Molecular Regulation of Cardiac Chamber-Specific Gene Expression

Tissue Usage Preference and Intrinsically Disordered Region Remodeling of Alternative Splicing Derived Proteoforms in the Heart

A computational analysis of mass spectrometry data was performed to uncover alternative splicing derived protein variants across chambers of the human heart. Evidence for 216 non-canonical isoforms was apparent in the atrium and the ventricle, including 52 isoforms not documented on SwissProt and recovered using an RNA sequencing derived database. Among non-canonical isoforms, 29 show signs of regulation based on statistically significant preferences in tissue usage, including a ventricular enriched protein isoform of tensin-1 (TNS1) and an atrium-enriched PDZ and LIM Domain 3 (PDLIM3) isoform 2 (PDLIM3-2/ALP-H). Examined variant regions that differ between alternative and canonical isoforms are highly enriched with intrinsically disordered regions. Moreover, over two-thirds of such regions are predicted to function in protein binding and RNA binding. The analysis here lends further credence to the notion that alternative splicing diversifies the proteome by rewiring intrinsically disordered regions, which are increasingly recognized to play important roles in the generation of biological function from protein sequences.

HAND factors regulate cardiac lineage commitment and differentiation from human pluripotent stem cells

BACKGROUND

Transcription factors HAND1 and HAND2 (HAND1/2) play significant roles in cardiac organogenesis. Abnormal expression and deficiency of HAND1/2 result in severe cardiac defects. However, the function and mechanism of HAND1/2 in regulating human early cardiac lineage commitment and differentiation are still unclear.

METHODS

With NKX2.5eGFP H9 human embryonic stem cells (hESCs), we established single and double knockout cell lines for HAND1 and HAND2, respectively, whose cardiomyocyte differentiation efficiency could be monitored by assessing NKX2.5-eGFP+ cells with flow cytometry. The expression of specific markers for heart fields and cardiomyocyte subtypes was examined by quantitative PCR, western blot and immunofluorescence staining. Microelectrode array and whole-cell patch clamp were performed to determine the electrophysiological characteristics of differentiated cardiomyocytes. The transcriptomic changes of HAND knockout cells were revealed by RNA sequencing. The HAND1/2 target genes were identified and validated experimentally by integrating with HAND1/2 chromatin immunoprecipitation sequencing data.

RESULTS

Either HAND1 or HAND2 knockout did not affect the cardiomyocyte differentiation kinetics, whereas depletion of HAND1/2 resulted in delayed differentiation onset. HAND1 knockout biased cardiac mesoderm toward second heart field progenitors at the expense of first heart field progenitors, leading to increased expression of atrial and outflow tract cardiomyocyte markers, which was further confirmed by the appearance of atrial-like action potentials. By contrast, HAND2 knockout cardiomyocytes had reduced expression of atrial cardiomyocyte markers and displayed ventricular-like action potentials. HAND1/2-deficient hESCs were more inclined to second heart field lineage and its derived cardiomyocytes with atrial-like action potentials than HAND1 single knockout during differentiation. Further mechanistic investigations suggested TBX5 as one of the downstream targets of HAND1/2, whose overexpression partially restored the abnormal cardiomyocyte differentiation in HAND1/2-deficient hESCs.

CONCLUSIONS

HAND1/2 have specific and redundant roles in cardiac lineage commitment and differentiation. These findings not only reveal the essential function of HAND1/2 in cardiac organogenesis, but also provide important information on the pathogenesis of HAND1/2 deficiency-related congenital heart diseases, which could potentially lead to new therapeutic strategies.

Human Genetics of Hypoplastic Left Heart Syndrome

Hypoplastic left heart syndrome (HLHS) is a severe congenital cardiovascular malformation characterized by hypoplasia of the left ventricle, aorta, and other structures on the left side of the heart. The pathologic definition includes atresia or stenosis of both the aortic and mitral valves. Despite considerable progress in clinical and surgical management of HLHS, mortality and morbidity remain concerns. One barrier to progress in HLHS management is poor understanding of its cause. Several lines of evidence point to genetic origins of HLHS. First, some HLHS cases have been associated with cytogenetic abnormalities (e.g., Turner syndrome). Second, studies of family clustering of HLHS and related cardiovascular malformations have determined HLHS is heritable. Third, genomic regions that encode genes influencing the inheritance of HLHS have been identified. Taken together, these diverse studies provide strong evidence for genetic origins of HLHS and related cardiac phenotypes. However, using simple Mendelian inheritance models, identification of single genetic variants that "cause" HLHS has remained elusive, and in most cases, the genetic cause remains unknown. These results suggest that HLHS inheritance is complex rather than simple. The implication of this conclusion is that researchers must move beyond the expectation that a single disease-causing variant can be found. Utilization of complex models to analyze high-throughput genetic data requires careful consideration of study design.

Tissue Usage Preference and Intrinsically Disordered Region Remodeling of Alternative Splicing Derived Proteoforms in the Heart

A computational analysis of mass spectrometry data was performed to uncover alternative splicing derived protein variants across chambers of the human heart. Evidence for 216 non-canonical isoforms was apparent in the atrium and the ventricle, including 52 isoforms not documented on SwissProt and recovered using an RNA sequencing derived database. Among non-canonical isoforms, 29 show signs of regulation based on statistically significant preferences in tissue usage, including a ventricular enriched protein isoform of tensin-1 (TNS1) and an atrium-enriched PDZ and LIM Domain 3 (PDLIM3) isoform 2 (PDLIM3-2/ALP-H). Examined variant regions that differ between alternative and canonical isoforms are highly enriched in intrinsically disordered regions, and over two-thirds of such regions are predicted to function in protein binding and/or RNA binding. The analysis here lends further credence to the notion that alternative splicing diversifies the proteome by rewiring intrinsically disordered regions, which are increasingly recognized to play important roles in the generation of biological function from protein sequences.

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.

Generation of Atrial-Specific Construct Using Sarcolipin Promoter-Associated CRM4 Enhancer

Cardiac gene therapy has been hampered by off-target expression of gene of interest irrespective of variety of delivery methods. To overcome this issue, cardiac-specific promoters provide target tissue specificity, although expression is often debilitated compared to that of ubiquitous promoters. We have previously shown that sarcolipin promoter with an enhancer calsequestrin cis-regulatory module 4 (CRM4) combination has an improved atrial specificity. Moreover, it showed a minimal extra-atrial expression, which is a significant advantage for AAV9-mediated cardiac gene therapy. Therefore, it can be a useful tool to study and treat atrial-specific diseases such as atrial fibrillation. In this chapter, we introduce practical and simple methodology for atrial-specific gene therapy using sarcolipin promoter with an enhancer CRM4.

Design and Production of Heart Chamber-Specific AAV9 Vectors

Adeno-associated virus serotype 9 (AAV9) is often used in heart research involving gene delivery due to its cardiotropism, high transduction efficiency, and little to no pathogenicity, making it highly applicable for gene manipulation, in vivo. However, current AAV9 technology is limited by the lack of strains that can selectively express and elucidate gene function in an atrial- and ventricular-specific manner. In fact, study of gene function in cardiac atria has been limited due to the lack of an appropriate tool to study atrial gene expression in vivo, hindering progress in the study of atrial-specific diseases such as atrial fibrillation, the most common cardiac arrhythmia in the USA.This chapter describes the method for the design and production of such chamber-specific AAV9 vectors, with the use of Nppa and Myl2 promoters to enhance atrial- and ventricular-specific expression. While several gene promoter candidates were considered and tested, Nppa and Myl2 were selected for use here because of their clearly defined regulatory elements that confer cardiac chamber-specific expression. Accordingly, Nppa (-425/+25) and Myl2 (-226/+36) promoter fragments are inserted into AAV9 vectors. The atrial- and ventricular-specific expression conferred by these new recombinant AAV9 was confirmed in a double-fluorescent Cre-dependent reporter mouse model. At only 450 and 262 base pairs of Nppa and Myl2 promoters, respectively, these AAV9 that drive chamber-specific AAV9 transgene expression address two major limitations of AAV9 technology, i.e., achieving chamber-specificity while maximizing space in the AAV genome for insertion of larger transgenes.

Maternal Thyroid Hormone Programs Cardiovascular Functions in the Offspring

Background: Maternal thyroid hormone (TH) plays an essential role for fetal development, especially for the cardiovascular system and its central control. However, the precise consequences of altered TH action during the different periods in pregnancy remain poorly understood. Methods: To address this question, we used mice heterozygous for a mutant thyroid hormone receptor α1 (TRα1) and wild-type controls that were born to wild-type mothers treated with 3,3',5-triiodothyronine (T3) during the first or the second half of pregnancy. We then phenotyped the offspring animals as adults by in vivo measurements and postmortem tissue analyses. Results: Maternal T3 treatment in either half of the pregnancy did not affect postnatal growth development. Serum thyroxine and hypophyseal thyrotropin subunit beta or deiodinase type II expression was also not affected in any group, only TRα1 mutant males exhibited a reduction in serum T3 levels after the treatment. Likewise, hepatic deiodinase type I was not altered, but serum selenium levels were reduced by the maternal treatment in wild-type offspring of both genders. Most interestingly, a significant increase in heart weight was found in adult wild-types born to mothers that received T3 during the first or second half of pregnancy, while TRα1 mutant males were protected from this effect. Moreover, we detected a significant increase in heart rate selectively in male mice that were exposed to elevated maternal T3 in the second half of the pregnancy. Conclusion: Taken together, our findings demonstrate that maternal TH is of particular relevance during the second half of pregnancy for establishing cardiac properties, with specific effects depending on TRα1 or gender. The data advocate routinely monitoring TH levels during pregnancy to avoid adverse cardiac effects in the offspring.

Developmental programming: Adipose depot-specific transcriptional regulation by prenatal testosterone excess in a sheep model of PCOS

Prenatal testosterone (T)-treated female sheep manifest adipose depot-specific disruptions in inflammatory/oxidative state, adipocyte differentiation and thermogenic adipocyte distribution. The objective of this study was to identify common and divergent gene pathways underlying prenatal T excess-induced adipose depot-specific disruptions. RNA sequencing and network analyses were undertaken with visceral (VAT), subcutaneous (SAT), epicardiac (ECAT) and perirenal (PRAT) adipose tissues from control and prenatal T-treated (100 mg T propionate twice a week from days 30-90 of gestation) female sheep at 21 months of age. Increased expression of adiposity and inflammation-related genes in VAT and genes that promote differentiation of white adipocytes in SAT were congruous with their metabolic roles with SAT favoring uptake/storage of free fatty acids and triglycerides and VAT favoring higher rate of fatty acid turnover and lipolysis. Selective upregulation of cardiac muscle and renoprotection genes in ECAT and PRAT respectively are suggestive of protective paracrine actions. Expression profile in prenatal T-treated sheep paralleled depot-specific dysfunctions with increased proinflammatory genes in VAT, reduced adipocyte differentiation genes in VAT and SAT and increased vascular related gene expression in PRAT. The high expression of genes involved in cardiomyocyte function in ECAT is suggestive of cardioprotective function being maintained to overcome the prenatal T-induced cardiac dysfunction and hypertension. These findings coupled with changes in gene pathways and networks involved in chromatin modification, extracellular matrix, immune and mitochondrial function, and endoplasmic reticulum to Golgi transport suggest that dysregulation in gene expression underlie prenatal T-treatment induced functional differences among adipose depots and manifestation of metabolic dysfunction.

Myocyte-specific enhancer factor 2c triggers transdifferentiation of adipose tissue-derived stromal cells into spontaneously beating cardiomyocyte-like cells

Cardiomyocyte regeneration is limited in adults. The adipose tissue-derived stromal vascular fraction (Ad-SVF) contains pluripotent stem cells that rarely transdifferentiate into spontaneously beating cardiomyocyte-like cells (beating CMs). However, the characteristics of beating CMs and the factors that regulate the differentiation of Ad-SVF toward the cardiac lineage are unknown. We developed a simple culture protocol under which the adult murine inguinal Ad-SVF reproducibly transdifferentiates into beating CMs without induction. The beating CMs showed the striated ventricular phenotype of cardiomyocytes and synchronised oscillation of the intracellular calcium concentration among cells on day 28 of Ad-SVF primary culture. We also identified beating CM-fated progenitors (CFPs) and performed single-cell transcriptome analysis of these CFPs. Among 491 transcription factors that were differentially expressed (≥ 1.75-fold) in CFPs and the beating CMs, myocyte-specific enhancer 2c (Mef2c) was key. Transduction of Ad-SVF cells with Mef2c using a lentiviral vector yielded CFPs and beating CMs with ~ tenfold higher cardiac troponin T expression, which was abolished by silencing of Mef2c. Thus, we identified the master gene required for transdifferentiation of Ad-SVF into beating CMs. These findings will facilitate the development of novel cardiac regeneration therapies based on gene-modified, cardiac lineage-directed Ad-SVF cells.

Quantitative Proteomics of Human Heart Samples Collected In Vivo Reveal the Remodeled Protein Landscape of Dilated Left Atrium Without Atrial Fibrillation

Genetic and genomic research has greatly advanced our understanding of heart disease. Yet, comprehensive, in-depth, quantitative maps of protein expression in hearts of living humans are still lacking. Using samples obtained during valve replacement surgery in patients with mitral valve prolapse (MVP), we set out to define inter-chamber differences, the intersect of proteomic data with genetic or genomic datasets, and the impact of left atrial dilation on the proteome of patients with no history of atrial fibrillation (AF).We collected biopsies from right atria (RA), left atria (LA) and left ventricle (LV) of seven male patients with mitral valve regurgitation with dilated LA but no history of AF. Biopsy samples were analyzed by high-resolution mass spectrometry (MS), where peptides were pre-fractionated by reverse phase high-pressure liquid chromatography prior to MS measurement on a Q-Exactive-HF Orbitrap instrument. We identified 7,314 proteins based on 130,728 peptides. Results were confirmed in an independent set of biopsies collected from three additional individuals. Comparative analysis against data from post-mortem samples showed enhanced quantitative power and confidence level in samples collected from living hearts. Our analysis, combined with data from genome wide association studies suggested candidate gene associations to MVP, identified higher abundance in ventricle for proteins associated with cardiomyopathies and revealed the dilated LA proteome, demonstrating differential representation of molecules previously associated with AF, in non-AF hearts.This is the largest dataset of cardiac protein expression from human samples collected in vivo It provides a comprehensive resource that allows insight into molecular fingerprints of MVP and facilitates novel inferences between genomic data and disease mechanisms. We propose that over-representation of proteins in ventricle is consequent not to redundancy but to functional need, and conclude that changes in abundance of proteins known to associate with AF are not sufficient for arrhythmogenesis.

Enhancing atrial-specific gene expression using a calsequestrin cis-regulatory module 4 with a sarcolipin promoter

Background

Cardiac gene therapy using the adeno‐associated virus serotype 9 vector is widely used because of its efficient transduction. However, the promoters used to drive expression often cause off‐target localization. To overcome this, studies have applied cardiac‐specific promoters, although expression is debilitated compared to that of ubiquitous promoters. To address these issues in the context of atrial‐specific gene expression, an enhancer calsequestrin cis‐regulatory module 4 (CRM4) and the highly atrial‐specific promoter sarcolipin were combined to enhance expression and minimize off tissue expression.

Methods

To observe expression and bio‐distribution, constructs were generated using two different reporter genes: luciferase and enhanced green fluorescent protein (EGFP). The ubiquitous cytomegalovirus (CMV), sarcolipin (SLN) and CRM4 combined with sarcolipin (CRM4.SLN) were compared and analyzed using the luciferase assay, western blotting, a quantitative polymerase chain reaction and fluorescence imaging.

Results

The CMV promoter containing vectors showed the strongest expression in vitro and in vivo. However, the module SLN combination showed enhanced atrial expression and a minimized off‐target effect even when compared with the individual SLN promoter.

Conclusions

For gene therapy involving atrial gene transfer, the CRM4.SLN combination is a promising alternative to the use of the CMV promoter. CRM4.SLN had significant atrial expression and minimized extra‐atrial expression.

Priming with oxytocin and relaxin improves cardiac differentiation of adipose tissue-derived stem cells

Previous studies have identified the heart as a source and a target tissue for oxytocin and relaxin hormones. These hormones play important roles in the regulation of cardiovascular function and repair of ischemic heart injury. In the current study, we examined the impact of oxytocin and relaxin on the development of cardiomyocytes from mesenchymal stem cells. For this purpose, mouse adipose tissue-derived stem cells (ADSCs) were treated with different concentrations of oxytocin or relaxin for 4 days. Three weeks after initiation of cardiac induction, differentiated ADSCs expressed cardiac-specific genes, Gata4, Mef2c, Nkx2.5, Tbx5, α- and β-Mhc, Mlc2v, Mlc2a and Anp, and cardiac proteins including connexin 43, desmin and α-actinin. 10 ^-7 M oxytocin and 50 ng/mL relaxin induced the maximum upregulation in the expression of cardiac markers. A combination of oxytocin and relaxin induced cardiomyocyte differentiation more potently than the individual factors. In our experiment, oxytocin-relaxin combination increased the population of cardiac troponin I-expressing cells to 6.84% as compared with 2.36% for the untreated ADSCs, 3.7% for oxytocin treatment and 3.41% for relaxin treatment groups. In summary, the results of this study indicated that oxytocin and relaxin hormones individually and in combination can improve cardiac differentiation of ADSCs, and treatment of the ADSCs and possibly other mesenchymal stem cells with these hormones may enhance their cardiogenic differentiation and survival after transplantation into the ischemic heart tissue.

The transcription factor MEF2A fine-tunes gene expression in the atrial and ventricular chambers of the adult heart

The distinct morphological and functional properties of the cardiac chambers arise from an elaborate developmental program involving cell lineage determination, morphogenesis, and dynamic spatiotemporal gene expression patterns. Although a number of transcription factors have been identified for proper gene regulation in the chambers, the complete transcriptional network that controls these patterns remains poorly defined. Previous studies have implicated the MEF2C transcription factor in the regulation of chamber-restricted enhancers. To better understand the mechanisms of MEF2-mediated regional gene regulation in the heart, we took advantage of MEF2A knock-out (KO) mice, a model that displays a predominantly ventricular chamber phenotype. Transcriptomic analysis of atrial and ventricular tissue from adult MEF2A KO hearts revealed a striking difference in chamber gene expression, with a larger proportion of dysregulated genes in the atrial chambers. Canonical pathway analysis of genes preferentially dysregulated in the atria and ventricles revealed distinct MEF2A-dependent cellular processes in each cardiac chamber. In the atria, MEF2A regulated genes involved in fibrosis and adhesion, whereas in the ventricles, it controlled inflammation and endocytosis. Finally, analysis of transcription factor-binding site motifs of differentially dysregulated genes uncovered distinct MEF2A co-regulators for the atrial and ventricular gene sets, and a subset of these was found to cooperate with MEF2A. In conclusion, our results suggest a mechanism in which MEF2 transcriptional activity is differentially recruited to fine-tune gene expression levels in each cardiac chamber. This regulatory mechanism ensures optimal output of these gene products for proper physiological function of the atrial and ventricular chambers.

Hypoplastic Left Heart Syndrome Sequencing Reveals a Novel NOTCH1 Mutation in a Family with Single Ventricle Defects

Hypoplastic left heart syndrome (HLHS) has been associated with germline mutations in 12 candidate genes and a recurrent somatic mutation in HAND1 gene. Using targeted and whole exome sequencing (WES) of heart tissue samples from HLHS patients, we sought to estimate the prevalence of somatic and germline mutations associated with HLHS. We performed Sanger sequencing of the HAND1 gene on 14 ventricular (9 LV and 5 RV) samples obtained from HLHS patients, and WES of 4 LV, 2 aortic, and 4 matched PBMC samples, analyzing for sequence discrepancy. We also screened for mutations in the 12 candidate genes implicated in HLHS. We found no somatic mutations in our HLHS cohort. However, we detected a novel germline frameshift/stop-gain mutation in NOTCH1 in a HLHS patient with a family history of both HLHS and hypoplastic right heart syndrome (HRHS). Our study, involving one of the first familial cases of single ventricle defects linked to a specific mutation, strengthens the association of NOTCH1 mutations with HLHS and suggests that the two morphologically distinct single ventricle conditions, HLHS and HRHS, may share a common molecular and cellular etiology. Finally, somatic mutations in the LV are an unlikely contributor to HLHS.

Differentiation and characterization of rhesus monkey atrial and ventricular cardiomyocytes from induced pluripotent stem cells

The combination of non-human primate animals and their induced pluripotent stem cell derived cardiomyocytes (iPSC-CMs) provides not only transplantation models for cell-based therapy of heart diseases, but also opportunities for heart-related drug research on both cellular and animal levels. However, the subtypes and electrophysiology properties of non-human primate iPSC-CMs hadn't been detailed characterized. In this study, we generated rhesus monkey induced pluripotent stem cells (riPSCs), and efficiently differentiated them into ventricular or atrial cardiomyocytes by modulating retinoic acid (RA) pathways. Our results revealed that the electrophysiological characteristics and response to canonical drugs of riPSC-CMs were similar with those of human pluripotent stem cell derived CMs. Therefore, rhesus monkeys and their iPSC-CMs provide a powerful and practicable system for heart related biomedical research.

Transcriptional heterogeneity in the lactase gene within cell-type is linked to the epigenome

Transcriptional variation in histologically- and genetically- identical cells is a widespread phenomenon in tissues, yet the processes conferring this heterogeneity are not well understood. To identify contributing factors, we analyzed epigenetic profiles associated with the in vivo transcriptional gradient of the mouse lactase gene (Lct), which occurs in enterocytes along the proximal-to-distal axis of the small intestine. We found that epigenetic signatures at enhancer and promoter elements aligns with transcriptional variation of Lct in enterocytes. Age and phenotype-specific environmental cues (lactose exposure after weaning) induced changes to epigenetic modifications and CTCF binding at select regulatory elements, which corresponded to the alterations in the intestinal Lct mRNA gradient. Thus, epigenetic modifications in combination with CTCF binding at regulatory elements account for the transcriptional gradient in Lct in cells of the same type. Epigenetic divergence within enterocytes may contribute to the functional specialization of intestinal subregions.

Same-Single-Cell Analysis of Pacemaker-Specific Markers in Human Induced Pluripotent Stem Cell-Derived Cardiomyocyte Subtypes Classified by Electrophysiology

Insights into the expression of pacemaker-specific markers in human induced pluripotent stem cell (hiPSC)-derived cardiomyocyte subtypes can facilitate the enrichment and track differentiation and maturation of hiPSC-derived pacemaker-like cardiomyocytes. To date, no study has directly assessed gene expression in each pacemaker-, atria-, and ventricular-like cardiomyocyte subtype derived from hiPSCs since currently the subtypes of these immature cardiomyocytes can only be identified by action potential profiles. Traditional acquisition of action potentials using patch-clamp recordings renders the cells unviable for subsequent analysis. We circumvented these issues by acquiring the action potential profile of a single cell optically followed by assessment of protein expression through immunostaining in that same cell. Our same-single-cell analysis for the first time revealed expression of proposed pacemaker-specific markers-hyperpolarization-activated cyclic nucleotide-modulated (HCN)4 channel and Islet (Isl)1-at the protein level in all three hiPSC-derived cardiomyocyte subtypes. HCN4 expression was found to be higher in pacemaker-like hiPSC-derived cardiomyocytes than atrial- and ventricular-like subtypes but its downregulation over time in all subtypes diminished the differences. Isl1 expression in pacemaker-like hiPSC-derived cardiomyocytes was initially not statistically different than the contractile subtypes but did become statistically higher than ventricular-like cells with time. Our observations suggest that although HCN4 and Isl1 are differentially expressed in hiPSC-derived pacemaker-like relative to ventricular-like cardiomyocytes, these markers alone are insufficient in identifying hiPSC-derived pacemaker-like cardiomyocytes. Stem Cells 2016;34:2670-2680.

Integrin Based Isolation Enables Purification of Murine Lineage Committed Cardiomyocytes

In contrast to mature cardiomyocytes which have limited regenerative capacity, pluripotent stem cells represent a promising source for the generation of new cardiomyocytes. The tendency of pluripotent stem cells to form teratomas and the heterogeneity from various differentiation stages and cardiomyocyte cell sub-types, however, are major obstacles to overcome before this type of therapy could be applied in a clinical setting. Thus, the identification of extracellular markers for specific cardiomyocyte progenitors and mature subpopulations is of particular importance. The delineation of cardiomyocyte surface marker patterns not only serves as a means to derive homogeneous cell populations by FACS, but is also an essential tool to understand cardiac development. By using single-cell expression profiling in early mouse embryonic hearts, we found that a combination of integrin alpha-1, alpha-5, alpha-6 and N-cadherin enables isolation of lineage committed murine cardiomyocytes. Additionally, we were able to separate trabecular cardiomyocytes from solid ventricular myocardium and atrial murine cells. These cells exhibit expected subtype specific phenotype confirmed by electrophysiological analysis. We show that integrin expression can be used for the isolation of living, functional and lineage-specific murine cardiomyocytes.

Transcriptome of human foetal heart compared with cardiomyocytes from pluripotent stem cells

Differentiated derivatives of human pluripotent stem cells (hPSCs) are often considered immature because they resemble foetal cells more than adult, with hPSC-derived cardiomyocytes (hPSC-CMs) being no exception. Many functional features of these cardiomyocytes, such as their cell morphology, electrophysiological characteristics, sarcomere organization and contraction force, are underdeveloped compared with adult cardiomyocytes. However, relatively little is known about how their gene expression profiles compare with the human foetal heart, in part because of the paucity of data on the human foetal heart at different stages of development. Here, we collected samples of matched ventricles and atria from human foetuses during the first and second trimester of development. This presented a rare opportunity to perform gene expression analysis on the individual chambers of the heart at various stages of development, allowing us to identify not only genes involved in the formation of the heart, but also specific genes upregulated in each of the four chambers and at different stages of development. The data showed that hPSC-CMs had a gene expression profile similar to first trimester foetal heart, but after culture in conditions shown previously to induce maturation, they cluster closer to the second trimester foetal heart samples. In summary, we demonstrate how the gene expression profiles of human foetal heart samples can be used for benchmarking hPSC-CMs and also contribute to determining their equivalent stage of development.

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.

Atrial and ventricular specification of ADSCs is stimulated by different doses of BMP4

To investigate the effect of BMP4 on cardiomyocyte differentiation of adipose tissue-derived stem cells (ADSCs), mouse ADSCs were treated with different concentrations of BMP4 in media containing fetal bovine serum (FBS) or Knockout™ Serum Replacement (KoSR). 3 weeks after cardiac induction, differentiated ADSCs expressed some cardiac-specific genes and proteins. BMP4 treatment upregulated the expression of cardiac transcription factors. In both FBS and KoSR-supplemented media, lower concentrations of BMP4 had a positive effect on the expression of MLC2A gene, while MLC2V was more expressed with higher concentrations of BMP4. BMP4 treatment in KoSR supplemented medium was more efficient for cardiac induction. Supplementation of culture media with insulin-transferrin-selenium improved the expression of MLC2A gene. The results of this study indicated that BMP4 is important for cardiac differentiation of the ADSCs. However, BMP4 was not enough for structural and functional maturation of the ADSC-derived cardiomyocytes.

Identification and purification of human induced pluripotent stem cell-derived atrial-like cardiomyocytes based on sarcolipin expression

The use of human stem cell-derived cardiomyocytes to study atrial biology and disease has been restricted by the lack of a reliable method for stem cell-derived atrial cell labeling and purification. The goal of this study was to generate an atrial-specific reporter construct to identify and purify human stem cell-derived atrial-like cardiomyocytes. We have created a bacterial artificial chromosome (BAC) reporter construct in which fluorescence is driven by expression of the atrial-specific gene sarcolipin (SLN). When purified using flow cytometry, cells with high fluorescence specifically express atrial genes and display functional calcium handling and electrophysiological properties consistent with atrial cardiomyocytes. Our data indicate that SLN can be used as a marker to successfully monitor and isolate hiPSC-derived atrial-like cardiomyocytes. These purified cells may find many applications, including in the study of atrial-specific pathologies and chamber-specific lineage development.

Atrial identity is determined by a COUP-TFII regulatory network

Atria and ventricles exhibit distinct molecular profiles that produce structural and functional differences between the two cardiac compartments. However, the factors that determine these differences remain largely undefined. Cardiomyocyte-specific COUP-TFII ablation produces ventricularized atria that exhibit ventricle-like action potentials, increased cardiomyocyte size, and development of extensive T tubules. Changes in atrial characteristics are accompanied by alterations of 2,584 genes, of which 81% were differentially expressed between atria and ventricles, suggesting that a major function of myocardial COUP-TFII is to determine atrial identity. Chromatin immunoprecipitation assays using E13.5 atria identified classic atrial-ventricular identity genes Tbx5, Hey2, Irx4, MLC2v, MLC2a, and MLC1a, among many other cardiac genes, as potential COUP-TFII direct targets. Collectively, our results reveal that COUP-TFII confers atrial identity through direct binding and by modulating expression of a broad spectrum of genes that have an impact on atrial development and function.

UTX, a histone H3-lysine 27 demethylase, acts as a critical switch to activate the cardiac developmental program

The removal of histone H3 lysine27 (H3K27) trimethylation mark is important for the robust induction of many cell type-specific genes during differentiation. Here we show that UTX, a H3K27 demethylase, acts as a critical switch to promote a cardiac-specific gene program. UTX-deficient ESCs failed to develop heart-like rhythmic contractions under a cardiac differentiation condition. UTX-deficient mice show severe defects in heart development and embryonic lethality. We found that UTX is recruited to cardiac-specific enhancers by associating with core cardiac transcription factors and demethylates H3K27 residues in cardiac genes. In addition, UTX facilitates the recruitment of Brg1 to the cardiac-specific enhancers. Together, our data reveal key roles for UTX in a timely transition from poised to active chromatin in cardiac genes during heart development and a fundamental mechanism by which a H3K27 demethylase triggers tissue-specific chromatin changes.

Adeno-associated viral vector mediated and cardiac-specific delivery of CD151 gene in ischemic rat hearts

Our previous studies demonstrated that CD151 gene promoted neovascularization in ischemic heart model. To improve the delivery efficacy and target specificity of CD151 gene to ischemic heart, we generated an adeno-associated virus (AAV) vector in which CD151 expression was controlled by the myosin light chain (MLC-2v) promoter to achieve the cardiac-specific expression of CD151 gene in ischemic myocardium and to limit unwanted CD151 expression in extracardiac organs. The function of this vector was examined in rat ischemic myocardium model. The protein expression of CD151 in the ischemic myocardium areas, liver and kidney was confirmed by using Western blot, while the microvessels within ischemic myocardium areas were detected by using immunohistochemistry. The results showed that MLC-2v significantly enhanced the expression of CD151 in ischemic myocardium, but attenuated its expression in other organs. The forced CD151 expression could increase the number of microvessels in the ischemic myocardium. This study demonstrates the AAV-mediated and MLC-2v regulated CD151 gene is highly expressed in the ischemic myocardium and cardiac-specific delivery that is more efficiently targets CD151 to the ischemia myocardium after myocardial infarction.

Differential gene expressions in atrial and ventricular myocytes: insights into the road of applying embryonic stem cell-derived cardiomyocytes for future therapies

Myocardial infarction has been the leading cause of morbidity and mortality in developed countries over the past few decades. The transplantation of cardiomyocytes offers a potential method of treatment. However, cardiomyocytes are in high demand and their supply is extremely limited. Embryonic stem cells (ESCs), which have been isolated from the inner cell mass of blastocysts, can self-renew and are pluripotent, meaning they have the ability to develop into any type of cell, including cardiomyocytes. This suggests that ESCs could be a good source of genuine cardiomyocytes for future therapeutic purposes. However, problems with the yield and purity of ESC-derived cardiomyocytes, among other hurdles for the therapeutic application of ESC-derived cardiomyocytes (e.g., potential immunorejection and tumor formation problems), need to be overcome before these cells can be used effectively for cell replacement therapy. ESC-derived cardiomyocytes consist of nodal, atrial, and ventricular cardiomyocytes. Specifically, for treatment of myocardial infarction, transplantation of a sufficient quantity of ventricular cardiomyocytes, rather than nodal or atrial cardiomyocytes, is preferred. Hence, it is important to find ways of increasing the yield and purity of specific types of cardiomyocytes. Atrial and ventricular cardiomyocytes have differential expression of genes (transcription factors, structural proteins, ion channels, etc.) and are functionally distinct. This paper presents a thorough review of differential gene expression in atrial and ventricular myocytes, their expression throughout development, and their regulation. An understanding of the molecular and functional differences between atrial and ventricular myocytes allows discussion of potential strategies for preferentially directing ESCs to differentiate into chamber-specific cells, or for fine tuning the ESC-derived cardiomyocytes into specific electrical and contractile phenotypes resembling chamber-specific cells.

Characterization of promoter elements required for cardiac chamber-specific expression

Salmon cardiac natriuretic peptide (sCP, an A-type natriuretic peptide) is an excellent model for the study of cardiac chamber-specific gene expression because it is uniquely specific to the heart and its promoter drives gene expression effectively in mammalian cardiac atrial but not in ventricular cells. We have now prepared hybrid luciferase constructs containing specific sequences from both sCP and BNP 5' promoters. According to our results the simple addition of a short rat BNP proximal promoter fragment to the inert 846 nucleotide sCP proximal promoter increases 100 times the basal activity of the sCP promoter in rat ventricular cardiomyocytes, and also conveys inducibility by mechanical load and endothelin-1. Thus, a small rBNP promoter fragment can transform the prototypical A-type natriuretic peptide regulation of sCP to B-type regulation, a result which argues against a major role of repressors causing the low expression level of A-type peptides in ventricular cardiomyocytes.

Transcriptional analysis of the mammalian heart with special reference to its endocrine function

Background

Pharmacological and gene ablation studies have demonstrated the crucial role of the endocrine function of the heart as mediated by the polypeptide hormones ANF and BNP in the maintenance of cardiovascular homeostasis. The importance of these studies lies on the fact that hypertension and chronic congestive heart failure are clinical entities that may be regarded as states of relative deficiency of ANF and BNP. These hormones are produced by the atrial muscle cells (cardiocytes), which display a dual secretory/muscle phenotype. In contrast, ventricular cardiocytes display mainly a muscle phenotype. Comparatively little information is available regarding the genetic background for this important phenotypic difference with particular reference to the endocrine function of the heart. We postulated that comparison of gene expression profiles between atrial and ventricular muscles would help identify gene transcripts that underlie the phenotypic differences associated with the endocrine function of the heart.

Results

Comparison of gene expression profiles in the rat heart revealed a total of 1415 differentially expressed genes between the atria and ventricles based on a 1.8 fold cut-off. The identification of numerous chamber specific transcripts, such as ANF for the atria and Irx4 for the ventricles among several others, support the soundness of the GeneChip data and demonstrates that the differences in gene expression profiles observed between the atrial and ventricular tissues were not spurious in nature. Pathway analysis revealed unique expression profiles in the atria for G protein signaling that included Gα_o1, Gγ₂ and Gγ₃, AGS1, RGS2, and RGS6 and the related K⁺ channels GIRK1 and GIRK4. Transcripts involved in vesicle trafficking, hormone secretion as well as mechanosensors (e.g. the potassium channel TREK-1) were identified in relationship to the synthesis, storage and secretion of hormones.

Conclusion

The data developed in this investigation describes for the first time data on gene expression particularly centred on the secretory function of the heart. This provides for a rational approach in the investigation of determinants of the endocrine of the heart in health and disease.

Transient and transgenic analysis of the zebrafish ventricular myosin heavy chain (vmhc) promoter: an inhibitory mechanism of ventricle-specific gene expression

The zebrafish ventricular myosin heavy chain (vmhc) gene exhibits restricted expression in the ventricle. However, the molecular mechanism underlying this chamber-specific expression is unclear. Here, we exploited both transient and transgenic technologies to dissect the zebrafish vmhc promoter. We demonstrated that a combination of two transient assays in this animal model quickly identified chamber-specific cis-elements, isolating a 2.2 kb fragment upstream from the vmhc gene that can drive ventricle-specific expression. Furthermore, deletion analysis identified multiple cis-elements that exhibited cardiac-specific expression. To achieve chamber specificity, a distal element was required to coordinate with and suppress a proximal enhancer element. Finally, we discovered that Nkx2.5-binding sites (NKE) were essential for this repressive function. In summary, our study of the zebrafish vmhc promoter suggests that ventricle-specific expression is achieved through an inhibitory mechanism that suppresses expression in the atrium. Developmental Dynamics 238:1564-1573, 2009. (c) 2009 Wiley-Liss, Inc.

Distinct function of 2 chromatin remodeling complexes that share a common subunit, Williams syndrome transcription factor (WSTF)

A number of nuclear complexes modify chromatin structure and operate as functional units. However, the in vivo role of each component within the complexes is not known. ATP-dependent chromatin remodeling complexes form several types of protein complexes, which reorganize chromatin structure cooperatively with histone modifiers. Williams syndrome transcription factor (WSTF) was biochemically identified as a major subunit, along with 2 distinct complexes: WINAC, a SWI/SNF-type complex, and WICH, an ISWI-type complex. Here, WSTF(-/-) mice were generated to investigate its function in chromatin remodeling in vivo. Loss of WSTF expression resulted in neonatal lethality, and all WSTF(-/-) neonates and approximately 10% of WSTF(+/-) neonates suffered cardiovascular abnormalities resembling those found in autosomal-dominant Williams syndrome patients. Developmental analysis of WSTF(-/-) embryos revealed that Gja5 gene regulation is aberrant from E9.5, conceivably because of inappropriate chromatin reorganization around the promoter regions where essential cardiac transcription factors are recruited. In vitro analysis in WSTF(-/-) mouse embryonic fibroblast (MEF) cells also showed impaired transactivation functions of cardiac transcription activators on the Gja5 promoter, but the effects were reversed by overexpression of WINAC components. Likewise in WSTF(-/-) MEF cells, recruitment of Snf2h, an ISWI ATPase, to PCNA and cell survival after DNA damage were both defective, but were ameliorated by overexpression of WICH components. Thus, the present study provides evidence that WSTF is shared and is a functionally indispensable subunit of the WICH complex for DNA repair and the WINAC complex for transcriptional control.

Mef2c is an essential regulatory element required for unique expression of the cardiac-specific CARK gene

The cardiac ankyrin repeat kinase (CARK) gene, also named TNNI3K for its interaction with cardiac troponin I, is both a unique expression and heart-enriched gene. To understand the mechanisms of CARK gene expression and regulation, we first cloned the full-length mRNA sequence and mapped the transcription start site of the mouse CARK gene and characterized its promoter regions. Two transcriptional isoforms of the CARK gene were identified in mouse heart tissue. Truncation analysis of the CARK promoter identified a minimal 151 bp region that has strong basal transcription activity. Mutational analysis revealed five conserved cis-acting elements in this 151-bp long minimal promoter. Mutational and loss-of-functional analysis and co-transfection studies indicated that MEF2 binding region is the most critical cis-acting element in the CARK promoter, and CARK transcription level can be down-regulated by MEF2C antisense. Binding to the MEF2 sites by Mef2c protein was confirmed by electrophoretic mobility shift assay and competition and supershift electrophoretic mobility shift assays.

Transcription factor Sp3 knockout mice display serious cardiac malformations

Mice lacking the zinc finger transcription factor specificity protein 3 (Sp3) die prenatally in the C57BL/6 background. To elucidate the cause of mortality we analyzed the potential role of Sp3 in embryonic heart development. Sp3 null hearts display defective looping at embryonic day 10.5 (E10.5), and at E14.5 the Sp3 null mutants have developed a range of severe cardiac malformations. In an attempt to position Sp3 in the cardiac developmental hierarchy, we analyzed the expression patterns of >15 marker genes in Sp3 null hearts. Expression of cardiac ankyrin repeat protein (Carp) was downregulated prematurely after E12.5, while expression of the other marker genes was not affected. Chromatin immunoprecipitation analysis revealed that Sp3 is bound to the Carp promoter region in vivo. Microarray analysis indicates that small-molecule metabolism and cell-cell interactions are the most significantly affected biological processes in E12.5 Sp3 null myocardium. Since the epicardium showed distension from the myocardium, we studied expression of Wt1, a marker for epicardial cells. Wt1 expression was diminished in epicardium-derived cells in the myocardium of Sp3 null hearts. We conclude that Sp3 is required for normal cardiac development and suggest that it has a crucial role in myocardial differentiation.

Hypoplastic left heart syndrome is heritable

OBJECTIVES

This study sought to determine the size of the genetic effect (heritability) in families identified by a hypoplastic left heart syndrome (HLHS) proband.

BACKGROUND

Hypoplastic left heart syndrome is a severe form of cardiovascular malformation (CVM), and it remains a leading cause of infant mortality and childhood morbidity. Familial clustering of HLHS and bicuspid aortic valve (BAV) has been observed, and pedigree analysis has suggested recessive inheritance. The genetic significance of these observations is unknown.

METHODS

In 38 probands with HLHS, a 3-generation family history was obtained; using a sequential sampling strategy, echocardiograms on family members were performed. A total of 235 participants were recruited. Heritability (h2) of HLHS and associated CVM was estimated using maximum-likelihood-based variance decomposition.

RESULTS

All HLHS probands had aortic valve hypoplasia and dysplasia; dysplasia of the mitral (94%), tricuspid (56%), and pulmonary (11%) valves was also noted. Overall, 21 of 38 (55%) families had more than 1 affected individual, and 36% of participants had CVM, including 11% with BAV. The heritability of HLHS alone and with associated CVM were 99% and 74% (p < 0.00001), respectively. The sibling recurrence risk for HLHS was 8%, and for CVM was 22%.

CONCLUSIONS

The high heritability of HLHS suggests that it is determined largely by genetic factors. The frequent occurrence of left- and right-sided valve dysplasia in HLHS probands and the increased prevalence of BAV in family members suggests that HLHS is a severe form of valve malformation.

Patterning the heart, a template for human cardiomyocyte development

Although in mice, the dynamics of gene expression during heart development is well characterized, information on humans is scarce due to the limited availability of material. Here, we analyzed the transcriptional distribution of Mlc-2a, Mlc-1v, Mlc-2v, and atrial natriuretic factor (ANF) in human embryonic hearts between 7 and 18 weeks of gestation and in healthy and hypertrophic adult hearts by in situ hybridization and compared expression with that in mice. Strikingly, Mlc-2a, Mlc-1v, and ANF, which are essentially chamber-restricted in mice by mid-gestation, showed a broader distribution in humans. On the other hand, Mlc-2v may prove to be an adequate ventricular marker in humans in contrast to mouse. This study emphasizes the importance of careful comparative human-animal analyses during embryonic development and adulthood, as avoiding erroneous extrapolations may be critical to develop new and successful myocardial replacement therapies.

Multimodal functional cardiac MRI in creatine kinase-deficient mice reveals subtle abnormalities in myocardial perfusion and mechanics

A decrease in the supply of ATP from the creatine kinase (CK) system is thought to contribute to the evolution of heart failure. However, previous studies on mice with a combined knockout of the mitochondrial and cytosolic CK (CK−/−) have not revealed overt left ventricular dysfunction. The aim of this study was to employ novel MRI techniques to measure maximal myocardial velocity ( Vmax) and myocardial perfusion and thus determine whether abnormalities in the myocardial phenotype existed in CK−/− mice, both at baseline and 4 wk after myocardial infarction (MI). As a result, myocardial hypertrophy was seen in all CK−/− mice, but ejection fraction (EF) remained normal. Vmax, however, was significantly reduced in the CK−/− mice [wild-type, 2.32 ± 0.09 vs. CK−/−, 1.43 ± 0.16 cm/s, P < 0.05; and wild-type MI, 1.53 ± 0.11 vs. CK−/− MI, 1.26 ± 0.11 cm/s, P = not significant (NS), P < 0.05 vs. baseline]. Myocardial perfusion was also lower in the CK−/− mice (wild-type, 6.68 ± 0.27 vs. CK−/−, 4.12 ± 0.63 ml/g·min, P < 0.05; and wild-type MI, 3.97 ± 0.65 vs. CK−/− MI, 3.71 ± 0.57 ml/g·min, P = NS, P < 0.05 vs. baseline), paralleled by a significantly reduced capillary density (histology). In conclusion, myocardial function in transgenic mice may appear normal when only gross indexes of performance such as EF are assessed. However, the use of a combination of novel MRI techniques to measure myocardial perfusion and mechanics allowed the abnormalities in the CK−/− phenotype to be detected. The myocardium in CK-deficient mice is characterized by reduced perfusion and reduced maximal contraction velocity, suggesting that the myocardial hypertrophy seen in these mice cannot fully compensate for the absence of the CK system.

Differential regional gene expression from cardiac dyssynchrony induced by chronic right ventricular free wall pacing in the mouse

Routine clinical right ventricular pacing generates left ventricular dyssynchrony manifested by early septal shortening followed by late lateral contraction, which, in turn, reciprocally stretches the septum. Dyssynchrony is disadvantageous to cardiac mechanoenergetics and worsens clinical prognosis, yet little is known about its molecular consequences. Here, we report the influence of cardiac dyssynchrony on regional cardiac gene expression in mice. Mice were implanted with a custom-designed miniature cardiac pacemaker and subjected to 1-wk overdrive right ventricular free wall pacing (720 beats/min, baseline heart rate 520-620 beats/min) to generate dyssynchrony (pacemaker: 3-V lithium battery, rate programmable, 1.5 g, bipolar lead). Electrical capture was confirmed by pulsed-wave Doppler and dyssynchrony by echocardiography. Gene expression from the left ventricular septal and lateral wall myocardium was assessed by microarray (dual-dye method, Agilent) using oligonucleotide probes and dye swap. Identical analysis was applied to four synchronously contracting controls. Of the 22,000 genes surveyed, only 18 genes displayed significant (P < 0.01) differential expression between septal/lateral walls >1.5 times that in synchronous controls. Gene changes were confirmed by quantitative PCR with excellent correlations. Most of the genes (n = 16) showed greater septal expression. Of particular interest were seven genes coding proteins involved with stretch responses, matrix remodeling, stem cell differentiation to myocyte lineage, and Purkinje fiber differentiation. One week of iatrogenic cardiac dyssynchrony triggered regional differential expression in relatively few select genes. Such analysis using a murine implantable pacemaker should facilitate molecular studies of cardiac dyssynchrony and help elucidate novel mechanisms by which stress/stretch stimuli due to dyssynchrony impact the normal and failing heart.

Building the mammalian heart from two sources of myocardial cells

Cardiogenesis is an exquisitely sensitive process. Any perturbation in the cells that contribute to the building of the heart leads to cardiac malformations, which frequently result in the death of the embryo. Previously, the myocardium was thought to be derived from a single source of cells. However, the recent identification of a second source of myocardial cells that make an important contribution to the cardiac chambers has modified the classical view of heart formation. It also has an important influence on the interpretation of mutant phenotypes in the mouse, with consequences for the classification and prognosis of human congenital heart defects.

Reference guide to the stages of chick heart embryology

Cardiac progenitors of the splanchnic mesoderm (primary and secondary heart field), cardiac neural crest, and the proepicardium are the major embryonic contributors to chick heart development. Their contribution to cardiac development occurs with precise timing and regulation during such processes as primary heart tube fusion, cardiac looping and accretion, cardiac septation, and the development of the coronary vasculature. Heart development is even more complex if one follows the development of the cardiac innervation, cardiac pacemaking and conduction system, endocardial cushions, valves, and even the importance of apoptosis for proper cardiac formation. This review is meant to provide a reference guide (Table 1) on the developmental timing according to the staging of Hamburger and Hamilton (1951) (HH) of these important topics in heart development for those individuals new to a chick heart research laboratory. Even individuals outside of the heart field, who are working on a gene that is also expressed in the heart, will gain information on what to look for during chick heart development. This reference guide provides complete and easy reference to the stages involved in heart development, as well as a global perspective of how these cardiac developmental events overlap temporally and spatially, making it a good bench top companion to the many recently written in-depth cardiac reviews of the molecular aspects of cardiac development.

Expression and regulation of mouse SERDIN1, a highly conserved cardiac-specific leucine-rich repeat protein

Despite recent progress, the precise mechanisms responsible for vertebrate cardiac development are still enigmatic. Better understanding of cardiac biology and disease necessitates identification and analysis of a full spectrum of regulatory and structural proteins specific to the developing heart. By performing an in silico screen, we identified a cardiac-specific gene we named Serdin1. The Serdin1 gene is conserved, and the message is restricted to the heart in several vertebrate species, thus implicating Serdin1 as an important gene in cardiac development. In situ hybridization confirmed that the Serdin1 message is cardiac-specific in mice as early as embryonic day 8.5. Antibody staining demonstrated predominantly nuclear staining in immortalized cardiac cell lines (P19 and HL-1) and proliferating cultured cardiomyocytes, whereas in vivo SERDIN1 localizes to I bands of the sarcomere. Seven kilobases of the upstream regulatory sequence of Serdin1 is sufficient for cardiac-specific expression. Computer analysis revealed an 80-bp homologous region between the mouse and the human Serdin genes that contains GATA, SRF, and MEF sites. Cardiac specificity and localization patterns suggest that SERDIN1 is intimately integrated with the molecular pathways controlling cardiogenesis in vertebrates.

Functional profiling of human atrial and ventricular gene expression

The purpose of our investigation was to identify the transcriptional basis for ultrastructural and functional specialization of human atria and ventricles. Using exploratory microarray analysis (Affymetrix U133A+B), we detected 11,740 transcripts expressed in human heart, representing the most comprehensive report of the human myocardial transcriptome to date. Variation in gene expression between atria and ventricles accounted for the largest differences in this data set, as 3.300 and 2.974 transcripts showed higher expression in atria and ventricles, respectively. Functional classification based on Gene Ontology identified chamber-specific patterns of gene expression and provided molecular insights into the regional specialization of cardiomyocytes, correlating important functional pathways to transcriptional activity: Ventricular myocytes preferentially express genes satisfying contractile and energetic requirements, while atrial myocytes exhibit specific transcriptional activities related to neurohumoral function. In addition, several pro-fibrotic and apoptotic pathways were concentrated in atrial myocardium, substantiating the higher susceptibility of atria to programmed cell death and extracellular matrix remodelling observed in human and experimental animal models of heart failure. Differences in transcriptional profiles of atrial and ventricular myocardium thus provide molecular insights into myocardial cell diversity and distinct region-specific adaptations to physiological and pathophysiological conditions. Moreover, as major functional classes of atrial- and ventricular-specific transcripts were common to human and murine myocardium, an evolutionarily conserved chamber-specific expression pattern in mammalian myocardium is suggested.

Cellular recruitment and the development of the myocardium

The vertebrate embryo experiences very rapid growth following fertilization. This necessitates the establishment of blood circulation, which is initiated during the early somite stages of development when the embryo begins to exhibit three-dimensional tissue organization. Accordingly, the contractile heart is the first functional organ that develops in both the bird and mammalian embryo. The vertebrate heart is quickly assembled as a simple two-layer tube consisting of an outer myocardium and inner endocardium. During embryogenesis, the heart undergoes substantial growth and remodeling to meet the increased circulatory requirements of an adult organism. Until recently, it was thought that all the cells that comprise the muscle of the mature heart could trace their roots back to two bilaterally distributed mesodermal fields within the early gastrula. It is now known that the cellular components that give rise to the myocardium have multiple ancestries and that de novo addition of cardiac myocytes to the developing heart occurs at various points during embryogenesis. In this article, we review what is presently known about the source of the cells that contribute to the myocardium and explore reasons why multiple myocardial cell sources exist.

Adeno-associated viral vector delivers cardiac-specific and hypoxia-inducible VEGF expression in ischemic mouse hearts

It has been shown that the adeno-associated virus (AAV) vector can deliver the VEGF gene efficiently into the ischemic mouse myocardium. However, the AAV genomes can be found in extracardiac organs after intramyocardial injection. To limit unwanted VEGF expression in organs other than the heart, we tested the use of the cardiac myosin light chain 2v (MLC-2v) promoter and the hypoxia-response element to mediate cardiac-specific and hypoxia-inducible VEGF expression. An AAV vector, MLCVEGF, with 250 bp of the MLC-2v promoter and nine copies of the hypoxia-response element driving VEGF expression, was constructed. Gene expression was studied in vitro by infection of rat cardiomyocytes, rat skeletal myocytes, and mouse fibroblasts with the vector and in vivo by direct injection of the vector into normal and ischemic mouse hearts. With MLCVEGF infection, VEGF expression was higher in cardiomyocytes than the other two cell lines and was hypoxiainducible. VEGF expression was also higher in ischemic hearts than in normal hearts. No VEGF expression was detectable in organs with detectable MLCVEGF vectors other than the heart. MLCVEGF-injected ischemic hearts had more capillaries and small vessels around the injection site, smaller infarct size, and better cardiac function than the negative controls. Hence, MLCVEGF can mediate cardiac-specific and hypoxia-inducible VEGF expression, neoangiogenesis, infarct-size reduction, and cardiac functional improvement.

Architectural Plan for the Heart: Early Patterning and Delineation of the Chambers and the Nodes

During folding of the embryo, lateroanterior visceral mesoderm forms the embryonic tubular heart at the midline, just ventral to the foregut. In mice, this nascent tube contains the future left ventricle and atrioventricular canal. Mesenchymal cells subsequently recruited to the cardiac lineage at the intake and the outflow of the tube will form the atria and the right ventricle and outflow tract, respectively. Shortly after its emergence, the embryonic heart tube starts to loop, and the first signs of left ventricular chamber differentiation become visible on the outer curvature of the middle portion of the tube. Subsequently, the right ventricle differentiates cranially, and the atria caudally, while the inflow tract, atrioventricular canal, inner curvatures, and outflow tract form recognizable components flanking the chambers. The latter, nonchamber regions in turn provide signals for the formation of the cushion mesenchyme, are involved in remodeling of the heart, and form the nodes of the conduction system. This review discusses how the patterning of the heart tube relates to the localized differentiation of atrial and ventricular chambers, why some parts of the heart do not form chambers, and how this relates to the formation of the conduction system.