Global but not myeloid lineage-directed Girk3 deletion increases bone mass in female mice

Germline and osteoblast-directed deletion of G protein-gated inwardly rectifying K⁺ channel 3 (Girk3) was recently shown to increase bone mass after 18 wk of age in male mice. Here, we show that germline Girk3 deletion also increases trabecular and cortical bone mass and increases the mechanical strength of the femur in female mice after 18 wk of age. Unlike male mice, however, osteoblast-directed Girk3 deletion using 2.3 kb-Col1a1-Cre does not increase bone mass in adult female mice. To discover mechanisms underlying high bone mass in female Girk3^-/- mice, bulk RNA-sequencing was performed on 2-d-old calvarial bone, revealing lower expression of proinflammatory cytokines such as IL-1β and IL-6 in Girk3^-/- mice. Accordingly, cytokines and chemokines are largely suppressed in the circulation of adult Girk3^-/- mice compared to WT littermates. The cytokines GM-CSF, IL-1β, IL-2, and IL-9 are reduced in the serum of both male and female Girk3^-/- mice, while eotaxin, IFNγ, MIP-1α, and others are sexually dimorphic. Histomorphometry reveals that osteoclast activity is modestly reduced in Girk3^-/- bone, which is supported by in vitro osteoclast resorption assays. However, deletion of Girk3 in myeloid-lineage cells with LysM-Cre is not sufficient to recapitulate high bone mass in either male or female mice. Moreover, female Girk3^-/- mice are not protected from ovariectomy-induced bone loss. Finally, single-cell screening using cytometry by time-of-flight in the BM revealed no differences in immune cell abundances due to global Girk3 deletion. Taken together, while Girk3 regulates inflammatory cytokine expression in the bone and serum, deletion of Girk3 in myeloid-lineage cells does not affect bone mass.