Sexual dimorphism of the synovial transcriptome underpins greater PTOA disease severity in male mice following joint injury

Objective: To elucidate sex differences in synovitis, mechanical sensitization, structural damage, bone remodeling, and the synovial transcriptome in the anterior cruciate ligament rupture(ACLR) mouse model of post-traumatic osteoarthritis (PTOA). Methods: Male and female 12 week-old C57Bl/6 mice were randomized to noninvasive ACLR or sham (n=9/sex/group/timepoint). Knee hyperalgesia, mechanical allodynia, and intra-articular MMP activity (via intravital imaging) were measured longitudinally. Trabecular and subchondral bone remodeling and osteophyte formation were assessed by microCT. Histological scoring of PTOA and synovitis and anti-MMP13 immunostaining was performed. NaV1.8-Cre;tdTom mice were used to document localization and sprouting of nociceptors. Bulk RNAseq of synovium in sham, 7d, and 28d post-ACLR, and contralateral joints (n=6) assessed injury-induced and sex-dependent synovial gene expression. Results: Male mice exhibited worse joint damage at 7d and 28d and worse synovitis at 28d, accompanied by greater MMP activity, knee hyperalgesia, and mechanical allodynia. Females had catabolic responses in trabecular and subchondral bone after injury, whereas males exhibited greater injury-induced osteophyte formation and sclerotic remodeling of trabecular and subchondral bone. Injury-induced NaV1.8+ nociceptor sprouting in subchondral bone and medial synovium was comparable between sexes. RNAseq of synovium demonstrated that both sexes had similar injury-induced gene expression at 7d, but only female mice exhibited synovial inflammatory resolution by 28d, whereas males had persistent pro-inflammatory, pro-fibrotic, pro-neurogenic, and pro-angiogenic gene expression. Conclusion: The worse overall joint pathology and pain behavior in male mice was associated with persistent activation of synovial inflammatory, fibrotic, and neuroangiogenic processes, implicating synovitis in driving sex differences in murine PTOA.