by western blot and confocal immunofluorescence. Dynamic changes were monitored by live cell imaging and total internal reflection fluorescence microscopy using MIN6B1 cells.

RESULTS

NM II and MLCK inhibition decreased GSIS, associated with shortening of peripheral actin stress fibres, and reduced numbers of FAs and insulin granules in close proximity to the basal membrane. By contrast, ROCK inhibition increased GSIS and caused disassembly of glucose-induced central actin stress fibres, resulting in large FAs without any effect on FA number. Only glucose-induced NM IIA reorganisation was blunted by MLCK inhibition. NM IIA knockdown decreased GSIS, levels of FA proteins and glucose-induced extracellular signal-regulated kinase 1/2 phosphorylation.

CONCLUSIONS/INTERPRETATION

Our data indicate that MLCK-NM IIA may modulate translocation of secretory granules, resulting in enhanced insulin secretion through actin and FA remodelling, and regulation of FA protein levels.

New insights into the control of secretion

Vesicular secretion is a fundamental process in the body with vesicle fusion releasing vesicle contents to the outside. This process called exocytosis is usually thought of as leading to an all-or-none release of content; regulation of secretory output dependent on regulating the numbers of fused vesicles. However, it is well established that the fusion pore that forms when the vesicle membrane fuses with the cell membrane is dynamic. More recent evidence indicates the dynamic opening and closing, and the size of the fusion pore, are limiting factors to the release of vesicle content. What remains unclear is whether these fusion pore behaviors are under cellular control and therefore relevant to cell physiology.Accumulating evidence over the last two years points to myosin 2 as one regulator of fusion pore behavior. This is interesting since myosin 2 activity is in turn controlled by kinases and phosphatases, well known to be under cellular control. We conclude that fusion pore behavior is likely a genuine control point for vesicle content release. This leads to a model for secretion with secretory output controlled not only by the numbers of vesicles fused but also by the regulation of the behavior of individual vesicles.

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.

High glucose predisposes gene expression and ERK phosphorylation to apoptosis and impaired glucose-stimulated insulin secretion via the cytoskeleton

Chronic high glucose (HG) inflicts glucotoxicity on vulnerable cell types such as pancreatic β cells and contributes to insulin resistance and impaired insulin secretion in diabetic patients. To identify HG-induced cellular aberrations that are candidate mediators of glucotoxicity in pancreatic β cells, we analyzed gene expression in ERoSHK6, a mouse insulin-secreting cell line after chronic HG exposure (six-day exposure to 33.3 mM glucose). Chronic HG exposure which reduced glucose-stimulated insulin secretion (GSIS) increased transcript levels of 185 genes that clustered primarily in 5 processes namely cellular growth and proliferation; cell death; cellular assembly and organization; cell morphology; and cell-to-cell signaling and interaction. The former two were validated by increased apoptosis of ERoSHK6 cells after chronic HG exposure and reaffirmed the vulnerability of β cells to glucotoxicity. The three remaining processes were partially substantiated by changes in cellular morphology and structure, and instigated an investigation of the cytoskeleton and cell-cell adhesion. These studies revealed a depolymerized actin cytoskeleton that lacked actin stress fibers anchored at vinculin-containing focal adhesion sites as well as loss of E-cadherin-mediated cell-cell adherence after exposure to chronic HG, and were concomitant with constitutive ERK1/2 phosphorylation that was refractory to serum and glucose deprivation. Although inhibition of ERK phosphorylation by PD98059 promoted actin polymerization, it increased apoptosis and GSIS impairment. These findings suggest that ERK phosphorylation is a proximate regulator of cellular processes targeted by chronic HG-induced gene expression and that dynamic actin polymerization and depolymerization is important in β cell survival and function. Therefore, chronic HG alters gene expression and signal transduction to predispose the cytoskeleton towards apoptosis and GSIS impairment.

Nonmuscle myosin-2: mix and match

Members of the nonmuscle myosin-2 (NM-2) family of actin-based molecular motors catalyze the conversion of chemical energy into directed movement and force thereby acting as central regulatory components of the eukaryotic cytoskeleton. By cyclically interacting with adenosine triphosphate and F-actin, NM-2 isoforms promote cytoskeletal force generation in established cellular processes like cell migration, shape changes, adhesion dynamics, endo- and exo-cytosis, and cytokinesis. Novel functions of the NM-2 family members in autophagy and viral infection are emerging, making NM-2 isoforms regulators of nearly all cellular processes that require the spatiotemporal organization of cytoskeletal scaffolding. Here, we assess current views about the role of NM-2 isoforms in these activities including the tight regulation of NM-2 assembly and activation through phosphorylation and how NM-2-mediated changes in cytoskeletal dynamics and mechanics affect cell physiological functions in health and disease.

Involvement of actin filament in the generation of Ca2+ mobilizing messengers in glucose-induced Ca2+ signaling in pancreatic β-cells

Glucose is a metabolic regulator of insulin secretion from pancreatic β-cells, which is regulated by intracellular Ca(2+) signaling. We and others previously demonstrated that glucose activates CD38/ADP-ribosyl cyclase (ADPR-cyclase) to produce two Ca(2+) second messengers, cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). Although F-actin remodeling is known to be an important step in glucose stimulated insulin secretion, the role of actin cytoskeleton in regulating Ca(2+) signaling in pancreatic β-cells remain to be solved. Here, we show that actin filaments are involved in the activation of CD38/ADPR-cyclase in pancreatic β-cells. Glucose induces a sequential formation of cADPR and NAADP. Pretreatment with jasplakinolide, an actin polymerizing agent, or a myosin heavy chain IIA (MHCIIA) blocker, blebbistatin, inhibited glucose-induced CD38 internalization, an essential step for cADPR formation. Blocking actin disassembly with jasplakinolide also abrogates glucose-induced cADPR and NAADP formation and sustained Ca(2+) signals. These results indicate that actin filaments along with MHCIIA play an important role in CD38 internalization for the generation of Ca(2+) mobilizing messengers for glucose-induced Ca(2+) signaling in pancreatic β-cells.

Overexpression of myosin-IIB in the brain of a rat model of streptozotocin-induced diabetes

The Ca(2+)/calmodulin complex interacts with and regulates various enzymes and target proteins known as calmodulin-binding proteins (CaMBPs). This group of proteins includes molecular motors such as myosins. In this study, we show that non-muscle myosin-IIB is overexpressed in the brains of diabetic rats. We isolated CaMBPs from the brains of non-diabetic rats and rats with streptozotocin-induced diabetes and purified them by immobilized-calmodulin affinity chromatography. The proteins were eluted with EGTA and urea, separated by SDS-PAGE, digested and submitted to peptide mass fingerprinting analysis. Thirteen intense bands were found in both types of brains, two were found exclusively in non-diabetic brains and four were found exclusively in diabetic brains. A large fraction of the eluted proteins contained putative IQ motifs or calmodulin-binding sites. The results of the myosin-IIB affinity chromatography elution, western blot and RT-PCR analyses suggest that myosin-IIB protein and mRNA are expressed at high levels in diabetic brains. This is the first study that has demonstrated differential expression of CaMBPs in diabetic and non-diabetic brain tissue through a comparative proteomic analysis, and it opens up a new approach to studying the relationship between the expression of myosins in the brain, hyperglycemia and intracellular calcium regulation.

Dynamics of insulin secretion and the clinical implications for obesity and diabetes

Insulin secretion is a highly dynamic process regulated by various factors including nutrients, hormones, and neuronal inputs. The dynamics of insulin secretion can be studied at different levels: the single β cell, pancreatic islet, whole pancreas, and the intact organism. Studies have begun to analyze cellular and molecular mechanisms underlying dynamics of insulin secretion. This review focuses on our current understanding of the dynamics of insulin secretion in vitro and in vivo and discusses their clinical relevance.

N-cadherin is dispensable for pancreas development but required for beta-cell granule turnover

The cadherin family of cell adhesion molecules mediates adhesive interactions that are required for the formation and maintenance of tissues. Previously, we demonstrated that N-cadherin, which is required for numerous morphogenetic processes, is expressed in the pancreatic epithelium at E9.5, but later becomes restricted to endocrine aggregates in mice. To study the role of N-cadherin during pancreas formation and function we generated a tissue-specific knockout of N-cadherin in the early pancreatic epithelium by inter-crossing N-cadherin-floxed mice with Pdx1Cre mice. Analysis of pancreas-specific ablation of N-cadherin demonstrates that N-cadherin is dispensable for pancreatic development, but required for beta-cell granule turnover. The number of insulin secretory granules is significantly reduced in N-cadherin-deficient beta-cells, and as a consequence insulin secretion is decreased.

Metabolic amplifying pathway increases both phases of insulin secretion independently of β-cell actin microfilaments

Two pathways control glucose-induced insulin secretion (IS) by β-cells. The triggering pathway involves ATP-sensitive potassium (KATP) channel-dependent depolarization, Ca2+ influx, and a rise in the cytosolic Ca2+ concentration ([Ca2+]c), which triggers exocytosis of insulin granules. The metabolic amplifying pathway augments IS without further increasing [Ca2+]c. The underlying mechanisms are unknown. Here, we tested the hypothesis that amplification implicates actin microfilaments. Mouse islets were treated with latrunculin B and cytochalasin B to depolymerize actin or jasplakinolide to polymerize actin. They were then perifused to measure [Ca2+]c and IS. Metabolic amplification was studied during imposed steady elevation of [Ca2+]c by tolbutamide or KCl or by comparing the magnitude of [Ca2+]c and IS changes produced by glucose and tolbutamide. Both actin polymerization and depolymerization augmented IS triggered by all stimuli without increasing (sometimes decreasing) [Ca2+]c, which indicates a predominantly inhibitory function of microfilaments in exocytosis at a step distal to [Ca2+]c increase. When [Ca2+]c was elevated and controlled by KCl or tolbutamide, the amplifying action of glucose was facilitated by actin depolymerization and unaffected by polymerization. Both phases of IS were larger in response to high-glucose than to tolbutamide in low-glucose, although triggering [Ca2+]c was lower. This difference in IS, due to amplification, persisted when the IS rate was doubled by actin depolymerization or polymerization. In conclusion, metabolic amplification is rapid and influences the first as well as the second phase of IS. It is a late step of stimulus-secretion coupling, which does not require functional actin microfilaments and could correspond to acceleration of the priming process conferring release competence to insulin granules.

Nephrin is expressed on the surface of insulin vesicles and facilitates glucose-stimulated insulin release

OBJECTIVE

Nephrin, an immunoglobulin-like protein essential for the function of the glomerular podocyte and regulated in diabetic nephropathy, is also expressed in pancreatic beta-cells, where its function remains unknown. The aim of this study was to investigate whether diabetes modulates nephrin expression in human pancreatic islets and to explore the role of nephrin in beta-cell function.

RESEARCH DESIGN AND METHODS

Nephrin expression in human pancreas and in MIN6 insulinoma cells was studied by Western blot, PCR, confocal microscopy, subcellular fractionation, and immunogold labeling. Islets from diabetic (n = 5) and nondiabetic (n = 7) patients were compared. Stable transfection and siRNA knockdown in MIN-6 cells/human islets were used to study nephrin function in vitro and in vivo after transplantation in diabetic immunodeficient mice. Live imaging of green fluorescent protein (GFP)-nephrin-transfected cells was used to study nephrin endocytosis.

RESULTS

Nephrin was found at the plasma membrane and on insulin vesicles. Nephrin expression was decreased in islets from diabetic patients when compared with nondiabetic control subjects. Nephrin transfection in MIN-6 cells/pseudoislets resulted in higher glucose-stimulated insulin release in vitro and in vivo after transplantation into immunodeficient diabetic mice. Nephrin gene silencing abolished stimulated insulin release. Confocal imaging of GFP-nephrin-transfected cells revealed nephrin endocytosis upon glucose stimulation. Actin stabilization prevented nephrin trafficking as well as nephrin-positive effect on insulin release.

CONCLUSIONS

Our data suggest that nephrin is an active component of insulin vesicle machinery that may affect vesicle-actin interaction and mobilization to the plasma membrane. Development of drugs targeting nephrin may represent a novel approach to treat diabetes.

Secretory control: evidence for agonist regulation of post-fusion vesicle behaviour

1. Here, we review recent work on vesicular secretion, with a focus on the control of post-fusion events as a means of regulating secretory output. 2. In the classical model of secretion, each fused vesicle releases the entirety of its content in an all-or-none manner. In this way, the secretory output of a cell is controlled by regulating the numbers of fused vesicles. The realisation that post-fusion events can control secretory output leads to a distinct model of partial release of vesicle content. 3. Recent work shows that post-fusion events are under cellular control. Further, new data from our laboratory demonstrates agonist-dependent regulation of fusion pore behaviour. 4. We conclude that post-fusion events are not epiphenomena, but are likely an important mechanism of secretory control.

Cholesterol regulates glucose-stimulated insulin secretion through phosphatidylinositol 4,5-bisphosphate

Membrane cholesterol modulates the ability of glucose to stimulate insulin secretion from pancreatic beta-cells. The molecular mechanism by which this occurs is not understood. Here, we show that in cultured beta-cells, cholesterol acts through phosphatidylinositol 4,5-bisphosphate (PIP(2)) to regulate actin dynamics, plasma membrane potential, and glucose-stimulated insulin secretion. Cholesterol-overloaded beta-cells exhibited decreased PIP(2) hydrolysis, with diminished glucose-induced actin reorganization, membrane depolarization, and insulin secretion. The converse findings were observed in cholesterol-depleted cells. These results support a model in which cholesterol depletion is coupled through PIP(2) to enhance both plasma membrane Ca2+ influx from the extracellular space, as well as inositol 1,4,5-triphosphate-stimulated Ca2+ efflux from intracellular stores. The inability to increase cytosolic Ca2+ may be the main underlying factor to account for impaired glucose-stimulated insulin secretion in cholesterol-overloaded beta-cells.

Mechanisms of biphasic insulin-granule exocytosis - roles of the cytoskeleton, small GTPases and SNARE proteins

The release of insulin from pancreatic islets requires negative regulation to ensure low levels of insulin release under resting conditions, as well as positive regulation to facilitate robust responsiveness to conditions of elevated fuel or glucose. The first phase of release involves the plasma-membrane fusion of a small pool of granules, termed the readily releasable pool; these granules are already at the membrane under basal conditions, and discharge their cargo in response to nutrient and also non-nutrient secretagogues. By contrast, second-phase secretion is evoked exclusively by nutrients, and involves the mobilization of intracellular granules to t-SNARE sites at the plasma membrane to enable the distal docking and fusion steps of insulin exocytosis. Nearly 40 years ago, the actin cytoskeleton was first recognized as a key mediator of biphasic insulin release, and was originally presumed to act as a barrier to block granule docking at the cell periphery. More recently, however, the discovery of cycling GTPases that are involved in F-actin reorganization in the islet beta-cell, combined with the availability of reagents that are more specific and tools with which to study the mechanisms that underlie granule movement, have contributed greatly to our understanding of the role of the cytoskeleton in regulating biphasic insulin secretion. Herein, we provide historical perspective and review recent progress that has been made towards integrating cytoskeletal reorganization and cycling of small Rho-, Rab- and Ras-family GTPases into our current models of stimulus-secretion coupling and second-phase insulin release.

Myosin 2 maintains an open exocytic fusion pore in secretory epithelial cells

Many studies have implicated F-actin and myosin 2 in the control of regulated secretion. Most recently, evidence suggests a role for the microfilament network in regulating the postfusion events of vesicle dynamics. This is of potential importance as postfusion behavior can influence the loss of vesicle content and may provide a new target for drug therapy. We have investigated the role of myosin 2 in regulating exocytosis in secretory epithelial cells by using novel assays to determine the behavior of the fusion pore in individual granules. We immunolocalize myosin 2A to the apical region of pancreatic acinar cells, suggesting it is this isoform that plays a role in granule exocytosis. We further show myosin 2 phosphorylation increased on cell stimulation, consistent with a regulatory role in secretion. Importantly, in a single-cell, single-granule secretion assay, neither the myosin 2 inhibitor (-)-blebbistatin nor the myosin light chain kinase inhibitor ML-9 had any effect on the numbers of granules stimulated to fuse after cell stimulation. These data indicate that myosin 2, if it has any action on secretion, must be targeting postfusion granule behavior. This interpretation is supported by direct study of fusion pore opening in which we show that (-)-blebbistatin and ML-9 promote fusion pore closure and decrease fusion pore lifetimes. Our work now adds to a growing body of evidence showing that myosin 2 is an essential regulator of postfusion granule behavior. In particular, in the case of the secretory epithelial cells, myosin 2 activity is necessary to maintain fusion pore opening.

Isoform B of myosin II heavy chain mediates actomyosin contractility during TNFalpha-induced apoptosis

Cells that are treated long-term with TNFalpha or short-term with TGFalpha together with cycloheximide (CHX) undergo apoptosis. Cell shrinkage and detachment during apoptosis is dependent on actomyosin contractility. Myosin II heavy chain (MHCII) isoforms have shared and distinct functions. Here, we investigated whether the involvement of MHCII isoforms A and B (MHCIIA and MHCIIB, respectively) in cell shrinkage and detachment differs during apoptosis. We show that TNFalpha induces caspase-dependent MHCIIA degradation, whereas MHCIIB levels and association with the cytoskeleton remained virtually unchanged in TtT/GF cells and NIH 3T3 fibroblasts. MHCIIA proteolysis also occurred in fibroblasts that lack MHCIIB when treated with TNFalpha and CHX together. The absence of MHCIIB did not affect cell death rate. However, MHCIIB-/- cells showed more resistance to TNFalpha-induced actin disassembly, cell shrinkage and detachment than wild-type fibroblasts, indicating the participation of MHCIIB in these events. Moreover, inhibition of atypical PKCzeta, which targets MHCIIB but not MHCIIA, blocked TNFalpha-induced shrinkage and detachment in TtT/GF cells and wild-type fibroblasts, but the inhibitory effect was significantly reduced in MHCIIB-/- fibroblasts. TNFalpha treatment increased cytoskeleton-associated myosin light chain (MLC) phosphorylation but did not induce actin cleavage. In conclusion, our results demonstrate that MHCIIB, together with MLC phosphorylation and actin, constitute the actomyosin cytoskeleton that mediates contractility during apoptosis.

The actin filament and dendritic peptide release

F-actin remodelling has been implicated in regulated secretion from many cell types, in particular secretion from neuron axon terminals and neuroendocrine cell types. Cortical F-actin has long been postulated to act as a barrier to vesicle movement and hence to inhibit secretion; however, more recent studies point to F-actin remodelling providing both supporting and restraining roles in secretion. Magnocellular neurons of the supraoptic nucleus secrete either oxytocin or vasopressin from their dendrites as well as their axon terminals; and peptide release from these two compartments can be differentially controlled to allow secretion from one compartment in isolation from the other. While oxytocin and vasopressin secretion can be provoked by F-actin depolymerization in both compartments, acutely stimulated secretion is dependent on F-actin remodelling in dendrites but not axon terminals, suggesting that F-actin plays a different role in regulating the readily releasable pool of secretory vesicles in the two compartments. In addition, activity-dependent secretion from the dendritic compartment can be primed by prior exposure to agents, including oxytocin, that stimulate release of Ca2+ from intracellular stores. While remodelling of F-actin is involved, it is not solely responsible for priming secretory responses.

[Morphologic evaluation and Ca2+ mobilization by glicose and acetylcholine in human pancreatic cells]

AIMS

The proposal of this study was to analyze morphology of the organelles and cytoskeleton in human pancreatic cells cultured and the mobilization of the cytosolic calcium ([Ca2+]c) in response to glucose and ACh by fluorimetry method.

MATERIAL AND METHODS

The cells were plated on glass coverslips, fixed and stained with a combination of fluorophores: the nuclei were stained with DAPI and mitochondria with Mytotracker Red. It was used phalloidin and the secondary antibodies Alexa Fluor conjugated green and red-fluorescent (488 and 594) to identify the protein cell actin F and type M3 muscarinic receptor respectively. The cells also were loaded with fura-2/AM to study Ca2+ mobilization.

RESULTS

The human pancreatic cells show characteristics morphologically preserved with great amount of mitochondria. In region major cell density was evidenced pseudo-islets and type M3 muscarinic receptors. Through increase of [Ca2+]c due to action of glucose and ACh were shown that the cells capacity to respond to these stimuli were conserved. The elevation of the [Ca2+]c depended on concentration by glucose-induced promoting sustained phase and ACh-induced a biphasic response.

CONCLUSION

The morphologic characteristics of human pancreatic cells cultured were preserved. The Ca2+ mobilization in response to glucose and ACh confirmed its functionality. The expression of the M3 muscarinic receptors in human pancreatic cell cultured was demonstrated.

The role of the actin cytoskeleton in oxytocin and vasopressin release from rat supraoptic nucleus neurons

Magnocellular neurons of the supraoptic nucleus (SON) can differentially control peptide release from the somato/dendritic and axon terminal compartment. Dendritic release can be selectively regulated through activation of intracellular calcium stores by calcium mobilizers such as thapsigargin (TG), resulting in preparation (priming) of somato/dendritic peptide pools for subsequent activity-dependent release. As dynamic modulation of the actin cytoskeleton is implicated in secretion from synaptic terminals and from several types of neuroendocrine cells, we studied its involvement in oxytocin and vasopressin release from SON neurons. Confocal image analysis of the somata revealed that the normally continuous cortical band of F-actin is disrupted after high potassium (K(+), 50 mm) or TG (200 nm) stimulation. The functional importance of actin remodelling was studied using cell-permeable actin polymerizing (jasplakinolide, 2 microm) or depolymerizing agents (latrunculin B, 5 microm) to treat SON and neural lobe (NL) explants in vitro and measure high K(+)-induced oxytocin and vasopressin release. Latrunculin significantly enhanced, and jasplakinolide inhibited, high-K(+)-evoked somato/dendritic peptide release, while release from axon terminals was not altered, suggesting that high-K(+)-evoked release in the SON, but not the NL, requires depolymerization of the actin cytoskeleton. TG-induced priming of somato/dendritic release was also blocked by jasplakinolide and latrunculin, suggesting that priming involves changes in actin remodelling.

Phosphorylation of nonmuscle myosin heavy chain IIA on Ser1917 is mediated by protein kinase C beta II and coincides with the onset of stimulated degranulation of RBL-2H3 mast cells

Dynamic remodeling of the actinomyosin cytoskeleton is integral to many biological processes. It is regulated, in part, by myosin phosphorylation. Nonmuscle myosin H chain IIA is phosphorylated by protein kinase C (PKC) on Ser(1917). Our aim was to determine the PKC isoform specificity of this phosphorylation event and to evaluate its potential role in regulated secretion. Using an Ab against the phosphorylated form of Ser(1917), we show that this site is not phosphorylated in unstimulated RBL-2H3 mast cells. The physiological stimulus, Ag, or the pharmacological activators, PMA plus A23187, induced Ser(1917) phosphorylation with a time course coincident with the onset of granule mediator secretion. Dephosphorylation at this site occurred as Ag-stimulated secretion declined from its peak, but dephosphorylation was delayed in cells activated with PMA plus A23187. Phosphate incorporation was also enhanced by PMA alone and by inhibition of protein phosphatase 2A. Gö6976, an inhibitor of conventional PKC isoforms, abolished secretion and Ser(1917) phosphorylation with similar dose dependencies consistent with involvement of either PKCalpha or PKCbeta. Phorbol ester-stimulated Ser(1917) phosphorylation was reconstituted in HEK-293 cells (which lack endogenous PKCbeta) by overexpression of both wild-type and constitutively active PKCbetaII but not the corresponding PKCbetaI or PKCalpha constructs. A similar selectivity for PKCbetaII overexpression was also observed in MIN6 insulinoma cells infected with recombinant PKC wild-type adenoviruses. Our results implicate PKC-dependent phosphorylation of myosin H chain IIA in the regulation of secretion in mast cells and suggest that Ser(1917) phosphorylation might be a marker of PKCbetaII activation in diverse cell types.

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

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

Regulation of two insulin granule populations within the reserve pool by distinct calcium sources

Insulin granule trafficking is a key step of glucose-stimulated insulin secretion from pancreatic beta cells. Using quantitative live cell imaging, we examined insulin granule movements within the reserve pool upon secretory stimulation in betaTC3 cells. For this study, we developed a custom image analysis program that permitted automatic tracking of the individual motions of over 20,000 granules. This analysis of a large sample size enabled us to study micro-populations of granules that were not quantifiable in previous studies. While over 90% of the granules depend on Ca2+ efflux from the endoplasmic reticulum for their mobilization, a small and fast-moving population of granules responds to extracellular Ca2+ influx after depolarization of the plasma membrane. We show that this differential regulation of the two granule populations is consistent with localized Ca2+ signals, and that the cytoskeletal network is involved in both types of granule movement. The fast-moving granules are correlated temporally and spatially to the replacement of the secreted insulin granules, which supports the hypothesis that these granules are responsible for replenishing the readily releasable pool. Our study provides a model by which glucose and other secretory stimuli can regulate the readily releasable pool through the same mechanisms that regulate insulin secretion.

A novel role for myosin II in insulin-stimulated glucose uptake in 3T3-L1 adipocytes

Insulin-stimulated glucose uptake requires the activation of several signaling pathways to mediate the translocation and fusion of GLUT4 vesicles from an intracellular pool to the plasma membrane. The studies presented here show that inhibition of myosin II activity impairs GLUT4-mediated glucose uptake but not GLUT4 translocation to the plasma membrane. We also show that adipocytes express both myosin IIA and IIB isoforms, and that myosin IIA is recruited to the plasma membrane upon insulin stimulation. Taken together, the data presented here represent the first demonstration that GLUT4-mediated glucose uptake is a myosin II-dependent process in adipocytes. Based on our findings, we hypothesize that myosin II is activated upon insulin stimulation and recruited to the cell cortex to facilitate GLUT4 fusion with the plasma membrane. The identification of myosin II as a key component of GLUT4-mediated glucose uptake represents an important advance in our understanding of the mechanisms regulating glucose homeostasis.

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

In pancreatic beta cells, insulin granule exocytosis is regulated by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein (SNAP) receptor) proteins, and this is coupled to cortical F-actin reorganization via the Rho family GTPase Cdc42 by an unknown mechanism. We investigated interactions among the target SNARE protein Syntaxin 1A and the vesicle-associated membrane SNARE protein (VAMP2) with Cdc42 and compared these structural interactions with their functional importance to glucose-stimulated insulin secretion in MIN6 beta cells. Subcellular fractionation analyses revealed a parallel redistribution of Cdc42 and VAMP2 from the granule fraction to the plasma membrane in response to glucose that temporally corresponded with the glucose-induced activation of Cdc42. Moreover, within these fractions Cdc42 and VAMP2 were found to co-immunoprecipitate under basal and glucose-stimulated conditions, suggesting that they moved as a complex. Furthermore, VAMP2 bound both GST-Cdc42-GTPgammaS and GST-Cdc42-GDP, indicating that the Cdc42-VAMP2 complex could form under both cytosolic GDP-bound Cdc42 and plasma membrane GTP-bound Cdc42 conformational conditions. In vitro binding analyses showed that VAMP2 bound directly to Cdc42 and that a heterotrimeric complex with Syntaxin 1A could also be formed. Deletion analyses of VAMP2 revealed that only the N-terminal 28 residues were required for Cdc42 binding. Expression of this 28-residue VAMP2 peptide in MIN6 beta cells resulted in the specific impairment of glucose-stimulated insulin secretion, indicating a functional importance for the Cdc42-VAMP2 interaction. Taken together, these data suggest a mechanism whereby glucose activates Cdc42 to induce the targeting of intracellular Cdc42-VAMP2-insulin granule complexes to Syntaxin 1A at the plasma membrane.

Hydrogen-deuterium exchange effects on beta-endorphin release from AtT20 murine pituitary tumor cells

Abundant evidences demonstrate that deuterium oxide (D2O) modulates various secretory activities, but specific mechanisms remain unclear. Using AtT20 cells, we examined effects of D2O on physiological processes underlying beta-endorphin release. Immunofluorescent confocal microscopy demonstrated that 90% D2O buffer increased the amount of actin filament in cell somas and decreased it in cell processes, whereas beta-tubulin was not affected. Ca2+ imaging demonstrated that high-K+-induced Ca2+ influx was not affected during D2O treatment, but was completely inhibited upon D2O washout. The H2O/D2O replacement in internal solutions of patch electrodes reduced Ca2+ currents evoked by depolarizing voltage steps, whereas additional extracellular H2O/D2O replacement recovered the currents, suggesting that D2O gradient across plasma membrane is critical for Ca2+ channel kinetics. Radioimmunoassay of high-K+-induced beta-endorphin release demonstrated an increase during D2O treatment and a decrease upon D(2)O washout. These results demonstrate that the H2O-to-D2O-induced increase in beta-endorphin release corresponded with the redistribution of actin, and the D2O-to-H2O-induced decrease in beta-endorphin release corresponded with the inhibition of voltage-sensitive Ca2+ channels. The computer modeling suggests that the differences in the zero-point vibrational energy between protonated and deuterated amino acids produce an asymmetric distribution of these amino acids upon D2O washout and this causes the dysfunction of Ca2+ channels.

Involvement of myosin II in dopamine-induced reorganization of the lactotroph cell's actin cytoskeleton

We have shown that dopamine (DA), an inhibitor of prolactin secretion from anterior pituitary lactotrophs, stabilizes the cortical actin cytoskeleton. DA-induced cortical actin stabilization is accompanied by cytoplasmic actin cable disassembly and cell rounding up. Our aim was to identify the mechanisms involved in DA-induced stabilization of the lactotroph's actin cytoskeleton. Here we show that DA increased the association of myosin II with the cell cortex, suggesting that DA facilitates actin-myosin interaction to stabilize cortical actin filaments. This notion was supported by the finding that inhibitors of actin-myosin interaction blocked DA-evoked morphological responses. In addition, our results showed that DA-induced myosin association with the cell periphery may be mediated by inhibition of Rac1/Cdc42-dependent pathways, whereas, DA-induced cytoplasmic actin filament disassembly may be mediated by the inhibition of MLCK- and RhoA-dependent pathways. In conclusion, the present results provide evidence that myosin II is involved in the DA-induced remodeling of actin filaments in lactotrophs, and that DA-induced cortical actin filament assembly and stabilization involve the translocation of myosin II to the cell cortex. This effect requires, among other things, inhibition of the Rac1/Cdc42-dependent signaling pathway.

Identification of an IQGAP1/AKAP79 complex in beta-cells

IQGAP1, is a recently discovered scaffold protein proposed to regulate membrane cytoskeleton events through protein-protein interactions with F-actin, E-cadherin, beta-catenin, and CLIP170. The binding of IQGAP1 to its partners is regulated by calcium/calmodulin (Ca(++)/CaM) and the small molecular weight guanine nucleotide triphosphatases (GTPases), Cdc42, and Rac1. Here we identify a novel IQGAP1 scaffolding function by isolating the cyclic AMP dependent kinase (PKA) with IQGAP1. IQGAP1 was co-purified with PKA using 5'-cyclic AMP (cAMP) affinity chromatography and PKA activity was co-immunoprecipitated with IQGAP1 using an anti-IQGAP1 antibody. The association of IQGAP1 with PKA was shown to occur through a direct interaction between A kinase anchoring protein 79 (AKAP79) and the carboxyl-terminal domain of IQGAP1. This suggests that cAMP/PKA may be coupled with Ca(++)/CaM and GTPases through an IQGAP1/AKAP79 complex.

Glucose regulates the cortical actin network through modulation of Cdc42 cycling to stimulate insulin secretion

Glucose-stimulated insulin granule exocytosis in pancreatic beta-cells involves cortical actin remodeling that results in the transient disruption of the interaction between polymerized actin with the plasma membrane t-SNARE (target membrane soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex. To examine the mechanism underlying the initiation of cortical actin remodeling, we have used the actin nucleating/stabilizing agent jasplakinolide to show that remodeling is initiated at a step proximal to the ATP-sensitive K+ channels in the stimulus-secretion pathway. Confocal immunofluorescent microscopy revealed that cortical actin remodeling was required for glucose-stimulated insulin secretion. Furthermore, glucose was found to mediate the endogenous activation state of the Rho family GTPase Cdc42, a positive proximal effector of actin polymerization, resulting in a net decrease of Cdc42-GTP within 5 min of stimulation. Intriguingly, glucose stimulation resulted in the rapid and reversible glucosylation of Cdc42, suggesting that glucose inactivated Cdc42 by selective glucosylation to induce cortical actin rearrangement. Moreover, expression of the constitutively active form of Cdc42 (Q61L) inhibited glucose-stimulated insulin secretion, whereas the dominant negative form (T17N) was without effect, suggesting that glucose-stimulated insulin secretion requires Cdc42 cycling to the GDP-bound state. In contrast, KCl-stimulated insulin secretion was unaffected by the expression of dominant negative or constitutively active Cdc42 and ceased to modulate endogenous Cdc42 activation, consistent with glucose-dependent cortical actin remodeling. These findings reveal that glucose regulates the cortical actin network through modulation of Cdc42 cycling to induce insulin secretion in pancreatic beta-cells.

Possible interrelationship between changes in F-actin and myosin II, protein phosphorylation, and cell volume regulation in Ehrlich ascites tumor cells

Osmotic shrinkage of Ehrlich ascites tumor cells (EATC) elicited translocation of myosin II from the cytosol to the cortical region, and swelling elicits concentration of myosin II in the Golgi region. Rho kinase and p38 both appeared to be involved in shrinkage-induced myosin II reorganization. In contrast, the previously reported shrinkage-induced actin polymerization [Pedersen et al. (1999) Exp. Cell Res. 252, 63-74] was independent of Rho kinase, p38, myosin light chain kinase (MLCK), and protein kinase C (PKC), which thus do not exert their effects on the shrinkage-activated transporters via effects on F-actin. The subsequent F-actin depolymerization, however, appeared MLCK- and PKC-dependent, and the initial swelling-induced F-actin depolymerization was MLCK-dependent; both effects were apparently secondary to kinase-mediated effects on cell volume changes. NHE1 in EATC is activated both by osmotic shrinkage and by the serine/threonine phosphatase inhibitor Calyculin A (CL-A). Both stimuli caused Rho kinase-dependent myosin II relocation to the cortical cytoplasm, but in contrast to the shrinkage-induced F-actin polymerization, CL-A treatment elicited a slight F-actin depolymerization. Moreover, Rho kinase inhibition did not significantly affect NHE1 activation, neither by shrinkage nor by CL-A. Implications for the possible interrelationship between changes in F-actin and myosin II, protein phosphorylation, and cell volume regulation are discussed.

Protein phosphatases 1 and 2A transiently associate with myosin during the peak rate of secretion from mast cells

Mast cells undergo cytoskeletal restructuring to allow secretory granules passage through the cortical actomyosin barrier to fuse with the plasma membrane and release inflammatory mediators. Protein phosphorylation is believed to regulate these rearrangements. Although some of the protein kinases implicated in this phosphorylation are known, the relevant protein phosphatases are not. At the peak rate of antigen-induced granule mediator release (2.5 min), protein phosphatases PP1 and PP2A, along with actin and myosin II, are transiently relocated to ruffles on the apical surface and a band at the peripheral edge of the cell. This leaves an area between the nucleus and the peripheral edge significantly depleted (3-5-fold) in these proteins. Phorbol 12-myristate 13-acetate (PMA) plus A23187 induces the same changes, at a time coincident with its slower rate of secretion. Coimmunoprecipitation experiments demonstrated a significantly increased association of myosin with PP1 and PP2A at the time of peak mediator release, with levels of association decreasing by 5 min. Jasplakinolide, an inhibitor of actin assembly, inhibits secretion and the cytoskeletal rearrangements. Surprisingly, jasplakinolide also affects myosin, inducing the formation of short rods throughout the cytoplasm. Inhibition of PP2A inhibited secretion, the cytoskeletal rearrangements, and led to increased phosphorylation of the myosin heavy and light chains at protein kinase C-specific sites. These findings indicate that a dynamic actomyosin cytoskeleton, partially regulated by both PP1 and PP2A, is required for mast cell secretion.

Differential localization of non-muscle myosin II isoforms and phosphorylated regulatory light chains in human MRC-5 fibroblasts

We investigated the localization of non-muscle myosin II isoforms and mono- (at serine 19) and diphosphorylated (at serine 19 and threonine 18) regulatory light chains (RLCs) in motile and non-motile MRC-5 fibroblasts. In migrating cells, myosin IIA localized to the lamella and throughout the posterior region. Myosin IIB colocalized with myosin IIA to the posterior region except at the very end. Diphosphorylated RLCs were detected in the restricted region where myosin IIA was enriched. In non-motile cells, myosin IIA was enriched in peripheral stress fibers with diphosphorylated RLCs, but myosin IIB was not. Our results suggest that myosin IIA may be highly activated by diphosphorylation of RLCs and primarily involved in cell migration.

Dephosphorylation of beta2-syntrophin and Ca2+/mu-calpain-mediated cleavage of ICA512 upon stimulation of insulin secretion

Islet cell autoantigen (ICA) 512 is a receptor-tyrosine phosphatase-like protein associated with the secretory granules of neuroendocrine cells, including pancreatic beta-cells. Binding of its cytoplasmic tail to beta2-syntrophin suggests that ICA512 connects secretory granules to the utrophin complex and the actin cytoskeleton. Here we show that stimulation of insulin secretion from INS-1 cells triggers the biosynthesis of pro-ICA512 and the degradation of its mature form. Inhibition of calpain, which is activated upon stimulation of insulin secretion, prevents the Ca2+-dependent proteolysis of ICA512. In vitro mu-calpain cleaves ICA512 between a putative PEST domain and the beta2-syntrophin binding site, whereas binding of ICA512 to beta2-syntrophin protects the former from cleavage. beta2-syntrophin and its F-actin-binding protein utrophin are enriched in subcellular fractions containing secretory granules. ICA512 preferentially binds phospho-beta2-syntrophin and stimulation of insulin secretion induces the Ca2+-dependent, okadaic acid-sensitive dephosphorylation of beta2-syntrophin. Similarly to calpeptin, okadaic acid inhibits ICA512 proteolysis and insulin secretion. Thus, stimulation of insulin secretion might promote the mobilization of secretory granules by inducing the dissociation of ICA512 from beta2-syntrophin-utrophin complexes and the cleavage of the ICA512 cytoplasmic tail by mu-calpain.