Amyloid Formation in Response to β Cell Stress Occurs In Vitro, but Not In Vivo, in Islets of Transgenic Mice Expressing Human Islet Amyloid Polypeptide

SORLA mediates endocytic uptake of proIAPP and protects against islet amyloid deposition

Objective

Sorting-related receptor with type A repeats (SORLA) is a neuronal sorting receptor that prevents accumulation of amyloid-beta peptides, the main constituent of senile plaques in Alzheimer disease. Recent transcriptomic studies show that SORLA transcripts are also found in beta cells of pancreatic islets, yet the role of SORLA in islets is unknown. Based on its protective role in reducing the amyloid burden in the brain, we hypothesized that SORLA has a similar function in the pancreas via regulation of amyloid formation from islet amyloid polypeptide (IAPP).

Methods

We generated human IAPP transgenic mice lacking SORLA (hIAPP:SORLA KO) to assess the consequences of receptor deficiency for islet histopathology and function in vivo. Using both primary islet cells and cell lines, we further investigated the molecular mechanisms whereby SORLA controls the cellular metabolism and accumulation of IAPP.

Results

Loss of SORLA activity in hIAPP:SORLA KO resulted in a significant increase in islet amyloid deposits and associated islet cell death compared to hIAPP:SORLA WT animals. Aggravated islet amyloid deposition was observed in mice fed a normal chow diet, not requiring high-fat diet feeding typically needed to induce islet amyloidosis in mouse models. In vitro studies showed that SORLA binds to and mediates the endocytic uptake of proIAPP, but not mature IAPP, delivering the propeptide to an endolysosomal fate.

Conclusions

SORLA functions as a proIAPP-specific clearance receptor, protecting against islet amyloid deposition and associated cell death caused by IAPP.

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SORLA is an endocytic receptor for amyloidogenic peptides expressed in islet beta cells.

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SORLA mediates cellular clearance of proIAPP.

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Loss of SORLA activity in mouse models causes spontaneous islet amyloid deposition.

Amylin at the interface between metabolic and neurodegenerative disorders

The pancreatic peptide amylin is best known for its role as a satiation hormone in the control of food intake and as the major component of islet amyloid deposits in the pancreatic islets of patients with type 2 diabetes mellitus (T2DM). Epidemiological studies have established a clear association between metabolic and neurodegenerative disorders in general, and between T2DM and Alzheimer's disease (AD) in particular. Here, we discuss that amylin may be an important player acting at the interface between these metabolic and neurodegenerative disorders. Abnormal amylin production is a hallmark peripheral pathology both in the early (pre-diabetic) and late phases of T2DM, where hyperamylinemic (early phase) and hypoamylinemic (late phase) conditions coincide with hyper- and hypo-insulinemia, respectively. Moreover, there are notable biochemical similarities between amylin and β-amyloids (Aβ), which are both prone to amyloid plaque formation and to cytotoxic effects. Amylin's propensity to form amyloid plaques is not restricted to pancreatic islet cells, but readily extends to the CNS, where it has been found to co-localize with Aβ plaques in at least a subset of AD patients. Hence, amylin may constitute a “second amyloid” in neurodegenerative disorders such as AD. We further argue that hyperamylinemic conditions may be more relevant for the early processes of amyloid formation in the CNS, whereas hypoamylinemic conditions may be more strongly associated with late stages of central amyloid pathologies. Advancing our understanding of these temporal relationships may help to establish amylin-based interventions in the treatment of AD and other neurodegenerative disorders with metabolic comorbidities.

Pros and cons of fatty acids in bone biology

Despite the growing interest in deciphering the causes and consequences of obesity-related disorders, the mechanisms linking fat intake to bone behaviour remain unclear. Since bone fractures are widely associated with increased morbidity and mortality, most notably in elderly and obese people, bone health has become a major social and economic issue. Consistently, public health system guidelines have encouraged low-fat diets in order to reduce associated complications. However, from a bone point of view, mechanisms linking fat intake to bone alteration remain quite controversial. Thus, after more than a decade of dedicated studies, this timely review offers a comprehensive overview of the relationships between bone and fatty acids. Using clinical evidences as a starting-point to more complex molecular elucidation, this work highlights the complexity of the system and reveals that bone alteration that cannot be solved simply by taking ω-3 pills. Fatty acid effects on bone metabolism can be both direct and indirect and require integrated investigations. Furthermore, even at the level of a single cell, one fatty acid is able to trigger several different independent pathways (receptors, metabolites…) which may all have a say in the final cellular metabolic response.

Body mass index and outcomes from pancreatic resection: a review and meta-analysis

INTRODUCTION

There are 1.6 billion adults worldwide who are overweight, with body mass indices (BMI) between 25 and 30, while more than 400 million are obese (BMI >30). Obesity predicts the incidence of and poor outcomes from pancreatic cancer. Obesity has also been linked to surgical complications in pancreatectomy, including increased length of hospital stay, surgical infections, blood loss, and decreased survival. However, BMI's impact on many complications following pancreatectomy remains controversial.

METHODS

We performed a MEDLINE search of all combinations of "BMI" with "pancreatectomy," "pancreatoduodenectomy," or "pancreaticoduodenectomy." From included studies, we created pooled and weighted estimates for quantitative and qualitative outcomes. We used the PRISMA criteria to ensure this project's validity.

RESULTS

Our primary cohort included 2,736 patients with BMI <30, 1,682 with BMI >25, and 546 with BMI between 25 and 30. Most outcomes showed no definitive differences across BMIs. Pancreatic fistula (PF) rates ranged from 4.7% to 31.0%, and four studies found multivariate association between BMI and PF (range odds ratio 1.6-4.2). Pooled analyses of PF by BMI showed significant association (p < 0.05).

CONCLUSION

BMI increases the operative complexity of pancreatectomy. However, with aggressive peri- and post-operative care, increases in BMI-associated morbidity and mortality may be mitigated.

Exendin-4 increases islet amyloid deposition but offsets the resultant beta cell toxicity in human islet amyloid polypeptide transgenic mouse islets

AIMS/HYPOTHESIS

In type 2 diabetes, aggregation of islet amyloid polypeptide (IAPP) into amyloid is associated with beta cell loss. As IAPP is co-secreted with insulin, we hypothesised that IAPP secretion is necessary for amyloid formation and that treatments that increase insulin (and IAPP) secretion would thereby increase amyloid formation and toxicity. We also hypothesised that the unique properties of the glucagon-like peptide-1 (GLP-1) receptor agonist exendin-4 to maintain or increase beta cell mass would offset the amyloid-induced toxicity.

METHODS

Islets from amyloid-forming human IAPP transgenic and control non-transgenic mice were cultured for 48 h in 16.7 mmol/l glucose alone (control) or with exendin-4, potassium chloride (KCl), diazoxide or somatostatin. Human IAPP and insulin release, amyloid deposition, beta cell area/islet area, apoptosis and AKT phosphorylation levels were determined.

RESULTS

In control human IAPP transgenic islets, amyloid formation was associated with increased beta cell apoptosis and beta cell loss. Increasing human IAPP release with exendin-4 or KCl increased amyloid deposition. However, while KCl further increased beta cell apoptosis and beta cell loss, exendin-4 did not. Conversely, decreasing human IAPP release with diazoxide or somatostatin limited amyloid formation and its toxic effects. Treatment with exendin-4 was associated with an increase in AKT phosphorylation compared with control and KCl-treated islets.

CONCLUSIONS/INTERPRETATION

IAPP release is necessary for islet amyloid formation and its toxic effects. Thus, use of insulin secretagogues to treat type 2 diabetes may result in increased islet amyloidogenesis and beta cell death. However, the AKT-associated anti-apoptotic effects of GLP-1 receptor agonists such as exendin-4 may limit the toxic effects of increased islet amyloid.

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.

Amyloid deposition in transplanted human pancreatic islets: a conceivable cause of their long-term failure

Following the encouraging report of the Edmonton group, there was a rejuvenation of the islet transplantation field. After that, more pessimistic views spread when long-term results of the clinical outcome were published. A progressive loss of the β-cell function meant that almost all patients were back on insulin therapy after 5 years. More than 10 years ago, we demonstrated that amyloid deposits rapidly formed in human islets and in mouse islets transgenic for human IAPP when grafted into nude mice. It is, therefore, conceivable to consider amyloid formation as one potential candidate for the long-term failure. The present paper reviews attempts in our laboratories to elucidate the dynamics of and mechanisms behind the formation of amyloid in transplanted islets with special emphasis on the impact of long-term hyperglycemia.

Ageing and amyloid fibrillogenesis: lessons from apolipoprotein AI, transthyretin and islet amyloid polypeptide

The age-associated (or senile) amyloidoses encompass a heterogeneous group of systemic or localized forms of amyloidosis. In this paper we present an overview of three age-associated amyloid forms derived from transthyretin, apolipoprotein AI and islet amyloid polypeptide. Mutations in the respective genes give rise to transthyretin and apolipoprotein AI forms of familial amyloidosis while senile forms of amyloid are associated with the wild-type proteins. Different mechanisms are probably of importance in the fibrillogenesis associated with these three amyloid types. It is also possible that different amyloidogenic pathways exist for a single amyloidogenic protein. Thus, limited proteolysis may be necessary in the fibrillogenesis in senile transthyretin amyloidosis but not in most familial transthyretin amyloidoses. Other factors in the pathogenesis of amyloidosis such as local concentration, nidus formation and glycation are also discussed.

Glucose- and time-dependence of islet amyloid formation in vitro

Islet amyloid contributes to the loss of beta-cell mass in type 2 diabetes. To examine the roles of glucose and time on amyloid formation, we developed a rapid in vitro model using isolated islets from human islet amyloid polypeptide (hIAPP) transgenic mice. Islets from hIAPP transgenic and non-transgenic mice were cultured for up to 7 days with either 5.5, 11.1, 16.7 or 33.3mmol/l glucose. At various time-points throughout the culture period, islets were harvested for determination of amyloid and beta-cell areas, and for measures of cell viability, insulin content, and secretion. Following culture of hIAPP transgenic islets in 16.7 or 33.3mmol/l glucose, amyloid formation was significantly increased compared to 5.5 or 11.1mmol/l glucose culture. Amyloid was detected as early as day 2 and increased in a time-dependent manner so that by day 7, a decrease in the proportion of beta-cell area in hIAPP transgenic islets was evident. When compared to non-transgenic islets after 7-day culture in 16.7mmol/l glucose, hIAPP transgenic islets were 24% less viable, had decreased beta-cell area and insulin content, but displayed no change in insulin secretion. Thus, we have developed a rapid in vitro model of light microscopy-visible islet amyloid formation that is both glucose- and time-dependent. Formation of amyloid in this model is associated with reduced cell viability and beta-cell loss but adequate functional adaptation. It thus enables studies investigating the mechanism(s) underlying the amyloid-associated loss of beta-cell mass in type 2 diabetes.

An in vitro model of early islet amyloid polypeptide (IAPP) fibrillogenesis using human IAPP-transgenic mouse islets

The mechanisms underlying insufficient insulin secretion and loss of beta-cell mass in feline and human type 2 diabetes mellitus are incompletely understood. However, islet amyloid polypeptide (IAPP)-derived islet amyloidosis (IA) has been linked to increased rates of beta-cell apoptosis and, therefore, our goal was to develop an in vitro model of IAPP fibrillogenesis using isolated pancreatic islets from mice transgenic for human IAPP (hIAPP Tg mice). Islets from hIAPP Tg mice, from mice transgenic for non-amyloidogenic murine IAPP (mIAPP Tg mice), and from the FVB background strain were exposed to normal (5.5 mM) or high (28 mM) glucose conditions in cell culture for 8 days. On days 0 and 8, islets were collected for electron microscopy (EM). EM showed no abnormalities in the mIAPP Tg or FVB islets at either time point. On day 8, hIAPP Tg islets cultured at high glucose concentration formed extracellular IAPP-derived flocculent deposits. No significant differences in rates of apoptosis were found between groups. Our findings, therefore, show that in vitro culture of hIAPP Tg mouse islets under high glucose conditions produces a readily available and rapidly inducible model of IAPP-derived fibrillogenesis and enables the study of early phases of the molecular pathogenesis of IA.

Aberrant processing of human proislet amyloid polypeptide results in increased amyloid formation

The amyloid present in the islets of Langerhans in type 2 diabetes is polymerized islet amyloid polypeptide (IAPP). The precursor protein proIAPP is posttranslationally modified, a process involving the removal of NH2- and COOH-terminal flanking peptides. This step is performed by the prohormone convertases PC2 and PC1/3. PC2 processes proIAPP preferably at the NH2-terminal processing site, and PC1/3 processes proIAPP exclusively at the COOH-terminal site. Little is known regarding the exact circumstances leading to islet amyloid formation. In this study, we have examined the possible significance of aberrant processing of proIAPP on amyloid formation in several in vitro cellular systems. In our studies, human (h)-proIAPP was transfected into beta-TC-6 cells expressing both prohormone convertases and in which proIAPP is processed into IAPP. Additionally, h-proIAPP was transfected into three different pituitary-derived cell lines with different prohormone convertase profiles: AtT-20 cells (deficient in PC2), GH3 cells (deficient in PC1/3), and GH4C1 cells (deficient in both convertases). We followed the processing of h-proIAPP with antibodies specific for the respective cleavage sites and stained the cells with Congo red to verify the accumulation of amyloid. Incomplete processing of h-proIAPP that occurs in AtT-20 and GH4C1 cells resulted in the formation of intracellular amyloid. No amyloid developed in beta-TC-6 and GH3 cells lines with full processing of proIAPP. An intracellular increase in proIAPP and/or its metabolic products may thus promote intracellular amyloid formation, thereby causing cell death. When extracellularly exposed, this amyloid might act as template for continuing amyloid formation from processed IAPP released from the surrounding beta-cells.

Long-term treatment with rosiglitazone and metformin reduces the extent of, but does not prevent, islet amyloid deposition in mice expressing the gene for human islet amyloid polypeptide

Islet amyloid deposition in type 2 diabetes is associated with reduced beta-cell mass. Therefore, interventions aimed at reducing islet amyloid formation may help preserve beta-cell mass in type 2 diabetes. Rosiglitazone and metformin act by different mechanisms to improve insulin sensitivity and thereby reduce beta-cell secretory demand, resulting in decreased release of insulin and islet amyloid polypeptide (IAPP), the unique constituent of islet amyloid deposits. We hypothesized that this reduced beta-cell secretory demand would lead to reduced islet amyloid formation. Human IAPP (hIAPP) transgenic mice, a model of islet amyloid, were treated for 12 months with rosiglitazone (1.5 mg.kg(-1).day(-1), n = 19), metformin (1 g.kg(-1).day(-1), n = 18), or control (n = 17). At the end of the study, islet amyloid prevalence (percent islets containing amyloid) and severity (percent islet area occupied by amyloid), islet mass, beta-cell mass, and insulin release were determined. Islet amyloid prevalence (44 +/- 8, 13 +/- 4, and 11 +/- 3% for control, metformin-, and rosiglitazone-treated mice, respectively) and severity (9.2 +/- 3.0, 0.22 +/- 0.11, and 0.10 +/- 0.05% for control, metformin-, and rosiglitazone-treated mice, respectively) were markedly reduced with both rosiglitazone (P < 0.001 for both measures) and metformin treatment (P < 0.001 for both measures). Both treatments were associated with reduced insulin release assessed as the acute insulin response to intravenous glucose (2,189 +/- 857, 621 +/- 256, and 14 +/- 158 pmol/l for control, metformin-, and rosiglitazone-treated mice, respectively; P < 0.05 for metformin vs. control and P < 0.005 for rosiglitazone vs. control), consistent with reduced secretory demand. Similarly, islet mass (33.4 +/- 7.0, 16.6 +/- 3.6, and 12.2 +/- 2.1 mg for control, metformin-, and rosiglitazone-treated mice, respectively) was not different with metformin treatment (P = 0.06 vs. control) but was significantly lower with rosiglitazone treatment (P < 0.05 vs. control). When the decreased islet mass was accounted for, the islet amyloid-related decrease in beta-cell mass (percent beta-cell mass/islet mass) was ameliorated in both rosiglitazone- and metformin-treated animals (57.9 +/- 3.1, 64.7 +/- 1.4, and 66.1 +/- 1.6% for control, metformin-, and rosiglitazone-treated mice, respectively; P < 0.05 for metformin or rosiglitazone vs. control). In summary, rosiglitazone and metformin protect beta-cells from the deleterious effects of islet amyloid, and this effect may contribute to the ability of these treatments to alleviate the progressive loss of beta-cell mass and function in type 2 diabetes.

Is aggregated IAPP a cause of beta-cell failure in transplanted human pancreatic islets?

Aggregation of the beta-cell product islet amyloid polypeptide (IAPP) is believed to be an important event in the development of the beta-cell lesion in type 2 diabetes. Preamyloidotic oligomeric IAPP assemblies exert toxic effects on beta cells that die, leading to reduced beta-cell mass. Normal human islets, when isolated and cultured in vitro or transplanted into nude mice, also develop amyloid deposits, which are associated with increased beta-cell death and reduced beta-cell mass. The possible role of IAPP aggregation and amyloid formation in loss of islet transplant function should be taken into consideration and studied further.

Biological importance of the peptides of the calcitonin family as revealed by disruption and transfer of corresponding genes

The hormone calcitonin (CT) of thyroid C-cell origin, the neuropeptides alpha- and beta-calcitonin gene-related peptide (CGRP), the widely expressed hormone and tissue factor adrenomedullin (AM), and amylin (AMY) that is co-produced with insulin in pancreatic beta-cells, are structurally related peptides. They have in common six or seven amino acid ring structures, linked by disulfide bridges between cysteine residues, and amidated carboxyl termini that are both required for biological activity. The actions of the peptides in vivo have traditionally been studied after intravenous and intracerebroventricular administration. As a result, CT lowers serum calcium and reduces pain perception. alpha- and beta CGRP and AM are highly potent vasodilatory peptides. AMY inhibits food intake through its action in the area postrema of the brain. Physiological actions of the peptides summarized in the present review have been defined through gene knockout and overexpression strategies.

Amyloidogenicity and cytotoxicity of islet amyloid polypeptide

Insoluble amyloid formation by islet amyloid polypeptide (IAPP) in the islets of Langerhans of the pancreas is a major pathophysiological feature of noninsulin dependent diabetes mellitus (NIDDM) or type II diabetes. Because in vivo formed amyloid colocalizes with areas of cell degeneration and IAPP amyloid aggregates are cytotoxic per se, the process of IAPP amyloid formation has been strongly associated with the progressive pancreatic cell degeneration and thus much of the pathology of type II diabetes. IAPP is a pancreatic polypeptide of 37 residues that, in its soluble form, is believed to play a role as a regulator of glucose homeostasis. The molecular cause and mechanism of the conversion of soluble IAPP into insoluble amyloid aggregates in vivo and its role in disease progress still remain to be clarified. Nevertheless, in the past few years significant progress has been made in understanding the amyloidogenesis pathway of IAPP in vitro and gaining insight into the structural and conformational "requirements" of IAPP amyloidogenesis and related cytotoxic effects. Importantly, several of the studies have revealed significant similarities of the above features of IAPP to other amyloidogenic polypeptides such as the beta-amyloid polypeptide Abeta. This suggests that, at the molecular level, amyloidogenesis, and possibly related cell degeneration and disease pathogenesis by completely different polypeptide sequences, may obey to common structural and conformational "rules" and follow similar molecular pathways. This review describes studies on the structural and conformational features of IAPP amyloid formation and cytotoxicity, and the application of the obtained knowledge for the understanding of the molecular mechanism of the IAPP amyloidogenesis pathway and the related cytotoxicity.

Islet amyloid and type 2 diabetes: from molecular misfolding to islet pathophysiology

Islet amyloid polypeptide (IAPP, amylin) is secreted from pancreatic islet beta-cells and converted to amyloid deposits in type 2 diabetes. Conversion from soluble monomer, IAPP 1-37, to beta-sheet fibrils involves changes in the molecular conformation, cellular biochemistry and diabetes-related factors. In addition to the recognised amyloidogenic region, human IAPP (hIAPP) 20-29, the peptides human or rat IAPP 30-37 and 8-20, assume beta-conformation and form fibrils. These three amyloidogenic regions of hIAPP can be modelled as a folding intermediate with an intramolecular beta-sheet. A hypothesis is proposed for co-secretion of proIAPP with proinsulin in diabetes and formation of a 'nidus' adjacent to islet capillaries for subsequent accumulation of secreted IAPP to form the deposit. Although intracellular fibrils have been identified in experimental systems, extracellular deposition predominates in animal models and man. Extensive fibril accumulations replace islet cells. The molecular species of IAPP that is cytotoxic remains controversial. However, since fibrils form invaginations in cell membranes, small non-toxic IAPP fibrillar or amorphous accumulations could affect beta-cell stimulus-secretion coupling. The level of production of hIAPP is important but not a primary factor in islet amyloidosis; there is little evidence for inappropriate IAPP hypersecretion in type 2 diabetes and amyloid formation is generated in transgenic mice overexpressing the gene for human IAPP only against a background of obesity. Animal models of islet amyloidosis suggest that diabetes is induced by the deposits whereas in man, fibril formation appears to result from diabetes-associated islet dysfunction. Islet secretory failure results from progressive amyloidosis which provides a target for new therapeutic interventions.

Islet amyloid polypeptide regulates multiple steps in stimulus-secretion coupling of beta cells in rat pancreatic islets

Islet amyloid polypeptide (IAPP) is produced in pancreatic beta cells. Intraislet function of IAPP is still uncertain. In the present study, we investigated effects of IAPP and somatostatin on stimulus-secretion coupling of beta cells in isolated rat pancreatic islets. Insulin secretion induced by 22.2 mM glucose was increased by an IAPP antiserum (0.1%) or an IAPP antagonist (IAPP8-37, 10 microM). Pretreatment of islets with pertussis toxin (PTX) abolished the stimulating effect of IAPP8-37 on glucose-induced insulin secretion. In contrast, IAPP antiserum and IAPP8-37 did not change insulin secretion induced by 30 mM KCl. Somatostatin (1 nM) inhibited insulin secretion induced by 22.2 mM glucose, 10 mM L-arginine, 25 microM forskolin, and 200 microM carbachol. IAPP (10 microM) enhanced the inhibitory effect of somatostatin on insulin secretion induced by L-arginine or forskolin. PTX pretreatment abolished the effects of somatostatin and IAPP on arginine-induced insulin secretion. In conclusion, IAPP regulates multiple steps in signal transductions of beta cells. The effects of IAPP on beta cells are mediated by PTX-sensitive regulatory G proteins.

Pro islet amyloid polypeptide (ProIAPP) immunoreactivity in the islets of Langerhans

Islet amyloid is typically found in type 2 diabetes mellitus and is believed to participate in the beta cell deterioration. The islet amyloid fibril consists of the 37-amino-acid islet amyloid polypeptide (IAPP) but its pathogenesis is only partly understood. We developed several different rabbit antisera against the flanking peptides of the IAPP precursor (proIAPP) and the proIAPP processing sites in order to study the possible occurrence of unprocessed proIAPP or parts thereof in islet amyloid. We applied these antisera in an immunohistochemical study on, islet amyloid deposits present in a newly generated mouse strain that over-expresses human IAPP but is devoid of mouse IAPP. Male mice of this strain develop severe islet amyloidosis when given a high fat diet. Generally, the antisera showed no immunoreactivity with the amyloid. However, in scattered single beta cells, where amyloid could be seen intracellularly, immunoreactivity with one or more of the antisera co-localized with the amyloid. Although virtually all amyloid in human islets of Langerhans is found extracellularly, we propose that the initial amyloid formation occurs intracellularly, perhaps by not fully processed or folded (pro)IAPP. This amyloid, which may develop rapidly under certain circumstances, probably leads to cell death. If not degraded these amyloid spots may then act as nidus for further amyloid formation from fully processed IAPP, secreted from surrounding beta cells.

Differences in amyloid deposition in islets of transgenic mice expressing human islet amyloid polypeptide versus human islets implanted into nude mice

Islet amyloid polypeptide (IAPP)-derived amyloid is frequently deposited in the islets of Langerhans in patients with chronic non-insulin-dependent diabetes mellitus (NIDDM). When human islets were implanted under the renal capsule in nude mice, amyloid occurred in 73% of the grafts within 2 weeks. In this study, we compare the deposition of amyloid in islets from a transgenic mouse strain expressing human IAPP (hIAPP) and in normal human islets after implantation in nude mice. The implantations were performed as follows: (1) nondiabetic recipients were given islets from transgenic mice alone, (2) human islets were implanted in the upper pole of the kidney and islets from transgenic mice were implanted in the lower pole of the kidney, (3) grafts containing a mixture of human and transgenic islets were implanted, and (4) transgenic islets and islets from nontransgenic littermates were implanted in therapeutic numbers into recipients made diabetic by a single injection of alloxan prior to implantation. The implants were removed after various periods from 4 days to 8 weeks. The implants were either fixed in Formalin, stained for amyloid, and viewed in polarized light, or processed for immunoelectron microscopy and studied after immunolabeling with specific antibodies against IAPP. We found that the course of amyloid deposition differed significantly between human islets and hIAPP-expressing mouse islets. In human islets, amyloid was mainly deposited intracellularly and only small amounts of amyloid were found extracellularly. In contrast, in islets from transgenic mice, amyloid was exclusively deposited extracellularly and deposition in this site was preceded by an aggregation of immunoreactive material along the basement membrane. These findings point to separate mechanisms for amyloid formation in these two models.

Islet amyloid polypeptide tonally inhibits beta-, alpha-, and delta-cell secretion in isolated rat pancreatic islets

Islet amyloid polypeptide (IAPP, or amylin) is produced in pancreatic beta-cells. The intraislet significance of IAPP is still uncertain. In the present study, paracrine effects of endogenous IAPP and somatostatin were investigated in isolated rat pancreatic islets. The intraislet IAPP activity was inhibited with an IAPP antiserum or a specific antagonist [IAPP-(8-37)]. Somatostatin activity was inhibited by immunoneutralization. Basal insulin and glucagon secretion were not affected by the somatostatin and/or IAPP blockade. Arginine-stimulated insulin and glucagon secretion were dose dependently increased by IAPP antiserum, IAPP-(8-37), and somatostatin antiserum, respectively. Arginine-stimulated somatostatin secretion was dose dependently potentiated by IAPP antiserum. Insulin secretion induced by 16.7 mM glucose was enhanced by IAPP antiserum and IAPP-(8-37), respectively. A combination of somatostatin antiserum with IAPP antiserum or IAPP-(8-37) further enhanced the arginine-stimulated insulin and glucagon secretion compared with effects when the blocking reagents were used individually. These results indicate that endogenously produced IAPP tonally inhibits stimulated insulin, glucagon, and somatostatin secretion. Furthermore, the paracrine effects of IAPP and somatostatin are additive.

Quantitative immunohistochemical analysis of islet amyloid polypeptide (IAPP) in normal, impaired glucose tolerant, and diabetic cats

Islet amyloid polypeptide (IAPP, "amylin") has been proposed as having important roles in the pathogenesis of type 2 diabetes mellitus via its biological activity and by forming islet amyloid. The domestic cat develops a type of diabetes that closely resembles type 2 diabetes in humans, including the frequent formation of islet amyloid deposits in the impaired glucose tolerant (IGT) and diabetic state. With the aid of computerized image analysis and immunohistochemistry, we examined the IAPP and insulin content in pancreatic islets of normal, IGT and diabetic cats. IAPP immunoreactivity in beta cells from IGT cats was significantly stronger (p < 0.01) as compared with cells from normal cats, while the insulin labelling strength was unchanged. Overtly diabetic cats were usually almost devoid of beta cells. As in humans, cellular IAPP but not IAPP in islet amyloid deposits was labelled by the newly developed monoclonal antibody to IAPP 4A5, thus providing further evidence that IAPP is modified by a yet unknown mechanism during the amyloidogenic process. The study provides evidence that an increased beta cell storage of IAPP independent of insulin may be an important factor in the early phase of the development of islet amyloid in this form of diabetes.

Effect of islet amyloid polypeptide on somatostatin inhibition of insulin secretion from isolated rat pancreatic islets

In this study, we investigated the presence of islet amyloid polypeptide (IAPP) in somatostatin cells of rat endocrine pancreas and the effect of exogenous IAPP and somatostatin, separate or combined, on in vitro insulin secretion. By immunocytochemistry, IAPP was found in both B and D cells of rat pancreatic islets. Furthermore, the labeling density of IAPP in D cells was nearly four times higher than in B cells. After a 2-day preincubation in RPMI 1640 (11.1 mM glucose), isolated rat pancreatic islets were exposed to IAPP and/or somatostatin for 90 min in modified Krebs-Ringer bicarbonate (KRB) buffers containing 11.1 or 22.2 mM glucose, or 11.1 mM glucose + 10 mM L-arginine, respectively. At 11.1 mM glucose, insulin secretion was not affected by IAPP and/or somatostatin at concentrations investigated. Insulin response to 22.2 mM glucose was inhibited by exogenous somatostatin. Arginine-stimulated insulin secretion was also inhibited by somatostatin, and the effect was significantly potentiated with additional 10(-5) M IAPP. The study shows that rat pancreatic D cells have higher IAPP density than B cells in the same islets and that IAPP and somatostatin may cooperate on rat pancreatic B cells to regulate the insulin secretion in response to potent stimulation.