Nociceptive Mechanisms Underlying Sickle Cell Pain

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Project Summary: The circulating lipid metabolite lysophosphatidylcholine (lysoPC) is profoundly upregulated in the blood of patients and mice with sickle cell disease (SCD); its over-abundance in RBC membranes drives RBC sickling. However, nothing is known about whether the elevated lysoPC levels contributes to pain in SCD. The objective of this proposal is to investigate the role lysoPC plays in acute and chronic SCD pain. Acute pain during vaso- occlusive crisis (VOC) and chronic pain are major comorbidities for patients with SCD that severely impact the patients' quality of life. Improved, mechanism-targeted pain treatments are desperately needed for SCD pain because the main treatment currently used is opioid-class drugs, which have serious adverse side effects, risk addiction, do not fully alleviate the pain, and in some patients, exacerbate the pain even further. The fact that acute and chronic SCD pain arise from many body sites, both deep and superficial, suggests that a circulating factor, such as lysoPC, may drive and maintain the pain in SCD. The Scientific Premise of this proposal is that in SCD, the elevated blood borne lysoPC which originates in RBCs gains access to sensory neurons via the permeable blood-nerve barrier, and then sensitizes a variety of receptors within these neurons that detect noxious stimuli. We hypothesize that during an acute VOC, soluble lysoPC sensitizes the G-protein Receptor 4 (GPR4) and/or Acid Sensing Ion Channel 3 (ASIC3) to protons that are released locally in tissue during ischemia, thereby driving the acute mechanical sensitization in SCD. Second, we hypothesize that lysoPC contributes to chronic mechanical steady state pain by activating the Transient Receptor Potential 5 (TRPC5) channel on sensory neurons thereby mediating the chronic mechanical pain in SCD. Three Specific Aims will interrogate this hypothesis: 1) Will global inhibition of lysoPC generation prevent acute crisis and chronic pain? 2) Does soluble lysoPC contribute to acute crisis pain by sensitizing the proton receptors GPR4 and/or ASIC3? 3) Does lysoPC contribute to chronic steady state pain by sensitizing the stretch-activated channel TRPC5? We hypothesize that therapeutically targeting one dysregulated lipid for the prevention of acute VOC and chronic pain, alleviation of VOC pain, and alleviation of chronic SCD pain will present a single, unique opportunity for novel drug development for SCD pain. To date, lysoPC has not yet been linked directly to pain mechanisms even though it has been shown to be dysregulated in various diseases, including rheumatoid arthritis, osteoarthritis, psoriasis, migraine, dysmenorrhea, multiple sclerosis and angina. Thus, this proposal may also reveal the mechanism through which lysoPC drives the pain in these disorders. More broadly, our experiments will uncover basic mechanisms by which lipids modulate ion channels and receptors in sensory neurons to alter their mechanical response properties.