Examining the Role of Arhgef3 in Regulating Bone Mass and Tissue Quality in Adult Mice

Document Type

Conference Proceeding

Publication Date

9-27-2024

Publication Title

J Bone Miner Res

Abstract

Genome-wide studies have found ARHGEF3 gene to be situated within a quantitative trait locus for bone mineral density (BMD) and has been identified as a strong positional candidate for the development of osteoporosis. However, the underlying role of Arhgef in regulating bone mass and tissue strength is entirely unknown. Given that Arhgef3 plays a key role in regulating the RhoA signaling pathway, a pathway that limits bone formation throughout adulthood, we hypothesized the absence of Arhgef3 in global knockout (KO) mice will increase bone mass and tissue quality compared to wild-type (WT) controls. Based on micro-CT analysis, cortical area and BMD in the tibia and femur were not statistically different between WT and KO mice (Fig 1A). However, the moment of inertia (MOI) was significantly higher in KO mice by 20% when compared to WT. This shift in MOI was attributed to an increase in periosteal expansion as well as endocortical expansion, which resulted in a cortical thickness that was significantly smaller in KO than WT. Despite differences in cortical bone, trabecular bone analysis in the femur found no significant differences between WT and KO mice. We then examined the mechanical properties of bone under four-point bending and found gains in ultimate load and stiffness, which on average were respectively 25% and 27% greater than WT mice (Fig 1B). Having used four-point bending, the tissue-level properties were then calculated and found significantly higher in KO mice compared to WT (Fig 1C). In particular, the ultimate stress and modulus of the bone were respectively 20% and 18% higher compared to WT. These findings suggest Arhgef3 not only regulates the architecture of the tissue being formed, but also the type of tissue that is deposited. To better understand how Arhgef3 impacts the type of matrix proteins expressed by osteoblasts, bone marrow stromal cells from WT and KO mice were isolated and differentiated into osteoblasts. After 14 days of culture in the presence of ascorbic acid and β-glycerophosphate, osteoblasts from KO mice expressed significantly higher levels of periostin, osteopontin, bone sialoprotein, along with alkaline phosphate expression (Fig 1D). Expression of osterix and Runt-related transcription factor 2 showed similar trends in WT and KO derived osteoblasts, indicating similar trends in osteoblast differentiation, but variations in the expression of matrix proteins and alkaline phosphate. Altogether these findings demonstrate that Arhgef3 plays a critical role in regulating cortical bone architecture and tissue properties. Furthermore, gains in tissue properties when deleting Arhgef3 are likely a function in the type of matrix produced by osteoblasts. To date, this is the first study to examine the role of Arhgef3 in regulating cortical bone architecture and tissue properties.

Volume

39

First Page

258

Last Page

259

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