Ectoderm to mesoderm transition by down-regulation of actomyosin contractility

Collective migration of cohesive tissues is a fundamental process in morphogenesis and is particularly well illustrated during gastrulation by the rapid and massive internalization of the mesoderm, which contrasts with the much more modest movements of the ectoderm. In the Xenopus embryo, the differences in morphogenetic capabilities of ectoderm and mesoderm can be connected to the intrinsic motility of individual cells, very low for ectoderm, high for mesoderm. Surprisingly, we find that these seemingly deep differences can be accounted for simply by differences in Rho-kinases (Rock)-dependent actomyosin contractility. We show that Rock inhibition is sufficient to rapidly unleash motility in the ectoderm and confer it with mesoderm-like properties. In the mesoderm, this motility is dependent on two negative regulators of RhoA, the small GTPase Rnd1 and the RhoGAP Shirin/Dlc2/ArhGAP37. Both are absolutely essential for gastrulation. At the cellular and tissue level, the two regulators show overlapping yet distinct functions. They both contribute to decrease cortical tension and confer motility, but Shirin tends to increase tissue fluidity and stimulate dispersion, while Rnd1 tends to favor more compact collective migration. Thus, each is able to contribute to a specific property of the migratory behavior of the mesoderm. We propose that the “ectoderm to mesoderm transition” is a prototypic case of collective migration driven by a down-regulation of cellular tension, without the need for the complex changes traditionally associated with the epithelial-to-mesenchymal transition.


Calculation of relative tensions and adhesiveness for cell doublets
Estimates of relative tensions were based on the principle that the geometry of the cell membranes at cell vertices reflects the equilibrium between the tensile forces exerted by the cell cortices [3,16]. For a doublet formed of cell A and cell B, the equilibrium involved the cortical tensions at the two free cell surfaces (CtA and CtB) and the contact tension at cell-cell interface (TAB). TAB is defined as the sum of the cortical tensions of each cell at the contact (CtA' and CtB') and of the negative contribution due to cell-cell adhesion (-ω).
The force equilibrium was expressed by two equations: (1) sin(a) * Ct A + sin(b) * Ct B + sin(c) * T AB = 0 (2) cos(a) * Ct A + cos (b) * Ct B + cos(c) * T AB = 0 Angles a, b and c corresponded to the orientation of each force vector. Each of these angles was measured as the tangent to an arc fitted to three points of the cell membrane, including the cell vertex [3].
Based on equations (1) and (2), we could use the three angles to calculate the ratios between the two cortical tensions (CtA/CtB) and the ratio between contact tension and each of the cortical tensions [3] using the equation: -sin(c)) and the ratio CtA/CtB between the two cortical tensions: CtA/CtB is expected to be on average close to 1.0 for homotypic doublets. CtA/CtB of heterotypic provided a direct readout of the relative cortical tension of two cell types, here Rnd1 or Shirinexpressing cells compared to control ectoderm cells (Fig.8J, see below the complete results of all homotypic and heterotypic doublets). Note that Ct varies broadly even for cell of the same type/condition (e.g. homotypic control ectoderm, Fig.8J, see [3]).
To compare the tensions between different types of doublets, we calculated a relative contact tension relT (T in the text and legends) expressed relative to the median value of control ectoderm contact tension set at 1. We also assumed that the average of the two cortical tensions (CtA+CtB)/2 was proportional to the median of the measured Cts. Thus, we defined a relative median cortical tension medCt for each condition, set to 1.0 for control ectoderm. relT could then be calculated using the following equation: (5) T AB = 2 * T A * ( Adhesiveness α is an absolute number between 0 and 1, defined as the tension reduction at the contacts relative to free surface [19] and can be directly calculated via the contact angles [19]. We adapted this calculation to asymmetrical doublets, thus Note that adhesiveness does not depend on absolute Ct or T, but on the balance of both. Accordingly, as illustrated here below, two pairs of cells may have the same adhesiveness despite displaying very different cortical tensions.

Examples of configurations:
Complete results from doublet measurements: Cortical tension, contact tension and adhesiveness of homotypic and heterotypic doublets of control ectoderm cells, and cells expressing Rnd1 or Shirin.