Publications of the Max-Planck-Zentrum für Physik und Medizin

In this manuscript, we carry out a systematic study on the mechanics and rheology of cellular aggregate formations. We study how cell–cell and cell–matrix interactions influence cellular aggregation, which can be described as an active phase separation process. Furthermore, we investigate the mechanisms underlying the coalescence of aggregates. The problem is analyzed in both Eulerian and Lagrangian frameworks and the computational intricacies for each approach are highlighted. Using our recently developed micromorphic-based artificial diffusion method, we circumvent the numerical instabilities arising from the convective nature of the problem. Finally, via a comprehensive numerical study, we investigate the dynamics of the cellular aggregate formation under various conditions. A notable agreement between the numerical and experimental results is observed. Our work provides significant insights into the mechanics of cellular aggregates which paves the way for better understanding the role of active mechanical forces in biological systems.

The mechanical properties of cells are dynamic, allowing them to adjust to different needs in different biological contexts. In recent years, advanced biophysical techniques have enabled the rapid, high-throughput assessment of single-cell mechanics, providing new insights into the regulation of the mechanical cell phenotype. However, the molecular mechanisms by which cells maintain and regulate their mechanical properties remain poorly understood. Here, we present a genome-scale RNA interference (RNAi) screen investigating the roles of kinase and phosphatase genes in regulating single-cell mechanics using Real-Time Fluorescence and Deformability Cytometry (RT-FDC). Our screen identified 82 known and novel mechanical regulators across diverse cellular functions from 214 targeted genes, leveraging RT-FDC’s unique capabilities for comprehensive, high-throughput mechanical phenotyping with single-cell and cell cycle resolution. These findings refine our understanding of how signaling pathways coordinate structural determinants of cell mechanical phenotypes and provide a starting point for uncovering new molecular targets involved in biomechanical regulation across diverse biological systems.

Intracellular transport of macromolecules is crucial for the proper functioning of most cellular processes. Although intracellular crowding is known to strongly alter macromolecule mobility, how cytoplasmic structures physically modulate diffusion remains largely unexplored. Here, we investigated the mechanisms by which cytoplasmic crowding controls diffusivity using live-cell experiments and porous media modeling approaches. Confocal microscopy combined with fluorescence recovery after photobleaching and fluorescence correlation spectroscopy measurements revealed an anticorrelation between free green fluorescent protein diffusivity and the heterogeneous cytoplasmic structure abundance in live mammalian cells. This motivated the development of a multiscale model, where the cytoplasm is treated as a hierarchical porous medium with nanometric and micrometric obstacles. Numerically solving the model allowed us to predict the effective cytoplasmic diffusion coefficient for various obstacle volume fractions and to identify tortuous and porous hydrodynamic hindrances as key diffusion reduction mechanisms. Comparison with our experimental results highlighted the importance of hydrodynamic interactions between diffusing molecules and nanometric obstacles. Importantly, we found that the effective cytoplasmic diffusivity was not dependent on specific intracellular regions but rather on the local intracellular obstacle volume fraction. Finally, the model was extended to predict the diffusivity of larger macromolecules, showing excellent agreement with literature data for several macromolecules and cell lines. This study provides insights into the physical mechanisms impeding intracellular diffusion, demonstrating the potential of porous media modeling approaches to predict transport mechanisms in dynamic or heterogeneous intracellular structures, as in cell motility, blebbing, and apoptosis.

Biological processes are regulated by chemical and mechanical signals, yet how these signalling modalities interact remains poorly understood. Here we identify a crosstalk between tissue stiffness and long-range chemical signalling in the developing Xenopus laevis brain. Targeted knockdown of the mechanosensitive ion channel Piezo1 in retinal ganglion cells or in the brain tissue surrounding retinal ganglion cells causes pathfinding errors in vivo. In the brain parenchyma, Piezo1 downregulation decreases the expression of the diffusive long-range chemical guidance cues Semaphorin3A (Sema3A) and Slit1, which instruct turning responses in distant cells. Furthermore, Piezo1 knockdown results in tissue softening due to reduced expression of the adhesion proteins NCAM1 and N-cadherin. Targeted depletion of NCAM1 and N-cadherin similarly reduces tissue stiffness and Sema3A expression. Conversely, increasing environmental stiffness ex vivo enhances tissue-level force generation and Slit1 and Sema3A expression. Finally, in vivo stiffening of soft brain regions induces ectopic Sema3A production via a Piezo1-dependent mechanism. Overall, these findings demonstrate that tissue mechanics locally modulates the availability of diffusive, long-range chemical signals, thus influencing cell function at sites distant from the mechanical cue.

Cells within biological tissue are constantly subjected to dynamic mechanical forces. Measuring the internal stress of tissues has proven crucial for our understanding of the role of mechanical forces in fundamental biological processes like morphogenesis, collective migration, cell division, or cell elimination and death. Previously, we have introduced Bayesian inversion stress microscopy (BISM), which relies on measuring cell-generated traction forces in vitro and has proven particularly useful to measure absolute stresses in confined cell monolayers. We further demonstrate the applicability and robustness of BISM across various experimental settings with different boundary conditions, ranging from confined tissues of arbitrary shape to monolayers composed of different cell types. Importantly, BISM does not require assumptions on cell rheology. Therefore, it can be applied to complex heterogeneous tissues consisting of different cell types, as long as they can be grown on a flat substrate. Finally, we compare BISM to other common stress measurement techniques using a coherent experimental setup, followed by a discussion on its limitations and further perspectives.

Red blood cell-derived extracellular vesicles (RBC-EVs) are emerging as promising biomaterials for next-generation drug delivery, owing to their intrinsic biocompatibility, immune evasion properties, and minimal oncogenic risk. However, their broader application is currently limited by unresolved challenges related to heterogeneity, reproducibility, and long-term storage stability. By combining discontinuous sucrose density gradient separation with high-resolution interferometric nanoparticle tracking analysis, we identified a sharp bimodal size distribution of the vesicles in freshly prepared samples. We then tracked how long-term storage at −80 °C drove its conversion into a monomodal distribution. To reproduce these conditions in a shorter time frame, we developed an “accelerated-ageing” protocol based on freeze–thaw cycles that generates RBC-EV samples with homogeneous density, size distribution, and biological activity, effectively replicating the properties of preparations stored for six months at −80 °C. This new vesicle population results stable and retains membrane integrity and cellular internalization capacity, as confirmed by surface-associated enzymatic activity assays and uptake tests in cancer cell lines. These results suggest that freezing-induced “accelerated ageing” represents an effective method for the optimization and standardization of RBC-EVs as building blocks for biomaterial and bioengineering applications.