Control of epithelial cell layer spreading in zebrafish

Epithelial cell layer spreading is a key process in various developmental and disease-related events, such as Drosophila dorsal closure, C.elegans ventral enclosure or wound healing. However, very little is yet known about the force-generating molecular and cellular mechanisms underlying this process. In zebrafish gastrulation, the enveloping cell layer (EVL), a surface epithelial cell layer formed at the animal pole of the gastrula, spreads over the entire yolk cell in a process termed epiboly. The objective of the proposal is to elucidate the force-generating mechanisms triggering EVL epiboly. Based on the results of our preliminary experiments and theoretical considerations, we propose that a contractile actomyosin band within the yolk syncytial layer (YSL), a thin cytoplasmic layer at the surface of the yolk cell, drives EVL epiboly by pulling on the edge of the EVL through contraction both around the circumference of the gastrula (cable-contraction motor) and perpendicular to it, along the animal-vegetal (A-V) axis of the gastrula (flow-friction-based motor). To address this hypothesis, we will (1) analyze how the actomyosin band within the YSL dynamically forms and structurally organizes to generate the pulling forces driving EVL epiboly; (2) formulate a theoretical hydrodynamic description of how actomyosin contraction within the YSL drives EVL epiboly; and (3) functionally test the role of actomyosin contraction within the YSL for EVL epiboly. We expect that these combined experimental and theoretical approaches will allow us to unravel the critical force-generating mechanisms underlying epithelial cell layer spreading in development.

In the past funding period, we have identified the main force-generating processes by which the enveloping cell layer (EVL), an simple squamous epithelial cell layer at the surface of the early zebrafish embryo, spreads over the spherical yolk cell during the course of gastrulation. Specifically, we were able to show that a ring-like accumulation of actomyosin within the yolk syncytial layer (YSL), a thin cytoplasmic layer at the yolk cell surface, exerts pulling forces on the margin of the EVL thereby pulling the EVL over the yolk cell. The YSL actomyosin ring exerts those pulling forces by two motor activities a cable-constriction motor, where circumferential contraction of the actomyosin ring couples to the spherical geometry of the yolk sac, thereby pulling on the EVL margin as a purse string; and a flow-friction motor, where retrograde actomyosin flows within the YSL, when resisted by friction to adjacent structures, directly pulls the EVL margin towards the vegetal pole. These findings provide insight into the force-generating mechanisms underlying EVL epiboly movements and, more generally, of actomyosin ring-like structures in various cell biological and disease-related processes, such as cell division and wound healing.