Formation of islet amyloid fibrils in beta-secretory granules of transgenic mice expressing human islet amyloid polypeptide/amylin.

To investigate the relationship between human islet amyloid polypeptide (IAPP)/amylin expression and islet amyloid deposits in the pathogenesis of human non-insulin-dependent diabetes mellitus (NIDDM), we developed transgenic mice using a human IAPP cDNA connected to an insulin promoter. Ribonucleic acid blotting and immunohistochemistry revealed the expression of the transgene in the pancreatic beta cells. Immunogold electron microscopy showed that beta-secretory granules contained the human C-terminal flanking peptide of the IAPP precursor. Reverse-phase HPLC demonstrated human and mouse IAPP amide in the pancreas. Electron microscopy showed the accumulation of fibril-like material in a considerable number of beta-secretory granules. These results suggest that in transgenic mice, the human IAPP precursor is expressed in beta cells and becomes normally sorted into beta-secretory granules in which normal conversion to mature human IAPP takes place. The human IAPP molecules, because of their amyloidogenesis, aggregate into amyloid fibrils in secretory granules. Glucose tolerance was normal at 7 months old and islet amyloid was not observed. A longer time may be required for islet amyloid deposits and hyperglycemia to develop in mice. Our working hypothesis is that in human NIDDM, IAPP aggregates into amyloid fibrils in beta-secretory granules, and that the fibrils are released into the extracellular space and islet amyloid deposits become substantial with time.