Differences in the regulation of fibroblast contraction of floating versus stressed collagen matrices

A review on static splinting therapy to prevent burn scar contracture: do clinical and experimental data warrant its clinical application?

BACKGROUND

Static splinting therapy is widely considered an essential part in burn rehabilitation to prevent scar contractures in the early phase of wound healing. However, scar contractures are still a common complication. In this article we review the information concerning the incidence of scar contracture, the effectiveness of static splinting therapy in preventing scar contractures, and specifically focus on the - possible - working mechanism of static-splinting, i.e. mechanical load, at the cellular and molecular level of the healing burn wound.

METHOD

A literature search was done including Pubmed, Cochrane library, CINAHL and PEDRO.

RESULTS

Incidence of scar contracture in patients with burns varied from 5% to 40%. No strong evidence for the effectiveness of static splinting therapy in preventing scar contracture was found, whereas in vitro and animal studies demonstrated that mechanical tension will stimulate the myofibroblast activity, resulting in the synthesis of new extracellular matrix and the maintenance of their contractile activity.

CONCLUSION

The effect of mechanical tension on the wound healing process suggests that static splinting therapy may counteract its own purpose. This review stresses the need for randomised controlled clinical trials to establish if static splinting to prevent contractures is a well-considered intervention or just wishful thinking.

Applications of knitted mesh fabrication techniques to scaffolds for tissue engineering and regenerative medicine

Knitting is an ancient and yet, a fresh technique. It has a history of no less than 1,000 years. The development of tissue engineering and regenerative medicine provides a new role for knitting. Several meshes knitted from synthetic or biological materials have been designed and applied, either alone, to strengthen materials for the patching of soft tissues, or in combination with other kinds of biomaterials, such as collagen and fibroin, to repair or replace damaged tissues/organs. In the latter case, studies have demonstrated that knitted mesh scaffolds (KMSs) possess excellent mechanical properties and can promote more effective tissue repair, ligament/tendon/cartilage regeneration, pipe-like-organ reconstruction, etc. In the process of tissue regeneration induced by scaffolds, an important synergic relationship emerges between the three-dimensional microstructure and the mechanical properties of scaffolds. This paper presents a comprehensive overview of the status and future prospects of knitted meshes and its KMSs for tissue engineering and regenerative medicine.

Pathological signaling via platelet-derived growth factor receptor {alpha} involves chronic activation of Akt and suppression of p53

In contrast to direct activation of platelet-derived growth factor (PDGF) receptor α (PDGFRα) via PDGF, indirect activation via growth factors outside the PDGF family failed to induce dimerization, internalization, and degradation of PDGFRα. Chronically activated, monomeric PDGFRα induced prolonged activation of Akt and suppressed the level of p53. These events were sufficient to promote both cellular responses (proliferation, survival, and contraction) that are intrinsic to proliferative vitreoretinopathy (PVR) and induce the disease itself. This signature signaling pathway appeared to extend beyond PVR since deregulating PDGFRα in ways that promote solid tumors also resulted in chronic activation of Akt and a decline in the level of p53.

Diminished type III collagen promotes myofibroblast differentiation and increases scar deposition in cutaneous wound healing

The repair of cutaneous wounds in the postnatal animal is associated with the development of scar tissue. Directing cell activities to efficiently heal wounds while minimizing the development of scar tissue is a major goal of wound management and the focus of intensive research efforts. Type III collagen (Col3), expressed in early granulation tissue, has been proposed to play a prominent role in cutaneous wound repair, although little is known about its role in this process. To establish the role of Col3 in cutaneous wound repair, we examined the healing of excisional wounds in a previously described murine model of Col3 deficiency. Col3 deficiency (Col3+/-) in aged mice resulted in accelerated wound closure with increased wound contraction. In addition, Col3-deficient mice had increased myofibroblast density in the wound granulation tissue as evidenced by an increased expression of the myofibroblast marker, α-smooth muscle actin. In vitro, dermal fibroblasts obtained from Col3-deficient embryos (Col3+/- and -/-) were more efficient at collagen gel contraction and also displayed increased myofibroblast differentiation compared to those harvested from wild-type (Col3+/+) embryos. Finally, wounds from Col3-deficient mice also had significantly more scar tissue area on day 21 post-wounding compared to wild-type mice. The effect of Col3 expression on myofibroblast differentiation and scar formation in this model suggests a previously undefined role for this ECM protein in tissue regeneration and repair.

Regulation of myofibroblast activities: calcium pulls some strings behind the scene

Myofibroblast-induced remodeling of collagenous extracellular matrix is a key component of our body's strategy to rapidly and efficiently repair damaged tissues; thus myofibroblast activity is considered crucial in assuring the mechanical integrity of vital organs and tissues after injury. Typical examples of beneficial myofibroblast activities are scarring after myocardial infarct and repair of damaged connective tissues including dermis, tendon, bone, and cartilage. However, deregulation of myofibroblast contraction causes the tissue deformities that characterize hypertrophic scars as well as organ fibrosis that ultimately leads to heart, lung, liver and kidney failure. The phenotypic features of the myofibroblast, within a spectrum going from the fibroblast to the smooth muscle cell, raise the question as to whether it regulates contraction in a fibroblast- or muscle-like fashion. In this review, we attempt to elucidate this point with a particular focus on the role of calcium signaling. We suggest that calcium plays a central role in myofibroblast biological activity not only in regulating contraction but also in mediating intracellular and extracellular mechanical signals, structurally organizing the contractile actin-myosin cytoskeleton, and establishing lines of intercellular communication.

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.

Mammalian Diaphanous (mDia) may be involved in the signal transduction of sphingosine-1-phosphate on developing actin stress fiber of human fibroblasts

Sphingosine 1-phosphate (S1P) is a biologically active lipid mediator with many pivotal roles in the regulation of cell growth, migration, differentiation, and apoptosis. However, signal transduction mediated by S1P in human fibroblasts is still unclear. We investigated signal transduction by S1P in human fibroblasts using collagen matrix contraction in order to explore whether or not S1P could be applied for the treatment of skin wound healing. We found that S1P promoted floating collagen matrices' contraction, for the in vitro model of initial phase wound contraction, in which some kinds of G protein, such as Gialpha, Rac 1, and Rho, were involved. However, Rho-associated coiled-coil forming kinase (ROCK) was partially involved in S1P-promoting floating collagen matrices contraction. Mammalian Diaphanous (mDia) as well as ROCK have been identified to be putative downstream target molecules of Rho. In mDia-silenced cells, the ROCK inhibitor suppressed actin stress fiber formation regardless of the presence or absence of S1P. Our results indicate that mDia as well as ROCK may be situated downstream of Gialpha, Rac1, and Rho to induce actin stress fiber development by human fibroblasts stimulated with SIP.

Controlled growth factor delivery for tissue engineering

Growth factors play a crucial role in information transfer between cells and their microenvironment in tissue engineering and regeneration. They initiate their action by binding to specific receptors on the surface of target cells and the chemical identity, concentration, duration, and context of these growth factors contain information that dictates cell fate. Hence, the importance of exogenous delivery of these molecules in tissue engineering is unsurprising, considering their importance for tissue regeneration. However, the short half-lives of growth factors, their relatively large size, slow tissue penetration, and their potential toxicity at high systemic levels, suggest that conventional routes of administration are unlikely to be effective. In this review, we provide an overview of the design criteria for growth factor delivery vehicles with respect to the growth factor itself and the microenvironment for delivery. We discuss various methodologies that could be adopted to achieve this localized delivery, and strategies using polymers as delivery vehicles in particular.

Growth factors outside the PDGF family drive experimental PVR

PURPOSE

Proliferative vitreoretinopathy (PVR) is a recurring and problematic disease for which there is no pharmacologic treatment. Platelet-derived growth factor (PDGF) in the vitreous is associated with experimental and clinical PVR. Furthermore, PDGF receptors (PDGFRs) are present and activated in epiretinal membranes of patient donors, and they are essential for experimental PVR. These observations suggest that PVR arises at least in part from PDGF/PDGFR-driven events. The goal of this study was to determine whether PDGFs were a potential therapeutic target for PVR.

METHODS

Experimental PVR was induced in rabbits by injecting fibroblasts. Vitreous specimens were collected from experimental rabbits or from patients undergoing vitrectomy to repair retinal detachment. A neutralizing PDGF antibody and a PDGF Trap were tested for their ability to prevent experimental PVR. Activation of PDGFR was monitored by antiphosphotyrosine Western blot analysis of immunoprecipitated PDGFRs. Contraction of collagen gels was monitored in vitro.

RESULTS

Neutralizing vitreal PDGFs did not effectively attenuate PVR, even though the reagents used potently blocked PDGF-dependent activation of the PDGF alpha receptor (PDGFRalpha). Vitreal growth factors outside the PDGF family modestly activated PDGFRalpha and appeared to do so without engaging the ligand-binding domain of PDGFRalpha. This indirect route to activate PDGFRalpha had profound functional consequences. It promoted the contraction of collagen gels and appeared sufficient to drive experimental PVR.

CONCLUSIONS

Although PDGF appears to be a poor therapeutic target, PDGFRalpha is particularly attractive because it can be activated by a much larger spectrum of vitreal growth factors than previously appreciated.

Regulation of matrix contraction in chronic venous disease

OBJECTIVE

The role of TGF-beta(1) in venous ulcer healing and the signalling cascades regulating dermal fibroblast function are poorly understood. To elucidate these processes, we hypothesized that TGF-beta(1) facilitates wound healing by increasing chronic venous insufficiency (CVI) induced matrix contraction via intracellular cross-talk between TGF-beta(1) and the ERK-1/2 MAP kinase signalling cascades.

METHODS

Fibroblasts isolated from calf biopsies (LC) of patients with different severity of CVI (CEAP, Clinical Etiological Anatomical Pathological classes) were seeded into 200 microl collagen gels under isometric conditions. Fibroblasts from neonatal foreskins (HS68), non-CVI patients (NC), and the ipsilateral normal thigh of each CVI patient (LT) served as controls. Thirteen patients with CVI (class 2, n=5; class 4, n=5; class 6, n=3) and 2 non-CVI controls (NC, n=2) were included in the study. All experimental conditions were determined by dose-response and time-course experiments. Gels were cultured with/without 0.1 ng/ml TGF-beta(1) and with/without 50 microM PD98059 (MEK and downstream-MAPK inhibitor). Additional patient fibroblasts were transfected with constitutively active Ras (pCMV-Ras) or an empty vector (pCMV-beta) with/without 0.1 ng/ml TGF-beta(1) and with/without 50 microm PD98059. The collagen gels were released after 4 days and the percent contraction was determined by area measurements using image analysis. Differences in alpha-smooth muscle actin (alpha-SMA) and ERK-1/2 MAPK (phosphorylated and total) protein levels were analyzed with western blotting.

RESULTS

Gels seeded with CVI fibroblasts contracted more than HS68, NC and LT fibroblasts. Inhibition of MAPK and/or stimulation with TGF-beta(1) increased the contraction of LC gels compared to unstimulated controls. Agonist induced gel contraction correlated with CVI disease severity. alpha-SMA protein expression in LC fibroblasts increased with MAPK inhibition with/without TGF-beta(1) stimulation, and correlated with the degree of gel contraction. Transfection with pCMV-Ras (activator of ERK-1/2) inhibited gel contraction; this inhibition was not reversed by addition of TGF-beta(1). Transfection with the pCMV-beta empty vector had no effect on gel contraction.

CONCLUSIONS

TGF-beta1 stimulation of CVI patient fibroblasts grown in 3D collagen gels results in conversion to a contractile phenotype through upregulation of alpha-SMA, and in enhanced gel contraction. Inhibition of MAPK further increases gel contraction, while Ras activation of ERK-1/2 inhibits TGF-beta1-induced gel contraction. These responses correlate with increasing CEAP severity. CVI fibroblast mediated gel contraction is therefore regulated through cross-talk between the ERK-1/2 MAPK and TGF-beta(1) signalling cascades. These data identify potentially clinically relevant therapeutic molecular targets that could enhance matrix contraction and thereby improve venous ulcer wound healing.

Fibroblast elongation and dendritic extensions in constrained versus unconstrained microtissues

Cytoskeletal tension is fundamental to many biological processes, including germ layer sorting during embryogenesis [Krieg et al., 2008]. In vitro, such tension influences cell sorting in self-assembled, 3D microtissues and can be of sufficient magnitude to cause complex-shaped microtissue failure [Dean et al., 2007]. To examine the process of failure under cell-derived tension, we subjected normal human fibroblasts (NHFs) to directed self-assembly [Dean et al., 2007] in micro-molds designed to yield self-constraining microtissues. As cells contracted in this assay, the constrained microtissues narrowed, thinned and ultimately failed at their midpoints. By adding small numbers of GFP+ cells, changes in cell movement and morphology were assessed and compared to those of unconstrained microtissues. We found that cells formed numerous dendritic extensions within an hour of self-assembly and retracted these extensions as they elongated up to 30 times their initial diameter ( approximately 600 microm) just prior to failure. Surprisingly, significant coordination in cell motility was observed over large distances within microtissues. Pharmacologic interventions showed that failure was myosin II and Rho kinase dependent and inhibition of failure resulted in shorter cells with greater numbers of extensions. These findings further our understanding of cellular self-assembly and introduce the use of GFP+ cells with directed self-assembly as a scaffold-free analogue to fibroblast-populated collagen gels (FPCGs).

Aortic carboxypeptidase-like protein is expressed in fibrotic human lung and its absence protects against bleomycin-induced lung fibrosis

The pathological hallmarks of idiopathic pulmonary fibrosis include proliferating fibroblasts and myofibroblasts, as well as excessive collagen matrix deposition. In addition, both myofibroblast contraction and remodeling of the collagen-rich matrix contribute to the abnormal structure and function of the fibrotic lung. Little is known, however, about collagen-associated proteins that promote fibroblast and myofibroblast retention, as well as the proliferation of these cells on the extracellular matrix. In this study, we demonstrate that aortic carboxypeptidase-like protein (ACLP), a collagen-associated protein with a discoidin-like domain, is expressed at high levels in human fibrotic lung tissue and human fibroblasts, and that its expression increases markedly in the lungs of bleomycin-injured mice. Importantly, ACLP-deficient mice accumulated significantly fewer myofibroblasts and less collagen in the lung after bleomycin injury, as compared with wild-type controls, despite equivalent levels of bleomycin-induced inflammation. ACLP that is secreted by lung fibroblasts was retained on fibrillar collagen, and ACLP-deficient lung fibroblasts that were cultured on collagen exhibited changes in cell spreading, proliferation, and contraction of the collagen matrix. Finally, the addition of recombinant discoidin-like domain of ACLP to cultured ACLP-deficient lung fibroblasts restored cell spreading and increased the contraction of collagen gels. Therefore, both ACLP and its discoidin-like domain may be novel targets for anti-myofibroblast-based therapies for the treatment of pulmonary fibrosis.

Mechanotransduction in development: a growing role for contractility

Mechanotransduction research has focused historically on how externally applied forces can affect cell signalling and function. A growing body of evidence suggests that contractile forces that are generated internally by the actomyosin cytoskeleton are also important in regulating cell behaviour, and suggest a broader role for mechanotransduction in biology. Although the molecular basis for these cellular forces in mechanotransduction is being pursued in cell culture, researchers are also beginning to appreciate their contribution to in vivo developmental processes. Here, we examine the role for mechanical forces and contractility in regulating cell and tissue structure and function during development.

Selective induction of integrin beta1 by hypoxia-inducible factor: implications for wound healing

Because of localized vascular damage and increased tissue oxygen demand, wound healing occurs in a relatively hypoxic microenvironment. These features are particularly relevant to wound healing and fibrosis in chronic inflammatory conditions, such as Crohn's disease and ulcerative colitis. In these studies, we sought to identify the contribution of hypoxia to mechanisms of wound repair in a model of the intestinal submucosa. Initial studies revealed that hypoxia promotes wound healing, as modeled by an increase in intestinal fibroblast-mediated collagen gel contraction. Guided by results from transcriptional profiling, we identified the selective induction of fibroblast integrin beta1 (ITGB1) by hypoxia. Further analysis revealed that hypoxia, as well as pharmacological activators of hypoxia-inducible factor (HIF), induce fibroblast beta1 integrin mRNA, protein, and function by as much as 4-fold. Cloning and analysis of the beta1 integrin gene promoter revealed a 10 +/- 0.8-fold increase in promoter activity in response to hypoxia, and subsequent studies identified a functional DNA binding region for HIF in the ITGB1 gene promoter. Mutational analysis of the HIF binding site within the ITGB1 promoter resulted in a significant loss of ITGB1 hypoxia-inducibility. As proof of principle, studies in a murine model of colitis revealed a correlation between colitic disease severity and tissue ITGB1 expression (R(2)=0.80). Taken together, these results demonstrate that hypoxia induces fibroblast ITGB1 expression and function by transcriptional mechanisms dependent on HIF.

Microarchitecture of three-dimensional scaffolds influences cell migration behavior via junction interactions

Cell migration plays a critical role in a wide variety of physiological and pathological phenomena as well as in scaffold-based tissue engineering. Cell migration behavior is known to be governed by biochemical stimuli and cellular interactions. Biophysical processes associated with interactions between the cell and its surrounding extracellular matrix may also play a significant role in regulating migration. Although biophysical properties of two-dimensional substrates have been shown to significantly influence cell migration, elucidating factors governing migration in a three-dimensional environment is a relatively new avenue of research. Here, we investigate the effect of the three-dimensional microstructure, specifically the pore size and Young's modulus, of collagen-glycosaminoglycan scaffolds on the migratory behavior of individual mouse fibroblasts. We observe that the fibroblast migration, characterized by motile fraction as well as locomotion speed, decreases as scaffold pore size increases across a range from 90 to 150 mum. Directly testing the effects of varying strut Young's modulus on cell motility showed a biphasic relationship between cell speed and strut modulus and also indicated that mechanical factors were not responsible for the observed effect of scaffold pore size on cell motility. Instead, in-depth analysis of cell locomotion paths revealed that the distribution of junction points between scaffold struts strongly modulates motility. Strut junction interactions affect local directional persistence as well as cell speed at and away from the junctions, providing a new biophysical mechanism for the governance of cell motility by the extracellular microstructure.

Absence of cyclooxygenase-2 exacerbates hypoxia-induced pulmonary hypertension and enhances contractility of vascular smooth muscle cells

BACKGROUND

Cyclooxygenase-2 (COX-2) is upregulated in pulmonary artery smooth muscle cells (PASMCs) during hypoxia and may play a protective role in the response of the lung to hypoxia. Selective COX-2 inhibition may have detrimental pulmonary vascular consequences during hypoxia.

METHODS AND RESULTS

To investigate the role of COX-2 in the pulmonary vascular response to hypoxia, we subjected wild-type and COX-2-deficient mice to a model of chronic normobaric hypoxia. COX-2-null mice developed severe pulmonary hypertension with exaggerated elevation of right ventricular systolic pressure, significant right ventricular hypertrophy, and striking vascular remodeling after hypoxia. Pulmonary vascular remodeling in COX-2-deficient mice was characterized by PASMC hypertrophy but not increased proliferation. Furthermore, COX-2-deficient mice had significant upregulation of the endothelin-1 receptor (ET(A)) in the lung after hypoxia. Similarly, selective pharmacological inhibition of COX-2 in wild-type mice exacerbated hypoxia-induced pulmonary hypertension and resulted in PASMC hypertrophy and increased ET(A) receptor expression in pulmonary arterioles. The absence of COX-2 in vascular smooth muscle cells during hypoxia in vitro augmented traction forces and enhanced contractility of an extracellular matrix. Treatment of COX-2-deficient PASMCs with iloprost, a prostaglandin I(2) analog, and prostaglandin E(2) abrogated the potent contractile response to hypoxia and restored the wild-type phenotype.

CONCLUSIONS

Our findings reveal that hypoxia-induced pulmonary hypertension and vascular remodeling are exacerbated in the absence of COX-2 with enhanced ET(A) receptor expression and increased PASMC hypertrophy. COX-2-deficient PASMCs have a maladaptive response to hypoxia manifested by exaggerated contractility, which may be rescued by either COX-2-derived prostaglandin I(2) or prostaglandin E(2).

Distinguishing fibroblast promigratory and procontractile growth factor environments in 3-D collagen matrices

Understanding growth factor function during wound repair is necessary for the development of therapeutic interventions to improve healing outcomes. In the current study, we compare the effects of serum and purified growth factors on human fibroblast function in three different collagen matrix models: cell migration in nested matrices, floating matrix contraction, and stressed-released matrix contraction. The results of these studies indicate that platelet-derived growth factor (PDGF) is unique in its capacity to promote cell migration. Serum, lysophosphatidic acid, sphingosine-1-phophate (S1P), and endothelin-1 promote stressed-released matrix contraction but not cell migration. In addition, we found that S1P inhibits fibroblast migration and treatment of serum to remove lipid growth factors or treatment of cells to interfere with S1P(2) receptor function increases serum promigratory activity. Our findings suggest that different sets of growth factors generate promigratory and procontractile tissue environments for fibroblasts and that the balance between PDGF and S1P is a key determinant of fibroblast promigratory activity.

Role of vitronectin and fibronectin receptors in oral mucosal and dermal myofibroblast differentiation

BACKGROUND INFORMATION

The activation of fibroblasts into myofibroblasts is a crucial event in healing that is linked to remodelling and scar formation, therefore we determined whether regulation of myofibroblast differentiation via integrins might affect wound healing responses in populations of patient-matched HOFs (human oral fibroblasts) compared with HDFs (human dermal fibroblasts).

RESULTS

Both the HOF and HDF cell types underwent TGF-beta1 (transforming growth factor-beta1)-induced myofibroblastic differentiation [upregulation of the expression of alpha-sma (alpha-smooth muscle actin)], although analysis of unstimulated cells indicated that HOFs contained higher basal levels of alpha-sma than HDFs (P<0.05). Functional blocking antibodies against the integrin subunits alpha 5 (fibronectin) or alpha v (vitronectin) were used to determine whether the effects of TGF-beta1 were regulated via integrin signalling pathways. alpha-sma expression in both HOFs and HDFs was down-regulated by antibodies against both alpha 5 and alpha v. Functionally, TGF-beta1 inhibited cell migration in an in vitro wound model and increased the contraction of collagen gels. Greater contraction was evident for HOFs compared with HDFs, both with and without stimulation by TGF-beta1 (P<0.05). When TGF-beta1-stimulated cells were incubated with blocking antibodies against alpha 5 and alpha v, gel contraction was decreased to that of non-stimulated cells; however, blocking alpha v or alpha 5 could not restore cellular migration in both HOFs and HDFs.

CONCLUSIONS

Despite intrinsic differences in their basal state, the cellular events associated with TGF-beta1-induced myofibroblastic differentiation are common to both HOFs and HDFs, and appear to require differential integrin usage; up-regulation of alpha-sma expression and increases in collagen gel contraction are vitronectin- and fibronectin-receptor-dependent processes, whereas wound re-population is not.

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.

A new technique for calculating individual dermal fibroblast contractile forces generated within collagen-GAG scaffolds

Cell-mediated contraction plays a critical role in many physiological and pathological processes, notably organized contraction during wound healing. Implantation of an appropriately formulated (i.e., mean pore size, chemical composition, degradation rate) three-dimensional scaffold into an in vivo wound site effectively blocks the majority of organized wound contraction and results in induced regeneration rather than scar formation. Improved understanding of cell contraction within three-dimensional constructs therefore represents an important area of study in tissue engineering. Studies of cell contraction within three-dimensional constructs typically calculate an average contractile force from the gross deformation of a macroscopic substrate by a large cell population. In this study, cellular solids theory has been applied to conventional column buckling relationships to quantify the magnitude of individual cell contraction events within a three-dimensional, collagen-glycosaminoglycan scaffold. This new technique can be used for studying cell mechanics with a wide variety of porous scaffolds that resemble low-density, open-cell foams. It extends previous methods for analyzing cell buckling of two-dimensional substrates to three-dimensional constructs. From data available in the literature, the mean contractile force (Fc) generated by individual dermal fibroblasts within the collagen-glycosaminoglycan scaffold was calculated to range between 11 and 41 nN (Fc=26+/-13 nN, mean+/-SD), with an upper bound of cell contractility estimated at 450 nN.

RNA interference in human foreskin fibroblasts within the three-dimensional collagen matrix

The technique of RNA interference (RNAi) was trialed in primary human foreskin fibroblasts, both in monolayer culture and in the fibroblast-populated collagen matrix. Knockdown of lamin A/C, p53, and FAK was possible with low-confluency (<50%) monolayer fibroblasts, a transfection vehicle concentration of 1%, and an siRNA concentration of 25-50 nM. Knockdown also was possible in the collagen matrix using similar reagent concentrations and a cellular density of one million fibroblasts per ml of matrix. Optimization of transfection conditions appeared to be important to increase knockdown efficiency. Consistent with prediction, knockdown of FAK induced apoptosis in the fibroblast-populated collagen matrix.

Delayed and deficient dermal maturation in mice lacking the CXCR3 ELR-negative CXC chemokine receptor

Replacement of wounded skin requires the initially florid cellular response to abate and even regress as the dermal layer returns to a relatively paucicellular state. The signals that direct this "stop and return" process have yet to be deciphered. CXCR3 chemokine receptor and its ligand CXCL11/IP-9/I-TAC are expressed by basal keratinocytes and CXCL10/IP-10 by keratinocytes and endothelial cells during wound healing in mice and humans. In vitro, these ligands limit motility in dermal fibroblasts and endothelial cells. To examine whether this signaling pathway contributes to wound healing in vivo, full-thickness excisional wounds were created on CXCR3 wild-type (+/+) or knockout (-/-) mice. Even at 90 days, long after wound closure, wounds in the CXCR3(-/-) mice remained hypercellular and presented immature matrix components. The CXCR3(-/-) mice also presented poor remodeling and reorganization of collagen, which resulted in a weakened healed dermis. This in vivo model substantiates our in vitro findings that CXCR3 signaling is necessary for inhibition of fibroblast and endothelial cell migration and subsequent redifferentiation of the fibroblasts to a contractile state. These studies establish a pathophysiologic role for CXCR3 and its ligand during wound repair.

Nitric oxide inhibits enterocyte migration through activation of RhoA-GTPase in a SHP-2-dependent manner

Diseases of intestinal inflammation like necrotizing enterocolitis (NEC) are associated with impaired epithelial barrier integrity and the sustained release of intestinal nitric oxide (NO). NO modifies the cytoskeletal regulator RhoA-GTPase, suggesting that NO could affect barrier healing by inhibiting intestinal restitution. We now hypothesize that NO inhibits enterocyte migration through RhoA-GTPase and sought to determine the pathways involved. The induction of NEC was associated with increased enterocyte NO release and impaired migration of bromodeoxyuridine-labeled enterocytes from terminal ileal crypts to villus tips. In IEC-6 enterocytes, NO significantly inhibited enterocyte migration and activated RhoA-GTPase while increasing the formation of stress fibers. In parallel, exposure of IEC-6 cells to NO increased the phosphorylation of focal adhesion kinase (pFAK) and caused a striking increase in cell-matrix adhesiveness, suggesting a mechanism by which NO could impair enterocyte migration. NEC was associated with increased expression of pFAK in the terminal ileal mucosa of wild-type mice and a corresponding increase in disease severity compared with inducible NO synthase knockout mice, confirming the dependence of NO for FAK phosphorylation in vivo and its role in the pathogenesis of NEC. Strikingly, inhibition of the protein tyrosine phosphatase SHP-2 in IEC-6 cells prevented the activation of RhoA by NO, restored focal adhesions, and reversed the inhibitory effects of NO on enterocyte migration. These data indicate that NO impairs mucosal healing by inhibiting enterocyte migration through activation of RhoA in a SHP-2-dependent manner and support a possible role for SHP-2 as a therapeutic target in diseases of intestinal inflammation like NEC.

Prostacyclin analogs inhibit fibroblast contraction of collagen gels through the cAMP-PKA pathway

Prostacyclin is an arachidonic acid metabolite that modulates vascular tone within the lung. The current study evaluated the hypothesis that prostacyclin can also modulate tissue remodeling by affecting fibroblast-mediated contraction of extracellular matrix. To accomplish this, fibroblasts were cultured in three-dimensional native type I collagen gels in the presence of prostacyclin analogs: carbaprostacyclin, iloprost, and beraprost. All three analogs significantly inhibited contraction of the three-dimensional collagen gels mediated by three different fibroblasts. All three analogs significantly inhibited fibronectin release and reduced fibroblast fibronectin mRNA expression. Addition of exogenous fibronectin restored the contractile activity to fibroblasts incubated in the presence of all three analogs. Iloprost and beraprost significantly activated cAMP-dependent protein kinase-A (PKA), and an action through this pathway was confirmed by blockade of the inhibitory effect on contraction and fibronectin release with the PKA inhibitor KT-5720. In contrast, carbaprostacyclin, which is not as selective for the prostacyclin (IP) receptor, did not activate PKA, and its effects on contraction and fibronectin release were not fully blocked by KT-5720. Finally, the cAMP analogs N(6)-Benzoyl- (6-Bnz-) cAMP and dibutyryl-cAMP inhibited contraction, and this contrasted with the activity of an Epac selective agonist 8-pCPT-2'-O-Me-cAMP, which had no effect. Taken together, these results indicate that prostacyclin, acting through the IP receptor and by activating PKA, can lead to inhibition of fibronectin release and can subsequently inhibit fibroblast-mediated collagen gel contraction. The ability of prostacyclin to modulate fibroblast function suggests that prostacyclin can contribute to tissue remodeling.

A new microrheometric approach reveals individual and cooperative roles for TGF-beta1 and IL-1beta in fibroblast-mediated stiffening of collagen gels

The stiffness of the extracellular matrix can profoundly influence cell and tissue behaviors. Thus there is an emerging emphasis on understanding how matrix mechanical environments are established, regulated, and modified. Here we develop a microrheometric assay to measure the mechanical properties of a model extracellular matrix (type I collagen gel) and use it to explore cytokine-induced, cell-mediated changes in matrix mechanical properties. The microrheometric assay uses micron-scale ferrimagnetic beads embedded within collagen gels during fibrillogenesis. The beads are magnetized, then subjected to a twisting field, with the aggregate rotation of the beads measured by a magnetometer. The degree of bead rotation reflects the stiffness of the surrounding matrix. We show that the microscale assay provides stiffness measures for collagen gels comparable to those obtained with standard macroscale rheometry. To demonstrate the utility of the assay for biological discovery, we measure stiffness changes in fibroblast-populated collagen gels exposed to three concentrations of six cytokines over 2 to 14 days. Among the cytokines tested, transforming growth factor-beta1 and interleukin-1beta enhanced matrix stiffness, and together exerted cooperative effects on cellular modulation of matrix mechanics. The microrheometry approach developed here should accelerate the discovery of biological pathways orchestrating cellular modulation of matrix mechanics.

Platelet-derived growth factor-beta receptor activation is essential for fibroblast and pericyte recruitment during cutaneous wound healing

Connective tissue remodeling provides mammals with a rapid mechanism to repair wounds after injury. Inappropriate activation of this reparative process leads to scarring and fibrosis. Here, we studied the effects of platelet-derived growth factor receptor-beta blockade in vivo using the platelet-derived growth factor receptor (PDGFR)-beta inhibitor imatinib mesylate on tissue repair. After 7 days, healing of wounds was delayed with significantly reduced wound closure and concomitant reduction in myofibroblast frequency, expression of fibronectin ED-A, and collagen type I. Using a collagen type I transgenic reporter mouse, we showed that inhibiting PDGFR-beta activation restricted the distribution of collagen-synthesizing cells to wound margins and dramatically reduced cell proliferation in vivo. By 14 days, treated wounds were fully closed. Blocking PDGFR-beta signaling did not prevent the differentiation of myofibroblasts in vitro but potently inhibited fibroblast proliferation and migration. In addition, PDGFR-beta inhibition in vivo was accompanied by abnormal microvascular morphogenesis reminiscent of that observed in PDGFR-beta-/- mice with significantly reduced immunostaining of the pericyte marker NG2. Imatinib treatment also inhibited pericyte proliferation and migration in vitro. This study highlights the significance of PDGFR-beta signaling for the recruitment, proliferation, and functional activities of fibro-blasts and pericytes during the early phases of wound healing.

Evidence that PI3K, Rac, Rho, and Rho kinase are involved in basic fibroblast growth factor-stimulated fibroblast–Collagen matrix contraction

Fibroblast-collagen matrix contraction has been used as a model system to study how cells organize connective tissue. Previous work showed that lysophosphatidic acid (LPA)-stimulated floating collagen matrix contraction is independent of Rho kinase while platelet-derived growth factor (PDGF)-stimulated contraction is Rho kinase-dependent. The current studies were carried out to determine the signaling mechanisms of basic fibroblast growth factor (bFGF)-stimulated fibroblast-collagen matrix contraction. Both bFGF and LPA promoted equally collagen matrix contraction well. Three different inhibitors, LY294002 for phosphatidylinositol-3-kinase (PI3K), C3 exotransferase for Rho and Y27632 for Rho kinase, suppressed the bFGF-stimulated fibroblast-collagen matrix contraction. With bFGF stimulation, fibroblasts spread with prominent stress fiber network formation and focal adhesions. In the presence of Rho kinase inhibitor, focal adhesions and stress fibers were mostly lost. We demonstrated that bFGF stimulation for fibroblast caused transient Rac and Rho activation but did not activate Cdc42. In addition, bFGF enhanced fibroblast migration in wound healing assay. The present study implicates PI3K, Rac, Rho, and Rho kinase as being involved in bFGF-stimulated collagen matrix contraction. The elucidation of bFGF-triggered signal transduction may be an important clue to understand the roles of bFGF in wound healing.

Fibroblast differentiation in wound healing and fibrosis

The contraction of granulation tissue from skin wounds was first described in the 1960s. Later it was discovered that during tissue repair, fibroblasts undergo a change in phenotype from their normal relatively quiescent state in which they are involved in slow turnover of the extracellular matrix, to a proliferative and contractile phenotype termed myofibroblasts. These cells show some of the phenotypic characteristics of smooth muscle cells and have been shown to contract in vitro. In the 1990s, a number of researchers in different fields showed that myofibroblasts are present during tissue repair or response to injury in a variety of other tissues, including the liver, kidney, and lung. During normal repair processes, the myofibroblastic cells are lost as repair resolves to form a scar. This cell loss is via apoptosis. In pathological fibroses, myofibroblasts persist in the tissue and are responsible for fibrosis via increased matrix synthesis and for contraction of the tissue. In many cases this expansion of the extracellular matrix impedes normal function of the organ. For this reason much interest has centered on the derivation of myofibroblasts and the factors that influence their differentiation, proliferation, extracellular matrix synthesis, and survival. Further understanding of how fibroblast differentiation and myofibroblast phenotype is controlled may provide valuable insights into future therapies that can control fibrosis and scarring.

ATP reduces gel compaction in osteoblast-populated collagen gels

Bone remodeling is a localized process, but regulated by systemic signals such as hormones, cytokines, and mechanical loading. The mechanism by which bone cells convert these systemic signals into local signals is not completely understood. It is broadly accepted that the "prestress" in cytoskeleton of cells affects the magnitude of cellular responses to mechanical stimuli. Prestress derives from stiff cytoskeletal proteins and their connections within the cell and from cell contractility upon attaching to matrix. In an in vitro model of three-dimensional gel compaction, the relative cellular prestress levels in the same matrix environment were determined by matrix compaction rate: a greater compaction rate resulted in a higher level of prestress. In the present study, the effects of ATP on the prestress of osteoblasts were studied using mouse MC3T3-E1 cells grown in three-dimensional bioartificial tissues (BATs). ATP (> or =100 microM) reduced the compaction rate of BATs in a dose-dependent manner. ADP, 2'-(or 3')-O-(4-benzoylbenzoyl) ATP, and UTP, but not alpha,beta-methylene ATP, also reduced the compaction rate but to a lesser extent. Pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid tetrasodium did not block the effect of ATP on BAT compaction rate. These results indicate that both P2X and P2Y receptors are involved in ATP-induced reduction of BAT compaction rate. Steady fluid flow and RT-PCR results showed that ATP reduced cell attachment on type I collagen by downregulating the expression of integrin alpha(1). These results suggest a potential role for P2 receptors in matrix remodeling and repair and as a potential drug target in treatment of bone diseases.

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.

Equibiaxial strain stimulates fibroblastic phenotype shift in smooth muscle cells in an engineered tissue model of the aortic wall

Many cells in the body reside in a complex three-dimensional (3D) environment stimulated by mechanical force. In vitro bioreactor systems have greatly improved our understanding of the mechanisms behind cell mechanotransduction. Current systems to impose strain in vitro are limited either by the lack of uniform strain profile or inability to strain 3D engineered tissues. In this study, we present a system capable of generating cyclic equibiaxial strain to an engineered vascular wall model. Type I collagen hydrogels populated with rat aortic smooth muscle cells (RASMCs) were created either as a compacting disk or constrained hemisphere. Both models were adhered to silicone membranes precoated with collagen I, fibronectin, or Cell-Tak and assayed for adhesion characteristics. The best performing model was then exposed to 48 h of 10% strain at 1Hz to simulate wall strain profiles found in vascular aneurysms, with static cultures serving as controls. The finite strain profile at the level of the membrane and the free surface of the construct was quantified using microbeads. The results indicate that the hemisphere model adhered with Cell-Tak had the most stable adhesion, followed by fibronectin and collagen I. Disk models did not adhere well under any coating condition. Uniform strain propagation was possible up to a maximum area strain of 20% with this system. RASMC responded to 10% equibiaxial strain by becoming less elongated, and immunohistochemistry suggested that stretched RASMC shifted to a more synthetic phenotype in comparison to static controls. These results suggest that equibiaxial strain may induce smooth muscle cell differentiation. We conclude that this system is effective in stimulating cells with cyclic equibiaxial strain in 3D cultures, and can be applied to a variety of biomaterial and tissue engineering applications.

Promotion of Human Dermal Fibroblast Migration, Matrix Remodelling and Modification of Fibroblast Morphology within a Novel 3D Model by Lucilia sericata Larval Secretions

Lucilia sericata larvae, or green bottle fly maggots are applied to chronic wounds to aid healing. Previously, our laboratory has characterized the enzymatic activities present within maggot excretions/secretions (ES). Since then, we have related these to the degradation of extracellular matrix components, alteration of human, dermal fibroblast adhesion to surfaces and the stimulation of fibroblast migration within a two-dimensional in vitro assay. In this study, we developed a novel three-dimensional in vitro assay in which to observe fibroblast migration and morphology in response to maggot ES. Here, primary human foreskin fibroblasts were embedded within collagen gels containing fibronectin. Phase contrast and confocal microscopy were used in conjunction with image analysis software to examine and quantify aspects of fibroblast behavior. Our results showed that maggot ES stimulated fibroblast migration through the matrix and induced altered cell morphologies. Remodelling of the extracellular matrix located between individual fibroblasts was also induced, providing a mechanism by which cells may detect each other's presence over considerable distances. Thus, mechanisms by which maggots enhance tissue formation within wounds may be via the promotion of fibroblast motility, acceleration of extracellular matrix remodelling and coordination of cellular responses.

Capturing complex 3D tissue physiology in vitro

The emergence of tissue engineering raises new possibilities for the study of complex physiological and pathophysiological processes in vitro. Many tools are now available to create 3D tissue models in vitro, but the blueprints for what to make have been slower to arrive. We discuss here some of the 'design principles' for recreating the interwoven set of biochemical and mechanical cues in the cellular microenvironment, and the methods for implementing them. We emphasize applications that involve epithelial tissues for which 3D models could explain mechanisms of disease or aid in drug development.

The effect of the amelogenin fraction of enamel matrix proteins on fibroblast-mediated collagen matrix reorganization

Enamel matrix proteins (EMP), extracted from developing porcine teeth, promote not only periodontal regeneration but also cutaneous wound healing presumably via the amelogenin fraction. Because it is unclear whether the effect of EMP can be ascribed to amelogenins, we compared EMP with recombinant amelogenin in the relaxed dermal equivalent (DE) in vitro model for early wound contraction. EMP and recombinant porcine amelogenin (rP172) at 1 mg/ml were incorporated into DEs composed of human dermal fibroblasts and a type I collagen matrix. The area reduction, as a measure of contraction, as well as fibroblast numbers and TGF-beta1 levels, were quantified over 7 days in culture in the presence of 10% foetal bovine serum. Both EMP and recombinant amelogenin increased contraction (p < 0.005) and fibroblast numbers (p < 0.005) compared with controls (acetic acid vehicle and 1mg/ml porcine serum albumin) and the positive control TGF-beta1 added at 10 ng/ml. Increased contraction with EMP and recombinant amelogenin was most pronounced after the first day of incubation and was associated with elevated (p < 0.005) TGF-beta1 levels in conditioned medium. In conclusion, the amelogenin component of EMP augmented fibroblast-driven collagen matrix remodelling, at least partially, by increasing the endogenous production of TGF-beta1. These effects of EMP/amelogenin may be beneficial for cutaneous wound healing.

P21-activated kinase 1: convergence point in PDGF- and LPA-stimulated collagen matrix contraction by human fibroblasts

Fibroblast three-dimensional collagen matrix culture provides a tissue-like model that can be used to analyze cell form and function. The physiological agonists platelet-derived growth factor (PDGF) and lysophosphatidic acid (LPA) both stimulate human fibroblasts to contract floating collagen matrices. In this study, we show that the PDGF and LPA signaling pathways required for matrix contraction converge on p21-activated kinase 1 (PAK1) and its downstream effector cofilin1 and that contraction depends on cellular ruffling activity, rather than on the protrusion and retraction of cellular dendritic extensions. We also show that, depending on the agonist, different Rho effectors cooperate with PAK1 to regulate matrix contraction, Rho kinase in the case of PDGF and mDia1 in the case of LPA. These findings establish a unified framework for understanding the cell signaling pathways involved in fibroblast contraction of floating collagen matrices.

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.

Matrix contraction by dermal fibroblasts requires transforming growth factor-beta/activin-linked kinase 5, heparan sulfate-containing proteoglycans, and MEK/ERK: insights into pathological scarring in chronic fibrotic disease

Scarring is characterized by excessive synthesis and contraction of extracellular matrix. Here, we show that fibroblasts from scarred (lesional) areas of patients with the chronic fibrotic disorder diffuse scleroderma [diffuse systemic sclerosis (dSSc)] show an enhanced ability to adhere to and contract extracellular matrix, relative to fibroblasts from unscarred (nonlesional) areas of dSSc patients and dermal fibroblasts from normal, healthy individuals. The contractile abilities of normal and dSSc dermal fibroblasts were suppressed by blocking heparin sulfate-containing proteoglycan biosynthesis or antagonizing transforming growth factor-beta receptor type I [activin-linked kinase (ALK5)] or ras/mitogen-activated protein kinase (MEK)/extracellular signal-regulated kinase (ERK). Compared with both normal and nonlesional fibroblasts, lesional dSSc fibroblasts overexpressed the heparin sulfate-containing proteoglycan syndecan 4. We also found that the procontractile signals from transforming growth factor (TGF)-beta were integrated through syndecan 4 and MEK/ERK because the ability of TGFbeta to induce contraction of dermal fibroblasts was prevented by MEK antagonism. TGFbeta could not induce a contractile phenotype or phosphorylate ERK in syndecan 4(-/-) dermal fibroblasts. These results suggest that integrating TGFbeta and ERK signals via syndecan 4 is essential for the contractile ability of dermal fibroblasts. We conclude that antagonizing MEK/ERK, TGFbeta1/ALK5, or syndecan 4 may alleviate scarring in chronic fibrotic disease.

Mechanobiology in the third dimension

Cells are mechanically coupled to their extracellular environments, which play critical roles in both communicating the state of the mechanical environment to the cell as well as in mediating cellular response to a variety of stimuli. Along with the molecular composition and mechanical properties of the extracellular matrix (ECM), recent work has demonstrated the importance of dimensionality in cell-ECM associations for controlling the sensitive communication between cells and the ECM. Matrix forces are generally transmitted to cells differently when the cells are on two-dimensional (2D) vs. within three-dimensional (3D) matrices, and cells in 3D environments may experience mechanical signaling that is unique vis-à-vis cells in 2D environments, such as the recently described 3D-matrix adhesion assemblies. This review examines how the dimensionality of the extracellular environment can affect in vitro cell mechanobiology, focusing on collagen and fibrin systems.

Horse Chestnut Extract Induces Contraction Force Generation in Fibroblasts through Activation of Rho/Rho Kinase

Contraction forces generated by non-muscle cells such as fibroblasts play important roles in determining cell morphology, vasoconstriction, and/or wound healing. However, few factors that induce cell contraction forces are known, such as lysophosphatidic acid and thrombin. Our study analyzed various plant extracts for ingredients that induce generation of cell contraction forces in fibroblasts populating collagen gels. We found that an extract of Horse chestnut (Aesculus hippocastanum) is able to induce such contraction forces in fibroblasts. The involvement of actin polymerization and stress fiber formation in the force generation was suggested by inhibition of this effect by cytochalasin D and by Rhodamine phalloidin. Rho kinase inhibitors (Y27632 and HA1077) and a Rho inhibitor (exoenzyme C3) significantly inhibited the force generation induced by the Horse chestnut extract. H7, which inhibits Rho kinase as well as other protein kinases, also significantly inhibited induction of force generation. However, inhibitors of other protein kinases such as myosin light chain kinase (ML-9), protein kinase C (Calphostin), protein kinase A (KT5720), and tyrosine kinase (Genistein, Herbimycin A) had no effect on force generation induced by Horse chestnut extract. These results suggest that the Horse chestnut extract induces generation of contraction forces in fibroblasts through stress fiber formation followed by activation of Rho protein and Rho kinase but not myosin light chain kinase or other protein kinases.

Cellular cross-linking of peptide modified hydrogels

Peptide modification of hydrogel-forming materials is being widely explored as a means to regulate the phenotype of cells immobilized within the gels. Alternatively, we hypothesized that the adhesive interactions between cells and peptides coupled to the gel-forming materials would also enhance the overall mechanical properties of the gels. To test this hypothesis, alginate polymers were modified with RGDSP-containing peptides and the resultant polymer was used to encapsulate C2C12 myoblasts. The mechanical properties of these gels were then assessed as a function of both peptide and cell density using compression and tensile tests. Overall, it was found that above a critical peptide and cell density, encapsulated myoblasts were able to provide additional mechanical integrity to hydrogels composed of peptide-modified alginate. This occurred presumably by means of cell-peptide cross-linking of the alginate polymers, in addition to the usual Ca++ cross-linking. These results are potentially applicable to other polymer systems and important for a range of tissue engineering applications.

p38 MAPK/HSP25 signaling mediates cadmium-induced contraction of mesangial cells and renal glomeruli

The environmental pollutant cadmium affects human health, with the kidney being a primary target. In addition to proximal tubules, glomeruli and their contractile mesangial cells have also been identified as targets of cadmium nephrotoxicity. Glomerular contraction is thought to contribute to reduced glomerular filtration, a characteristic of cadmium nephrotoxicity. Because p38 MAPK/HSP25 signaling has been implicated in smooth muscle contraction, we examined its role in cadmium-induced contraction of mesangial cells. We report that exposure of mesangial cells to cadmium resulted in 1) cell contraction, 2) activation of MAP kinases, 3) increased HSP25 phosphorylation coincident with p38 MAP kinase activation, 4) sequential phosphorylation of the two phosphorylation sites of mouse HSP25 with Ser15 being phosphorylated before Ser86, 5) reduction of oligomeric size of HSP25, and 6) association of HSP25 with microfilaments. Exposure of isolated rat glomeruli to cadmium also resulted in contraction and increased HSP25 phosphorylation. The cadmium-induced responses were inhibited by the specific p38 MAP kinase inhibitor SB-203580, and cadmium-induced phosphorylation of HSP25 was inhibited by expression of a dominant-negative p38 MAP kinase mutant. These findings tentatively suggest that cadmium-induced nephrotoxicity results, in part, from glomerular contraction due to p38 MAP kinase/HSP25 signaling-dependent contraction of mesangial cells. With regard to the cellular action of HSP25, these data support a change in paradigm: in addition to its well-established cytoprotective function, HSP25 may also be involved in processes that ultimately lead to adverse effects, as is observed in the response of mesangial cells to cadmium.

Bladder acellular matrix as a substrate for studying in vitro bladder smooth muscle-urothelial cell interactions

The objective of this study was to evaluate the ability of bladder acellular matrix (BAM) to support the individual and combined growth of primary porcine bladder smooth muscle (SMC) and urothelial (UEC) cells. An in vitro co-culture system was devised to evaluate the effect of UEC on (i) SMC-mediated contraction of BAM discs, and (ii) SMC invasiveness into BAM. Cells were seeded onto BAM discs under 4 different culture conditions. Constructs were incubated for 1, 7, 14 and 28 days. Samples were then harvested for evaluation of matrix contraction. Immunohistochemistry (IHC) was utilized to examine cellular organization within the samples and conditioned media supernatants analyzed for net gelatinase activity. BAM contraction was significantly increased with co-culture. The same side co-culture configuration lead to a greater reduction in surface area than opposite side co-culture. IHC revealed enhanced SMC infiltration into BAM when co-culture was utilized. A significant increase in net gelatinase activity was also observed with the co-culture configuration. Enhanced infiltration and contractile ability of bladder SMCs with UEC co-culture may, in part, be due to an increase in gelatinase activity. The influence of bladder UECs on SMC behaviour in vitro indicates that BAM may contain some key inductive factors that serve to promote important bladder cell-cell and cell-matrix interactions.

HSP27 regulates fibroblast adhesion, motility, and matrix contraction

Heat shock protein 27 (HSP27) modulates actin-dependent cell functions in several systems. We hypothesized that HSP27 modulates wound contraction. Stably transfected fibroblast cell lines that overexpress HSP27 (SS12) or underexpress HSP27 (AS10) were established, and cell behaviors related to wound contraction were examined. First, fibroblast-populated collagen lattice (FPCL) contraction was examined because it has been studied as a wound-healing model. In floating FPCL contraction assays, SS12 cells caused increased contraction, whereas AS10 cells caused reduced contraction. Because floating matrix contraction is thought to be mediated by the tractional force of the cells, cell behaviors related to tractional force were examined. In collagen matrix, SS12 cells elongated faster and to a greater extent and contained longer stress fibers than control cells, whereas AS10 cells were slower to elongate than control cells. SS12 cells attached to the dishes more efficiently than the control, whereas AS10 cells attached less efficiently. Migration of SS12 cells on collagen-coated dishes was also enhanced, although AS10 cells did not differ from the control cells. In summary, HSP27 regulates fibroblast adhesion, elongation, and migration and the contraction of the floating matrix in a manner dependent on the level of its expression.

Blockade of platelet-derived growth factor receptor-beta by CDP860, a humanized, PEGylated di-Fab', leads to fluid accumulation and is associated with increased tumor vascularized volume

PURPOSE

CDP860 is an engineered Fab' fragment-polyethylene glycol conjugate, which binds to and blocks the activity of the beta-subunit of the platelet-derived growth factor receptor (PDGFR-beta). Studies in animals have suggested that PDGFR-beta inhibition reduces tumor interstitial fluid pressure, and thus increases the uptake of concomitantly administered drugs. The purpose of this study was to determine whether changes in tumor vascular parameters could be detected in humans, and to assess whether CDP860 would be likely to increase the uptake of a concurrently administered small molecule in future studies.

PATIENTS AND METHODS

Patients with advanced ovarian or colorectal cancer and good performance status received intravenous infusions of CDP860 on days 0 and 28. Patients had serial dynamic contrast-enhanced magnetic resonance imaging studies to measure changes in tumor vascular parameters.

RESULTS

Three of eight patients developed significant ascites, and seven of eight showed evidence of fluid retention. In some patients, the ratio of vascular volume to total tumor volume increased significantly (P < .001) within 24 hours following CDP860 administration, an effect suggestive of recruitment of previously non-functioning vessels.

CONCLUSION

These observations suggest that inhibition of PDGFR-beta might improve delivery of a concurrently administered therapy. However, in cancer patients, further exploration of the dosing regimen of CDP860 is required to dissociate adverse effects from beneficial effects. The findings challenge the view that inhibition of PDGF alone is beneficial, and confirm that effects of PDGFR kinase inhibition mediate, to some extent, the fluid retention observed in patients treated with mixed tyrosine kinase inhibitors.

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.

Expression of pro-inflammatory markers by human dermal fibroblasts in a three-dimensional culture model is mediated by an autocrine interleukin-1 loop

In vivo, fibroblasts reside in connective tissues, with which they communicate in a reciprocal way. Such cell--extracellular matrix interactions can be studied in vitro by seeding fibroblasts in collagen lattices. Depending upon the mechanical properties of the system, fibroblasts are activated to assume defined phenotypes. In the present study, we examined a transcriptional profile of primary human dermal fibroblasts cultured in a relaxed collagen environment and found relative induction (>2-fold) of 393 out of approx. 7100 transcripts when compared with the same system under mechanical tension. Despite down-regulated proliferation and matrix synthesis, cells did not become generally quiescent, since they induced transcription of numerous other genes including matrix metalloproteinases (MMPs) and growth factors/cytokines. Of particular interest was the induction of gene transcripts encoding pro-inflammatory mediators, e.g. cyclo-oxygenase-2 (COX-2), and interleukins (ILs)-1 and -6. These are apparently regulated in a hierarchical fashion, since the addition of IL-1 receptor antagonist prevented induction of COX-2, IL-1 and IL-6, but not that of MMP-1 or keratinocyte growth factor (KGF). Our results suggest strongly that skin fibroblasts are versatile cells, which adapt to their extracellular environment by displaying specific phenotypes. One such phenotype, induced by a mechanically relaxed collagen environment, is the 'pro-inflammatory' fibroblast. We propose that fibroblasts that are embedded in a matrix environment can actively participate in the regulation of inflammatory processes.

Valvular myofibroblast activation by transforming growth factor-beta: implications for pathological extracellular matrix remodeling in heart valve disease

The pathogenesis of cardiac valve disease correlates with the emergence of muscle-like fibroblasts (myofibroblasts). These cells display prominent stress fibers containing alpha-smooth muscle actin (alpha-SMA) and are believed to differentiate from valvular interstitial cells (VICs). However, the biological factors that initiate myofibroblast differentiation and activation in valves remain unidentified. We show that transforming growth factor-beta1 (TGF-beta1) mediates differentiation of VICs into active myofibroblasts in vitro in a dose-dependent manner, as determined by a significant increase in alpha-SMA and the dramatic augmentation of stress fiber formation and alignment. Additionally, TGF-beta1 and increased mechanical stress function synergistically to enhance contractility. In turn, contractile valve myofibroblasts exert tension on the extracellular matrix, resulting in a dramatic realignment of extracellular fibronectin fibrils. TGF-beta1 also inhibits valve myofibroblast proliferation without enhancing apoptosis. Our results are consistent with activation of a highly contractile myofibroblast phenotype by TGF-beta1 and are the first to connect valve myofibroblast contractility with pathological valve matrix remodeling. We suggest that the activation of contractile myofibroblasts by TGF-beta1 may be a significant first step in promoting alterations to the valve matrix architecture that are evident in valvular heart disease.

Epidermal Growth Factor Induces Fibroblast Contractility and Motility via a Protein Kinase C δ-dependent Pathway

Myosin-based cell contractile force is considered to be a critical process in cell motility. However, for epidermal growth factor (EGF)-induced fibroblast migration, molecular links between EGF receptor (EGFR) activation and force generation have not been clarified. Herein, we demonstrate that EGF stimulation increases myosin light chain (MLC) phosphorylation, a marker for contractile force, concomitant with protein kinase C (PKC) activity in mouse fibroblasts expressing human EGFR constructs. Interestingly, PKCdelta is the most strongly phosphorylated isoform, and the preferential PKCdelta inhibitor rottlerin largely prevented EGF-induced phosphorylation of PKC substrates and MARCKS. The pathway through which EGFR activates PKCdelta is suggested by the fact that the MEK-1 inhibitor U0126 and the phosphatidylinositol 3-kinase inhibitor LY294002 had no effect on PKCdelta activation, whereas lack of PLCgamma signaling resulted in delayed PKCdelta activation. EGF-enhanced MLC phosphorylation was prevented by a specific MLC kinase inhibitor ML-7 and the PKC inhibitors chelerythrine chloride and rottlerin. Further indicating that PKCdelta is required, a dominant-negative PKCdelta construct or RNAi-mediated PKCdelta depletion also prevented MLC phosphorylation. In the absence of PLC signaling, MLC phosphorylation and cell force generation were delayed similarly to PKCdelta activation. All of the interventions that blocked PKCdelta activation or MLC phosphorylation abrogated EGF-induced cell contractile force generation and motility. Our results suggest that PKCdelta activation is responsible for a major part of EGF-induced fibroblast contractile force generation. Hence, we identify here a new pathway helping to govern cell motility, with PLC signaling playing a role in activation of PKCdelta to promote the acute phase of EGF-induced MLC activation.