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Interaction and feedback between cell mechanics and fate specification in vertebrate gastrulation

Periodic Reporting for period 3 - MECSPEC (Interaction and feedback between cell mechanics and fate specification in vertebrate gastrulation)

Reporting period: 2020-07-01 to 2021-12-31

In the past funding periods, we have been working on the question of how cell/tissue morphogenesis and mechanics interact with cell fate specification and differentiation during early zebrafish embryogenesis. In developmental biology, enormous progress has been made on deciphering the gene regulatory pathways controlling cell fate specification and embryo patterning. Likewise, research at the interface between developmental biology and physics has provided novel insight into the key mechanisms by which mechanical forces function in cell and tissue morphogenesis during development. However, remarkably little is yet known about how cell/tissue mechanics influence the gene regulatory pathways involved in cell fate specification and vice versa. To address this important question, we have chosen to study early development of zebrafish, a vertebrate model organism amendable to a wide range of experimental techniques from different disciplines, ranging from genetics to biophysics.

In this funding period (P3), we have predominantly focussed on the mechanisms underlying tissue fluidization at the onset of zebrafish gastrulation, more broadly addressing the interaction between mesendoderm specification and blastoderm epiboly movements (objective 1). We have also continued working on the interplay between cell fate specification and morphogenesis during EVL epiboly movements (objective 2) and in blastoderm explants in vitro (objective 3).
For objective 1, we have followed up on our previous observations that at the onset of zebrafish morphogenesis, the blastoderm undergoes rapid, pronounced and spatially patterned tissue fluidization, which is important for blastoderm spreading, and that this fluidization is temporally controlled by mitotic cell rounding-dependent cell-cell contact disassembly during the last rounds of cell cleavages (Petridou et al., 2019, Nature Cell Biology). To understand how the blastoderm tissue undergoes fluidization, we have used – in collaboration with Prof. Hannezo at IST Austria – percolation theory to theoretically describe how changes in blastoderm cell-cell contacts can lead to an abrupt and strong tissue fluidization, as observed within the blastoderm at the onset of gastrulation. We found that there is a critical point in cell connectivity within the blastoderm, at which the blastoderm undergoes a rigidity transition leading to its fluidization (Petridou et al, 2021, Cell). This provides a conceptual framework for explaining how cell connectivity within tissues relate to tissue material properties, and what role such relationship might play in embryonic development.

For objective 2, we have analysed how enveloping layer (EVL) morphogenesis/spreading and EVL cell fate specification/differentiation are related to each other. Specifically, we have analysed the interplay between intermediate filament expression, hallmark of EVL specification, and EVL spreading during zebrafish gastrulation. We found that during the spreading process the EVL tissue becomes increasingly stiffer, and that this tissue stiffening process relies on keratin intermediate filament expression and organization into a filamentous network within EVL cells. We also found that interfering with keratin expression within the EVL leads to reduced EVL stiffness and impaired epiboly movements. Finally, we observed that tension within the plane of the EVL tissue, as a result of the yolk syncytial layer (YSL) pulling on the margin of the EVL, triggers keratin expression within the EVL, pointing at the possibility of keratin expression being regulated by mechanical signals. Collectively, these findings suggest that EVL cell specification and differentiation, as evident by their expression of keratin intermediate filaments, is controlled by tissue tension and leads to tissue stiffening, thereby revealing a mechanism by which mechanical tension control cell fate specification and tissue stiffening.

For objective 3, we have focussed on understanding how cell-cell contact formation functions in germ layer self-organisation by studying the influence of actomyosin cell cortex tension on the formation of adhesive contacts between zebrafish progenitor cells. We found that when cortex tension was massively up-regulated, cell-cell contacts did not grow big, as expected from previous models, but, instead, remained very small. This unexpected effect, by which the cell-cell contacts size limits rather than promotes contact expansion, is due to cortical tension reducing turnover of cadherin adhesion molecules at the contact, required for fast contact growth (Slovakova et al., 2022, PNAS).
Our findings so far have unravelled a number of novel and unexpected feedback mechanisms determining the interplay between cell/tissue mechanics and cell fate specification. In particular, we discovered an important function of mesendoderm specification in modulating tissue fluidity, and, conversely, tissue fluidization in directing proper tissue morphogenesis leading to mesendoderm cell fate specification (Petridou et al., 2018 Nature Cell Biology & 2021, Cell). We also identified how differential cell growth can trigger lateral inhibition in cell fate specification by modulating TAZ-dependent gene transcription through mechanical compression (Xia et al., 2019, Cell). These findings provide novel insight of the role of cell/tissue mechanics in tissue rheology and cell fate specification that go far beyond the state of the art. In addition, we made several still preliminary findings on the role of tissue tension and EVL cell fate specification and, vice versa, on the role of EVL cell differentiation, and the expression of intermediate filaments, on EVL tissue stiffness. Together, we expect that these projects will decisively advance our understanding of the interplay and feedback between physical mechanisms driving cell/tissue morphogenesis and the gene regulatory pathways determining cell fate specification in development.