The Nociceptor Primary Cilium Contributes to Mechanical Nociceptive Threshold and Inflammatory and Neuropathic Pain

Abstract

The primary cilium, a single microtubule-based organelle protruding from the cell surface and critical for neural development, also functions in adult neurons. While some dorsal root ganglion neurons elaborate a primary cilium, whether it is expressed by and functional in nociceptors is unknown. Recent studies have shown a role of Hedgehog, whose canonical signaling is primary cilium dependent, in nociceptor sensitization. We establish the presence of primary cilia in soma of rat nociceptors, where they contribute to mechanical threshold, prostaglandin E2(PGE2)-induced hyperalgesia, and chemotherapy-induced neuropathic pain (CIPN). Intrathecal administration of siRNA targetingIft88, a primary cilium-specific intra-flagellar transport (IFT) protein required for ciliary integrity, resulted in attenuation ofIft88mRNA and nociceptor primary cilia. Attenuation of primary cilia was associated with an increase in mechanical nociceptive thresholdin vivo,and decrease in nociceptor excitabilityin vitro, abrogation of hyperalgesia, and nociceptor sensitization induced by both a prototypical pronociceptive inflammatory mediator PGE2and paclitaxel CIPN, in a sex-specific fashion. siRNA targetingIft52,another IFT protein, and knockdown of NompB, theDrosophila Ift88ortholog, also abrogated CIPN and reduced baseline mechanosensitivity, respectively, providing independent confirmation for primary cilia control of nociceptor function. Hedgehog-induced hyperalgesia is attenuated byIft88siRNA, supporting a role for primary cilia in Hedgehog-induced hyperalgesia. Attenuation of CIPN by cyclopamine (intradermal and intra-ganglion), which inhibits Hedgehog signaling, supports a role of Hedgehog in CIPN. Our findings support a role of the nociceptor primary cilium in control of mechanical nociceptive threshold and inflammatory and neuropathic pain, the latter Hedgehog-dependent.Significance statementMany neurons have a tiny antenna-like structure, the primary cilium, protruding from their cell body, which processes information about the extracellular environment as well as regulates intracellular signaling. We report experiments aimed at understanding the role of the primary cilium in pain sensory neurons (nociceptors), including in the setting of inflammatory and neuropathic pain. We establish that nociceptors bear a primary cilium and show that this organelle regulates detection of noxious stimuli and contributes to nociceptor sensitization, using both the rat and the fruit fly as model organisms to manipulate primary cilia in pain states. We also identify primary cilium dependence of the contribution of Hedgehog to pain states.