Epicardial spindle orientation controls cell entry into the myocardium

MYRF is Essential in Mesothelial Cells to Promote Lung Development and Maturation

The mesothelium is a squamous monolayer that ensheathes internal organs, lines the body cavity, and the diaphragm. It serves as a protective barrier, coated in glycocalyx, and secretes lubricants to facilitate tissue movement. How the mesothelium forms is poorly understood. Here, we investigate Myrf , a transcription factor gene expressed in the mesothelium, because it carries variants in patients with Congenital Diaphragmatic Hernia (CDH), a disorder that affects the diaphragm, lung, and other organs. In mice, inactivation of Myrf early in organogenesis resulted in CDH and defective mesothelial specification, compromising its function as a signaling center for lung growth. Inactivation of Myrf later led to enhanced mesothelium differentiation into mesenchymal cell types through partial epithelial-to-mesenchymal transition (EMT), resulting in a unique accumulation of smooth muscle encasing the lung. In this role, MYRF functions in parallel with YAP/TAZ. Together, these findings establish MYRF as a critical regulator of mesothelium development, and when mutated, causes CDH.

The molecular mechanisms of cardiac development and related diseases

Cardiac development is a complex and intricate process involving numerous molecular signals and pathways. Researchers have explored cardiac development through a long journey, starting with early studies observing morphological changes and progressing to the exploration of molecular mechanisms using various molecular biology methods. Currently, advancements in stem cell technology and sequencing technology, such as the generation of human pluripotent stem cells and cardiac organoids, multi-omics sequencing, and artificial intelligence (AI) technology, have enabled researchers to understand the molecular mechanisms of cardiac development better. Many molecular signals regulate cardiac development, including various growth and transcription factors and signaling pathways, such as WNT signaling, retinoic acid signaling, and Notch signaling pathways. In addition, cilia, the extracellular matrix, epigenetic modifications, and hypoxia conditions also play important roles in cardiac development. These factors play crucial roles at one or even multiple stages of cardiac development. Recent studies have also identified roles for autophagy, metabolic transition, and macrophages in cardiac development. Deficiencies or abnormal expression of these factors can lead to various types of cardiac development abnormalities. Nowadays, congenital heart disease (CHD) management requires lifelong care, primarily involving surgical and pharmacological treatments. Advances in surgical techniques and the development of clinical genetic testing have enabled earlier diagnosis and treatment of CHD. However, these technologies still have significant limitations. The development of new technologies, such as sequencing and AI technologies, will help us better understand the molecular mechanisms of cardiac development and promote earlier prevention and treatment of CHD in the future.

β1 integrins regulate cellular behaviour and cardiomyocyte organization during ventricular wall formation

AIMS

The mechanisms regulating the cellular behaviour and cardiomyocyte organization during ventricular wall morphogenesis are poorly understood. Cardiomyocytes are surrounded by extracellular matrix (ECM) and interact with ECM via integrins. This study aims to determine whether and how β1 integrins regulate cardiomyocyte behaviour and organization during ventricular wall morphogenesis in the mouse.

METHODS AND RESULTS

We applied mRNA deep sequencing and immunostaining to determine the expression repertoires of α/β integrins and their ligands in the embryonic heart. Integrin β1 subunit (β1) and some of its ECM ligands are asymmetrically distributed and enriched in the luminal side of cardiomyocytes, and fibronectin surrounds cardiomyocytes, creating a network for them. Itgb1, which encodes the β1, was deleted via Nkx2.5Cre/+ to generate myocardial-specific Itgb1 knockout (B1KO) mice. B1KO hearts display an absence of a trabecular zone but a thicker compact zone. The levels of hyaluronic acid and versican, essential for trabecular initiation, were not significantly different between control and B1KO. Instead, fibronectin, a ligand of β1, was absent in the myocardium of B1KO hearts. Furthermore, B1KO cardiomyocytes display a random cellular orientation and fail to undergo perpendicular cell division, be organized properly, and establish the proper tissue architecture to form trabeculae. Mosaic clonal lineage tracing showed that Itgb1 regulates cardiomyocyte transmural migration and proliferation autonomously.

CONCLUSION

β1 is asymmetrically localized in the cardiomyocytes, and some of its ECM ligands are enriched along the luminal side of the myocardium, and fibronectin surrounds cardiomyocytes. β1 integrins are required for cardiomyocytes to attach to the ECM network. This engagement provides structural support for cardiomyocytes to maintain shape, undergo perpendicular division, and establish cellular organization. Deletion of Itgb1 leads to loss of β1 and fibronectin and prevents cardiomyocytes from engaging the ECM network, resulting in failure to establish tissue architecture to form trabeculae.

Cardiac Fibroblastic Niches in Homeostasis and Inflammation

Fibroblasts are essential for building and maintaining the structural integrity of all organs. Moreover, fibroblasts can acquire an inflammatory phenotype to accommodate immune cells in specific niches and to provide migration, differentiation, and growth factors. In the heart, balancing of fibroblast activity is critical for cardiac homeostasis and optimal organ function during inflammation. Fibroblasts sustain cardiac homeostasis by generating local niche environments that support housekeeping functions and by actively engaging in intercellular cross talk. During inflammatory perturbations, cardiac fibroblasts rapidly switch to an inflammatory state and actively communicate with infiltrating immune cells to orchestrate immune cell migration and activity. Here, we summarize the current knowledge on the molecular landscape of cardiac fibroblasts, focusing on their dual role in promoting tissue homeostasis and modulating immune cell-cardiomyocyte interaction. In addition, we discuss potential future avenues for manipulating cardiac fibroblast activity during myocardial inflammation.

miR-194-3p regulates epithelial-mesenchymal transition in embryonic epicardial cells via p120/β-catenin signaling

The epicardium is integral to cardiac development and facilitates endogenous heart regeneration and repair. While miR-194-3p is associated with cellular migration and invasion, its impact on epicardial cells remains uncharted. In this work we use gain-of-function and loss-of-function methodologies to investigate the function of miR-194-3p in cardiac development. We culture embryonic epicardial cells in vitro and subject them to transforming growth factor β (TGF-β) treatment to induce epithelial-mesenchymal transition (EMT) and monitor miR-194-3p expression. In addition, the effects of miR-194-3p mimics and inhibitors on epicardial cell development and changes in EMT are investigated. To validate the binding targets of miR-194-3p and its ability to recover the target gene-phenotype, we produce a mutant vector p120-catenin-3'UTR-MUT. In epicardial cells, TGF-β-induced EMT results in a notable overexpression of miR-194-3p. The administration of miR-194-3p mimics promotes EMT, which is correlated with elevated levels of mesenchymal markers. Conversely, miR-194-3p inhibitor attenuates EMT. Further investigations reveal a negative correlation between miR-194-3p and p120-catenin, which influences β-catenin level in the cell adhesion pathway. The suppression of EMT caused by the miR-194-3p inhibitor is balanced by silencing of p120-catenin. In conclusion, miR-194-3p directly targets p120-catenin and modulates its expression, which in turn alters β-catenin expression, critically influencing the EMT process in the embryonic epicardial cells via the cell adhesion mechanism.

Septin 7 interacts with Numb to preserve sarcomere structural organization and muscle contractile function

Here, we investigated the mechanisms by which aging-related reductions of the levels of Numb in skeletal muscle fibers contribute to loss of muscle strength and power, two critical features of sarcopenia. Numb is an adaptor protein best known for its critical roles in development, including asymmetric cell division, cell-type specification, and termination of intracellular signaling. Numb expression is reduced in old humans and mice. We previously showed that, in mouse skeletal muscle fibers, Numb is localized to sarcomeres where it is concentrated near triads; conditional inactivation of Numb and a closely related protein Numb-like (Numbl) in mouse myofibers caused weakness, disorganization of sarcomeres, and smaller mitochondria with impaired function. Here, we found that a single knockout of Numb in myofibers causes reduction in tetanic force comparable to a double Numb, Numbl knockout. We found by proteomics analysis of protein complexes isolated from C2C12 myotubes by immunoprecipitation using antibodies against Numb that Septin 7 is a potential Numb-binding partner. Septin 7 is a member of the family of GTP-binding proteins that organize into filaments, sheets, and rings, and is considered part of the cytoskeleton. Immunofluorescence evaluation revealed a partial overlap of staining for Numb and Septin 7 in myofibers. Conditional, inducible knockouts of Numb led to disorganization of Septin 7 staining in myofibers. These findings indicate that Septin 7 is a Numb-binding partner and suggest that interactions between Numb and Septin 7 are critical for structural organization of the sarcomere and muscle contractile function.

Epigenetic determinants and non-myocardial signaling pathways contributing to heart growth and regeneration

Congenital heart disease is the most common birth defect worldwide. Defective cardiac myogenesis is either a major presentation or associated with many types of congenital heart disease. Non-myocardial tissues, including endocardium and epicardium, function as a supporting hub for myocardial growth and maturation during heart development. Recent research findings suggest an emerging role of epigenetics in nonmyocytes supporting myocardial development. Understanding how growth signaling pathways in non-myocardial tissues are regulated by epigenetic factors will likely identify new disease mechanisms for congenital heart diseases and shed lights for novel therapeutic strategies for heart regeneration.

Epicardium and Coronary Vessels

Congenital anomalies and acquired diseases of the coronary blood vessels are of great clinical relevance. The early diagnosis of these conditions remains, however, challenging. In order to improve our knowledge of these ailments, progress has to be achieved in the research of the molecular and cellular mechanisms that control development of the coronary vascular bed. The aim of this chapter is to provide a succint account of the key elements of coronary blood vessel development, especially in the context of the role played by the epicardium and epicardial cellular derivatives. We will discuss the importance of the epicardium in coronary blood vessel morphogenesis, from the contribution of the epicardially derived mesenchyme to these blood vessels to its role as an instructive signaling center, attempting to relate these concepts to the origin of coronary disease.

Septin 7 Interacts With Numb To Preserve Sarcomere Structural Organization And Muscle Contractile Function

Here, we investigated mechanisms by which aging-related reductions of the levels of Numb in skeletal muscle fibers contribute to loss of muscle strength and power, two critical features of sarcopenia. Numb is an adaptor protein best known for its critical roles in development including asymmetric cell division, cell-type specification and termination of intracellular signaling. Numb expression is reduced in old humans and mice. We previously showed that, in mouse skeletal muscle fibers, Numb is localized to sarcomeres where it is concentrated near triads; conditional inactivation of Numb and a closely related protein Numb-like (NumbL) in mouse myofibers caused weakness, disorganization of sarcomeres and smaller mitochondria with impaired function. Here, we found that a single knockout of Numb in myofibers causes reduction in tetanic force comparable to a double Numb, NumbL knockout. We found by proteomics analysis of protein complexes isolated from C2C12 myotubes by immunoprecipitation using antibodies against Numb, that Septin 7 is a potential Numb binding partner. Septin 7 is a member of the family of GTP-binding proteins that organize into filaments, sheets and rings, and is considered part of the cytoskeleton. Immunofluorescence evaluation revealed a partial overlap of staining for Numb and Septin 7 in myofibers. Conditional, inducible knockouts of Numb led to disorganization of Septin 7 staining in myofibers. These findings indicate that Septin 7 is a Numb binding partner and suggest that interactions between Numb and Septin 7 are critical for structural organization of the sarcomere and muscle contractile function.

Hypoplastic Left Heart Syndrome: Signaling & Molecular Perspectives, and the Road Ahead

Hypoplastic left heart syndrome (HLHS) is a lethal congenital heart disease (CHD) affecting 8-25 per 100,000 neonates globally. Clinical interventions, primarily surgical, have improved the life expectancy of the affected subjects substantially over the years. However, the etiological basis of HLHS remains fundamentally unclear to this day. Based upon the existing paradigm of studies, HLHS exhibits a multifactorial mode of etiology mediated by a complicated course of genetic and signaling cascade. This review presents a detailed outline of the HLHS phenotype, the prenatal and postnatal risks, and the signaling and molecular mechanisms driving HLHS pathogenesis. The review discusses the potential limitations and future perspectives of studies that can be undertaken to address the existing scientific gap. Mechanistic studies to explain HLHS etiology will potentially elucidate novel druggable targets and empower the development of therapeutic regimens against HLHS in the future.

β1 integrins regulate cellular behaviors and cardiomyocyte organization during ventricular wall formation

Aims

The mechanisms regulating the cellular behavior and cardiomyocyte organization during ventricular wall morphogenesis are poorly understood. Cardiomyocytes are surrounded by extracellular matrix (ECM) and interact with ECM via integrins. This study aims to determine whether and how β1 integrins regulate cardiomyocyte behavior and organization during ventricular wall morphogenesis in the mouse.

Methods and Results

We applied mRNA deep sequencing and immunostaining to determine the expression repertoires of α/β integrins and their ligands in the embryonic heart. Integrin β1 subunit (β1) and some of its ECM ligands are asymmetrically distributed and enriched in the luminal side of cardiomyocytes, while fibronectin surrounds cardiomyocytes, creating a network for them. Itgb1 , which encodes the β1 integrin subunit, was deleted via Nkx2.5 Cre/+ to generate myocardial-specific Itgb1 knockout (B1KO) mice. B1KO hearts display an absence of trabecular zone but a thicker compact zone. The abundances of hyaluronic acid and versican are not significantly different. Instead, fibronectin, a ligand of β1, was absent in B1KO. We examined cellular behaviors and organization via various tools. B1KO cardiomyocytes display a random cellular orientation and fail to undergo perpendicular cell division, be organized properly, and establish the proper tissue architecture to form trabeculae. The reduction of Notch1 activation was not the cause of the abnormal cellular organization in B1KO hearts. Mosaic clonal lineage tracing shows that Itgb1 regulates cardiomyocyte transmural migration and proliferation autonomously.

Conclusions

β1 is asymmetrically localized in the cardiomyocytes, and its ECM ligands are enriched in the luminal side of the myocardium and surrounding cardiomyocytes. β1 integrins are required for cardiomyocytes to attach to the ECM network. This engagement provides structural support for cardiomyocytes to maintain shape, undergo perpendicular division, and establish cellular organization. Deletion of Itgb1 , leading to ablation of β1 integrins, causes the dissociation of cardiomyocytes from the ECM network and failure to establish tissue architecture to form trabeculae.

EZH2 controls epicardial cell migration during heart development

Enhancer of zeste homolog 2 (EZH2) is an important transcriptional regulator in development that catalyzes H3K27me3. The role of EZH2 in epicardial development is still unknown. In this study, we show that EZH2 is expressed in epicardial cells during both human and mouse heart development. Ezh2 epicardial deletion resulted in impaired epicardial cell migration, myocardial hypoplasia, and defective coronary plexus development, leading to embryonic lethality. By using RNA sequencing, we identified that EZH2 controls the transcription of tissue inhibitor of metalloproteinase 3 (TIMP3) in epicardial cells during heart development. Loss-of-function studies revealed that EZH2 promotes epicardial cell migration by suppressing TIMP3 expression. We also found that epicardial Ezh2 deficiency-induced TIMP3 up-regulation leads to extracellular matrix reconstruction in the embryonic myocardium by mass spectrometry. In conclusion, our results demonstrate that EZH2 is required for epicardial cell migration because it blocks Timp3 transcription, which is vital for heart development. Our study provides new insight into the function of EZH2 in cell migration and epicardial development.

Regulation of Epicardial Cell Fate during Cardiac Development and Disease: An Overview

The epicardium is the outermost cell layer in the vertebrate heart that originates during development from mesothelial precursors located in the proepicardium and septum transversum. The epicardial layer plays a key role during cardiogenesis since a subset of epicardial-derived cells (EPDCs) undergo an epithelial–mesenchymal transition (EMT); migrate into the myocardium; and differentiate into distinct cell types, such as coronary vascular smooth muscle cells, cardiac fibroblasts, endothelial cells, and presumably a subpopulation of cardiomyocytes, thus contributing to complete heart formation. Furthermore, the epicardium is a source of paracrine factors that support cardiac growth at the last stages of cardiogenesis. Although several lineage trace studies have provided some evidence about epicardial cell fate determination, the molecular mechanisms underlying epicardial cell heterogeneity remain not fully understood. Interestingly, seminal works during the last decade have pointed out that the adult epicardium is reactivated after heart damage, re-expressing some embryonic genes and contributing to cardiac remodeling. Therefore, the epicardium has been proposed as a potential target in the treatment of cardiovascular disease. In this review, we summarize the previous knowledge regarding the regulation of epicardial cell contribution during development and the control of epicardial reactivation in cardiac repair after damage.

CDH18 is a fetal epicardial biomarker regulating differentiation towards vascular smooth muscle cells

The epicardium is a mesothelial layer covering the myocardium serving as a progenitor source during cardiac development. The epicardium reactivates upon cardiac injury supporting cardiac repair and regeneration. Fine-tuned balanced signaling regulates cell plasticity and cell-fate decisions of epicardial-derived cells (EPCDs) via epicardial-to-mesenchymal transition (EMT). However, powerful tools to investigate epicardial function, including markers with pivotal roles in developmental signaling, are still lacking. Here, we recapitulated epicardiogenesis using human induced pluripotent stem cells (hiPSCs) and identified type II classical cadherin CDH18 as a biomarker defining lineage specification in human active epicardium. The loss of CDH18 led to the onset of EMT and specific differentiation towards cardiac smooth muscle cells. Furthermore, GATA4 regulated epicardial CDH18 expression. These results highlight the importance of tracing CDH18 expression in hiPSC-derived epicardial cells, providing a model for investigating epicardial function in human development and disease and enabling new possibilities for regenerative medicine.

Spindle positioning and its impact on vertebrate tissue architecture and cell fate

In multicellular systems, oriented cell divisions are essential for morphogenesis and homeostasis as they determine the position of daughter cells within the tissue and also, in many cases, their fate. Early studies in invertebrates led to the identification of conserved core mechanisms of mitotic spindle positioning centred on the Gαi-LGN-NuMA-dynein complex. In recent years, much has been learnt about the way this complex functions in vertebrate cells. In particular, studies addressed how the Gαi-LGN-NuMA-dynein complex dynamically crosstalks with astral microtubules and the actin cytoskeleton, and how it is regulated to orient the spindle according to cellular and tissue-wide cues. We have also begun to understand how dynein motors and actin regulators interact with mechanosensitive adhesion molecules sensing extracellular mechanical stimuli, such as cadherins and integrins, and with signalling pathways so as to respond to extracellular cues instructing the orientation of the division axis in vivo. In this Review, with the focus on epithelial tissues, we discuss the molecular mechanisms of mitotic spindle orientation in vertebrate cells, and how this machinery is regulated by epithelial cues and extracellular signals to maintain tissue cohesiveness during mitosis. We also outline recent knowledge of how spindle orientation impacts tissue architecture in epithelia and its emerging links to the regulation of cell fate decisions. Finally, we describe how defective spindle orientation can be corrected or its effects eliminated in tissues under physiological conditions, and the pathological implications associated with spindle misorientation.

The Spatiotemporal Expression of Notch1 and Numb and Their Functional Interaction during Cardiac Morphogenesis

Numb family proteins (NFPs), including Numb and Numblike (Numbl), are commonly known for their role as cell fate determinants for multiple types of progenitor cells, mainly due to their function as Notch inhibitors. Previous studies have shown that myocardial NFP double knockout (MDKO) hearts display an up-regulated Notch activation and various defects in cardiac progenitor cell differentiation and cardiac morphogenesis. Whether enhanced Notch activation causes these defects in MDKO is not fully clear. To answer the question, we examined the spatiotemporal patterns of Notch1 expression, Notch activation, and Numb expression in the murine embryonic hearts using multiple approaches including RNAScope, and Numb and Notch reporter mouse lines. To further interrogate the interaction between NFPs and Notch signaling activation, we deleted both Notch1 or RBPJk alleles in the MDKO. We examined and compared the phenotypes of Notch1 knockout, NFPs double knockout, Notch1; Numb; Numbl and RBPJk; Numb; Numbl triple knockouts. Our study showed that Notch1 is expressed and activated in the myocardium at several stages, and Numb is enriched in the epicardium and did not show the asymmetric distribution in the myocardium. Cardiac-specific Notch1 deletion causes multiple structural defects and embryonic lethality. Notch1 or RBPJk deletion in MDKO did not rescue the structural defects in the MDKO but partially rescued the defects of cardiac progenitor cell differentiation, cardiomyocyte proliferation, and trabecular morphogenesis. Our study concludes that NFPs regulate progenitor cell differentiation, cardiomyocyte proliferation, and trabecular morphogenesis partially through Notch1 and play more roles than inhibiting Notch1 signaling during cardiac morphogenesis.

Implications of the Wilms' Tumor Suppressor Wt1 in Cardiomyocyte Differentiation

The Wilms’ tumor suppressor Wt1 is involved in multiple developmental processes and adult tissue homeostasis. The first phenotypes recognized in Wt1 knockout mice were developmental cardiac and kidney defects. Wt1 expression in the heart has been described in epicardial, endothelial, smooth muscle cells, and fibroblasts. Expression of Wt1 in cardiomyocytes has been suggested but remained a controversial issue, as well as the role of Wt1 in cardiomyocyte development and regeneration after injury. We determined cardiac Wt1 expression during embryonic development, in the adult, and after cardiac injury by quantitative RT-PCR and immunohistochemistry. As in vitro model, phenotypic cardiomyocyte differentiation, i.e., the appearance of rhythmically beating clones from mouse embryonic stem cells (mESCs) and associated changes in gene expression were analyzed. We detected Wt1 in cardiomyocytes from embryonic day (E10.5), the first time point investigated, until adult age. Cardiac Wt1 mRNA levels decreased during embryonic development. In the adult, Wt1 was reactivated in cardiomyocytes 48 h and 3 weeks following myocardial infarction. Wt1 mRNA levels were increased in differentiating mESCs. Overexpression of Wt1(-KTS) and Wt1(+KTS) isoforms in ES cells reduced the fraction of phenotypically cardiomyocyte differentiated clones, which was preceded by a temporary increase in c-kit expression in Wt1(-KTS) transfected ES cell clones and induction of some cardiomyocyte markers. Taken together, Wt1 shows a dynamic expression pattern during cardiomyocyte differentiation and overexpression in ES cells reduces their phenotypical cardiomyocyte differentiation.

A Cascade of 2.5D CNN and Bidirectional CLSTM Network for Mitotic Cell Detection in 4D Microscopy Image

Mitosis detection is one of the challenging steps in biomedical imaging research, which can be used to observe the cell behavior. Most of the already existing methods that are applied in detecting mitosis usually contain many nonmitotic events (normal cell and background) in the result (false positives, FPs). In order to address such a problem, in this study, we propose to apply 2.5-dimensional (2.5D) networks called CasDetNet_CLSTM, which can accurately detect mitotic events in 4D microscopic images. This CasDetNet_CLSTM involves a 2.5D faster region-based convolutional neural network (Faster R-CNN) as the first network, and a convolutional long short-term memory (CLSTM) network as the second network. The first network is used to select candidate cells using the information from nearby slices, whereas the second network uses temporal information to eliminate FPs and refine the result of the first network. Our experiment shows that the precision and recall of our networks yield better results than those of other state-of-the-art methods.

Accurate and fast mitotic detection using an anchor-free method based on full-scale connection with recurrent deep layer aggregation in 4D microscopy images

BACKGROUND

To effectively detect and investigate various cell-related diseases, it is essential to understand cell behaviour. The ability to detection mitotic cells is a fundamental step in diagnosing cell-related diseases. Convolutional neural networks (CNNs) have been successfully applied to object detection tasks, however, when applied to mitotic cell detection, most existing methods generate high false-positive rates due to the complex characteristics that differentiate normal cells from mitotic cells. Cell size and orientation variations in each stage make detecting mitotic cells difficult in 2D approaches. Therefore, effective extraction of the spatial and temporal features from mitotic data is an important and challenging task. The computational time required for detection is another major concern for mitotic detection in 4D microscopic images.

RESULTS

In this paper, we propose a backbone feature extraction network named full scale connected recurrent deep layer aggregation (RDLA++) for anchor-free mitotic detection. We utilize a 2.5D method that includes 3D spatial information extracted from several 2D images from neighbouring slices that form a multi-stream input.

CONCLUSIONS

Our proposed technique addresses the scale variation problem and can efficiently extract spatial and temporal features from 4D microscopic images, resulting in improved detection accuracy and reduced computation time compared with those of other state-of-the-art methods.

Recapturing embryonic potential in the adult epicardium: Prospects for cardiac repair

Research into potential targets for cardiac repair encompasses recognition of tissue‐resident cells with intrinsic regenerative properties. The adult vertebrate heart is covered by mesothelium, named the epicardium, which becomes active in response to injury and contributes to repair, albeit suboptimally. Motivation to manipulate the epicardium for treatment of myocardial infarction is deeply rooted in its central role in cardiac formation and vasculogenesis during development. Moreover, the epicardium is vital to cardiac muscle regeneration in lower vertebrate and neonatal mammalian‐injured hearts. In this review, we discuss our current understanding of the biology of the mammalian epicardium in development and injury. Considering present challenges in the field, we further contemplate prospects for reinstating full embryonic potential in the adult epicardium to facilitate cardiac regeneration.

Analysis of wt1a reporter line expression levels during proepicardium formation in the zebrafish

The epicardium is the outer mesothelial layer of the heart. It covers the myocardium and plays important roles in both heart development and regeneration. It is derived from the proepicardium (PE), groups of cells that emerges at early developmental stages from the dorsal pericardial layer (DP) close to the atrio-ventricular canal and the venous pole of the heart-tube. In zebrafish, PE cells extrude apically into the pericardial cavity as a consequence of DP tissue constriction, a process that is dependent on Bmp pathway signaling. Expression of the transcription factor Wilms tumor-1, Wt1, which is a leader of important morphogenetic events such as apoptosis regulation or epithelial-mesenchymal cell transition, is also necessary during PE formation. In this study, we used the zebrafish model to compare intensity level of the wt1a reporter line epi:GFP in PE and its original tissue, the DP. We found that GFP is present at higher intensity level in the PE tissue, and differentially wt1 expression at pericardial tissues could be involved in the PE formation process. Our results reveal that bmp2b overexpression leads to enhanced GFP level both in DP and in PE tissues.

Epicardial TGFβ and BMP Signaling in Cardiac Regeneration: What Lesson Can We Learn from the Developing Heart?

The epicardium, the outer layer of the heart, has been of interest in cardiac research due to its vital role in the developing and diseased heart. During development, epicardial cells are active and supply cells and paracrine cues to the myocardium. In the injured adult heart, the epicardium is re-activated and recapitulates embryonic behavior that is essential for a proper repair response. Two indispensable processes for epicardial contribution to heart tissue formation are epithelial to mesenchymal transition (EMT), and tissue invasion. One of the key groups of cytokines regulating both EMT and invasion is the transforming growth factor β (TGFβ) family, including TGFβ and Bone Morphogenetic Protein (BMP). Abundant research has been performed to understand the role of TGFβ family signaling in the developing epicardium. However, less is known about signaling in the adult epicardium. This review provides an overview of the current knowledge on the role of TGFβ in epicardial behavior both in the development and in the repair of the heart. We aim to describe the presence of involved ligands and receptors to establish if and when signaling can occur. Finally, we discuss potential targets to improve the epicardial contribution to cardiac repair as a starting point for future investigation.

Epicardium in Heart Development

The epicardium, the outermost tissue layer that envelops all vertebrate hearts, plays a crucial role in cardiac development and regeneration and has been implicated in potential strategies for cardiac repair. The heterogenous cell population that composes the epicardium originates primarily from a transient embryonic cell cluster known as the proepicardial organ (PE). Characterized by its high cellular plasticity, the epicardium contributes to both heart development and regeneration in two critical ways: as a source of progenitor cells and as a critical signaling hub. Despite this knowledge, there are many unanswered questions in the field of epicardial biology, the resolution of which will advance the understanding of cardiac development and repair. We review current knowledge in cross-species epicardial involvement, specifically in relation to lineage specification and differentiation during cardiac development.

The Role of the Epicardium During Heart Development and Repair

The heart is lined by a single layer of mesothelial cells called the epicardium that provides important cellular contributions for embryonic heart formation. The epicardium harbors a population of progenitor cells that undergo epithelial-to-mesenchymal transition displaying characteristic conversion of planar epithelial cells into multipolar and invasive mesenchymal cells before differentiating into nonmyocyte cardiac lineages, such as vascular smooth muscle cells, pericytes, and fibroblasts. The epicardium is also a source of paracrine cues that are essential for fetal cardiac growth, coronary vessel patterning, and regenerative heart repair. Although the epicardium becomes dormant after birth, cardiac injury reactivates developmental gene programs that stimulate epithelial-to-mesenchymal transition; however, it is not clear how the epicardium contributes to disease progression or repair in the adult. In this review, we will summarize the molecular mechanisms that control epicardium-derived progenitor cell migration, and the functional contributions of the epicardium to heart formation and cardiomyopathy. Future perspectives will be presented to highlight emerging therapeutic strategies aimed at harnessing the regenerative potential of the fetal epicardium for cardiac repair.

Role of carotenoids and retinoids during heart development

The nutritional requirements of the developing embryo are complex. In the case of dietary vitamin A (retinol, retinyl esters and provitamin A carotenoids), maternal derived nutrients serve as precursors to signaling molecules such as retinoic acid, which is required for embryonic patterning and organogenesis. Despite variations in the composition and levels of maternal vitamin A, embryonic tissues need to generate a precise amount of retinoic acid to avoid congenital malformations. Here, we summarize recent findings regarding the role and metabolism of vitamin A during heart development and we survey the association of genes known to affect retinoid metabolism or signaling with various inherited disorders. A better understanding of the roles of vitamin A in the heart and of the factors that affect retinoid metabolism and signaling can help design strategies to meet nutritional needs and to prevent birth defects and disorders associated with altered retinoid metabolism. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

Epicardial cell shape and maturation are regulated by Wt1 via transcriptional control of Bmp4

The epicardium plays a crucial role in embryonic heart development and adult heart repair; however, the molecular events underlying its maturation remain unknown. Wt1, one of the main markers of the embryonic epicardium, is essential for epicardial development and function. Here, we analyse the transcriptomic profile of epicardial-enriched cells at different stages of development and from control and epicardial-specific Wt1 knockout (Wt1KO) mice. Transcriptomic and cell morphology analyses of epicardial cells from epicardial-specific Wt1KO mice revealed a defect in the maturation process of the mutant epicardium, including sustained upregulation of Bmp4 expression and the inability of mutant epicardial cells to transition into a mature squamous phenotype. We identified Bmp4 as a transcriptional target of Wt1, thus providing a molecular basis for the retention of the cuboidal cell shape observed in the Wt1KO epicardium. Accordingly, inhibition of the Bmp4 signalling pathway both ex vivo and in vivo rescued the cuboidal phenotype of the mutant epicardium. Our findings indicate the importance of the cuboidal-to-squamous transition in epicardial maturation, a process regulated by Wt1.

Cardiomyocyte orientation modulated by the Numb family proteins-N-cadherin axis is essential for ventricular wall morphogenesis

The roles of cellular orientation during trabecular and ventricular wall morphogenesis are unknown, and so are the underlying mechanisms that regulate cellular orientation. Myocardial-specific Numb and Numblike double-knockout (MDKO) hearts display a variety of defects, including in cellular orientation, patterns of mitotic spindle orientation, trabeculation, and ventricular compaction. Furthermore, Numb- and Numblike-null cardiomyocytes exhibit cellular behaviors distinct from those of control cells during trabecular morphogenesis based on single-cell lineage tracing. We investigated how Numb regulates cellular orientation and behaviors and determined that N-cadherin levels and membrane localization are reduced in MDKO hearts. To determine how Numb regulates N-cadherin membrane localization, we generated an mCherry:Numb knockin line and found that Numb localized to diverse endocytic organelles but mainly to the recycling endosome. Consistent with this localization, cardiomyocytes in MDKO did not display defects in N-cadherin internalization but rather in postendocytic recycling to the plasma membrane. Furthermore, N-cadherin overexpression via a mosaic model partially rescued the defects in cellular orientation and trabeculation of MDKO hearts. Our study unravels a phenomenon that cardiomyocytes display spatiotemporal cellular orientation during ventricular wall morphogenesis, and its disruption leads to abnormal trabecular and ventricular wall morphogenesis. Furthermore, we established a mechanism by which Numb modulates cellular orientation and consequently trabecular and ventricular wall morphogenesis by regulating N-cadherin recycling to the plasma membrane.

Actin dynamics and the Bmp pathway drive apical extrusion of proepicardial cells

The epicardium, the outer mesothelial layer enclosing the myocardium, plays key roles in heart development and regeneration. During embryogenesis, the epicardium arises from the proepicardium (PE), a cell cluster that appears in the dorsal pericardium (DP) close to the venous pole of the heart. Little is known about how the PE emerges from the pericardial mesothelium. Using a zebrafish model and a combination of genetic tools, pharmacological agents and quantitative in vivo imaging, we reveal that a coordinated collective movement of DP cells drives PE formation. We found that Bmp signaling and the actomyosin cytoskeleton promote constriction of the DP, which enables PE cells to extrude apically. We provide evidence that cell extrusion, which has been described in the elimination of unfit cells from epithelia and the emergence of hematopoietic stem cells, is also a mechanism for PE cells to exit an organized mesothelium and fulfil their developmental fate to form a new tissue layer, the epicardium.

Summary: Proepicardial cells emerge from the pericardial mesothelium through apical extrusion, a process that depends on BMP signaling and actomyosin rearrangements.

Decreased expression of cell polarity protein Scribble correlated with altered subcellular localization of the Crumbs homologue 3 protein in human adenomyotic endometrial cells

AIM

Previous studies have revealed that loss of cell apical-basal polarity contributed to the early stages of tumorigenesis. Adenomyosis involves a down-growth and aberrant implantation of the endometrial basalis into the myometrium. This study discovered aberrant expression of polarity protein Scribble (Scrib) and Crumbs homologue 3 protein (CRB3) in epithelial cells of diffuse adenomyosis.

METHODS

This was a case-controlled study, including 39 patients with histologic evidence of adenomyosis, and 48 patients with carcinoma in situ of the uterine cervix without adenomyosis or endometriosis as control. Adenomyotic foci, eutopic endometrium of adenomyotic patients as well as normal endometrium were collected. Reverse Transcription Polymerase Chain Reaction (RT-PCR), Immunoreactivity, confocal microscopy and immune electron microscopy were conducted to evaluate Scribble expression and localization of Scribble and CRB3.

RESULTS

Scrib was screen out as an abnormally expressed polarity protein in adenomyotic eutopic endometrium (ADM-EU) at messenger RNA (mRNA) level. The ADM-EU and adenomyotic ectopic endometrium showed a significantly decreased expression of Scrib compared with normal endometrium (all P-values <0.05). Scrib decreased significantly in ADM-EU than normal endometrium only in patients at proliferative phase and with severe dysmenorrhea (P-values <0.01, P-values <0.001 respectively). In ADM-EU, Scrib expression significantly lowered in patients with severe dysmenorrhea than mild dysmenorrhea (P-values <0.05). Aberrant redistribution of CRB3 from apical to basal lateral membrane portion was also detected in experiments by confocal microscopy immune electron microscopy (all P-values <0.01).

CONCLUSION

Basolateral polarity protein Scrib was found decreased significantly in endometrial cells of adenomyosis at mRNA and protein level, compared with normal endometrium. Menstrual phase and severity of dysmenorrhea has an impact on Scrib expression. Scrib decrease was accompanied by aberrant redistribution of CRB3 from apical to basal lateral membrane portion.

Altered haemodynamics causes aberrations in the epicardium

During embryo development, the heart is the first functioning organ. Although quiescent in the adult, the epicardium is essential during development to form a normal four‐chambered heart. Epicardial‐derived cells contribute to the heart as it develops with fibroblasts and vascular smooth muscle cells. Previous studies have shown that a heartbeat is required for epicardium formation, but no study to our knowledge has shown the effects of haemodynamic changes on the epicardium. Since the aetiologies of many congenital heart defects are unknown, we suggest that an alteration in the heart's haemodynamics might provide an explanatory basis for some of them. To change the heart's haemodynamics, outflow tract (OFT) banding using a double overhang knot was performed on HH21 chick embryos, with harvesting at different developmental stages. The epicardium of the heart was phenotypically and functionally characterised using a range of techniques. Upon alteration of haemodynamics, the epicardium exhibited abnormal morphology at HH29, even though migration of epicardial cells along the surface of the heart was found to be normal between HH24 and HH28. The abnormal epicardial phenotype was exacerbated at HH35 with severe changes in the structure of the extracellular matrix (ECM). A number of genes tied to ECM production were also differentially expressed in HH29 OFT‐banded hearts, including DDR2 and collagen XII. At HH35, the differential expression of these genes was even greater with additional downregulation of collagen I and TCF21. In this study, the epicardium was found to be severely impacted by altered haemodynamics upon OFT banding. The increased volume of the epicardium at HH29, upon OFT‐banding, and the expression changes of ECM markers were the first indicative signs of aberrations in epicardial architecture; by HH35, the phenotype had progressed. The decrease in epicardial thickness at HH35 suggests an increase in tension, with a force acting perpendicular to the surface of the epicardium. Although the developing epicardium and the blood flowing through the heart are separated by the endocardium and myocardium, the data presented here demonstrate that altering the blood flow affects the structure and molecular expression of the epicardial layer. Due to the intrinsic role the epicardium in cardiogenesis, defects in epicardial formation could have a role in the formation of a wide range of congenital heart defects.

Covering and Re-Covering the Heart: Development and Regeneration of the Epicardium

The epicardium, a mesothelial layer that envelops vertebrate hearts, has become a therapeutic target in cardiac repair strategies because of its vital role in heart development and cardiac injury response. Epicardial cells serve as a progenitor cell source and signaling center during both heart development and regeneration. The importance of the epicardium in cardiac repair strategies has been reemphasized by recent progress regarding its requirement for heart regeneration in zebrafish, and by the ability of patches with epicardial factors to restore cardiac function following myocardial infarction in mammals. The live surveillance of epicardial development and regeneration using zebrafish has provided new insights into this topic. In this review, we provide updated knowledge about epicardial development and regeneration.

Mechanisms of Trabecular Formation and Specification During Cardiogenesis

Trabecular morphogenesis is a key morphologic event during cardiogenesis and contributes to the formation of a competent ventricular wall. Lack of trabeculation results in embryonic lethality. The trabecular morphogenesis is a multistep process that includes, but is not limited to, trabecular initiation, proliferation/growth, specification, and compaction. Although a number of signaling molecules have been implicated in regulating trabeculation, the cellular processes underlying mammalian trabecular formation are not fully understood. Recent works show that the myocardium displays polarity, and oriented cell division (OCD) and directional migration of the cardiomyocytes in the monolayer myocardium are required for trabecular initiation and formation. Furthermore, perpendicular OCD is an extrinsic asymmetric cell division that contributes to trabecular specification, and is a mechanism that causes the trabecular cardiomyocytes to be distinct from the cardiomyocytes in compact zone. Once the coronary vasculature system starts to function in the embryonic heart, the trabeculae will coalesce with the compact zone to thicken the heart wall, and abnormal compaction will lead to left ventricular non-compaction (LVNC) and heart failure. There are many reviews about compaction and LVNC. In this review, we will focus on the roles of myocardial polarity and OCD in trabecular initiation, formation, and specification.

The ontogeny, activation and function of the epicardium during heart development and regeneration

The epicardium plays a key role during cardiac development, homeostasis and repair, and has thus emerged as a potential target in the treatment of cardiovascular disease. However, therapeutically manipulating the epicardium and epicardium-derived cells (EPDCs) requires insights into their developmental origin and the mechanisms driving their activation, recruitment and contribution to both the embryonic and adult injured heart. In recent years, studies of various model systems have provided us with a deeper understanding of the microenvironment in which EPDCs reside and emerge into, of the crosstalk between the multitude of cardiovascular cell types that influence the epicardium, and of the genetic programmes that orchestrate epicardial cell behaviour. Here, we review these discoveries and discuss how technological advances could further enhance our knowledge of epicardium-based repair mechanisms and ultimately influence potential therapeutic outcomes in cardiovascular regenerative medicine.

Notch signaling regulates Hey2 expression in a spatiotemporal dependent manner during cardiac morphogenesis and trabecular specification

Hey2 gene mutations in both humans and mice have been associated with multiple cardiac defects. However, the currently reported localization of Hey2 in the ventricular compact zone cannot explain the wide variety of cardiac defects. Furthermore, it was reported that, in contrast to other organs, Notch doesn’t regulate Hey2 in the heart. To determine the expression pattern and the regulation of Hey2, we used novel methods including RNAscope and a Hey2^CreERT2 knockin line to precisely determine the spatiotemporal expression pattern and level of Hey2 during cardiac development. We found that Hey2 is expressed in the endocardial cells of the atrioventricular canal and the outflow tract, as well as at the base of trabeculae, in addition to the reported expression in the ventricular compact myocardium. By disrupting several signaling pathways that regulate trabeculation and/or compaction, we found that, in contrast to previous reports, Notch signaling and Nrg1/ErbB2 regulate Hey2 expression level in myocardium and/or endocardium, but not its expression pattern: weak expression in trabecular myocardium and strong expression in compact myocardium. Instead, we found that FGF signaling regulates the expression pattern of Hey2 in the early myocardium, and regulates the expression level of Hey2 in a Notch1 dependent manner.

CDC42 is required for epicardial and pro-epicardial development by mediating FGF receptor trafficking to the plasma membrane

The epicardium contributes to multiple cardiac lineages and is essential for cardiac development and regeneration. However, the mechanism of epicardium formation is unclear. This study aimed to establish the cellular and molecular mechanisms underlying the dissociation of pro-epicardial cells (PECs) from the pro-epicardium (PE) and their subsequent translocation to the heart to form the epicardium. We used lineage tracing, conditional deletion, mosaic analysis and ligand stimulation in mice to determine that both villous protrusions and floating cysts contribute to PEC translocation to myocardium in a CDC42-dependent manner. We resolved a controversy by demonstrating that physical contact of the PE with the myocardium constitutes a third mechanism for PEC translocation to myocardium, and observed a fourth mechanism in which PECs migrate along the surface of the inflow tract to reach the ventricles. Epicardial-specific Cdc42 deletion disrupted epicardium formation, and Cdc42 null PECs proliferated less, lost polarity and failed to form villous protrusions and floating cysts. FGF signaling promotes epicardium formation in vivo, and biochemical studies demonstrated that CDC42 is involved in the trafficking of FGF receptors to the cell membrane to regulate epicardium formation.

Highlighted article: During epicardial formation in mice, four different mechanisms of pro-epicardial cell translocation to the myocardium can be identified, with CDC42 playing a key role.

Proteomic characterization of epicardial-myocardial signaling reveals novel regulatory networks including a role for NF-κB in epicardial EMT

Signaling between the epicardium and underlying myocardium is crucial for proper heart development. The complex molecular interactions and regulatory networks involved in this communication are not well understood. In this study, we integrated mass spectrometry with bioinformatics to systematically characterize the secretome of embryonic chicken EPDC-heart explant (EHE) co-cultures. The 150-protein secretome dataset established greatly expands the knowledge base of the molecular players involved in epicardial-myocardial signaling. We identified proteins and pathways that are implicated in epicardial-myocardial signaling for the first time, as well as new components of pathways that are known to regulate the crosstalk between epicardium and myocardium. The large size of the dataset enabled bioinformatics analysis to deduce networks for the regulation of specific biological processes and predicted signal transduction nodes within the networks. We performed functional analysis on one of the predicted nodes, NF-κB, and demonstrate that NF-κB activation is an essential step in TGFβ2/PDGFBB-induced cardiac epithelial-to-mesenchymal transition. In summary, we have generated a global perspective of epicardial-myocardial signaling for the first time, and our findings open exciting new avenues for investigating the molecular basis of heart development and regeneration.

Biological characteristics of embryonic epicardial cells in vitro correlate with embryonic day

The epicardial cell (EpiC) culture system plays an important role in investigating the specific mechanisms and signaling molecules that are involved in the development of EpiCs. From this early formation until adulthood, EpiCs undergo dynamic changes in the expression of embryonic genes that correlate with changes in the embryonic EpiC properties. The differences of embryonic EpiC properties may affect the related results of experiments in which EpiC culture system is used; however, these differences have not been explored. Therefore, in this study we examined the differences in the biological characteristics of EpiCs on different embryonic days in vitro EpiCs were isolated from embryonic ventricle explants on embryonic day (E) 11.5, E13.5, and E15.5. The differences in the migration, proliferation and differentiation were studied in EpiCs of different embryonic day by scratch assay, cell cycle analysis and platelet derived growth factor-bb (PDGF-BB) treatment. The results showed that EpiCs were successfully cultured from E11.5, E13.5, and E15.5 embryonic ventricle explants. The time windows of E11.5, E13.5, and E15.5 EpiC isolation out of the explants were different. The migration abilities of E11.5, E13.5, and E15.5 EpiCs decreased during embryonic development. Smooth muscle cell differentiation potential of early stage EpiCs was better than that of the later stage EpiCs. Although the proliferation ability of E11.5 EpiCs was significantly weaker than those of E13.5 and E15.5 EpiCs, the proliferation abilities of E13.5 and E15.5 EpiCs did not differ. These results suggest that the biological characteristics of EpiCs correlate with the timing of embryonic development, and different embryonic stage of ventricle should be properly chosen for culturing EpiCs depending on the purposes of the specific experiments.

Lamin-B1 contributes to the proper timing of epicardial cell migration and function during embryonic heart development

Lamin proteins form a meshwork beneath the nuclear envelope and contribute to many different cellular processes. Mutations in lamins cause defective organogenesis in mouse models and human diseases that affect adipose tissue, brain, skeletal muscle, and the heart. In vitro cell culture studies have shown that lamins help maintain nuclear shape and facilitate cell migration. However, whether these defects contribute to improper tissue building in vivo requires further clarification. By studying the heart epicardium during embryogenesis, we show that Lb1-null epicardial cells exhibit in vivo and in vitro migratory delay. Transcriptome analyses of these cells suggest that Lb1 influences the expression of cell adhesion genes, which could affect cell migration during epicardium development. These epicardial defects are consistent with incomplete development of both vascular smooth muscle and compact myocardium at later developmental stages in Lb1-null embryos. Further, we found that Lb1-null epicardial cells have a delayed nuclear morphology change in vivo, suggesting that Lb1 facilitates morphological changes associated with migration. These findings suggest that Lb1 contributes to nuclear shape maintenance and migration of epicardial cells and highlights the use of these cells for in vitro and in vivo study of these classic cell biological phenomena.

Discovering cardiac pericyte biology: From physiopathological mechanisms to potential therapeutic applications in ischemic heart disease

Microvascular pericytes and the more recently discovered adventitial pericyte-like progenitor cells are a subpopulation of vascular stem cells closely associated with small and large blood vessels respectively. These populations of perivascular cells are remarkably abundant in the heart. Pericytes control important physiological processes such as angiogenesis, blood flow and vascular permeability. In the heart, this pleiotropic activity makes pericytes extremely interesting for applications in regenerative medicine. On the other hand, dysfunction of pericytes could participate in the pathogenesis of cardiovascular disease, such as arterial hypertension, fibro-calcific cardiovascular remodeling, myocardial edema and post-ischemic coronary no-reflow. On a therapeutic standpoint, preclinical studies in small animal models of myocardial infarction have demonstrated the healing potential of pericytes transplantation, which has been ascribed to direct vascular incorporation and paracrine pro-angiogenic and anti-apoptotic activities. These promising findings open the door to the clinical use of pericytes for the treatment of cardiovascular diseases.

Imaging Cleared Embryonic and Postnatal Hearts at Single-cell Resolution

Single clonal tracing and analysis at the whole-heart level can determine cardiac progenitor cell behavior and differentiation during cardiac development, and allow for the study of the cellular and molecular basis of normal and abnormal cardiac morphogenesis. Recent emerging technologies of retrospective single clonal analyses make the study of cardiac morphogenesis at single cell resolution feasible. However, tissue opacity and light scattering of the heart as imaging depth is increased hinder whole-heart imaging at single cell resolution. To overcome these obstacles, a whole-embryo clearing system that can render the heart highly transparent for both illumination and detection must be developed. Fortunately, in the last several years, many methodologies for whole-organism clearing systems such as CLARITY, Scale, SeeDB, ClearT, 3DISCO, CUBIC, DBE, BABB and PACT have been reported. This lab is interested in the cellular and molecular mechanisms of cardiac morphogenesis. Recently, we established single cell lineage tracing via the ROSA26-Cre^ERT2; ROSA26-Confetti system to sparsely label cells during cardiac development. We adapted several whole embryo-clearing methodologies including Scale and CUBIC (clear, unobstructed brain imaging cocktails and computational analysis) to clear the embryo in combination with whole mount staining to image single clones inside the heart. The heart was successfully imaged at single cell resolution. We found that Scale can clear the embryonic heart, but cannot effectively clear the postnatal heart, while CUBIC can clear the postnatal heart, but damages the embryonic heart by dissolving the tissue. The methods described here will permit the study of gene function at a single clone resolution during cardiac morphogenesis, which, in turn, can reveal the cellular and molecular basis of congenital heart defects.

Three-Dimensional Analysis of Cell Division Orientation in Epidermal Basal Layer Using Intravital Two-Photon Microscopy

Epidermal structures are different among body sites, and proliferative keratinocytes in the epidermis play an important role in the maintenance of the epidermal structures. In recent years, intravital skin imaging has been used in mammalian skin research for the investigation of cell behaviors, but most of these experiments were performed with rodent ears. Here, we established a non-invasive intravital imaging approach for dorsal, ear, hind paw, or tail skin using R26H2BEGFP hairless mice. Using four-dimensional (x, y, z, and time) imaging, we successfully visualized mitotic cell division in epidermal basal cells. A comparison of cell division orientation relative to the basement membrane in each body site revealed that most divisions in dorsal and ear epidermis occurred in parallel, whereas the cell divisions in hind paw and tail epidermis occurred both in parallel and oblique orientations. Based on the quantitative analysis of the four-dimensional images, we showed that the epidermal thickness correlated with the basal cell density and the rate of the oblique divisions.

NUMB negatively regulates the epithelial-mesenchymal transition of triple-negative breast cancer by antagonizing Notch signaling

Triple-negative breast cancer (TNBC), an aggressive subtype of breast cancer with higher rates of early relapse and metastasis, is frequently associated with aberrant activation of epithelial-mesenchymal transition (EMT). Nonetheless, how EMT is initiated and regulated during TNBC progression is not well understood. Here, we report that NUMB is a negative regulator of EMT in both human mammary epithelial cells and breast cancer cells. Reduced NUMB expression was significantly associated with elevated EMT in TNBC. Conversely, overexpression of NUMB strongly attenuated the EMT program and metastasis of TNBC cell lines. Interestingly, we showed that NUMB employs different molecular mechanisms to regulate EMT. In normal mammary epithelial cells and breast cancer cells expressing wild-type p53, NUMB suppressed EMT by stabilizing p53. However, in TNBC cells, loss of NUMB facilitated the EMT program by activating Notch signaling. Consistent with these findings, low NUMB expression and high Notch activity were significantly correlated with the TNBC subtype in patients. Collectively, these findings reveal novel molecular mechanisms of NUMB in the regulation of breast tumor EMT, especially in TNBC.

Hippo Signaling Mediators Yap and Taz Are Required in the Epicardium for Coronary Vasculature Development

Formation of the coronary vasculature is a complex and precisely coordinated morphogenetic process that begins with the formation of epicardium. The epicardium gives rise to many components of the coronary vasculature, including fibroblasts, smooth muscle cells, and endothelium. Hippo signaling components have been implicated in cardiac development and regeneration. However, a role of Hippo signaling in the epicardium has not been explored. Employing a combination of genetic and pharmacological approaches, we demonstrate that inhibition of Hippo signaling mediators Yap and Taz leads to impaired epicardial epithelial-to-mesenchymal transition (EMT) and a reduction in epicardial cell proliferation and differentiation into coronary endothelial cells. We provide evidence that Yap and Taz control epicardial cell behavior, in part by regulating Tbx18 and Wt1 expression. Our findings show a role for Hippo signaling in epicardial cell proliferation, EMT, and cell fate specification during cardiac organogenesis.

Single-Cell Lineage Tracing Reveals that Oriented Cell Division Contributes to Trabecular Morphogenesis and Regional Specification

The cardiac trabeculae are sheet-like structures extending from the myocardium that function to increase surface area. A lack of trabeculation causes embryonic lethality due to compromised cardiac function. To understand the cellular and molecular mechanisms of trabecular formation, we genetically labeled individual cardiomyocytes prior to trabeculation via the brainbow multicolor system and traced and analyzed the labeled cells during trabeculation by whole-embryo clearing and imaging. The clones derived from labeled single cells displayed four different geometric patterns that are derived from different patterns of oriented cell division (OCD) and migration. Of the four types of clones, the inner, transmural, and mixed clones contributed to trabecular cardiomyocytes. Further studies showed that perpendicular OCD is an extrinsic asymmetric cell division that putatively contributes to trabecular regional specification. Furthermore, N-Cadherin deletion in labeled clones disrupted the clonal patterns. In summary, our data demonstrate that OCD contributes to trabecular morphogenesis and specification.

Epicardial Outgrowth Culture Assay and Ex Vivo Assessment of Epicardial-derived Cell Migration

A single layer of epicardial cells lines the heart, providing paracrine factors that stimulate cardiomyocyte proliferation and directly contributing cardiovascular progenitors during development and disease. While a number of factors have been implicated in epicardium-derived cell (EPDC) mobilization, the mechanisms governing their subsequent migration and differentiation are poorly understood. Here, we present in vitro and ex vivo strategies to study EPDC motility and differentiation. First, we describe a method of obtaining primary epicardial cells by outgrowth culture from the embryonic mouse heart. We also introduce a detailed protocol to assess three-dimensional migration of labeled EPDC in an organ culture system. We provide evidence using these techniques that genetic deletion of myocardin-related transcription factors in the epicardium attenuates EPDC migration. This approach serves as a platform to evaluate candidate modifiers of EPDC biology and could be used to develop genetic or chemical screens to identify novel regulators of EPDC mobilization that might be useful for cardiac repair.

Cell adhesion molecule control of planar spindle orientation

Polarized epithelial cells align the mitotic spindle in the plane of the sheet to maintain tissue integrity and to prevent malignant transformation. The orientation of the spindle apparatus is regulated by the immobilization of the astral microtubules at the lateral cortex and depends on the precise localization of the dynein-dynactin motor protein complex which captures microtubule plus ends and generates pulling forces towards the centrosomes. Recent developments indicate that signals derived from intercellular junctions are required for the stable interaction of the dynein-dynactin complex with the cortex. Here, we review the molecular mechanisms that regulate planar spindle orientation in polarized epithelial cells and we illustrate how different cell adhesion molecules through distinct and non-overlapping mechanisms instruct the cells to align the mitotic spindle in the plane of the sheet.

Atypical Protein Kinase C-Dependent Polarized Cell Division Is Required for Myocardial Trabeculation

A hallmark of cardiac development is the formation of myocardial trabeculations exclusively from the luminal surface of the primitive heart tube. Although a number of genetic defects in the endocardium and cardiac jelly disrupt myocardial trabeculation, the role of cell polarization remains unclear. Here, we demonstrate that atypical protein kinase C iota (Prkci) and its interacting partners are localized primarily to the luminal side of myocardial cells of early murine embryonic hearts. A subset of these cells undergoes polarized cell division with the cell division plane perpendicular to the heart's lumen. Disruption of the cell polarity complex by targeted gene mutations results in aberrant mitotic spindle alignment, loss of polarized cardiomyocyte division, and loss of normal myocardial trabeculation. Collectively, these results suggest that, in response to inductive signals, Prkci and its downstream partners direct polarized cell division of luminal myocardial cells to drive trabeculation in the nascent heart.