Moderate Effects of the Arginine to Histidine R47H Variant of the Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) on Bone Structure in Male and Female Mice: Insights from the Four Core Genotypes mice

Background The Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) gene is expressed in cells of the hematopoietic lineage, like microglia and osteoclasts. A TREM2 gene variant known as TREM2-R47H is associated with an increased risk of developing Alzheimer’s disease (AD). Previous studies have shown sex-dimorphic bone and muscle consequences that are associated with the TREM2 variant. Sex chromosomes have also been shown to play a key contributor to skeletal mass and bone strength. Due to the sex-dimorphic bone and skeletal muscle phenotype exhibited by mice expressing the TREM2 gene variant, we investigated the role of chromosomal (XX vs XY) or gonadal (ovaries vs testes) sex. Methods Four Core Genotypes (FCG) C57Bl/6J mice expressing the TREM2-R47H variant were mated to obtain TREM2 wildtype (TREM2 ^+/+ , WT) and TREM2 ^R47H/+ FCG mice. Four to 5.5-month-old gonadal male (XXT and XYT) and female (XXO and XYT) mice were analyzed. Body weight and bone mineral density were initially measured at baseline and endpoint (5.5 months of age) by DXA/Piximus. Micro-computed tomography, dynamic histomorphometry, 3-point bending test (mechanical properties), and bone turnover markers were measured at the endpoint. Two-way ANOVA analyses were performed through Prism 10 to identify the contributions of chromosome sex, the presence of the TREM2-R47H variant, and their interaction, separately for each gonadal sex. Results Gonadal males: chromosome sex (XX/XY) effects are found for several bone structural parameters in femur and lumbar vertebra 5, whereas there was an interaction between gonadal sex and chromosome sex for other structural measurements in both bones by µCT. Overall, values are higher for TREM2 ^R47H/+ than WT for XYT, but not XXT mice, suggesting that the TREM2 genotype effects depend on the presence of the Y chromosome. Mechanical testing shows chromosome sex effects, with higher overall values for XXT mice. Bone formation on the femur cortex and serum formation/resorption markers were unchanged, suggesting that structural changes result from bone modeling/remodeling at an earlier age. Gonadal females: Chromosome sex affects body weight gain (higher in XYO than XXO mice), but no bone mineral density accrual. Chromosome sex affects total lean mass (XYO > XXO) with chromosome sex x TREM2 genotype interaction and differences in total/%fat mass (TREM2 ^R47H/+ <WT), and %lean mass (TREM2 ^R47H/+ >WT) only for XYO mice. Chromosome sex affects distal femur volumetric bone mass (XYO > XXO), but the TREM2 genotype influences lumbar vertebra trabecular number and separation, which trended higher in TREM2 ^R47H/+ vs WT mice for sex complement. Chromosome sex influences femur cortical bone, with overall higher values in XXO mice, independent of TREM2 genotype. Mechanical testing parameters also were XXO > XYO mice. Femur cortical bone formation is higher on the endocortical but lower on the periosteal surface in XXO vs XYO (chromosome sex effect). The opposite effects on the bone surfaces might explain the unchanged serum bone formation marker, Procollagen Type 1 N-terminal propeptide (P1NP). Yet, chromosome sex affects the levels of the resorption marker, C-terminal telopeptide of type 1 collagen (CTX-1), which were lower in XXO mice. Conclusion Our findings suggest that chromosome sex partially affects the consequences of expression of the TREM2-R47H variant on bone structure, whereas the outcomes of the gene variant depend on the mouse gonadal sex.