Altered cellular localisation and expression, together with unconventional protein trafficking, of prion protein, PrP, in type 1 diabetes.

Aims/hypothesis: Normal cellular prion protein (PrP) is a conserved mammalian glycoprotein found on the outer plasma membrane leaflet through a glycophosphatidylinositol anchor. Although PrP is expressed by a wide range of tissues throughout the body, the complete repertoire of its functions has not been fully determined. The misfolded pathogenic isoform PrP (the scrapie form of PrP) is a causative agent of neurodegenerative prion diseases. The aim of this study is to evaluate PrP localisation, expression and trafficking in pancreases from organ donors with and without type 1 diabetes and to infer PrP function through studies on interacting protein partners.

Methods: In order to evaluate localisation and trafficking of PrP in the human pancreas, 12 non-diabetic, 12 type 1 diabetic and 12 autoantibody-positive organ donor tissue samples were analysed using immunofluorescence analysis. Furthermore, total RNA was isolated from 29 non-diabetic, 29 type 1 diabetic and 24 autoantibody-positive donors to estimate PrP expression in the human pancreas. Additionally, we performed PrP-specific immunoblot analysis on total pancreatic protein from non-diabetic and type 1 diabetic organ donors to test whether changes in PrP mRNA levels leads to a concomitant increase in PrP protein levels in human pancreases.

Results: In non-diabetic and type 1 diabetic pancreases (the latter displaying both insulin-positive [INS(+)] and -negative [INS(-)] islets), we found PrP in islets co-registering with beta cells in all INS(+) islets and, strikingly, unexpected activation of PrP in alpha cells within diabetic INS(-) islets. We found PrP localised to the plasma membrane and endoplasmic reticulum (ER) but not the Golgi, defining two cellular pools and an unconventional protein trafficking mechanism bypassing the Golgi. We demonstrate PrP co-registration with established protein partners, neural cell adhesion molecule 1 (NCAM1) and stress-inducible phosphoprotein 1 (STI1; encoded by STIP1) on the plasma membrane and ER, respectively, linking PrP function with cyto-protection, signalling, differentiation and morphogenesis. We demonstrate that both PRNP (encoding PrP) and STIP1 gene expression are dramatically altered in type 1 diabetic and autoantibody-positive pancreases.

Conclusions/interpretation: As the first study to address PrP expression in non-diabetic and type 1 diabetic human pancreas, we provide new insights for PrP in the pathogenesis of type 1 diabetes. We evaluated the cell-type specific expression of PrP in the human pancreas and discovered possible connections with potential interacting proteins that we speculate might address mechanisms relevant to the role of PrP in the human pancreas.