Expression of CTGF/CCN2 in response to LPA is stimulated by fibrotic extracellular matrix via the integrin/FAK axis

Pharmacological targeting of ECM homeostasis, fibroblast activation and invasion for the treatment of pulmonary fibrosis

INTRODUCTION

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive interstitial lung disease with a dismal prognosis. While the standard-of-care (SOC) drugs approved for IPF represent a significant advancement in antifibrotic therapies, they primarily slow disease progression and have limited overall efficacy and many side effects. Consequently, IPF remains a condition with high unmet medical and pharmacological needs.

AREAS COVERED

A wide variety of molecules and mechanisms have been implicated in the pathogenesis of IPF, many of which have been targeted in clinical trials. In this review, we discuss the latest therapeutic targets that affect extracellular matrix (ECM) homeostasis and the activation of lung fibroblasts, with a specific focus on ECM invasion.

EXPERT OPINION

A promising new approach involves targeting ECM invasion by fibroblasts, a process that parallels cancer cell behavior. Several cancer drugs are now being tested in IPF for their ability to inhibit ECM invasion, offering significant potential for future treatments. The delivery of these therapies by inhalation is a promising development, as it may enhance local effectiveness and minimize systemic side effects, thereby improving patient safety and treatment efficacy.

Involvement of lysophosphatidic acid-LPA1-YAP signaling in healthy and pathological FAPs migration

Skeletal muscle fibrosis is defined as the excessive accumulation of extracellular matrix (ECM) components and is a hallmark of muscular dystrophies. Fibro-adipogenic progenitors (FAPs) are the main source of ECM, and thus have been strongly implicated in fibrogenesis. In skeletal muscle fibrotic models, including muscular dystrophies, FAPs undergo dysregulations in terms of proliferation, differentiation, and apoptosis, however few studies have explored the impact of FAPs migration. Here, we studied fibroblast and FAPs migration and identified lysophosphatidic acid (LPA), a signaling lipid central to skeletal muscle fibrogenesis, as a significant migration inductor. We identified LPA receptor 1 (LPA1) mediated signaling as crucial for this effect through a mechanism dependent on the Hippo pathway, another pathway implicated in fibrosis across diverse tissues. This cross-talk favors the activation of the Yes-associated protein 1 (YAP) and Transcriptional coactivator with PDZ-binding motif (TAZ), leading to increased expression of fibrosis-associated genes. This study reveals the role of YAP in LPA-mediated fibrotic responses as inhibition of YAP transcriptional coactivator activity hinders LPA-induced migration in fibroblasts and FAPs. Moreover, we found that FAPs derived from the mdx4cv mice, a murine model of Duchenne muscular dystrophy, display a heightened migratory phenotype due to enhanced LPA signaling compared to wild-type FAPs. Remarkably, we found that the inhibition of LPA1 or YAP transcriptional coactivator activity in mdx4cv FAPs reverts this phenotype. In summary, the identified LPA-LPA1-YAP pathway emerges as a critical driver of skeletal muscle FAPs migration and provides insights into potential novel targets to mitigate fibrosis in muscular dystrophies.

LPA-induced expression of CCN2 in muscular fibro/adipogenic progenitors (FAPs): Unraveling cellular communication networks

Cellular Communication Network Factor 2, CCN2, is a profibrotic cytokine implicated in physiological and pathological processes in mammals. The expression of CCN2 is markedly increased in dystrophic muscles. Interestingly, diminishing CCN2 genetically or inhibiting its function improves the phenotypes of chronic muscular fibrosis in rodent models. Elucidating the cell-specific mechanisms behind the induction of CCN2 is a fundamental step in understanding its relevance in muscular dystrophies. Here, we show that the small lipids LPA and 2S-OMPT induce CCN2 expression in fibro/adipogenic progenitors (FAPs) through the activation of the LPA1 receptor and, to a lower extent, by also the LPA6 receptor. These cells show a stronger induction than myoblasts or myotubes. We show that the LPA/LPARs axis requires ROCK kinase activity and organized actin cytoskeleton upstream of YAP/TAZ signaling effectors to upregulate CCN2 levels, suggesting that mechanical signals are part of the mechanism behind this process. In conclusion, we explored the role of the LPA/LPAR axis on CCN2 expression, showing a strong cytoskeletal-dependent response in muscular FAPs.

Drugs targeting CTGF in the treatment of pulmonary fibrosis

Pulmonary fibrosis represents the final alteration seen in a wide variety of lung disorders characterized by increased fibroblast activity and the accumulation of substantial amounts of extracellular matrix, along with inflammatory damage and the breakdown of tissue architecture. This condition is marked by a significant mortality rate and a lack of effective treatments. The depositing of an excessive quantity of extracellular matrix protein follows the damage to lung capillaries and alveolar epithelial cells, leading to pulmonary fibrosis and irreversible damage to lung function. It has been proposed that the connective tissue growth factor (CTGF) plays a critical role in the advancement of pulmonary fibrosis by enhancing the accumulation of the extracellular matrix and exacerbating fibrosis. In this context, the significance of CTGF in pulmonary fibrosis is examined, and a summary of the development of drugs targeting CTGF for the treatment of pulmonary fibrosis is provided.

Inulin-like polysaccharide ABWW may impede CCl4 induced hepatic stellate cell activation through mediating the FAK/PI3K/AKT signaling pathway in vitro & in vivo

Studies have shown that terrestrial acidic polysaccharides containing carboxyl groups and seaweed sulfated polysaccharides have strong potential in anti-liver fibrosis. However, there is no investigation on the anti-liver fibrosis of fructan, a ubiquitous natural polysaccharide. The present study aimed to understand the effect of fructan in ameliorating carbon tetrachloride (CCl4)-induced liver fibrosis in mice. Here, an inulin-like fructan ABWW from Achyranthes bidentata Bl. was characterized by fructose enzymatic hydrolysis, methylation analysis, ESI-MS, and NMR. It was composed of →2)-β-d-Fruf-(1→ and →2)-β-d-Fruf-(1, 6→, terminated with →1)-α-d-Glcp and →2)-β-d-Fruf residues. The biological studies showed that ABWW could improve liver damage and liver fibrosis induced by CCl4in vivo and inhibit hepatic stellate cell (HSC) activation and migration in vitro. We further demonstrated that ABWW inhibited LX2 activation via suppressing the FAK/PI3K/AKT signaling pathway. Hence, ABWW might be a potential novel active compound for anti-fibrosis new drug development.

From outside to inside and back again: the lysophosphatidic acid-CCN axis in signal transduction

CCN1 and CCN2 are matricellular proteins that are transcriptionally induced by various stimuli, including growth factors. CCN proteins act to facilitate signaling events involving extracellular matrix proteins. Lysophosphatidic acid (LPA) is a lipid that activates G protein-coupled receptors (GPCRs), enhancing proliferation, adhesion, and migration in many types of cancer cells. Our group previously reported that LPA induces production of CCN1 protein in human prostate cancer cell lines within 2-4 h. In these cells, the mitogenic activity of LPA is mediated by LPA Receptor 1 (LPAR1), a GPCR. There are multiple examples of the induction of CCN proteins by LPA, and by the related lipid mediator sphingosine-1-phosphate (S1P), in various cellular models. The signaling pathways responsible for LPA/S1P-induced CCN1/2 typically involve activation of the small GTP-binding protein Rho and the transcription factor YAP. Inducible CCNs can potentially play roles in downstream signal transduction events required for LPA and S1P-induced responses. Specifically, CCNs secreted into the extracellular space can facilitate the activation of additional receptors and signal transduction pathways, contributing to the biphasic delayed responses typically seen in response to growth factors acting via GPCRs. In some model systems, CCN1 and CCN2 play key roles in LPA/S1P-induced cell migration and proliferation. In this way, an extracellular signal (LPA or S1P) can activate GPCR-mediated intracellular signaling to induce the production of extracellular modulators (CCN1 and CCN2) that in turn initiate another round of intracellular signaling.

Connective tissue growth factor: Role in trabecular meshwork remodeling and intraocular pressure lowering

Connective tissue growth factor (CTGF) is a distinct signaling molecule modulating many physiological and pathophysiological processes. This protein is upregulated in numerous fibrotic diseases that involve extracellular matrix (ECM) remodeling. It mediates the downstream effects of transforming growth factor beta (TGF-β) and is regulated via TGF-β SMAD-dependent and SMAD-independent signaling routes. Targeting CTGF instead of its upstream regulator TGF-β avoids the consequences of interfering with the pleotropic effects of TGF-β. Both CTGF and its upstream mediator, TGF-β, have been linked with the pathophysiology of glaucomatous optic neuropathy due to their involvement in the regulation of ECM homeostasis. The excessive expression of these growth factors is associated with glaucoma pathogenesis via elevation of the intraocular pressure (IOP), the most important risk factor for glaucoma. The raised in the IOP is due to dysregulation of ECM turnover resulting in excessive ECM deposition at the site of aqueous humor outflow. It is therefore believed that CTGF could be a potential therapeutic target in glaucoma therapy. This review highlights the CTGF biology and structure, its regulation and signaling, its association with the pathophysiology of glaucoma, and its potential role as a therapeutic target in glaucoma management.

Activation of skeletal muscle FAPs by LPA requires the Hippo signaling via the FAK pathway

Lysophosphatidic acid (LPA) is a lysophospholipid that signals through six G-protein coupled receptors (LPARs), LPA₁ to LPA₆. LPA has been described as a potent modulator of fibrosis in different pathologies. In skeletal muscle, LPA increases fibrosis-related proteins and the number of fibro/adipogenic progenitors (FAPs). FAPs are the primary source of ECM-secreting myofibroblasts in acute and chronic damage. However, the effect of LPA on FAPs activation in vitro has not been explored. This study aimed to investigate FAPs' response to LPA and the downstream signaling mediators involved. Here, we demonstrated that LPA mediates FAPs activation by increasing their proliferation, expression of myofibroblasts markers, and upregulation of fibrosis-related proteins. Pretreatment with the LPA₁/LPA₃ antagonist Ki16425 or genetic deletion of LPA₁ attenuated the LPA-induced FAPs activation, resulting in decreased expression of cyclin e1, α-SMA, and fibronectin. We also evaluated the activation of the focal adhesion kinase (FAK) in response to LPA. Our results showed that LPA induces FAK phosphorylation in FAPs. Treatment with the P-FAK inhibitor PF-228 partially prevented the induction of cell responses involved in FAPs activation, suggesting that this pathway mediates LPA signaling. FAK activation controls downstream cell signaling within the cytoplasm, such as the Hippo pathway. LPA induced the dephosphorylation of the transcriptional coactivator YAP (Yes-associated protein) and promoted direct expression of target pathway genes such as Ctgf/Ccn2 and Ccn1. The blockage of YAP transcriptional activity with Super-TDU further confirmed the role of YAP in LPA-induced FAPs activation. Finally, we demonstrated that FAK is required for LPA-dependent YAP dephosphorylation and the induction of Hippo pathway target genes. In conclusion, LPA signals through LPA₁ to regulate FAPs activation by activating FAK to control the Hippo pathway.

Differential Fibrotic Response of Muscle Fibroblasts, Myoblasts, and Myotubes to Cholic and Deoxycholic Acids

Fibrosis is a condition characterized by an increase in the components of the extracellular matrix (ECM). In skeletal muscle, the cells that participate in the synthesis of ECM are fibroblasts, myoblasts, and myotubes. These cells respond to soluble factors that increase ECM. Fibrosis is a phenomenon that develops in conditions of chronic inflammation, extensive lesions, or chronic diseases. A pathological condition with muscle weakness and increased bile acids (BA) in the blood is cholestatic chronic liver diseases (CCLD). Skeletal muscle expresses the membrane receptor for BA called TGR5. To date, muscle fibrosis in CCLD has not been evaluated. This study aims to assess whether BA can induce a fibrotic condition in muscle fibroblasts, myoblasts, and myotubes. The cells were incubated with deoxycholic (DCA) and cholic (CA) acids, and fibronectin protein levels were evaluated by Western blot. In muscle fibroblasts, both DCA and CA induced an increase in fibronectin protein levels. The same response was found in fibroblasts when activating TGR5 with the specific receptor agonist (INT-777). Interestingly, DCA reduced fibronectin protein levels in both myoblasts and myotubes, while CA did not show changes in fibronectin protein levels in myoblasts and myotubes. These results suggest that DCA and CA can induce a fibrotic phenotype in muscle-derived fibroblasts. On the other hand, DCA decreased the fibronectin in myoblasts and myotubes, whereas CA did not show any effect in these cell populations. Our results show that BA has different effects depending on the cell population to be analyzed.

Single-cell transcriptome dynamics of the autotaxin-lysophosphatidic acid axis during muscle regeneration reveal proliferative effects in mesenchymal fibro-adipogenic progenitors

Lysophosphatidic acid is a growth factor-like bioactive phospholipid recognising LPA receptors and mediating signalling pathways that regulate embryonic development, wound healing, carcinogenesis, and fibrosis, via effects on cell migration, proliferation and differentiation. Extracellular LPA is generated from lysophospholipids by the secreted hydrolase-ectonucleotide pyrophosphatase/phosphodiesterase 2 (ENPP2; also, AUTOTAXIN/ATX) and metabolised by different membrane-bound phospholipid phosphatases (PLPPs). Here, we use public bulk and single-cell RNA sequencing datasets to explore the expression of Lpar _1-6, Enpp2, and Plpp genes under skeletal muscle homeostasis and regeneration conditions. We show that the skeletal muscle system dynamically expresses the Enpp2-Lpar-Plpp gene axis, with Lpar1 being the highest expressed member among LPARs. Lpar1 was expressed by mesenchymal fibro-adipogenic progenitors and tenocytes, whereas FAPs mainly expressed Enpp2. Clustering of FAPs identified populations representing distinct cell states with robust Lpar1 and Enpp2 transcriptome signatures in homeostatic cells expressing higher levels of markers Dpp4 and Hsd11b1. However, tissue injury induced transient repression of Lpar genes and Enpp2. The role of LPA in modulating the fate and differentiation of tissue-resident FAPs has not yet been explored. Ex vivo, LPAR1/3 and ENPP2 inhibition significantly decreased the cell-cycle activity of FAPs and impaired fibro-adipogenic differentiation, implicating LPA signalling in the modulation of the proliferative and differentiative fate of FAPs. Together, our results demonstrate the importance of the ENPP2-LPAR-PLPP axis in different muscle cell types and FAP lineage populations in homeostasis and injury, paving the way for further research on the role of this signalling pathway in skeletal muscle homeostasis and regeneration, and that of other organs and tissues, in vivo.

Connective Tissue Growth Factor in Idiopathic Pulmonary Fibrosis: Breaking the Bridge

CTGF is upregulated in patients with idiopathic pulmonary fibrosis (IPF), characterized by the deposition of a pathological extracellular matrix (ECM). Additionally, many omics studies confirmed that aberrant cellular senescence-associated mitochondria dysfunction and metabolic reprogramming had been identified in different IPF lung cells (alveolar epithelial cells, alveolar endothelial cells, fibroblasts, and macrophages). Here, we reviewed the role of the CTGF in IPF lung cells to mediate anomalous senescence-related metabolic mechanisms that support the fibrotic environment in IPF.

ATX/LPA axis regulates FAK activation, cell proliferation, apoptosis, and motility in human pancreatic cancer cells

Previous studies implicated ATX/LPA axis as a potential driver of tumorigenesis and progression in pancreatic cancer. This study aimed to determine the existence of the autocrine pathway of ATX/LPA action in pancreatic cancer cells, and to elucidate its influence on focal adhesion kinase (FAK) activation, cellular proliferation, apoptosis, and migration. Firstly, we identified the lysophosphatidic acid (LPA) concentrations in cultured cell supernatant by ELISA and observed the effect of the autotaxin (ATX)-specific inhibitor S32826 on LPA concentrations. We found the existence of a certain concentration of LPA in cellular supernatant, which was significantly decreased by S32826 in a dose- and time-dependent manner. A maximum response was observed at 50 μM for 72 h. Secondly, the effect of S32826 on the protein expression and intracellular sublocalization of total FAK and phosphorylated FAK (pY397 FAK) was determined by Western blot and immunofluorescence staining. It was found that the expression of total FAK and pY397 FAK and their distribution along the cell membrane where adhesion structures are located were significantly decreased by S32826. Finally, we observed the influence of S32826 on cell proliferation, apoptosis, and migration by CCK-8 assay, flow cytometric analysis, and transwell migration assay. Results showed that cell viability and migration were significantly declined, and the proportions of apoptotic cells were significantly increased by S32826. This study verified the existence of autocrine regulation of LPA secretion via producing ATX by pancreatic cancer cells in vitro and the important role of LPA/ATX axis on FAK activation, cell proliferation, apoptosis, and motility.

Therapeutic potential of targeting regulatory mechanisms of hepatic stellate cell activation in liver fibrosis

Hepatic fibrosis is a manifestation of different etiologies of liver disease with the involvement of multiple mediators in complex network interactions. Activated hepatic stellate cells (aHSCs) are the central driver of hepatic fibrosis, given their potential to induce connective tissue formation and extracellular matrix (ECM) protein accumulation. Therefore, identifying the cellular and molecular pathways involved in the activation of HSCs is crucial in gaining mechanistic and therapeutic perspectives to more effectively target the disease. In addition to a comprehensive summary of our current understanding of the role of HSCs in liver fibrosis, we also discuss here the proposed therapeutic strategies based on targeting HSCs.

Driving fibrosis in neuromuscular diseases: Role and regulation of Connective tissue growth factor (CCN2/CTGF)

Connective tissue growth factor or cellular communication network 2 (CCN2/CTGF) is a matricellular protein member of the CCN family involved in several crucial biological processes. In skeletal muscle, CCN2/CTGF abundance is elevated in human muscle biopsies and/or animal models for diverse neuromuscular pathologies, including muscular dystrophies, neurodegenerative disorders, muscle denervation, and muscle overuse. In this context, CCN2/CTGF is deeply involved in extracellular matrix (ECM) modulation, acting as a strong pro-fibrotic factor that promotes excessive ECM accumulation. Reducing CCN2/CTGF levels or biological activity in pathological conditions can decrease fibrosis, improve muscle architecture and function. In this work, we summarize information about the role of CCN2/CTGF in fibrosis associated with neuromuscular pathologies and the mechanisms and signaling pathways that regulate their expression in skeletal muscle.

Current Pharmacological Strategies for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a lethal, X-linked neuromuscular disorder caused by the absence of dystrophin protein, which is essential for muscle fiber integrity. Loss of dystrophin protein leads to recurrent myofiber damage, chronic inflammation, progressive fibrosis, and dysfunction of muscle stem cells. There is still no cure for DMD so far and the standard of care is principally limited to symptom relief through glucocorticoids treatments. Current therapeutic strategies could be divided into two lines. Dystrophin-targeted therapeutic strategies that aim at restoring the expression and/or function of dystrophin, including gene-based, cell-based and protein replacement therapies. The other line of therapeutic strategies aims to improve muscle function and quality by targeting the downstream pathological changes, including inflammation, fibrosis, and muscle atrophy. This review introduces the important developments in these two lines of strategies, especially those that have entered the clinical phase and/or have great potential for clinical translation. The rationale and efficacy of each agent in pre-clinical or clinical studies are presented. Furthermore, a meta-analysis of gene profiling in DMD patients has been performed to understand the molecular mechanisms of DMD.

Dietary salt exacerbates intestinal fibrosis in chronic TNBS colitis via fibroblasts activation

Intestinal fibrosis is a frequent complication in inflammatory bowel diseases (IBD). It is a challenge to identify environmental factors such as diet that may be driving this risk. Intestinal fibrosis result from accumulation of extracellular matrix (ECM) proteins secreted by myofibroblasts. Factors promoting intestinal fibrosis are unknown, but diet appears to be a critical component in its development. Consumption of salt above nutritional recommendations can exacerbate chronic inflammation. So far, high salt diet (HSD) have not been thoroughly investigated in the context of intestinal fibrosis associated to IBD. In the present study, we analyze the role of dietary salt in TNBS chronic colitis induced in rat, an intestinal fibrosis model, or in human colon fibroblast cells. Here, we have shown that high-salt diet exacerbates undernutrition and promoted ECM-associated proteins in fibroblasts. Taken together, our results suggested that dietary salt can activate intestinal fibroblasts, thereby contributing to exacerbation of intestinal fibrosis. Dietary salt may be considered as a putative environmental factor that drives intestinal fibrosis risk.

MYOC Promotes the Differentiation of C2C12 Cells by Regulation of the TGF-β Signaling Pathways via CAV1

Simple Summary

MYOC is a secreted glycoprotein and it expresses at high levels in skeletal muscle cells. However, the function of MYOC in muscle is still unclear. Accordingly, in this study, we examined that MYOC expression increased gradually during C2C12 differentiation and it could promote the differentiation of C2C12. Furthermore, we demonstrated that MYOC could bind to CAV1. We further confirmed that CAV1 could positively regulate C2C12 differentiation through the TGF-β pathway. At last, we determined the relationship among MYOC, CAV1 and TGF-β. We found that MYOC promoted the differentiation of C2C12 cells by regulation of the TGF-β signaling pathways via CAV1. The present study is the first to demonstrate the mechanism of action of MYOC in C2C12 cells. It provides a novel method of exploring the mechanism of muscle differentiation and represents a potential novel method for the treatment of muscle diseases.

Abstract

Myocilin (MYOC) is a glycoprotein encoded by a gene associated with glaucoma pathology. In addition to the eyes, it also expresses at high transcription levels in the heart and skeletal muscle. MYOC affects the formation of the murine gastrocnemius muscle and is associated with the differentiation of mouse osteoblasts, but its role in the differentiation of C2C12 cells has not yet been reported. Here, MYOC expression was found to increase gradually during the differentiation of C2C12 cells. Overexpression of MYOC resulted in enhanced differentiation of C2C12 cells while its inhibition caused reduced differentiation. Furthermore, immunoprecipitation indicated that MYOC binds to Caveolin-1 (CAV1), a protein that influences the TGF-β pathway. Laser confocal microscopy also revealed the common sites of action of the two during the differentiation of C2C12 cells. Additionally, CAV1 was upregulated significantly as C2C12 cells differentiated, with CAV1 able to influence the differentiation of the cells. Furthermore, the Western blotting analysis demonstrated that the expression of MYOC affected the TGF-β pathway. Finally, MYOC was overexpressed while CAV1 was inhibited. The results indicate that reduced CAV1 expression blocked the promotion of C2C12 cell differentiation by MYOC. In conclusion, the results demonstrated that MYOC regulates TGF-β by influencing CAV1 to promote the differentiation of C2C12 cells.

Role of Matricellular CCN Proteins in Skeletal Muscle: Focus on CCN2/CTGF and Its Regulation by Vasoactive Peptides

The Cellular Communication Network (CCN) family of matricellular proteins comprises six proteins that share conserved structural features and play numerous biological roles. These proteins can interact with several receptors or soluble proteins, regulating cell signaling pathways in various tissues under physiological and pathological conditions. In the skeletal muscle of mammals, most of the six CCN family members are expressed during embryonic development or in adulthood. Their roles during the adult stage are related to the regulation of muscle mass and regeneration, maintaining vascularization, and the modulation of skeletal muscle fibrosis. This work reviews the CCNs proteins' role in skeletal muscle physiology and disease, focusing on skeletal muscle fibrosis and its regulation by Connective Tissue Growth factor (CCN2/CTGF). Furthermore, we review evidence on the modulation of fibrosis and CCN2/CTGF by the renin-angiotensin system and the kallikrein-kinin system of vasoactive peptides.

PDGF-PDGFR network differentially regulates the fate, migration, proliferation, and cell cycle progression of myogenic cells

Platelet-derived growth factors (PDGFs) regulate embryonic development, tissue regeneration, and wound healing through their binding to PDGF receptors, PDGFRα and PDGFRβ. However, the role of PDGF signaling in regulating muscle development and regeneration remains elusive, and the cellular and molecular responses of myogenic cells are understudied. Here, we explore the PDGF-PDGFR gene expression changes and their involvement in skeletal muscle myogenesis and myogenic fate. By surveying bulk RNA sequencing and single-cell profiling data of skeletal muscle stem cells, we show that myogenic progenitors and muscle stem cells differentially express PDGF ligands and PDGF receptors during myogenesis. Quiescent adult muscle stem cells and myoblasts preferentially express PDGFRβ over PDGFRα. Remarkably, cell culture- and injury-induced muscle stem cell activation altered PDGF family gene expression. In myoblasts, PDGF-AB and PDGF-BB treatments activate two pro-chemotactic and pro-mitogenic downstream transducers, RAS-ERK1/2 and PI3K-AKT. PDGFRs inhibitor AG1296 inhibited ERK1/2 and AKT activation, myoblast migration, proliferation, and cell cycle progression induced by PDGF-AB and PDGF-BB. We also found that AG1296 causes myoblast G0/G1 cell cycle arrest. Remarkably, PDGF-AA did not promote a noticeable ERK1/2 or AKT activation, myoblast migration, or expansion. Also, myogenic differentiation reduced the expression of both PDGFRα and PDGFRβ, whereas forced PDGFRα expression impaired myogenesis. Thus, our data highlight PDGF signaling pathway to stimulate satellite cell proliferation aiming to enhance skeletal muscle regeneration and provide a deeper understanding of the role of PDGF signaling in non-fibroblastic cells.

The linkage between inflammation and fibrosis in muscular dystrophies: The axis autotaxin-lysophosphatidic acid as a new therapeutic target?

Muscular dystrophies (MDs) are a diverse group of severe disorders characterized by increased skeletal muscle feebleness. In many cases, respiratory and cardiac muscles are also compromised. Skeletal muscle inflammation and fibrosis are hallmarks of several skeletal muscle diseases, including MDs. Until now, several keys signaling pathways and factors that regulate inflammation and fibrosis have been identified. However, no curative treatments are available. Therefore, it is necessary to find new therapeutic targets to fight these diseases and improve muscle performance. Lysophosphatidic acid (LPA) is an active glycerophospholipid mainly synthesized by the secreted enzyme autotaxin (ATX), which activates six different G protein-coupled receptors named LPA₁ to LPA₆ (LPARs). In conjunction, they are part of the ATX/LPA/LPARs axis, involved in the inflammatory and fibrotic response in several organs-tissues. This review recapitulates the most relevant aspects of inflammation and fibrosis in MDs. It analyzes experimental evidence of the effects of the ATX/LPA/LPARs axis on inflammatory and fibrotic responses. Finally, we speculate about its potential role as a new therapeutic pharmacological target to treat these diseases.

Connective tissue growth factor promotes chemotaxis of preosteoblasts through integrin α5 and Ras during tensile force-induced intramembranous osteogenesis

In vertebrates, new bone formation via intramembranous osteogenesis is a critical biological event for development, remodeling, and fracture healing of bones. Chemotaxis of osteoblast lineage cells is an essential cellular process in new bone formation. Connective tissue growth factor (CTGF) is known to exert chemotactic properties on various cells; however, details of CTGF function in the chemotaxis of osteoblast lineage cells and underlying molecular biological mechanisms have not been clarified. The aim of the present study was to evaluate the chemotactic properties of CTGF and its underlying mechanisms during active bone formation through intramembranous osteogenesis. In our mouse tensile force-induced bone formation model, preosteoblasts were aggregated at the osteogenic front of calvarial bones. CTGF was expressed at the osteogenic front, and functional inhibition of CTGF using a neutralizing antibody suppressed the aggregation of preosteoblasts. In vitro experiments using μ-slide chemotaxis chambers showed that a gradient of CTGF induced chemotaxis of preosteoblastic MC3T3-E1 cells, while a neutralizing integrin α5 antibody and a Ras inhibitor inhibited the CTGF-induced chemotaxis of MC3T3-E1 cells. These findings suggest that the CTGF-integrin α5-Ras axis is an essential molecular mechanism to promote chemotaxis of preosteoblasts during new bone formation through intramembranous osteogenesis.

TGF-β-driven downregulation of the Wnt/β-Catenin transcription factor TCF7L2/TCF4 in PDGFRα+ fibroblasts

Mesenchymal stromal/stem cells (MSCs) are multipotent progenitors essential for organogenesis, tissue homeostasis, regeneration, and scar formation. Tissue injury upregulates TGF-β signaling, which modulates myofibroblast fate, extracellular matrix remodeling, and fibrosis. However, the molecular determinants of MSCs differentiation and survival remain poorly understood. The canonical Wnt Tcf/Lef transcription factors regulate development and stemness, but the mechanisms by which injury-induced cues modulate their expression remain underexplored. Here, we studied the cell-specific gene expression of Tcf/Lef and, more specifically, we investigated whether damage-induced TGF-β impairs the expression and function of TCF7L2, using several models of MSCs, including skeletal muscle fibro-adipogenic progenitors. We show that Tcf/Lefs are differentially expressed and that TGF-β reduces the expression of TCF7L2 in MSCs but not in myoblasts. We also found that the ubiquitin-proteasome system regulates TCF7L2 proteostasis and participates in TGF-β-mediated TCF7L2 protein downregulation. Finally, we show that TGF-β requires HDACs activity to repress the expression of TCF7L2. Thus, our work found a novel interplay between TGF-β and Wnt canonical signaling cascades in PDGFRα+ fibroblasts and suggests that this mechanism could be targeted in tissue repair and regeneration.

Adherent muscle connective tissue fibroblasts are phenotypically and biochemically equivalent to stromal fibro/adipogenic progenitors

Extracellular matrix (ECM) gives structure, support, and is the niche for several cells found in skeletal muscle. ECM is mainly produced by muscle connective tissue (CT) fibroblasts during development and regeneration. Stromal fibroadipogenic progenitors (FAPs) are CT fibroblasts-like mesenchymal progenitors (MPs) with important roles in regeneration and degeneration. Chronic damage restrains the normal regenerative behavior of muscle fibroblasts/FAPs. Thus, the isolation and study of these mesenchymal progenitors are of crucial importance for understanding their behavior and biology. We investigated whether adult muscle CT fibroblasts (hereafter referred to as adherent fibroblasts [aFbs]) cultured via pre-plating strategy belong to a heterogeneous population of FAPs. By combining microscopy, western blot analyses, flow cytometry, and FACS we determined that aFbs isolated from skeletal muscle largely overlap with FAPs. In addition, we used the PDGFRαEGFP mice in order to corroborate our results with EGFP+ FAPs. Moreover, our strategy allows the isolation of activated EGFP+ FAPs from the murine DMD model PDGFRαEGFP; mdx and PDGFRαEGFP denervated mice. Here we report that 1 h 30 min of pre-plating strategy allows the isolation and culture of a highly enriched population of aFbs. These cells are phenotypically and biochemically a FAPs-like population of adherent cells. In addition, aFbs respond in the same fashion as FAPs to Nilotinib, an inducer of FAPs apoptosis. Moreover, flow cytometry characterization of these aFbs suggests that 85% of them express the MP marker PDGFRα, and isolation of aFbs from the PDGFRαEGFP mice suggests that 75% of them show high EGFP expression. Furthermore, TGF-β1 induces aFbs proliferation, myofibroblast differentiation, and ECM production. We were also able to isolate activated aFbs from skeletal muscle of the DMD mice and from the PDGFRαEGFP mice 2-days after denervation. Our findings suggest that the in vitro pre-plating strategy allows the isolation and culture of a relatively pure aFbs population, which resembles FAPs in vitro.

LPA receptor 4 deficiency attenuates experimental atherosclerosis

The widely expressed lysophosphatidic acid (LPA) selective receptor 4 (LPAR4) contributes to vascular development in mice and zebrafish. LPAR4 regulates endothelial permeability, lymphocyte migration, and hematopoiesis, which could contribute to atherosclerosis. We investigated the role of LPAR4 in experimental atherosclerosis elicited by adeno-associated virus expressing PCSK9 to lower LDL receptor levels. After 20 weeks on a Western diet, cholesterol levels and lipoprotein distribution were similar in WT male and Lpar4Y/- mice (P = 0.94). The atherosclerotic lesion area in the proximal aorta and arch was ∼25% smaller in Lpar4Y/- mice (P = 0.009), and less atherosclerosis was detected in Lpar4Y/- mice at any given plasma cholesterol. Neutral lipid accumulation in aortic root sections occupied ∼40% less area in Lpar4Y/- mice (P = 0.001), and CD68 expression was ∼25% lower (P = 0.045). No difference in α-smooth muscle actin staining was observed. Bone marrow-derived macrophages isolated from Lpar4Y/- mice displayed significantly increased upregulation of the M2 marker Arg1 in response to LPA compared with WT cells. In aortic root sections from Lpar4Y/- mice, heightened M2 "repair" macrophage marker expression was detected by CD206 staining (P = 0.03). These results suggest that LPAR4 may regulate the recruitment of specific sets of macrophages or their phenotypic switching in a manner that could influence the development of atherosclerosis.

Mechanical regulation of myofibroblast phenoconversion and collagen contraction

Activated fibroblasts promote physiological wound repair following tissue injury. However, dysregulation of fibroblast activation contributes to the development of fibrosis by enhanced production and contraction of collagen-rich extracellular matrix. At the peak of their activities, fibroblasts undergo phenotypic conversion into highly contractile myofibroblasts by developing muscle-like features, including formation of contractile actin-myosin bundles. The phenotype and function of fibroblasts and myofibroblasts are mechanically regulated by matrix stiffness using a feedback control system that is integrated with the progress of tissue remodelling. The actomyosin contraction machinery and cell-matrix adhesion receptors are critical elements that are needed for mechanosensing by fibroblasts and the translation of mechanical signals into biological responses. Here, we focus on mechanical and chemical regulation of collagen contraction by fibroblasts and the involvement of these factors in their phenotypic conversion to myofibroblasts.

The cross-talk between TGF-β and PDGFRα signaling pathways regulates stromal fibro/adipogenic progenitors’ fate

Fibro/adipogenic progenitors (FAPs) are tissue-resident mesenchymal stromal cells (MSCs) required for proper skeletal muscle development, regeneration, and maintenance. However, FAPs are also responsible for fibro-fatty scar deposition following chronic damage. We aimed to study a functional cross-talk between TGF-β and PDGFRα signaling pathways in FAPs’ fate. Here, we show that the number of FAPs correlates with TGF-β levels and with extracellular matrix deposition during regeneration and repair. Interestingly, the expression of PDGFRα changed dynamically in the stromal/fibroblast lineage after injury. Furthermore, PDGFRα-dependent immediate early gene expression changed during regeneration and repair. We also found that TGF-β signaling reduces PDGFRα expression in FAPs, mouse dermal fibroblasts, and in two related mesenchymal/fibroblast cell lines. Moreover, TGF-β promotes myofibroblast differentiation of FAPs but inhibits their adipogenicity. Accordingly, TGF-β impairs the expression of PDGFRα-dependent immediate early genes in a TGF-BR1-dependent manner. Finally, pharmacological inhibition of PDGFRα activity with AG1296 impaired TGF-β-induced extracellular matrix remodeling, Smad2 signaling, myofibroblast differentiation, and migration of MSCs. Thus, our work establishes a functional cross-talk between TGF-β and PDGFRα signaling pathways that is involved in regulating the biology of FAPs/MSCs.