Role of GRP78 in beta cell adaptation in obesity and diabetes

胰腺β细胞衰竭导致胰岛素缺乏和糖尿病。由于细胞损伤如肽错误折叠、免疫攻击、脂毒性或氧化应激而发生的β细胞ER应激是胰岛素产生失败和β细胞死亡的有据可查的原因。保护β细胞免受ER应激或使平衡远离ER应激死亡反应可以维持代谢健康并预防糖尿病。因此，一个重要的目标是提高对β细胞内质网应激临界点的理解，以找到将β细胞保持在功能范围内并避免因失代偿应激而失去它们的方法。Grp 78是该项目的重点，是ER健康的中央调节器。在正常的非应激细胞功能期间，Grp 78充当伴侣，并且还与三种ER跨膜未折叠蛋白反应（UPR）激活剂相关联并抑制其。Grp 78是内质网应激水平的重要传感器和细胞应激反应的调节因子。当ER应激增加时，Grp 78被未折叠的肽滴定，去抑制UPR激活剂，允许下游信号传导扩大ER容量以适应增加的工作量并缓解应激。Grp 78本身是UPR的转录靶点;对胁迫的适应导致Grp 78丰度增加。因此，Grp 78是维持分泌肽合成装置的仔细平衡机制的核心。该项目的目标是描述β细胞中Grp 78的生物学，以便更多地了解可耐受压力和失代偿之间的关键临界点。初步数据显示，Grp 78是β细胞稳态所必需的。缺乏Grp 78的小鼠在生命的最初几周内发展为糖尿病;胰岛素产生失败是由加速的β细胞死亡引起的，与β细胞分化状态的不明原因的丧失有关。对于这个项目，我们组建了一个团队，其中包括Amy Lee，他是在20世纪80年代克隆Grp 78的科学家，并产生了许多相关工具，包括Grp 78-floxed小鼠，以及Jason Kim，代谢专家，他拥有最先进的体内胰岛素分泌定量工具。在三个重点目标中，我们将仔细评估在基础和胰岛素需求应激条件下，在β细胞胎儿发育、出生后成熟和成年期间，Grp 78的一个或两个等位基因的β细胞缺失的代谢和分子影响。我们将研究小鼠和人类胰岛中适应性UPR和失代偿之间的临界点，详细探索细胞死亡是如何触发的，以及Grp 78与β细胞成熟和身份维持之间新颖而令人兴奋的关系。这些研究完成后，将阐明决定β细胞在Grp 78减少期间是否成功适应或屈服于失代偿的分子应激途径。

Pancreatic beta cell failure leads to insulin deficiency and diabetes. Beta cell ER stress, which occurs due to cellular insults like peptide misfolding, immune attack, lipotoxicity or oxidative stress, is a well-documented cause of insulin production failure and beta cell death. Protecting beta cells from ER stress or tipping the balance away from the ER stress death response can maintain metabolic health and prevent diabetes. An important goal is therefore to improve understanding of the beta cell ER stress tipping point, in order to find ways to keep beta cells in the functional range and avoid losing them to decompensated stress. Grp78, the focus of this project, is a central regulator of ER health. During normal unstressed cell function, Grp78 acts as a chaperone and also associates with, and inhibits, the three ER transmembrane unfolded protein response (UPR) activators. Grp78 is a critical sensor of ER stress level and regulator of the cell response to stress. When ER stress increases, Grp78 is titrated away by unfolded peptide, de-repressing the UPR activators, allowing downstream signaling to expand ER capacity to accommodate the increased workload and relieve stress. Grp78 is itself a transcriptional target of UPR; adaptation to stress results in increased Grp78 abundance. Thus, Grp78 is the centerpiece of a carefully balanced mechanism to maintain the secretory peptide synthesis apparatus. The goal of this project is to describe the biology of Grp78 in beta cells, so as to learn more about the critical tipping-point between tolerable stress and decompensation. Preliminary data shows that Grp78 is absolutely required for beta cell homeostasis. Mice lacking Grp78 develop diabetes in the first few weeks of life; insulin production failure is caused by accelerated beta cell death, associated with an unexplained loss of beta cell differentiation status. For this project we have assembled a team including Amy Lee, the scientist who cloned Grp78 in the 1980s and generated many related tools including the Grp78-floxed mice, and Jason Kim, a metabolism expert with the tools to perform state-of-the-art in vivo quantification of insulin secretion. In three focused aims we will carefully assess the metabolic and molecular impact of beta cell deletion of one or both alleles of Grp78 during beta cell fetal development, postnatal maturation and adulthood, under basal and insulin-demand stressed conditions. We will examine the tipping-point between adaptive UPR and decompensation in both mice and human islets, with a detailed exploration of how cell death is triggered, and the novel and exciting relationship between Grp78 and beta cell maturation and identity maintenance. When completed, these studies will clarify the molecular stress pathways that determine whether beta cells successfully adapt or succumb to decompensation during Grp78 reduction.