Amyloid deposits are found in the islets of Langerhans of up to 96 % of patients with Type II (non-insulin-dependent) diabetes mellitus [1,2] but the relation of islet amyloid to the syndrome of diabetes is not clear. Amyloid deposition precedes the onset of hyperglycaemia in cats and monkeys [3,4] suggesting a primary aetiological role of amyloid in these species. The severity of islet amyloidosis in diabetic patients at autopsy is, however, greatest in those who have progressed from sulphonylurea to insulin treatment [5] and the degree of amyloid deposition is associated with a decrease of beta-cell function in monkeys [4] implicating islet amyloid in islet dysfunction in the later stages of the disease. An increased incidence of amyloid deposition in vivo in some strains Diabetologia (1999) Abstract Aims/hypothesis. Amyloid fibrils are formed in islets isolated from transgenic mice expressing the gene for human islet amyloid polypeptide (IAPP) by an unknown mechanism. This model of islet amyloidosis in Type II (non-insulin-dependent) diabetes mellitus has been used to investigate the temporal and glucose dependency of fibril formation. Methods. To determine the time course and nature of amyloid-like accumulations and the role of glucose, transgenic mouse islets were cultured for 2±12 days in medium containing glucose (4.2 mmol/l, 11.1 mmol/l or 16.7 mmol/l) or 3.3 mmol/l glucose plus non-glucose secretagogues, 10 mmol/l leucine, 10 mmol/l leucine + 0.1 mmol/l tolbutamide, 10 mmol/l alpha-ketoisocaproic acid + 10 mmol/l glutamine. The extent of fibril formation was determined by quantitative immuno-electron microscopy. Insulin and islet amyloid polypeptide secretion into the media was measured by radioimmunoassay. Results. Extracellular amyloid fibrils immunoreactive for islet amyloid polypeptide were visible initially after 6 days of culture in 11.1 mmol/l glucose and formed 2.3 ± 0.8 % of the islet area after 12 days; small accumulations of intracellular fibrils and amorphous extracellular islet amyloid polypeptide-immunoreactive material were present at 6±12 days. Beta-cell secretion was increased significantly by 16.7 mmol/l glucose and by alpha-ketoisocaproic acid + glutamine. The proportion of fibrillar amyloid (amyloid area/islet area%) correlated with the amount of insulin (r = 0.55, p < 0.05) and IAPP (r = 0.5, p < 0.05) in the culture media. Evidence of cellular damage was present in less than 10 % cells and correlated with the degree of fibril deposition (r = 0.8, p < 0.0001). Conclusion/interpretation. These data suggest that islet amyloid polypeptide amyloid is formed primarily at extracellular sites in isolated transgenic mouse islets and progressive fibril formation correlates with beta-cell secretion. [Diabetologia (1999[Diabetologia ( ) 42: 1219± 1227

Islet amyloid, formed from islet amyloid polypeptide (JAPP), 'amylin', is deposited between the islet cells and capillaries in Type 2 (non-insulin dependent) diabetes: these deposits accumulate during the course of the disease and are likely to play a role in the progressive decrease in islet function in Type 2 diabetes. IAPP is co-localised and co-secreted with insulin. The causal factors for the formation of islet amyloid in are largely unknown; the amino acid sequence of IAPP is important since species-specific changes in molecular structure explain the lack of amyloid in rodent models of diabetes [1]. Transgenic mice overexpressing the gene for human islet amyloid polypeptide, (hIAPP) have elevated circulating concentrations of JAPP [2]. Although many different transgenic lines have been created with variable amounts of overexpression, few have shown formation of islet amyloid in vivo [2][3][4][5]. In addition, amyloid was not formed when cellular secretion of n-cell products was chronically stimulated in hIAPP transgenic mice (hIAPP TM) by treatment with dexamethasone or partial pancreatectomy. Recent studies in hIAPP TM have shown that amyloid is deposited in vivo when the levels of transgene expression is increased in homozygous hIAPP TM [6], or by induction of obesity [7][8][9].Amyloid is formed in vitro within three days in islets isolated from hIAPP TM and cultured in the presence of 13-cell secretagogues [10]; amyloid fibrils are present in hIAPP TM islets cultured in 16.7 or 28 mM glucose for three or more days. These fibrils are short and unbranching and IAPP immunoreactive (IAPP-ir) and appear identical in structure to amyloid in islets of Type 2 diabetic subjects. To determine if glucose is involved in fibril formation from IAPP, islets were cultured in the presence of non-glucose secretagogues leucine, ct-ketoisocaproic acid and tolbutamide. Amyloid was formed with non-glucose stimulation of islets and the amount of amyloid was correlated with the induced insulin and JAPP secretion. This suggests that glycation of IAPP is not necessary for fibril formation.In diabetes, amyloid deposits replace islet endocrine cells. Synthetic IAPP fibrils have been shown to be cytotoxic to islet cells in culture by a process of apoptosis (11). However, we have found that synthetic hIAPP (1.2-12 .tM) in fibrillar form was not cytotoxic to intact mouse or human islets but did cause death of cultured mouse 13-TC cells. The mechanism by which fibrils induce cytotoxicty in isolated cells but not in islet-cell lines is unclear; we have been unable to confirm cytotoxicity by apoptosis and suggest that fibrils interact with the cell membrane and cause necrosis rather than apoptosis. Amyloid fibrils in vivo disrupt the surface of islet 13-cells and create deep membrane invaginations filled with fibrils. This would interrupt the normal function of the cell membrane for glucose signalling and insulin secretion.Deposition of amyloid in vivo is irreversible and progressive. Many other pathological processes are associated with...