Quantitative characterization of cell invasion in vitro: formulation and validation of a mathematical model of the collagen gel invasion assay

Application of Temporary, Cell-Containing Alginate Microcarriers to Facilitate the Fabrication of Spatially Defined Cell Pockets in 3D Collagen Hydrogels

Mimicking the complexity of natural tissue is a major challenge in the field of tissue engineering. Here, a facile 2-step fabrication method to prepare 3D constructs with distinct regions of high cell concentrations and without the need for elaborate equipment is proposed. The initial incorporation of cells in a sacrificial alginate matrix allows the addition of other, cell relevant biopolymers, such as, collagen to form a spatially confined, interpenetrating network at the microscale. A layered structure at the macroscale can be achieved by incorporating these cell-containing microspheres in thin collagen layers. Cells are locally released by de-gelling the alginate matrix and their attachment to the collagen hydrogel layers has been studied. The use of the murine pre-osteoblast cell line MC3T3-E1 as an example cell line shows that the cells behave differently in their cell migration pattern based on the initial composition of the alginate microspheres.

A microfluidic platform for modeling metastatic cancer cell matrix invasion

Invasion of the extracellular matrix is a critical step in the colonization of metastatic tumors. The invasion process is thought to be driven by both chemokine signaling and interactions between invading cancer cells and physical components of the metastatic niche, including endothelial cells that line capillary walls and serve as a barrier to both diffusion and invasion of the underlying tissue. Transwell chambers, a tool for generating artificial chemokine gradients to induce cell migration, have facilitated recent work to investigate the chemokine contributions to matrix invasion. These chambers, however, are poorly designed for imaging, which limits their use in investigating the physical cell-cell and cell-matrix interactions driving matrix invasion. Microfluidic devices offer a promising model in which the invasion process can be imaged. Many current designs, however, have limited surface areas and possess intricate geometries that preclude the use of standard staining protocols to visualize cells and matrix proteins. In this work, we present a novel microfluidic platform for imaging cell-cell and cell-matrix interactions driving metastatic cancer cell matrix invasion. Our model is applied to investigate how endothelial cell-secreted matrix proteins and the physical endothelial monolayer itself interact with invading metastatic breast cancer cells to facilitate invasion of an underlying type I collagen gel. The results show that matrix invasion of metastatic breast cancer cells is significantly enhanced in the presence of live endothelial cells. Probing this interaction further, our platform revealed that, while the fibronectin-rich matrix deposited by endothelial cells was not sufficient to drive invasion alone, metastatic breast cancer cells were able to exploit components of energetically inactivated endothelial cells to gain entry into the underlying matrix. These findings reveal novel cell-cell interactions driving a key step in the colonization of metastatic tumors and have important implications for designing drugs targeted at preventing cancer metastasis.

Displacement correlations between a single mesenchymal-like cell and its nucleus effectively link subcellular activities and motility in cell migration analysis

Cell migration is an essential process in organism development and physiological maintenance. Although current methods permit accurate comparisons of the effects of molecular manipulations and drug applications on cell motility, effects of alterations in subcellular activities on motility cannot be fully elucidated from those methods. Here, we develop a strategy termed cell-nuclear (CN) correlation to parameterize represented dynamic subcellular activities and to quantify their contributions in mesenchymal-like migration. Based on the biophysical meaning of the CN correlation, we propose a cell migration potential index (CMPI) to measure cell motility. When the effectiveness of CMPI was evaluated with respect to one of the most popular cell migration analysis methods, Persistent Random Walk, we found that the cell motility estimates among six cell lines used in this study were highly consistent between these two approaches. Further evaluations indicated that CMPI can be determined using a shorter time period and smaller cell sample size, and it possesses excellent reliability and applicability, even in the presence of a wide range of noise, as might be generated from individual imaging acquisition systems. The novel approach outlined here introduces a robust strategy through an analysis of subcellular locomotion activities for single cell migration assessment.

The role of engineering approaches in analysing cancer invasion and metastasis

In the past decade, novel materials, probes and tools have enabled fundamental and applied cancer researchers to take a fresh look at the complex problem of tumour invasion and metastasis. These new tools, which include imaging modalities, controlled but complex in vitro culture conditions, and the ability to model and predict complex processes in vivo, represent an integration of traditional with novel engineering approaches; and their potential effect on quantitatively understanding tumour progression and invasion looks promising.

Quantification and regulation of cell migration

Cell migration is essential for many physiological and pathological processes that include embryonic development, the immune response, wound healing, angiogenesis, and cancer metastasis. It is also important for emerging tissue engineering applications such as tissue reconstitution and the colonization of biomedical implants. By summarizing results from recent experimental and theoretical studies, this review outlines the role played by growth factors or substrate-adhesion molecules in modulating cell motility and shows that cell motility can be an important factor in determining the rates of tissue formation. The application of cell motility assays and the use of theoretical models for analyzing cell migration and proliferation are also discussed.

Fluorescence-based assays for in vitro analysis of cell adhesion and migration

Cell adhesion and cell migration are two primary cellular phenomena for which in vitro approaches may be exploited to effectively dissect the individual events and underlying molecular mechanisms. The use of assays dedicated to the analysis of cell adhesion and migration in vitro also afford an efficient way of conducting larger basic and applied research screenings on the factors affecting these processes and are potentially exploitable in the context of routine diagnostic, prognostic, and predictive tests in the biological and medical fields. Therefore, there is a longstanding continuum in the interest in devising more rationale such assays and major contributions in this direction have been provided by the advent of procedures based on fluorescence cell tagging, the design of instruments capable of detecting fluorescent signals with high sensitivity, and informatic tools allowing sophisticated elaboration of data generated through these instruments. In this report, we describe three representative fluorescence-based model assays for the qualitative and quantitative assessment of cell adhesion and cell locomotion in static and dynamic conditions. The assays are easily performed, accurate and reproducible, and can be automated for high-to-medium throughput screenings of cell behavior in vitro. Performance of the assays involves the use of certain dedicated disposable accessories, which are commercially available, and a few instruments that, due to their versatility, can be regarded as constituents of a more generic laboratory setup.

Internet-based image analysis quantifies contractile behavior of individual fibroblasts inside model tissue

In a cell-populated collagen gel, intrinsic fiber structure visible in differential interference contrast images can provide markers for an in situ strain gauge to quantify cell-gel mechanics, while optical sections of fluorescent protein distribution capture cytoskeletal kinematics. Mechanics quantification can be derived automatically from timelapse differential interference contrast images using a Deformation Quantification and Analysis software package accessible online at http://dqa.web.cmu.edu. In our studies, fibroblast contractile machinery was observed to function entirely within pseudopods, while GFP-alpha-actinin concentrated in pseudopod tips and cortex. Complex strain patterns around individual cells showed instances of both elastic and inelastic strain transmission, suggesting a role in observed long-range alignment of cells.

Expression and regulation of CD97 in colorectal carcinoma cell lines and tumor tissues

The expression of CD97, a member of the EGF-TM7 family with adhesive properties, is proportional to the aggressiveness and lymph node involvement in thyroid tumors. CD97 has never been systematically investigated in other tumors. First, we examined colorectal carcinoma cell lines (n = 18) for CD97 expression and regulation. All cell lines were CD97-positive. The level of CD97 in each line correlated with migration and invasion in vitro. This result was confirmed in CD97-inducible Tet-off HT1080 cells. Transforming growth factor-beta, which inhibits proliferation in transforming growth factor-beta-sensitive LS513 and LS1034 cells, down-regulated CD97 in these cell lines. Examining CD97 during sodium butyrate-induced cell differentiation of Caco-2 cells, we could demonstrate a CD97-decreasing effect. Second, we screened 81 colorectal adenocarcinomas by immunohistology for expression of CD97. Normal colorectal epithelium is CD97-negative. Seventy-five of 81 of the carcinomas expressed CD97. The strongest staining for CD97 occurred in scattered tumor cells at the invasion front compared to cells located within solid tumor formations of the same tumor. Carcinomas with more strongly CD97-stained scattered tumor cells showed a poorer clinical stage as well as increased lymph vessel invasion compared to cases with uniform CD97 staining. In summary, CD97 expression correlates with dedifferentiation, migration, and invasion in colorectal tumor cell lines. Moreover, more strongly CD97-stained tumor cells at the invasion front of colorectal carcinomas indicate the involvement of the molecule in tumor migration and invasion.

Cell motility is inhibited by the antiepileptic compound, valproic acid and its teratogenic analogues

Valproic acid (VPA) is an established human teratogen that causes neural tube defects in 1-2% of human foetuses exposed to the drug during early pregnancy. In this study, individual cell motility was evaluated using short- and long-term time-lapse video-recording and computer assisted image analysis, and it was found that VPA and selected VPA-analogues inhibited individual cell motility of L-cells in a dose-dependent manner. The compounds caused a decrease in the root-mean-square speed, S, and in the rate of diffusion, R, but an increase in the time of persistence in direction, P. Using short-term recordings and measurements of mean-cell speed, the reduction in the motile behaviour was shown to correlate with the teratogenic potency of the tested compounds. The observed effects of VPA on cell motility was independent of the employed L-cell clone, and could be reproduced in cells containing the neuronal marker NCAM and in the neuronal cell line N2a. Furthermore, the observed effect was independent of culture substratum, being observed for L-cells grown on fibronectin as well as on plastic. Immunofluorescence microscopy revealed that VPA-treatment of mouse L-cells caused a redistribution of F-actin and of a series of focal adhesion proteins, indicating that the effect of VPA on cell motility may be causally related to increased cell-substratum interactions or to alterations in the organisation or dynamics of the actin cytoskeleton.

A fibrin or collagen gel assay for tissue cell chemotaxis: assessment of fibroblast chemotaxis to GRGDSP

Fibroblast chemotaxis is implicated in many physiological processes, including wound healing and morphogenesis. We present a novel assay for chemotaxis of fibroblasts (and other slow-moving tissue cells) in a direct-viewing chamber containing a physiologically relevant three-dimensional fibrin or collagen gel in which long-lasting, spatially continuous gradients have been sustained for at least 24 h, long enough for significant fibroblast migration. This combination of features is not available in any alternative assay of comparable setup simplicity. Using a putative fibroblast chemotactic factor, the fibronectin peptide GRGDSP, we measured human foreskin fibroblast alignment in the direction along the gradient, which followed a biphasic dependence on GRGDSP concentration with an optimal concentration of about 10 nM. Time-lapse video microscopy revealed that cell migration was up the soluble GRGDSP gradient, confirming positive chemotaxis to GRGDSP and rejecting the possibility of dominant haptotaxis down the soluble GRGDSP gradient, that is, up a putative gradient of integrin-mediated adhesion induced by the soluble GRGDSP gradient.

The fibroblast-populated collagen microsphere assay of cell traction force--Part 2: Measurement of the cell traction parameter

In Part 1 of this work, we formulated and analyzed a mathematical model for our fibroblast-populated collagen microsphere (FPCM) assay of cell traction forces (Moon and Tranquillo, 1993). In this assay, the FPCM diameter decreases with time as the cells compact the gel by exerting traction on collagen fibrils. In Part 1 we demonstrated that the diameter reduction profiles for varied initial cell concentration and varied initial FPCM diameter are qualitatively consistent with the model predictions. We show here in Part 2 how predictions of a model similar to that of Part 1, along with the determination of the growth parameters of the cells and the viscoelastic parameters of the gel, allow us to estimate the magnitude of a cell traction parameter, the desired objective index of cell traction forces. The model is based on a monophasic continuum-mechanical theory of cell-extracellular matrix (ECM) mechanical interactions, with a species conservation equation for cells (1), a mass conservation equation for ECM (2), and a mechanical force balance for the cell/ECM composite (3). Using a constant-stress rheometer and a fluids spectrometer in creep and oscillatory shear modes, respectively, we establish and characterize the linear viscoelastic regime for the reconstituted type 1 collagen gel used in our FPCM traction assay and in other assays of cell-collagen mechanical interactions. Creep tests are performed on collagen gel specimens in a state resembling that in our FPCM traction assay (initially uncompacted, and therefore nearly isotropic and at a relatively low collagen concentration of 2.1 mg/ml), yielding measurements of the zero shear viscosity, mu 0 7.4 x 10(6) Poise), and the steady-state creep compliance, J0e. The shear modulus, G (155 dynes/cm2), is then determined from the inverse of J0e in the linear viscoelastic regime. Oscillatory shear tests are performed in strain sweep mode, indicating linear viscoelastic behavior up to shear strains of approximately 10 percent. We discuss the estimation of Poisson's ratio, v, which along with G and mu 0 specifies the assumed isotropic, linear viscoelastic stress tensor for the cell/collagen gel composite which appears in (3). The proliferation rate of fibroblasts in free floating collagen gel (appearing in (1)) is characterized by direct cell counting, yielding an estimate of the first-order growth rate constant, k (5.3 x 10(-6) s-1). These independently measured and estimated parameter values allow us to estimate that the cell traction parameter, tau 0, defined in the active stress tensor which also appears in (3), is in the range of 0.00007-0.0002 dyne.cm4/mg collagen.cell.(ABSTRACT TRUNCATED AT 400 WORDS)

Cytokine-stimulated chemotaxis of human neutrophils in a 3-D conjoined fibrin gel assay

The ability of neutrophils to migrate through three-dimensional (3-D) tissues in response to chemical stimuli is critical to their host defense function. However, studies characterizing stimulated migration in vitro have been largely limited to two-dimensional (2-D) surfaces. In this study, we have employed direct observation methods to quantify human neutrophil migration in 3-D fibrin gel using time-lapse video microscopy and automated cell tracking methods. A novel 3-D conjoined gel assay was developed to establish experimentally quantifiable and theoretically predictable diffusion gradients of chemotactic factors. This assay was used to measure objective migration parameters, namely the random motility and chemotaxis coefficients, in response to the cytokine, interleukin-8 (IL-8). The random motility coefficient, mu, showed a biphasic dependence on IL-8 concentration with a maximum of 1.1 x 10(-8) cm2/s at 5 x 10(-8) M IL-8; no significant motility was observed in the absence of IL-8. We further established the dependence of cell orientation bias, phi, on the concentration and gradient steepness (i.e., specific gradient, SG) of IL-8. Results indicate that phi increases with increasing SG, provided the concentration is maintained sufficiently low, which we conjecture to result from minimizing IL-8 receptor down-regulation. The chemotaxis coefficient, chi, was maximum at an intermediate SG for both IL-8 concentrations studied. We also examined the applicability of this assay to estimate mu and chi from indirect measurements of chemotaxis, namely the simpler measurement of cell redistribution after a prescribed incubation time, as opposed to direct cell tracking measurements. By virtue of measuring chi, this is the first quantitatively objective study of mammalian cell chemotaxis in a physiologically relevant 3-D gel and, in particular, of neutrophil chemotaxis on any substratum in response to the physiologically relevant chemotactic factor, IL-8.

Biased cell migration of fibroblasts exhibiting contact guidance in oriented collagen gels

We present here the first quantitative correlation for cell contact guidance in an oriented fibrillar network in terms of biased cell migration. The correlation is between the anisotropic cell diffusion parameter, DA = Dx/Dy, and the collagen gel birefringence, delta n, a measure of axially biased collagen fibril orientation in the x-direction. The cell diffusion coefficients, Dx and Dy, measure the dispersal of cells in the directions coincident with and normal to the axis of fibril orientation, respectively. Three essential methodological components are involved: (i) exploiting the orienting effect of a magnetic field on collagen fibrils during fibrillogenesis to systematically prepare uniform axially oriented collagen gels; (ii) using a microscope/image analysis workstation with precise, computer-controlled rotating and translating stages to automate birefringence measurement and, along with rapid "coarse optical sectioning" via digital image processing, to enable 3-D cell tracking of many cells in multiple samples simultaneously; and (iii) employing a rigorous statistical analysis of the cell tracks to estimate the magnitude and precision of the direction-dependent cell diffusion coefficients, Dx and Dy, that define DA. We find that this measure of biased migration in contact guidance (DA) increases with increasing collagen fibril orientation (delta n) due mainly to a rapid enhancement of migration along the axis of fibril orientation at low levels of fibril orientation, and to a continued suppression of migration normal to the axis of fibril orientation at high levels of fibril orientation.