Conference Agenda

Although the biochemical pathways of blood coagulation are well understood, the role of platelet mechanobiology in clot remodeling and the initiation of tissue repair remains less clear. Platelets not only secrete biochemical factors essential for the rapid formation of fibrin-rich clots, but they also contribute to wound contraction. Studies have demonstrated that platelets are instrumental in assembling the first fibronectin fibers, setting the stage for the subsequent migration of other cell types.

It is now widely accepted that cell motility is driven by the polymerization of actin—the most abundant protein in eukaryotic cells—into a network of interconnected filaments. We describe this mechanism within a continuum mechanics framework, suggesting that actin polymerization induces mechanical swelling in a localized region near the nucleation sites, which ultimately drives movement in cells or bacteria.

To explore the mechanobiology behind these phenomena, cutting-edge microscopy techniques [1,2] have been integrated with a novel theoretical approach, resulting in more sophisticated models and simulations. Departing from the commonly assumed incompressibility of all components in many existing mixture theories [3], we have extended the classical Larché-Cahn framework for chemo-transport-mechanics [4].

This new theoretical model has shown promising results in simulating cell motility [5] and could potentially be applied to other areas of mechanobiology as well.

References

[1] M. Burkhardt, et al. (2016). Synergistic interactions of blood-​borne immune cells, fibroblasts and extracellular matrix drive repair in an in vitro peri-​implant wound healing model. Sci Rep 6, 21071.

[2] S. Lickertet. al. (2022). Platelets drive fibronectin fibrillogenesis using integrin αIIbβ3. Science Advances, 8(10), eabj8331

[3] F.J. Vernerey and M. Farsad. A constrained mixture approach to mechano-sensing and force generation in contractile cells. J MECH BEHAV BIOMED, 4(8):1683–1699, 2011.

[4] M Arricca, L Cabras, M Serpelloni, C Bonanno, R M. McMeeking, and A Salvadori. A coupled model of transport-reaction-mechanics with trapping, Part II: Large strain analysis. J MECH PHYS SOLIDS, 181:105425, 2023

[5] A. Salvadori, C. Bonanno, M. Serpelloni, R.M. McMeeking, (2024), On the generation of force required for actin-based motility., Scientific Reports, 14:18384, https://doi.org/10.1038/s41598-024-69422-3.

Abstract: Cells can sense the mechanical properties, such as stiffness of extracellular matrix (ECM), and this mechanosensing capability can affect cell behaviors, tissue development, and pathological changes. Most ECMs exhibit dissipative mechanical properties, i.e., viscosity and plasticity. The dissipative properties of ECMs can regulate stem cell differentiation, cell spreading, and migration. However, the mechanism of the cell mechanosensing, especially sensing the dissipative properties, remains unclear. This report mainly introduces our exploration of the cell mechanosensing mechanism in recent years. Combining chemo-mechanical model development and biochemical experiments, we found that the contractile force generated by intracellular myosin can be transmitted to ECMs through binding dynamics of adhesion molecules. The competition between stress relaxation of the viscoelastic matrix and the adhesion binding dynamics determines the effective stiffness sensed by cells, thereby influencing the cell spreading and migration. Next, we studied how the ECM plasticity affects the dynamic behaviors of cancer cell invadopodia in 3D cell culture. We found that the competition between myosin contractility and actin polymerization drives the periodic oscillatory growth of invadopodia; during the invadopodia oscillation, the matrix plastic deformation gradually accumulates, forming a cavity in the invadopodial front and further promoting the invadopodia growth. In addition to the cancer cell invadopodia, we explored the mechanism of wave-like dynamics in immune cells’ podosome clusters, and our study shows that cells can utilize the podosome wave-like dynamics to sense the ECM stiffness. Overall, these studies reveal the previously unrecognized impact of the ECMs’ dissipative properties on cell dynamics, offering new clues for the design and optimization of biological materials.

Interaction of cells and their corresponding extracellular matrix (ECM) are known to regulate and control many different biological processes such as cancer progression and embryonic formation. A key component instructing cell-matrix interactions are the ECM’s material properties, i.e. its average, bulk response to cell generated stresses and strains. While cell-matrix forces as effectors of molecular signaling pathways and mechanical drivers of biological processes has been extensively studied for elastic ECM’s, native ECM properties are viscoelastic and characterized by including both elastic and dissipative components. Importantly, recent studies have shown that viscoelastic properties also moderate cellular behavior. Therefore, accounting for viscous properties in addition to the elastic properties is essential.

TFM is a widely established technique capable of quantifying cell-matrix forces by measuring strains applied on the ECM. Traditional elastic TFM algorithms (eTFM) make use of the analytical Boussinesq solution and Fourier analysis to provide a semi-analytical and computationally efficient algorithm for flat substrates with finite thickness. Most importantly, traditional TFM algorithms only assume that the substrate is elastic, neglecting the dissipative behavior of native and in vitro ECM. TFM approaches based on Finite Element models (FEM) can include viscoelasticity, but their numerical complexity and high computational cost restrict their practical application.

Here, we propose a viscoelastic TFM algorithm (veTFM) generalizing the semi-analytical elastic algorithm to viscoelastic substrates with finite thickness while maintaining the computational efficiency of traditional TFM algorithms. veTFM makes use of Fourier and Laplace analysis to reduce the complexity of the problem, while considering viscoelastic materials characterized by a two-component generalized Maxwell model. veTFM is validated in silico against analytical and FEM simulations, proving its validity in cases where the temporal variation of cell-generated strains on the ECM can be captured. Furthermore, veTFM’s applicability is experimentally evaluated in three different in vitro cases corresponding to muscle, epithelial and connective cell types. Comparing tractions obtained by veTFM to those obtained by traditional elastic algorithms shows that a viscoelastic ECM effectively behaves stiffer under rapid cellular force application and softer if forces are applied slowly. This confirms and quantifies previous observations on rate-dependent cellular response to viscoelastic substrates. Virtually extending this analysis to materials with a wide range of viscoelastic properties, including those of tissues, further shows that viscoelastic dissipation, i.e. the magnitude of stress dissipated by the viscoelastic ECM, in conjunction with the applied strain rate are the key factors differentiating viscoelastic from elastic traction force inference.

In conclusion, our method provides a practical, efficient framework for accurately characterizing how cells dynamically adapt their mechanosensing to viscoelastic mechanical environments, extending beyond the current methods restricted to elastic materials.

Notch is an evolutionarily conserved cell-cell signaling pathway central for several morphogenesis processes, such as cardiac development and angiogenesis. In humans, this pathway features several receptors (Notch1-4) and ligands (Dll1, Dll3, Dll4, Jag1, Jag2) that elicit different cell responses. For example, while the interaction of Dll4 with Notch1 inhibits the angiogenic response of endothelial cells, the expression of Jag1 promotes blood vessel formation instead. The presence of multiple ligands thus enables the tight control of morphogenic processes over time and space. When the ratio between the different ligands or their competition mechanisms are dysregulated, pathological morphogenesis arises. This is for example the case of cancer angiogenesis, characterized by an overexpression of Jag1 and exuberant blood vessel formation. Identifying the determinants of the effects of the different ligands and their competition can therefore increase our understanding of aberrant morphogenesis and lead to the development of appropriate therapeutic strategies. Recent studies indicate that a minimum amount of force needs to be exerted on the Notch receptor-ligand complex to elicit Notch activation. The role of this phenomenon in determining the ligands' effects is currently unclear.

Here, we propose a new computational model of Notch signaling to investigate the role of the force-mediated Notch activation for the different ligands' roles. An agent-based cellular automata approach was chosen, to account for the spatial features of the system in exam and to capture possible steric hindrance effects. The model featured a squared lattice representing the interface between two cells: one expressing Notch receptors (receiving cell) and the other expressing Notch ligands (signaling cell). These proteins could perform different actions at each time-step, mimicking the main characteristics of Notch signaling such as random protein movement across the cell membrane and protein endocytosis. When a Notch receptor and ligand co-localized on the same grid, the Notch receptor-ligand complex formed, followed by either Notch activation or protein dissociation. The likelihood of the two events in the stochastic model was assumed to be influenced by the catch-bond behavior characterizing the Notch protein complexes, as well as by the minimum force required for Notch activation.

A simplified version of the model with only one type of receptor and ligand was first validated against ordinary differential equations, usually adopted to model Notch signaling. The agent-based model was able to predict the same results of the ordinary differential equation. Next, the simulations were adopted to investigate whether differences in magnitudes of force necessary for Notch activation can explain differences in the roles of the Notch ligands. In agreement with literature, by assuming that a higher force is necessary to elicit Jag1-mediated Notch1 activation compared to Dll4, we were able to predict a stronger Notch1 response to Dll4 versus Jag1 exposure. These simulations therefore point at the magnitude of force necessary to induce the activation of a Notch complex as determinant for the role of Notch ligands. In future studies, the model will be extended to simulate cells expressing different ligands interacting among each other, thereby more accurately capturing their competition.

The migration of epithelial cells plays a critical role in physiological processes such as wound healing. In this context, cells utilize distinct migration modes based on the geometric properties of gaps: lamellipodial crawling at convex edges and purse-string-like movements at concave edges. Despite advances in identifying biochemical pathways, the underlying mechanisms determining these mode switches in response to curvature remain unclear. In several studies, we identified a critical link between the morphologies of the endoplasmic reticulum (ER), the mitochondria and the whole cell, and together, these morphologies can dictate various cell functions including cell migration and metabolic processes.

Specifically, through a combination of experimental data and theoretical modeling, we show that the ER undergoes curvature-specific morphological reorganizations that act as a determinant of migration modes. At convex edges, the ER forms tubular networks that align perpendicularly, facilitating lamellipodial crawling. At concave edges, the ER reorganizes into dense sheet-like structures favoring actomyosin-driven purse-string contractions. Our mathematical model describes the ER as a flexible fiber whose morphology-dependent strain energy guides these transitions, revealing a lower energy state when ER tubules or sheets form in accordance with local edge curvature.

This study positions the ER as a critical player in cellular mechanotransduction, providing a mechanistic link between subcellular organization and cellular migration strategies. Our findings offer insights into how cellular and subcellular geometries dynamically influence the physical properties and behaviors of cells, forming a basis for understanding migration regulation in complex tissues.