Biology subjects

Richert, A.

Publications and source records attributed to Richert, A..

4 recordsLinked to original sources

bioRxiv2025

Allometric spreading and focal adhesion collective organization are coordinated by cell-scale geometrical constraints

Focal adhesions are protein complexes that transmit actin cytoskeleton forces to the extracellular matrix and serve as signaling hubs that regulate cell physiology. While their growth is achieved through a local force-dependent process, the requirement of sustaining stress at the cell scale suggests a global regulation of the collective organization of focal adhesions. To investigate evidence of such large-scale regulation, we compared changes in cell shape and the organization of focal adhesion-like structures during the early spreading of fibroblasts either on a two-dimensional substrate or confined between two parallel plates, and for cells of different volumes. In this way, we reveal that the areal density of focal adhesions is conserved regardless of cell size or third-dimensional confinement, despite different absolute values of the surface covered by adhesion clusters. In particular, the width of the focal adhesions ring, which fills the flat lamella at the cell front, adapts to cell size and third-dimensional confinement and scales with cell-substrate contact radius. We find that this contact radius also adapts in the parallel-plate geometry so that the cumulated area of cellsubstrate contact is conserved at the cell scale. We suggest that this behavior is the result of 3D cell shape changes which govern spreading transitions. Indeed, because of volume conservation constraints, the evolution of cell-body contact angle adjusts according to cell size and confinement, whereas the rate of early spreading at the cell-substrate contact is not affected by thirddimensional geometry. Overall, our data suggest that a coordination between global and local scales mediates the adaptation of cell-substrate contacts and focal adhesions distribution to large scale geometrical constraints, which allows an invariant cell-substrate adhesive energy.

biophysics↗

bioRxiv2024

Disentangling the contributions of stress fibres and the unbundled actin meshwork to the anisotropy of cortical tension in response to cell shape

Many fundamental biological processes, in particular development and morphogenetic movements, involve tissue and cell deformation, as well as the generation of anisotropic mechanical stresses. They are often accompanied by the appearance of oriented contractile actomyosin structures resembling the stress fibres (SF) observed in vitro. Here, we investigate, at the single cell level, how cell shape -- by itself -- could control the structure and tension of the actomyosin cortex. Using a unique combination of 3D micropatterning, single peripheral SF (PSF) tension measurement, laser ablation and image analysis, we show that cell shape anisotropy, e.g. its 2D aspect ratio, is indeed sufficient to induce anisotropy of the cortical structure and tension. In particular, taking into account the experimentally measured anisotropy of the cortical meshwork, we could quantify cortical tension and decouple the contribution originating from bundled actin (oriented cortical stress fibres, CSF) and the contribution of the unbundled actin meshwork (UAM). We show that the increase of cortical tension anisotropy with the cells aspect ratio depends on the CSF alignment and orientation, the contribution of the isotropic mesh being independent of cell shape. Remarkably, while experimental data from single stress fibre measurements and laser ablation were analysed through different theoretical frameworks, namely that of negative pressure in nematics and hole drilling in prestressed materials, we found quantitatively the same composite material behaviour. In sum, we decipher here the very material properties of the actomyosin cortex, and its sensitivity to cell shape which is at the root of many mechanobiological processes, in particular morphogenesis.

biophysics↗

bioRxiv2024

Mechanical constraints organize 3D tissues and orchestrate muscle differentiation

Biological tissues achieve proper shape and ordered structures during development through responses to internal and external signals, with mechanical cues playing a crucial role. These forces guide cellular organization, leading to complex self-organizing structures that are foundational to embryonic patterns. Emerging theories and experiments suggest that "topological morphogens" drive these processes. Despite the predominance of three-dimensional (3D) structures in biology, studying 3D tissues remains challenging due to limited model systems and the complexity of modeling. Here, we address these challenges by using self-organized cellular aggregates, specifically spindle-shaped C2C12 myoblasts, subjected to controlled mechanical stretching. Our findings reveal that these cells form a multilayered, actin-oriented tissue structure, where mechanical forces drive long-range 3D organization and muscle differentiation. Notably, tissue surface emerges as a hotspot for differentiation, correlating with directional order as shown by single molecule fluorescent in situ hybridization. Significance StatementWe explore how cells work together to form complex structures, particularly in 3D, using muscle precursors cells (C2C12 myoblasts) as a model. By applying controlled stretching forces, we found that these cells self-organize into layered tissues that guide their transformation into muscle. This research highlights the critical role of physical forces in shaping tissues, suggesting that the way cells are physically arranged and stretched in three dimensions can significantly influence their behavior and function. Our findings offer new insights into how tissues develop and could have implications for tissue engineering, where creating the right 3D environment is key to successful tissue growth and repair.

biophysics↗

bioRxiv2020

The impact of cell geometry and the cytoskeleton on the nucleo-cytoplasmic localisation of the SMYD3 methyltransferase suggests that the epigenetic machinery is mechanosensitive

Mechanical cues from the cellular microenvironment are converted into biochemical signals controlling diverse cell behaviours, including growth and differentiation. But it is still unclear how mechanotransduction ultimately affects nuclear readouts, genome function and transcriptional programs. Key signaling pathways and transcription factors can be activated, and can relocalize to the nucleus, upon mechanosensing. Here, we tested the hypothesis that epigenetic regulators, such as methyltransferase enzymes, might also contribute to mechanotransduction. We found that the SMYD3 lysine methyltransferase is spatially redistributed dependent on cell geometry (cell shape and aspect ratio) in murine myoblasts. Specifically, elongated rectangles were less permissive than square shapes to SMYD3 nuclear accumulation, via reduced nuclear import. Notably, SMYD3 has both nuclear and cytoplasmic substrates. The distribution of SMYD3 in response to cell geometry correlated with cytoplasmic and nuclear lysine tri-methylation (Kme3) levels, but not Kme2. Moreover, drugs targeting cytoskeletal acto-myosin induced nuclear accumulation of Smyd3. We also observed that square vs rectangular geometry impacted the nuclear-cytoplasmic relocalisation of several mechano-sensitive proteins, notably YAP/TAZ proteins and the SETDB1 methyltransferase. Thus, mechanical cues from cellular geometric shapes are transduced by a combination of transcription factors and epigenetic regulators shuttling between the cell nucleus and cytoplasm.

biophysics↗