# 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

## TL;DR

This study explores how a genetic variant linked to Alzheimer's disease affects bone structure differently in male and female mice, depending on their sex chromosomes.

## Contribution

The study reveals sex chromosome-specific effects of the TREM2-R47H variant on bone structure in mice, highlighting interactions between genetic and gonadal sex.

## Key findings

- TREM2-R47H genotype effects on bone structure depend on the presence of the Y chromosome in gonadal males.
- Chromosome sex influences bone mineral density and mechanical properties in a sex-specific manner.
- TREM2-R47H mice show altered trabecular bone parameters in lumbar vertebrae, with trends toward structural differences.

## Abstract

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.

Four Core Genotypes (FCG) C57Bl/6J mice expressing the TREM2-R47H variant were mated to obtain TREM2 wildtype (TREM2+/+, WT) and TREM2R47H/+ 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.

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 TREM2R47H/+ 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 (TREM2R47H/+<WT), and %lean mass (TREM2R47H/+>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 TREM2R47H/+ 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.

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.

Alzheimer’s disease (AD) is the most common form of dementia and currently affects 6.9 million Americans. AD has also been associated with affecting women at a higher rate compared to men, and recent studies have suggested the Trigger Receptor Expressed on Myeloid Cells 2 (TREM2) R47H point mutation gene may play a key role in AD and in bone integrity. To uncover the role of the TREM2 variant, a transgenic mouse model was used to tease out the effects of sex chromosomes on the consequences of the presence of the TREM2-R47H gene within each gonadal sex.