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Heart valve disease affects up to 30% of the population and has been shown to have origins during embryonic development. Valvulogenesis begins with formation of endocardial cushions in the atrioventricular canal and outflow tract regions. Subsequently, endocardial cushions remodel, elongate and progressively form mature valve structures composed of a highly organized connective tissue that provides the necessary biomechanical function throughout life. While endocardial cushion formation has been well studied, the processes required for valve remodeling are less well understood. The transcription factor Scleraxis (Scx) is detected in mouse valves from E15.5 during initial stages of remodeling, and expression remains high until birth when formation of the highly organized mature structure is complete. Heart valves from Scx-/- mice are abnormally thick and develop fibrotic phenotypes similar to human disease by juvenile stages. These phenotypes begin around E15.5 and are associated with defects in connective tissue organization and valve interstitial cell differentiation. In order to understand the etiology of this phenotype, we analyzed the transcriptome of remodeling valves isolated from E15.5 Scx-/- embryos using RNA-seq. From this, we have identified a profile of protein and non-protein mRNAs that are dependent on Scx function and using bioinformatics we can predict the molecular functions and biological processes affected by these genes. These include processes and functions associated with gene regulation (methyltransferase activity, DNA binding, Notch signaling), vitamin A metabolism (retinoic acid biosynthesis) and cellular development (cell morphology, cell assembly and organization). In addition, several mRNAs are affected by alternative splicing events in the absence of Scx, suggesting additional roles in post-transcriptional modification. In summary, our findings have identified transcriptome profiles from abnormal heart valves isolated from E15.5 Scx-/- embryos that could be used in the future to understand mechanisms of heart valve disease in the human population.

There is limited information about age-specific structural and functional properties of human heart valves, while this information is key to the development and evaluation of living valve replacements for pediatric and adolescent patients. Here, we present an extended data set of structure-function properties of cryopreserved human pulmonary and aortic heart valves, providing age-specific information for living valve replacements. Tissue composition, morphology, mechanical properties, and maturation of leaflets from 16 pairs of structurally unaffected aortic and pulmonary valves of human donors (fetal-53 years) were analyzed. Interestingly, no major differences were observed between the aortic and pulmonary valves. Valve annulus and leaflet dimensions increase throughout life. The typical three-layered leaflet structure is present before birth, but becomes more distinct with age. After birth, cell numbers decrease rapidly, while remaining cells obtain a quiescent phenotype and reside in the ventricularis and spongiosa. With age and maturation-but more pronounced in aortic valves-the matrix shows an increasing amount of collagen and collagen cross-links and a reduction in glycosaminoglycans. These matrix changes correlate with increasing leaflet stiffness with age. Our data provide a new and comprehensive overview of the changes of structure-function properties of fetal to adult human semilunar heart valves that can be used to evaluate and optimize future therapies, such as tissue engineering of heart valves. Changing hemodynamic conditions with age can explain initial changes in matrix composition and consequent mechanical properties, but cannot explain the ongoing changes in valve dimensions and matrix composition at older age.

Mechanical forces are well known for modulating heart valve developmental programs. Yet, it is still unclear how genetic programs and mechanosensation interact during heart valve development. Here, we assessed the mechanosensitive pathways involved during zebrafish outflow tract (OFT) valve development in vivo. Our results show that the hippo effector Yap1, Klf2, and the Notch signaling pathway are all essential for OFT valve morphogenesis in response to mechanical forces, albeit active in different cell layers. Furthermore, we show that Piezo and TRP mechanosensitive channels are important factors modulating these pathways. In addition, live reporters reveal that Piezo controls Klf2 and Notch activity in the endothelium and Yap1 localization in the smooth muscle progenitors to coordinate OFT valve morphogenesis. Together, this work identifies a unique morphogenetic program during OFT valve formation and places Piezo as a central modulator of the cell response to forces in this process.

Endothelial-mesenchymal transformation (EMT) is a critical event for the embryonic morphogenesis of cardiac valves. Inducers of EMT during valvulogenesis include VEGF, TGF-β1, and wnt/β-catenin (where wnt refers to the wingless-type mammary tumor virus integration site family of proteins), that are regulated in a spatiotemporal manner. EMT has also been observed in diseased, strain-overloaded valve leaflets, suggesting a regulatory role for mechanical strain. Although the preponderance of studies have focused on the role of soluble mitogens, we asked if the valve tissue microenvironment contributed to EMT. To recapitulate these microenvironments in a controlled, in vitro environment, we engineered 2D valve endothelium from sheep valve endothelial cells, using microcontact printing to mimic the regions of isotropy and anisotropy of the leaflet, and applied cyclic mechanical strain in an attempt to induce EMT. We measured EMT in response to both low (10%) and high strain (20%), where low-strain EMT occurred via increased TGF-β1 signaling and high strain via increased wnt/β-catenin signaling, suggesting dual strain-dependent routes to distinguish EMT in healthy versus diseased valve tissue. The effect was also directionally dependent, where cyclic strain applied orthogonal to axis of the engineered valve endothelium alignment resulted in severe disruption of cell microarchitecture and greater EMT. Once transformed, these tissues exhibited increased contractility in the presence of endothelin-1 and larger basal mechanical tone in a unique assay developed to measure the contractile tone of the engineered valve tissues. This finding is important, because it implies that the functional properties of the valve are sensitive to EMT. Our results suggest that cyclic mechanical strain regulates EMT in a strain magnitude and directionally dependent manner.

A fundamental event in the formation of heart valves involves the transformation of endocardial cells within the outflow tract (OFT) and atrioventricular canal (AVC) cushions through a process known as endothelial-to-mesenchymal transition (EndMT). Aberrant EndMT is a primary cause of congenital valvular malformations. Manic Fringe (MFNG) has been previously associated with cardiovascular development, although its role in heart valve development remains underexplored. In this study, we seek to enhance our understanding of MFNG’s involvement in valve formation and its association with EndMT. Staining results of histological section revealed the expression of MFNG in the AVC and OFT from embryonic day 9.5 to 10.5 (E9.5–E10.5), when EndMT takes place. Cellular data demonstrated that MFNG exerts a positive regulatory influence on the EndMT process, promoting endothelial cell (EC) migration by enhancing the activity of the Notch signalling pathway. MFNG knockdown mediated by antisense morpholino oligonucleotides (MO) injection caused abnormal development of the heart and valves in zebrafish. Furthermore, through whole-exome sequencing (WES), we identified a heterozygous MFNG mutation in patients diagnosed with tetralogy of Fallot-pulmonary valve stenosis (TOF-PS). Cellular and molecular assays confirmed that this deleterious mutation reduced MFNG expression and hindered the EndMT process. In summary, our study verifies that MFNG plays a role in promoting EndMT mediated by the Notch signalling pathway during the heart and valve development. The MFNG deleterious variant induces MFNG loss of function, potentially elucidating the underlying molecular mechanisms of MFNG’s involvement in the pathogenesis of congenital heart valve defects. These observations contribute to our current genetic understanding of congenital heart valve disease and may provide a potential target for prenatal diagnosis and treatment. Key messages Our examination revealed, for the first time, that MFNG exhibited high expression levels during EndMT of heart valve development in mice. Our findings provide compelling evidence that MFNG plays a role in promoting EndMT mediated by the Notch signalling pathway. Our results identified, for the first time, a deleterious MFNG p. T77M variant that inhibited the EndMT process by downregulating the activity of the Notch signalling pathway, thereby preventing the normal valve formation. MFNG may serve as an early diagnostic marker and an effective therapeutic target for the clinical treatment of congenital heart valve defects.

Tissue development and homeostasis are dependent upon the concerted synthesis, maintenance, and degradation of extracellular matrix (ECM) molecules. Cardiac fibrosis is now recognized as a primary contributor to incidence of heart failure, particularly heart failure with preserved ejection fraction, wherein cardiac filling in diastole is compromised. Periostin is a cell-associated protein involved in cell fate determination, proliferation, tumorigenesis, and inflammatory responses. As a non-structural component of the ECM, secreted 90 kDa periostin is emerging as an important matricellular factor in cardiac mesenchymal tissue development. In addition, periostin’s role as a mediator in cell–matrix crosstalk has also garnered attention for its association with fibroproliferative diseases in the myocardium, and for its association with TGF-β/BMP signaling. This review summarizes the phylogenetic history of periostin, its role in cardiac development, and the major signaling pathways influencing its expression in cardiovascular pathology. Further, we provide a synthesis of the current literature to distinguish the multiple roles of periostin in cardiac health, development and disease. As periostin may be targeted for therapeutic treatment of cardiac fibrosis, these insights may shed light on the putative timing for application of periostin-specific therapies.

Chemotactic cytokines (chemokines) attract immune cells, although their original evolutionary role may relate more closely with embryonic development. We noted differential expression of the chemokine receptor CXCR7 (RDC-1) on marginal zone B cells, a cell type associated with autoimmune diseases. We generated Cxcr7⁻/⁻ mice but found that CXCR7 deficiency had little effect on B cell composition. However, most Cxcr7⁻/⁻ mice died at birth with ventricular septal defects and semilunar heart valve malformation. Conditional deletion of Cxcr7 in endothelium, using Tie2-Cre transgenic mice, recapitulated this phenotype. Gene profiling of Cxcr7⁻/⁻ heart valve leaflets revealed a defect in the expression of factors essential for valve formation, vessel protection, or endothelial cell growth and survival. We confirmed that the principal chemokine ligand for CXCR7 was CXCL12/SDF-1, which also binds CXCR4. CXCL12 did not induce signaling through CXCR7; however, CXCR7 formed functional heterodimers with CXCR4 and enhanced CXCL12-induced signaling. Our results reveal a specialized role for CXCR7 in endothelial biology and valve development and highlight the distinct developmental role of evolutionary conserved chemokine receptors such as CXCR7 and CXCR4.

Heart valve anomalies are some of the most common congenital heart defects, yet neither the genetic nor the epigenetic forces guiding heart valve development are well understood. When functioning normally, mature heart valves prevent intracardiac retrograde blood flow; before valves develop, there is considerable regurgitation, resulting in reversing (or oscillatory) flows between the atrium and ventricle. As reversing flows are particularly strong stimuli to endothelial cells in culture, an attractive hypothesis is that heart valves form as a developmental response to retrograde blood flows through the maturing heart. Here, we exploit the relationship between oscillatory flow and heart rate to manipulate the amount of retrograde flow in the atrioventricular (AV) canal before and during valvulogenesis, and find that this leads to arrested valve growth. Using this manipulation, we determined that klf2a is normally expressed in the valve precursors in response to reversing flows, and is dramatically reduced by treatments that decrease such flows. Experimentally knocking down the expression of this shear-responsive gene with morpholine antisense oligonucleotides (MOs) results in dysfunctional valves. Thus, klf2a expression appears to be necessary for normal valve formation. This, together with its dependence on intracardiac hemodynamic forces, makes klf2a expression an early and reliable indicator of proper valve development. Together, these results demonstrate a critical role for reversing flows during valvulogenesis and show how relatively subtle perturbations of normal hemodynamic patterns can lead to both major alterations in gene expression and severe valve dysgenesis.

Congenital valve defects account for a substantial proportion of cardiovascular malformations, yet the molecular mechanisms orchestrating cardiac valve development remain incompletely elucidated. While Bone morphogenetic protein (BMP) signaling is essential for valvulogenesis, the specific contributions of individual BMP ligands, particularly the teleost-specific bmp16, have not been characterized. Using the CRISPR/Cas9 system, we generated a bmp16 null knockout and delineated critical roles of this ligand in valvular morphogenesis. bmp16 knockout embryos display a significant reduction in Sox9-positive valvular cells and exhibit severely dysplastic arterial valves, characterized by increased interleaflet distance, thickened leaflets, and shortened leaflet lengths. These morphological abnormalities correlate with impaired valve function, culminating in progressive blood regurgitation, ventricular dilation, and pericardial edema. Mechanistically, loss of bmp16 or pharmacological inhibition of BMP signaling significantly downregulates notch1b expression in developing valves, while pharmacological activation of Notch signaling rescues the regurgitation phenotype in bmp16 mutants. Collectively, our findings establish bmp16 as a novel regulator of valve development and uncover a functional BMP-Notch signaling axis required for vertebrate valvulogenesis, providing new insights into the molecular mechanisms that govern cardiac valve formation and pathogenesis.

Epithelial–mesenchymal transition (EMT) is an essential process during morphogenesis. Dissecting the signalling strategies that orchestrate EMT have shown that a complex signalling network, which controls adhesion, motility, survival and differentiation, also regulates the initiation and execution of EMT during embryonic development. Key Points Epithelial and mesenchymal cells have distinct characteristics. These cell types can be partially or fully interconverted through the processes of epithelial–mesenchymal transition (EMT) and mesenchymal–epithelial transition (MET). EMT is controlled by various extracellular triggers. The intracellular pathways that are activated by these triggers exhibit extensive crosstalk and have many common endpoints. Biophysics as well as cell- and molecular-biology approaches have been combined to provide many novel insights into the process of EMT. EMT has an important role in many embryological processes. Examples include gastrulation, neural-crest development and heart-valve formation. There is mounting evidence that EMT processes are involved in several pathological processes, including would healing, fibrosis and cancer. Better model systems and the identification of genes that mark specific EMT events are required for further progress in this field. Epithelial–mesenchymal transition is an indispensable mechanism during morphogenesis, as without mesenchymal cells, tissues and organs will never be formed. However, epithelial-cell plasticity, coupled to the transient or permanent formation of mesenchyme, goes far beyond the problem of cell-lineage segregation. Understanding how mesenchymal cells arise from an epithelial default status will also have a strong impact in unravelling the mechanisms that control fibrosis and cancer progression.