A dual genetic tracing system identifies diverse and dynamic origins of cardiac valve mesenchyme

Multi-modal refinement of the human heart atlas during the first gestational trimester

Forty first-trimester human hearts were studied to lay groundwork for further studies of the mechanisms underlying congenital heart defects. We first sampled 49,227 cardiac nuclei from three fetuses at 8.6, 9.0, and 10.7 post-conceptional weeks (pcw) for single-nucleus RNA sequencing, enabling the distinction of six classes comprising 21 cell types. Improved resolution led to the identification of previously unappreciated cardiomyocyte populations and minority autonomic and lymphatic endothelial transcriptomes, among others. After integration with 5-7 pcw heart single-cell RNA-sequencing data, we identified a human cardiomyofibroblast progenitor preceding the diversification of cardiomyocyte and stromal lineages. Spatial transcriptomic analysis (six Visium sections from two additional hearts) was aided by deconvolution, and key spatial markers validated on sectioned and whole hearts in two- and three-dimensional space and over time. Altogether, anatomical-positional features, including innervation, conduction and subdomains of the atrioventricular septum, translate latent molecular identity into specialized cardiac functions. This atlas adds unprecedented spatial and temporal resolution to the characterization of human-specific aspects of early heart formation.

Synchronized lineage tracing of cell membranes and nuclei by dual recombinases and dual fluorescent

Genetic lineage tracing has been widely employed to investigate cell lineages and fate. However, conventional reporting systems often label the entire cytoplasm, making it challenging to discern cell boundaries. Additionally, single Cre-loxP recombination systems have limitations in tracing specific cell populations. This study proposes three reporting systems utilizing Cre, Dre, and Dre+Cre mediated recombination. These systems incorporate tdTomato expression on the cell membrane and PhiYFP expression within the nucleus, allowing for clear observation of the nucleus and membrane. The efficacy of these systems is successfully demonstrated by labeling cardiomyocytes and hepatocytes. The potential for dynamic visualization of the cell membrane is showcased using intravital imaging microscopy or three-dimensional imaging. Furthermore, by combining this dual recombinase system with the ProTracer system, hepatocyte proliferation is traced with enhanced precision. This reporting system holds significant importance for advancing the understanding of cell fate studies in development, homeostasis, and diseases.

Application of New Lineage Tracing Techniques in Cardiovascular Development and Physiology

Cardiovascular disease has been the leading cause of mortality and morbidity worldwide in the past 3 decades. Multiple cell lineages undergo dynamic alternations in gene expression, cell state determination, and cell fate conversion to contribute, adapt, and even modulate the pathophysiological processes during disease progression. There is an urgent need to understand the intricate cellular and molecular underpinnings of cardiovascular cell development in homeostasis and pathogenesis. Recent strides in lineage tracing methodologies have revolutionized our understanding of cardiovascular biology with the identification of new cellular origins, fates, plasticity, and heterogeneity within the cardiomyocyte, endothelial, and mesenchymal cell populations. In this review, we introduce the new technologies for lineage tracing of cardiovascular cells and summarize their applications in studying cardiovascular development, diseases, repair, and regeneration.

Feeder-free generation and characterization of endocardial and cardiac valve cells from human pluripotent stem cells

Valvular heart disease presents a significant health burden, yet advancements in valve biology and therapeutics have been hindered by the lack of accessibility to human valve cells. In this study, we have developed a scalable and feeder-free method to differentiate human induced pluripotent stem cells (iPSCs) into endocardial cells, which are transcriptionally and phenotypically distinct from vascular endothelial cells. These endocardial cells can be challenged to undergo endothelial-to-mesenchymal transition (EndMT), after which two distinct populations emerge-one population undergoes EndMT to become valvular interstitial cells (VICs), while the other population reinforces their endothelial identity to become valvular endothelial cells (VECs). We then characterized these populations through bulk RNA-seq transcriptome analyses and compared our VIC and VEC populations to pseudobulk data generated from normal valve tissue of a 15-week-old human fetus. By increasing the accessibility to these cell populations, we aim to accelerate discoveries for cardiac valve biology and disease.

Fate-Mapping Macrophages: From Ontogeny to Functions

Macrophages are vital to the physiological function of most tissues, but also contribute to disease through a multitude of pathological roles. They are thus highly plastic and heterogeneous. It is now well recognized that macrophages develop from several distinct progenitors from embryogenesis onwards and extending throughout life. Tissue-resident macrophages largely originate from embryonic sources and in many cases self-maintain independently without monocyte input. However, in certain tissues, monocyte-derived macrophages replace these over time or as a result of tissue injury and inflammation. This additional layer of heterogeneity has introduced many questions regarding the influence of origin on fate and function of macrophages in health and disease. To comprehensively address these questions, appropriate methods of tracing macrophage ontogeny are required. This chapter explores why ontogeny is of vital importance in macrophage biology and how to delineate macrophage populations by origin through genetic fate mapping. First, we summarize the current view of macrophage ontogeny and briefly discuss how origin may influence macrophage function in homeostasis and pathology. We go on to make the case for genetic fate mapping as the gold standard and briefly review different fate-mapping models. We then put forward our recommendations for fate-mapping strategies best suited to answer specific research questions and finally discuss the strengths and limitations of currently available models.

Tissue-specific Cre driver mice to study vascular diseases

Vascular diseases, including atherosclerosis and abdominal aneurysms, are the primary cause of mortality and morbidity among the elderly worldwide. The life quality of patients is significantly compromised due to inadequate therapeutic approaches and limited drug targets. To expand our comprehension of vascular diseases, gene knockout (KO) mice, especially conditional knockout (cKO) mice, are widely used for investigating gene function and mechanisms of action. The Cre-loxP system is the most common method for generating cKO mice. Numerous Cre driver mice have been established to study the main cell types that compose blood vessels, including endothelial cells, smooth muscle cells, and fibroblasts. Here, we first discuss the characteristics of each layer of the arterial wall. Next, we provide an overview of the representative Cre driver mice utilized for each of the major cell types in the vessel wall and their most recent applications in vascular biology. We then go over Cre toxicity and discuss the practical methods for minimizing Cre interference in experimental outcomes. Finally, we look into the future of tissue-specific Cre drivers by introducing the revolutionary single-cell RNA sequencing and dual recombinase system.

Cell diversity and plasticity during atrioventricular heart valve EMTs

Epithelial-to-mesenchymal transitions (EMTs) of both endocardium and epicardium guide atrioventricular heart valve formation, but the cellular complexity and small scale of this tissue have restricted analyses. To circumvent these issues, we analyzed over 50,000 murine single-cell transcriptomes from embryonic day (E)7.75 hearts to E12.5 atrioventricular canals. We delineate mesenchymal and endocardial bifurcation during endocardial EMT, identify a distinct, transdifferentiating epicardial population during epicardial EMT, and reveal the activation of epithelial-mesenchymal plasticity during both processes. In Sox9-deficient valves, we observe increased epithelial-mesenchymal plasticity, indicating a role for SOX9 in promoting endothelial and mesenchymal cell fate decisions. Lastly, we deconvolve cell interactions guiding the initiation and progression of cardiac valve EMTs. Overall, these data reveal mechanisms of emergence of mesenchyme from endocardium or epicardium at single-cell resolution and will serve as an atlas of EMT initiation and progression with broad implications in regenerative medicine and cancer biology.

Heterogeneity and Functional Analysis of Cardiac Fibroblasts in Heart Development

Background

As one of the major cell types in the heart, fibroblasts play critical roles in multiple biological processes. Cardiac fibroblasts are known to develop from multiple sources, but their transcriptional profiles have not been systematically compared. Furthermore, while the function of a few genes in cardiac fibroblasts has been studied, the overall function of fibroblasts as a cell type remains uninvestigated.

Methods

Single-cell mRNA sequencing (scRNA-seq) and bioinformatics approaches were used to analyze the genome-wide genes expression and extracellular matrix genes expression in fibroblasts, as well as the ligand-receptor interactions between fibroblasts and cardiomyocytes. Single molecular in situ hybridization was employed to analyze the expression pattern of fibroblast subpopulation-specific genes. The Diphtheria toxin fragment A (DTA) system was utilized to ablate fibroblasts at each developmental phase.

Results

Using RNA staining of Col1a1 at different stages, we grouped cardiac fibroblasts into four developmental phases. Through the analysis of scRNA-seq profiles of fibroblasts at 18 stages from two mouse strains, we identified significant heterogeneity, preserving lineage gene expression in their precursor cells. Within the main fibroblast population, we found differential expressions of Wt1, Tbx18, and Aldh1a2 genes in various cell clusters. Lineage tracing studies showed Wt1- and Tbx18-positive fibroblasts originated from respective epicardial cells. Furthermore, using a conditional DTA system-based elimination, we identified the crucial role of fibroblasts in early embryonic and heart growth, but not in neonatal heart growth. Additionally, we identified the zone- and stage-associated expression of extracellular matrix genes and fibroblast-cardiomyocyte ligand-receptor interactions. This comprehensive understanding sheds light on fibroblast function in heart development.

Conclusion

We observed cardiac fibroblast heterogeneity at embryonic and neonatal stages, with preserved lineage gene expression. Ablation studies revealed their distinct roles during development, likely influenced by varying extracellular matrix genes and ligand-receptor interactions at different stages.

Identifying molecular and functional similarities and differences between human primary cardiac valve interstitial cells and ventricular fibroblasts

Introduction: Fibroblasts are mesenchymal cells that predominantly produce and maintain the extracellular matrix (ECM) and are critical mediators of injury response. In the heart, valve interstitial cells (VICs) are a population of fibroblasts responsible for maintaining the structure and function of heart valves. These cells are regionally distinct from myocardial fibroblasts, including left ventricular cardiac fibroblasts (LVCFBs), which are located in the myocardium in close vicinity to cardiomyocytes. Here, we hypothesize these subpopulations of fibroblasts are transcriptionally and functionally distinct. Methods: To compare these fibroblast subtypes, we collected patient-matched samples of human primary VICs and LVCFBs and performed bulk RNA sequencing, extracellular matrix profiling, and functional contraction and calcification assays. Results: Here, we identified combined expression of SUSD2 on a protein-level, and MEOX2, EBF2 and RHOU at a transcript-level to be differentially expressed in VICs compared to LVCFBs and demonstrated that expression of these genes can be used to distinguish between the two subpopulations. We found both VICs and LVCFBs expressed similar activation and contraction potential in vitro, but VICs showed an increase in ALP activity when activated and higher expression in matricellular proteins, including cartilage oligomeric protein and alpha 2-Heremans-Schmid glycoprotein, both of which are reported to be linked to calcification, compared to LVCFBs. Conclusion: These comparative transcriptomic, proteomic, and functional studies shed novel insight into the similarities and differences between valve interstitial cells and left ventricular cardiac fibroblasts and will aid in understanding region-specific cardiac pathologies, distinguishing between primary subpopulations of fibroblasts, and generating region-specific stem-cell derived cardiac fibroblasts.

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.

Lineage tracing clarifies the cellular origin of tissue-resident macrophages in the developing heart

Tissue-resident macrophages play essential functions in the maintenance of tissue homeostasis and repair. Recently, the endocardium has been reported as a de novo hemogenic site for the contribution of hematopoietic cells, including cardiac macrophages, during embryogenesis. These observations challenge the current consensus that hematopoiesis originates from the hemogenic endothelium within the yolk sac and dorsal aorta. Whether the developing endocardium has such a hemogenic potential requires further investigation. Here, we generated new genetic tools to trace endocardial cells and reassessed their potential contribution to hematopoietic cells in the developing heart. Fate-mapping analyses revealed that the endocardium contributed minimally to cardiac macrophages and circulating blood cells. Instead, cardiac macrophages were mainly derived from the endothelium during primitive/transient definitive (yolk sac) and definitive (dorsal aorta) hematopoiesis. Our findings refute the concept of endocardial hematopoiesis, suggesting that the developing endocardium gives rise minimally to hematopoietic cells, including cardiac macrophages.

Generation of Piezo1-CreER transgenic mice for visualization and lineage tracing of mechanical force responsive cells in vivo

Cells and tissues are exposed to a wide range of mechanical stimuli during development, tissue homeostasis, repair, and regeneration. Over the past few decades, mechanosensitive ion channels (MSCs), as force-sensing integral membrane proteins, have attracted great attention with regard to their structural dynamics and mechanics at the molecular level and functions in various cells. Piezo-type MSC component 1 (Piezo1) is a newly discovered MSC; it is inherently mechanosensitive. However, which type of cells express Piezo1 in vivo remains unclear. To detect and trace Piezo1-expressing cells, we generated and characterized a novel tamoxifen-inducible Cre knock-in mouse line, Piezo1-CreER, which expresses CreER recombinase under the control of the endogenous Piezo1 promoter. Using this genetic tool, we detected the expression of Piezo1 in various cell types at the embryonic, neonatal, and adult stages. Our data showed that Piezo1 was highly expressed in endothelial cells in all the three stages, while the Piezo1 expression in epithelial cells was dynamic during development and growth. In summary, we established a new genetic tool, Piezo1-CreER, to study Piezo1-expressing cells in vivo during development, injury response, and tissue repair and regeneration.

Lineage Tracing Methods to Study Mammary Epithelial Hierarchies In Vivo

Lineage tracing is now considered the gold standard approach to study cellular hierarchies and cell fate in vivo (McKenna and Gagnon, Development 146:dev169730, 2019; Kretzschmar and Watt, Cell 148:33-45, 2012). This type of clonal analysis consists of genetically labeling defined cells and following their destiny and progeny in vivo and in situ.Here we will describe different existing in vivo systems to clonally trace targeted cells and will discuss their respective advantages and inconveniences; we will then provide stepwise instructions for setting up and evaluate lineage tracing experiments, listing the most common downstream analyses and read-out assays.

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.

Dual role for CXCL12 signaling in semilunar valve development

Cxcl12-null embryos have dysplastic, misaligned, and hyperplastic semilunar valves (SLVs). In this study, we show that CXCL12 signaling via its receptor CXCR4 fulfills distinct roles at different stages of SLV development, acting initially as a guidance cue to pattern cellular distribution within the valve primordia during the endocardial-to-mesenchymal transition (endoMT) phase and later regulating mesenchymal cell proliferation during SLV remodeling. Transient, anteriorly localized puncta of internalized CXCR4 are observed in cells undergoing endoMT. In vitro, CXCR4+ cell orientation in response to CXCL12 requires phosphatidylinositol 3-kinase (PI3K) signaling and is inhibited by suppression of endocytosis. This dynamic intracellular localization of CXCR4 during SLV development is related to CXCL12 availability, potentially enabling activation of divergent downstream signaling pathways at key developmental stages. Importantly, Cxcr7-/- mutants display evidence of excessive CXCL12 signaling, indicating a likely role for atypical chemokine receptor CXCR7 in regulating ligand bioavailability and thus CXCR4 signaling output during SLV morphogenesis.

Role of the Epicardium in the Development of the Atrioventricular Valves and Its Relevance to the Pathogenesis of Myxomatous Valve Disease

This paper is dedicated to the memory of Dr. Adriana "Adri" Gittenberger-de Groot and in appreciation of her work in the field of developmental cardiovascular biology and the legacy that she has left behind. During her impressive career, Dr. Gittenberger-de Groot studied many aspects of heart development, including aspects of cardiac valve formation and disease and the role of the epicardium in the formation of the heart. In this contribution, we review some of the work on the role of epicardially-derived cells (EPDCs) in the development of the atrioventricular valves and their potential involvement in the pathogenesis of myxomatous valve disease (MVD). We provide an overview of critical events in the development of the atrioventricular junction, discuss the role of the epicardium in these events, and illustrate how interfering with molecular mechanisms that are involved in the epicardial-dependent formation of the atrioventricular junction leads to a number of abnormalities. These abnormalities include defects of the AV valves that resemble those observed in humans that suffer from MVD. The studies demonstrate the importance of the epicardium for the proper formation and maturation of the AV valves and show that the possibility of epicardial-associated developmental defects should be taken into consideration when determining the genetic origin and pathogenesis of MVD.

The Role of Cell Tracing and Fate Mapping Experiments in Cardiac Outflow Tract Development, New Opportunities through Emerging Technologies

Whilst knowledge regarding the pathophysiology of congenital heart disease (CHDs) has advanced greatly in recent years, the underlying developmental processes affecting the cardiac outflow tract (OFT) such as bicuspid aortic valve, tetralogy of Fallot and transposition of the great arteries remain poorly understood. Common among CHDs affecting the OFT, is a large variation in disease phenotypes. Even though the different cell lineages contributing to OFT development have been studied for many decades, it remains challenging to relate cell lineage dynamics to the morphologic variation observed in OFT pathologies. We postulate that the variation observed in cellular contribution in these congenital heart diseases might be related to underlying cell lineage dynamics of which little is known. We believe this gap in knowledge is mainly the result of technical limitations in experimental methods used for cell lineage analysis. The aim of this review is to provide an overview of historical fate mapping and cell tracing techniques used to study OFT development and introduce emerging technologies which provide new opportunities that will aid our understanding of the cellular dynamics underlying OFT pathology.

Dual recombinases-based genetic lineage tracing for stem cell research with enhanced precision

Stem cell research has become a hot topic in biology, as the understanding of stem cell biology can provide new insights for both regenerative medicine and clinical treatment of diseases. Accurately deciphering the fate of stem cells is the basis for understanding the mechanism and function of stem cells during tissue repair and regeneration. Cre-loxP-mediated recombination has been widely applied in fate mapping of stem cells for many years. However, nonspecific labeling by conventional cell lineage tracing strategies has led to discrepancies or even controversies in multiple fields. Recently, dual recombinase-mediated lineage tracing strategies have been developed to improve both the resolution and precision of stem cell fate mapping. These new genetic strategies also expand the application of lineage tracing in studying cell origin and fate. Here, we review cell lineage tracing methods, especially dual genetic approaches, and then provide examples to describe how they are used to study stem cell fate plasticity and function in vivo.

Strategies for site-specific recombination with high efficiency and precise spatiotemporal resolution

Site-specific recombinases (SSRs) are invaluable genome engineering tools that have enormously boosted our understanding of gene functions and cell lineage relationships in developmental biology, stem cell biology, regenerative medicine, and multiple diseases. However, the ever-increasing complexity of biomedical research requires the development of novel site-specific genetic recombination technologies that can manipulate genomic DNA with high efficiency and fine spatiotemporal control. Here, we review the latest innovative strategies of the commonly used Cre-loxP recombination system and its combinatorial strategies with other site-specific recombinase systems. We also highlight recent progress with a focus on the new generation of chemical- and light-inducible genetic systems and discuss the merits and limitations of each new and established system. Finally, we provide the future perspectives of combining various recombination systems or improving well-established site-specific genetic tools to achieve more efficient and precise spatiotemporal genetic manipulation.

A most formidable arsenal: genetic technologies for building a better mouse

The mouse is one of the most widely used model organisms for genetic study. The tools available to alter the mouse genome have developed over the preceding decades from forward screens to gene targeting in stem cells to the recent influx of CRISPR approaches. In this review, we first consider the history of mice in genetic study, the development of classic approaches to genome modification, and how such approaches have been used and improved in recent years. We then turn to the recent surge of nuclease-mediated techniques and how they are changing the field of mouse genetics. Finally, we survey common classes of alleles used in mice and discuss how they might be engineered using different methods.

Simultaneous quantitative assessment of two distinct cell lineages with a nuclear-localized dual genetic reporter

Genetic lineage tracing has been widely used for studying in vivo cell fate plasticity during embryogenesis, tissue homeostasis, and disease development. Recent applications with multiple site-specific recombinases have been used in complex and sophisticated genetic fate mapping studies. However, the previous multicolor reporters for dual recombinases had limitations of precise in situ quantification of cell number, which is mainly due to the intermingling of cells in condensed tissues. Here, we generated a dual recombinase-mediated nuclear-localized GFP and tdTomato reporter line, which enables clear, simultaneous quantification of two distinct cell lineages in vivo. Combining this dual genetic reporter with Tbx18-Cre and Cdh5-Dre lines, which genetically trace epicardial and endothelial cells, respectively, we obtained high-resolution images for the anatomic distribution of the descendants of these two distinct cell lineages in the valve mesenchyme during development, remodeling, and maturation stages. This new dual genetic reporter is expected to facilitate fate tracing of two cell lineages and their objective quantification in vivo.

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.

Human pluripotent stem cell-derived cardiac stromal cells and their applications in regenerative medicine

Coronary heart disease is one of the leading causes of death in the United States. Recent advances in stem cell biology have led to the development and engineering of human pluripotent stem cell (hPSC)-derived cardiac cells and tissues for application in cellular therapy and cardiotoxicity studies. Initial studies in this area have largely focused on improving differentiation efficiency and maturation states of cardiomyocytes. However, other cell types in the heart, including endothelial and stromal cells, play crucial roles in cardiac development, injury response, and cardiomyocyte function. This review discusses recent advances in differentiation of hPSCs to cardiac stromal cells, identification and classification of cardiac stromal cell types, and application of hPSC-derived cardiac stromal cells and tissues containing these cells in regenerative and drug development applications.

Genetic lineage tracing with multiple DNA recombinases: A user's guide for conducting more precise cell fate mapping studies

Site-specific recombinases, such as Cre, are a widely used tool for genetic lineage tracing in the fields of developmental biology, neural science, stem cell biology, and regenerative medicine. However, nonspecific cell labeling by some genetic Cre tools remains a technical limitation of this recombination system, which has resulted in data misinterpretation and led to many controversies in the scientific community. In the past decade, to enhance the specificity and precision of genetic targeting, researchers have used two or more orthogonal recombinases simultaneously for labeling cell lineages. Here, we review the history of cell-tracing strategies and then elaborate on the working principle and application of a recently developed dual genetic lineage-tracing approach for cell fate studies. We place an emphasis on discussing the technical strengths and caveats of different methods, with the goal to develop more specific and efficient tracing technologies for cell fate mapping. Our review also provides several examples for how to use different types of DNA recombinase-mediated lineage-tracing strategies to improve the resolution of the cell fate mapping in order to probe and explore cell fate-related biological phenomena in the life sciences.

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.

Triple-cell lineage tracing by a dual reporter on a single allele

Genetic lineage tracing is widely used to study organ development and tissue regeneration. Multicolor reporters are a powerful platform for simultaneously tracking discrete cell populations. Here, combining Dre-rox and Cre-loxP systems, we generated a new dual-recombinase reporter system, called Rosa26 traffic light reporter (R26-TLR), to monitor red, green, and yellow fluorescence. Using this new reporter system with the three distinct fluorescent reporters combined on one allele, we found that the readouts of the two recombinases Cre and Dre simultaneously reflect Cre⁺Dre^-, Cre^-Dre⁺, and Cre⁺Dre⁺ cell lineages. As proof of principle, we show specific labeling in three distinct progenitor/stem cell populations, including club cells, AT2 cells, and bronchoalveolar stem cells, in Sftpc-DreER;Scgb1a1-CreER;R26-TLR mice. By using this new dual-recombinase reporter system, we simultaneously traced the cell fate of these three distinct cell populations during lung repair and regeneration, providing a more comprehensive picture of stem cell function in distal airway repair and regeneration. We propose that this new reporter system will advance developmental and regenerative research by facilitating a more sophisticated genetic approach to studying in vivo cell fate plasticity.

Developmental analyses of mouse embryos and adults using a non-overlapping tracing system for all three germ layers

Genetic lineage-tracing techniques are powerful tools for studying specific cell populations in development and pathogenesis. Previous techniques have mainly involved systems for tracing a single gene, which are limited in their ability to facilitate direct comparisons of the contributions of different cell lineages. We have developed a new combinatorial system for tracing all three germ layers using self-cleaving 2A peptides and multiple site-specific recombinases (SSRs). In the resulting TRiCK (TRiple Coloured germ layer Knock-in) mice, the three germ layers are conditionally and simultaneously labelled with distinct fluorescent proteins via embryogenesis. We show that previously reported ectopic expressions of lineage markers are the outcome of secondary gene expression. The results presented here also indicate that the commitment of caudal axial stem cells to neural or mesodermal fate proceeds without lineage fluctuations, contrary to the notion of their bi-potency. Moreover, we developed IMES, an optimized tissue clearing method that is highly compatible with a variety of fluorescent proteins and immunostaining, and the combined use of TRiCK mice and IMES can facilitate comprehensive analyses of dynamic contributions of all three germ layers.

Smooth Muscle Cell Phenotypic Diversity

Vascular smooth muscle cells (SMC) play a critical role in controlling blood pressure and blood distribution, as well as maintaining the structural integrity of the blood vessel. SMC also participate in physiological and pathological vascular remodeling due to their remarkable ability to dynamically modulate their phenotype. During the past decade, the development of in vivo fate mapping systems for unbiased identification and tracking of SMC and their progeny has led to major discoveries as well as the reevaluation of well-established concepts about the contribution of vascular SMC in major vascular diseases including atherosclerosis. Lineage tracing studies revealed that SMC undergoes multiple phenotypic transitions characterized by the expression of markers of alternative cell types (eg, macrophage-like and mesenchymal-stem cell-like) and populate injured or diseased vessels by oligoclonal expansion of a limited number of medial SMC. With the development of high-throughput transcriptomics and single-cell RNA sequencing (scRNAseq), the field is moving forward towards in-depth SMC phenotypic characterization. Herein, we review the major observations put forth by lineage and clonality tracing studies and the evidence in support for SMC phenotypic diversity in healthy and diseased vascular tissue. We will also discuss the opportunities and remaining challenges of combining lineage tracing and single-cell transcriptomics technologies, as well as studying the functional relevance of SMC phenotypic transitions and identifying the mechanisms controlling them.

Dual lineage tracing identifies intermediate mesenchymal stage for endocardial contribution to fibroblasts, coronary mural cells, and adipocytes

Early embryonic endocardium undergoes endothelial-to-mesenchymal transition to form cardiac cushion mesenchymal cells (MCs). Embryonic endocardium also gives rise to fibroblasts, intramyocardial adipocytes, and coronary mural cells, including smooth muscle cells and pericytes, in development. Whether endocardial cells directly differentiate into fibroblasts, coronary mural cells, and adipocytes or indirectly via an intermediate stage of endocardial-derived cushion MCs remains unknown. In addition to endocardium, epicardium and neural crest also contribute to cardiac cushion MCs. Given the developmental heterogeneity of cushion MCs and the lack of specific markers for endocardial-derived cushion MCs, conventional genetic lineage tracing utilizing Cre recombinase driven by one specific regulatory element is not sufficient to examine the fates of endocardial-derived cushion MCs. Intersectional genetic targeting approaches, which combine regulatory elements from two or more genes, have been employed to increase the specificity of cell targeting. Here, we developed a dual-recombinase intersectional targeting approach using Nfatc1-Dre, Sox9-CreER, and Cre/Dre double-dependent reporter Ai66 to specifically label endocardial-derived cushion MCs. Taking advantage of intersectional lineage tracing, we found that a subset of cardiac cells including fibroblasts, coronary mural cells, and intramyocardial adipocytes in adult hearts were derived from endocardial-derived cushion MCs. Our study suggests that embryonic endocardium contributes to cushion MCs first, and then endocardial-derived cushion MCs migrate into myocardium and differentiate into fibroblasts, coronary mural cells, and adipocytes in development. Understanding developmental origins of cardiac cell lineages will provide us more insights into cardiac development, regeneration, and diseases.