Conformational modulation of slow skeletal muscle troponin T by an NH(2)-terminal metal-binding extension

Role of the interaction between troponin T and AMP deaminase by zinc bridge in modulating muscle contraction and ammonia production

The N-terminal region of troponin T (TnT) does not bind any protein of the contractile machinery and the role of its hypervariability remains uncertain. In this review we report the evidence of the interaction between TnT and AMP deaminase (AMPD), a regulated zinc enzyme localized on the myofibril. In periods of intense muscular activity, a decrease in the ATP/ADP ratio, together with a decrease in the tissue pH, is the stimulus for the activation of the enzyme that deaminating AMP to IMP and NH3 displaces the myokinase reaction towards the formation of ATP. In skeletal muscle subjected to strong tetanic contractions, a calpain-like proteolytic activity produces the removal in vivo of a 97-residue N-terminal fragment from the enzyme that becomes desensitized towards the inhibition by ATP, leading to an unrestrained production of NH3. When a 95-residue N-terminal fragment is removed from AMPD by trypsin, simulating in vitro the calpain action, rabbit fast TnT or its phosphorylated 50-residue N-terminal peptide binds AMPD restoring the inhibition by ATP. Taking in consideration that the N-terminus of TnT expressed in human as well as rabbit white muscle contains a zinc-binding motif, we suggest that TnT might mimic the regulatory action of the inhibitory N-terminal domain of AMPD due to the presence of a zinc ion connecting the N-terminal and C-terminal regions of the enzyme, indicating that the two proteins might physiologically associate to modulate muscle contraction and ammonia production in fast-twitching muscle under strenuous conditions.

Motor neurons and endothelial cells additively promote development and fusion of human iPSC-derived skeletal myocytes

BACKGROUND

Neurovascular cells have wide-ranging implications on skeletal muscle biology regulating myogenesis, maturation, and regeneration. Although several in vitro studies have investigated how motor neurons and endothelial cells interact with skeletal myocytes independently, there is limited knowledge about the combined effect of neural and vascular cells on muscle maturation and development.

METHODS

Here, we report a triculture system comprising human-induced pluripotent stem cell (iPSC)-derived skeletal myocytes, human iPSC-derived motor neurons, and primary human endothelial cells maintained under controlled media conditions. Briefly, iPSCs were differentiated to generate skeletal muscle progenitor cells (SMPCs). These SMPCs were seeded at a density of 5 × 104 cells/well in 12-well plates and allowed to differentiate for 7 days before adding iPSC-derived motor neurons at a concentration of 0.5 × 104 cells/well. The neuromuscular coculture was maintained for another 7 days in coculture media before addition of primary human umbilical vein endothelial cells (HUVEC) also at 0.5 × 104 cells/well. The triculture was maintained for another 7 days in triculture media comprising equal portions of muscle differentiation media, coculture media, and vascular media. Extensive morphological, genetic, and molecular characterization was performed to understand the combined and individual effects of neural and vascular cells on skeletal muscle maturation.

RESULTS

We observed that motor neurons independently promoted myofiber fusion, upregulated neuromuscular junction genes, and maintained a molecular niche supportive of muscle maturation. Endothelial cells independently did not support myofiber fusion and downregulated expression of LRP4 but did promote expression of type II specific myosin isoforms. However, neurovascular cells in combination exhibited additive increases in myofiber fusion and length, enhanced production of Agrin, along with upregulation of several key genes like MUSK, RAPSYN, DOK-7, and SLC2A4. Interestingly, more divergent effects were observed in expression of genes like MYH8, MYH1, MYH2, MYH4, and LRP4 and secretion of key molecular factors like amphiregulin and IGFBP-4.

CONCLUSIONS

Neurovascular cells when cultured in combination with skeletal myocytes promoted myocyte fusion with concomitant increase in expression of various neuromuscular genes. This triculture system may be used to gain a deeper understanding of the effects of the neurovascular niche on skeletal muscle biology and pathophysiology.

High efficiency preparation of skinned mouse cardiac muscle strips from cryosections for contractility studies

NEW FINDINGS

What is the central question of this study? Can frozen cardiac papillary muscles and cryosectioning be used to reliably obtain uniform cardiac muscle strips with high yields? What is the main finding and its importance? A new method was developed using frozen cardiac papillary muscles and cryosectioning to reliably obtain uniform cardiac muscle strips with high yields. Experimental results demonstrate that this new methodology significantly increases the efficiency and application of quantitative biomechanical studies using skinned muscle fibres with an additional advantage of no need for transferring live animals.

ABSTRACT

Skinned cardiac muscle preparations are widely used to study contractile function of myofilament proteins and pathophysiological changes. The current methods applied in these biomechanical studies include detergent permeabilization of freshly isolated papillary muscle, ventricular trabeculae, surgically dissected ventricular muscle strips, mechanically blended cardiac muscle bundles or myocytes, and enzymatically isolated single cardiomyocytes. To facilitate and expand the skinned cardiac muscle approach, we have developed an efficient and readily practical method for mechanical studies of skinned mouse cardiac papillary muscle strips prepared from cryosections. Longitudinal papillary muscle strips of 120-150 µm width cut from 35-70 µm-thick cryosections are mounted to a force transducer and chemically skinned for the studies of force-pCa and sarcomere length-tension relationship and rate of tension redevelopment. In addition to more effective skinning and perfusion than with whole papillary muscle and much higher yield of useful preparations than that from trabeculae, this new methodology has two more major advantages. One is to allow for the use of frozen cardiac muscle in storage to maximize the value of muscle samples, facilitating resource sharing among research institutions without the need of transferring live animals or fresh biopsies. The other is that the integrity of the muscle strips is well preserved during the preparation and mechanical studies, allowing coupled characterization of myofilament proteins. The combined power of biomechanics and protein biochemistry can provide novel insights into integrative physiological and pathophysiological mechanisms of cardiac muscle contraction while the high yield of high-quality muscle strips also provides an efficient platform for development of therapeutic reagents.

The loss of slow skeletal muscle isoform of troponin T in spindle intrafusal fibres explains the pathophysiology of Amish nemaline myopathy

KEY POINTS

The pathogenic mechanism and the neuromuscular reflex-related phenotype (e.g. tremors accompanied by clonus) of Amish nemaline myopathy, as well as of other recessively inherited TNNT1 myopathies, remain to be clarified. The truncated slow skeletal muscle isoform of troponin T (ssTnT) encoded by the mutant TNNT1 gene is unable to incorporate into myofilaments and is degraded in muscle cells. By contrast to extrafusal muscle fibres, spindle intrafusal fibres of normal mice contain a significant level of cardiac TnT and a low molecular weight splice form of ssTnT. Intrafusal fibres of ssTnT-knockout mice have significantly increased cardiac TnT. Rotarod and balance beam tests have revealed abnormal neuromuscular co-ordination in ssTnT-knockout mice and a blunted response to a spindle sensitizer, succinylcholine. The loss of ssTnT and a compensatory increase of cardiac TnT in intrafusal nuclear bag fibres may increase myofilament Ca2+ -sensitivity and tension, impairing spindle function, thus identifying a novel mechanism for the development of targeted treatment.

ABSTRACT

A nonsense mutation at codon Glu180 of TNNT1 gene causes Amish nemaline myopathy (ANM), a recessively inherited disease with infantile lethality. TNNT1 encodes the slow skeletal muscle isoform of troponin T (ssTnT). The truncated ssTnT is unable to incorporate into myofilament and is degraded in muscle cells. The symptoms of ANM include muscle weakness, atrophy, contracture and tremors accompanied by clonus. An ssTnT-knockout (KO) mouse model recapitulates key features of ANM such as atrophy of extrafusal slow muscle fibres and increased fatigability. However, the neuromuscular reflex-related symptoms of ANM have not been explained. By isolating muscle spindles from ssTnT-KO and control mice aiming to examine the composition of myofilament proteins, we found that, in contrast to extrafusal fibres, intrafusal fibres contain a significant level of cardiac TnT and the low molecular weight splice form of ssTnT. Intrafusal fibres from ssTnT-KO mice have significantly increased cardiac TnT. Rotarod and balance beam tests revealed impaired neuromuscular co-ordination in ssTnT-KO mice, indicating abnormality in spindle functions. Unlike the wild-type control, the beam running ability of ssTnT-KO mice had a blunted response to a spindle sensitizer, succinylcholine. Immunohistochemistry detected ssTnT and cardiac TnT in nuclear bag fibres, whereas fast skeletal muscle TnT was detected in nuclear chain fibres, and cardiac α-myosin was present in one of the two nuclear bag fibres. The loss of ssTnT and a compensatory increase of cardiac TnT in nuclear bag fibres would increase myofilament Ca2+ -sensitivity and tension, thus affecting spindle activities. This mechanism provides an explanation for the pathophysiology of ANM, as well as a novel target for treatment.

TNNT1 nemaline myopathy: natural history and therapeutic frontier

We describe the natural history of 'Amish' nemaline myopathy (ANM), an infantile-onset, lethal disease linked to a pathogenic c.505G>T nonsense mutation of TNNT1, which encodes the slow fiber isoform of troponin T (TNNT1; a.k.a. TnT). The TNNT1 c.505G>T allele has a carrier frequency of 6.5% within Old Order Amish settlements of North America. We collected natural history data for 106 ANM patients born between 1923 and 2017. Over the last two decades, mean age of molecular diagnosis was 16 ± 27 days. TNNT1 c.505G>T homozygotes were normal weight at birth but failed to thrive by age 9 months. Presenting neonatal signs were axial hypotonia, hip and shoulder stiffness, and tremors, followed by progressive muscle weakness, atrophy and contractures. Affected children developed thoracic rigidity, pectus carinatum and restrictive lung disease during infancy, and all succumbed to respiratory failure by 6 years of age (median survival 18 months, range 0.2-66 months). Muscle histology from two affected children showed marked fiber size variation owing to both Type 1 myofiber smallness (hypotrophy) and Type 2 fiber hypertrophy, with evidence of nemaline rods, myofibrillar disarray and vacuolar pathology in both fiber types. The truncated slow TNNT1 (TnT) fragment (p.Glu180Ter) was undetectable in ANM muscle, reflecting its rapid proteolysis and clearance from sarcoplasm. Similar functional and histological phenotypes were observed in other human cohorts and two transgenic murine models (Tnnt1-/- and Tnnt1 c.505G>T). These findings have implications for emerging molecular therapies, including the suitably of TNNT1 gene replacement for newborns with ANM or other TNNT1-associated myopathies.

Myofilaments: Movers and Rulers of the Sarcomere

Striated cardiac and skeletal muscles play very different roles in the body, but they are similar at the molecular level. In particular, contraction, regardless of the type of muscle, is a precise and complex process involving the integral protein myofilaments and their associated regulatory components. The smallest functional unit of muscle contraction is the sarcomere. Within the sarcomere can be found a sophisticated ensemble of proteins associated with the thick filaments (myosin, myosin binding protein-C, titin, and obscurin) and thin myofilaments (actin, troponin, tropomyosin, nebulin, and nebulette). These parallel thick and thin filaments slide across one another, pulling the two ends of the sarcomere together to regulate contraction. More specifically, the regulation of both timing and force of contraction is accomplished through an intricate network of intra- and interfilament interactions belonging to each myofilament. This review introduces the sarcomere proteins involved in striated muscle contraction and places greater emphasis on the more recently identified and less well-characterized myofilaments: cardiac myosin binding protein-C, titin, nebulin, and obscurin. © 2017 American Physiological Society. Compr Physiol 7:675-692, 2017.

Protein Structure-Function Relationship at Work: Learning from Myopathy Mutations of the Slow Skeletal Muscle Isoform of Troponin T

Troponin T (TnT) is the sarcomeric thin filament anchoring subunit of the troponin complex in striated muscles. A nonsense mutation in exon 11 of the slow skeletal muscle isoform of TnT (ssTnT) gene (TNNT1) was found in the Amish populations in Pennsylvania and Ohio. This single nucleotide substitution causes a truncation of the ssTnT protein at Glu¹⁸⁰ and the loss of the C-terminal tropomyosin (Tm)-binding site 2. As a consequence, it abolishes the myofilament integration of ssTnT and the loss of function causes an autosomal recessive nemaline myopathy (NM). More TNNT1 mutations have recently been reported in non-Amish ethnic groups with similar recessive NM phenotypes. A nonsense mutation in exon 9 truncates ssTnT at Ser¹⁰⁸, deleting Tm-binding site 2 and a part of the middle region Tm-binding site 1. Two splicing site mutations result in truncation of ssTnT at Leu²⁰³ or deletion of the exon 14-encoded C-terminal end segment. Another splicing mutation causes an internal deletion of the 39 amino acids encoded by exon 8, partially damaging Tm-binding site 1. The three splicing mutations of TNNT1 all preserve the high affinity Tm-binding site 2 but still present recessive NM phenotypes. The molecular mechanisms for these mutations to cause myopathy provide interesting models to study and understand the structure-function relationship of TnT. This focused review summarizes the current knowledge of TnT isoform regulation, structure-function relationship of TnT and how various ssTnT mutations cause recessive NM, in order to promote in depth studies for further understanding the pathogenesis and pathophysiology of TNNT1 myopathies toward the development of effective treatments.

TNNT1, TNNT2, and TNNT3: Isoform genes, regulation, and structure-function relationships

Troponin T (TnT) is a central player in the calcium regulation of actin thin filament function and is essential for the contraction of striated muscles. Three homologous genes have evolved in vertebrates to encode three muscle type-specific TnT isoforms: TNNT1 for slow skeletal muscle TnT, TNNT2 for cardiac muscle TnT, and TNNT3 for fast skeletal muscle TnT. Alternative splicing and posttranslational modifications confer additional structural and functional variations of TnT during development and muscle adaptation to various physiological and pathological conditions. This review focuses on the TnT isoform genes and their molecular evolution, alternative splicing, developmental regulation, structure-function relationships of TnT proteins, posttranslational modifications, and myopathic mutations and abnormal splicing. The goal is to provide a concise summary of the current knowledge and some perspectives for future research and translational applications.

Increases of desmin and α-actinin in mouse cardiac myofibrils as a response to diastolic dysfunction

Up-regulation of desmin has been reported in cardiac hypertrophy and failure but the pathophysiological cause and significance remain to be investigated. By examining genetically modified mouse models representative for diastolic or systolic heart failure, we found significantly increased levels of desmin and α-actinin in the myofibrils of hearts with impaired diastolic function but not hearts with weakened systolic function. The increased desmin and α-actinin are mainly found in myofibrils at the Z-disks. Two weeks of transverse aortic constriction (TAC) induced increases of desmin and α-actinin in mouse hearts of occult diastolic failure but not in wild type or transgenic mouse hearts with mildly lowered systolic function or with increased diastolic function. The chronic or TAC-induced increase of desmin showed no proportional increase in phosphorylation, implicating an up-regulated expression rather than a decreased protein turnover. The data demonstrate a novel early response specifically to diastolic heart failure, indicating a function of the Z-disk in the challenging clinical condition of heart failure with preserved ejection fraction (HFpEF).

NH2-terminal truncations of cardiac troponin I and cardiac troponin T produce distinct effects on contractility and calcium homeostasis in adult cardiomyocytes

Cardiac troponin I (TnI) has an NH2-terminal extension that is an adult heart-specific regulatory structure. Restrictive proteolytic truncation of the NH2-terminal extension of cardiac TnI occurs in normal hearts and is upregulated in cardiac adaptation to hemodynamic stress or β-adrenergic deficiency. NH2-terminal truncated cardiac TnI (cTnI-ND) alters the conformation of the core structure of cardiac TnI similarly to that produced by PKA phosphorylation of Ser(23/24) in the NH2-terminal extension. At organ level, cTnI-ND enhances ventricular diastolic function. The NH2-terminal region of cardiac troponin T (TnT) is another regulatory structure that can be selectively cleaved via restrictive proteolysis. Structural variations in the NH2-terminal region of TnT also alter the molecular conformation and function. Transgenic mouse hearts expressing NH2-terminal truncated cardiac TnT (cTnT-ND) showed slower contractile velocity to prolong ventricular rapid-ejection time, resulting in higher stroke volume. Our present study compared the effects of cTnI-ND and cTnT-ND in cardiomyocytes isolated from transgenic mice on cellular morphology, contractility, and calcium kinetics. Resting cTnI-ND, but not cTnT-ND, cardiomyocytes had shorter length than wild-type cells with no change in sarcomere length. cTnI-ND, but not cTnT-ND, cardiomyocytes produced higher contractile amplitude and faster shortening and relengthening velocities in the absence of external load than wild-type controls. Although the baseline and peak levels of cytosolic Ca(2+) were not changed, Ca(2+) resequestration was faster in both cTnI-ND and cTnT-ND cardiomyocytes than in wild-type control. The distinct effects of cTnI-ND and cTnT-ND demonstrate their roles in selectively modulating diastolic or systolic functions of the heart.

Distinct conformational and functional effects of two adjacent pathogenic mutations in cardiac troponin I at the interface with troponin T

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Mutations in the TnT-interface of cardiac TnI cause cardiomyopathies.

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Mutations A117G and K118C resulted in distinct changes in protein conformation.

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K118C, but not A117G, decreased the binding affinity for TnT.

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K118C decreased binding affinity for TnC in a Ca²⁺-dependent manner – A117G had a similar but less profound effect.

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PKA treatment or N-terminal truncation produces similar changes in cardiac TnI.

The α-helix in troponin I (TnI) at the interface with troponin T (TnT) is a highly conserved structure. A point mutation in this region, A116G, was found in human cardiac TnI in a case of cardiomyopathy. An adjacent dominantly negative mutation found in turkey cardiac TnI (R111C, equivalent to K117C in human and K118C in mouse) decreased diastolic function and blunted beta-adrenergic response in transgenic mice. To investigate the functional importance of the TnI–TnT interface and pathological impact of the cardiac TnI mutations, we engineered K118C and A117G mutations in mouse cardiac TnI for functional studies. Despite their adjacent locations, A117G substitution results in faster mobility of cardiac TnI in SDS–PAGE whereas K118C decreases gel mobility, indicating significant and distinct changes in overall protein conformation. Consistently, monoclonal antibody epitope analysis demonstrated distinct local and remote conformational alterations in the two mutant proteins. Protein binding assays showed that K118C, but not A117G, decreased the relative binding affinity of cardiac TnI for TnT. K118C mutation decreased binding affinity for troponin C in a Ca²⁺-dependent manner, whereas A117G had a similar but less profound effect. Protein kinase A phosphorylation or truncation to remove the cardiac specific N-terminal extension of cardiac TnI resulted in similar conformational changes in the region interfacing with TnT and minimized the functional impacts of the mutations. The data demonstrate potent conformational and functional impacts of the TnT-interfacing helix in TnI and suggest a role of the N-terminal extension of cardiac TnI in modulating TnI–TnT interface functions.

Abnormal splicing in the N-terminal variable region of cardiac troponin T impairs systolic function of the heart with preserved Frank-Starling compensation

Abnormal splice‐out of the exon 7‐encoded segment in the N‐terminal variable region of cardiac troponin T (cTnT‐ΔE7) was found in turkeys and, together with the inclusion of embryonic exon (eTnT), in adult dogs with a correlation with dilated cardiomyopathy. Overexpression of these cTnT variants in transgenic mouse hearts significantly decreased cardiac function. To further investigate the functional effect of cTnT‐ΔE7 or ΔE7+eTnT in vivo under systemic regulation, echocardiography was carried out in single and double‐transgenic mice. No atrial enlargement, ventricular hypertrophy or dilation was detected in the hearts of 2‐month‐old cTnT‐ΔE7 and ΔE7+eTnT mice in comparison to wild‐type controls, indicating a compensated state. However, left ventricular fractional shortening and ejection fraction were decreased in ΔE7 and ΔE7+eTnT mice, and the response to isoproterenol was lower in ΔE7+eTnT mice. Left ventricular outflow tract velocity and gradient were decreased in the transgenic mouse hearts, indicating decreased systolic function. Ex vivo working heart function showed that high afterload or low preload resulted in more severe decreases in the systolic function and energetic efficiency of cTnT‐ΔE7 and ΔE7+eTnT hearts. On the other hand, increases in preload demonstrated preserved Frank‐Starling responses and minimized the loss of cardiac function and efficiency. The data demonstrate that the N‐terminal variable region of cardiac TnT regulates systolic function of the heart.

Using transgenic mouse models expressing myopathic splicing variants of cardiac troponin T, we demonstrated that abnormality in the N‐terminal variable region of troponin T selectively affects the systolic function of the heart, whereas the Frank‐Starling response is preserved.

A dominantly negative mutation in cardiac troponin I at the interface with troponin T causes early remodeling in ventricular cardiomyocytes

We previously reported a point mutation substituting Cys for Arg(111) in the highly conserved troponin T (TnT)-contacting helix of cardiac troponin I (cTnI) in wild turkey hearts (Biesiadecki et al. J Biol Chem 279: 13825-13832, 2004). This dominantly negative TnI-TnT interface mutation decreases the binding affinity of cTnI for TnT, impairs diastolic function, and blunts the β-adrenergic response of cardiac muscle (Wei et al. J Biol Chem 285: 27806-27816, 2010). Here we further investigate cellular phenotypes of transgenic mouse cardiomyocytes expressing the equivalent mutation cTnI-K118C. Functional studies were performed on single adult cardiomyocytes after recovery in short-term culture from isolation stress. The amplitude of contraction and the velocities of shortening and relengthening were lower in cTnI-K118C cardiomyocytes than wild-type controls. The intracellular Ca(2+) transient was slower in cTnI-K118C cardiomyocytes than wild-type cells. cTnI-K118C cardiomyocytes also showed a weaker β-adrenergic response. The resting length of cTnI-K118C cardiomyocytes was significantly greater than that of age-matched wild-type cells, with no difference in cell width. The resting sarcomere was not longer, but slightly shorter, in cTnI-K118C cardiomyocytes than wild-type cells, indicating longitudinal addition of sarcomeres. More tri- and quadrinuclei cardiomyocytes were found in TnI-K118C than wild-type hearts, suggesting increased nuclear divisions. Whole-genome mRNA array and Western blots detected an increased expression of leukemia inhibitory factor receptor-β in the hearts of 2-mo-old cTnI-K118C mice, suggesting a signaling pathway responsible for the potent effect of cTnI-K118C mutation on early remodeling in cardiomyocytes.

Gene regulation, alternative splicing, and posttranslational modification of troponin subunits in cardiac development and adaptation: a focused review

Troponin plays a central role in regulating the contraction and relaxation of vertebrate striated muscles. This review focuses on the isoform gene regulation, alternative RNA splicing, and posttranslational modifications of troponin subunits in cardiac development and adaptation. Transcriptional and posttranscriptional regulations such as phosphorylation and proteolysis modifications, and structure-function relationships of troponin subunit proteins are summarized. The physiological and pathophysiological significances are discussed for impacts on cardiac muscle contractility, heart function, and adaptations in health and diseases.

Deficiency of slow skeletal muscle troponin T causes atrophy of type I slow fibres and decreases tolerance to fatigue

The total loss of slow skeletal muscle troponin T (ssTnT encoded by TNNT1 gene) due to a nonsense mutation in codon Glu(180) causes a lethal form of recessively inherited nemaline myopathy (Amish nemaline myopathy, ANM). To investigate the pathogenesis and muscle pathophysiology of ANM, we studied the phenotypes of partial and total loss of ssTnT in Tnnt1 gene targeted mice. An insertion of neomycin resistance cassette in intron 10 of Tnnt1 gene caused an approximately 60% decrease in ssTnT protein expression whereas cre-loxP-mediated deletion of exons 11-13 resulted in total loss of ssTnT, as seen in ANM muscles. In diaphragm and soleus muscles of the knockdown and knockout mouse models, we demonstrated that ssTnT deficiency resulted in significantly decreased levels of other slow fibre-specific myofilament proteins whereas fast fibre-specific myofilament proteins were increased correspondingly. Immunohistochemical studies revealed that ssTnT deficiency produced significantly smaller type I slow fibres and compensatory growth of type II fast fibres. Along with the slow fibre atrophy and the changes in myofilament protein isoform contents, ssTnT deficiency significantly reduced the tolerance to fatigue in soleus muscle. ssTnT-deficient soleus muscle also contains significant numbers of small-sized central nuclei type I fibres, indicating active regeneration. The data provide strong support for the essential role of ssTnT in skeletal muscle function and the causal effect of its loss in the pathology of ANM. This observation further supports the hypothesis that the function of slow fibres can be restored in ANM patients if a therapeutic supplement of ssTnT is achieved.

Increasing O-GlcNAcylation level on organ culture of soleus modulates the calcium activation parameters of muscle fibers

O-N-acetylglucosaminylation is a reversible post-translational modification which presents a dynamic and highly regulated interplay with phosphorylation. New insights suggest that O-GlcNAcylation might be involved in striated muscle physiology, in particular in contractile properties such as the calcium activation parameters. By the inhibition of O-GlcNAcase, we investigated the effect of the increase of soleus O-GlcNAcylation level on the contractile properties by establishing T/pCa relationships. We increased the O-GlcNAcylation level on soleus biopsies performing an organ culture of soleus treated or not with PUGNAc or Thiamet-G, two O-GlcNAcase inhibitors. The enhancement of O-GlcNAcylation pattern was associated with an increase of calcium affinity on slow soleus skinned fibers. Analysis of the glycoproteins pattern showed that this effect is solely due to O-GlcNAcylation of proteins extracted from skinned biopsies. We also characterized the O-GlcNAcylated contractile proteins using a proteomic approach, and identified among others troponin T and I as being O-GlcNAc modified. We quantified the variation of O-GlcNAc level on all these identified proteins, and showed that several regulatory contractile proteins, predominantly fast isoforms, presented a drastic increase in their O-GlcNAc level. Since the only slow isoform of contractile protein presenting an increase of O-GlcNAc level was MLC2, the effect of enhanced O-GlcNAcylation pattern on calcium activation parameters could involve the O-GlcNAcylation of sMLC2, without excluding that an unidentified O-GlcNAc proteins, such as TnC, could be potentially involved in this mechanism. All these data strongly linked O-GlcNAcylation to the modulation of contractile activity of skeletal muscle.

Toad heart utilizes exclusively slow skeletal muscle troponin T: an evolutionary adaptation with potential functional benefits

The three isoforms of vertebrate troponin T (TnT) are normally expressed in a muscle type-specific manner. Here we report an exception that the cardiac muscle of toad (Bufo) expresses exclusively slow skeletal muscle TnT (ssTnT) together with cardiac forms of troponin I and myosin as determined using immunoblotting, cDNA cloning, and/or LC-MS/MS. Using RT-PCR and 3'- and 5'-rapid amplification of cDNA ends on toad cardiac mRNA, we cloned full-length cDNAs encoding two alternatively spliced variants of ssTnT. Expression of the cloned cDNAs in Escherichia coli confirmed that the toad cardiac muscle expresses solely ssTnT, predominantly the low molecular weight variant with the exon 5-encoded NH(2)-terminal segment spliced out. Functional studies were performed in ex vivo working toad hearts and compared with the frog (Rana) hearts. The results showed that toad hearts had higher contractile and relaxation velocities and were able to work against a significantly higher afterload than that of frog hearts. Therefore, the unique evolutionary adaptation of utilizing exclusively ssTnT in toad cardiac muscle corresponded to a fitness value from improving systolic function of the heart. The data demonstrated a physiological importance of the functional diversity of TnT isoforms. The structure-function relationship of TnT may be explored for the development of new treatment of heart failure.

The heart-specific NH2-terminal extension regulates the molecular conformation and function of cardiac troponin I

In addition to the core structure conserved in all troponin I isoforms, cardiac troponin I (cTnI) has an ∼30 amino acids NH(2)-terminal extension. This peptide segment is a heart-specific regulatory structure containing two Ser residues that are substrates of PKA. Under β-adrenergic regulation, phosphorylation of cTnI in the NH(2)-terminal extension increases the rate of myocardial relaxation. The NH(2)-terminal extension of cTnI is also removable by restrictive proteolysis to produce functional adaptation to hemodynamic stresses. The molecular mechanism for the NH(2)-terminal modifications to regulate the function of cTnI is not fully understood. In the present study, we tested a hypothesis that the NH(2)-terminal extension functions by modulating the conformation of other regions of cTnI. Monoclonal antibody epitope analysis and protein binding experiments demonstrated that deletion of the NH(2)-terminal segment altered epitopic conformation in the middle, but not COOH-terminal, region of cTnI. PKA phosphorylation produced similar effects. This targeted long-range conformational modulation corresponded to changes in the binding affinities of cTnI for troponin T and for troponin C in a Ca(2+)-dependent manner. The data suggest that the NH(2)-terminal extension of cTnI regulates cardiac muscle function through modulating molecular conformation and function of the core structure of cTnI.

Structure of the NH2-terminal variable region of cardiac troponin T determines its sensitivity to restrictive cleavage in pathophysiological adaptation

We previously reported that the NH(2)-terminal variable region of cardiac troponin T (cTnT) is removed by restrictive μ-calpain cleavage in myocardial ischemia-reperfusion [24]. Selective removal of the NH(2)-terminal variable region of cTnT had a compensatory effect on myocardial contractility [25]. Here we further studied this posttranslational modification under pathophysiological conditions. Thrombin perfusion of isolated mouse hearts and cardiomyocytes induced the production of NH(2)-terminal truncated cTnT (cTnT-ND), suggesting a role of calcium overloading. Ouabain treatment of primary cultures of mouse cardiomyocytes in hypokalemic media, another calcium overloading condition, also produced cTnT-ND. Exploring the molecular mechanisms, we found that cTnT phosphorylation was primarily in the NH(2)-terminal region and the level of cTnT phosphorylation did not change under the calcium overloading conditions. However, alternatively spliced cTnT variants differing in the NH(2)-terminal primary structure produced significantly different levels of cTnT-ND in vivo in transgenic mouse hearts. The results suggest that stress conditions involving calcium overloading may convey an increased sensitivity of cTnT to the restrictive μ-calpain proteolysis, in which structure of the NH(2)-terminal variable region may play a determining role.

Calcium-regulated conformational change in the C-terminal end segment of troponin I and its binding to tropomyosin

The troponin complex plays an essential role in the thin filament regulation of striated muscle contraction. Of the three subunits of troponin, troponin I (TnI) is the actomyosin ATPase inhibitory subunit and its effect is released upon Ca(2+) binding to troponin C. The exon-8-encoded C-terminal end segment represented by the last 24 amino acids of cardiac TnI is highly conserved and is critical to the inhibitory function of troponin. Here, we investigated the function and calcium regulation of the C-terminal end segment of TnI. A TnI model molecule was labeled with Alexa Fluor 532 at a Cys engineered at the C-terminal end and used to reconstitute the tertiary troponin complex. A Ca(2+) -regulated conformational change in the C-terminus of TnI was shown by a sigmoid-shape fluorescence intensity titration curve similar to that of the CD calcium titration curve of troponin C. Such corresponding Ca(2+) responses are consistent with the function of troponin as a coordinated molecular switch. Reconstituted troponin complex containing a mini-troponin T lacking its two tropomyosin-binding sites showed a saturable binding to tropomyosin at pCa 9 but not at pCa 4. This Ca(2+) -regulated binding was diminished when the C-terminal 19 amino acids of cardiac TnI were removed. These results provided novel evidence for suggesting that the C-terminal end segment of TnI participates in the Ca(2+) regulation of muscle thin filament through interaction with tropomyosin.

Identification and analysis of muscle-related protein isoforms expressed in the white muscle of the mandarin fish (Siniperca chuatsi)

To identify muscle-related protein isoforms expressed in the white muscle of the mandarin fish Siniperca chuatsi, we analyzed 5,063 high-quality expressed sequence tags (ESTs) from white muscle cDNA library and predicted the integrity of the clusters annotated to these genes and the physiochemical properties of the putative polypeptides with full length. Up to about 33% of total ESTs were annotated to muscle-related proteins: myosin, actin, tropomyosin/troponin complex, parvalbumin, and Sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCa). Thirty-two isoforms were identified and more than one isoform existed in each of these proteins. Among these isoforms, 14 putative polypeptides were with full length. In addition, about 2% of total ESTs were significantly homologous to "glue" molecules such as alpha-actinins, myosin-binding proteins, myomesin, tropomodulin, cofilin, profilin, twinfilins, coronin-1, and nebulin, which were required for the integrity and maintenance of the muscle sarcomere. The results demonstrated that multiple isoforms of major muscle-related proteins were expressed in S. chuatsi white muscle. The analysis on these isoforms and other proteins sequences will greatly aid our systematic understanding of the high flexibility of mandarin fish white muscle at molecular level and expand the utility of fish systems as models for the muscle genetic control and function.

Localization of the two tropomyosin-binding sites of troponin T

Troponin T (TnT) binds to tropomyosin (Tm) to anchor the troponin complex in the thin filament, and it thus serves as a vital link in the Ca(2+) regulation of striated muscle contraction. Pioneer work three decades ago determined that the T1 and T2 chymotryptic fragments of TnT each contains a Tm-binding site. A more precise localization of the two Tm-binding sites of TnT remains to be determined. In the present study, we tested serial deletion constructs of TnT and carried out monoclonal antibody competition experiments to show that the T1 region Tm-binding site involves mainly a 39 amino acids segment in the N-terminal portion of the conserved middle region of TnT. We further employed another set of TnT fragments to locate the T2 region Tm-binding site to a segment of 25 amino acids near the beginning of the T2 fragment. The localization of the two Tm-binding sites of TnT provided new information for the structure-function relationship of TnT and the anchoring of troponin complex on muscle thin filament.

Mutual rescues between two dominant negative mutations in cardiac troponin I and cardiac troponin T

Troponin T (TnT) and troponin I (TnI) are two evolutionarily and functionally linked subunits of the troponin complex that regulates striated muscle contraction. We previously reported a single amino acid substitution in the highly conserved TnT-binding helix of cardiac TnI (cTnI) in wild turkey hearts in concurrence with an abnormally spliced myopathic cardiac TnT (cTnT) (Biesiadecki, B. J., Schneider, K. L., Yu, Z. B., Chong, S. M., and Jin, J. P. (2004) J. Biol. Chem. 279, 13825-13832). To investigate the functional effect of this cTnI mutation and its potential value in compensating for the cTnT abnormality, we developed transgenic mice expressing the mutant cTnI (K118C) in the heart with or without the deletion of the endogenous cTnI gene to mimic the homozygote and heterozygote of wild turkeys. Double and triple transgenic mice were created by crossing the cTnI-K118C lines with transgenic mice overexpressing the myopathic cTnT (exon 7 deletion). Functional studies of ex vivo working hearts found that cTnI-K118C alone had a dominantly negative effect on diastolic function and blunted the inotropic responses of cardiac muscle to beta-adrenergic stimuli without abolishing the protein kinase A-dependent phosphorylation of cTnI. When co-expressed with the cTnT mutation, cTnI-K118C corrected the significant depression of systolic function caused by cTnT exon 7 deletion, and the co-existence of exon 7-deleted cTnT minimized the diastolic abnormality of cTnI-K118C. Characterization of this naturally selected pair of mutually rescuing mutations demonstrated that TnI-TnT interaction is a critical link in the Ca(2+) signaling and beta-adrenergic regulation in cardiac muscle, suggesting a potential target for the treatment of troponin cardiomyopathies and heart failure.

Coexistence of cardiac troponin T variants reduces heart efficiency

Corresponding to the synchronized contraction of the myocardium and rhythmic pumping function of the heart, a single form of cardiac troponin T (cTnT) is present in the adult cardiac muscle of humans and most other vertebrate species. Alternative splicing variants of cTnT are found in failing human hearts and animal dilated cardiomyopathies. Biochemical analyses have shown that these cTnT variants are functional and produce shifted myofilament Ca(2+) sensitivity. We proposed a hypothesis that the coexistence of two or more functionally distinct TnT variants in the adult ventricular muscle that is normally activated as a syncytium may decrease heart function and cause cardiomyopathy (Huang et al., Am J Physiol Cell Physiol 294: C213-C222, 2008). In the present study, we studied transgenic mouse hearts expressing one or two cTnT variants in addition to normal adult cTnT to investigate whether desynchronized myofilament activation decreases ventricular efficiency. The function of ex vivo working hearts was examined in the absence of systemic neurohumoral influence. The results showed that the transgenic mouse hearts produced lower maximum left ventricular pressure, slower contractile and relaxation velocities, and decreased stroke volume compared with wild-type controls. Ventricular pumping efficiency, calculated by the ejection integral versus total systolic integral and cardiac work versus oxygen consumption, was significantly lower in transgenic mouse hearts and corresponded to the number of cTnT variants present. The results indicated a pathogenic mechanism in which the coexistence of functionally different cTnT variants in cardiac muscle reduces myocardial efficiency due to desynchronized thin filament activation.

To investigate protein evolution by detecting suppressed epitope structures

Material remains of ancestor nucleotides and proteins are largely unavailable, thus sequence comparison among homologous genes in present-day organisms forms the core of current knowledge of molecular evolution. Variation in protein three-dimensional structure is a basis for functional diversity. To study the evolution of three-dimensional structures in related proteins would significantly improve our understanding of protein evolution and function. A protein may contain ancestor conformations that have been allosterically suppressed by evolutionarily additive structures. Using monoclonal antibody probes to detect such conformation in proteins after removing the suppressor structure, our study demonstrates three-dimensional structure evidence for the evolutionary relationship between troponin I and troponin T, two subunits of the troponin complex in the Ca(2+)-regulatory system of striated muscle, and among their muscle type-specific isoforms. The experimental data show the feasibility of detecting evolutionarily suppressed history-telling structural states in proteins by removing conformational modulator segments added during evolution. In addition to identifying structural modifications that were critical to the emergence of diverged proteins, investigating this novel mode of evolution will help us to understand the origin and functional potential of protein structures.

Phosphorylation of cardiac troponin I by mammalian sterile 20-like kinase 1

Mst1 (mammalian sterile 20-like kinase 1) is a ubiquitously expressed serine/threonine kinase and its activation in the heart causes cardiomyocyte apoptosis and dilated cardiomyopathy. Its myocardial substrates, however, remain unknown. In a yeast two-hybrid screen of a human heart cDNA library with a dominant-negative Mst1 (K59R) mutant used as bait, cTn [cardiac Tn (troponin)] I was identified as an Mst1-interacting protein. The interaction of cTnI with Mst1 was confirmed by co-immunoprecipitation in both co-transfected HEK-293 cells (human embryonic kidney cells) and native cardiomyocytes, in which cTnI interacted with full-length Mst1, but not with its N-terminal kinase fragment. in vitro phosphorylation assays demonstrated that cTnI is a sensitive substrate for Mst1. In contrast, cTnT was phosphorylated by Mst1 only when it was incorporated into the Tn complex. MS analysis indicated that Mst1 phosphorylates cTnI at Thr(31), Thr(51), Thr(129) and Thr(143). Substitution of Thr(31) with an alanine residue reduced Mst1-mediated cTnI phosphorylation by 90%, whereas replacement of Thr(51), Thr(129) or Thr(143) with alanine residues reduced Mst1-catalysed cTnI phosphorylation by approx. 60%, suggesting that Thr(31) is a preferential phosphorylation site for Mst1. Furthermore, treatment of cardiomyocytes with hydrogen peroxide rapidly induced Mst1-dependent phosphorylation of cTnI at Thr(31). Protein epitope analysis and binding assays showed that Mst1-mediated phosphorylation modulates the molecular conformation of cTnI and its binding affinity to TnT and TnC, thus indicating functional significances. The results of the present study suggest that Mst1 is a novel mediator of cTnI phosphorylation in the heart and may contribute to the modulation of myofilament function under a variety of physiological and pathophysiological conditions.

Adaptation by alternative RNA splicing of slow troponin T isoforms in type 1 but not type 2 Charcot-Marie-Tooth disease

Slow troponin T (TnT) plays an indispensable role in skeletal muscle function. Alternative RNA splicing in the NH(2)-terminal region produces high-molecular-weight (HMW) and low-molecular-weight (LMW) isoforms of slow TnT. Normal adult slow muscle fibers express mainly HMW slow TnT. Charcot-Marie-Tooth disease (CMT) is a group of inherited peripheral polyneuropathies caused by various neuronal defects. We found in the present study that LMW slow TnT was significantly upregulated in demyelination form type 1 CMT (CMT1) but not axonal form type 2 CMT (CMT2) muscles. Contractility analysis showed an increased specific force in single fibers isolated from CMT1 but not CMT2 muscles compared with control muscles. However, an in vitro motility assay showed normal velocity of the myosin motor isolated from CMT1 and CMT2 muscle biopsies, consistent with their unchanged myosin isoform contents. Supporting a role of slow TnT isoform regulation in contractility change, LMW and HMW slow TnT isoforms showed differences in the molecular conformation in conserved central and COOH-terminal regions with changed binding affinity for troponin I and tropomyosin. In addition to providing a biochemical marker for the differential diagnosis of CMT, the upregulation of LMW slow TnT isoforms under the distinct pathophysiology of CMT1 demonstrates an adaptation of muscle function to neurological disorders by alternative splicing modification of myofilament proteins.

Restricted N-terminal truncation of cardiac troponin T: a novel mechanism for functional adaptation to energetic crisis

The N-terminal variable region of cardiac troponin T (TnT) is a regulatory structure that can be selectively removed during myocardial ischaemia reperfusion by mu-calpain proteolysis. Here we investigated the pathophysiological significance of this post-translational modification that removes amino acids 1-71 of cardiac TnT. Working heart preparations were employed to study rat acute myocardial infarction and transgenic mouse hearts over-expressing the N-terminal truncated cardiac TnT (cTnT-ND). Ex vivo myocardial infarction by ligation of the left anterior descending coronary artery induced heart failure and produced cTnT-ND not only in the infarct but also in remote zones, including the right ventricular free wall, indicating a whole organ response in the absence of systemic neurohumoral mechanisms. Left ventricular pressure overload in mouse working hearts produced increased cTnT-ND in both ventricles, suggesting a role of haemodynamic stress in triggering an acute whole organ proteolytic regulation. Transgenic mouse hearts in which the endogenous intact cardiac TnT was partially replaced by cTnT-ND showed lowered contractile velocity. When afterload increased from 55 mmHg to 90 mmHg, stroke volume decreased in the wild type but not in the transgenic mouse hearts. Correspondingly, the left ventricular rapid-ejection time of the transgenic mouse hearts was significantly longer than that of wild type hearts, especially at high afterload. The restricted deletion of the N-terminal variable region of cardiac troponin T demonstrates a novel mechanism by which the thin filament regulation adapts to sustain cardiac function under stress conditions.

Co-expression of skeletal and cardiac troponin T decreases mouse cardiac function

In contrast to skeletal muscles that simultaneously express multiple troponin T (TnT) isoforms, normal adult human cardiac muscle contains a single isoform of cardiac TnT. To understand the significance of myocardial TnT homogeneity, we examined the effect of TnT heterogeneity on heart function. Transgenic mouse hearts overexpressing a fast skeletal muscle TnT together with the endogenous cardiac TnT was investigated in vivo and ex vivo as an experimental system of concurrent presence of two classes of TnT in the adult cardiac muscle. This model of myocardial TnT heterogeneity produced pathogenic phenotypes: echocardiograph imaging detected age-progressive reductions of cardiac function; in vivo left ventricular pressure analysis showed decreased myocardial contractility; ex vivo analysis of isolated working heart preparations confirmed an intrinsic decrease of cardiac function in the absence of neurohumoral influence. The transgenic mice also showed chronic myocardial hypertrophy and degeneration. The dominantly negative effects of introducing a fast TnT into the cardiac thin filaments to produce two classes of Ca(2+) regulatory units in the adult myocardium suggest that TnT heterogeneity decreases contractile function by disrupting the synchronized action during ventricular contraction that is normally activated as an electrophysiological syncytium.

Troponin T core structure and the regulatory NH2-terminal variable region

The conserved central and COOH-terminal regions of troponin T (TnT) interact with troponin C, troponin I, and tropomyosin to regulate striated muscle contraction. Phylogenic data show that the NH2-terminal region has evolved as an addition to the conserved core structure of TnT. This NH2-terminal region does not bind other thin filament proteins, and its sequence is hypervariable between fiber type and developmental isoforms. Previous studies have demonstrated that NH2-terminal modifications alter the COOH-terminal conformation of TnT and thin filament Ca2+-activation, yet the functional core structure of TnT and the mechanism of NH2-terminal modulation are not well understood. To define the TnT core structure and investigate the regulatory role of the NH2-terminal variable region, we investigated two classes of model TnT molecules: (1) NH2-terminal truncated cardiac TnT and (2) chimera proteins consisting of an acidic or basic skeletal muscle TnT NH2-terminus spliced to the cardiac TnT core. Deletion of the TnT hypervariable NH2-terminus preserved binding to troponin I and tropomyosin and sustained cardiac muscle contraction in the heart of transgenic mice. Further deletion of the conserved central region diminished binding to tropomyosin. The reintroduction of differently charged NH2-terminal domains in the chimeric molecules produced long-range conformational changes in the central and COOH-terminal regions to alter troponin I and tropomyosin binding. Similar NH2-terminal charge effects are demonstrated in naturally occurring cardiac TnT isoforms, indicating a physiological significance. These results suggest that the hypervariable NH2-terminal region modulates the conformation and function of the TnT core structure to fine-tune muscle contractility.

Differential regulation of myofilament protein isoforms underlying the contractility changes in skeletal muscle unloading

Weight-bearing skeletal muscles change phenotype in response to unloading. Using the hindlimb suspension rat model, we investigated the regulation of myofilament protein isoforms in correlation to contractility. Four weeks of continuous hindlimb unloading produced progressive atrophy and contractility changes in soleus but not extensor digitorum longus muscle. The unloaded soleus muscle also had decreased fatigue resistance. Along with the decrease of myosin heavy chain isoform I and IIa and increase of IIb and IIx, coordinated regulation of thin filament regulatory protein isoforms were observed: gamma- and beta-tropomyosin decreased and alpha-tropomyosin increased, resulting in an alpha/beta ratio similar to that in normal fast twitch skeletal muscle; troponin I and troponin T (TnT) both showed decrease in the slow isoform and increases in the fast isoform. The TnT isoform switching began after 7 days of unloading and TnI isoform showed detectable changes at 14 days while other protein isoform changes were not significant until 28 days of treatment. Correlating to the early changes in contractility, especially the resistance to fatigue, the early response of TnT isoform regulation may play a unique role in the adaptation of skeletal muscle to unloading. When the fast TnT gene expression was upregulated in the unloaded soleus muscle, alternative RNA splicing switched to produce more high molecular weight acidic isoforms, reflecting a potential compensation for the decrease of slow TnT that is critical to skeletal muscle function. The results demonstrate that differential regulation of TnT isoforms is a sensitive mechanism in muscle adaptation to functional demands.

Selective deletion of the NH2-terminal variable region of cardiac troponin T in ischemia reperfusion by myofibril-associated mu-calpain cleavage

The structure of the NH2-terminal region of troponin T (TnT) is hypervariable among the muscle type-specific isoforms and is also regulated by alternative RNA splicing. This region does not contain binding sites for other thin filament proteins, but alteration of its structure affects the Ca2+ regulation of muscle contraction. Here we report a truncated cardiac TnT produced during myocardial ischemia reperfusion. Amino acid sequencing and protein fragment reconstruction determined that it is generated by a posttranslational modification selectively removing the NH2-terminal variable region and preserving the conserved core structure of TnT. Triton X-100 extraction of cardiac muscle fibers promoted production of the NH2-terminal truncated cardiac TnT (cTnT-ND), indicating a myofibril-associated proteolytic activity. Mu-calpain is a myofibril-associated protease and is known to degrade TnT. Supporting a role of mu-calpain in producing cTnT-ND in myocardial ischemia reperfusion, calpain inhibitors decreased the level of cTnT-ND in Triton-extracted myofibrils. Mu-calpain treatment of the cardiac myofibril and troponin complex specifically reproduced cTnT-ND. In contrast, mu-calpain treatment of isolated cardiac TnT resulted in nonspecific degradation, suggesting that this structural modification is relevant to physiological structures of the myofilament. Triton X-100 treatment of transgenic mouse cardiac myofibrils overexpressing fast skeletal muscle TnT produced similar NH2-terminal truncations of the endogenous and exogenous TnT, despite different amino acid sequences at the cleavage site. With the functional consequences of removing the NH2-terminal variable region of TnT, the mu-calpain-mediated proteolytic modification of TnT may act as an acute mechanism to adjust muscle contractility under stress conditions.

Intracellular distributions of essential elements in cardiomyocytes

We describe the intracellular distributions of nine essential elements (P, S, Cl, K, Ca, Mn, Fe, Cu, and Zn) found in cardiomyocytes imaged using synchrotron X-ray induced fluorescence. Cardiomyocytes were isolated from rat hearts, flash frozen on Si(3)N(4) windows, freeze-dried, and imaged with approximately 300 nm spatial resolution. Distinct longitudinal patterns in cardiomyocytes were most apparent for the elements Fe and Cu, which clearly colocalized. Transverse striations were apparent for P, S, Fe, and Zn, while those for Zn were consistently the most prominent ( approximately 10(-3)M) and appeared with a periodicity in the range 1.63-1.75 microm, the expected length of a sarcomere. Transverse striations for high concentrations of P, Fe, and Zn and low concentrations of S colocalized and coincided with the I-band of the intact cardiomyocyte. Fluorescence microscopy using FluoZin-3 in intact cardiomyocytes suggests that Zn(2+) influx is through sarcolemmal calcium channels and that significant stores of intracellular Zn(2+) may be released quickly (<1s) into the cytosol. These data collectively suggest that Zn(2+) is buffered by structures associated near the T-tubules and/or in the sarcoplasmic reticulum and is found in relative abundance sufficient to act as a modifier of Ca(2+) regulation or as a possible signaling messenger for gene expression.

Role of the fetal and alpha/beta exons in the function of fast skeletal troponin T isoforms: correlation with altered Ca2+ regulation associated with development

In mammalian fast skeletal muscle, constitutive and alternative splicing from a single troponin T (TnT) gene produce multiple developmentally regulated and tissue specific TnT isoforms. Two exons, alpha (exon 16) and beta (exon 17), located near the 3' end of the gene and coding for two different 14 amino acid residue peptides are spliced in a mutually exclusive manner giving rise to the adult TnTalpha and the fetal TnTbeta isoforms. In addition, an acidic peptide coded by a fetal (f) exon located between exons 8 and 9 near the 5' end of the gene, is specifically present in TnTbeta and absent in the adult isoforms. To define the functional role of the f and alpha/beta exons, we constructed combinations of TnT cDNAs from a single human fetal fast skeletal TnTbeta cDNA clone in order to circumvent the problem of N-terminal sequence heterogeneity present in wild-type TnT isoforms, irrespective of the stage of development. Nucleotide sequences of these constructs, viz. TnTalpha, TnTalpha + f, TnTbeta - f and TnTbeta are identical, except for the presence or absence of the alpha or beta and f exons. Our results, using the recombinant TnT isoforms in different functional in vitro assays, show that the presence of the f peptide in the N-terminal T1 region of TnT, has a strong inhibitory effect on binary interactions between TnT and other thin filament proteins, TnI, TnC and Tm. The presence of the f peptide led to reduced Ca2+-dependent ATPase activity in a reconstituted thin filament, whereas the contribution of the alpha and beta peptides in the biological activity of TnT was primarily modulatory. These results indicate that the f peptide confers an inhibitory effect on the biological function of fast skeletal TnT and this can be correlated with changes in the Ca2+ regulation associated with development in fast skeletal muscle.

Cellular Fate of Truncated Slow Skeletal Muscle Troponin T Produced by Glu180 Nonsense Mutation in Amish Nemaline Myopathy

A nonsense mutation at codon Glu180 in exon 11 of slow skeletal muscle troponin T (TnT) gene (TNNT1) causes an autosomal-recessive inherited nemaline myopathy. We previously reported the absence of intact or prematurely terminated slow TnT polypeptide in Amish nemaline myopathy (ANM) patient muscle. The present study further investigates the expression and fate of mutant slow TnT in muscle cells. Intact slow TnT mRNA was readily detected in patient muscle, indicating unaffected transcription and RNA splicing. Sequence of the cloned cDNAs revealed the single nucleotide mutation in two alternatively spliced isoforms of slow TnT mRNA. Mutant TNNT1 cDNA is translationally active in Escherichia coli and non-muscle eukaryotic cells, producing the expected truncated slow TnT protein. The mutant mRNA was expressed at significant levels in differentiated C2C12 myotubes, but unlike intact exogenous TnT, truncated slow TnT protein was not detected. Transfective expression in undifferentiated myoblasts produced slow TnT mRNA but not a detectable amount of truncated or intact slow TnT proteins, indicating a muscle cell-specific proteolysis of TnT when it is not integrated into myofilaments. The slow TnT-(1-179) fragment has substantially lower affinity for binding to tropomyosin, in keeping with the loss of one of two tropomyosin-binding sites. Our findings suggest that inefficient incorporation into myofilament is responsible for the instability of mutant slow TnT in ANM muscle. Rapid degradation of the truncated slow TnT protein, rather than instability of the nonsense mRNA, provides the protective mechanism against the potential dominant negative effect of the mutant TnT fragment.

Troponin T isoforms alter the tolerance of transgenic mouse cardiac muscle to acidosis

Troponin T (TnT) is an essential protein in the Ca2+ regulatory system of striated of muscle. Three fiber type-specific TnT genes have evolved in higher vertebrates to encode cardiac, slow and fast skeletal muscle TnT isoforms. To understand the functional significance of TnT isoforms, we studied the effects of acidosis on the contractility of transgenic mouse cardiac muscle that expresses fast skeletal muscle TnT. Contractility analysis of intact cardiac muscle strips showed that while no differences were detected at physiological pH, the transgenic cardiac muscle had significantly greater decreases in +dF/dtmax at acidic pH than that of the wild-type control. Contractility of skinned cardiac muscles demonstrated that the presence of fast TnT resulted in significantly larger decreases in force and Ca2+ sensitivity at acidic pH than that of the wild-type control. The effect of TnT isoforms on the tolerance of muscle to acidosis may explain the higher tolerance of embryonic versus adult cardiac muscles. The results are consistent with the hypothesis that charge differences in TnT isoforms contribute to the contractility of muscle. The data further support a hypothesis that slow TnT is similar to the cardiac, but not fast, and TnT may contribute to the higher tolerance of slow muscles to stress conditions. Therefore, TnT isoform diversity may contribute to the compatibility of muscle thin filaments to cellular environments in different fiber types, during development and functional adaptation.

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.

Yin Yang 1 is increased in human heart failure and represses the activity of the human alpha-myosin heavy chain promoter

Yin Yang 1 (YY1) is a transcription factor that can repress or activate transcription of the genes with which it interacts. In this report we show that YY1 is a negative regulator of the alpha-myosin heavy chain (alphaMyHC) gene, which, with betaMyHC are the molecular motors of the heart. AlphaMyHC mRNA and protein levels are down-regulated in hypertrophy and heart failure, and this is thought to be detrimental for cardiac contractility. We show that YY1 specifically interacts with the alphaMyHC promoter and that overexpression of YY1 in cardiac cells represses the activity of the alphaMyHC promoter. We also show that the 170-200-amino acid region of YY1, important for its interaction with histone acetyl transferases and histone deacetylases, is important for its repressive activity and that YY1 deleted in this region is an activator of the alphaMyHC promoter. Moreover, we show that YY1 levels and DNA binding activity are increased in failing human left ventricles and in a mouse model of hypertrophic cardiomyopathy, where alphaMyHC levels are decreased. These results suggest that YY1 is a negative regulator of alphaMyHC gene expression.

Truncation by Glu180 nonsense mutation results in complete loss of slow skeletal muscle troponin T in a lethal nemaline myopathy

A lethal form of nemaline myopathy, named "Amish Nemaline Myopathy" (ANM), is linked to a nonsense mutation at codon Glu180 in the slow skeletal muscle troponin T (TnT) gene. We found that neither the intact nor the truncated slow TnT protein was present in the muscle of patients with ANM. The complete loss of slow TnT is consistent with the observed recessive pattern of inheritance of the disease and indicates a critical role of the COOH-terminal T2 domain in the integration of TnT into myofibrils. Expression of slow and fast isoforms of TnT is fiber-type specific. The lack of slow TnT results in selective atrophy of type 1 fibers. Slow TnT confers a higher Ca2+ sensitivity than does fast TnT in single fiber contractility assays. Despite the lack of slow TnT, individuals with ANM have normal muscle power at birth. The postnatal onset and infantile progression of ANM correspond to a down-regulation of cardiac and embryonic splice variants of fast TnT in normal developing human skeletal muscle, suggesting that the fetal TnT isoforms complement slow TnT. These results lay the foundation for understanding the molecular pathophysiology and the potential targeted therapy of ANM.

Current and prospective applications of metal ion-protein binding

Since immobilized metal ion affinity chromatography (IMAC) was first introduced, several variants of this method and many other metal affinity-based techniques have been devised. IMAC quickly established itself as a highly reliable purification procedure, showing rapid expansion in the number of preparative and analytical applications while not remaining confined to protein separation. It was soon applied to protein refolding (matrix-assisted refolding), evaluation of protein folding status, protein surface topography studies and biosensor development. In this review, applications in protein processing are described of IMAC as well as other metal affinity-based technologies.

Cardiac troponin T variants produced by aberrant splicing of multiple exons in animals with high instances of dilated cardiomyopathy

Adult cardiac muscle normally expresses a single cardiac troponin T (cTnT). As a potential pathogenic mechanism for turkey dilated cardiomyopathy, the splice-out of a normally constitutive exon generates an additional low molecular weight cTnT with altered conformation and function. We further found that aberrant splicing of cTnT also occurs in several mammals correlating to dilated cardiomyopathy. Skipping of the same exon as that in the turkey was found in the canine cTnT. Splice-out of the adjacent exon 6 occurred in the guinea pig cTnT. Retention of the embryonic exon 5 was found in the cTnT of cat, dog, and guinea pig. These aberrant splicing variants significantly altered the structure of cTnT to sustain functional effects as that in the myopathic turkey cTnT. The genomic sequence of canine cTnT gene shows no specific alterations. However, the alternative splicing patterns of canine cTnT are different in developing cardiac and skeletal muscles, suggesting abnormality of trans-regulatory factors. Transgenic expression of the aberrant cTnT variants resulted in contractile changes in mouse cardiomyocytes. The findings support the hypothesis that thin filament heterogeneity due to the co-expression of alternatively spliced cTnT variants may desynchronize myocardial contraction and contribute to the pathogenesis and pathophysiology of cardiomyopathy and heart failure.

Cardiac troponin T isoforms affect the Ca2+ sensitivity and inhibition of force development. Insights into the role of troponin T isoforms in the heart

At least four isoforms of troponin T (TnT) exist in the human heart, and they are expressed in a developmentally regulated manner. To determine whether the different N-terminal isoforms are functionally distinct with respect to structure, Ca(2+) sensitivity, and inhibition of force development, the four known human cardiac troponin T isoforms, TnT1 (all exons present), TnT2 (missing exon 4), TnT3 (missing exon 5), and TnT4 (missing exons 4 and 5), were expressed, purified, and utilized in skinned fiber studies and in reconstituted actomyosin ATPase assays. TnT3, the adult isoform, had a slightly higher alpha-helical content than the other three isoforms. The variable region in the N terminus of cardiac TnT was found to contribute to the determination of the Ca(2+) sensitivity of force development in a charge-dependent manner; the greater the charge the higher the Ca(2+) sensitivity, and this was primarily because of exon 5. These studies also demonstrated that removal of either exon 4 or exon 5 from TnT increased the cooperativity of the pCa force relationship. Troponin complexes reconstituted with the four TnT isoforms all yielded the same maximal actin-tropomyosin-activated myosin ATPase activity. However, troponin complexes containing either TnT1 or TnT2 (both containing exon 5) had a reduced ability to inhibit this ATPase activity when compared with wild type troponin (which contains TnT3). Interestingly, fibers containing these isoforms also showed less relaxation suggesting that exon 5 of cardiac TnT affects the ability of Tn to inhibit force development and ATPase activity. These results suggest that the different N-terminal TnT isoforms would produce different functional properties in the heart that would directly affect myocardial contraction.

Troponin T isoforms modulate calcium dependence of the kinetics of the cross-bridge cycle: studies using a transgenic mouse line

Alternative splicing of troponin T (TnT) in striated muscle during development results in expression of different isoforms, with the splicing of a 5(') exon of TnT resulting in the expression of low-molecular-weight basic adult TnT isoforms and high-molecular-weight acidic embryonic TnT isoforms. Although other differences exist, the main differences between cardiac TnT (cTnT) and fast skeletal muscle TnT (fTnT) are in the NH(2) terminus, with fTnT being less acidic than cTnT. A transgenic mouse line expressing chicken fTnT in the heart was used to investigate the functional significance of TnT NH(2)-terminal charge differences on cardiac muscle contractility. The rates of force redevelopment (k(tr)) at four levels of Ca(2+) activation were recorded for skinned left ventricular trabeculae from control and transgenic mice. The k(tr) vs Ca(2+) relationship was different in control mice and transgenic mice, suggesting that the structure of TnT, and possibly the NH(2)-terminal region, is involved in determining the kinetics of cross-bridge cycle. These results suggest that isoform shifts in TnT may be an important molecular mechanism for determining the Ca(2+) dependence of cardiac muscle contractility.

Exon skipping in cardiac troponin T of turkeys with inherited dilated cardiomyopathy

Troponin T is a central component of the thin filament-associated troponin-tropomyosin system and plays an essential role in the Ca(2+) regulation of striated muscle contraction. The importance of the structure and function of troponin T is evident in the regulated isoform expression during development and the point mutations resulting in familial hypertrophic and dilated cardiomyopathies. We report here that turkeys with inherited dilated cardiomyopathy and heart failure express an unusual low molecular weight cardiac troponin T missing 11 amino acids due to the splice out of the normally conserved exon 8-encoded segment. The deletion of a 9-bp segment from intron 7 of the turkey cardiac troponin T gene may be responsible for the weakened splicing of the downstream exon 8 during mRNA processing. The exclusion of the exon 8-encoded segment results in conformational changes in cardiac troponin T, an altered binding affinity for troponin I and tropomyosin, and an increased calcium sensitivity of the actomyosin ATPase. Expression of the exon 8-deleted cardiac troponin T prior to the development of cardiomyopathy in turkeys indicates a novel RNA splicing disease and provides evidence for the role of troponin T structure-function variation in myocardial pathogenesis and heart failure.

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.

A proteomic approach to mimic fibrosis disease evolvement by an in vitro cell line

Subepithelial fibrosis in asthma involves an increase in the thickening of the lamina reticularis and is due to increased deposition of collagen I, III and V, and fibronectin. The cause of the thickening of the reticular layer is not known in detail, however, it is proposed to be caused by bronchial myofibroblasts. The transformation of fibroblasts to myofibroblasts may be contributed by inflammatory cytokines. In this paper we have studied and compared in vivo tissue material with a human fibroblast target cell. A normal primary fetal fibroblast cell line and HFL-1 (human fibroblast lurg cells) were used as a comparison between fibroblasts from human central biopsies regarding morphology and cell proliferation. Both cell morphology and cell proliferation rate was similar between the different set of cell cultures. Furthermore, it could be concluded that fibroblasts cultures from patients with asthma were surrounded by more extracellular matrix molecules compared to the primary cell line HFL-1, which may mimic the in vivo situation during formation of fibrosis. We wanted to investigate if differential protein display by two-dimensional (2-D) gel electrophoresis and subsequent protein identification by matrix assisted laser desorption/ionization-time of flight (MALDI-TOF)-mass spectrometry could reveal proteins induced by cytokine stimulation that can be correlated to the transformation of normal human fetal lungs cells into a more myofibroblast like phenotype. After stimulation with transforming growth factor-beta (TGF-beta) several myofibroblast markers were found to be regulated. Especially cytoskeletal and cytoskeletal-associated proteins like actin isoforms and tropomyosin, proteins that are responsible for contraction as well as transportation of extra cellular matrix proteins, which are overproduced in the formation of fibrosis. These results indicate that TGF-beta, which is increased in a fibrotic process, participates in the transformation of fibroblasts to myofibroblasts.