Heterotopic expression of the Xl-Fli transcription factor during Xenopus embryogenesis: modification of cell adhesion and engagement in the apoptotic pathway.

In the Xenopus laevis embryo, the overexpression of the Xl-FLI protein, a transcription factor of the ETS family, provokes severe developmental anomalies, which affect anteroposterior and dorsoventral polarities, optic cup formation, head cartilage morphogenesis, and erythrocyte differentiation. It has been proposed that these effects could be correlated to modifications of cell adhesion properties and/or to an increased engagement of cells in the apoptotic pathway during early development (Remy et al., Int. J. Dev. Biol. 40, 577-589, 1996). To address these questions, we have first analyzed the behavior of cells overexpressing the protein in both aggregation and adhesion assays. We observe perturbations of cell-cell interactions as well as perturbations of cell adhesion and spreading on fibronectin and extracellular matrix (ECM). Second, we have analyzed apoptosis of cells overexpressing the Xl-FLI protein, by testing DNA fragmentation, caspase-3 activity and by performing TUNEL assay. We show that Xl-Fli overexpression results in the appearance of hallmarks of apoptosis, including exclusion of cells from the interior of the embryo, internucleosomal fragmentation of DNA and dose-dependent induction of caspase-3, resulting in the hydrolysis of poly(ADP-ribose) polymerase. In addition, a dominant-negative mutation of BMPs receptors decreases the effects of Xl-Fli overexpression, suggesting that a modification of the BMP signalling could be responsible for increased apoptosis. The latter appears to affect predominantly ventral and ventrolateral regions of the embryo.