Pharmacologic therapy for common forms of chronic pain is ineffective and plagued with side effects. Our long- term goal is to reveal mechanisms of pain/nociceptive signaling and define drug targets. G protein-coupled receptors (GPCRs) control most patho-physiological processes, including pain, and are the target of 34% of therapeutic drugs. GPCRs are considered to function solely at the plasma membrane, where they interact with extracellular ligands and couple to intracellular G proteins. However, agonists released from injured and diseased tissues evoke redistribution of GPCRs to endosomes in neurons. These endosomal GPCRs (eGPCRs) generate sustained signals in subcellular compartments that control the ion channel activity that underlies chronic pain. The central hypothesis is that activation of pronociceptive eGPCRs produces nociceptive signaling and most forms of chronic pain; antagonists of eGPCRs block nociceptive signaling and are anti-nociceptive. The rationale for this proposal is that discovery of eGPCR pain mechanisms will facilitate development of drugs that selectively antagonize eGPCRs in neurons and provide superior pain relief with fewer side effects. The overall objectives are to discover mechanisms underlying chronic pain and validate a therapeutic target. The central hypothesis will be tested by pursuing three specific aims: 1) Discover the mechanisms of eGPCR signaling in subcellular compartments of neurons; biophysical and imaging approaches will be used; nanoparticles (NPs) will be designed with components that target neurons, promote endocytosis and release eGPCR ligands in the acidic endosome; 2) Discover the mechanisms by which eGPCRs regulate ion channels that control neuron activity; ion channel activity and excitability of neurons will be studied with electrophysiology. NP-encapsulated drug probes will define the role of eGPCRs in neuronal excitation; 3) Validate eGPCRs as a therapeutic target for chronic inflammatory, neuropathic and cancer pain; NP-encapsulated eGPCR ligands will be compared to conventional therapy in three pain models. The proposed pain mechanism is a novel explanation that resolves the enigma of widespread clinical trial failures of GPCR-targeted drugs. Innovation in the proposal extends to the NP approach to probe the mechanism and validate the target. The proposal is clinically significant because it validates an eGPCR-target that offers superior pain relief with fewer side-effects and is applicable to most patients with intractable chronic pain.
The proposed research is relevant to public health because it seeks to discover a novel pain mechanism that underlies the most common types of chronic pain and validate a therapeutic target within that mechanism. This work will lay the foundation for the development of new types of medication that alleviates chronic pain including chronic inflammatory, neuropathic and cancer pain. Thus, the proposal directly addresses part of the mission of the NIH - to reduce illness and disability.
