Aims and Objective: A transmembrane cell-adhesion protein, the Coxsackie-adenovirus receptor (CAR) serves as an entry receptor for enteroviruses and may be essential for their ability to infect cells. Since beta-cell enteroviral infection could contribute to the development of Type 1 diabetes, it is important that CAR expression is analysed in the human pancreas. CAR exists as at least 5 isoforms and we have studied the expression of these different isoforms in human pancreas.
Methods: Formalin-fixed paraffin embedded pancreatic sections from 17 non-diabetic controls, and 6 Type 1 diabetes patients were studied, together with a human tissue microarray. Immunohistochemistry, confocal immunofluorescence microscopy and western blotting with isoform-specific antisera were employed to examine the expression and cellular localisation of each CAR isoform. Isoform specific qRT-PCR was performed on RNA extracted from isolated human islets.
Results: An isoform of CAR having a terminal SIV motif and a unique PDZ binding domain was preferentially expressed in human beta cells at the protein level. This was also the major isoform amplified by RT-PCR from RNA extracted from isolated human islets. Surprisingly, this protein was distributed mainly within the cytoplasm of beta cells whereas it was primarily localised to the plasma membrane in tissues such as testis and bladder. Co-immunofluorescence analysis revealed significant subcellular co-localisation with ZnT8, PC1/3 and insulin, but not with pro-insulin, in beta-cells.
Conclusion: The restricted expression of the SIV protein may contribute to the selective infection of beta cells by enteroviruses under conditions when it is translocated to the cell surface.