Endoplasmic Reticulum Stress Responses in Pain

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Summary Peripheral nerve injury activates pattern recognition receptors (i.e. Toll-like receptors) in immune cells, thus triggering and maintaining inflammation, and ultimately determining the perpetuation of pain. Immune cell activation demands high levels of protein synthesis, folding and secretion, which are regulated by the endoplasmic reticulum (ER). An excessive demand in protein handling can evoke ER stress (accumulation of misfolded proteins) and consequently trigger robust activation of the unfolded protein response (UPR). IRE1α- XBP1 is the most evolutionarily conserved arm of the UPR and can be directly activated via Toll-like receptor engagement to promote the expression of pro-inflammatory factors. We have unveiled that conditional knockout (cKO) mice devoid of IRE1α/XBP1 in immune cells (Ern1/Xbp1f/f-Vav1cre), display decreased PGE2 production in vivo, reduced nociceptor responsiveness, and faster resolution of non-reflexive pain-related behaviors following paw incision. Similarly, these cKO mice exhibit improved recovery after partial sciatic nerve ligation (PSNL). Through unbiased genome-wide transcriptomic analyses we found that IRE1α-XBP1 signaling in leukocytes is critically required for the induction of prostanoids, cytokines and other novel factors such as Nupr1 (associated with chronic inflammatory diseases in humans). Of note, IRE1α-XBP1 overactivation has been correlated with painful or inflammatory conditions in humans. Therefore, we hypothesize that IRE1α-XBP1 signaling in leukocytes governs peripheral neuro-immune interactions and the development of chronic pain. Using cutting-edge experimental approaches, we will accomplish the following specific aims: 1) Determine how IRE1α-XBP1 signaling dictates the dynamics of immune cell infiltration and molecular changes that drive behavioral non-reflexive hypersensitivity following peripheral nerve injury. We postulate that the IRE1α-XBP1 arm operates as a key modulator of pro-algesic factors in immune cells, and that Nupr1 is a novel XBP1- dependent factor (using ChIP PCR) implicated in PSNL. 2) Establish how IRE1α-XBP1 activation governs individualized immune cellular reprogramming and how this drives the cross-talk with nociceptor afferents following peripheral nerve injury. Our hypothesis is that the reduced hypersensitivity observed in our cKO mice following PSNL is determined by discrete gene signatures in specific injury-infiltrating leukocyte subsets (using single cell RNA sequencing), and by the acquisition of a less responsive phenotype in nociceptors (via in vivo intracellular DRG recordings). 3) Define the therapeutic potential of inhibiting IRE1a to accelerate recovery from PSNL. We posit that pharmacological inhibition of IRE1α using MKC8866 (RNAase domain inhibitor) or KIRA6 (kinase domain inhibitor) will prevent or treat chronic neuropathic pain. Our team has expertise in pain biology and neuroimmune interactions (Romero-Sandoval), immunology and ER stress biology (Cubillos-Ruiz), in vivo electrophysiology (Boada), and scRNAseq and bioinformatics (Miller). Thus, we are uniquely positioned to test our innovative hypothesis and contribute to the development of novel non-narcotic treatments for chronic pain.