Defining the Central Role of ER-Associated Degradation (ERAD) in Neuroendocrine Cells SUMMARY My laboratory has a long-standing interest in protein folding and degradation within the endoplasmic reticulum (ER) by defining the physiological and pathological importance of mammalian ER quality-control machineries in vivo. ER-associated degradation (ERAD) is the principal protein quality-control mechanism responsible for targeting misfolded proteins in the ER for cytosolic proteasomal degradation. Failure to clear misfolded proteins in the ER presumably activates the unfolded protein response, or UPR. We showed that Sel1L-Hrd1 ERAD modulates the activation of UPR sensor IRE1a by mediating its turnover (Sun et al. 2015 Nat Cell Biol). In two recent studies, we reported that impaired ERAD function may be directly linked to the pathogenesis of metabolic diseases, thereby holding significant therapeutic potential (Shi et al. 2017 and Kim et al. 2018 J CIin Invest). Specifically, we reported that mice with Sel1L deficiency in either AVP or POMC neurons exhibit diabetes insipidus and early-onset obesity, respectively. We showed that Sel1L-Hrd1 ERAD controls the maturation of two prohormones, proAVP and POMC, within the ER by targeting the misfolded forms for proteasomal degradation, thereby preventing the aggregation of a large proportion of native prohormones. We now propose to test the overarching hypothesis that the Sel1L-Hrd1 ERAD protein complex plays a critical role in neuroendocrine cells by directly recruiting misfolded prohormones for proteasomal degradation and by coordinating the activation of other ER quality-control machineries, such as UPR and autophagy, to ensure proper prohormone maturation and neuronal homeostasis. This model challenges the current paradigm in ER biology by placing ERAD at the center of cellular function in normal physiology and disease pathogenesis. Using POMC neurons as a model system, we will accomplish the following Aims: (1) determine the underlying molecular mechanisms and therapeutic potential of ERAD-POMC interactions; (2) demonstrate the pathophysiological importance and mechanisms underlying the crosstalk between ERAD and autophagy in POMC neurons; and (3) demonstrate the pathophysiological importance and mechanisms underlying the crosstalk between ERAD and IRE1α in POMC neurons. This study will provide unprecedented insights into ERAD function and prohormone biology in neuroendocrine cells, which has direct clinical implications for human diseases that are associated with defects of prohormone folding and export. RELEVANCE TO HUMAN HEALTH: All neuropeptides are synthesized as precursor proteins known as “prohormones” in the ER; however, molecular mechanisms underlying their maturation within the ER remain poorly understood. This study will establish the pathophysiological significance of ERAD in coordinating ER homeostasis during prohormone maturation and explore the therapeutic potential of targeting ERAD in the treatment of diseases attributed to defects in prohormone maturation, such as diabetes insipidus, early-onset obesity, and other rare diseases such as Prader-Willi Syndrome.

NARRATIVE Endoplasmic reticulum (ER)-associated degradation (ERAD) is the principal ER quality control machinery responsible for the clearance of misfolded ER proteins for cytosolic proteasomal degradation. Recent studies from our laboratory have revealed the significance of ERAD in the maturation of, a least a subset of, prohormones within the ER; however, the underlying molecular mechanism and significance remain unclear. This study will test an exciting hypothesis that Sel1L-Hrd1 ERAD protein complex plays a critical role in POMC neurons by directly recruiting misfolded POMC (prohormone) for proteasomal degradation and by coordinating the activation of other ER quality-control machineries such as UPR and autophagy.