Structure and regulation of the mouse cardiac troponin I gene

Evolution of the N-Terminal Regulation of Cardiac Troponin I for Heart Function of Tetrapods: Lungfish Presents an Example of the Emergence of Novel Submolecular Structure to Lead the Capacity of Adaptation

Troponin-based Ca2+ regulation of striated muscle contraction emerged approximately 700 million years ago with largely conserved functions during evolution. Troponin I (TnI) is the inhibitory subunit of troponin and has evolved into three muscle type-specific isoforms in vertebrates. Cardiac TnI is specifically expressed in the adult heart and has a unique N-terminal extension implicating a specific value during natural selection. The N-terminal extension of cardiac TnI in higher vertebrates contains β-adrenergic-regulated protein kinase A (PKA) phosphorylation sites as a mechanism to enhance cardiac muscle relaxation and facilitate ventricular filling. Phylogenic studies showed that the N-terminal extension of cardiac TnI first emerged in the genomes of early tetrapods as well as primordial lobe-finned fishes such as the coelacanth whereas it is absent in ray-finned fish. This apparently rapid evolution of β-adrenergic regulation of cardiac function suggests a high selection value for the heart of vertebrate animals on land to work under higher metabolic demands. Sequencing and PKA phosphorylation data showed that lungfish cardiac TnI has evolved with an amphibian-like N-terminal extension with prototype PKA phosphorylation sites while its overall structure remained fish like. The data demonstrate that the submolecular structure of TnI may evolve ahead of the whole protein for cardiac muscle contractility to adapt to new environmental conditions. Understanding the evolution of the β-adrenergic regulation of TnI and cardiac adaptation to the increased energetic demands of life on land adds knowledge for the treatment of human heart diseases and failure.

Augmentation of myocardial If dysregulates calcium homeostasis and causes adverse cardiac remodeling

HCN channels underlie the depolarizing funny current (If) that contributes importantly to cardiac pacemaking. If is upregulated in failing and infarcted hearts, but its implication in disease mechanisms remained unresolved. We generated transgenic mice (HCN4tg/wt) to assess functional consequences of HCN4 overexpression-mediated If increase in cardiomyocytes to levels observed in human heart failure. HCN4tg/wt animals exhibit a dilated cardiomyopathy phenotype with increased cellular arrhythmogenicity but unchanged heart rate and conduction parameters. If augmentation induces a diastolic Na+ influx shifting the Na+/Ca2+ exchanger equilibrium towards 'reverse mode' leading to increased [Ca2+]i. Changed Ca2+ homeostasis results in significantly higher systolic [Ca2+]i transients and stimulates apoptosis. Pharmacological inhibition of If prevents the rise of [Ca2+]i and protects from ventricular remodeling. Here we report that augmented myocardial If alters intracellular Ca2+ homeostasis leading to structural cardiac changes and increased arrhythmogenicity. Inhibition of myocardial If per se may constitute a therapeutic mechanism to prevent cardiomyopathy.

Megalobrama amblycephala cardiac troponin T variants: molecular cloning, expression and response to nitrite

Cardiac troponin T (TNNT2), as a member of troponin superfamily, plays important roles during early cardiogenesis, and contraction and relaxation of myocardial cells. In this study, two alternatively spliced variants of Megalobrama amblycephala TNNT2 were identified showing a difference of 19 amino acids in the N-terminal hypervariable region. The longer cDNA (TNNT2-1) was 1,118 bp, encoding 284 amino acid residues, contained conserved central tropomyosin-binding region, cardiac specific signal and C-terminal segments except the N-terminal hypervariable region. The TNNT2 transcripts first appeared at 16 hours post-fertilization (hpf) peaking at 28 hpf (onset of heartbeat). In addition, strong expression of TNNT2 was found in the cardiac muscle. After nitrite exposure, the increased TNNT2 expression levels in the heart indicated that nitrite might induce cardiac injury. Results of semi-quantitative RT-PCR indicated that the two alternatively spliced variants existed in early development stages since their first appearance at 16 hpf and heart, spleen, headkiney of M. amblycephala. The shorter transcript (TNNT2-2) was proved to be dominant in the embryos and heart of M. amblycephala, furthermore, the increase of TNNT2 expression level in the heart after nitrite exposure was mainly caused by TNNT2-2.

Differential effects of retinoids and inhibitors of ERK and p38 signaling on adipogenic and myogenic differentiation of P19 stem cells

All-trans-retinoic acid (atRA) is an essential signaling molecule in embryonic development. It regulates cell differentiation by activating nuclear retinoic acid receptors (RAR) and retinoid-X receptors (RXR), which both control gene expression. In addition, atRA could act in the cytoplasm by modulating the activity of mitogen-activated protein kinases (MAPK) ERK and p38, which also have a role in cell differentiation. AtRA can induce the differentiation of P19 embryonic carcinoma stem cells into adipocytes, cardiomyocytes, and skeletal muscle cells, concurrently, in the same culture. We postulated that combinations of atRA, atRA analogs exhibiting selectivity for RAR or RXR, and inhibitors of ERK and p38 signaling (ERKi and p38i) could be used to favor one mesodermal fate over the others in the P19 model. In a first series of experiments, we replaced atRA by an agonist of RXR (LG100268) or RAR (TTNPB) to preferentially stimulate one group of receptors over the other. LG100268 was as adipogenic and myogenic as atRA, whereas TTNPB strongly induced adipogenesis, but not myogenesis. ERKi enhanced the myogenic action of atRA, and p38i increased both adipogenesis and myogenesis. In a second series of experiments, we combined atRA with an RAR or RXR antagonist (RARatg or RXRatg) to preferentially deactivate each receptor group in turn. The combinations atRA+RXRatg and atRA+RARatg, including or not ERKi, had similar mesodermal actions as atRA. In contrast, there was no myogenesis with atRA+RXRatg+p38i treatment, and there were no myogenesis and no adipogenesis with the atRA+RARatg+p38i combination. Overall, the results indicate that p38 has a role in mesodermal differentiation that depends on the retinoid context. Indeed, p38 in conjunction with RXR is important in myogenesis, and p38 and RAR in adipogenesis. Under the conditions tested, it was possible to stimulate adipogenesis with a block on myogenesis, whereas increased myogenesis was accompanied by adipogenesis.

Real-time reverse transcription-PCR expression profiling of porcine troponin I family in three different types of muscles during development

In this study, the expression profiling of three troponin I isoforms (TNNI1, TNNI2 and TNNI3) was investigated in two pig breeds differing in muscularity (Yorkshire and Meishan) at six stages (fetal 60 days and postnatal 3, 35, 60, 120, and 180 days) and three types of muscles (longissimus dorsi muscle, LD; semitendinosus, ST; cardiac muscle, CM) using relative real-time quantitative PCR. Significant differences of troponin I expression in three muscles were found between Yorkshire and Meishan breeds at some stages. The expression peak of TNNI1 and TNNI2 in LD and ST was at postnatal 35 or 60 days in Yorkshire and at postnatal 120 or 180 days in Meishan pigs, while it occurred in CM at postnatal 3 days in two pig breeds. The relative expression values of TNNI1 and TNNI2 were significantly higher in LD than ST at most of stages after birth. The expression ratio of TNNI2 versus TNNI1 favoured TNNI2 expression in ST and LD, but on the contrary in CM. The expression peak of TNNI3 occurred at postnatal 60 and 120 days in Yorkshire and Meishan pigs, respectively. TNNI1 and TNNI3 were co-expressed in CM during the fetal and earlier stages after birth.

Molecular cloning and comparative characterization of the porcine troponin I family

Troponin I (TnI) is a family of three muscle-specific myofibrillar proteins involved in calcium-sensitive regulation of contraction in cardiac and skeletal muscle. In this study, the full-length cDNA and genomic sequence of three genes of porcine TnI family were cloned and sequenced. The full-length cDNA of TNNI1, TNNI2, and TNNI3 genes were 989 bp, 734 bp, and 831 bp in length, which contained an open reading frame of 564, 549, and 636 nucleotides, respectively. Three Troponin I shared 54.4 approximately 58.3% similarity with each other in their predicted amino acid sequences. The TNNI1, TNNI2, and TNNI3 displayed the same genomic structure as other vertebrates and spanned over 9785 bp, 2373 bp, and 3648 bp genomic regions, respectively. The regulatory elements in the proximal promoter of TNNI2 and TNNI3 were conserved among human, mouse, and pig, but regulatory element differences existed in the TNNI1 promoter among them. Expression profiling showed that TnI genes were widely expressed in the tissues studied, with the highest expression level of TNNI1 and TNNI2 in skeletal muscle, and TNNI3 in cardiac muscle.

Manipulating the cell differentiation through lentiviral vectors

The manipulation of cell differentiation is important to create new sources for the treatment of degenerative diseases or solve cell depletion after aggressive therapy against cancer. In this chapter, the use of a tissue-specific promoter lentiviral vector to obtain a myocardial pure lineage from murine embryonic stem cells (mES) is described in detail. Since the cardiac isoform of troponin I gene product is not expressed in skeletal or other muscle types, short mouse cardiac troponin proximal promoter is used to drive reporter genes. Cells are infected simultaneously with two lentiviral vectors, the first expressing EGFP to monitor the transduction efficiency, and the other expressing a puromycin resistance gene to select the specific cells of interest. This technical approach describes a method to obtain a pure cardiomyocyte population and can be applied to other lineages of interest.

A lentiviral vector with a short troponin-I promoter for tracking cardiomyocyte differentiation of human embryonic stem cells

Human embryonic stem cells (hESCs) may become important for cardiac repair due to their potentially unlimited ability to generate cardiomyocytes (CMCs). Moreover, genetic manipulation of hESC-derived CMCs would be a very promising technique for curing myocardial disorders. At the present time, however, inducing the differentiation of hESCs into CMCs is extremely difficult and, therefore, an easy and standardizable technique is needed to evaluate differentiation strategies. Vectors driving cardiac-specific expression may represent an important tool not only for monitoring new cardiac-differentiation strategies, but also for the manipulation of cardiac differentiation of ESCs. To this aim, we generated cardiac-specific lentiviral vectors (LVVs) in which expression is driven by a short fragment of the cardiac troponin-I proximal promoter (TNNI3) with a human cardiac alpha-actin enhancer, and tested its suitability in inducing tissue-specific gene expression and ability to track the CMC lineage during differentiation of ESCs. We determined that (1) TNNI3-LVVs efficiently drive cardiac-specific gene expression and mark the cardiomyogenic lineage in human and mouse ESC differentiation systems (2) the cardiac alpha-actin enhancer confers a further increase in gene-expression specificity of TNNI3-LVVs in hESCs. Although this technique may not be useful in tracking small numbers of cells, data suggested that TNNI3-based LVVs are a powerful tool for manipulating human ESCs and modifying hESC-derived CMCs.

Cardiac-specific activation of Cre expression at late fetal development

In a first step towards dissecting molecular mechanisms that contribute to the development of cardiac diseases, we have generated transgenic mice that express a Cre-GFP fusion protein under the transcriptional control of a 4.3kb murine cardiac Troponin I gene (cTnI) promoter. Cre-GFP expression, similar in three transgenic lines, is described in one line. In mouse embryos, transgenic for the Cre-GFP and ROSA lacZ reporter allele, first Cre-mediated recombination appeared at 16.5 dpc selectively at the heart. Like the endogenous cTnI gene, transgenic Cre expression showed a slow rise through fetal development that increased neonatally. Bitransgenic hearts, stained at 30 days of age, showed intense signals in ventricular and atrial myocytes while no recombination occurred in other tissues. The delayed onset of Cre activity in cTnI-Cre mice could provide a useful genetic tool to evaluate the function of loxP targeted cardiac genes without interference of recombination during early heart development.

Three-dimensional measurement and visualization of morphogenesis applied to cardiac embryology

Volume growth and proliferation are key processes in heart morphogenesis, yet their regionalization during development of the heart has been described only anecdotally. To study the contribution of cardiomyocyte proliferation to heart development, a quantitative reconstruction method was designed, allowing the local mapping of this morphogenetic process. First, a morphological surface reconstruction is made of the heart, using sections stained specifically for cardiomyocytes. Then, by a comprehensive series of image processing steps, local three-dimensional (3D) information of proliferation is obtained. These local quantitative data are then mapped onto the morphological surface reconstruction, resulting in a reconstruction that not only provides morphological information (qualitative), but also displays local information on proliferation rate (quantitative). The resulting 3D quantitative reconstructions revealed novel observations regarding the morphogenesis of the heart.

Regionalized sequence of myocardial cell growth and proliferation characterizes early chamber formation

Increase in cell size and proliferation of myocytes are key processes in cardiac morphogenesis, yet their regionalization during development of the heart has been described only anecdotally. We have made quantitative reconstructions of embryonic chicken hearts ranging in stage from the fusion of the heart-forming fields to early formation of the chambers. These reconstructions reveal that the early heart tube is recruited from a pool of rapidly proliferating cardiac precursor cells. The proliferation of these small precursor cells ceases as they differentiate into overt cardiomyocytes, producing a slowly proliferating straight heart tube composed of cells increasing in size. The largest cells were found at the ventral side of the heart tube, which corresponds to the site of the forming ventricle, as well as the site where proliferation is reinitiated. The significance of these observations is 2-fold. First, they support a model of early cardiac morphogenesis in 2 stages. Second, they demonstrate that regional increase in size of myocytes contributes significantly to chamber formation.

Drastic Ca2+ sensitization of myofilament associated with a small structural change in troponin I in inherited restrictive cardiomyopathy

Six missense mutations in human cardiac troponin I (cTnI) were recently found to cause restrictive cardiomyopathy (RCM). We have bacterially expressed and purified these human cTnI mutants and examined their functional and structural consequences. Inserting the human cTnI into skinned cardiac muscle fibers showed that these mutations had much greater Ca2+-sensitizing effects on force generation than the cTnI mutations in hypertrophic cardiomyopathy (HCM). The mutation K178E in the second actin-tropomyosin (Tm) binding region showed a particularly potent Ca2+-sensitizing effect among the six RCM-causing mutations. Circular dichroism and nuclear magnetic resonance spectroscopy revealed that this mutation does not extensively affect the structure of the whole cTnI molecule, but induces an unexpectedly subtle change in the structure of a region around the mutated residue. The results indicate that the K178E mutation has a localized effect on a structure that is critical to the regulatory function of the second actin-Tm binding region of cTnI. The present study also suggests that both HCM and RCM involving cTnI mutations share a common feature of increased Ca2+ sensitivity of cardiac myofilament, but more severe change in Ca2+ sensitivity is associated with the clinical phenotype of RCM.

Biology of the troponin complex in cardiac myocytes

Troponin is the regulatory complex of the myofibrillar thin filament that plays a critical role in regulating excitation-contraction coupling in the heart. Troponin is composed of three distinct gene products: troponin C (cTnC), the 18-kD Ca(2+)-binding subunit; troponin I (cTnI), the approximately 23-kD inhibitory subunit that prevents contraction in the absence of Ca2+ binding to cTnC; and troponin T (cTnT), the approximately 35-kD subunit that attaches troponin to tropomyosin (Tm) and to the myofibrillar thin filament. Over the past 45 years, extensive biochemical, biophysical, and structural studies have helped to elucidate the molecular basis of troponin function and thin filament activation in the heart. At the onset of systole, Ca2+ binds to the N-terminal Ca2+ binding site of cTnC initiating a conformational change in cTnC, which catalyzes protein-protein associations activating the myofibrillar thin filament. Thin filament activation in turn facilitates crossbridge cycling, myofibrillar activation, and contraction of the heart. The intrinsic length-tension properties of cardiac myocytes as well as the Frank-Starling properties of the intact heart are mediated primarily through Ca(2+)-responsive thin filament activation. cTnC, cTnI, and cTnT are encoded by distinct single-copy genes in the human genome, each of which is expressed in a unique cardiac-restricted developmentally regulated fashion. Elucidation of the transcriptional programs that regulate troponin transcription and gene expression has provided insights into the molecular mechanisms that regulate and coordinate cardiac myocyte differentiation and provided unanticipated insights into the pathogenesis of cardiac hypertrophy. Autosomal dominant mutations in cTnI and cTnT have been identified and are associated with familial hypertrophic and restrictive cardiomyopathies.

Amphibian cardiac troponin I gene's organization, developmental expression, and regulatory properties are different from its mammalian homologue

In mammals, the expression of the troponin I-slow (TnIs) isoform is predominant in the heart during embryogenesis and, shortly after birth, is replaced by the cardiac-specific isoform, TnIc; a developmental switch thought to be mediated by thyroid hormone. Whereas, in Xenopus, TnIc is expressed at the onset of heart formation and is the only TnI isoform expressed in the heart. Herein, we demonstrate that the expression patterns of these genes appear to be common within the anuran lineage and, unlike their mammalian counterparts, are not affected by thyroid hormone. To elucidate the regulatory mechanism(s) governing the expression of the amphibian TnIc gene, we characterized the TnIc gene from Rana catesbeiana and used its 5'-flanking region to drive expression of green fluorescent protein in the Xenopus transgenic system. Our results demonstrate that a 300-bp minimal promoter containing intact GATA and CArG-box elements is sufficient to drive expression of this reporter gene in a pattern that mimics, both spatially and temporally, the expression of the endogenous Xenopus TnIc gene.

An R111C polymorphism in wild turkey cardiac troponin I accompanying the dilated cardiomyopathy-related abnormal splicing variant of cardiac troponin T with potentially compensatory effects

Cardiac muscle contraction is regulated by Ca(2+) through the troponin complex consisting of three subunits: troponin C (TnC), troponin T (TnT), and troponin I (TnI). We reported previously that the abnormal splicing of cardiac TnT in turkeys with dilated cardiomyopathy resulted in a greater binding affinity to TnI. In the present study, we characterized a polymorphism of cardiac TnI in the heart of wild turkeys. cDNA cloning and sequencing of the novel turkey cardiac TnI revealed a single amino acid substitution, R111C. Arg(111) in avian cardiac TnI corresponds to a Lys in mammals. This residue is conserved in cardiac and skeletal muscle TnIs across the vertebrate phylum, implying a functional importance. In the partial crystal structure of cardiac troponin, this amino acid resides in an alpha-helix that directly contacts with TnT. Structural modeling indicates that the substitution of Cys for Arg or Lys at this position would not disrupt the global structure of troponin. To evaluate the functional significance of the different size and charge between the Arg and Cys side chains, protein-binding assays using purified turkey cardiac TnI expressed in Escherichia coli were performed. The results show that the R111C substitution lowered binding affinity to TnT, which is potentially compensatory to the increased TnI-binding affinity of the cardiomyopathy-related cardiac TnT splicing variant. Therefore, the fixation of the cardiac TnI Cys(111) allele in the wild turkey population and the corresponding functional effect reflect an increased fitness value, suggesting a novel target for the treatment of TnT myopathies.

Requirement of transcription factor NFAT in developing atrial myocardium

Nuclear factor of activated T cell (NFAT) is a ubiquitous regulator involved in multiple biological processes. Here, we demonstrate that NFAT is temporally required in the developing atrial myocardium between embryonic day 14 and P0 (birth). Inhibition of NFAT activity by conditional expression of dominant-negative NFAT causes thinning of the atrial myocardium. The thin myocardium exhibits severe sarcomere disorganization and reduced expression of cardiac troponin-I (cTnI) and cardiac troponin-T (cTnT). Promoter analysis indicates that NFAT binds to and regulates transcription of the cTnI and the cTnT genes. Thus, regulation of cytoskeletal protein gene expression by NFAT may be important for the structural architecture of the developing atrial myocardium.

A genomewide survey of developmentally relevant genes in Ciona intestinalis. IX. Genes for muscle structural proteins

Ascidians are simple chordates that are related to, and may resemble, vertebrate ancestors. Comparison of ascidian and vertebrate genomes is expected to provide insight into the molecular genetic basis of chordate/vertebrate evolution. We annotated muscle structural (contractile protein) genes in the completely determined genome sequence of the ascidian Ciona intestinalis, and examined gene expression patterns through extensive EST analysis. Ascidian muscle protein isoform families are generally of similar, or lesser, complexity in comparison with the corresponding vertebrate isoform families, and are based on gene duplication histories and alternative splicing mechanisms that are largely or entirely distinct from those responsible for generating the vertebrate isoforms. Although each of the three ascidian muscle types - larval tail muscle, adult body-wall muscle and heart - expresses a distinct profile of contractile protein isoforms, none of these isoforms are strictly orthologous to the smooth-muscle-specific, fast or slow skeletal muscle-specific, or heart-specific isoforms of vertebrates. Many isoform families showed larval-versus-adult differential expression and in several cases numerous very similar genes were expressed specifically in larval muscle. This may reflect different functional requirements of the locomotor larval muscle as opposed to the non-locomotor muscles of the sessile adult, and/or the biosynthetic demands of extremely rapid larval development.

Three-dimensional reconstruction of gene expression patterns during cardiac development

The study of the genetic regulation of embryonic development requires the three-dimensional (3D) mapping of gene expression at the microscopic level. Despite the recent burst in the number of methods focusing on 3D reconstruction of embryonic specimens, an adequate and accessible 3D reconstruction protocol for the visualization of patterns of gene expression is lacking. In this communication we describe a protocol that was developed for the 3D visualization of patterns of gene expression determined by in situ hybridization (ISH) on serial sections. The method still requires tissue sectioning, due to penetration limits of the specific staining agents into whole embryo preparations. With regard to expenditure of resources, i.e., hardware, software, and time, the protocol is relatively undemanding. Because the variation between specimens requires the visualization of multiple specimens per stage, it was decided to "do more, less well." The current protocol, therefore, results in reconstructions of sufficient, but not the highest, quality. The use of the protocol is demonstrated on a series of serially sectioned mouse hearts, ranging from embryonic day 8.5 to 14.5. The myocardium of the hearts was identified by ISH using a mixture of specific mRNA probes and reconstructed.

T-box genes and cardiac development

BACKGROUND

T-box genes play roles in vertebrate gastrulation and in later organogenesis. Their existence in all metazoans examined so far indicates that this is an evolutionarily ancient gene family. Drosophila melanogaster has eight T-box genes, whereas Caenorhabditis elegans has 22. Mammals appear to have at least 18 T-box genes, comprising five subfamilies.

METHODS

A full range of cytological, developmental, molecular and genetic methodologies have recently been applied to the study of T-box genes.

RESULTS

Over the last 5 years, mutations in TBX1 and TBX5 have been implicated in two human disorders with haplo-insufficient cardiovascular phenotypes, DiGeorge/velocardiofacial syndrome and Holt-Oram ("heart-hand") syndrome. Interestingly, the number of T-box gene family members discovered to have cardiac or pharyngeal arch expression domains during vertebrate embryonic development has steadily grown. In addition, various Tbx5 loss-of-function models in organisms as distant as the mouse and zebrafish do indeed phenocopy Holt-Oram syndrome. Finally, the intriguing discovery earlier this year that a T-box gene is expressed in a subset of cardioblasts in D. melanogaster suggests that members of this gene family may have fundamental, conserved roles in cardiovascular pattern formation.

CONCLUSIONS

These developments prompted us to review the current understanding of the contribution of T-box genes to cardiovascular morphogenesis.

Structural consequences of cardiac troponin I phosphorylation

beta-Adrenergic stimulation of the heart results in bisphosphorylation of the N-terminal extension of cardiac troponin I (TnI). Bisphosphorylation of TnI reduces the affinity of the regulatory site on troponin C (TnC) for Ca(2+) by increasing the rate of Ca(2+) dissociation. What remains unclear is how the phosphorylation signal is transmitted from one subunit of troponin to another. We have produced a series of mutations in the N-terminal extension of TnI designed to further our understanding of the mechanisms involved. The ability of phosphorylation of the mutant TnIs to affect Ca(2+) sensitivity has been assessed. We find that the Pro residues found in a conserved (Xaa-Pro)(4) motif N-terminal to the phosphorylation sites are not required for the effect of the N-terminal extension on Ca(2+) binding in the presence or absence of phosphorylation. Our experiments also reveal that the full effects of phosphorylation are seen even when residues 1-15 of TnI are deleted. If further residues are removed, not only does the effect of phosphorylation diminish but deletion of the N-terminal extension mimics phosphorylation. We propose that TnI residues 16-29 bind to TnC stabilizing the "open" Ca(2+)-bound state. Phosphorylation (or deletion) prevents this binding, accelerating Ca(2+) release.

Cardiomyocytes induce endothelial cells to trans-differentiate into cardiac muscle: implications for myocardium regeneration

The concept of tissue-restricted differentiation of postnatal stem cells has been challenged by recent evidence showing pluripotency for hematopoietic, mesenchymal, and neural stem cells. Furthermore, rare but well documented examples exist of already differentiated cells in developing mammals that change fate and trans-differentiate into another cell type. Here, we report that endothelial cells, either freshly isolated from embryonic vessels or established as homogeneous cells in culture, differentiate into beating cardiomyocytes and express cardiac markers when cocultured with neonatal rat cardiomyocytes or when injected into postischemic adult mouse heart. Human umbilical vein endothelial cells also differentiate into cardiomyocytes under similar experimental conditions and transiently coexpress von Willebrand factor and sarcomeric myosin. In contrast, neural stem cells, which efficiently differentiate into skeletal muscle, differentiate into cardiomyocytes at a low rate. Fibroblast growth factor 2 and bone morphogenetic protein 4, which activate cardiac differentiation in embryonic cells, do not activate cardiogenesis in endothelial cells or stimulate trans-differentiation in coculture, suggesting that different signaling molecules are responsible for cardiac induction during embryogenesis and in successive periods of development. The fact that endothelial cells can generate cardiomyocytes sheds additional light on the plasticity of endothelial cells during development and opens perspectives for cell autologous replacement therapies.

A proteolytic NH2-terminal truncation of cardiac troponin I that is up-regulated in simulated microgravity

In a tail suspension rat model, we investigated changes in myofilament protein during cardiac adaptation in simulated microgravity. Contractile force and velocity of cardiac muscle were decreased in the tail suspension rats as compared with the control. Ca(2+)-dependent actomyosin ATPase activity was also decreased; however, sensitivity of cardiac muscle to Ca(2+) activation was unchanged. There was no change in expression of myosin heavy chain, tropomyosin, troponin T, or troponin I isoforms in hearts of tail suspension rats. A novel finding is a fragment of cardiac troponin I (cTnI) that had increased amounts in the heart of tail suspension rats. Binding of this cTnI fragment by a monoclonal antibody that specifically recognizes the COOH terminus indicates an intact COOH terminus. NH(2)-terminal sequence analysis of the cTnI fragment revealed truncations primarily of amino acids 1-26 and 1-27 and smaller amounts of 1-30, including Ser(23) and Ser(24), which are substrates of protein kinase A phosphorylation. This cTnI fragment is present in normal cardiac muscle and incorporated into myofibrils, indicating a role in regulating contractility. This proteolytic modification of cTnI up-regulated during simulated microgravity suggests a potential role of the NH(2)-terminal segment of cTnI in functional adaptations of cardiac muscle.

The highly conserved COOH terminus of troponin I forms a Ca2+-modulated allosteric domain in the troponin complex

The primary structure of the COOH-terminal region of troponin I (TnI) is highly conserved among the cardiac, slow, and fast skeletal muscle TnI isoforms and across species. Although no binding site for the other thin filament proteins is found at the COOH terminus of TnI, truncations of the last 19-23 amino acid residues reduce the activity of TnI in the inhibition of actomyosin ATPase and result in cardiac muscle malfunction. We have developed a specific monoclonal antibody (mAb), TnI-1, against the conserved COOH terminus of TnI. Using this mAb, isolation of the troponin complex by immunoaffinity chromatography from muscle homogenate and immunofluorescence microscopic staining of myofibrils indicate that the COOH terminus of TnI forms an exposed structure in the muscle thin filament. Binding of this mAb to the COOH terminus of cardiac TnI induced extensive conformational changes in the protein, suggesting an allosteric role of this region in the functional integrity of troponin. In the absence of Ca2+, the binding of troponin C and troponin T to TnI had very little effect on the conformation of the COOH terminus of TnI as indicated by the unaffected mAb affinity for the TnI-1 epitope. However, Ca2+ significantly increased the accessibility of the TnI-1 epitope on TnI in the presence of troponin C and troponin T. The results provide evidence that the COOH terminus is an essential structure in TnI and participates in the allosteric switch during Ca2+ activation of contraction.

mRNA 5'-leader trans-splicing in the chordates

We report the discovery of mRNA 5'-leader trans-splicing (SL trans-splicing) in the chordates. In the ascidian protochordate Ciona intestinalis, the mRNAs of at least seven genes undergo trans-splicing of a 16-nucleotide 5'-leader apparently derived from a 46-nucleotide RNA that shares features with previously characterized splice donor SL RNAs. SL trans-splicing was known previously to occur in several protist and metazoan phyla, however, this is the first report of SL trans-splicing within the deuterostome division of the metazoa. SL trans-splicing is not known to occur in the vertebrates. However, because ascidians are primitive chordates related to vertebrate ancestors, our findings raise the possibility of ancestral SL trans-splicing in the vertebrate lineage.

Clinical features of hypertrophic cardiomyopathy caused by a Lys183 deletion mutation in the cardiac troponin I gene

BACKGROUND

Mutations that cause hypertrophic cardiomyopathy (HCM) have been identified in 9 genes that code proteins in the sarcomere. Previous reports have demonstrated that cardiac troponin I (cTnI) gene mutations may account for familial HCM; however, the clinical characteristics and prognosis of patients with HCM caused by cTnI gene mutations are not known.

METHODS AND RESULTS

We analyzed cTnI gene mutations in 130 unrelated probands with HCM and their families to clarify the genotype-phenotype correlations. We identified 25 individuals in 7 families with a Lys183 deletion (Lys183 del) mutation in exon 7 of the cTnI gene. The disease penetrance in subjects aged >20 years was 88% by echocardiography and 96% by ECG. Sudden death occurred in 7 individuals of 4 families at any age. Overall, 7 (43.8%) of 16 individuals aged >40 years had left ventricular systolic dysfunction, and 3 (18.8%) displayed dilated cardiomyopathy-like features. Of affected individuals, 4 of 5 individuals aged >40 years followed by echocardiography showed septal thinning and decreased fractional shortening during >5 years of follow-up.

CONCLUSIONS

The Lys183 del mutation in the cTnI gene in patients with HCM is associated with variable clinical features and outcomes. HCM caused by the Lys183 del mutation has a significant disease penetrance. This mutation is associated with sudden death at any age and dilated cardiomyopathy-like features in those aged >40 years. However, it remains unclear whether screening of families with HCM for this mutation will be useful in patient management and counseling.

Structural characterization of the human fast skeletal muscle troponin I gene (TNNI2)

Three troponin I genes have been identified in vertebrates that encode the isoforms expressed in adult cardiac muscle (TNNI3), slow skeletal muscle (TNNI1) and fast skeletal muscle (TNNI2), respectively. While the organization and regulation of human cardiac and slow skeletal muscle genes have been investigated in detail, the fast skeletal troponin I gene has to date only been examined in birds. Here, we describe the structure and complete sequence of the human fast skeletal muscle troponin I gene (TNNI2) and identify putative regulatory elements within both the 5' flanking region and the first intron. In particular, a region containing MEF-2, E-box, CCAC and CAGG elements was identified in intron 1 that closely resembles the fast internal regulatory element (FIRE) of the quail intronic enhancer. We have previously shown that the fast skeletal muscle troponin I gene is located at 11p15.5 and noted potential close linkage with the fast skeletal muscle troponin T gene (TNNT3). Here, we have isolated two independent human PAC genomic clones that contain either TNNI2 or TNNT3 and demonstrate by interphase FISH mapping that they are less than 100 kb apart in the genome. The results demonstrate that the human TNNI2 gene is closely related to its avian counterparts with conserved elements within both the putative promoter and first intron. Our data further confirm close physical linkage of TNNI2 and TNNI3 on 11p15.5.

Identification and genomic cloning of CMHC1. A unique myosin heavy chain expressed exclusively in the developing chicken heart

We report the identification and cloning of a unique chick myosin heavy chain (CMHC1) that is expressed exclusively in the heart during embryogenesis. Using primers specific to myosin heavy chains, we used reverse transcriptase-polymerase chain reaction to clone and isolate CMHC1 from embryonic day 10 chicken heart RNA. Sequence analysis indicated that CMHC1 was a novel member of the myosin heavy chain family. Expression of the CMHC1 transcripts was detected in Hamburger Hamilton stage 10 chick embryos in the fusing myocardium. Expression of CMHC1 was maintained at high levels throughout the tubular heart of later stage embryos. Reverse transcriptase-polymerase chain reaction and in situ hybridizations failed to detect CMHC1 transcripts in the developing somites, limb buds, or skeletal musculature at any stage of chick development. Genomic CMHC1 clones have been isolated that contain sequences approximately 5.2 kilobase upstream of the presumptive CMHC1 transcription start site. Portions of the upstream regulatory region induced a 21-fold increase in reporter gene expression in primary cardiomyocytes. Because of its unique cardiac-restricted expression, CMHC1 will provide an excellent model system to study the molecular mechanisms required for the early developmental regulation of heart-specific genes.

Tissue-restricted expression of the cardiac alpha-myosin heavy chain gene is controlled by a downstream repressor element containing a palindrome of two ets-binding sites

The expression of the alpha-myosin heavy chain (MHC) gene is restricted primarily to cardiac myocytes. To date, several positive regulatory elements and their binding factors involved in alpha-MHC gene regulation have been identified; however, the mechanism restricting the expression of this gene to cardiac myocytes has yet to be elucidated. In this study, we have identified by using sequential deletion mutants of the rat cardiac alpha-MHC gene a 30-bp purine-rich negative regulatory (PNR) element located in the first intronic region that appeared to be essential for the tissue-specific expression of the alpha-MHC gene. Removal of this element alone elevated (20- to 30-fold) the expression of the alpha-MHC gene in cardiac myocyte cultures and in heart muscle directly injected with plasmid DNA. Surprisingly, this deletion also allowed a significant expression of the alpha-MHC gene in HeLa and other nonmuscle cells, where it is normally inactive. The PNR element required upstream sequences of the alpha-MHC gene for negative gene regulation. By DNase I footprint analysis of the PNR element, a palindrome of two high-affinity Ets-binding sites (CTTCCCTGGAAG) was identified. Furthermore, by analyses of site-specific base-pair mutation, mobility gel shift competition, and UV cross-linking, two different Ets-like proteins from cardiac and HeLa cell nuclear extracts were found to bind to the PNR motif. Moreover, the activity of the PNR-binding factor was found to be increased two- to threefold in adult rat hearts subjected to pressure overload hypertrophy, where the alpha-MHC gene is usually suppressed. These data demonstrate that the PNR element plays a dual role, both downregulating the expression of the alpha-MHC gene in cardiac myocytes and silencing the muscle gene activity in nonmuscle cells. Similar palindromic Ets-binding motifs are found conserved in the alpha-MHC genes from different species and in other cardiac myocyte-restricted genes. These results are the first to reveal a role of the Ets class of proteins in controlling the tissue-specific expression of a cardiac muscle gene.

Combinatorial cis-acting elements control tissue-specific activation of the cardiac troponin I gene in vitro and in vivo

The cardiac troponin I gene is one of the few sarcomeric protein genes exclusively expressed in cardiac muscle. We show here that this specificity is controlled by a proximal promoter (-230/+16) in transfected cardiac cells in culture, in the adult hearts, and in transgenic animals. Functional analysis indicates that MEF2/Oct-1, Sp1, and GATA regulatory elements are required for optimal gene activation because selective mutations produce weak or inactive promoters. MEF2 and Oct-1 transcription factors bind to the same A/T-rich element. A mutation that blocks this binding markedly reduces gene activation in vivo and in vitro, and overexpression of MEF2A, MEF2C, and MEF2D in noncardiac cells transactivates the cardiac troponin I promoter. Disruption of these elements inactivates the cardiac troponin I promoter in cultured cardiac cells but has a less important role in transfected adult heart. Moreover, nuclear extracts from an almost pure population of adult cardiac cells contain much lower levels of GATA binding activity compared with fetal cardiac cells. These findings point to a differential role of GATA factors in the maintenance of gene expression in the adult heart as compared with the activation of cardiac genes in fetal cardiomyocytes. Overexpression of GATA family members transactivates the cardiac troponin I promoter, and GATA-5 and GATA-6 are stronger transactivators than GATA-4, a property apparently unique to the cardiac troponin I promoter. Transgenic mice carrying the -230/+126 base pair promoter express beta-galactosidase reporter gene in the heart both at early stages of cardiogenesis and in the adult animals. These results indicate that the ability of the cardiac troponin I proximal promoter to target expression of a downstream gene in the heart is also maintained when the transgene is integrated into the genome.

Tissue-specific alternative splicing of ascidian troponin I isoforms. Redesign of a protein isoform-generating mechanism during chordate evolution

In vertebrates, troponin I (TnI) exists as shorter and longer isoforms encoded by distinct genes expressed in skeletal and cardiac muscle, respectively. We report that the protochordate ascidian Ciona intestinalis expresses a homologous set of shorter and longer TnI isoforms in body wall muscle and heart, respectively. The heart-specific segment of the ascidian longer TnI isoform shares several sequence features with vertebrate cardiac TnI but lacks the protein kinase A phosphorylation sites implicated in sympatho-adrenal control of cardiac function. In contrast with vertebrates, the ascidian longer and shorter TnI isoforms are produced from a single gene by tissue-specific alternative RNA splicing; remarkably, the molecular mechanism of TnI isoform generation has been entirely reworked during ascidian/vertebrate evolution. Because alternative splicing is the more probable chordate ancestral condition, the long/cardiac versus short/somatic muscle pattern of TnI isoforms likely existed before the occurrence of the gene duplication events that created the vertebrate TnI gene family. Thus, gene duplication was apparently not the primary engine of isoform diversity in this aspect of TnI gene family evolution; rather, it simply provided an alternative (transcriptional) means of maintaining a previously established system of isoform diversity and tissue specificity based on alternative RNA splicing.

Interactions of structural C and regulatory N domains of troponin C with repeated sequence motifs in troponin I

The actomyosin ATPase inhibitory protein troponin I (TnI) plays a central regulatory role in skeletal and cardiac muscle contraction and relaxation through its calcium-dependent interactions with troponin C (TnC) and actin. Previously we have demonstrated the utility of F29W and F105W mutants of TnC for measurement of binding affinities of inhibitory peptide TnI(96-116) to its regulatory N and structural C domains, both in isolation and in the intact TnC molecule [Pearlstone, J. R. & Smillie, L. B. (1995) Biochemistry 34, 6932-6940]. This approach is now extended to fragment TnI(96-148). Curve-fitting analyses of fluorescence changes induced in the intact TnC mutants and the isolated N and C domains by increasing [TnI(96-148)] have permitted the assignments of K(D) values (designated K(D,N) and K(D,C)) to the interaction of TnI(96-148) with the N and C domains, respectively, of intact TnC. Taken together with the previous data for TnI(96-116) binding, it can be concluded that, within TnI(96-148), residues 96-116 are primarily responsible for binding to C domain of intact TnC and residues 117-148 to its N domain. Inspection of the available mammalian and avian skeletal muscle TnI amino acid sequences reveals a previously unrecognized conserved motif repeated 3-fold, once in the inhibitory peptide region (approximately residues 101-114; designated alpha) and twice more in the region of residues approximately 121-132 (beta) and approximately 135-146 (gamma). The number and distribution of these motifs have important structural implications for the TnI x C complex. In the beta motif of cardiac TnI, as compared with skeletal, several changes in charged amino acids are suggested as candidates responsible for the greater sensitivity of cardiac Ca2+-regulated actomyosin to acidic pH as in ischemia.

Strong evolutionary conservation of broadly expressed protein isoforms in the troponin I gene family and other vertebrate gene families

It is well established that different protein classes undergo molecular evolution at different rates, presumably reflecting differing functional constraints. However, it is also the case that different isoforms of the "same" protein, encoded by a multigene family, may evolve at different rates. Here I report a relationship within gene families between isoform evolutionary rate and gene expression profile: Broadly expressed isoforms show stronger sequence conservation than do narrowly expressed isoforms. This observation emerged initially from cDNA cloning and sequencing studies, described here, of a vertebrate gene family encoding three differentially expressed isoforms of the muscle protein troponin I. However, the expression breadth/sequence conservation relationship applies to vertebrate gene families in general. In a broad and arbitrary survey sampling of sequence data on well-characterized vertebrate gene families, I found that in 14/15 families the most strongly conserved isoform was the most broadly expressed isoform, or one of several similarly broadly expressed isoforms. Broadly expressed isoforms are presumably subjected to greater negative selection pressure because they must function in a more diverse biochemical environment than do narrowly expressed isoforms. The expression breadth/evolutionary rate relationship has several interesting implications regarding the overall process of gene family evolution by duplication/divergence from ancestral genes.

Isolation, characterization and in vivo analysis of the murine calbindin-D28K upstream regulatory region

The genomic locus containing the murine calbindin-D28K gene has been isolated and partially characterized. Genomic cloning revealed an exon/intron chromosomal structure very similar to the avian gene previously described. The ability of the calbindin-D28K upstream region to direct cell-specific expression was tested in vivo. Varying lengths of upstream sequence were used to drive expression of lacZ in transgenic mice. Characterization of 23 transgenic mouse lines revealed that even as much as 3.0 kb of upstream sequence was unable to direct expression independently of integration site effects, suggesting the absence of important elements. Despite the small number of expressing transgenic lines and the great variability, there was a tendency of cell specificity of transgene expression exhibited in distinct brain regions. In the cerebellum, Purkinje cell-specific expression was observed with the shortest (1.0 kb) upstream sequence tested. Specificity of transgene expression in Purkinje cells was abolished with longer portions of upstream sequence. The same observation was made for transgene expression in granule cells of the dentate gyrus, while the opposite effect was observed for expression in CA1 hippocampal cells. The absence of any transgenic lines exhibiting appropriate transgene expression in the kidney suggested that the VDREs described previously for the murine calbindin gene are not sufficient to direct kidney expression in vivo. It is concluded that 3.0 kb of calbindin upstream sequence includes the regulatory elements dictating a portion of cell-specificity in the CNS of transgenic mice, albeit lacking regions that allow expression independently of chromosomal effects.

Selective binding of specific mouse genomic DNA fragments by mouse vimentin filaments in vitro

Mouse vimentin intermediate filaments (IFs) reconstituted in vitro were analyzed for their capacity to select certain DNA sequences from a mixture of about 500-bp-long fragments of total mouse genomic DNA. The fragments preferentially bound by the IFs and enriched by several cycles of affinity binding and polymerase chain reaction (PCR) amplification were cloned and sequenced. In general, they were G-rich and highly repetitive in that they often contained Gn, (GT)n, and (GA)n repeat elements. Other, more complex repeat sequences were identified as well. Apart from the capacity to adopt a Z-DNA and triple helix configuration under superhelical tension, many fragments were potentially able to form cruciform structures and contained consensus binding sites for various transcription factors. All of these sequence elements are known to occur in introns and 5'/3'-flanking regions of genes and to play roles in DNA transcription, recombination and replication. A FASTA search of the EMBL data bank indeed revealed that sequences homologous to the mouse repetitive DNA fragments are commonly associated with gene-regulatory elements. Unexpectedly, vimentin IFs also bound a large number of apparently overlapping, AT-rich DNA fragments that could be aligned into a composite sequence highly homologous to the 234-bp consensus centromere repeat sequence of gamma-satellite DNA. Previous experiments have shown a high affinity of vimentin for G-rich, repetitive telomere DNA sequences, superhelical DNA, and core histones. Taken together, these data support the hypothesis that, after penetration of the double nuclear membrane via an as yet unidentified mechanism, vimentin IFs cooperatively fix repetitive DNA sequence elements in a differentiation-specific manner in the nuclear periphery subjacent to the nuclear lamina and thus participate in the organization of chromatin and in the control of transcription, replication, and recombination processes. This includes aspects of global regulation of gene expression such as the position effects associated with translocation of genes to heterochromatic centromere and telomere regions of the chromosomes.

Cardiac and skeletal muscle troponin I isoforms are encoded by a dispersed gene family on mouse chromosomes 1 and 7

We mapped the locations of the genes encoding the slow skeletal muscle, fast skeletal muscle, and cardiac isoforms of troponin I (Tnni) in the mouse genome by interspecific hybrid backcross analysis of species-specific (C57BL/6 vs Mus spretus) restriction fragment length polymorphisms (RFLPs). The slow skeletal muscle troponin I locus (Tnni1) mapped to Chromosome (Chr) 1. The fast skeletal muscle troponin I locus (Tnni2), mapped to Chr 7, approximately 70 cM from the centromere. The cardiac troponin I locus (Tnni3) also mapped to Chr 7, approximately 5-10 cM from the centromere and unlinked to the fast skeletal muscle troponin I locus. Thus, the troponin I gene family is dispersed in the mouse genome.