Guillaume Charras

Genome stability depends on the accurate repair of DNA double-strand breaks (DSBs). While chromatin-based mechanisms have been well studied, emerging evidence reveals that nuclear mechanics actively shape repair outcomes, although the mechanisms remain unclear. Here we identify microtubule acetylation as a regulator of nuclear mechanics that promotes DSB repair via homologous recombination (HR). DNA damage induces progressive tubulin acetylation, leading to displacement of the perinuclear vimentin cage. Loss of the vimentin cage softens the nucleus and renders the nuclear envelope responsive to microtubule forces, which drives transient nuclear shape fluctuations, localized DSB mobility and repair. Inhibiting tubulin acetylation abolishes these fluctuations, restricts DSB mobility and negatively impacts the homology search during HR, leaving non-homologous end joining intact. Consequently, persistent DSBs coalesce and cluster at the nuclear periphery, triggering cell-cycle arrest and senescence and increasing sensitivity to DNA-damaging agents. Removal of the vimentin cage is sufficient to rescue these effects, with restored nuclear fluctuations and repair efficiency. Thus, we propose that nuclear fluctuations act as a mechanical strategy to accelerate error-free repair. These findings reveal how cytoskeletal forces shape nuclear function and establish nuclear fluctuations as a fundamental principle for maintaining genome stability.

There is broad consensus that intermediate filaments, such as keratin, play a key role in protecting cells and tissues from large deformations. However, little is known about how they fulfil this function. Here we show that epithelial cells slowly adapt to stretching through a coupling of a star-bundling transition of keratin filaments with the escape of the nucleus from its keratin cage. The bundling transition begins with a depletion of keratin filaments at tricellular junctions followed by a progressive accumulation in thick bundles that bisect cell–cell junctions. Bundling is a cooperative process that initiates in a few scattered cells and propagates to their neighbours, leading to the growth of multicellular clusters that contain a percolated network of thick keratin bundles. Bundling dynamics are slow and strongly influenced by the interaction between actin and keratin. Informed by a computational model, we provide evidence that keratin bundling generates a compressive stress on the nucleus, which is relaxed by nuclear escape from the keratin cage. The topological transitions identified here provide epithelia with a multiscale mechanism to adapt to sustained stretching.

Cell extrusion is a fundamental process in tissue homeostasis, morphogenesis, and cancer progression, facilitating the removal of cells either alive or through apoptosis. While biochemical signaling pathways are known to regulate extrusion, recent advances have underscored the importance of mechanical forces in this process. Here, using optogenetic control of RhoA activation in epithelial monolayers combined with Bayesian Inversion Stress Microscopy (BISM) and three-dimensional cell-based modeling, we uncover a counterintuitive mechanism whereby elevated tension, instead of stabilizing the monolayer, actively drives extrusion in highly contractile cells. We show that local RhoA activation enhances myosin II–dependent contractility and F-actin reorganization, which promotes cell stiffening, resulting in localized tension buildup. The ensuing tensile stress amplifies vertical mechanical fluctuations, which in turn trigger cell extrusion. Remarkably, these tension-induced extrusions occur both apically and basally. Furthermore, our findings show that RhoA-mediated contractility is not merely an effector of extrusion but also an active promoter of basal extrusion, independently of caspase activation. Our study demonstrates that tensile stress can directly initiate extrusion events and bias their outcome toward apical or basal fates. By identifying tension as a driver rather than a suppressor of extrusion, this work revises current models of epithelial homeostasis and highlights mechanical control as a targetable axis in disease and regeneration.

The robust patterning of cell fates during embryonic development requires precise coordination of signalling gradients within defined spatial constraints. Using a geometrically confined in vitro system derived from human embryonic stem cells, we demonstrate that patterning of neuromesodermal progenitors (NMPs) during axial elongation is driven by boundary-dependent mechanisms. Despite extensive work on radial fate patterning in confined 2D systems, the quantitative role of boundary conditions in shaping spatiotemporal dynamics remains unclear. Here, we show that a minimal reaction-diffusion model coupled with a simplified gene regulatory network accurately predicts spatial patterns across diverse geometries. Guided by its predictions, we identify Wnt signalling as a key component of the activator signal. Inhibition of Wnt secretion preserved initiation of patterning but disrupted subsequent morphogenesis, indicating distinct mechanisms govern pattern establishment versus maintenance. Our findings reveal how geometry encodes positional information that directs molecular patterning, providing insight into how spatial constraints and signalling dynamics guide robust tissue self-organisation during development.

Understanding the physical basis of cellular shape change in response to both internal and external mechanical stresses requires characterisation of cytoplasmic rheology. At subsecond time-scales and micron length-scales, cells behave as fluid-filled sponges in which shape changes necessitate intracellular fluid redistribution. However, whether these cytoplasmic poroelastic properties play an important role in cellular mechanical response over length- and time-scales relevant to cell physiology remains unclear. Here, we investigated whether and how a localised deformation of the cell surface gives rise to transient intracellular flows spanning several microns and lasting seconds. Next, we showed that pressure gradients induced in the cytoplasm can be sustained over several minutes. We found that stable pressure gradients can arise from the combination of cortical tension, cytoplasmic poroelasticity, and water flows across the membrane. Overall our data indicate that intracellular cytosolic flows and pressure gradients may play a much greater role than currently appreciated, acting over time- and length-scales relevant to mechanotransduction and cell migration, signifying that poroelastic properties need to be accounted for in models of the cell.

Epithelial tissues are often exposed to cyclic deformations in their physiological environment. The mechanical integrity of epithelial tissues relies on intercellular adhesion proteins which link neighbouring cells and transmit forces across the cellular network. The stability of the intercellular adhesive bonds is critical. Under sustained stress, bond failure leads to cumulative damage that progressively weakens the material and ultimately causes failure at the tissue scale. Although the collective behaviour of adhesion bonds under static loading has been characterised to some extent, their response to dynamic loading remains largely unknown, despite its physiological importance. Here we combine quantitative experiments on MDCK monolayers with analytical and stochastic modeling of slip-bond intercellular linkers to investigate tissue resilience under cyclic loading. We find that cyclic loading significantly prolongs tissue lifetime and deformation before failure compared to constant tension, due to intervals of low stress facilitating bond repair. Our model identifies intrinsic rupture and repair timescales governing adhesion stability, revealing three regimes of tissue behavior: rupture, slow damage accumulation and stable equilibrium. Normalizing loading parameters by the intrinsic molecular timescales collapses experimental and simulated data into universal stability maps. These findings demonstrate that epithelial resilience emerges from stochastic adhesion bond dynamics, providing a predictive framework linking molecular adhesion turnover to macroscopic tissue mechanics under physiological cyclic forces. SIGNIFICANCE: Epithelial tissues, the body’s primary mechanical barriers, experience continuous mechanical stress from physiological movements such as breathing or peristalsis. Through experiments and modeling, we show that a tissue’s ability to resist and repair damage under cyclic loading comes from the natural binding and unbinding of the molecules that link neighboring cells. We identify fundamental timescales governing rupture and recovery in cell-cell junctions, showing that tissue stability can be universally predicted from molecular adhesion kinetics. This work establishes a quantitative framework connecting microscopic adhesion kinetics to macroscopic tissue mechanics. It provides a tool for studying resilience in diverse biophysical soft matter systems and for understanding how living materials maintain integrity under fluctuating forces.

There is broad consensus that intermediate filaments play a key role in protecting cells and tissues from large deformations. However, little is known about how they fulfil this function. Here we demonstrate that epithelial cells undergo a slow adaptation to stretch that couples a star- bundling transition of keratin filaments and the escape of the nucleus from its keratin cage. The bundling transition begins with a depletion of keratin filaments at tri-cellular junctions followed by a progressive accumulation in thick bundles that bisect cell-cell junctions. Bundling is a cooperative process that initiates in a few scattered cells and propagates to their neighbours through desmosomes, leading to the growth of multicellular clusters that contain a percolated network of thick keratin bundles. Bundling dynamics are slow and strongly influenced by the interaction between actin and keratin. Informed by a computational model, we provide evidence that keratin bundling generates a compressive stress on the nucleus, which is relaxed by nuclear escape from the keratin cage. The topological transitions identified here provide epithelia with a multiscale mechanism to adapt to sustained stretch.