Estrogen deficiency from ovariectomy enhances the formation of osteocyte plasma membrane disruptions from treadmill exercise in vivo

Abstract

Plasma membrane disruptions (PMD) form in cortical bone osteocytes with mechanical loading, and may be involved in mechanisms underlying how osteocytes sense their mechanical environment and drive bone adaptation. Our group recently reported that osteocyte PMD repair is sensitive to oxidative stress, with reactive oxygen species inhibiting repair and antioxidants promoting repair. Estrogen is a physiologically relevant antioxidant, and exercise increases oxidative stress. The goal of the current study was to determine whether exercise‐induced osteocyte PMD formation would be affected by estrogen depletion from ovariectomy (OVX), and if this process would affect cortical bone adaptation to exercise.Female CD‐1 mice were subjected to OVX or sham surgeries. Beginning 4 weeks after surgery, mice were run downhill on treadmills daily (12m/min, 30 min/day) for 4 weeks and compared against sedentary controls (n=10 sham sedentary, 10 sham exercise, 10 OVX sedentary, 10 OVX exercise). Immunohistochemistry against an endogenous PMD tracer (albumin) was used to quantify the number of osteocyte PMD via confocal microscopy in the femur diaphysis. Cortical bone mass was monitored longitudinally throughout the study by DXA and by micro‐computed tomography (microCT) at the conclusion of the study. Groups were compared with 2‐factor ANOVA with interaction (factor 1 = surgery: OVX or sham, factor 2 = exercise group: exercise or sedentary).Uterine weight and trabecular bone mass were decreased by OVX, confirming successful depletion of estrogen. When compared against sedentary controls, exercise‐induced cortical bone osteocyte PMD formation was more substantial in OVX mice (+180%) than in sham‐operated mice (+90%) (interaction effect: p=0.028). Bone mineral density of the femur diaphysis was lower in the OVX mice but was not significantly increased by exercise in either OVX or sham mice (p>0.285). MicroCT‐based measurements of cortical bone geometry in the femur including tissue area, bone area, and moments of inertia were generally unaffected by exercise or ovariectomy (p>0.183).As exercise‐induced osteocyte PMD formation was enhanced by ovariectomy, data from this study suggest that endogenous estrogen may blunt the formation of osteocyte PMD during repetitive mechanical loading. Despite differences in PMD formation, the ovariectomized mice did not demonstrate altered cortical bone adaptation to exercise as compared to sham controls. The lack of cortical bone adaptation to exercise in either sham or OVX mice, as seen here, may indicate that the loading duration was not sufficient to induce changes in cortical bone geometry, suggesting that longer‐term studies may be warranted. More in‐depth analyses of cortical bone turnover (via histology) are ongoing, which may be a more sensitive measurement to determine whether the increased formation of exercise‐induced osteocyte PMD in OVX mice is associated with an altered cortical bone adaptation response.Support or Funding InformationNational Science Foundation (CMMI 1727949), National Institute on Aging (P01‐AG036675).This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.