Age-associated changes in knee osteoarthritis, pain-related behaviors, and dorsal root ganglia immunophenotyping of male and female mice

Abstract

Objective: Osteoarthritis (OA) is a leading cause of chronic pain, yet OA pain management remains poor. Age is the strongest predictor of OA development, and mechanisms driving OA pain are unclear. While injury-induced OA models are useful, only a subset of OA is linked to traumatic injury. Here, we aimed to characterize age-associated joint damage, mechanical sensitization, and dorsal root ganglia (DRG) immune phenotypes in mice of both sexes. Methods: Male or female mice aged 6- or 20-months old were evaluated for histopathologic knee OA, pain-related behaviors, and L3-L5 dorsal root ganglia (DRG) immune characterization via flow cytometry. DRG gene expression in aged mice and humans was also examined. Results: Twenty-month old male mice had worse cartilage degeneration than 6-month old mice. Older female knees showed increased cartilage degeneration, but to a lesser degree than males. Older mice of both sexes had worse mechanical allodynia, knee hyperalgesia, and grip strength compared to younger mice. For both sexes, DRGs from older mice showed decreased CD45+ cells, and a significant increase in F4/80+ macrophages and CD11c+ dendritic cells. Older male DRGs showed increased expression of Ccl2 and Ccl5 and older female DRGs showed increased Cxcr4 and Ccl3 compared to 6-month DRGs, among other differentially expresssed genes. Human DRG analysis from six individuals >80 years old revealed elevated CCL2 in male DRGs compared to females, whereas CCL3 was higher in female DRGs. Conclusions: Here we show that aging in male and female mice is accompanied by mild knee OA, mechanical sensitization, and changes to immune cell populations in the DRG, suggesting novel avenues for development of analgesic therapies.

Using Mice to Model Human Osteoarthritis: Age, Biologic Sex, and Pain Matter

“…Similar to the findings of sex differences in spontaneous age‐related OA and pain‐like behavior reported by Geraghty et al (4), a study of posttraumatic knee OA in 10‐week‐old C57BL/6 male and female mice noted more severe histologic OA in male mice, yet female mice exhibited similar levels of pain behavior (13). In a gene knockout study of cartilage acidic protein 1 using a DMM model, female but not male mice exhibited less severe OA and gait abnormalities used as a measure of pain‐like behavior (14).…”

“…In humans, the severity of radiographic OA correlates more strongly with pain in men than in women (9). Geraghty et al (4) found that despite less severe histologic OA in female mice, they exhibited similar, if not more severe, pain‐like behavior than the male mice. In both sexes, pain was associated with increased percentages of macrophages and dendritic cells in the DRGs, while the DRG immunophenotypes of male and female mice differed.…”

“…The good news is that there has been a growing movement in the field to include female animals in OA studies and to include measures of pain‐like behavior. In this issue, Geraghty et al (4) report results of a comprehensive study of spontaneous, naturally occurring knee OA in both male and female C57BL/6 mice that included analysis of joint histology, measures of pain‐like behavior, and examination of inflammatory mediators in the dorsal root ganglion (DRG), a key location for the propagation of nociceptive joint pain signals. Mice were examined at 6 months of age, roughly equivalent to a 30‐ to 40‐year‐old human, and at 20 months of age, roughly the equivalent of a 60‐ to 70‐year‐old human (5).…”

Recommendations For a Standardized Approach to Histopathologic Evaluation of Synovial Membrane in Murine Models of Experimental Osteoarthritis

“…These 2 models are induced by either surgical cutting of the MMTL or both the MMTL and medial meniscus respectively, which likely explains the predominantly medial side changes. In comparison, mild age associated changes in the synovium were previously observed in both medial and lateral joint compartments (35,36).…”

“…Our recommendations, which were informed by currently published methods, are summarized in ( Table 4 ). Of note, we have utilized this system in a previous studies (35,36) with reproducible results to gain insight into aging-related changes to the synovium in mice.…”