Type 2 diabetes, insulin secretion and beta-cell mass

In nondiabetic subjects, insulin secretion is sufficiently increased as a compensatory adaptation to insulin resistance whereas in subjects with type 2 diabetes, the adaptation is insufficient. Evidences for the islet dysfunction in type 2 diabetes are a)impaired insulin response to various challenges such as glucose, arginine and isoproterenol, b)defective dynamic of insulin secretion resulting in preferential reduction on first phase insulin secretion and irregular oscillations of plasma insulin and c)defective conversion of proinsulin to insulin leading to elevated proinsulin to insulin ratio. In addition, recent studies have also presented evidence of a reduced beta cell mass in diabetes, caused predominantly by enhanced islet apoptosis, although this needs to be confirmed in more studies. These defects may be caused by primary beta cell defects, such as seen in the monogenic diabetes forms of MODY, or by secondary beta cell defects, caused by glucotoxicity, lipotoxicity or islet amyloid aggregation. The defects may also be secondary to defective beta cell stimulation by incretin hormones or the autonomic nerves. The appreciation of islet dysfunction as a key factor underlying the progression from an insulin resistant state into type 2 diabetes has therapeutic implications, since besides improvement of insulin sensitivity, treatment should also aim at improving the islet compensation. This may possibly be achieved by stimulating insulin secretion, supporting islet stimulating mechanisms, removing toxic beta-cell insults and inhibiting beta cell apoptosis.     

Intra- and inter-islet synchronization of metabolically driven insulin secretion

Insulin secretion from pancreatic beta-cells is pulsatile with a period of 5-10 min and is believed to be responsible for plasma insulin oscillations with similar frequency. To observe an overall oscillatory insulin profile it is necessary that the insulin secretion from individual beta-cells is synchronized within islets, and that the population of islets is also synchronized. We have recently developed a model in which pulsatile insulin secretion is produced as a result of calcium-driven electrical oscillations in combination with oscillations in glycolysis. We use this model to investigate possible mechanisms for intra-islet and inter-islet synchronization. We show that electrical coupling is sufficient to synchronize both electrical bursting activity and metabolic oscillations. We also demonstrate that islets can synchronize by mutually entraining each other by their effects on a simple model "liver," which responds to the level of insulin secretion by adjusting the blood glucose concentration in an appropriate way. Since all islets are exposed to the blood, the distributed islet-liver system can synchronize the individual islet insulin oscillations. Thus, we demonstrate how intra-islet and inter-islet synchronization of insulin oscillations may be achieved.     

Negative Feedback Synchronizes Islets of Langerhans

Insulin is released from the pancreas in pulses with a period of ∼ 5 min. These oscillatory insulin levels are essential for proper liver utilization and perturbed pulsatility is observed in type 2 diabetes. What coordinates the many islets of Langerhans throughout the pancreas to produce unified oscillations of insulin secretion? One hypothesis is that coordination is achieved through an insulin-dependent negative feedback action of the liver onto the glucose level. This hypothesis was tested in an in vitro setting using a microfluidic system where the population response from a group of islets was input to a model of hepatic glucose uptake, which provided a negative feedback to the glucose level. This modified glucose level was then delivered back to the islet chamber where the population response was again monitored and used to update the glucose concentration delivered to the islets. We found that, with appropriate parameters for the model, oscillations in islet activity were synchronized. This approach demonstrates that rhythmic activity of a population of physically uncoupled islets can be coordinated by a downstream system that senses islet activity and supplies negative feedback. In the intact animal, the liver can play this role of the coordinator of islet activity.     

Negative Feedback Synchronizes Islets of Langerhans

Insulin is released from the pancreas in pulses with a period of ∼ 5 min. These oscillatory insulin levels are essential for proper liver utilization and perturbed pulsatility is observed in type 2 diabetes. What coordinates the many islets of Langerhans throughout the pancreas to produce unified oscillations of insulin secretion? One hypothesis is that coordination is achieved through an insulin-dependent negative feedback action of the liver onto the glucose level. This hypothesis was tested in an in vitro setting using a microfluidic system where the population response from a group of islets was input to a model of hepatic glucose uptake, which provided a negative feedback to the glucose level. This modified glucose level was then delivered back to the islet chamber where the population response was again monitored and used to update the glucose concentration delivered to the islets. We found that, with appropriate parameters for the model, oscillations in islet activity were synchronized. This approach demonstrates that rhythmic activity of a population of physically uncoupled islets can be coordinated by a downstream system that senses islet activity and supplies negative feedback. In the intact animal, the liver can play this role of the coordinator of islet activity.     

Low Level Pro-inflammatory Cytokines Decrease Connexin36 Gap Junction Coupling in Mouse and Human Islets through Nitric Oxide-mediated Protein Kinase Cδ

Pro-inflammatory cytokines contribute to the decline in islet function during the development of diabetes. Cytokines can disrupt insulin secretion and calcium dynamics; however, the mechanisms underlying this are poorly understood. Connexin36 gap junctions coordinate glucose-induced calcium oscillations and pulsatile insulin secretion across the islet. Loss of gap junction coupling disrupts these dynamics, similar to that observed during the development of diabetes. This study investigates the mechanisms by which pro-inflammatory cytokines mediate gap junction coupling. Specifically, as cytokine-induced NO can activate PKCδ, we aimed to understand the role of PKCδ in modulating cytokine-induced changes in gap junction coupling. Isolated mouse and human islets were treated with varying levels of a cytokine mixture containing TNF-α, IL-1β, and IFN-γ. Islet dysfunction was measured by insulin secretion, calcium dynamics, and gap junction coupling. Modulators of PKCδ and NO were applied to determine their respective roles in modulating gap junction coupling. High levels of cytokines caused cell death and decreased insulin secretion. Low levels of cytokine treatment disrupted calcium dynamics and decreased gap junction coupling, in the absence of disruptions to insulin secretion. Decreases in gap junction coupling were dependent on NO-regulated PKCδ, and altered membrane organization of connexin36. This study defines several mechanisms underlying the disruption to gap junction coupling under conditions associated with the development of diabetes. These mechanisms will allow for greater understanding of islet dysfunction and suggest ways to ameliorate this dysfunction during the development of diabetes.     

New insights into the role of connexins in pancreatic islet function and diabetes

Multi-cellular systems require complex signaling mechanisms for proper tissue function, to mediate signaling between cells in close proximity and at distances. This holds true for the islets of Langerhans, which are multicellular micro-organs located in the pancreas responsible for glycemic control, through secretion of insulin and other hormones. Coupling of electrical and metabolic signaling between islet β-cells is required for proper insulin secretion and effective glycemic control. β-cell specific coupling is established through gap junctions composed of connexin36, which results in coordinated insulin release across the islet. Islet connexins have been implicated in both Type-1 and Type-2 diabetes; however a clear link remains to be determined. The goal of this review is to discuss recent discoveries regarding the role of connexins in regulating insulin secretion, the regulation of connexins within the islet, and recent studies which support a role for connexins in diabetes. Further studies which investigate the regulation of connexins in the islet and their role in diabetes may lead to novel diabetes therapies which regulate islet function and β-cell survival through modulation of gap junction coupling.     

Measurement of pulsatile insulin secretion in the rat: direct sampling from the hepatic portal vein

It has previously been shown that insulin is secreted in discrete secretory bursts by sampling directly from the portal vein in the dog and humans. Deficient pulsatile insulin secretion is the basis for impaired insulin secretion in type 2 diabetes. However, while novel genetically modified disease models of diabetes are being developed in rodents, no validated method for quantifying pulsatile insulin secretion has been established for rodents. To address this we 1) developed a novel rat model with chronically implanted portal vein catheters, 2) established the parameters to permit deconvolution of portal vein insulin concentrations profiles to measure insulin secretion and resolve its pulsatile components, and 3) measured total and pulsatile insulin secretion compared with that in the dog, the species in which this sampling and deconvolution approach was validated for quantifying pulsatile insulin secretion. In rats, portal vein catheter patency and function were maintained for periods up to 2-3 wk with no postoperative complications such as catheter tract infection. Rat portal vein insulin concentration profiles in the fasting state revealed distinct insulin oscillations with a periodicity of approximately 5 min and an amplitude of up to 600 pmol/l, which was remarkably similar to that in the dogs and in humans. Deconvolution analysis of portal vein insulin concentrations revealed that the majority of insulin ( approximately 70%) in the rat is secreted in distinct insulin pulses occurring at approximately 5-min intervals. This model therefore permits direct accurate measurements of pulsatile insulin secretion in a relatively inexpensive animal. With increased introduction of genetically modified rat models will be an important tool in elucidating the underlying mechanisms of impaired pulsatile insulin secretion in diabetes.     

Role of islet amyloid in type 2 diabetes mellitus

Diabetes mellitus is one of the most common metabolic diseases worldwide and its prevalence is rapidly increasing. Due to its chronic nature (diabetes mellitus can be treated but as yet not cured) and its serious complications, it is one of the most expensive diseases with regard to total health care costs per patient. The elevated blood glucose levels in diabetes mellitus are caused by a defect in production and/or secretion of the polypeptide hormone insulin, which normally promotes glucose-uptake in cells. Insulin is produced by the pancreatic 'beta-cells' in the 'islets of Langerhans', which lie distributed within the exocrine pancreatic tissue. In type 2 diabetes mellitus, the initial defect in the pathogenesis of the disease in most of the patients is believed to be 'insulin resistance'. Hyperglycemia (clinically overt diabetes mellitus) will not develop as long as the body is able to produce enough insulin to compensate for the reduced insulin action. When this compensation fails ('beta-cell failure') blood glucose levels will become too high. In this review, we discuss one of the mechanisms that have been implicated in the development of beta-cell failure, i.e. amyloid formation in the pancreatic islets. This islet amyloid is a characteristic histopathological feature of type 2 diabetes mellitus and both in vitro and in vivo studies have revealed that its formation causes death of islet beta-cells. Being a common pathogenic factor in an otherwise heterogeneous disease, islet amyloidosis is an attractive novel target for therapeutic intervention in type 2 diabetes mellitus.     

Mitochondrial metabolism reveals a functional architecture in intact islets of Langerhans from normal and diabetic Psammomys obesus

The cells within the intact islet of Langerhans function as a metabolic syncytium, secreting insulin in a coordinated and oscillatory manner in response to external fuel. With increased glucose, the oscillatory amplitude is enhanced, leading to the hypothesis that cells within the islet are secreting with greater synchronization. Consequently, non-insulin-dependent diabetes mellitus (NIDDM; type 2 diabetes)-induced irregularities in insulin secretion oscillations may be attributed to decreased intercellular coordination. The purpose of the present study was to determine whether the degree of metabolic coordination within the intact islet was enhanced by increased glucose and compromised by NIDDM. Experiments were performed with isolated islets from normal and diabetic Psammomys obesus. Using confocal microscopy and the mitochondrial potentiometric dye rhodamine 123, we measured mitochondrial membrane potential oscillations in individual cells within intact islets. When mitochondrial membrane potential was averaged from all the cells in a single islet, the resultant waveform demonstrated clear sinusoidal oscillations. Cells within islets were heterogeneous in terms of cellular synchronicity (similarity in phase and period), sinusoidal regularity, and frequency of oscillation. Cells within normal islets oscillated with greater synchronicity compared with cells within diabetic islets. The range of oscillatory frequencies was unchanged by glucose or diabetes. Cells within diabetic (but not normal) islets increased oscillatory regularity in response to glucose. These data support the hypothesis that glucose enhances metabolic coupling in normal islets and that the dampening of oscillatory insulin secretion in NIDDM may result from disrupted metabolic coupling.     

Voluntary running exercise prevents β-cell failure in susceptible islets of the Zucker diabetic fatty rat

Physical activity improves glycemic control in type 2 diabetes (T2D), but its contribution to preserving β-cell function is uncertain. We evaluated the role of physical activity on β-cell secretory function and glycerolipid/fatty acid (GL/FA) cycling in male Zucker diabetic fatty (ZDF) rats. Six-week-old ZDF rats engaged in voluntary running for 6 wk (ZDF-A). Inactive Zucker lean and ZDF (ZDF-I) rats served as controls. ZDF-I rats displayed progressive hyperglycemia with β-cell failure evidenced by falling insulinemia and reduced insulin secretion to oral glucose. Isolated ZDF-I rat islets showed reduced glucose-stimulated insulin secretion expressed per islet and per islet protein. They were also characterized by loss of the glucose regulation of fatty acid oxidation and GL/FA cycling, reduced mRNA expression of key β-cell genes, and severe reduction of insulin stores. Physical activity prevented diabetes in ZDF rats through sustaining β-cell compensation to insulin resistance shown in vivo and in vitro. Surprisingly, ZDF-A islets had persistent defects in fatty acid oxidation, GL/FA cycling, and β-cell gene expression. ZDF-A islets, however, had preserved islet insulin mRNA and insulin stores compared with ZDF-I rats. Physical activity did not prevent hyperphagia, dyslipidemia, or obesity in ZDF rats. In conclusion, islets of ZDF rats have a susceptibility to failure that is possibly due to altered β-cell fatty acid metabolism. Depletion of pancreatic islet insulin stores is a major contributor to islet failure in this T2D model, preventable by physical activity.     

Model for glucagon secretion by pancreatic α-cells

Glucagon hormone is synthesized and released by pancreatic α-cells, one of the islet-cell types. This hormone, along with insulin, maintains blood glucose levels within the physiological range. Glucose stimulates glucagon release at low concentrations (hypoglycemia). However, the mechanisms involved in this secretion are still not completely clear. Here, using experimental calcium time series obtained in mouse pancreatic islets at low and high glucose conditions, we propose a glucagon secretion model for α-cells. Our model takes into account that the resupply of releasable granules is not only controlled by cytoplasmic Ca2+, as in other neuroendocrine and endocrine cells, but also by the level of extracellular glucose. We found that, although calcium oscillations are highly variable, the average secretion rates predicted by the model fall into the range of values reported in the literature, for both stimulated and non-stimulated conditions. For low glucose levels, the model predicts that there would be a well-controlled number of releasable granules refilled slowly from a large reserve pool, probably to ensure a secretion rate that could last for several minutes. Studying the α-cell response to the addition of insulin at low glucose, we observe that the presence of insulin reduces glucagon release by decreasing the islet Ca2+ level. This observation is in line with previous work reporting that Ca2+ dynamics, mainly frequency, is altered by insulin. Thus, the present results emphasize the main role played by Ca2+ and glucose in the control of glucagon secretion by α-cells. Our modeling approach also shows that calcium oscillations potentiate glucagon secretion as compared to constant levels of this cellular messenger. Altogether, the model sheds new light on the subcellular mechanisms involved in α-cell exocytosis, and provides a quantitative predictive tool for studying glucagon secretion modulators in physiological and pathological conditions.     

Regulated autophagy controls hormone content in secretory-deficient pancreatic endocrine beta-cells

Endocrine cells are continually regulating the balance between hormone biosynthesis, secretion, and intracellular degradation to ensure that cellular hormone stores are maintained at optimal levels. In pancreatic beta-cells, intracellular insulin stores in beta-granules are mostly upheld by efficiently up-regulating proinsulin biosynthesis at the translational level to rapidly replenish the insulin lost via exocytosis. Under normal circumstances, intracellular degradation of insulin plays a relatively minor janitorial role in retiring aged beta-granules, apparently via crinophagy. However, this mechanism alone is not sufficient to maintain optimal insulin storage in beta-cells when insulin secretion is dysfunctional. Here, we show that despite an abnormal imbalance of glucose/glucagon-like peptide 1 regulated insulin production over secretion in Rab3A(-/-) mice compared with control animals, insulin storage levels were maintained due to increased intracellular beta-granule degradation. Electron microscopy analysis indicated that this was mediated by a significant 12-fold up-regulation of multigranular degradation vacuoles in Rab3A(-/-) mouse islet beta-cells (P 相似文献   

The mass, but not the frequency, of insulin secretory bursts in isolated human islets is entrained by oscillatory glucose exposure

Insulin is secreted in discrete insulin secretory bursts. Regulation of insulin release is accomplished almost exclusively by modulation of insulin pulse mass, whereas the insulin pulse interval remains stable at approximately 4 min. It has been reported that in vivo insulin pulses can be entrained to a pulse interval of approximately 10 min by infused glucose oscillations. If oscillations in glucose concentration play an important role in the regulation of pulsatile insulin secretion, abnormal or absent glucose oscillations, which have been described in type 2 diabetes, might contribute to the defective insulin secretion. Using perifused human islets exposed to oscillatory vs. constant glucose, we questioned 1) whether the interval of insulin pulses released by human islets is entrained to infused glucose oscillations and 2) whether the exposure of islets to oscillating vs. constant glucose confers an increased signal for insulin secretion. We report that oscillatory glucose exposure does not entrain insulin pulse frequency, but it amplifies the mass of insulin secretory bursts that coincide with glucose oscillations (P < 0.001). Dose-response analyses showed that the mode of glucose drive does not influence total insulin secretion (P = not significant). The apparent entrainment of pulsatile insulin to infused glucose oscillations in nondiabetic humans in vivo might reflect the amplification of underlying insulin secretory bursts that are detected as entrained pulses at the peripheral sampling site, but without changes in the underlying pacemaker activity.     

Islet amyloid polypeptide inhibits glucose-stimulated insulin secretion from isolated rat pancreatic islets

Islet amyloid polypeptide has 37 amino acids and is a major component of amyloid deposition in pancreatic islets of patients with type 2 diabetes mellitus. To determine whether the peptide is involved in the impaired insulin secretion in this type of diabetes mellitus, we synthesized islet amyloid polypeptide and its fragments and examined its effect on insulin secretion. Islet amyloid polypeptide inhibited the glucose-stimulated insulin secretion from isolated rat pancreatic islets, as calcitonin gene-related peptide did, but the fragments failed to inhibit the secretion. Thus, we propose that amyloid deposition may be an important factor in the impairment of insulin secretion in type 2 diabetes mellitus.     

Rat maternal diabetes impairs pancreatic beta-cell function in the offspring

It has been shown that maternal diabetes increases the risk for obesity, glucose intolerance, and Type 2 diabetes mellitus in the adult life of the offspring. Mechanisms for these effects on the offspring are not well understood, and little information is available to reveal the mechanisms. We studied the effect of maternal diabetes on beta-cell function in the offspring of streptozotocin (STZ)-induced diabetic rat mothers (STZ-offspring). STZ-offspring did not become glucose intolerant up to 15 wk of age. At this age, however, insulin secretion was significantly impaired, as measured by in vivo and in vitro studies. Consistent with these changes, islet glucose metabolism and some important glucose metabolic enzyme activities were reduced. No significant changes were found in islet morphological analysis. These data indicate that beta-cell function is impaired in adult STZ-offspring; these changes may contribute to the development of type 2 diabetes mellitus in adulthood.     

Unraveling pancreatic islet biology by quantitative proteomics

The pancreatic islets of Langerhans play a critical role in maintaining blood glucose homeostasis by secreting insulin and several other important peptide hormones. Impaired insulin secretion due to islet dysfunction is linked to the pathogenesis underlying both Type 1 and Type 2 diabetes. Over the past 5 years, emerging proteomic technologies have been applied to dissect the signaling pathways that regulate islet functions and gain an understanding of the mechanisms of islet dysfunction relevant to diabetes. Herein, we briefly review some of the recent quantitative proteomic studies involving pancreatic islets geared towards gaining a better understanding of islet biology relevant to metabolic diseases.     

CART is a novel islet regulatory peptide

CART peptides have emerged as important islet regulators. CART is expressed both in islet endocrine cells and in parasympathetic and sensory nerves innervating the islets. In adult rats the intra-islet expression of CART is limited to the somatostatin producing delta-cells, while in adult mice CART is mainly expressed in nerve fibers. During development islet CART is upregulated; in rats in almost all types of islet endocrine cells, including the insulin-producing beta-cells, and in mice mainly in the alpha-cells. This pattern of expression peaks around birth. CART is also expressed in human pancreatic nerves and in islet tumours where the expression level of CART may be related to the degree of differentiation of the tumour. Interestingly, in several rat models of type 2 diabetes CART expression is robustly upregulated in the beta-cells, and is prominent during the phase of beta cell proliferation and hypertrophy. While CART inhibits glucose stimulated insulin secretion from rat islets it augments insulin secretion amplified by cAMP. Mice lacking CART, on the other hand, have islet dysfunction, and humans with a missense mutation in the cart gene are prone to develop type 2 diabetes. These data favor a role of CART in normal islet function and in the pathophysiology of type 2 diabetes.     

Dual Role of VAMP8 in Regulating Insulin Exocytosis and Islet β Cell Growth

Optimal insulin secretion required to maintain glucose homeostasis is the summation of total pancreatic islet β cell mass and intrinsic secretory capacity of individual β cells, which are regulated by distinct mechanisms that could be amplified by glucagon-like-peptide-1 (GLP-1). Because of these actions of GLP-1 on islet β cells, GLP-1 has been deployed to?treat diabetes. We employed SNARE protein VAMP8-null mice to demonstrate that VAMP8 mediates insulin granule recruitment to the plasma membrane, which partly accounts for GLP-1 potentiation of glucose-stimulated insulin secretion. VAMP8-null mice also exhibited increased islet β cell mass from increased β cell mitosis, with β cell proliferative activity greatly amplified by GLP-1. Thus, despite the β cell exocytotic defect, VAMP8-null mice have an increased total insulin secretory capacity, which improved glucose homeostasis. We conclude that these VAMP8-mediated events partly underlie the therapeutic actions of GLP-1 on insulin secretion and β cell growth.