As small regulatory transcripts, microRNAs (miRs) act as genetic ‘fine tuners’ of posttranscriptional events, and as genetic switches to promote phenotypic switching. The miR miR26a targets the BMP signalling effector, smad1 . We show that loss of miR26a leads to hemorrhage (a loss of vascular stability) in vivo , suggesting altered vascular differentiation. Reduction in miR26a levels increases smad1 mRNA and phospho-Smad1 (pSmad1) levels. We show that increasing BMP signalling by overexpression of smad1 also leads to hemorrhage. Normalization of Smad1 levels through double knockdown of miR26a and smad1 rescues hemorrhage, suggesting a direct relationship between miR26a , smad1 and vascular stability. Using an in vivo BMP genetic reporter and pSmad1 staining, we show that the effect of miR26a on smooth muscle differentiation is non-autonomous; BMP signalling is active in embryonic endothelial cells, but not in smooth muscle cells. Nonetheless, increased BMP signalling due to loss of miR26a results in an increase in acta2 -expressing smooth muscle cell numbers and promotes a differentiated smooth muscle morphology. Similarly, forced expression of smad1 in endothelial cells leads to an increase in smooth muscle cell number and coverage. Furthermore, smooth muscle phenotypes caused by inhibition of the BMP pathway are rescued by loss of miR26a . Taken together, our data suggest that miR26a modulates BMP signalling in endothelial cells and indirectly promotes a differentiated smooth muscle phenotype. Our data highlights how crosstalk from BMP-responsive endothelium to smooth muscle is important for smooth muscle differentiation.

Vascular smooth muscle cells (vSMC) are essential to the integrity of blood vessels, and therefore an attractive target of therapeutics aimed at improving vascular function. Smooth muscle cells are one of the few cell types that maintain plasticity and can switch phenotypes from differentiated (contractile) to de-differentiated (synthetic) and vice versa. As small regulatory transcripts, miRNAs act as genetic 'fine tuners' of posttranscriptional events and can act as genetic switches promoting phenotypic switching. The microRNA miR26a targets the BMP signalling effector, smad1. We show that loss of miR26 leads to hemorrhage (a loss of vascular stability) in vivo, suggesting altered vascular differentiation. Reduction in miR26a levels increases smad1 mRNA and phospho-Smad1 (pSmad1) levels. We show that increasing BMP signalling by overexpression of smad1 also leads to hemorrhage and that normalization of Smad1 levels through double knockdown of miR26 and smad1 rescues hemorrhage suggesting a direct relationship between miR26 and smad1 and vascular stability. Using a BMP genetic reporter and pSmad1 staining we show that the effect of miR26 on vascular instability is non-autonomous; BMP signalling is active in embryonic endothelial cells, but not in smooth muscle cells. Nonetheless, increased BMP signalling due to loss of miR26 results in an increase in acta2-expressing smooth muscle cell numbers and promotes a differentiated smooth muscle morphology. Taken together our data suggests that miR26 modulates BMP signalling in endothelial cells and indirectly promotes a differentiated smooth muscle phenotype. Our data also suggests that crosstalk from BMP-responsive endothelium to smooth muscle is important for its differentiation.