Osteopenia in Twisted Gastrulation-Deficient Mice Results from Enhanced Osteoclast Function

Understanding how regulation of skeletal homeostasis is mediated by twisted gastrulation (Twsg1), an extracellular protein that regulates bone morphogenetic proteins (BMPs), is essential for revealing mechanisms of normal and pathological bone remodeling. Furthermore, knowledge of such regulation will aid in determining optimal concentration, appropriate release and regulation of BMP activity that augments therapies against diseases causing bone loss, long and craniofacial bone defects, and maxillofacial procedures such as sinus and alveolar ridge augmentation. Objectives: To assess the function of Twsg1 in skeletal homeostasis and its importance in the regulation of cellular activity sustaining bone remodeling. Methods: Using a gene replacement vector, mice deficient in Twsg1 (Twsg1^-/-) were generated. Micro-CT and histomorphometric analysis were used to assess the skeletal phenotype. Osteoblast (OB) and osteoclast (OC) precursors were isolated and differentiated from wild-type (Twsg1^+/+) and Twsg1^-/- mice. In vitro and in vivo indices of OB and OC function were determined by real-time RT-PCR, histological staining and ELISA. OC resorption activity was assessed on dentine discs. Results: Studies show that Twsg1^-/- mice are severely osteopenic. The skeletal deficiency exhibited by Twsg1^-/- mice (decreased cortical and trabecular architecture) suggested an imbalance in OCs, OBs or both. Indices of OC function, including markers of differentiation, in vitro and in vivo TRAP staining and resorptive ability show that the osteopenia is due to increased OC number, extensively larger OCs, and enhanced OC activity. Conversely, the phenotype is not explained by defects in OB function, as markers of OB differentiation and bone formation rate were not significantly different between Twsg1^+/+ and Twsg1^-/- mice. Conclusion: Our results indicate that the osteopenia exhibited in Twsg1-deficiency is mediated through increased OC function and not through distorted OB activity. These novel findings suggest Twsg1 is a key regulator of osteoclast function. (NIH/NIDCR: T32DE007288 (J.S.R.); K08HD043138 and R01DE016601 (A.P.); AHC (R.G.))