Clarifying pathogenesis of MIDY - a new form of diabetes mellitus

摘要 糖尿病(DM)是一种由胰岛素分泌不足引起的代谢综合征 胰腺β细胞分泌的控制血糖水平的激素。为了启动胰岛素分泌， 前体胰岛素原转移到内质网(ER)，在那里它经历氧化折叠。 正确折叠的胰岛素原离开内质网，分类到高尔基体，然后分泌颗粒，在 胰岛素原被加工成胰岛素，准备分泌。人类胰岛素基因的最新分析 发现了近30个导致内质网胰岛素原错误折叠的错义突变，导致了一种新的综合征 被称为突变INS基因诱导的青年糖尿病(Midy)。这些突变的胰岛素原产生一种有毒的获得性- 野生型胰岛素与野生型胰岛素形成高分子聚集体对其分泌功能的影响 内质网中的胰岛素原-这种结合阻止了WT胰岛素原内质网的退出、成熟和分泌。减少了 胰岛素分泌导致更多的WT和突变的胰岛素原上调，从而加剧β细胞 由于错误折叠蛋白的积累而导致的内质网应激。这些突变的胰岛素原是否被降解，以及 因此，它们的选择性降解是否会释放允许其分泌的WT胰岛素原，目前尚不清楚。 在此背景下，我们最近发现内质网相关降解(ERAD)途径负责 降解经典Midy突变体Akita；ERAD是处理错误折叠的关键ER质量控制过程 内质网蛋白进入胞浆进行蛋白酶体降解。我们未发表的研究结果现在表明，一个急诊室- 名为Grp170的常驻因素以秋田为目标，通过ERAD进行处置。我们假设Grp170通过 将秋田从高相对分子质量聚集体中分离出来，生成更小的秋田低聚物，并进行ERAD。 重要的是，由于WT胰岛素原在这个反应中也是解缠的，它有效地退出ER，成熟，并 秘而不宣。Grp170诱导的秋田降解因此恢复了胰岛素的分泌，减轻了Midy病。 为了验证这一假设，我们将澄清Grp170如何促进Akita进行ERAD依赖的降解(目标1)， 并阐明是否可以通过刺激Grp170诱导的秋田降解来恢复WT胰岛素的分泌(目标2)。 总而言之，这项提议试图揭示一种名为Midy的DM的基本致病机制。

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 Our unpublished findings now suggest that an ER- resident factor called Grp170 targets Akita for disposal via ERAD.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.