Precocious appearance of cardiac troponin T pre-mRNAs during early avian embryonic skeletal muscle development in ovo

Troponin T isoforms and posttranscriptional modifications: Evolution, regulation and function

Troponin-mediated Ca²(+)-regulation governs the actin-activated myosin motor function which powers striated (skeletal and cardiac) muscle contraction. This review focuses on the structure-function relationship of troponin T, one of the three protein subunits of the troponin complex. Molecular evolution, gene regulation, alternative RNA splicing, and posttranslational modifications of troponin T isoforms in skeletal and cardiac muscles are summarized with emphases on recent research progresses. The physiological and pathophysiological significances of the structural diversity and regulation of troponin T are discussed for impacts on striated muscle function and adaptation in health and diseases.

Characterization of cis-regulatory elements and transcription factor binding: gel mobility shift assay

To understand how cardiac gene expression is regulated, the identification and characterization of cis-regulatory elements and their trans-acting factors by gel mobility shift assay (GMSA) or gel retardation assay are essential and common steps. In addition to providing a general protocol for GMSA, this chapter describes some applications of this assay to characterize cardiac-specific and ubiquitous trans-acting factors bound to regulatory elements [novel TCTG(G/C) direct repeat and A/T-rich region] of the rat cardiac troponin T promoter. In GMSA, the specificity of the binding of trans-acting factor to labeled DNA probe should be verified by the addition of unlabeled probe in the reaction mixture. The migratory property of DNA-protein complexes formed by protein extracts prepared from different tissues can be compared to determine the tissue specificity of trans-acting factors. GMSA, coupled with specific antibody to trans-acting factor (antibody supershift assay), is used to identify proteins present in the DNA-protein complex. The gel-shift competition assay with an unlabeled probe containing a slightly different sequence is a powerful technique used to assess the sequence specificity and relative binding affinity of a DNA-protein interaction. GMSA with SDS-PAGE fractionated proteins allows for the determination of the apparent molecular mass of bound trans-acting factor.

A point mutation in bioactive RNA results in the failure of mutant heart correction in Mexican axolotls

Ambystoma mexicanum is an intriguing animal model for studying heart development because it carries a mutation in gene c. Hearts of homozygous recessive (c/c) mutant embryos do not contain organized myofibrils and fail to beat. The defect can be corrected by organ-culturing the mutant heart in the presence of RNA from anterior endoderm or endoderm/mesoderm-conditioned medium. By screening a cDNA library made of total conditioned medium RNA from normal axolotl embryonic endoderm, we isolated a single clone (MIR), the synthetic RNA from which corrects the mutant heart defect by promoting myofibrillogenesis and thus was named MIR (myofibrillogenesis inducing RNA). In the present study, we have examined MIR gene expression in mutant axolotl hearts at early pre-heart-beat developmental stages and found its quantitative expression, as detected by RT-PCR, to be the same as in normal hearts. However, careful analysis of sequence data revealed a G-->U point mutation in the mutant MIR RNA. Further computational analyses, using GENEBEE software to compare normal and mutant MIR RNAs show a significant alteration in RNA secondary structure of the point-mutated MIR RNA. The results from bioassay and confocal microscopy immunofluorescent studies demonstrate that, unlike MIR RNA derived from normal embryos, the mutated MIR RNA does not promote myofibrillogenesis in mutant embryonic hearts and fails to rescue/correct the mutant heart defect.

Precocious expression of cardiac troponin T in early chick embryos is independent of bone morphogenetic protein signaling

Cardiac troponin T (cTNT) is a component of the troponin complex, which confers calcium sensitivity to contraction in skeletal and cardiac muscle. Although it is thought that most components of the contractile myofibril are expressed exclusively in differentiated muscle cells, we observed that mRNAs coding for cTNT were detectable in explanted late gastrula mesoderm at least 12 hr before cardiac myocyte differentiation. We therefore conducted a detailed analysis of cTNT gene expression in the early chick embryo. Whole-mount in situ hybridization studies showed that by Hamburger and Hamilton stage 5, cTNT mRNAs are detectable in lateral mesoderm and, by stage 6, are observed throughout the lateral embryonic and extraembryonic mesoderm in a distribution that is much broader than the recognized heart field. As myocardial cell differentiation commences, cTNT transcripts become progressively localized to the forming heart and, by stage 14, are completely restricted to heart muscle cells. Western blot analyses demonstrated that cTNT protein expression is under translational control, as cTNT protein is not detectable until stage 9, concomitant with myocardial cell differentiation. Removal of endoderm at stage 5 had no effect on cTNT mRNA levels, and the bone morphogenetic protein (BMP) inhibitor noggin failed to block cTNT expression, even in the heart-forming region and in cases where heart formation was inhibited. Implantation of noggin-expressing CHO cells at the anterior midline of stage 7 embryos resulted in cardia bifida. These findings demonstrate the precocious, BMP-independent expression of a gene coding for a myofibrillar protein and suggest that an additional regulatory pathway exists for activation of some cardiogenic genes.

Comparative studies on the expression patterns of three troponin T genes during mouse development

In vertebrates, three troponin T (TnT) genes, cardiac TnT (cTnT), skeletal muscle fast-twitch TnT (fTnT), and slow-twitch TnT (sTnT), have evolved for the regulation of striated muscle contraction. To understand the mechanism for muscle fiber-specific expression of the TnT genes, we compared their expression patterns during mouse development. Our data revealed that the TnT expression in the developing embryo was not as restricted as that in the adult. In addition to a strong expression in the developing heart beginning at day 7.5 p.c (postcoitum), the cTnT transcript was detected at later stages in some skeletal muscles, where beginning at day 11.75 p.c. the fTnT and sTnT genes were also expressed. Only sTnT but not fTnT was found transiently in the developing heart. At day 13.5 p.c., expressions of all three genes were detected in the developing tongue and this co-expression continued to day 16.5 p.c. with the fTnT isoform being predominant. At this stage, overlapping and distinct expression patterns of both sTnT and fTnT genes were also evident in many developing skeletal muscles. These data suggest that different muscles during development undergo a complex change in TnT isoforms resulting in different contractile properties. Unexpectedly, the cTnT transcript was persistently found in the developing bladder, where presumably smooth muscle is present. In transgenic mice, expression of a LacZ gene driven by a rat cTnT promoter (-497 to +192 bp) was very similar to that of the endogenous cTnT gene, suggesting that this promoter contained regulatory elements sufficient for the control of tissue-specific cTnT expression during development.

Evidence that translation of smooth muscle alpha-actin mRNA is delayed in the chick promyocardium until fusion of the bilateral heart-forming regions

Heart development in the chick embryo proceeds from bilateral mesodermal primordia established during gastrulation. These primordia migrate to the midline and fuse into a single heart trough. During their migration as a cohesive sheet, the cells of the paired heart fields become epithelial and undergo cardiac differentiation, exhibiting organized myofibrils and rhythmic contractions near the time of their fusion. Between the stages of cardiomyoblast commitment and overt differentiation of cardiomyocytes, a significant time interval exists. Using a new riboprobe (usmaar) for whole-mount in situ hybridization in chick embryos, we report the earliest phases of smooth muscle alpha-actin (smaa) mRNA distribution during the precontractile developmental window. We show that ingressed heart-forming regions express smaa by the head-process stage (Hamburger and Hamilton stage 5). In addition, we used usmaar to study the formation and early morphogenesis of the heart. Consistent with fate mapping studies (Garcia-Martinez and Schoenwolf [1993] Dev. Biol. 159:706-719; Schoenwolf and Garcia-Martinez [1995] Cell Mol. Biol. Res. 41:233-240; Garcia-Martinez et al., in preparation), our results with this probe, combined with detailed histological and SEM analyses of the so-called cardiac crescent, demonstrate unequivocally that the heart arises from separated and paired heart rudiments, rather than from a single crescent-shaped rudiment (that is, prior to fusion of the paired heart rudiments to establish the straight-heart tube, the rostral midline of the cardiac crescent lacks mesodermal cells and consequently fails to label with usmaar). Smaa is also expressed in the splanchnic and somatic mesoderm, marking the earliest step in coelom formation. Consequently, we also used usmaar to describe formation of the pericardium. Finally, we provide evidence of a post-transcriptional level of control of smaa gene expression in the heart fields. Our results suggest that the expression of smaa may mark a primitive mesodermal state from which definitive cell types can be derived through inductive events.

The human MLL gene: nucleotide sequence, homology to the Drosophila trx zinc-finger domain, and alternative splicing

We have previously reported the cloning of several cDNAs corresponding to the MLL gene. The predicted primary amino acid sequence of two of these clones, 14p-18B and 14-7, reveals nearly complete identity with parts of the sequences of HRX, ALL-1, and Htrx-1, including a Zinc-finger region with homology to the Drosophila trithorax gene. However, we found that there is a stretch of 39 amino acids that is absent from 14p-18B when compared to ALL-1 and HRX. Another sequence of three amino acids is present in ALL-1, but is absent from 14p-18B and HRX. Nucleotide sequence examination reveals that these differences arise from alternative splicing, suggesting that MLL, HRX, and ALL-1 each represents a different alternative splicing product from the same gene. At least two cDNA clones, 14-7 and 14p-18C, correspond to incompletely processed transcripts including intron sequences. Northern blots using a subclone of 14p-18B revealed mRNA species of 14-16 kb in size in various human tissues. RNase protection assays show that the splice variant containing exon 8 and lacking a 9-bp extension 3' of exon 12 is predominantly expressed in hematopoietic cell lines.

Repeating developmental expression of G-Hox 7, a novel homeobox-containing gene in the chicken

Here we describe the isolation and characterization of a new chicken homeobox-containing gene, G-Hox 7, which is related to Drosophila msh. The deduced amino acid sequence of the cDNA shows greater than 96% homology to the homeo domain of other vertebrate msh-like genes. As for other species, the amino and carboxy termini of the protein are, however, greatly divergent when compared phylogenetically. In situ hybridization studies revealed the early and wide-spread expression of G-Hox 7 during chick development. This includes its expression in the primitive streak and extraembryonic cells undergoing epiboly, and its expression along the neural axis, including the forebrain. Expression was also observed in the neural crest, neural crest-derived facial and branchial structures, the otocyst, limb, and heart valves. This widespread and recurrent expression of the transcript suggests that the gene may play an essential role at multiple sites during the initiation of new developmental pathways.

Identification and characterization of a ventricular-specific avian myosin heavy chain, VMHC1: expression in differentiating cardiac and skeletal muscle

To investigate the initial differentiative processes of avian cardiac and skeletal myogenesis, we have isolated and characterized a molecular marker of the cardiac myocyte cell lineage, ventricular myosin heavy chain 1 (VMHC1). Our goal in this initial study was to use a gene-specific probe to analyze the expression pattern of VMHC1 RNA during development. DNA sequence analysis confirmed that VMHC1 represented a novel member of the MHC gene family. PCR analysis using gene-specific primers determined that the VMHC1 RNA is first expressed in the stage 7 cardiac primordia, much earlier than the appearance of a tubular beating heart. RNA blot analyses determined that the VMHC1 message was present in the embryonic and adult ventricles but not in the embryonic or adult atria or skeletal muscle tissues of either the fast or slow type after definitive muscle structures were formed. Still, PCR and in situ hybridization analyses of the initial phases of cardiac and skeletal myogenic differentiation determined that VMHC1 was expressed in both progenitor populations at the initiation of myogenesis regardless of the source of myoblast or site of initial differentiation. The transient expression in skeletal muscle precursors coincided with the onset of differentiation in these cells. These data suggest that the differentiative programs of cardiac and skeletal myocytes overlap during their initial phases, then quickly become distinct. The VMHC1 gene should provide a model for identification of transcription factors involved in cardiac myocyte differentiation.