Myocardin-related transcription factors regulate morphogenetic events in vertebrate embryos by controlling F-actin organization and apical constriction

Myocardin-related transcription factors (Mrtfa and Mrtfb), also known as megakaryoblastic leukemia proteins (Mkl1/MAL and Mkl2), associate with serum response factor (Srf) to regulate transcription in response to actin dynamics, however, the functions of Mrtfs in early vertebrate embryos remain largely unknown. Here we document the requirement of Mrtfs for blastopore closure at gastrulation and neural plate folding in Xenopus early embryos. Both stimulation and inhibition of Mrtf activity caused similar gross morphological phenotypes, yet the effects on F-actin distribution and cell behavior were different. Suppressing Mrtf-dependent transcription reduced overall F-actin levels and inhibited apical constriction during gastrulation and neurulation. By contrast, constitutively active Mrtf caused tricellular junction remodeling and induced apical constriction in superficial ectoderm. The underlying mechanism appeared distinct from the one utilized by known apical constriction inducers. We propose that the regulation of apical constriction is among the primary cellular responses to Mrtf. Our findings highlight a dedicated role of specific transcription factors, Mrtfs, in early morphogenetic processes.