Novel cell lines promote the discovery of genes involved in early heart development

Multicellular In vitro Models of Cardiac Arrhythmias: Focus on Atrial Fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia in clinical practice with a large socioeconomic impact due to its associated morbidity, mortality, reduction in quality of life and health care costs. Currently, antiarrhythmic drug therapy is the first line of treatment for most symptomatic AF patients, despite its limited efficacy, the risk of inducing potentially life-threating ventricular tachyarrhythmias as well as other side effects. Alternative, in-hospital treatment modalities consisting of electrical cardioversion and invasive catheter ablation improve patients' symptoms, but often have to be repeated and are still associated with serious complications and only suitable for specific subgroups of AF patients. The development and progression of AF generally results from the interplay of multiple disease pathways and is accompanied by structural and functional (e.g., electrical) tissue remodeling. Rational development of novel treatment modalities for AF, with its many different etiologies, requires a comprehensive insight into the complex pathophysiological mechanisms. Monolayers of atrial cells represent a simplified surrogate of atrial tissue well-suited to investigate atrial arrhythmia mechanisms, since they can easily be used in a standardized, systematic and controllable manner to study the role of specific pathways and processes in the genesis, perpetuation and termination of atrial arrhythmias. In this review, we provide an overview of the currently available two- and three-dimensional multicellular in vitro systems for investigating the initiation, maintenance and termination of atrial arrhythmias and AF. This encompasses cultures of primary (animal-derived) atrial cardiomyocytes (CMs), pluripotent stem cell-derived atrial-like CMs and (conditionally) immortalized atrial CMs. The strengths and weaknesses of each of these model systems for studying atrial arrhythmias will be discussed as well as their implications for future studies.

Dysregulation of TBX1 dosage in the anterior heart field results in congenital heart disease resembling the 22q11.2 duplication syndrome

Non-allelic homologous recombination events on chromosome 22q11.2 during meiosis can result in either the deletion (22q11.2DS) or duplication (22q11.2DupS) syndrome. Although the spectrum and frequency of congenital heart disease (CHD) are known for 22q11.2DS, there is less known for 22q11.2DupS. We now evaluated cardiac phenotypes in 235 subjects with 22q11.2DupS including 102 subjects we collected and 133 subjects that were previously reported as a confirmation and found 25% have CHD, mostly affecting the cardiac outflow tract (OFT). Previous studies have shown that global loss or gain of function (LOF; GOF) of mouse Tbx1, encoding a T-box transcription factor mapping to the region of synteny to 22q11.2, results in similar OFT defects. To further evaluate Tbx1 function in the progenitor cells forming the cardiac OFT, termed the anterior heart field, Tbx1 was overexpressed using the Mef2c-AHF-Cre driver (Tbx1 GOF). Here we found that all resulting conditional GOF embryos had a persistent truncus arteriosus (PTA), similar to what was previously reported for conditional Tbx1 LOF mutant embryos. To understand the basis for the PTA in the conditional GOF embryos, we found that proliferation in the Mef2c-AHF-Cre lineage cells before migrating to the heart, was reduced and critical genes were oppositely changed in this tissue in Tbx1 GOF embryos versus conditional LOF embryos. These results suggest that a major function of TBX1 in the AHF is to maintain the normal balance of expression of key cardiac developmental genes required to form the aorta and pulmonary trunk, which is disrupted in 22q11.2DS and 22q11.2DupS.

Hic-5 regulates epithelial to mesenchymal transition in ovarian cancer cells in a TGFβ1-independent manner

The molecular basis of epithelial ovarian cancer (EOC) dissemination is still poorly understood. We have previously identified the hydrogen peroxide-inducible clone-5 (Hic-5) gene as hypomethylated in high-grade (HG) serous EOC tumors, compared to normal ovarian tissues. Hic-5 is a focal adhesion scaffold protein and has been primarily studied for its role as a key mediator of TGF-β–induced epithelial-to-mesenchymal transition (EMT) in epithelial cells of both normal and malignant origin; however, its role in EOC has been never investigated.

Here we demonstrate that Hic-5 is overexpressed in advanced EOC, and that Hic-5 is upregulated upon TGFβ1 treatment in the EOC cell line with epithelial morphology (A2780s), associated with EMT induction. However, ectopic expression of Hic-5 in A2780s cells induces EMT independently of TGFβ1, accompanied with enhancement of cellular proliferation rate and migratory/invasive capacity and increased resistance to chemotherapeutic drugs. Moreover, Hic-5 knockdown in the EOC cells with mesenchymal morphology (SKOV3) was accompanied by induction of mesenchymal-to-epithelial transition (MET), followed by a reduction of their proliferative, migratory/invasive capacity, and increased drugs sensitivity in vitro, as well as enhanced tumor cell colonization and metastatic growth in vivo. The modulation of Hic-5 expression in EOC cells resulted in altered regulation of numerous EMT-related canonical pathways and was indicative for a possible role of Hic-5 in controlling EMT through a RhoA/ROCK mediated mechanism.

To our knowledge, this is the first report examining the role of Hic-5 in EOC, and its role in maintaining the mesenchymal phenotype of EOC cells independently of exogenous TGFβ1 treatment.

GATA4 transgenic mice as an in vivo model of congenital heart disease

Our previous study indicated that 8 patients from a family with a history of congenital heart disease had simple atrial septal defect (ASD) and carried the same mutation at codon 310 in the GATA4 gene. In the present study, to identify the functional defects caused by this mutation in an in vivo model, the transgene DNA constructs were microinjected into mice to generate a transgenic mouse model. The mice were genotyped using PCR and DNA sequencing. Protein expression was measured by western blot analysis. qPCR was used to determine the copy number of the transgenes. The heart tissue was fixed and sectioned by conventional procedures. The Vevo 2000 system was used to perform echocardiography on the mice. The expression of GATA4 target genes was measured using the real-time PCR system. The incidence of ASD in the heterozygous transgenic mice was found to be greater than that in the wild-type control mice (P<0.05). In addition, the expression of α-myosin heavy chain (α-MHC) in the heart tissues from the homozygous mice was lower than that in the heart tissues from their wild-type littermates (P<0.05). In conclusion, these results suggest that the introduction of GATA4 M310V negatively affects the normal expression of α-MHC. In accordance with previous findings on GATA4 mutation screening and in vitro experiments, this study confirms that GATA4 M310V mutation may lead to the development of the congenital heart defect, ASD.

Transforming growth factor-β1-induced transcript 1 protein, a novel marker for smooth muscle contractile phenotype, is regulated by serum response factor/myocardin protein

Serum response factor (SRF) plays a central role in regulating expression of smooth muscle-specific genes partly by associating with the potent tissue-specific cofactor myocardin. Previous studies have shown that transforming growth factor-β1-induced transcript 1 (TGFB1I1, also known as Hic-5) is a TGF-β-responsive gene and is involved in the cellular response to vascular injury, but the regulation of TGFB1I1 expression remains elusive. In this report, we demonstrated that TGFB1I1 is a novel marker for the smooth muscle contractile phenotype and is regulated by SRF/myocardin. We found that TGFB1I1 is specifically expressed in smooth muscle cells (SMCs) and in smooth muscle-rich tissues. Furthermore, TGFB1I1 expression is significantly down-regulated in a variety of models for smooth muscle phenotypic modulation. The TGFB1I1 promoter contains an evolutionarily conserved CArG element, and this element is indispensible for myocardin-induced transactivation of TGFB1I1 promoter. By oligonucleotide pulldown and chromatin immunoprecipitation assays, we found that SRF binds to this CArG element in vitro and in vivo. Ectopic expression of myocardin is sufficient to induce endogenous TGFB1I1 expression in multiple cell lines whereas knocking-down myocardin or SRF significantly attenuated TGFB1I1 expression in SMCs. Furthermore, our data demonstrated that SRF is essential for TGF-β-mediated induction of TGFB1I1. Finally, silencing of TGFB1I1 expression significantly promotes SMC proliferation. Collectively, this study provides the first evidence that TGFB1I1 is not only an SRF/myocardin-regulated smooth muscle marker but also critical for maintaining smooth muscle contractile phenotype by inhibiting smooth muscle proliferation.

Expression of focal adhesion proteins in the developing rat kidney

Focal adhesions play a critical role as centers that transduce signals by cell-matrix interactions and regulate fundamental processes such as proliferation, migration, and differentiation. Focal adhesion kinase (FAK), paxillin, integrin-linked kinase (ILK), and hydrogen peroxide-inducible clone-5 (Hic-5) are major proteins that contribute to these events. In this study, we investigated the expression of focal adhesion proteins in the developing rat kidney. Western blotting analysis revealed that the protein levels of FAK, p-FAK(397), paxillin, p-paxillin(118), and Hic-5 were high in embryonic kidneys, while ILK expression persisted from the embryonic to the mature stage. Immunohistochemistry revealed that FAK, p-FAK(397), paxillin, and p-paxillin(118) were strongly expressed in condensed mesenchymal cells and the ureteric bud. They were detected in elongating tubules and immature glomerular cells in the nephrogenic zone. Hic-5 was predominantly expressed in mesenchymal cells as well as immature glomerular endothelial and mesangial cells, suggesting that Hic-5 might be involved in mesenchymal cell development. ILK expression was similar to that of FAK in the developmental stages. Interestingly, ILK was strongly expressed in podocytes in mature glomeruli. ILK might play a role in epithelial cell differentiation as well as kidney growth and morphogenesis. In conclusion, the temporospatially regulated expression of focal adhesion proteins during kidney development might play a role in morphogenesis and cell differentiation.

Hic-5 is required for fetal gene expression and cytoskeletal organization of neonatal cardiac myocytes

Cardiac costameres link the extracellular matrix to the sarcomere at the z-disc and contain proteins such as integrins and other signaling molecules implicated in the regulation of pathological hypertrophy. Paxillin family members, hic-5 and paxillin, are scaffolding proteins associated with the integrin complex that have been shown to mediate numerous protein interactions in other cell types. While paxillin has been described in postnatal heart, hic-5 has not been identified. Our results provide evidence of hic-5 in neonatal cardiac myocytes co-localized with paxillin and alpha-actinin at the z-discs and the ends of actin filaments. Treatment with the hypertrophic agonist phenylephrine resulted in increased hic-5 expression while having no effect on paxillin levels. To see if increased hic-5 expression was sufficient to induce changes in cytoskeletal organization, hic-5 was overexpressed in myocytes by adenoviral infection. Hic-5 overexpression significantly increased the number of cells with organized cytoskeleton. Using siRNA mediated knockdown, we examined the requirement of hic-5 and paxillin in regulation of phenylephrine induced gene expression and cytoskeletal organization. Our results indicate that hic-5, not paxillin is required for upregulation of ANF and alpha-skeletal actin genes as well as in cytoskeletal reorganization. Finally, we demonstrated that hic-5 upregulation occurs downstream of MEK1/2-ERK1/2 signaling as inhibition of MEK1/2 using U0126 inhibitor completely inhibited hic-5 upregulation by PE. In a complimentary study, we showed that hic-5 knockdown had no effect on PE induced ERK1/2 phosphorylation. These findings demonstrate a novel role for hic-5 in the regulation of actin cytoskeleton and fetal gene expression.

Tissue specific promoters improve specificity of AAV9 mediated transgene expression following intra-vascular gene delivery in neonatal mice

The AAV9 capsid displays a high natural affinity for the heart following a single intravenous (IV) administration in both newborn and adult mice. It also results in substantial albeit relatively lower expression levels in many other tissues. To increase the overall safety of this gene delivery method we sought to identify which one of a group of promoters is able to confer the highest level of cardiac specific expression and concurrently, which is able to provide a broad biodistribution of expression across both cardiac and skeletal muscle. The in vivo behavior of five different promoters was compared: CMV, desmin (Des), alpha-myosin heavy chain (α-MHC), myosin light chain 2 (MLC-2) and cardiac troponin C (cTnC). Following IV administration to newborn mice, LacZ expression was measured by enzyme activity assays. Results showed that rAAV2/9-mediated gene delivery using the α-MHC promoter is effective for focal transgene expression in the heart and the Des promoter is highly suitable for achieving gene expression in cardiac and skeletal muscle following systemic vector administration. Importantly, these promoters provide an added layer of control over transgene activity following systemic gene delivery.

Hic-5 promotes endothelial cell migration to lysophosphatidic acid

Endothelial cell migration is critical for proper blood vessel development. Signals from growth factors and matrix proteins are integrated through focal adhesion proteins to alter cell migration. Hydrogen peroxide-inducible clone 5 (Hic-5), a paxillin family member, is enriched in the focal adhesions in bovine pulmonary artery endothelial (BPAE) cells, which migrate to lysophosphatidic acid (LPA) on denatured collagen. In this study, we investigate the role of Hic-5 in LPA-stimulated endothelial cell migration. LPA recruits Hic-5 to the focal adhesions and to the pseudopodia in BPAE cells plated on collagen, suggesting that recruitment of Hic-5 to focal adhesions is associated with endothelial cell migration. Knockdown of endogenous Hic-5 significantly decreases migration toward LPA, confirming involvement of Hic-5 in migration. To address the role of Hic-5 in endothelial cell migration, we exogenously expressed wild-type (WT) Hic-5 and green fluorescent protein Hic-5 C369A/C372A (LIM3 mutant) constructs in BPAE cells. WT Hic-5 expression increases chemotaxis of BPAE cells to LPA, whereas migration toward LPA of the green fluorescent protein Hic-5 C369A/C372A-expressing cells is similar to that shown in vector control cells. Additionally, ERK phosphorylation is enhanced in the presence of LPA in WT Hic-5 cells. A pharmacological inhibitor of MEK activity inhibits LPA-stimulated WT Hic-5 cell migration and ERK phosphorylation, suggesting Hic-5 enhances migration via MEK activation of ERK. Together, these studies indicate that Hic-5, a focal adhesion protein in endothelial cells, is recruited to the pseudopodia in the presence of LPA and enhances migration.

Hic-5 contributes to epithelial-mesenchymal transformation through a RhoA/ROCK-dependent pathway

Epithelial-mesenchymal transformation (EMT) in response to TGFbeta1 is a coordinated process of tissue morphogenesis that occurs during embryonic development as well as during certain pathologic events including kidney tubulointerstitial fibrosis. It is characterized by the disassembly of cell-cell junctions and dramatic alterations in the actin cytoskeleton that facilitates cell-matrix adhesion and stimulates migration. The focal adhesion adapter protein, Hic-5, has previously been reported to be upregulated during TGFbeta1-induced EMT in mouse mammary epithelial cells and the current study recapitulates this result in both mouse kidney proximal tubule epithelial, MCT, cells and human mammary epithelial, MCF10A, cells. To evaluate a causative role for Hic-5 in EMT, Hic-5 RNA interference (siRNA) was used to prevent Hic-5 expression in response to TGFbeta1 stimulation and was shown to suppress cell migration and actin stress fiber formation. It also resulted in the retention of a robust epithelial cell morphology characterized by elevated E-cadherin protein expression and well-organized adherens junctions. In addition, Hic-5 siRNA treatment led to the suppression of TGFbeta1 induction of RhoA activation. In contrast, forced expression of Hic-5 led to the formation of ROCK-dependent actin stress fibers. Furthermore, the induction of Hic-5 expression in response to TGFbeta1 was shown to be a RhoA/ROCK I-dependent process. Together, these data implicate Hic-5 as a key regulator of EMT and suggest that RhoA stimulated Hic-5 expression in response to TGFbeta1 may be functioning in a feed forward mechanism whereby Hic-5 maintains the mesenchymal phenotype through sustained RhoA activation and signaling.

Microarray analysis of Tbx5-induced genes expressed in the developing heart

Tbx5 is a member of the T-box family of transcription factors and is associated with Holt-Oram syndrome (HOS), a congenital disorder characterized by heart and limb defects. Although implicated in several processes during development, only a few genes regulated by Tbx5 have been reported. To identify candidate genes regulated by Tbx5 during heart development, a microarray approach was used. A cardiac-derived mouse cell line (1H) was infected with adenoviruses expressing Tbx5 or beta-galactosidase and RNA was isolated for analysis using an Affymetrix gene chip representing over 39,000 transcripts. Real-time reverse transcriptase-polymerase chain reaction confirmed Tbx5 induction of a subset of the genes, including nppa, photoreceptor cadherin, brain creatine kinase, hairy/enhancer-of-split related 2, and gelsolin. In situ hybridization analysis indicated overlapping expression of these genes with tbx5 in the embryonic mouse heart. In addition, the effect of HOS-associated mutations on the ability of Tbx5 to induce target gene expression was evaluated. Together, these data identify several genes induced by Tbx5 that are potentially important during cardiac development. These genes represent new candidate gene targets of Tbx5 that may be related to congenital heart malformations associated with HOS.

Identification and isolation of embryonic stem cell-derived target cells by adenoviral conditional targeting

The technical limitations of isolating target cells have restricted the utility of pluripotent embryonic stem (ES) cells. For example, early cardiac (i.e., precontractile) cells have not been isolated from ES cells. Here, we find that direct expression of reporter genes under cell-specific promoters-the currently available strategy for isolating cells lacking cell-specific surface markers-is ineffective for isolating progenitor cells. This was due to the weak activity of cell-specific promoters, particularly in ES cells at early stages. We show that adenoviral conditional targeting efficiently isolates viable ES cell-derived target cells without harmful effects. In this strategy, we employ the alpha-myosin heavy chain and Nkx2.5 promoter to visualize and purify efficiently differentiated and primitive cells of the cardiac lineage, respectively. While the former cells predominantly expressed sarcomeric proteins and maintained contractile function, the latter demonstrated neither of these features, but rather exhibited expression patterns characteristic of a mixture of primitive cells and cardiomyocytes. Interestingly, smooth muscle actin was predominantly expressed in the latter cells, and both functionally known and unknown genes were systematically identified, demonstrating the benefits of this system. Thus, our method facilitates molecular and cellular studies of development and ES cell-derived cell therapy.

Proposal to improve vertebrate cell cultures to establish them as substitutes for the regulatory testing of chemicals and effluents using fish

Cultures of vertebrate cells are widely applied in mechanistic studies in human toxicology as well as in toxicity identification in ecotoxicology. As in vitro models, they display many advantages over whole animal experimentation, pertaining to such characteristics as availability, reproducibility and costs. As well, they satisfy the societal desire to reduce the number of animals in toxicology. For these reasons vertebrate cell models also appear to be a desirable replacement for animals in regulatory tests. Several vertebrate cell models are now accepted for regulatory purposes in human health sciences, with the test for photocytotoxicity using the 3T3 mouse cell line being one example. However, an in vitro alternative to whole animal tests has not yet been established for regulatory risk assessment in ecotoxicology. This review sets out to outline why such a replacement has not yet been possible and explores avenues to improve vertebrate cell cultures so that a replacement of whole animal tests could more likely be achieved. Inasmuch as fish is the most widely used non-mammalian vertebrate in risk assessment and regulation, focus will be on the replacement, by in vitro vertebrate models, of fish.

Comprehensive microarray analysis of Hoxa11/Hoxd11 mutant kidney development

The Hox11 paralogous genes play critical roles in kidney development. They are expressed in the early metanephric mesenchyme and are required for the induction of ureteric bud formation and its subsequent branching morphogenesis. They are also required for the normal nephrogenesis response of the metanephric mesenchyme to inductive signals from the ureteric bud. In this report, we use microarrays to perform a comprehensive gene expression analysis of the Hoxa11/Hoxd11 mutant kidney phenotype. We examined E11.5, E12.5, E13.5 and E16.5 developmental time points. A novel high throughput strategy for validation of microarray data is described, using additional biological replicates and an independent microarray platform. The results identified 13 genes with greater than 3-fold change in expression in early mutant kidneys, including Hoxa11s, GATA6, TGFbeta2, chemokine ligand 12, angiotensin receptor like 1, cytochrome P450, cadherin5, and Lymphocyte antigen 6 complex, Iroquois 3, EST A930038C07Rik, Meox2, Prkcn, and Slc40a1. Of interest, many of these genes, and others showing lower fold expression changes, have been connected to processes that make sense in terms of the mutant phenotype, including TGFbeta signaling, iron transport, protein kinase C function, growth arrest and GDNF regulation. These results identify the multiple molecular pathways downstream of Hox11 function in the developing kidney.

Niche-Dependent Translineage Commitment of Endothelial Progenitor Cells, Not Cell Fusion in General, Into Myocardial Lineage Cells

Objective— Previous studies from our laboratory have shown therapeutic potential of ex vivo expanded endothelial progenitor cells (EPCs) for myocardial ischemia. Our purpose was to investigate the mechanisms regulating EPC contribution to myocardial regeneration.

Methods and Results— To evaluate niche-dependent expression profiles of EPCs in vitro, we performed coculture using cultured EPCs derived from human peripheral blood and rat cardiac myoblast cell line (H9C2). Reverse-transcription polymerase chain reaction (PCR) disclosed the expression of human-specific cardiac markers as well as human-specific smooth muscle markers. Cytoimmunochemistry presented several cocultured cells stained with human specific cardiac antibody. To prove this translineage differentiation in vivo, human cultured EPCs were injected into nude rat myocardial infarction model. Reverse-transcription PCR as well as immunohistochemistry of rat myocardial samples demonstrated the expression of human specific cardiac, vascular smooth muscle, and endothelial markers. We observed the distribution of colors (Qtracker; Quantum Dot Corp) in coculture to detect the fused cells, and the frequency of cell fusion was <1%.

Conclusions— EPCs can contribute to not only vasculogenesis but also myogenesis in the ischemic myocardium in vivo. Transdifferentiation, not cell fusion, is dominant for EPCs commitment to myocardial lineage cells. Ex vivo expanded EPCs transplantation might have enhanced therapeutic potential for myocardial regeneration.

Uni-axial stretching regulates intracellular localization of Hic-5 expressed in smooth-muscle cells in vivo

Hic-5 is a focal adhesion protein belonging to the paxillin LIM family that shuttles in and out of the nucleus. In the present study, we examined the expression of Hic-5 among mouse tissues by immunohistochemistry and found its expression only in smooth-muscle cells in several tissues. This result is consistent with a previous report on adult human tissues and contradicts the relatively ubiquitous expression of paxillin, the protein most homologous to Hic-5. One factor characterizing smooth-muscle cells in vivo is a continuous exposure to mechanical stretching in the organs. To study the involvement of Hic-5 in cellular responses to mechanical stress, we exposed mouse embryo fibroblasts to a uni-axial cyclic stretching and found that Hic-5 was relocalized from focal adhesions to stress fibers through its C-terminal LIM domains during the stress. In sharp contrast to this, paxillin did not change its focal-adhesion-based localization. Of the factors tested, which included interacting partners of Hic-5, only CRP2 (an only-LIM protein expressed in vascular smooth-muscle cells) and GIT1 were, like Hic-5, localized to stress fibers during the cyclic stretching. Interestingly, Hic-5 showed a suppressive effect on the contractile capability of cells embedded in three-dimensional collagen gels, and the effect was further augmented when CRP2 co-localized with Hic-5 to fiber structures of those cells. These results suggested that Hic-5 was a mediator of tensional force, translocating directly from focal adhesions to actin stress fibers upon mechanical stress and regulating the contractile capability of cells in the stress fibers.

Paxillin: adapting to change

Molecular scaffold or adaptor proteins facilitate precise spatiotemporal regulation and integration of multiple signaling pathways to effect the optimal cellular response to changes in the immediate environment. Paxillin is a multidomain adaptor that recruits both structural and signaling molecules to focal adhesions, sites of integrin engagement with the extracellular matrix, where it performs a critical role in transducing adhesion and growth factor signals to elicit changes in cell migration and gene expression.

Identification of the TBX5 transactivating domain and the nuclear localization signal

TBX5 is a member of the T-box gene family and encodes a transcription factor involved in cardiac and limb development. Mutations of TBX5 cause Holt-Oram syndrome (HOS), an autosomal-dominant condition with congenital cardiac defects and forelimb anomalies. Here, we used a GAL4-TBX5 fusion protein in a modified yeast-one hybrid system to elucidate the TBX5 transactivating domain. Using a series of deletion mutations of TBX5, we narrowed down its functional domain to amino acids 339-379 of its C-terminal half; point mutagenesis analysis then showed that the loss of amino acids 349-351 abolished transactivation. This result was confirmed in mammalian cells. Furthermore, wild-type TBX5, but not TBX5 with mutations at the amino acids 349-351, has ability to inhibit NCI-H1299 cell growth also suggesting that these amino acids are crucial for the TBX5 function in mammalian cells. In addition, to identify the nuclear localization signal of TBX5, we searched for cluster of basic amino acids. We found that the deletion of the KRK sequence at amino acids 325-327 mislocalizes TBX5 to cytoplasm, suggesting that these amino acids serve as a nuclear localization signal. These studies enhance our understanding of the structure-function relationship of TBX5 and suggest that truncation mutations of TBX5 could cause HOS through the loss of its transactivating domain and/or the nuclear localization signal.

TBX5, a gene mutated in Holt-Oram syndrome, is regulated through a GC box and T-box binding elements (TBEs)

TBX5 is a member of the T-box gene family and encodes a transcription factor that regulates the expression of other gene(s) in the developing heart and limbs. Mutations of TBX5 cause Holt-Oram syndrome (HOS), an autosomal dominant condition characterized by congenital heart defects and limb anomalies. How TBX5 gene expression is regulated is still largely unknown. In order to identify transcription factors regulating TBX5 expression, we examined the 5'-flanking region of the human TBX5 gene. We determined that up to 300 bp of the 5'-flanking region of the TBX5 gene was necessary for promoter activity in mouse cardiomyocyte ECL2 cells. One GC box, three potential T-box-like binding elements (TBE-A, -B, and -C), and one NKX2.5 binding site were identified. Site-directed mutagenesis of the potential binding sites revealed that the GC box, TBE-B, TBE-C, and NKX2.5 are functionally positive for the expression of TBX5. DNA footprint analysis showed that these binding regions are resistant to DNaseI digestion. Electrophoretic mobility shift assays (EMSAs) further demonstrated the protein-DNA interactions at the GC box and the potential TBE-B, TBE-C, and NKX2.5 sites in a sequence-specific manner. The ability of TBX5 to regulate its own promoter was demonstrated by the ability of ectopically expressed human TBX5 to increase reporter expression. We conclude that the GC box, T-box-like binding elements, and NKX2.5 binding site play important roles in the regulation of TBX5 expression, and that TBX5 is likely to be autoregulated as part of the mechanism of its transcription.

Adult cardiac Sca-1-positive cells differentiate into beating cardiomyocytes

Although somatic stem cells have been reported to exist in various adult organs, there have been few reports concerning stem cells in the heart. We here demonstrate that Sca-1-positive (Sca-1+) cells in adult hearts have some of the features of stem cells. Sca-1+ cells were isolated from adult murine hearts by a magnetic cell sorting system and cultured on gelatin-coated dishes. A fraction of Sca-1+ cells stuck to the culture dish and proliferated slowly. When treated with oxytocin, Sca-1+ cells expressed genes of cardiac transcription factors and contractile proteins and showed sarcomeric structure and spontaneous beating. Isoproterenol treatment increased the beating rate, which was accompanied by the intracellular Ca(2+) transients. The cardiac Sca-1+ cells expressed oxytocin receptor mRNA, and the expression was up-regulated after oxytocin treatment. Some of the Sca-1+ cells expressed alkaline phosphatase after osteogenic induction and were stained with Oil-Red O after adipogenic induction. These results suggest that Sca-1+ cells in the adult murine heart have potential as stem cells and may contribute to the regeneration of injured hearts.

Cell cycle regulation to repair the infarcted myocardium

Lower vertebrates such as newt and zebrafish are able to reactivate high levels of cardiomyocyte cell cycle activity in response to experimental injury resulting in apparent regeneration. In contrast, damaged myocardium is replaced by fibrotic scar tissue in higher vertebrates. This process compromises the contractile function of the surviving myocardium, ultimately leading to heart failure. Various strategies are being pursued to augment myocyte number in the diseased hearts. One approach entails the reactivation of cell cycle in surviving cardiomyocytes. Here, we provide a summary of methods to monitor cell cycle activity, and interventions demonstrating positive cell cycle effects in cardiomyocytes as well as discuss the potential utility of cell cycle regulation to augment myocyte number in diseased hearts.

Conditional expression of SV40 T-antigen in mouse cardiomyocytes facilitates an inducible switch from proliferation to differentiation

Studies of cardiac muscle gene expression and signaling have been hampered by the lack of immortalized cardiomyocyte cell lines capable of proliferation and irreversible withdrawal from the cell cycle. With the goal of creating such cell lines, we generated transgenic mice using cardiac-specific cis-regulatory elements from the mouse Nkx2.5 gene to drive the expression of a simian virus 40 large T-antigen (TAg) gene flanked by sites for recombination by Cre recombinase. These transgenic mice developed tumors within the ventricular myocardium. Cells isolated from these tumors expressed cardiac markers and proliferated rapidly during serial passage in culture, without apparent senescence. However, they were unable to exit the cell cycle and failed to exhibit morphological features of terminal differentiation. Introduction of Cre recombinase to these cardiac cell lines by adenoviral delivery resulted in the elimination of TAg expression, accompanied by rapid cessation of cell division, and increase in cell size without an apparent induction of cellular differentiation. Incubation of cells lacking TAg in serum-deficient media with various pharmacological agents (norepinephrine, phenylephrine, or bone morphogenetic protein-2/4) or constitutively active calcium/calmodulin-dependent protein kinase I and/or calcineurin led to the formation of sarcomeres and up-regulation of cardiac genes involved in excitation-contraction coupling. The combination of TAg expression under the control of an early cardiac promoter and Cre-mediated recombination allowed us to derive an immortal cell line from the ventricular myocardium that could be controllably withdrawn from the cell cycle. The conditional expression of TAg in this manner permits propagation and regulated growth termination of cell types that are otherwise unable to be maintained in cell culture and may have applications for cardiac repair technologies.

Expression of the LIM proteins paxillin and Hic-5 in human tissues

The LIM domain is a protein-protein interaction motif critically involved in a variety of fundamental biological processes, including cytoskeletal organization, cell lineage specification, and organ development. In this study we examined the expression of the LIM proteins paxillin and Hic-5 in adult human tissues by immunohistochemistry and immunoblotting. Paxillin expression was widespread and observed both in non-muscle and muscle tissues. Of the latter, paxillin was mainly expressed in multinuclear striated muscle. In contrast, Hic-5 showed restricted expression and was expressed in muscle tissues, mainly in mononuclear smooth muscle. Taken together with previous findings, it appears likely that the counterbalance between paxillin and Hic-5 may be deeply involved in muscle differentiation.

Mouse DTEF-1 (ETFR-1, TEF-5) is a transcriptional activator in alpha 1-adrenergic agonist-stimulated cardiac myocytes

alpha(1)-Adrenergic signaling in cardiac myocytes activates the skeletal muscle alpha-actin gene through an MCAT cis-element, the binding site of the transcriptional enhancer factor-1 (TEF-1) family of transcription factors. TEF-1 accounts for more than 85% of the MCAT binding activity in neonatal rat cardiac myocytes. Other TEF-1 family members account for the rest. Although TEF-1 itself has little effect on the alpha(1)-adrenergic activation of skeletal muscle alpha-actin, the related factor RTEF-1 augments the response and is a target of alpha(1)-adrenergic signaling. Here, we examined another TEF-1 family member expressed in cardiac muscle, DTEF-1, and observed that it also augmented the alpha(1)-adrenergic response of skeletal muscle alpha-actin. A DTEF-1 peptide-specific antibody revealed that endogenous DTEF-1 accounts for up to 5% of the MCAT binding activity in neonatal rat cardiac myocytes. A TEF-1/DTEF-1 chimera suggests that alpha(1)-adrenergic signaling modulates DTEF-1 function. Orthophosphate labeling and immunoprecipitation of an epitope-tagged DTEF-1 showed that DTEF-1 is phosphorylated in vivo. alpha(1)-Adrenergic stimulation increased while phosphatase treatment lowered the MCAT binding by DTEF-1 and the endogenous non-TEF-1 MCAT-binding factor. In contrast, alpha(1)-adrenergic stimulation did not alter, and phosphatase treatment increased, MCAT binding of TEF-1 and RTEF-1. Taken together, these results suggest that DTEF-1 is a target for alpha(1)-adrenergic activation of the skeletal muscle alpha-actin gene in neonatal rat cardiac myocytes.

Cardiomyocyte cell cycle regulation

Although rapid progress is being made in many areas of molecular cardiology, issues pertaining to the origins of heart-forming cells, the mechanisms responsible for cardiogenic induction, and the pathways that regulate cardiomyocyte proliferation during embryonic and adult life remain unanswered. In the present study, we review approaches and studies that have shed some light on cardiomyocyte cell cycle regulation. For reference, an initial description of cardiomyogenic induction and morphogenesis is provided, which is followed by a summary of published cell cycle analyses during these stages of cardiac ontology. A review of studies examining cardiomyocyte cell cycle analysis and de novo cardiomyogenic induction in the adult heart is then presented. Finally, studies in which cardiomyocyte cell cycle activity was experimentally manipulated in vitro and in vivo are reviewed. It is hoped that this compilation will serve to stimulate thought and experimentation in this intriguing area of cardiomyocyte cell biology.