Identification of Sortilin Alternatively Spliced Variants in 3T3L1 Adipocytes

Abstract

Type 2 diabetes mellitus (T2DM) is a chronic and progressive metabolic disease with no cure. Adipocytes play a crucial role in glycemic regulation and take up circulating glucose in response to insulin signaling. In T2DM, translocation of major glucose transporter 4 (Glut4) from cytoplasmic locations to the plasma membrane is impaired. Sortilin is an important constituent of Glut4 storage vesicles and interacts with guiding proteins to determine location of Glut4 in the trans-Golgi network. Sortilin levels are shown to affect adipocyte function. Using mouse 3T3L1 adipocytes, we demonstrate that alternative splicing of sortilin pre-mRNA results in an inclusion of an exon (17b) between exons 17 and 18 in the 10CC motif of the VPS10p domain crucial for ligand interaction. Sort17b expression correlates to insulin resistance and over-expression of Sort17b decreases glucose uptake in adipocytes. Using co-immunoprecipitation assays, we demonstrate that Sort17b is a strong binding partner of Glut4. Using bioinformatic analysis, we show that this insertion results in a novel intrinsically disordered region and has potential sites of proteolytic cleavage. Our study is the first to describe sortilin’s alternatively spliced variants in adipocytes and their effects on glucose uptake. As a broader approach, the research demonstrates the impact of a post-transcriptional event on the metabolic fate of adipocytes in conditions of insulin resistance.