Myocardin-related transcription factor regulates actomyosin contractility and apical junction remodeling during vertebrate neural tube closure.

Myocardin-related transcription factor A (Mrtfa), also known as megakaryoblastic leukemia protein (Mkl1/MAL), associates with serum response factor (Srf) to regulate transcription in response to actin dynamics; however, the functions of Mrtfa in vertebrate embryos remain largely unknown. Here, we show that Mrtfa is required for neural plate folding in early Xenopus embryos. Mrtfa knockdown reduced F-actin levels and inhibited apical constriction in the neural and non-neural ectoderm. By contrast, overexpression of constitutively active Mrtfa induced apical constriction in ectodermal cells via remodeling of tricellular junctions and junctional recruitment of Myosin II. We also identify potential Mrtfa target genes in embryonic ectoderm that encode actins and many regulators of actomyosin networks and junction remodeling. Our findings suggest a role for Mrtfa in the control of morphogenetic movements during neurulation. We propose that the regulation of actomyosin contractility is an essential cellular response to Mrtfa-dependent transcriptional activation.