Mechanics regulate human embryonic stem cell self-organization to specify mesoderm
Preprint posted on 11 February 2020 https://www.biorxiv.org/content/10.1101/2020.02.10.943076v1
Article now published in Developmental Cell at http://dx.doi.org/10.1016/j.devcel.2020.10.015
Gastrulation is the process whereby the embryonic germ layers (endoderm, mesoderm, and ectoderm) are coordinately specified and patterned. The combination of differentiation and morphogenesis establishes the future body plan of the adult organism and is often considered the most important event during embryonic development. The first step in gastrulation is establishment of the primitive streak (PS), a transient structure through which pluripotent cells ingress and subsequently differentiate into endoderm and mesoderm. PS formation is driven by BMP, Nodal, WNT and FGF signaling and is marked by the expression of the T-box transcription factor Brachyury (T).
During gastrulation, the embryo changes dramatically in size and shape, which almost certainly influences the mechanical properties of the embryo. In a wide range of organisms, from the Cnidaria, Nematostella vectensis, to invertebrates such as Drosophila and vertebrates such as zebrafish, mechanotransduction of physical forces activates WNT signaling and subsequent expression of T (Pukhlyakova, et al., 2018; Brunet, et al., 2013). However, the technical challenges of manipulating and measuring mechanical forces in mammalian embryos means that it remains unclear whether these mechanisms are conserved in higher organisms.
The Weaver lab previously investigated the association between mechanical forces, signaling, and cell fate specification in a mammalian model of gastrulation. Using differentiating human embryonic stem cells (hESCs), which are amenable to genetic, pharmacological and mechanical perturbations, they demonstrated that reducing substrate stiffness enhances mesoderm formation (Przybyla, et al., 2016).
What we like about this pre-print
In the lab’s latest pre-print, Muncie et al. further explore this relationship. Here, they show that distinct clusters of T+ mesoderm cells form at high tension regions of differentiating hESC (+BMP4) colonies. Furthermore, when differentiating hESCs are spatially confined within triangle or square colonies (generated using micropatterned surfaces), T expression is induced specifically at colony corners and edges, which are under the highest tension.
The tension force within the colonies is driven by cell-cell adhesion. Partial knock-down of the adherens junction component E-cadherin reduces regionalized tension, and consequently T expression. The association between Cadherins and signaling pathway activity has been extensively studied (see one of our previous pre-lights here: https://prelights.biologists.com/highlights/n-cadherin-stabilises-neural-identity-by-dampening-anti-neural-signals/). For example, the downstream effector of WNT signaling, β-catenin, is directly associated with adherens junctions and thus, changes in cell-cell adhesion may lead to release of β-catenin from cell junctions where it is then free to participate in WNT signaling. Src-family kinases (SFKs) phosphorylate junctional β-catenin, releasing it into the cytoplasm. The authors found that there is an increase in SFK activity and a reduction in junctional β-catenin at high tension regions. This suggests that tension may stimulate WNT signaling activity, which may then promote the increase in T expression and PS-like formation.
We like this paper as it adds to the growing evidence that mechanical forces are a key modulator of signaling and cell fate decisions during gastrulation, previously underappreciated due to the difficulty and lack of technologies to address this in vivo. Moreover, it raises the question whether mechanotransduction by β-catenin and subsequent induction of T is a conserved evolutionary mechanism of mesoderm formation across bilaterians.
This work raises a lot of interesting questions about the role of mechanical forces within the embryo during gastrulation. Including:
- How do mechanical forces change over time during gastrulation and do these play any role in regulating this process?
- What might cause changes in the mechanical properties of embryos at gastrulation?
- Technically, how can we measure mechanical forces within an embryo at this stage? See this recent article (Duch, et al., 2020) measuring forces from within an oocyte using nanodevices!
- The author’s show that both reduced substrate stiffness and elevated intracellular tension as associated with increased mesoderm/PS induction. Typically, stiffer substrates promote actin polymerization and actomyosin force generation, which leads to elevated intracellular tension. Thus, the relationship between these two parameters in this context is still unclear.
- What is the connection between tension, E-CADHERIN and activation of SFKs?
Brunet, T., Bouclet, A., Ahmadi, P., Mitrossilis, D., Driquez, B., Brunet, A.C., Henry, L., Serman, F., Bealle, G., Menager, C., et al. (2013). Evolutionary conservation of early mesoderm specification by mechanotransduction in Bilateria. Nat Commun 4, 2821.
Duch, M., Torras, N., Asami, M., Suzuki, T., Arjona, M.I., Gomez-Martinez, R., VerMilyea, M.D., Castilla, R., Plaza, J.A., and Perry, A.C.F. (2020). Tracking intracellular forces and mechanical property changes in mouse one-cell embryo development. Nat Mater.
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Przybyla, L., Lakins, J.N., and Weaver, V.M. (2016). Tissue Mechanics Orchestrate Wnt-Dependent Human Embryonic Stem Cell Differentiation. Cell Stem Cell 19, 462-475.
Pukhlyakova, E., Aman, A.J., Elsayad, K., and Technau, U. (2018). beta-Catenin-dependent mechanotransduction dates back to the common ancestor of Cnidaria and Bilateria. Proc Natl Acad Sci U S A 115, 6231-6236.
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Posted on: 30 June 2020Read preprint