The overall goal of this program is to improve pain management in sickle cell disease (SCD) patients. Our work aims to identify objective pain sensitivity measurements to serve as clinical endpoints to test the efficacy of novel analgesic agents, to conduct mechanistic studies in mouse models of SCD and to conduct pre-clinical screen of potential therapeutic agents able to reduce SCD pain. Pain phenotypes in sickle cell disease (SCD) patients are highly variable. A small percentage of SCD patients experience many vaso-occlusive crises/year, 5% of patients account for over 30% of pain episodes, while 39% report few episodes of severe pain. The underlying mechanisms of such variability of pain phenotype is incompletely understood, which in turn partially explains the high readmission rates among SCD patients. Clearly, a better understanding of the pathobiology of SCD is needed to improve its therapy. Humanized sickle cell mice recapitulate several phenotypes of SCD patients and provide models for the study of SCD pain. In a large cross-sectional study of SCD mice, we examined thermosensory response and sensory nerve fiber function using sine-wave electrical stimulation at 2000, 250, and 5 Hz to preferentially stimulate A, A, and C sensory nerve fibers respectively. We use two strains of humanized SCD mice (BERKs and Townes), which have been studied extensively and shown to display the hematologic abnormalities seen in SCD. We are now investigating the role of novel approaches to treat pain in SCD. Specifically we are examining the roles of the mTOR inhibitor rapamycin, which in erythroid precursor cells from normal human subjects has been shown to increase fetal hemoglobin. We are also evaluating the effect of the 2 adrenoreceptor agonist, dexmedetomidine in the nociception phenotype SCD mice. We have shown that BERK and Townes mice, compared to respective controls, have decreases in 2000, 250, and 5Hz current vocalization thresholds in patterns that suggest sensitization of a broad spectrum of sensory nerve fibers during basal conditions. In addition, the pattern and degree of sensitization of sensory fibers varied according to strain, sex, age, and genotype of the mice. In a similarly variable pattern, both Townes and BERKs also had significantly altered sensitivity to noxious thermal stimuli in agreement with what has been shown by others. Therefore, the analysis of somatosensory function using sine-wave electrical stimulation in humanized sickle cell mice suggests that in SCD, both myelinated and unmyelinated, fibers are sensitized. The pattern of sensory fiber sensitization is distinct from that observed in pain models of neuropathic and inflammatory pain. Using the same paradigm of quantitative sensory testing used in sickle cell mice, we have shown that sickle cell disease patients who display a severe pain phenotype, have sensitization of myelinated sensory nerve fibers. These findings raise the possibility that sensitization of a broad spectrum of sensory fibers might contribute to the altered and variable nociception phenotype in SCD. This past year, we have also characterized the neurocognitive behavior profile of humanized SCD mice (Townes, BERK) and identified the hematologic and neuropathologic abnormalities associated with the behavioral alterations observed in these animals. Both heterozygous and homozygous Townes mice displayed severe cognitive deficits characterized by significant deficits in spatial learning compared to controls. Homozygous Townes also had increased depression- and anxiety-like behaviors as well as reduced performance on voluntary wheel running compared to controls. Behavior deficits observed in Townes were also seen in BERK sickling mice. Interestingly, most deficits in homozygotes were observed in older mice and were associated with worsening anemia and neuropathologic abnormalities including the presence of large bands of dark/pyknotic (shrunken) neurons in CA1 and CA3 fields of hippocampus and evidence of neuronal dropout in cerebellum. These observations suggest that altered nociception and cognitive and behavioral deficits in SCD mice mirror those described in SCD patients and that aging, anemia, and profound neuropathologic changes in hippocampus and cerebellum are possible biologic correlates of those deficits. These findings also support using SCD mice for studies of cognitive deficits in SCD and point to vulnerable brain areas with susceptibility to neuronal injury in SCD and to mechanisms that potentially underlie those deficits.
