Upregulation of the expression of tight and adherens junction-associated proteins during maturation of neonatal pancreatic islets in vitro.

Cell-cell contacts mediated by intercellular junctions are crucial for proper insulin secretion in the endocrine pancreas. The biochemical composition of the intercellular junctions in this organ and the role of junctional proteins in endocrine pancreatic dysfunctions are still unclear. In this study, we investigated the expression and cellular location of junctional and cytoskeletal proteins in cultured neonatal rat pancreatic islets. Neonatal B-cells had an impaired insulin secretion compared to adult cells. Cultured neonatal islets showed a time-dependent increase in the glucose-induced secretory response. The maturation of B-cells in vitro was accompanied by upregulation of the expression of some junctional proteins in islet cells. Neonatal islets cultured for only 24 h showed a low expression and a diffuse cytoplasmic location of the tight junctional proteins occludin and ZO-1 and of the adherens junctional proteins alpha- and beta-catenins, as demonstrated by immunoblotting and immunocytochemistry. Culturing islets for up to 8 days significantly increased the cell expression of these junctional proteins but not of the cytoskeletal proteins vinculin and alpha-actinin. A translocation of ZO-1 and catenins to the cell-cell contact region, as well as a higher association of F-actin with the intercellular junction, were also observed in neonatal islets following prolonged culturing. ZO-1 and beta-catenin were immunolocated in the endocrine pancreas of adult rats indicating that these junctional proteins are also expressed in this organ in situ. In conclusion, endocrine pancreatic cells express several junctional proteins that are upregulated following differentiation of the endocrine pancreas in vitro.