REGULATED EXPRESSION OF THE RETINOBLASTOMA GENE-PRODUCT BY FIBROBLASTGROWTH-FACTOR BUT NOT BY ACTIVIN DURING MESODERM INDUCTION IN XENOPUS

The retinoblastoma (RE) gene is a tumor suppressor gene that plays an important role in cell cycle arrest and in the terminal differentiatio n of skeletal myoblasts. Differentiation into muscle occurs in Xenopus embryo explants during mesoderm induction by fibroblast growth factor (FGF) or activin A. We examined expression of the RE gene product (pR B) during mesoderm induction in vivo and in vitro. We show that hypo- and hyper-phosphorylated forms of pRB are present during early develop ment and that expression of both forms increases significantly during the blastula stage, concomitant with mesoderm induction. Further inves tigation revealed that pRB is enriched in the presumptive mesoderm of the blastula stage embryo. In animal cap explants induced by Xenopus b FGF (XbFGF), pRB expression levels increased approximately tenfold whi le no increase was observed in explants induced by activin. However, w hen explants were induced by XbFGF in the presence of sodium orthovana date, a compound previously shown to synergize with FGF to produce mor e dorsal ''activin-like'' inductions than FGF alone, only a slight inc rease in pRB expression was observed. Furthermore, upregulation of pRB during mesoderm induction in vitro displayed an inverse correlation w ith expression of XFKH1, a marker for notochord. These results suggest that pRB may be important for patterning along the dorsoventral axis.