Regulation of headache pain responses by the Trigeminal ganglion clock - PROJECT SUMMARY / ABSTRACT Pain, often referred to as the “fifth vital sign,” is a universal experience and a major reason to seek medical intervention. Growing evidence indicates a regulatory role of our biological timer, the circadian clock, in pain timing and thresholds. Particularly, several headache disorders have been found to display distinct circadian features, including cluster headache (CH) and migraine. CH is an extremely painful disorder (nicknamed “suicide headache”), affecting 1 out of every 1,000 people, and is known to exhibit a precise daily rhythmicity of headache attacks. However, the molecular mechanisms underlying rhythmic pain responses in headache disorders remain poorly understood. This significant gap in knowledge hinders the development of novel chronotherapeutic strategies to optimize pain management. To elucidate the circadian regulation of pain responses, we first examined the trigeminal ganglion (TG), the primary somatosensory ganglion for the canonical headache pathway, and discovered strong circadian rhythms in TG ex vivo cultures. Using a nitroglycerin (NTG) induced mouse headache model, we demonstrated circadian rhythms of pain sensitivity which were abolished when the core clock genes Per1/Per2 were disrupted. TG RNA-sequencing analysis revealed a robust circadian transcriptome including core clock genes and clock-regulated pain neurotransmitters. Among the most rhythmically expressed genes in the TG circadian transcriptome, Mertk (MER proto-oncogene, tyrosine receptor kinase) was recently identified as a CH susceptibility gene in three independent large-scale GWAS studies; however, its role in pain pathways is completely unknown. We found that NTG induced the phosphorylation of MERTK (which is required for its tyrosine kinase activity), and importantly circadian pain sensitivity was abolished in Mertk KO mice, suggesting a role for Mertk to confer circadian timing to pain pathways. Based on these exciting preliminary findings, we hypothesize that the TG circadian clock governs rhythmic headache responses via regulation of pain genes including the novel regulator Mertk. Our two specific aims are as follows. In Aim 1, combining genetic, environmental and pharmacological approaches, we will delineate the regulatory role of the nociceptive neuron clock in headache pain rhythms and a potential chronotherapeutic application. In Aim 2, we will investigate the role of MERTK in rhythmic headache responses and identify downstream mechanistic pathways. This grant proposal embodies conceptual and experimental innovations including circadian regulation of headache pain and a novel regulatory role of MERTK in the clock-headache crosstalk. With the unique combination of complementary expertise in circadian biology and headache/CH, we are ideally suited to interrogate key molecular mechanisms at the interface of circadian rhythms and headache pain. Given the increasing appreciation of the fundamental function of the clock in pain pathophysiology, our study will have strong basic and translational impact, and may ultimately lead to a new paradigm of manipulating the circadian machinery to ameliorate headache pain.
