Mechanisms underlying sex differences in emergence of advanced osteoarthritis pain

Abstract OA pain is heterogeneous with distinct mechanisms across pain phenotypes that are poorly understood and that likely underlie the variability in therapeutic efficacy across OA pain patients. Many OA patients report mid- stage OA pain that occurs during joint use and dissipates during rest. Others develop advanced OA pain characterized by persistent joint pain that does not dissipate during rest and that is resistant to current treatments. Females have a higher incidence of OA pain and report more severe pain compared to males. This grant application uses an innovative mouse model of mid- and advanced OA pain that allows analysis of mechanisms underlying the transition between mid-stage and advanced OA pain. Female mice develop advanced OA pain with less joint damage compared to males allowing analysis of sex differences that underlie enhanced susceptibility for females to develop advanced OA pain compared to males. The proposed studies will investigate pathological changes in innervation of the joint and adjacent muscle by comparing controls to mid-stage and advanced OA pain. The specific aims will explore the hypothesis that mice with advanced OA have sensory nerve sprouting into joint tissues and adjacent muscle as well as sensory nerve damage and that females develop nerve sprouting and nerve injury in the context of less joint tissue damage compared to males. Aim 1 will determine pathological sprouting of sensory nerves in the knee joint and surrounding muscle of male and female mice with mid-stage and advanced OA pain. This aim will examine whether males and females with advanced OA develop pathological sprouting of small diameter CGRP positive fibers and NF-200 positive myelinated fibers into joint tissues and adjacent muscle, and whether females with advanced OA pain show pathological sprouting in the context of less joint tissue damage. Aim 2 will determine potential differences in axonal nerve injury through induction of ATF3 in sensory neuron cell bodies of male and female mice with mid-stage and advanced OA pain. Retrograde labeling from the knee joint or tibialis anterior muscle will be used to examine whether there are differences in ATF3 within sensory neurons innervating the joint compared to the adjacent muscle. Co-labeling of ATF3 with CGRP and NF200 will be used to analyze whether there are differences in nerve injury across the different populations of sensory fibers. These techniques will elucidate whether nerve injury occurs specifically in the context of advanced OA pain and whether it is observed in females with less joint pathology compared to males. Improved understanding of the pathophysiological changes within the joint that differ between mid-stage and advanced OA pain is a critical step in improving pain management across this heterogeneous pain condition. Direct analysis of sexual dimorphism in these changes will provide insights into potential increase susceptibility for females to develop neuropathic pain and require different therapeutic treatment strategies compared to males.