Overexpression of developmentally regulated GTP-binding protein-2 increases bone loss

Abstract

The developmentally regulated GTP-binding protein-2 (DRG2) is a novel subclass of GTPbinding proteins. Many functional characteristics of osteoclasts (OC) are associated with small GTPases. We hypothesized that DRG2 affects bone mass via modulating OC activity. Using DRG2 transgenic mice, we investigated the role of DRG2 in bone remodeling. DRG2 overexpression caused a decrease in bone mass and an increase in the number and activity of OC in vivo. DRG2 overexpression increased fusion, spreading, survival, and resorption activity of OC in vitro. Downregulation of DRG2 by siRNA decreased fusion, spreading, and survival of OC, supporting the observations found in DRG2 transgenic OC. Transgenic mature OCs were larger, with actin rings and higher ERK, Akt, Rac1 and Rho activities than wild-type OCs. Inhibition of these proteins abolished the effects of DRG2 on formation of large OCs with actin rings, implying that DRG2 affects cytoskeleton reorganization in a Rac1/Rho/ERK/Akt-dependent manner. In summary, DRG2 is associated with survival and cytoskeleton organization of OC under influence of macrophage colony-stimulating factor, and its overexpression leads to elevated bone resorptive activity of OC, resulting in bone loss. DRG2; osteoclast; fusion; spreading; bone resorption BONE UNDERGOES DYNAMIC REMODELING, and maintenance of bone mass and integrity requires a precise balance between bone resorption by osteoclasts (OC) and bone formation by osteoblasts. In osteoporosis, resorption exceeds formation, whereas osteopetrosis is due to defective OC formation and function. OC differentiation depends on at least two factors derived from stromal cells/osteoblasts: macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor-B ligand (RANKL) (3, 12). During differentiation, OCs become multinucleated by fusion, which plays a critical role in bone resorption (13). To actively resorb bone, OC forms an actin ring that is a complex molecular attachment structure that seals off an extracellular compartment located between the cell and resorbing bone (5). By virtue of tight sealing, the extracellular compartment is characterized by maintaining a low pH and enriching lysosomal enzymes that are required for digestion of the bone matrix.Many OC properties that are necessary for efficient bone resorption, such as actin ring formation, fusion, and cell survival, depend on the activity of small GTPases (6,15,16,36). The Rho family of GTPases plays a critical role in actin organization, with RhoA, Rac1, Cdc42, and RhoU being responsible for cytoskeletal organization and the polarization of OCs (6, 15, 36). The ARF GTPase family, known to regulate membrane trafficking and actin dynamics, is responsible for the regulation of the OC cytoskeleton and formation of the sealing zone (11). Decrease in Rac2 impairs migration and fusion required for OC and results in bone defects found in Shwachman-Diamond syndrome (24). The activity of Rac1 (9) and Ras (4) is essential for OC survival. Although several small GTPases have...

DRG2 Deficient Mice Exhibit Impaired Motor Behaviors with Reduced Striatal Dopamine Release

“…To provide insight into the functional roles of DRG2 in mouse development, we report here, for the first time, that DRG2-deficient mice exhibit a significant growth delay and reduced body weight accompanied by defective osteogenesis and a short life span. Interestingly, DRG2 overexpression caused a decrease in bone mass and increases in the number and activity of osteoclasts in vivo in a previous work [31]. Conversely, in micro-CT analyses of DRG2-deficient mice, it was clearly identified that unossified areas were significantly increased in the skull and that bone formation was dramatically reduced in the appendicular skeleton.…”

Fine mapping of bone structure and strength QTLs in heterogeneous stock rat

“…Cops3 is an oncogene residing in the human chromosomal region 17p11.2-p12 - the copy number and expression level of Cops3 was significantly associated with the development of osteosarcoma, the most common primary malignancy of bone [29,30]. Drg2 , a GTP binding protein, overexpression of which in transgenic mice leads to increased number and activity of osteoclasts and bone loss [31]. Map2k3 is increased by RANKL, which in turn aids in osteoclastogenesis from bone marrow precursor cells [39].…”