Clarifying pathogenesis of MIDY - a new form of diabetes mellitus

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Abstract Diabetes mellitus (DM) is a metabolic syndrome caused by the insufficient secretion of insulin, a peptide hormone secreted by pancreatic β-cells to control blood glucose levels. To initiate insulin secretion, the precursor proinsulin translocates into the endoplasmic reticulum (ER) where it undergoes oxidative folding. Properly folded proinsulin exits the ER, sorts to the Golgi apparatus, and then secretory granules where proinsulin is processed to form insulin poised for secretion. Recent analysis of the human insulin gene revealed nearly 30 missense mutations that cause proinsulin to misfold in the ER, resulting in a new syndrome called Mutant INS-gene-induced Diabetes of Youth (MIDY). These mutant proinsulins exert a toxic gain-of- function effect on wildtype (WT) insulin secretion by forming high molecular weight (MW) aggregates with WT proinsulin in the ER – this engagement blocks WT proinsulin ER exit, maturation, and secretion. Decreases in insulin secretion result in upregulation of even more WT and mutant proinsulin, thereby exacerbating beta cell ER stress due to accumulation of misfolded proteins. Whether these mutant proinsulins are degraded, and if so, whether their selective degradation might liberate WT proinsulin that allows for its secretion, are unknown. In this context, we recently identified the ER-associated degradation (ERAD) pathway as responsible for degrading the classic MIDY mutant Akita; ERAD is a key ER quality control process that disposes misfolded ER proteins to the cytosol for proteasomal degradation. Our unpublished findings now suggest that an ER- resident factor called Grp170 targets Akita for disposal via ERAD. We hypothesize that Grp170 does so by untangling Akita from the high MW aggregates, generating smaller Akita oligomers that undergo ERAD. Importantly, as WT proinsulin is also disentangled in this reaction, it efficiently exits the ER, matures, and is secreted. Grp170-induced Akita degradation thus restores insulin secretion and alleviates the MIDY disease. To test this hypothesis, we will clarify how Grp170 promotes Akita for ERAD-dependent degradation (Aim 1), and elucidate if WT insulin secretion can be restored by stimulating Grp170-induced Akita degradation (Aim 2). In sum, this proposal seeks to reveal the basic pathogenic mechanism underlying a form of DM called MIDY.