SUMMARY Skeletal dimorphism can be seen under physiological conditions and following genetic manipulations at the structural, cellular, and molecular levels. Understanding the basis for these differences is crucial to developing approaches to prevent and reverse the loss of bone mass and strength, tailored specifically for females and males. We have shown lower total, femoral, and spinal BMD in female wild type C57BL/6J mice compared to male littermates from 1 to 4 months of age. Further, µCT analysis in 4-months mice showed lower TbN, and higher TbSp in distal femur of female compared to male mice. The differences are further enhanced in 20-months mice, with changes in these parameters and lower BV/TV in female mice versus male littermates. Similarly, the consequences of genetic manipulations differ between male and female skeleton. The difference in bone struc- ture and composition between sexes has been ascribed to disparate sex steroid actions. Sex steroids interact with their receptors in bone cells to directly modify gene expression and intracellular kinase activation, and indi- rectly engage other signaling pathways and mechanical signals. Yet, sex steroid receptors deletion from bone cells has not provided a clear understanding of the mechanisms underlying sexual dimorphism. This evidence suggests that factors other than sex steroids such as chromosome sex contribute to the differences in bone between males and females. Yet, a key question that remains unclear in our field is whether chromosome sex contributes to sex-specific skeletal differences. The four-core genotype (FCG) mouse model, in which the testis- determining gene Sry was deleted from the Y chromosome and/or expressed as a transgene (TG) comprises mice of 2 gonadal sexes: male XXM (Sry TG) or XYM (YSry-/Sry TG), and female XXF (wild type) or XYF (YSry-). Studies with these mice uncovered chromosome sex effects on the brain, on lifespan, and on adiposity, unrelated to the sex hormone present. Yet, gonadal versus chromosome sex contribution to bone development and mainte- nance with aging is unknown. Our long-term goal is to understand the basis for bone sexual dimorphism, and how sex differences affect the growing and aging skeleton. We will test the hypothesis that bone mass and strength accrual and decline result from a combination of gonadal and chromosome sex-dependent mechanisms, by pursuing 2 Aims. Aim 1 we will determine the contribution of gonadal versus chromosome sex to bone mass and strength in 2-, 4-, and 20-months FCG mice. Aim 2, will identify the molecular mech- anism for sexual dimorphism in adult bone tissue in 4-months FCG mice, by testing the contribution of the gonadal/chromosome sex to protein secretion and gene expression in bone cells. These studies will advance our knowledge of the mechanisms controlling bone cell function in a sex-dependent manner during growth, skel- etal maturation, and aging. Further, the results of the studies could inform on potential ways to improve bone health in a gender dependent manner and might provide the basis for improved therapies to maintain bone health in people with gender dysphoria, with circulating sex hormones that do not correspond to the chromosome sex.

PROJECT NARRATIVE The skeletal phenotype differs between males and females under physiological conditions as well as following genetic manipulations not only at the structural, but also at the cellular and molecular levels. However, whether the sex differences stem from presence of ovaries versus testis (and the corresponding sex steroids) or the presence of two X chromosomes versus one X and one Y chromosomes (or a combination of both gonadal and chromosome sex) remains unknown. Understanding the basis for these differences is crucial to developing approaches to prevent and reverse the loss of bone mass and strength, tailored specifically for females and males.