Mechanisms of force generation and transmission by myofibroblasts

Urokinase receptor cleavage: a crucial step in fibroblast-to-myofibroblast differentiation

Fibroblasts migrate into and repopulate connective tissue wounds. At the wound edge, fibroblasts differentiate into myofibroblasts, and they promote wound closure. Regulated fibroblast-to-myofibroblast differentiation is critical for regenerative healing. Previous studies have focused on the role in fibroblasts of urokinase plasmingen activator/urokinase plasmingen activator receptor (uPA/uPAR), an extracellular protease system that promotes matrix remodeling, growth factor activation, and cell migration. Whereas fibroblasts have substantial uPA activity and uPAR expression, we discovered that cultured myofibroblasts eventually lost cell surface uPA/uPAR. This led us to investigate the relevance of uPA/uPAR activity to myofibroblast differentiation. We found that fibroblasts expressed increased amounts of full-length cell surface uPAR (D1D2D3) compared with myofibroblasts, which had reduced expression of D1D2D3 but increased expression of the truncated form of uPAR (D2D3) on their cell surface. Retaining full-length uPAR was found to be essential for regulating myofibroblast differentiation, because 1) protease inhibitors that prevented uPAR cleavage also prevented myofibroblast differentiation, and 2) overexpression of cDNA for a noncleavable form of uPAR inhibited myofibroblast differentiation. These data support a novel hypothesis that maintaining full-length uPAR on the cell surface regulates the fibroblast to myofibroblast transition and that down-regulation of uPAR is necessary for myofibroblast differentiation.

Lack of MK2 inhibits myofibroblast formation and exacerbates pulmonary fibrosis

Fibroblasts play a major role in tissue repair and remodeling. Their differentiation into myofibroblasts, marked by increased expression of smooth muscle-specific alpha-actin (alpha-SMA), is believed to be important in wound healing and fibrosis. We have recently described a role for MK2 in this phenotypic differentiation in culture. In this article, we demonstrate that MK2 also regulates myofibroblasts in vivo. Disruption of MK2 in mice prevented myofibroblast formation in a model of pulmonary fibrosis. However, MK2 disruption and consequent lack of myofibroblast formation exacerbated fibrosis rather than ameliorated it as previously postulated. When mice lacking MK2 (MK2-/-) were exposed to bleomycin, more collagen accumulated and more fibroblasts populated fibrotic regions in their lungs than in similarly treated wild-type mice. While there were many vimentin-positive cells in the bleomycin-treated MK2-/- mouse lungs, few alpha-SMA-positive cells were observed in these lungs compared with wild-type mouse lungs. siRNA against MK2 reduced alpha-SMA expression in wild-type mouse embryonic fibroblasts (MEF), consistent with its suppression in MK2-/- MEF. On the other hand expressing constitutively active MK2 in MK2-/- MEF significantly increased alpha-SMA expression. MK2-/-MEF proliferated at a faster rate and produced more collagen; however, they migrated at a slower rate than wild-type MEF. Overexpressing phosphomimicking HSP27, a target of MK2, did not reverse the effect of MK2 disruption on fibroblast migration. MK2 disruption did not affect Smad2 activation by transforming growth factor-beta. Thus, MK2 appears to mediate myofibroblast differentiation, and inhibiting that differentiation might contribute to fibrosis rather than protect against it.

Fibrocytes from burn patients regulate the activities of fibroblasts

Wound healing requires an elaborate interplay between numerous cell types that orchestrate a series of regulated and overlapping events. Fibrocytes are a unique leukocyte subpopulation implicated in this process. One role proposed for these cells in wound healing is to synthesize extracellular matrix. Interestingly, using mass spectrometry to quantify hydroxyproline, we discovered that the capacity of fibrocytes from normal subjects or from burn patients to produce collagen is much less than that of dermal fibroblasts. Therefore, we investigated whether fibrocytes could play an indirect, regulatory, role in the healing of burn wounds by affecting the functions of dermal fibroblasts. Dermal fibroblasts treated with medium conditioned by burn patient fibrocytes, but not by those derived from normal subjects, showed an increase in cell proliferation and migration. Using confocal microscopy, flow cytometry, and immunoblotting, we found the level of alpha-smooth muscle actin (alpha-SMA) expression to be increased in these treated dermal fibroblasts, which also showed an enhanced ability to contract collagen lattices. To determine whether these effects could be attributed to transforming growth factor beta (TGF-beta1) or to connective tissue growth factor (CTGF), we measured total TGF-beta1 levels in the conditioned medium by an enzyme-linked immunosorbtion assay and assessed levels of CTGF mRNA and protein in fibroblasts and fibrocytes by reverse transcription-polymerase chain reaction and Western blotting. The results showed significantly higher levels of TGF-beta1 and CTGF produced by burn patient fibrocytes. In addition, the application of a TGF-beta1 neutralizing antibody significantly reduced the effect of burn patient fibrocyte medium on dermal fibroblast proliferation, migration, and collagen lattice contraction. Our results suggest that in healing burn wounds, fibrocytes could regulate the activities of local fibroblasts.

Maintenance of radiation-induced intestinal fibrosis: cellular and molecular features

Recent advances in cell and molecular radiobiology clearly showed that tissue response to radiation injury cannot be restricted to a simple cell-killing process, but depends upon continuous and integrated pathogenic processes, involving cell differentiation and crosstalk between the various cellular components of the tissue within the extracellular matrix. Thus, the prior concept of primary cell target in which a single-cell type (whatever it's epithelial or endothelial cells) dictates the whole tissue response to radiation injury has to be replaced by the occurrence of coordinated multicellular response that may either lead to tissue recovery or to sequel development. In this context, the present review will focus on the maintenance of the radiation-induced wound healing and fibrogenic signals triggered by and through the microenvironment toward the mesenchymal cell compartment, and will highlight how sequential and sustained modifications in cell phenotypes will in cascade modify cell-to-cell interactions and tissue composition.

The effect of cell-matrix interactions and aging on the malignant process

The malignant process, transformation of normal cells, proliferation, and metastasis formation, was considered as if originating from one single cell. Although the intrinsic mechanisms of transformation from the normal to the malignant state were both confirmed, an increasing body of evidence points to the surrounding matrix and cell-matrix interactions as major players in this process. Some of the most important arguments in favor of this contention are cited and commented in this chapter. Another important question concerns the relationship between the aging process and malignant transformation. A few decades ago, the frequency of clinically manifest tumors of several organs and tissues appeared to increase with age. As, however, average life expectancy increased rapidly over the last decades, clinical frequency of malignant tumors did not follow this tendency. It was argued that late in life the malignant process appears to decline. This justly inspired several teams to study the relationship between cellular senescence and malignant transformation. This is now an actively growing field which deserves special attention. Some of the pertinent experimental and theoretical arguments in favor of an antioncogene-mediated switch between these two processes are also reviewed with the caveat that this important and new subject of basic and clinical research on the malignant process is just at its beginning. It will certainly take an increasing importance during the coming years and decades with the hope to contribute to answer one of the most burning questions concerning the aging process: will life expectancy continue to increase linearly as predicted by some gerontologists, or will life expectancy level off or even decline as predicted by other epidemiologists. The relationship between cellular senescence and malignant transformation will play in this respect an important role.

Basic fibroblast growth factor in an artificial dermis promotes apoptosis and inhibits expression of ?-smooth muscle actin, leading to reduction of wound contraction

To clarify the mechanisms underlying declines in wound contraction caused by basic fibroblast growth factor (bFGF) and the role of autologous fibroblasts in modulating wound healing, we have examined the expression of alpha-smooth muscle actin (alpha-SMA) and apoptosis in a model of wound healing using collagen sponges with and without bFGF (1 microg) and/or fibroblasts (1 x 10(6) cells/cm(2)) applied to experimentally produced full-thickness skin wounds in rats (n=10 for each group). At 7 days postoperatively, wounds filled with a fibroblast-seeded collagen sponge (fibroblast-seeded group) displayed a greater area of collagen sponge and a smaller area of fibroblasts compared with control wounds filled with collagen sponge alone (control group). Therefore, seeding of fibroblasts in the dermal substitute might retard degradation of the collagen sponge, inhibiting fibroblast infiltration into the substitute. By day 14, wounds filled with bFGF-treated collagen sponge without fibroblast seeding (bFGF group) displayed decreased alpha-SMA expression and significantly increased apoptosis compared with other wounds. Double staining revealed that apoptosis in alpha-SMA-positive fibroblastic cells was significantly increased in the bFGF group, suggesting that bFGF treatment is a potent stimulator of myofibroblast apoptosis. Furthermore, morphometric analysis demonstrated the significant decrease in the level of wound contraction and the degree of mature collagen bundle formation in the bFGF group by day 42. The bFGF group also showed increased bFGF expression in macrophages by day 28. These results suggest that bFGF administration to an artificial dermis promotes apoptosis of alpha-SMA-positive fibroblastic cells and inhibits alpha-SMA expression in the treated wound, thus reducing wound contraction.

FAK is required for TGFbeta-induced JNK phosphorylation in fibroblasts: implications for acquisition of a matrix-remodeling phenotype

Transforming growth factor beta (TGFbeta) plays a critical role in connective tissue remodeling by fibroblasts during development, tissue repair, and fibrosis. We investigated the molecular pathways in the transmission of TGFbeta signals that lead to features of connective tissue remodeling, namely formation of an alpha-smooth muscle actin (alpha-SMA) cytoskeleton, matrix contraction, and expression of profibrotic genes. TGFbeta causes the activation of focal adhesion kinase (FAK), leading to JNK phosphorylation. TGFbeta induces JNK-dependent actin stress fiber formation, matrix contraction, and expression of profibrotic genes in fak+/+, but not fak-/-, fibroblasts. Overexpression of MEKK1, a kinase acting upstream of JNK, rescues TGFbeta responsiveness of JNK-dependent transcripts and actin stress fiber formation in FAK-deficient fibroblasts. Thus we propose a FAK-MEKK1-JNK pathway in the transmission of TGFbeta signals leading to the control of alpha-SMA cytoskeleton reorganization, matrix contraction, and profibrotic gene expression and hence to the physiological and pathological effects of TGFbeta on connective tissue remodeling by fibroblasts.

Epithelial-mesenchymal interactions in pulmonary fibrosis

Pulmonary fibrosis represents the sequelae of a variety of acute and chronic lung injuries of known and unknown etiologies. Tissue specimens obtained from patients with pulmonary fibrosis, regardless of the etiology, consistently show evidence of an ongoing wound-repair response. Epithelial-mesenchymal interactions have critical roles in normal lung development, tissue repair processes, and fibrosis. Current hypotheses propose that dysregulated function of, and impaired communication between, epithelial and mesenchymal cells prevent resolution of the wound-repair response and contribute to the pathobiology of pulmonary fibrosis. This hypothesis is supported by abundant evidence from patients, animal models, and cell-culture studies demonstrating abnormalities in epithelial cell and mesenchymal cell activities including proliferation, differentiation, and survival. This article reviews the aberrant epithelial and mesenchymal cellular phenotypes found in the context of pulmonary fibrosis and discusses the mechanisms that perpetuate these cellular phenotypes.

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.

Expression of proteins associated with cell-matrix adhesion in proliferative vitreoretinopathy designed by Dispase model

PURPOSE

During recent years, the interaction of cell surface molecule, extracellular matrix proteins, and cytoskeletal elements has been a topic for research for the purpose of understanding the mechanisms of pathologic conditions. This study aims to evaluate the expression of CD44, as a cell surface adhesion molecule; fibronectin (FN), as an extracellular and a cell surface protein; vinculin and actin/á-smooth muscle actin (alfa-SMA), as cytoskeletal elements; and the interactions of these proteins in the microenvironment of proliferative vitreoretinopathy (PVR).

METHODS

This experimental study was designed by the intravitreal Dispase model in rabbits and proteins' expression were evaluated via immunohistochemical staining.

RESULTS

As a cell surface protein, CD44 expression was determined in only four eyes focally and weakly, but in a small number of cells. Among the cytoskeletal proteins, vinculin expression was the most extensive and the strongest in intensity in epi- and subretinal membranes. Alpha-SMA expression was mostly present within small foci of cells. Fibronectin expression was determined in some of the eyes only faintly.

CONCLUSIONS

Vinculin seems to be involved in PVR pathogenesis. Variability in co-distribution of the expression of vinculin, FN, and alfa-SMA reflects the dynamic interactions evolving between cell and extracellular matrix during the epi- and subretinal membrane formations. The results of this study were determined not to be in support of the assumption that CD44 has a functional role in the pathogenesis of PVR.

Mechanoregulation of gene expression in fibroblasts

Mechanical loads placed on connective tissues alter gene expression in fibroblasts through mechanotransduction mechanisms by which cells convert mechanical signals into cellular biological events, such as gene expression of extracellular matrix components (e.g., collagen). This mechanical regulation of ECM gene expression affords maintenance of connective tissue homeostasis. However, mechanical loads can also interfere with homeostatic cellular gene expression and consequently cause the pathogenesis of connective tissue diseases such as tendinopathy and osteoarthritis. Therefore, the regulation of gene expression by mechanical loads is closely related to connective tissue physiology and pathology. This article reviews the effects of various mechanical loading conditions on gene regulation in fibroblasts and discusses several mechanotransduction mechanisms. Future research directions in mechanoregulation of gene expression are also suggested.

Filamin links cell shape and cytoskeletal structure to Rho regulation by controlling accumulation of p190RhoGAP in lipid rafts

Cytoskeleton-dependent changes in the activity of the small GTPase Rho mediate the effects of cell shape on cell function; however, little is known about how cell spreading and related distortion of the cytoskeleton regulate Rho activity. Here we show that rearrangements of the actin cytoskeleton associated with early phases of cell spreading in human microvascular endothelial (HMVE) cells suppress Rho activity by promoting accumulation of p190RhoGAP in lipid rafts where it exerts its Rho inhibitory activity. p190RhoGAP is excluded from lipid rafts and Rho activity increases when cell rounding is induced or the actin cytoskeleton is disrupted, and p190RhoGAP knockdown using siRNA prevents Rho inactivation by cell spreading. Importantly, cell rounding fails to prevent accumulation of p190RhoGAP in lipid rafts and to increase Rho activity in cells that lack the cytoskeletal protein filamin. Moreover, filamin is degraded in spread cells and cells that express a calpain-resistant form of filamin exhibit high Rho activity even when spread. Filamin may therefore represent the missing link that connects cytoskeleton-dependent changes of cell shape to Rho inactivation during the earliest phases of cell spreading by virtue of its ability to promote accumulation of p190RhoGAP in lipid rafts.

Collagen gel contractility is enhanced in human endometriotic stromal cells: a possible mechanism underlying the pathogenesis of endometriosis-associated fibrosis

BACKGROUND

Excessive fibrosis is frequently associated with endometriosis. To evaluate the involvement of the extracellular matrix contractility of endometriotic stromal cells (ECSCs) in the pathogenesis of endometriosis-associated fibrosis, we compared the collagen gel contractility of cultured ECSCs with that of normal endometrial stromal cells. To clarify the mechanism underlying collagen gel contraction by ECSCs, we also evaluated the effect of (+)-(R)-trans-4-(1-aminoethyl)-N-(4-pyridyl) cyclohexanecarboxamide dihydrochloride, monohydrate (Y-27632), a selective Rho-associated coiled-coil-forming protein kinase (ROCK) inhibitor, on the collagen gel contraction by ECSCs.

METHODS AND RESULTS

ECSCs showed enhanced collagen gel contractility in comparison with NESCs. Myofibroblastic differentiation and the increased expression of fibronectin, RhoA, ROCK-I and ROCK-II proteins were observed with ECSCs using the 3D culture. Y-27632 significantly inhibited the collagen gel contractility of ECSCs without cytotoxicity.

CONCLUSIONS

The present findings suggest that the enhanced collagen contractility in ECSCs is associated with myofibroblastic differentiation, the increased expression of fibronectin and the activation of the Rho-ROCK-mediated signalling pathway, all of which may be involved in the pathogenesis of endometriosis-associated fibrosis. These results suggest that the inhibition of the Rho-ROCK-mediated signalling pathway may provide a novel strategy for the treatment of this disease. In addition, our experimental system of ECSCs using 3D collagen gel culture would be suitable for evaluating novel treatments for endometriosis.

The plant pathogenesis related protein GLIPR-2 is highly expressed in fibrotic kidney and promotes epithelial to mesenchymal transition in vitro

Fibrosis is the accumulation of extracellular matrix proteins and is a common end pathway in many chronic diseases. To identify novel mediators of fibrosis we used transcript profiling in a mouse model of kidney fibrosis, the COL4A3 knockout (alport) mouse. One gene that we found up-regulated in fibrotic kidney was GLIPR-2, also known as GAPR-1 and C9orf19, a member of the plant pathogenesis-related proteins family 1. We have found that GLIPR-2 protein expression is significantly increased in fibrotic kidney compared to healthy controls. Examination of the expression pattern of GLIPR-2 indicated that the protein is selectively expressed in epithelial cells. Co-staining with antibodies for alpha-smooth muscle actin expression, a marker of myofibroblasts, showed that GLIPR-2 expressing cells are closely apposed to areas of strong alpha-smooth muscle actin expression. The origin of these myofibroblasts is not known, but in vitro studies have shown that GLIPR-2 can induce epithelial to mesenchymal transition (EMT) in a renal epithelial cell line. We propose that increased GLIPR-2 expression in kidney contributes to development of fibrosis by increasing the pool of activated fibroblasts, possibly through the induction of EMT.

Expression and function of alpha-smooth muscle actin during embryonic-stem-cell-derived cardiomyocyte differentiation

Three alpha-muscle actin isoforms are sequentially expressed during in vivo cardiac development. alpha-Smooth muscle actin is first and transiently expressed, followed by alpha-skeletal and finally alpha-cardiac actin. The significance of these transitions in actin gene expression during myogenesis remains to be determined. To understand whether actin isoforms have specific functions during cardiac development and cardiomyocyte contractility, we have hampered alpha-smooth muscle and alpha-skeletal actin expression and organization during embryonic stem cell differentiation towards cardiomyocyte. We show that the sequence of actin isoform expression displays similar pattern in the in vitro model and in mouse heart embryogenesis. Treatment with an interfering fusion peptide containing the N-terminal sequence of alpha-smooth muscle actin during a time window preceding spontaneous beating, prevents proper cardiac sarcomyogenesis, whereas alpha-skeletal actin-fusion peptide has no effect. Knockdown of alpha-smooth muscle actin in embryonic stem cells using RNA interference also affects cardiac differentiation. The application of both fusion peptides on beating embryoid bodies impairs frequency. These results suggest specific functional activities for actin isoforms in cardiogenesis and cardiomyocyte contractility.

Fibrosin, a novel fibrogenic cytokine, modulates expression of myofibroblasts

Granulation tissue fibroblasts, or myofibroblasts are characterized by the presence of alpha smooth muscle actin fibers (alpha SMA). These specialized cells are involved in wound contraction and in retractile phenomena observed during fibrotic disease. Myofibroblasts have also been shown to play a role in embryonic development. Growth factors such as Transforming growth factor beta TGFbeta and Nerve growth factor (NGF) can modulate the differentiation of myofibroblasts. In this report, we show that in vitro application of fibrosin, a novel fibrogenic cytokine, stimulates expression of alpha SMA-producing cells at least four-fold above that observed in control cultures. In addition, administration of fibrosin in a wound healing model in mice stimulates increased numbers of myofibroblasts 7 days after injury, when compared with untreated, or, control, wounded mice. These results suggest that fibrosin plays an important role in up regulating the appearance of myofibroblasts during wound healing, and possibly in fibrotic diseases. It may, therefore, be important in the process of scarring.

Microstructural characteristics of extracellular matrix produced by stromal fibroblasts

The overall objective of this investigation was to characterize the extracellular matrix deposited by the stromal fibroblasts as a function of time in culture and matrix microstructure. Stromal fibroblasts were seeded onto collagen matrices and cultured for up to 5 weeks. The collagen matrices contained collagen fibrils with an average diameter of 215 +/- 20 nm. When cultured on a collagen film, an average fibril diameter of 62 +/- 39 nm was observed for single layer films with only slight variations with time in culture, and after 1 week of culture between two film layers 67 +/- 47 nm fibrils were observed after 1 week. When the film surface was molded into 1 and 2 microm microgrooves, the initial average fibril diameter of the extracellular matrix was 73 +/- 21 and 73 +/- 31 nm respectively. When cultured on a collagen sponge, an average fibril diameter of 107 +/- 20 nm was initially observed and decreased to 47.5 +/- 17 nm after 1 week in culture. For cells cultured on a collagen sponge, Western blotting showed an increase in myofibroblast phenotype expression with time in culture. Shifts in phenotype were less distinct for cells cultured on collagen films. The microstructure, rather than geometry, of the matrix substrate appeared to influence the newly synthesized extracellular matrix and cell phenotype.

Isoform-specific regulation of the actin-organizing protein palladin during TGF-beta1-induced myofibroblast differentiation

Contractile myofibroblasts are responsible for remodeling of extracellular matrix during wound healing; however, their continued activity results in various fibrocontractive diseases. Conversion of fibroblasts into myofibroblasts is induced by transforming growth factor-beta1 (TGF-beta1) and is hallmarked by the neo-expression of alpha-smooth muscle actin (alpha-SMA), a commonly used myofibroblast marker. Moreover, myofibroblast differentiation and acquisition of the contractile phenotype involves functionally important alterations in the expression of actin-organizing proteins. We investigated whether myofibroblast differentiation is accompanied by changes in the expression of palladin, a cytoskeletal protein that controls stress fiber integrity. Palladin is expressed as several isoforms, including major 3Ig (90 kDa) and 4Ig (140 kDa) forms that differ in their N-terminal sequence. Expression of the 4Ig isoform is strongly induced in fibroblast stress fibers upon TGF-beta1 treatment preceding alpha-SMA upregulation. TGF-beta1 induced upregulation of palladin is mediated both by Smad and mitogen-activated protein kinase pathways. Furthermore, palladin 4Ig-isoform is co-expressed with alpha-SMA in vivo in experimental rat wounds and in human myofibroblast-containing lesions. Taken together these results identify palladin 4Ig as a novel marker of myofibroblast conversion in vitro and in vivo. They also provide for the first time information about the signaling cascades involved in the regulation of palladin expression.

The common bile duct ligation in rat: A relevant in vivo model to study the role of mechanical stress on cell and matrix behaviour

Common bile duct ligation leads to bile accumulation and liver fibrosis. In this model, little attention has been dedicated to the modification of the common bile duct. We have studied by histochemistry and immunohistochemistry, 3 and 5 days after ligation, the connective tissue modifications of the common bile duct wall. After bile duct ligation, compared with normal bile duct, a strong increase of the bile duct diameter, due to bile stasis, and a thickness of the bile duct wall were observed; numerous myofibroblasts expressing alpha-smooth muscle actin appeared in parallel with the detection of many proliferating connective tissue cells. These myofibroblasts secreted very early high amount of elastic fibre components, elastin and fibrillin-1. Elastic fibre increase was also observed close to the epithelial cell layer. Procollagen type III deposition was also induced 3 days after ligation but decreased thereafter, underlining that myofibroblasts modify their synthesis of extracellular matrix components to comply with the request. We show here that common bile duct ligation represents an invaluable model to study myofibroblastic differentiation and extracellular matrix adaptation produced by an acute mechanical stress.

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.

CCN2 is necessary for adhesive responses to transforming growth factor-beta1 in embryonic fibroblasts

CCN2 is induced by transforming growth factor-beta (TGFbeta) in fibroblasts and is overexpressed in connective tissue disease. CCN2 has been proposed to be a downstream mediator of TGFbeta action in fibroblasts; however, the role of CCN2 in regulating this process unclear. By using embryonic fibroblasts isolated from ccn2-/- mice, we showed that CCN2 is required for a subset of responses to TGFbeta. Affymetrix genome-wide expression profiling revealed that 942 transcripts were induced by TGFbeta greater than 2-fold in ccn2+/+ fibroblasts, of which 345 were not induced in ccn2-/- fibroblasts, including pro-adhesive and matrix remodeling genes. Whereas TGFbeta properly induced a generic Smad3-responsive promoter in ccn2-/- fibroblasts, TGFbeta-induced activation of focal adhesion kinase (FAK) and Akt was reduced in ccn2-/- fibroblasts. Emphasizing the importance of FAK and Akt activation in CCN2-dependent transcriptional responses to TGFbeta in fibroblasts, CCN2-dependent transcripts were not induced by TGFbeta in fak-/- fibroblasts and were reduced by wortmannin in wild-type fibroblasts. Akt1 overexpression in ccn2-/- fibroblasts rescued the TGFbeta-induced transcription of CCN2-dependent mRNA. Finally, induction of TGFbeta-induced fibroblast adhesion to fibronectin and type I collagen was significantly diminished in ccn2-/- fibroblasts. Thus in embryonic fibroblasts, CCN2 is a necessary cofactor required for TGFbeta to activate the adhesive FAK/Akt/phosphatidylinositol 3-kinase cascade, FAK/Akt-dependent genes, and adhesion to matrix.

Comment modéliser les événements de la fibrose cutanée ?

Skin fibrosis is classically seen as the consequence of chronic inflammation and altered healing response that is characterized by the differentiation of fibroblasts into secretory myofibroblasts and accumulation of connective tissue. Although fibrosis severely affects organ function and causes esthetic defects, no effective therapy is currently available to attenuate the fibrogenic process probably because the fibrogenic process is more complex than previously thought. Indeed, it might involve several interacting and mutually dependent cell types (fibroblasts, keratinocytes, endothelial cells, inflammatory cells), numerous paracrine factors, bio-active molecules and micro-environmental stimuli (growth factors, vasoactive peptides, balance between pro- and anti-inflammatory cytokines, coagulation system, reactive oxygen species, extracellular matrix...). In this perspective, the traditional approach that model individual cell response in simple cell culture system is probably inadequate and too simplistic. This article reviews the new models used to study skin fibrosis in vitro, in organotypic culture systems and in vivo and examines how these different models might be used to identify new molecular pathways involved in fibrogenesis. The monolayer cultures allow the study of fibrogenic signals induced by a single factor on a single cell type. Isolation of cells from fibrotic tissue allows to define the fibrogenic differentiation acquired in vivo. The organotypic models allow cell to cell and cell to matrix interaction and the experimental models in pigs and mice allowed studies in integrated physiological systems. These various and complementary models would also provide new tools to develop and test new drugs and treatments.

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.

Cardiac support device modifies left ventricular geometry and myocardial structure after myocardial infarction

BACKGROUND

Whether mechanical restraint of the left ventricle (LV) can influence remodeling after myocardial infarction (MI) remains poorly understood. This study surgically placed a cardiac support device (CSD) over the entire LV and examined LV and myocyte geometry and function after MI.

METHODS AND RESULTS

Post-MI sheep (35 to 45 kg; MI size, 23+/-2%) were randomized to placement of the CorCap CSD (Acorn Cardiovascular, Inc) (MI+CSD; n=6) or remained untreated (MI only; n=5). Uninstrumented sheep (n=10) served as controls. At 3 months after MI, LV end-diastolic volume (by MRI) was increased in the MI only group compared with controls (98+/-8 versus 43+/-4 mL; P<0.05). In the MI+CSD group, LV end-diastolic volume was lower than MI only values (56+/-7 mL; P<0.05) but remained higher than controls (P<0.05). Isolated LV myocyte shortening velocity was reduced by 35% from control values (P<0.05) in both MI groups. LV myocyte beta-adrenergic response was reduced with MI but normalized in the MI+CSD group. LV myocyte length increased in the MI group and was reduced in the MI+CSD group. Relative collagen content was increased and matrix metalloproteinase-9 was decreased within the MI border region of the CSD group.

CONCLUSIONS

A CSD beneficially modified LV and myocyte remodeling after MI through both cellular and extracellular mechanisms. These findings provide evidence that nonpharmacological strategies can interrupt adverse LV remodeling after MI.

Phenotypic Modulation of Intima and Media Smooth Muscle Cells in Fatal Cases of Coronary Artery Lesion

OBJECTIVE

Characterize the phenotypic features of media and intima coronary artery smooth muscle cells (SMCs) in mildly stenotic plaques, erosions, stable plaques, and in-stent restenosis.

METHODS AND RESULTS

Expression of alpha-smooth muscle actin (alpha-SMA), smooth muscle myosin heavy chains (SMMHCs), and smoothelin was investigated by immunohistochemistry followed by morphometric quantification. The cross-sectional area and the expression of cytoskeletal proteins in the media were lower in restenotic lesions and, to a lesser extent, in stable plaques compared with mildly stenotic plaques and erosions. An important expression of alpha-SMA was detected in the intima of the different lesions; moreover, alpha-SMA staining was significantly larger in erosions compared with all other conditions. In the same location, a striking decrease of SMMHCs and a disappearance of smoothelin were observed in all situations.

CONCLUSIONS

Medial atrophy is prevalent in restenotic lesions and stable plaques compared with mildly stenotic plaques and erosions. Intimal SMCs of all situations exhibit a phenotypic profile, suggesting that they have modulated into myofibroblasts (MFs). The high accumulation of alpha-SMA-positive MFs in erosions compared with stable plaques correlates with the higher appearance of thrombotic complications in this situation.

Vesicoamniotic shunt for complete urinary tract obstruction is partially effective

PURPOSE

The long-term outcome for children after antenatal intervention for obstructive uropathies is disappointing. We reported that renal dysplastic changes are well established 3 weeks after obstruction in a fetal lamb model. We used this model to explore renal development and bladder function after fetal intervention.

METHODS

We created an obstructive uropathy in fetal lambs at 60 days gestation by ligating the urethra and urachus. A vesicostomy (female) or urethrostomy (male) were performed 21 days later. The fetuses were killed at term (145 days) and bladder volume and compliance were measured. The urinary tract was processed for histologic examination.

RESULTS

Twenty two fetuses were shunted. Nine were miscarried or were still-born. Thirteen survived, and 11 had a successful shunt with a small bladder (8 +/- 5 mL) compared with controls (71 +/- 19 mL) (P < .05). Shunted bladders had poor compliance. Histologically, they had thickened submucosal connective tissue with hypertrophied muscle. Histology of the renal tissue demonstrated relatively well-preserved renal architecture with reduced nephron mass (oligonephronia) in 2 lambs and multicystic dysplastic change in 3. Six (55%) had normal nephrogenesis.

CONCLUSIONS

In our model, shunt operations after obstructive uropathy fail to preserve bladder function. Shunting ameliorated the development of cystic dysplasia, but half of the lambs had oligonephronia or multicystic dysplastic kidney. These might develop renal failure later in life.

Transforming growth factor-beta promotes pro-fibrotic behavior by serosal fibroblasts via PKC and ERK1/2 mitogen activated protein kinase cell signaling

OBJECTIVE

To assess the role of fibroblasts, transforming growth factor (TGF)-beta, and cell signal pathways in promoting fibrosis in Crohn's disease (CD).

SUMMARY BACKGROUND DATA

Intestinal strictures are a major source of morbidity in CD. Fibroblasts found at sites of stricture promote fibrogenesis. The mechanisms underlying this pro-fibrotic behavior remain elusive.

METHODS

Fibroblasts were isolated from strictured and macroscopically normal serosa in patients with CD and from normal serosa in patients with colorectal cancer. Whole cell connective tissue growth factor (CTGF) and fibronectin expression were determined by Western blot analysis. Fibroblast type I collagen expression was evaluated by real-time PCR, while fibroblast contractile activity was measured using fibroblast populated collagen lattices. Cells were stimulated with TGF-beta1 and inhibitors of the protein kinase C (PKC) and ERK 1/2 mitogen activated protein (MAP) kinase cell signaling pathways.

RESULTS

Stricture fibroblasts displayed enhanced constitutive expression of fibronectin. TGF-beta promoted fibroblast CTGF, fibronectin, and type I collagen expression and enhanced fibroblast contractile activity. Inhibition of PKC reduced basal collagen expression and contractile activity in Crohn's fibroblasts and attenuated the effect of TGF-beta on fibroblast CTGF, fibronectin, and collagen I expression as well as fibroblast contractility. ERK 1/2 inhibition had a similar effect on TGF-beta-induced CTGF and fibronectin expression.

CONCLUSIONS

TGF-beta is a critical pro-fibrotic growth factor in CD, and its effects are mediated via PKC and ERK 1/2 MAP kinase cell signaling. These pathways may represent novel therapeutic targets for patients with CD characterized by recurrent intestinal stricture formation.

Focal adhesion size controls tension-dependent recruitment of alpha-smooth muscle actin to stress fibers

Expression of α-smooth muscle actin (α-SMA) renders fibroblasts highly contractile and hallmarks myofibroblast differentiation. We identify α-SMA as a mechanosensitive protein that is recruited to stress fibers under high tension. Generation of this threshold tension requires the anchoring of stress fibers at sites of 8–30-μm-long “supermature” focal adhesions (suFAs), which exert a stress approximately fourfold higher (∼12 nN/μm²) on micropatterned deformable substrates than 2–6-μm-long classical FAs. Inhibition of suFA formation by growing myofibroblasts on substrates with a compliance of ≤11 kPa and on rigid micropatterns of 6-μm-long classical FA islets confines α-SMA to the cytosol. Reincorporation of α-SMA into stress fibers is established by stretching 6-μm-long classical FAs to 8.1-μm-long suFA islets on extendable membranes; the same stretch producing 5.4-μm-long classical FAs from initially 4-μm-long islets is without effect. We propose that the different molecular composition and higher phosphorylation of FAs on supermature islets, compared with FAs on classical islets, accounts for higher stress resistance.

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.

Cooperative effects of Rho and mechanical stretch on stress fiber organization

The small GTPase Rho regulates the formation of actin stress fibers in adherent cells through activation of its effector proteins Rho-kinase and mDia. We found in bovine aortic endothelial cells that inhibitions of Rho, Rho-kinase, and mDia (with C3, Y27632, and F1F2Delta1, respectively) suppressed stress fiber formation, but fibers appeared after 10% cyclic uniaxial stretch (1-Hz frequency). In contrast to the predominately perpendicular alignment of stress fibers to the stretch direction in normal cells, the stress fibers in cells with Rho pathway inhibition became oriented parallel to the stretch direction. In cells with normal Rho activity, the extent of perpendicular orientation of stress fibers depended on the magnitude of stretch. Expressing active RhoV14 plasmid in these cells enhanced the stretch-induced stress fiber orientation by an extent equivalent to an additional approximately 3% stretch. This augmentation of the stretch-induced perpendicular orientation by RhoV14 was blocked by Y27632 and by F1F2Delta1. Thus, the activity of the Rho pathway plays a critical role in determining both the direction and extent of stretch-induced stress fiber orientation in bovine aortic endothelial cells. Our results demonstrate that the stretch-induced stress fiber orientation is a function of the interplay between Rho pathway activity and the magnitude of stretching.

Endothelin-1 stimulates contraction and migration of rat pancreatic stellate cells

AIM: To examine the ability of ET-1 to affect the cell functions of PSCs and the underlying molecular mechanisms.

METHODS: PSCs were isolated from the pancreas of male Wistar rats after perfusion with collagenase, and cells between passages two and five were used. Expression of ET-1 and ET receptors was assessed by reverse transcription-PCR and immunostaining. Phosphorylation of myosin regulatory light chain (MLC), extracellular-signal regulated kinase (ERK), and Akt was examined by Western blotting. Contraction of PSCs was assessed on hydrated collagen lattices. Cell migration was examined using modified Boyden chambers. Cell proliferation was assessed by measuring the incorporation of 5-bromo-2×deoxyuridine.

RESULTS: Culture-activated PSCs expressed ET_A and ET_B receptors, and ET-1. ET-1 induced phosphorylation of MLC and ERK, but not Akt. ET-1 induced contraction and migration, but did not alter proliferation of PSCs. ET-1-induced contraction was inhibited by an ET_A receptor antagonist BQ-123 and an ET_B receptor antagonist BQ-788, whereas migration was inhibited by BQ-788 but not by BQ-123. A Rho kinase inhibitor Y-27632 abolished both contraction and migration.

CONCLUSION: ET-1 induced contraction and migration of PSCs through ET receptors and activation of Rho-Rho kinase. ET_A and ET_B receptors play different roles in the regulation of these cellular functions in response to ET-1.

Interstitial fluid flow induces myofibroblast differentiation and collagen alignment in vitro

The differentiation of fibroblasts to contractile myofibroblasts, which is characterized by de novo expression of alpha-smooth muscle actin (alpha-SMA), is crucial for wound healing and a hallmark of tissue scarring and fibrosis. These processes often follow inflammatory events, particularly in soft tissues such as skin, lung and liver. Although inflammatory cells and damaged epithelium can release transforming growth factor beta1 (TGF-beta1), which largely mediates myofibroblast differentiation, the biophysical environment of inflammation and tissue regeneration, namely increased interstitial flow owing to vessel hyperpermeability and/or angiogenesis, may also play a role. We demonstrate that low levels of interstitial (3D) flow induce fibroblast-to-myofibroblast differentiation as well as collagen alignment and fibroblast proliferation, all in the absence of exogenous mediators. These effects were associated with TGF-beta1 induction, and could be eliminated with TGF-beta1 blocking antibodies. Furthermore, alpha1beta1 integrin was seen to play an important role in the specific response to flow, as its inhibition prevented fibroblast differentiation and subsequent collagen alignment but did not block their ability to contract the gel in a separate floating gel assay. This study suggests that the biophysical environment that often precedes fibrosis, such as swelling, increased microvascular permeability and increased lymphatic drainage--all which involve interstitial fluid flow--may itself play an important role in fibrogenesis.

Substratum roughness alters the growth, area, and focal adhesions of epithelial cells, and their proximity to titanium surfaces

Epithelial (E) cells were cultured on smooth tissue culture plastic (TCP), TCP-Ti, polished Ti (P), and rough grit-blasted Ti (B), acid-etched Ti (AE), and grit-blasted and acid-etchedTi (SLA) surfaces and their growth, area, adhesion, and membrane-Ti proximity assessed. Rough surfaces decreased the growth of E cells compared to smooth surfaces in cultures up to 28 days. In general rough surfaces decreased the spreading of E cells as assessed by their area with the most pronounced affect for the SLA surface. On the other hand, the strength of E cells adhesion as inferred by immunofluorescence staining of vinculin in focal adhesions indicated that E cells formed more and larger focal adhesions on the smooth P surface compared to the rougher AE surface. As this finding indicates a stronger adhesion to smooth surfaces, it is likely that E cells on rough surfaces are more susceptible to mechanical removal. An immunogold labeling method was developed to visualize focal adhesions using back-scattered electron imaging with a scanning electron microscope (SEM). On rough surfaces focal adhesions were primarily localized on to the ridges rather than the valleys and the cells tended to bridge over the valleys. Transmission electron microscopy (TEM) measurements of membrane proximity to the Ti surface indicated that average distance of cell to the Ti increased as the Ti surface roughness increased. Therefore, the size and shape of surface features are important determinants of epithelial adhesive behavior and epithelial coverage of rough surfaces would be difficult to attain if such surfaces become exposed.

YB-1 coordinates vascular smooth muscle alpha-actin gene activation by transforming growth factor beta1 and thrombin during differentiation of human pulmonary myofibroblasts

Profibrotic regulatory mechanisms for tissue repair after traumatic injury have developed under strong evolutionary pressure to rapidly stanch blood loss and close open wounds. We have examined the roles played by two profibrotic mediators, transforming growth factor beta1 (TGFbeta1) and thrombin, in directing expression of the vascular smooth muscle alpha-actin (SMalphaA) gene, an important determinant of myofibroblast differentiation and early protein marker for stromal cell response to tissue injury. TGFbeta1 is a well known transcriptional activator of the SMalphaA gene in myofibroblasts. In contrast, thrombin independently elevates SMalphaA expression in human pulmonary myofibroblasts at the posttranscriptional level. A common feature of SMalphaA up-regulation mediated by thrombin and TGFbeta1 is the involvement of the cold shock domain protein YB-1, a potent repressor of SMalphaA gene transcription in human fibroblasts that also binds mRNA and regulates translational efficiency. YB-1 dissociates from SMalphaA enhancer DNA in the presence of TGFbeta1 or its Smad 2, 3, and 4 coregulatory mediators. Thrombin does not effect SMalphaA gene transcription but rather displaces YB-1 from SMalphaA exon 3 coding sequences previously shown to be required for mRNA translational silencing. The release of YB-1 from promoter DNA coupled with its ability to bind RNA and shuttle between the nucleus and cytoplasm is suggestive of a regulatory loop for coordinating SMalphaA gene output in human pulmonary myofibroblasts at both the transcriptional and translational levels. This loop may help restrict organ-destructive remodeling due to excessive myofibroblast differentiation.

Tissue repair, contraction, and the myofibroblast

After the first description of the myofibroblast in granulation tissue of an open wound by means of electron microscopy, as an intermediate cell between the fibroblast and the smooth muscle cell, the myofibroblast has been identified both in normal tissues, particularly in locations where there is a necessity of mechanical force development, and in pathological tissues, in relation with hypertrophic scarring, fibromatoses and fibrocontractive diseases as well as in the stroma reaction to epithelial tumors. It is now accepted that fibroblast/myofibroblast transition begins with the appearance of the protomyofibroblast, whose stress fibers contain only beta- and gamma-cytoplasmic actins and evolves, but not necessarily always, into the appearance of the differentiated myofibroblast, the most common variant of this cell, with stress fibers containing alpha-smooth muscle actin. Myofibroblast differentiation is a complex process, regulated by at least a cytokine (the transforming growth factor-beta1), an extracellular matrix component (the ED-A splice variant of cellular fibronectin), as well as the presence of mechanical tension. The myofibroblast is a key cell for the connective tissue remodeling that takes place during wound healing and fibrosis development. On this basis, the myofibroblast may represent a new important target for improving the evolution of such diseases as hypertrophic scars, and liver, kidney or pulmonary fibrosis.

Regulation of alpha-smooth muscle actin expression in granulation tissue myofibroblasts is dependent on the intronic CArG element and the transforming growth factor-beta1 control element

Myofibroblasts are specialized contractile fibroblasts that are critical in wound closure and tissue contracture. Generation of contractile force is correlated with the expression of alpha-smooth muscle actin (alpha-SMA); however, little is known regarding molecular mechanisms that control activation of alpha-SMA in myofibroblasts in granulation tissue. The aims of the present studies were to identify sufficient promoter regions required for alpha-SMA expression in myofibroblasts in vivo and to determine whether activation of alpha-SMA expression in myofibroblasts in vivo is dependent on an intronic CArG [CC(A/T)6GG] and a transforming growth factor-beta1 control element (TCE) that are required for alpha-SMA expression in smooth muscle cells. A Lac Z transgene construct from -2600 through the first intron was expressed in myofibroblasts within granulation tissue of cutaneous wounds in a pattern that closely mimicked endogenous alpha-SMA expression. Mutation of either the intronic CArG element or the TCE completely inhibited transgene expression in myofibroblasts in granulation tissue and responsiveness to transforming growth factor-beta1 in cultured transgenic fibroblasts. These same elements were also critical in regulating alpha-SMA expression during skeletal muscle repair but not during skeletal muscle development. Taken together, these results provide the first in vivo evidence for the importance of the intronic CArG and TCE cis-elements in the regulation of alpha-SMA expression in myofibroblasts in granulation tissue.

The expression and function of the endothelin system in contractile properties of vaginal myofibroblasts of women with uterovaginal prolapse

OBJECTIVE

The endothelin-1 system regulates (myo)fibroblast contraction in wound healing. Our aim was to determine endothelin-1 system expression and function in contractile properties of vaginal myofibroblasts of women with uterovaginal prolapse.

STUDY DESIGN

Cultures of alpha-smooth muscle actin-positive myofibroblasts that were established at the time of repair surgery for prolapse (n = 30; mean age, 56 +/- 14 years) were analyzed and compared for their expression of the endothelin-1 system and contractile properties to myofibroblasts from primiparous women.

RESULTS

Myofibroblasts expressed the complete endothelin system but did not secrete endothelin-1. Endothelin-1 binding was mediated exclusively by the endothelin B-receptor. In 3-dimensional collagen gels, spontaneous contraction of myofibroblasts from estrogen-treated women with prolapse was statistically significantly lower than from young primiparous women. Exogenous addition of endothelin-1 decreased the spontaneous contraction of myofibroblasts.

CONCLUSION

Genital myofibroblasts of women with uterovaginal prolapse are poorly contractile, and endothelin-1 further decreases vaginal myofibroblast contraction, which is opposite to observations in skin myofibroblasts.

Active fascial contractility: Fascia may be able to contract in a smooth muscle-like manner and thereby influence musculoskeletal dynamics

Dense connective tissue sheets, commonly known as fascia, play an important role as force transmitters in human posture and movement regulation. Fascia is usually seen as having a passive role, transmitting mechanical tension which is generated by muscle activity or external forces. However, there is some evidence to suggest that fascia may be able to actively contract in a smooth muscle-like manner and consequently influence musculoskeletal dynamics. General support for this hypothesis came with the discovery of contractile cells in fascia, from theoretical reflections on the biological advantages of such a capacity, and from the existence of pathological fascial contractures. Further evidence to support this hypothesis is offered by in vitro studies with fascia which have been reported in the literature: the biomechanical demonstration of an autonomous contraction of the human lumbar fascia, and the pharmacological induction of temporary contractions in normal fascia from rats. If verified by future research, the existence of an active fascial contractility could have interesting implications for the understanding of musculoskeletal pathologies with an increased or decreased myofascial tonus. It may also offer new insights and a deeper understanding of treatments directed at fascia, such as manual myofascial release therapies or acupuncture. Further research to test this hypothesis is suggested.

Tissue Reorganization in Response to Mechanical Load Increases Functionality

In the rapidly growing field of tissue engineering, the functional properties of tissue substitutes are recognized as being of the utmost importance. The present study was designed to evaluate the effects of static mechanical forces on the functionality of the produced tissue constructs. Living tissue sheets reconstructed by the self-assembly approach from human cells, without the addition of synthetic material or extracellular matrix (ECM), were subjected to mechanical load to induce cell and ECM alignment. In addition, the effects of alignment on the function of substitutes reconstructed from these living tissue sheets were evaluated. Our results show that tissue constructs made from living tissue sheets, in which fibroblasts and ECM were aligned, presented higher mechanical resistance. This was assessed by the modulus of elasticity and ultimate strength as compared with tissue constructs in which components were randomly oriented. Moreover, tissue-engineered vascular media made from a prealigned living tissue sheet, produced with smooth muscle cells, possessed greater contractile capacity compared with those produced from living tissue sheets that were not prealigned. These results show that the mechanical force generated by cells during tissue organization is an asset for tissue component alignment. Therefore, this work demonstrates a means to improve the functionality (mechanical and vasocontractile properties) of tissues reconstructed by tissue engineering by taking advantage of the biomechanical forces generated by cells under static strain.

Smooth muscle actin determines mechanical force-induced p38 activation

The mitogen-activated protein kinase p38 is activated by mechanical force, but the cellular elements that mediate force-induced p38 phosphorylation are not defined. As alpha-smooth muscle actin (SMA) is an actin isoform associated with force generation in fibroblasts, we asked if SMA participates in the activation of p38 by force. Tensile forces (0.65 pn/mum(2)) generated by magnetic fields were applied to collagen-coated magnetite beads bound to Rat-2 cells. Immunoblotting showed that p38alpha was the predominant p38 isoform. Analysis of bead-associated proteins demonstrated that SMA enrichment of collagen receptor complexes required the alpha2beta1 integrin. SMA was present almost entirely as filaments. Swinholide depolymerized SMA filaments and blocked force-induced p38 phosphorylation and force-induced increases of SMA. Knockdown of SMA (70% reduction) using RNA interference did not affect beta-actin but inhibited force-induced p38 phosphorylation by 50%. Inhibition of Rho kinase blocked SMA filament assembly, force-induced increases of SMA, and force-induced p38 activation. Force application increased SMA content and enhanced the association of phosphorylated p38 with SMA filaments. Blockade of p38 phosphorylation by SB203586 abrogated force-induced increases of SMA. In cells transfected with SMA promoter-beta-galactosidase fusion constructs, co-transfection with constitutively active p38 or MKK6 increased SMA promoter activity by 2.5-3-fold. Dominant negative p38 blocked force-induced activation of the SMA promoter. In SMA negative cells, there was no force-induced p38 phosphorylation. We conclude that force-induced p38 phosphorylation is dependent on an SMA filament-dependent pathway that uses a feed-forward amplification loop to synergize force-induced SMA expression with p38 activation.

Myofibroblast development is characterized by specific cell-cell adherens junctions

Myofibroblasts of wound granulation tissue, in contrast to dermal fibroblasts, join stress fibers at sites of cadherin-type intercellular adherens junctions (AJs). However, the function of myofibroblast AJs, their molecular composition, and the mechanisms of their formation are largely unknown. We demonstrate that fibroblasts change cadherin expression from N-cadherin in early wounds to OB-cadherin in contractile wounds, populated with alpha-smooth muscle actin (alpha-SMA)-positive myofibroblasts. A similar shift occurs during myofibroblast differentiation in culture and seems to be responsible for the homotypic segregation of alpha-SMA-positive and -negative fibroblasts in suspension. AJs of plated myofibroblasts are reinforced by alpha-SMA-mediated contractile activity, resulting in high mechanical resistance as demonstrated by subjecting cell pairs to hydrodynamic forces in a flow chamber. A peptide that inhibits alpha-SMA-mediated contractile force causes the reorganization of large stripe-like AJs to belt-like contacts as shown for enhanced green fluorescent protein-alpha-catenin-transfected cells and is associated with a reduced mechanical resistance. Anti-OB-cadherin but not anti-N-cadherin peptides reduce the contraction of myofibroblast-populated collagen gels, suggesting that AJs are instrumental for myofibroblast contractile activity.

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.