Rapid and sustained stimulation of beta-cells with glucose induces biphasic insulin secretion. The two phases appear to reflect a characteristic of stimulus-secretion coupling in each beta-cell rather than heterogeneity in the time-course of the response between beta-cells or islets. There is no evidence indicating that biphasic secretion can be attributed to an intrinsically biphasic metabolic signal. In contrast, the biphasic rise in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) induced by glucose is important to shape the two phases of secretion. The first phase requires a rapid and marked elevation of [Ca(2+)](i) and corresponds to the release of insulin granules from a limited pool. The magnitude of the second phase is determined by the elevation of [Ca(2+)](i), but its development requires production of another signal. This signal corresponds to the amplifying action of glucose and may serve to replenish the pool of granules that are releasable at the prevailing [Ca(2+)](i). The species characteristics of biphasic insulin secretion and its perturbations in pathological situations are discussed.

The cellular and molecular mechanisms of insulin secretion are being intensively investigated, yet most researchers are seemingly unaware of the complexity of the dynamic regulation of the secretion. In this article, we summarize studies of the physiology of insulin secretion performed over several decades. The insulin response of perifused islets of rats, perfused rat pancreas, or that of a human, to a square-wave glucose stimulus is biphasic, a transient first-phase response of 4- to 10-min duration followed by a gradual rise in secretion rates (second-phase response). Several hypotheses have been proposed to account for the phasic nature of insulin secretion; they are briefly discussed in this review. We have favored the hypothesis that nutrient stimulators such as glucose, in addition to a primary and almost immediate secretory signal, with time induce both stimulatory and inhibitory messages in the beta-cell, and those messages modulate the primary insulinogenic signal. Indeed, studies in the rat pancreas and in humans have demonstrated that short stimulations with glucose generate a state of refractoriness of the insulin secretion, which we have termed time-dependent inhibition (TDI). Nonnutrient secretagogues such as arginine induce strong TDI independent of the duration of stimulation. Once the agent is removed, TDI persists for a considerable period. In contrast, prolonged stimulations with glucose (and other nutrients) lead to the amplification of the insulin response to subsequent stimuli; this can be demonstrated in the perfused rat pancreas, in perifused islets from several rodents, and in humans. We have termed this stimulatory signal time-dependent potentiation (TDP). The generation of TDP requires higher glucose concentrations and prolonged stimulation; the effect is retained for some time after cessation of the stimulus. Of major interest is the observation that, while the acute insulin response to glucose is severely reduced in glucose-intolerant animals and humans, TDP seems to be intact. The cellular mechanisms of TDI and TDP are poorly understood, but data reviewed here suggest that they are distinct from those that lead to the acute insulin response to stimuli. A model is proposed whereby the magnitude and kinetics of the insulin response to a given stimulus reflect the balance between TDP and TDI. Researchers studying the cellular and molecular mechanisms of insulin release are urged to take into consideration these complex and opposing factors which regulate insulin secretion.

The beta-cell mitochondria are known to generate metabolic coupling factors, or messengers, that mediate plasma membrane depolarization and the increase in cytosolic Ca(2+), the triggering event in glucose-stimulated insulin secretion. Accordingly, ATP closes nucleotide-sensitive K(+) channels necessary for the opening of voltage-gated Ca(2+) channels. ATP also exerts a permissive action on insulin exocytosis. In contrast, GTP directly stimulates the exocytotic process. cAMP is considered to have a dual function: on the one hand, it renders the beta-cell more responsive to glucose; on the other, it mediates the effect of glucagon and other hormones that potentiate insulin secretion. Mitochondrial shuttles contribute to the formation of pyridine nucleotides, which may also participate in insulin exocytosis. Among the metabolic factors generated by glucose, citrate-derived malonyl-CoA has been endorsed, but recent results have questioned its role. We have proposed that glutamate, which is also formed by mitochondrial metabolism, stimulates insulin exocytosis in conditions of permissive, clamped cytosolic Ca(2+) concentrations. The evidence for the implication of these and other putative messengers in metabolism-secretion coupling is discussed in this review.

Several reports indicate that hypoglycemic sulfonylureas augment Ca(2+)-dependent insulin secretion via mechanisms other than inhibition of the ATP-sensitive K(+) channel. The effect involves a 65-kd protein in the granule membrane and culminates in intragranular acidification. Lowering of granule pH is necessary for the insulin granule to gain release competence. Proton pumping into the granule is driven by a v-type H(+)-ATPase, but requires simultaneous Cl(-) uptake into the granule via metabolically regulated ClC-3 Cl(-) channels to maintain electroneutrality. Here we discuss the possibility that modulation of granule ClC-3 channels represents the mechanism whereby sulfonylureas directly potentiate the beta-cell exocytotic machinery.

Regulated exocytosis forms the basis for many intercellular signaling processes, for example, in hormone secretion or neurotransmitter release. During regulated exocytosis, the membrane of a secretory vesicle fuses with the plasma membrane in a tightly controlled reaction that is most often triggered by calcium. Recent advances have allowed major insights into the molecular mechanisms that mediate regulated exocytosis. In the present review, we will briefly discuss two key features of regulated exocytosis that have been particularly well studied recently. First, we will examine the current understanding of the membrane fusion reaction that underlies regulated exocytosis and that is effected by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) and munc18-like proteins similar to other membrane fusion reactions. Second, we will describe the role of the major candidates for the calcium sensors that trigger exocytosis, a protein family called synaptotagmins. Although our understanding of regulated exocytosis is as yet incomplete, the results from the studies of SNAREs, munc18s, and synaptotagmins have provided a molecular anchor for a more complete future description.

A workshop on autoreactive T-cell responses in NOD mice was held to optimize autoreactive T-cell detection methodologies. Using different proliferation assay protocols, 1 of the 11 participating laboratories detected spontaneous T-cell responses to GAD(524-543) and insulin(9-23) in their NOD mice. Two other laboratories were able to detect autoreactive responses when using enzyme-linked immunospot assay (ELISPOT) and enzyme-linked immunosorbent assay (ELISA) analysis of cytokines in culture supernatants, suggesting that these assays provided greater sensitivity. To address the divergent findings, a follow-up mini-workshop tested NOD mice from four different colonies side-by-side for T-cell proliferative responses to an expanded panel of autoantigens, using the protocol that had enabled detection of responses in the 1st International NOD Mouse T-Cell Workshop. Under these assay conditions, 16 of 16 NOD mice displayed proliferative responses to whole GAD65, 13 of 16 to GAD(524-543), 9 of 16 to GAD(217-236), 7 of 16 to insulin(9-23), and 5 of 16 to HSP277. Thus, spontaneous proliferative T-cell responses can be consistently detected to some beta-cell autoantigens and peptides thereof. Overall, the results suggest that more sensitive assays (e.g., ELISPOT, ELISA analysis of cytokines in supernatants, or tetramer staining) may be preferred for the detection of autoreactive T-cells.

Several self-antigens have been reported as targets of the autoimmune response in nonobese diabetic (NOD) mice. The aim of this workshop was to identify autoantibody assays that could provide useful markers of autoimmunity in this animal model for type 1 diabetes. More than 400 serum samples from NOD (4, 8, and 12 weeks of age and at diabetes onset), BALB/c, and B6 mice were collected from six separate animal facilities, coded, and distributed to five laboratories for autoantibody measurement. Insulin autoantibodies (IAA) were measured by radiobinding assay (RBA) by four laboratories and by enzyme-linked immunosorbent assay (ELISA) in one laboratory. Using the 99th percentile of BALB/c and B6 control mice as the threshold definition of positivity, IAA by RBA were detected in NOD mice at frequencies ranging from 10 to 30% at age 4 weeks, from 26 to 56% at 8 weeks, from 42 to 56% at 12 weeks, and from 15 to 75% at diabetes onset. With ELISA, IAA signals differed significantly between control mouse strains and increased with age in both control and NOD mice, with frequencies in NOD animals being 0% at 4 weeks, 14% at 8 weeks, 19% at 12 weeks, and 42% at diabetes onset. For IAA, the ELISA results were relatively discordant with those of RBA. GAD autoantibody (GADA) and IA-2 autoantibody (IA-2A) signals obtained by RBA were low (maximum 2.5% of total) but were increased in NOD mice compared with control mice at diabetes onset (GADA 29-50%; IA-2A 36-47%). ELISA also detected GADA (42%) and IA-2A (50%) at diabetes onset, with results concordant with those of RBA. Remarkably, GADA and IA-2A frequencies varied significantly with respect to the source colony of NOD mice. Furthermore, whereas neither GADA nor IA-2A correlated with IAA, there was strong concordance between GADA and IA-2A in individual mice. Sera with increased binding to GAD and IA-2 also had increased binding to the unrelated antigen myelin oligodendrocyte glycoprotein, and binding to GAD could not be inhibited with excess unlabeled antigen, suggesting nonspecific interactions. In sum, this workshop demonstrated that IAA measured by sensitive RBA are a marker of autoimmunity in NOD mice and draw into question the true nature of GADA and IA-2A in this animal model.

Recent advances in molecular and cell biology may allow for the development of novel strategies for the treatment and cure of type 1 diabetes. In particular, it is now possible to envisage restoration of insulin secretion by gene or cell-replacement therapy. The beta-cell is, however, remarkably sophisticated, and many of the features of this highly differentiated secretory cell will have to be faithfully mimicked in surrogate cells. In particular, insulin is normally secreted in a well-regulated fashion in rapid response to the metabolic needs of the individual and most specifically (but not exclusively) to changes in circulating levels of glucose. Such regulated secretion will be indispensable in order to avoid both hyper- and hypoglycemic episodes and depends on the ability of cells to store insulin in secretory granules before exocytosis in response to physiological stimuli. Furthermore, any newly created insulin-secreting cell will have to be able to adapt to alterations in insulin requirements that accompany changes with exercise, body weight, and aging. Fine tuning of insulin secretion over the longer term will also be important to avoid "clinical shifting" that could be caused by over-insulinization, including increased adiposity and cardiovascular disease. Finally, it will be necessary to ensure that newly created or implanted (surrogate) beta-cells are protected in some way from recognition by the immune system and in particular from autoimmune destruction.

The neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) is ubiquitously distributed in both the central and peripheral nervous systems and exerts a variety of effects. PACAP is a neuropeptide in pancreatic islets, where it has been suggested as a parasympathetic and sensory neurotransmitter. PACAP stimulates insulin secretion in a glucose-dependent manner, by an effect executed mainly through augmenting the formation of cAMP and stimulating the uptake of calcium. Accumulating evidence in animal studies points to a physiological importance of PACAP in the regulation of the insulin response to feeding. This review summarizes the current knowledge of islet actions and mechanisms and the function of PACAP.

Pancreatic beta-cells are sensitive to a number of proapoptotic stimuli. Thus, apoptosis is an important part of the physiological neonatal remodeling of the endocrine pancreas, and a number of pathological stimuli involved in type 1 and type 2 diabetes have been shown to elicit beta-cell apoptosis. Factors of relevance to type 1 diabetes include proinflammatory cytokines, nitric oxide, and reactive oxygen species as well as Fas ligand. Recent findings that free fatty acids, glucose, sulfonylurea, and amylin cause beta-cell apoptosis in vitro suggest that programmed cell death may also be involved in the pathogenesis of type 2 diabetes. Furthermore, there is evidence favoring a convergence in signaling pathways toward common effectors of beta-cell apoptosis elicited by stimuli implicated in the pathogenesis of type 1 and type 2 diabetes. Therefore, recent studies involving the stimuli and signaling pathways of beta-cell apoptosis-in particular, mitogen- and stress-activated protein kinases-will be reviewed. It is concluded that immunological, inflammatory, and metabolic signals cause beta-cell apoptosis, and the possibility that these signals converge toward a common beta-cell death signaling pathway should be investigated further.

Studies on the pathogenesis of type 1 diabetes have mainly focused on the role of the immune system in the destruction of pancreatic beta-cells. Lack of data on the cellular and molecular events at the beta-cell level is caused by the inaccessibility of these cells during development of the disease. Indirect information has been collected from isolated rodent and human islet cell preparations that were exposed to cytotoxic conditions. This article reviews in vitro experiments that investigated the role of beta-cells in the process of beta-cell death. beta-Cells rapidly die in necrosis because of toxic levels of oxidizing radicals or of nitric oxide; they progressively become apoptotic after prolonged culture at low glucose or with proinflammatory cytokines. Their susceptibility to necrosis or apoptosis varies with their functional state and thus with the environmental conditions. A change in cellular phenotype can alter its recognition of potentially cytotoxic agents and its defense mechanisms against cell death. These observations support the view that beta-cells are not necessarily passive victims of a cytotoxic process but can actively participate in a process of beta-cell death. Their role will be influenced by neighboring non-beta-cells, which can make the islet internal milieu more protective or toxic for the beta-cells. We consider duct cells as potentially important contributors to this local process.

All pancreatic cell types (endocrine, exocrine, and ductal) are derived from the same endodermal dorsal and ventral anlage, which grow together to form the definitive pancreas. Golosow and Grobstein were pioneers in the field of pancreatic developmental research, as were Wessells and Cohen, who already in the 1960s performed classic embryological experiments describing the morphogenesis of the pancreas and the epithelio-mesenchymal interactions that are instrumental for proper pancreas development. Recent findings suggest that follistatin and fibroblast growth factors represent some of these key mesenchymal factors that actively promote at least pancreatic exocrine development. The true endodermal origin of the pancreatic endocrine cells became evident by experiments performed by the groups of LeDouarin and Rutter in the 1970s. The newly acquired insights regarding the specification of pancreatic endocrine cells as controlled by the notch signaling pathway (i.e., similar to the mechanisms by which neurons are specified during neurogenesis) have provided a novel understanding of the long acknowledged similarities between neurons and the pancreatic endocrine cells. Last, the identification of a number of distinct transcription factors operating at various levels of pancreatic development and in different cell types has provided useful information both on pancreas development and on various pancreatic disorders such as diabetes. Interestingly, four of the hitherto defined five different maturity-onset diabetes of the young (MODY) genes correspond to transcription factors, and, in addition, several transcription factors have also been linked to type 2 diabetes.

Endocrine pancreas plasticity may be defined as the ability of the organ to adapt the beta-cell mass to the variations in insulin demand. For example, during late pregnancy and obesity, the increase of the beta-cell mass, in association with beta-cell hyperactivity, contributes to insulin oversecretion in response to insulin resistance. There is increasing evidence that the ability of the beta-cell mass to expand in adult mammals is much higher than previously thought. During pregnancy, placental hormones, especially placental lactogens, are mainly responsible for the changes in beta-cell mass. The factors involved in beta-cell growth in obesity are far from clear, although increased free fatty acids seem to be the main candidate. Many data suggest that the impairment of insulin secretion in type 2 diabetes is partly related to reduction of beta-cell mass, at least relative to prevailing insulin demand. This defect may originate from genetic predisposition, but the situation is likely worsened by environmental factors such as hyperglycemia (glucotoxicity) and hyperlipidemia (lipotoxicity). Better understanding of beta-cell growth and regeneration mechanisms may allow new strategies in the treatment of type 2 diabetes based on early limitation of beta-cell damage and/or restoration of a functional beta-cell mass.

Substantial new information has accumulated on molecular mechanisms of pancreas development, regulation of beta-cell gene expression, and the role of growth factors in the differentiation, growth, and regeneration of beta-cells. The present review focuses on some recent studies on the mechanism of action of cytokines such as growth hormone (GH) and prolactin (PRL) in beta-cell proliferation and gene expression-in particular, the role of signal transducers and activators of transcription (STAT) proteins. The implication of the discovery of suppressors of cytokine signaling (SOCS) proteins for the interaction between stimulatory and inhibitory cytokines, including GH, PRL, leptin, and the proinflammatory cytokines interleukin-1 and interferon-gamma, in beta-cell survival is not yet clear. Recent studies indicate a role of cell adhesion molecules and the delta-like protein preadipocyte factor 1/fetal antigen 1 (Pref-1/FA-1) in cytokine-induced beta-cell growth and development. Surprisingly, glucagon-like peptide-1 (GLP-1) was recently found to stimulate not only insulin secretion but also beta-cell replication and differentiation, which may present a new perspective in treatment of type 2 diabetes. Together with the intriguing reports on positive effects of insulin on both beta-cell growth and function, a picture is emerging of an integrated network of signaling events acting in concert to control beta-cell mass adaptation to insulin demand.

Type 2 diabetes is characterized by a progressive loss of beta-cell function throughout the course of the disease. The pattern of loss is an initial defect in early or first-phase insulin secretion, followed by a decreasing maximal capacity of glucose to potentiate all nonglucose signals. Last, a defective steady-state and basal insulin secretion develops, leading to complete beta-cell failure requiring insulin treatment. This functional loss exceeds the expected impact of a 20-50% loss of beta-cells reported at autopsy, which has been associated with amyloid deposits. This review summarizes the nature of the amyloid deposition process and its association with disproportionate hyperproinsulinemia. It reviews recent studies in IAPP (islet-amyloid polypeptide, or amylin) transgenic mice developing islet amyloid deposits and hyperglycemia to suggest that the process of amyloid fibril formation impairs function early and leads to beta-cell failure and eventual death. Based on the known association of amyloid deposits and relative hyperproinsulinemia, it is hypothesized that fibril formation begins during impaired glucose tolerance after other factors cause the initial defects in early insulin secretion and insulin action. Thus, the process that leads to beta-cell loss is implicated in the deposition of amyloid and the late unrelenting progressive hyperglycemia now found in all patients despite current therapies.

Inadequate beta-cell function is an essential component of all forms of diabetes. The most obvious problem is a failure to maintain sufficient beta-cell mass and function to cope with whatever insulin resistance is present. The most striking functional defect is a loss of acute glucose-induced insulin secretion (GIIS). This review discusses the ways in which beta-cells successfully adapt to increased demand and then decompensate as diabetes develops. Successful adaptation is achieved through increased beta-cell mass and increased insulin secretion. The hypothesis is explored that beta-cells exposed to the diabetic milieu lose their differentiation, which leads to loss of specialized functions such as GIIS. This concept has been strengthened by the finding of dedifferentiation of beta-cells in a rat model of partial pancreatectomy that includes a reduction of insulin gene expression, which may further contribute to decreased insulin production. Another finding was increased expression of c-Myc, which probably contributes to an increase in the expression of lactate dehydrogenase and the development of beta-cell hypertrophy. Arguments are developed that the beta-cell changes found in diabetes are better correlated with increased glucose levels than with non-esterified fatty acid levels, thus supporting the importance of glucose toxicity.

Insulin receptor substrate (IRS) proteins mediate a variety of the metabolic and growth-promoting actions of insulin and IGF-1. After phosphorylation by activated receptors, these intracellular signaling molecules recruit various downstream effector pathways including phosphatidylinositol 3-kinase and Grb2. Ablation of the IRS-2 gene produces a diabetic phenotype; mice lacking IRS-2 display peripheral insulin resistance and beta-cell dysfunction characterized by a 50% reduction in beta-cell mass. In contrast, deletion of IRS-1 retards somatic growth and enhances beta-cell mass. IRS1-/- mice are 50% smaller than controls but have a twofold increase in pancreatic beta-cell mass. Thus, observations from these recently developed animal models implicate the IRS signaling systems in the response of classical insulin target tissues, and they suggest a critical role for these proteins in the regulation of beta-cell function. In humans, type 2 diabetes generally occurs when insulin-secretory reserves fail to compensate for peripheral insulin resistance. Study and identification of the signals downstream of IRS proteins in beta-cells may provide unique insights into the compensatory mechanisms by which these cells respond to insulin resistance. Therefore, the intent of this review is to summarize recent observations regarding the regulation of beta-cell function by members of the IRS protein family.

Nutrient homeostasis is known to be regulated by pancreatic islet tissue. The function of islet beta-cells is controlled by a glucose sensor that operates at physiological glucose concentrations and acts in synergy with signals that integrate messages originating from hypothalamic neurons and endocrine cells in gut and pancreas. Evidence exists that the extrapancreatic cells producing and secreting these (neuro)endocrine signals also exhibit a glucose sensor and an ability to integrate nutrient and (neuro)hormonal messages. Similarities in these cellular and molecular pathways provide a basis for a network of coordinated functions between distant cell groups, which is necessary for an appropriate control of nutrient homeostasis. The glucose sensor seems to be a fundamental component of these control mechanisms. Its molecular characterization is most advanced in pancreatic beta-cells, with important roles for glucokinase and mitochondrial oxidative fluxes in the regulation of ATP-sensitive K+ channels. Other glucose-sensitive cells in the endocrine pancreas, hypothalamus, and gut were found to share some of these molecular characteristics. We propose that similar metabolic signaling pathways influence the function of pancreatic alpha-cells, hypothalamic neurons, and gastrointestinal endocrine and neural cells.

Glucose is stored in mammalian tissues in the form of glycogen. Glycogen levels are markedly reduced in liver or muscle cells of patients with insulin-resistant or insulin-deficient forms of diabetes, suggesting that impaired glycogen synthesis may contribute to development of hyperglycemia. Recently, interest in this area has been further stimulated by new insights into the spatial organization of metabolic enzymes within cells and the importance of such organization in regulation of glycogen metabolism. It is now clear that a four-member family of glycogen targeting subunits of protein phosphatase-1 (PP1) plays a major role in coordinating these events. These proteins target PP1 to the glycogen particle and also bind differentially to glycogen synthase, glycogen phosphorylase, and phosphorylase kinase, thereby serving as molecular scaffolds. Moreover, the various glycogen-targeting subunits have distinct tissue expression patterns and can influence regulation of glycogen metabolism in response to glycogenic and glycogenolytic signals. The purpose of this article is to summarize new insights into the structure, function, regulation, and metabolic effects of the glycogen-targeting subunits of PP1 and to evaluate the possibility that these molecules could serve as therapeutic targets for lowering of blood glucose in diabetes.

Glucose stimulates insulin secretion by generating triggering and amplifying signals in beta-cells. The triggering pathway is well characterized. It involves the following sequence of events: entry of glucose by facilitated diffusion, metabolism of glucose by oxidative glycolysis, rise in the ATP-to-ADP ratio, closure of ATP-sensitive K+ (KATP) channels, membrane depolarization, opening of voltage-operated Ca2+ channels, Ca2+ influx, rise in cytoplasmic free Ca2+ concentration ([Ca2+]i), and activation of the exocytotic machinery. The amplifying pathway can be studied when beta-cell [Ca2+]i is elevated and clamped by a depolarization with either a high concentration of sulfonylurea or a high concentration of K+ in the presence of diazoxide (K(ATP) channels are then respectively blocked or held open). Under these conditions, glucose still increases insulin secretion in a concentration-dependent manner. This increase in secretion is highly sensitive to glucose (produced by as little as 1-6 mmol/l glucose), requires glucose metabolism, is independent of activation of protein kinases A and C, and does not seem to implicate long-chain acyl-CoAs. Changes in adenine nucleotides may be involved. The amplification consists of an increase in efficacy of Ca2+ on exocytosis of insulin granules. There exists a clear hierarchy between both pathways. The triggering pathway predominates over the amplifying pathway, which remains functionally silent as long as [Ca2+]i has not been raised by the first pathway; i.e., as long as glucose has not reached its threshold concentration. The alteration of this hierarchy by long-acting sulfonylureas or genetic inactivation of K(ATP) channels may lead to inappropriate insulin secretion at low glucose. The amplifying pathway serves to optimize the secretory response not only to glucose but also to nonglucose stimuli. It is impaired in beta-cells of animal models of type 2 diabetes, and indirect evidence suggests that it is altered in beta-cells of type 2 diabetic patients. Besides the available drugs that act on K(ATP) channels and increase the triggering signal, novel drugs that correct a deficient amplifying pathway would be useful to restore adequate insulin secretion in type 2 diabetic patients.