Exploring the transcriptome of resident spinal microglia after collagen antibody-induced arthritis

Microglia have emerged as crucial players in the maintenance of mechanical hypersensitivity in models of chronic pain, including rheumatoid arthritis. Recent studies have suggested that there is a sexually dimorphic microglial involvement in chronic pain, but the debate is still ongoing. Here, we have used the collagen antibody-induced arthritis (CAIA) mouse model to ascertain possible differences between male and female microglia in the context of arthritis-induced pain. We have focused on the late phase of this arthritis model, when joint inflammation has resolved but mechanical hypersensitivity and microglial activation persist. We found that intrathecal administration of minocycline reversed mechanical thresholds to control levels in male, but not female mice. Moreover, we isolated resident microglia from the lumbar dorsal horns of male and female mice and observed a significantly lower number of microglial cells in females by flow cytometry analysis. Furthermore, genome-wide RNA sequencing results pointed to several transcriptional differences between male and female microglia, but no convincing differences were identified between control and CAIA groups. Taken together, these findings suggest that there are significant but subtle sex differences in microglial expression profiles independent of treatment. To what extent they help bring about the behavioural sexual dimorphism observed after minocycline administration remains to be explored. Finally, our experiments failed to identify the underlying biological correlates of the microglial activation that is present in the late phase of the CAIA model. It is likely that transcriptional changes are either subtle and highly localised and therefore difficult to identify with bulk isolation techniques or that other factors, such as changes in protein expression or epigenetic modifications are at play. Overall design: RNA-seq of male and female saline or CAIA treated mice

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Fernandez-Zafra T, Gao T, Jurczak A, Sandor K, Hore Z, Agalave NM, Su J, Estelius J, Lampa J, Hokfelt T, Wiesenfeld-Hallin Z, Xu X, Denk F, Svensson CI