Rad GTPase is essential for the regulation of bone density and bone marrow adipose tissue in mice

Bone. 2017 Oct;103:270-280. doi: 10.1016/j.bone.2017.07.018. Epub 2017 Jul 18.

Abstract

The small GTP-binding protein Rad (RRAD, Ras associated with diabetes) is the founding member of the RGK (Rad, Rem, Rem2, and Gem/Kir) family that regulates cardiac voltage-gated Ca2+ channel function. However, its cellular and physiological functions outside of the heart remain to be elucidated. Here we report that Rad GTPase function is required for normal bone homeostasis in mice, as Rad deletion results in significantly lower bone mass and higher bone marrow adipose tissue (BMAT) levels. Dynamic histomorphometry in vivo and primary calvarial osteoblast assays in vitro demonstrate that bone formation and osteoblast mineralization rates are depressed, while in vitro osteoclast differentiation is increased, in the absence of Rad. Microarray analysis revealed that canonical osteogenic gene expression (Runx2, osterix, etc.) is not altered in Rad-/- calvarial osteoblasts; instead robust up-regulation of matrix Gla protein (MGP, +11-fold), an inhibitor of extracellular matrix mineralization and a protein secreted during adipocyte differentiation, was observed. Strikingly, Rad deficiency also resulted in significantly higher marrow adipose tissue levels in vivo and promoted spontaneous in vitro adipogenesis of primary calvarial osteoblasts. Adipogenic differentiation of wildtype calvarial osteoblasts resulted in the loss of endogenous Rad protein, further supporting a role for Rad in the control of BMAT levels. These findings reveal a novel in vivo function for Rad and establish a role for Rad signaling in the complex physiological control of skeletal homeostasis and bone marrow adiposity.

Keywords: Adipogenesis; Bone marrow adipose tissue; Matrix Gla protein; Osteoblasts; Osteogenesis; Ras GTPase.

MeSH terms

  • Adipogenesis / physiology*
  • Adipose Tissue / pathology
  • Animals
  • Bone Density / physiology*
  • Bone Marrow / enzymology*
  • Bone Marrow / pathology
  • Mice
  • Mice, Inbred C57BL
  • Mice, Knockout
  • Osteogenesis / physiology*
  • ras Proteins / metabolism*

Substances

  • Rrad protein, mouse
  • ras Proteins