PROJECT SUMMARY/ABSTRACT The primary objective of this proposal is to determine the conserved mechanism that underlies the development of different types of chronic pain and identify a tractable target with broad implications for therapy. Despite diverse pathological triggers and different upstream signaling pathways, nociceptive activity-induced functional and structural plasticity in the spinal dorsal horn serves as the common neural substrate for the different types of chronic pain. However, it remains unclear which molecular mechanisms orchestrate structural and functional plasticity in the spinal dorsal horn and whether these mechanisms are conserved across the different types of chronic pain. Rho GTPases (e.g., Rac1 and RhoA) play essential roles in dendritic spine morphogenesis and synaptic plasticity by controlling actin cytoskeleton organization. In particular, Rac1 promotes the formation, growth, and stabilization of spines and synapses. We previously identified Tiam1 as a critical regulator of Rac1- dependent spine morphogenesis in brain development. Tiam1 is activated by synaptic NMDARs and TrkB receptors and mediates their effects on actin and spine remodeling. During the pain processing, NMDARs and TrkB receptors-mediated central sensitization in the spinal dorsal horn are critically involved in chronic pain hypersensitivity, and Rac1-dependent increases in the size and density of dendritic spines account for the long- term nature of chronic pain. Our preliminary studies found that Tiam1 was activated in the spinal dorsal horn under neuropathic pain conditions and modulated synaptic remodeling by promoting peripheral nerve injury- induced actin polymerization and synaptic NMDAR stabilization. Moreover, Tiam1 deletion from excitatory neurons or spinal dorsal horn neurons prevented chronic pain development triggered by peripheral nerve injury, chemotherapy, diabetes, and inflammation. In this proposal, we will test our central hypothesis that Tiam1 links nociceptive activity-activated NMDARs and TrkB receptors to Rac1 signaling, orchestrating synaptic structural plasticity via actin cytoskeleton reorganization and functional plasticity via synaptic NMDAR stabilization in excitatory neuron populations in the spinal dorsal horn, which serves as a conserved mechanism underlying the development of different types of chronic pain and can be targeted for therapeutic chronic pain intervention. We will pursue the following three specific aims: 1) Identify Tiam1’s convergent function in different types of chronic pain; 2) Elucidate the mechanisms by which Tiam1 contributes to different types of chronic pain; 3) Validate spinal Tiam1 as a therapeutic target for the treatment of chronic pain. At the completion of this project, we will uncover a conserved mechanism that underlies the development of different types of chronic pain and identify a novel therapeutic target that could be translated into the clinic to treat chronic pain with broad impl...