Novel developmentally regulated exon identified in the rat fast skeletal muscle troponin T gene

Structure and sequence of the human fast skeletal troponin T (TNNT3) gene: insight into the evolution of the gene and the origin of the developmentally regulated isoforms

We describe the cloning, sequencing and structure of the human fast skeletal troponin T (TNNT3) gene located on chromosome 11p15.5. The single-copy gene encodes 19 exons and 18 introns. Eleven of these exons, 1–3, 9–15 and 18, are constitutively spliced, whereas exons 4–8 are alternatively spliced. The gene contains an additional subset of developmentally regulated and alternatively spliced exons, including a foetal exon located between exon 8 and 9 and exon 16 or α (adult) and 17 or β (foetal and neonatal). Exon phasing suggests that the majority of the alternatively spliced exons located at the 5′ end of the gene may have evolved as a result of exon shuffling, because they are of the same phase class. In contrast, the 3′ exons encoding an evolutionarily conserved heptad repeat domain, shared by both TnT and troponin I (TnI), may be remnants of an ancient ancestral gene. The sequence of the 5′ flanking region shows that the putative promoter contains motifs including binding sites for MyoD, MEF-2 and several transcription factors which may play a role in transcriptional regulation and tissue-specific expression of TnT. The coding region of TNNT3 exhibits strong similarity to the corresponding rat sequence. However, unlike the rat TnT gene, TNNT3 possesses two repeat regions of CCA and TC. The exclusive presence of these repetitive elements in the human gene indicates divergence in the evolutionary dynamics of mammalian TnT genes. Homologous muscle-specific splicing enhancer motifs are present in the introns upstream and downstream of the foetal exon, and may play a role in the developmental pattern of alternative splicing of the gene. The genomic correlates of TNNT3 are relevant to our understanding of the evolution and regulation of expression of the gene, as well as the structure and function of the protein isoforms. The nucleotide sequence of TNNT3 has been submitted to EMBL/GenBank under Accession No. AF026276.

Regulation of troponin T expression during muscle development in sea bream Sparus auratus Linnaeus: the potential role of thyroid hormones

In the sea bream Sparus auratus three stage-specific fast troponin T (fTnT) isoforms have been cloned and correspond to embryonic-, larval- and adult-specific isoforms. Characterisation, using database searches, of the putative genomic organisation of Fugu rubripes and Tetraodon nigroviridis fTnT indicates that alternative exon splicing in the 5 region of the gene generates the different isoforms. Moreover, comparison of teleost fTnTs suggests that alternative splicing of fTnT appears to be common in teleosts. A different temporal expression pattern for each fTnT splice varotnt is found during sea bream development and probably relates to differing functional demands, as a highly acidic embryonic form (pI 5.16) is substituted by a basic larval form (pI 9.57). Thyroid hormones (THs), which play an important regulatory role in muscle development in flatfish and tetrapods, appear also to influence TnT gene expression in the sea bream. However, THs have a divergent action on different sea bream TnT genes and although the slow isoform (sTnT1) is TH-responsive, fTnT, sTnT2 and the itronless isoform (iTnT) are unaffected. The present results taken together with those published for flatfish seem to suggest differences may exist in the regulation of larval muscle development in teleosts.

Troponin T isoform expression is modulated during Atlantic halibut metamorphosis

BACKGROUND

Flatfish metamorphosis is a thyroid hormone (TH) driven process which leads to a dramatic change from a symmetrical larva to an asymmetrical juvenile. The effect of THs on muscle and in particular muscle sarcomer protein genes is largely unexplored in fish. The change in Troponin T (TnT), a pivotal protein in the assembly of skeletal muscles sarcomeres and a modulator of calcium driven muscle contraction, during flatfish metamophosis is studied.

RESULTS

In the present study five cDNAs for halibut TnT genes were cloned; three were splice variants arising from a single fast TnT (fTnT) gene; a fourth encoded a novel teleost specific fTnT-like cDNA (AfTnT) expressed exclusively in slow muscle and the fifth encoded the teleost specific sTnT2. THs modified the expression of halibut fTnT isoforms which changed from predominantly basic to acidic isoforms during natural and T4 induced metamorphosis. In contrast, expression of red muscle specific genes, AfTnT and sTnT2, did not change during natural metamorphosis or after T4 treatment. Prior to and after metamorphosis no change in the dorso-ventral symmetry or temporal-spatial expression pattern of TnT genes and muscle fibre organization occurred in halibut musculature.

CONCLUSION

Muscle organisation in halibut remains symmetrical even after metamorphosis suggesting TH driven changes are associated with molecular adaptations. We hypothesize that species specific differences in TnT gene expression in teleosts underlies different larval muscle developmental programs which better adapts them to the specific ecological constraints.

Alternative splicing, muscle calcium sensitivity, and the modulation of dragonfly flight performance

Calcium sensitivity of myosin cross-bridge activation in striated muscles commonly varies during ontogeny and in response to alterations in muscle usage, but the consequences for whole-organism physiology are not well known. Here we show that the relative abundances of alternatively spliced transcripts of the calcium regulatory protein troponin T (TnT) vary widely in flight muscle of Libellula pulchella dragonflies, and that the mixture of TnT splice variants explains significant portions of the variation in muscle calcium sensitivity, wing-beat frequency, and an index of aerodynamic power output during free flight. Two size-distinguishable morphs differ in their maturational pattern of TnT splicing, yet they show the same relationship between TnT transcript mixture and calcium sensitivity and between calcium sensitivity and aerodynamic power output. This consistency of effect in different developmental and physiological contexts strengthens the hypothesis that TnT isoform variation modulates muscle calcium sensitivity and whole-organism locomotor performance. Modulating muscle power output appears to provide the ecologically important ability to operate at different points along a tradeoff between performance and energetic cost.

Primary structure and developmental acidic to basic transition of 13 alternatively spliced mouse fast skeletal muscle troponin T isoforms

Large samples of original cDNAs encoding neonatal and adult mouse fast skeletal muscle troponin T (fTnT) have been isolated and characterized. The results demonstrate expression relationships of 8 alternatively spliced exons of the fTnT gene and reveal the primary structure of as many as 13 fTnT isoforms that diverge into acidic and basic classes due to differential mRNA splicing in the N-terminal variable region. In the C-terminal variable region encoded by the mutually exclusive exons 16 and 17, the splicing pathway and structure of exon 16 appears to be adult fTnT-specific, suggesting an adaptation to the functional demands of mature fast skeletal muscle. The cloned cDNAs were expressed in E. coli as standards to identify a high M(r) to low M(r), acidic to basic fTnT isoform transition in postnatal developing skeletal muscles. Different from the developmental cardiac TnT switch generated by alternative splicing of a single exon, the fTnT isoform transition is an additive effect of alternative splicing of multiple N-terminal-coding exons, especially exons 4, 8 and fetal that are expressed at higher frequencies in the neonatal than in the adult muscle. The developmental fTnT isoform primary structure transition in both N- and C-terminal variable regions suggest a physiological importance of the apparently complex TnT isoform expression.

Changes in some troponin and insulin-like growth factor messenger ribonucleic acids in regenerating and denervated skeletal muscles

To investigate the role of innervation and to determine if the process of muscle differentiation is preprogrammed, the expression of insulin-like growth factors (IGF-I and IGF-II), troponin I and troponin T mRNAs was studied in regenerating transplants of rat Extensor digitorum longus muscle in the presence and absence of nerve. The role of innervation was further investigated by denervating some adult fast (Gastrocnemius and Plantaris) and slow (Soleus) skeletal muscles. In normal adult skeletal muscles, IGF-I, IGF-II and developmental fast troponin T mRNA containing exon y, are undetectable or present at very low levels. Induction of all these mRNAs was observed in regenerating muscles in both the presence and absence of nerve as well as following denervation of adult fast and slow skeletal muscles. Their low level expression was maintained in adult denervated skeletal muscles but gradually suppressed in both innervated and noninnervated regenerating extensor digitorum longus muscle transplants after 2 months. Fast troponin T mRNA was synthesized in both innervated and noninnervated EDL transplants although the level of this transcript changed markedly in response to denervation of both adult fast and slow skeletal muscles. The fast troponin T mRNA containing exon 17 was also initially expressed in both regenerating muscles but its level was reduced with time in both transplants and in all adult denervated skeletal muscles. Fast and slow troponin I mRNAs were synthesised during EDL muscle regeneration in both the presence and absence of nerve but the slow troponin I expression was not maintained in noninnervated transplants. The level of fast troponin I mRNA decreased in denervated fast skeletal muscles but markedly increased in denervated Soleus. The level of slow troponin I mRNA was slightly increased in denervated fast skeletal muscles but considerably reduced in denervated Soleus.

Diminished Ca2+ sensitivity of skinned cardiac muscle contractility coincident with troponin T-band shifts in the diabetic rat

We have measured the apparent Ca2+ sensitivities of force development in skinned cardiac trabeculae at different sarcome lengths together with shifts in troponin (Tn) T subunits on specimens from the same hearts and drawn insights into the pathogenesis of myocardial dysfunction in the diabetic rat. The Ca(2+)-force relations were measured at a long (2.4-microns) and a short (1.9-microns) sarcomere length. In disease, compared with the control condition, the apparent Ca2+ sensitivity was greatly diminished at a sarcomere length of 1.9 microns but not affected at all at the long length (2.4 microns). We also examined the alterations in contractile regulatory proteins TnT and TnI by both sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blots. The TnI band was largely unperturbed, but major changes were discerned in TnT. The normal rat heart indicated two major bands (TnT1 and TnT2) and a faint third band (TnT3); in the diabetic rat heart, there was a significant shift in intensity from TnT1 to TnT3. Since myosin isozyme shifts also accompany diabetes in the rat, we used a prototypical hypothyroid rat as well to evaluate the myosin influence in the length-induced effects on Ca2+ sensitivity. Myosin shifts during hypothyroidism were unaccompanied by significant changes in TnT, and there were also no length-dependent modifications in Ca2+ sensitivity. The findings raise the possibility that diabetic Ca(2+)-sensitivity changes in the myocardium are coupled with TnT alterations. A plausible explanation is offered whereby these TnT alterations modify the length dependence of Ca2+ sensitivity.

Identification of a fetal exon in the human fast troponin T gene

A developmentally regulated exon has been identified in the 5'-alternatively spliced region of the human fast Troponin T (TnT) gene. Expressed in fetal (but not adult) muscle, this exon is homologous with the fetal exons recently described in the rabbit and rat fast TnT genes. They all exhibit a split codon organization and encode a highly acidic peptide. To determine if the splicing pathways, including the human fetal exon, are also conserved, we defined the major TnT splicing patterns in fetal muscle. They generate fetal TnT 1, fetal TnT 3, and TnT1f, and TnT3f, species previously described in rabbit and rat skeletal muscles.