Nematbakhsh, A. | bioRxiv Science

bioRxiv2020

Role of combined cell membrane and wall mechanicalproperties regulated by polarity signals in cell budding

The budding yeast, Saccharomyces cerevisiae, is a prime biological model to study mechanisms underlying asymmetric growth. Previous studies have shown that, prior to yeast bud emergence, polarization of a conserved small GTPase, Cdc42, must be established. Additionally, hydrolase changes the mechanical properties of the cell wall and plasma membrane with the periplasm between them (cell surface). However, how the surface mechanical properties in the emerging bud are different from the properties of the mother cell and their role in bud formation are not well understood. We hypothesize that the polarized chemical signal alters the local dimensionless ratio of stretching to bending stiffness of the cell surface of the emerging yeast bud. To test this hypothesis, a novel three-dimensional coarse-grained particle-based model has been developed which describes inhomogeneous mechanical properties of the cell surface. Model simulations suggest that regulation of the dimensionless ratio of stretching to bending stiffness of the cell surface is necessary to initiate bud formation. Furthermore, model simulations predict that bud shape depends strongly on the experimentally observed molecular distribution of the polarized signaling molecule Cdc42, while the neck shape of the emerging bud is strongly impacted by the properties of the chitin and septin ring. This 3D model of asymmetric cell growth can also be used for studying viral budding and other vegetative reproduction processes performed via budding.

biophysics↗

bioRxiv2020

Epithelial organ shape is generated by patterned actomyosin contractility and maintained by the extracellular matrix

Epithelial sheets play important roles in defining organ architecture during development. Here, we employed an iterative experimental and multi-scale computational modeling approach to decouple direct and indirect effects of actomyosin-generated forces, nuclear positioning, extracellular matrix (ECM), and cell-cell adhesion in shaping Drosophila wing imaginal discs, a powerful system for elucidating general principles of epithelial morphogenesis. Basally generated actomyosin forces are found to regulate apically biased nuclear positioning and are required for generating epithelial bending and cell elongation of the wing disc pouch. Surprisingly, however, short-term pharmacological inhibition of ROCK-driven actomyosin contractility does not impact the maintenance of tissue height or curved shape. In comparison, the relative tautness of the extracellular basement membrane is also patterned between regions of the wing disc. However, computational simulations show that patterning of ECM tautness provides only a minor contribution to modulating tissue shape. Instead, the buildup of a passive ECM pre-strain serves a principle role in shape maintenance. Surprisingly, this is independent from the maintenance of actomyosin contractility. Furthermore, localized apical adhesion between the two cell layers within the wing disc requires ROCK-driven actomyosin activity in the absence of the basal extracellular matrix. This apical adhesion between the two cell layers provides additional mechanical support to help maintain tissue integrity. The combined experimental and computational approach provides general insight into how the subcellular forces are generated and maintained within individual cells to induce tissue curvature and suggests an important design principle of epithelial organogenesis whereby forces generated by actomyosin followed by maintenance as pre-strain within the ECM are interconnected, but functionally separable. Significance statementA major outstanding question in developmental biology is the elucidation of general principles of organ shape formation and maintenance. Here, an iterative experimental and multi-scale computational modeling approach reveals that actomyosin contractility generates the bent profile along the anterior-posterior axis while tension within the ECM is sufficient and necessary for preserving the bent shape even in the absence of continued actomyosin contractility once the shape is generated. The mechanisms tested in this study define the necessary factors for establishing the shape of the wing disc, which later everts to form the adult wing during pupal development. The method can be extended to test novel mechanisms of other epithelial systems that consist of several cellular and ECM layers.

biophysics↗