The characterization of Mustang in chondrogenesis in vitro

The Musculoskeletal Temporally Activated Novel Gene (Mustang) was identified by suppressive subtractive hybridization comparing bone fracture callus to intact bone. Mustang´s peak in expression (50-fold over intact levels) was localized temporally to early time points during the Bone Fracture Repair (BFR) process that correspond to proliferation of chondrocyte progenitor cells and chondrogenesis at the fracture site. Further, this small protein contains a nuclear localization sequence and spatially localized to proliferating chondrocytes within the adult rat knee joint. These data suggests that Mustang may act as a transcriptional co-activator or co-repressor. The elucidation of Mustang´s expression during the proliferation and differentiation of chondrocytes in vitro could help shed light on Mustang´s function. To this end, Mustang was over expressed by stable transfection and silenced via RNAi in the RCJ 5.18 chondrocytic cell line. The over expression of Mustang up to six-fold in the RCJ cells showed no significant effect on either cell growth or matrix production. However, reduction of Mustang expression by 52-66% resulted in both a significant decrease in proliferation rate as well as diminished matrix production. This suggests that Mustang is necessary for chondrocyte proliferation as well as differentiation and possibly functions as a regulator of chondrogenesis.