Keratins act as global coordinators of tissue spreading through mechanosensitive feedback

Epithelial spreading plays a pivotal role in the development of organisms especially those such as zebrafish which require the epithelial enveloping layer (EVL) to spread to cover the substantial yolk surface during gastrulation. Epiboly requires the transition of the epithelium with cuboidal cells to form a thin, flat squamous epithelial sheet. During this transition, the cells show tissue-scale mechanosensation with mechanisms such as direct mechanical control over the axis of cell division. Cytoskeletal intermediate filaments play a crucial role in vertebrate cells, not only facilitating mechanical stability but also helping facilitate the mechanosensitive response of the cell. Mechanosenstivity displayed by intermediate filaments is due not just to their interesting physical properties but also to their interactions with other cytoskeletal elements such as actin and microtubules. Keratin is the predominant intermediate filament expressed in the EVL. It expresses concomitantly with the gastrulation movements of the developing embryo. Our work focuses on understanding the role and dynamics of the keratin cytoskeletal network in modulating the physical aspects of EVL spreading. We demonstrated with the combination of physical characterisation and manipulations of the EVL, utilising a variety of biophysical tools and microscopy, the mechanistic role of keratin in tissue spreading. Generating novel genetic morphants and mutants, we probe the effect that the loss of the keratin network has on the physiology of the epithelium and the developing embryo. We show that the changing organisation of the keratin network is important for changing EVL physical properties as the stress imposed on the EVL increases during epiboly. By modelling the epithelium, we study how the mechanical heterogeneity in an epithelium can feed back into a mechanical loop to the maturation of the keratin network and hence affect the mechanics of the epithelium. However, unlike what would be predicted by the effect of intermediate filaments in acting as a security belt and increasing the resistance of the epithelium, we observe that loss of keratin leads to a delay in the EVL movement. Using both local aspirations of the YSL and EVL ablations, we demonstrate the mechanistic facilitation of actin mechanosensation in a keratin-dependent manner. Furthermore, using chemical inhibitors of microtubule polymerisation, we provide insight into the mechanisms underlying the organisation and distribution of keratin. Interestingly, the phenotype observed upon this loss of microtubules shows that keratins interact with the nucleus through microtubular interactions. Together with these diverse observations, we describe the mechanosensory feedback between resilience and that is critical for uniform and robust spreading of the epithelium.