Cell spheroid viscoelasticity is deformation-dependent.

Tissue surface tension influences cell sorting and tissue fusion. Earlier mechanical studies suggest that multicellular spheroids actively reinforce their surface tension with applied force. Here we study this open question through high-throughput microfluidic micropipette aspiration measurements on cell spheroids to identify the role of force duration and spheroid deformability. In particular, we aspirate spheroid protrusions of mice fibroblast NIH3T3 and human embryonic HEK293T homogeneous cell spheroids into micron-sized capillaries for different pressures and monitor their viscoelastic creep behavior. We find that larger spheroid deformations lead to faster cellular retraction once the pressure is released, regardless of the applied force. Additionally, less deformable NIH3T3 cell spheroids with an increased expression level of alpha-smooth muscle actin, a cytoskeletal protein upregulating cellular contractility, also demonstrate slower cellular retraction after pressure release for smaller spheroid deformations. Moreover, HEK293T cell spheroids only display cellular retraction at larger pressures with larger spheroid deformations, despite an additional increase in viscosity at these larger pressures. These new insights demonstrate that spheroid viscoelasticity is deformation-dependent and challenge whether surface tension truly reinforces at larger aspiration pressures.

How cytoskeletal crosstalk makes cells move: Bridging cell-free and cell studies.

Cell migration is a fundamental process for life and is highly dependent on the dynamical and mechanical properties of the cytoskeleton. Intensive physical and biochemical crosstalk among actin, microtubules, and intermediate filaments ensures their coordination to facilitate and enable migration. In this review, we discuss the different mechanical aspects that govern cell migration and provide, for each mechanical aspect, a novel perspective by juxtaposing two complementary approaches to the biophysical study of cytoskeletal crosstalk: live-cell studies (often referred to as top-down studies) and cell-free studies (often referred to as bottom-up studies). We summarize the main findings from both experimental approaches, and we provide our perspective on bridging the two perspectives to address the open questions of how cytoskeletal crosstalk governs cell migration and makes cells move.

Elucidating the role of water in collagen self-assembly by isotopically modulating collagen hydration.

Water is known to play an important role in collagen self-assembly, but it is still largely unclear how water-collagen interactions influence the assembly process and determine the fibril network properties. Here, we use the H[Formula: see text]O/D[Formula: see text]O isotope effect on the hydrogen-bond strength in water to investigate the role of hydration in collagen self-assembly. We dissolve collagen in H[Formula: see text]O and D[Formula: see text]O and compare the growth kinetics and the structure of the collagen assemblies formed in these water isotopomers. Surprisingly, collagen assembly occurs ten times faster in D[Formula: see text]O than in H[Formula: see text]O, and collagen in D[Formula: see text]O self-assembles into much thinner fibrils, that form a more inhomogeneous and softer network, with a fourfold reduction in elastic modulus when compared to H[Formula: see text]O. Combining spectroscopic measurements with atomistic simulations, we show that collagen in D[Formula: see text]O is less hydrated than in H[Formula: see text]O. This partial dehydration lowers the enthalpic penalty for water removal and reorganization at the collagen-water interface, increasing the self-assembly rate and the number of nucleation centers, leading to thinner fibrils and a softer network. Coarse-grained simulations show that the acceleration in the initial nucleation rate can be reproduced by the enhancement of electrostatic interactions. These results show that water acts as a mediator between collagen monomers, by modulating their interactions so as to optimize the assembly process and, thus, the final network properties. We believe that isotopically modulating the hydration of proteins can be a valuable method to investigate the role of water in protein structural dynamics and protein self-assembly.