A HCN4+ cardiomyogenic progenitor derived from the first heart field and human pluripotent stem cells

Segregation of Central Ventricular Conduction System Lineages in Early SMA+ Cardiomyocytes Occurs Prior to Heart Tube Formation

The cardiac conduction system (CCS) transmits electrical activity from the atria to the ventricles to coordinate heartbeats. Atrioventricular conduction diseases are often associated with defects in the central ventricular conduction system comprising the atrioventricular bundle (AVB) and right and left branches (BBs). Conducting and contractile working myocytes share common cardiomyogenic progenitors, however the time at which the CCS lineage becomes specified is unclear. In order to study the fate and the contribution to the CCS of cardiomyocytes during early heart tube formation, we performed a genetic lineage analysis using a Sma-CreERT2 mouse line. Lineage tracing experiments reveal a sequential contribution of early Sma expressing cardiomyocytes to different cardiac compartments, labeling at embryonic day (E) 7.5 giving rise to the interventricular septum and apical left ventricular myocardium. Early Sma expressing cardiomyocytes contribute to the AVB, BBs and left ventricular Purkinje fibers. Clonal analysis using the R26-confetti reporter mouse crossed with Sma-CreERT2 demonstrates that early Sma expressing cardiomyocytes include cells exclusively fated to give rise to the AVB. In contrast, lineage segregation is still ongoing for the BBs at E7.5. Overall this study highlights the early segregation of the central ventricular conduction system lineage within cardiomyocytes at the onset of heart tube formation.

Direct nkx2-5 transcriptional repression of isl1 controls cardiomyocyte subtype identity

During cardiogenesis, most myocytes arise from cardiac progenitors expressing the transcription factors Isl1 and Nkx2-5. Here, we show that a direct repression of Isl1 by Nkx2-5 is necessary for proper development of the ventricular myocardial lineage. Overexpression of Nkx2-5 in mouse embryonic stem cells (ESCs) delayed specification of cardiac progenitors and inhibited expression of Isl1 and its downstream targets in Isl1(+) precursors. Embryos deficient for Nkx2-5 in the Isl1(+) lineage failed to downregulate Isl1 protein in cardiomyocytes of the heart tube. We demonstrated that Nkx2-5 directly binds to an Isl1 enhancer and represses Isl1 transcriptional activity. Furthermore, we showed that overexpression of Isl1 does not prevent cardiac differentiation of ESCs and in Xenopus laevis embryos. Instead, it leads to enhanced specification of cardiac progenitors, earlier cardiac differentiation, and increased cardiomyocyte number. Functional and molecular characterization of Isl1-overexpressing cardiomyocytes revealed higher beating frequencies in both ESC-derived contracting areas and Xenopus Isl1-gain-of-function hearts, which associated with upregulation of nodal-specific genes and downregulation of transcripts of working myocardium. Immunocytochemistry of cardiomyocyte lineage-specific markers demonstrated a reduction of ventricular cells and an increase of cells expressing the pacemaker channel Hcn4. Finally, optical action potential imaging of single cardiomyocytes combined with pharmacological approaches proved that Isl1 overexpression in ESCs resulted in normally electrophysiologically functional cells, highly enriched in the nodal subtype at the expense of the ventricular lineage. Our findings provide an Isl1/Nkx2-5-mediated mechanism that coordinately regulates the specification of cardiac progenitors toward the different myocardial lineages and ensures proper acquisition of myocyte subtype identity.

Mechanisms underlying the cardiac pacemaker: the role of SK4 calcium-activated potassium channels

The proper expression and function of the cardiac pacemaker is a critical feature of heart physiology. The sinoatrial node (SAN) in human right atrium generates an electrical stimulation approximately 70 times per minute, which propagates from a conductive network to the myocardium leading to chamber contractions during the systoles. Although the SAN and other nodal conductive structures were identified more than a century ago, the mechanisms involved in the generation of cardiac automaticity remain highly debated. In this short review, we survey the current data related to the development of the human cardiac conduction system and the various mechanisms that have been proposed to underlie the pacemaker activity. We also present the human embryonic stem cell-derived cardiomyocyte system, which is used as a model for studying the pacemaker. Finally, we describe our latest characterization of the previously unrecognized role of the SK4 Ca(2+)-activated K(+) channel conductance in pacemaker cells. By exquisitely balancing the inward currents during the diastolic depolarization, the SK4 channels appear to play a crucial role in human cardiac automaticity.

Improving Cell Engraftment in Cardiac Stem Cell Therapy

Myocardial infarction (MI) affects millions of people worldwide. MI causes massive cardiac cell death and heart function decrease. However, heart tissue cannot effectively regenerate by itself. While stem cell therapy has been considered an effective approach for regeneration, the efficacy of cardiac stem cell therapy remains low due to inferior cell engraftment in the infarcted region. This is mainly a result of low cell retention in the tissue and poor cell survival under ischemic, immune rejection and inflammatory conditions. Various approaches have been explored to improve cell engraftment: increase of cell retention using biomaterials as cell carriers; augmentation of cell survival under ischemic conditions by preconditioning cells, genetic modification of cells, and controlled release of growth factors and oxygen; and enhancement of cell survival by protecting cells from excessive inflammation and immune surveillance. In this paper, we review current progress, advantages, disadvantages, and potential solutions of these approaches.

Single-Cell Expression Profiling Reveals a Dynamic State of Cardiac Precursor Cells in the Early Mouse Embryo

In the early vertebrate embryo, cardiac progenitor/precursor cells (CPs) give rise to cardiac structures. Better understanding their biological character is critical to understand the heart development and to apply CPs for the clinical arena. However, our knowledge remains incomplete. With the use of single-cell expression profiling, we have now revealed rapid and dynamic changes in gene expression profiles of the embryonic CPs during the early phase after their segregation from the cardiac mesoderm. Progressively, the nascent mesodermal gene Mesp1 terminated, and Nkx2-5+/Tbx5+ population rapidly replaced the Tbx5low+ population as the expression of the cardiac genes Tbx5 and Nkx2-5 increased. At the Early Headfold stage, Tbx5-expressing CPs gradually showed a unique molecular signature with signs of cardiomyocyte differentiation. Lineage-tracing revealed a developmentally distinct characteristic of this population. They underwent progressive differentiation only towards the cardiomyocyte lineage corresponding to the first heart field rather than being maintained as a progenitor pool. More importantly, Tbx5 likely plays an important role in a transcriptional network to regulate the distinct character of the FHF via a positive feedback loop to activate the robust expression of Tbx5 in CPs. These data expands our knowledge on the behavior of CPs during the early phase of cardiac development, subsequently providing a platform for further study.

Isolation of contractile cardiomyocytes from human pluripotent stem-cell-derived cardiomyogenic cultures using a human NCX1-EGFP reporter

The prospective isolation of defined contractile human pluripotent stem cell (hPSC)-derived cardiomyocytes is advantageous for regenerative medicine and drug screening applications. Currently, enrichment of cardiomyocyte populations from such cultures can be achieved by combinations of cell surface markers or the labor-intensive genetic modification of cardiac developmental genes, such as NKX2.5 or MYH6, with fluorescent reporters. To create a facile, portable method for the isolation of contractile cardiomyocytes from cardiomyogenic hPSC cultures, we employed a highly conserved cardiac enhancer sequence in the SLC8A1 (NCX1) gene to generate a lentivirally deliverable, antibiotic-selectable NCX1cp-EGFP reporter. We show that human embryonic stem cells (and induced pluripotent stem cells) transduced with the NCX1cp-EGFP reporter cassette exhibit enhanced green fluorescent protein (EGFP) expression in cardiac progenitors from 5 days into the directed cardiac hPSC differentiation protocol, with all reporter-positive cells transitioning to spontaneously contracting foci 3 days later. In subsequent stages of cardiomyocyte maturation, NCX1cp-EGFP expression was exclusively limited to contractile cells expressing high levels of cardiac troponin T (CTNT), MLC2a/v, and α-actinin proteins, and was not present in CD90/THY1(+) cardiac stromal cells or CD31/PECAM(+) endothelial cells. Flow-assisted cytometrically sorted EGFP(+) fractions of differentiated cultures were highly enriched in both early (NKX2.5 and TBX5) and late (CTNT/TNNI2, MYH6, MYH7, NPPA, and MYL2) cardiomyocyte markers, with a significant proportion of cells displaying a ventricular-like action potential pattern in patch-clamp recordings. We conclude that the use of the cardiac-specific promoter of the human SLC8A1(NCX1) gene is an effective strategy to isolate contractile cardiac cells and their progenitors from hPSC-derived cardiomyogenic cultures.

Human fetal cardiac progenitors: The role of stem cells and progenitors in the fetal and adult heart

The human fetal heart is formed early during embryogenesis as a result of cell migrations, differentiation, and formative blood flow. It begins to beat around gestation day 22. Progenitor cells are derived from mesoderm (endocardium and myocardium), proepicardium (epicardium and coronary vessels), and neural crest (heart valves, outflow tract septation, and parasympathetic innervation). A variety of molecular disturbances in the factors regulating the specification and differentiation of these cells can cause congenital heart disease. This review explores the contribution of different cardiac progenitors to the embryonic heart development; the pathways and transcription factors guiding their expansion, migration, and functional differentiation; and the endogenous regenerative capacity of the adult heart including the plasticity of cardiomyocytes. Unfolding these mechanisms will become the basis for understanding the dynamics of specific congenital heart disease as well as a means to develop therapy for fetal as well as postnatal cardiac defects and heart failure.

Chamber identity programs drive early functional partitioning of the heart

The vertebrate heart muscle (myocardium) develops from the first heart field (FHF) and expands by adding second heart field (SHF) cells. While both lineages exist already in teleosts, the primordial contributions of FHF and SHF to heart structure and function remain incompletely understood. Here we delineate the functional contribution of the FHF and SHF to the zebrafish heart using the cis-regulatory elements of the draculin (drl) gene. The drl reporters initially delineate the lateral plate mesoderm, including heart progenitors. Subsequent myocardial drl reporter expression restricts to FHF descendants. We harnessed this unique feature to uncover that loss of tbx5a and pitx2 affect relative FHF versus SHF contributions to the heart. High-resolution physiology reveals distinctive electrical properties of each heart field territory that define a functional boundary within the single zebrafish ventricle. Our data establish that the transcriptional program driving cardiac septation regulates physiologic ventricle partitioning, which successively provides mechanical advantages of sequential contraction.

The heart forms from combining the first with the second heart field, which in mammals creates left and right ventricle. Here transgenic zebrafish and physiology studies reveal that transcription factors controlling septation in mammals already in teleosts guide muscle coupling by controlling the relative contribution of the two fields to the heart.

HEART DEVELOPMENT. Integration of Bmp and Wnt signaling by Hopx specifies commitment of cardiomyoblasts

Cardiac progenitor cells are multipotent and give rise to cardiac endothelium, smooth muscle, and cardiomyocytes. Here, we define and characterize the cardiomyoblast intermediate that is committed to the cardiomyocyte fate, and we characterize the niche signals that regulate commitment. Cardiomyoblasts express Hopx, which functions to coordinate local Bmp signals to inhibit the Wnt pathway, thus promoting cardiomyogenesis. Hopx integrates Bmp and Wnt signaling by physically interacting with activated Smads and repressing Wnt genes. The identification of the committed cardiomyoblast that retains proliferative potential will inform cardiac regenerative therapeutics. In addition, Bmp signals characterize adult stem cell niches in other tissues where Hopx-mediated inhibition of Wnt is likely to contribute to stem cell quiescence and to explain the role of Hopx as a tumor suppressor.

Expansion and patterning of cardiovascular progenitors derived from human pluripotent stem cells

The inability of multipotent cardiovascular progenitor cells (CPCs) to undergo multiple divisions in culture has precluded stable expansion of precursors of cardiomyocytes and vascular cells. This contrasts with neural progenitors, which can be expanded robustly and are a renewable source of their derivatives. Here we use human pluripotent stem cells bearing a cardiac lineage reporter to show that regulated MYC expression enables robust expansion of CPCs with insulin-like growth factor-1 (IGF-1) and a hedgehog pathway agonist. The CPCs can be patterned with morphogens, recreating features of heart field assignment, and controllably differentiated to relatively pure populations of pacemaker-like or ventricular-like cardiomyocytes. The cells are clonogenic and can be expanded for >40 population doublings while retaining the ability to differentiate into cardiomyocytes and vascular cells. Access to CPCs will allow precise recreation of elements of heart development in vitro and facilitate investigation of the molecular basis of cardiac fate determination. This technology is applicable for cardiac disease modeling, toxicology studies and tissue engineering.

Transcription factor ISL1 is essential for pacemaker development and function

The sinoatrial node (SAN) maintains a rhythmic heartbeat; therefore, a better understanding of factors that drive SAN development and function is crucial to generation of potential therapies, such as biological pacemakers, for sinus arrhythmias. Here, we determined that the LIM homeodomain transcription factor ISL1 plays a key role in survival, proliferation, and function of pacemaker cells throughout development. Analysis of several Isl1 mutant mouse lines, including animals harboring an SAN-specific Isl1 deletion, revealed that ISL1 within SAN is a requirement for early embryonic viability. RNA-sequencing (RNA-seq) analyses of FACS-purified cells from ISL1-deficient SANs revealed that a number of genes critical for SAN function, including those encoding transcription factors and ion channels, were downstream of ISL1. Chromatin immunoprecipitation assays performed with anti-ISL1 antibodies and chromatin extracts from FACS-purified SAN cells demonstrated that ISL1 directly binds genomic regions within several genes required for normal pacemaker function, including subunits of the L-type calcium channel, Ank2, and Tbx3. Other genes implicated in abnormal heart rhythm in humans were also direct ISL1 targets. Together, our results demonstrate that ISL1 regulates approximately one-third of SAN-specific genes, indicate that a combination of ISL1 and other SAN transcription factors could be utilized to generate pacemaker cells, and suggest ISL1 mutations may underlie sick sinus syndrome.

Microfluidic organs-on-chips

An organ-on-a-chip is a microfluidic cell culture device created with microchip manufacturing methods that contains continuously perfused chambers inhabited by living cells arranged to simulate tissue- and organ-level physiology. By recapitulating the multicellular architectures, tissue-tissue interfaces, physicochemical microenvironments and vascular perfusion of the body, these devices produce levels of tissue and organ functionality not possible with conventional 2D or 3D culture systems. They also enable high-resolution, real-time imaging and in vitro analysis of biochemical, genetic and metabolic activities of living cells in a functional tissue and organ context. This technology has great potential to advance the study of tissue development, organ physiology and disease etiology. In the context of drug discovery and development, it should be especially valuable for the study of molecular mechanisms of action, prioritization of lead candidates, toxicity testing and biomarker identification.

Molecular regulation of cardiomyocyte differentiation

The heart is the first organ to form during embryonic development. Given the complex nature of cardiac differentiation and morphogenesis, it is not surprising that some form of congenital heart disease is present in ≈1 percent of newborns. The molecular determinants of heart development have received much attention over the past several decades. This has been driven in large part by an interest in understanding the causes of congenital heart disease coupled with the potential of using knowledge from developmental biology to generate functional cells and tissues that could be used for regenerative medicine purposes. In this review, we highlight the critical signaling pathways and transcription factor networks that regulate cardiomyocyte lineage specification in both in vivo and in vitro models. Special focus will be given to epigenetic regulators that drive the commitment of cardiomyogenic cells from nascent mesoderm and their differentiation into chamber-specific myocytes, as well as regulation of myocardial trabeculation.

Programming and reprogramming a human heart cell

The latest discoveries and advanced knowledge in the fields of stem cell biology and developmental cardiology hold great promise for cardiac regenerative medicine, enabling researchers to design novel therapeutic tools and approaches to regenerate cardiac muscle for diseased hearts. However, progress in this arena has been hampered by a lack of reproducible and convincing evidence, which at best has yielded modest outcomes and is still far from clinical practice. To address current controversies and move cardiac regenerative therapeutics forward, it is crucial to gain a deeper understanding of the key cellular and molecular programs involved in human cardiogenesis and cardiac regeneration. In this review, we consider the fundamental principles that govern the "programming" and "reprogramming" of a human heart cell and discuss updated therapeutic strategies to regenerate a damaged heart.

Human stem cells for modeling heart disease and for drug discovery

A major research focus in the field of cardiovascular medicine is the prospect of using stem cells and progenitor cells for cardiac regeneration. With the advent of induced pluripotent stem cell (iPSC) technology, major efforts are also underway to use iPSCs to model heart disease, to screen for new drugs, and to test candidate drugs for cardiotoxicity. Here, we discuss recent advances in the exciting fields of stem cells and cardiovascular disease.

Pluripotent stem cell derived cardiovascular progenitors--a developmental perspective

Human pluripotent stem cells can now be routinely differentiated into cardiac cell types including contractile cardiomyocytes, enabling the study of heart development and disease in vitro, and creating opportunities for the development of novel therapeutic interventions for patients. Our grasp of the system, however, remains partial, and a significant reason for this has been our inability to effectively purify and expand the intermediate cardiovascular progenitor cells (CPCs) equivalent to those studied in heart development. Doing so could facilitate the construction of a cardiac lineage cell fate map, boosting our capacity to more finely control stem cell lineage commitment to functionally distinct cardiac identities, as well as providing a model for identifying which genes confer cardiac potential on CPCs. This review offers a perspective on CPC development as understood from model organisms and pluripotent stem cell systems, focusing on issues of identity as well as the signalling implicated in inducing, expanding and patterning these cells.

Derivation of cardiac progenitor cells from embryonic stem cells

Cardiac progenitor cells (CPCs) have the capacity to differentiate into cardiomyocytes, smooth muscle cells (SMC), and endothelial cells and hold great promise in cell therapy against heart disease. Among various methods to isolate CPCs, differentiation of embryonic stem cell (ESC) into CPCs attracts great attention in the field since ESCs can provide unlimited cell source. As a result, numerous strategies have been developed to derive CPCs from ESCs. In this protocol, differentiation and purification of embryonic CPCs from both mouse and human ESCs is described. Due to the difficulty of using cell surface markers to isolate embryonic CPCs, ESCs are engineered with fluorescent reporters activated by CPC-specific cre recombinase expression. Thus, CPCs can be enriched by fluorescence-activated cell sorting (FACS). This protocol illustrates procedures to form embryoid bodies (EBs) from ESCs for CPC specification and enrichment. The isolated CPCs can be subsequently cultured for cardiac lineage differentiation and other biological assays. This protocol is optimized for robust and efficient derivation of CPCs from both mouse and human ESCs.

Insights into cardiac conduction system formation provided by HCN4 expression

Specialized myocytes of the cardiac conduction system (CCS) are essential to coordinate sequential contraction of cardiac atria and ventricles. Anomalies of the CCS can result in lethal cardiac arrhythmias, including sick sinus syndrome and atrial or ventricular fibrillation. To develop future therapies and regenerative medicine aimed at cardiac arrhythmias, it is important to understand formation and function of distinct components of the CCS. Essential to this understanding is the development of CCS-specific markers. In this review, we briefly summarize available mouse models of CCS markers and focus on those involving the hyperpolarization cation-selective nucleotide-gated cation channel, HCN4, which selectively marks all components of the specialized CCS in adult heart. Recent studies have revealed, however, that HCN4 expression during development is highly dynamic in cardiac precursors. These studies have offered insights into the contributions of the first and second heart field to myocyte and conduction system lineages and suggested the timing of allocation of specific conduction system precursors during development. Altogether, they have highlighted the utility of HCN4 as a cell surface marker for distinct components of the CCS at distinct stages of development, which can be utilized to facilitate purification and characterization of CCS precursors in mouse and human model systems and pave the way for regenerative therapies.

Persistent Noggin arrests cardiomyocyte morphogenesis and results in early in utero lethality

BACKGROUND

Multiple bone morphogenetic protein (BMP) genes are expressed in the developing heart from the initiation to late-differentiation stages, and play pivotal roles in cardiovascular development. In this study, we investigated the requirement of BMP activity in heart development by transgenic over-expression of extracellular BMP antagonist Noggin.

RESULTS

Using Nkx2.5-Cre to drive lineage-restricted Noggin within cardiomyocyte progenitors, we show persistent Noggin arrests cardiac development at the linear heart stage. This is coupled with a significantly reduced cell proliferation rate, subsequent cardiomyocyte programmed cell death and reduction of downstream intracellular pSMAD1/5/8 expression. Noggin mutants exhibit reduced heartbeat which likely results in subsequent fully penetrant in utero lethality. Significantly, confocal and electron micrographic examination revealed considerably fewer contractile elements, as well as a lack of maturation of actin-myosin microfilaments. Molecular analysis demonstrated that ectopic Noggin-expressing regions in the early heart's pacemaker region, failed to express the potassium/sodium hyperpolarization-activated cyclic nucleotide-gated channel 4 (Hcn4), resulting in an overall decrease in Hcn4 levels.

CONCLUSIONS

Combined, our results reveal a novel role for BMP signaling in the progression of heart development from the tubular heart stage to the looped stage by means of regulation of proliferation and promotion of maturation of the in utero heart's contractile apparatus and pacemaker.

Prion protein- and cardiac troponin T-marked interstitial cells from the adult myocardium spontaneously develop into beating cardiomyocytes

Atypically-shaped cardiomyocytes (ACMs) constitute a novel subpopulation of beating heart cells found in the cultures of cardiac myocyte-removed crude fraction cells obtained from adult mouse cardiac ventricles. Although ~500 beating ACMs are observed under microscope in the cell cultures obtained from the hearts of either male or female mice, the origin of these cells in cardiac tissue has yet to be elucidated due to the lack of exclusive markers. In the present study, we demonstrate the efficacy of cellular prion protein (PrP) as a surface marker of ACMs. Cells expressing PrP at the plasma membrane in the culture of the crude fraction cells were found to develop into beating ACMs by themselves or fuse with each other to become larger multinuclear beating ACMs. Combining PrP with a cardiac-specific contractile protein cardiac troponin T (cTnT) allowed us to identify native ACMs in the mouse cardiac ventricles as either clustered or solitary cells. PrP- and cTnT-marked cells were also found in the adult, even aged, human cardiac ventricles. These findings suggest that interstitial cells marked by PrP and cTnT, native ACMs, exhibit life-long survival in the cardiac ventricles of both mice and humans.

How to make a cardiomyocyte

During development, cardiogenesis is orchestrated by a family of heart progenitors that build distinct regions of the heart. Each region contains diverse cell types that assemble to form the complex structures of the individual cardiac compartments. Cardiomyocytes are the main cell type found in the heart and ensure contraction of the chambers and efficient blood flow throughout the body. Injury to the cardiac muscle often leads to heart failure due to the loss of a large number of cardiomyocytes and its limited intrinsic capacity to regenerate the damaged tissue, making it one of the leading causes of morbidity and mortality worldwide. In this Primer we discuss how insights into the molecular and cellular framework underlying cardiac development can be used to guide the in vitro specification of cardiomyocytes, whether by directed differentiation of pluripotent stem cells or via direct lineage conversion. Additional strategies to generate cardiomyocytes in situ, such as reactivation of endogenous cardiac progenitors and induction of cardiomyocyte proliferation, will also be discussed.

Regenerative medicine for the heart: perspectives on stem-cell therapy

SIGNIFICANCE

Despite decades of progress in cardiovascular biology and medicine, heart disease remains the leading cause of death, and there is no cure for the failing heart. Since heart failure is mostly caused by loss or dysfunction of cardiomyocytes (CMs), replacing dead or damaged CMs with new CMs might be an ideal way to reverse the disease. However, the adult heart is composed mainly of terminally differentiated CMs that have no significant self-regeneration capacity.

RECENT ADVANCES

Stem cells have tremendous regenerative potential and, thus, current cardiac regenerative research has focused on developing stem cell sources to repair damaged myocardium.

CRITICAL ISSUES

In this review, we examine the potential sources of cells that could be used for heart therapies, including embryonic stem cells and induced pluripotent stem cells, as well as alternative methods for activating the endogenous regenerative mechanisms of the heart via transdifferentiation and cell reprogramming. We also discuss the current state of knowledge of cell purification, delivery, and retention.

FUTURE DIRECTIONS

Efforts are underway to improve the current stem cell strategies and methodologies, which will accelerate the development of innovative stem-cell therapies for heart regeneration.

Induction of diverse cardiac cell types by reprogramming fibroblasts with cardiac transcription factors

Various combinations of cardiogenic transcription factors, including Gata4 (G), Hand2 (H), Mef2c (M) and Tbx5 (T), can reprogram fibroblasts into induced cardiac-like myocytes (iCLMs) in vitro and in vivo. Given that optimal cardiac function relies on distinct yet functionally interconnected atrial, ventricular and pacemaker (PM) cardiomyocytes (CMs), it remains to be seen which subtypes are generated by direct reprogramming and whether this process can be harnessed to produce a specific CM of interest. Here, we employ a PM-specific Hcn4-GFP reporter mouse and a spectrum of CM subtype-specific markers to investigate the range of cellular phenotypes generated by reprogramming of primary fibroblasts. Unexpectedly, we find that a combination of four transcription factors (4F) optimized for Hcn4-GFP expression does not generate beating PM cells due to inadequate sarcomeric protein expression and organization. However, applying strict single-cell criteria to GHMT-reprogrammed cells, we observe induction of diverse cellular phenotypes, including those resembling immature forms of all three major cardiac subtypes (i.e. atrial, ventricular and pacemaker). In addition, we demonstrate that cells induced by GHMT are directly reprogrammed and do not arise from an Nxk2.5(+) progenitor cell intermediate. Taken together, our results suggest a remarkable degree of plasticity inherent to GHMT reprogramming and provide a starting point for optimization of CM subtype-specific reprogramming protocols.

The symptom complex of familial sinus node dysfunction and myocardial noncompaction is associated with mutations in the HCN4 channel

BACKGROUND

Inherited arrhythmias were originally considered isolated electrical defects. There is growing evidence that ion channel dysfunction also contributes to myocardial disorders, but genetic overlap has not been reported for sinus node dysfunction (SND) and noncompaction cardiomyopathy (NCCM).

OBJECTIVES

The study sought to investigate a familial electromechanical disorder characterized by SND and NCCM, and to identify the underlying genetic basis.

METHODS

The index family and a cohort of unrelated probands with sinus bradycardia were examined by electrocardiography, Holter recording, exercise stress test, echocardiography, and/or cardiac magnetic resonance imaging. Targeted next-generation and direct sequencing were used for candidate gene analysis and mutation scanning. Ion channels were expressed in HEK293 cells and studied using patch-clamp recordings.

RESULTS

SND and biventricular NCCM were diagnosed in multiple members of a German family. Segregation analysis suggested autosomal-dominant inheritance of the combined phenotype. When looking for potentially disease-causing gene variants with cosegregation, a novel hyperpolarization-activated cyclic nucleotide channel 4 (HCN4)-G482R mutation and a common cysteine and glycine-rich protein 3 (CSRP3)-W4R variant were identified. HCN4-G482R is located in the highly conserved channel pore domain. Mutant subunits were nonfunctional and exerted dominant-negative effects on wild-type current. CSRP3-W4R has previously been linked to dilated and hypertrophic cardiomyopathy, but was also found in healthy subjects. Moreover, different truncation (695X) and missense (P883R) HCN4 mutations segregated with a similar combined phenotype in an additional, unrelated family and a single unrelated proband respectively, which both lacked CSRP3-W4R.

CONCLUSIONS

The symptom complex of SND and NCCM is associated with heritable HCN4 defects. The NCCM phenotype may be aggravated by a common CSRP3 variant in one of the families.

Rac1 signaling is critical to cardiomyocyte polarity and embryonic heart development

BACKGROUND

Defects in cardiac septation are the most common form of congenital heart disease, but the mechanisms underlying these defects are still poorly understood. The small GTPase Rac1 is implicated in planar cell polarity of epithelial cells in Drosophila; however, its role in mammalian cardiomyocyte polarity is not clear. We tested the hypothesis that Rac1 signaling in the second heart field regulates cardiomyocyte polarity, chamber septation, and right ventricle development during embryonic heart development.

METHODS AND RESULTS

Mice with second heart field-specific deficiency of Rac1 (Rac1(SHF)) exhibited ventricular and atrial septal defects, a thinner right ventricle myocardium, and a bifid cardiac apex. Fate-mapping analysis showed that second heart field contribution to the interventricular septum and right ventricle was deficient in Rac1(SHF) hearts. Notably, cardiomyocytes had a spherical shape with disrupted F-actin filaments in Rac1(SHF) compared with elongated and well-aligned cardiomyocytes in littermate controls. Expression of Scrib, a core protein in planar cell polarity, was lost in Rac1(SHF) hearts with decreased expression of WAVE and Arp2/3, leading to decreased migratory ability. In addition, Rac1-deficient neonatal cardiomyocytes displayed defects in cell projections, lamellipodia formation, and cell elongation. Furthermore, apoptosis was increased and the expression of Gata4, Tbx5, Nkx2.5, and Hand2 transcription factors was decreased in the Rac1(SHF) right ventricle myocardium.

CONCLUSIONS

Deficiency of Rac1 in the second heart field impairs elongation and cytoskeleton organization of cardiomyocytes and results in congenital septal defects, thin right ventricle myocardium, and a bifid cardiac apex. Our study suggests that Rac1 signaling is critical to cardiomyocyte polarity and embryonic heart development.

Early patterning and specification of cardiac progenitors in gastrulating mesoderm

Mammalian heart development requires precise allocation of cardiac progenitors. The existence of a multipotent progenitor for all anatomic and cellular components of the heart has been predicted but its identity and contribution to the two cardiac progenitor 'fields' has remained undefined. Here we show, using clonal genetic fate mapping, that Mesp1+ cells in gastrulating mesoderm are rapidly specified into committed cardiac precursors fated for distinct anatomic regions of the heart. We identify Smarcd3 as a marker of early specified cardiac precursors and identify within these precursors a compartment boundary at the future junction of the left and right ventricles that arises prior to morphogenesis. Our studies define the timing and hierarchy of cardiac progenitor specification and demonstrate that the cellular and anatomical fate of mesoderm-derived cardiac cells is specified very early. These findings will be important to understand the basis of congenital heart defects and to derive cardiac regeneration strategies.

High efficiency differentiation of human pluripotent stem cells to cardiomyocytes and characterization by flow cytometry

There is an urgent need to develop approaches for repairing the damaged heart, discovering new therapeutic drugs that do not have toxic effects on the heart, and improving strategies to accurately model heart disease. The potential of exploiting human induced pluripotent stem cell (hiPSC) technology to generate cardiac muscle "in a dish" for these applications continues to generate high enthusiasm. In recent years, the ability to efficiently generate cardiomyogenic cells from human pluripotent stem cells (hPSCs) has greatly improved, offering us new opportunities to model very early stages of human cardiac development not otherwise accessible. In contrast to many previous methods, the cardiomyocyte differentiation protocol described here does not require cell aggregation or the addition of Activin A or BMP4 and robustly generates cultures of cells that are highly positive for cardiac troponin I and T (TNNI3, TNNT2), iroquois-class homeodomain protein IRX-4 (IRX4), myosin regulatory light chain 2, ventricular/cardiac muscle isoform (MLC2v) and myosin regulatory light chain 2, atrial isoform (MLC2a) by day 10 across all human embryonic stem cell (hESC) and hiPSC lines tested to date. Cells can be passaged and maintained for more than 90 days in culture. The strategy is technically simple to implement and cost-effective. Characterization of cardiomyocytes derived from pluripotent cells often includes the analysis of reference markers, both at the mRNA and protein level. For protein analysis, flow cytometry is a powerful analytical tool for assessing quality of cells in culture and determining subpopulation homogeneity. However, technical variation in sample preparation can significantly affect quality of flow cytometry data. Thus, standardization of staining protocols should facilitate comparisons among various differentiation strategies. Accordingly, optimized staining protocols for the analysis of IRX4, MLC2v, MLC2a, TNNI3, and TNNT2 by flow cytometry are described.

Next-generation models of human cardiogenesis via genome editing

Cardiogenesis is one of the earliest and most important steps during human development and is orchestrated by discrete families of heart progenitors, which build distinct regions of the fetal heart. For the past decade, a lineage map for the distinct subsets of progenitors that generate the embryonic mammalian heart has begun to lay a foundation for the development of new strategies for rebuilding the adult heart after injury, an unmet clinical need for the vast majority of patients with end-stage heart failure who are not heart transplant recipients. The studies also have implications for the root causes of congenital heart disease, which affects 1 in 50 live births, the most prevalent malformations in children. Although much of this insight has been generated in murine models, it is becoming increasingly clear that there can be important divergence with principles and pathways for human cardiogenesis, as well as for regenerative pathways. The development of human stem cell models, coupled with recent advances in genome editing with RNA-guided endonucleases, offers a new approach for the primary study of human cardiogenesis. In addition, application of the technology to the in vivo setting in large animal models, including nonhuman primates, has opened the door to genome-edited large animal models of adult and congenital heart disease, as well as a detailed mechanistic dissection of the more diverse and complex set of progenitor families and pathways, which guide human cardiogenesis. Implications of this new technology for a new generation of human-based, genetically tractable systems are discussed, along with potential therapeutic applications.

Generation of a Cre knock-in into the Myocardin locus to mark early cardiac and smooth muscle cell lineages

The molecular events that control cell fate determination in cardiac and smooth muscle lineages remain elusive. Myocardin is an important transcription cofactor that regulates cell proliferation, differentiation, and development of the cardiovascular system. Here, we describe the construction and analysis of a dual Cre and enhanced green fluorescent protein (EGFP) knock-in mouse line in the Myocardin locus (Myocd(KI)). We report that the Myocd(KI) allele expresses the Cre enzyme and the EGFP in a manner that recapitulates endogenous Myocardin expression patterns. We show that Myocardin expression marks the earliest cardiac and smooth muscle lineages. Furthermore, this genetic model allows for the identification of a cardiac cell population, which maintains both Myocardin and Isl-1 expression, in E7.75-E8.0 embryos, highlighting the contribution and merging of the first and second heart fields during cardiogenesis. Therefore, the Myocd(KI) allele is a unique tool for studying cardiovascular development and lineage-specific gene manipulation.

Cardiac cell lineages that form the heart

Myocardial cells ensure the contractility of the heart, which also depends on other mesodermal cell types for its function. Embryological experiments had identified the sources of cardiac precursor cells. With the advent of genetic engineering, novel tools have been used to reconstruct the lineage tree of cardiac cells that contribute to different parts of the heart, map the development of cardiac regions, and characterize their genetic signature. Such knowledge is of fundamental importance for our understanding of cardiogenesis and also for the diagnosis and treatment of heart malformations.

Early lineage restriction in temporally distinct populations of Mesp1 progenitors during mammalian heart development

Cardiac development arises from two sources of mesoderm progenitors, the first heart field (FHF) and the second (SHF). Mesp1 has been proposed to mark the most primitive multipotent cardiac progenitors common for both heart fields. Here, using clonal analysis of the earliest prospective cardiovascular progenitors in a temporally controlled manner during early gastrulation, we found that Mesp1 progenitors consist of two temporally distinct pools of progenitors restricted to either the FHF or the SHF. FHF progenitors were unipotent, whereas SHF progenitors were either unipotent or bipotent. Microarray and single-cell PCR with reverse transcription analysis of Mesp1 progenitors revealed the existence of molecularly distinct populations of Mesp1 progenitors, consistent with their lineage and regional contribution. Together, these results provide evidence that heart development arises from distinct populations of unipotent and bipotent cardiac progenitors that independently express Mesp1 at different time points during their specification, revealing that the regional segregation and lineage restriction of cardiac progenitors occur very early during gastrulation.

Baf250a orchestrates an epigenetic pathway to repress the Nkx2.5-directed contractile cardiomyocyte program in the sinoatrial node

The sinoatrial node (SAN) is essential for rhythmic beating of the heart; however, our understanding of what controls proper functioning of the SAN remains primitive. To explore molecular control of SAN function, we specifically deleted Baf250a, a key regulatory component of the ATP-dependent chromatin remodeling complex SWI/SNF, in the SAN. Deletion of Baf250a in the SAN led to sinus bradycardia. Time series analysis of dysregulated genes after deletion of Baf250a reveals a transcriptional hierarchy maintaining pacemaker cell identity, i.e., Baf250a activates the expression of Tbx3, and Baf250a, Tbx3 and histone deacetylase 3 coordinately repress the expression of Nkx2.5. Disruption of this repressive pathway switches on expression of Nkx2.5, which stimulates expression of Gata4 and Tbx5. These three cardiac transcription factors further turn on a contractile cardiomyocyte program in the SAN, which eventually leads to sick sinus disease (SSD). Our study suggests that disruption of key genetic pathways regulating cardiac lineage segregation may cause SSD and cardiac arrhythmias in general.

Tissue engineering approaches to heart repair

The heart is a large organ containing many cell types, each of which is necessary for normal function. Because of this, cardiac regenerative medicine presents many unique challenges. Because each of the many types of cells within the heart has unique physiological and electrophysiological characteristics, donor cells must be well matched to the area of the heart into which they are grafted to avoid mechanical dysfunction or arrhythmia. In addition, grafted cells must be functionally integrated into host tissue to effectively repair cardiac function. Because of its size and physiological function, the metabolic needs of the heart are considerable. Therefore grafts must contain not only cardiomyocytes but also a functional vascular network to meet their needs for oxygen and nutrition. In this article we review progress in the use of pluripotent stem cells as a source of donor cardiomyocytes and highlight current unmet needs in the field. We also examine recent tissue engineering approaches integrating cells with various engineered materials that should address some of these unmet needs.

Inducible gene deletion in the entire cardiac conduction system using Hcn4-CreERT2 BAC transgenic mice

Developmental defects and disruption of molecular pathways of the cardiac conduction system (CCS) can cause life-threatening cardiac arrhythmias. Despite decades of effort, knowledge about the development and molecular control of the CCS remains primitive. Mouse genetics, complementary to other approaches such as human genetics, has become a key tool for exploring the developmental processes of various organs and associated diseases. Genetic analysis using mouse models will likely provide great insights about the development of the CCS, which can facilitate the development of novel therapeutic strategies to treat arrhythmias. To enable genetic studies of the CCS, CCS-associated Cre mouse models are essential. However, existing mouse models with Cre activity reported in the CCS have various limitations such as Cre leak, haploinsufficiency, and inadequate specificity of the Cre activity. To circumvent those limitations, we successfully generated Hcn4-CreERT2 bacterial artificial chromosome (BAC) transgenic mice using BAC recombineering in which Cre activity was specifically detected in the entire CCS after tamoxifen induction. Our Hcn4-CreERT2 BAC transgenic line will be an invaluable genetic tool with which to dissect the developmental control of CCS and arrhythmias.