Cell adaptation to a physiologically relevant ECM mimic with different viscoelastic properties

Microsystems for biomechanical measurements

The use of microtechnology to make biomechanical measurements allows for the study of cellular and subcellular scale mechanical forces. Forces generated by cells are in the few nanoNewton to several microNewton range and can change spatially over subcellular size scales. Transducing forces at such small size and force scales is a challenging task. Methods of microfabrication developed in the integrated circuit industry have allowed researchers to build platforms with cellular and subcellular scale parts with which individual cells can interact. These parts act as transducers of stresses and forces generated by the cell during migration or in the maintenance of physical equilibrium. Due to the size and sensitivity of such devices, quantitative studies of single cell and even single molecule biomechanics have become possible. In this review we focus on two classes of cellular force transducers: silicon-based devices and soft-polymer platforms. We concentrate on the biomechanical discoveries made with these devices and less so on the engineering behind their development because this is covered in great detail elsewhere.

Modeling mechanosensing and its effect on the migration and proliferation of adherent cells

The behavior of normal adherent cells is influenced by the stiffness of the substrate they are anchored to. Cells are able to detect substrate mechanical properties by actively generating contractile forces and use this information to migrate and proliferate. In particular, the speed and direction of cell crawling, as well as the rate of cell proliferation, vary with the substrate compliance and prestrain. In this work, we present an active mechanosensing model based on an extension of the classical Hill's model for skeletal muscle behavior. We also propose a thermodynamical approach to model cell migration regulated by mechanical stimuli and a proliferation theory also depending on the mechanical environment. These contributions give rise to a conceptually simple mathematical formulation with a straightforward and inexpensive computational implementation, yielding results consistent with numerous experiments. The model can be a useful tool for practical applications in biology and medicine in situations where cell-substrate interaction as well as substrate mechanical behavior play an important role, such as the design of tissue engineering applications.

Defining the role of matrix compliance and proteolysis in three-dimensional cell spreading and remodeling

Recent studies have identified extracellular matrix (ECM) compliance as an influential factor in determining the fate of anchorage-dependent cells. We explore a method of examining the influence of ECM compliance on cell morphology and remodeling in three-dimensional culture. For this purpose, a biological ECM analog material was developed to pseudo-independently alter its biochemical and physical properties. A set of 18 material variants were prepared with shear modulus ranging from 10 to 700 Pa. Smooth muscle cells were encapsulated in these materials and time-lapse video microscopy was used to show a relationship between matrix modulus, proteolytic biodegradation, cell spreading, and cell compaction of the matrix. The proteolytic susceptibility of the matrix, the degree of matrix compaction, and the cell morphology were quantified for each of the material variants to correlate with the modulus data. The initial cell spreading into the hydrogel matrix was dependent on the proteolytic susceptibility of the materials, whereas the extent of cell compaction proved to be more correlated to the modulus of the material. Inhibition of matrix metalloproteinases profoundly affected initial cell spreading and remodeling even in the most compliant materials. We concluded that smooth muscle cells use proteolysis to form lamellipodia and tractional forces to contract and remodel their surrounding microenvironment. Matrix modulus can therefore be used to control the extent of cellular remodeling and compaction. This study further shows that the interconnection between matrix modulus and proteolytic resistance in the ECM may be partly uncoupled to provide insight into how cells interpret their physical three-dimensional microenvironment.

Engineering a clinically-useful matrix for cell therapy

The design criteria for matrices for encapsulation of cells for cell therapy include chemical, biological, engineering, marketing, regulatory, and financial constraints. What is required is a biocompatible material for culture of cells in three-dimensions (3-D) that offers ease of use, experimental flexibility to alter composition and compliance, and a composition that would permit a seamless transition from in vitro to in vivo use. The challenge is to replicate the complexity of the native extracellular matrix (ECM) environment with the minimum number of components necessary to allow cells to rebuild a given tissue. Our approach is to deconstruct the ECM to a few modular components that can be reassembled into biomimetic materials that meet these criteria. These semi-synthetic ECMs (sECMs) employ thiol-modified derivatives of hyaluronic acid (HA) that can form covalently crosslinked, biodegradable hydrogels. These sECMs are "living" biopolymers, meaning that they can be crosslinked in the presence of cells or tissues to enable cell therapy and tissue engineering. Moreover, the sECMs allow inclusion of the appropriate biological cues needed to simulate the complexity of the ECM of a given tissue. Taken together, the sECM technology offers a manufacturable, highly reproducible, flexible, FDA-approvable, and affordable vehicle for cell expansion and differentiation in 3-D.

Evaluating drug efficacy and toxicology in three dimensions: using synthetic extracellular matrices in drug discovery

The acceptance of the new paradigm of 3-D cell culture is currently constrained by the lack of a biocompatible material in the marketplace that offers ease of use, experimental flexibility, and a seamless transition from in vitro to in vivo applications. I describe the development of a covalently cross-linked mimic of the extracellular matrix (sECM), now commercially available, for 3-D culture of cells in vitro and for translational use in vivo. These bio-inspired, biomimetic materials can be used "as is" in drug discovery, toxicology, cell banking, and, ultimately, medicine. For cell therapy and the development of clinical combination products, the sECM biomaterials must be highly reproducible, manufacturable, approvable, and affordable. To obtain integrated, functional, multicellular systems that recapitulate tissues and organs, the needs of the true end users, physicians and patients, must dictate the key design criteria. In chemical terms, the sECM consists of chemically-modified hyaluronan (HA), other glycosaminoglycans (GAGs), and ECM polypeptides containing thiol residues that are cross-linked using biocompatible polyvalent electrophiles. For example, co-cross-linking the semisynthetic thiol-modified HA-like GAG with thiol-modified gelatin produces Extracel as a hydrogel. This hydrogel may be formed in situ in the presence of cells or tissues to provide an injectable cell-delivery vehicle. Alternately, an Extracel hyrogel can be lyophilized to create a macroporous scaffold, which can then be employed for 3-D cell culture. In this Account, we describe four applications of sECMs that are relevant to the evaluation of drug efficacy and drug toxicity. First, the uses of sECMs to promote both in vitro and in vivo growth of healthy cellularized 3-D tissues are summarized. Primary or cell-line-derived cells, including fibroblasts, chondrocytes, hepatocytes, adult and embryonic stem cells, and endothelial and epithelial cells have been used. Second, primary hepatocytes retain their biochemical phenotypes and achieve greater longevity in 3-D culture in Extracel. This constitutes a new 3-D method for rapid evaluation of hepatotoxicity in vitro. Third, cancer cell lines are readily grown in 3-D culture in Extracel, offering a method for rapid evaluation of new anticancer agents in a more physiological ex vivo tumor model. This system has been used to evaluate signal transduction modifiers obtained from our research on lipid signaling. Fourth, a new "tumor engineering" xenograft model uses orthotopic injection of Extracel-containing tumor cells in nude mice. This approach allows production of patient-specific mice using primary human tumor samples and offers a superior metastatic cancer model. Future applications of the injectable cell delivery and 3-D cell culture methods include chemoattractant and angiogenesis assays, high-content automated screening of chemical libraries, pharmacogenomic and toxicogenomic studies with cultured organoids, and personalized treatment models. In summary, the sECM technology offers a versatile "translational bridge" from in vitro to in vivo to facilitate drug discovery in both academic and pharmaceutical laboratories.

Effects of extracellular matrix analogues on primary human fibroblast behavior

In vitro cell culture is a vital research tool for cell biology, pharmacology, toxicology, protein production, systems biology and drug discovery. Traditional culturing methods on plastic surfaces do not accurately represent the in vivo environment, and a paradigm shift from two-dimensional to three-dimensional (3-D) experimental techniques is underway. To enable this change, a variety of natural, synthetic and semi-synthetic extracellular matrix (ECM) equivalents have been developed to provide an appropriate cellular microenvironment. We describe herein an investigation of the properties of four commercially available ECM equivalents on the growth and proliferation of primary human tracheal scar fibroblast behavior, both in 3-D and pseudo-3-D conditions. We also compare subcutaneous tissue growth of 3-D encapsulated fibroblasts in vivo in two of these materials, Matrigel and Extracel. The latter shows increased cell proliferation and remodeling of the ECM equivalent. The results provide researchers with a rational basis for selection of a given ECM equivalent based on its biological performance in vitro and in vivo, as well as the practicality of the experimental protocols. Biomaterials that use a customizable glycosaminoglycan-based hydrogel appear to offer the most convenient and flexible system for conducting in vitro research that accurately translates to in vivo physiology needed for tissue engineering.

Fibroblast mechanics in 3D collagen matrices

Connective tissues provide mechanical support and frameworks for the other tissues of the body. Type 1 collagen is the major protein component of ordinary connective tissue, and fibroblasts are the cell type primarily responsible for its biosynthesis and remodeling. Research on fibroblasts interacting with collagen matrices explores all four quadrants of cell mechanics: pro-migratory vs. pro-contractile growth factor environments on one axis; high tension vs. low tension cell-matrix interactions on the other. The dendritic fibroblast - probably equivalent to the resting tissue fibroblast - can be observed only in the low tension quadrant and generally has not been appreciated from research on cells incubated with planar culture surfaces. Fibroblasts in the low tension quadrant require microtubules for formation of dendritic extensions, whereas fibroblasts in the high tension quadrant require microtubules for polarization but not for spreading. Ruffling of dendritic extensions rather than their overall protrusion or retraction provides the mechanism for remodeling of floating collagen matrices, and floating matrix remodeling likely reflects a model of tissue mechanical homeostasis.

Modeling tissue morphogenesis and cancer in 3D

Three-dimensional (3D) in vitro models span the gap between two-dimensional cell cultures and whole-animal systems. By mimicking features of the in vivo environment and taking advantage of the same tools used to study cells in traditional cell culture, 3D models provide unique perspectives on the behavior of stem cells, developing tissues and organs, and tumors. These models may help to accelerate translational research in cancer biology and tissue engineering.

Cell morphology and migration linked to substrate rigidity

A mathematical model, based on thermodynamics, was developed to demonstrate how substrate rigidity influences cell morphology and migration. The mechanisms by which substrate rigidity are translated into cell-morphological changes and cell movement are described. The model takes into account the competition between the elastic energies in the cell-substrate system and work of adhesion at the cell periphery. The cell morphology and migration are dictated by the minimum of the total free energy of the cell-substrate system. By using this model, reported experimental observations on cell morphological changes and migration can be better understood with a theoretical basis. In addition, these observations can be more accurately correlated with the variation of substrate rigidity. This study indicates that the activity of the adherent cell is dependent not only on the substrate rigidity but also is correlated with the relative rigidity between the cell and substrate. Moreover, the study suggests that the cell stiffness can be estimated based on the substrate stiffness corresponding to the change of trend in morphological stability.

TNF-alpha suppresses alpha-smooth muscle actin expression in human dermal fibroblasts: an implication for abnormal wound healing

Abnormal wound healing encompasses a wide spectrum, from chronic wounds to hypertrophic scars. Both conditions are associated with an abnormal cytokine profile in the wound bed. In this study, we sought to understand the dynamic relationships between myofibroblast differentiation and mechanical performance of the collagen matrix under tissue growth factor-beta (TGF-beta) and tumor necrosis factor-alpha (TNF-alpha) stimulation. We found TGF-beta increased alpha-smooth muscle actin (alpha-SMA) and TNF-alpha alone decreased the basal alpha-SMA expression. When TGF-beta1 and TNF-alpha were both added, the alpha-SMA expression was suppressed below the baseline. Real-time PCR showed that TNF-alpha suppresses TGF-beta1-induced myofibroblast (fibroproliferative) phenotypic genes, for example, alpha-SMA, collagen type 1A, and fibronectin at the mRNA level. TNF-alpha suppresses TGF-beta1-induced gene expression by affecting its mRNA stability. Our results further showed that TNF-alpha inhibits TGF-beta1-induced Smad-3 phosphorylation via Jun N-terminal kinase signaling. Mechanical testing showed that TNF-alpha decreases the stiffness and contraction of the lattices after 5 days in culture. We proposed that changes in alpha-SMA, collagen, and fibronectin expression result in decreased contraction and stiffness of collagen matrices. Therefore, the balance of cytokines in a wound defines the mechanical properties of the extracellular matrix and optimal wound healing.

Tissue Engineering for Cutaneous Wounds

Skin, the largest organ in the body, protects against toxins and microorganisms in the environment and serves to prevent dehydration of all non-aquatic animals. Immune surveillance, sensory detection, and self-healing are other critical functions of the skin. Loss of skin integrity because of injury or illness may result acutely in substantial physiologic imbalance and ultimately in significant disability or even death. It is estimated that, in 1992, there were 35.2 million cases of significant skin loss (US data) that required major therapeutic intervention. Of these, approximately 7 million wounds become chronic. Regardless of the specific advanced wound care product, the ideal goal would be to regenerate tissues such that both the structural and functional properties of the wounded tissue are restored to the levels before injury. The advent of tissue-engineered skin replacements revolutionized the therapeutic potential for recalcitrant wounds and for wounds that are not amenable to primary closure. This article will introduce the reader to the field of tissue engineering, briefly review tissue-engineered skin replacement from a historical perspective and then review current state-of-the-art concepts from our vantage point.

Simplifying the extracellular matrix for 3-D cell culture and tissue engineering: A pragmatic approach

The common technique of growing cells on tissue culture plastic (TCP) is gradually being supplanted by methods for culturing cells in two-dimensions (2-D) on matrices with more appropriate physical and biological properties or by encapsulation of cells in three-dimensions (3-D). The universal acceptance of the new 3-D paradigm is currently constrained by the lack of a biocompatible material in the marketplace that offers ease of use, experimental flexibility, and a seamless transition from in vitro to in vivo applications. In this Prospect, I argue that the standard for 3-D cell culture should be bio-inspired, biomimetic materials that can be used "as is" in drug discovery, toxicology, cell banking, and ultimately in medicine. Such biomaterials must therefore be highly reproducible, manufacturable, approvable, and affordable. To obtain integrated, functional, multicellular systems that recapitulate tissues and organs, the needs of the true end-users-physicians and patients-must dictate the key design criteria. Herein I describe the development of one such material that meets these requirements: a covalently crosslinked, biodegradable, simplified mimic of the extracellular matrix (ECM) that permits 3-D culture of cells in vitro and enables tissue formation in vivo. In contrast to materials that were designed for in vitro cell culture and then found unsuitable for clinical use, these semi-synthetic hyaluronan-derived materials were developed for in vivo tissue repair, and are now being re-engineered for in vitro applications in research.