An Investigation of Coxsackie and Adenovirus Receptor in the Human Pancreatic Beta Cells

Abstract

Human pancreatic beta cells are susceptible to infection by enteroviruses, especially Coxsackie B viruses, and such infections could contribute to the development of Type 1 diabetes. Enteroviruses gain entry via cell surface receptors, one of which, the Coxsackie and Adenovirus receptor (CAR), is a transmembrane cell adhesion protein which serves as a key entry receptor for Coxsackie B viruses and is thought to be localised mainly within regions where contacts are formed between adjacent cells. CAR exists as at least 5 isoforms and this study has examined their expression profile and distribution in the human pancreas utilising; formalin-fixed paraffin-embedded pancreatic sections from non-diabetic individuals, type 1 diabetes patients and a human tissue microarray. Isolated human islets, human pancreatic beta and ductal cell lines were also studied. Immunological and molecular approaches were employed to examine the expression and cellular localisation of the known CAR isoforms in human pancreas. One specific isoform of CAR (CAR-SIV) with a unique C terminal PDZ binding domain, was highly expressed in human beta cells at the protein level. Surprisingly, it was distributed in a punctate manner mainly within the cytoplasm of the cells, rather than at the cell surface. In human beta cells, within the cytoplasm CAR-SIV co-localised with ZnT8, PC1/3 and insulin but less so with proinsulin suggesting that CAR-SIV is associated with insulin secretory granules. Immunogold labelling and electron microscopic analysis revealed that CAR-SIV is localised both to maturing insulin secretory granules and to fully mature, dense-core (insulin) secretory granules. Intriguingly, CAR-SIV colocalises and interacts with a cytosolic protein, PICK1, which plays a role in the budding, maturation and trafficking of insulin secretory granules. On this basis, a model is proposed whereby CAR-SIV and PICK1 interact to regulate the maturation and trafficking of insulin secretory granules. Overall, this study suggests that the specialised role and subcellular localisation of CAR-SIV in human beta cells may contribute to their sensitivity to enteroviral infection following externalisation of the protein at the cell surface, during insulin exocytosis.