Bending toward differentiation

Nuclear deformation by microtubule molecular motors

We present a model to calculate the displacement and extension of deformable cellular cargo pulled by molecular motors stepping along cytoskeletal filaments. We consider the case of a single type of molecular motor and cytoskeletal filaments oriented in one dimension in opposite directions on either side of a cargo. We model a deformable cargo as a simple elastic spring. We simulate this tug-of-war simple exclusion process model using a Monte Carlo Gillespie algorithm and calculate the displacement and extension of the cargo for different configurations of motors and filaments. We apply our model to kinesin-1 motors on microtubules to investigate whether they are strong enough to translocate and deform the largest cellular cargo, the nucleus. We show that the extension caused by motors on a single microtubule saturates for larger numbers of motors but that the extension and displacement scales linearly with the number of microtubules. We also show how the binding and unbinding behaviors of molecular motors on microtubule filaments affect the nuclear deformation. Our modelling results correspond to experiments on cells treated with the drug kinesore, which is thought to increase rescue events resulting in more stable microtubules and more active kinesin-1 molecular motors bound to them. Both the experiments and our simulations result in nuclear deformation.

Appreciating the role of cell shape changes in the mechanobiology of epithelial tissues

The wide range of epithelial cell shapes reveals the complexity and diversity of the intracellular mechanisms that serve to construct their morphology and regulate their functions. Using mechanosensitive steps, epithelial cells can sense a variety of different mechanochemical stimuli and adapt their behavior by reshaping their morphology. These changes of cell shape rely on a structural reorganization in space and time that generates modifications of the tensional state and activates biochemical cascades. Recent studies have started to unveil how the cell shape maintenance is involved in mechanical homeostatic tasks to sustain epithelial tissue folding, identity, and self-renewal. Here, we review relevant works that integrated mechanobiology to elucidate some of the core principles of how cell shape may be conveyed into spatial information to guide collective processes such as epithelial morphogenesis. Among many other parameters, we show that the regulation of the cell shape can be understood as the result of the interplay between two counteracting mechanisms: actomyosin contractility and intercellular adhesions, and that both do not act independently but are functionally integrated to operate on molecular, cellular, and tissue scales. We highlight the role of cadherin-based adhesions in force-sensing and mechanotransduction, and we report recent developments that exploit physics of liquid crystals to connect cell shape changes to orientational order in cell aggregates. Finally, we emphasize that the further intermingling of different disciplines to develop new mechanobiology assays will lead the way toward a unified picture of the contribution of cell shape to the pathophysiological behavior of epithelial tissues.