Abstract

Protein misfolding is a contributor to the development of type 2 diabetes (T2D), but the specific role of impaired proteostasis is unclear. Here we show a robust accumulation of misfolded proteins in the mitochondria of human pancreatic islets from patients with T2D and elucidate its impact on β cell viability through the mitochondrial matrix protease LONP1. Quantitative proteomics studies of protein aggregates reveal that islets from donors with T2D have a signature resembling mitochondrial rather than endoplasmic reticulum protein misfolding. Loss of LONP1, a vital component of the mitochondrial proteostatic machinery, with reduced expression in the β cells of donors with T2D, yields mitochondrial protein misfolding and reduced respiratory function, leading to β cell apoptosis and hyperglycaemia. LONP1 gain of function ameliorates mitochondrial protein misfolding and restores human β cell survival after glucolipotoxicity via a protease-independent effect requiring LONP1-mitochondrial HSP70 chaperone activity. Thus, LONP1 promotes β cell survival and prevents hyperglycaemia by facilitating mitochondrial protein folding. These observations provide insights into the nature of proteotoxicity that promotes β cell loss during the pathogenesis of T2D, which could be considered as future therapeutic targets. LONP1, whose expression is downregulated in islets from donors with type 2 diabetes, is vital to mediate efficient mitochondrial protein folding, thus preventing proteotoxicity and promoting islet β cell survival and function.