Sex hormone deficiency in male and female mice expressing the Alzheimer's disease-associated risk-factor TREM2 R47H variant impacts the musculoskeletal system in a sex- and genotype-dependent manner

The R47H variant of the triggering receptor expressed on myeloid cells 2 (TREM2) is a risk factor for Alzheimer's disease in humans and leads to lower bone mass accrual in female but not male 12-mo-old mice. To determine whether, as with aging, gonadectomy results in sex-specific musculoskeletal effects, gonad removal or SHAM surgery was performed in 4-mo-old TREM2^R47H/+ mice and WT male and female littermates (n = 10-12/group), with sexes analyzed separately. Body weight was lower in males, but higher in females after gonadectomy, independently of their genotype. Gonadectomy also leads to decreased BMD in males at all sites and in the whole body (total) and spine in female mice for both genotypes. Total and femur BMD was lower in gonadectomized male mice 6-wk post-surgery, independently of the genotype. On the other hand, BMD was only lower in ovariectomized WT but not TREM2^R47H/+ mice in all sites measured at this time point. Bone formation and resorption marker levels were not affected by orchiectomy, whereas CTX was higher 3 wk after surgery and P1NP showed a tendency toward lower values at the 6-wk time point only in ovariectomized WT mice. Micro-CT analyses showed no differences resulting from gonadectomy in structural parameters in femoral cortical bone for either sex, but lower tissue mineral density in males of either genotype 6-wk post-surgery. Nevertheless, biomechanical properties were overall lower in gonadectomized males of either genotype, and only for WT ovariectomized mice. Distal femur cancellous bone structure was also affected by gonadectomy in a genotype- and sex-dependent manner, with genotype-independent changes in males, and only in WT female mice. Thus, expression of the TREM2 R47H variant minimally alters the impact of gonadectomy in the musculoskeletal system in males, whereas it partially ameliorates the consequences of ovariectomy in female mice.