Brain tissue mechanics is governed by microscale relations of the tissue constituents

Local mechanical tissue properties are a critical regulator of cell function in the central nervous system (CNS) during development and disorder. However, we still don't fully understand how the mechanical properties of individual tissue constituents, such as cell nuclei or myelin, determine tissue mechanics. Here we developed a model predicting local tissue mechanics, which induces non-affine deformations of the tissue components. Using the mouse hippocampus and cerebellum as model systems, we show that considering individual tissue components alone, as identified by immunohistochemistry, is not sufficient to reproduce the local mechanical properties of CNS tissue. Our results suggest that brain tissue shows a universal response to applied forces that depends not only on the amount and stiffness of the individual tissue constituents but also on the way how they assemble. Our model may unify current incongruences between the mechanics of soft biological tissues and the underlying constituents and facilitate the design of better biomedical materials and engineered tissues. To this end, we provide a freely-available platform to predict local tissue elasticity upon providing immunohistochemistry images and stiffness values for the constituents of the tissue

A time-dependent phenomenological model for cell mechano-sensing

Adherent cells normally apply forces as a generic means of sensing and responding to the mechanical nature of their surrounding environment. How these forces vary as a function of the extracellular rigidity is critical to understanding the regulatory functions that drive important phenomena such as wound healing or muscle contraction. In recognition of this fact, experiments have been conducted to understand cell rigidity-sensing properties under known conditions of the extracellular environment, opening new possibilities for modeling this active behaviour. In this work, we provide a physics-based constitutive model taking into account the main structural components of the cell to reproduce its most significant contractile properties such as the traction forces exerted as a function of time and the extracellular stiffness. This model shows how the interplay between the time-dependent response of the acto-myosin contractile system and the elastic response of the cell components determine the mechano-sensing behaviour of single cells

A time-dependent phenomenological model for cell mechano-sensing

Adherent cells normally apply forces as a generic means of sensing and responding to the mechanical nature of their surrounding environment. How these forces vary as a function of the extracellular rigidity is critical to understanding the regulatory functions that drive important phenomena such as wound healing or muscle contraction. In recognition of this fact, experiments have been conducted to understand cell rigidity-sensing properties under known conditions of the extracellular environment, opening new possibilities for modeling this active behaviour. In this work, we provide a physics-based constitutive model taking into account the main structural components of the cell to reproduce its most significant contractile properties such as the traction forces exerted as a function of time and the extracellular stiffness. This model shows how the interplay between the time-dependent response of the acto-myosin contractile system and the elastic response of the cell components determine the mechano-sensing behaviour of single cells

Degradation of extracellular matrix regulates osteoblast migration: A microfluidic-based study

Bone regeneration is strongly dependent on the capacity of cells to move in a 3D microenvironment, where a large cascade of signals is activated. To improve the understanding of this complex process and to advance in the knowledge of the role of each specific signal, it is fundamental to analyze the impact of each factor independently. Microfluidic-based cell culture is an appropriate technology to achieve this objective, because it allows recreating realistic 3D local microenvironments by taking into account the extracellular matrix, cells and chemical gradients in an independent or combined scenario. The main aim of this work is to analyze the impact of extracellular matrix properties and growth factor gradients on 3D osteoblast movement, as well as the role of cell matrix degradation. For that, we used collagen-based hydrogels, with and without crosslinkers, under different chemical gradients, and eventually inhibiting metalloproteinases to tweak matrix degradation. We found that osteoblast''s 3D migratory patterns were affected by the hydrogel properties and the PDGF-BB gradient, although the strongest regulatory factor was determined by the ability of cells to remodel the matrix

Mechano-sensing and cell migration: A 3D model approach

Cell migration is essential for tissue development in different physiological and pathological conditions. It is a complex process orchestrated by chemistry, biological factors, microstructure and surrounding mechanical properties. Focusing on the mechanical interactions, cells do not only exert forces on the matrix that surrounds them, but they also sense and react to mechanical cues in a process called mechano-sensing. Here, we hypothesize the involvement of mechano-sensing in the regulation of directional cell migration through a three-dimensional (3D) matrix. For this purpose, we develop a 3D numerical model of individual cell migration, which incorporates the mechano-sensing process of the cell as the main mechanism regulating its movement. Consistent with this hypothesis, we found that factors, such as substrate stiffness, boundary conditions and external forces, regulate specific and distinct cell movements

Cell Cytoskeleton Dynamics: Mechano-Sensing Properties

`The actin cytoskeleton network is the dominant structure of eukaryotic cells. It is highlydynamic and plays a central role in a wide range of mechanical and biological functions.Cytoskeleton is composed mainly of actin filaments (F-actin) resulting from the self-assemblyof monomeric actin (G-actin) and cross-linked by actin cross-linking proteins (ACPs) whosenature and concentration determine the morphological and rheological properties of thenetwork. These actin filaments are reversibly coupled to membrane proteins (critical to theresponse of cells to external stress) and in conjunction with motor proteins from the myosinfamily, are able to generate contractile force during cell migration. Knowledge of actincytoskeleton and its rheological properties is therefore indispensable for understanding theunderlying mechanics and various biological processes of cells. Here, we present a 3-DBrownian dynamics (BD) computational model in which actin monomers polymerize andbecome cross-linked by two types of ACPs, forming either parallel filament bundles ororthogonal networks. Also, the active and dynamic behaviour of motors is included. In thissimulation, actin monomers, filaments, ACPs, and motors experience thermal motion andinteract with each other with binding probabilities and defined potentials. Displacements aregoverned by the Langevin equation, and positions of all elements are updated using the Eulerintegration scheme.In this first part of the work, the mechano-sensing properties of active networks are investigatedby evaluating stress and strain rate in response to different substrate stiffness

Quantification of sprouting angiogenesis under the effect of different growth factors involved in the tumor microenvironmen

One of the most important problems in tumor control is the management of metastatic process. Angiogenesis or the formation of new blood vessels from preexisting ones plays a crucial role in the expansion of the tumor by providing oxygen, nutrition and conduits for cancer cells to invade and metastasize new tissues¹. Abnormalities of growth factors (GFs) released such as PDGFs (Platelet Derived Growth Factor) could be involved in malignant human diseases2,3. Inflammation and cancer present similar mechanisms of development including angiogenesis or cell proliferation4. In order to know the effect on sprouting promotion of GFs existent in the tumor environment such as VEGF (Vascular Endothelial Growth Factor), PDGF, BMP2 (Bone Morphogenetic Protein 2) or TGF-ß (Transforming Growth Factor-ß), we have developed a microfluidic-based test based on devices designed by Farahat et al. (2012)5, which allows to the user the quantification of sprouting formation under the effect of these GFs. TGF-ß pathway involved in tumor progression in multiple human cancers, instigates phenotypical changes affecting to the cell growth, differentiation and migration6. Knowing the overexpression of GFs such as VEGF or BMP2 in tumors7,8, we aimed to compare its effect on endothelial cells in angiogenesis. Analyzing the promotion of sprout in normal conditions under GFs addition would be possible to determine which of these molecules could decrease or promote the advance of the endothelial cells. The results obtained in this work indicated that VEGF is the most important factor to enhance the angiogenic process while non-specific factors such as BMP2 or TGF-ß show a low effectiveness. In the case of PDGF, the negative effect of this molecule observed in our assays could be explained by the non-optimal balance of concentration. Furthermore, we are currently working to quantify the effect of fluid flow on the sprouting promotion

Morphological Transformation and Force Generation of Active Cytoskeletal Networks

Cells assemble numerous types of actomyosin bundles that generate contractile forces for biological processes, such as cytokinesis and cell migration. One example of contractile bundles is a transverse arc that forms via actomyosin-driven condensation of actin filaments in the lamellipodia of migrating cells and exerts significant forces on the surrounding environments. Structural reorganization of a network into a bundle facilitated by actomyosin contractility is a physiologically relevant and biophysically interesting process. Nevertheless, it remains elusive how actin filaments are reoriented, buckled, and bundled as well as undergo tension buildup during the structural reorganization. In this study, using an agent-based computational model, we demonstrated how the interplay between the density of myosin motors and cross-linking proteins and the rigidity, initial orientation, and turnover of actin filaments regulates the morphological transformation of a cross-linked actomyosin network into a bundle and the buildup of tension occurring during the transformation

Microfluidic model of monocyte extravasation reveals the role of hemodynamics and subendothelial matrix mechanics in regulating endothelial integrity

Extravasation of circulating cells is an essential process that governs tissue inflammation and the body's response to pathogenic infection. To initiate anti-inflammatory and phagocytic functions within tissues, immune cells must cross the vascular endothelial barrier from the vessel lumen to the subluminal extracellular matrix. In this work, we present a microfluidic approach that enables the recreation of a three-dimensional, perfused endothelial vessel formed by human endothelial cells embedded within a collagen-rich matrix. Monocytes are introduced into the vessel perfusate, and we investigate the role of luminal flow and collagen concentration on extravasation. In vessels conditioned with the flow, increased monocyte adhesion to the vascular wall was observed, though fewer monocytes extravasated to the collagen hydrogel. Our results suggest that the lower rates of extravasation are due to the increased vessel integrity and reduced permeability of the endothelial monolayer. We further demonstrate that vascular permeability is a function of collagen hydrogel mass concentration, with increased collagen concentrations leading to elevated vascular permeability and increased extravasation. Collectively, our results demonstrate that extravasation of monocytes is highly regulated by the structural integrity of the endothelial monolayer. The microfluidic approach developed here allows for the dissection of the relative contributions of these cues to further understand the key governing processes that regulate circulating cell extravasation and inflammation

Fibroblast migration in 3D is controlled by haptotaxis in a non-muscle myosin II-dependent manner

Cell migration in 3D is a key process in many physiological and pathological processes. Although valuable knowledge has been accumulated through analysis of various 2D models, some of these insights are not directly applicable to migration in 3D. In this study, we have confined biomimetic hydrogels within microfluidic platforms in the presence of a chemoattractant (platelet-derived growth fac- tor-BB). We have characterized the migratory responses of human fibroblasts within them, particularly focusing on the role of non-muscle myosin II. Our results indicate a prominent role for myosin II in the integration of chemo- tactic and haptotactic migratory responses of fibroblasts in 3D confined environments