Biology subjects

Betz, T.

Publications and source records attributed to Betz, T..

At least 19 recordsLinked to original sources

bioRxiv2026

Structural order emerges without mechanical order inside living cells

Intracellular organization is vital for cell function, and disruption of organelle positioning is connected to severe diseases, such as cancer. Given the small length scale of cells and the importance of Brownian motion, maintaining intracellular organization requires mechanical interactions to remain stable. Here, we test the common view that intracellular organization is reflected in intracellular mechanical properties. Using the recently introduced active mechanical fingerprint, we quantify intracellular mechanical properties and active forces to create mechanical maps of patterned RPE1 cells. The organization of actin, microtubules, the Golgi apparatus, the endoplasmic reticulum, and the nucleus reveals pronounced structural polarization across the cell population. To overcome limitations of simple averages, we use a Wasserstein barycenter to calculate an optimal intracellular organization that explains all measurements with minimal cellular distortion. To our surprise, and in contrast to previous reports, we find neither a statistically significant spatial mechanical pattern nor any relevant correlation between the tested structures and intracellular mechanical properties. While there is clear local mechanical anisotropy, the large heterogeneity of mechanical values outweighs any intracellular mechanical gradient. This is further supported by large cell-to-cell variability. Thus, despite having the same genome, cells function properly while their intracellular mechanical properties vary over two orders of magnitude.

biophysics↗

bioRxiv2026

Stretched Exponential Modeling Reveals Drug-Specific Kinetics in Human Engineered Skeletal Muscle

Understanding muscle tissue is key not only for advancing our knowledge in biophysical function of the human body, but can also open new paths to better description of pathological states, improve treatment and training and even provide insights for drug development. While using excised muscles is an excellent way to study muscular function, this limits access to non-human tissue, as human muscle tissue donation is very limited. The past decade has seen a rise in 3D in vitro models of human skeletal muscle, however many current experimental approaches to create such models often struggle with the high variability of primary cells or the limited translational relevance in cases where murine lines are used. Furthermore, the state-of-the-art functional analysis is typically focusing at peak force, which overlooks critical kinetic information. In this study, we present an systematic approach to generate functional engineered skeletal muscle tissues (ESMs), measure the contraction force dynamics and model these with a new approach to extract the effect of a series of standard pharmacological modulators. Within 14 days, LHCN-M2 ESMs mature into aligned, multinucleated myofibers expressing key sarcomeric markers and exhibiting robust excitation-contraction coupling. To decode these dynamics, we introduce a novel mathematical model based on stretched exponential functions. This framework accurately captures the heterogeneous contraction and relaxation phases across diverse phenotypes using only six interpretable parameters. We validated the platforms sensitivity using a library of pharmacological modulators. Our kinetic modeling revealed a distinct parameter fingerprint for different drug classes such as the specific changes of contraction kinetics that peak force alone could not detect. Additionally the presence of the necessary drug targets as well as the state of maturation were investigated by proteomic profiling. Together, study provides a scalable, high-fidelity human platform for high-content pharmacological screening and muscle biophysics.

biophysics↗

bioRxiv2026

Zebrafish embryos are robust to mechanical perturbations

The investigation of morphogenesis during organism development has massively benefited from the interaction between developmental biology and biophysics, which gave new quantitative insights into natures physical working principles. The process of zebrafish epiboly provides a beautiful model system as cells exhibit collective migration, meanwhile requiring symmetry breaking for gastrulation and the subsequent tissue differentiation. These elegantly robust processes are facilitated by both biochemical and mechanical cues. Here we test the hypothesis that the future body axis is robust against mechanical perturbations of tissue flow and tissue stress fields. For this, we trigger tissue flow by photoablation, which leads to an externally enforced symmetry breaking before natural symmetry breaking of gastrulation occurs. Using 3D light sheet microscopy, we obtained nuclei trajectories as a proxy for the migration of individual cells. Analyzing the derived tissue velocity field, we observed that epiboly is highly robust and mostly unaffected by ablation damage. However, the photoablation induced a convergent motion of cells towards an azimuthal angle resembling shield formation. We extensively characterized this convergent motion and checked if this tissue flow is able to reorient natural symmetry breaking during shield formation. While ablations can transiently disrupt or reorient the convergent motion, we cannot confirm any redirection of the location where the future body axis of the animal develops. The observations were also highly dependent on the developmental stage of the embryo, which cast a greater and more permanent impact, suggesting strong control exerted by factors other than mechanical signals. Significance statementMorphogenesis emerges from the interplay between biochemical patterning and mechanical forces, yet the extent to which mechanical cues influence embryo developmental remains unclear. Using zebrafish epiboly as model system, we imposed controlled mechanical perturbations through targeted photoablation and tracking resulting 3D cell movements. We show that although ablations generate transient convergent motions, epiboly is remarkably robust to externally induced tissue flows. Our findings indicate that the future body axis is strongly buffered against mechanical disturbances, highlighting the dominant role of developmental stage and non-mechanical cues in guiding development.

biophysics↗

bioRxiv2025

Direct mechanical communication of cellular to nuclear shape in oocytes

The mechanical properties of the cytoplasm and nucleoplasm are crucial for the correct and robust functioning of a cell and play a key role in understanding how mechanical signals are transferred to the nucleus. Here, we demonstrate remarkable shape mimicry between the cellular and nuclear shape of oocytes, following the externally applied deformation without direct contact between the cell cortex and the nucleus. This effect arises from a surprisingly soft and fluid-like nucleoplasm that is barely resisting any external strain, while the viscoelastic cytoplasm drives shape transmission. Comparative studies in jellyfish, starfish, and mouse oocytes reveal that lower cytoplasmic elasticity in jellyfish leads to reduced nuclear shape mimicry, highlighting the role of cytoplasmic mechanics in nuclear deformation. Significance StatementMechanosensing of the nucleus is, in addition to chemical signalling, an important factor in gene expression. Although nuclei are often thought to be rigid inclusions in the cytoplasm of a cell, we show that in oocytes nuclei are much more deformable. Using a combination of intranuclear, intracellular and extracellular measurements, we attribute our findings to a fine balance between the soft nucleoplasm surrounded by an elastic shell and the viscoelastic properties of the cytoplasm.

biophysics↗

bioRxiv2025

3D dynamic multiscale force and shape analysis of in-vivo elastic stress sensors

The measurement of stresses and forces at the tissue level has proven to be an indispensable tool for the understanding of complex biological phenomena such as cancer invasion, embryo development or wound healing. One of the most versatile tools for force inference at the cell and tissue level are elastic force sensors, whose biocompatibility and tunable material properties make them suitable for many different experimental scenarios. The evaluation of those forces, however, is still a bottleneck due to the numerical methods seen in literature until now, which are usually slow and render low experimental yield. Here we present Bead-Buddy, a ready-to-use platform for the evaluation of deformation and stresses from fluorescently labelled sensors within seconds. The strengths of BeadBuddy lie in the pre-computed analytical solutions of the elastic problem, the abstraction of data into Spherical Harmonics, and a simple user interface that creates a smooth workflow for force inference. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=174 SRC="FIGDIR/small/633835v3_ufig1.gif" ALT="Figure 1"> View larger version (54K): org.highwire.dtl.DTLVardef@80c95corg.highwire.dtl.DTLVardef@123d712org.highwire.dtl.DTLVardef@1fd1ce0org.highwire.dtl.DTLVardef@72c688_HPS_FORMAT_FIGEXP M_FIG C_FIG

biophysics↗

bioRxiv2024

Membrane tension regulation is required for wound repair

Disruptions of the eukaryotic plasma membrane due to chemical and mechanical challenges are frequent and detrimental, and thus need to be repaired to maintain proper cell function and avoid cell death. However, the cellular mechanisms involved in wound resealing and restoration of homeostasis are diverse and contended. Here, we show that clathrin-mediated endocytosis is induced at later stages of plasma membrane wound repair following the actual resealing of the wound. This compensatory endocytosis occurs near the wound, predominantly at sites of previous early endosome exocytosis which is required in the initial stage of membrane resealing, suggesting a spatio-temporal co-ordination of exo- and endocytosis during wound repair. Using cytoskeletal alterations and modulation of membrane tension and membrane area, we identify membrane tension as a major regulator of the wounding-associated exo- and endocytic events that mediate efficient wound repair. Thus, membrane tension changes are a universal trigger for plasma membrane wound repair modulating the exocytosis of early endosomes required for resealing and subsequent clathrin-mediated endocytosis acting at later stages to restore cell homeostasis and function.

cell biology↗

bioRxiv2023

Analytical method for reconstructing the stress on a spherical particle from its surface deformation

The mechanical forces that cells experience from the tissue surrounding them are crucial for their behavior and development. Experimental studies of such mechanical forces require a method for measuring them. A widely used approach in this context is bead deformation analysis, where spherical particles are embedded into the tissue. The deformation of the particles then allows to reconstruct the mechanical stress acting on them. Existing approaches for this reconstruction are either very time-consuming or not sufficiently general. In this article, we present an analytical approach to this problem based on an expansion in solid spherical harmonics that allows us to find the complete stress tensor describing the stress acting on the tissue. Our approach is based on the linear theory of elasticity and uses an ansatz derived by Love. We clarify the conditions under which this ansatz can be used, making our results useful also for other contexts in which this ansatz is employed. Our method can be applied to arbitrary radial particle deformations and requires a very low computational effort. The usefulness of the method is demonstrated by an application to experimental data. STATEMENT OF SIGNIFICANCEMeasurements of mechanical forces acting on cells in a tissue are important for understanding the physical behavior of biological systems, but they are also quite challenging. A common strategy is to place a spherical bead inside the tissue and to then reconstruct the mechanical stress from the bead deformation that this stress causes. Here, we introduce a novel analytical method using which this reconstruction can be achieved. This method is significantly faster than numerical approaches and significantly more general than existing analytical techniques, such that it can be expected to find a broad range of applications in mechanobiology.

biophysics↗

bioRxiv2023

CFM: Confinement Force Microscopy-a dynamic, precise and stable microconfiner for traction force microscopy in spatial confinement

Cells migrating through tissues experience changing physical confinement, yet methods to dynamically control confinement while quantifying the resulting forces remain limited. Here, we present a microconfiner platform for live-cell imaging that enables programmable confinement, allowing real-time control over the level, timing and frequency of confinement while measuring traction forces exerted on the microenvironment, a method we term confinement force microscopy (CFM). Using CFM, we find that cells respond to confinement in two phases: a rapid passive stress rise caused by compression of the cell body and nucleus against the substrate, followed by an active stress increase associated with enhanced contractility, intracellular pressure buildup and bleb formation. Bleb expansion can partially relieve pressure and reduce stress on the surroundings. ROCK and myosin II inhibition both reduce stress generation, but with distinct effects on blebbing. Overall, CFM provides a versatile approach to study dynamic mechanical adaptation in tissue-like environments.

biophysics↗

bioRxiv2023

Intracellular mechanical fingerprint reveals cell type specific mechanical tuning

Living cells are complex entities that perform various tasks with astonishing robustness. While the direct dependence of biological processes on controlled protein expression is well established, we have only begun to understand how intracellular mechanical characteristics guide and support biological function. This is in stark contrast to the expected functional role that intracellular mechanical properties should have for many core cellular functions such as organization, homeostasis, and transport. From a mechanical point of view, cells are complex viscoelastic materials that are continuously driven out of thermodynamic equilibrium, which makes both a physical measurement and mathematical modeling of their properties difficult. Here, we define a "mechanical fingerprint" that can not only characterize the intracellular mechanical state, but also carve out the mechanical differences between cell types with the potential to relate these to proper cell function. By analyzing the frequency-dependent viscoelastic properties and intracellular activity of cells using microrheology, we distilled the complex active mechanical state into just 6 parameters that comprise the mechanical fingerprint. The systematic investigation of the fingerprint illustrates a parameter tuning that could be related to functional cellular requirements. However, the full potential of the mechanical fingerprint is given by a statistical analysis of its parameters across all investigated cell types, which suggests that cells can modify mechanical parameters in a correlated way to position their intracellular mechanical properties within a well-defined phase-space that is spanned between activity, mechanical resistance, and fluidity. This paves the way for a systematic study of the interdependence of biological function and intracellular active mechanics.

biophysics↗

bioRxiv2023

Culture substrate stiffness impacts human myoblast contractility-dependent proliferation and nuclear envelope wrinkling

The remarkable self-repair ability of skeletal muscle tissues is driven by muscle stem cells, whose activities are orchestrated by a variety of transient cues present in their microenvironment. Understanding how the cross-interactions of these biophysical and biochemical microenvironmental cues influences muscle stem cells and their progeny is crucial in strategizing remedies for pathological dysregulation of these cues in aging and disease. In this study, we investigated the cell-level influences of extracellular matrix ligands in the context of mechanically tuned culture substrates on primary human myoblast contractility and proliferation within 16 hours of plating. We found that fibronectin led to stronger stiffness-dependent responses in myoblasts compared to laminin and collagen as revealed by cell spreading area, focal adhesion morphology, traction stresses, and proliferation. While we did not observe stiffness-dependent changes in cell fate signatures at this early time-point, an analysis of the stiffness-sensitive myoblast proteome uncovered an enrichment for differentiation associated signatures on 17 kPa culture substrates, in addition to distinct profiles with relation to metabolism, cytoskeletal regulation, ECM interactions, and nuclear component enrichment. Interestingly, we found that softer substrates increased the incidence of myoblasts with a wrinkled nucleus, and that the extent of wrinkling could predict Ki67 expression. While nuclear wrinkling and Ki67 expression could be controlled by pharmacological manipulation of cellular contractility, cellular traction stresses did not correlate with nuclear envelope wrinkling level. These results provide new insights into the regulation of human myoblast stiffness-dependent contractility response by ECM ligands and also highlight a link between myoblast contractility and proliferation.

bioengineering↗

bioRxiv2022

Dystrophin is a mechanical tension modulator

Duchenne muscular dystrophy (DMD) represents the most common inherited muscular disease, where progressing muscle weakness leads to loss of ambulation and premature death. DMD is caused by mutations in the dystrophin gene, and is known to reduce the contractile capacity of muscle tissue both in vivo, and in reconstituted systems in vitro. However, these observations are based on mechanical studies that focused on stimulated contractions of skeletal muscle tissue. Seemingly paradoxical, upon evaluating bioengineered skeletal muscles produced from DMD patient derived myoblasts we observe an increase in myosin motor mediated homeostatic tissue tension that strongly correlates with decreased stimulated tissue strength, suggesting the involvement of dystrophin in regulating the baseline homeostatic tension level of tissue. This was confirmed by three independent pairs of isogenic cell lines, one of each lacking dystrophin compared to the isogenic dystrophin-expressing control. From this we speculate that the protective function of dystrophin also supports cellular fitness via active participation in the mechanosensation to achieve and sustain an ideal level of tissue tension. Hence, this study reveals fundamental novel insights into skeletal muscle biomechanics and into a new key mechanical aspect of DMD pathogenesis, provided by increased homeostatic tissue tension.

biophysics↗

bioRxiv2022

Onsager regression characterizes living systems in passive measurements

Understanding life is arguably among the most complex scientific problems faced in modern research. From a physics perspective, living systems are complex dynamic entities that operate far from thermodynamic equilibrium.1-3 This active, non-equilibrium behaviour, with its constant hunger for energy, allows life to overcome the ever dispersing forces of entropy, and drives cellular organisation.4, 5 Unfortunately, most analysis methods provided by the toolbox of statistical mechanics cannot be used in such non-equilibrium situations, forcing researchers to use sophisticated and often invasive approaches to study the mechanistic processes inside living organisms. Here we introduce a new observable coined the mean back relaxation. Based on three-point probabilities, and exploiting Onsagers regression hypothesis, it extracts additional information from passively observed trajectories compared to classical observables such as the mean squared displacement. We mathematically prove that the mean back relaxation is able to detect broken detailed balance in systems confined in stationary or actively diffusing potentials. We show in experiment and theory that it gives access to the non-equilibrium generating energy and the viscoelastic material properties of a well controlled artificial system, and we experimentally demonstrate that it does so even for a variety of living systems, revealing an astonishing relation between the mean back relaxation and the active mechanical energy. Based on these findings, we conclude that it acts as a new marker of non-equilibrium dynamics. Combining, in a next step, passive fluctuations with the extracted active energy allows to overcome a fundamental barrier in the study of living systems; it gives access to the viscoelastic material properties from passive measurements.

biophysics↗

bioRxiv2022

Human endothelial cells display a rapid and fluid flow dependent tensional stress increase in response to tumor necrosis factor-α

As endothelial cells form the inner layer of blood vessels they display the first barrier to interstitial tissues, which results in a crucial role for inflammation. On the global, systemic level an important element of the complex process controlling the inflammatory response is the release of the cytokine tumor necrosis factor- (TNF-). While other pro-inflammatory agents like thrombin or histamine are known to induce acute but transient changes in endothelial cells which have been well studied biologically as well as mechanically, TNF- is primarily known for its sustained effects on permeability and leukocyte recruitment. These functions are associated with transcriptional changes that take place on the timescale of hours and days. Here we show that already 4 minutes after the addition of TNF- onto monolayers of human umbilical vein endothelial cells, a striking rise in mechanical substrate traction force and internal monolayer tension can be recorded. As expected, the traction forces act primarily at the boundary of the monolayer. While the traction forces increase monotonically during the initial cellular response, we find that the internal monolayer tension displays a rapid peak that can be abolished when applying a shear flow to the cells. The increased internal monolayer tension may provide a mechanical signal for the cells to prepare for the recruitment of leukocytes, additionally to the well studied biochemical response.

biophysics↗

bioRxiv2021

2.5D Tractions in monocytes reveal mesoscale mechanics of podosomes during substrate indenting cell protrusion

Degradation and protrusion are key to cellular barrier breaching in cancer metastasis and leukocyte extravasation. Cancerous invadopodia and myelomonocytic podosomes are widely considered as structural tools facilitating these processes and are thus summarized under the term invadosomes. Despite similar behaviour on the individual scale, substantial differences have been reported to arise on the collective scale. They are considered to be a result of podosome mesoscale-connectivity. In this study, we investigated global in-plane and out-of-plane mechanical forces of podosome clusters in ER-Hoxb8 cell derived monocytes. We are able to correlate these forces with the interpodosomal connectivity. The observed traction and protrusion patterns fail to be explained by summation of single podosome mechanics. Instead, they appear to originate from superimposed mesoscale effects. Based on mechanistic and morphological similarities with epithelial monolayer mechanics, we propose a spatiotemporal model of podosome cluster mechanics capable of relating single to collective podosome mechanical behaviour. Our results suggest that network contraction-driven (in-plane) tractions lead to a buckling instability that contributes to the out-of-plane indentation into the substrate. First assigning an active mechanical role to the dorsal podosome actomyosin network, we aim at translating actomyosin hierarchy into scale dependency of podosome mechanics.

biophysics↗

bioRxiv2021

Pressure drives rapid burst-like collective migration from 3D cancer aggregates

Collective migration of cells is a key behaviour observed during morphogenesis, wound healing and cancer cell invasion. Hence, understanding the different aspects of collective migration is at the core of further progress in describing and treating cancer and other pathological defects. The standard dogma in cell migration is that cells exert forces on the environment to move and cell-cell adhesion-based forces provide the coordination for collective migration. Here, we report a new collective migration mechanism that is independent of pulling forces on the extra-cellular matrix (ECM), as it is driven by the pressure difference generated inside model tumours. We observe a striking collective migration phenotype, where a rapid burst-like stream of HeLa cervical cancer cells emerges from the 3D aggregate embedded in matrices with low collagen concentration (0.5 mg ml-1). This invasion-like behaviour is recorded within 8 hours post embedding (hpe), and is characterised by high cell velocity and super-diffusive collective motion. We show that cellular swelling, triggered by the soft matrix, leads to a rise in intrinsic pressure, which eventually drives an invasion-like phenotype of HeLa cancer aggregates. These dynamic observations provide new evidence that pressure-driven effects need to be considered for a complete description of the mechanical forces involved in collective migration and invasion.

biophysics↗

bioRxiv2021

Designing a high-resolution, LEGO-based microscope for an educational setting

Microscopy is an essential tool in many fields of science. However, due to their costs and fragility, the usage of microscopes is limited in classroom settings and nearly absent at home. In this article we present the construction of a microscope using LEGO(R) bricks and low-cost, easily available lenses. We demonstrate that the obtained magnification and resolution are sufficient to resolve micrometer-sized objects and propose a series of experiments that explore various biophysical principles. Finally, a study with students in the age range of 9 to 13 shows that the understanding of microscopy increases significantly after working with the LEGO microscope.

scientific communication and education↗

bioRxiv2021

The force loading rate drives cell mechanosensing through both reinforcement and fluidization

Cell response to force regulates essential processes in health and disease. However, the fundamental mechanical variables that cells sense and respond to remain unclear. Here we show that the rate of force application (loading rate) drives mechanosensing, as predicted by a molecular clutch model. By applying dynamic force regimes to cells through substrate stretching, optical tweezers, and atomic force microscopy, we find that increasing loading rates trigger talin-dependent mechanosensing, leading to adhesion growth and reinforcement, and YAP nuclear localization. However, above a given threshold the actin cytoskeleton undergoes fluidization and softens, decreasing loading rates and preventing reinforcement. By stretching rat lungs in vivo, we show that a similar phenomenon occurs at the organ level. Our results show that cell sensing of external forces and of passive mechanical parameters (like tissue stiffness) can be understood through the same mechanisms, driven by the properties under force of the mechanosensing molecules involved.

biophysics↗

bioRxiv2021

Intracellular softening and fluidification reveals a mechanical switch of cytoskeletal material contributions during division

The life and death of an organism depends largely on correct cell division. While the overall biochemical signaling and morphological processes during mitosis are well understood, the importance of mechanical forces and material properties is only just starting to be discovered. Recent studies of global cell stiffening during cell division may imply an understanding of the cytosol mechanics that is mistaken. Here we show that in contrast to the stiffening process in the cell cortex, the interior of the cell undergoes a softening and fluidification that is accompanied by a decrease of active forces driving particle mobility. Using optical tweezers-based microrheology we capture the complex active and passive material state of the cytoplasm using only six relevant parameters. We demonstrate that the softening occurs because of a surprising role switch between microtubules and actin, where the intracellular, actin-based mechanics is largely controlled by a formin-mediated network.

biophysics↗