Abstract 449: Dimerization of Sortilin Regulates its Trafficking to Extracellular Vesicles and Leads to Secretion of Soluble Sortilin Homodimers, a Potential Novel Biomarker

Objective: Extracellular vesicles (EVs) play a critical role in intercellular communication by transferring microRNAs, lipids and proteins to neighboring cells. Sortilin, a sorting receptor that directs target proteins to the secretary or endocytic compartments of cells, is found in both EVs and cells. In many diseases, including cardiovascular calcification and cancer, sortilin expression levels are atypically high. We previously demonstrated that sortilin promotes vascular calcification via its trafficking of tissue-nonspecific alkaline phosphatase to EVs. Therefore, suppressing trafficking of sortilin to EVs may act as a novel therapy against EV-associated diseases. However, the precise mechanisms of sortilin trafficking remain to be determined. In this study, we hypothesized that dimerization of sortilin regulates its trafficking to EVs. Approach and Results: We found that sortilin forms a homodimer with an intermolecular disulfide bond (IDB) at Cysteine783 (Cys783) residue. Palmitoylation and the IDB formation compete for Cys783. Formation of the IDB led to trafficking of sortilin to EVs by preventing palmitoylation, which further promoted sortilin trafficking to the Golgi apparatus. Moreover, addition of sortilin-derived propeptide decreased sortilin homodimers within EVs, suggesting that the propeptide binding suppresses sortilin trafficking to EVs through prevention of its dimerization. We also found that soluble sortilin secreted by cells overexpressing full-length sortilin forms homodimers with IDBs using various commercially available antibodies against sortilin, thus providing an opportunity for diagnosis of EV-associated diseases by detection of sortilin homodimers in patients. Conclusions: Sortilin is transported to EVs via the formation of homodimers with an IDB, which is endogenously regulated by its own propeptide. Therefore, inhibiting sortilin dimerization may provide a novel therapy against EV-associated diseases, including cardiovascular calcification and cancer. In addition, soluble sortilin homodimer with IDBs could be detected and used as a novel diagnostic biomarker for EV-associated diseases.