The alternative splicing regulator Nova2 constrains vascular Erk signalling to limit specification of the lymphatic lineage

The correct assignment of cell fate within fields of multipotent progenitors is essential for accurate tissue diversification. The first lymphatic vessels arise from pre-existing veins after venous endothelial cells become specified as lymphatic progenitors. Prox1 specifies lymphatic fate and labels these progenitors, however the mechanisms restricting Prox1 expression and limiting the progenitor pool remain unknown. We identified a zebrafish mutant that displayed premature, expanded and prolonged lymphatic specification. The gene responsible encodes the regulator of alternative splicing, Nova2. In zebrafish and human endothelial cells, Nova2 selectively regulates pre-mRNA splicing for components of signalling pathways and phosphoproteins. Nova2-deficient endothelial cells display increased Mapk/Erk signalling and Prox1 expression is dynamically controlled by Erk signalling. We identify a mechanism whereby Nova2 regulated splicing constrains Erk signalling, thus limiting lymphatic progenitor cell specification. This identifies the capacity of a factor that tunes mRNA splicing to control assignment of cell fate during vascular differentiation.