Pancreatic beta-cell glucose toxicity and mRNA translation

ABSTRACT Pancreatic β-cells are specialized to carry out high capacity mRNA translation that is tightly regulated by the nutrient environment. However, these features expose β-cells to several vulnerabilities, including deleterious endoplasmic reticulum stress and translation errors that can can result in production of defective ribosome products (DRIPs), which have been implicated in autoimmunity in type 1 diabetes (T1D). Whereas, acute glucose exposure robustly increases translation of insulin and other secretory granule proteins, chronic high glucose decreases insulin translation and secretion even prior to impact on global translation or upregulation of endoplasmic reticulum stress. To determine the extent of translational changes at this early stage of β-cell dysfunction, I used ribosome profiling and nascent proteomics in MIN6 insulinoma cells to elucidate the genome- wide impact of sustained high glucose on β-cell mRNA translation. Sustained high glucose conditions that suppressed insulin secretion downregulated translation of insulin and proteins, such as SCGN, IDH2, VPS41, SLC2A2, IGF2, SLC30A8 and PFKFB3, which are involved in insulin secretory granule formation, exocytosis, and metabolism-coupled insulin secretion. Translation of these mRNAs was also downregulated in primary rat and human islets following ex-vivo incubation with chronic high glucose, and in an in vivo partial pancreatectomy model of chronic hyperglycemia. Translational downregulation decreased cellular abundance of these proteins. Subsequent analysis of actively translating ribosomes and pathways that regulate mRNA quality during translation showed that sustained high glucose changes the protein composition of translating ribosomes and suppresses nonsense mediated RNA decay (NMD). Altered ribosome composition and activity of RNA decay pathways could result in changes in the abundance and fidelity of the proteins produced. I hypothesize that hyperglycemia-induced remodeling of ribosomes and suppression of NMD alter expression of key β-cell genes and increase neo-antigen production. In this proposal I will investigate the extent to which sustained high glucose-remodeled ribosomes contribute to dysregulation of mRNA translation in β-cells and determine the impact of reduced RNA surveillance and altered translation on neoantigen production and autoimmunity in T1D. These studies could uncover novel therapeutic targets for prevention of progressive β-cell failure in T1D and for optimizing functionality of ex vivo generated β-cells for cell replacement therapy.