Resumen de Integration between actomyosin dynamics and cell cell adhesion during drosophila dorsal closure
Morphogenesis refers to a wide number of processes that usually take place during embryonic development, but also during adulthood when tissue and organ damage occur. During morphogenesis, several cells coordinate their behavior in time and space to give rise to gastrulation movements, convergent and extension, and tissue spreading among others. The study of morphogenesis gives the opportunity to link processes occurring at different scales putting together the understanding on what happens at the molecular level with the cellular level (i.e. cell shape changes) and the tissue level (i.e. proper arrangement of cells).
Dorsal closure (DC) is an example of a morphogenetic process taking place during mid-late development of the Drosophila embryo, in which two epithelial sheet monolayers, interact to give rise to the final form of the embryo. Specifically, the cells of the epidermis elongate towards the dorsal midline to cover the gap where an extra-embryonic tissue, the amnioserosa (AS), is placed. Different forces are coordinated to drive this process but the major one is the force generated by the apical contraction of AS cells. Apical contraction is a common cellular process in morphogenesis in which nonmuscle Myosin-II, by the cyclical interaction between ATP and Actin, reduces the apical surface area of epithelial cells. In several tissues undergoing morphogenesis, including the AS, it has been found that cells undergo apical area oscillations, which correlate with transient contractions of an apical actomyosin network. However, how these periodic contractions occur, how they are regulated during DC, and how they give rise to cell and tissue level changes remains to be elucidated.
To better understand the relationship between Myosin activity and cell and tissue deformation, in this work we have made use of phosphomimetic and non-phosphorylatable mutant forms of Myosin and analyzed how they affect Myosin dynamics, cell area oscillations and tissue closure, using a combination of time-lapse imaging, quantitative image analysis and genetics. We have also studied how Myosin phosphorylation is regulated during dorsal closure by analyzing Myosin, amnioserosa cell dynamics and tissue closure in embryos in which Myosin kinase and Myosin phosphatase activities are perturbed. Altogether, the results shown in this thesis reflect how Myosin phosphorylation promotes tissue contraction through an increase in the frequency of appearance of Myosin contractile networks. This results in more persistent contractile networks, that generate cells that spend more time in the contracted state relative to the expanded state during an oscillatory cycle and are thus more contracted. In contrast, a non-phosphorylatable form of Myosin decreases the frequency of appearance of Myosin contractile networks as well as their persistence, which produces cells that spend more time in the expanded state and have a higher apical surface area than wild type cells. Finally, we have shown that Mbs, the targeting subunit of Myosin phosphatase, not only regulates Myosin phosphorylation but also adhesion dynamics. Interestingly, she finds that in mbs mutant embryos, the amnioserosa tears apart due to defective junction remodelling at sites where cells extrude from the plane of the epithelium. This is due to a mislocalization of E-cadherin at these sites, through a process that is likely to involve Moesin.
In the last part of this work, we show the work done in the lab of Guillaume Charras, in London. We made use of a novel in culture system to study how external applied forces and changes in cells contractility affect tissue rearrangements. Our results show that Myosin phosphorylation and external applied stretch play an important role in oriented junctional rearrangements within an epithelial monolayer.
