The neuropeptide pituitary adenylate cyclase-activating polypeptide and islet function

Control of myeloid cell functions by nociceptors

The immune system has evolved to protect the host from infectious agents, parasites, and tumor growth, and to ensure the maintenance of homeostasis. Similarly, the primary function of the somatosensory branch of the peripheral nervous system is to collect and interpret sensory information about the environment, allowing the organism to react to or avoid situations that could otherwise have deleterious effects. Consequently, a teleological argument can be made that it is of advantage for the two systems to cooperate and form an “integrated defense system” that benefits from the unique strengths of both subsystems. Indeed, nociceptors, sensory neurons that detect noxious stimuli and elicit the sensation of pain or itch, exhibit potent immunomodulatory capabilities. Depending on the context and the cellular identity of their communication partners, nociceptors can play both pro- or anti-inflammatory roles, promote tissue repair or aggravate inflammatory damage, improve resistance to pathogens or impair their clearance. In light of such variability, it is not surprising that the full extent of interactions between nociceptors and the immune system remains to be established. Nonetheless, the field of peripheral neuroimmunology is advancing at a rapid pace, and general rules that appear to govern the outcomes of such neuroimmune interactions are beginning to emerge. Thus, in this review, we summarize our current understanding of the interaction between nociceptors and, specifically, the myeloid cells of the innate immune system, while pointing out some of the outstanding questions and unresolved controversies in the field. We focus on such interactions within the densely innervated barrier tissues, which can serve as points of entry for infectious agents and, where known, highlight the molecular mechanisms underlying these interactions.

The PACAP Paradox: Dynamic and Surprisingly Pleiotropic Actions in the Central Regulation of Energy Homeostasis

Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP), a pleiotropic neuropeptide, is widely distributed throughout the body. The abundance of PACAP expression in the central and peripheral nervous systems, and years of accompanying experimental evidence, indicates that PACAP plays crucial roles in diverse biological processes ranging from autonomic regulation to neuroprotection. In addition, PACAP is also abundantly expressed in the hypothalamic areas like the ventromedial and arcuate nuclei (VMN and ARC, respectively), as well as other brain regions such as the nucleus accumbens (NAc), bed nucleus of stria terminalis (BNST), and ventral tegmental area (VTA) - suggesting that PACAP is capable of regulating energy homeostasis via both the homeostatic and hedonic energy balance circuitries. The evidence gathered over the years has increased our appreciation for its function in controlling energy balance. Therefore, this review aims to further probe how the pleiotropic actions of PACAP in regulating energy homeostasis is influenced by sex and dynamic changes in energy status. We start with a general overview of energy homeostasis, and then introduce the integral components of the homeostatic and hedonic energy balance circuitries. Next, we discuss sex differences inherent to the regulation of energy homeostasis via these two circuitries, as well as the activational effects of sex steroid hormones that bring about these intrinsic disparities between males and females. Finally, we explore the multifaceted role of PACAP in regulating homeostatic and hedonic feeding through its actions in regions like the NAc, BNST, and in particular the ARC, VMN and VTA that occur in sex- and energy status-dependent ways.

Analysis of Half a Billion Datapoints Across Ten Machine-Learning Algorithms Identifies Key Elements Associated With Insulin Transcription in Human Pancreatic Islet Cells

Machine learning (ML)-workflows enable unprejudiced/robust evaluation of complex datasets. Here, we analyzed over 490,000,000 data points to compare 10 different ML-workflows in a large (N=11,652) training dataset of human pancreatic single-cell (sc-)transcriptomes to identify genes associated with the presence or absence of insulin transcript(s). Prediction accuracy/sensitivity of each ML-workflow was tested in a separate validation dataset (N=2,913). Ensemble ML-workflows, in particular Random Forest ML-algorithm delivered high predictive power (AUC=0.83) and sensitivity (0.98), compared to other algorithms. The transcripts identified through these analyses also demonstrated significant correlation with insulin in bulk RNA-seq data from human islets. The top-10 features, (including IAPP, ADCYAP1, LDHA and SST) common to the three Ensemble ML-workflows were significantly dysregulated in scRNA-seq datasets from Ire-1αβ-/- mice that demonstrate dedifferentiation of pancreatic β-cells in a model of type 1 diabetes (T1D) and in pancreatic single cells from individuals with type 2 Diabetes (T2D). Our findings provide direct comparison of ML-workflows in big data analyses, identify key elements associated with insulin transcription and provide workflows for future analyses.

Therapeutic potential of vasoactive intestinal peptide and its receptor VPAC2 in type 2 diabetes

Owing to the increasing prevalence of type 2 diabetes, the development of novel hypoglycemic drugs has become a research hotspot, with the ultimate goal of developing therapeutic drugs that stimulate glucose-induced insulin secretion without inducing hypoglycemia. Vasoactive intestinal peptide (VIP), a 28-amino-acid peptide, can stimulate glucose-dependent insulin secretion, particularly by binding to VPAC2 receptors. VIP also promotes islet β-cell proliferation through the forkhead box M1 pathway, but the specific molecular mechanism remains to be studied. The clinical application of VIP is limited because of its short half-life and wide distribution in the human body. Based on the binding properties of VIP and VPAC2 receptors, VPAC2-selective agonists have been developed to serve as novel hypoglycemic drugs. This review summarizes the physiological significance of VIP in glucose homeostasis and the potential therapeutic value of VPAC2-selective agonists in type 2 diabetes.

Pancreas-Brain Crosstalk

The neural regulation of glucose homeostasis in normal and challenged conditions involves the modulation of pancreatic islet-cell function. Compromising the pancreas innervation causes islet autoimmunity in type 1 diabetes and islet cell dysfunction in type 2 diabetes. However, despite the richly innervated nature of the pancreas, islet innervation remains ill-defined. Here, we review the neuroanatomical and humoral basis of the cross-talk between the endocrine pancreas and autonomic and sensory neurons. Identifying the neurocircuitry and neurochemistry of the neuro-insular network would provide clues to neuromodulation-based approaches for the prevention and treatment of diabetes and obesity.

Neurotransmitters and Neuropeptides: New Players in the Control of Islet of Langerhans' Cell Mass and Function

Islets of Langerhans control whole body glucose homeostasis, as they respond, releasing hormones, to changes in nutrient concentrations in the blood stream. The regulation of hormone secretion has been the focus of attention for a long time because it is related to many metabolic disorders, including diabetes mellitus. Endocrine cells of the islet use a sophisticate system of endocrine, paracrine and autocrine signals to synchronize their activities. These signals provide a fast and accurate control not only for hormone release but also for cell differentiation and survival, key aspects in islet physiology and pathology. Among the different categories of paracrine/autocrine signals, this review highlights the role of neurotransmitters and neuropeptides. In a manner similar to neurons, endocrine cells synthesize, accumulate, release neurotransmitters in the islet milieu, and possess receptors able to decode these signals. In this review, we provide a comprehensive description of neurotransmitter/neuropetide signaling pathways present within the islet. Then, we focus on evidence supporting the concept that neurotransmitters/neuropeptides and their receptors are interesting new targets to preserve β-cell function and mass. A greater understanding of how this network of signals works in physiological and pathological conditions would advance our knowledge of islet biology and physiology and uncover potentially new areas of pharmacological intervention. J. Cell. Physiol. 231: 756-767, 2016. © 2015 Wiley Periodicals, Inc.

Beta cell connectivity in pancreatic islets: a type 2 diabetes target?

Beta cell connectivity describes the phenomenon whereby the islet context improves insulin secretion by providing a three-dimensional platform for intercellular signaling processes. Thus, the precise flow of information through homotypically interconnected beta cells leads to the large-scale organization of hormone release activities, influencing cell responses to glucose and other secretagogues. Although a phenomenon whose importance has arguably been underappreciated in islet biology until recently, a growing number of studies suggest that such cell-cell communication is a fundamental property of this micro-organ. Hence, connectivity may plausibly be targeted by both environmental and genetic factors in type 2 diabetes mellitus (T2DM) to perturb normal beta cell function and insulin release. Here, we review the mechanisms that contribute to beta cell connectivity, discuss how these may fail during T2DM, and examine approaches to restore insulin secretion by boosting cell communication.

Pleiotropic mechanisms for the glucose-lowering action of DPP-4 inhibitors

Dipeptidyl peptidase (DPP)-4 inhibition is a glucose-lowering treatment for type 2 diabetes. The classical mechanism for DPP-4 inhibitors is that they inhibit DPP-4 activity in peripheral plasma, which prevents the inactivation of the incretin hormone glucagon-like peptide (GLP)-1 in the peripheral circulation. This in turn increases circulating intact GLP-1, which results in stimulated insulin secretion and inhibited glucagon secretion, in turn increasing glucose utilization and diminishing hepatic glucose production, which, through reduction in postprandial and fasting glucose, reduces HbA1c. However, recent experimental studies in mainly rodents but also to a limited degree in humans have found additional mechanisms for DPP-4 inhibitors that may contribute to their glucose-lowering action. These nonclassical mechanisms include 1) inhibition of gut DPP-4 activity, which prevents inactivation of newly released GLP-1, which in turn augments GLP-1-induced activations of autonomic nerves and results in high portal GLP-1 levels, resulting in inhibited glucose production through portal GLP-1 receptors; 2) inhibition of islet DPP-4 activity, which prevents inactivation of locally produced intact GLP-1 in the islets, thereby augmenting insulin secretion and inhibiting glucagon secretion and possibly preventing islet inflammation; and 3) prevention of the inactivation of other bioactive peptides apart from GLP-1, such as glucose-dependent insulinotropic polypeptide, stromal-derived factor-1α, and pituitary adenylate cyclase-activating polypeptide, which may improve islet function. These pleiotropic effects may contribute to the effects of DPP-4 inhibition. This Perspectives in Diabetes outlines and discusses these nonclassical mechanisms of DPP-4 inhibition.

Colonic delivery of docosahexaenoic acid improves impaired glucose tolerance via GLP-1 secretion and suppresses pancreatic islet hyperplasia in diabetic KK-A(y) mice

Glucagon-like peptide-1 (GLP-1) is an incretin hormone that regulates the insulin secretion depending on blood glucose level. Recent studies show that the unsaturated fatty acids can promote GLP-1 secretion from intestinal L-cells. We have shown previously that docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) administered into a mouse closed intestinal loop, especially into the colonic segment, stimulate GLP-1 and insulin secretion and have a hypoglycemic effect, suggesting that DHA and EPA have potential as antidiabetic agents. The present study examined the antidiabetic effect of DHA following long-term in vivo delivery to the colon using normal ddY and diabetic KK-A(y) mice. The plasma GLP-1 concentration of KK-A(y) mice increased after long-term DHA administration, and this had a significant hypoglycemic effect. In contrast, although GLP-1 secretion in ddY mice tended to increase after DHA administration, blood glucose concentration did not differ between vehicle- and DHA-treated ddY mice. Immunostaining of the pancreas after long-term DHA administration showed that continuous DHA treatment stimulated β-cell apoptosis and accordingly suppressed islet cell growth in KK-A(y) mice. Colon targeting of DHA may provide a new strategy for improving impaired glucose tolerance in type 2 diabetes mellitus by stimulating GLP-1 secretion, which may subsequently suppress the compensatory hyperplasia of pancreatic islets.

Cyclic AMP dynamics in the pancreatic β-cell

Insulin secretion from pancreatic β-cells is tightly regulated by glucose and other nutrients, hormones, and neural factors. The exocytosis of insulin granules is triggered by an elevation of the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) and is further amplified by cyclic AMP (cAMP). Cyclic AMP is formed primarily in response to glucoincretin hormones and other G(s)-coupled receptor agonists, but generation of the nucleotide is critical also for an optimal insulin secretory response to glucose. Nutrient and receptor stimuli trigger oscillations of the cAMP concentration in β-cells. The oscillations arise from variations in adenylyl cyclase-mediated cAMP production and phosphodiesterase-mediated degradation, processes controlled by factors like cell metabolism and [Ca(2+)](i). Protein kinase A and the guanine nucleotide exchange factor Epac2 mediate the actions of cAMP in β-cells and operate at multiple levels to promote exocytosis and pulsatile insulin secretion. The cAMP signaling system contains important targets for pharmacological improvement of insulin secretion in type 2 diabetes.

Trophic effects of PACAP on pancreatic islets: a mini-review

Progressive beta-cell insufficiency in the pancreas is a hallmark of both types I and II diabetes, and agents that protect against beta-cell dysfunction are potential drug targets for diabetes mellitus. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a strong secretagogue of insulin from pancreatic islets and is suggested to be involved in physiological blood glucose homeostasis and the pathology of diabetes. Recent studies in genetically engineered animal models have shown that PACAP stimulates pancreatic functions, especially in cooperation with other regulatory factors including glucose. Furthermore, chronic activation of PACAP signaling regulates pancreatic islet mass in a context-dependent manner. Accumulating in vivo and in vitro evidence suggest that PACAP has trophic effects and regulates both proliferation and cell viability of beta-cells and thereby contributes to protection against diabetes. This review focuses on such trophic actions of PACAP on pancreatic beta-cells and discusses the pathophysiological significance of pancreatic PACAP, with the aim to provide information for future development of treatment for diabetes.

A novel cyclopeptide from the cyclization of PACAP(1-5) with potent activity towards PAC1 attenuates STZ-induced diabetes

The N-terminal deletion of pituitary adenylate cyclase-activating polypeptide (PACAP)(1-5) generates its own antagonist. The cyclopeptide C*HSDGIC*, which results from the cyclization of PACAP(1-5) with disulfide, was designed and synthesized. CHO cells expressing a PAC1 N/R splice variant (PAC1-CHO) were used to detect the potent activation of PAC1 by C*HSDGIC*. In vitro cell assays showed that C*HSDGIC* stimulated cAMP production and increased the viability of PAC1-CHO cells at micromolar concentrations, about 1000 fold that of PACAP. PACAP(6-38) blocked the effects of PACAP on the proliferation of PAC1-CHO cells but did not interfere with the effects of C*HSDGIC*, suggesting that the activation of PAC1 by C*HSDGIC* was independent of the binding of PAC1 to the C-terminus of PACAP. In experiments in vivo, 10mumol/kg C*HSDGIC* decreased the plasma glucose level, increased the plasma insulin level and improved glucose tolerance significantly (P<0.01) when co-injected with STZ for 5 days. The results of these in vitro and in vivo studies of the biological characteristics of C*HSDGIC* reveal that it is a potent activator of PAC1.

Effect of pituitary adenylate cyclase-activating polypeptide in islet transplantation

INTRODUCTION

Pituitary adenylate cyclase-activating polypeptide (PACAP) is an islet substance serving as an intra-islet amplifier of glucose-induced insulin secretion similar to exendin-4. It has been reported that systemic administration of PACAP maintained beta-cell mass, delayed the onset of hyperglycemia, and protected beta cells from glucose toxicity. Moreover, PACAP increases glucose-stimulated insulin release in vitro and in vivo. In this study, we investigated the possibility of PACAP use in human islet transplantation.

METHODS

Human islets were cultured in the presence or absence of PACAP (10(-12) mol/L) for 48 hours. We assessed beta-cell viability using FACS, cellular composition analysis by iCys/LSC, and glucose-stimulated insulin secretion. In vivo, islets were transplanted beneath the kidney capsule of Streptozotocin-induced diabetic immunodeficient mice. An intravenous glucose tolerance test (IVGTT) was also performed in the presence or absence of PACAP (Peptide International, Louisville, Ky, United States; 1.3 nmol/kg).

RESULTS

There were significant improvements in terms of beta-cell viability and cellular composition between islets cultured with or without PACAP, respectively (P < .05). Moreover, glucose-stimulated insulin secretion significantly improved in islets cultured with PACAP compared with controls, respectively (P < .05). Treatment of recipient mice with PACAP resulted in beneficial effects on insulin secretion (PACAP vs control, 13.2 vs 1.9 mU/L), in IVGTT. However, no significant difference was observed in glucose levels between the 2 groups.

CONCLUSIONS

Our study indicated that PACAP significantly improved beta-cell viability and survival during culture, and increased insulin secretion in vitro and in vivo. However, blood glucose levels in vivo after an IVGTT did not significantly improve, probably due to increased glucagon secretion from alpha cells. PACAP supplementation to culture medium could be of assistance to improve clinical islet transplantation outcomes.

Islet G protein-coupled receptors as potential targets for treatment of type 2 diabetes

Islet dysfunction - characterized by a combination of defective insulin secretion, inappropriately high glucagon secretion and reduced beta-cell mass - has a central role in the pathophysiology of type 2 diabetes. Several G protein-coupled receptors (GPCRs) expressed in islet beta-cells are known to be involved in the regulation of islet function, and therefore are potential therapeutic targets. This is evident from the recent success of glucagon-like peptide 1 (GLP1) mimetics and dipeptidyl peptidase 4 (DPP4) inhibitors, which promote activation of the GLP1 receptor to stimulate insulin secretion and inhibit glucagon secretion, and also have the potential to increase beta-cell mass. Other islet beta-cell GPCRs that are involved in the regulation of islet function include the glucose-dependent insulinotropic peptide (GIP) receptor, lipid GPCRs, pleiotropic peptide GPCRs and islet biogenic amine GPCRs. This Review summarizes islet GPCR expression, signalling and function, and highlights their potential as targets for the treatment of type 2 diabetes.

Role of pituitary adenylate cyclase-activating polypeptide in the pancreatic endocrine system

In the pancreatic islets, pituitary adenylate cyclase-activating polypeptide (PACAP) is expressed in beta cells and autonomic nerve terminals; the majority of these nerve terminals are parasympathetic. PACAP binds to three types of G protein-coupled receptors (GPCRs): VPAC1 receptors, VPAC2 receptors, and PAC1 receptors. All these receptor types are expressed in pancreatic islets. PACAP stimulates insulin and glucagon secretion. These actions are achieved in part through increased formation of cAMP after activation of adenylate cyclase and in part through increase in cytosolic calcium, achieved through increase in calcium uptake and release from intracellular calcium stores. Deletion of PAC1 receptors or VPAC2 receptors results in impaired insulin secretion and glucose intolerance. Studies in PAC1 receptor gene deleted mice have suggested that PACAP may be of physiological importance in mediating prandial insulin secretion and in contributing to the glucagon response to hypoglycemia. Animal studies have also suggested that activation of the receptors, in particular VPAC2 receptors, may be used as a therapeutic approach for the treatment of type 2 diabetes. Hence, PACAP is an islet neuropeptide with a potential role in islet physiology and as a basis for development of islet-promoting therapy in type 2 diabetes.

Feeding and metabolism in mice lacking pituitary adenylate cyclase-activating polypeptide

Disruption of the pituitary adenylate cyclase-activating polypeptide (PACAP) gene in mice has demonstrated a role for this highly conserved neuropeptide in the regulation of metabolism and temperature control. Localization of PACAP neurons within hypothalamic nuclei that regulate appetite suggest PACAP may affect feeding and thus energy balance. We used PACAP-null mice to address this question, examining both food intake and energy expenditure. PACAP-null mice were leaner than wild-type littermates due to decreased adiposity and displayed increased insulin sensitivity. The lean phenotype in the PACAP-null mice was completely eliminated if animals were fed a high-fat diet or housed near thermoneutrality (28 C). Further metabolic analyses of PACAP-null mice housed at 21 C indicated that the reduced body weight could not be explained by decreased food intake, increased metabolic rate, or increased locomotor activity. The thyroid hormone axis of PACAP-null mice was affected, because mRNA levels of hypothalamic TRH and brown adipose tissue type 2 deiodinase were reduced in PACAP-null mice housed at room temperature, and brain deiodinase activity was lower in PACAP-null mice after an acute cold challenge compared with wild-type controls. These results demonstrate that PACAP is not required for the regulation of food intake yet is necessary to maintain normal energy homeostasis, likely playing a role in central cold-sensing mechanisms.

Genome-wide search for susceptibility genes to type 2 diabetes in West Africans: potential role of C-peptide

C-peptide is a substance that the pancreas releases into the circulation in equimolar amounts to insulin and has demonstrated important physiological effects which relate to the vascular field, in particular the microcirculation. For this analysis, we included 321 full and 36 half sibling pairs affected with type 2 diabetes (T2D) from West Africa. A genome-wide panel of 390 tri-nucleotide and tetra-nucleotide repeats with an average distance of 8.9 cM was performed on a total of 691 persons. Variance components based on multipoint linkage approach as implemented in SOLAR were performed for log C-peptide. Significant linkage evidences were observed on 10q23 at D10S2327 with a LOD score of 4.04 (nominal p-value=0.000008, empirical p-value=0.0004); and on 4p15 at D4S2632 with a LOD score of 3.48 (nominal p-value=0.000031, empirical p-value=0.0013). Other suggestive evidence of linkage were observed on 15q14 at D15S659 with a LOD score 2.41 (nominal p-value=0.000435, empirical p-value=0.0068), and on 18p11 near D18S976 with a LOD score 2.18 (nominal p-value=0.000771 and empirical p-value=0.0094). Interestingly, five positional candidate genes for diabetes and related complications are located in our linkage region (the pituitary adenylate cyclase activating polypeptide (PACAP in 18p11); the peroxisome proliferator-activated receptor gamma coactivator 1 (PPARGC1 in 4p15); PTEN, PPP1R5, and IDE located in 10q23. In conclusion, we identified four major genetic loci (10q23, 4p15, 15q14, and 18p11) influencing C-peptide concentration in West Africans with T2D.

Knocked down and out: PACAP in development, reproduction and feeding

One approach to understanding the role of PACAP in vivo is to knockdown the translation of PACAP mRNA to protein or to knock out the PACAP gene by targeted disruption. In this paper, we review the effect of PACAP knockdown with morpholinos on early brain development in zebrafish. Also reviewed is the role of PACAP at several stages of reproduction as assessed in mice with a disrupted PACAP gene. New data are presented to analyze PACAP's action in energy homeostasis (body mass, food intake, endocrine parameters) using female PACAP-null mice. The evidence suggests PACAP is important for brain development in zebrafish and is required for normal reproduction, but not for body mass or food intake in mice maintained near thermoneutrality.

Role of VIP and PACAP in islet function

Vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are two closely related neuropeptides that are expressed in islets and in islet parasympathetic nerves. Both peptides bind to their common G-protein-coupled receptors, VPAC1 and VPAC2, and PACAP, in addition to the specific receptor PAC1, all three of which are expressed in islets. VIP and PACAP stimulate insulin secretion in a glucose-dependent manner and they both also stimulate glucagon secretion. This action is achieved through increased formation of cAMP after activation of adenylate cyclase and stimulation of extracellular calcium uptake. Deletion of PAC1 receptors or VPAC2 receptors results in glucose intolerance. These peptides may be of importance in mediating prandial insulin secretion and the glucagon response to hypoglycemia. Animal studies have also suggested that activation of the receptors, in particular VPAC2 receptors, may be used as a therapeutic approach for the treatment of type 2 diabetes. This review summarizes the current knowledge of the potential role of VIP and PACAP in islet function.

G-protein-coupled receptors and islet function-implications for treatment of type 2 diabetes

Islet function is regulated by a number of different signals. A main signal is generated by glucose, which stimulates insulin secretion and inhibits glucagon secretion. The glucose effects are modulated by many factors, including hormones, neurotransmitters and nutrients. Several of these factors signal through guanine nucleotide-binding protein (G protein)-coupled receptors (GPCR). Examples of islet GPCR are GPR40 and GPR119, which are GPCR with fatty acids as ligands, the receptors for the incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), the receptors for the islet hormones glucagon and somatostatin, the receptors for the classical neurotransmittors acetylcholine (ACh; M(3) muscarinic receptors) and noradrenaline (beta(2)- and alpha(2)-adrenoceptors) and for the neuropeptides pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP; PAC(1) and VPAC(2) receptors), cholecystokinin (CCK(A) receptors) and neuropeptide Y (NPY Y1 receptors). Other islet GPCR are the cannabinoid receptor (CB(1) receptors), the vasopressin receptors (V1(B) receptors) and the purinergic receptors (P(2Y) receptors). The islet GPCR couple mainly to adenylate cyclase and to phospholipase C (PLC). Since important pharmacological strategies for treatment of type 2 diabetes are stimulation of insulin secretion and inhibition of glucagon secretion, islet GPCR are potential drug targets. This review summarizes knowledge on islet GPCR.

Expression of islet inducible nitric oxide synthase and inhibition of glucose-stimulated insulin release after long-term lipid infusion in the rat is counteracted by PACAP27

Chronic exposure of pancreatic islets to elevated plasma lipids (lipotoxicity) can lead to beta-cell dysfunction, with overtime becoming irreversible. We examined, by confocal microscopy and biochemistry, whether the expression of islet inducible nitric oxide synthase (iNOS) and the concomitant inhibition of glucose-stimulated insulin release seen after lipid infusion in rats was modulated by the islet neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP)27. Lipid infusion for 8 days induced a strong expression of islet iNOS, which was mainly confined to beta-cells and was still evident after incubating islets at 8.3 mmol/l glucose. This was accompanied by a high iNOS-derived NO generation, a decreased insulin release, and increased cyclic GMP accumulation. No iNOS expression was found in control islets. Addition of PACAP27 to incubated islets from lipid-infused rats resulted in loss of iNOS protein expression, increased cyclic AMP, decreased cyclic GMP, and suppression of the activities of neuronal constitutive (nc)NOS and iNOS and increased glucose-stimulated insulin response. These effects were reversed by the PKA inhibitor H-89. The suppression of islet iNOS expression induced by PACAP27 was not affected by the proteasome inhibitor MG-132, which by itself induced the loss of iNOS protein, making a direct proteasomal involvement less likely. Our results suggest that PACAP27 through its cyclic AMP- and PKA-stimulating capacity strongly suppresses not only ncNOS but, importantly, also the lipid-induced stimulation of iNOS expression, possibly by a nonproteasomal mechanism. Thus PACAP27 restores the impairment of glucose-stimulated insulin release and additionally might induce cytoprotection against deleterious actions of iNOS-derived NO in beta-cells.

beta-cell failure in diabetes and preservation by clinical treatment

There is a progressive deterioration in beta-cell function and mass in type 2 diabetics. It was found that islet function was about 50% of normal at the time of diagnosis, and a reduction in beta-cell mass of about 60% was shown at necropsy. The reduction of beta-cell mass is attributable to accelerated apoptosis. The major factors for progressive loss of beta-cell function and mass are glucotoxicity, lipotoxicity, proinflammatory cytokines, leptin, and islet cell amyloid. Impaired beta-cell function and possibly beta-cell mass appear to be reversible, particularly at early stages of the disease where the limiting threshold for reversibility of decreased beta-cell mass has probably not been passed. Among the interventions to preserve or "rejuvenate" beta-cells, short-term intensive insulin therapy of newly diagnosed type 2 diabetes will improve beta-cell function, usually leading to a temporary remission time. Another intervention is the induction of beta-cell "rest" by selective activation of ATP-sensitive K+ (K(ATP)) channels, using drugs such as diazoxide. A third type of intervention is the use of antiapoptotic drugs, such as the thiazolidinediones (TZDs), and incretin mimetics and enhancers, which have demonstrated significant clinical evidence of effects on human beta-cell function. The TZDs improve insulin secretory capacity, decrease beta-cell apoptosis, and reduce islet cell amyloid with maintenance of neogenesis. The TZDs have indirect effects on beta-cells by being insulin sensitizers. The direct effects are via peroxisome proliferator-activated receptor gamma activation in pancreatic islets, with TZDs consistently improving basal beta-cell function. These beneficial effects are sustained in some individuals with time. There are several trials on prevention of diabetes with TZDs. Incretin hormones, which are released from the gastrointestinal tract in response to nutrient ingestion to enhance glucose-dependent insulin secretion from the pancreas, aid the overall maintenance of glucose homeostasis through slowing of gastric emptying, inhibition of glucagon secretion, and control of body weight. From the two major incretins, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), only the first one or its mimetics or enhancers can be used for treatment because the diabetic beta-cell is resistant to GIP action. Because of the rapid inactivation of GLP-1 by dipeptidyl peptidase (DPP)-IV, several incretin analogs were developed: GLP-1 receptor agonists (incretin mimetics) exenatide (synthetic exendin-4) and liraglutide, by conjugation of GLP-1 to circulating albumin. The acute effect of GLP-1 and GLP-1 receptor agonists on beta-cells is stimulation of glucose-dependent insulin release, followed by enhancement of insulin biosynthesis and stimulation of insulin gene transcription. The chronic action is stimulating beta-cell proliferation, induction of islet neogenesis, and inhibition of beta-cell apoptosis, thus promoting expansion of beta-cell mass, as observed in rodent diabetes and in cultured beta-cells. Exenatide and liraglutide enhanced postprandial beta-cell function. The inhibition of the activity of the DPP-IV enzyme enhances endogenous GLP-1 action in vivo, mediated not only by GLP-1 but also by other mediators. In preclinical studies, oral active DPP-IV inhibitors (sitagliptin and vildagliptin) also promoted beta-cell proliferation, neogenesis, and inhibition of apoptosis in rodents. Meal tolerance tests showed improvement in postprandial beta-cell function. Obviously, it is difficult to estimate the protective effects of incretin mimetics and enhancers on beta-cells in humans, and there is no clinical evidence that these drugs really have protective effects on beta-cells.

Uncovering conserved patterns in bioactive peptides in Metazoa

Bioactive (neuro)peptides play critical roles in regulating most biological processes in animals. Peptides belonging to the same family are characterized by a typical sequence pattern that is conserved among the family's peptide members. Such a conserved pattern or motif usually corresponds to the functionally important part of the biologically active peptide. In this paper, all known bioactive (neuro)peptides annotated in Swiss-Prot and TrEMBL protein databases are collected, and the pattern searching program Pratt is used to search these unaligned peptide sequences for conserved patterns. The obtained patterns are then refined by combining the information on amino acids at important functional sites collected from the literature. All the identified patterns are further tested by scanning them against Swiss-Prot and TrEMBL protein databases. The diagnostic power of each pattern is validated by the fact that any annotated protein from Swiss-Prot and TrEMBL that contains one of the established patterns, is indeed a known (neuro)peptide precursor. We discovered 155 novel peptide patterns in addition to the 56 established ones in the PROSITE database. All the patterns cover 110 peptide families. Fifty-five of these families are not characterized by the PROSITE signatures, and 12 are also not identified by other existing motif databases, such as Pfam and SMART. Using the newly identified peptide signatures as a search tool, we predicted 95 hypothetical proteins as putative peptide precursors.

Neuropeptide signaling and hydrocephalus: SCO with the flow

Congenital hydrocephalus affects 0.1-0.3% of live births, with a high mortality rate (approximately 50%) in the absence of surgical intervention. Although the insertion of shunts alleviates the symptoms of the majority of congenital cases, the molecular basis of hydrocephalus and the mechanisms of cerebrospinal fluid (CSF) circulation remain largely unknown. Two important players are the subcommissural organ/Reissner's fiber (SCO/RF) complex and the ventricular ependymal (vel) cells that together facilitate the flow of the CSF through the narrow canals of the ventricular system. In this issue of the JCI, Lang et al. demonstrate that overexpression of the pituitary adenylate cyclase-activating polypeptide (PACAP) type I (PAC1) receptor gene results in abnormal development of the SCO and vel cells, leading to congenital hydrocephalus (see the related article beginning on page 1924). The ligand for the PAC1 receptor is the neuropeptide PACAP, which uncovers what the authors believe to be a novel role for this signaling cascade in the regulation of CSF circulation.

Glucagon-like peptide-1-based therapies for the treatment of type 2 diabetes mellitus

The 'incretin effect' describes the phenomenon of an enhanced insulin response following oral ingestion of glucose compared with that after intravenous administration of glucose, leading to identical postprandial plasma glucose excursions. It accounts for up to 60% of the postprandial insulin secretion, but is diminished in patients with type 2 diabetes mellitus. Gastrointestinal hormones that promote the incretin effect are called incretins. Glucagon-like peptide-1 (GLP-1) is an important incretin. Under hyperglycemic conditions in humans, it stimulates insulin secretion and normalizes blood glucose levels. GLP-1 does not stimulate insulin secretion at normal glucose levels; therefore, it does not cause hypoglycemia. Furthermore, it inhibits glucagon secretion and delays gastric emptying. In vitro and animal data have demonstrated that GLP-1 increases beta-cell mass by stimulating islet cell neogenesis and by inhibiting the apoptosis of islet cells. The improvement of beta-cell function due to GLP-1 can be indirectly observed from the increased insulin secretory capacity of humans receiving such treatment. GLP-1 may represent an attractive therapeutic method for patients with type 2 diabetes because of its multiple effects, including the simulation of satiety in the CNS by acting as a transmitter or by crossing the blood brain barrier. Native GLP-1 is degraded rapidly upon intravenous or subcutaneous administration and is therefore not feasible for routine therapy. Long-acting GLP-1 analogs (e.g. liraglutide) and exendin-4 (exenatide) that are resistant to degradation, called 'incretin mimetics', are being investigated in clinical trials. Dipeptidyl peptidase-IV inhibitors (e.g. vildagliptin, sitagliptin, and saxagliptin) that inhibit the enzyme responsible for incretin degradation are also being studied.

The insulin response to gastric glucose is reduced in PAC1 and GRP receptor gene deleted mice

BACKGROUND AND AIMS

Islet function is regulated by islet autonomic nerves. These nerves harbour not only the classical neurotransmitters, acetyl choline and noradrenaline, but also neuropeptides. This study examined whether the neuropeptides, pituitary adenylate cyclase activating polypeptide (PACAP) and gastrin releasing polypeptide (GRP) contribute to the regulation of insulin secretion in model experiments by using receptor gene deleted mice.

METHODS

Anaesthetized mice with genetic deletion of one of the PACAP receptors (PAC1 receptors) or one of the GRP receptors (GRP receptor) or their wildtype counterparts were given glucose through a gastric gavage (150 mg) or intravenously (0.25, 0.50 or 1 g/kg). Blood samples were taken regularly during the following 120 min (after gastric glucose) or at 1 min (after intravenous glucose) for analysis of glucose and insulin.

RESULTS

The insulin response to gastric glucose was suppressed by 66% in PAC1 receptor gene deleted mice in association with impaired glucose elimination, whereas the insulin response to intravenous glucose was impaired by 36% only. The insulin response to glucose was suppressed in GRP receptor gene deleted mice by 24% together with impaired glucose elimination, whereas the insulin response to intravenous glucose was not significantly suppressed.

CONCLUSIONS

The insulin responses to gastric versus intravenous glucose in receptor gene deleted mice show that PACAP, and to a lesser extent GRP, contributes to the insulin response to gastric administration of glucose.

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

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

Dipeptidyl peptidase IV inhibitors: a promising new therapeutic approach for the management of type 2 diabetes

Glucagon-like peptide-1 is an insulinotropic hormone with antidiabetic potential due to its spectrum of effects, which include glucose-dependent stimulation of insulin and inhibition of glucagon secretion, tropic effects on the pancreatic beta-cells, inhibition of gastric emptying and the reduction of appetite. Glucagon-like peptide-1 is, however, extremely rapidly inactivated by the serine peptidase, dipeptidyl peptidase IV, so that the native peptide is not useful clinically. A new approach to utilise the beneficial effects of glucagon-like peptide-1 in the treatment of type 2 diabetes has been the development of orally active dipeptidyl peptidase IV inhibitors. Preclinical studies have demonstrated that this approach is effective in enhancing endogenous levels of glucagon-like peptide-1, resulting in improved glucose tolerance in glucose-intolerant and diabetic animal models. In recent studies of 3-12 months duration in patients with type 2 diabetes, dipeptidyl peptidase IV inhibitors have proved efficacious, both as monotherapy and when given in combination with metformin. Fasting and postprandial glucose concentrations were reduced, leading to reductions in glycosylated haemoglobin levels, while beta-cell function was preserved. Current information suggests dipeptidyl peptidase IV inhibitors are body weight neutral and are well tolerated. A number of dipeptidyl peptidase IV inhibitors are now in the late stages of clinical development. These have different properties, in terms of their duration of action and anticipated dosing frequency, but data from protracted dosing studies is presently not available to allow comparison of their clinical efficacy.

New therapeutic strategies for the treatment of type 2 diabetes mellitus based on incretins

Orally ingested glucose leads to a greater insulin response compared to intravenously administered glucose leading to identical postprandial plasma glucose excursions, a phenomenon referred to as the "incretin effect". The incretin effect comprises up to 60% of the postprandial insulin secretion and is diminished in type 2 diabetes. One of the very important gastrointestinal hormones promoting this effect is glucagon-like peptide 1 (GLP-1). It only stimulates insulin secretion and normalizes blood glucose in humans under hyperglycemic conditions, therefore it does not cause hypoglycemia. Other important physiological actions of GLP-1 are the inhibition of glucagon secretion and gastric emptying. It further acts as a neurotransmitter in the hypothalamus stimulating satiety. In vitro and animal data demonstrated that GLP-1 increases beta-cell mass by stimulating islet cell neogenesis and by inhibiting apoptosis of islets. In humans, the improvement of beta-cell function can be indirectly observed from the increased insulin secretory capacity after GLP-1 infusions. GLP-1 represents an attractive therapeutic principle for type 2 diabetes. However, native GLP-1 is degraded rapidly upon exogenous administration and is therefore not feasible for routine therapy. The first long-acting GLP-1 analog ("incretin mimetic") Exenatide (Byetta) has just been approved for type 2 diabetes therapy. Other compounds are being investigated in clinical trials (e.g. liraglutide, CJC1131). Dipeptidyl-peptidase IV inhibitors (DPP-IV inhibitors; e.g. Vildagliptin, Sitagliptin) that inhibit the enzyme responsible for incretin degradation are also under study.

Systemic administration of pituitary adenylate cyclase-activating polypeptide maintains beta-cell mass and retards onset of hyperglycaemia in beta-cell-specific calmodulin-overexpressing transgenic mice

OBJECTIVE

Pituitary adenylate cyclase-activating polypeptide (PACAP) has been shown to play an important role in the regulation of islet function. We investigated its effects in beta-cell-specific calmodulin-overexpressing diabetic (CaMTg) mice, in which we consider that apoptosis of beta cells is the primary defect leading to basal hyperglycaemia.

METHODS

CaMTg mice were treated with continuous s.c. infusions of PACAP from 2 to 4 weeks after birth, and were evaluated against littermate non-transgenic (nTg) and saline-treated CaMTg mice as to plasma glucose levels, insulin content, islet function and morphological features.

RESULTS

Remarkable and progressive hyperglycaemia was observed in CaMTg mice, and PACAP treatment blunted this elevation. Insulin secretion from isolated islets demonstrated an impaired response to glucose in CaMTg mice, and PACAP treatment did not cause any improvement. The total pancreatic insulin content in CaMTg mice decreased significantly to 19.1% of that in nTg mice. PACAP treatment of CaMTg mice increased the content to 158% of the value in saline-treated CaMTg mice. The insulin content in isolated islets from CaMTg mice also decreased to 15.9% of that in nTg mice, while PACAP treatment caused a doubling of the value. Immunohistochemical investigation revealed that the insulin-positive islet area was markedly smaller in CaMTg mice and that PACAP treatment significantly expanded the insulin-positive islet area.

CONCLUSIONS

These findings indicate that PACAP treatment retards the onset of hyperglycaemia in CaMTg mice by maintaining beta-cell mass and PACAP treatment may potentially be a therapeutic measure for preventing beta-cell exhaustion during hyperglycaemia.

Therapeutic assessment of glucagon-like peptide-1 agonists compared with dipeptidyl peptidase IV inhibitors as potential antidiabetic drugs

The most prevalent form of diabetes is non-insulin-dependent or Type 2 diabetes. Innovative strategies to enhance insulin secretion and thereby improve glucose tolerance in patients with this type of diabetes are currently under preclinical and clinical investigation. These therapies include the applications of incretin hormones; gut hormones released postprandially that stimulate insulin secretion in pancreatic beta-cells. Because incretin actions are rapidly terminated by N-terminal cleavage of these peptide hormones by the amino-peptidase dipeptidyl peptidase IV (DPP IV, CD26), the utility of DPP IV inhibitors for the treatment of Type 2 diabetes is also under investigation. This review compares the therapeutic potential and possible side effects of metabolically stable analogues/peptide agonists of the incretin glucagon-like peptide-1 (GLP-1) with the application of DPP IV inhibitors that reduce the rate of endogenous degradation of GLP-1 and other incretins. GLP-1 analogues have been shown to be highly efficacious in the treatment of Type 2 diabetes, with minimal side effects. Of particular importance is the fact that they do not induce hypoglycaemia. However, they are currently available only in an injectable form. In contrast, DPP IV inhibitors have the clear advantage of oral application resulting in better patient compliance. Furthermore, they also potentiate the actions of other incretins normally degraded by the action of DPP IV. However, they possess more potential side effects. Taken together, both approaches offer promising new drugs for the treatment of Type 2 diabetes.

Cytosolic Ca2+ responses to sub-picomolar and nanomolar PACAP in pancreatic beta-cells are mediated by VPAC2 and PAC1 receptors

Pituitary adenylate cyclase-activating polypeptide (PACAP) potentiates glucose-induced insulin release and increases cytosolic Ca2+ concentration ([Ca2+]i) in islet beta-cells in a concentration-dependent manner with two peaks at 10(-13) and 10(-9) M. PAC1 receptor (PAC1-R) and VPAC2 receptor (VPAC2-R) are expressed in pancreatic beta-cells and thought to be involved in insulin release. We aimed to determine the receptor types involved in the [Ca2+]i responses to 10(-13) and 10(-9) M PACAP. We measured [Ca2+]i in beta-cells and examined comparative effects of PAC1-R-selective agonist maxadilan, its antagonist M65, VPAC2-R-selective agonist Ro25-1553, and native ligands of PACAP and VIP. In the presence of 8.3 mM glucose, maxadilan, Ro25-1553, PACAP, and VIP at 10(-13) and 10(-9) M all increased [Ca2+]i. PACAP and maxadilan elicited greater effects at 10(-9) M than at 10(-13) M both in the incidence and amplitude of [Ca2+]i responses. For VIP and Ro25-1553, in contrast, the effects at 10(-9) and 10(-13) M were comparable. Furthermore, the amplitude of [Ca2+]i responses to 10(-9) M PACAP, but not 10(-13) M PACAP, was suppressed by M65. The results suggest that VPAC2-R and PAC1-R contribute equally to [Ca2+]i responses to sub-picomolar concentrations of PACAP, while PAC1-R has greater contribution to [Ca2+]i responses to nanomolar concentrations of this peptide.

Targeting beta-cell cyclic 3'5' adenosine monophosphate for the development of novel drugs for treating type 2 diabetes mellitus. A review

Cyclic 3'5'AMP is an important physiological amplifier of glucose-induced insulin secretion by the pancreatic islet beta-cell, where it is formed by the activity of adenylyl cyclase, especially in response to the incretin hormones GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic peptide). These hormones are secreted from the small intestine during and following a meal, and are important in producing a full insulin secretory response to nutrient stimuli. Cyclic AMP influences many steps involved in glucose-induced insulin secretion and may be important in regulating pancreatic islet beta-cell differentiation, growth and survival. Cyclic AMP (cAMP) itself is rapidly degraded in the pancreatic islet beta-cell by cyclic nucleotide phosphodiesterase (PDE) enzymes. This review discusses the possibility of targeting cAMP mechanisms in the treatment of type 2 diabetes mellitus, in which insulin release in response to glucose is impaired. This could be achieved by the use of GLP-1 or GIP to elevate cAMP in the pancreatic islet beta-cell. However, these peptides are normally rapidly degraded by dipeptidyl peptidase IV (DPP IV). Thus longer-acting analogues of GLP-1 and GIP, resistant to enzymic degradation, and orally active inhibitors of DPP IV have also been developed, and these agents were found to improve metabolic control in experimentally diabetic animals and in patients with type 2 diabetes. The use of selective inhibitors of type 3 phosphodiesterase (PDE3B), which is probably the important pancreatic islet beta-cell PDE isoform, would require their targeting to the islet beta-cell, because inhibition of PDE3B in adipocytes and hepatocytes would induce insulin resistance.

Milrinone efficiently potentiates insulin secretion induced by orally but not intravenously administered glucose in C57BL6J mice

To study the effect of phosphodiesterase (PDE) 3 inhibition on plasma insulin and glucose levels, the selective PDE 3 inhibitor milrinone (0.25, 1.0, and 2.5 mg/kg) was given orally to anesthetized CL57Bl/6J mice 10 min before a gastric glucose gavage (150 mg/mouse). It was found that milrinone augmented the glucose-mediated increase in plasma insulin at 1.0 and 2.5 mg/kg without, however, any improvement in glucose elimination. In contrast, when given 10 min before intravenous glucose (1 g/kg), milrinone (1 mg/kg) did not affect the insulin response to glucose. The increase in glucagon-like peptide-1 (GLP-1) levels after gastric glucose was not altered by milrinone. However, the PDE3 inhibitor augmented the insulin response to intravenous GLP-1 (2.8 nmol/kg). We therefore conclude that PDE3 inhibition by milrinone augments insulin secretion in vivo in mice after oral but not after intravenous glucose, which may be explained by enhanced response to the cAMP-dependent insulinotropic action of endogenously released GLP-1.

Inhibitors of dipeptidyl peptidase IV: a novel approach for the prevention and treatment of Type 2 diabetes?

Inhibitors of the enzyme dipeptidyl peptidase IV (DPP IV) are of increasing interest to both diabetologists and the pharmaceutical industry alike, as they may become established as the next member of the oral antidiabetic class of therapeutic agents, designed to lower blood glucose and, possibly, prevent the progressive impairment of glucose metabolism in patients with impaired glucose tolerance and Type 2 diabetes. DPP IV has become a focus of attention for drug design, as it has a pivotal role in the rapid degradation of at least two of the hormones released during food ingestion, a property that has warranted the design of inhibitor-based drugs. At the molecular level, DPP IV cleaves two amino acids from the N-terminus of the intact, biologically active forms of both so-called incretin hormones, glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide (formerly known as gastric inhibitory polypeptide), resulting in truncated metabolites, which are largely inactive. Inhibition of the enzyme, therefore, is thought to increase levels of the active forms of both incretin hormones, culminating in an increase in insulin release after a meal, in a fully glucose-dependent manner. DPP IV inhibitors combine several features of interest to the drug design process. They can be readily optimised for their target and be designed as low molecular weight, orally active entities compatible with once-daily administration.

Pathways in beta-cell stimulus-secretion coupling as targets for therapeutic insulin secretagogues

Physiologically, insulin secretion is subject to a dual, hierarchal control by triggering and amplifying pathways. By closing ATP-sensitive K+ channels (KATP channels) in the plasma membrane, glucose and other metabolized nutrients depolarize beta-cells, stimulate Ca2+ influx, and increase the cytosolic concentration of free Ca2+ ([Ca2+]i), which constitutes the indispensable triggering signal to induce exocytosis of insulin granules. The increase in beta-cell metabolism also generates amplifying signals that augment the efficacy of Ca2+ on the exocytotic machinery. Stimulatory hormones and neurotransmitters modestly increase the triggering signal and strongly activate amplifying pathways biochemically distinct from that set into operation by nutrients. Many drugs can increase insulin secretion in vitro, but only few have a therapeutic potential. This review identifies six major pathways or sites of stimulus-secretion coupling that could be aimed by potential insulin-secreting drugs and describes several strategies to reach these targets. It also discusses whether these perspectives are realistic or theoretical only. These six possible beta-cell targets are 1) stimulation of metabolism, 2) increase of [Ca2+]i by closure of K+ ATP channels, 3) increase of [Ca2+]i by other means, 4) stimulation of amplifying pathways, 5) action on membrane receptors, and 6) action on nuclear receptors. The theoretical risk of inappropriate insulin secretion and, hence, of hypoglycemia linked to these different approaches is also envisaged.

Impaired glucose homeostasis in transgenic mice expressing the human transient neonatal diabetes mellitus locus, TNDM

Transient neonatal diabetes mellitus (TNDM) is a rare inherited diabetic syndrome apparent in the first weeks of life and again during early adulthood. The relative contributions of reduced islet beta cell number and impaired beta cell function to the observed hypoinsulinemia are unclear. The inheritance pattern of this imprinted disorder implicates overexpression of one or both genes within the TNDM locus: ZAC, which encodes a proapoptotic zinc finger protein, and HYMAI, which encodes an untranslated mRNA. To investigate the consequences for pancreatic function, we have developed a high-copy transgenic mouse line, TNDM29, carrying the human TNDM locus. TNDM29 neonates display hyperglycemia, and older adults, impaired glucose tolerance. Neonatal hyperglycemia occurs only on paternal transmission, analogous to paternal dependence of TNDM in humans. Embryonic pancreata of TNDM29 mice showed reductions in expression of endocrine differentiation factors and numbers of insulin-staining structures. By contrast, beta cell mass was normal or elevated at all postnatal stages, whereas pancreatic insulin content in neonates and peak serum insulin levels after glucose infusion in adults were reduced. Expression of human ZAC and HYMAI in these transgenic mice thus recapitulates key features of TNDM and implicates impaired development of the endocrine pancreas and beta cell function in disease pathogenesis.

Impaired glucose homeostasis in transgenic mice expressing the human transient neonatal diabetes mellitus locus, TNDM

Transient neonatal diabetes mellitus (TNDM) is a rare inherited diabetic syndrome apparent in the first weeks of life and again during early adulthood. The relative contributions of reduced islet beta cell number and impaired beta cell function to the observed hypoinsulinemia are unclear. The inheritance pattern of this imprinted disorder implicates overexpression of one or both genes within the TNDM locus: ZAC, which encodes a proapoptotic zinc finger protein, and HYMAI, which encodes an untranslated mRNA. To investigate the consequences for pancreatic function, we have developed a high-copy transgenic mouse line, TNDM29, carrying the human TNDM locus. TNDM29 neonates display hyperglycemia, and older adults, impaired glucose tolerance. Neonatal hyperglycemia occurs only on paternal transmission, analogous to paternal dependence of TNDM in humans. Embryonic pancreata of TNDM29 mice showed reductions in expression of endocrine differentiation factors and numbers of insulin-staining structures. By contrast, beta cell mass was normal or elevated at all postnatal stages, whereas pancreatic insulin content in neonates and peak serum insulin levels after glucose infusion in adults were reduced. Expression of human ZAC and HYMAI in these transgenic mice thus recapitulates key features of TNDM and implicates impaired development of the endocrine pancreas and beta cell function in disease pathogenesis.