fus/TLS orchestrates splicing of developmental regulators during gastrulation.

Here we investigated the function of the atypical RNA-binding protein fus/TLS (fused in sarcoma/translocated in sarcoma) during early frog development. We found that fus is necessary for proper mRNA splicing of a set of developmental regulatory genes during early frog development and gastrulation. Upon fus knockdown, embryos fail to gastrulate and show mesodermal differentiation defects that we connect to intron retention in fgf8 (fibroblast growth factor 8) and fgfr2 (fgf receptor 2) transcripts. During gastrulation, the animal and marginal regions dissociate, and we show that this is caused, at least in part, by intron retention in cdh1 transcripts. We confirm the specificity of splicing defects at a genomic level using analysis of RNA sequencing (RNA-seq) and show that 3%-5% of all transcripts display intron retention throughout the pre-mRNA. By analyzing gene ontology slim annotations, we show that the affected genes are enriched for developmental regulators and therefore represent a biologically coherent set of targets for fus regulation in embryogenesis. This shows that fus is central to embryogenesis and may provide information on its function in neurodegenerative disease.