Transitions from fetal to fast troponin T isoforms are coordinated with changes in tropomyosin and alpha-actinin isoforms in developing rabbit skeletal muscle

Troponins and Skeletal Muscle Pathologies

Skeletal muscles account for ~30-40% of the total weight of human body and are responsible for its most important functions, including movement, respiration, thermogenesis, and glucose and protein metabolism. Skeletal muscle damage negatively impacts the whole-body functioning, leading to deterioration of the quality of life and, in severe cases, death. Therefore, timely diagnosis and therapy for skeletal muscle dysfunction are important goals of modern medicine. In this review, we focused on the skeletal troponins that are proteins in the thin filaments of muscle fibers. Skeletal troponins play a key role in regulation of muscle contraction. Biochemical properties of these proteins and their use as biomarkers of skeletal muscle damage are described in this review. One of the most convenient and sensitive methods of protein biomarker measurement in biological liquids is immunochemical analysis; hence, we examined the factors that influence immunochemical detection of skeletal troponins and should be taken into account when developing diagnostic test systems. Also, we reviewed the available data on the skeletal troponin mutations that are considered to be associated with pathologies leading to the development of diseases and discussed utilization of troponins as drug targets for treatment of the skeletal muscle disorders.

Troponin Variants as Markers of Skeletal Muscle Health and Diseases

Ca2 +-regulated contractility is a key determinant of the quality of muscles. The sarcomeric myofilament proteins are essential players in the contraction of striated muscles. The troponin complex in the actin thin filaments plays a central role in the Ca2+-regulation of muscle contraction and relaxation. Among the three subunits of troponin, the Ca2+-binding subunit troponin C (TnC) is a member of the calmodulin super family whereas troponin I (TnI, the inhibitory subunit) and troponin T (TnT, the tropomyosin-binding and thin filament anchoring subunit) are striated muscle-specific regulatory proteins. Muscle type-specific isoforms of troponin subunits are expressed in fast and slow twitch fibers and are regulated during development and aging, and in adaptation to exercise or disuse. TnT also evolved with various alternative splice forms as an added capacity of muscle functional diversity. Mutations of troponin subunits cause myopathies. Owing to their physiological and pathological importance, troponin variants can be used as specific markers to define muscle quality. In this focused review, we will explore the use of troponin variants as markers for the fiber contents, developmental and differentiation states, contractile functions, and physiological or pathophysiological adaptations of skeletal muscle. As protein structure defines function, profile of troponin variants illustrates how changes at the myofilament level confer functional qualities at the fiber level. Moreover, understanding of the role of troponin modifications and mutants in determining muscle contractility in age-related decline of muscle function and in myopathies informs an approach to improve human health.

Impact of tropomyosin isoform composition on fast skeletal muscle thin filament regulation and force development

Tropomyosin (Tm) plays a central role in the regulation of muscle contraction and is present in three main isoforms in skeletal and cardiac muscles. In the present work we studied the functional role of α- and βTm on force development by modifying the isoform composition of rabbit psoas skeletal muscle myofibrils and of regulated thin filaments for in vitro motility measurements. Skeletal myofibril regulatory proteins were extracted (78%) and replaced (98%) with Tm isoforms as homogenous ααTm or ββTm dimers and the functional effects were measured. Maximal Ca(2+) activated force was the same in ααTm versus ββTm myofibrils, but ββTm myofibrils showed a marked slowing of relaxation and an impairment of regulation under resting conditions compared to ααTm and controls. ββTm myofibrils also showed a significantly shorter slack sarcomere length and a marked increase in resting tension. Both these mechanical features were almost completely abolished by 10 mM 2,3-butanedione 2-monoxime, suggesting the presence of a significant degree of Ca(2+)-independent cross-bridge formation in ββTm myofibrils. Finally, in motility assay experiments in the absence of Ca(2+) (pCa 9.0), complete regulation of thin filaments required greater ββTm versus ααTm concentrations, while at full activation (pCa 5.0) no effect was observed on maximal thin filament motility speed. We infer from these observations that high contents of ββTm in skeletal muscle result in partial Ca(2+)-independent activation of thin filaments at rest, and longer-lasting and less complete tension relaxation following Ca(2+) removal.

Alterations in thin filament length during postnatal skeletal muscle development and aging in mice

The lengths of the sarcomeric thin filaments vary in a skeletal muscle-specific manner and help specify the physiological properties of skeletal muscle. Since the extent of overlap between the thin and thick filaments determines the amount of contractile force that a sarcomere can actively produce, thin filament lengths are accurate predictors of muscle-specific sarcomere length-tension relationships and sarcomere operating length ranges. However, the striking uniformity of thin filament lengths within sarcomeres, specified during myofibril assembly, has led to the widely held assumption that thin filament lengths remain constant throughout an organism's lifespan. Here, we rigorously tested this assumption by using computational super-resolution image analysis of confocal fluorescence images to explore the effects of postnatal development and aging on thin filament length in mice. We found that thin filaments shorten in postnatal tibialis anterior (TA) and gastrocnemius muscles between postnatal days 7 and 21, consistent with the developmental program of myosin heavy chain (MHC) gene expression in this interval. By contrast, thin filament lengths in TA and extensor digitorum longus (EDL) muscles remained constant between 2 mo and 2 yr of age, while thin filament lengths in soleus muscle became shorter, suggestive of a slow-muscle-specific mechanism of thin filament destabilization associated with aging. Collectively, these data are the first to show that thin filament lengths change as part of normal skeletal muscle development and aging, motivating future investigations into the cellular and molecular mechanisms underlying thin filament adaptation across the lifespan.

The role of tropomyosin isoforms and phosphorylation in force generation in thin-filament reconstituted bovine cardiac muscle fibres

The thin filament extraction and reconstitution protocol was used to investigate the functional roles of tropomyosin (Tm) isoforms and phosphorylation in bovine myocardium. The thin filament was extracted by gelsolin, reconstituted with G-actin, and further reconstituted with cardiac troponin together with one of three Tm varieties: phosphorylated alphaTm (alphaTm.P), dephosphorylated alphaTm (alphaTm.deP), and dephosphorylated betaTm (betaTm.deP). The effects of Ca, phosphate, MgATP and MgADP concentrations were examined in the reconstituted fibres at pH 7.0 and 25 degrees C. Our data show that Ca(2+) sensitivity (pCa(50): half saturation point) was increased by 0.19 +/- 0.07 units when betaTm.deP was used instead of alphaTm.deP (P < 0.05), and by 0.27 +/- 0.06 units when phosphorylated alphaTm was used (P < 0.005). The cooperativity (Hill factor) decreased (but insignificantly) from 3.2 +/- 0.3 (5) to 2.8 +/- 0.2 (7) with phosphorylation. The cooperativity decreased significantly from 3.2 +/- 0.3 (5) to 2.1 +/- 0.2 (9) with isoform change from alphaTm.deP to betaTm.deP. There was no significant difference in isometric tension or stiffness between alphaTm.P, alphaTm.deP, and betaTm.deP muscle fibres at saturating [Ca(2+)] or after rigor induction. Based on the six-state cross-bridge model, sinusoidal analysis indicated that the equilibrium constants of elementary steps differed up to 1.7x between alphaTm.deP and betaTm.deP, and up to 2.0x between alphaTm.deP and alphaTm.P. The rate constants differed up to 1.5x between alphaTm.deP and betaTm.deP, and up to 2.4x between alphaTm.deP and alphaTm.P. We conclude that tension and stiffness per cross-bridge are not significantly different among the three muscle models.

The superfast extraocular myosin (MYH13) is localized to the innervation zone in both the global and orbital layers of rabbit extraocular muscle

Extraocular muscles (EOMs) are the most molecularly heterogeneous and physiologically diverse mammalian striated muscles. They express the entire array of striated muscle myosins, including a specialized myosin heavy chain MYH13, which is restricted to extraocular and laryngeal muscles. EOMs also exhibit a breadth of contractile activity, from superfast saccades to slow tracking and convergence movements. These movements are accomplished by the action of six ultrastructurally defined fiber types that differ from the type IIa, IIb, IIx and I fibers found in other skeletal muscles. Attempts to associate different eye movements with either the expression of different myosins or the activity of particular EOM fiber types are complicated by the molecular heterogeneity of several of the fiber types, and by electromyography studies showing that the majority of extraocular motor units participate in both fast and slow eye movements. To better understand the role of MYH13 in ocular motility, we generated MYH13-sequence-specific antibodies and used SDS-PAGE to quantify the regional distribution of myosin in EOM and to characterize its heterogeneity in single fibers. These studies demonstrate that MYH13 is preferentially expressed in the majority of orbital and global fibers in the central innervation zone of rabbit EOM. Many individual fibers express MYH13 with the fast IIb myosin and varying amounts of IIx myosin. The differential localization of MYH13, coupled with specialization of the sarcoplasmic reticulum and thin filament systems, probably explains how activation of the endplate band region enables the majority of EOM fibers to contribute to superfast contractions.

Alternative splicing, muscle contraction and intraspecific variation: associations between troponin T transcripts, Ca2+ sensitivity and the force and power output of dragonfly flight muscles during oscillatory contraction

The flight muscles of Libellula pulchella dragonflies contain a mixture of six alternatively spliced transcripts of a single troponin T (TnT) gene. Here, we examine how intraspecific variation in the relative abundance of different TnT transcripts affects the Ca2+ sensitivity of skinned muscle fibers and the performance of intact muscles during work-loop contraction regimes that approximate in vivo conditions during flight. The relative abundance of one TnT transcript, or the pooled relative abundance of two TnT transcripts, showed a positive correlation with a 10-fold range of variation in Ca2+ sensitivity of skinned fibers (r2=0.77, P&lt;0.0001) and a threefold range in peak specific force (r2=0.74, P&lt;0.0001), specific work per cycle (r2=0.54; P&lt;0.0001) and maximum specific power output (r2=0.48, P=0.0005) of intact muscle. Using these results to reanalyze previously published data for wing kinematics during free flight, we show that the relative abundances of these particular transcripts are also positively correlated with wingbeat frequency and amplitude. TnT variation alone may be responsible for these effects, or TnT variation may be a marker for changes in a suite of co-regulated molecules. Dragonflies from two ponds separated by 16 km differed significantly in both TnT transcript composition and muscle contractile performance, and within each population there are two distinct morphs that showed different maturational trajectories of TnT transcript composition and muscle contractility. Thus, there is broad intraspecific variability and a high degree of population structure for contractile performance phenotypes, TnT ribotypes and ontogenetic patterns involving these traits that affect locomotor performance.

Structure and evolution of the alternatively spliced fast troponin T isoform gene

The vertebrate fast skeletal muscle troponin T gene, TnTf, produces a complexity of isoforms through differential mRNA splicing. The mechanisms that regulate splicing and the physiological significance of TnTf isoforms are poorly understood. To investigate these questions, we have determined the complete sequence structure of the quail TnTf gene, and we have characterized the developmental expression of alternatively spliced TnTf mRNAs in quail embryonic muscles. We report the following: 1) the quail TnTf gene is significantly larger than the rat TnTf gene and has 8 non-homologous exons, including a pectoral muscle-specific set of alternatively spliced exons; 2) specific sequences are implicated in regulated exon splicing; 3) a 900-base pair sequence element, composed primarily of intron sequence flanking the pectoral muscle-specific exons, is tandemly repeated 4 times and once partially, providing direct evidence that the pectoral-specific TnT exon domain arose by intragenic duplications; 4) a chicken repeat 1 retrotransposon element resides upstream of this repeated intronic/pectoral exon sequence domain and is implicated in transposition of this element into an ancestral genome; and 5) a large set of novel isoforms, produced by regulated exon splicing, is expressed in quail muscles, providing insights into the developmental regulation, physiological function, and evolution of the vertebrate TnTf isoforms.

The N-terminal region of troponin T is essential for the maximal activation of rat cardiac myofilaments

Troponin T (TnT) is an essential protein in the transduction of the Ca2+-binding signal that triggers striated muscle contraction. Functional diversity among various TnT isoforms found in cardiac and skeletal muscles has been correlated with the sequence heterogeneity at the amino (N-) and the carboxyl (C-) terminal regions. The most striking difference between cardiac TnT (cTnT) and skeletal TnT (sTnT) is that cTnT has an extended N-terminus, which is rich in negatively charged amino acids. To investigate the role of this region in cTnT, we deleted the first 76 amino acids in rat cTnT (cTnT77-289) by site-directed mutagenesis. We exchanged the native troponin complex in rat cardiac myofibrillar preparations and detergent skinned cardiac fiber bundles by treatment with excess cTnT or cTnT77-289. After reconstituting the cTnT77-289 containing myofibrils with cardiac troponin I-cardiac troponin C (cTnI-cTnC), the MgATPase activity was 70% of the cTnT treated myofibrils in the relaxed state and 83% of the cTnT treated myofibrils in the maximal Ca2+-activated state. These observations were supported by force measurements in which cTnT and cTnT77-289 were exchanged into skinned fiber bundles. Prior to reconstitution with cTnI-cTnC, the Ca2+-independent maximal force developed by the cTnT77-289 containing fiber was 45% of the force developed by the cTnT containing fiber. After reconstituting with cTnI-cTnC, the Ca2+-activated maximal force of the cTnT77-289 containing fiber was 62% of the force developed by the cTnT containing +cTnI-cTnC reconstituted fiber. In both assays, no significant changes in the normalized Ca2+-activity relation or in co-operativity were observed. Fluorescence experiments using pyrene-labeled Tm demonstrated that the binding of cTnT77-289 to Tm was 3-4 fold stronger than that of cTnT. Our results suggest that strong interactions between cTnT77-289 and Tm stabilize cardiac myofilaments in a sub-maximally activated state. Our findings also indicate that the N-terminus of cTnT is essential for maximal activation of cardiac myofilaments.

Characterisation of troponin-T from salmonid fish

Five major troponin-T isoforms were isolated from the myotomal muscles of Atlantic salmon: three from fast muscle (Tn-T1F, Tn-T2F and Tn-T3F) and two from slow muscle (Tn-T1S and Tn-T2S). In addition to their presence in troponin preparations, these proteins were also recognised to be Tn-T on the basis of immunoreaction with anti-troponin-T antibodies and partial amino acid sequence. The electrophoretic mobility in the presence of SDS of the various Tn-Ts increases in the order: 1S < 1F < 2S < 2F < or = 3F. Compositional analysis shows that the higher M(r) forms (1F and 1S) contain considerably more proline, glutamic acid and alanine than the lower-M(r) forms (2F, 3F and 2S). Every isoform lacks cysteine and phosphoserine is present only in isoforms 2F and 3F. All of the Tn-Ts, with the exception of isoform 1F, are N-terminally blocked. CNBr fragments from same cell type Tn-Ts yield identical sequences over at least fifteen Edman cycles. Two full-length cDNA sequences, presumed to represent 1S and 3F, or isoforms that are highly similar, are reported. As documented for higher vertebrate Tn-Ts, the predicted primary structures display a non-uniform distribution of charged amino acids and greater divergence at each end than in the central section. The most striking difference between the two salmonid proteins is the presence of a N-terminal (proline-, glutamic acid- and alanine-rich) extension of about fifty amino acids in Tn-T1s (278 amino acids) that is missing from the fast muscle Tn-T (223 amino acids). The sequences also differ in that 1S lacks the known phosphorylation site while the fast-type isoform contains serine next to the initiating methionine. Of the two, the slow isoform has accumulated the greater number of substitutions.

Mammalian skeletal muscle fiber type transitions

Mammalian skeletal muscle is an extremely heterogeneous tissue, composed of a large variety of fiber types. These fibers, however, are not fixed units but represent highly versatile entities capable of responding to altered functional demands and a variety of signals by changing their phenotypic profiles. This adaptive responsiveness is the basis of fiber type transitions. The fiber population of a given muscle is in a dynamic state, constantly adjusting to the current conditions. The full range of adaptive ability spans fast to slow characteristics. However, it is now clear that fiber type transitions do not proceed in immediate jumps from one extreme to the other, but occur in a graded and orderly sequential manner. At the molecular level, the best examples of these stepwise transitions are myofibrillar protein isoform exchanges. For the myosin heavy chain, this entails a sequence going from the fastest (MHCIIb) to the slowest (MHCI) isoform, and vice-versa. Depending on the basal protein isoform profile and hence the position within the fast-slow spectrum, the adaptive ranges of different fibers vary. A simple transition scheme has emerged from the multitude of data collected on fiber type conversions under a variety of conditions.

Physiologically regulated alternative splicing patterns of fast troponin T RNA are conserved in mammals

NH2-terminal isoforms of fast troponin T (TnT) are generated by alternative splicing of fast TnT RNA transcripts. Significantly different estimates for the number of isoforms have been obtained by nucleic acid and protein chemical studies. To resolve this controversy and to determine whether specific 5'-splicing patterns correlate with fiber phenotype, we generated representative populations of 5'-TnT cDNAs from the TnT mRNAs expressed in a set of physiologically and anatomically diverse skeletal muscles. Sequencing and restriction enzyme analyses revealed a total of nine cDNAs that encode the six adult and three perinatal NH2-terminal TnT variants previously identified. Three major 5'-splicing pathways (the TnT1f, TnT2f, and TnT3f patterns) account for more than 90% of the TnT mRNAs and proteins in adult rabbit skeletal muscle. Comparative studies in rats, mice, and humans show that these splicing patterns are conserved and that fast-twitch fibers that are primarily glycolytic utilize the TnT1f and TnT2f patterns preferentially, whereas fast-twitch fibers that are primarily oxidative use the TnT1f and TnT3f patterns preferentially.

Changes in the molecular types of connectin and nebulin during development of chicken skeletal muscle

Changes in the molecular types of connectin and nebulin during development of chicken breast and leg muscles were determined by an improved SDS-polyacrylamide gel electrophoresis (PAGE) using 2% polyacrylamide slab gel. The adult leg-type alpha-connectin (alpha L-connectin) and nebulin (L-nebulin) appeared in embryonic breast muscle, and changed into the adult breast-type ones (alpha B-connectin, B-nebulin) specific for adult breast muscle after hatching. In leg muscle, alpha L-connectin and L-nebulin appeared in an embryonic stage, and remained unchanged in molecular types throughout the entire process of development. alpha-Connectin and nebulin seemed to be regulated by a similar mechanism during development. On the other hand, beta-connectin appeared in an earlier stage of development of the embryonic breast muscle, independently of alpha-connectin.

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.

Investigation of the effects of phosphorylation of rabbit striated muscle alpha alpha-tropomyosin and rabbit skeletal muscle troponin-T

FPLC has been employed to prepare the phosphorylated and unphosphorylated forms of rabbit striated muscle alpha alpha-tropomyosin (TM), and the major isoform of rabbit fast-skeletal-muscle troponin-T (Tn-T2f) and corresponding chymotryptic fragment T1 (residues 1-158), in order to investigate the effects which these in vivo modifications have on thin filament function. In all instances, no significance could be attributed to the presence of a phosphate moiety on acetyl serine 1 of Tn-T (or fragment T1). As expected, fragment T1 increased the relative viscosities of solutions of unphosphorylated alpha alpha-TM, but this induction was noticeably lower for phosphorylated alpha alpha-TM. In affinity chromatography experiments, fragment T1 bound equally well to either form of alpha alpha-TM, but the interaction between fragment T2 (residues 159-259) and phosphorylated alpha alpha-TM was strengthened relative to the control. In the presence of alpha alpha-TM (unphosphorylated), fragment T1 was found to down regulate the actin-activated myosin-S1 MgATPase activity, indicating that this portion of Tn-T possesses modulatory properties. Under the same conditions, less inhibition was observed with phosphorylated alpha alpha-TM. When the two different forms of alpha alpha-TM were reconstituted into a complete regulatory system, the activation of myosin-S1 was double for those thin filaments containing the phosphorylated molecule. Dephosphorylation of the phospho alpha alpha-TM reduced the rates to control values. In ATPase Ca2+ titrations, these systems exhibited no difference in the co-operativity of activation and little or no difference in the pCa2+ 1/2 value. Developmentally linked changes in the steady-state phosphorylation of alpha alpha-TM could be a mechanism to increase the activating propensity of thin filaments, by modifying the functional properties of the T1 section of Tn-T.

Role of innervation for development and maintenance of troponin subunit isoform patterns in fast- and slow-twitch muscles of the rabbit

This study investigates the neural influence on the establishment and maintenance of muscle type-specific expression patterns of the three troponin (Tn) subunits, troponin T (TnT), troponin C (TnC), and troponin I (TnI) during postnatal development and in the adult rabbit. For this purpose, we followed changes in the expression of fast and slow TnT, TnC, and TnI isoforms at the protein and mRNA level in slow- and fast-twitch muscles. During postnatal development all fast Tn isoforms increased in fast-twitch muscle. Sequential transitions (TnTs-->TnT3f-->TnT1f) occurred in the TnT isoform pattern. These changes occurred in parallel with sequential transitions in the pattern of myosin heavy chain (HC) isoforms. Neonatal slow-twitch muscle displayed more mature (slow) isoform patterns for both TnT subunits and myosin HCs than fast-twitch muscle. Although the expression of slow TnC in slow-twitch muscle required innervation, denervation had little effect on slow TnT and TnI which seemed to be controlled by an intrinsic program. In fast-twitch muscle, denervation enhanced the expression of all slow Tn subunit isoforms. In addition, it led to a pronounced increase of the slow TnT2s isoform such that the amount of TnT2s exceeded that of TnT1s. The effects of denervation together with previous data on low-frequency stimulated muscle indicate that the expression of fast Tn isoforms in fast-twitch muscle is neurally controlled. The pattern of slow Tn isoforms in slow-twitch muscle seems to be regulated by an intrinsic program and, in addition, by neural influences.

Identification of the 30 kDa polypeptide in post mortem skeletal muscle as a degradation product of troponin-T

Although a 30 kDa polypeptide frequently is seen by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of post mortem (pm) skeletal muscle and in turn is used as an indicator of proteolysis, its origin has not been conclusively identified. We used antibodies to selected myofibrillar proteins, including some known to be degraded pm, to identify this polypeptide. The left side of eight beef carcasses was electrically stimulated (ES) within 1 h after slaughter, and the right side served as the non-stimulated (NS) control. The longissimus lumborum (LL) muscle was removed from the carcass at 24 h pm and was stored at 2 degrees C. Myofibrils were prepared from the LL muscle immediately after stimulation (0 day) and from the stored muscle sample at 1, 3, 7, 14 and 28 days pm for analysis of SDS-PAGE and Western blots. By SDS-PAGE, troponin-T (TN-T) decreased in amount more rapidly pm in ES samples than in NS samples. By SDS-PAGE, a 30 kDa band increased and became a prominent band by 7 days pm in both NS and ES samples. A monoclonal antibody (mAb) to TN-T labeled purified TN-T, as well as the TN-T in myofibrils, a prominent 30 kDa polypeptide and a family of lower molecular mass polypeptides in pm muscle. This mAb also labeled a 30 kDa band that had been electrophoretically purified from pm muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of a program of contractile protein gene expression initiated upon skeletal muscle differentiation

The functional diversity of skeletal muscle is largely determined by the combinations of contractile protein isoforms that are expressed in different fibers. Just how the developmental expression of this large array of genes is regulated to give functional phenotypes is thus of great interest. In the present study, we performed a comprehensive analysis of contractile protein isoform mRNA profiles in skeletal muscle systems representing each generation of fiber formed: primary, secondary, and regenerating fibers. We find that in each system examined there is a common pattern of isoform gene expression during early differentiation for 5 of the 6 gene families we have investigated: myosin light chain (MLC)1, MLC2, tropomyosin, troponin (Tn)C, and TnI. We suggest that the common isoform patterns observed together represent a genetic program of skeletal muscle differentiation that is independent of the mature fiber phenotype and is found in all newly formed myotubes. Within each of these contractile protein gene families the program is independent of the isoforms of myosin heavy chain (MHC) expressed. The maintenance of such a program may reflect a specific requirement of the initial differentiation process.

Contractile protein isoforms in muscle development

The contractile proteins of skeletal muscle are often represented by families of very similar isoforms. Protein isoforms can result from the differential expression of multigene families or from multiple transcripts from a single gene via alternative splicing. In many cases the regulatory mechanisms that determine the accumulation of specific isoforms via alternative splicing or differential gene expression are being unraveled. However, the functional significance of expressing different proteins during muscle development remains a key issue that has not been resolved. It is widely believed that distinct isoforms within a family are uniquely adapted to muscles with different physiological properties, since separate isoform families are often coordinately regulated within functionally distinct muscle fiber types. It is also possible that different isoforms are functionally indistinguishable and represent an inherent genetic redundancy among critically important muscle proteins. The goal of this review is to assess the evidence that muscle proteins which exist as different isoforms in developing and mature skeletal and cardiac muscles are functionally unique. Since regulation of both transcription and alternative splicing within multigene families may also be an important factor determining the accumulation of specific protein isoforms, evidence that genetic regulation rather than protein coding information provides the functional basis of isoform diversity is also examined.

Identification and pattern of transitions of some developmental and adult isoforms of fast troponin T in some human and rat skeletal muscles

Using a monoclonal antibody (F24) in an immunoblotting procedure, the composition of fast troponin T in several adult and developing skeletal muscles of rat and human was studied. With the exception of diaphragm, four isoforms of fast troponin T (HF1-HF4) were detected in all the adult human skeletal muscles investigated. Another isoform of fast troponin T undetectable in the adult human skeletal muscles, designated the fetal isoform (HFF1), was found to be present in all fetal skeletal muscles at 20 weeks of gestation except the diaphragm. Unlike isoform HF4 that was undetectable in all the fetal skeletal muscles, isoforms HF1-HF3 were present in all the human fetal skeletal muscles including the diaphragm. At least five isoforms of fast troponin T (AF1-AF5) could be detected in adult rat skeletal muscles. An additional isoform designated (D) appeared to be present in the rat diaphragm. In some muscles one of the isoforms, AF1, could be further resolved into two to three variants. The proportions and the level of expression of AF1-AF5 isoforms varied not only in different muscles but in some cases also in different parts of the same muscle. In addition to the adult isoforms, four other developmental isoforms termed fetal (FF1 and FF2) and neonatal (NF1 and NF2), were detected during the early development in the rat skeletal muscles. Their presence was first detected during the late fetal to early neonatal period and these isoforms were generally undetectable in a majority of the muscles after 1-2 months of age although their low level of expression persisted in a small number of muscles.