Control of Epithelial Cell Layer Spreading in Zebrafish

Epithelial cell layer spreading is a common and fundamental process in various developmental and disease-related events, such as dorsal closure in Drosophila and wound healing. In the previous funding period, we have dissected the force- generating mechanisms driving epithelial cell layer spreading during zebrafish gastrulation1. Specifically, we showed that spreading of the enveloping cell layer (EVL) over the yolk cell during gastrulation is triggered by contraction of the actomyosin cortex forming a band-like structure within the yolk syncytial layer (YSL), a thin cytoplasmic layer on the yolk cell surface close to the margin of the EVL. While these findings provided novel insight into the mechanisms by which the YSL actomyosin band drives EVL spreading, important questions remain as to the initiation of actomyosin band formation within the YSL and the mechanical linkage between EVL and YSL allowing the YSL to pull on the EVL. To address these questions, we will use an interdisciplinary approach employing methods and tools from developmental biology, cell biology, biophysics and theoretical modeling to analyze how microtubules within the YSL promote tight junction formation between the YSL and EVL, and how this process directs actomyosin band assembly within the YSL required for EVL spreading. We expect that this approach will provide insight into the mechanisms of force-generation, transduction and mechansensation during epithelial cell layer spreading.

In this project, we have analyzed the molecular, cellular and biophysical mechanisms underlying epithelial cell layer spreading in early vertebrate development. Epithelial cell layer spreading is a common process in various developmental and disease-related processes, such as wound healing. Yet, how this process is achieved across different scales, ranging from molecular interactions to collective cell behaviour on a tissue level, is still not fully understood. The results of this project revealed three important features of this process that are important for epithelial cell layer spreading to proceed normally: (1) the ability of surface layer cells to undergo active spreading, thereby reducing tissue surface tension; (2) the ability of the spreading tissue to change its material properties (tissue viscosity), thereby facilitating the spreading process; and (3) the ability of junctions between the tissue and its substrate to strengthen in response to forces applied on these junctions. These findings considerably advance our understanding of the mechanochemical principles by which epithelial cell layers undergo spreading.