Antibody-based proteomics for discovery and exploration of proteins expressed in pancreatic islets

Abnormal glucose tolerance and deviant blood glucose levels are late stage clinical parameters that signify diabetes mellitus. To be able to diagnose the disease at an earlier stage and develop new tools for beta cell imaging, new molecular markers are needed. In the present study, five proteins highly expressed in pancreatic islets with no expression in the surrounding exocrine glandular cells of pancreas, and one protein with the opposite expression pattern, were identified by searches in the Human Protein Atlas (www.proteinatlas.org). The proteins were analyzed immunohistochemically on a specially designed tissue microarray, containing isolated human islets and pancreatic tissues with different characteristics, and compared to the expression of previously known markers of endocrine and exocrine pancreatic cells. Of the five novel endocrine markers, tetraspanin-7 was identified as a membrane-bound protein with exclusive positivity in islet cells. Also beta-2-microglobulin and ubiquitin carboxyl-terminal hydrolase isozyme L1 were expressed in a majority of islet cells, whereas sad1/unc-84 domain-containing protein 1 and beta-1,3-glucuronyltransferase 1 were positive in a smaller subset of islet cells. The potential exocrine marker galectin-2 was expressed in both exocrine acinary cells and pancreatic ductal cells, with no or low positivity in islet cells. In conclusion, antibody-based proteomics and specially designed tissue microarrays enable identification and exploration of novel proteins with differential expression in pancreatic islets. Here we describe 5 candidate proteins for further investigation of their physiological role and potential involvement in the pathogenesis of diabetes. One of these proteins, tetraspanin-7, is expressed on the cell membrane and could thus be a potential candidate for future development of tracers for beta cell imaging.