Sexual Dimorphism in Differentiating Osteoclast Precursors Demonstrates Enhanced Inflammatory Pathway Activation in Female Cells

Abstract

Sexual dimorphism of the skeleton is well documented. At maturity, the male skeleton is typically larger and has a higher bone density than the female skeleton. However, the underlying mechanisms for these differences are not completely understood. In this study, we examined sexual dimorphism in the formation of osteoclasts between cells from female and male mice. We found that the number of osteoclasts in bones was greater in females. Similarly, in vitro osteoclast differentiation was accelerated in female osteoclast precursor (OCP) cells. To further characterize sex differences between female and male osteoclasts, we performed gene expression profiling of cultured, highly purified, murine bone marrow OCPs that had been treated for 3 days with macrophage colony‐stimulating factor (M‐CSF) and receptor activator of NF‐κB ligand (RANKL). We found that 125 genes were differentially regulated in a sex‐dependent manner. In addition to genes that are contained on sex chromosomes, transcriptional sexual dimorphism was found to be mediated by genes involved in innate immune and inflammatory response pathways. Furthermore, the NF‐κB‐NFATc1 axis was activated earlier in female differentiating OCPs, which partially explains the differences in transcriptomic sexual dimorphism in these cells. Collectively, these findings identify multigenic sex‐dependent intrinsic difference in differentiating OCPs, which results from an altered response to osteoclastogenic stimulation. In humans, these differences could contribute to the lower peak bone mass and increased risk of osteoporosis that females demonstrate relative to males. © 2021 American Society for Bone and Mineral Research (ASBMR).

Sexual Dimorphism in Osteoclasts

“…These discrepancies probably reflect significant differences in the culture assays that were employed or the origins of the cells. Significantly, our results correlate with measurements of the number of osteoclasts in the bones of mice [14]. However, more work is clearly needed to better understand the reasons for the discrepancies between female and male cultures in the various assays.…”

“…MKP-1 is an important negative regulator of the MAPK pathways of the innate immune system [36]. In contrast to these models of inflammatory osteolysis, we found that in mice in homeostasis bone marrow-derived osteoclastogenesis was greater in cells from female than from males [14]. Hence, the model in which osteoclast sexual dimorphism is examined seems to influence what outcome is observed.…”

“…Our original studies used cultures of bone marrow macrophage (BMM), which are a mixed culture [12]. We have seen similar differences between female and male osteoclastogenesis in cultures of murine bone marrow cells that were directly isolated by fluorescent-activated cell sorting (FACS) as CD11b lo/neg , CD3 neg , CD45R neg , CD115 (CSF-1Receptor) pos [13] and then immediately cultured with M-CSF and RANKL for 6 days [14]. The latter assay did not pretreat cells with M-CSF or M-CSF + RANKL to enhance commitment to the macrophage/osteoclast lineage, as is done by some investigators.…”

Hairy and enhancer of split 1 is a primary effector of NOTCH2 signaling and induces osteoclast differentiation and function

“…So that an increase in the BV of Notch2 tm1.1Ecan would not necessarily represent a rescue of the osteopenic phenotype and that HES1 was a mediator of NOTCH2. It is not readily apparent why the Hes1 inactivation caused a phenotype in female but not in male mice, and the observation stresses the importance of examining phenotypes in mice of different sexes independently ( 56 , 57 ). It is not unusual to observe sex-specific phenotypes in genetically engineered mice ( 58 , 59 , 60 ).…”

“…It is not unusual to observe sex-specific phenotypes in genetically engineered mice ( 58 , 59 , 60 ). Possible explanations for the prevalence of a phenotype in female mice include genetic influences, a loss of the inhibitory actions of estrogens on osteoclastogenesis in the context of the Hes1 inactivation as well as the earlier NF-κB–NFATc1 activation and osteoclastogenesis that occurs in female mice ( 57 , 61 , 62 ).…”