The ECM as a driver of heart development and repair

Extracellular matrix in cardiac morphogenesis, fibrosis, and regeneration

The extracellular matrix (ECM) plays a crucial role in providing structural integrity and regulating cell communication essential for organ development, homeostasis, and regeneration, including hearts. Evidence indicates that disruptions in the spatiotemporal expression or alterations in ECM components lead to cardiac malformations, including a wide range of congenital heart diseases (CHDs). Furthermore, research on injured hearts across various vertebrate species, some of which show effective regeneration while others experience irreversible fibrosis, underscores the significance of ECM molecules in cardiac regeneration. This review presents an overview of heart development and the dynamics of ECM during cardiac morphogenesis, beginning with the formation of the contractile heart tube and advancing to the development of distinct chambers separated by valves to facilitate unidirectional blood flow. Furthermore, we discuss research emphasizing the multifaceted roles of secreted molecules in mediating fibrosis and regeneration following myocardial injury.

BubR1 Controls Heart Development by Promoting Expression of Cardiogenesis Regulators

BACKGROUND

Congenital heart defects are structural anomalies present at birth that can affect the function of the heart. Aneuploidy is a significant risk factor for congenital heart defects. Mosaic variegated aneuploidy syndrome, caused by mutations in Bub1b (encoding BubR1, a mitotic checkpoint protein), leads to congenital heart defects such as septal defects. However, the molecular rationale for how Bub1b mutations promote congenital heart defects associated with mosaic variegated aneuploidy syndrome remains unresolved.

METHODS

To study morphological, structural, and cellular consequences of BubR1 deletion in the heart, we crossed mice carrying conditional alleles of Bub1b with Nkx2.5-cre mice. Single-cell RNA sequencing was carried out to determine differentially expressed genes and biological processes in various cell types present in the developing heart. Trajectory analysis was carried out to determine the differentiation trajectory of BubR1 knockout embryonic hearts. Finally, CellChat analysis provided details on the major signaling interactions that were either absent or hyperactive in the BubR1 knockout heart.

RESULTS

Here, we show that cardiac-specific BubR1 deletion causes embryonic lethality due to developmental stalling after cardiac looping with defects in cardiac maturation including chamber wall thickness, septation, and trabeculation. Single-cell transcriptomic profiling further revealed that the differentiation trajectory of cardiomyocytes is severely impacted with suppression of critical cardiogenesis genes. Hyperactivation of Wnt signaling in BubR1 knockout hearts indicated a disturbed homeostasis in cellular pathways essential for proper tissue morphogenesis of the heart.

CONCLUSIONS

Taken together, these findings reveal that BubR1 is a crucial regulator of cardiac development in vivo, which ensures the proper timing of heart morphogenesis.

Molecular Regulation of Cardiomyocyte Maturation

PURPOSE OF THE REVIEW

This review aims to discuss the process of cardiomyocyte maturation, with a focus on the underlying molecular mechanisms required to form a fully functional heart. We examine both long-standing concepts associated with cardiac maturation and recent developments, and the overall complexity of molecularly integrating all the processes that lead to a mature heart.

RECENT FINDINGS

Cardiac maturation, defined here as the sequential changes that occurring before the heart reaches full maturity, has been a subject of investigation for decades. Recently, there has been a renewed, highly focused interest in this process, driven by clinically motivated research areas where enhancing maturation may lead to improved therapeutic opportunities. These include using pluripotent stem cell models for cell therapy and disease modeling, as well as recent advancements in adult cardiac regeneration approaches. We highlight key processes underlying maturation of the heart, including cellular and organ growth, and electrophysiological, metabolic, and contractile maturation. We further discuss how these processes integrate and interact to contribute to the overall complexity of the developing heart. Finally, we emphasize the transformative potential for translating relevant maturation concepts to emerging models of heart disease and regeneration.

Versican controlled by Lmx1b regulates hyaluronate density and hydration for semicircular canal morphogenesis

During inner ear semicircular canal morphogenesis in zebrafish, patterned canal-genesis zones express genes for extracellular matrix component synthesis. These include hyaluronan and the hyaluronan-binding chondroitin sulfate proteoglycan Versican, which are abundant in the matrices of many developing organs. Charged hyaluronate polymers play a key role in canal morphogenesis through osmotic swelling. However, the developmental factor(s) that pattern the synthesis of the matrix components and regulation of hyaluronate density and swelling are unknown. Here, we identify the transcription factor Lmx1b as a positive transcriptional regulator of hyaluronan, Versican, and chondroitin synthesis genes crucial for canal morphogenesis. We show that Versican regulates hyaluronan density through its protein core, whereas the charged chondroitin side chains contribute to the hydration of hyaluronate-containing extracellular matrices. Versican-tuned properties of hyaluronate matrices may be a broadly used mechanism in morphogenesis with important implications for understanding diseases in which these matrices are impaired, and for hydrogel engineering for tissue regeneration.

Self-organizing human heart assembloids with autologous and developmentally relevant cardiac neural crest-derived tissues

Neural crest cells (NCCs) are a multipotent embryonic cell population of ectodermal origin that extensively migrate during early development and contribute to the formation of multiple tissues. Cardiac NCCs play a critical role in heart development by orchestrating outflow tract septation, valve formation, aortic arch artery patterning, parasympathetic innervation, and maturation of the cardiac conduction system. Abnormal migration, proliferation, or differentiation of cardiac NCCs can lead to severe congenital cardiovascular malformations. However, the complexity and timing of early embryonic heart development pose significant challenges to studying the molecular mechanisms underlying NCC-related cardiac pathologies. Here, we present a sophisticated functional model of human heart assembloids derived from induced pluripotent stem cells, which, for the first time, recapitulates cardiac NCC integration into the human embryonic heart in vitro . NCCs successfully integrated at developmentally relevant stages into heart organoids, and followed developmental trajectories known to occur in the human heart. They demonstrated extensive migration, differentiated into cholinergic neurons capable of generating nerve impulses, and formed mature glial cells. Additionally, they contributed to the mesenchymal populations of the developing outflow tract. Through transcriptomic analysis, we revealed that NCCs acquire molecular features of their cardiac derivatives as heart assembloids develop. NCC-derived parasympathetic neurons formed functional connections with cardiomyocytes, promoting the maturation of the cardiac conduction system. Leveraging this model's cellular complexity and functional maturity, we uncovered that early exposure of NCCs to antidepressants harms the development of NCC derivatives in the context of the developing heart. The commonly prescribed antidepressant Paroxetine disrupted the expression of a critical early neuronal transcription factor, resulting in impaired parasympathetic innervation and functional deficits in cardiac tissue. This advanced heart assembloid model holds great promise for high-throughput drug screening and unraveling the molecular mechanisms underlying NCC-related cardiac formation and congenital heart defects.

IN BRIEF

Human neural crest heart assembloids resembling the major directions of neural crest differentiation in the human embryonic heart, including parasympathetic innervation and the mesenchymal component of the outflow tract, provide a human-relevant embryonic platform for studying congenital heart defects and drug safety.

Single-cell RNA sequencing reveals key regulators and differentiation trajectory of iPSC-derived cardiomyocytes

Cardiac differentiation of human pluripotent stem cells is not only a new strategy of regenerative therapy for cardiovascular disease treatment but also provides unique opportunities for the study of in vitro disease models and human heart development. To elucidate the dynamic gene regulatory networks and pivotal regulators involved in the cardiomyocyte differentiation process, we conducted an analysis of single-cell RNA sequencing data obtained from the reprogramming of two human induced pluripotent stem cell (iPSC) lines into cardiomyocytes. The data were collected from 32,365 cells at 4 stages of this process. We successfully identified cardiomyocyte clusters and several other cell clusters with different molecular characteristics derived from iPSC and described the differentiation trajectory of cardiomyocytes during differentiation in vitro. Through differential gene analysis and SCENIC analysis, we identified several candidate genes including CREG and NR2F2 that play an important regulatory role in cardiomyocyte lineage commitment. This study provides the key differentiation trajectory of heart differentiation in vitro at single-cell resolution and reveals the molecular basis of heart development and differentiation of iPSC-derived cardiomyocytes.

Integrated multi-omics analysis identifies features that predict human pluripotent stem cell-derived progenitor differentiation to cardiomyocytes

Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) are advancing cardiovascular development and disease modeling, drug testing, and regenerative therapies. However, hPSC-CM production is hindered by significant variability in the differentiation process. Establishment of early quality markers to monitor lineage progression and predict terminal differentiation outcomes would address this robustness and reproducibility roadblock in hPSC-CM production. An integrated transcriptomic and epigenomic analysis assesses how attributes of the cardiac progenitor cell (CPC) affect CM differentiation outcome. Resulting analysis identifies predictive markers of CPCs that give rise to high purity CM batches, including TTN, TRIM55, DGKI, MEF2C, MAB21L2, MYL7, LDB3, SLC7A11, and CALD1. Predictive models developed from these genes provide high accuracy in determining terminal CM purities at the CPC stage. Further, insights into mechanisms of batch failure and dominant non-CM cell types generated in failed batches are elucidated. Namely EMT, MAPK, and WNT signaling emerge as significant drivers of batch divergence, giving rise to off-target populations of fibroblasts/mural cells, skeletal myocytes, epicardial cells, and a non-CPC SLC7A11+ subpopulation. This study demonstrates how integrated multi-omic analysis of progenitor cells can identify quality attributes of that progenitor and predict differentiation outcomes, thereby improving differentiation protocols and increasing process robustness.

Metabolic and Proteomic Profiling of Coronary Microvascular Dysfunction: Insights from Rat Models

Coronary microvascular dysfunction (CMD) represents a principal etiological factor in ischemic heart disease. Nonetheless, a considerable subset of CMD patients experiences diagnostic delays attributable to the inadequacy of current diagnostic methodologies; which in turn results in deferred therapeutic interventions and elevated mortality rates. This study seeks to elucidate the distinct metabolic profile associated with CMD in rat models and to identify specific diagnostic markers that could enhance the diagnostic accuracy for CMD. In this study, 18 Wistar rats were randomly allocated into two groups: the sham group and the CMD group. The CMD group received injections of embolic microspheres into the left ventricle to establish a CMD model. Subsequently, non-targeted metabolomics and acetylated proteomics analyses were conducted. Machine-learning techniques were employed to identify the co-diagnostic markers of the disease. This study identified 53 key proteins through differential expression proteins (DEPs) and modular proteins analysis. Subsequently, four core proteins (Emc1; Ank1; Fbln2; and Hp) were determined as diagnostic markers for CMD using lasso regression, support vector machine, and random forest methodologies. Receiver operating characteristic curve analysis further demonstrated robust diagnostic performance. Gene ontology and kyoto encyclopedia of genes and genome enrichment analyses indicated that the DEPs were predominantly associated with metabolic pathways. Ultimately, the integrative analysis of proteomics and metabolomics suggested that the central metabolic mechanism underlying CMD pathogenesis may be linked to the tricarboxylic acid cycle. This study revealed specific changes in the proteomic and metabolic profiles of CMD rats and identified four diagnostic markers, which are proteins and metabolites that could be potential diagnostic biomarkers for CMD.

Cardiac tissue engineering: an emerging approach to the treatment of heart failure

Heart failure is a major health problem in which the heart is unable to pump enough blood to meet the body's needs. It is a progressive disease that becomes more severe over time and can be caused by a variety of factors, including heart attack, cardiomyopathy and heart valve disease. There are various methods to cure this disease, which has many complications and risks. The advancement of knowledge and technology has proposed new methods for many diseases. One of the promising new treatments for heart failure is tissue engineering. Tissue engineering is a field of research that aims to create living tissues and organs to replace damaged or diseased tissue. The goal of tissue engineering in heart failure is to improve cardiac function and reduce the need for heart transplantation. This can be done using the three important principles of cells, biomaterials and signals to improve function or replace heart tissue. The techniques for using cells and biomaterials such as electrospinning, hydrogel synthesis, decellularization, etc. are diverse. Treating heart failure through tissue engineering is still under development and research, but it is hoped that there will be no transplants or invasive surgeries in the near future. In this study, based on the most important research in recent years, we will examine the power of tissue engineering in the treatment of heart failure.

Isomer-specific cardiotoxicity induced by tricresyl phosphate in zebrafish embryos/larvae

Tricresyl phosphate (TCP) has received extensive attentions due to its potential adverse effects, while the toxicological information of TCP isomers is limited. In this study, 2 h post-fertilization zebrafish embryos were exposed to tri-o-cresyl phosphate (ToCP), tri-m-cresyl phosphate (TmCP) or tri-p-cresyl phosphate (TpCP) at concentrations of 0, 100, 300 and 600 μg/L until 120 hpf, and the cardiotoxicity and mechanism of TCP isomers in zebrafish embryos/larvae were evaluated. The results showed that ToCP or TmCP exposure induced cardiac morphological defects and dysfunction in zebrafish, characterized by increased distance between sinus venosus and bulbus arteriosis, increased atrium and pericardial sac area, trabecular defects, and decreased heart rate and blood flow velocity, while no adverse effects of TpCP on zebrafish heart were found. Transcriptomic results revealed that extracellular matrix (ECM) and motor proteins, as well as PPAR signaling pathways, were included in the cardiac morphological defects and dysfunction induced by ToCP and TmCP. Co-exposure test with D-mannitol indicated that the inhibition of energy metabolism by ToCP and TmCP affected cardiac morphology and function by decreasing osmoregulation. This study is the first to report the cardiotoxicity induced by TCP in zebrafish from an isomer perspective, providing a new insight into the toxicity of TCP isomers and highlighting the importance of evaluating the toxicity of different isomers.

Mechanical forces remodel the cardiac extracellular matrix during zebrafish development

The cardiac extracellular matrix (cECM) is fundamental for organ morphogenesis and maturation, during which time it undergoes remodeling, yet little is known about whether mechanical forces generated by the heartbeat regulate this remodeling process. Using zebrafish as a model and focusing on stages when cardiac valves and trabeculae form, we found that altering cardiac contraction impairs cECM remodeling. Longitudinal volumetric quantifications in wild-type animals revealed region-specific dynamics: cECM volume decreases in the atrium but not in the ventricle or atrioventricular canal. Reducing cardiac contraction resulted in opposite effects on the ventricular and atrial ECM, whereas increasing the heart rate affected the ventricular ECM but had no effect on the atrial ECM, together indicating that mechanical forces regulate the cECM in a chamber-specific manner. Among the ECM remodelers highly expressed during cardiac morphogenesis, we found one that was upregulated in non-contractile hearts, namely tissue inhibitor of matrix metalloproteinase 2 (timp2). Loss- and gain-of-function analyses of timp2 revealed its crucial role in cECM remodeling. Altogether, our results indicate that mechanical forces control cECM remodeling in part through timp2 downregulation.

Versican controlled by Lmx1b regulates hyaluronate density and hydration for semicircular canal morphogenesis

During inner ear semicircular canal morphogenesis in zebrafish, patterned canal-genesis zones express genes for extracellular matrix component synthesis. These include hyaluronan and the hyaluronan-binding chondroitin sulfate proteoglycan Versican, which are abundant in the matrices of many developing organs. Charged hyaluronate polymers play a key role in canal morphogenesis through osmotic swelling. However, the developmental factor(s) that control the synthesis of the matrix components and regulation of hyaluronate density and swelling are unknown. Here, we identify the transcription factor, Lmx1b, as a positive transcriptional regulator of hyaluronan, Versican, and chondroitin synthesis genes crucial for canal morphogenesis. We show that Versican regulates hyaluronan density through its protein core, whereas the charged chondroitin side chains contribute to the osmotic swelling of hyaluronate. Versican-tuned properties of hyaluronate matrices may be a broadly used mechanism in morphogenesis with important implications for understanding diseases where these matrices are impaired, and for hydrogel engineering for tissue regeneration.

Parenchymal cues define Vegfa-driven venous angiogenesis by activating a sprouting competent venous endothelial subtype

Formation of organo-typical vascular networks requires cross-talk between differentiating parenchymal cells and developing blood vessels. Here we identify a Vegfa driven venous sprouting process involving parenchymal to vein cross-talk regulating venous endothelial Vegfa signaling strength and subsequent formation of a specialized angiogenic cell, prefabricated with an intact lumen and pericyte coverage, termed L-Tip cell. L-Tip cell selection in the venous domain requires genetic interaction between vascular Aplnra and Kdrl in a subset of venous endothelial cells and exposure to parenchymal derived Vegfa and Apelin. Parenchymal Esm1 controls the spatial positioning of venous sprouting by fine-tuning local Vegfa availability. These findings may provide a conceptual framework for understanding how Vegfa generates organo-typical vascular networks based on the selection of competent endothelial cells, induced via spatio-temporal control of endothelial Kdrl signaling strength involving multiple parenchymal derived cues generated in a tissue dependent metabolic context.

Versican Promotes Cardiomyocyte Proliferation and Cardiac Repair

BACKGROUND

The adult mammalian heart is incapable of regeneration, whereas a transient regenerative capacity is maintained in the neonatal heart, primarily through the proliferation of preexisting cardiomyocytes. Neonatal heart regeneration after myocardial injury is accompanied by an expansion of cardiac fibroblasts and compositional changes in the extracellular matrix. Whether and how these changes influence cardiomyocyte proliferation and heart regeneration remains to be investigated.

METHODS

We used apical resection and myocardial infarction surgical models in neonatal and adult mice to investigate extracellular matrix components involved in heart regeneration after injury. Single-cell RNA sequencing and liquid chromatography-mass spectrometry analyses were used for versican identification. Cardiac fibroblast-specific Vcan deletion was achieved using the mouse strains Col1a2-2A-CreER and Vcanfl/fl. Molecular signaling pathways related to the effects of versican were assessed through Western blot, immunostaining, and quantitative reverse transcription polymerase chain reaction. Cardiac fibrosis and heart function were evaluated by Masson trichrome staining and echocardiography, respectively.

RESULTS

Versican, a cardiac fibroblast-derived extracellular matrix component, was upregulated after neonatal myocardial injury and promoted cardiomyocyte proliferation. Conditional knockout of Vcan in cardiac fibroblasts decreased cardiomyocyte proliferation and impaired neonatal heart regeneration. In adult mice, intramyocardial injection of versican after myocardial infarction enhanced cardiomyocyte proliferation, reduced fibrosis, and improved cardiac function. Furthermore, versican augmented the proliferation of human induced pluripotent stem cell-derived cardiomyocytes. Mechanistically, versican activated integrin β1 and downstream signaling molecules, including ERK1/2 and Akt, thereby promoting cardiomyocyte proliferation and cardiac repair.

CONCLUSIONS

Our study identifies versican as a cardiac fibroblast-derived pro-proliferative proteoglycan and clarifies the role of versican in promoting adult cardiac repair. These findings highlight its potential as a therapeutic factor for ischemic heart diseases.

Cardiac regeneration: Pre-existing cardiomyocyte as the hub of novel signaling pathway

In the mammalian heart, cardiomyocytes are forced to withdraw from the cell cycle shortly after birth, limiting the ability of the heart to regenerate and repair. The development of multimodal regulation of cardiac proliferation has verified that pre-existing cardiomyocyte proliferation is an essential driver of cardiac renewal. With the continuous development of genetic lineage tracking technology, it has been revealed that cell cycle activity produces polyploid cardiomyocytes during the embryonic, juvenile, and adult stages of cardiogenesis, but newly formed mononucleated diploid cardiomyocytes also elevated sporadically during myocardial infarction. It implied that adult cardiomyocytes have a weak regenerative capacity under the condition of ischemia injury, which offers hope for the clinical treatment of myocardial infarction. However, the regeneration frequency and source of cardiomyocytes are still low, and the mechanism of regulating cardiomyocyte proliferation remains further explained. It is noteworthy to explore what force triggers endogenous cardiomyocyte proliferation and heart regeneration. Here, we focused on summarizing the recent research progress of emerging endogenous key modulators and crosstalk with other signaling pathways and furnished valuable insights into the internal mechanism of heart regeneration. In addition, myocardial transcription factors, non-coding RNAs, cyclins, and cell cycle-dependent kinases are involved in the multimodal regulation of pre-existing cardiomyocyte proliferation. Ultimately, awakening the myocardial proliferation endogenous modulator and regeneration pathways may be the final battlefield for the regenerative therapy of cardiovascular diseases.

Molecular Pathways and Animal Models of Semilunar Valve and Aortic Arch Anomalies

The great arteries of the vertebrate carry blood from the heart to the systemic circulation and are derived from the pharyngeal arch arteries. In higher vertebrates, the pharyngeal arch arteries are a symmetrical series of blood vessels that rapidly remodel during development to become the asymmetric aortic arch arteries carrying oxygenated blood from the left ventricle via the outflow tract. At the base of the aorta, as well as the pulmonary trunk, are the semilunar valves. These valves each have three leaflets and prevent the backflow of blood into the heart. During development, the process of aortic arch and valve formation may go wrong, resulting in cardiovascular defects, and these may, at least in part, be caused by genetic mutations. In this chapter, we will review models harboring genetic mutations that result in cardiovascular defects affecting the great arteries and the semilunar valves.

Alpha-1 adrenergic signaling drives cardiac regeneration via extracellular matrix remodeling transcriptional program in zebrafish macrophages

The autonomic nervous system plays a pivotal role in cardiac repair. Here, we describe the mechanistic underpinning of adrenergic signaling in fibrotic and regenerative response of the heart to be dependent on immunomodulation. A pharmacological approach identified adrenergic receptor alpha-1 as a key regulator of macrophage phenotypic diversification following myocardial damage in zebrafish. Genetic manipulation and single-cell transcriptomics showed that the receptor signals activation of an "extracellular matrix remodeling" transcriptional program in a macrophage subset, which serves as a key regulator of matrix composition and turnover. Mechanistically, adrenergic receptor alpha-1-activated macrophages determine activation of collagen-12-expressing fibroblasts, a cellular determinant of cardiac regenerative niche, through midkine-mediated paracrine crosstalk, allowing lymphatic and blood vessel growth and cardiomyocyte proliferation at the lesion site. These findings identify the mechanism of adrenergic signaling in macrophage phenotypic and functional determination and highlight the potential of neural modulation for regulation of fibrosis and coordination of myocardial regenerative response.

Cardiomyocyte-fibroblast crosstalk in the postnatal heart

During the postnatal period in mammals, the heart undergoes significant remodeling in response to increased circulatory demands. In the days after birth, cardiac cells, including cardiomyocytes and fibroblasts, progressively lose embryonic characteristics concomitant with the loss of the heart’s ability to regenerate. Moreover, postnatal cardiomyocytes undergo binucleation and cell cycle arrest with induction of hypertrophic growth, while cardiac fibroblasts proliferate and produce extracellular matrix (ECM) that transitions from components that support cellular maturation to production of the mature fibrous skeleton of the heart. Recent studies have implicated interactions of cardiac fibroblasts and cardiomyocytes within the maturing ECM environment to promote heart maturation in the postnatal period. Here, we review the relationships of different cardiac cell types and the ECM as the heart undergoes both structural and functional changes during development. Recent advances in the field, particularly in several recently published transcriptomic datasets, have highlighted specific signaling mechanisms that underlie cellular maturation and demonstrated the biomechanical interdependence of cardiac fibroblast and cardiomyocyte maturation. There is increasing evidence that postnatal heart development in mammals is dependent on particular ECM components and that resulting changes in biomechanics influence cell maturation. These advances, in definition of cardiac fibroblast heterogeneity and function in relation to cardiomyocyte maturation and the extracellular environment provide, support for complex cell crosstalk in the postnatal heart with implications for heart regeneration and disease mechanisms.

Exposure to the herbicide fluridone induces cardiovascular toxicity in early developmental stages of zebrafish

Fluridone is a systemic herbicide used to control a range of invasive aquatic plants in irrigation systems, lake, and reservoirs. Since aquatic herbicides are more likely to have a hazardous impact on ecosystems than terrestrially applied herbicides, a risk assessment is needed to determine whether to expand or limit their use. The aim of this study was to investigate the developmental toxicity of fluridone using zebrafish. Diverse toxicological results were observed for the sub-lethal endpoints, including lack of hatching, reduced heartbeat and disturbed blood circulation through dysmorphic heart, and edema formation. Abnormal apoptosis was observed in the brain and yolk sac of fluridone-exposed larvae. A computational analysis was used to predict chemical properties in non-target organisms and revealed that fluridone was highly relevant in the cardiovascular system. Double transgenic zebrafish (fli1a:EGFP;cmlc2:dsRed) were used to evaluate the effects of fluridone on the cardiovascular system during embryonic development. Ectopic growth of sub-intestinal vessels and sprouting angiogenesis in the hindbrain region were highly inhibited. Additionally, essential genes involved in the VEGF signaling and heart development were differentially expressed in dose-dependent manner. Collectively, our toxicological findings in fluridone exposure highlight defects in the cardiovascular development causing embryonic lethality that could damage aquatic communities and natural ecosystems.

Delineation of the healthy rabbit heart by immunohistochemistry - A technical note

Heart failure poses a big health problem and may result from obesity, smoking, alcohol and/or growing age. Studying pathological heart tissue demands accurate histological and immunohistochemical stainings in animal models, including chromogenic and fluorescent approaches. Moreover, a reliable set of healthy heart stainings and labeling are required, in order to provide a reference for the pathological situation. Heart and brain tissue of a healthy rabbit were collected, and different histological key steps were compared, such as paraffin embedding after formalin fixation versus cryopreservation; an antigen retrieval (AR) step in processing paraffin sections versus the same procedure without AR; or a chromogenic with a fluorescent detection system, respectively. Using serial sections, we stained the same morphological structure with classic approaches (HE, Masson Goldner Trichrome (GT) and Elastica van Gieson (EL)) and with different markers, including collagen I, collagen III, fibronectin, α-SMA, protease-activated receptor-2 (PAR-2) which is an inflammation-related marker, and ki67 for proliferating cells. Differences between conditions were quantitatively assessed by measuring the color intensity. Generally, cryosections exhibited a more prominent signal intensity in immunohistochemically labeled sections than in paraffin sections, but the strong staining was slurry, which sometimes impeded proper identification of morphological structures, particularly at higher magnifications. In addition, the advantage of an AR step was observed when compared to the condition without AR, where signal intensities were significantly lower. Different stainings of the heart arteries and the myocardium revealed a clear distribution of extracellular matrix components, with prominent collagen III in the artery wall, but an absence of collagen III in the myocardium. Moreover, paraffin-embedded sections provided more distinct structures compared to cryosections after collagen III, ki67, fibronectin, and α-SMA labeling. As for the Purkinje cells that were depicted in the heart and the cerebellum (Purkinje neurons), we found GT staining most suitable to depict them in the heart, while HE as well as EL staining was ideal to depict Purkinje neurons in the cerebellum. In sum, we provide useful reference images with different stainings for researchers using the rabbit heart or brain model. Such images can help to decide which of the immunohistochemical protocols are valuable to reach a specific aim. Recommendations are given for the best visualization of the target structures and specific (immunohistochemical) staining.

Macrophage-mediated extracellular matrix remodeling after fat grafting in nude mice

Clinical unpredictability and variability following fat grafting remain non-negligible problems due to the unknown mechanism of grafted fat retention. The role of the extracellular matrix (ECM), which renders cells with structural and biochemical support, has been ignored. This study aimed to clarify the ECM remodeling process, related cellular events, and the spatiotemporal relationship between ECM remodeling and adipocyte survival and adipogenesis after fat grafting. Labeled Coleman fat by the matrix-tracing technique was grafted in nude mice. The ECM remodeling process and cellular events were assessed in vivo. The related cytokines were evaluated by qRT-PCR. An in vitro cell migration assay was performed to verify the chemotactic effect of M2-like macrophages on fibroblasts. The results demonstrated that in the periphery, most of the adipocytes of the graft survived or regenerated, and the graft-derived ECM was gradually replaced by the newly-formed ECM. In the central parts, most adipocytes in the grafts died shortly after, and a small part of the graft-derived and newly-formed ECM was expressed with irregular morphology. Adipose ECM remodeling is associated with increased infiltration of macrophages and fibroblasts, as well as up-regulated expression of cytokines in the adipose tissue. To sum up, our results describe the various preservation mode of fat grafts after transplantation and underscore the importance of macrophage-mediated ECM remodeling in graft preservation after fat grafting. The appreciation and manipulation of underlying mechanisms that are operant in this setting stand to explore new therapeutic approaches and improve clinical outcomes of fat grafting.

Targeting the cytoskeleton and extracellular matrix in cardiovascular disease drug discovery

INTRODUCTION

Currently, cardiovascular disease (CVD) drug discovery has focused primarily on addressing the inflammation and immunopathology aspects inherent to various CVD phenotypes such as cardiac fibrosis and coronary artery disease. However, recent findings suggest new biological pathways for cytoskeletal and extracellular matrix (ECM) regulation across diverse CVDs, such as the roles of matricellular proteins (e.g. tenascin-C) in regulating the cellular microenvironment. The success of anti-inflammatory drugs like colchicine, which targets microtubule polymerization, further suggests that the cardiac cytoskeleton and ECM provide prospective therapeutic opportunities.

AREAS COVERED

Potential therapeutic targets include proteins such as gelsolin and calponin 2, which play pivotal roles in plaque development. This review focuses on the dynamic role that the cytoskeleton and ECM play in CVD pathophysiology, highlighting how novel target discovery in cytoskeletal and ECM-related genes may enable therapeutics development to alter the regulation of cellular architecture in plaque formation and rupture, cardiac contractility, and other molecular mechanisms.

EXPERT OPINION

Further research into the cardiac cytoskeleton and its associated ECM proteins is an area ripe for novel target discovery. Furthermore, the structural connection between the cytoskeleton and the ECM provides an opportunity to evaluate both entities as sources of potential therapeutic targets for CVDs.

A Novel Missense Variant in Actin Binding Domain of MYH7 Is Associated With Left Ventricular Noncompaction

Cardiomyopathies are a group of common heart disorders that affect numerous people worldwide. Left ventricular non-compaction (LVNC) is a structural disorder of the ventricular wall, categorized as a type of cardiomyopathy that mostly caused by genetic disorders. Genetic variations are underlying causes of developmental deformation of the heart wall and the resultant contractile insufficiency. Here, we investigated a family with several affected members exhibiting LVNC phenotype. By whole-exome sequencing (WES) of three affected members, we identified a novel heterozygous missense variant (c.1963C>A:p.Leu655Met) in the gene encoding myosin heavy chain 7 (MYH7). This gene is evolutionary conserved among different organisms. We identified MYH7 as a highly enriched myosin, compared to other types of myosin heavy chains, in skeletal and cardiac muscles. Furthermore, MYH7 was among a few classes of MYH in mouse heart that highly expresses from early embryonic to adult stages. In silico predictions showed an altered actin-myosin binding, resulting in weaker binding energy that can cause LVNC. Moreover, CRISPR/Cas9 mediated MYH7 knockout in zebrafish caused impaired cardiovascular development. Altogether, these findings provide the first evidence for involvement of p.Leu655Met missense variant in the incidence of LVNC, most probably through actin-myosin binding defects during ventricular wall morphogenesis.

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

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

Therapeutic Acellular Scaffolds for Limiting Left Ventricular Remodelling-Current Status and Future Directions

Myocardial infarction (MI) is one of the leading causes of heart-related deaths worldwide. Following MI, the hypoxic microenvironment triggers apoptosis, disrupts the extracellular matrix and forms a non-functional scar that leads towards adverse left ventricular (LV) remodelling. If left untreated this eventually leads to heart failure. Besides extensive advancement in medical therapy, complete functional recovery is never accomplished, as the heart possesses limited regenerative ability. In recent decades, the focus has shifted towards tissue engineering and regenerative strategies that provide an attractive option to improve cardiac regeneration, limit adverse LV remodelling and restore function in an infarcted heart. Acellular scaffolds possess attractive features that have made them a promising therapeutic candidate. Their application in infarcted areas has been shown to improve LV remodelling and enhance functional recovery in post-MI hearts. This review will summarise the updates on acellular scaffolds developed and tested in pre-clinical and clinical scenarios in the past five years with a focus on their ability to overcome damage caused by MI. It will also describe how acellular scaffolds alone or in combination with biomolecules have been employed for MI treatment. A better understanding of acellular scaffolds potentialities may guide the development of customised and optimised therapeutic strategies for MI treatment.

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.

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

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

Whole-body clonal mapping identifies giant dominant clones in zebrafish skin epidermis

Skin expansion during development is predominantly driven by growth of basal epithelial cell (BEC)-derived clonal populations, which often display varied sizes and shapes. However, little is known about the causes of clonal heterogeneity and the maximum size to which a single clone can grow. Here, we created a zebrafish model, basebow, for capturing clonal growth behavior in the BEC population on a whole-body, centimeter scale. By tracking 222 BECs over the course of a 28-fold expansion of body surface area, we determined that most BECs survive and grow clonal populations with an average size of 0.013 mm2. An extensive survey of 742 sparsely labeled BECs further revealed that giant dominant clones occasionally arise on specific body regions, covering up to 0.6% of the surface area. Additionally, a growth-induced extracellular matrix component, Lamb1a, mediates clonal growth in a cell-autonomous manner. Altogether, our findings demonstrate how clonal heterogeneity and clonal dominance may emerge to enable post-embryonic growth of a vertebrate organ, highlighting key cellular mechanisms that may only become evident when visualizing single cell behavior at the whole-animal level.

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

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

The role of basement membranes in cardiac biology and disease

Basement membranes are highly specialised extracellular matrix structures that within the heart underlie endothelial cells and surround cardiomyocytes and vascular smooth muscle cells. They generate a dynamic and structurally supportive environment throughout cardiac development and maturation by providing physical anchorage to the underlying interstitium, structural support to the tissue, and by influencing cell behaviour and signalling. While this provides a strong link between basement membrane dysfunction and cardiac disease, the role of the basement membrane in cardiac biology remains under-researched and our understanding regarding the mechanistic interplay between basement membrane defects and their morphological and functional consequences remain important knowledge-gaps. In this review we bring together emerging understanding of basement membrane defects within the heart including in common cardiovascular pathologies such as contractile dysfunction and highlight some key questions that are now ready to be addressed.