Transgenic overexpression of human islet amyloid polypeptide inhibits insulin secretion and glucose elimination after gastric glucose gavage in mice

Amylin and beta amyloid proteins interact to form amorphous heterocomplexes with enhanced toxicity in neuronal cells

Human pancreatic islet amyloid polypeptide (hIAPP) and beta amyloid (Aβ) can accumulate in Type 2 diabetes (T2D) and Alzheimer's disease (AD) brains and evidence suggests that interaction between the two amyloidogenic proteins can lead to the formation of heterocomplex aggregates. However, the structure and consequences of the formation of these complexes remains to be determined. The main objective of this study was to characterise the different types and morphology of Aβ-hIAPP heterocomplexes and determine if formation of such complexes exacerbate neurotoxicity. We demonstrate that hIAPP promotes Aβ oligomerization and formation of small oligomer and large aggregate heterocomplexes. Co-oligomerized Aβ42-hIAPP mixtures displayed distinct amorphous structures and a 3-fold increase in neuronal cell death as compared to Aβ and hIAPP alone. However, in contrast to hIAPP, non-amyloidogenic rat amylin (rIAPP) reduced oligomer Aβ-mediated neuronal cell death. rIAPP exhibited reductions in Aβ induced neuronal cell death that was independent of its ability to interact with Aβ and form heterocomplexes; suggesting mediation by other pathways. Our findings reveal distinct effects of IAPP peptides in modulating Aβ aggregation and toxicity and provide new insight into the potential pathogenic effects of Aβ-IAPP hetero-oligomerization and development of IAPP based therapies for AD and T2D.

Sub-Toxic Human Amylin Fragment Concentrations Promote the Survival and Proliferation of SH-SY5Y Cells via the Release of VEGF and HspB5 from Endothelial RBE4 Cells

Human amylin is a 37-residue peptide hormone (hA1-37) secreted by β-cells of the pancreas and, along with insulin, is directly associated with type 2 diabetes mellitus (T2DM). Amyloid deposits within the islets of the pancreas represent a hallmark of T2DM. Additionally, amylin aggregates have been found in blood vessels and/or brain of patients with Alzheimer’s disease, alone or co-deposited with β-amyloid. The purpose of this study was to investigate the neuroprotective potential of human amylin in the context of endothelial-neuronal “cross-talk”. We initially performed dose-response experiments to examine cellular toxicity (quantified by the [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] MTT assay) of different hA17–29 concentrations in endothelial cells (RBE4). In the culture medium of these cells, we also measured heat shock protein B5 (HspB5) levels by ELISA, finding that even a sub-toxic concentration of hA17–29 (3 µM) produced an increase of HspB5. Using a cell medium of untreated and RBE4 challenged for 48 h with a sub-toxic concentration of hA17–29, we determined the potential beneficial effect of their addition to the medium of neuroblastoma SH-SY5Y cells. These cells were subsequently incubated for 48 h with a toxic concentration of hA17–29 (20 µM). We found a complete inhibition of hA17–29 toxicity, potentially related to the presence in the conditioned medium not only of HspB5, but also of vascular endothelial growth factor (VEGF). Pre-treating SH-SY5Y cells with the anti-Flk1 antibody, blocking the VEGF receptor 2 (VEGFR2), significantly decreased the protective effects of the conditioned RBE4 medium. These data, obtained by indirectly measuring VEGF activity, were strongly corroborated by the direct measurement of VEGF levels in conditioned RBE4 media as detected by ELISA. Altogether, these findings highlighted a novel role of sub-toxic concentrations of human amylin in promoting the secretion of proteic factors by endothelial cells (HspB5 and VEGF) that support the survival and proliferation of neuron-like cells.

Unfolding Novel Mechanisms of Polyphenol Flavonoids for Better Glycaemic Control: Targeting Pancreatic Islet Amyloid Polypeptide (IAPP)

Type 2 diabetes (T2D) is characterised by hyperglycaemia resulting from defective insulin secretion, insulin resistance, or both. The impact of over-nutrition and reduced physical activity, evidenced by the exponential rise in obesity and the prevalence of T2D, strongly supports the implementation of lifestyle modification programs. Accordingly, an increased consumption of fruits and plant-derived foods has been advocated, as their intake is inversely correlated with T2D prevalence; this has been attributed, in part, to their contained polyphenolic compounds. Over the last decade, a body of work has focussed on establishing the mechanisms by which polyphenolic compounds exert beneficial effects to limit carbohydrate digestion, enhance insulin-mediated glucose uptake, down-regulate hepatic gluconeogenesis and decrease oxidative stress; the latter anti-oxidative property being the most documented. Novel effects on the inhibition of glucocorticoid action and the suppression of amylin misfolding and aggregation have been identified more recently. Amyloid fibrils form from spontaneously misfolded amylin, depositing in islet cells to elicit apoptosis, beta cell degeneration and decrease insulin secretion, with amyloidosis affecting up to 80% of pancreatic islet cells in T2D. Therefore, intervening with polyphenolic compounds offers a novel approach to suppressing risk or progression to T2D. This review gives an update on the emerging mechanisms related to dietary polyphenol intake for the maintenance of glycaemic control and the prevention of T2D.

Amylin-like immunoreactivity in pancreatic X cells of the black-spotted frog Rana (Pelophylax) nigromaculata

tIn this study, we investigated the presence of ovoid or ellipsoidal amylin-immunoreactive cells of the pancreatic islets of the black-spotted frog Rana (Pelophylax) nigromaculata. Using double immunofluorescent staining, all amylin-immunoreactive cells were shown to be immuno-negative for insulin, glucagon, and somatostatin, and they were often observed in peripheral regions of clusters of insulin-immunoreactive cells. Under immunoelectron microscopy, amylin-immunoreactive signals were detected on the secretory granules in a specific type of endocrine cells. From our results, we conclude that the amylin-immunoreactive cells correspond to X cells among the 4 distinct types of endocrine cells (B, A/PP, D, and X) previously identified in the frog. Amylin secreted from X cells may regulate the hormone secretion from A/PP cells and/or B cells through a paracrine mechanism.

Amylin and its analogs: a friend or foe for the treatment of Alzheimer's disease?

Amylin, a gut-brain axis hormone, and amyloid-beta peptides (Aβ), a major component of the Alzheimer's disease (AD) brain, share several features, including similar β-sheet secondary structures, binding to the same receptor and being degraded by the same protease, insulin degrading enzyme (IDE). However, while amylin readily crosses the blood brain barrier (BBB) and mediates several activities including improving glucose metabolism, relaxing cerebrovascular structure, modulating inflammatory reaction and perhaps enhancing neural regeneration, Aβ has no known physiological functions. Thus, abundant Aβ in the AD brain could block or interfere with the binding of amylin to its receptor and hinder its functions. Recent studies using animal models for AD demonstrate that amylin and its analog reduce the AD pathology in the brain and improve cognitive impairment in AD. Given that, in addition to amyloid plaques and neurofibrillary tangles, perturbed cerebral glucose metabolism and cerebrovascular damage are the hallmarks of the AD brain, we propose that giving exogenous amylin type peptides have the potential to become a new avenue for the diagnosis and therapeutic of AD. Although amylin's property of self-aggregation may be a limitation to developing it as a therapeutic for AD, its clinical analog, pramlintide containing 3 amino acid differences from amylin, does not aggregate like human amylin, but more potently mediates amylin's activities in the brain. Pramlintide is an effective drug for diabetes with a favorable profile of safety. Thus a randomized, double-blind, placebo-controlled clinical trial should be conducted to examine the efficacy of pramlintide for AD. This review summarizes the knowledge and findings on amylin type peptides and discuss pros and cons for their potential for AD.

Human IAPP-induced pancreatic β cell toxicity and its regulation by autophagy

Pancreatic islets in patients with type 2 diabetes mellitus (T2DM) are characterized by loss of β cells and formation of amyloid deposits derived from islet amyloid polypeptide (IAPP). Here we demonstrated that treatment of INS-1 cells with human IAPP (hIAPP) enhances cell death, inhibits cytoproliferation, and increases autophagosome formation. Furthermore, inhibition of autophagy increased the vulnerability of β cells to the cytotoxic effects of hIAPP. Based on these in vitro findings, we examined the pathogenic role of hIAPP and its relation to autophagy in hIAPP-knockin mice. In animals fed a standard diet, hIAPP had no toxic effects on β cell function; however, hIAPP-knockin mice did not exhibit a high-fat-diet-induced compensatory increase in β cell mass, which was due to limited β cell proliferation and enhanced β cell apoptosis. Importantly, expression of hIAPP in mice with a β cell-specific autophagy defect resulted in substantial deterioration of glucose tolerance and dispersed cytoplasmic expression of p62-associated toxic oligomers, which were otherwise sequestrated within p62-positive inclusions. Together, our results indicate that increased insulin resistance in combination with reduced autophagy may enhance the toxic potential of hIAPP and enhance β cell dysfunction and progression of T2DM.

Autocrine regulation of insulin secretion

Impaired insulin secretion from pancreatic β-cells is a major factor in the pathogenesis of type 2 diabetes. The main regulator of insulin secretion is the plasma glucose concentration. Insulin secretion is modified by other nutrients, circulating hormones and the autonomic nervous system, as well as local paracrine and autocrine signals. Autocrine signalling involves diffusible molecules that bind to receptors on the same cell from which they have been released. The first transmitter to be implicated in the autocrine regulation of β-cell function was insulin itself. The importance of autocrine insulin signalling is underscored by the finding that mice lacking insulin receptors in β-cells are glucose intolerant. In addition to insulin, β-cells secrete a variety of additional substances, including peptides (e.g. amylin, chromogranin A and B and their cleavage products), neurotransmitters (ATP and γ-aminobutyric acid) and ions (e.g. zinc). Here we review the autocrine effects of substances secreted from β-cells, with a focus on acute effects in stimulus-secretion coupling, present some novel data and discuss the general significance of autocrine signals for the regulation of insulin secretion.

Fibril structure of human islet amyloid polypeptide

Misfolding and amyloid fibril formation by human islet amyloid polypeptide (hIAPP) are thought to be important in the pathogenesis of type 2 diabetes, but the structures of the misfolded forms remain poorly understood. Here we developed an approach that combines site-directed spin labeling with continuous wave and pulsed EPR to investigate local secondary structure and to determine the relative orientation of the secondary structure elements with respect to each other. These data indicated that individual hIAPP molecules take up a hairpin fold within the fibril. This fold contains two β-strands that are much farther apart than expected from previous models. Atomistic structural models were obtained using computational refinement with EPR data as constraints. The resulting family of structures exhibited a left-handed helical twist, in agreement with the twisted morphology observed by electron microscopy. The fibril protofilaments contain stacked hIAPP monomers that form opposing β-sheets that twist around each other. The two β-strands of the monomer adopt out-of-plane positions and are staggered by about three peptide layers (∼15 Å). These results provide a mechanism for hIAPP fibril formation and could explain the remarkable stability of the fibrils. Thus, the structural model serves as a starting point for understanding and preventing hIAPP misfolding.

Involvement of ATP-sensitive potassium (K(ATP)) channels in the loss of beta-cell function induced by human islet amyloid polypeptide

Islet amyloid polypeptide (IAPP) is a major component of amyloid deposition in pancreatic islets of patients with type 2 diabetes. It is known that IAPP can inhibit glucose-stimulated insulin secretion; however, the mechanisms of action have not yet been established. In the present work, using a rat pancreatic beta-cell line, INS1E, we have created an in vitro model that stably expressed human IAPP gene (hIAPP cells). These cells showed intracellular oligomers and a strong alteration of glucose-stimulated insulin and IAPP secretion. Taking advantage of this model, we investigated the mechanism by which IAPP altered beta-cell secretory response and contributed to the development of type 2 diabetes. We have measured the intracellular Ca(2+) mobilization in response to different secretagogues as well as mitochondrial metabolism. The study of calcium signals in hIAPP cells demonstrated an absence of response to glucose and also to tolbutamide, indicating a defect in ATP-sensitive potassium (K(ATP)) channels. Interestingly, hIAPP showed a greater maximal respiratory capacity than control cells. These data were confirmed by an increased mitochondrial membrane potential in hIAPP cells under glucose stimulation, leading to an elevated reactive oxygen species level as compared with control cells. We concluded that the hIAPP overexpression inhibits insulin and IAPP secretion in response to glucose affecting the activity of K(ATP) channels and that the increased mitochondrial metabolism is a compensatory response to counteract the secretory defect of beta-cells.

Amylin effect in extrapancreatic tissues participating in glucose homeostasis, in normal, insulin-resistant and type 2 diabetic state

Amylin is co-secreted with insulin, responds to the same stimuli, is anorectic, lowers body weight by reducing fat mass, and is proposed for diabetes treatment. We examined the effect of a 3-day constant infusion of close to physiological doses of amylin in Wistar rats, on glucotransporter expression, glycogen content (G), glycogen synthase a activity (GSa) and glucose transport (GT), in liver, muscle and fat from insulin resistant (IR) and type 2 diabetic (T2D) models, compared to normal (N) animals; plasma glucose and insulin were measured. Plasma insulin in IR was higher than in N or T2D, and amylin normalized the value. In both, IR and T2D, liver G was lower than normal, accompanied by GLUT-2, mRNA and protein, higher and lower, respectively, than in N; amylin normalized G in both groups, without changes in GLUT-2, except for an mRNA increase in T2D. In IR and T2D, muscle GSa was reduced, together with respective over- and under-GLUT-4 expression; amylin induced only a trend toward GSa normalization in both groups. In isolated adipocytes, GT and GLUT-4 in IR and T2D were lower and higher, respectively, than in N; after amylin, not only GT was normalized in both groups but also the response to insulin was much more pronounced, including that in N, without major changes in GLUT-4. This suggests that the beneficial effect of amylin in states running with altered glucose homeostasis could occur by partially acting on the hexose metabolism of the liver and mainly on that of the adipose tissue.

Gut microbiota in health and disease

Gut microbiota is an assortment of microorganisms inhabiting the length and width of the mammalian gastrointestinal tract. The composition of this microbial community is host specific, evolving throughout an individual's lifetime and susceptible to both exogenous and endogenous modifications. Recent renewed interest in the structure and function of this "organ" has illuminated its central position in health and disease. The microbiota is intimately involved in numerous aspects of normal host physiology, from nutritional status to behavior and stress response. Additionally, they can be a central or a contributing cause of many diseases, affecting both near and far organ systems. The overall balance in the composition of the gut microbial community, as well as the presence or absence of key species capable of effecting specific responses, is important in ensuring homeostasis or lack thereof at the intestinal mucosa and beyond. The mechanisms through which microbiota exerts its beneficial or detrimental influences remain largely undefined, but include elaboration of signaling molecules and recognition of bacterial epitopes by both intestinal epithelial and mucosal immune cells. The advances in modeling and analysis of gut microbiota will further our knowledge of their role in health and disease, allowing customization of existing and future therapeutic and prophylactic modalities.

Disturbed alpha-cell function in mice with beta-cell specific overexpression of human islet amyloid polypeptide

Exogenous administration of islet amyloid polypeptide (IAPP) has been shown to inhibit both insulin and glucagon secretion. This study examined α-cell function in mice with β-cell specific overexpression of human IAPP (hIAPP) after an oral protein gavage (75 mg whey protein/mouse). Baseline glucagon levels were higher in transgenic mice (41 ± 4.0 pg/mL, n = 6) than in wildtype animals (19 ± 5.1 pg/mL, n = 5, P = .015). In contrast, the glucagon response to protein was impaired in transgenic animals (21 ± 2.7 pg/mL in transgenic mice versus 38 ± 5.7 pg/mL in wildtype mice at 15 minutes; P = .027). Baseline insulin levels did not differ between the groups, while the insulin response, as the glucagon response, was impaired after protein challenge (P = .018). Glucose levels were not different between the groups and did not change significantly after protein gavage. Acetaminophen was given through gavage to the animals (2 mg/mouse) to estimate gastric emptying. The plasma acetaminophen profile was similar in the two groups of mice. We conclude that disturbances in glucagon secretion exist in mice with β-cell specific overexpression of human IAPP, which are not secondary to changes in gastric emptying. The reduced glucagon response to protein challenge may reflect a direct inhibitory influence of hIAPP on glucagon secretion.

Human islet amyloid polypeptide transgenic mice: in vivo and ex vivo models for the role of hIAPP in type 2 diabetes mellitus

Human islet amyloid polypeptide (hIAPP), a pancreatic islet protein of 37 amino acids, is the main component of islet amyloid, seen at autopsy in patients with type 2 diabetes mellitus (DM2). To investigate the roles of hIAPP and islet amyloid in DM2, we generated transgenic mice expressing hIAPP in their islet beta cells. In this study, we found that after a long-term, high-fat diet challenge islet amyloid was observed in only 4 of 19 hIAPP transgenic mice. hIAPP transgenic females exhibited severe glucose intolerance, which was associated with a downregulation of GLUT-2 mRNA expression. In isolated islets from hIAPP males cultured for 3 weeks on high-glucose medium, the percentage of amyloid containing islets increased from 5.5% to 70%. This ex vivo system will allow a more rapid, convenient, and specific study of factors influencing islet amyloidosis as well as of therapeutic strategies to interfere with this pathological process.

Environmental factors differently affect human and rat IAPP: conformational preferences and membrane interactions of IAPP17-29 peptide derivatives

Interest in the 37-residue human islet amyloid polypeptide (hIAPP) is related to its ability to form amyloid deposits in patients affected by type II diabetes. Attempts to unravel the molecular features of this disease have indicated several regions of this polypeptide to be responsible for either the ability to form insoluble fibrils or the abnormal interaction with membranes. To extend these studies to peptides that enclose His18, whose ionization state is believed to play a key role in the aggregation of hIAPP, we report on the synthesis of two peptides, hIAPP17-29 and rIAPP17-29, encompassing the 17-29 sequences of human and rat IAPP, respectively, as well as on their conformational features in water and in several membrane-mimicking environments as revealed by circular dichroism (CD) and 2D-NMR studies. hIAPP17-29 adopts a beta-sheet structure in water and its solubility increases at low pH. Anionic sodium dodecyl sulfate (SDS) micelles promoted the formation of an alpha-helical structure in the peptide chain, which was poorly influenced by pH variations. rIAPP17-29 was soluble and unstructured in all the environments investigated, with a negligible effect of pH. The membrane interactions of hIAPP17-29 and rIAPP17-29 were assessed by recording differential scanning calorimetry (DSC) measurements aimed at elucidating the peptide-induced changes in the thermotropic behaviour of zwitterionic (DPPC) and negatively charged (DPPC/DPPS 3:1) model membranes (DPPC=1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPS=1,2-dipalmitoyl-sn-glycero-3-phosphoserine). Results of DSC experiments demonstrated the high potential of hIAPP17-29 to interact with DPPC membranes. hIAPP17-29 exhibited a negligible affinity for negatively charged DPPC/DPPS model membranes at neutral pH. On the other hand, rIAPP17-29 did not interact with neutral or negatively charged membranes. The role played by His18 in the modulation of the biophysical properties of this hIAPP region was assessed by synthesising and studying the R18HrIAPP17-29 peptide; the replacement of a single Arg with a His residue is not sufficient to induce either amyloidogenic propensity or membrane interaction in this region. The results show that the 17-29 domain of hIAPP has many properties of the full-length protein "in vitro" and this opens up new perspectives for both research and eventually therapy.

DPP-4 inhibition improves glucose tolerance and increases insulin and GLP-1 responses to gastric glucose in association with normalized islet topography in mice with beta-cell-specific overexpression of human islet amyloid polypeptide

Inhibition of dipeptidyl peptidase-4 (DPP-4) is currently explored as a novel therapy of type 2 diabetes. The strategy has been shown to improve glycemia in most, but not all, rodent forms of glucose intolerance. In this study, we explored the effects of DPP-4 inhibition in mice with beta-cell overexpression of human islet amyloid polypeptide (IAPP). We therefore administered the orally active and highly selective DPP-4 inhibitor, vildagliptin (3 micromol/mouse daily) to female mice with beta-cell overexpression of human IAPP. Controls were given plain water, and a series of untreated wildtype mice was also included. After five weeks, an intravenous glucose tolerance test showed improved glucose disposal and a markedly enhanced insulin response in mice treated with vildagliptin. After eight weeks, a gastric tolerance test showed that vildagliptin improved glucose tolerance and markedly (approximately ten-fold) augmented the insulin response in association with augmented (approximately five-fold) levels of intact glucagon-like peptide-1 (GLP-1). Furthermore, after nine weeks, islets were isolated. Islets from vildagliptin-treated mice showed augmented glucose-stimulated insulin response and a normalization of the islet insulin content, which was reduced by approximately 50% in transgenic controls versus wildtype animals. Double immunostaining of pancreatic islets for insulin and glucagon revealed that transgenic islets displayed severely disturbed intra-islet topography with frequently observed centrally located alpha-cells. Treatment with vildagliptin restored the islet topography. We therefore conclude that DPP-4 inhibition improves islet function and islet topography in mice with beta-cell specific transgenic overexpression of human IAPP.

Islet amyloid polypeptide (IAPP) transgenic rodents as models for type 2 diabetes

Blood glucose concentrations are maintained by insulin secreted from beta-cells located in the islets of Langerhans. There are approximately 2000 beta-cells per islet, and approximately one million islets of Langerhans scattered throughout the pancreas. The islet in type 2 diabetes mellitus (T2D) has deficient beta-cell mass due to increased beta-cell apoptosis and islet amyloid derived from islet amyloid polypeptide (IAPP). Accumulating evidence implicates toxic IAPP oligomers in the mediation of beta-cell apoptosis in T2D. Humans, monkeys, and cats express an amyloidogenic toxic form of IAPP and spontaneously develop diabetes characterized by islet amyloid deposits. However, longitudinal studies of islet pathology in humans are impossible, and studies in nonhuman primates and cats are costly and impractical. Rodent IAPP is not amyloidogenic, thus commonly used rodent models of diabetes do not recapitulate islet pathology in humans. To investigate the diabetogenic role of human IAPP (h-IAPP), several mouse models and, more recently, a rat model transgenic for h-IAPP have been developed. Studies in these models have revealed that the toxic effect of h-IAPP on beta-cell apoptosis demonstrates a threshold-dependent effect. Specifically, increasing h-IAPP transgene expression by breeding or induction of insulin resistance leads to increased beta-cell apoptosis and diabetes. These transgenic rodent models for h-IAPP provide an opportunity to elucidate the mechanisms responsible for h-IAPP-induced beta-cell apoptosis further and to test novel approaches to the prevention and treatment of T2D.

Chronically administered islet amyloid polypeptide in rats serves as an adiposity inhibitor and regulates energy homeostasis

AIMS/HYPOTHESIS

Islet amyloid polypeptide (IAPP) reduces food intake and body weight in laboratory animals. In addition, IAPP appears to regulate nutrient metabolism. In the present studies, we investigated the effect of chronic IAPP treatment on different aspects of energy homeostasis.

METHODS

IAPP was infused (25 pmol/kg/min) from subcutaneous osmotic pumps for 2-7 days. Rats in 2 saline-infused control groups were fed ad libitum (AF) or pair-fed (PF) against the IAPP-treated rats.

RESULTS

As expected, the IAPP infusion reduced food intake and body weight gain. In addition, the IAPP treatment decreased the epididymal fat pad (vs. PF rats, p < 0.05) and lowered circulating levels of triglycerides (vs. PF rats, p < 0.05), free fatty acids (vs. PF rats, p < 0.05), leptin (vs. both AF and PF rats, p < 0.05) and insulin (vs. AF rats, p < 0.05). In contrast, glucose and protein metabolism in the IAPP-treated rats was largely unchanged, as shown in results regarding serum glucose, glucose transport in skeletal muscle, blood urea nitrogen, and glycogen and protein content in the liver and in skeletal muscle.

CONCLUSION/INTERPRETATION

In summary, chronic IAPP exposure led to a changed lipid metabolism, which was characterized by decreased adiposity, hypolipidemia and hypoleptinemia, and to unchanged glucose and protein homeostasis. These results were similar to those seen in rodents during chronic exposure to another satiety/adiposity regulator, leptin. In conclusion, chronically administered IAPP plays a role as a satiety and adiposity signal in rats, and helps regulate energy homeostasis.

Inhibition of insulin secretion

Reports of the effects of amylin and amylin agonists on insulin secretion have varied widely. Some confusion can be attributed to the use of human amylin, which has been shown to readily fall out of solution resulting in low estimates of bioactivity. Some confusion can be resolved by assessing the probability that this had happened. The view taken here, supported by authors using reliable and well-characterized ligands (representing the preponderance of recent studies), is that exogenously administered amylin agonists inhibit insulin secretion, at least partly via activation of an amylin-like receptor linked to Gi-mediated inhibition of cAMP in islets. There may additionally be autonomic extrapancreatic effects of amylin on insulin secretion that derive from its action at the area postrema. Studies with amylin receptor antagonists, including human studies, indicate that endogenously secreted amylin may physiologically inhibit beta-cell secretion (insulin and amylin) via feedback inhibition that is characteristic of many other hormones. Part of this inhibition may be local (paracrine), as indicated by the amylin sensitivity of isolated preparations and the fact that the concentration of secreted products in the islet interstitium can be over 100-fold higher than in the circulation (Bendayan, 1993).

Neurodegenerative disorders associated with diabetes mellitus

More than 20 syndromes among the significant and increasing number of degenerative diseases of neuronal tissues are known to be associated with diabetes mellitus, increased insulin resistance and obesity, disturbed insulin sensitivity, and excessive or impaired insulin secretion. This review briefly presents such syndromes, including Alzheimer disease, ataxia-telangiectasia, Down syndrome/trisomy 21, Friedreich ataxia, Huntington disease, several disorders of mitochondria, myotonic dystrophy, Parkinson disease, Prader-Willi syndrome, Werner syndrome, Wolfram syndrome, mitochondrial disorders affecting oxidative phosphorylation, and vitamin B(1) deficiency/inherited thiamine-responsive megaloblastic anemia syndrome as well as their respective relationship to malignancies, cancer, and aging and the nature of their inheritance (including triplet repeat expansions), genetic loci, and corresponding functional biochemistry. Discussed in further detail are disturbances of glucose metabolism including impaired glucose tolerance and both insulin-dependent and non-insulin-dependent diabetes caused by neurodegeneration in humans and mice, sometimes accompanied by degeneration of pancreatic beta-cells. Concordant mouse models obtained by targeted disruption (knock-out), knock-in, or transgenic overexpression of the respective transgene are also described. Preliminary conclusions suggest that many of the diabetogenic neurodegenerative disorders are related to alterations in oxidative phosphorylation (OXPHOS) and mitochondrial nutrient metabolism, which coincide with aberrant protein precipitation in the majority of affected individuals.

Islet amyloid: a complication of islet dysfunction or an aetiological factor in Type 2 diabetes?

The role of islet amyloidosis in the onset and progression of Type 2 diabetes remains obscure. Islet amyloid polypeptide is a 37 amino-acid, beta-cell peptide which is co-stored and co-released with insulin. Human islet amyloid polypeptide refolds to a beta-conformation and oligomerises to form insoluble fibrils; proline substitutions in rodent islet amyloid polypeptide prevent this molecular transition. Pro-islet amyloid polypeptide (67 amino acids in man) is processed in secretory granules. Refolding of islet amyloid polypeptide may be prevented by intragranular heterodimer formation with insulin (but not proinsulin). Diabetes-associated abnormal proinsulin processing could contribute to de-stabilisation of granular islet amyloid polypeptide. Increased pro-islet amyloid polypeptide secretion as a consequence of islet dysfunction could promote fibrillogenesis; the propeptide forms fibrils and binds to basement membrane glycosamino-glycans. Islet amyloid polypeptide gene polymorphisms are not universally associated with Type 2 diabetes. Transgenic mice expressing human islet amyloid polypeptide gene have increased islet amyloid polypeptide concentrations but develop islet amyloid only against a background of obesity and/or high fat diet. In transgenic mice, obese monkeys and cats, initially small perivascular deposits progressively increase to occupy 80% islet mass; the severity of amyloidosis in animal models is related to the onset of hyperglycaemia, suggesting that islet amyloid and the associated destruction of islet cells cause diabetes. In human diabetes, islet amyloid can affect less than 1% or up to 80% of islets indicating that islet amyloidosis largely results from diabetes-related pathologies and is not an aetiological factor for hyperglycaemia. However, the associated progressive beta-cell destruction leads to severe islet dysfunction and insulin requirement.

Postnatally disturbed pancreatic islet cell distribution in human islet amyloid polypeptide transgenic mice

OBJECTIVE

Islet amyloid polypeptide (IAPP)/amylin is produced by the pancreatic islet beta-cells, which also produce insulin. To study potential functions of IAPP, we have generated transgenic mice overexpressing human IAPP (hIAPP) in the beta-cells. These mice show a diabetic phenotype when challenged with an oral glucose load. In this study, we examined the islet cytoarchitecture in the hIAPP mice by examining islet cell distribution in the neonatal period, as well as 1, 3 and 6 months after birth.

RESULTS

Neonatal transgenic mice exhibited normal islet cell distribution with beta-cells constituting the central islet portion, whereas glucagon and somatostatin-producing cells constituted the peripheral zone. In contrast, in hIAPP transgenic mice at the age of 1 month, the glucagon-immunoreactive (IR) cells were dispersed throughout the islets. Furthermore, at the age of 3 and 6 months, the islet organisation was similarly severely disturbed as at 1 month. Expression of both endogenous mouse IAPP and transgenic hIAPP was clearly higher in 6-month-old mice as compared to newborns, as revealed by mRNA in situ hybridisation.

CONCLUSIONS

Mice transgenic for hIAPP have islets with disrupted islet cytoarchitecture in the postnatal period, particularly affecting the distribution of glucagon-IR cells. This islet cellular phenotype of hIAPP transgenic mice is similar to that of other mouse models of experimental diabetes and might contribute to the impaired glucose homeostasis.

Role of islet amyloid in type 2 diabetes mellitus: consequence or cause?

Type 2 diabetes mellitus (DM2) is characterized metabolically by defects in both insulin secretion and insulin action, resulting in hyperglycemia. Histopathologically, DM2 is characterized by depositions of protein in the pancreatic islets. This 'islet amyloid' is present in >90% of patients with DM2, as well as in monkeys and cats with DM2. The pathogenesis of DM2 is heterogeneous and multifactorial, although insulin resistance seems to be the predominant initiating factor for development of the disease. In the longer term, an insulin secretion defect is also revealed (referred to as 'beta-cell failure'), resulting in clinically manifest diabetes. Recent data, particularly from transgenic mouse studies, indicate that islet amyloidosis is a diabetogenic factor, which is both consequence (of insulin resistance) and cause (of beta-cell failure) of DM2. Available transgenic mouse models with islet amyloid formation in vivo will provide the opportunity to assess the effectiveness of novel anti-amyloidogenic therapies, for which promising results are emerging.

The pathophysiology of type 2 diabetes mellitus: an overview

Type 2 diabetes mellitus is a heterogeneous disorder characterized by chronic hyperglycaemia. The aetiological heterogeneity is suggested by genetic inheritance and its interplay with environmental factors. Impaired insulin secretion and decreased insulin sensitivity are the main pathophysiological features, responsible for development of hyperglycaemia in type 2 diabetes. However, the genetic basis of these defects has been demonstrated only in small subgroups of patients. Whether impaired secretion or action of insulin is the primary defect in the majority of patients is not known, although it is generally agreed that defective insulin release is a requirement for the disease to develop.

PACAP contributes to insulin secretion after gastric glucose gavage in mice

Pituitary adenylate cyclase-activating polypeptide (PACAP) is localized to pancreatic ganglia governing the parasympathetic nerves, which contribute to prandial insulin secretion. We hypothesized that this contribution involves PACAP and show here that the PACAP receptor antagonist PACAP-(6---27) (1.5 nmol/kg iv) reduces the 15-min insulin response to gastric glucose (150 mg/mouse) by 18% in anesthetized mice (P = 0.041). The reduced insulinemia was not due to inhibited release of the incretin factor glucagon-like peptide 1 (GLP-1) because PACAP-(6---27) enhanced the GLP-1 response to gastric glucose. Furthermore, the GLP-1 antagonist exendin-3-(9---39) (30 nmol/kg) exerted additive inhibitory effect on the insulin response when combined with PACAP-(6---27). The PACAP antagonism was specific because intravenous PACAP-(6---27) inhibited the insulin response to intravenous PACAP-27 plus glucose without affecting the insulin response to intravenous glucose alone (1 g/kg) or glucose together with other insulin secretagogues of potential incretin relevance of intestinal (GLP-1, gastric inhibitory polypeptide, cholecystokinin) and neural (vasoactive intestinal peptide, gastrin-releasing peptide, cholinergic agonism) origin. We conclude that PACAP contributes to the insulin response to gastric glucose in mice and suggest that PACAP is involved in the regulation of prandial insulin secretion.

Diurnal variation in circulating leptin is dependent on gender, food intake and circulating insulin in mice

Leptin is an adipocyte hormone involved in the regulation of energy homeostasis. Its circulating levels show a diurnal rhythm with a nocturnal peak. We examined the influences of gender, feeding state, and plasma insulin and glucose on the diurnal rhythm in normal mice. Plasma was sampled at 4-h interval for 24 h in female (n=80) and male (n=80) mice, which were freely fed or fasted. In both genders, plasma leptin displayed a diurnal rhythm with a nadir at 8 or 10 AM and a nocturnal peak at 10 PM to 2 AM. The nocturnal increase in leptin was higher in females (+160+/-18%) than in males (61+/- 16%; P<0.001), completely abolished by fasting, and correlated significantly to the diurnal variation in plasma insulin both in females (r=0.44, P=0.003) and males (r=0.46, P<0.001). Baseline plasma leptin in non-fasted animals were not different between the genders, whereas during fasting, the reduction in leptin was more pronounced in males than in females, resulting in a higher plasma leptin after fasting in females. Plasma insulin was higher in males under non-fasted conditions (P=0.003), but not significantly different between genders in fasted animals. In conclusion, plasma leptin displays a nocturnal increase in mice, which is more pronounced in female mice than in male mice, is completely abolished by fasting and correlates to the diurnal variation in circulating insulin. It is suggested that the nocturnal rise in leptin shows gender dependency and is caused by the increase in plasma insulin caused by food intake.