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Molecular and Cellular Mechanisms of Pain and Itch in the Mammalian Peripheral Nervous System


Two main questions drove my PhD research: 1) what are the molecules in the peripheral nervous system that are important for touch, pain, and itch; and 2) how do different cell types in the periphery (namely immune cells, skin cells, and neurons) interact in chronic pain and itch. To address the first question, I mined several transcriptomic datasets from our lab and others, and uncovered the sphingosine 1-phosphate (S1P) signaling pathway as a candidate mediator of somatosensation. I uncovered an essential role for endogenous, baseline levels of S1P and S1P Receptor 3 (S1PR3) signaling in setting mechanical pain thresholds. I demonstrated that S1PR3 modulates activity KCNQ2/3 potassium channels in mechanical pain neurons to increase neuronal excitability. This study was the first to identify a molecule selectively required for baseline mechanical pain sensation. In addition to this necessary role for S1P in setting mechanical pain thresholds, I found that elevated levels of S1P, observed in inflammatory itch and pain disorders, triggers nociceptor activation and itch/pain behaviors in mice. Using a variety of genetic, physiological, pharmacological, and behavioral approaches, I showed that elevated S1P evokes itch and pain in a dose-dependent manner via two distinct molecular pathways dependent upon Transient Receptor Potential (TRP) ion channels in discrete subsets of itch- and pain-sensing somatosensory neurons. This study revealed a detailed molecular understanding of how a single ligand/receptor can engage in distinct pain and itch signaling pathways in the periphery. Moreover, using an engineered S1P molecule containing an azobenzene photoswitch, I showed that S1P-dependent nociceptor activation and pain hypersensitivity behaviors could be rapidly activated and inactivated with light.

With respect to the second research question, I sought to unravel the complex molecular and cellular pathways underlying the development of chronic itch in atopic dermatitis (eczema). Using RNA-seq of tissues at key time points in a mouse model of chronic itch, I identified molecules and cells associated with the development of atopic dermatitis in skin, spinal cord, and somatosensory ganglia. These data implicate neutrophils in the pathogenesis of atopic dermatitis. Antibody-mediated depletion of neutrophils significantly attenuated itch-evoked scratching behaviors in our mouse model of chronic itch. Neutrophils were also required for induction of several key hallmarks of chronic itch, including inflammatory cytokines, skin hyperinnervation, itch signaling molecules, and activity-induced genes and markers of neuropathic itch. Finally, I demonstrated that induction of CXCL10, a chemokine and ligand of the CXCR3 receptor that promotes itch via activation of sensory neurons, is neutrophil-dependent. Moreover, CXCR3 antagonism attenuated chronic itch across multiple time points of the model. Taken together, these findings show that neutrophils promote the transition from acute to chronic itch via induction of CXCL10/CXCR3 signaling.

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