Investigating the role of ER-phagy in proinsulin quality control

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， 一种由胰腺β细胞分泌的调节血糖摄取的多肽激素。胰岛素 前体胰岛素原在β细胞内质网(ER)中折叠。当正确折叠时，胰岛素原退出 内质网和交通流到高尔基体，进入分泌颗粒，在那里被蛋白质分解处理形成 生物活性胰岛素，注定要分泌的。这一过程中的缺陷会直接导致DM。这是例证 在一种名为突变INS基因诱导的青年糖尿病(Midy)的情况下，这是一种早发性糖尿病 因表达突变的胰岛素原而引起的疾病。Midy突变的胰岛素原发挥有毒的作用 野生型(WT)胰岛素原的折叠和成熟，因为当它们错误折叠时，它们形成高分子 重量，洗涤剂-不可溶聚集体，也将WT胰岛素原包裹在内质网中，从而降低胰岛素 分泌物。胰岛素分泌减少导致更多的WT和突变的代偿性上调 胰岛素原，导致内质网压力和β细胞死亡。我们最近发现内质网偶联自噬(ER- 吞噬)途径是降解Midy胰岛素原聚集体所必需的。这一过程取决于ER- 吞噬受体RTN3。我们未发表的发现现在表明，内质网膜蛋白PGRMC1是一种 RTN3结合伙伴，起到货物受体的作用，将Midy胰岛素原招募到RTN3进行处置。 PGRMC1与RTN3和突变的胰岛素原在物理上相互作用，证明了PGRMC1-RTN3 复合体是Midy胰岛素原和内质网吞噬途径之间的纽带。令人惊讶的是，药理上的 PGRMC1的损伤增加了胰岛素原的分泌。因此我们假设PGRMC1复合体 利用RTN3将突变的胰岛素原重新招募到ER吞噬途径中(目标1)，并损害RTN3- 在生理上重要的Midy模型中，PGRMC1复合体触发WT胰岛素原的分泌(目标2)。

Abstract Diabetes mellitus (DM) is a metabolic syndrome that is caused by deficiency in the secretion of insulin, which is a peptide hormone that is secreted by pancreatic β-cells to regulate the uptake of blood glucose. The insulin precursor proinsulin is folded in the β-cell endoplasmic reticulum (ER). When properly folded, proinsulin exits the ER and traffics to the Golgi and into secretory granules where it is proteolytically processed to form bioactive insulin, destined for secretion. Defects in this process can directly result in DM. This is exemplified during a condition called Mutant INS-gene-induced Diabetes Youth (MIDY), which is an early-onset diabetic condition caused by expression of a mutant proinsulin. MIDY mutant proinsulins exert a toxic gain-of-function on wildtype (WT) proinsulin folding and maturation because when they misfold, they form high-molecular weight, detergent-insoluble aggregates that also entrap WT proinsulin in the ER, thereby decreasing insulin secretion. Decreased insulin secretion results in compensatory upregulation in even more WT and mutant proinsulin, causing ER stress and β-cell demise. We have recently found that the ER-coupled autophagy (ER- phagy) pathway is required for degradation of MIDY proinsulin aggregates. This process depends on the ER- phagy receptor RTN3. Our unpublished findings now suggest that the ER membrane protein PGRMC1 is a RTN3-binding partner that functions as a cargo receptor to recruit MIDY proinsulin to RTN3 for disposal. PGRMC1 physically interacts with both RTN3 and mutant proinsulin, demonstrating that the PGRMC1-RTN3 complex acts as the nexus between MIDY proinsulin and the ER-phagy pathway. Strikingly, pharmacological impairment of PGRMC1 increases proinsulin secretion. We therefore hypothesize that PGRMC1 complexes with RTN3 to recruit mutant proinsulin into the ER-phagy pathway (Aim 1), and that impairing the RTN3- PGRMC1 complex triggers WT proinsulin secretion in physiologically important MIDY models (Aim 2).