Dynamic Cortical Actin Contractions During Convergence and Extension of Frog Embryo

Actomyosin networks linked to the micro-environment through the plasma membrane are thought to be key players regulating cell behaviors. Even after decades of studies dedicated to reveal the role of actin cytoskeleton in single cell migrations, we do not understand how actomyosin networks drive cell movements within multicellular tissues, like those in developing embryos. Here we use the converging and extending mesoderm tissues of Xenopus embryo as a model system to understand the role of cortical actin cytoskeleton during cell rearrangement within a multicellular organism. Using high resolution live imaging of F-actin, we observe the unique dynamic actin contractions called "punctuated actin contractions" in the mid-cell body of mesenchymal cells. These contraction dynamics are a common feature of Xenopus embryonic cells and mediate cell shape changes during morphogenesis. Quantitative morphological analysis of these F-actin dynamics indicates that frequent and aligned movements of multiple actin contractions accompany mesoderm cells as they intercalate and elongate. Moreover, we found levels of punctuated actin contractions are permissively regulated by the non canonical Wnt-signaling pathway. Over-expression on Xfz7 can induce early maturation of actin contractions in mesoderm and produce mesoderm-like actin contractions in ectoderm cells. In contrast, expression of the dominant negative Disheveled constructs Xdd1 block the progression of actin contractions to their late mesoderm dynamics, it but has no effect in ectoderm. Additionally, using inhibitors combined with FRAP analysis we find the dynamics of actin contractions are regulated by both myosin contractility and F-actin polymerization. We also show the actin contractions can be induced by microtubules via RhoGEFs in mesoderm tissue. In summary, this study reveals punctuated actin contractions within converging and extending mesoderm, and uncovers permissive roles for non-canonical Wnt-signaling, myosin contractility, and F-actin polymerization which play a role in regulating those dynamics