Myosin light chain 2a and 2v identifies the embryonic outflow tract myocardium in the developing rodent heart

Noninvasive analysis of contractility during identical maturations revealed two phenotypes in ventricular but not in atrial iPSC-CM

Patient-derived induced pluripotent stem cells (iPSCs) can be differentiated into atrial and ventricular cardiomyocytes to allow for personalized drug screening. A hallmark of differentiation is the manifestation of spontaneous beating in a two-dimensional (2-D) cell culture. However, an outstanding observation is the high variability in this maturation process. We valued that contractile parameters change during differentiation serving as an indicator of maturation. Consequently, we recorded noninvasively spontaneous motion activity during the differentiation of male iPSC toward iPSC cardiomyocytes (iPSC-CMs) to further analyze similar maturated iPSC-CMs. Surprisingly, our results show that identical differentiations into ventricular iPSC-CMs are variable with respect to contractile parameters resulting in two distinct subpopulations of ventricular-like cells. In contrast, differentiation into atrial iPSC-CMs resulted in only one phenotype. We propose that the noninvasive and cost-effective recording of contractile activity during maturation using a smartphone device may help to reduce the variability in results frequently reported in studies on ventricular iPSC-CMs.NEW & NOTEWORTHY Differentiation of induced pluripotent stem cells (iPSCs) into iPSC-derived cardiomyocytes (iPSC-CMs) exhibits a high variability in mature parameters. Here, we monitored noninvasively contractile parameters of iPSC-CM during full-time differentiation using a smartphone device. Our results show that parallel maturations of iPSCs into ventricular iPSC-CMs, but not into atrial iPSC-CMs, resulted in two distinct subpopulations of iPSC-CMs. These findings suggest that our cost-effective method may help to compare iPSC-CMs at the same maturation level.

miR-195b is required for proper cellular homeostasis in the elderly

Over the last decade we have witnessed an increasing number of studies revealing the functional role of non-coding RNAs in a multitude of biological processes, including cellular homeostasis, proliferation and differentiation. Impaired expression of non-coding RNAs can cause distinct pathological conditions, including herein those affecting the gastrointestinal and cardiorespiratory systems, respectively. miR-15/miR-16/miR-195 family members have been broadly implicated in multiple biological processes, including regulation of cell proliferation, apoptosis and metabolism within distinct tissues, such as heart, liver and lungs. While the functional contribution of miR-195a has been reported in multiple biological contexts, the role of miR-195b remains unexplored. In this study we dissected the functional role of miR-195b by generating CRISPR-Cas9 gene edited miR-195b deficient mice. Our results demonstrate that miR-195b is dispensable for embryonic development. miR-195b-/- mice are fertile and displayed no gross anatomical and/or morphological defects. Mechanistically, cell cycle regulation, metabolism and oxidative stress markers are distinctly impaired in the heart, liver and lungs of aged mice, a condition that is not overtly observed at midlife. The lack of overt functional disarray during embryonic development and early adulthood might be due to temporal and tissue-specific compensatory mechanisms driven by selective upregulation miR-15/miR-16/miR-195 family members. Overall, our data demonstrated that miR-195b is dispensable for embryonic development and adulthood but is required for cellular homeostasis in the elderly.

Utilizing human induced pluripotent stem cells to study atrial arrhythmias in the short QT syndrome

BACKGROUND

Among the monogenic inherited causes of atrial fibrillation is the short QT syndrome (SQTS), a rare channelopathy causing atrial and ventricular arrhythmias. One of the limitations in studying the mechanisms and optimizing treatment of SQTS-related atrial arrhythmias has been the lack of relevant human atrial tissues models.

OBJECTIVE

To generate a unique model to study SQTS-related atrial arrhythmias by combining the use of patient-specific human induced pluripotent stem cells (hiPSCs), atrial-specific differentiation schemes, two-dimensional tissue modeling, optical mapping, and drug testing.

METHODS AND RESULTS

SQTS (N588K KCNH2 mutation), isogenic-control, and healthy-control hiPSCs were coaxed to differentiate into atrial cardiomyocytes using a retinoic-acid based differentiation protocol. The atrial identity of the cells was confirmed by a distinctive pattern of MLC2v downregulation, connexin 40 upregulation, shorter and triangular-shaped action potentials (APs), and expression of the atrial-specific acetylcholine-sensitive potassium current. In comparison to the healthy- and isogenic control cells, the SQTS-hiPSC atrial cardiomyocytes displayed abbreviated APs and refractory periods along with an augmented rapidly activating delayed-rectifier potassium current (IKr). Optical mapping of a hiPSC-based atrial tissue model of the SQTS displayed shortened APD and altered biophysical properties of spiral waves induced in this model, manifested by accelerated spiral-wave frequency and increased rotor curvature. Both AP shortening and arrhythmia irregularities were reversed by quinidine and vernakalant treatment, but not by sotalol.

CONCLUSIONS

Patient-specific hiPSC-based atrial cellular and tissue models of the SQTS were established, which provide examples on how this type of modeling can shed light on the pathogenesis and pharmacological treatment of inherited atrial arrhythmias.

Combined method of whole mount and block-face imaging: Acquisition of 3D data of gene expression pattern from conventional in situ hybridization

Visualization of spatiotemporal expression of a gene of interest is a fundamental technique for analyzing the involvements of genes in organ development. In situ hybridization (ISH) is one of the most popular methods for visualizing gene expression. When conventional ISH is performed on sections or whole-mount specimens, the gene expression pattern is represented in 2-dimensional (2D) microscopic images or in the surface view of the specimen. To obtain 3-dimensional (3D) data of gene expression from conventional ISH, the "serial section method" has traditionally been employed. However, this method requires an extensive amount of time and labor because it requires researchers to collect a tremendous number of sections, label all sections by ISH, and image them before 3D reconstruction. Here, we proposed a rapid and low-cost 3D imaging method that can create 3D gene expression patterns from conventional ISH-labeled specimens. Our method consists of a combination of whole-mount ISH and Correlative Microscopy and Blockface imaging (CoMBI). The whole-mount ISH-labeled specimens were sliced using a microtome or cryostat, and all block-faces were imaged and used to reconstruct 3D images by CoMBI. The 3D data acquired using our method showed sufficient quality to analyze the morphology and gene expression patterns in the developing mouse heart. In addition, 2D microscopic images of the sections can be obtained when needed. Correlating 2D microscopic images and 3D data can help annotate gene expression patterns and understand the anatomy of developing organs. These results indicated that our method can be useful in the field of developmental biology.

Effective derivation of ventricular cardiomyocytes from hPSCs using ascorbic acid-containing maturation medium

Cardiomyocytes derived from human pluripotent stem cells (hPSCs) can be used in various applications including disease modeling, drug safety screening, and novel cell-based cardiac therapies. Here, we report an optimized selection and maturation method to induce maturation of cardiomyocytes into a specific subtype after differentiation driven by the regulation of Wnt signaling. The medium used to optimize selection and maturation was in a glucose starvation conditions, supplemented with either a nutrition complex or ascorbic acid. Following optimized selection and maturation, more cardiac Troponin T (cTnT)-positive cardiomyocytes were detected using albumin and ascorbic acid than B27. In addition, ascorbic acid enriched maturation of ventricular cardiomyocytes. We compared cardiomyocyte-specific gene expression patterns under different selection and maturation conditions by next-generation sequencing (NGS) analysis. Our optimized conditions will enable simple and efficient maturation and specification of the desired cardiomyocyte subtype, facilitating both biomedical research and clinical applications.

Myosin light chain 2 marks differentiating ventricular cardiomyocytes derived from human embryonic stem cells

Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) have great value for studies of human cardiac development, drug discovery, disease modeling, and cell therapy. However, the mixed cardiomyocyte subtypes (ventricular-, atrial-, and nodal-like myocytes) and the maturation heterogeneity of hPSC-CMs restrain their application in vitro and in vivo. Myosin light chain 2 (MYL2, encoding the ventricular/cardiac muscle isoform MLC2v protein) is regarded as a ventricular-specific marker of cardiac myocardium; however, its restricted localization to ventricles during human heart development has been questioned. Consequently, it is currently unclear whether MYL2 definitively marks ventricular hESC-CMs. Here, by using a MYL2-Venus hESC reporter line, we characterized a time-dependent increase of the MYL2-Venus positive (MLC2v-Venus⁺) hESC-CMs during differentiation. We also compared the molecular, cellular, and functional properties between the MLC2v-Venus⁺ and MYL2-Venus negative (MLC2v-Venus^-) hESC-CMs. At early differentiation stages of hESC-CMs, we reported that both MLC2v-Venus^- and MLC2v-Venus⁺ CMs displayed ventricular-like traits but the ventricular-like cells from MLC2v-Venus⁺ hESC-CMs displayed more developed action potential (AP) properties than that from MLC2v-Venus^- hESC-CMs. Meanwhile, about a half MLC2v-Venus^- hESC-CM population displayed atrial-like AP properties, and a half showed ventricular-like AP properties, whereas only ~ 20% of the MLC2v-Venus^- hESC-CMs expressed the atrial marker nuclear receptor subfamily 2 group F member 2 (NR2F2, also named as COUPTFII). At late time points, almost all MLC2v-Venus⁺ hESC-CMs exhibited ventricular-like AP properties. Further analysis demonstrates that the MLC2v-Venus⁺ hESC-CMs had enhanced Ca²⁺ transients upon increase of the MLC2v level during cultivation. Concomitantly, the MLC2v-Venus⁺ hESC-CMs showed more defined sarcomeric structures and better mitochondrial function than those in the MLC2v-Venus^- hESC-CMs. Moreover, the MLC2v-Venus⁺ hESC-CMs were more sensitive to hypoxic stimulus than the MLC2v-Venus^- hESC-CMs. These results provide new insights into the development of human ventricular myocytes and reveal a direct correlation between the expression profile of MLC2v and ventricular hESC-CM development. Our findings that MLC2v is predominantly a ventricular marker in developmentally immature hESC-CMs have implications for human development, drug screening, and disease modeling, and this marker should prove useful in overcoming issues associated with hESC-CM heterogeneity.

Regulatory Light Chains in Cardiac Development and Disease

The role of regulatory light chains (RLCs) in cardiac muscle function has been elucidated progressively over the past decade. The RLCs are among the earliest expressed markers during cardiogenesis and persist through adulthood. Failing hearts have shown reduced RLC phosphorylation levels and that restoring baseline levels of RLC phosphorylation is necessary for generating optimal force of muscle contraction. The signalling mechanisms triggering changes in RLC phosphorylation levels during disease progression remain elusive. Uncovering this information may provide insights for better management of heart failure patients. Given the cardiac chamber-specific expression of RLC isoforms, ventricular RLCs have facilitated the identification of mature ventricular cardiomyocytes, opening up possibilities of regenerative medicine. This review consolidates the standing of RLCs in cardiac development and disease and highlights knowledge gaps and potential therapeutic advancements in targeting RLCs.

Cardiac Development: A Glimpse on Its Translational Contributions

Cardiac development is a complex developmental process that is initiated soon after gastrulation, as two sets of precardiac mesodermal precursors are symmetrically located and subsequently fused at the embryonic midline forming the cardiac straight tube. Thereafter, the cardiac straight tube invariably bends to the right, configuring the first sign of morphological left–right asymmetry and soon thereafter the atrial and ventricular chambers are formed, expanded and progressively septated. As a consequence of all these morphogenetic processes, the fetal heart acquired a four-chambered structure having distinct inlet and outlet connections and a specialized conduction system capable of directing the electrical impulse within the fully formed heart. Over the last decades, our understanding of the morphogenetic, cellular, and molecular pathways involved in cardiac development has exponentially grown. Multiples aspects of the initial discoveries during heart formation has served as guiding tools to understand the etiology of cardiac congenital anomalies and adult cardiac pathology, as well as to enlighten novels approaches to heal the damaged heart. In this review we provide an overview of the complex cellular and molecular pathways driving heart morphogenesis and how those discoveries have provided new roads into the genetic, clinical and therapeutic management of the diseased hearts.

Purification of Pluripotent Stem Cell-Derived Cardiomyocytes Using CRISPR/Cas9-Mediated Integration of Fluorescent Reporters

Human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes have become critically important for the detailed study of cardiac development, disease modeling, and drug screening. However, directed differentiation of hiPSCs into cardiomyocytes often results in mixed populations of cardiomyocytes and other cell types, which may confound experiments that require pure populations of cardiomyocytes. Here, we detail the use of a CRISPR/Cas9 genome editing strategy to develop cardiomyocyte-specific reporters that allow for the isolation of hiPSC-derived cardiomyocytes and chamber-specific myocytes. Moreover, we describe a cardiac differentiation protocol to derive cardiomyocytes from hiPSCs, as well as a strategy to use fluorescence-activated cell sorting to isolate pure populations of fluorescently labeled cardiomyocytes for downstream applications.

Single-cell protein expression of hiPSC-derived cardiomyocytes using Single-Cell Westerns

The ability to reprogram human somatic cells into human induced pluripotent stem cells (hiPSCs) has enabled researchers to generate cell types in vitro that have the potential to faithfully recapitulate patient-specific disease processes and phenotypes. hiPSC-derived cardiomyocytes (hiPSC-CMs) offer the promise of in vitro patient- and disease-specific models for drug testing and the discovery of novel therapeutic approaches for treating cardiovascular diseases. While methods to differentiate hiPSCs into cardiomyocytes have been demonstrated, the heterogeneity and immaturity of these differentiated populations have restricted their potential in reproducing human disease and the associated target cell phenotypes. These barriers may be overcome through comprehensive single-cell characterization to dissect the rich heterogeneity of hiPSC-CMs and to study the source of varying cell fates. In this study, we optimized and validated a new Single-Cell Western method to assess protein expression in hiPSC-CMs. To better understand distinct subpopulations generated from cardiomyocyte differentiations and to track populations at single-cell resolution over time, we measured and quantified the expression of cardiomyocyte subtype-specific proteins (MLC2V and MLC2A) using Single-Cell Westerns. By understanding their heterogeneity through single-cell protein expression and quantification, we may improve upon current cardiomyocyte differentiation protocols, generate hiPSC-CMs that are more representative of in vivo derived cardiomyocytes for disease modeling, and utilize hiPSC-CMs for regenerative medicine purposes. Single-Cell Westerns provide a robust platform for protein expression analysis at single-cell resolution.

Delta-like ligand 4-mediated Notch signaling controls proliferation of second heart field progenitor cells by regulating Fgf8 expression

The role played by the Notch pathway in cardiac progenitor cell biology remains to be elucidated. Delta-like ligand 4 (Dll4), the arterial-specific Notch ligand, is expressed by second heart field (SHF) progenitors at time-points that are crucial in SHF biology. Dll4-mediated Notch signaling is required for maintaining an adequate pool of SHF progenitors, such that Dll4 knockout results in a reduction in proliferation and an increase in apoptosis. A reduced SHF progenitor pool leads to an underdeveloped right ventricle (RV) and outflow tract (OFT). In its most severe form, there is severe RV hypoplasia and poorly developed OFT resulting in early embryonic lethality. In its milder form, the OFT is foreshortened and misaligned, resulting in a double outlet right ventricle. Dll4-mediated Notch signaling maintains Fgf8 expression by transcriptional regulation at the promoter level. Combined heterozygous knockout of Dll4 and Fgf8 demonstrates genetic synergy in OFT alignment. Exogenous supplemental Fgf8 rescues proliferation in Dll4 mutants in ex-vivo culture. Our results establish a novel role for Dll4-mediated Notch signaling in SHF biology. More broadly, our model provides a platform for understanding oligogenic inheritance that results in clinically relevant OFT malformations.

Cyclooxygenases Inhibitors Efficiently Induce Cardiomyogenesis in Human Pluripotent Stem Cells

Application of human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) is limited by the challenges in their efficient differentiation. Recently, the Wingless (Wnt) signaling pathway has emerged as the key regulator of cardiomyogenesis. In this study, we evaluated the effects of cyclooxygenase inhibitors on cardiac differentiation of hPSCs. Cardiac differentiation was performed by adherent monolayer based method using 4 hPSC lines (HES3, H9, IMR90, and ES4SKIN). The efficiency of cardiac differentiation was evaluated by flow cytometry and RT-qPCR. Generated hPSC-CMs were characterised using immunocytochemistry, electrophysiology, electron microscopy, and calcium transient measurements. Our data show that the COX inhibitors Sulindac and Diclofenac in combination with CHIR99021 (GSK-3 inhibitor) efficiently induce cardiac differentiation of hPSCs. In addition, inhibition of COX using siRNAs targeted towards COX-1 and/or COX-2 showed that inhibition of COX-2 alone or COX-1 and COX-2 in combination induce cardiomyogenesis in hPSCs within 12 days. Using IMR90-Wnt reporter line, we showed that inhibition of COX-2 led to downregulation of Wnt signalling activity in hPSCs. In conclusion, this study demonstrates that COX inhibition efficiently induced cardiogenesis via modulation of COX and Wnt pathway and the generated cardiomyocytes express cardiac-specific structural markers as well as exhibit typical calcium transients and action potentials. These cardiomyocytes also responded to cardiotoxicants and can be relevant as an in vitro cardiotoxicity screening model.

Analysis of Uncharacterized mKiaa1211 Expression during Mouse Development and Cardiovascular Morphogenesis

Mammalian Kiaa1211 and Kiaa1211-like are a homologous pair of uncharacterized, highly conserved genes cloned from fetal and adult brain cDNA libraries. Herein we map the in utero spatiotemporal expression of mKiaa1211 and mKiaa1211L mRNA and their expression patterns in postnatal testis, skin, gastrointestinal, and adipose progenitor tissues. Significantly, mKiaa1211 is present throughout the early stages of mouse heart development, particularly in the second heart field (SHF) lineage as it differentiates from mesenchymal cells into cardiomyocytes. We also show that mKiaa1211 is expressed within several early neuronal tissues destined to give rise to central, peripheral, and sympathetic nervous system structures. Expression profiling revealed that the paralog mKiaa1211L is not expressed during the normal developmental process and that mKiaa1211 expression was noticeably absent from most adult terminally differentiated tissues. Finally, we confirm that a previously uncharacterized CRISPR/CAS-generated mKiaa1211 mouse mutant allele is hypomorphic.

Comparison of Cultured Human Cardiomyocyte Clusters Obtained from Embryos/Fetuses or Derived from Human Embryonic Stem Cells

Cardiomyocytes (CMs) derived from human embryonic stem cells (hESCs) or induced pluripotent stem cells (iPSCs) are used to study cardiogenesis and mechanisms of heart disease, and are being used in methods for toxiological screening of drugs. The phenotype of stem-cell-derived CMs should ideally resemble native CMs. Here, we compare embryonic/fetal CMs with hESC-derived CMs according to function and morphology. CM clusters were obtained from human embryonic/fetal hearts from elective terminated pregnancies before gestational week 12, and separated into atrial and ventricular tissues. Specific markers for embryonic CMs and primary cilia were visualized using immunofluorescence microscopy analysis. Contracting human embryonic cardiomyocyte (hECM) clusters morphologically and phenotypically resemble CMs in the embryonic/fetal heart. In addition, the contracting hECM clusters expressed primary cilia similar to that of cells in the embryonic/fetal heart. The electrophysiological characteristics of atrial and ventricular CMs were established by recording action potentials (APs) using sharp electrodes. In contrast to ventricular APs, atrial APs displayed a marked early repolarization followed by a plateau phase. hESC-CMs displayed a continuum of AP shapes. In all embryonic/fetal clusters, both atrial and ventricular, AP duration was prolonged by exposure to the K_V11.1 channel inhibitor dofetilide (50 nM); however, the prolongation was not significant, possibly due to the relatively small number of experiments. This study provides novel information on APs and functional characteristics of atrial and ventricular CMs in first trimester hearts, and demonstrates that K_v11.1 channels play a functional role already at these early stages. These results provide information needed to validate methods being developed on the basis of in vitro-derived CMs from either hESC or iPSC, and although there was a good correlation between the morphology of the two types of CMs, differences in electrophysiological characteristics exist.

The biochemical and electrophysiological profiles of amniotic fluid-derived stem cells following Wnt signaling modulation cardiac differentiation

Owing to the beneficial properties of amniotic fluid-derived stem cells (AFSCs), including pluripotency and the lack of ethical issues associated with embryonic stem cells (ESCs), they should be a promising cell source for regenerative medicine. However, how to differentiate AFSCs into contracting cardiomyocytes has not been established. In this study, a well-established, direct cardiac differentiation protocol involving the modulation of Wnt signaling was used to differentiate Oct 3/4⁺ AFSCs into cardiomyocytes. By day 14 of cardiomyocyte differentiation, these AFSCs expressed cardiac-specific genes (i.e., cardiac troponin T and myosin light chain 2v) and proteins but could not spontaneously contract. Using the patch-clamp technique, we further characterized the electrophysiological properties of human ESC-derived cardiomyocytes (hESC-CMs) and differentiated AFSCs. We used different configurations to investigate membrane potentials and ion currents in differentiated AFSCs and hESC-CMs. Under cell-attached voltage- or whole-cell current-clamp modes, we recorded spontaneous action currents (ACs) or action potentials (APs) in hESC-CMs but not in differentiated AFSCs. Compared to hESC-CMs, differentiated AFSCs showed significantly diminished activity of both BK_Ca and IK_Ca channels, which might lead to a lack of spontaneous ACs and APs in differentiated AFSCs. These results indicated that this well-established Wnt signaling modulating cardiac differentiation protocol was insufficient to induce the differentiation of functional cardiomyocytes from Oct 3/4⁺ AFSCs. Therefore, AFSC may not be an ideal candidate for cardiomyocyte differentiation.

Thick Filament Protein Network, Functions, and Disease Association

Sarcomeres consist of highly ordered arrays of thick myosin and thin actin filaments along with accessory proteins. Thick filaments occupy the center of sarcomeres where they partially overlap with thin filaments. The sliding of thick filaments past thin filaments is a highly regulated process that occurs in an ATP-dependent manner driving muscle contraction. In addition to myosin that makes up the backbone of the thick filament, four other proteins which are intimately bound to the thick filament, myosin binding protein-C, titin, myomesin, and obscurin play important structural and regulatory roles. Consistent with this, mutations in the respective genes have been associated with idiopathic and congenital forms of skeletal and cardiac myopathies. In this review, we aim to summarize our current knowledge on the molecular structure, subcellular localization, interacting partners, function, modulation via posttranslational modifications, and disease involvement of these five major proteins that comprise the thick filament of striated muscle cells. © 2018 American Physiological Society. Compr Physiol 8:631-709, 2018.

Cardiac outflow morphogenesis depends on effects of retinoic acid signaling on multiple cell lineages

BACKGROUND

Retinoic acid (RA), the bioactive derivative of vitamin A, is essential for vertebrate heart development. Both excess and reduced RA signaling lead to cardiovascular malformations affecting the outflow tract (OFT). To address the cellular mechanisms underlying the effects of RA signaling during OFT morphogenesis, we used transient maternal RA supplementation to rescue the early lethality resulting from inactivation of the murine retinaldehyde dehydrogenase 2 (Raldh2) gene.

RESULTS

By embryonic day 13.5, all rescued Raldh2(-/-) hearts exhibit severe, reproducible OFT septation defects, although wild-type and Raldh2(+/-) littermates have normal hearts. Cardiac neural crest cells (cNCC) were present in OFT cushions of Raldh2(-/-) mutant embryos but ectopically located in the periphery of the endocardial cushions, rather than immediately underlying the endocardium. Excess mesenchyme was generated by Raldh2(-/-) mutant endocardium, which displaced cNCC derivatives from their subendocardial, medial position.

CONCLUSIONS

RA signaling affects not only cNCC numbers but also their position relative to endocardial mesenchyme during the septation process. Our study shows that inappropriate coordination between the different cell types of the OFT perturbs its morphogenesis and leads to a severe congenital heart defect, persistent truncus arteriosus.

Heparan Sulfate Biosynthesis Enzyme, Ext1, Contributes to Outflow Tract Development of Mouse Heart via Modulation of FGF Signaling

Glycosaminoglycans are important regulators of multiple signaling pathways. As a major constituent of the heart extracellular matrix, glycosaminoglycans are implicated in cardiac morphogenesis through interactions with different signaling morphogens. Ext1 is a glycosyltransferase responsible for heparan sulfate synthesis. Here, we evaluate the function of Ext1 in heart development by analyzing Ext1 hypomorphic mutant and conditional knockout mice. Outflow tract alignment is sensitive to the dosage of Ext1. Deletion of Ext1 in the mesoderm induces a cardiac phenotype similar to that of a mutant with conditional deletion of UDP-glucose dehydrogenase, a key enzyme responsible for synthesis of all glycosaminoglycans. The outflow tract defect in conditional Ext1 knockout(Ext1^f/f:Mesp1Cre) mice is attributable to the reduced contribution of second heart field and neural crest cells. Ext1 deletion leads to downregulation of FGF signaling in the pharyngeal mesoderm. Exogenous FGF8 ameliorates the defects in the outflow tract and pharyngeal explants. In addition, Ext1 expression in second heart field and neural crest cells is required for outflow tract remodeling. Our results collectively indicate that Ext1 is crucial for outflow tract formation in distinct progenitor cells, and heparan sulfate modulates FGF signaling during early heart development.

Prospects for In Vitro Myofilament Maturation in Stem Cell-Derived Cardiac Myocytes

Cardiomyocytes derived from human stem cells are quickly becoming mainstays of cardiac regenerative medicine, in vitro disease modeling, and drug screening. Their suitability for such roles may seem obvious, but assessments of their contractile behavior suggest that they have not achieved a completely mature cardiac muscle phenotype. This could be explained in part by an incomplete transition from fetal to adult myofilament protein isoform expression. In this commentary, we review evidence that supports this hypothesis and discuss prospects for ultimately generating engineered heart tissue specimens that behave similarly to adult human myocardium. We suggest approaches to better characterize myofilament maturation level in these in vitro systems, and illustrate how new computational models could be used to better understand complex relationships between muscle contraction, myofilament protein isoform expression, and maturation.

Proteomic Analysis of Human Pluripotent Stem Cell-Derived, Fetal, and Adult Ventricular Cardiomyocytes Reveals Pathways Crucial for Cardiac Metabolism and Maturation

BACKGROUND

Differentiation of pluripotent human embryonic stem cells (hESCs) to the cardiac lineage represents a potentially unlimited source of ventricular cardiomyocytes (VCMs), but hESC-VCMs are developmentally immature. Previous attempts to profile hESC-VCMs primarily relied on transcriptomic approaches, but the global proteome has not been examined. Furthermore, most hESC-CM studies focus on pathways important for cardiac differentiation, rather than regulatory mechanisms for CM maturation. We hypothesized that gene products and pathways crucial for maturation can be identified by comparing the proteomes of hESCs, hESC-derived VCMs, human fetal and human adult ventricular and atrial CMs.

METHODS AND RESULTS

Using two-dimensional-differential-in-gel electrophoresis, 121 differentially expressed (>1.5-fold; P<0.05) proteins were detected. The data set implicated a role of the peroxisome proliferator-activated receptor α signaling in cardiac maturation. Consistently, WY-14643, a peroxisome proliferator-activated receptor α agonist, increased fatty oxidative enzyme level, hyperpolarized mitochondrial membrane potential and induced a more organized morphology. Along this line, treatment with the thyroid hormone triiodothyronine increased the dynamic tension developed in engineered human ventricular cardiac microtissue by 3-fold, signifying their maturation.

CONCLUSIONS

We conclude that the peroxisome proliferator-activated receptor α and thyroid hormone pathways modulate the metabolism and maturation of hESC-VCMs and their engineered tissue constructs. These results may lead to mechanism-based methods for deriving mature chamber-specific CMs.

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.

AcvR1-mediated BMP signaling in second heart field is required for arterial pole development: implications for myocardial differentiation and regional identity

BMP signaling plays an essential role in second heart field-derived heart and arterial trunk development, including myocardial differentiation, right ventricular growth, and interventricular, outflow tract and aortico-pulmonary septation. It is mediated by a number of different BMP ligands, and receptors, many of which are present simultaneously. The mechanisms by which they regulate morphogenetic events and degree of redundancy amongst them have still to be elucidated. We therefore assessed the role of BMP Type I receptor AcvR1 in anterior second heart field-derived cell development, and compared it with that of BmpR1a. By removing Acvr1 using the driver Mef2c[AHF]-Cre, we show that AcvR1 plays an essential role in arterial pole morphogenesis, identifying defects in outflow tract wall and cushion morphology that preceded a spectrum of septation defects from double outlet right ventricle to common arterial trunk in mutants. Its absence caused dysregulation in gene expression important for myocardial differentiation (Isl1, Fgf8) and regional identity (Tbx2, Tbx3, Tbx20, Tgfb2). Although these defects resemble to some degree those in the equivalent Bmpr1a mutant, a novel gene knock-in model in which Bmpr1a was expressed in the Acvr1 locus only partially restored septation in Acvr1 mutants. These data show that both BmpR1a and AcvR1 are needed for normal heart development, in which they play some non-redundant roles, and refine our understanding of the genetic and morphogenetic processes underlying Bmp-mediated heart development important in human congenital heart disease.

Induced pluripotent stem cell-derived cardiomyocytes for cardiovascular disease modeling and drug screening

Human induced pluripotent stem cells (hiPSCs) have emerged as a novel tool for drug discovery and therapy in cardiovascular medicine. hiPSCs are functionally similar to human embryonic stem cells (hESCs) and can be derived autologously without the ethical challenges associated with hESCs. Given the limited regenerative capacity of the human heart following myocardial injury, cardiomyocytes derived from hiPSCs (hiPSC-CMs) have garnered significant attention from basic and translational scientists as a promising cell source for replacement therapy. However, ongoing issues such as cell immaturity, scale of production, inter-line variability, and cell purity will need to be resolved before human clinical trials can begin. Meanwhile, the use of hiPSCs to explore cellular mechanisms of cardiovascular diseases in vitro has proven to be extremely valuable. For example, hiPSC-CMs have been shown to recapitulate disease phenotypes from patients with monogenic cardiovascular disorders. Furthermore, patient-derived hiPSC-CMs are now providing new insights regarding drug efficacy and toxicity. This review will highlight recent advances in utilizing hiPSC-CMs for cardiac disease modeling in vitro and as a platform for drug validation. The advantages and disadvantages of using hiPSC-CMs for drug screening purposes will be explored as well.

Myosin light chain 2-based selection of human iPSC-derived early ventricular cardiac myocytes

Applications of human induced pluripotent stem cell derived-cardiac myocytes (hiPSC-CMs) would be strengthened by the ability to generate specific cardiac myocyte (CM) lineages. However, purification of lineage-specific hiPSC-CMs is limited by the lack of cell marking techniques. Here, we have developed an iPSC-CM marking system using recombinant adenoviral reporter constructs with atrial- or ventricular-specific myosin light chain-2 (MLC-2) promoters. MLC-2a and MLC-2v selected hiPSC-CMs were purified by fluorescence-activated cell sorting and their biochemical and electrophysiological phenotypes analyzed. We demonstrate that the phenotype of both populations remained stable in culture and they expressed the expected sarcomeric proteins, gap junction proteins and chamber-specific transcription factors. Compared to MLC-2a cells, MLC-2v selected CMs had larger action potential amplitudes and durations. In addition, by immunofluorescence, we showed that MLC-2 isoform expression can be used to enrich hiPSC-CM consistent with early atrial and ventricular myocyte lineages. However, only the ventricular myosin light chain-2 promoter was able to purify a highly homogeneous population of iPSC-CMs. Using this approach, it is now possible to develop ventricular-specific disease models using iPSC-CMs while atrial-specific iPSC-CM cultures may require additional chamber-specific markers.

Heavy and light roles: myosin in the morphogenesis of the heart

Myosin is an essential component of cardiac muscle, from the onset of cardiogenesis through to the adult heart. Although traditionally known for its role in energy transduction and force development, recent studies suggest that both myosin heavy-chain and myosin light-chain proteins are required for a correctly formed heart. Myosins are structural proteins that are not only expressed from early stages of heart development, but when mutated in humans they may give rise to congenital heart defects. This review will discuss the roles of myosin, specifically with regards to the developing heart. The expression of each myosin protein will be described, and the effects that altering expression has on the heart in embryogenesis in different animal models will be discussed. The human molecular genetics of the myosins will also be reviewed.

Robust cardiomyocyte differentiation from human pluripotent stem cells via temporal modulation of canonical Wnt signaling

Human pluripotent stem cells (hPSCs) offer the potential to generate large numbers of functional cardiomyocytes from clonal and patient-specific cell sources. Here we show that temporal modulation of Wnt signaling is both essential and sufficient for efficient cardiac induction in hPSCs under defined, growth factor-free conditions. shRNA knockdown of β-catenin during the initial stage of hPSC differentiation fully blocked cardiomyocyte specification, whereas glycogen synthase kinase 3 inhibition at this point enhanced cardiomyocyte generation. Furthermore, sequential treatment of hPSCs with glycogen synthase kinase 3 inhibitors followed by inducible expression of β-catenin shRNA or chemical inhibitors of Wnt signaling produced a high yield of virtually (up to 98%) pure functional human cardiomyocytes from multiple hPSC lines. The robust ability to generate functional cardiomyocytes under defined, growth factor-free conditions solely by genetic or chemically mediated manipulation of a single developmental pathway should facilitate scalable production of cardiac cells suitable for research and regenerative applications.

The expression of Visinin-like 1 during mouse embryonic development

Visinin like 1 (Vsnl1) encodes a calcium binding protein which is well conserved between species. It was originally found in the brain and its biological functions in central nervous system have been addressed in several studies. Low expression levels have also been found in some peripheral organs, but very little information is available regarding its physiological roles in non-neuronal tissues. Except for the kidney, the expression pattern of Vsnl1 mRNA and protein has not yet been addressed during embryogenesis. By in situ hybridization and immunolabeling we have extensively analyzed the expression pattern of Vsnl1 during murine development. Vsnl1 specifies the cardiac primordia and its expression becomes restricted to the atrial myocardium after heart looping. However, in the adult heart, Vsnl1 is expressed by all four cardiac chambers. It also serves as a specific marker for the cardiomyocyte-derived structures in the systemic and pulmonary circulation. Vsnl1 is dynamically expressed also by many other organs during development e.g. taste buds, cochlea, thyroid, tooth, salivary and adrenal gland. The stage specific expression pattern of Vsnl1 makes it a potentially useful marker particularly in studies of cardiac and vascular morphogenesis.

Formal genesis of the outflow tracts of the heart revisited: previous works in the light of recent observations

The formal genesis of the great arteries continues to be controversial due to the lack of consensus of septation of the developing outflow tract. In order to make it clear how the great arteries are generated, we have re-examined our previous papers which emphasized the formation of the aorta and pulmonary trunk, concept of the aorticopulmonary septum, formation of the leaflets of semilunar valves, morphogenesis of the crista supraventricularis, programmed cell death and rotation of the outflow tract. In the present paper, we compare outcomes gained from the re-examination of our previous papers with prevalent interpretations of the arterial trunk. We obtained conclusions as follows: (i) The elongation of the fourth and sixth aortic arch arteries, which sprout from the wall of the aortic sac at the expense of the distal truncus, contributes to the formation of the aorta and pulmonary trunk; (ii) Smooth muscle cells of the tunica media of the arterial trunks do not arise from the transformation of the myocardial cells of the truncus wall (not 'arterialization'); (iii) Truncus swellings are divided into two parts: distal and proximal. The former contributes to the separation of the orifices of arterial trunks ('aorticopulmonary septum'). The latter contributes to the formation of the leaflets of the semilunar valves of the aorta and pulmonary trunk; (iv) The origin of the myocardial cells of the crista supraventricularis is a wall of the conus originated from secondary/anterior heart fields; and (v) There has been no acceptable proof that rotation and counterclockwise rotation are involved.

Embryonic stem cell transplantation: promise and progress in the treatment of heart disease

Cardiovascular diseases remain the leading cause of death worldwide, and the burden is equally shared between men and women around the globe. Cardiomyocytes that die in response to disease processes or aging are replaced by scar tissue instead of new muscle cells. Although recent reports suggest an intrinsic capacity for the mammalian myocardium to regenerate via endogenous stem/progenitor cells, the magnitude of such a response appears to be minimal and has yet to be realized fully in cardiovascular patients. Despite the advances in pharmacotherapy and new biomedical technologies, the prognosis for patients diagnosed with end-stage heart failure appears to be grave. While heart transplantation is a viable option, this life-saving intervention suffers from an acute shortage of cardiac organ donors. In view of these existing issues, donor cell transplantation is emerging as a promising strategy to regenerate diseased myocardium. Studies from multiple laboratories have shown that transplantation of donor cells (e.g. fetal cardiomyocytes, skeletal myoblasts, smooth muscle cells, and adult stem cells) can improve the function of diseased hearts over a short period of time (1-4 weeks). While long-term follow-up studies are warranted, it is generally perceived that the beneficial effects of transplanted cells are mainly due to increased angiogenesis or favorable scar remodeling in the engrafted myocardium. Although skeletal myoblasts and bone marrow stem cells hold the highest potential for implementation of autologous therapies, initial results from phase I trials are not promising. In contrast, transplantation of fetal cardiomyocytes has been shown to confer protection against the induction of ventricular tachycardia in experimental myocardial injury models. Furthermore, results from multiple laboratories suggest that fetal cardiomyocytes can couple functionally with host myocytes, stimulate formation of new blood vessels, and improve myocardial function. While it is neither practical nor ethical to test the potential of fetal cardiomyocytes in clinical trials, embryonic stem (ES) cells serve as a novel source for generation of unlimited quantities of cardiomyocytes for myocardial repair. The initial success in the application of ES cells to partially repair and improve myocardial function in experimental models of heart disease has been quite promising. However, multiple hurdles need to be crossed before the potential benefits of ES cells can be translated to the clinic. In this review, we summarize the current knowledge of cardiomyocyte derivation and enrichment from ES-cell cultures and provide a brief survey of factors increasing cardiomyogenic induction in both mouse and human ES cultures. Subsequently, we summarize the current state of research using mouse and human ES cells for the treatment of heart disease in various experimental models. Furthermore, we discuss the challenges that need to be overcome prior to the successful clinical utilization of ES-derived cardiomyocytes for the treatment of end-stage heart disease. While we are optimistic that the researchers in this field will sail across the hurdles, we also suggest that a more cautious approach to the validation of ES cardiomyocytes in experimental models would certainly prevent future disappointments, as seen with skeletal myoblast studies.

Differentially expressed genes in embryonic cardiac tissues of mice lacking Folr1 gene activity

BACKGROUND

Heart anomalies are the most frequently observed among all human congenital defects. As with the situation for neural tube defects (NTDs), it has been demonstrated that women who use multivitamins containing folic acid peri-conceptionally have a reduced risk for delivering offspring with conotruncal heart defects 123. Cellular folate transport is mediated by a receptor or binding protein and by an anionic transporter protein system. Defective function of the Folr1 (also known as Folbp1; homologue of human FRalpha) gene in mice results in inadequate transport, accumulation, or metabolism of folate during cardiovascular morphogenesis.

RESULTS

We have observed cardiovascular abnormalities including outflow tract and aortic arch arterial defects in genetically compromised Folr1 knockout mice. In order to investigate the molecular mechanisms underlying the failure to complete development of outflow tract and aortic arch arteries in the Folr1 knockout mouse model, we examined tissue-specific gene expression difference between Folr1 nullizygous embryos and morphologically normal heterozygous embryos during early cardiac development (14-somite stage), heart tube looping (28-somite stage), and outflow track septation (38-somite stage). Microarray analysis was performed as a primary screening, followed by investigation using quantitative real-time PCR assays. Gene ontology analysis highlighted the following ontology groups: cell migration, cell motility and localization of cells, structural constituent of cytoskeleton, cell-cell adhesion, oxidoreductase, protein folding and mRNA processing. This study provided preliminary data and suggested potential candidate genes for further description and investigation.

CONCLUSION

The results suggested that Folr1 gene ablation and abnormal folate homeostasis altered gene expression in developing heart and conotruncal tissues. These changes affected normal cytoskeleton structures, cell migration and motility as well as cellular redox status, which may contribute to cardiovascular abnormalities in mouse embryos lacking Folr1 gene activity.

Required, tissue-specific roles for Fgf8 in outflow tract formation and remodeling

Fibroblast growth factor 8 (Fgf8) is a secreted signaling protein expressed in numerous temporospatial domains that are potentially relevant to cardiovascular development. However, the pathogenesis of complex cardiac and outflow tract defects observed in Fgf8-deficient mice, and the specific source(s) of Fgf8 required for outflow tract formation and subsequent remodeling are unknown. A detailed examination of the timing and location of Fgf8 production revealed previously unappreciated expression in a subset of primary heart field cells; Fgf8 is also expressed throughout the anterior heart field (AHF) mesoderm and in pharyngeal endoderm at the crescent and early somite stages. We used conditional mutagenesis to examine the requirements for Fgf8 function in these different expression domains during heart and outflow tract morphogenesis. Formation of the primary heart tube and the addition of right ventricular and outflow tract myocardium depend on autocrine Fgf8 signaling in cardiac crescent mesoderm. Loss of Fgf8 in this domain resulted in decreased expression of the Fgf8 target gene Erm, and aberrant production of Isl1 and its target Mef2c in the anterior heart field, thus linking Fgf8 signaling with transcription factor networks that regulate survival and proliferation of the anterior heart field. We further found that mesodermal- and endodermal-derived Fgf8 perform specific functions during outflow tract remodeling: mesodermal Fgf8 is required for correct alignment of the outflow tract and ventricles, whereas activity of Fgf8 emanating from pharyngeal endoderm regulates outflow tract septation. These findings provide a novel insight into how the formation and remodeling of primary and anterior heart field-derived structures rely on Fgf8 signals from discrete temporospatial domains.

Bmp2 instructs cardiac progenitors to form the heart-valve-inducing field

A hallmark of heart-valve development is the swelling and deposition of extracellular matrix in the heart-valve region. Only myocardium overlying this region can signal to underlying endothelium and cause it to lose cell-cell contacts, delaminate, and invade the extracellular space abutting myocardium and endocardium to form endocardial cushions (EC) in a process known as epithelial to mesenchymal transformation (EMT). The heart-valve myocardium expresses bone morphogenetic protein-2 (Bmp2) coincident with development of valve mesenchyme. BMPs belong to the transforming growth factor beta superfamily (TGF-beta) and play a wide variety of roles during development. We show that conditional ablation of Bmp2 in cardiac progenitors results in cell fate changes in which the heart-valve region adopts the identity of differentiated chamber myocardium. Moreover, Bmp2-deficient hearts fail to induce production and deposition of matrix at the heart-valve-forming region, resulting in the inability of the endothelium to swell and impairing the development of ECs. Furthermore, in collagen invasion assays, Bmp2 mutant endothelium is incapable of undergoing EMT, and addition of BMP2 protein to mutant heart explants rescues this phenotype. Our results demonstrate that Bmp2 is both necessary and sufficient to specify a field of cardiac progenitor cells as the heart-valve-inducing region amid developing atria and ventricles.

Transcriptional regulation of the cardiac-specificMLC2gene duringXenopusembryonic development

The mechanisms by which transcription factors, which are not themselves tissue restricted, establish cardiomyocyte-specific patterns of transcription in vivo are unknown. Nor do we understand how positional cues are integrated to provide regionally distinct domains of gene expression within the developing heart. We describe regulation of the Xenopus XMLC2 gene,which encodes a regulatory myosin light chain of the contractile apparatus in cardiac muscle. This gene is expressed from the onset of cardiac differentiation in the frog embryo and is expressed throughout all the myocardium, both before and after heart chamber formation. Using transgenesis in frog embryos, we have identified an 82 bp enhancer within the proximal promoter region of the gene that is necessary and sufficient for heart-specific expression of an XMLC2 transgene. This enhancer is composed of two GATA sites and a composite YY1/CArG-like site. We show that the low-affinity SRF site is essential for transgene expression and that cardiac-specific expression also requires the presence of at least one adjacent GATA site. The overlapping YY1 site within the enhancer appears to act primarily as a repressor of ectopic expression, although it may also have a positive role. Finally, we show that the frog MLC2 promoter drives pan myocardial expression of a transgene in mice, despite the more restricted patterns of expression of murine MLC2 genes. We speculate that a common regulatory mechanism may be responsible for pan-myocardial expression of XMLC2 in both the frog and mouse, modulation of which could have given rise to more restricted patterns of expression within the heart of higher vertebrates.

Isl1 identifies a cardiac progenitor population that proliferates prior to differentiation and contributes a majority of cells to the heart

Hearts of mice lacking Isl1, a LIM homeodomain transcription factor, are completely missing the outflow tract, right ventricle, and much of the atria. isl1 expression and lineage tracing of isl1-expressing progenitors demonstrate that Isl1 is a marker for a distinct population of undifferentiated cardiac progenitors that give rise to the cardiac segments missing in isl1 mutants. Isl1 function is required for these progenitors to contribute to the heart. In isl1 mutants, isl1-expressing progenitors are progressively reduced in number, and FGF and BMP growth factors are downregulated. Our studies define two sets of cardiogenic precursors, one of which expresses and requires Isl1 and the other of which does not. Our results have implications for the development of specific cardiac lineages, left-right asymmetry, cardiac evolution, and isolation of cardiac progenitor cells.

Cardiac chamber formation: development, genes, and evolution

Concepts of cardiac development have greatly influenced the description of the formation of the four-chambered vertebrate heart. Traditionally, the embryonic tubular heart is considered to be a composite of serially arranged segments representing adult cardiac compartments. Conversion of such a serial arrangement into the parallel arrangement of the mammalian heart is difficult to understand. Logical integration of the development of the cardiac conduction system into the serial concept has remained puzzling as well. Therefore, the current description needed reconsideration, and we decided to evaluate the essentialities of cardiac design, its evolutionary and embryonic development, and the molecular pathways recruited to make the four-chambered mammalian heart. The three principal notions taken into consideration are as follows. 1) Both the ancestor chordate heart and the embryonic tubular heart of higher vertebrates consist of poorly developed and poorly coupled "pacemaker-like" cardiac muscle cells with the highest pacemaker activity at the venous pole, causing unidirectional peristaltic contraction waves. 2) From this heart tube, ventricular chambers differentiate ventrally and atrial chambers dorsally. The developing chambers display high proliferative activity and consist of structurally well-developed and well-coupled muscle cells with low pacemaker activity, which permits fast conduction of the impulse and efficacious contraction. The forming chambers remain flanked by slowly proliferating pacemaker-like myocardium that is temporally prevented from differentiating into chamber myocardium. 3) The trabecular myocardium proliferates slowly, consists of structurally poorly developed, but well-coupled, cells and contributes to the ventricular conduction system. The atrial and ventricular chambers of the formed heart are activated and interconnected by derivatives of embryonic myocardium. The topographical arrangement of the distinct cardiac muscle cells in the forming heart explains the embryonic electrocardiogram (ECG), does not require the invention of nodes, and allows a logical transition from a peristaltic tubular heart to a synchronously contracting four-chambered heart. This view on the development of cardiac design unfolds fascinating possibilities for future research.

Elucidating the molecular mechanism of cardiac remodeling using a comparative genomic approach

It is proposed that analysis of global gene expression would provide an understanding of the molecular mechanisms of cardiac remodeling. However, previous studies have only provided "snapshots" of differential gene expression. Furthermore, the differences in gene expression between regions of the heart that can result in sampling variability have not been characterized. In this study, we employed the Affymetrix GeneChip technology to evaluate the patterns of expression in two different in vivo models of cardiac remodeling and in two different regions (left ventricle free wall and intraventricular septum) of the heart. Mice underwent transverse aortic constriction (TAC), myocardial infarction (MI), or sham operation, and RNA from the left ventricle free wall and the septum was isolated 1 wk later. Histological analysis showed profound myocyte hypertrophy and fibrosis in both the septum and the left ventricle free wall of the TAC model, whereas, in the MI model, only the left ventricle exhibited hypertrophy. These differences were also reflected in the expression analysis. In conclusion, our analysis shows that regional differences in gene expression exist in the heart. Moreover, common pathways that are coregulated in both models exist, and these might be central to the hypertrophic phenotype regardless of the initial hypertrophic stimuli.

Intracellular calcium plays an essential role in cardiac development

Intracellular calcium signaling plays an essential role in cardiac physiology and modulates cardiac gene expression. However, the role that intracellular calcium signaling plays during cardiac development is not known. To address this issue, we examined the effects of altered intracellular calcium levels on cardiac morphogenesis. In acutely cultured mouse embryos, L-type calcium channel blockade decreased resting intracellular calcium levels and inhibited calcium transients. Embryos cultured at embryonic day (E) 7.5-8.5 in the presence of the L-type calcium channel blockers nifedipine and verapamil developed hearts that had a large left ventricle, lacked a right ventricle and had a long, thin outflow tract. If embryos were cultured at E7.5, calcium channel blockade also induced an abnormal, anterior cardiac loop. These alterations in development were not due to altered cardiac function, as heart rates at the end of the culture period were not affected by calcium channel blockade and blood flow was observed. Treatment with nifedipine altered the mRNA expression of the transcription factor Gata4, which was absent in the developing ventricles, and the sarcomeric protein Mylpc (myosin light chain 2V), which was decreased distal to the left ventricle and was absent at the site of the developing right ventricle. In contrast, the expression pattern of other cardiac transcription factor (Hand1, Hand2, Mef2c, Nkx2-5) and cytoskeletal protein (Myhca, Tagln) mRNA did not change with calcium channel blockade. These data demonstrate that proper intracellular calcium signaling is essential for normal cardiac looping, gene expression, and organ development.

Species-specific differences of myosin content in the developing cardiac chambers of fish, birds, and mammals

Key morphogenetic events during heart ontogenesis are similar in different vertebrate species. We report that in primitive vertebrates, i.e., cartilaginous fishes, both the embryonic and the adult heart show a segmental subdivision similar to that of the embryonic mammalian heart. Early morphogenetic events during cardiac development in the dogfish are long-lasting, providing a suitable model to study changes in pattern of gene expression during these stages. We performed a comparative study among dogfish, chicken, rat, and mouse to assess whether species-specific qualitative and/or quantitative differences in myosin heavy chain (MyHC) distribution arise during development, indicative of functional differences between species. MyHC RNA content was investigated by means of in situ hybridisation using an MyHC probe specific for a highly conserved domain, and MyHC protein content was assessed by immunohistochemistry. MyHC transcripts were found to be homogeneously distributed in the myocardium of the tubular and embryonic heart of dogfish and rodents. A difference between atrial and ventricular MyHC content (mRNA and protein) was observed in the adult stage. Interestingly, differences in the MyHC content were observed at the tubular heart stage in chicken. These differences in MyHC content illustrate the distinct developmental profiles of avian and mammalian species, which might be ascribed to distinct functional requirements of the myocardial segments during ontogenesis. The atrial myocardium showed the highest MyHC content in the adult heart of all species analysed (dogfish (S. canicula), mouse (M. musculus), rat (R. norvegicus), and chicken (G. gallus)). These observations indicate that in the adult heart of vertebrates the atrial myocardium contains more myosin than the ventricular myocardium.

Calreticulin reveals a critical Ca(2+) checkpoint in cardiac myofibrillogenesis

Calreticulin (crt) is an ubiquitously expressed and multifunctional Ca(2+)-binding protein that regulates diverse vital cell functions, including Ca(2+) storage in the ER and protein folding. Calreticulin deficiency in mice is lethal in utero due to defects in heart development and function. Herein, we used crt(-/-) embryonic stem (ES) cells differentiated in vitro into cardiac cells to investigate the molecular mechanisms underlying heart failure of knockout embryos. After 8 d of differentiation, beating areas were prominent in ES-derived wild-type (wt) embryoid bodies (EBs), but not in ES-derived crt(-/-) EBs, despite normal expression levels of cardiac transcription factors. Crt(-/-) EBs exhibited a severe decrease in expression and a lack of phosphorylation of ventricular myosin light chain 2 (MLC2v), resulting in an impaired organization of myofibrils. Crt(-/-) phenotype could be recreated in wt cells by chelating extracellular or cytoplasmic Ca(2+) with EGTA or BAPTA, or by inhibiting Ca(2+)/calmodulin-dependent kinases (CaMKs). An imposed ionomycin-triggered cystolic-free Ca(2+) concentration ([Ca(2+)](c)) elevation restored the expression, phosphorylation, and insertion of MLC2v into sarcomeric structures and in turn the myofibrillogenesis. The transcription factor myocyte enhancer factor C2 failed to accumulate into nuclei of crt(-/-) cardiac cells in the absence of ionomycin-triggered [Ca(2+)](c) increase. We conclude that the absence of calreticulin interferes with myofibril formation. Most importantly, calreticulin deficiency revealed the importance of a Ca(2+)-dependent checkpoint critical for early events during cardiac myofibrillogenesis.

Structure of the conus arteriosus of the sturgeon (Acipenser naccarii) heart. I: the conus valves and the subendocardium

Sturgeons are bony fish that retain structural traits typical of the more primitive Chondrostei. From an evolutionary viewpoint, sturgeons are considered relic fish. However, they show remarkable ecological plasticity and are well adapted to contemporary environmental conditions. Although development of the cardiovascular system is critical for all organs and systems, and is affected by evolutionary changes, the structure of the sturgeon heart has been mostly overlooked. This is also true for the conus arteriosus, which, as in Chondrostei, is endowed with several rows of valves and a layer of contractile myocardium. This work reports on the structure of the valves, the endocardium, and the subendocardium of the conus arteriosus of the sturgeon (Acipenser naccarii) heart. It is part of a broader study that aims to cover the entire structure of the sturgeon heart. The conus arteriosus of 15 A. naccarii hearts, ranging in age from juveniles to sexually-differentiated adults, has been studied by conventional light, transmission (TEM), and scanning electron microscopy (SEM). In addition, maceration of the soft tissues with NaOH, and actin localization by fluorescent phalloidin has been used. The conus is a tubular chamber that arises from the right ventricular side and presents two constrictions at the conus-ventricle and conus-aorta junctions. The conus is endowed with three rows of valves: one distal and two proximal. The segment of the conus located between the distal and the two proximal rows is devoid of valvular structures. The distal row has four leaflets, while the two proximal rows show the greatest variation in leaflet number, size, and shape. All leaflets have collagenous chordae tendineae arising from the free border and from the parietal side of the leaflets. The endocardium is a flat endothelium which shows a thick, irregular basement membrane. The leaflet body is formed by a loose connective tissue which blends with the subendocardium. The subendocardium is a connective tissue consisting of myofibroblasts, collagen, and elastin. It is divided into two distinct areas: one proximal, which shows little elastin and poorly organized collagen; and one distal, which is rich in elastin, with cells and extracellular fibers organized into layers that are oriented in alternative circumferential and longitudinal directions. The present report is the first systematic analysis of the structure of the sturgeon conus. Descriptions of the conus valves should recognize the existence of three valve rows only. The variability in valve morphology, and the loose structure of the leaflet tissue make it unlikely that the valves play an effective role in preventing blood backflow. In this regard, the ventricle-conus constriction may act as a sphincter. The subendocardium is an elastic coat capable of actively sustaining the tissue deformation that accompanies the heart contractile cycle. Further comparative studies are needed to provide deeper insight into the structural changes that accompany phyletic diversification.

[Regulation of myocardial gene expression during heart development]

The heart is an organ with special significance in medicine and developmental biology. The development of the heart and its vessels during embryogenesis is the result of numerous and complex processes. At present, our understanding is based on decades of meticulous anatomical studies. However, the spectacular progress of modern molecular biology and developmental biology has marked the beginning of a new era in embryology. The molecular bases for cardiogenesis are just emerging. Several families of genes with restricted expression to the heart have been identified in the last years, including genes encoding for contractile proteins, ion channels as well as transcription factors involved in tissue specific gene expression. Likewise, the analyses of regulatory elements have increased our understanding of the molecular mechanisms directing gene expression. In this review, we illustrate the different patterns of gene and transgene expression in the developing myocardium. These data demonstrate that the wide molecular heterogeneity observed in the developing myocardium is not restricted to embryogenesis but it also remains in the adulthood. Therefore, such molecular diversity should be taken into account on the design of future gene therapy approaches, having thus direct clinical implications.

Elevated transforming growth factor β2 enhances apoptosis and contributes to abnormal outflow tract and aortic sac development in retinoic X receptor α knockout embryos

Septation of the single tubular embryonic outflow tract into two outlet segments in the heart requires the precise integration of proliferation, differentiation and apoptosis during remodeling. Lack of proper coordination between these processes would result in a variety of congenital cardiac defects such as those seen in the retinoid X receptor α knockout (Rxra–/–) mouse. Rxra–/– embryos exhibit lethality between embryonic day (E) 13.5 and 15.5 and harbor a variety of conotruncal and aortic sac defects making it an excellent system to investigate the molecular and morphogenic causes of these cardiac malformations. At E12.5, before the embryonic lethality, we found no qualitative difference between wild type and Rxra–/– proliferation (BrdU incorporation) in outflow tract cushion tissue but a significant increase in apoptosis as assessed by both TUNEL labeling in paraffin sections and caspase activity in trypsin-dispersed hearts. Additionally, E12.5 embryos demonstrated elevated levels of transforming growth factor β2 (TGFβ2) protein in multiple cell lineages in the heart. Using a whole-mouse-embryo culture system, wild-type E11.5 embryos treated with TGFβ2 protein for 24 hours displayed enhanced apoptosis in both the sinistroventralconal cushion and dextrodorsalconal cushion in a manner analogous to that observed in the Rxra–/–. TGFβ2 protein treatment also led to malformations in both the outflow tract and aortic sac. Importantly, Rxra–/– embryos that were heterozygous for a null mutation in the Tgfb2 allele exhibited a partial restoration of the elevated apoptosis and of the malformations. This was evident at both E12.5 and E13.5. The data suggests that elevated levels of TGFβ2 can (1) contribute to abnormal outflow tract morphogenesis by enhancing apoptosis in the endocardial cushions and (2) promote aortic sac malformations by interfering with the normal development of the aorticopulmonary septum.

MLC3F transgene expression iniv mutant mice reveals the importance of left-right signalling pathways for the acquisition of left and right atrial but not ventricular compartment identity

Abstract Transcriptional differences between left and right cardiac chambers are revealed by an nlacZ reporter transgene controlled by regulatory sequences of the MLC3F gene, which is expressed in the left ventricle (LV), atrioventricular canal (AVC), and right atrium (RA). To examine the role of left-right signalling in the acquisition of left and right chamber identity, we have investigated MLC3F transgene expression in iv mutant mice. iv/iv mice exhibit randomised direction of heart looping and an elevated frequency of associated laterality defects, including atrial isomerism. At fetal stages, 3F-nlacZ-2E transgene expression remains confined to the morphological LV, AVC, and RA in L-loop hearts, although these appear on the opposite side of the body. In cases of morphologically distinguishable right atrial appendage isomerism, both atrial appendages show strong transgene expression. Conversely, specimens with morphological left atrial appendage isomerism show only weak expression in both atrial appendages. The earliest left-right atrial differences in the expression of the 3F-nlacZ-2E transgene are observed at E8.5. DiI labelling experiments confirmed that transcriptional regionalisation of the 3F-nlacZ-2E transgene at this stage reflects future atrial chamber identity. In some iv/iv embryos at E8.5, the asymmetry of 3F-nlacZ-2E expression was lost, suggesting atrial isomerism at the transcriptional level prior to chamber formation. These data suggest that molecular specification of left and right atrial but not ventricular chambers is dependent on left-right axial cues.

Multiple transcriptional domains, with distinct left and right components, in the atrial chambers of the developing heart

During heart development, 2 fast-conducting regions of working myocardium balloon out from the slow-conducting primary myocardium of the tubular heart. Three regions of primary myocardium persist: the outflow tract, atrioventricular canal, and inflow tract, which are contiguous throughout the inner curvature of the heart. The contribution of the inflow tract to the definitive atrial chambers has remained enigmatic largely because of the lack of molecular markers that permit unambiguous identification of this myocardial domain. We now report that the genes encoding atrial natriuretic factor, myosin light chain (MLC) 3F, MLC2V, and Pitx-2, and transgenic mouse lines expressing nlacZ under the control of regulatory sequences of the mouse MLC1F/3F gene, display regionalized patterns of expression in the atrial component of the developing mouse heart. These data distinguish 4 broad transcriptional domains in the atrial myocardium: (1) the atrioventricular canal that will form the smooth-walled lower atrial rim proximal to the ventricles; (2) the atrial appendages; (3) the caval vein myocardium (systemic inlet); and (4) the mediastinal myocardium (pulmonary inlet), including the atrial septa. The pattern of expression of Pitx-2 reveals that each of these transcriptional domains has a distinct left and right component. This study reveals for the first time differential gene expression in the systemic and pulmonary inlets, which is not shared by the contiguous atrial appendages and provides evidence for multiple molecular compartments within the atrial chambers. Furthermore, this work will allow the contribution of each of these myocardial components to be studied in congenitally malformed hearts, such as those with abnormal venous return.

Patterning the embryonic heart: identification of five mouse Iroquois homeobox genes in the developing heart

We isolated cDNAs of mouse Iroquois-related homeobox genes Irx1, -2, -3, -4, and -5 and characterized their patterns of expression in the developing heart. Irx1 and Irx2 were found to be expressed specifically in the ventricular septum from the onset of its formation onward. In fetal stages, the expression of both genes appeared to gradually become confined to the myocardium of the atrioventricular bundle and bundle branches of the forming ventricular conduction system. Irx3 was found to be expressed specifically in the trabeculated myocardium of the ventricles. Irx4 expression was observed in a segment of the linear heart tube and the atrioventricular canal and ventricular myocardium including the inner curvature after looping, resembling the pattern of MLC2V. Transcripts for Irx5 were detected specifically in the endocardium lining the ventricular and atrial working myocardium that also expressed von Willebrand factor, but were absent from the endocardium of the endocardial cushions, i.e., the atrioventricular canal, inner curvature, and outflow tract. The spatiodevelopmental pattern of Irx5 matched that of ANF, a marker for the forming working myocardium of the chambers. Taken together, all members of the Irx gene family were found to be expressed in highly specific patterns in the developing mouse heart, suggesting a critical role in the specification of the distinct components of the four-chambered heart.

Presence of functional sarcoplasmic reticulum in the developing heart and its confinement to chamber myocardium

During development fast-contracting atrial and ventricular chambers develop from a peristaltic-contracting heart tube. This study addresses the question of whether chamber formation is paralleled by a matching expression of the sarcoplasmic reticulum (SR) Ca(2+) pump. We studied indo-1 Ca(2+) transients elicited by field stimulation of linear heart tube stages and of explants from atria and outflow tracts of the prototypical preseptational E13 rat heart. Ca(2+) transients of H/H 11+ chicken hearts, which constitute the prototypic linear heart tube stage, were sensitive to verapamil only, indicating a minor contribution of Ca(2+)-triggered SR Ca(2+) release. Outflow tract transients displayed sensitivity to the inhibitors similar to that of the linear heart tube stages. Atrial Ca(2+) transients disappeared upon addition of ryanodine, tetracaine, or verapamil, indicating the presence of Ca(2+)-triggered SR Ca(2+) release. Quantitative radioactive in situ hybridization on sections of E13 rat hearts showed approximately 10-fold higher SERCA2a mRNA levels in the atria compared to nonmyocardial tissue and approximately 5-fold higher expression in compact ventricular myocardium. The myocardium of atrioventricular canal, outflow tract, inner curvature, and ventricular trabecules displayed weak expression. Immunohistochemistry on sections of rat and human embryos showed a similar pattern. The significance of these findings is threefold. (i) A functional SR is present long before birth. (ii) SR development is concomitant with cardiac chamber development, explaining regional differences in cardiac function. (iii) The pattern of SERCA2a expression underscores a manner of chamber development by differentiation at the outer curvature, rather than by segmentation of the linear heart tube.

Chamber formation and morphogenesis in the developing mammalian heart

In this study we challenge the generally accepted view that cardiac chambers form from an array of segmental primordia arranged along the anteroposterior axis of the linear and looping heart tube. We traced the spatial pattern of expression of genes encoding atrial natriuretic factor, sarcoplasmic reticulum calcium ATPase, Chisel, Irx5, Irx4, myosin light chain 2v, and beta-myosin heavy chain and related these to morphogenesis. Based on the patterns we propose a two-step model for chamber formation in the embryonic heart. First, a linear heart forms, which is composed of "primary" myocardium that nonetheless shows polarity in phenotype and gene expression along its anteroposterior and dorsoventral axes. Second, specialized ventricular chamber myocardium is specified at the ventral surface of the linear heart tube, while distinct left and right atrial myocardium forms more caudally on laterodorsal surfaces. The process of looping aligns these primordial chambers such that they face the outer curvature. Myocardium of the inner curvature, as well as that of inflow tract, atrioventricular canal, and outflow tract, retains the molecular signature originally found in linear heart tube myocardium. Evidence for distinct transcriptional programs which govern compartmentalization in the forming heart is seen in the patterns of expression of Hand1 for the dorsoventral axis, Irx4 and Tbx5 for the anteroposterior axis, and Irx5 for the distinction between primary and chamber myocardium.

Suppression of atrial myosin gene expression occurs independently in the left and right ventricles of the developing mouse heart

Many cardiac genes are broadly expressed in the early heart and become restricted to the atria or ventricles as development proceeds. Additional transcriptional differences between left and right compartments of the embryonic heart have been described recently, in particular for a number of transgenes containing cardiac regulatory elements. We now demonstrate that three myosin genes which become transcriptionally restricted to the atria between embryonic day (E) 12.5 and birth, alpha-myosin heavy chain (MHC), myosin light chain (MLC) 1A and MLC2A, are coordinately downregulated in the compact myocardium of the left ventricle before that of the right ventricle. alpha-MHC protein also accumulates in the right, but not left, compact ventricular myocardium during this period, suggesting that this transient regionalization contributes to fktal heart function. dHAND and eHAND, basic helix-loop-helix transcription factors known to be expressed in the right and left ventricles respectively at E10. 5, remain regionalized between E12.5 and E14.5. Downregulation of alpha-MHC, MLC1A, and MLC2A in iv/iv embryos, which have defective left/right patterning, initiates in the morphological left (systemic) ventricle regardless of its anatomical position on the right or left hand side of the heart. This points to the importance of left/right ventricular differences in sarcomeric gene expression patterns during fktal cardiogenesis and indicates that these differences originate in the embryo in response to anterior-posterior patterning of the heart tube rather than as a result of cardiac looping. Dev Dyn 2000;217:75-85.

Myocardialization of the cardiac outflow tract

During development, the single-circuited cardiac tube transforms into a double-circuited four-chambered heart by a complex process of remodeling, differential growth, and septation. In this process the endocardial cushion tissues of the atrioventricular junction and outflow tract (OFT) play a crucial role as they contribute to the mesenchymal components of the developing septa and valves in the developing heart. After fusion, the endocardial ridges in the proximal portion of the OFT initially form a mesenchymal outlet septum. In the adult heart, however, this outlet septum is basically a muscular structure. Hence, the mesenchyme of the proximal outlet septum has to be replaced by cardiomyocytes. We have dubbed this process "myocardialization." Our immunohistochemical analysis of staged chicken hearts demonstrates that myocardialization takes place by ingrowth of existing myocardium into the mesenchymal outlet septum. Compared to other events in cardiac septation, it is a relatively late process, being initialized around stage H/H28 and being basically completed around stage H/H38. To unravel the molecular mechanisms that are responsible for the induction and regulation of myocardialization, an in vitro culture system in which myocardialization could be mimicked and manipulated was developed. Using this in vitro myocardialization assay it was observed that under the standard culture conditions (i) whole OFT explants from stage H/H20 and younger did not spontaneously myocardialize the collagen matrix, (ii) explants from stage H/H21 and older spontaneously formed extensive myocardial networks, (iii) the myocardium of the OFT could be induced to myocardialize and was therefore "myocardialization-competent" at all stages tested (H/H16-30), (iv) myocardialization was induced by factors produced by, most likely, the nonmyocardial component of the outflow tract, (v) at none of the embryonic stages analyzed was ventricular myocardium myocardialization-competent, and finally, (vi) ventricular myocardium did not produce factors capable of supporting myocardialization.