klf2a couples mechanotransduction and zebrafish valve morphogenesis through fibronectin synthesis

egr3 is a mechanosensitive transcription factor gene required for cardiac valve morphogenesis

Biomechanical forces, and their molecular transducers, including key mechanosensitive transcription factor genes, such as KLF2, are required for cardiac valve morphogenesis. However, klf2 mutants fail to completely recapitulate the valveless phenotype observed under no-flow conditions. Here, we identify the transcription factor EGR3 as a conserved biomechanical force transducer critical for cardiac valve formation. We first show that egr3 null zebrafish display a complete and highly penetrant loss of valve leaflets, leading to severe blood regurgitation. Using tissue-specific loss- and gain-of-function tools, we find that during cardiac valve formation, Egr3 functions cell-autonomously in endothelial cells, and identify one of its effectors, the nuclear receptor Nr4a2b. We further find that mechanical forces up-regulate egr3/EGR3 expression in the developing zebrafish heart and in porcine valvular endothelial cells, as well as during human aortic valve remodeling. Altogether, these findings reveal that EGR3 is necessary to transduce the biomechanical cues required for zebrafish cardiac valve morphogenesis, and potentially for pathological aortic valve remodeling in humans.

Endocardial primary cilia and blood flow are required for regulation of EndoMT during endocardial cushion development

Blood flow is critical for heart valve formation, and cellular mechanosensors are essential to translate flow into transcriptional regulation of development. Here, we identify a role for primary cilia in vivo in the spatial regulation of cushion formation, the first stage of valve development, by regionally controlling endothelial to mesenchymal transition (EndoMT) via modulation of Kruppel-like Factor 4 (Klf4) . We find that high shear stress intracardiac regions decrease endocardial ciliation over cushion development, correlating with KLF4 downregulation and EndoMT progression. Mouse embryos constitutively lacking cilia exhibit a blood-flow dependent accumulation of KLF4 in these regions, independent of upstream left-right abnormalities, resulting in impaired cushion cellularization. snRNA-seq revealed that cilia KO endocardium fails to progress to late-EndoMT, retains endothelial markers and has reduced EndoMT/mesenchymal genes that KLF4 antagonizes. Together, these data identify a mechanosensory role for endocardial primary cilia in cushion development through regional regulation of KLF4.

Should I shrink or should I grow: cell size changes in tissue morphogenesis

Cells change shape, move, divide, and die to sculpt tissues. Common to all these cell behaviours are cell size changes, which have recently emerged as key contributors to tissue morphogenesis. Cells can change their mass-the number of macromolecules they contain-or their volume-the space they encompass. Changes in cell mass and volume occur through different molecular mechanisms and at different timescales, slow for changes in mass and rapid for changes in volume. Therefore, changes in cell mass and cell volume, which are often linked, contribute to the development and shaping of tissues in different ways. Here, we review the molecular mechanisms by which cells can control and alter their size, and we discuss how changes in cell mass and volume contribute to tissue morphogenesis. The role that cell size control plays in developing embryos is only starting to be elucidated. Research on the signals that control cell size will illuminate our understanding of the cellular and molecular mechanisms that drive tissue morphogenesis.

Hemodynamics regulate spatiotemporal artery muscularization in the developing circle of Willis

Vascular smooth muscle cells (VSMCs) envelop vertebrate brain arteries, playing a crucial role in regulating cerebral blood flow and neurovascular coupling. The dedifferentiation of VSMCs is implicated in cerebrovascular diseases and neurodegeneration. Despite its importance, the process of VSMC differentiation on brain arteries during development remains inadequately characterized. Understanding this process could aid in reprogramming and regenerating differentiated VSMCs in cerebrovascular diseases. In this study, we investigated VSMC differentiation on the zebrafish circle of Willis (CoW), comprising major arteries that supply blood to the vertebrate brain. We observed that the arterial expression of CoW endothelial cells (ECs) occurs after their migration from the cranial venous plexus to form CoW arteries. Subsequently, acta2+ VSMCs differentiate from pdgfrb+ mural cell progenitors upon recruitment to CoW arteries. The progression of VSMC differentiation exhibits a spatiotemporal pattern, advancing from anterior to posterior CoW arteries. Analysis of blood flow suggests that earlier VSMC differentiation in anterior CoW arteries correlates with higher red blood cell velocity wall shear stress. Furthermore, pulsatile blood flow is required for differentiation of human brain pdgfrb+ mural cells into VSMCs as well as VSMC differentiation on zebrafish CoW arteries. Consistently, the flow-responsive transcription factor klf2a is activated in ECs of CoW arteries prior to VSMC differentiation, and klf2a knockdown delays VSMC differentiation on anterior CoW arteries. In summary, our findings highlight the role of blood flow activation of endothelial klf2a as a mechanism regulating the initial VSMC differentiation on vertebrate brain arteries.

Dimethyl fumarate induces cardiac developmental toxicity in zebrafish via down-regulation of oxidative stress

Dimethyl fumarate (DMF) is an immunosuppressant commonly used to treat multiple sclerosis and other autoimmune diseases. Despite known side effects such as lymphopenia, the effect of DMF on cardiac development remains unclear. To assess this, we used zebrafish to evaluate the cardiac developmental toxicity of DMF. Our study showed that DMF reduced the survival rate of zebrafish embryos, with those exposed to 1, 1.3, and 1.6 mg/L exhibiting heart rate reduction, shortened body length, delayed yolk sac absorption, pericardial edema, increased distance from sinus venous to bulbus arteriosus, and separation of cardiomyocytes and endocardial cells at 72 hpf. Heart development-related genes showed disorder, apoptosis-related genes were up-regulated, and the oxidative stress response was down-regulated. Treatment with cysteamine ameliorated the heart development defects. Our study demonstrates that DMF induces cardiac developmental toxicity in zebrafish, possibly by down-regulating oxidative stress responses. This study provides a certain research basis for further study of DMF-induced cardiac developmental toxicity, and provides some experimental evidence for future clinical application and study of DMF.

Multiple pkd and piezo gene family members are required for atrioventricular valve formation

Cardiac valves ensure unidirectional blood flow through the heart, and altering their function can result in heart failure. Flow sensing via wall shear stress and wall stretching through the action of mechanosensors can modulate cardiac valve formation. However, the identity and precise role of the key mechanosensors and their effectors remain mostly unknown. Here, we genetically dissect the role of Pkd1a and other mechanosensors in atrioventricular (AV) valve formation in zebrafish and identify a role for several pkd and piezo gene family members in this process. We show that Pkd1a, together with Pkd2, Pkd1l1, and Piezo2a, promotes AV valve elongation and cardiac morphogenesis. Mechanistically, Pkd1a, Pkd2, and Pkd1l1 all repress the expression of klf2a and klf2b, transcription factor genes implicated in AV valve development. Furthermore, we find that the calcium-dependent protein kinase Camk2g is required downstream of Pkd function to repress klf2a expression. Altogether, these data identify, and dissect the role of, several mechanosensors required for AV valve formation, thereby broadening our understanding of cardiac valvulogenesis.

Krüppel-like factor 2a (KLF2A) suppresses GCRV replication by upregulating serpinc1 expression in Ctenopharyngodon idellus kidney (CIK) cells

Krüppel-like factor 2a (KLF2A), a transcription factor of the krüppel-like family, is involved in regulating the immune molecules and is associated with viral infection. However, the function of KLF2A during viral infections in fish remains unclear. In this study, grass carp (Ctenopharyngodon idellus) was used to predict the target genes regulated by KLF2A. The results showed that the candidate target genes included four members of the serpin gene family (serpinb1l2, serpinc1, serpinh1a, and serpinh1b). Dual-luciferase experiments showed that klf2a positively regulates serpinc1 expression. Dose-dependent klf2a overexpression in C. idellus kidney (CIK) cells significantly upregulated the expression of serpinc1. Overexpressing klf2a or serpinc1 in CIK cells activated interferon responses and suppressed grass carp reovirus (GCRV) replication. Klf2a and serpinc1 co-expression inhibited GCRV replication. These results show that klf2a upregulates serpinc1 mRNA expression, promotes type 1 interferon responses, and suppresses GCRV infection. This study provides insights into the regulatory role and biological functions of KLF2A in host-virus interactions in fish.

Diflubenzuron Induces Cardiotoxicity in Zebrafish Embryos

Diflubenzuron is an insecticide that serves as a chitin inhibitor to restrict the growth of many harmful larvae, including mosquito larvae, cotton bollworm and flies. The residue of diflubenzuron is often detected in aquaculture, but its potential toxicity to aquatic organisms is still obscure. In this study, zebrafish embryos (from 6 h to 96 h post-fertilization, hpf) were exposed to different concentrations of diflubenzuron (0, 0.5, 1.5, 2.5, 3.5 and 4.5 mg/L), and the morphologic changes, mortality rate, hatchability rate and average heart rate were calculated. Diflubenzuron exposure increased the distance between the venous sinus and bulbar artery (SV-BA), inhibited proliferation of myocardial cells and damaged vascular development. In addition, diflubenzuron exposure also induced contents of reactive oxygen species (ROS) and malondialdehyde (MDA) and inhibited the activity of antioxidants, including SOD (superoxide dismutase) and CAT (catalase). Moreover, acridine orange (AO) staining showed that diflubenzuron exposure increased the apoptotic cells in the heart. Q-PCR also indicated that diflubenzuron exposure promoted the expression of apoptosis-related genes (bax, bcl2, p53, caspase3 and caspase9). However, the expression of some heart-related genes were inhibited. The oxidative stress-induced apoptosis damaged the cardiac development of zebrafish embryos. Therefore, diflubenzuron exposure induced severe cardiotoxicity in zebrafish embryos. The results contribute to a more comprehensive understanding of the safety use of diflubenzuron.

Effect of Blood Flow on Cardiac Morphogenesis and Formation of Congenital Heart Defects

Congenital heart disease (CHD) affects about 1 in 100 newborns and its causes are multifactorial. In the embryo, blood flow within the heart and vasculature is essential for proper heart development, with abnormal blood flow leading to CHD. Here, we discuss how blood flow (hemodynamics) affects heart development from embryonic to fetal stages, and how abnormal blood flow solely can lead to CHD. We emphasize studies performed using avian models of heart development, because those models allow for hemodynamic interventions, in vivo imaging, and follow up, while they closely recapitulate heart defects observed in humans. We conclude with recommendations on investigations that must be performed to bridge the gaps in understanding how blood flow alone, or together with other factors, contributes to CHD.

Emerging Role of Mechanical Forces in Cell Fate Acquisition

Mechanical forces are now recognized as key cellular effectors that together with genetic and cellular signals physically shape and pattern tissues and organs during development. Increasing efforts are aimed toward understanding the less explored role of mechanical forces in controlling cell fate decisions in embryonic development. Here we discuss recent examples of how differential forces feedback into cell fate specification and tissue patterning. In particular, we focus on the role of actomyosin-contractile force generation and transduction in affecting tissue morphogenesis and cell fate regulation in the embryo.

Mechanical control of tissue shape: Cell-extrinsic and -intrinsic mechanisms join forces to regulate morphogenesis

During vertebrate development, cells must proliferate, move, and differentiate to form complex shapes. Elucidating the mechanisms underlying the molecular and cellular processes involved in tissue morphogenesis is essential to understanding developmental programmes. Mechanical stimuli act as a major contributor of morphogenetic processes and impact on cell behaviours to regulate tissue shape and size. Specifically, cell extrinsic physical forces are translated into biochemical signals within cells, through the process of mechanotransduction, activating multiple mechanosensitive pathways and defining cell behaviours. Physical forces generated by tissue mechanics and the extracellular matrix are crucial to orchestrate tissue patterning and cell fate specification. At the cell scale, the actomyosin network generates the cellular tension behind the tissue mechanics involved in building tissue. Thus, understanding the role of physical forces during morphogenetic processes requires the consideration of the contribution of cell intrinsic and cell extrinsic influences. The recent development of multidisciplinary approaches, as well as major advances in genetics, microscopy, and force-probing tools, have been key to push this field forward. With this review, we aim to discuss recent work on how tissue shape can be controlled by mechanical forces by focusing specifically on vertebrate organogenesis. We consider the influences of mechanical forces by discussing the cell-intrinsic forces (such as cell tension and proliferation) and cell-extrinsic forces (such as substrate stiffness and flow forces). We review recently described processes supporting the role of intratissue force generation and propagation in the context of shape emergence. Lastly, we discuss the emerging role of tissue-scale changes in tissue material properties, extrinsic forces, and shear stress on shape establishment.

Extracellular mechanical forces drive endocardial cell volume decrease during zebrafish cardiac valve morphogenesis

Organ morphogenesis involves dynamic changes of tissue properties while cells adapt to their mechanical environment through mechanosensitive pathways. How mechanical cues influence cell behaviors during morphogenesis remains unclear. Here, we studied the formation of the zebrafish atrioventricular canal (AVC) where cardiac valves develop. We show that the AVC forms within a zone of tissue convergence associated with the increased activation of the actomyosin meshwork and cell-orientation changes. We demonstrate that tissue convergence occurs with a reduction of cell volume triggered by mechanical forces and the mechanosensitive channel TRPP2/TRPV4. Finally, we show that the extracellular matrix component hyaluronic acid controls cell volume changes. Together, our data suggest that multiple force-sensitive signaling pathways converge to modulate cell volume. We conclude that cell volume reduction is a key cellular feature activated by mechanotransduction during cardiovascular morphogenesis. This work further identifies how mechanical forces and extracellular matrix influence tissue remodeling in developing organs.

Cardiac forces regulate zebrafish heart valve delamination by modulating Nfat signaling

In the clinic, most cases of congenital heart valve defects are thought to arise through errors that occur after the endothelial–mesenchymal transition (EndoMT) stage of valve development. Although mechanical forces caused by heartbeat are essential modulators of cardiovascular development, their role in these later developmental events is poorly understood. To address this question, we used the zebrafish superior atrioventricular valve (AV) as a model. We found that cellularized cushions of the superior atrioventricular canal (AVC) morph into valve leaflets via mesenchymal–endothelial transition (MEndoT) and tissue sheet delamination. Defects in delamination result in thickened, hyperplastic valves, and reduced heart function. Mechanical, chemical, and genetic perturbation of cardiac forces showed that mechanical stimuli are important regulators of valve delamination. Mechanistically, we show that forces modulate Nfatc activity to control delamination. Together, our results establish the cellular and molecular signature of cardiac valve delamination in vivo and demonstrate the continuous regulatory role of mechanical forces and blood flow during valve formation.

Why do developing zebrafish atrioventricular heart valves become hyperplastic under certain hemodynamic conditions? This study suggests that part of the answer lies in how the mechanosensitive Nfat pathway regulates the valve mesenchymal-to-endothelial transition.

Myocardial Afterload Is a Key Biomechanical Regulator of Atrioventricular Myocyte Differentiation in Zebrafish

Heart valve development is governed by both genetic and biomechanical inputs. Prior work has demonstrated that oscillating shear stress associated with blood flow is required for normal atrioventricular (AV) valve development. Cardiac afterload is defined as the pressure the ventricle must overcome in order to pump blood throughout the circulatory system. In human patients, conditions of high afterload can cause valve pathology. Whether high afterload adversely affects embryonic valve development remains poorly understood. Here we describe a zebrafish model exhibiting increased myocardial afterload, caused by vasopressin, a vasoconstrictive drug. We show that the application of vasopressin reliably produces an increase in afterload without directly acting on cardiac tissue in zebrafish embryos. We have found that increased afterload alters the rate of growth of the cardiac chambers and causes remodeling of cardiomyocytes. Consistent with pathology seen in patients with clinically high afterload, we see defects in both the form and the function of the valve leaflets. Our results suggest that valve defects are due to changes in atrioventricular myocyte signaling, rather than pressure directly acting on the endothelial valve leaflet cells. Cardiac afterload should therefore be considered a biomechanical factor that particularly impacts embryonic valve development.

Cyclosporine A Induces Cardiac Developmental Toxicity in Zebrafish by Up-Regulation of Wnt Signaling and Oxidative Stress

Due to the widely application of Cyclosporine A (CsA) as an immunosuppressant in clinic, it is necessary to study its potential toxicity. Therefore, we used zebrafish as a model animal to evaluate the toxicity of CsA on embryonic development. Exposure of zebrafish embryos to CsA at concentrations of 5 mg/L, 10 mg/L, and 15 mg/L from 12 hpf to 72 hpf resulted in abnormal embryonic development, including cardiac malformation, pericardial edema, decreased heart rate, decreased blood flow velocity, deposition at yolk sac, shortened body length, and increased distance between venous sinus and arterial bulb (SV-BA). The expression of genes related to cardiac development was disordered, and the apoptotic genes were up-regulated. Oxidative stress level was up-regulated and accumulated in pericardium in a dose-dependent manner. Astaxanthin (ATX) treatment could significantly alleviate zebrafish heart defects. CsA induced up-regulation of Wnt signaling in zebrafish, and IWR-1, an inhibitor of Wnt signaling pathway, could effectively rescue the heart defects induced by CsA. Together, our study indicated that CsA induced cardiac developmental toxicity in zebrafish larvae through up-regulating oxidative stress and Wnt signaling, contributing to a more comprehensive evaluation of the safety of the drug.

Sculpting the heart: Cellular mechanisms shaping valves and trabeculae

The transformation of the heart from a simple tube to a complex organ requires the orchestration of several morphogenetic processes. Two structures critical for cardiac function, the cardiac valves and the trabecular network, are formed through extensive tissue morphogenesis-endocardial cell migration, deadhesion and differentiation into fibroblast-like cells during valve formation, and cardiomyocyte delamination and apico-basal depolarization during trabeculation. Here, we review current knowledge of how these specialized structures acquire their shape by focusing on the underlying cellular behaviors and molecular mechanisms, highlighting findings from in vivo models and briefly discussing the recent advances in cardiac cell culture and organoids.

Bioelectric signaling and the control of cardiac cell identity in response to mechanical forces

[Figure: see text].

Lamb1a regulates atrial growth by limiting second heart field addition during zebrafish heart development

During early vertebrate heart development, the heart transitions from a linear tube to a complex asymmetric structure, a morphogenetic process that occurs simultaneously with growth of the heart. Cardiac growth during early heart morphogenesis is driven by deployment of cells from the second heart field (SHF) into both poles of the heart. Laminin is a core component of the extracellular matrix and, although mutations in laminin subunits are linked with cardiac abnormalities, no role for laminin has been identified in early vertebrate heart morphogenesis. We identified tissue-specific expression of laminin genes in the developing zebrafish heart, supporting a role for laminins in heart morphogenesis. Analysis of heart development in lamb1a zebrafish mutant embryos reveals mild morphogenetic defects and progressive cardiomegaly, and that Lamb1a functions to limit heart size during cardiac development by restricting SHF addition. lamb1a mutants exhibit hallmarks of altered haemodynamics, and blocking cardiac contractility in lamb1a mutants rescues heart size and atrial SHF addition. Together, these results suggest that laminin mediates interactions between SHF deployment and cardiac biomechanics during heart morphogenesis and growth in the developing embryo.

Mechanosensitive Notch-Dll4 and Klf2-Wnt9 signaling pathways intersect in guiding valvulogenesis in zebrafish

In the zebrafish embryo, the onset of blood flow generates fluid shear stress on endocardial cells, which are specialized endothelial cells that line the interior of the heart. High levels of fluid shear stress activate both Notch and Klf2 signaling, which play crucial roles in atrioventricular valvulogenesis. However, it remains unclear why only individual endocardial cells ingress into the cardiac jelly and initiate valvulogenesis. Here, we show that lateral inhibition between endocardial cells, mediated by Notch, singles out Delta-like-4-positive endocardial cells. These cells ingress into the cardiac jelly, where they form an abluminal cell population. Delta-like-4-positive cells ingress in response to Wnt9a, which is produced in parallel through an Erk5-Klf2-Wnt9a signaling cascade also activated by blood flow. Hence, mechanical stimulation activates parallel mechanosensitive signaling pathways that produce binary effects by driving endocardial cells toward either luminal or abluminal fates. Ultimately, these cell fate decisions sculpt cardiac valve leaflets.

Making Blood from the Vessel: Extrinsic and Environmental Cues Guiding the Endothelial-to-Hematopoietic Transition

It is increasingly recognized that specialized subsets of endothelial cells carry out unique functions in specific organs and regions of the vascular tree. Perhaps the most striking example of this specialization is the ability to contribute to the generation of the blood system, in which a distinct population of “hemogenic” endothelial cells in the embryo transforms irreversibly into hematopoietic stem and progenitor cells that produce circulating erythroid, myeloid and lymphoid cells for the lifetime of an animal. This review will focus on recent advances made in the zebrafish model organism uncovering the extrinsic and environmental factors that facilitate hemogenic commitment and the process of endothelial-to-hematopoietic transition that produces blood stem cells. We highlight in particular biomechanical influences of hemodynamic forces and the extracellular matrix, metabolic and sterile inflammatory cues present during this developmental stage, and outline new avenues opened by transcriptomic-based approaches to decipher cell–cell communication mechanisms as examples of key signals in the embryonic niche that regulate hematopoiesis.

Strong as a Hippo's Heart: Biomechanical Hippo Signaling During Zebrafish Cardiac Development

The heart is comprised of multiple tissues that contribute to its physiological functions. During development, the growth of myocardium and endocardium is coupled and morphogenetic processes within these separate tissue layers are integrated. Here, we discuss the roles of mechanosensitive Hippo signaling in growth and morphogenesis of the zebrafish heart. Hippo signaling is involved in defining numbers of cardiac progenitor cells derived from the secondary heart field, in restricting the growth of the epicardium, and in guiding trabeculation and outflow tract formation. Recent work also shows that myocardial chamber dimensions serve as a blueprint for Hippo signaling-dependent growth of the endocardium. Evidently, Hippo pathway components act at the crossroads of various signaling pathways involved in embryonic zebrafish heart development. Elucidating how biomechanical Hippo signaling guides heart morphogenesis has direct implications for our understanding of cardiac physiology and pathophysiology.

Pyridaben induced cardiotoxicity during the looping stages of zebrafish (Danio rerio) embryos

Pyridaben is a widely used acaricide in agriculture and reaches a high concentration (97 μg/L) in paddy water for a short time when pyridaben was applied to rice. However, its toxicity to aquatic organisms is still poorly understood. Therefore, we assessed the pyridaben cardiotoxicity to aquatic organisms using the zebrafish (Danio rerio) model. We found that pyridaben is highly toxic to aquatic organisms, and LC₅₀ of pyridaben for zebrafish at 72 hpf was 100.6 μg/L. Pyridaben caused severe cardiac malformations and functional abnormalities. Morphologic abnormity included severe pericardial edema, cardiomegaly, decreased cardiomyocytes, thinning of the myocardial layer, linear heart, and increased the distance between sinus venous and bulbus arteriosus (SV-BA). Functional failure included arrhythmia, heart failure, and reduced pumping efficiency. The genes involved in heart development, WNT signaling, BMP signaling, ATPase, and cardiac troponin C were abnormally expressed in the pyridaben treatment group. Exposure to pyridaben increased oxidative stress and induced cell apoptosis. The above causes may lead to cardiac toxicity. The results suggest that pyridaben exposure induced elevated oxidative stress through the WNT signaling pathway, which in turn led to apoptosis in the heart and cardiotoxicity. Besides, pyridaben exposure at the critical stage of cardiac looping (24-36 hpf) resulted in the greatest cardiotoxicity. The chorion reduced the entry of pyridaben and protected zebrafish embryos, resulting in cardiotoxicity second only to the stage of cardiac looping. The study should provide valuable information that pyridaben exposure causes cardiotoxicity in zebrafish embryos and have potential health risks for other aquatic organisms and humans.

Fetal Blood Flow and Genetic Mutations in Conotruncal Congenital Heart Disease

In congenital heart disease, the presence of structural defects affects blood flow in the heart and circulation. However, because the fetal circulation bypasses the lungs, fetuses with cyanotic heart defects can survive in utero but need prompt intervention to survive after birth. Tetralogy of Fallot and persistent truncus arteriosus are two of the most significant conotruncal heart defects. In both defects, blood access to the lungs is restricted or non-existent, and babies with these critical conditions need intervention right after birth. While there are known genetic mutations that lead to these critical heart defects, early perturbations in blood flow can independently lead to critical heart defects. In this paper, we start by comparing the fetal circulation with the neonatal and adult circulation, and reviewing how altered fetal blood flow can be used as a diagnostic tool to plan interventions. We then look at known factors that lead to tetralogy of Fallot and persistent truncus arteriosus: namely early perturbations in blood flow and mutations within VEGF-related pathways. The interplay between physical and genetic factors means that any one alteration can cause significant disruptions during development and underscore our need to better understand the effects of both blood flow and flow-responsive genes.

Mechanical Patterning in Animal Morphogenesis

Morphogenesis is one of the most remarkable examples of biological pattern formation. Despite substantial progress in the field, we still do not understand the organizational principles responsible for the robust convergence of the morphogenesis process across scales to form viable organisms under variable conditions. Achieving large-scale coordination requires feedback between mechanical and biochemical processes, spanning all levels of organization and relating the emerging patterns with the mechanisms driving their formation. In this review, we highlight the role of mechanics in the patterning process, emphasizing the active and synergistic manner in which mechanical processes participate in developmental patterning rather than merely following a program set by biochemical signals. We discuss the value of applying a coarse-grained approach toward understanding this complex interplay, which considers the large-scale dynamics and feedback as well as complementing the reductionist approach focused on molecular detail. A central challenge in this approach is identifying relevant coarse-grained variables and developing effective theories that can serve as a basis for an integrated framework for understanding this remarkable pattern-formation process. Expected final online publication date for the Annual Review of Cell and Developmental Biology, Volume 37 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Cerebral Cavernous Malformation: From Mechanism to Therapy

Cerebral cavernous malformations are acquired vascular anomalies that constitute a common cause of central nervous system hemorrhage and stroke. The past 2 decades have seen a remarkable increase in our understanding of the pathogenesis of this vascular disease. This new knowledge spans genetic causes of sporadic and familial forms of the disease, molecular signaling changes in vascular endothelial cells that underlie the disease, unexpectedly strong environmental effects on disease pathogenesis, and drivers of disease end points such as hemorrhage. These novel insights are the integrated product of human clinical studies, human genetic studies, studies in mouse and zebrafish genetic models, and basic molecular and cellular studies. This review addresses the genetic and molecular underpinnings of cerebral cavernous malformation disease, the mechanisms that lead to lesion hemorrhage, and emerging biomarkers and therapies for clinical treatment of cerebral cavernous malformation disease. It may also serve as an example for how focused basic and clinical investigation and emerging technologies can rapidly unravel a complex disease mechanism.

Endothelial cell mechanics and blood flow forces in vascular morphogenesis

The vertebrate cardiovascular system is made up by a hierarchically structured network of highly specialised blood vessels. This network emerges during early embryogenesis and evolves in size and complexity concomitant with embryonic growth and organ formation. Underlying this plasticity are actin-driven endothelial cell behaviours, which allow endothelial cells to change their shape and move within the vascular network. In this review, we discuss the cellular and molecular mechanisms involved in vascular network formation and how these intrinsic mechanisms are influenced by haemodynamic forces provided by pressurized blood flow. While most of this review focusses on in vivo evidence from zebrafish embryos, we also mention complementary findings obtained in other experimental systems.

Getting to the Heart of Left-Right Asymmetry: Contributions from the Zebrafish Model

The heart is laterally asymmetric. Not only is it positioned on the left side of the body but the organ itself is asymmetric. This patterning occurs across scales: at the organism level, through left-right axis patterning; at the organ level, where the heart itself exhibits left-right asymmetry; at the cellular level, where gene expression, deposition of matrix and proteins and cell behaviour are asymmetric; and at the molecular level, with chirality of molecules. Defective left-right patterning has dire consequences on multiple organs; however, mortality and morbidity arising from disrupted laterality is usually attributed to complex cardiac defects, bringing into focus the particulars of left-right patterning of the heart. Laterality defects impact how the heart integrates and connects with neighbouring organs, but the anatomy of the heart is also affected because of its asymmetry. Genetic studies have demonstrated that cardiac asymmetry is influenced by left-right axis patterning and yet the heart also possesses intrinsic laterality, reinforcing the patterning of this organ. These inputs into cardiac patterning are established at the very onset of left-right patterning (formation of the left-right organiser) and continue through propagation of left-right signals across animal axes, asymmetric differentiation of the cardiac fields, lateralised tube formation and asymmetric looping morphogenesis. In this review, we will discuss how left-right asymmetry is established and how that influences subsequent asymmetric development of the early embryonic heart. In keeping with the theme of this issue, we will focus on advancements made through studies using the zebrafish model and describe how its use has contributed considerable knowledge to our understanding of the patterning of the heart.

When form meets function: the cells and signals that shape the lymphatic vasculature during development

The lymphatic vasculature is an integral component of the cardiovascular system. It is essential to maintain tissue fluid homeostasis, direct immune cell trafficking and absorb dietary lipids from the digestive tract. Major advances in our understanding of the genetic and cellular events important for constructing the lymphatic vasculature during development have recently been made. These include the identification of novel sources of lymphatic endothelial progenitor cells, the recognition of lymphatic endothelial cell specialisation and heterogeneity, and discovery of novel genes and signalling pathways underpinning developmental lymphangiogenesis. Here, we review these advances and discuss how they inform our understanding of lymphatic network formation, function and dysfunction.

Effects of extended pharmacological disruption of zebrafish embryonic heart biomechanical environment on cardiac function, morphology, and gene expression

BACKGROUND

Biomechanical stimuli are known to be important to cardiac development, but the mechanisms are not fully understood. Here, we pharmacologically disrupted the biomechanical environment of wild-type zebrafish embryonic hearts for an extended duration and investigated the consequent effects on cardiac function, morphological development, and gene expression.

RESULTS

Myocardial contractility was significantly diminished or abolished in zebrafish embryonic hearts treated for 72 hours from 2 dpf with 2,3-butanedione monoxime (BDM). Image-based flow simulations showed that flow wall shear stresses were abolished or significantly reduced with high oscillatory shear indices. At 5 dpf, after removal of BDM, treated embryonic hearts were maldeveloped, having disrupted cardiac looping, smaller ventricles, and poor cardiac function (lower ejected flow, bulboventricular regurgitation, lower contractility, and slower heart rate). RNA sequencing of cardiomyocytes of treated hearts revealed 922 significantly up-regulated genes and 1,698 significantly down-regulated genes. RNA analysis and subsequent qPCR and histology validation suggested that biomechanical disruption led to an up-regulation of inflammatory and apoptotic genes and down-regulation of ECM remodeling and ECM-receptor interaction genes. Biomechanics disruption also prevented the formation of ventricular trabeculation along with notch1 and erbb4a down-regulation.

CONCLUSIONS

Extended disruption of biomechanical stimuli caused maldevelopment, and potential genes responsible for this are identified.

Low doses of BPF-induced hypertrophy in cardiomyocytes derived from human embryonic stem cells via disrupting the mitochondrial fission upon the interaction between ERβ and calcineurin A-DRP1 signaling pathway

Bisphenol F (BPF) is a replacement to bisphenol A, which has been extensively used in industrial manufacturing. Its wide detection in various human samples raises increasing concern on its safety. Currently, whether a low dose of BPF compromises cardiac function is still unknown. This study provides the first evidence that low-dose BPF can induce cardiac hypertrophy by using cardiomyocytes derived from human embryonic stem cells (hES). Non-cytotoxic BPF increased cytosolic Ca 2+ influx ([Ca2+ ]c), which was most remarkable at low dose (7 ng/ml) rather than at higher doses. Significant changes in the morphological parameters of mitochondria and significant decreases in ATP production were induced by 7 ng/ml BPF, representing a classic hypertrophic cardiomyocyte. After eliminating the direct effects on mitochondrial fission-related DRP1 by administration of the DRP1 inhibitor Mdivi-1, we examined the changes in [Ca 2+ ]c levels induced by BPF, which enhanced the calcineurin (Cn) activity and induced the abnormal mitochondrial fission via the CnAβ-DRP1 signaling pathway. BPF triggered excessive Ca 2+ influx by disrupting the L-type Ca 2+channel in cardiomyocytes. The interaction between ERβ and CnAβ cooperatively involved in the BPF-induced Ca 2+ influx, which resulted in the abnormal mitochondrial fission and compromised the cardiac function. Our findings provide a feasible molecular mechanism for explaining low-dose BPF-induced cardiac hypertrophy in vitro, preliminarily suggesting that BPF may not be as safe as assumed in humans.

Abortive intussusceptive angiogenesis causes multi-cavernous vascular malformations

Mosaic inactivation of CCM2 in humans causes cerebral cavernous malformations (CCMs) containing adjacent dilated blood-filled multi-cavernous lesions. We used CRISPR-Cas9 mutagenesis to induce mosaic inactivation of zebrafish ccm2 resulting in a novel lethal multi-cavernous lesion in the embryonic caudal venous plexus (CVP) caused by obstruction of blood flow by intraluminal pillars. These pillars mimic those that mediate intussusceptive angiogenesis; however, in contrast to the normal process, the pillars failed to fuse to split the pre-existing vessel in two. Abortive intussusceptive angiogenesis stemmed from mosaic inactivation of ccm2 leading to patchy klf2a overexpression and resultant aberrant flow signaling. Surviving adult fish manifested histologically typical hemorrhagic CCM. Formation of mammalian CCM requires the flow-regulated transcription factor KLF2; fish CCM and the embryonic CVP lesion failed to form in klf2a null fish indicating a common pathogenesis with the mammalian lesion. These studies describe a zebrafish CCM model and establish a mechanism that can explain the formation of characteristic multi-cavernous lesions.

A zebrafish toolbox for biomechanical signaling in cardiovascular development and disease

PURPOSE OF REVIEW

The zebrafish embryo has emerged as a powerful model organism to investigate the mechanisms by which biophysical forces regulate vascular and cardiac cell biology during development and disease. A versatile arsenal of methods and tools is available to manipulate and analyze biomechanical signaling. This review aims to provide an overview of the experimental strategies and tools that have been utilized to study biomechanical signaling in cardiovascular developmental processes and different vascular disease models in the zebrafish embryo. Within the scope of this review, we focus on work published during the last two years.

RECENT FINDINGS

Genetic and pharmacological tools for the manipulation of cardiac function allow alterations of hemodynamic flow patterns in the zebrafish embryo and various types of transgenic lines are available to report endothelial cell responses to biophysical forces. These tools have not only revealed the impact of biophysical forces on cardiovascular development but also helped to establish more accurate models for cardiovascular diseases including cerebral cavernous malformations, hereditary hemorrhagic telangiectasias, arteriovenous malformations, and lymphangiopathies.

SUMMARY

The zebrafish embryo is a valuable vertebrate model in which in-vivo manipulations of biophysical forces due to cardiac contractility and blood flow can be performed. These analyses give important insights into biomechanical signaling pathways that control endothelial and endocardial cell behaviors. The technical advances using this vertebrate model will advance our understanding of the impact of biophysical forces in cardiovascular pathologies.

A biomechanical switch regulates the transition towards homeostasis in oesophageal epithelium

Epithelial cells rapidly adapt their behaviour in response to increasing tissue demands. However, the processes that finely control these cell decisions remain largely unknown. The postnatal period covering the transition between early tissue expansion and the establishment of adult homeostasis provides a convenient model with which to explore this question. Here, we demonstrate that the onset of homeostasis in the epithelium of the mouse oesophagus is guided by the progressive build-up of mechanical strain at the organ level. Single-cell RNA sequencing and whole-organ stretching experiments revealed that the mechanical stress experienced by the growing oesophagus triggers the emergence of a bright Krüppel-like factor 4 (KLF4) committed basal population, which balances cell proliferation and marks the transition towards homeostasis in a yes-associated protein (YAP)-dependent manner. Our results point to a simple mechanism whereby mechanical changes experienced at the whole-tissue level are integrated with those sensed at the cellular level to control epithelial cell fate.

Antagonistic Activities of Vegfr3/Flt4 and Notch1b Fine-tune Mechanosensitive Signaling during Zebrafish Cardiac Valvulogenesis

The formation of cardiac valves depends on mechanical forces exerted by blood flow. Endocardial cells lining the interior of the heart are sensitive to these stimuli and respond by rearranging into luminal cells subjected to shear stress and abluminal cells not exposed to it. The mechanisms by which endocardial cells sense these dynamic biomechanical stimuli and how they evoke different cellular responses are largely unknown. Here, we show that blood flow activates two parallel mechanosensitive pathways, one mediated by Notch and the other by Klf2a. Both pathways negatively regulate the angiogenesis receptor Vegfr3/Flt4, which becomes restricted to abluminal endocardial cells. Its loss disrupts valve morphogenesis and results in the occurrence of Notch signaling within abluminal endocardial cells. Our work explains how antagonistic activities by Vegfr3/Flt4 on the abluminal side and by Notch on the luminal side shape cardiac valve leaflets by triggering unique differences in the fates of endocardial cells.

The ECM as a driver of heart development and repair

The developing heart is formed of two tissue layers separated by an extracellular matrix (ECM) that provides chemical and physical signals to cardiac cells. While deposition of specific ECM components creates matrix diversity, the cardiac ECM is also dynamic, with modification and degradation playing important roles in ECM maturation and function. In this Review, we discuss the spatiotemporal changes in ECM composition during cardiac development that support distinct aspects of heart morphogenesis. We highlight conserved requirements for specific ECM components in human cardiac development, and discuss emerging evidence of a central role for the ECM in promoting heart regeneration.

Basic Biology of Extracellular Matrix in the Cardiovascular System, Part 1/4: JACC Focus Seminar

The extracellular matrix (ECM) is the noncellular component of tissues in the cardiovascular system and other organs throughout the body. It is formed of filamentous proteins, proteoglycans, and glycosaminoglycans, which extensively interact and whose structure and dynamics are modified by cross-linking, bridging proteins, and cleavage by matrix degrading enzymes. The ECM serves important structural and regulatory roles in establishing tissue architecture and cellular function. The ECM of the developing heart has unique properties created by its emerging contractile nature; similarly, ECM lining blood vessels is highly elastic in order to sustain the basal and pulsatile forces imposed on their walls throughout life. In this part 1 of a 4-part JACC Focus Seminar, we focus on the role, function, and basic biology of the ECM in both heart development and in the adult.

Forced to communicate: Integration of mechanical and biochemical signaling in morphogenesis

Morphogenesis is a physical process that requires the generation of mechanical forces to achieve dynamic changes in cell position, tissue shape, and size as well as biochemical signals to coordinate these events. Mechanical forces are also used by the embryo to transmit detailed information across space and detected by target cells, leading to downstream changes in cellular properties and behaviors. Indeed, forces provide signaling information of complementary quality that can both synergize and diversify the functional outputs of biochemical signaling. Here, we discuss recent findings that reveal how mechanical signaling and biochemical signaling are integrated during morphogenesis and the possible context-specific advantages conferred by the interactions between these signaling mechanisms.

Oscillatory fluid-induced mechanobiology in heart valves with parallels to the vasculature

Forces generated by blood flow are known to contribute to cardiovascular development and remodeling. These hemodynamic forces induce molecular signals that are communicated from the endothelium to various cell types. The cardiovascular system consists of the heart and the vasculature, and together they deliver nutrients throughout the body. While heart valves and blood vessels experience different environmental forces and differ in morphology as well as cell types, they both can undergo pathological remodeling and become susceptible to calcification. In addition, while the plaque morphology is similar in valvular and vascular diseases, therapeutic targets available for the latter condition are not effective in the management of heart valve calcification. Therefore, research in valvular and vascular pathologies and treatments have largely remained independent. Nonetheless, understanding the similarities and differences in development, calcific/fibrous pathologies and healthy remodeling events between the valvular and vascular systems can help us better identify future treatments for both types of tissues, particularly for heart valve pathologies which have been understudied in comparison to arterial diseases.

Fluid flow as a driver of embryonic morphogenesis

Fluid flow is a powerful morphogenic force during embryonic development. The physical forces created by flowing fluids can either create morphogen gradients or be translated by mechanosensitive cells into biological changes in gene expression. In this Primer, we describe how fluid flow is created in different systems and highlight the important mechanosensitive signalling pathways involved for sensing and transducing flow during embryogenesis. Specifically, we describe how fluid flow helps establish left-right asymmetry in the early embryo and discuss the role of flow of blood, lymph and cerebrospinal fluid in sculpting the embryonic cardiovascular and nervous system.

Mechanisms of heart valve development and disease

The valves of the heart are crucial for ensuring that blood flows in one direction from the heart, through the lungs and back to the rest of the body. Heart valve development is regulated by complex interactions between different cardiac cell types and is subject to blood flow-driven forces. Recent work has begun to elucidate the important roles of developmental pathways, valve cell heterogeneity and hemodynamics in determining the structure and function of developing valves. Furthermore, this work has revealed that many key genetic pathways involved in cardiac valve development are also implicated in diseased valves. Here, we review recent discoveries that have furthered our understanding of the molecular, cellular and mechanosensitive mechanisms of valve development, and highlight new insights into congenital and acquired valve disease.

Blood Flow Forces in Shaping the Vascular System: A Focus on Endothelial Cell Behavior

The endothelium is the cell monolayer that lines the interior of the blood vessels separating the vessel lumen where blood circulates, from the surrounding tissues. During embryonic development, endothelial cells (ECs) must ensure that a tight barrier function is maintained whilst dynamically adapting to the growing vascular tree that is being formed and remodeled. Blood circulation generates mechanical forces, such as shear stress and circumferential stretch that are directly acting on the endothelium. ECs actively respond to flow-derived mechanical cues by becoming polarized, migrating and changing neighbors, undergoing shape changes, proliferating or even leaving the tissue and changing identity. It is now accepted that coordinated changes at the single cell level drive fundamental processes governing vascular network morphogenesis such as angiogenic sprouting, network pruning, lumen formation, regulation of vessel caliber and stability or cell fate transitions. Here we summarize the cell biology and mechanics of ECs in response to flow-derived forces, discuss the latest advances made at the single cell level with particular emphasis on in vivo studies and highlight potential implications for vascular pathologies.

Biomechanical Cues Direct Valvulogenesis

The vertebrate embryonic heart initially forms with two chambers, a ventricle and an atrium, separated by the atrioventricular junction. Localized genetic and biomechanical information guides the development of valves, which function to ensure unidirectional blood flow. If the valve development process goes awry, pathology associated with congenital valve defects can ensue. Congenital valve defects (CVD) are estimated to affect 1–2% of the population and can often require a lifetime of treatment. Despite significant clinical interest, molecular genetic mechanisms that direct valve development remain incompletely elucidated. Cells in the developing valve must contend with a dynamic hemodynamic environment. A growing body of research supports the idea that cells in the valve are highly sensitive to biomechanical forces, which cue changes in gene expression required for normal development or for maintenance of the adult valve. This review will focus on mechanotransductive pathways involved in valve development across model species. We highlight current knowledge regarding how cells sense physical forces associated with blood flow and pressure in the forming heart, and summarize how these changes are transduced into genetic and developmental responses. Lastly, we provide perspectives on how altered biomechanical cues may lead to CVD pathogenesis.

Using Zebrafish to Analyze the Genetic and Environmental Etiologies of Congenital Heart Defects

Congenital heart defects (CHDs) are among the most common human birth defects. However, the etiology of a large proportion of CHDs remains undefined. Studies identifying the molecular and cellular mechanisms that underlie cardiac development have been critical to elucidating the origin of CHDs. Building upon this knowledge to understand the pathogenesis of CHDs requires examining how genetic or environmental stress changes normal cardiac development. Due to strong molecular conservation to humans and unique technical advantages, studies using zebrafish have elucidated both fundamental principles of cardiac development and have been used to create cardiac disease models. In this chapter we examine the unique toolset available to zebrafish researchers and how those tools are used to interrogate the genetic and environmental contributions to CHDs.

Primary cilia mediate Klf2-dependant Notch activation in regenerating heart

Unlike adult mammalian heart, zebrafish heart has a remarkable capacity to regenerate after injury. Previous study has shown Notch signaling activation in the endocardium is essential for regeneration of the myocardium and this activation is mediated by hemodynamic alteration after injury, however, the molecular mechanism has not been fully explored. In this study we demonstrated that blood flow change could be perceived and transmitted in a primary cilia dependent manner to control the hemodynamic responsive klf2 gene expression and subsequent activation of Notch signaling in the endocardium. First we showed that both homologues of human gene KLF2 in zebrafish, klf2a and klf2b, could respond to hemodynamic alteration and both were required for Notch signaling activation and heart regeneration. Further experiments indicated that the upregulation of klf2 gene expression was mediated by endocardial primary cilia. Overall, our findings reveal a novel aspect of mechanical shear stress signal in activating Notch pathway and regulating cardiac regeneration.

Nfatc1 Promotes Interstitial Cell Formation During Cardiac Valve Development in Zebrafish

RATIONALE

The transcription factor NFATC1 (nuclear factor of activated T-cell 1) has been implicated in cardiac valve formation in humans and mice, but we know little about the underlying mechanisms. To gain mechanistic understanding of cardiac valve formation at single-cell resolution and insights into the role of NFATC1 in this process, we used the zebrafish model as it offers unique attributes for live imaging and facile genetics.

OBJECTIVE

To understand the role of Nfatc1 in cardiac valve formation.

METHODS AND RESULTS

Using the zebrafish atrioventricular valve, we focus on the valve interstitial cells (VICs), which confer biomechanical strength to the cardiac valve leaflets. We find that initially atrioventricular endocardial cells migrate collectively into the cardiac jelly to form a bilayered structure; subsequently, the cells that led this migration invade the ECM (extracellular matrix) between the 2 endocardial cell monolayers, undergo endothelial-to-mesenchymal transition as marked by loss of intercellular adhesion, and differentiate into VICs. These cells proliferate and are joined by a few neural crest-derived cells. VIC expansion and a switch from a promigratory to an elastic ECM drive valve leaflet elongation. Functional analysis of Nfatc1 reveals its requirement during VIC development. Zebrafish nfatc1 mutants form significantly fewer VICs due to reduced proliferation and impaired recruitment of endocardial and neural crest cells during the early stages of VIC development. With high-speed microscopy and echocardiography, we show that reduced VIC formation correlates with valvular dysfunction and severe retrograde blood flow that persist into adulthood. Analysis of downstream effectors reveals that Nfatc1 promotes the expression of twist1b-a well-known regulator of epithelial-to-mesenchymal transition.

CONCLUSIONS

Our study sheds light on the function of Nfatc1 in zebrafish cardiac valve development and reveals its role in VIC formation. It also further establishes the zebrafish as a powerful model to carry out longitudinal studies of valve formation and function.

TGF-β Signaling Promotes Tissue Formation during Cardiac Valve Regeneration in Adult Zebrafish

Cardiac valve disease can lead to severe cardiac dysfunction and is thus a frequent cause of morbidity and mortality. Its main treatment is valve replacement, which is currently greatly limited by the poor recellularization and tissue formation potential of the implanted valves. As we still lack suitable animal models to identify modulators of these processes, here we used adult zebrafish and found that, upon valve decellularization, they initiate a rapid regenerative program that leads to the formation of new functional valves. After injury, endothelial and kidney marrow-derived cells undergo cell cycle re-entry and differentiate into new extracellular matrix-secreting valve cells. The TGF-β signaling pathway promotes the regenerative process by enhancing progenitor cell proliferation as well as valve cell differentiation. These findings reveal a key role for TGF-β signaling in cardiac valve regeneration and establish the zebrafish as a model to identify and test factors promoting cardiac valve recellularization and growth.

Reactivation of Notch signaling is required for cardiac valve regeneration

Cardiac Valve Disease is one of the most common heart disorders with an emerging epidemic of cardiac valve degeneration due to aging. Zebrafish can regenerate most of their organs, including their heart. We aimed to explore the regenerative potential of cardiac valves and the underlying molecular mechanisms involved. We used an inducible, tissue-specific system of chemogenetic ablation and showed that zebrafish can also regenerate their cardiac valves. Upon valvular damage at larval stages, the intracardiac flow pattern becomes reminiscent of the early embryonic stages, exhibiting an increase in the retrograde flow fraction through the atrioventricular canal. As a result of the altered hemodynamics, notch1b and klf2a expression are ectopically upregulated, adopting the expression pattern of earlier developmental stages. We find that Notch signaling is re-activated upon valvular damage both at larval and adult stages and that it is required during the initial regeneration phase of cardiac valves. Our results introduce an animal model of cardiac valve specific ablation and regeneration.

Underlying mechanisms of reproductive toxicity caused by multigenerational exposure of 2, bromo-4, 6-dinitroaniline (BDNA) to Zebrafish (Danio rerio) at environmental relevant levels

2-bromo-4, 6-dinitroaniline (BDNA) is a mutagenic aromatic amine involved in the production and degradation of Disperse blue 79, one of the most extensively used brominated azo dyes. In our previous study, a multigenerational exposure of BDNA (0.5, 5, 50 and 500 μg/L) to zebrafish from F₀ adult to F₂ larvae including a recovery group in F₂ larvae was conducted. The effects on apical points observed in individuals and the long-term effects predicted on population were all related to reproduction. In this study, we performed molecular analysis to elucidate the underlying mechanisms of the reproductive toxicity of BDNA. In F₁ generation, measurement of vitellogenin and transcription levels of genes associated with hypothalamus-pituitary-gland (HPG) axis, estrogen receptor (ER) and androgen receptor (AR) were conducted. There was a decrease in VTG level in the blood of F₁ female fish and transcription of genes related to ER was more affected than that of genes related to AR. These results were consistent with adverse effects that sexual differentiation was biased towards males and fecundity was impaired in a concentration-dependent manner in adults of F₁ generation after 150 days exposure. In F₂ generation, global gene transcriptions of F₂ larvae were investigated. It was uncovered that processes related to apoptosis, development and DNA damage were strongly affected. Alterations to these biological pathways accounted for the irreversible parental influence on a significant decrease in hatchability and increase in abnormality of F₂ larvae. All evidence suggested that the multigenerational exposure of BDNA posed lasting effects transmitted from parents to offspring that persisted after exposure ceased.

Mechanotransduction in cardiovascular morphogenesis and tissue engineering

Cardiovascular morphogenesis involves cell behavior and cell identity changes that are activated by mechanical forces associated with heart function. Recently, advances in in vivo imaging, methods to alter blood flow, and computational modelling have greatly advanced our understanding of how forces produced by heart contraction and blood flow impact different morphogenetic processes. Meanwhile, traditional genetic approaches have helped to elucidate how endothelial cells respond to forces at the cellular and molecular level. Here we discuss the principles of endothelial mechanosensitity and their interplay with cellular processes during cardiovascular morphogenesis. We then discuss their implications in the field of cardiovascular tissue engineering.

Mechanically activated piezo channels modulate outflow tract valve development through the Yap1 and Klf2-Notch signaling axis

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.