A Matter of the Heart: The African Clawed Frog Xenopus as a Model for Studying Vertebrate Cardiogenesis and Congenital Heart Defects

Fbrsl1 is required for heart development in Xenopus laevis and de novo variants in FBRSL1 can cause human heart defects

De novo truncating variants in fibrosin-like 1 (FBRSL1), a member of the AUTS2 gene family, cause a disability syndrome, including organ malformations such as heart defects. Here, we use Xenopus laevis to investigate whether Fbrsl1 plays a role in heart development. Xenopus laevis fbrsl1 is expressed in tissues relevant for heart development, and morpholino-mediated knockdown of Fbrsl1 results in severely hypoplastic hearts. Our data suggest that Fbrsl1 is required for the development of the first heart field, which contributes to the ventricle and the atria, but not for the second heart field, which gives rise to the outflow tract. The morphant heart phenotype could be rescued using a human N-terminal FBRSL1 isoform that contains an alternative exon, but lacks the AUTS2 domain. N-terminal isoforms carrying patient variants failed to rescue. Interestingly, a long human FBRSL1 isoform, harboring the AUTS2 domain, also did not rescue the morphant heart defects. Thus, our data suggest that different FBRSL1 isoforms may have distinct functions and that only the short N-terminal isoform, appears to be critical for heart development.

Rare variants in CAPN2 increase risk for isolated hypoplastic left heart syndrome

Hypoplastic left heart syndrome (HLHS) is a severe congenital heart defect (CHD) characterized by hypoplasia of the left ventricle and aorta along with stenosis or atresia of the aortic and mitral valves. HLHS represents only ∼4%-8% of all CHDs but accounts for ∼25% of deaths. HLHS is an isolated defect (i.e., iHLHS) in 70% of families, the vast majority of which are simplex. Despite intense investigation, the genetic basis of iHLHS remains largely unknown. We performed exome sequencing on 331 families with iHLHS aggregated from four independent cohorts. A Mendelian-model-based analysis demonstrated that iHLHS was not due to single, large-effect alleles in genes previously reported to underlie iHLHS or CHD in >90% of families in this cohort. Gene-based association testing identified increased risk for iHLHS associated with variation in CAPN2 (p = 1.8 × 10-5), encoding a protein involved in functional adhesion. Functional validation studies in a vertebrate animal model (Xenopus laevis) confirmed CAPN2 is essential for cardiac ventricle morphogenesis and that in vivo loss of calpain function causes hypoplastic ventricle phenotypes and suggest that human CAPN2707C>T and CAPN21112C>T variants, each found in multiple individuals with iHLHS, are hypomorphic alleles. Collectively, our findings show that iHLHS is typically not a Mendelian condition, demonstrate that CAPN2 variants increase risk of iHLHS, and identify a novel pathway involved in HLHS pathogenesis.

Use of Frogs as a Model to Study the Etiology of HLHS

A frog is a classical model organism used to uncover processes and regulations of early vertebrate development, including heart development. Recently, we showed that a frog also represents a useful model to study a rare human congenital heart disease, hypoplastic left heart syndrome. In this review, we first summarized the cellular events and molecular regulations of vertebrate heart development, and the benefit of using a frog model to study congenital heart diseases. Next, we described the challenges in elucidating the etiology of hypoplastic left heart syndrome and discussed how a frog model may contribute to our understanding of the molecular and cellular bases of the disease. We concluded that a frog model offers its unique advantage in uncovering the cellular mechanisms of hypoplastic left heart syndrome; however, combining multiple model organisms, including frogs, is needed to gain a comprehensive understanding of the disease.

Anuran heart development and critical developmental periods: a comparative analysis of three Neotropical anuran species

The heart begins to form early during vertebrate development and is the first functional organ of the embryo. This study aimed to describe and compare the heart development in three Neotropical anuran species, Physalaemus albonotatus, Elachistocleis bicolor, and Scinax nasicus. Different Gosner Stages (GS) of embryos (GS 18-20) and premetamorphic (GS 21-25), prometamorphic (GS 26-41) and metamorphic (GS 42-46) tadpoles were analyzed using stereoscopic microscopy and Scanning Electronic Microscopy. Heart development was similar in the three analyzed species; however, some heterochronic events were identified between P. albonotatus and S. nasicus compared to E. bicolor. In addition, different patterns of melanophores arrangement were observed. During the embryonic and metamorphic periods, the main morphogenetic events occur: formation of the heart tube, regionalization of the heart compartments, development of spiral valve, onset of heartbeat, looping, and final displacement of the atrium and its complete septation. Both periods are critical for the normal morphogenesis and the correct functioning of the anuran heart. These results are useful to characterize the normal anuran heart morphology and to identify possible abnormalities caused by exposure to environmental contaminants.

Modified Xenopus laevis approach (R-FETAX) as an alternative test for the evaluation of foetal valproate spectrum disorder

In compliance to animal welfare 3Rs principle there is a great demand for refined tests alternative to classical mammal teratogenicity tests. We propose a refined alternative amphibian method (R-FETAX) to evaluate chemical induced embryotoxicity. The human foetal valproate spectrum disorder (FVSD) characteristics are morphological defects (including cranio-facial, neural tube defects) and behavioural alterations due to valproate (VPA) exposure in pregnancy. Vertebrate assays to evaluate FVSD include classical and alternative mammal (implying adult sacrifice), and non-mammal developmental models (zebrafish, amphibians, chick). Among these latter only zebrafish assays report in the same test both morphological and behavioural examinations. Compared to zebrafish, the amphibian Xenopus laevis excels having a more comparable organ development and morphology to mammalian systems. We used X. laevis embryos exposed during developmental specific windows to VPA therapeutic concentrations. Different VPA effects were observed depending on the exposure window: concentration-related embryo-lethal and teratogenic effects (neural tube, facial, tail defects) were observed in groups exposed at the organogenetic phylotypic stages. Neurobehavioral deficits were described using a functional swimming test at the highest VPA concentration exposure during the phylotypic stages and at any concentration during neurocognitive competent stages. Malformations were compared to those obtained in a mammalian assay (the rat post-implantation whole embryo culture method, WEC), that we used in the past to evaluate VPA teratogenicity. R-FETAX and WEC data were modelled and their relative sensitivity was calculated. We suggest the amphibian R-FETAX as a refined windowed alternative test for the evaluation of chemicals inducing both morphological and behavioural anomalies, including VPA.

Modeling endoderm development and disease in Xenopus

The endoderm is the innermost germ layer that forms the linings of the respiratory and gastrointestinal tracts, and their associated organs, during embryonic development. Xenopus embryology experiments have provided fundamental insights into how the endoderm develops in vertebrates, including the critical role of TGFβ-signaling in endoderm induction,elucidating the gene regulatory networks controlling germ layer development and the key molecular mechanisms regulating endoderm patterning and morphogenesis. With new genetic, genomic, and imaging approaches, Xenopus is now routinely used to model human disease, discover mechanisms underlying endoderm organogenesis, and inform differentiation protocols for pluripotent stem cell differentiation and regenerative medicine applications. In this chapter, we review historical and current discoveries of endoderm development in Xenopus, then provide examples of modeling human disease and congenital defects of endoderm-derived organs using Xenopus.

Heart Development and Regeneration in Non-mammalian Model Organisms

Cardiovascular disease is a serious threat to human health and a leading cause of mortality worldwide. Recent years have witnessed exciting progress in the understanding of heart formation and development, enabling cardiac biologists to make significant advance in the field of therapeutic heart regeneration. Most of our understanding of heart development and regeneration, including the genes and signaling pathways, are driven by pioneering works in non-mammalian model organisms, such as fruit fly, fish, frog, and chicken. Compared to mammalian animal models, non-mammalian model organisms have special advantages in high-throughput applications such as disease modeling, drug discovery, and cardiotoxicity screening. Genetically engineered animals of cardiovascular diseases provide valuable tools to investigate the molecular and cellular mechanisms of pathogenesis and to evaluate therapeutic strategies. A large number of congenital heart diseases (CHDs) non-mammalian models have been established and tested for the genes and signaling pathways involved in the diseases. Here, we reviewed the mechanisms of heart development and regeneration revealed by these models, highlighting the advantages of non-mammalian models as tools for cardiac research. The knowledge from these animal models will facilitate therapeutic discoveries and ultimately serve to accelerate translational medicine.

Retinoic Acid Signaling and Heart Development

As the first organ to form and function in all vertebrates, the heart is crucial to development. Tightly-regulated levels of retinoic acid (RA) are critical for the establishment of the regulatory networks that drive normal cardiac development. Thus, the heart is an ideal organ to investigate RA signaling, with much work remaining to be done in this area. Herein, we highlight the role of RA signaling in vertebrate heart development and provide an overview of the field's inception, its current state, and in what directions it might progress so that it may yield fruitful insight for therapeutic applications within the domain of regenerative medicine.

Experimental models of cardiac physiology and pathology

Experimental models of cardiac disease play a key role in understanding the pathophysiology of the disease and developing new therapies. The features of the experimental models should reflect the clinical phenotype, which can have a wide spectrum of underlying mechanisms. We review characteristics of commonly used experimental models of cardiac physiology and pathophysiology in all translational steps including in vitro, small animal, and large animal models. Understanding their characteristics and relevance to clinical disease is the key for successful translation to effective therapies.

Cardiovascular shunting in vertebrates: a practical integration of competing hypotheses

This review explores the long-standing question: 'Why do cardiovascular shunts occur?' An historical perspective is provided on previous research into cardiac shunts in vertebrates that continues to shape current views. Cardiac shunts and when they occur is then described for vertebrates. Nearly 20 different functional reasons have been proposed as specific causes of shunts, ranging from energy conservation to improved gas exchange, and including a plethora of functions related to thermoregulation, digestion and haemodynamics. It has even been suggested that shunts are merely an evolutionary or developmental relic. Having considered the various hypotheses involving cardiovascular shunting in vertebrates, this review then takes a non-traditional approach. Rather than attempting to identify the single 'correct' reason for the occurrence of shunts, we advance a more holistic, integrative approach that embraces multiple, non-exclusive suites of proposed causes for shunts, and indicates how these varied functions might at least co-exist, if not actually support each other as shunts serve multiple, concurrent physiological functions. It is argued that deposing the 'monolithic' view of shunting leads to a more nuanced view of vertebrate cardiovascular systems. This review concludes by suggesting new paradigms for testing the function(s) of shunts, including experimentally placing organ systems into conflict in terms of their perfusion needs, reducing sources of variation in physiological experiments, measuring possible compensatory responses to shunt ablation, moving experiments from the laboratory to the field, and using cladistics-related approaches in the choice of experimental animals.

Preventing Ethanol-Induced Brain and Eye Morphology Defects Using Optogenetics

Background: Embryonic exposure to the teratogen ethanol leads to dysmorphias, including eye and brain morphology defects associated with fetal alcohol spectrum disorder (FASD). Exposure of Xenopus laevis embryos to ethanol leads to similar developmental defects, including brain and eye dysmorphism, confirming our work and the work of others showing Xenopus as a useful system for studies of the brain and eye birth defects associated with FASD. Several targets of ethanol action have been hypothesized, one being regulation of Kir2.1 potassium channel. Endogenous ion fluxes and membrane voltage variation (bioelectric signals) have been shown to be powerful regulators of embryonic cell behaviors that are required for correct brain and eye morphology. Disruptions to these voltage patterns lead to spatially correlated disruptions in gene expression patterns and corresponding morphology. Materials and Methods: Here, we use controlled membrane voltage modulation to determine when and where voltage modulation is sufficient to rescue ethanol-induced brain and eye defects in Xenopus embryos. Results: We found (1) that modulating membrane voltage using light activation of the channelrhodopsin-2 variant D156A rescues ethanol exposed embryos, resulting in normal brain and eye morphologies; (2) hyperpolarization is required for the full duration of ethanol exposure; (3) hyperpolarization of only superficial ectoderm is sufficient for this effect; and(4) the rescue effect acts at a distance. Conclusions: These results, particularly the last, raise the exciting possibility of using bioelectric modulation to treat ethanol-induced brain and eye birth defects, possibly with extant ion channel drugs already prescribed to pregnant women. This may prove to be a simple and cost-effective strategy for reducing the impact of FASD.

Conservation and divergence of protein pathways in the vertebrate heart

Heart disease is the leading cause of death in the western world. Attaining a mechanistic understanding of human heart development and homeostasis and the molecular basis of associated disease states relies on the use of animal models. Here, we present the cardiac proteomes of 4 model vertebrates with dual circulatory systems: the pig (Sus scrofa), the mouse (Mus musculus), and 2 frogs (Xenopus laevis and Xenopus tropicalis). Determination of which proteins and protein pathways are conserved and which have diverged within these species will aid in our ability to choose the appropriate models for determining protein function and to model human disease. We uncover mammalian- and amphibian-specific, as well as species-specific, enriched proteins and protein pathways. Among these, we find and validate an enrichment in cell-cycle-associated proteins within Xenopus laevis. To further investigate functional units within cardiac proteomes, we develop a computational approach to profile the abundance of protein complexes across species. Finally, we demonstrate the utility of these data sets for predicting appropriate model systems for studying given cardiac conditions by testing the role of Kielin/chordin-like protein (Kcp), a protein found as enriched in frog hearts compared to mammals. We establish that germ-line mutations in Kcp in Xenopus lead to valve defects and, ultimately, cardiac failure and death. Thus, integrating these findings with data on proteins responsible for cardiac disease should lead to the development of refined, species-specific models for protein function and disease states.

The Many Faces of Xenopus: Xenopus laevis as a Model System to Study Wolf-Hirschhorn Syndrome

Wolf–Hirschhorn syndrome (WHS) is a rare developmental disorder characterized by intellectual disability and various physical malformations including craniofacial, skeletal, and cardiac defects. These phenotypes, as they involve structures that are derived from the cranial neural crest, suggest that WHS may be associated with abnormalities in neural crest cell (NCC) migration. This syndrome is linked with assorted mutations on the short arm of chromosome 4, most notably the microdeletion of a critical genomic region containing several candidate genes. However, the function of these genes during embryonic development, as well as the cellular and molecular mechanisms underlying the disorder, are still unknown. The model organism Xenopus laevis offers a number of advantages for studying WHS. With the Xenopus genome sequenced, genetic manipulation strategies can be readily designed in order to alter the dosage of the WHS candidate genes. Moreover, a variety of assays are available for use in Xenopus to examine how manipulation of WHS genes leads to changes in the development of tissue and organ systems affected in WHS. In this review article, we highlight the benefits of using X. laevis as a model system for studying human genetic disorders of development, with a focus on WHS.

Functional Morphology of the Cardiac Jelly in the Tubular Heart of Vertebrate Embryos

The early embryonic heart is a multi-layered tube consisting of (1) an outer myocardial tube; (2) an inner endocardial tube; and (3) an extracellular matrix layer interposed between the myocardium and endocardium, called "cardiac jelly" (CJ). During the past decades, research on CJ has mainly focused on its molecular and cellular biological aspects. This review focuses on the morphological and biomechanical aspects of CJ. Special attention is given to (1) the spatial distribution and fiber architecture of CJ; (2) the morphological dynamics of CJ during the cardiac cycle; and (3) the removal/remodeling of CJ during advanced heart looping stages, which leads to the formation of ventricular trabeculations and endocardial cushions. CJ acts as a hydraulic skeleton, displaying striking structural and functional similarities with the mesoglea of jellyfish. CJ not only represents a filler substance, facilitating end-systolic occlusion of the embryonic heart lumen. Its elastic components antagonize the systolic deformations of the heart wall and thereby power the refilling phase of the ventricular tube. Non-uniform spatial distribution of CJ generates non-circular cross sections of the opened endocardial tube (initially elliptic, later deltoid), which seem to be advantageous for valveless pumping. Endocardial cushions/ridges are cellularized remnants of non-removed CJ.

The Wnt inhibitor Dkk1 is required for maintaining the normal cardiac differentiation program in Xenopus laevis

Wnt proteins can activate different intracellular signaling pathways. These pathways need to be tightly regulated for proper cardiogenesis. The canonical Wnt/β-catenin inhibitor Dkk1 has been shown to be sufficient to trigger cardiogenesis in gain-of-function experiments performed in multiple model systems. Loss-of-function studies however did not reveal any fundamental function for Dkk1 during cardiogenesis. Using Xenopus laevis as a model we here show for the first time that Dkk1 is required for proper differentiation of cardiomyocytes, whereas specification of cardiomyocytes remains unaffected in absence of Dkk1. This effect is at least in part mediated through regulation of non-canonical Wnt signaling via Wnt11. In line with these observations we also found that Isl1, a critical regulator for specification of the common cardiac progenitor cell (CPC) population, acts upstream of Dkk1.

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Dkk1 is required for cardiac development in Xenopus laevis.

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The Wnt inhibitor Dkk1 acts downstream of Isl1 during cardiac development in vivo.

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Loss of Dkk1 has no impact on cardiac specification in Xenopus.

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Normal cardiac differentiation is impaired upon Dkk1 inhibition in Xenopus.

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Dkk1 regulates canonical Wnt/β-catenin signaling during Xenopus cardiogenesis.

Persistent fibrosis, hypertrophy and sarcomere disorganisation after endoscopy-guided heart resection in adult Xenopus

Models of cardiac repair are needed to understand mechanisms underlying failure to regenerate in human cardiac tissue. Such studies are currently dominated by the use of zebrafish and mice. Remarkably, it is between these two evolutionary separated species that the adult cardiac regenerative capacity is thought to be lost, but causes of this difference remain largely unknown. Amphibians, evolutionary positioned between these two models, are of particular interest to help fill this lack of knowledge. We thus developed an endoscopy-based resection method to explore the consequences of cardiac injury in adult Xenopus laevis. This method allowed in situ live heart observation, standardised tissue amputation size and reproducibility. During the first week following amputation, gene expression of cell proliferation markers remained unchanged, whereas those relating to sarcomere organisation decreased and markers of inflammation, fibrosis and hypertrophy increased. One-month post-amputation, fibrosis and hypertrophy were evident at the injury site, persisting through 11 months. Moreover, cardiomyocyte sarcomere organisation deteriorated early following amputation, and was not completely recovered as far as 11 months later. We conclude that the adult Xenopus heart is unable to regenerate, displaying cellular and molecular marks of scarring. Our work suggests that, contrary to urodeles and teleosts, with the exception of medaka, adult anurans share a cardiac injury outcome similar to adult mammals. This observation is at odds with current hypotheses that link loss of cardiac regenerative capacity with acquisition of homeothermy.