Investigating the role of ER-phagy in proinsulin quality control

摘要 糖尿病（DM）是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， ER和运输到高尔基体和分泌颗粒，在那里它被蛋白水解加工形成 生物活性胰岛素，用于分泌。这个过程中的缺陷会直接导致DM。运被示例 在称为突变INS基因诱导的糖尿病青年（MIDY）的情况下，这是一种早发性糖尿病， 由于胰岛素原突变而引起的疾病。MIDY突变胰岛素原发挥毒性功能获得性 对野生型（WT）胰岛素原折叠和成熟的影响，因为当它们错误折叠时，它们形成高分子 重量，洗涤剂不溶性聚集体，也将WT胰岛素原截留在ER中，从而降低胰岛素 分泌物胰岛素分泌减少导致甚至更多WT和突变体中的代偿性上调。 胰岛素原，引起ER应激和β细胞死亡。我们最近发现，ER偶联自噬（ER- 吞噬）途径是降解MIDY胰岛素原聚集体所必需的。这个过程取决于ER- 噬菌体受体RTN 3。我们未发表的研究结果现在表明，ER膜蛋白PGRMC 1是一种 RTN 3结合配偶体，其作为货物受体发挥功能，以将MIDY胰岛素原募集至RTN 3进行处置。 PGRMC 1与RTN 3和突变胰岛素原两者物理相互作用，表明PGRMC 1-RTN 3 复合物充当MIDY胰岛素原和ER-吞噬途径之间的联系。引人注目的是， PGRMC 1的损伤增加胰岛素原分泌。因此，我们假设PGRMC 1复合物 与RTN 3一起招募突变的胰岛素原进入ER-吞噬途径（Aim 1），并且损害RTN 3- PGRMC 1复合物在生理学上重要的MIDY模型中触发WT胰岛素原分泌（目的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 Our unpublished findings now suggest that the ER membrane protein PGRMC1 is a RTN3-binding partner that functions as a cargo receptor to recruit MIDY proinsulin to RTN3 for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).