Amyloid fibril formation is progressive and correlates with beta-cell secretion in transgenic mouse isolated islets

Aims/hypothesis. Amyloid fibrils are formed in islets isolated from transge nic mice expressing the gene for human islet amyloid polypeptide (IAPP) by an unknown mechanism. This model of islet amyloidosis in Type II (non-insul in-dependent) diabetes mellitus has been used to investigate the temporal a nd glucose dependency of fibril formation. Methods. To determine the time course and nature of amyloid-like accumulati ons and the role of glucose, transgenic mouse islets were cultured for 2-12 days in medium containing glucose (4.2 mmol/l, 1.1 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 qu antitative immune-electron microscopy. Insulin and islet amyloid polypeptid e secretion into the media was measured by radioimmunoassay. Results. Extracellular amyloid fibrils immunoreactive for islet amyloid pol ypeptide were visible initially after 6 days of culture in 1.1 mmol/l gluco se and formed 2.3 +/- 0.8% of the islet area after 12 days; small accumulat ions of intracellular fibrils and amorphous extracellular islet amyloid pol ypeptide-immunoreactive material were present at 6-12 days. Beta-cell secre tion was increased significantly by 16.7 mmol/l glucose and by alpha-ketois ocaproic acid + glutamine. The proportion of fibrillar amyloid (amyloid are a/islet area%) correlated with the amount of insulin (r = 0.55, p < 0.05) a nd IAPP (r = 0.5, p < 0.05) in the culture media. Evidence of cellular dama ge was present in less than 10% cells and correlated with the degree of fib ril deposition (r = 0.8,p < 0.0001). Conclusion/interpretation. These data suggest that islet amyloid polypeptid e amyloid is formed primarily at extracellular sites in isolated transgenic mouse islets and progressive fibril formation correlates with beta-cell se cretion.