Transforming growth factor beta stimulates fibroblast-collagen matrix contraction by different mechanisms in mechanically loaded and unloaded matrices

Collagen stiffness regulates cellular contraction and matrix remodeling gene expression

Cell-level mechanical and 3D spatial cues are essential to the organization and architecture of new tissues that form during growth, repair or in bioreactors. Fibroblast-seeded 3D collagen constructs have been used as bioartifical extracellular matrix (ECM) providing a 3D environment to embedded resident cells. As cells attach to scaffold fibrils, they generate quantifiable contractile forces which depend on cell type, cell attachment, cell density, growth factors, and matrix stiffness. The aim of this study was to quantify the cytomechanical and molecular responses of human dermal (HDF) and neonatal foreskin fibroblasts (HNFF) seeded in constructs of increased stiffness. We also tested the effect of blocking early attachment using serum starvation on these outputs. Constructs were placed under uniaxial strains of 0-10% to increase scaffold stiffness, prior to gel contraction, and force generation was monitored using a tensional culture force monitor (t-CFM). Increased matrix stiffness reduced generation of quantifiable cellular force (up to 70%) over 24 h in both cell types and delayed the onset of measurable contraction (upto sevenfold). The delay of measurable force generation was cell lineage dependent but not FCS dependent. Gene expression of MMP-2, TIMP-2, and collagen type III expression in HDFs were significantly upregulated in constructs of increased stiffness. HNFFs did not show any significant changes in these gene expressions indicating a lineage specific response.

Use of an in vitro model of tissue-engineered skin to investigate the mechanism of skin graft contraction

Skin graft contraction leading to loss of joint mobility and cosmetic deformity remains a major clinical problem. In this study we used a tissue-engineered model of human skin, based on sterilized human adult dermis seeded with keratinocytes and fibroblasts, which contracts by up to 60% over 28 days in vitro, as a model to investigate the mechanism of skin contraction. Pharmacologic agents modifying collagen synthesis, degradation, and cross-linking were examined for their effect on contraction. Collagen synthesis and degradation were determined using immunoassay techniques. The results show that skin contraction was not dependent on inhibition of collagen synthesis or stimulation of collagen degradation, but was related to collagen remodelling. Thus, reducing dermal pliability with glutaraldehyde inhibited the ability of cells to contract the dermis. So did inhibition of matrix metalloproteinases and inhibition of lysyl oxidase-mediated collagen cross-linking, but not transglutaminase-mediated cross-linking. In summary, this in vitro model of human skin has allowed us to identify specific cross-linking pathways as possible pharmacologic targets for prevention of graft contracture in vivo.

Force activates smooth muscle α-actin promoter activity through the Rho signaling pathway

In pressure or volume overload, hypertrophic growth of the myocardium is associated with myofibroblast differentiation, a process in which cardiac fibroblasts express smooth muscle α-actin (SMA). The signaling mechanisms that mediate force-induced myofibroblast differentiation and SMA expression are not defined. We examined the role of the Rho–Rho-kinase pathway in force-induced SMA expression in fibroblasts using an in vitro model system that applies static tensile forces (0.65 pN/μm2) to integrins via collagen-coated magnetite beads. Force maximally induced RhoA activation at 10 minutes that was localized to force application sites and required intact actin filaments. Force application induced phosphorylation of LIM kinase (5-10 minutes) and an early dephosphorylation of cofilin (5 minutes) that was followed by prolonged cofilin phosphorylation. These responses were blocked by Y27632, an inhibitor of Rho kinase. Force promoted actin filament assembly at force application sites (10-20 minutes), a process that required Rho kinase and cofilin. Force application induced nuclear translocation of the transcriptional co-activator MRTF-A but not MRTF-B. Nuclear translocation of MRTF-A required Rho kinase and intact actin filaments. Force caused 3.5-fold increases of SMA promoter activity that were completely blocked by transfection of cells with dominant-negative MRTF-A or by inhibition of Rho kinase or by actin filament disassembly. These data indicate that mechanical forces mediate actin assembly through the Rho–Rho-kinase–LIMK cofilin pathway. Force-mediated actin filament assembly promotes nuclear translocation of MRTF and subsequent activation of the SMA promoter to enhance SMA expression.

Fractal and image analysis of morphological changes in the actin cytoskeleton of neonatal cardiac fibroblasts in response to mechanical stretch

Cardiac fibroblasts are the most numerous cells in the heart and are critical in the formation and normal functioning of the organ. Cardiac fibroblasts are firmly attached to and surrounded by extracellular matrix (ECM). Mechanical forces transmitted through interaction with the ECM can result in changes of overall cellular shape, cytoskeletal organization, proliferation, and gene expression of cardiac fibroblasts. These responses may be different in the normally functioning heart, when compared with various pathological conditions, including inflammation or hypertrophy. It is apparent that cellular phenotype and physiology, in turn, are affected by multiple signal transduction pathways modulated directly by the state of polymerization of the actin cytoskeleton. Morphological changes in actin organization resulting from response to adverse conditions in fibroblasts and other cell types are basically descriptive. Some studies have approached quantifying changes in actin cytoskeletal morphology, but these have involved complex and difficult procedures. In this study, we apply image analysis and non-Euclidian geometrical fractal analysis to quantify and describe changes induced in the actin cytoskeleton of cardiac fibroblasts responding to mechanical stress. Characterization of these rapid responses of fibroblasts to mechanical stress may provide insight into the regulation of fibroblasts behavior and gene expression during heart development and disease.

Collagen Gel Anisotropy Measured by 2-D Laser Trap Microrheometry

Collagen gels can serve as biomaterials ideal for tissue equivalents, especially if they are remodeled to have fibril anisotropy mimicking native tissue. Type I collagen gel remodeling was studied microscopically to investigate the changes caused by fibroblasts in collagen gel structures, with and without the growth factors PDGF-BB and TGF-beta1. A bidirectional laser trap microrheometry technique was developed that revealed a high degree of local heterogeneity and anisotropy in the structure of the collagen gels during active fibroblast contraction. The use of the growth factors increased not only the gel anisotropy, but the heterogeneity as well, indicating further changes in the collagen fibril orientations. This work shows the ability to influence the remodeling capabilities of fibroblasts by using growth factors in order to begin to elucidate the changes in the local mechanical environment of contracting collagen gels. We present this experimental technique as a method for probing changes in the fibroblast-driven anisotropy of collagen gels as a basis for understanding microstructural tissue organization important in the development of collagen-based tissue equivalents.

Formation and function of the myofibroblast during tissue repair

It is generally accepted that fibroblast-to-myofibroblast differentiation represents a key event during wound healing and tissue repair. The high contractile force generated by myofibroblasts is beneficial for physiological tissue remodeling but detrimental for tissue function when it becomes excessive such as in hypertrophic scars, in virtually all fibrotic diseases and during stroma reaction to tumors. Specific molecular features as well as factors that control myofibroblast differentiation are potential targets to counteract its development, function, and survival. Such targets include alpha-smooth muscle actin and more recently discovered markers of the myofibroblast cytoskeleton, membrane surface proteins, and the extracellular matrix. Moreover, intervening with myofibroblast stress perception and transmission offers novel strategies to reduce tissue contracture; stress release leads to the instant loss of contraction and promotes apoptosis.

Detection of epithelial to mesenchymal transition in airways of a bleomycin induced pulmonary fibrosis model derived from an alpha-smooth muscle actin-Cre transgenic mouse

BACKGROUND

Epithelial to mesenchymal transition (EMT) in alveolar epithelial cells (AECs) has been widely observed in patients suffering interstitial pulmonary fibrosis. In vitro studies have also demonstrated that AECs could convert into myofibroblasts following exposure to TGF-beta1. In this study, we examined whether EMT occurs in bleomycin (BLM) induced pulmonary fibrosis, and the involvement of bronchial epithelial cells (BECs) in the EMT. Using an alpha-smooth muscle actin-Cre transgenic mouse (alpha-SMA-Cre/R26R) strain, we labelled myofibroblasts in vivo. We also performed a phenotypic analysis of human BEC lines during TGF-beta1 stimulation in vitro.

METHODS

We generated the alpha-SMA-Cre mouse strain by pronuclear microinjection with a Cre recombinase cDNA driven by the mouse alpha-smooth muscle actin (alpha-SMA) promoter. alpha-SMA-Cre mice were crossed with the Cre-dependent LacZ expressing strain R26R to produce the double transgenic strain alpha-SMA-Cre/R26R. beta-galactosidase (betagal) staining, alpha-SMA and smooth muscle myosin heavy chains immunostaining were carried out simultaneously to confirm the specificity of expression of the transgenic reporter within smooth muscle cells (SMCs) under physiological conditions. BLM-induced peribronchial fibrosis in alpha-SMA-Cre/R26R mice was examined by pulmonary betagal staining and alpha-SMA immunofluorescence staining. To confirm in vivo observations of BECs undergoing EMT, we stimulated human BEC line 16HBE with TGF-beta1 and examined the localization of the myofibroblast markers alpha-SMA and F-actin, and the epithelial marker E-cadherin by immunofluorescence.

RESULTS

betagal staining in organs of healthy alpha-SMA-Cre/R26R mice corresponded with the distribution of SMCs, as confirmed by alpha-SMA and SM-MHC immunostaining. BLM-treated mice showed significantly enhanced betagal staining in subepithelial areas in bronchi, terminal bronchioles and walls of pulmonary vessels. Some AECs in certain peribronchial areas or even a small subset of BECs were also positively stained, as confirmed by alpha-SMA immunostaining. In vitro, addition of TGF-beta1 to 16HBE cells could also stimulate the expression of alpha-SMA and F-actin, while E-cadherin was decreased, consistent with an EMT.

CONCLUSION

We observed airway EMT in BLM-induced peribronchial fibrosis mice. BECs, like AECs, have the capacity to undergo EMT and to contribute to mesenchymal expansion in pulmonary fibrosis.

Cell responses to biomimetic protein scaffolds used in tissue repair and engineering

Basic science research in tissue engineering and regenerative medicine aims to investigate and understand the deposition, growth, and remodeling of tissues by drawing together approaches from a range of disciplines. This review discusses approaches that use biomimetic proteins and cellular therapies, both in the development of clinical products and of model platforms for scientific investigation. Current clinical approaches to repairing skin, bone, nerve, heart valves, blood vessels, ligaments, and tendons are described and their limitations identified. Opportunities and key questions for achieving clinical goals are discussed through commonly used examples of biomimetic scaffolds: collagen, fibrin, fibronectin, and silk. The key questions addressed by three-dimensional culture models, biomimetic materials, surface chemistry, topography, and their interaction with cells in terms of durotaxis, mechano-regulation, and complex spatial cueing are reviewed to give context to future strategies for biomimetic technology.

Collagenase-3 (MMP-13) Enhances Remodeling of Three-Dimensional Collagen and Promotes Survival of Human Skin Fibroblasts

Collagenase-3 (MMP-13) is a matrix metalloproteinase capable of cleaving a multitude of extracellular matrix proteins in addition to fibrillar collagens. Human MMP-13 is expressed by fibroblasts in chronic cutaneous ulcers, but not in normally healing adult skin wounds. However, MMP-13 is produced by fibroblasts in adult gingival and in fetal skin wounds characterized by rapid collagen remodeling and scarless healing. Here, we have examined the role of human MMP-13 in remodeling of three-dimensional (3D) collagenous matrix by primary adult human skin fibroblasts. The high level of human MMP-13 expression by fibroblasts achieved by adenoviral gene delivery resulted in potent enhancement of remodeling and contraction of 3D collagen. Fibroblasts expressing MMP-13 in 3D collagen possessed altered filamentous actin morphology with patch-like actin distribution in cell extensions. The expression of MMP-13 promotes survival and proliferation of fibroblasts in floating collagen gel, and results in activation of Akt and extracellular signal-regulated kinase-1/2 by these cells. The results provide evidence for a novel role for human MMP-13 in regulating dermal fibroblast survival, proliferation, and interaction in 3D collagen, which may be an important survival mechanism for fibroblasts in chronic skin ulcers and contribute to scarless healing of adult gingival and fetal skin wounds.

Periostin promotes atrioventricular mesenchyme matrix invasion and remodeling mediated by integrin signaling through Rho/PI 3-kinase

Recent evidence suggests that extracellular matrix components may play a signaling role in embryonic valve development. We have previously identified the spatiotemporal expression patterns of periostin in developing valves, but its function during this process is largely unknown. To evaluate the functional role periostin plays during valvulogenesis, two separate three-dimensional culture assay systems, which model chick atrioventricular cushion development, were employed. These assays demonstrated that cushion mesenchymal cells adhered and spread on purified periostin in a dose-responsive manner, similar to collagen I and fibronectin via alpha(v)beta(3) and beta(1) integrin pairs. Periostin overexpression resulted in enhanced mesenchyme invasion through 3D collagen gels and increased matrix compaction. This invasion was dependent on alpha(v)beta(3) more than beta(1) integrin signaling, and was mediated differentially by Rho kinase and PI 3-kinase. Both matrix invasion and compaction were associated with a colocalization of periostin and beta(1) integrin expression to migratory cell phenotype in both surface and deep cells. The Rho/PI 3-kinase pathway also differentially mediated matrix compaction. Both Rho and PI 3-kinase were involved in normal cushion mesenchyme matrix compaction, but only PI 3-kinase was required for the enhanced matrix compaction due to periostin. Taken together, these results highlight periostin as a mediator of matrix remodeling by cushion mesenchyme towards a mature valve structure.

Enhanced myofibroblastic differentiation and survival in Thy-1(-) lung fibroblasts

Thy-1 is a glycosylphosphatidyl-inositol-linked cell surface glycoprotein whose exact biological role remains unclear. Differential expression of Thy-1 affects fibroblast proliferation and fibrogenic signaling. In idiopathic pulmonary fibrosis, the proliferating myofibroblasts within the fibroblastic foci are Thy-1(-), whereas normal lung fibroblasts are predominantly Thy-1(+). In this study, we used rat lung fibroblasts sorted for Thy-1 expression to examine myofibroblastic differentiation in response to fibrogenic stimuli. We examined the effects of transforming growth factor-beta, endothelin-1, and connective tissue growth factor on the expression of myofibroblast proteins and myogenic regulatory factors by real-time RT-PCR and immunoblotting. Thy-1(-) cells have significantly higher myofibroblast and myogenic regulatory factor gene and protein expression compared with Thy-1(+) cells, confirmed by immunofluorescence. We also used floating collagen matrix contraction assays to assess the functional differentiation of the fibroblasts. At baseline and after stimulation with transforming growth factor-beta and endothelin-1, Thy-1(-) cells caused significantly greater collagen contraction than did Thy-1(+) cells, supporting the hypothesis that Thy-1(-) cells are more fully differentiated myofibroblasts. Because apoptosis has been implicated in the regression of myofibroblasts, we examined the percentage of apoptotic cells in the contracted collagen matrices at baseline and after stimulation with fibrogenic agents. A significantly greater proportion of Thy-1(+) cells underwent apoptosis in all conditions compared with Thy-1(-) fibroblasts. Transfection of Thy-1 into Thy-1(-) cells inhibits collagen matrix contraction and reduces cell survival. Our data indicate that Thy-1 regulates myogenic gene expression, myofibroblastic differentiation, and survival in lung fibroblasts.

Promotion by fibronectin of collagen gel contraction mediated by human corneal fibroblasts

Collagen contraction mediated by corneal fibroblasts (CFs) is implicated in the maintenance of corneal shape. Given that fibronectin is expressed at sites of corneal stromal wounding, we investigated the effect of fibronectin on CF-mediated collagen gel contraction. Human CFs were cultured in a three-dimensional gel of type I collagen in the absence or presence of various extracellular matrix (ECM) components. The contraction of collagen gels mediated by CFs was evaluated by measurement of changes in gel diameter. The formation of stress fibers and focal adhesions in CFs was examined by fluorescence microscopy. The abundance of paxillin, phosphorylated paxillin, integrins alpha5, beta1, and alpha2, and alpha-smooth muscle actin in CFs was examined by immunoblot analysis. Fibronectin promoted CF-mediated collagen gel contraction in a concentration- and time-dependent manner. Other ECM proteins or glycosaminoglycans did not exhibit such an effect. Fibronectin also induced cell spreading, the formation of stress fibers, and the establishment of focal adhesions containing paxillin in CFs cultured in three-dimensional collagen gels. In addition, it increased the amounts of paxillin, phosphorylated paxillin, and integrins alpha5 and beta1 in these cells. The expression of integrin alpha2 and alpha-smooth muscle actin was not affected by fibronectin, however. Furthermore, the peptide GRGDSP (an antagonist of fibronectin receptors) blocked the stimulatory effect of fibronectin on CF-mediated collagen gel contraction. These results suggest that fibronectin promoted CF-mediated collagen gel contraction in a manner dependent on the formation of stress fibers and focal adhesions, the activation of paxillin, and the up-regulation of integrin alpha5beta1. Fibronectin may therefore contribute to the maintenance of corneal shape by CFs during the healing of stromal wounds.

Urokinase plasminogen activator in injured adventitia increases the number of myofibroblasts and augments early proliferation

Myofibroblasts are involved in vessel remodeling during the development of hypertension as well as after angioplasty and aortocoronary grafting, but the mechanisms of myofibroblastic phenotypic modulation are not fully elucidated. We assessed the role of urokinase plasminogen activator (uPA) and its proteolytic activity in myofibroblast differentiation and the early proliferation following mechanical injury of the rat carotid adventitia. The effects of perivascular application of recombinant uPA (r-uPA), proteolytically inactive r-uPA(H/Q) and uPA neutralizing antibody were evaluated 4 days after surgical injury to the adventitia. The phenotype of adventitial cells was assessed using anti-alpha-smooth muscle actin (alpha-SM actin) antibody, anti-SM heavy chain myosin, anti-high-molecular-weight caldesmon, anti-smoothelin and anti-ED-1 antibodies, proliferation by the expression of proliferating cell nuclear antigen, and the size of the adventitia by quantitative morphometry. Four days after injury, the intensive immunostaining for urokinase appeared in the rat carotid artery adventitia. At the same time, the frequency of alpha-SM actin-positive adventitial cells was 1.8+/-1.1% in uninjured arteries and 25.2+/-5.4% in injured arteries (p<0.05), and the respective frequency of ED-1-positive cells 1.5+/-1.1 and 25.0+/-5.2%. The application of exogenous r-uPA doubled the numbers of alpha-SM actin-positive adventitial cells to 55.7+/-6.8% (p<0.05). ED-1-positive cells and proliferating cell nuclear antigen-positive cells as well as the size of the adventitia were also significantly increased after r-uPA compared with injury alone. In contrast, the proteolytically inactive r-uPA(H/Q) did not affect any parameters. The application of uPA neutralizing antibody attenuated the frequency of alpha-SM actin-positive cells to 12.6+/-3.5% (p<0.05), the frequency of ED-1-positive cells, and the numbers of adventitial cells. r-uPA stimulation of cultured human skin fibroblasts significantly increased the alpha-SM actin content in a concentration-dependent manner. In contrast, r-uPAH/Q did not induce changes in alpha-SM actin content. We conclude that uPA, which is upregulated in the injured adventitia, can augment adventitial cell accumulation, including myofibroblasts, and adventitia growth early after injury of the rat carotid artery adventitia by mechanisms involving proteolysis.

Multiple signaling pathways mediate compaction of collagen matrices by EGF-stimulated fibroblasts

Fibroblasts stimulated by EGF within collagen matrices generate contraction forces that are likely of importance to cell migration and matrix compaction during wound healing. We have employed an in vitro fibroblast-embedded collagen matrix compaction assay to ascertain signaling pathway components downstream of EGFR activation leading to generation and transmission of contractile force. EGF compacts this floating collagen matrix to a similar extent as PDGF. We demonstrate that compaction requires EGFR kinase activity, yet is maximal in magnitude at intermediate EGF concentrations. This suggests that transmission of EGFR-induced contractile force to the matrix can be mitigated by consequent anti-adhesive effects of EGFR signaling in a dose-dependent manner. Treatment with pharmacological inhibitors demonstrated involvement of the signaling components extracellular signal-regulated kinase (ERK), Rho kinase, and myosin light chain kinase (MLCK) in the force generation and/or transmission process. Moreover, treatment with the pan-calpain inhibitor ALLN and isoform-specific downregulation of m-calpain (CAPN2) using RNA interference determined m-calpain to be a key component of the EGF-induced force response. ALLN treatment modulated the compaction response in a biphasic manner, enhancing matrix deformation to the greatest extent at intermediate concentrations. Our findings have thus identified key signals downstream of EGFR, which integrate in a complex manner to generate and transmit contractile forces to yield matrix deformation.

Mechanical strain alters gene expression in an in vitro model of hypertrophic scarring

Fibroblasts represent a highly mechanoresponsive cell type known to play key roles in normal and pathologic processes such as wound healing, joint contracture, and hypertrophic scarring. In this study, we used a novel fibroblast-populated collagen lattice (FPCL) isometric tension model, allowing us to apply graded biaxial loads to dermal fibroblasts in a 3-dimensional matrix. Cell morphology demonstrated dose-dependent transition from round cells lacking stress fibers in nonloaded lattices to a broad, elongated morphology with prominent actin stress fibers in 800-mg-loaded lattices. Using quantitative real-time RT-PCR, a dose dependent induction of both collagen-1 and collagen-3 mRNA up to 2.8- and 3-fold, respectively, as well as a 2.5-fold induction of MMP-1 (collagenase) over unloaded FPCLs was observed. Quantitative expression of the proapoptotic gene Bax was down-regulated over 4-fold in mechanically strained FPCLs. These results suggest that mechanical strain up-regulates matrix remodeling genes and down-regulates normal cellular apoptosis, resulting in more cells, each of which produces more matrix. This "double burden" may underlie the pathophysiology of hypertrophic scars and other fibrotic processes in vivo.

Contribution of activin receptor–like kinase 5 (transforming growth factor β receptor type I) signaling to the fibrotic phenotype of scleroderma fibroblasts

OBJECTIVE

To use a specific transforming growth factor beta receptor type I (TGFbetaRI; activin receptor-like kinase 5 [ALK-5]) kinase inhibitor (SD208) to determine the role of activation of the TGFbetaRI kinase (ALK-5) in maintaining the profibrotic phenotype of dermal fibroblasts in systemic sclerosis (SSc).

METHODS

The effect of SD208 on the expression of key biochemical markers of the fibrotic phenotype was compared in fibroblasts cultured from clinically involved (lesional) and clinically uninvolved skin of patients with diffuse cutaneous SSc (dcSSc) and in fibroblasts from healthy controls matched for age, sex, and anatomic site. Protein expression was compared together with the ability of fibroblasts to adhere to the extracellular matrix and to remodel and contract a free-floating fibroblast-populated type I collagen lattice.

RESULTS

Inhibiting TGFbetaRI kinase reduced the expression of a cohort of fibrotic markers by dermal fibroblasts from patients with dcSSc, including type I collagen and beta1 integrin. Moreover, inhibition also attenuated the elevated adhesive and contractile abilities of dcSSc fibroblasts.

CONCLUSION

Our data suggest that some of the key profibrotic features of lesional SSc fibroblasts are dependent upon ALK-5 activity. Thus, TGFbetaRI kinase-mediated signaling may contribute to dermal fibrosis in dcSSc.

The origins and regulation of tissue tension: identification of collagen tension-fixation process in vitro

The absence of a controllable in vitro model of soft tissue remodeling is a major impediment, limiting our understanding of collagen pathologies, tissue repair and engineering. Using 3D fibroblast-collagen lattice model, we have quantified changes in matrix tension and material properties following remodeling by blockade of cell-generated tension with cytochalasin D. This demonstrated a time-dependent shortening of the collagen network, progressively stabilized into a built-in tension within the matrix. This was differentially enhanced by TGFB1 and mechanical loading to give subtle control of the new, remodeled matrix material properties. Through this model, we have been able to identify the 'tension remodeling' process, by which cells control material properties in response to environmental factors.

Stromagenesis: the changing face of fibroblastic microenvironments during tumor progression

During tumorigenesis, reciprocal changes in stromal fibroblasts and tumor cells induce changes to the neoplastic microenvironmental landscape. In stromagenesis, both the complex network of bi-directional stromal fibroblastic signaling pathways and the stromal extracellular matrix are modified. The presence of a 'primed' stroma during the early, reversible stage of tumorigenesis is optimal for stromal-directed therapeutic intervention. Three-dimensional (3D) cell culture systems have been developed that mimic the in vivo microenvironment. These systems provide unique experimental tools to identify early alterations in stromagenesis that are supportive of tumor progression with the ultimate goal of blocking neoplastic permissiveness and restoring normal phenotypes.

Smad3 mediates TGF-beta1-induced collagen gel contraction by human lung fibroblasts

Transforming growth factor-beta1 (TGF-beta1) is a key mediator in tissue repair and fibrosis. Using small interference RNA (siRNA), the role of Smad2 and Smad3 in TGF-beta stimulation of human lung fibroblast contraction of collagenous matrix and induction of alpha-SMA and the role of alpha-SMA in contraction were assessed. HFL-1 cells were transfected with Smad2, Smad3 or control-siRNA, and cultured in floating Type I collagen gels +/- -TGF-beta1. TGF-beta1 augmented gel contraction in Smad2-siRNA- and control-siRNA-treated cells, but had no effect in Smad3-siRNA-treated cells. Similarly, TGF-beta1 upregulated alpha-SMA in Smad2-siRNA- and control-siRNA-treated cells, but had no effect on Smad3-siRNA-treated cells. Alpha-SMA-siRNA-treated cells did not contact the collagen gels with or without TGF-beta1, suggesting alpha-SMA is required for gel contraction. Thus, Smad3 mediates TGF-beta1-induced contraction and alpha-SMA induction in human lung fibroblasts. Smad3, therefore, could be a target for blocking contraction of human fibrotic tissue induced by TGF-beta1.

The fibroblast-populated collagen matrix as a model of wound healing: a review of the evidence

The fibroblast-populated collagen matrix (FPCM) has been utilized as an in vitro model of wound healing for more than 2 decades. It offers a reasonable approximation of the healing wound during the phases of established granulation tissue and early scar. The gross and microscopic morphology of the FPCM and the healing wound are similar at analogous phases. The processes of proliferation, survival/apoptosis, protein synthesis, and contraction act in similar directions in these two models, and the response to exogenous agents also is consistent between them. If its limitations are respected, then the FPCM can be used as a model of the healing wound.

The relative magnitudes of endothelial force generation and matrix stiffness modulate capillary morphogenesis in vitro

When suspended in collagen gels, endothelial cells elongate and form capillary-like networks containing lumens. Human blood outgrowth endothelial cells (HBOEC) suspended in relatively rigid 3 mg/ml floating collagen gels, formed in vivo-like, thin, branched multi-cellular structures with small, thick-walled lumens, while human umbilical vein endothelial cells (HUVEC) formed fewer multi-cellular structures, had a spread appearance, and had larger lumens. HBOEC exert more traction on collagen gels than HUVEC as evidenced by greater contraction of floating gels. When the stiffness of floating gels was decreased by decreasing the collagen concentration from 3 to 1.5 mg/ml, HUVEC contracted gels more and formed thin, multi-cellular structures with small lumens, similar in appearance to HBOEC in floating 3 mg/ml gels. In contrast to floating gels, traction forces exerted by cells in mechanically constrained gels encounter considerable resistance. In constrained collagen gels (3 mg/ml), both cell types appeared spread, formed structures with fewer cells, had larger, thinner-walled lumens than in floating gels, and showed prominent actin stress fibers, not seen in floating gels. These results suggest that the relative magnitudes of cellular force generation and apparent matrix stiffness modulate capillary morphogenesis in vitro and that this balance may play a role in regulating angiogenesis in vivo.

Mechanisms of force generation and transmission by myofibroblasts

The myofibroblast has been shown to have a key role in tissue reconstruction after injury and pathological changes characterized by fibrosis. Force generation by the myofibroblast depends on the isometric contraction of stress fibers containing alpha-smooth muscle actin, and is mediated through Rho/Rho-kinase. The force is transmitted by vinculin and tensin-containing "supermature" focal adhesions, which connect stress fibers with the extracellular matrix. Force production and transmission by the myofibroblast are modulated by the coordinated action of cytokines, extracellular matrix components and mechanical tension. Regulation of these phenomena will be important for therapeutic strategies aimed at influencing fibrocontractive diseases.

Endothelin-1 promotes myofibroblast induction through the ETA receptor via a rac/phosphoinositide 3-kinase/Akt-dependent pathway and is essential for the enhanced contractile phenotype of fibrotic fibroblasts

The endothelins are a family of endothelium-derived peptides that possess a variety of functions, including vasoconstriction. Endothelin-1 (ET-1) is up-regulated during tissue repair and promotes myofibroblast contraction and migration, hence contributing to matrix remodeling during tissue repair. Here, we show that addition of ET-1 to normal lung fibroblasts induces expression of proteins that contribute to a contractile phenotype, including alpha-smooth muscle actin (alpha-SMA), ezrin, moesin, and paxillin. We confirm that ET-1 enhances the ability of lung fibroblasts to contract extracellular matrix, a function essential for tissue repair, through induction of de novo protein synthesis. Blockade of the Akt/phosphoinositide 3-kinase (PI3-kinase) pathway with LY294002 and wortmannin prevents the ability of ET-1 to induce alpha-SMA, ezrin, paxillin, and moesin and to promote matrix contraction. Dominant negative rac and Akt blocked the ability of ET-1 to promote formation of alpha-SMA stress fibers. Using specific ET-1 receptor inhibitors, we show that ET-1 induces collagen matrix contraction through the ETA, but not the ETB, receptor. Relative to normal pulmonary fibroblasts, fibroblasts cultured from scars of patients with the fibrotic disease systemic sclerosis (scleroderma) show enhanced ET-1 expression and binding. Systemic sclerosis lung fibroblasts show increased ability to contract a collagen matrix and elevated expression of the procontractile proteins alpha-SMA, ezrin, paxillin, and moesin, which are greatly reduced by antagonizing endogenous ET-1 signaling. Thus, blocking ET-1 or the PI3-kinase/Akt cascades might be beneficial in reducing scar formation in pulmonary fibrosis.

Human adipose tissue-derived cells delay re-epithelialization in comparison with skin fibroblasts in organotypic skin culture

BACKGROUND

Wound healing of deep and extensive burns can induce hypertrophic scar formation. During the early steps of wound healing fibroblasts migrate into the wounded area. Fibroblastic cells present in tissues other than dermis may also migrate into the wounded area and participate in the wound healing process.

OBJECTIVES

To examine the influence of human fibroblastic cells derived from subcutaneous fat or dermis on epidermal morphogenesis in vitro.

METHODS

We prepared human skin equivalents (HSEs) made of a collagen type I matrix populated either with dermal fibroblasts or adipose tissue-derived cells (ADCs), on top of which keratinocytes were seeded and subsequently grown at the air-liquid interface.

RESULTS

A fully differentiated epidermis was formed on matrices populated with ADCs. However, the HSE formed differed in a number of features from HSE generated with dermal fibroblasts. The major differences included: marked contraction of the dermal matrix, low lateral migration of keratinocytes, high keratin 17 expression indicating increased keratinocyte activation, delayed deposition of collagen IV at the epidermal/matrix junction, accumulation of alpha-smooth muscle actin-positive cells only underneath the epidermal compartment and positioning of these cells in a direction parallel to the epidermal compartment. The latter two phenomena have also been found in scar tissue.

CONCLUSIONS

The possibility of generating HSEs with different cell types represents an attractive approach for in vitro studies focusing on the mechanism of wound healing.

Heart valve and arterial tissue engineering

In the industrialized world, cardiovascular disease alone is responsible for almost half of all deaths. Many of the conditions can be treated successfully with surgery, often using transplantation techniques; however, autologous vessels or human-donated organs are in short supply. Tissue engineering aims to create specific, matching grafts by growing cells on appropriate matrices, but there are many steps between the research laboratory and the operating theatre. Neo-tissues must be effective, durable, non-thrombogenic and non-immunogenic. Scaffolds should be bio-compatible, porous (to allow cell/cell communication) and amenable to surgery. In the early days of cardiovascular tissue engineering, autologous or allogenic cells were grown on inert matrices, but patency and thrombogenicity of grafts were disappointing. The current ethos is toward appropriate cell types grown in (most often) a polymeric matrix that degrades at a rate compatible with the cells' production of their own extracellular matrical proteins, thus gradually replacing the graft with a living counterpart. The geometry is crucial. Computer models have been made of valves, and these are used as three-dimensional patterns for mass-production of implant scaffolds. Vessel walls have integral connective tissue architecture, and application of physiological level mechanical forces conditions bio-engineered components to align in precise orientation. This article reviews the concepts involved and successes achieved to date.

Dimensions and dynamics in integrin function

Integrins play crucial roles in cell adhesion, migration, and signaling by providing transmembrane links between the extracellular matrix and the cytoskeleton. Integrins cluster in macromolecular complexes to generate cell-matrix adhesions such as focal adhesions. In this mini-review, we compare certain integrin-based biological responses and signaling during cell interactions with standard 2D cell culture versus 3D matrices. Besides responding to the composition of the matrix, cells sense and react to physical properties that include three-dimensionality and rigidity. In routine cell culture, fibroblasts and mesenchymal cells appear to use focal adhesions as anchors. They then use intracellular actomyosin contractility and dynamic, directional integrin movements to stretch cell-surface fibronectin and to generate characteristic long fibrils of fibronectin in "fibrillar adhesions". Some cells in culture proceed to produce dense, three-dimensional matrices similar to in vivo matrix, as opposed to the flat, rigid, two-dimensional surfaces habitually used for cell culture. Cells within such more natural 3D matrices form a distinctive class of adhesion termed "3D-matrix adhesions". These 3D adhesions show distinctive morphology and molecular composition. Their formation is heavily dependent on interactions between integrin alpha 5 beta 1 and fibronectin. Cells adhere much more rapidly to 3D matrices. They also show more rapid morphological changes, migration, and proliferation compared to most 2D matrices or 3D collagen gels. Particularly notable are low levels of tyrosine phosphorylation of focal adhesion kinase and moderate increases in activated mitogen-activated protein kinase. These findings underscore the importance of the dimensionality and dynamics of matrix substrates in cellular responses to the extracellular matrix.

Smad3 mediates the TGF-beta-induced contraction of type I collagen gels by mouse embryo fibroblasts

TGF-beta signals through TGF-beta receptors and Smad proteins. TGF-beta also augments fibroblast-mediated collagen gel contraction, an in vitro model of connective tissue remodeling. To investigate the importance of Smad2 or Smad3 in this augmentation process, embryo-derived fibroblasts from mice lacking expression of Smad2 or Smad3 genes were cast into native type I collagen gels. Fibroblast-populated gels were then released into 0.2% FCS-DMEM alone or with recombinant human TGF-beta1, beta2, beta3, or recombinant rat PDGF-BB. Gel contraction was determined using an image analyzer. All three isoforms of TGF-beta significantly augmented contraction of collagen gels mediated by fibroblasts with genotypes of Smad2 knockout (S2KO), Smad2 wildtype (S2WT), and Smad3 wildtype (S3WT), but not Smad3 knockout (S3KO) mice. PDGF-BB augmented collagen gel contraction by all fibroblast types. These results suggest that expression of Smad3 but not Smad2 may be critical in TGF-beta augmentation of fibroblast-mediated collagen gel contraction. Thus, the Smad3 gene could be a target for blocking contraction of fibrotic tissue induced by TGF-beta.

Mechanobiology of force transduction in dermal tissue

BACKGROUND/AIMS

The influence of mechanical forces on skin has been examined since 1861 when Langer first reported the existence of lines of tension in cadaver skin. Internal tension in the dermis is not only passively transferred to the epidermis but also gives rise to active cell-extracellular matrix and cell-cell mechanical interactions that may be an important part of the homeostatic processes that are involved in normal skin metabolism. The purpose of this review is to analyse how internal and external mechanical loads are applied at the macromolecular and cellular levels in the epidermis and dermis.

METHODS

A review of the literature suggests that internal and external forces applied to dermal cells appear to be involved in mechanochemical transduction processes involving both cell-cell and cell-extra-cellular matrix (ECM) interactions. Internal forces present in dermis are the result of passive tension that is incorporated into the collagen fiber network during development. Active tension generated by fibroblasts involves specific interactions between cell membrane integrins and macromolecules found in the ECM, especially collagen fibrils. Forces appear to be transduced at the cell-ECM interface via re-arrangement of cytoskeletal elements, activation of stretch-induced changes in ion channels, cell contraction at adherens junctions, activation of cell membrane-associated secondary messenger pathways and through growth factor-like activities that influence cellular proliferation and protein synthesis.

CONCLUSIONS

Internal and external mechanical loading appears to affect skin biology through mechanochemical transduction processes. Further studies are needed to understand how mechanical forces, energy storage and conversion of mechanical energy into changes in chemical potential of small and large macromolecules may occur and influence the metabolism of dermal cells.

Synovial Remodeling Process Induced by Type V Collagen Immunization in Rabbits

The pathogenesis of diffuse connective tissue diseases is still unknown despite studies of the autoimmunity aspects related to extracellular matrix elements, mainly the collagens. Articulations are frequently affected by the synovitis process in these diseases. The objective of the present study was to verify the morphologic aspects of the synovial membrane of rabbits immunized with type V collagen, which has some particular characteristics 75 days after the first antigen inoculation and when compared to control animals. The synovial membrane of the animals sacrificed after 75 days of immunization presented an intense remodeling phenomenon along the connective tissue screen and interlobular septa of the adipose-muscle tissue screen compartment. The remodeling process determined type I and III collagen fiber depositions in the vascular and connective tissue compartments of the synovial membrane. The nutrient vessels of the adipose-muscle compartment showed a similar remodeling process, which resulted in small vessel occlusion. Few residual inflammatory foci consisting of monocytes and eosinophils were observed. Thus, our experimental model reproduces morphologic changes in different tissues, characterized by an extracellular matrix remodeling process similar to those observed in many diffuse connective tissue diseases such as systemic lupus erytematosus and scleroderma. Therefore, this model could be useful in understanding the pathogenesis and the treatment of these diseases.

Phenotypic differences between dermal fibroblasts from different body sites determine their responses to tension and TGFbeta1

BACKGROUND

Wounds in the nonglabrous skin of keloid-prone individuals tend to cause large disordered accumulations of collagen which extend beyond the original margins of the wound. In addition to abnormalities in keloid fibroblasts, comparison of dermal fibroblasts derived from nonwounded glabrous or nonglabrous skin revealed differences that may account for the observed location of keloids.

METHODS

Fibroblast apoptosis and the cellular content of alpha-smooth-muscle actin, TGFbeta1 receptorII and ED-A fibronectin were estimated by FACS analysis. The effects of TGFbeta1 and serum were examined.

RESULTS

In monolayer cultures non-glabrous fibroblasts were slower growing, had higher granularity and accumulated more alpha-smooth-muscle actin than fibroblasts from glabrous tissues. Keloid fibroblasts had the highest level of alpha-smooth-muscle actin in parallel with their expression level of ED-A fibronectin. TGFbeta1 positively regulated alpha-smooth-muscle actin expression in all fibroblast cultures, although its effects on apoptosis in fibroblasts from glabrous and non-glabrous tissues were found to differ. The presence of collagen I in the ECM resulted in reduction of alpha-smooth-muscle actin. A considerable percentage of the apoptotic fibroblasts in attached gels were alpha-smooth-muscle actin positive. The extent of apoptosis correlated positively with increased cell and matrix relaxation. TGFbeta1 was unable to overcome this apoptotic effect of matrix relaxation.

CONCLUSION

The presence of myofibroblasts and the apoptosis level can be regulated by both TGFbeta1 and by the extracellular matrix. However, reduction of tension in the matrix is the critical determinant. This predicts that the tension in the wound bed determines the type of scar at different body sites.

Cell interactions with three-dimensional matrices

Signaling and other cellular functions differ in three-dimensional compared with two-dimensional systems. Cell adhesion structures can evolve in vitro towards in-vivo-like adhesions with distinct biological activities. In this review, we examine recent advances in studies of interactions of fibroblasts with collagen gels and fibronectin-containing matrices that mimic in vivo three-dimensional microenvironments. These three-dimensional systems are illuminating mechanisms of cell-matrix interactions in living organisms.

Myofibroblasts and mechano-regulation of connective tissue remodelling

During the past 20 years, it has become generally accepted that the modulation of fibroblastic cells towards the myofibroblastic phenotype, with acquisition of specialized contractile features, is essential for connective-tissue remodelling during normal and pathological wound healing. Yet the myofibroblast still remains one of the most enigmatic of cells, not least owing to its transient appearance in association with connective-tissue injury and to the difficulties in establishing its role in the production of tissue contracture. It is clear that our understanding of the myofibroblast its origins, functions and molecular regulation will have a profound influence on the future effectiveness not only of tissue engineering but also of regenerative medicine generally.

Tissue engineering of biological cardiovascular system surrogates

Cardiovascular diseases are common in ageing communities globally. This fact is most striking in the industrialised world where the aged population makes up a large proportion of society. Elderly patients are frequently treated surgically with grafts to replace damaged tissues and vessels. The number of human-donated components is insufficient and synthetic surrogates are sought. These might be wholly mechanical, wholly biological, or tissue engineered complexes of cells and their products growing in a scaffold. At present, many such composites exist with potential for use as substitutes for specific blood vessels. The challenges of producing tissue engineered heart valves are now being widely explored. Neotissues must provide an effective, durable, non-thrombogenic and non-immunogenic substitute that will fulfil the purpose of the natural tissue. The aims and scope of this paper are to review current and novel concepts in the field of tissue engineering of biological cardiovascular system surrogates. Mechanical stresses and strains on cardiovascular cells in vitro have been recognised and can be measured by a culture force monitor. Physiological stresses can be generated by a tensioning culture force monitor and applied to engineered tissue, aligning the cells and mimicking arterial wall architecture. The hydrostatic forces a vessel experiences and mechanical parameters of blood vessels can be studied in the tubular culture system of a multi-cue bioreactor.