Rotation and asymmetric development of the zebrafish heart requires directed migration of cardiac progenitor cells

Primary cilia, A-kinase anchoring proteins and constitutive activity at the orphan G protein-coupled receptor GPR161: A tale about a tail

Primary cilia are non-motile antennae-like structures responsible for sensing environmental changes in most mammalian cells. Ciliary signalling is largely mediated by the Sonic Hedgehog (Shh) pathway, which acts as a master regulator of ciliary protein transit and is essential for normal embryonic development. One particularly important player in primary cilia is the orphan G protein-coupled receptor, GPR161. In this review, we introduce GPR161 in the context of Shh signalling and describe the unique features on its C-terminus such as PKA phosphorylation sites and an A-kinase anchoring protein motif, which may influence the function of the receptor, cAMP compartmentalisation and/or trafficking within primary cilia. We discuss the recent putative pairing of GPR161 and spexin-1, highlighting the additional steps needed before GPR161 could be considered 'deorphanised'. Finally, we speculate that the marked constitutive activity and unconventional regulation of GPR161 may indicate that the receptor may not require an endogenous ligand. LINKED ARTICLES: This article is part of a themed issue Therapeutic Targeting of G Protein-Coupled Receptors: hot topics from the Australasian Society of Clinical and Experimental Pharmacologists and Toxicologists 2021 Virtual Annual Scientific Meeting. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.14/issuetoc.

Zebrafish Congenital Heart Disease Models: Opportunities and Challenges

Congenital heart defects (CHDs) are common human birth defects. Genetic mutations potentially cause the exhibition of various pathological phenotypes associated with CHDs, occurring alone or as part of certain syndromes. Zebrafish, a model organism with a strong molecular conservation similar to humans, is commonly used in studies on cardiovascular diseases owing to its advantageous features, such as a similarity to human electrophysiology, transparent embryos and larvae for observation, and suitability for forward and reverse genetics technology, to create various economical and easily controlled zebrafish CHD models. In this review, we outline the pros and cons of zebrafish CHD models created by genetic mutations associated with single defects and syndromes and the underlying pathogenic mechanism of CHDs discovered in these models. The challenges of zebrafish CHD models generated through gene editing are also discussed, since the cardiac phenotypes resulting from a single-candidate pathological gene mutation in zebrafish might not mirror the corresponding human phenotypes. The comprehensive review of these zebrafish CHD models will facilitate the understanding of the pathogenic mechanisms of CHDs and offer new opportunities for their treatments and intervention strategies.

Cardiac construction-Recent advances in morphological and transcriptional modeling of early heart development

During human embryonic development the early establishment of a functional heart is vital to support the growing fetus. However, forming the embryonic heart is an extremely complex process, requiring spatiotemporally controlled cell specification and differentiation, tissue organization, and coordination of cardiac function. These complexities, in concert with the early and rapid development of the embryonic heart, mean that understanding the intricate interplay between these processes that help shape the early heart remains highly challenging. In this review I focus on recent insights from animal models that have shed new light on the earliest stages of heart development. This includes specification and organization of cardiac progenitors, cell and tissue movements that make and shape the early heart tube, and the initiation of the first beat in the developing heart. In addition I highlight relevant in vitro models that could support translation of findings from animal models to human heart development. Finally I discuss challenges that are being addressed in the field, along with future considerations that together may help move us towards a deeper understanding of how our hearts are made.

Cooperation between Nodal and FGF signals regulates zebrafish cardiac cell migration and heart morphogenesis

Asymmetric vertebrate heart development is driven by an intricate sequence of morphogenetic cell movements, the coordination of which requires precise interpretation of signaling cues by heart primordia. Here we show that Nodal functions cooperatively with FGF during heart tube formation and asymmetric placement. Both pathways act as migratory stimuli for cardiac progenitor cells (CPCs), but FGF is dispensable for directing heart tube asymmetry, which is governed by Nodal. We further find that Nodal controls CPC migration by inducing left-right asymmetries in the formation of actin-based protrusions in CPCs. Additionally, we define a developmental window in which FGF signals are required for proper heart looping and show cooperativity between FGF and Nodal in this process. We present evidence FGF may promote heart looping through addition of the secondary heart field. Finally, we demonstrate that loss of FGF signaling affects proper development of the atrioventricular canal (AVC), which likely contributes to abnormal chamber morphologies in FGF-deficient hearts. Together, our data shed insight into how the spatiotemporal dynamics of signaling cues regulate the cellular behaviors underlying organ morphogenesis.

Establishment of Cardiac Laterality

Formation of the vertebrate heart with its complex arterial and venous connections is critically dependent on patterning of the left-right axis during early embryonic development. Abnormalities in left-right patterning can lead to a variety of complex life-threatening congenital heart defects. A highly conserved pathway responsible for left-right axis specification has been uncovered. This pathway involves initial asymmetric activation of a nodal signaling cascade at the embryonic node, followed by its propagation to the left lateral plate mesoderm and activation of left-sided expression of the Pitx2 transcription factor specifying visceral organ asymmetry. Intriguingly, recent work suggests that cardiac laterality is encoded by intrinsic cell and tissue chirality independent of Nodal signaling. Thus, Nodal signaling may be superimposed on this intrinsic chirality, providing additional instructive cues to pattern cardiac situs. The impact of intrinsic chirality and the perturbation of left-right patterning on myofiber organization and cardiac function warrants further investigation. We summarize recent insights gained from studies in animal models and also some human clinical studies in a brief overview of the complex processes regulating cardiac asymmetry and their impact on cardiac function and the pathogenesis of congenital heart defects.

Toxic effects of benfluralin on zebrafish embryogenesis via the accumulation of reactive oxygen species and apoptosis

The dinitroaniline herbicide benfluralin is used weed control in conventional systems and poses a high risk of accumulation in aquatic systems. Previous studies have shown the toxic effects of benfluralin on non-target organisms; however, its developmental toxicity in vertebrates has not yet been reported. This study demonstrated the developmental toxicity of benfluralin and its mechanism of action, using zebrafish as an aquatic vertebrate model. Benfluralin induces morphological and physiological alterations in body length, yolk sac, and heart edema. We also demonstrated a reactive oxygen species (ROS) increase of approximately 325.53 % compared with the control group after 20 μM benfluralin-treatment. In addition, the malformation of the heart and vascular structures was identified using transgenic flk1:eGFP zebrafish models at 20 μM concentration benfluralin exposure. Moreover, benfluralin induced small livers, approximately 59.81 % of normal liver size, via abnormal development of the liver as observed in the transgenic L-fabp:dsRed zebrafish. Benfluralin also inhibits normal growth via abnormal expression of cell cycle regulatory genes and increases oxidative stress, inflammation, and apoptosis. Collectively, we elucidated the mechanisms associated with benfluralin toxicity, which lead to various abnormalities and developmental toxicities in zebrafish. Therefore, this study provides information on the parameters used to assess developmental toxicity in other aquatic organisms, such as herbicides, pesticides, and environmental contaminants.

A bioelectrical phase transition patterns the first vertebrate heartbeats

A regular heartbeat is essential to vertebrate life. In the mature heart, this function is driven by an anatomically localized pacemaker. By contrast, pacemaking capability is broadly distributed in the early embryonic heart1-3, raising the question of how tissue-scale activity is first established and then maintained during embryonic development. The initial transition of the heart from silent to beating has never been characterized at the timescale of individual electrical events, and the structure in space and time of the early heartbeats remains poorly understood. Using all-optical electrophysiology, we captured the very first heartbeat of a zebrafish and analysed the development of cardiac excitability and conduction around this singular event. The first few beats appeared suddenly, had irregular interbeat intervals, propagated coherently across the primordial heart and emanated from loci that varied between animals and over time. The bioelectrical dynamics were well described by a noisy saddle-node on invariant circle bifurcation with action potential upstroke driven by CaV1.2. Our work shows how gradual and largely asynchronous development of single-cell bioelectrical properties produces a stereotyped and robust tissue-scale transition from quiescence to coordinated beating.

Wnt9 directs zebrafish heart tube assembly via a combination of canonical and non-canonical pathway signaling

During zebrafish heart formation, cardiac progenitor cells converge at the embryonic midline where they form the cardiac cone. Subsequently, this structure transforms into a heart tube. Little is known about the molecular mechanisms that control these morphogenetic processes. Here, we use light-sheet microscopy and combine genetic, molecular biological and pharmacological tools to show that the paralogous genes wnt9a/b are required for the assembly of the nascent heart tube. In wnt9a/b double mutants, cardiomyocyte progenitor cells are delayed in their convergence towards the embryonic midline, the formation of the heart cone is impaired and the transformation into an elongated heart tube fails. The same cardiac phenotype occurs when both canonical and non-canonical Wnt signaling pathways are simultaneously blocked by pharmacological inhibition. This demonstrates that Wnt9a/b and canonical and non-canonical Wnt signaling regulate the migration of cardiomyocyte progenitor cells and control the formation of the cardiac tube. This can be partly attributed to their regulation of the timing of cardiac progenitor cell differentiation. Our study demonstrates how these morphogens activate a combination of downstream pathways to direct cardiac morphogenesis.

Understanding laterality disorders and the left-right organizer: Insights from zebrafish

Vital internal organs display a left-right (LR) asymmetric arrangement that is established during embryonic development. Disruption of this LR asymmetry-or laterality-can result in congenital organ malformations. Situs inversus totalis (SIT) is a complete concordant reversal of internal organs that results in a low occurrence of clinical consequences. Situs ambiguous, which gives rise to Heterotaxy syndrome (HTX), is characterized by discordant development and arrangement of organs that is associated with a wide range of birth defects. The leading cause of health problems in HTX patients is a congenital heart malformation. Mutations identified in patients with laterality disorders implicate motile cilia in establishing LR asymmetry. However, the cellular and molecular mechanisms underlying SIT and HTX are not fully understood. In several vertebrates, including mouse, frog and zebrafish, motile cilia located in a "left-right organizer" (LRO) trigger conserved signaling pathways that guide asymmetric organ development. Perturbation of LRO formation and/or function in animal models recapitulates organ malformations observed in SIT and HTX patients. This provides an opportunity to use these models to investigate the embryological origins of laterality disorders. The zebrafish embryo has emerged as an important model for investigating the earliest steps of LRO development. Here, we discuss clinical characteristics of human laterality disorders, and highlight experimental results from zebrafish that provide insights into LRO biology and advance our understanding of human laterality disorders.

Modeling Movement Disorders via Generation of hiPSC-Derived Motor Neurons

Generation of motor neurons (MNs) from human-induced pluripotent stem cells (hiPSCs) overcomes the limited access to human brain tissues and provides an unprecedent approach for modeling MN-related diseases. In this review, we discuss the recent progression in understanding the regulatory mechanisms of MN differentiation and their applications in the generation of MNs from hiPSCs, with a particular focus on two approaches: induction by small molecules and induction by lentiviral delivery of transcription factors. At each induction stage, different culture media and supplements, typical growth conditions and cellular morphology, and specific markers for validation of cell identity and quality control are specifically discussed. Both approaches can generate functional MNs. Currently, the major challenges in modeling neurological diseases using iPSC-derived neurons are: obtaining neurons with high purity and yield; long-term neuron culture to reach full maturation; and how to culture neurons more physiologically to maximize relevance to in vivo conditions.

Nodal signaling regulates asymmetric cellular behaviors, driving clockwise rotation of the heart tube in zebrafish

Clockwise rotation of the primitive heart tube, a process regulated by restricted left-sided Nodal signaling, is the first morphological manifestation of left-right asymmetry. How Nodal regulates cell behaviors to drive asymmetric morphogenesis remains poorly understood. Here, using high-resolution live imaging of zebrafish embryos, we simultaneously visualized cellular dynamics underlying early heart morphogenesis and resulting changes in tissue shape, to identify two key cell behaviors: cell rearrangement and cell shape change, which convert initially flat heart primordia into a tube through convergent extension. Interestingly, left cells were more active in these behaviors than right cells, driving more rapid convergence of the left primordium, and thereby rotating the heart tube. Loss of Nodal signaling abolished the asymmetric cell behaviors as well as the asymmetric convergence of the left and right heart primordia. Collectively, our results demonstrate that Nodal signaling regulates the magnitude of morphological changes by acting on basic cellular behaviors underlying heart tube formation, driving asymmetric deformation and rotation of the heart tube.

Single-cell transcriptomic analysis of vascular endothelial cells in zebrafish embryos

Vascular endothelial cells exhibit substantial phenotypic and transcriptional heterogeneity which is established during early embryogenesis. However, the molecular mechanisms involved in establishing endothelial cell diversity are still not well understood. Zebrafish has emerged as an advantageous model to study vascular development. Despite its importance, the single-cell transcriptomic profile of vascular endothelial cells during zebrafish development is still missing. To address this, we applied single-cell RNA-sequencing (scRNA-seq) of vascular endothelial cells isolated from zebrafish embryos at the 24 hpf stage. Six distinct clusters or subclusters related to vascular endothelial cells were identified which include arterial, two venous, cranial, endocardial and endothelial progenitor cell subtypes. Furthermore, we validated our findings by characterizing novel markers for arterial, venous, and endocardial cells. We experimentally confirmed the presence of two transcriptionally different venous cell subtypes, demonstrating heterogeneity among venous endothelial cells at this early developmental stage. This dataset will be a valuable resource for future functional characterization of vascular endothelial cells and interrogation of molecular mechanisms involved in the establishment of their heterogeneity and cell-fate decisions.

Endocardial identity is established during early somitogenesis by Bmp signalling acting upstream of npas4l and etv2

The endocardium plays important roles in the development and function of the vertebrate heart; however, few molecular markers of this tissue have been identified and little is known about what regulates its differentiation. Here, we describe the Gt(SAGFF27C); Tg(4xUAS:egfp) line as a marker of endocardial development in zebrafish. Transcriptomic comparison between endocardium and pan-endothelium confirms molecular distinction between these populations and time-course analysis suggests differentiation as early as eight somites. To investigate what regulates endocardial identity, we employed npas4l, etv2 and scl loss-of-function models. Endocardial expression is lost in npas4l mutants, significantly reduced in etv2 mutants and only modestly affected upon scl loss-of-function. Bmp signalling was also examined: overactivation of Bmp signalling increased endocardial expression, whereas Bmp inhibition decreased expression. Finally, epistasis experiments showed that overactivation of Bmp signalling was incapable of restoring endocardial expression in etv2 mutants. By contrast, overexpression of either npas4l or etv2 was sufficient to rescue endocardial expression upon Bmp inhibition. Together, these results describe the differentiation of the endocardium, distinct from vasculature, and place npas4l and etv2 downstream of Bmp signalling in regulating its differentiation.

Summary: A zebrafish transgenic reporter of the endocardium is identified, permitting transcriptomic analysis and identification of new endocardial markers. Epistasis experiments demonstrate npas4l and etv2 act downstream of Bmp signalling to regulate endocardial differentiation.

Sequential action of JNK genes establishes the embryonic left-right axis

The establishment of the left-right axis is crucial for the placement, morphogenesis and function of internal organs. Left-right specification is proposed to be dependent on cilia-driven fluid flow in the embryonic node. Planar cell polarity (PCP) signalling is crucial for patterning of nodal cilia, yet downstream effectors driving this process remain elusive. We have examined the role of the JNK gene family, a proposed downstream component of PCP signalling, in the development and function of the zebrafish node. We show jnk1 and jnk2 specify length of nodal cilia, generate flow in the node and restrict southpaw to the left lateral plate mesoderm. Moreover, loss of asymmetric southpaw expression does not result in disturbances to asymmetric organ placement, supporting a model in which nodal flow may be dispensable for organ laterality. Later, jnk3 is required to restrict pitx2c expression to the left side and permit correct endodermal organ placement. This work uncovers multiple roles for the JNK gene family acting at different points during left-right axis establishment. It highlights extensive redundancy and indicates JNK activity is distinct from the PCP signalling pathway.

Asymmetric Hapln1a drives regionalized cardiac ECM expansion and promotes heart morphogenesis in zebrafish development

AIMS

Vertebrate heart development requires the complex morphogenesis of a linear tube to form the mature organ, a process essential for correct cardiac form and function, requiring coordination of embryonic laterality, cardiac growth, and regionalized cellular changes. While previous studies have demonstrated broad requirements for extracellular matrix (ECM) components in cardiac morphogenesis, we hypothesized that ECM regionalization may fine tune cardiac shape during heart development.

METHODS AND RESULTS

Using live in vivo light sheet imaging of zebrafish embryos, we describe a left-sided expansion of the ECM between the myocardium and endocardium prior to the onset of heart looping and chamber ballooning. Analysis using an ECM sensor revealed the cardiac ECM is further regionalized along the atrioventricular axis. Spatial transcriptomic analysis of gene expression in the heart tube identified candidate genes that may drive ECM expansion. This approach identified regionalized expression of hapln1a, encoding an ECM cross-linking protein. Validation of transcriptomic data by in situ hybridization confirmed regionalized hapln1a expression in the heart, with highest levels of expression in the future atrium and on the left side of the tube, overlapping with the observed ECM expansion. Analysis of CRISPR-Cas9-generated hapln1a mutants revealed a reduction in atrial size and reduced chamber ballooning. Loss-of-function analysis demonstrated that ECM expansion is dependent upon Hapln1a, together supporting a role for Hapln1a in regionalized ECM modulation and cardiac morphogenesis. Analysis of hapln1a expression in zebrafish mutants with randomized or absent embryonic left-right asymmetry revealed that laterality cues position hapln1a-expressing cells asymmetrically in the left side of the heart tube.

CONCLUSION

We identify a regionalized ECM expansion in the heart tube which promotes correct heart development, and propose a novel model whereby embryonic laterality cues orient the axis of ECM asymmetry in the heart, suggesting these two pathways interact to promote robust cardiac morphogenesis.

Modeling Human Cardiac Arrhythmias: Insights from Zebrafish

Cardiac arrhythmia, or irregular heart rhythm, is associated with morbidity and mortality and is described as one of the most important future public health challenges. Therefore, developing new models of cardiac arrhythmia is critical for understanding disease mechanisms, determining genetic underpinnings, and developing new therapeutic strategies. In the last few decades, the zebrafish has emerged as an attractive model to reproduce in vivo human cardiac pathologies, including arrhythmias. Here, we highlight the contribution of zebrafish to the field and discuss the available cardiac arrhythmia models. Further, we outline techniques to assess potential heart rhythm defects in larval and adult zebrafish. As genetic tools in zebrafish continue to bloom, this model will be crucial for functional genomics studies and to develop personalized anti-arrhythmic therapies.

Live imaging of adult zebrafish cardiomyocyte proliferation ex vivo

Zebrafish are excellent at regenerating their heart by reinitiating proliferation in pre-existing cardiomyocytes. Studying how zebrafish achieve this holds great potential in developing new strategies to boost mammalian heart regeneration. Nevertheless, the lack of appropriate live-imaging tools for the adult zebrafish heart has limited detailed studies into the dynamics underlying cardiomyocyte proliferation. Here, we address this by developing a system in which cardiac slices of the injured zebrafish heart are cultured ex vivo for several days while retaining key regenerative characteristics, including cardiomyocyte proliferation. In addition, we show that the cardiac slice culture system is compatible with live timelapse imaging and allows manipulation of regenerating cardiomyocytes with drugs that normally would have toxic effects that prevent their use. Finally, we use the cardiac slices to demonstrate that adult cardiomyocytes with fully assembled sarcomeres can partially disassemble their sarcomeres in a calpain- and proteasome-dependent manner to progress through nuclear division and cytokinesis. In conclusion, we have developed a cardiac slice culture system, which allows imaging of native cardiomyocyte dynamics in real time to discover cellular mechanisms during heart regeneration.

Twisting of the zebrafish heart tube during cardiac looping is a tbx5-dependent and tissue-intrinsic process

Organ laterality refers to the left-right asymmetry in disposition and conformation of internal organs and is established during embryogenesis. The heart is the first organ to display visible left-right asymmetries through its left-sided positioning and rightward looping. Here, we present a new zebrafish loss-of-function allele for tbx5a, which displays defective rightward cardiac looping morphogenesis. By mapping individual cardiomyocyte behavior during cardiac looping, we establish that ventricular and atrial cardiomyocytes rearrange in distinct directions. As a consequence, the cardiac chambers twist around the atrioventricular canal resulting in torsion of the heart tube, which is compromised in tbx5a mutants. Pharmacological treatment and ex vivo culture establishes that the cardiac twisting depends on intrinsic mechanisms and is independent from cardiac growth. Furthermore, genetic experiments indicate that looping requires proper tissue patterning. We conclude that cardiac looping involves twisting of the chambers around the atrioventricular canal, which requires correct tissue patterning by Tbx5a.

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.

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.

Anterior lateral plate mesoderm gives rise to multiple tissues and requires tbx5a function in left-right asymmetry, migration dynamics, and cell specification of late-addition cardiac cells

In this study, we elucidate a single cell resolution fate map in the zebrafish in a sub-section of the anterior Lateral Plate Mesoderm (aLPM) at 18 hpf. Our results show that this tissue is not organized into segregated regions but gives rise to intermingled pericardial sac, peritoneum, pharyngeal arch and cardiac precursors. We further report upon asymmetrical contributions of lateral aLPM-derived heart precursors-specifically that twice as many heart precursors arise from the right side versus the left side of the embryo. Cell tracking analyses and large-scale cell labeling of the lateral aLPM corroborate these differences and show that the observed asymmetries are dependent upon Tbx5a expression. Previously, it was shown that cardiac looping was affected in Tbx5a knock-down and knock-out zebrafish (Garrity et al., 2002; Parrie et al., 2013); our present data also implicate tbx5a function in cell specification, establishment and maintenance of cardiac left-right asymmetry.

Cardiac Morphogenesis: Specification of the Four-Chambered Heart

Early heart morphogenesis involves a process in which embryonic precursor cells are instructed to form a cyclic contracting muscle tube connected to blood vessels, pumping fluid. Subsequently, the heart becomes structurally complex and its size increases several orders of magnitude to functionally keep up with the demands of the growing organism. Programmed transcriptional regulatory networks control the early steps of cardiac development. However, already during the early stages of its assembly, the heart tube starts to produce electrochemical potentials, contractions, and flow, which are transduced into signals that feed back into the process of morphogenesis itself. Heart morphogenesis, thus, involves the interplay between progressively changing genetic networks, function, and shape. Morphogenesis is evolutionarily conserved, but species-specific differences occur and in mouse, for instance, distinct phases of development become overlapping and compounded in an extremely fast gestation. Here, we review the early morphogenesis of the chambered heart that maintains a circulation supporting development of an organism rapidly growing in size and requirements.

Integrin α5 and Integrin α4 cooperate to promote endocardial differentiation and heart morphogenesis

Endocardium is critically important for proper function of the cardiovascular system. Not only does endocardium connect the heart to blood vasculature, it also plays an important role in heart morphogenesis, valve formation, and ventricular trabeculation. The extracellular protein Fibronectin (Fn1) promotes endocardial differentiation, but the signaling pathways downstream of Fn1 that regulate endocardial development are not understood. Here, we analyzed the role of the Fibronectin receptors Integrin alpha5 (Itga5) and Integrin alpha4 (Itga4) in zebrafish heart development. We show that itga5 mRNA is expressed in both endocardium and myocardium during early stages of heart development. Through analysis of both itga5 single mutants and itga4;itga5 double mutants, we show that loss of both itga5 and itga4 results in enhanced defects in endocardial differentiation and morphogenesis compared to loss of itga5 alone. Loss of both itga5 and itga4 results in cardia bifida and severe myocardial morphology defects. Finally, we find that loss of itga5 and itga4 results in abnormally narrow anterior endodermal sheet morphology. Together, our results support a model in which Itga5 and Itga4 cooperate to promote endocardial differentiation, medial migration of endocardial and myocardial cells, and morphogenesis of anterior endoderm.

Swimming toward solutions: Using fish and frogs as models for understanding RASopathies

The RAS signaling pathway regulates cell growth, survival, and differentiation, and its inappropriate activation is associated with disease in humans. The RASopathies, a set of developmental syndromes, arise when the pathway is overactive during development. Patients share a core set of symptoms, including congenital heart disease, craniofacial anomalies, and neurocognitive delay. Due to the conserved nature of the pathway, animal models are highly informative for understanding disease etiology, and zebrafish and Xenopus are emerging as advantageous model systems. Here we discuss these aquatic models of RASopathies, which recapitulate many of the core symptoms observed in patients. Craniofacial structures become dysmorphic upon expression of disease-associated mutations, resulting in wider heads. Heart defects manifest as delays in cardiac development and changes in heart size, and behavioral deficits are beginning to be explored. Furthermore, early convergence and extension defects cause elongation of developing embryos: this phenotype can be quantitatively assayed as a readout of mutation strength, raising interesting questions regarding the relationship between pathway activation and disease. Additionally, the observation that RAS signaling may be simultaneously hyperactive and attenuated suggests that downregulation of signaling may also contribute to etiology. We propose that models should be characterized using a standardized approach to allow easier comparison between models, and a better understanding of the interplay between mutation and disease presentation.

Molecular and cellular basis of left-right asymmetry in vertebrates

Although the human body appears superficially symmetrical with regard to the left-right (L-R) axis, most visceral organs are asymmetric in terms of their size, shape, or position. Such morphological asymmetries of visceral organs, which are essential for their proper function, are under the control of a genetic pathway that operates in the developing embryo. In many vertebrates including mammals, the breaking of L-R symmetry occurs at a structure known as the L-R organizer (LRO) located at the midline of the developing embryo. This symmetry breaking is followed by transfer of an active form of the signaling molecule Nodal from the LRO to the lateral plate mesoderm (LPM) on the left side, which results in asymmetric expression of Nodal (a left-side determinant) in the left LPM. Finally, L-R asymmetric morphogenesis of visceral organs is induced by Nodal-Pitx2 signaling. This review will describe our current understanding of the mechanisms that underlie the generation of L-R asymmetry in vertebrates, with a focus on mice.

Left-right asymmetric heart jogging increases the robustness of dextral heart looping in zebrafish

Building a left-right (L-R) asymmetric organ requires asymmetric information. This comes from various sources, including asymmetries in embryo-scale genetic cascades (including the left-sided Nodal cascade), organ-intrinsic mechanical forces, and cell-level chirality, but the relative influence of these sources and how they collaborate to drive asymmetric morphogenesis is not understood. During zebrafish heart development, the linear heart tube extends to the left of the midline in a process known as jogging. The jogged heart then undergoes dextral (i.e. rightward) looping to correctly position the heart chambers relative to one another. Left lateralized jogging is governed by the left-sided expression of Nodal in mesoderm tissue, while looping laterality is mainly controlled by heart-intrinsic cell-level asymmetries in the actomyosin cytoskeleton. The purpose of lateralized jogging is not known. Moreover, after jogging, the heart tube returns to an almost midline position and so it is not clear whether or how jogging may impact the dextral loop. Here, we characterize a novel loss-of-function mutant in the zebrafish Nodal homolog southpaw (spaw) that appears to be a true null. We then assess the relationship between jogging and looping laterality in embryos lacking asymmetric Spaw signals. We found that the probability of a dextral loop occurring, does not depend on asymmetric Spaw signals per se, but does depend on the laterality of jogging. Thus, we conclude that the role of leftward jogging is to spatially position the heart tube in a manner that promotes robust dextral looping. When jogging laterality is abnormal, the robustness of dextral looping decreases. This establishes a cooperation between embryo-scale Nodal-dependent L-R asymmetries and organ-intrinsic cellular chirality in the control of asymmetric heart morphogenesis and shows that the transient laterality of the early heart tube has consequences for later heart morphogenetic events.

Morphogenetic control of zebrafish cardiac looping by Bmp signaling

Cardiac looping is an essential and highly conserved morphogenetic process that places the different regions of the developing vertebrate heart tube into proximity of their final topographical positions. High-resolution 4D live imaging of mosaically labelled cardiomyocytes reveals distinct cardiomyocyte behaviors that contribute to the deformation of the entire heart tube. Cardiomyocytes acquire a conical cell shape, which is most pronounced at the superior wall of the atrioventricular canal and contributes to S-shaped bending. Torsional deformation close to the outflow tract contributes to a torque-like winding of the entire heart tube between its two poles. Anisotropic growth of cardiomyocytes based on their positions reinforces S-shaping of the heart. During cardiac looping, bone morphogenetic protein pathway signaling is strongest at the future superior wall of the atrioventricular canal. Upon pharmacological or genetic inhibition of bone morphogenetic protein signaling, myocardial cells at the superior wall of the atrioventricular canal maintain cuboidal cell shapes and S-shaped bending is impaired. This description of cellular rearrangements and cardiac looping regulation may also be relevant for understanding the etiology of human congenital heart defects.

Versican is crucial for the initiation of cardiovascular lumen development in medaka (Oryzias latipes)

Versican is an evolutionary conserved extracellular matrix proteoglycan, and versican expression loss in mice results in embryonic lethality owing to cardiovascular defects. However, the in utero development of mammals limits our understanding of the precise role of versican during cardiovascular development. Therefore, the use of evolutionarily distant species that develop ex utero is more suitable for studying the mechanistic basis of versican activity. We performed ENU mutagenesis screening to identify medaka mutants with defects in embryonic cardiovascular development. In this study, we described a recessive point mutation in the versican 3'UTR resulting in reduced versican protein expression. The fully penetrant homozygous mutant showed termination of cardiac development at the linear heart tube stage and exhibited absence of cardiac looping, a constricted outflow tract, and no cardiac jelly. Additionally, progenitor cells did not migrate from the secondary source towards the arterial pole of the linear heart tube, resulting in a constricted outflow tract. Furthermore, mutants lacked blood flow and vascular lumen despite continuous peristaltic heartbeats. These results enhance our understanding of the mechanistic basis of versican in cardiac development, and this mutant represents a novel genetic model to investigate the mechanisms of vascular tubulogenesis.

Myosin Vb is required for correct trafficking of N-cadherin and cardiac chamber ballooning

BACKGROUND

During heart morphogenesis, the cardiac chambers undergo ballooning: a process involving regionalized elongation of cardiomyocytes. Cardiomyocyte shape changes require reorganization of the actin cytoskeleton; however, the genetic regulation of this process is not well understood.

RESULTS

From a forward genetic screen, we identified the zebrafish uq ^23ks mutant which manifests chamber ballooning defects. Whole-genome sequencing-mapping identified a truncating mutation in the gene, myo5b. myo5b encodes an atypical myosin required for endosome recycling and, consistent with this, increased vesicles were observed in myo5b mutant cardiomyocytes. Expression of RFP-Rab11a (a recycling endosome marker) confirmed increased recycling endosomes in cardiomyocytes of myo5b mutants. To investigate potential cargo of MyoVb-associated vesicles, we examined the adherens junction protein, N-cadherin. N-cadherin appeared mispatterned at cell junctions, and an increase in the number of intracellular particles was also apparent. Co-localization with RFP-Rab11a confirmed increased N-cadherin-positive recycling endosomes, demonstrating N-cadherin trafficking is perturbed in myo5b mutants. Finally, phalloidin staining showed disorganized F-actin in myo5b cardiomyocytes, suggesting the cytoskeleton fails to remodel, obstructing chamber ballooning.

CONCLUSIONS

MyoVb is required for cardiomyocyte endosomal recycling and appropriate N-cadherin localization during the onset of chamber ballooning. Cardiomyocytes lacking MyoVb are unable to reorganize their actin cytoskeleton, resulting in failed chamber ballooning. Developmental Dynamics 248:284-295, 2019. © 2019 Wiley Periodicals, Inc.

Screening drugs for myocardial disease in vivo with zebrafish: an expert update

INTRODUCTION

Our understanding of the complexity of cardiovascular disease pathophysiology remains very incomplete and has hampered cardiovascular drug development over recent decades. The prevalence of cardiovascular diseases and their increasing global burden call for novel strategies to address disease biology and drug discovery. Areas covered: This review describes the recent history of cardiovascular drug discovery using in vivo phenotype-based screening in zebrafish. The rationale for the use of this model is highlighted and the initial efforts in the fields of disease modeling and high-throughput screening are illustrated. Finally, the advantages and limitations of in vivo zebrafish screening are discussed, highlighting newer approaches, such as genome editing technologies, to accelerate our understanding of disease biology and the development of precise disease models. Expert opinion: Full understanding and faithful modeling of specific cardiovascular disease is a rate-limiting step for cardiovascular drug discovery. The resurgence of in vivo phenotype screening together with the advancement of systems biology approaches allows for the identification of lead compounds which show efficacy on integrative disease biology in the absence of validated targets. This strategy bypasses current gaps in knowledge of disease biology and paves the way for successful drug discovery and downstream molecular target identification.

Intrinsic cellular chirality regulates left-right symmetry breaking during cardiac looping

The vertebrate body plan is overall symmetrical but left-right (LR) asymmetric in the shape and positioning of internal organs. Although several theories have been proposed, the biophysical mechanisms underlying LR asymmetry are still unclear, especially the role of cell chirality, the LR asymmetry at the cellular level, on organ asymmetry. Here with developing chicken embryos, we examine whether intrinsic cell chirality or handedness regulates cardiac C looping. Using a recently established biomaterial-based 3D culture platform, we demonstrate that chick cardiac cells before and during C looping are intrinsically chiral and exhibit dominant clockwise rotation in vitro. We further show that cells in the developing myocardium are chiral as evident by a rightward bias of cell alignment and a rightward polarization of the Golgi complex, correlating with the direction of cardiac tube rotation. In addition, there is an LR polarized distribution of N-cadherin and myosin II in the myocardium before the onset of cardiac looping. More interestingly, the reversal of cell chirality via activation of the protein kinase C signaling pathway reverses the directionality of cardiac looping, accompanied by a reversal in cellular biases on the cardiac tube. Our results suggest that myocardial cell chirality regulates cellular LR symmetry breaking in the heart tube and the resultant directionality of cardiac looping. Our study provides evidence of an intrinsic cellular chiral bias leading to LR symmetry breaking during directional tissue rotation in vertebrate development.

Lineage-Specific Chiral Biases of Human Embryonic Stem Cells during Differentiation

Left-right symmetry breaking is a complex developmental process and an important part of embryonic axis development. As of yet, the biophysical mechanism behind LR asymmetry establishment remains elusive for the overall asymmetry of embryos as well as for the organ-specific asymmetry. Here, we demonstrate that inherent cellular chirality is observable in the cells of early embryonic stages using a 3D Matrigel bilayer system. Differentiation of human embryonic stem cells to three lineages corresponding to heart, intestine, and neural tissues demonstrates phenotype-specific inherent chiral biases, complementing the current knowledge regarding organ development. The existence of inherent cellular chirality early in development and its correlation with organ asymmetry implicate cell chirality as a possible regulator in LR symmetry breaking.

Left-right asymmetry in heart development and disease: forming the right loop

Extensive studies have shown how bilateral symmetry of the vertebrate embryo is broken during early development, resulting in a molecular left-right bias in the mesoderm. However, how this early asymmetry drives the asymmetric morphogenesis of visceral organs remains poorly understood. The heart provides a striking model of left-right asymmetric morphogenesis, undergoing rightward looping to shape an initially linear heart tube and align cardiac chambers. Importantly, abnormal left-right patterning is associated with severe congenital heart defects, as exemplified in heterotaxy syndrome. Here, we compare the mechanisms underlying the rightward looping of the heart tube in fish, chick and mouse embryos. We propose that heart looping is not only a question of direction, but also one of fine-tuning shape. This is discussed in the context of evolutionary and clinical perspectives.

Nppa and Nppb act redundantly during zebrafish cardiac development to confine AVC marker expression and reduce cardiac jelly volume

Atrial natriuretic peptide (nppa/anf) and brain natriuretic peptide (nppb/bnp) form a gene cluster with expression in the chambers of the developing heart. Despite restricted expression, a function in cardiac development has not been demonstrated by mutant analysis. This is attributed to functional redundancy; however, their genomic location in cis has impeded formal analysis. Using genome editing, we have generated mutants for nppa and nppb, and found that single mutants were indistinguishable from wild type, whereas nppa/nppb double mutants displayed heart morphogenesis defects and pericardial oedema. Analysis of atrioventricular canal (AVC) markers show expansion of bmp4, tbx2b, has2 and versican expression into the atrium of double mutants. This expanded expression correlates with increased extracellular matrix in the atrium. Using a biosensor for hyaluronic acid to measure the cardiac jelly (cardiac extracellular matrix), we confirmed cardiac jelly expansion in nppa/nppb double mutants. Finally, bmp4 knockdown rescued the expansion of has2 expression and cardiac jelly in double mutants. This definitively shows that nppa and nppb function redundantly during cardiac development to restrict gene expression to the AVC, preventing excessive cardiac jelly synthesis in the atrial chamber.

Planar cell polarity signalling coordinates heart tube remodelling through tissue-scale polarisation of actomyosin activity

Development of a multiple-chambered heart from the linear heart tube is inherently linked to cardiac looping. Although many molecular factors regulating the process of cardiac chamber ballooning have been identified, the cellular mechanisms underlying the chamber formation remain unclear. Here, we demonstrate that cardiac chambers remodel by cell neighbour exchange of cardiomyocytes guided by the planar cell polarity (PCP) pathway triggered by two non-canonical Wnt ligands, Wnt5b and Wnt11. We find that PCP signalling coordinates the localisation of actomyosin activity, and thus the efficiency of cell neighbour exchange. On a tissue-scale, PCP signalling planar-polarises tissue tension by restricting the actomyosin contractility to the apical membranes of outflow tract cells. The tissue-scale polarisation of actomyosin contractility is required for cardiac looping that occurs concurrently with chamber ballooning. Taken together, our data reveal that instructive PCP signals couple cardiac chamber expansion with cardiac looping through the organ-scale polarisation of actomyosin-based tissue tension.

The molecular mechanisms underlying cardiac chamber formation are not well understood. Here, the authors show that planar cell polarity signalling through Wnt5b and Wnt11 coordinates localised and tissue-scale polarised actomyosin contractility in the zebrafish heart, regulating cardiac chamber formation and looping.

TGF-β Family Signaling in Early Vertebrate Development

TGF-β family ligands function in inducing and patterning many tissues of the early vertebrate embryonic body plan. Nodal signaling is essential for the specification of mesendodermal tissues and the concurrent cellular movements of gastrulation. Bone morphogenetic protein (BMP) signaling patterns tissues along the dorsal-ventral axis and simultaneously directs the cell movements of convergence and extension. After gastrulation, a second wave of Nodal signaling breaks the symmetry between the left and right sides of the embryo. During these processes, elaborate regulatory feedback between TGF-β ligands and their antagonists direct the proper specification and patterning of embryonic tissues. In this review, we summarize the current knowledge of the function and regulation of TGF-β family signaling in these processes. Although we cover principles that are involved in the development of all vertebrate embryos, we focus specifically on three popular model organisms: the mouse Mus musculus, the African clawed frog of the genus Xenopus, and the zebrafish Danio rerio, highlighting the similarities and differences between these species.

Distinct myocardial lineages break atrial symmetry during cardiogenesis in zebrafish

The ultimate formation of a four-chambered heart allowing the separation of the pulmonary and systemic circuits was key for the evolutionary success of tetrapods. Complex processes of cell diversification and tissue morphogenesis allow the left and right cardiac compartments to become distinct but remain poorly understood. Here, we describe an unexpected laterality in the single zebrafish atrium analogous to that of the two atria in amniotes, including mammals. This laterality appears to derive from an embryonic antero-posterior asymmetry revealed by the expression of the transcription factor gene meis2b. In adult zebrafish hearts, meis2b expression is restricted to the left side of the atrium where it controls the expression of pitx2c, a regulator of left atrial identity in mammals. Altogether, our studies suggest that the multi-chambered atrium in amniotes arose from a molecular blueprint present before the evolutionary emergence of cardiac septation and provide insights into the establishment of atrial asymmetry.

Uncovering the regeneration strategies of zebrafish organs: a comprehensive systems biology study on heart, cerebellum, fin, and retina regeneration

Background

Regeneration is an important biological process for the restoration of organ mass, structure, and function after damage, and involves complex bio-physiological mechanisms including cell differentiation and immune responses. We constructed four regenerative protein-protein interaction (PPI) networks using dynamic models and AIC (Akaike’s Information Criterion), based on time-course microarray data from the regeneration of four zebrafish organs: heart, cerebellum, fin, and retina. We extracted core and organ-specific proteins, and proposed a recalled-blastema-like formation model to uncover regeneration strategies in zebrafish.

Results

It was observed that the core proteins were involved in TGF-β signaling for each step in the recalled-blastema-like formation model and TGF-β signaling may be vital for regeneration. Integrins, FGF, and PDGF accelerate hemostasis during heart injury, while Bdnf shields retinal neurons from secondary damage and augments survival during the injury response. Wnt signaling mediates the growth and differentiation of cerebellum and fin neural stem cells, potentially providing a signal to trigger differentiation.

Conclusion

Through our analysis of all four zebrafish regenerative PPI networks, we provide insights that uncover the underlying strategies of zebrafish organ regeneration.

Nodal signalling and asymmetry of the nervous system

The role of Nodal signalling in nervous system asymmetry is still poorly understood. Here, we review and discuss how asymmetric Nodal signalling controls the ontogeny of nervous system asymmetry using a comparative developmental perspective. A detailed analysis of asymmetry in ascidians and fishes reveals a critical context-dependency of Nodal function and emphasizes that bilaterally paired and midline-unpaired structures/organs behave as different entities. We propose a conceptual framework to dissect the developmental function of Nodal as asymmetry inducer and laterality modulator in the nervous system, which can be used to study other types of body and visceral organ asymmetries. Using insights from developmental biology, we also present novel evolutionary hypotheses on how Nodal led the evolution of directional asymmetry in the brain, with a particular focus on the epithalamus. We intend this paper to provide a synthesis on how Nodal signalling controls left-right asymmetry of the nervous system.This article is part of the themed issue 'Provocative questions in left-right asymmetry'.

Twists and turns

Computational modelling of the heart tube during development reveals the interplay between tissue asymmetry and growth that helps our hearts take shape.

A predictive model of asymmetric morphogenesis from 3D reconstructions of mouse heart looping dynamics

How left-right patterning drives asymmetric morphogenesis is unclear. Here, we have quantified shape changes during mouse heart looping, from 3D reconstructions by HREM. In combination with cell labelling and computer simulations, we propose a novel model of heart looping. Buckling, when the cardiac tube grows between fixed poles, is modulated by the progressive breakdown of the dorsal mesocardium. We have identified sequential left-right asymmetries at the poles, which bias the buckling in opposite directions, thus leading to a helical shape. Our predictive model is useful to explore the parameter space generating shape variations. The role of the dorsal mesocardium was validated in Shh^-/- mutants, which recapitulate heart shape changes expected from a persistent dorsal mesocardium. Our computer and quantitative tools provide novel insight into the mechanism of heart looping and the contribution of different factors, beyond the simple description of looping direction. This is relevant to congenital heart defects.

Gdf3 is required for robust Nodal signaling during germ layer formation and left-right patterning

Vertebrate embryonic patterning depends on signaling from Nodal, a TGFβ superfamily member. There are three Nodal orthologs in zebrafish; southpaw directs left-right asymmetries, while squint and cyclops function earlier to pattern mesendoderm. TGFβ member Vg1 is implicated in mesoderm formation but the role of the zebrafish ortholog, Growth differentiation factor 3 (Gdf3), has not been fully explored. We show that zygotic expression of gdf3 is dispensable for embryonic development, while maternally deposited gdf3 is required for mesendoderm formation and dorsal-ventral patterning. We further show that Gdf3 can affect left-right patterning at multiple stages, including proper development of regional cell morphology in Kupffer’s vesicle and the establishment of southpaw expression in the lateral plate mesoderm. Collectively, our data indicate that gdf3 is critical for robust Nodal signaling at multiple stages in zebrafish embryonic development.

hace1 Influences zebrafish cardiac development via ROS-dependent mechanisms

BACKGROUND

In this study, we reveal a previously undescribed role of the HACE1 (HECT domain and Ankyrin repeat Containing E3 ubiquitin-protein ligase 1) tumor suppressor protein in normal vertebrate heart development using the zebrafish (Danio rerio) model. We examined the link between the cardiac phenotypes associated with hace1 loss of function to the expression of the Rho small family GTPase, rac1, which is a known target of HACE1 and promotes ROS production via its interaction with NADPH oxidase holoenzymes.

RESULTS

We demonstrate that loss of hace1 in zebrafish via morpholino knockdown results in cardiac deformities, specifically a looping defect, where the heart is either tubular or "inverted". Whole-mount in situ hybridization of cardiac markers shows distinct abnormalities in ventricular morphology and atrioventricular valve formation in the hearts of these morphants, as well as increased expression of rac1. Importantly, this phenotype appears to be directly related to Nox enzyme-dependent ROS production, as both genetic inhibition by nox1 and nox2 morpholinos or pharmacologic rescue using ROS scavenging agents restores normal cardiac structure.

CONCLUSIONS

Our study demonstrates that HACE1 is critical in the normal development and proper function of the vertebrate heart via a ROS-dependent mechanism. Developmental Dynamics 247:289-303, 2018. © 2017 Wiley Periodicals, Inc.

Establishment of lateral organ asymmetries in the invertebrate chordate, Ciona intestinalis

BACKGROUND

The evolutionary emergence and diversification of the chordates appear to involve dramatic changes in organ morphogenesis along the left/right axis. However, the ancestral chordate mechanism for establishing lateral asymmetry remains ambiguous. Additionally, links between the initial establishment of lateral asymmetry and subsequent asymmetries in organ morphogenesis are poorly characterized.

RESULTS

To explore asymmetric organ morphogenesis during chordate evolution, we have begun to characterize left/right patterning of the heart and endodermal organs in an invertebrate chordate, Ciona intestinalis. Here, we show that Ciona has a laterally asymmetric, right-sided heart. Our data indicate that cardiac lateral asymmetry requires H+/K+ ion flux, but is independent of Nodal signaling. Our pharmacological inhibitor studies show that ion flux is required for polarization of epidermal cilia and neurula rotation and suggest that ion flux functions synergistically with chorion contact to drive cardiac laterality. Live imaging analysis revealed that larval heart progenitor cells undergo a lateral shift without displaying any migratory behaviors. Furthermore, we find that this passive shift corresponds with the emergence of lateral asymmetry in the endoderm, which is also ion flux dependent.

CONCLUSIONS

Our data suggest that ion flux promotes laterally asymmetric morphogenesis of the larval endoderm rudiment leading to a passive, Nodal-independent shift in the position of associated heart progenitor cells. These findings help to refine hypotheses regarding ancestral chordate left/right patterning mechanisms and how they have diverged within invertebrate and vertebrate chordate lineages.

Left-Right Patterning: Breaking Symmetry to Asymmetric Morphogenesis

Vertebrates exhibit striking left-right (L-R) asymmetries in the structure and position of the internal organs. Symmetry is broken by motile cilia-generated asymmetric fluid flow, resulting in a signaling cascade - the Nodal-Pitx2 pathway - being robustly established within mesodermal tissue on the left side only. This pathway impinges upon various organ primordia to instruct their side-specific development. Recently, progress has been made in understanding both the breaking of embryonic L-R symmetry and how the Nodal-Pitx2 pathway controls lateralized cell differentiation, migration, and other aspects of cell behavior, as well as tissue-level mechanisms, that drive asymmetries in organ formation. Proper execution of asymmetric organogenesis is critical to health, making furthering our understanding of L-R development an important concern.

Tmem2 Regulates Embryonic Vegf Signaling by Controlling Hyaluronic Acid Turnover

Angiogenesis is responsible for tissue vascularization during development, as well as in pathological contexts, including cancer and ischemia. Vascular endothelial growth factors (VEGFs) regulate angiogenesis by acting through VEGF receptors to induce endothelial cell signaling. VEGF is processed in the extracellular matrix (ECM), but the complexity of ECM control of VEGF signaling and angiogenesis remains far from understood. In a forward genetic screen, we identified angiogenesis defects in tmem2 zebrafish mutants that lack both arterial and venous Vegf/Vegfr/Erk signaling. Strikingly, tmem2 mutants display increased hyaluronic acid (HA) surrounding developing vessels. Angiogenesis in tmem2 mutants was rescued, or restored after failed sprouting, by degrading this increased HA. Furthermore, oligomerized HA or overexpression of Vegfc rescued angiogenesis in tmem2 mutants. Based on these data, and the known structure of Tmem2, we find that Tmem2 regulates HA turnover to promote normal Vegf signaling during developmental angiogenesis.

The ADAMTS hyalectanase family: biological insights from diverse species

The a disintegrin-like and metalloproteinase with thrombospondin type-1 motifs (ADAMTS) family of metzincins are complex secreted proteins that have diverse functions during development. The hyalectanases (ADAMTS1, 4, 5, 8, 9, 15 and 20) are a subset of this family that have enzymatic activity against hyalectan proteoglycans, the processing of which has important implications during development. This review explores the evolution, expression and developmental functions of the ADAMTS family, focusing on the ADAMTS hyalectanases and their substrates in diverse species. This review gives an overview of how the family and their substrates evolved from non-vertebrates to mammals, the expression of the hyalectanases and substrates in different species and their functions during development, and how these functions are conserved across species.

Current Perspectives in Cardiac Laterality

The heart is the first organ to break symmetry in the developing embryo and onset of dextral looping is the first indication of this event. Looping is a complex process that progresses concomitantly to cardiac chamber differentiation and ultimately leads to the alignment of the cardiac regions in their final topology. Generation of cardiac asymmetry is crucial to ensuring proper form and consequent functionality of the heart, and therefore it is a highly regulated process. It has long been known that molecular left/right signals originate far before morphological asymmetry and therefore can direct it. The use of several animal models has led to the characterization of a complex regulatory network, which invariably converges on the Tgf-β signaling molecule Nodal and its downstream target, the homeobox transcription factor Pitx2. Here, we review current data on the cellular and molecular bases of cardiac looping and laterality, and discuss the contribution of Nodal and Pitx2 to these processes. A special emphasis will be given to the morphogenetic role of Pitx2 and to its modulation of transcriptional and functional properties, which have also linked laterality to atrial fibrillation.