Department of Molecular and Cellular Biochemistry, University of Kentucky College of Medicine, BBSRB, 741 S Limestone Street, Lexington, KY 40536-0509, USA.
Department of Anatomy and Cell Biology, Indiana University School of Medicine, 635 Barnhill Drive, Indianapolis, IN 46202-5120, USA.
Bone. 2017 Oct;103:270-280. doi: 10.1016/j.bone.2017.07.018. Epub 2017 Jul 18.
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 Ca 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.
小分子 GTP 结合蛋白 Rad（RRAD，与糖尿病相关的 Ras）是 RGK（Rad、Rem、Rem2 和 Gem/Kir）家族的创始成员，该家族调节心脏电压门控 Ca 通道功能。然而，其在心脏以外的细胞和生理功能仍有待阐明。在这里，我们报告 Rad GTP 酶功能对于小鼠的正常骨稳态是必需的，因为 Rad 缺失导致骨量明显降低和骨髓脂肪组织（BMAT）水平升高。体内动态组织形态计量学和体外原代颅骨成骨细胞测定表明，在没有 Rad 的情况下，骨形成和成骨细胞矿化率降低，而体外破骨细胞分化增加。微阵列分析显示，Rad 颅骨成骨细胞中的典型成骨基因表达（Runx2、osterix 等）没有改变；相反，细胞外基质矿化抑制剂和脂肪细胞分化过程中分泌的基质 Gla 蛋白（MGP）的表达显著上调（+11 倍）。引人注目的是，Rad 缺乏也导致体内骨髓脂肪组织水平显著升高，并促进原代颅骨成骨细胞的自发体外脂肪生成。野生型颅骨成骨细胞的脂肪分化导致内源性 Rad 蛋白的丢失，进一步支持 Rad 在控制 BMAT 水平中的作用。这些发现揭示了 Rad 在体内的新功能，并确立了 Rad 信号在骨骼稳态和骨髓脂肪组织生理控制的复杂过程中的作用。
