Protein secretion is an essential function in all cells. It plays a particularly prominent role in complex multiceltular organisms with hormonal, neural and immunological systems for intercellular communication. The endoplasmic reticulum (ER), which handles the early post-translational processing of secreted proteins tries to maintain a balance between the influx of client proteins and the capacity of the folding machinery. This equilibrium is easily perturbed by fluctuations in client protein load and by pathological events that impair ER function. Thus ER stress, a consequence of perturbing this equilibrium, occurs during hypoxia, nutrient deprivation or exposure to endogenous toxins such as homocysteine. ER stress is also likely to occur in secretory cells handling heavy client protein loads, such as 13cells in islets of Langerhans in insulin resistant humans. Signaling pathways responsive to ER stress are referred to collectively as the unfolded protein response (UPR) and they protect cells against ER stress by reducing client protein synthesis and by upregulating genes that increase the capacity of the secretory apparatus to process its client proteins. At the same time the UPR also activates signals and genes that promote cell death. The long term goals of this project are to understand what controls the survival versus death decision in the UPR and to understand how signaling in the UPR promotes secretory capacity. We will focus on three specific aims: To define the role of translational repression and recovery in ER stress and metabolic regulation by studying the impact of mutations that impair elF2(_ dephosphorylation on cell survival in ER stress and on the function of tissues such as liver and pancreas that engage in heavy secretion. To define the role of PERK signaling in survival or death of ER stressed cells by utilizing conditionally active forms of PERK that are uncoupled from ER stress. To define the role of IRE1 in development and maintenance of the secretory apparatus by studying secretion and gene expression in cells and tissues with experimentally-altered IRE1 function. A better understanding of the UPR will permit rational selection of targets for therapeutic interventions in diseases and pathological states associated with ER stress such as diabetes mellitus, ischemia and hyperhomocysteinemia. Tools to manipulate cells'secretory capacity may be used to enhance production of recombinant proteins in cultured cells or in gene therapy or cell therapy in vivo.
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