The Unfolded Protein Response and Neuroprotection in Stroke

摘要

Abstract Ischemic stroke is a devastating medical condition for which no pharmacologic intervention is available, except thrombolysis that can be used only for a small percentage of stroke patients. To improve stroke outcome, new pharmacologic approaches must be considered, such as boosting endogenous pro-survival pathways. Here, the unfolded protein response (UPR) is a promising target, because the UPR restores endoplasmic reticulum (ER) function, which is critical for survival of stressed cells. The ER plays a pivotal role in folding and processing newly synthesized proteins. ER function is impaired in a variety of stress conditions, including stroke, which results in accumulation of unfolded/misfolded proteins in the ER, a condition called ER stress. To resolve ER stress, the UPR activates adaptive responses that are mediated by 3 stress sensors in the ER membrane – activating transcription factor-6 (ATF6), inositol-requiring enzyme-1 (IRE1), and protein kinase RNA-like ER kinase (PERK). These UPR branches have 3 primary functions: 1) increase protein-folding capacity, 2) decrease the ER load, and 3) eliminate accumulated unfolded/misfolded proteins from the ER. The UPR also modulates other pro-survival pathways including O-linked β-N-acetylglucosamine (O-GlcNAc) modification. Although we know that stroke impairs ER function and activates the UPR, we do not yet know how the individual UPR branches define the fate and function of post-ischemic neurons in stroke, nor which UPR branch or branches play a predominant role in stroke outcome. Such knowledge is essential to developing a novel strategy to harness UPR pro-survival pathways for therapeutic benefits in stroke. Our long- term goal is to develop strategies to boost UPR pro-survival pathways for therapeutic purposes in stroke. The objective of this application is to establish the mechanistic link between the UPR and stroke outcome, and to identify the UPR branch or branches that critically define recovery of neurologic function after stroke. Our central hypothesis is that boosting pro-survival UPR and related pathways facilitates restoration of impaired ER function and cellular homeostasis in post-ischemic neurons, thereby improving stroke outcome. Based on our new unique UPR-selective and neuron-specific genetically modified mouse models, the hypothesis will be tested in the following specific aims: 1) Determine the role of ATF6 activation in stroke outcome; 2) Determine the contribution of the IRE1/XBP1/O-GlcNAc axis to stroke outcome; 3) Determine the role of the PERK branch in post-ischemic protein synthesis and stroke outcome. The proposed research is significant because we expect to uncover the mechanisms that link the UPR and downstream pathways to stroke outcome. Such knowledge will be a pivotal platform for future studies aimed at establishing new and innovative approaches to improve recovery of neurologic function after stroke, which critically defines quality of life for stroke patients.