Gax, a temporally expressed homeobox gene in myogenic lineages, is up-regulated upon terminal differentiation

Gax, a growth arrest-specific homeobox gene, was recently cloned from an adult vascular smooth muscle cDNA library. Since homeobox genes have been shown to be important in pattern formation and organogenesis, we performed in situ hybridization and immunohistochemistry to establish its embryonic pattern of expression. Gax expression in C57B1/6J mouse embryos at day 7.5 was limited to lateral plate mesoderm. Subsequently, Gax was restricted to derivatives of the mesoderm, ectoderm and neural crest. Among these, Gax was expressed in atrial and ventricular cardiomyocytes through day 12.5. Thereafter, protein expression became localized to a rim of ventricular myocytes. Gax was detected throughout muscle differentiation in somites, both truncal and apendicular myoblasts and in mature skeletal myocytes. To further elucidate the role of Gax during myogenesis. Western blot analysis was performed in C2C12 skeletal myoblasts undergoing terminal differentiation. Significant up-regulation of Gax protein expression was observed during the transition from myoblasts to myotubes. Consistent with this observation, MyoD was shown to transactivate the Gax promotor in vitro. Based on its known function as a transcription factor and role as a growth arrest gene, we postulate that Gax expression is important in the developmental differentiation of myocytes.