A role for GATA-6 in vertebrate chondrogenesis.

The GATA family of transcription factors are known to play multiple critical roles in vertebrate developmental processes, including erythropoiesis, endoderm formation and cardiogenesis. There have been no previous demonstrations of a functional role for any GATA family member being associated with musculoskeletal development but we now identify a possible role for GATA-6 in chondrogenesis. We detect abundant levels of GATA-6 mRNA in precartilaginous condensations (PCCs) in both the axial and appendicular skeleton of mouse embryos and in committed primary chondrocyte precursors. We also show that the G-protein coupled receptor, Gpr49, is a target of GATA-6 regulation in differentiating embryonal carcinoma cells and that, in vivo, the expression domains of the two genes overlap within PCCs. Finally, we have identified conserved, canonical GATA binding sites within the Gpr49 gene locus, and show by EMSAs that GATA-6 can bind to these sites in vitro. These data therefore suggest that GATA-6 also plays a role in chondrogenesis and that Gpr49 is a potential direct target of GATA regulation in this process.

Wnt2 is a direct downstream target of GATA6 during early cardiogenesis.

The GATA4, 5 and 6 subfamily of transcription factors are potent transactivators of transcription expressed within the precardiac mesoderm. However, little is known of the immediate downstream targets of GATA-factor regulation during the earliest stages of cardiogenesis. Using the P19-CL6 embryonal carcinoma (EC) cell line as an in vitro model of cardiogenesis, we show that GATA6 is the most abundantly expressed of the GATA factors in presumptive cardiac cells. Consequently, we performed a microarray screen comparing mRNA from control EC cells, early in the cardiac differentiation pathway, with those in which GATA6 had been overexpressed. These studies identified 103 genes whose expression changed significantly and this was verified in a representative array of these genes by real-time RT-PCR. We show that early cardiac expression of one of these genes, Wnt2, mirrors that of GATA6 in vitro and in vivo. In addition, its upregulation by GATA6 in differentiating EC cells is mediated by the direct binding of GATA-factor(s) to the cognate Wnt2 promoter, suggesting Wnt2 is an immediate downstream target of GATA-factor regulation during early cardiogenesis.

GATA factors lie upstream of Nkx 2.5 in the transcriptional regulatory cascade that effects cardiogenesis.

Members of the GATA-4, -5, and -6 subfamily of transcription factors are co-expressed with the homeoprotein Nkx 2.5 in the precardiac mesoderm during the earliest stages of its specification and are known to be important determinants of cardiac gene expression. Ample evidence suggests that GATA factors and Nkx 2.5 cross-regulate each other's expression; however, the temporal order of the expression of these transcription factors in vivo remains unresolved, and thus precise definition of the role of the products of the genes they transcribe in early development has been difficult to assess. We employed P19 CL6 mouse embryonic carcinoma cells as a model to investigate this problem, because these cells, like embryonic stem cells, can be induced to differentiate along multiple lineages. Here we demonstrate that when P19 CL6 cells are induced to differentiate to a cardiogenic lineage, the expression of GATA-4 and GATA-6 is up-regulated prior to the transcriptional activation of Nkx 2.5. Moreover, over-expression of GATA-4 or -6 at the time of Nkx 2.5 induction results in a significant up-regulation of endogenous Nkx 2.5 transcription. Finally, it is known that a Nkx-dependent enhancer is necessary for GATA-6 expression within cardiomyocytes of the developing mouse embryo. We demonstrate that within undifferentiated P19 CL6 cells, GATA-6 expression is subject to active repression by a novel upstream element that possesses binding sites for factors involved in transcriptional repression that are conserved between mammalian species.

Neurotrophin-independent attraction of growing sensory and motor axons towards developing Xenopus limb buds in vitro.

The mechanisms for directing axons to their targets in developing limbs remain largely unknown though recent studies in mice have demonstrated the importance of neurotrophins in this process. We now report that in co-cultures of larval Xenopus laevis limb buds with spinal cords and dorsal root ganglia of Xenopus and axolotl (Ambystoma mexicanum) axons grow directly to the limb buds over distances of up to 800 microm and in particular to sheets of epidermal cells which migrate away from the limb buds and also tail segments in culture. This directed axonal growth persists in the presence of trk-IgG chimeras, which sequester neurotrophins, and k252a, which blocks their actions mediated via trk receptors. These findings indicate that developing limb buds in Xenopus release diffusible factors other than neurotrophins, able to attract growth of sensory and motor axons over long distances.