Collagen fibril flow and tissue translocation coupled to fibroblast migration in 3D collagen matrices

Mechanical interactions of mouse mammary gland cells with collagen in a three-dimensional construct

An effort to understand the development of breast cancer motivates the study of mammary gland cells and their interactions with the extracellular matrix. A mixture of mammary gland epithelial cells (normal murine mammary gland), collagen, and fluorescent beads was loaded into microchannels and observed via four-dimensional imaging. Collagen concentrations of 1.3, 2, and 3 mg/mL were used. The displacements of the beads were used to calculate strains in the 3D matrix. To ensure physiologically relevant materials properties for analysis, the collagen was characterized using independent tensile testing with strain rates in the range of those measured in the cell-gel constructs. 3D elastic theory for an isotropic material was employed to calculate the stress. The technique presented adds to the field of measuring cell-generated stresses by providing the capability of measuring 3D stresses locally around a single cell and using physiologically relevant materials properties for analysis. The highest strains were observed in the most compliant matrix. Additionally, the stresses fluctuated over time due to the cells' interaction with the collagen matrix.

Fibroblasts in three dimensional matrices: cell migration and matrix remodeling

Fibroblast-collagen matrix culture has facilitated the analysis of cell physiology under conditions that more closely resemble an in vivo-like environment compared to conventional 2-dimensional (2D) cell culture. Furthermore, it has led to significant progress in understanding reciprocal and adaptive interactions between fibroblasts and the collagen matrix, which occur in tissue. Recent studies on fibroblasts in 3-dimensional (3D) collagen matrices have revealed the importance of biomechanical conditions in addition to biochemical cues for cell signaling and migration. Depending on the surrounding mechanical conditions, cells utilize specific cytoskeletal proteins to adapt to their environment. More specifically, cells utilize microtubule dependent dendritic extensions to provide mechanical structure for matrix contraction under a low cell-matrix tension state, whereas cells in a high cell-matrix tension state utilize conventional acto-myosin activity for matrix remodeling. Results of collagen matrix contraction and cell migration in a 3D collagen matrix revealed that the use of appropriate growth factors led to promigratory and procontractile activity for cultured fibroblasts. Finally, the relationship between cell migration and tractional force for matrix remodeling was discussed.

Plasticity of cell migration: a multiscale tuning model

Cell migration underlies tissue formation, maintenance, and regeneration as well as pathological conditions such as cancer invasion. Structural and molecular determinants of both tissue environment and cell behavior define whether cells migrate individually (through amoeboid or mesenchymal modes) or collectively. Using a multiparameter tuning model, we describe how dimension, density, stiffness, and orientation of the extracellular matrix together with cell determinants—including cell–cell and cell–matrix adhesion, cytoskeletal polarity and stiffness, and pericellular proteolysis—interdependently control migration mode and efficiency. Motile cells integrate variable inputs to adjust interactions among themselves and with the matrix to dictate the migration mode. The tuning model provides a matrix of parameters that control cell movement as an adaptive and interconvertible process with relevance to different physiological and pathological contexts.

Promigratory and procontractile growth factor environments differentially regulate cell morphogenesis

Three-dimensional (3D) cell-matrix cultures provide a useful model to analyze and dissect the structural, functional, and mechanical aspects of cell-matrix interactions and motile behavior important for cell and tissue morphogenesis. In the current studies we tested the effects of serum and physiological growth factors on the morphogenetic behavior of human fibroblasts cultured on the surfaces of 3D collagen matrices. Fibroblasts in medium containing serum contracted into clusters, whereas cells in medium containing platelet-derived growth factor (PDGF) were observed to migrate as individuals. The clustering activity of serum appeared to depend on lysophosphatidic acid, required cell contraction based on inhibition by blocking Rho kinase or myosin II, and was reversed upon switching to PDGF. Oncogenic Ras transformed human fibroblasts did not exhibit serum-stimulated cell clustering. Our findings emphasize the importance of cell-specific promigratory and procontractile growth factor environments in the differential regulation of cell motile function and cell morphogenesis.

Proteolytic interstitial cell migration: a five-step process

Cell migration is a multi-step process that leads to the actin-driven translocation of cells on or through tissue substrate. Basic steps involved in cell migration have been defined for two-dimensional haptokinetic migration which, however, does not provide physical constraints imposed by three-dimensional interstitial tissues. We here describe the process of pericellular proteolysis that leads to extracellular matrix (ECM) degradation and realignment during cell movement and integrate it into established steps of cell migration. After actin-driven leading edge protrusion (step I) and anterior formation of integrin-mediated focal interactions to the substrate (step II), ECM breakdown is focalized towards physical ECM barriers several micrometer rearward of the leading edge (step III). Actomyosin-mediated cell contraction (step IV) then leads to rear-end retraction and forward sliding of cell body and nucleus so that a small tube-like matrix defect bordered by realigned ECM fibers becomes apparent (step V). Pericellular proteolysis is thus integral to the migration cycle and serves to widen ECM gaps and thereby lowers physical stress upon the cell body, which ultimately leads to aligned higher-oder ECM patterns.

Filamin A-beta1 integrin complex tunes epithelial cell response to matrix tension

The physical properties of the extracellular matrix (ECM) regulate the behavior of several cell types; yet, mechanisms by which cells recognize and respond to changes in these properties are not clear. For example, breast epithelial cells undergo ductal morphogenesis only when cultured in a compliant collagen matrix, but not when the tension of the matrix is increased by loading collagen gels or by increasing collagen density. We report that the actin-binding protein filamin A (FLNa) is necessary for cells to contract collagen gels, and pull on collagen fibrils, which leads to collagen remodeling and morphogenesis in compliant, low-density gels. In stiffer, high-density gels, cells are not able to contract and remodel the matrix, and morphogenesis does not occur. However, increased FLNa-beta1 integrin interactions rescue gel contraction and remodeling in high-density gels, resulting in branching morphogenesis. These results suggest morphogenesis can be "tuned" by the balance between cell-generated contractility and opposing matrix stiffness. Our findings support a role for FLNa-beta1 integrin as a mechanosensitive complex that bidirectionally senses the tension of the matrix and, in turn, regulates cellular contractility and response to this matrix tension.

An experimental model for assessing fibroblast migration in 3-D collagen matrices

The purpose of this study was to develop and test a novel culture model for studying fibroblast migration in 3-D collagen matrices. Human corneal fibroblasts were seeded within dense, randomly oriented compressed collagen matrices. A 6 mm diameter button of this cell-seeded matrix was placed in the middle of an acellular, less dense outer collagen matrix. These constructs were cultured for 1, 3, 5 or 7 days in serum-free media, 10% fetal bovine serum (FBS), or 50 ng/ml PDGF. Constructs were then fixed and labeled with AlexaFluor 546 phalloidin (for f-actin) and TOTO-3 (for nuclei). Cell-matrix interactions were assessed using a combination of fluorescent and reflected light confocal imaging. Human corneal fibroblasts in serum-free media showed minimal migration into the outer (non-compressed) matrix. In contrast, culture in serum or PDGF stimulated cell migration. Cell-induced collagen matrix reorganization in the outer matrix could be directly visualized using reflected light imaging, and was highest following culture in 10% FBS. Cellular contraction in 10% FBS often led to alignment of cells parallel to the outer edge of the inner matrix, similar to the pattern observed during corneal wound healing following incisional surgery. Overall, this 3-D model allows the effects of different culture conditions on cell migration and matrix remodeling to be assessed simultaneously. In addition, the design allows for ECM density, geometry and mechanical constraints to be varied in a controlled fashion. These initial results demonstrate differences in cell and matrix patterning during migration in response to serum and PDGF.

Contact guidance mediated three-dimensional cell migration is regulated by Rho/ROCK-dependent matrix reorganization

Cells generate mechanical force to organize the extracellular matrix (ECM) and drive important developmental and reparative processes. Likewise, tumor cells invading into three-dimensional (3D) matrices remodel the ECM microenvironment. Importantly, we previously reported a distinct radial reorganization of the collagen matrix surrounding tumors that facilitates local invasion. Here we describe a mechanism by which cells utilize contractility events to reorganize the ECM to provide contact guidance that facilitates 3D migration. Using novel assays to differentially organize the collagen matrix we show that alignment of collagen perpendicular to the tumor-explant boundary promotes local invasion of both human and mouse mammary epithelial cells. In contrast, organizing the collagen matrix to mimic the ECM organization associated with noninvading regions of tumors suppresses 3D migration/invasion. Moreover, we demonstrate that matrix reorganization is contractility-dependent and that the Rho/Rho kinase pathway is necessary for collagen alignment to provide contact guidance. Yet, if matrices are prealigned, inhibiting neither Rho nor Rho kinase inhibits 3D migration, which supports our conclusion that Rho-mediated matrix alignment is an early step in the invasion process, preceding and subsequently facilitating 3D migration.

Oncogenic Ras-transformed human fibroblasts exhibit differential changes in contraction and migration in 3D collagen matrices

Tractional force exerted by tissue cells in 3D collagen matrices can be utilized for matrix remodeling or cell migration. The interrelationship between these motile processes is not well understood. The current studies were carried out to test the consequences of oncogenic Ras (H-Ras(V12)) transformation on human fibroblast contraction and migration in 3D collagen matrices. Beginning with hTERT-immortalized cells, we prepared fibroblasts stably transformed with E6/E7 and with the combination HPV16 E6/E7 and H-Ras(V12). Oncogenic Ras-transformed cells lost contact inhibition of cell growth, formed colonies in soft agar and were unable to make adherens junctions. We observed no changes in the extent or growth factor dependence of collagen matrix contraction (floating or stress-relaxation) by oncogenic Ras-transformed cells. On the other hand, transformed cells in nested collagen matrices lost not only growth factor selectivity, but also cell-matrix density-dependent inhibition of migration. These findings demonstrate differential regulation of collagen matrix contraction and cell migration in 3D collagen matrices.

Mapping local matrix remodeling induced by a migrating tumor cell using three-dimensional multiple-particle tracking

Mesenchymal cell migration through a three-dimensional (3D) matrix typically involves major matrix remodeling. The direction of matrix deformation occurs locally in all three dimensions, which cannot be measured by current techniques. To probe the local, 3D, real-time deformation of a collagen matrix during tumor cell migration, we developed an assay whereby matrix-embedded beads are tracked simultaneously in all three directions with high resolution. To establish a proof of principle, we investigated patterns of collagen I matrix deformation near fibrosarcoma cells in the absence and presence of inhibitors of matrix metalloproteinases and acto-myosin contractility. Our results indicate that migrating cells show patterns of local matrix deformation toward the cell that are symmetric in magnitude with respect to the axis of cell movement. In contrast, patterns of matrix release from the cell are asymmetric: the matrix is typically relaxed first at the back of the cell, allowing forward motion, and then at the cell's leading edge. Matrix deformation in regions of the matrix near the cell's leading edge is elastic and mostly reversible, but induces irreversible matrix rupture events near the trailing edge. Our results also indicate that matrix remodeling spatially correlates with protrusive activity. This correlation is mediated by myosin II and Rac1, and eliminated after inhibition of pericellular proteolysis or ROCK. We have developed an assay based on high-resolution 3D multiple-particle tracking that allows us to probe local matrix remodeling during mesenchymal cell migration through a 3D matrix and simultaneously monitor protrusion dynamics.

Fibroblast mechanics in three-dimensional collagen matrices

Fascia provides mechanical support and frameworks for the other tissues of the body. Type 1 collagen is the major protein component of fascia, and fibroblasts are the cell type primarily responsible for its biosynthesis and remodeling. Research on fibroblasts interacting with collagen matrices provides new insights regarding how cell-matrix tension state and growth factor specificity regulate cell migration and matrix remodeling.