Role of MRTF in endo-mesoderm specification and gastrulation in Nematostella vectensis

MRTF (Myocardin-Related Transcription Factor) is a unique transcriptional cofactor whose activity is regulated by cytosolic G-actin/F-actin ratio. As part of the Rock pathway, this actin binding cofactor is a bridge between cytoskeleton rearrangements and gene expression. In bilaterians, MRTF plays an important role in Epithelial to Mesenchymal transition (EMT) and muscle cells differentiation. However, its function in non-bilaterian organisms, such as Nematostella vectensis, is yet to be solved. In this cnidarian model, after germ layer specification, gastrulation is initiated by partial EMT of mesodermal cells involving a morphological cell change associated with reorganization of the cytoskeleton. Thus, based on MRTF function in bilaterians, this work questions whether MRTF is involved in these processes in Nematostella vectensis as well. In this study, we investigated the role of MRTF in germ layer specification and gastrulation during the embryonic development of Nematostella vectensis. In order to explore its activity, we have realized loss-of-function experiments using MRTF-protein inhibitor (CCG-203791) and shRNA-mediated knockdown. As a complement, we also realized a gain-of-function experiment with MRTF-protein activator (ISX-9) treatment. Perturbations of MRTF protein activity have been followed by morphological and gene expression analyses. Our results show that loss of MRTF function leads to altered mesodermal behavior and gastrulation arrest. This phenotype is associated with complete loss of expression of endodermal genes wnt3 and foxA. Surprisingly, expression of mesodermal markers appears unchanged. Simultaneously, ISX-9-mediated MRTF activation induces a more severely altered phenotype linked to a complete inhibition of mesodermal invagination and no gastrulation. This morphological effect is associated with a downregulation of endodermal markers and alteration of mesodermal markers, but also with an overexpression of the mesoderm transcription factor, ERG. Taken together, these results highlight the essential role of MRTF in germ-layer specification and partial EMT of mesodermal cells during gastrulation of Nematostella vectensis.