Blumenfeld, R. | Search bioRxiv

bioRxiv2020

Cell surface fluctuations regulate early embryonic lineage sorting

In development, lineage segregation of multiple lineages must be coordinated in time and space. An important example is the mammalian inner cell mass (ICM), in which the primitive endoderm (PrE, founder of the yolk sac) physically segregates from the epiblast (EPI, founder of the foetus). The physical mechanisms that determine this spatial segregation between EPI and PrE are still poorly understood. Here, we identify an asymmetry in cell-cell affinity, a mechanical property thought to play a significant role in tissue sorting in other systems, between EPI and PrE precursors (pEPI and pPrE). However, a computational model of cell sorting indicated that these differences alone appeared insufficient to explain the spatial segregation. We also observed significantly greater surface fluctuations in pPrE compared to pEPI. Including the enhanced surface fluctuation in pPrE in our simulation led to robust cell sorting. We identify phospho-ERM regulated membrane tension as an important mediator of the increased surface fluctuations in pPrE. Using aggregates of engineered cell lines with different surface fluctuation levels cells with higher surface fluctuations were consistently excluded to the outside of the aggregate. These cells behaved similarly when incorporated in the embryo. Surface fluctuations-driven segregation is reminiscent of activity-induced phase separation, a sorting phenomenon in colloidal physics. Together, our experiments and model identify dynamic cell surface fluctuations, in addition to static mechanical properties, as a key factor for orchestrating the correct spatial positioning of the founder embryonic lineages.

developmental biology↗

bioRxiv2018

Force-based three-dimensional model predicts mechanical drivers of cell sorting

Many biological processes, including tissue morphogenesis, are driven by mechanical sorting. However, the primary mechanical drivers of cell sorting remain controversial, in part because there remains a lack of appropriate threedimensional computational methods to probe the mechanical interactions that drive sorting. To address this important issue, we developed a three-dimensional, local force-based simulation method to enable such investigation into the sorting mechanisms of multicellular aggregates. Our method utilises the subcellular element method, in which cells are modeled as collections of locally-interacting force-bearing elements, accommodating cell growth and cell division. We define two different types of intracellular elements, assigning different attributes to boundary elements to model a cell cortex, which mediates the interfacial interaction between different cells. By tuning interfacial adhesion and tension in each cell cortex, we can control and predict the degree of sorting in cellular aggregates. The method is validated by comparing the interface areas of simulated cell doublets to experimental data and to previous theoretical work. We then define numerical measures of sorting and investigate the effects of mechanical parameters on the extent of sorting in multicellular aggregates. Using this method, we find that a minimum adhesion is required for differential interfacial tension to produce inside-out sorting of two cell types with different mechanical phenotypes. We predict the value of the minimum adhesion, which is in excellent agreement with observations in several developmental systems. We also predict the level of tension asymmetry needed for robust sorting. The generality and flexibility of the method make it applicable to tissue self-organization in a myriad of biological processes, such as tumorigenesis and embryogenesis.

biophysics↗