Cell traction forces in 3-D microenvironments

Las células son capaces de sentir y responder activamente frente a los estímulos mecánicos de su entorno. Los estímulos mecánicos que provienen de la matriz extracelular, tales como la rigidez, la topología de la superficie o la deformación, son traducidos en señales bioquímicas a través de las interacciones entre la célula y la matriz. Para poder sobrevivir y crecer las células necesitan adherirse y propagarse sobre el sustrato que las rodea. Una vez adheridas, las células generan fuerzas contráctiles a través de la interacción actina-miosina, ejerciendo de este modo tracción sobre el sustrato subyacente. Es por ello, que las fuerzas de tracción ejercidas por las células son reguladores críticos de la adhesión, la señalización y la función celular, y por tanto son muy importantes en numerosos procesos biológicos tales como la inflamación, la cicatrización de heridas, la angiogénesis e incluso la metástasis. Pese a su importancia, la medición de las fuerzas celulares en un contexto fisiológico así como entender su contribución en los procesos biológicos sigue siendo todavía un reto. Además, debido a que las interacciones célula-matriz varían considerablemente entre ambientes bidimensionales y tridimensionales, entender su influencia sobre las respuestas celulares normales y patológicas en sistemas tridimensionales es esencial para poder traducir de manera eficiente dichos conocimientos en terapias médicas. El principal objetivo de esta Tesis es, por tanto, el desarrollo de modelos computacionales enfocados al estudio de diferentes aspectos de las interacciones célula-matriz, que permitan entender mejor los fenómenos específicos y que sirvan como referencia para el desarrollo de nuevos experimentos y de técnicas de modelado in vitro. Además, todos los modelos y experimentos contenidos en esta tesis se centran en el estudio de células individuales. En primer lugar, debido a la complejidad y a las grandes diferencias que presentan con respecto a la migración celular colectiva, y en segundo lugar debido a la importancia que supone el estudio de la migración celular individual en procesos tan importantes como es la invasión de células tumorales. Además, debido a la relevancia que suponen fisiológicamente los entornos tridimensionales, en la mayoría de los modelos in silico desarrollados en esta Tesis, se han considerado aproximaciones tridimensionales para poder así imitar mejor las condiciones in vivo de células y tejidos.En primer lugar, se ha investigado la dinámica de unión de los sitios de adhesión célula-matriz, más en particular cómo las células transmiten las fuerzas a través de estas uniones a la matriz extracelular. Para ello, se ha desarrollado un modelo numérico mediante el uso del método de los elementos finitos [1]. En segundo lugar, se ha desarrollado un modelo in vitro para el estudio de las interacciones célula-matriz tanto a nivel celular como a nivel de tejido. En particular, se presentan diferentes dispositivos de microfluídica, los cuales están siendo utilizados en la actualidad para el estudio de diferentes procesos biológicos. Estos han sido utilizados para estudiar los procesos de formación de gradientes químicos a través de una matriz tridimensional [2]. Investigaciones recientes han indicado que las fuerzas de tracción celular son reguladores críticos de la invasión de las células tumorales, las cuales dependen en gran medida de las propiedades mecánicas tanto de las células como de la matriz que las rodea. Debido a que surge la necesidad de tener un conocimiento mucho más profundo sobre este mecanismo, la segunda parte de esta Tesis se ha centrado en el desarrollo de diferentes experimentos para cuantificar las fuerzas celulares, así como en el desarrollo de un modelo in silico basado en elementos finitos para reconstruir las fuerzas ejercidas por las células durante su migración, permitiendo de este modo estudiar la dependencia de las propiedades mecánicas de las células sobre la solución de fuerzas obtenida [3]. En resumen, una mejor comprensión de los mecanismos subyacentes a las interacciones célula-matriz, aportados en parte por la aparición de nuevas tecnologías para estudiar la mecánica celular a alta resolución espacial y temporal, no sólo resulta en una mejor comprensión del comportamiento de células normales, sino que también conduce al desarrollo de terapias novedosas para tratar enfermedades relacionadas con los defectos en las interacciones mecánicas celulares.<br /

The interplay between tissue growth and scaffold degradation in engineered tissue constructs

In vitro tissue engineering is emerging as a potential tool to meet the high demand for replacement tissue, caused by the increased incidence of tissue degeneration and damage. A key challenge in this field is ensuring that the mechanical properties of the engineered tissue are appropriate for the in vivo environment. Achieving this goal will require detailed understanding of the interplay between cell proliferation, extracellular matrix (ECM) deposition and scaffold degradation.\ud \ud In this paper, we use a mathematical model (based upon a multiphase continuum framework) to investigate the interplay between tissue growth and scaffold degradation during tissue construct evolution in vitro. Our model accommodates a cell population and culture medium, modelled as viscous fluids, together with a porous scaffold and ECM deposited by the cells, represented as rigid porous materials. We focus on tissue growth within a perfusion bioreactor system, and investigate how the predicted tissue composition is altered under the influence of (i) differential interactions between cells and the supporting scaffold and their associated ECM, (ii) scaffold degradation, and (iii) mechanotransduction-regulated cell proliferation and ECM deposition.\ud \ud Numerical simulation of the model equations reveals that scaffold heterogeneity typical of that obtained from μCT scans of tissue engineering scaffolds can lead to significant variation in the flow-induced mechanical stimuli experienced by cells seeded in the scaffold. This leads to strong heterogeneity in the deposition of ECM. Furthermore, preferential adherence of cells to the ECM in favour of the artificial scaffold appears to have no significant influence on the eventual construct composition; adherence of cells to these supporting structures does, however, lead to cell and ECM distributions which mimic and exaggerate the heterogeneity of the underlying scaffold. Such phenomena have important ramifications for the mechanical integrity of engineered tissue constructs and their suitability for implantation in vivo

Modeling of Mechanosensing Mechanisms Reveals Distinct Cell Migration Modes to Emerge From Combinations of Substrate Stiffness and Adhesion Receptor–Ligand Affinity

Mesenchymal cell migration is an integral process in development and healing. The process is regulated by both mechanical and biochemical properties. Mechanical properties of the environment are sensed through mechanosensing, which consists of molecular responses mediated by mechanical signals. We developed a computational model of a deformable 3D cell on a flat substrate using discrete element modeling. The cell is polarized in a single direction and thus moves along the long axis of the substrate. By modeling discrete focal adhesions and stress fibers, we implement two mechanosensing mechanisms: focal adhesion stabilization by force and stress fiber strengthening upon contraction stalling. Two substrate-associated properties, substrate (ligand) stiffness and adhesion receptor–ligand affinity (in the form of focal adhesion disassembly rate), were varied for different model setups in which the mechanosensing mechanisms are set as active or inactive. Cell displacement, focal adhesion number, and cellular traction were quantified and tracked in time. We found that varying substrate stiffness (a mechanical property) and adhesion receptor–ligand affinity (a biochemical property) simultaneously dictate the mode in which cells migrate; cells either move in a smooth manner reminiscent of keratocytes or in a cyclical manner reminiscent of epithelial cells. Mechanosensing mechanisms are responsible for the range of conditions in which a cell adopts a particular migration mode. Stress fiber strengthening, specifically, is responsible for cyclical migration due to build-up of enough force to elicit rupture of focal adhesions and retraction of the cellular rear. Together, both mechanisms explain bimodal dependence of cell migration on substrate stiffness observed in the literature

Microscale Measurements of Cell and Tissue Mechanics in Three Dimensions

Two-dimensional (2D) studies have revealed that mechanical forces drive cell migration and can feedback to regulate proliferation, differentiation and the synthesis/remodeling of extracellular matrix (ECM) proteins. Whether these observations can be translated to clinical settings or be utilized for tissue engineering will depend critically on our ability to translate these findings into physiologically relevant three-dimensional (3D) environments. The general goal of this dissertation has been to develop and apply new technologies capable of extending studies of cell and tissue mechanics into 3D environments. In the first project, we measured both shear and normal traction forces exerted by cells cultured on planar substrates. We observed that focal adhesions serve as pivots about which cells generate rotational moments. In the second project, we combined enzymatically degradable synthetic hydrogels with finite element models to measure the mechanical tractions exerted by cells fully encapsulated within 3D matrices. We found that cells reach out thin protrusions and pull back inward towards the cell body with the highest forces at the tip. Cellular extensions that were invading into the surrounding matrix displayed a strong inward force 10-15 microns behind the leading tip, suggesting that growing extensions may establish a contractile waypoint, before invading further. To study the forces cells exert during tissue remodeling, we utilized photolithograpy to generate arrays of microtissues consisting of cells encapsulated in 3D collagen matrices. Microcantilevers were used to constrain the remodeling of the collagen gel and to report the forces generated during this process. We used this technique to explore the effects of boundary stiffness and matrix density within model tendon and cardiac tissues. Finally, we combined this system with a Foerster radius energy transfer (FRET) based biosensor of fibronectin conformation to reveal how tissue geometry and cell-genereated tractions cooperate to pattern matrix conformation during tissue remodeling. Together, these studies highlight novel approaches to understand the nature of cell-ECM interactions in 3D matrices. Such mechanical insights will help us to understand how physical forces drive cell migration and behavior within physiologically relevant environments

Quantifying forces in cell biology

Cells exert, sense, and respond to physical forces through an astounding diversity of mechanisms. Here we review recently developed tools to quantify the forces generated by cells. We first review technologies based on sensors of known or assumed mechanical properties, and discuss their applicability and limitations. We then proceed to draw an analogy between these human-made sensors and force sensing in the cell. As mechanics is increasingly revealed to play a fundamental role in cell function we envisage that tools to quantify physical forces may soon become widely applied in life-sciences laboratories

Three-Dimensional Traction Force Microscopy: A New Tool for Quantifying Cell-Matrix Interactions

The interactions between biochemical processes and mechanical signaling play important roles during various cellular processes such as wound healing, embryogenesis, metastasis, and cell migration. While traditional traction force measurements have provided quantitative information about cell matrix interactions in two dimensions, recent studies have shown significant differences in the behavior and morphology of cells when placed in three-dimensional environments. Hence new quantitative experimental techniques are needed to accurately determine cell traction forces in three dimensions. Recently, two approaches both based on laser scanning confocal microscopy have emerged to address this need. This study highlights the details, implementation and advantages of such a three-dimensional imaging methodology with the capability to compute cellular traction forces dynamically during cell migration and locomotion. An application of this newly developed three-dimensional traction force microscopy (3D TFM) technique to single cell migration studies of 3T3 fibroblasts is presented to show that this methodology offers a new quantitative vantage point to investigate the three-dimensional nature of cell-ECM interactions

Unravelling cell migration: defining movement from the cell surface

Cell motility is essential for life and development. Unfortunately, cell migration is also linked to several pathological processes, such as cancer metastasis. Cells’ ability to migrate relies on many actors. Cells change their migratory strategy based on their phenotype and the properties of the surrounding microenvironment. Cell migration is, therefore, an extremely complex phenomenon. Researchers have investigated cell motility for more than a century. Recent discoveries have uncovered some of the mysteries associated with the mechanisms involved in cell migration, such as intracellular signaling and cell mechanics. These findings involve different players, including transmembrane receptors, adhesive complexes, cytoskeletal components , the nucleus, and the extracellular matrix. This review aims to give a global overview of our current understanding of cell migration

Myosin IIA-mediated forces regulate multicellular integrity during vascular sprouting

Angiogenic sprouting is a critical process involved in vascular network formation within tissues. During sprouting, tip cells and ensuing stalk cells migrate collectively into the extracellular matrix while preserving cell-cell junctions, forming patent structures that support blood flow. Although several signaling pathways have been identified as controlling sprouting, it remains unclear to what extent this process is mechanoregulated. To address this question, we investigated the role of cellular contractility in sprout morphogenesis, using a biomimetic model of angiogenesis. Three-dimensional maps of mechanical deformations generated by sprouts revealed that mainly leader cells, not stalk cells, exert contractile forces on the surrounding matrix. Surprisingly, inhibiting cellular contractility with blebbistatin did not affect the extent of cellular invasion but resulted in cell-cell dissociation primarily between tip and stalk cells. Closer examination of cell-cell junctions revealed that blebbistatin impaired adherens-junction organization, particularly between tip and stalk cells. Using CRISPR/Cas9-mediated gene editing, we further identified NMIIA as the major isoform responsible for regulating multicellularity and cell contractility during sprouting. Together, these studies reveal a critical role for NMIIA-mediated contractile forces in maintaining multicellularity during sprouting and highlight the central role of forces in regulating cell-cell adhesions during collective motility.R01 EB000262 - NIBIB NIH HHS; R01 HL115553 - NHLBI NIH HHSPublished versio

Influence of ECM Composition and Intracellular Calcium on Endothelial Biomechanics and Prediction of Cellular Stresses Using Machine Learning

Endothelial cells, which form the inner layer of the vasculature, constantly interact with their external microenvironment called the extracellular matrix (ECM) by exerting contractile cell-substrate stresses called tractions and cell-cell stresses called intercellular stresses. This cellular mechanosensing can become aberrant and act as a precursor for many vascular pathological and physiological processes such as cancer metastasis, atherosclerosis, cell differentiation, migration, and morphogenesis. Also, intracellular calcium signalling plays an important role in endothelial cell motility and in maintaining vascular tone. Alteration in ECM composition has been linked to several pathologies, in fact, a transition to a fibronectin-rich matrix from a type I collagen-rich and elastin-rich matrix in coronary artery disease, for example. However, the influence of ECM compositions and intracellular calcium levels on cell mechanics is not clearly understood. The first study will shed light on ECM composition and its influence on endothelial mechanical properties including traction, intercellular stresses, cell velocity, and various morphological parameters. The second study will enhance our knowledge on the role calcium signaling plays on cellular tractions. The final chapters will focus on the development and utilization of Machine Learning (ML) models for the predictions of tractions and intercellular stresses with morphological and pharmacological predictors, which to our knowledge is the first work in the field. The results yielded from this work will further our understanding of cellular mechanics at the mesoscale by: i) Identifying the role of specific ECM molecules in mechanical signaling, ii) Understanding the influence of transient calcium signaling on tractions, and iii) Providing a machine learning framework that can be used for the prediction of tractions and intercellular stresses as a dose dependent response to a drug that is known to influence cell mechanics. These findings will be beneficial to drug development studies and targeted drug therapy for treating various vascular-related pathologies

The Biochemical and Biophysical Mechanisms of Macrophage Migration

The ability of macrophages to migrate is critical for a proper immune response. During an innate immune response, macrophages migrate to sites of infection or inflammation where they clear pathogens through phagocytosis and activate an adaptive immune response by releasing cytokines and acting as antigen-presenting cells. Unfortunately, improper regulation of macrophage migration is associated with a variety of dieases including cancer, atherosclerosis, wound-healing, and rheumatoid arthritis. In this thesis, engineered substrates were used to study the chemical and physical mechanisms of macrophage migration. We first used microcontact printing to generate surfaces specifically functionalized with fibronectin and functionally blocked against cell adhesion to study the migration of RAW/LR5 murine macrophages. Using these surfaces we found that macrophage migration is biphasic with respect to increasing surface ligand or soluble chemokine concentration, and that RAW/LR5 migration is dependent on PI3K and ROCK signaling. We then used traction force microscopy to measure the force generation capabilities of primary human macrophages and found that these cells generate strong forces at their leading edge in a stiffness-dependent manner. Through the use of chemical inhbitors we showed that force generation is dependent on myosin II contraction, PI3K signaling, and Rac signaling downstream of the GEF Vav1, but not the GEF Tiam1. Finally, we investigated the motility and force generation of M1 and M2 polarized primary human macrophages. We found that M1 macrophages are less motile and generate less force than M0 or M2 macrophages, and that M2 macrophages are more motile but do not have any change in force generation compared to M0 macrophages. We have been able to show that both chemical signals and mechanical mechanisms contribute to macrophage migration. This work contributes to the growing understanding of the mechanisms that govern macrophage migration and demonstrates the importance of mechanics when studying leukocyte migration