Short- and Long-Term Effects of Social Isolation on Adult Murine Bone are Sex-Dependent

Abstract Social isolation is a known modifiable risk factor for many chronic diseases including cardiovascular, metabolic, and neurological disorders. Recent research has demonstrated that social isolation is similarly detrimental to skeletal health, but these effects may be sexually dimorphic. In rodents, isolation negatively affects bone in adult male mice, but not in females. However, these sex differences have not been systematically investigated, and it is unknown if they persist with long-term social isolation. The goal of our study was to investigate if isolation-induced bone loss may occur on different timescales between female and male mice, as well as investigate the potential roles of estrogen and testosterone. We examined bone changes in grouped (4 mice/cage) or isolated (1 mouse/cage) female and male 16-week-old C57BL/6J mice after 2, 4, or 8 weeks of treatment. We found that social isolation through single housing significantly reduced bone parameters across treatment lengths in male mice (20% average reduction in Tb.BV/TV; 8% average reduction in Ct.Th.) but not in females even with prolonged isolation. In trabecular bone, isolation affected male bone parameters in as little as 2 weeks. Isolation also decreased biomechanical properties in the femur of male but not female mice. While the females’ overall bone phenotype was unaffected, isolated females did show an increase in bone turnover markers with 2 weeks of isolation. Isolation also altered estrogen-related gene expression in male mice isolated for 4 or 8 weeks. Overall, our results demonstrate that both short- and long-term social isolation has sexually dimorphic effects on murine bone. These findings have important clinical implications for individuals at risk for social isolation, as well as for pre-clinical rodent models utilizing single housing. Lay Summary Social isolation may have worse effects on bone in males than females, but this has not been thoroughly explored. We investigated if different lengths of isolation had different effects on the bones of male and female mice. We found that both short- and long-term isolation had a rapid and negative impact on the bones of male but not female mice, although isolated females did show some changes in markers of bone formation and resorption. These findings have important implications for humans at risk for social isolation, as well as for studies with rodents that use single housing. Competing Interest Statement The authors have declared no competing interest. Footnotes Declarations of Interest: None Results, figures, and discussion updated to clarify timing of bone changes; Supplemental file order updated.