Extracellular fibrillar structure of latent TGF beta binding protein-1: role in TGF beta-dependent endothelial-mesenchymal transformation during endocardial cushion tissue formation in mouse embryonic heart

Loss of myocardial Hey2/Hrt2 function disrupts rightward shift of atrioventricular cushion tissue and causes tricuspid atresia

BACKGROUND

Endocardial cushion tissue is primordia of the valves and septa of the adult heart, and its malformation causes various congenital heart diseases (CHDs). Tricuspid atresia (TA) is defined as congenital absence or agenesis of the tricuspid valve caused by endocardial cushion defects. However, little is known about what type of endocardial cushion defect causes TA.

RESULTS

Using three-dimensional volume rendering image analysis, we demonstrated morphological changes of endocardial cushion tissue in developing Hey2/Hrt2 KO mouse embryos that showed malformation of the tricuspid valve, which resembled human TA at neonatal period. In control embryos, atrioventricular (AV) endocardial cushion tissues showed rightward shift to form a tricuspid valve. However, the rightward shift of endocardial cushion tissue was disrupted in Hey2/Hrt2 KO embryos, leading to the misalignment of AV cushions. We also found that muscular tissue filled up the space between the right atrium and ventricle, resulting in the absence of the tricuspid valve. Moreover, analysis using tissue-specific conditional KO mice showed that HEY2/HRT2-expressing myocardium may physically regulate the AV shift.

CONCLUSION

Disruption of rightward cushion movement is an initial cue of TA phenotype, and myocardial HEY2/HRT2 is necessary for the regulation of proper alignment of AV endocardial cushion tissue.

Latent Transforming Growth Factor β Binding Protein 3 Controls Adipogenesis

Transforming growth factor-beta (TGFβ) is released from cells as part of a trimeric latent complex consisting of TGFβ, the TGFβ propeptides, and either a latent TGFβ binding protein (LTBP) or glycoprotein-A repetitions predominant (GARP) protein. LTBP1 and 3 modulate latent TGFβ function with respect to secretion, matrix localization, and activation and, therefore, are vital for the proper function of the cytokine in a number of tissues. TGFβ modulates stem cell differentiation into adipocytes (adipogenesis), but the potential role of LTBPs in this process has not been studied. We observed that 72 h post adipogenesis initiation Ltbp1, 2, and 4 expression levels decrease by 74-84%, whereas Ltbp3 expression levels remain constant during adipogenesis. We found that LTBP3 silencing in C3H/10T1/2 cells reduced adipogenesis, as measured by the percentage of cells with lipid vesicles and the expression of the transcription factor peroxisome proliferator-activated receptor gamma (PPARγ). Lentiviral mediated expression of an Ltbp3 mRNA resistant to siRNA targeting rescued the phenotype, validating siRNA specificity. Knockdown (KD) of Ltbp3 expression in 3T3-L1, M2, and primary bone marrow stromal cells (BMSC) indicated a similar requirement for Ltbp3. Epididymal and inguinal white adipose tissue fat pad weights of Ltbp3^-/- mice were reduced by 62% and 57%, respectively, compared to wild-type mice. Inhibition of adipogenic differentiation upon LTBP3 loss is mediated by TGFβ, as TGFβ neutralizing antibody and TGFβ receptor I kinase blockade rescue the LTBP3 KD phenotype. These results indicate that LTBP3 has a TGFβ-dependent function in adipogenesis both in vitro and possibly in vivo.

Post-Transcriptional Regulation of Molecular Determinants during Cardiogenesis

Cardiovascular development is initiated soon after gastrulation as bilateral precardiac mesoderm is progressively symmetrically determined at both sides of the developing embryo. The precardiac mesoderm subsequently fused at the embryonic midline constituting an embryonic linear heart tube. As development progress, the embryonic heart displays the first sign of left-right asymmetric morphology by the invariably rightward looping of the initial heart tube and prospective embryonic ventricular and atrial chambers emerged. As cardiac development progresses, the atrial and ventricular chambers enlarged and distinct left and right compartments emerge as consequence of the formation of the interatrial and interventricular septa, respectively. The last steps of cardiac morphogenesis are represented by the completion of atrial and ventricular septation, resulting in the configuration of a double circuitry with distinct systemic and pulmonary chambers, each of them with distinct inlets and outlets connections. Over the last decade, our understanding of the contribution of multiple growth factor signaling cascades such as Tgf-beta, Bmp and Wnt signaling as well as of transcriptional regulators to cardiac morphogenesis have greatly enlarged. Recently, a novel layer of complexity has emerged with the discovery of non-coding RNAs, particularly microRNAs and lncRNAs. Herein, we provide a state-of-the-art review of the contribution of non-coding RNAs during cardiac development. microRNAs and lncRNAs have been reported to functional modulate all stages of cardiac morphogenesis, spanning from lateral plate mesoderm formation to outflow tract septation, by modulating major growth factor signaling pathways as well as those transcriptional regulators involved in cardiac development.

The role of LTBPs in TGF beta signaling

The purpose of this review is to discuss the transforming growth factor beta (TGFB) binding proteins (LTBP) with respect to their participation in the activity of TGFB. We first describe pertinent aspects of the biology and cell function of the LTBPs. We then summarize the physiological consequences of LTBP loss in humans and mice. Finally, we consider a number of outstanding questions relating to LTBP function.

Re-Programming Autoreactive T Cells Into T-Regulatory Type 1 Cells for the Treatment of Autoimmunity

Systemic delivery of peptide-major histocompatibility complex (pMHC) class II-based nanomedicines can re-program cognate autoantigen-experienced CD4+ T cells into disease-suppressing T-regulatory type 1 (TR1)-like cells. In turn, these TR1-like cells trigger the formation of complex regulatory cell networks that can effectively suppress organ-specific autoimmunity without impairing normal immunity. In this review, we summarize our current understanding of the transcriptional, phenotypic and functional make up of TR1-like cells as described in the literature. The true identity and direct precursors of these cells remain unclear, in particular whether TR1-like cells comprise a single terminally-differentiated lymphocyte population with distinct transcriptional and epigenetic features, or a collection of phenotypically different subsets sharing key regulatory properties. We propose that detailed transcriptional and epigenetic characterization of homogeneous pools of TR1-like cells will unravel this conundrum.

Latent TGF-beta binding protein-1 plays an important role in craniofacial development

OBJECTIVE

This study aims to replicate the phenotype of Ltbp1 knockout mice in zebrafish, and to address the function of LTBP1 in craniofacial development.

METHODS

Whole mount in situ hybridization (WISH) of ltbp1 was performed at critical periods of zebrafish craniofacial development to explore the spatial-temporal expression pattern. Furthermore, we generated morpholino based knockdown model of ltbp1 to study the craniofacial phenotype.

RESULTS

WISH of ltbp1 was mainly detected in the mandibular jaw region, brain trunk, and internal organs such as pancreas and gallbladder. And ltbp1 colocalized with both sox9a and ckma in mandibular region. Morpholino based knockdown of ltbp1 results in severe jaw malformation. Alcian blue staining revealed severe deformity of Meckel's cartilage along with the absence of ceratobranchial. Three-dimension measurements of ltbp1 morphants jaws showed decrease in both mandible length and width and increase in open mouth distance. Expression of cartilage marker sox9a and muscle marker ckma was decreased in ltbp1 morphants.

CONCLUSIONS

Our experiments found that ltbp1 was expressed in zebrafish mandibular jaw cartilages and the surrounding muscles. The ltbp1 knockdown zebrafish exhibited phenotypes consistent with Ltbp1 knockout mice. And loss of ltbp1 function lead to significant mandibular jaw defects and affect both jaw cartilages and surrounding muscles.

Extracellular Matrix Analysis of Human Renal Arteries in Both Quiescent and Active Vascular State

In vascular tissue engineering strategies, the addition of vascular-specific extracellular matrix (ECM) components may better mimic the in vivo microenvironment and potentially enhance cell-matrix interactions and subsequent tissue growth. For this purpose, the exact composition of the human vascular ECM first needs to be fully characterized. Most research has focused on characterizing ECM components in mature vascular tissue; however, the developing fetal ECM matches the active environment required in vascular tissue engineering more closely. Consequently, we characterized the ECM protein composition of active (fetal) and quiescent (mature) renal arteries using a proteome analysis of decellularized tissue. The obtained human fetal renal artery ECM proteome dataset contains higher levels of 15 ECM proteins versus the mature renal artery ECM proteome, whereas 16 ECM proteins showed higher levels in the mature tissue compared to fetal. Elastic ECM proteins EMILIN1 and FBN1 are significantly enriched in fetal renal arteries and are mainly produced by cells of mesenchymal origin. We functionally tested the role of EMILIN1 and FBN1 by anchoring the ECM secreted by vascular smooth muscle cells (SMCs) to glass coverslips. This ECM layer was depleted from either EMILIN1 or FBN1 by using siRNA targeting of the SMCs. Cultured endothelial cells (ECs) on this modified ECM layer showed alterations on the transcriptome level of multiple pathways, especially the Rho GTPase controlled pathways. However, no significant alterations in adhesion, migration or proliferation were observed when ECs were cultured on EMILIN1- or FNB1-deficient ECM. To conclude, the proteome analysis identified unique ECM proteins involved in the embryonic development of renal arteries. Alterations in transcriptome levels of ECs cultured on EMILIN1- or FBN1-deficient ECM showed that these candidate proteins could affect the endothelial (regenerative) response.

TGF-β1 - A truly transforming growth factor in fibrosis and immunity

'Jack of all trades, master of everything' is a fair label for transforming growth factor β1 (TGF-β) - a cytokine that controls our life at many levels. In the adult organism, TGF-β1 is critical for the development and maturation of immune cells, maintains immune tolerance and homeostasis, and regulates various aspects of immune responses. Following acute tissue damages, TGF-β1 becomes a master regulator of the healing process with impacts on about every cell type involved. Divergence from the tight control of TGF-β1 actions, for instance caused by chronic injury, severe trauma, or infection can tip the balance from regulated physiological to excessive pathological repair. This condition of fibrosis is characterized by accumulation and stiffening of collagenous scar tissue which impairs organ functions to the point of failure. Fibrosis and dysregulated immune responses are also a feature of cancer, in which tumor cells escape immune control partly by manipulating TGF-β1 regulation and where immune cells are excluded from the tumor by fibrotic matrix created during the stroma 'healing' response. Despite the obvious potential of TGF-β-signalling therapies, globally targeting TGF-β1 receptor, downstream pathways, or the active growth factor have proven to be extremely difficult if not impossible in systemic treatment regimes. However, TGF-β1 binding to cell receptors requires prior activation from latent complexes that are extracellularly presented on the surface of immune cells or within the extracellular matrix. These different locations have led to some divergence in the field which is often either seen from the perspective of an immunologists or a fibrosis/matrix researcher. Despite these human boundaries, there is considerable overlap between immune and tissue repair cells with respect to latent TGF-β1 presentation and activation. Moreover, the mechanisms and proteins employed by different cells and spatiotemporal control of latent TGF-β1 activation provide specificity that is amenable to drug development. This review aims at synthesizing the knowledge on TGF-β1 extracellular activation in the immune system and in fibrosis to further stimulate cross talk between the two research communities in solving the TGF-β conundrum.

Isoform-specific effects of transforming growth factor β on endothelial-to-mesenchymal transition

Endothelial-to-mesenchymal transition (EndMT) was first reported in the embryogenesis. Recent studies show that EndMT also occurs in the disease progression of atherosclerosis, cardiac and pulmonary fibrosis, pulmonary hypertension, diabetic nephropathy, and cancer. Although transforming growth factor β (TGFβ) is crucial for EndMT, it is not clear which isoform elicits a predominant effect. The current study aims to directly compare the dose-dependent effects of TGFβ1, TGFβ2, and TGFβ3 on EndMT and characterize the underlying mechanisms. In our results, all three TGFβ isoforms induced EndMT in human microvascular endothelial cells after 72 hr, as evidenced by the increased expression of mesenchymal markers N-cadherin and α-smooth muscle actin as well as the decreased expression of endothelial nitric oxide synthase. Interestingly, the effect of TGFβ2 was the most pronounced. At 1 ng/ml, only TGFβ2 treatment resulted in significantly increased phosphorylation (activation) of Smad2/3 and p38-MAPK and increased expression of mesenchymal transcription factors Snail and FoxC2. Intriguingly, we observed that treatment with 1 ng/ml TGFβ1 and TGFβ3, but not TGFβ2, resulted in an increased expression of TGFβ2, thus indicating that EndMT with TGFβ1 and TGFβ3 treatments was due to the secondary effects through TGFβ2 secretion. Furthermore, silencing TGFβ2 using small interfering RNA blunted the expression of EndMT markers in TGFβ1- and TGFβ3-treated cells. Together, our results indicate that TGFβ2 is the most potent inducer of EndMT and that TGFβ1- and TGFβ3-induced EndMT necessitates a paracrine loop involving TGFβ2.

TGF-β Signaling in Control of Cardiovascular Function

Genetic studies in animals and humans indicate that gene mutations that functionally perturb transforming growth factor β (TGF-β) signaling are linked to specific hereditary vascular syndromes, including Osler-Rendu-Weber disease or hereditary hemorrhagic telangiectasia and Marfan syndrome. Disturbed TGF-β signaling can also cause nonhereditary disorders like atherosclerosis and cardiac fibrosis. Accordingly, cell culture studies using endothelial cells or smooth muscle cells (SMCs), cultured alone or together in two- or three-dimensional cell culture assays, on plastic or embedded in matrix, have shown that TGF-β has a pivotal effect on endothelial and SMC proliferation, differentiation, migration, tube formation, and sprouting. Moreover, TGF-β can stimulate endothelial-to-mesenchymal transition, a process shown to be of key importance in heart valve cushion formation and in various pathological vascular processes. Here, we discuss the roles of TGF-β in vasculogenesis, angiogenesis, and lymphangiogenesis and the deregulation of TGF-β signaling in cardiovascular diseases.

LTBPs in biology and medicine: LTBP diseases

The latent transforming growth factor (TGF) β binding proteins (LTBP) are crucial mediators of TGFβ function, as they control growth factor secretion, matrix deposition, presentation and activation. Deficiencies in specific LTBP isoforms yield discrete phenotypes representing defects in bone, lung and cardiovascular development mediated by loss of TGFβ signaling. Additional phenotypes represent loss of unique TGFβ-independent features of LTBP effects on elastogenesis and microfibril assembly. Thus, the LTBPs act as sensors for the regulation of both growth factor activity and matrix function.

Egr2 and Egr3 in regulatory T cells cooperatively control systemic autoimmunity through Ltbp3-mediated TGF-β3 production

Systemic lupus erythematosus (SLE) is a prototypical autoimmune disease characterized by multiorgan inflammation induced by autoantibodies. Early growth response gene 2 (Egr2), a transcription factor essential for T-cell anergy induction, controls systemic autoimmunity in mice and humans. We have previously identified a subpopulation of CD4⁺ regulatory T cells, CD4⁺CD25^-LAG3⁺ cells, that characteristically express both Egr2 and LAG3 and control mice model of lupus via TGF-β3 production. However, due to the mild phenotype of lymphocyte-specific Egr2-deficient mice, the presence of an additional regulator has been speculated. Here, we show that Egr2 and Egr3 expressed in T cells cooperatively prevent humoral immune responses by supporting TGF-β3 secretion. T cell-specific Egr2/Egr3 double-deficient (Egr2/3DKO) mice spontaneously developed an early onset lupus-like disease that was more severe than in T cell-specific Egr2-deficient mice. In accordance with the observation that CD4⁺CD25^-LAG3⁺ cells from Egr2/3DKO mice completely lost the capacity to produce TGF-β3, the excessive germinal center reaction in Egr2/3DKO mice was suppressed by the adoptive transfer of WT CD4⁺CD25^-LAG3⁺ cells or treatment with a TGF-β3-expressing vector. Intriguingly, latent TGF-β binding protein (Ltbp)3 expression maintained by Egr2 and Egr3 was required for TGF-β3 production from CD4⁺CD25^-LAG3⁺ cells. Because Egr2 and Egr3 did not demonstrate cell intrinsic suppression of the development of follicular helper T cells, Egr2- and Egr3-dependent TGF-β3 production by CD4⁺CD25^-LAG3⁺ cells is critical for controlling excessive B-cell responses. The unique attributes of Egr2/Egr3 in T cells may provide an opportunity for developing novel therapeutics for autoantibody-mediated diseases including SLE.

Endothelial-to-mesenchymal transition: Cytokine-mediated pathways that determine endothelial fibrosis under inflammatory conditions

During the last decade, the endothelial-to-mesenchymal transition (EndMT) process has attracted considerable attention due to associations with the onset of certain diseases, such as organ fibrosis and cancer. Several studies have assessed the mechanisms and signaling pathways that regulate endothelial fibrosis in the context of human pathologies. A number of inflammatory mediators, including pro-inflammatory cytokines, growth factors, oxidative stress, and toxins, induce the conversion of endothelial cells into mesenchymal fibroblast-like cells that promote disease progression. This review is separated into five chapters that critically present current knowledge on EndMT in the context of pathology. First, the main characteristics of EndMT are summarized, with a focus on the endothelial protein pattern changes that modulate the expressions of endothelial/fibrotic markers and extracellular matrix proteins. These expressions could serve as mechanisms for explaining potential EndMT contributions to human pathologies in adults. Second, the main findings supporting a connection between EndMT-mediated endothelial fibrosis and inflammatory conditions are presented. These connections could be linked to the onset and progression of pathological conditions. Third, EndMT inducers are described in detail. This includes considerations on the actions of the first and most well-known EndMT inducer, TGF-β; of the most prominent pro-inflammatory cytokines released during inflammation, such as IL 1-β and TNF-α; and of the NF-κB transcription factor, a common player during inflammation-induced EndMT. Furthermore, thorough attention is given to EndMT induction by endotoxins in the context of bacterial infectious diseases. Additionally, the participation of the inflammatory oxidative stress environment in the EndMT induction was also reviewed. Fourth, the pathophysiological findings of inflammation-induced EndMT are presented, and, fifth, special focus is placed on associations with cancer onset and development. Altogether, this review highlights the important role of EndMT-mediated endothelial fibrosis during inflammation in human pathologies.

Aggregatibacter actinomycetemcomitans outer membrane protein 29 (Omp29) induces TGF-β-regulated apoptosis signal in human gingival epithelial cells via fibronectin/integrinβ1/FAK cascade

Gingival junctional epithelial cell apoptosis caused by periodontopathic bacteria exacerbates periodontitis. This pathological apoptosis is involved in the activation of transforming growth factor β (TGF-β). However, the molecular mechanisms by which microbes induce the activation of TGF-β remain unclear. We previously reported that Aggregatibacter actinomycetemcomitans (Aa) activated TGF-β receptor (TGF-βR)/smad2 signalling to induce epithelial cell apoptosis, even though Aa cannot bind to TGF-βR. Additionally, outer membrane protein 29 kDa (Omp29), a member of the Aa Omps family, can induce actin rearrangements via focal adhesion kinase (FAK) signalling, which also plays a role in the activation of TGF-β by cooperating with integrin. Accordingly, we hypothesized that Omp29-induced actin rearrangements via FAK activity would enhance the activation of TGF-β, leading to gingival epithelial cell apoptosis in vitro. By using human gingival epithelial cell line OBA9, we found that Omp29 activated TGF-βR/smad2 signalling and decreased active TGF-β protein levels in the extracellular matrix (ECM) of cell culture, suggesting the transactivation of TGF-βR. Inhibition of actin rearrangements by cytochalasin D or blebbistatin and knockdown of FAK or integrinβ1 expression by siRNA transfection attenuated TGF-βR/smad2 signalling activity and reduction of TGF-β levels in the ECM caused by Omp29. Furthermore, Omp29 bound to fibronectin (Fn) to induce its aggregation on integrinβ1, which is associated with TGF-β signalling activity. All the chemical inhibitors and siRNAs tested blocked Omp29-induced OBA9 cells apoptosis. These results suggest that Omp29 binds to Fn in order to facilitate Fn/integrinβ1/FAK signalling-dependent TGF-β release from the ECM, thereby inducing gingival epithelial cell apoptosis via TGF-βR/smad2 pathway.

Fibrillin-containing microfibrils are key signal relay stations for cell function

Fibrillins constitute the backbone of microfibrils in the extracellular matrix of elastic and non-elastic tissues. Mutations in fibrillins are associated with a wide range of connective tissue disorders, the most common is Marfan syndrome. Microfibrils are on one hand important for structural stability in some tissues. On the other hand, microfibrils are increasingly recognized as critical mediators and drivers of cellular signaling. This review focuses on the signaling mechanisms initiated by fibrillins and microfibrils, which are often dysregulated in fibrillin-associated disorders. Fibrillins regulate the storage and bioavailability of growth factors of the TGF-β superfamily. Cells sense microfibrils through integrins and other receptors. Fibrillins potently regulate pathways of the immune response, inflammation and tissue homeostasis. Emerging evidence show the involvement of microRNAs in disorders caused by fibrillin deficiency. A thorough understanding of fibrillin-mediated cell signaling pathways will provide important new leads for therapeutic approaches of the underlying disorders.

Structure and function of the mammalian fibrillin gene family: implications for human connective tissue diseases

Fibrillins and latent transforming growth factor β binding proteins (LTBPs) are components of the extracellular matrix of connective tissue. While fibrillins are integral to the 10nm microfibrils, and often associated with elastin, all family members are likely to have an additional role in regulating the bioavailability of transforming growth factor β (TGBβ). Both fibrillins and LTBPs are large glycoproteins, containing a series of calcium binding epidermal growth factor domains as well as a number of copies of a unique 8 cysteine domain found only in this protein superfamily. There are three mammalian fibrillins and four LTBPs. Fibrillin monomers link head to tail in microfibrils which can then form two and three dimensional structures. In some tissues elastin is recruited to the fibrillin microfibrils to provide elasticity to the tissue. LTBPs are part of the TGBβ large latent complex which sequesters TGBβ in the extracellular matrix. Fibrillin-1 appears to bind to LTBPs to assist in this process and is thus involved in regulating the bioavailability of TGBβ. Mutation of fibrillin genes results in connective tissue phenotypes which reflect both the increased level of active TGBβ and the structural failure of the extracellular matrix due to the absence or abnormality of fibrillin protein. Fibrillinopathies include Marfan syndrome, familial ectopia lentis, familial thoracic aneurysm (mutations of FBN1) and congenital contractural arachnodactyly (mutation of FBN2). There are no diseases currently associated with mutation of FBN3 in humans, and this gene is no longer active in rodents. Expression patterns of fibrillin genes are consistent with their role in extracellular matrix structure of connective tissue. FBN1 expression is high in most cell types of mesenchymal origin, particularly bone. Human and mouse FBN2 expression is high in fetal cells and has more restricted expression in mesenchymal cell types postnatally. FBN3 is expressed early in development (embryonic and fetal tissues) in humans. The fibrillins are thus important in maintaining the structure and integrity of the extracellular matrix and, in combination with their sequence family members the LTBPs, also contribute to the regulation of the TGFβ family of major growth factors.

New insights into the regulation of epithelial-mesenchymal transition and tissue fibrosis

Tissue fibrosis often presents as the final outcome of chronic disease and is a significant cause of morbidity and mortality worldwide. Fibrosis is driven by continuous expansion of fibroblasts and myofibroblasts. Epithelial-mesenchymal transition (EMT) is a form of cell plasticity in which epithelia acquire mesenchymal phenotypes and is increasingly recognized as an integral aspect of tissue fibrogenesis. In this review, we describe recent insight into the molecular and cellular factors that regulate EMT and its underlying signaling pathways. We also consider how mechanical cues from the microenvironment affect the regulation of EMT. Finally, we discuss the role of EMT in fibrotic diseases and propose approaches for detecting and treating fibrogenesis by targeting EMT.

Loss of muscleblind-like 1 promotes invasive mesenchyme formation in endocardial cushions by stimulating autocrine TGFβ3

BACKGROUND

Valvulogenesis and septation in the developing heart depend on the formation and remodeling of endocardial cushions in the atrioventricular canal (AVC) and outflow tract (OFT). These cushions are invaded by a subpopulation of endocardial cells that undergo an epithelial-mesenchymal transition in response to paracrine and autocrine transforming growth factor β (TGFβ) signals. We previously demonstrated that the RNA binding protein muscleblind-like 1 (MBNL1) is expressed specifically in the cushion endocardium, and knockdown of MBNL1 in stage 14 embryonic chicken AVC explants enhances TGFβ-dependent endocardial cell invasion.

RESULTS

In this study, we demonstrate that the effect of MBNL1 knockdown on invasion remains dependent on TGFβ3 after it is no longer required to induce basal levels of invasion. TGFβ3, but not TGFβ2, levels are elevated in medium conditioned by MBNL1-depleted AVC explants. TGFβ3 is elevated even when the myocardium is removed, indicating that MBNL1 modulates autocrine TGFβ3 production in the endocardium. More TGFβ3-positive cells are observed in the endocardial monolayer following MBNL1 knockdown. Addition of exogenous TGFβ3 to AVC explants recapitulates the effects of MBNL1 knockdown. Time course experiments demonstrate that knockdown of MBNL1 induces precocious TGFβ3 secretion, and early exposure to excess TGFβ3 induces precocious invasion. MBNL1 expression precedes TGFβ3 in the AVC endocardium, consistent with a role in preventing precocious autocrine TGFβ3 signaling. The stimulatory effects of MBNL1 knockdown on invasion are lost in stage 16 AVC explants. Knockdown of MBNL1 in OFT explants similarly enhances cell invasion, but not activation. TGFβ is necessary and sufficient to mediate this effect.

CONCLUSIONS

Taken together, these data support a model in which MBNL1 negatively regulates cell invasion in the endocardial cushions by restricting the magnitude and timing of endocardial-derived TGFβ3 production.

Serotonin potentiates transforming growth factor-beta3 induced biomechanical remodeling in avian embryonic atrioventricular valves

Embryonic heart valve primordia (cushions) maintain unidirectional blood flow during development despite an increasingly demanding mechanical environment. Recent studies demonstrate that atrioventricular (AV) cushions stiffen over gestation, but the molecular mechanisms of this process are unknown. Transforming growth factor-beta (TGFβ) and serotonin (5-HT) signaling modulate tissue biomechanics of postnatal valves, but less is known of their role in the biomechanical remodeling of embryonic valves. In this study, we demonstrate that exogenous TGFβ3 increases AV cushion biomechanical stiffness and residual stress, but paradoxically reduces matrix compaction. We then show that TGFβ3 induces contractile gene expression (RhoA, aSMA) and extracellular matrix expression (col1α2) in cushion mesenchyme, while simultaneously stimulating a two-fold increase in proliferation. Local compaction increased due to an elevated contractile phenotype, but global compaction appeared reduced due to proliferation and ECM synthesis. Blockade of TGFβ type I receptors via SB431542 inhibited the TGFβ3 effects. We next showed that exogenous 5-HT does not influence cushion stiffness by itself, but synergistically increases cushion stiffness with TGFβ3 co-treatment. 5-HT increased TGFβ3 gene expression and also potentiated TGFβ3 induced gene expression in a dose-dependent manner. Blockade of the 5HT2b receptor, but not 5-HT2a receptor or serotonin transporter (SERT), resulted in complete cessation of TGFβ3 induced mechanical strengthening. Finally, systemic 5-HT administration in ovo induced cushion remodeling related defects, including thinned/atretic AV valves, ventricular septal defects, and outflow rotation defects. Elevated 5-HT in ovo resulted in elevated remodeling gene expression and increased TGFβ signaling activity, supporting our ex-vivo findings. Collectively, these results highlight TGFβ/5-HT signaling as a potent mechanism for control of biomechanical remodeling of AV cushions during development.

Long form of latent TGF-β binding protein 1 (Ltbp1L) regulates cardiac valve development

Transforming Growth Factor β (TGF-β) is crucial for valve development and homeostasis. The long form of Latent TGF-β binding protein 1 (LTBP1L) covalently binds all TGF-β isoforms and regulates their bioavailability. Ltbp1L expression analysis during valvulogenesis revealed two patterns of Ltbp1L production: an early one (E9.5-11.5) associated with endothelial-to-mesenchymal transformation (EMT); and a late one (E12.5 to birth) contemporaneous with valve remodeling. Similarly, histological analysis of Ltbp1L(-/-) developing valves identified two different pathologies: generation of hypoplastic endocardial cushions in early valvulogenesis, followed by development of hyperplastic valves in late valvulogenesis. Ltbp1L promotes valve EMT, as Ltbp1L absence yields hypoplastic endocardial cushions in vivo and attenuated EMT in vitro. Ltbp1L(-/-) valve hyperplasia in late valvuogenesis represents a consequence of prolonged EMT. We demonstrate that Ltbp1L is a major regulator of Tgf-β activity during valvulogenesis since its absence results in a perturbed Tgf-β pathway that causes all Ltbp1L(-/-) valvular defects.

An ADAMTSL2 founder mutation causes Musladin-Lueke Syndrome, a heritable disorder of beagle dogs, featuring stiff skin and joint contractures

Background

Musladin-Lueke Syndrome (MLS) is a hereditary disorder affecting Beagle dogs that manifests with extensive fibrosis of the skin and joints. In this respect, it resembles human stiff skin syndrome and the Tight skin mouse, each of which is caused by gene defects affecting fibrillin-1, a major component of tissue microfibrils. The objective of this work was to determine the genetic basis of MLS and the molecular consequence of the identified mutation.

Methodology and Principal Findings

We mapped the locus for MLS by genome-wide association to a 3.05 Mb haplotype on canine chromosome 9 (CFA9 (50.11–54.26; p_raw <10⁻⁷)), which was homozygous and identical-by-descent among all affected dogs, consistent with recessive inheritance of a founder mutation. Sequence analysis of a candidate gene at this locus, ADAMTSL2, which is responsible for the human TGFβ dysregulation syndrome, Geleophysic Dysplasia (GD), uncovered a mutation in exon 7 (c.660C>T; p.R221C) perfectly associated with MLS (p-value = 10⁻¹²). Murine ADAMTSL2 containing the p.R221C mutation formed anomalous disulfide-bonded dimers when transiently expressed in COS-1, HEK293F and CHO cells, and was present in the medium of these cells at lower levels than wild-type ADAMTSL2 expressed in parallel.

Conclusions/Significance

The genetic basis of MLS is a founder mutation in ADAMTSL2, previously shown to interact with latent TGF-β binding protein, which binds fibrillin-1. The molecular effect of the founder mutation on ADAMTSL2 is formation of disulfide-bonded dimers. Although caused by a distinct mutation, and having a milder phenotype than human GD, MLS nevertheless offers a new animal model for study of GD, and for prospective insights on mechanisms and pathways of skin fibrosis and joint contractures.

Endothelial cell-derived endothelin-1 promotes cardiac fibrosis in diabetic hearts through stimulation of endothelial-to-mesenchymal transition

BACKGROUND

Persistently high plasma endothelin-1 (ET-1) levels in diabetic patients have been associated with the development of cardiac fibrosis, which results from the deposition of extracellular matrix and fibroblast recruitment from an as-yet unknown source. The underlying mechanism, however, remains elusive. Here, we hypothesize that ET-1 might contribute to the accumulation of cardiac fibroblasts through an endothelial-to-mesenchymal transition in diabetic hearts.

METHODS AND RESULTS

We induced diabetes mellitus in vascular endothelial cell-specific ET-1 knockout [ET-1(f/f);Tie2-Cre (+)] mice and their wild-type littermates using the toxin streptozotocin. Gene expression and histological and functional parameters were examined at 8, 24, and 36 weeks after the induction of diabetes mellitus. Diabetes mellitus increased cardiac ET-1 expression in wild-type mice, leading to mitochondrial disruption and myofibril disarray through the generation of superoxide. Diabetic mice also showed impairment of cardiac microvascularization and a decrease in cardiac vascular endothelial growth factor expression. ET-1 further promotes cardiac fibrosis and heart failure through the accumulation of fibroblasts via endothelial-to-mesenchymal transition. All of these features were abolished in ET-1(f/f);Tie2-Cre (+) hearts. Targeted ET-1 gene silencing by small interfering RNA in cultured human endothelial cells ameliorated high glucose-induced phenotypic transition and acquisition of a fibroblast marker through the inhibition of transforming growth factor-beta signaling activation and preservation of the endothelial cell-to-cell contact regulator VE-cadherin.

CONCLUSIONS

These results provide new insights suggesting that diabetes mellitus-induced cardiac fibrosis is associated with the emergence of fibroblasts from endothelial cells and that this endothelial-to-mesenchymal transition process is stimulated by ET-1. Targeting endothelial cell-derived ET-1 might be beneficial in the prevention of diabetic cardiomyopathy.

Noonan syndrome cardiac defects are caused by PTPN11 acting in endocardium to enhance endocardial-mesenchymal transformation

Noonan syndrome (NS), the most common single-gene cause of congenital heart disease, is an autosomal dominant disorder that also features proportionate short stature, facial abnormalities, and an increased risk of myeloproliferative disease. Germline-activating mutations in PTPN11, which encodes the protein tyrosine phosphatase SHP2, cause about half of NS cases; other causative alleles include KRAS, SOS1, and RAF1 mutants. We showed previously that knock-in mice bearing the NS mutant Ptpn11(D61G) on a mixed 129S4/SvJae X C57BL6/J background exhibit all major NS features, including a variety of cardiac defects, with variable penetrance. However, the cellular and molecular mechanisms underlying NS cardiac defects and whether genetic background and/or the specific NS mutation contribute to the NS phenotype remained unclear. Here, using an inducible knock-in approach, we show that all cardiac defects in NS result from mutant Shp2 expression in the endocardium, not in the myocardium or neural crest. Furthermore, the penetrance of NS defects is affected by genetic background and the specific Ptpn11 allele. Finally, ex vivo assays and pharmacological approaches show that NS mutants cause cardiac valve defects by increasing Erk MAPK activation, probably downstream of ErbB family receptor tyrosine kinases, extending the interval during which cardiac endocardial cells undergo endocardial-mesenchymal transformation. Our data provide a mechanistic underpinning for the cardiac defects in this disorder.

Transforming growth factor-beta signaling in epithelial-mesenchymal transition and progression of cancer

Transforming growth factor-beta (TGF-beta) is a multifunctional cytokine that induces growth arrest, tissue fibrosis, and epithelial-mesenchymal transition (EMT) through activation of Smad and non-Smad signaling pathways. EMT is the differentiation switch by which polarized epithelial cells differentiate into contractile and motile mesenchymal cells. Cell motility and invasive capacity are activated upon EMT. Multiple transcription factors, including deltaEF1/ZEB1, SIP1/ZEB2, and Snail/SNAI1, are induced by TGF-beta-Smad signaling and play critical roles in TGF-beta-induced EMT. In addition, both non-Smad signaling activated by TGF-beta and cross-talk with other signaling pathways play important roles in induction of EMT. Of these, Ras signaling synergizes with TGF-beta-Smad signaling, and plays an important role in the induction of EMT. TGF-beta inhibitors prevent invasion and metastasis of advanced cancer through multiple mechanisms, including inhibition of EMT. The discovery of molecules that inhibit TGF-beta-induced EMT but not TGF-beta-induced growth arrest may be an ideal strategy for treatment of invasion and metastasis of cancer.

Latent transforming growth factor-beta-binding protein-4 regulates transforming growth factor-beta1 bioavailability for activation by fibrogenic lung fibroblasts in response to bleomycin

Recent evidence suggests that subsets of lung fibroblasts differentially contribute to fibrogenic progression. We have previously shown that a subset of rat lung fibroblasts with fibrogenic characteristics [Thy-1 (-) fibroblasts] responds to stimuli (bleomycin, interleukin-4, etc) with increased latent transforming growth factor (TGF)-beta activation, whereas non-fibrogenic Thy-1-expressing [Thy-1 (+)] fibroblasts do not. Activation of latent TGF-beta1 by interstitial lung fibroblasts is critical for fibrogenic responses. To better understand the susceptibility of fibrogenic fibroblasts to the stimulation of TGF-beta activation, we examined the role of latent TGF-beta-binding proteins (LTBPs), key regulators of TGF-beta bioavailability and activation, in TGF-beta1 activation by these fibroblasts. Treatment of fibroblasts with bleomycin up-regulated LTBP-4 mRNA, protein, and soluble LTBP-4-bound large latent TGF-beta1 complexes in Thy-1 (-) fibroblasts to significantly higher levels than in Thy-1 (+) fibroblasts. Bleomycin-induced TGF-beta1 activation required LTBP-4, since lung fibroblasts deficient in LTBP-4 did not activate TGF-beta1. Expression of LTBP-4 restored TGF-beta1 activation in response to bleomycin, but expression either of LTBP-4 lacking the TGF-beta-binding site or only the TGF-beta-binding domain did not. Bleomycin treatment of mice increased LTBP-4 expression in the lung. Thy-1 knockout mice had increased levels of both LTBP-4 expression and TGF-beta activation, as well as enhanced Smad3 phosphorylation compared with wild-type mice. Together, these data identify a critical role for LTBP-4 in the regulation of latent TGF-beta1 activation in bleomycin-induced lung fibrosis.

Absence of alphavbeta6 integrin is linked to initiation and progression of periodontal disease

Integrin alphavbeta6 is generally not expressed in adult epithelia but is induced in wound healing, cancer, and certain fibrotic disorders. Despite this generalized absence, we observed that alphavbeta6 integrin is constitutively expressed in the healthy junctional epithelium linking the gingiva to tooth enamel. Moreover, expression of alphavbeta6 integrin was down-regulated in human periodontal disease, a common medical condition causing tooth loss and also contributing to the development of cardiovascular diseases by increasing the total systemic inflammatory burden. Remarkably, integrin beta6 knockout mice developed classic signs of spontaneous, chronic periodontal disease with characteristic inflammation, epithelial down-growth, pocket formation, and bone loss around the teeth. Integrin alphavbeta6 acts as a major activator of transforming growth factor-beta1 (TGF-beta1), a key anti-inflammatory regulator in the immune system. Co-expression of TGF-beta1 and alphavbeta6 integrin was observed in the healthy junctional epithelium. Moreover, an antibody that blocks alphavbeta6 integrin-mediated activation of TGF-beta1 initiated inflammatory periodontal disease in a rat model of gingival inflammation. Thus, alphavbeta6 integrin is constitutively expressed in the epithelium sealing the gingiva to the tooth and plays a central role in protection against inflammatory periodontal disease through activation of TGF-beta1.

Long form of latent TGF-beta binding protein 1 (Ltbp1L) is essential for cardiac outflow tract septation and remodeling

Latent TGF-beta binding protein 1 (LTBP1) is a member of the LTBP/fibrillin family of extracellular proteins. Due to the usage of different promoters, LTBP1 exists in two major forms, long (L) and short (S), each expressed in a temporally and spatially unique fashion. Both LTBP1 molecules covalently interact with latent TGF-beta and regulate its function, presumably via interaction with the extracellular matrix (ECM). To explore the in vivo role of Ltbp1 in mouse development, at the time when only the L isoform is expressed, we mutated the Ltbp1L locus by gene targeting. Ltbp1L-null animals die shortly after birth from defects in heart development, consisting of the improper septation of the cardiac outflow tract (OFT) and remodeling of the associated vessels. These cardiac anomalies present as persistent truncus arteriosus (PTA) and interrupted aortic arch (IAA), which are associated with the faulty function of cardiac neural crest cells (CNCCs). The lack of Ltbp1L in the ECM of the septating OFT and associated vessels results in altered gene expression and function of CNCCs and decreased Tgf-beta activity in the OFT. This phenotype reveals a crucial role for Ltbp1L and matrix as extracellular regulators of Tgf-beta activity in heart organogenesis.

Multiple transforming growth factor-beta isoforms and receptors function during epithelial-mesenchymal cell transformation in the embryonic heart

Epithelial-mesenchymal cell transformation (EMT) is a critical process during development of the heart valves. Transition of endothelial cells into mesenchymal cells in the atrioventricular (AV) canal and the outflow tract regions of the heart form the cardiac cushions that eventually form the heart valves. Collagen gel invasion assay has aided in the identification of molecules that regulate EMT. Among those, transforming growth factor-beta (TGF-beta) ligands and receptors demonstrate a critical role during EMT. In the chick, TGF-beta ligands and some receptors have specific functions during EMT. TGF-beta2 mediates endothelial cell-cell activation and separation, and TGF-beta3 mediates cell invasion into the extracellular matrix. Receptors involved in the EMT process include TGF-beta receptor type II (TBRII), TBRIII, endoglin and the TBRI receptors, ALK2 and ALK5. In contrast, in the mouse model, TGF-beta2 is the only ligand involved in EMT. The TGF-beta2 null mouse has either increased EMT or a mesenchymal cell proliferation after EMT. However, functional studies of TGF-beta1 in vivo and in vitro showed that TGF-beta1 functions in the EMT of the mouse AV canal. Latent TGF-beta-binding protein (LTBP-1) and endoglin have a role in the EMT process. Therefore, TGF-betas mediate cardiac EMT in both embryonic species. Further studies will reveal the identification of ligand and receptor-specific activities.

Potential role for heparan sulfate proteoglycans in regulation of transforming growth factor-beta (TGF-beta) by modulating assembly of latent TGF-beta-binding protein-1

Latent transforming growth factor-beta-binding proteins (LTBPs) are extracellular matrix (ECM) glycoproteins that play a major role in storage of latent TGF-beta in the ECM and regulate its availability. We have previously identified fibronectin as a key molecule for incorporation of LTBP1 and TGF-beta into the ECM of osteoblasts and fibroblasts. Here we provide evidence that heparan sulfate proteoglycans may mediate binding between LTBP1 and fibronectin. We have localized critical domains in the N terminus of LTBP1 that are required for co-localization with fibronectin in osteoblast cultures and have identified heparin binding sites in the N terminus of LTBP1 between residues 345 and 487. Solid-phase binding assays suggest that LTBP1 does not bind directly to fibronectin but that the binding is indirect. Heparin coupled to bovine serum albumin (heparin-BSA) was able to mediate binding between fibronectin and LTBP1. Treatment of primary osteoblast cultures with heparin or heparin-BSA but not with chondroitin sulfate impaired LTBP1 deposition onto fibronectin without inhibiting expression of LTBP1. Inhibition of LTBP1 incorporation was accompanied by reduced incorporation of latent TGF-beta into the ECM, with increased amounts of soluble latent TGF-beta. Inhibition of attachment of glycosaminoglycans to the core proteins of proteoglycans by beta-d-xylosides also reduced incorporation of LTBP1 into the ECM. These studies suggest that heparan sulfate proteoglycans may play a critical role in regulating TGF-beta availability by controlling the deposition of LTBP1 into the ECM in association with fibronectin.

Increased expression of latent TGF-ß binding protein-1 and fibrillin-1 in human uterine leiomyomata

We compared latent TGF-ss binding protein-1 (LTBP-1) and fibrillin-1 (FBN-1) expression in leiomyomata and myometrium, correlated with leiomyomata size. We studied in vivo and in vitro effects of ovarian steroids using matched leiomyomata and myometrium samples from both phases of the menstrual cycle. Leiomyomata were divided into small (<or=2 cm), medium (3-5 cm) and large (>or=6 cm) groups. We validated LTBP-1 and FBN-1 expression using QPCR, western blot and immunohistochemistry. LTBP-1 and FBN-1 mRNA and protein expressions were higher in the medium-sized group compared with myometrium in the proliferative phase (P = 0.01; P = 0.01). FBN-1 mRNA expression was higher in the secretory phase (P = 0.01). LTBP-1 mRNA and protein expression was higher in the medium group compared with the small and large groups in the proliferative phase (P = 0.04; P = 0.04). No differences between groups were seen in FBN-1 expression in either phase. 17Beta-estradiol (E2) increased mRNA and protein expression of LTBP-1 and FBN-1 in cultured leiomyoma smooth muscle cells (LSMC) (P < 0.05). No change in FBN-1 and LTBP-1 expression was observed when cells were treated with E2 plus progesterone. Estrogen may be involved in LTBP-1 and FBN-1 expression in leiomyomata. Extracellular matrix metabolism may be different in medium-sized leiomyoma.

FOG-2 attenuates endothelial-to-mesenchymal transformation in the endocardial cushions of the developing heart

Development of the heart valves is a complex process that relies on the successful remodeling of endocardial cushions. This process is dependent on a number of transcriptional regulators, including GATA4 and its interacting partner FOG-2. We have previously shown that the endocardial cushions in FOG-2 deficient mice are hyperplastic and fail to remodel appropriately, suggesting a defect late in endocardial cushion development. To elucidate this defect, we examined the later steps in endocardial cushion development including mesenchymal cell proliferation, differentiation, and apoptosis. We also measured myocardialization and endothelial-to-mesenchymal transformation (EMT) using previously described in vitro assays. We found no difference in the ability of the endocardial cushions to undergo myocardialization or in the rates of mesenchymal cell proliferation, differentiation, or apoptosis in the FOG-2 deficient cushions when compared to wild-type controls. However, using a collagen gel invasion assay, we found a 78% increase in outflow tract cushion EMT and a 35% increase in atrioventricular cushion EMT in the FOG-2 deficient mice when compared with wild-type mice. Taken together with GATA4's known role in promoting EMT, these results suggest that FOG-2 functions in cardiac valve formation as an attenuator of EMT by repressing GATA4 activity within the developing endocardial cushions.

BMP1 controls TGFbeta1 activation via cleavage of latent TGFbeta-binding protein

Transforming growth factor beta1 (TGFbeta1), an important regulator of cell behavior, is secreted as a large latent complex (LLC) in which it is bound to its cleaved prodomain (latency-associated peptide [LAP]) and, via LAP, to latent TGFbeta-binding proteins (LTBPs). The latter target LLCs to the extracellular matrix (ECM). Bone morphogenetic protein 1 (BMP1)-like metalloproteinases play key roles in ECM formation, by converting precursors into mature functional proteins, and in morphogenetic patterning, by cleaving the antagonist Chordin to activate BMP2/4. We provide in vitro and in vivo evidence that BMP1 cleaves LTBP1 at two specific sites, thus liberating LLC from ECM and resulting in consequent activation of TGFbeta1 via cleavage of LAP by non-BMP1-like proteinases. In mouse embryo fibroblasts, LAP cleavage is shown to be predominantly matrix metalloproteinase 2 dependent. TGFbeta1 is a potent inducer of ECM formation and of BMP1 expression. Thus, a role for BMP1-like proteinases in TGFbeta1 activation completes a novel fast-forward loop in vertebrate tissue remodeling.

Human pulmonary valve progenitor cells exhibit endothelial/mesenchymal plasticity in response to vascular endothelial growth factor-A and transforming growth factor-beta2

In situ analysis of fetal semilunar valve leaflets has revealed cells coexpressing endothelial and mesenchymal markers along the endothelium, with diminished frequency seen in adult valves. To determine whether such cells are progenitor cells, we isolated clonal populations from human pulmonary valves. The clones expressed endothelial markers but showed potential to further differentiate into endothelium in response to vascular endothelial growth factor (VEGF)-A. When exposed to transforming growth factor (TGF)-beta2, individual clones adopted a mesenchymal phenotype to varying degrees and expressed markers of endothelial to mesenchymal transformation (EMT). Both VEGF- and TGFbeta2-induced phenotypic changes were partially reversible, indicating the plasticity of these cells. When challenged with VEGF or TGFbeta2, a hierarchy of endothelial/mesenchymal potential could be seen among the clonal populations: cells initially closer to an endothelial phenotype showed a strong response to TGFbeta2 that could be inhibited by VEGF, whereas cells closer to a mesenchymal phenotype responded to TGFbeta2 but were resistant to endothelial-inducing effects of VEGF. These findings suggest the presence of bipotential valve progenitor cells with ability to differentiate into either endothelial or interstitial cells of the valve leaflet. Understanding the differentiation potential and function of these cells may be important for understanding heart valve disease and may also be applied to current paradigms for creating tissue-engineered heart valves.

Fibrillins: from biogenesis of microfibrils to signaling functions

Fibrillins are large proteins that form extracellular microfibril suprastructures ubiquitously found in elastic and nonelastic tissues. Mutations in fibrillin-1 and -2 lead to a number of heritable connective tissue disorders generally termed fibrillinopathies. Clinical symptoms in fibrillinopathies manifest in the skeletal, ocular, and cardiovascular systems and highlight the importance of fibrillins in development and homeostasis of tissues and organs, including blood vessels, bone, and eye. Microfibrils appear to have dual roles in (1) conferring mechanical stability and limited elasticity to tissues, and (2) modulating the activity of growth factors of the transforming growth factor beta (TGF-beta) superfamily. This chapter's focus is on the biogenesis of microfibrils, developmental expression patterns of fibrillins, signaling functions of microfibrils, and mouse models deficient in fibrillins.

Understanding heart development and congenital heart defects through developmental biology: a segmental approach

ABSTRACT The heart is the first organ to form and function during development. In the pregastrula chick embryo, cells contributing to the heart are found in the postero-lateral epiblast. During the pregastrula stages, interaction between the posterior epiblast and hypoblast is required for the anterior lateral plate mesoderm (ALM) to form, from which the heart will later develop. This tissue interaction is replaced by an Activin-like signal in culture. During gastrulation, the ALM is committed to the heart lineage by endoderm-secreted BMP and subsequently differentiates into cardiomyocyte. The right and left precardiac mesoderms migrate toward the ventral midline to form the beating primitive heart tube. Then, the heart tube generates a right-side bend, and the d-loop and presumptive heart segments begin to appear segmentally: outflow tract (OT), right ventricle, left ventricle, atrioventricular (AV) canal, atrium and sinus venosus. T-box transcription factors are involved in the formation of the heart segments: Tbx5 identifies the left ventricle and Tbx20 the right ventricle. After the formation of the heart segments, endothelial cells in the OT and AV regions transform into mesenchyme and generate valvuloseptal endocardial cushion tissue. This phenomenon is called endocardial EMT (epithelial-mesenchymal transformation) and is regulated mainly by BMP and TGFbeta. Finally, heart septa that have developed in the OT, ventricle, AV canal and atrium come into alignment and fuse, resulting in the completion of the four-chambered heart. Altered development seen in the cardiogenetic process is involved in the pathogenesis of congenital heart defects. Therefore, understanding the molecular nature regulating the 'nodal point' during heart development is important in order to understand the etiology of congenital heart defects, as well as normal heart development.

Autoantibodies to Fibrillin-1 Activate Normal Human Fibroblasts in Culture through the TGF-β Pathway to Recapitulate the “Scleroderma Phenotype”

Fibroblasts from patients with systemic sclerosis (SSc) are activated producing excessive amounts of extracellular matrix (ECM) components. Recently, we identified a new SSc-specific autoantibody against portions of fibrillin-1, a major component of ECM microfibrils and regulator of TGF-beta1 signaling. To examine a potential pathogenic role of anti-fibrillin-1 autoantibodies, normal human fibroblasts were treated with affinity-purified autoantibodies isolated from SSc sera and then examined for alterations in gene and protein expression levels using microarrays, quantitative RT-PCR, immunoblots, and immunofluorescence. Compared with fibroblasts cultured in normal medium or in medium containing normal human IgG, anti-fibrillin-1 autoantibody-treated normal dermal fibroblasts showed increased expression of COL and several other ECM components characteristically overexpressed in SSc fibroblasts. This was accompanied by phosphorylation and nuclear translocation of Smad3. Neutralization of TGF-beta1 with anti-TGF-beta1 Abs significantly diminished the activation of fibroblasts by anti-fibrillin-1 autoantibodies. These data indicate that anti-fibrillin-1 autoantibodies can induce the activation of normal dermal fibroblasts into a profibrotic phenotype resembling that of SSc by potentially causing the release of sequestered TGF-beta1 from fibrillin-1-containing microfibrils in the ECM.

Enhanced expression of fibrillin-1, a constituent of the myocardial extracellular matrix in fibrosis

Fibrillin-1 localization in the myocardium and the modulation of its expression in cardiac fibrosis were examined. In normal rat hearts, fibrillin-1 was abundant throughout the myocardium as thin fibers that crossed over the perimysium and around arteries. After cardiac fibrosis was induced in rats by either 14-day ANG II infusion or 21-day DOCA-salt treatment [a high endothelin-1 (ET-1) model], fibrillin-1 immunostaining was stronger in the interstitium (2.8-fold and 4.4-fold increases, respectively, in each model), extended between myocytes, and accumulated in microscopic scars and in the perivascular area of both ventricles. mRNA analysis confirmed its enhanced ventricular expression in both groups of rats (2.5-fold and 6.6-fold increments, respectively, in each model). In 1B normotensive and 2C hypertensive transgenic mice, two lines expressing an ANG II fusion protein in cardiac myocytes, strong fibrillin-1 immunoreactivity was observed in the interstitium and around arteries (3.7-fold and 7-fold increases, respectively). ANG II and transforming growth factor-beta1 enhanced fibrillin-1 synthesis by cardiac fibroblasts. Some fibrillin-1 fragments interacted with RGD-dependent integrins, including alpha(8)beta(1)-integrin, of cardiac fibroblasts but not necessarily through the RGD motif. Our findings illustrate that fibrillin-1 is an important constituent of the myocardium. In vitro and in vivo evidence suggests that ANG II can directly induce fibrillin-1 expression in cardiac fibroblasts. This protein can thus contribute to reactive and reparative processes.

Cardiac myocyte differentiation: the Nkx2.5 and Cripto target genes in P19 clone 6 cells

Genetic evidence has implicated several genes as being critical for the development of cardiomyocytes. Whereas a few of the targets of these genes and the pathways they constitute are known the majority of targets and the interrelationships of the pathways involved still remains largely unknown. The power of high-throughput analytical techniques like microarrays and real-time RT-PCR combined with the ability to selectively silence specific mRNA in model tissue culture systems can begin to fill in these gaps and increase our understanding of the molecular mechanisms of cell commitment and terminal differentiation. We have used microarray analysis and siRNA directed against the cardiac-specific transcription factor Nkx2.5 and one of its targets Cripto in P19 clone 6 (P19Cl6) cells to identify potential targets for these genes. We demonstrate Nkx2.5 affects genes that have been shown to be controlled by the canonical Wnt or TGFbeta/BMP signaling pathways. We also show that Cripto can regulate the critical stem cell gene Nanog and two Oct 4-regulated genes: Dppa2 and 4. Cripto also affects the formation of nitric oxide, a small signaling molecule that has been reported to be important for growth and development of cardiac and smooth muscle. It affects the nitric oxide system by regulating genes that control the levels of nitric oxide synthase mRNA concentration as well as the activation and bioavailability of the protein.

Frzb modulates Wnt-9a-mediated beta-catenin signaling during avian atrioventricular cardiac cushion development

Normal development of the cardiac atrioventricular (AV) endocardial cushions is essential for proper ventricular septation and morphogenesis of the mature mitral and tricuspid valves. In this study, we demonstrate spatially restricted expression of both Wnt-9a (formerly Wnt-14) and the secreted Wnt antagonist Frzb in AV endocardial cushions of the developing chicken heart. Wnt-9a expression is detected only in AV canal endocardial cells, while Frzb expression is detected in both endocardial and transformed mesenchymal cells of the developing AV cardiac cushions. We present evidence that Wnt-9a promotes cell proliferation in the AV canal and overexpression of Wnt-9a in ovo results in enlarged endocardial cushions and AV inlet obstruction. Wnt-9a stimulates beta-catenin-responsive transcription in AV canal cells, duplicates the embryonic axis upon ventral injections in Xenopus embryos and appears to regulate cell proliferation by activating a Wnt/beta-catenin signaling pathway. Additional functional studies reveal that Frzb inhibits Wnt-9a-mediated cell proliferation in cardiac cushions. Together, these data argue that Wnt-9a and Frzb regulate mesenchymal cell proliferation leading to proper AV canal cushion outgrowth and remodeling in the developing avian heart.

Microfibrils at basement membrane zones interact with perlecan via fibrillin-1

Mutational defects in fibrillin-rich microfibrils give rise to a number of heritable connective tissue disorders, generally termed microfibrillopathies. To understand the pathogenesis of these microfibrillopathies, it is important to elucidate the supramolecular composition of microfibrils and their interaction properties with extracellular matrix components. Here we demonstrate that the proteoglycan perlecan is an associated component of microfibrils typically close to basement membrane zones. Double immunofluorescence studies demonstrate colocalization of fibrillin-1, the major backbone component of microfibrils, with perlecan in fibroblast cultures as well as in dermal and ocular tissues. Double immunogold labeling further confirms colocalization of perlecan to microfibrils in various tissues at the ultrastructural level. Extraction studies revealed that perlecan is not covalently associated with microfibrils. High affinity interactions between fibrillin-1 and perlecan were found by kinetic binding studies with dissociation constants in the low nanomolar range. A detailed mapping study of the interaction epitopes by solid phase binding assays primarily revealed interactions of perlecan domains I and II with a central region of fibrillin-1. Analysis of perlecan null embryos showed less microfibrils at the dermal-epidermal junction as compared with wild-type littermates. The data presented indicate a functional significance for perlecan in anchoring microfibrils to basement membranes and in the biogenesis of microfibrils.

Cell Biology of Cardiac Cushion Development

The valves of the heart develop in the embryo from precursor structures called endocardial cushions. After cardiac looping, endocardial cushion swellings form and become populated by valve precursor cells formed by an epithelial-mesenchymal transition (EMT). Endocardial cushions subsequently undergo directed growth and remodeling to form the valvular structures and the membranous septa of the mature heart. The developmental processes that mediate cushion formation include many prototypic cellular actions including adhesion, signaling, migration, secretion, replication, differentiation, and apoptosis. Cushion morphogenesis is unique in that these cellular possesses occur in a functioning organ where the cushions act as valves even while developing into definitive valvular structures. Cardiovascular defects are the most common congenital defects, and one of the most common causes of death during infancy. Thus, there is significant interest in understanding the mechanisms that underlie this complex developmental process. In this regard, substantial progress has been made by incorporating an understanding of cardiac morphology and cell biology with the rapidly expanding repertoire of molecular mechanisms gained through human genetics and research using animal models. This article reviews cardiac morphogenesis as it relates to heart valve formation and highlights selected growth factors, intracellular signaling mediators, and extracellular matrix components involved in the creation and remodeling of endocardial cushions into mature cardiac structures.

Immunolocalization of chick periostin protein in the developing heart

The process that cardiac cushions undergo to form the mature septa and valves of the adult heart is poorly understood. Periostin is an extracellular molecule that is expressed during cushion mesenchyme formation and throughout valvulogenesis. Once thought to be an osteoblast-specific factor, studies have shown this molecule is antiosteogenic. We have produced an antibody to chicken periostin and examined periostin's localization in the developing avian heart. This antibody recognized proteins from chick heart lysates around 90 kD molecular weight as predicted from the chick periostin mRNA and other periostin orthologs. Periostin immunolocalization was first evident as fibrous strands in the cushion mesenchyme. At HH25, periostin was detected on the basal surface of the trabecular endothelium and also on the endocardial epithelium of the atrioventricular cushion. We hypothesize that periostin may function in the organization of extracellular matrix molecules, providing cues necessary for attachment and spreading during the epithelial-to-mesenchymal transitions of the endocardial epithelium. Enhanced secretion of periostin in the region of delamination may directly or indirectly promote change in the myocardium that precedes or mediates delamination of the leaflet. At later stages of development (HH34-38), periostin was seen predominantly in the fibrous regions of the heart, such as the left atrioventricular valve (LAV), annulus, cardiac skeleton, and adventitia. We propose that periostin is induced by sheer stress and may be an essential molecular component for structures of the heart that undergo mechanical stress or tension during the cardiac cycle.

Integrin alphaVbeta6-mediated activation of latent TGF-beta requires the latent TGF-beta binding protein-1

Transforming growth factor-βs (TGF-β) are secreted as inactive complexes containing the TGF-β, the TGF-β propeptide, also called the latency-associated protein (LAP), and the latent TGF-β binding protein (LTBP). Extracellular activation of this complex is a critical but incompletely understood step in TGF-β regulation. We have investigated the role of LTBP in modulating TGF-β generation by the integrin α_Vβ₆. We show that even though α_vβ₆ recognizes an RGD on LAP, LTBP-1 is required for α_Vβ₆-mediated latent TGF-β activation. The domains of LTBP-1 necessary for activation include the TGF-β propeptide-binding domain and a basic amino acid sequence (hinge domain) with ECM targeting properties. Our results demonstrate an LTBP-1 isoform-specific function in α_Vβ₆-mediated latent TGF-β activation; LTBP-3 is unable to substitute for LTBP-1 in this assay. The results reveal a functional role for LTBP-1 in latent TGF-β activation and suggest that activation of specific latent complexes is regulated by distinct mechanisms that may be determined by the LTBP isoform and its potential interaction with the matrix.

Essential roles of Meltrin beta (ADAM19) in heart development

Morphogenesis of the heart requires development of the endocardial cushion tissue that gives rise to the membranous septa and valves. Here we show that Meltrin beta/ADAM19, a novel metalloprotease-disintegrin, participates in the development of the endocardial cushion. Mice lacking Meltrin beta exhibit ventricular septal defect (VSD) and immature valves, and most of the animals die soon after birth. During development of the endocardial cushion, epithelial-mesenchymal transformation (EMT) of endocardial epithelial cells generates most of the cushion mesenchymes that constitute the main components of the septa and valves. Meltrin beta is expressed in both the epithelia and the mesenchymes of the endocardial cushion. In the absence of Meltrin beta, the cushion is small or thin in the septum-forming region and show poor remodeling of cardiac jelly components; both of these characteristics suggest impaired growth and differentiation of the endocardial cushion. When embryonic fibroblasts are cultured sparsely, Meltrin beta-lacking cells exhibit aberrant ectodomain shedding of type I Neuregulin, one of the ErbB ligands expressed in endocardial cells. These results suggest the necessity of proteolytic regulation of ErbB ligands by Meltrin beta for proper heart development.