Induction and Myofibrillar Targeting of CARP, and Suppression of the Nkx2.5 Pathway in the MDM Mouse with Impaired Titin-based Signaling

Ankrd1 inhibits the FAK/Rho-GTPase/F-actin pathway by downregulating ITGA6 transcriptional to regulate myoblast functions

Skeletal muscle constitutes the largest percentage of tissue in the animal body and plays a pivotal role in the development of normal life activities in the organism. However, the regulation mechanism of skeletal muscle growth and development remains largely unclear. This study investigated the effects of Ankrd1 on the proliferation and differentiation of C2C12 myoblasts. Here, we identified Ankrd1 as a potential regulator of muscle cell development, and found that Ankrd1 knockdown resulted in the proliferation ability decrease but the differentiation level increase of C2C12 cells. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyzes as well as RNA-seq results showed that Ankrd1 knockdown activated focal adhesion kinase (FAK)/F-actin signal pathway with most genes significantly enriched in this pathway upregulated. The integrin subunit Itga6 promoter activity is increased when Ankrd1 knockdown, as demonstrated by a dual-luciferase reporter assay. This study revealed the molecular mechanism by which Ankrd1 knockdown enhanced FAK phosphorylation activity through the alteration of integrin subunit levels, thus activating FAK/Rho-GTPase/F-actin signal pathway, eventually promoting myoblast differentiation. Our data suggested that Ankrd1 might serve as a potential regulator of muscle cell development. Our findings provide new insights into skeletal muscle growth and development and valuable references for further study of human muscle-related diseases.

ANKRD1 expression is aberrantly upregulated in the mdm mouse model of muscular dystrophy and induced by stretch through NFκB

The muscular dystrophy with myositis (mdm) mouse model results in a severe muscular dystrophy due to an 83-amino-acid deletion in the N2A region of titin, an expanded sarcomeric protein that functions as a molecular spring which senses and modulates the response to mechanical forces in cardiac and skeletal muscles. ANKRD1 is one of the muscle ankyrin repeat domain proteins (MARPs) a family of titin-associated, stress-response molecules and putative transducers of stretch-induced signaling in skeletal muscle. The aberrant over-activation of Nuclear factor Kappa B (NF-κB) and the Ankyrin-repeat domain containing protein 1 (ANKRD1) occurs in several models of progressive muscle disease including Duchenne muscular dystrophy. We hypothesized that mechanical regulation of ANKRD1 is mediated by NF-κB activation in skeletal muscles and that this mechanism is perturbed by small deletion of the stretch-sensing titin N2A region in the mdm mouse. We applied static mechanical stretch of the mdm mouse diaphragm and cyclic mechanical stretch of C2C12 myotubes to examine the interaction between NF-κΒ and ANKRD1 expression utilizing Western blot and qRTPCR. As seen in skeletal muscles of other severe muscular dystrophies, an aberrant increased basal expression of NF-κB and ANKRD1 were observed in the diaphragm muscles of the mdm mice. Our data show that in the mdm diaphragm, basal levels of NF-κB are increased, and pharmacological inhibition of NF-κB does not alter basal levels of ANKRD1. Alternatively, NF-κB inhibition did alter stretch-induced ANKRD1 upregulation. These data show that NF-κB activity is at least partially responsible for the stretch-induced expression of ANKRD1.

Exploring TTN variants as genetic insights into cardiomyopathy pathogenesis and potential emerging clues to molecular mechanisms in cardiomyopathies

The giant protein titin (TTN) is a sarcomeric protein that forms the myofibrillar backbone for the components of the contractile machinery which plays a crucial role in muscle disorders and cardiomyopathies. Diagnosing TTN pathogenic variants has important implications for patient management and genetic counseling. Genetic testing for TTN variants can help identify individuals at risk for developing cardiomyopathies, allowing for early intervention and personalized treatment strategies. Furthermore, identifying TTN variants can inform prognosis and guide therapeutic decisions. Deciphering the intricate genotype-phenotype correlations between TTN variants and their pathologic traits in cardiomyopathies is imperative for gene-based diagnosis, risk assessment, and personalized clinical management. With the increasing use of next-generation sequencing (NGS), a high number of variants in the TTN gene have been detected in patients with cardiomyopathies. However, not all TTN variants detected in cardiomyopathy cohorts can be assumed to be disease-causing. The interpretation of TTN variants remains challenging due to high background population variation. This narrative review aimed to comprehensively summarize current evidence on TTN variants identified in published cardiomyopathy studies and determine which specific variants are likely pathogenic contributors to cardiomyopathy development.

Transcriptomic profiles of muscular dystrophy with myositis (mdm) in extensor digitorum longus, psoas, and soleus muscles from mice

Background

Titinopathies are inherited muscular diseases triggered by genetic mutations in the titin gene. Muscular dystrophy with myositis (mdm) is one such disease caused by a LINE repeat insertion, leading to exon skipping and an 83-amino acid residue deletion in the N2A-PEVK region of mouse titin. This region has been implicated in a number of titin—titin ligand interactions, hence are important for myocyte signaling and health. Mice with this mdm mutation develop a severe and progressive muscle degeneration. The range of phenotypic differences observed in mdm mice shows that the deletion of this region induces a cascade of transcriptional changes extending to numerous signaling pathways affected by the titin filament. Previous research has focused on correlating phenotypic differences with muscle function in mdm mice. These studies have provided understanding of the downstream physiological effects resulting from the mdm mutation but only provide insights on processes that can be physiologically observed and measured. We used differential gene expression (DGE) to compare the transcriptomes of extensor digitorum longus (EDL), psoas and soleus muscles from wild-type and mdm mice to develop a deeper understand of these tissue-specific responses.

Results

The overall expression pattern observed shows a well-differentiated transcriptional signature in mdm muscles compared to wild type. Muscle-specific clusters observed within the mdm transcriptome highlight the level of variability of each muscle to the deletion. Differential gene expression and weighted gene co-expression network analysis showed a strong directional response in oxidative respiration-associated mitochondrial genes, which aligns with the poor shivering and non-shivering thermogenesis previously observed. Sln, which is a marker associated with shivering and non-shivering thermogenesis, showed the strongest expression change in fast-fibered muscles. No drastic changes in MYH expression levels were reported, which indicated an absence of major fiber-type switching events. Overall expression shifts in MYH isoforms, MARPs, and extracellular matrix associated genes demonstrated the transcriptional complexity associated with mdm mutation. The expression alterations in mitochondrial respiration and metabolism related genes in the mdm muscle dominated over other transcriptomic changes, and likely account for the late stage cellular responses in the mdm muscles.

Conclusions

We were able to demonstrate that the complex nature of mdm mutation extends beyond a simple rearrangement in titin gene. EDL, psoas and soleus exemplify unique response modes observed in skeletal muscles with mdm mutation. Our data also raises the possibility that failure to maintain proper energy homeostasis in mdm muscles may contribute to the pathogenesis of the degenerative phenotype in mdm mice. Understanding the full disease-causing molecular cascade is difficult using bulk RNA sequencing techniques due to intricate nature of the disease. The development of the mdm phenotype is temporally and spatially regulated, hence future studies should focus on single fiber level investigations.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08873-2.

Residual force enhancement is reduced in permeabilized fiber bundles from mdm muscles

Residual force enhancement (RFE) is the increase in steady-state force after active stretch relative to the force during isometric contraction at the same final length. The mdm mutation in mice, characterized by a small deletion in N2A titin, has been proposed to prevent N2A titin-actin interactions so that active mdm muscles are more compliant than WT. This decrease in active muscle stiffness is associated with reduced RFE. We investigated RFE in permeabilized soleus (SOL) and extensor digitorum longus (EDL) fiber bundles from wild type and mdm mice. On each fiber bundle, we performed active and passive stretches from an average sarcomere length of 2.6 - 3.0 µm at a slow rate of 0.04 µm/s, as well as isometric contractions at the initial and final lengths. One-way ANOVA showed that SOL and EDL fiber bundles from mdm mice exhibited significantly lower RFE than WT (P<0.0001). This result is consistent with previous observations in single myofibrils and intact muscles. However, it contradicts the results from a previous study which appeared to show that compensatory mechanisms could restore titin force enhancement in single fibers from mdm psoas. We suggest that residual force enhancement measured previously in mdm single fibers was an artifact of the high variability in passive tension found in degenerating fibers, which begins after ∼24 days of age. The results are consistent with the hypothesis that RFE is reduced in mdm skeletal muscles due to impaired Ca2+ dependent titin-actin interactions resulting from the small deletion in N2A titin.

The titin N2B and N2A regions: biomechanical and metabolic signaling hubs in cross-striated muscles

Muscle specific signaling has been shown to originate from myofilaments and their associated cellular structures, including the sarcomeres, costameres or the cardiac intercalated disc. Two signaling hubs that play important biomechanical roles for cardiac and/or skeletal muscle physiology are the N2B and N2A regions in the giant protein titin. Prominent proteins associated with these regions in titin are chaperones Hsp90 and αB-crystallin, members of the four-and-a-half LIM (FHL) and muscle ankyrin repeat protein (Ankrd) families, as well as thin filament-associated proteins, such as myopalladin. This review highlights biological roles and properties of the titin N2B and N2A regions in health and disease. Special emphasis is placed on functions of Ankrd and FHL proteins as mechanosensors that modulate muscle-specific signaling and muscle growth. This region of the sarcomere also emerged as a hotspot for the modulation of passive muscle mechanics through altered titin phosphorylation and splicing, as well as tethering mechanisms that link titin to the thin filament system.

Muscle ankyrin repeat protein 1 (MARP1) locks titin to the sarcomeric thin filament and is a passive force regulator

Muscle ankyrin repeat protein 1 (MARP1) is frequently up-regulated in stressed muscle, but its effect on skeletal muscle function is poorly understood. Here, we focused on its interaction with the titin–N2A element, found in titin’s molecular spring region. We show that MARP1 binds to F-actin, and that this interaction is stronger when MARP1 forms a complex with titin–N2A. Mechanics and super-resolution microscopy revealed that MARP1 “locks” titin–N2A to the sarcomeric thin filament, causing increased extension of titin’s elastic PEVK element and, importantly, increased passive force. In support of this mechanism, removal of thin filaments abolished the effect of MARP1 on passive force. The clinical relevance of this mechanism was established in diaphragm myofibers of mechanically ventilated rats and of critically ill patients. Thus, MARP1 regulates passive force by locking titin to the thin filament. We propose that in stressed muscle, this mechanism protects the sarcomere from mechanical damage.

The N2A region of titin has a unique structural configuration

The N2A segment of titin is a main signaling hub in the sarcomeric I-band that recruits various signaling factors and processing enzymes. It has also been proposed to play a role in force production through its Ca²⁺-regulated association with actin. However, the molecular basis by which N2A performs these functions selectively within the repetitive and extensive titin chain remains poorly understood. Here, we analyze the structure of N2A components and their association with F-actin. Specifically, we characterized the structure of its Ig domains by elucidating the atomic structure of the I81-I83 tandem using x-ray crystallography and computing a homology model for I80. Structural data revealed these domains to present heterogeneous and divergent Ig folds, where I81 and I83 have unique loop structures. Notably, the I81-I83 tandem has a distinct rotational chain arrangement that confers it a unique multi-domain topography. However, we could not identify specific Ca²⁺-binding sites in these Ig domains, nor evidence of the association of titin N2A components with F-actin in transfected C2C12 myoblasts or C2C12-derived myotubes. In addition, F-actin cosedimentation assays failed to reveal binding to N2A. We conclude that N2A has a unique architecture that predictably supports its selective recruitment of binding partners in signaling, but that its mechanical role through interaction with F-actin awaits validation.

N2A Titin: Signaling Hub and Mechanical Switch in Skeletal Muscle

Since its belated discovery, our understanding of the giant protein titin has grown exponentially from its humble beginning as a sarcomeric scaffold to recent recognition of its critical mechanical and signaling functions in active muscle. One uniquely useful model to unravel titin’s functions, muscular dystrophy with myositis (mdm), arose spontaneously in mice as a transposon-like LINE repeat insertion that results in a small deletion in the N2A region of titin. This small deletion profoundly affects hypertrophic signaling and muscle mechanics, thereby providing insights into the function of this specific region and the consequences of its dysfunction. The impact of this mutation is profound, affecting diverse aspects of the phenotype including muscle mechanics, developmental hypertrophy, and thermoregulation. In this review, we explore accumulating evidence that points to the N2A region of titin as a dynamic “switch” that is critical for both mechanical and signaling functions in skeletal muscle. Calcium-dependent binding of N2A titin to actin filaments triggers a cascade of changes in titin that affect mechanical properties such as elastic energy storage and return, as well as hypertrophic signaling. The mdm phenotype also points to the existence of as yet unidentified signaling pathways for muscle hypertrophy and thermoregulation, likely involving titin’s PEVK region as well as the N2A signalosome.

Single-Molecule Force Spectroscopy on the N2A Element of Titin: Effects of Phosphorylation and CARP

Titin is a large filamentous protein that forms a sarcomeric myofilament with a molecular spring region that develops force in stretched sarcomeres. The molecular spring has a complex make-up that includes the N2A element. This element largely consists of a 104-residue unique sequence (N2A-Us) flanked by immunoglobulin domains (I80 and I81). The N2A element is of interest because it assembles a signalosome with CARP (Cardiac Ankyrin Repeat Protein) as an important component; CARP both interacts with the N2A-Us and I81 and is highly upregulated in response to mechanical stress. The mechanical properties of the N2A element were studied using single-molecule force spectroscopy, including how these properties are affected by CARP and phosphorylation. Three protein constructs were made that consisted of 0, 1, or 2 N2A-Us elements with flanking I80 and I81 domains and with specific handles at their ends for study by atomic force microscopy (AFM). The N2A-Us behaved as an entropic spring with a persistence length (Lp) of ∼0.35 nm and contour length (Lc) of ∼39 nm. CARP increased the Lp of the N2A-Us and the unfolding force of the Ig domains; force clamp experiments showed that CARP reduced the Ig domain unfolding kinetics. These findings suggest that CARP might function as a molecular chaperone that protects I81 from unfolding when mechanical stress is high. The N2A-Us was found to be a PKA substrate, and phosphorylation was blocked by CARP. Mass spectrometry revealed a PKA phosphosite (Ser-9895 in NP_001254479.2) located at the border between the N2A-Us and I81. AFM studies showed that phosphorylation affected neither the Lp of the N2A-Us nor the Ig domain unfolding force (Funfold). Simulating the force-sarcomere length relation of a single titin molecule containing all spring elements showed that the compliance of the N2A-Us only slightly reduces passive force (1.4%) with an additional small reduction by CARP (0.3%). Thus, it is improbable that the compliance of the N2A element has a mechanical function per se. Instead, it is likely that this compliance has local effects on binding of signaling molecules and that it contributes thereby to strain- and phosphorylation- dependent mechano-signaling.

Differences in stability and calcium sensitivity of the Ig domains in titin's N2A region

Titin is a large filamentous protein that spans half a sarcomere, from Z-disk to M-line. The N2A region within the titin molecule exists between the proximal immunoglobulin (Ig) region and the PEVK region and protein-protein interactions involving this region are required for normal muscle function. The N2A region consists of four Ig domains (I80-I83) with a 105 amino acid linker region between I80 and I81 that has a helical nature. Using chemical stability measurements, we show that predicted differences between the adjacent Ig domains (I81-I83) correlate with experimentally determined differences in chemical stability and refolding kinetics. Our work further shows that I83 has the lowest ΔG_unfolding , which is increased in the presence of calcium (pCa 4.3), indicating that Ca²⁺ plays a role in stabilizing this immunoglobulin domain. The characteristics of N2A's three Ig domains provide insight into the stability of the binding sites for proteins that interact with the N2A region. This work also provides insights into how Ca²⁺ might influence binding events involving N2A.

Effects of a titin mutation on force enhancement and force depression in mouse soleus muscles

The active isometric force produced by muscles varies with muscle length in accordance with the force-length relationship. Compared with isometric contractions at the same final length, force increases after active lengthening (force enhancement) and decreases after active shortening (force depression). In addition to cross-bridges, titin has been suggested to contribute to force enhancement and depression. Although titin is too compliant in passive muscles to contribute to active tension at short sarcomere lengths on the ascending limb and plateau of the force-length relationship, recent evidence suggests that activation increases titin stiffness. To test the hypothesis that titin plays a role in force enhancement and depression, we investigated isovelocity stretching and shortening in active and passive wild-type and mdm (muscular dystrophy with myositis) soleus muscles. Skeletal muscles from mdm mice have a small deletion in the N2A region of titin and show no increase in titin stiffness during active stretch. We found that: (1) force enhancement and depression were reduced in mdm soleus compared with wild-type muscles relative to passive force after stretch or shortening to the same final length; (2) force enhancement and force depression increased with amplitude of stretch across all activation levels in wild-type muscles; and (3) maximum shortening velocity of wild-type and mdm muscles estimated from isovelocity experiments was similar, although active stress was reduced in mdm compared with wild-type muscles. The results of this study suggest a role for titin in force enhancement and depression, which contribute importantly to muscle force during natural movements.

Optimal length, calcium sensitivity and twitch characteristics of skeletal muscles from mdm mice with a deletion in N2A titin

During isometric contractions, the optimal length of skeletal muscles increases with decreasing activation. The underlying mechanism for this phenomenon is thought to be linked to length dependence of Ca²⁺ sensitivity. Muscular dystrophy with myositis (mdm), a recessive titin mutation in mice, was used as a tool to study the role of titin in activation dependence of optimal length and length dependence of Ca²⁺ sensitivity. We measured the shift in optimal length between tetanic and twitch stimulation in mdm and wild-type muscles, and the length dependence of Ca²⁺ sensitivity at short and long sarcomere lengths in mdm and wild-type fiber bundles. The results indicate that the mdm mutation leads to a loss of activation dependence of optimal length without the expected change in length dependence of Ca²⁺ sensitivity, demonstrating that these properties are not linked, as previously suggested. Furthermore, mdm muscles produced maximum tetanic stress during sub-optimal filament overlap at lengths similar to twitch contractions in both genotypes, but the difference explains less than half of the observed reduction in active force of mdm muscles. Mdm muscles also exhibited increased electromechanical delay, contraction and relaxation times, and decreased rate of force development in twitch contractions. We conclude that the small deletion in titin associated with mdm in skeletal muscles alters force production, suggesting an important regulatory role for titin in active force production. The molecular mechanisms for titin's role in regulating muscle force production remain to be elucidated.

Titin mutations and muscle disease

The introduction of next-generation sequencing technology has revealed that mutations in the gene that encodes titin (TTN) are linked to multiple skeletal and cardiac myopathies. The most prominent of these myopathies is dilated cardiomyopathy (DCM). Over 60 genes are linked to the etiology of DCM, but by far, the leading cause of DCM is mutations in TTN with truncating variants in TTN (TTNtvs) associated with familial DCM in ∼ 20% of the cases. Titin is a large (3-4 MDa) and abundant protein that forms the third myofilament type of striated muscle where it spans half the sarcomere, from the Z-disk to the M-line. The underlying mechanisms by which titin mutations induce disease are poorly understood and targeted therapies are not available. Here, we review what is known about TTN mutations in muscle disease, with a major focus on DCM. We highlight that exon skipping might provide a possible therapeutic avenue to address diseases that arise from TTNtvs.

Splicing Factor RBM20 Regulates Transcriptional Network of Titin Associated and Calcium Handling Genes in The Heart

RNA binding motif 20 (RBM20) regulates pre-mRNA splicing of over thirty genes, among which titin is a major target. With RBM20 expression, titin expresses a larger isoform at fetal stage to a smaller isoform at adult resulting from alternative splicing, while, without RBM20, titin expresses exclusively a larger isoform throughout all ages. In addition to splicing regulation, it is unknown whether RBM20 also regulates gene expression. In this study, we employed Rbm20 knockout rats to investigate gene expression profile using Affymetrix expression array. We compared wild type to Rbm20 knockout at day1, 20 and 49. Bioinformatics analysis showed RBM20 regulates fewer genes expression at younger age and more at older age and commonly expressed genes have the same trends. GSEA indicated up-regulated genes are associated with heart failure. We examined titin binding partners. All titin direct binding partners are up-regulated and their increased expression is associated with dilated cardiomyopathy. Particularly, we found that genes involving calcium handling and muscle contraction are changed by RBM20. Intracellular calcium level measurement with individual cardiomyocytes further confirmed that changes of these proteins impact calcium handling. Selected genes from titin binding partners and calcium handling were validated with QPCR and western blotting. These data demonstrate that RBM20 regulates gene splicing as well as gene expression. Altered gene expression by RBM20 influences protein-protein interaction, calcium releasing and thus muscle contraction. Our results first reported gene expression impacted by RBM20 with heart maturation, and provided new insights into the role of RBM20 in the progression of heart failure.

Small-molecule inhibition of MuRF1 attenuates skeletal muscle atrophy and dysfunction in cardiac cachexia

BACKGROUND

Muscle ring finger 1 (MuRF1) is a muscle-specific ubiquitin E3 ligase activated during clinical conditions associated with skeletal muscle wasting. Yet, there remains a paucity of therapeutic interventions that directly inhibit MuRF1 function, particularly in vivo. The current study, therefore, developed a novel compound targeting the central coiled coil domain of MuRF1 to inhibit muscle wasting in cardiac cachexia.

METHODS

We identified small molecules that interfere with the MuRF1-titin interaction from a 130 000 compound screen based on Alpha Technology. A subset of nine prioritized compounds were synthesized and administrated during conditions of muscle wasting, that is, to C2C12 muscle cells treated with dexamethasone and to mice treated with monocrotaline to induce cardiac cachexia.

RESULTS

The nine selected compounds inhibited MuRF1-titin complexation with IC₅₀ values <25 μM, of which three were found to also inhibit MuRF1 E3 ligase activity, with one further showing low toxicity on cultured myotubes. This last compound, EMBL chemical core ID#704946, also prevented atrophy in myotubes induced by dexamethasone and attenuated fibre atrophy and contractile dysfunction in mice during cardiac cachexia. Proteomic and western blot analyses showed that stress pathways were attenuated by ID#704946 treatment, including down-regulation of MuRF1 and normalization of proteins associated with apoptosis (BAX) and protein synthesis (elF2B-delta). Furthermore, actin ubiquitinylation and proteasome activity was attenuated.

CONCLUSIONS

We identified a novel compound directed to MuRF1's central myofibrillar protein recognition domain. This compound attenuated in vivo muscle wasting and contractile dysfunction in cardiac cachexia by protecting de novo protein synthesis and by down-regulating apoptosis and ubiquitin-proteasome-dependent proteolysis.

The histone code reader Spin1 controls skeletal muscle development

While several studies correlated increased expression of the histone code reader Spin1 with tumor formation or growth, little is known about physiological functions of the protein. We generated Spin1^M5 mice with ablation of Spin1 in myoblast precursors using the Myf5-Cre deleter strain. Most Spin1^M5 mice die shortly after birth displaying severe sarcomere disorganization and necrosis. Surviving Spin1^M5 mice are growth-retarded and exhibit the most prominent defects in soleus, tibialis anterior, and diaphragm muscle. Transcriptome analyses of limb muscle at embryonic day (E) 15.5, E16.5, and at three weeks of age provided evidence for aberrant fetal myogenesis and identified deregulated skeletal muscle (SkM) functional networks. Determination of genome-wide chromatin occupancy in primary myoblast revealed direct Spin1 target genes and suggested that deregulated basic helix-loop-helix transcription factor networks account for developmental defects in Spin1^M5 fetuses. Furthermore, correlating histological and transcriptome analyses, we show that aberrant expression of titin-associated proteins, abnormal glycogen metabolism, and neuromuscular junction defects contribute to SkM pathology in Spin1^M5 mice. Together, we describe the first example of a histone code reader controlling SkM development in mice, which hints at Spin1 as a potential player in human SkM disease.

Anisotropic mechanosensitive pathways in the diaphragm and their implications in muscular dystrophies

The diaphragm is the "respiratory pump;" the muscle that generates pressure to allow ventilation. Diaphragm muscles play a vital function and thus are subjected to continuous mechanical loading. One of its peculiarities is the ability to generate distinct mechanical and biochemical responses depending on the direction through which the mechanical forces applied to it. Contractile forces originated from its contractile components are transmitted to other structural components of its muscle fibers and the surrounding connective tissue. The anisotropic mechanical properties of the diaphragm are translated into biochemical signals that are directionally mechanosensitive by mechanisms that appear to be unique to this muscle. Here, we reviewed the current state of knowledge on the biochemical pathways regulated by mechanical signals emphasizing their anisotropic behavior in the normal diaphragm and analyzed how they are affected in muscular dystrophies.

Multifunctional protein: cardiac ankyrin repeat protein

Cardiac ankyrin repeat protein (CARP) not only serves as an important component of muscle sarcomere in the cytoplasm, but also acts as a transcription co-factor in the nucleus. Previous studies have demonstrated that CARP is up-regulated in some cardiovascular disorders and muscle diseases; however, its role in these diseases remains controversial now. In this review, we will discuss the continued progress in the research related to CARP, including its discovery, structure, and the role it plays in cardiac development and heart diseases.

Genetic epidemiology of titin-truncating variants in the etiology of dilated cardiomyopathy

Heart failure (HF) is a complex clinical syndrome defined by the inability of the heart to pump enough blood to meet the body's metabolic demands. Major causes of HF are cardiomyopathies (diseases of the myocardium associated with mechanical and/or electrical dysfunction), among which the most common form is dilated cardiomyopathy (DCM). DCM is defined by ventricular chamber enlargement and systolic dysfunction with normal left ventricular wall thickness, which leads to progressive HF. Over 60 genes are linked to the etiology of DCM. Titin (TTN) is the largest known protein in biology, spanning half the cardiac sarcomere and, as such, is a basic structural and functional unit of striated muscles. It is essential for heart development as well as mechanical and regulatory functions of the sarcomere. Next-generation sequencing (NGS) in clinical DCM cohorts implicated truncating variants in titin (TTNtv) as major disease alleles, accounting for more than 25% of familial DCM cases, but these variants have also been identified in 2-3% of the general population, where these TTNtv blur diagnostic and clinical utility. Taking into account the published TTNtv and their association to DCM, it becomes clear that TTNtv harm the heart with position-dependent occurrence, being more harmful when present in the A-band TTN, presumably with dominant negative/gain-of-function mechanisms. However, these insights are challenged by the depiction of position-independent toxicity of TTNtv acting via haploinsufficient alleles, which are sufficient to induce cardiac pathology upon stress. In the current review, we provide an overview of TTN and discuss studies investigating various TTN mutations. We also present an overview of different mechanisms postulated or experimentally validated in the pathogenicity of TTNtv. DCM-causing genes are also discussed with respect to non-truncating mutations in the etiology of DCM. One way of understanding pathogenic variants is probably to understand the context in which they may or may not affect protein-protein interactions, changes in cell signaling, and substrate specificity. In this regard, we also provide a brief overview of TTN interactions in situ. Quantitative models in the risk assessment of TTNtv are also discussed. In summary, we highlight the importance of gene-environment interactions in the etiology of DCM and further mechanistic studies used to delineate the pathways which could be targeted in the management of DCM.

Titin force enhancement following active stretch of skinned skeletal muscle fibres

In actively stretched skeletal muscle sarcomeres, titin-based force is enhanced, increasing the stiffness of active sarcomeres. Titin force enhancement in sarcomeres is vastly reduced in mdm, a genetic mutation with a deletion in titin. Whether loss of titin force enhancement is associated with compensatory mechanisms at higher structural levels of organization, such as single fibres or entire muscles, is unclear. The aim of this study was to determine whether mechanical deficiencies in titin force enhancement are also observed at the fibre level, and whether mechanisms compensate for the loss of titin force enhancement. Single skinned fibres from control and mutant mice were stretched actively and passively beyond filament overlap to observe titin-based force. Mutant fibres generated lower contractile stress (force divided by cross-sectional area) than control fibres. Titin force enhancement was observed in control fibres stretched beyond filament overlap, but was overshadowed in mutant fibres by an abundance of collagen and high variability in mechanics. However, titin force enhancement could be measured in all control fibers and most mutant fibres following short stretches, accounting for ∼25% of the total stress following active stretch. Our results show that the partial loss of titin force enhancement in myofibrils is not preserved in all mutant fibres and this mutation likely affects fibres differentially within a muscle. An increase in collagen helps to reestablish total force at long sarcomere lengths with the loss in titin force enhancement in some mutant fibres, increasing the overall strength of mutant fibres.

Effects of a titin mutation on negative work during stretch-shortening cycles in skeletal muscles

Negative work occurs in muscles during braking movements such as downhill walking or landing after a jump. When performing negative work during stretch-shortening cycles, viscoelastic structures within muscles store energy during stretch, return a fraction of this energy during shortening, and dissipate the remaining energy as heat. Because tendons and extracellular matrix are relatively elastic rather than viscoelastic, energy is mainly dissipated by cross bridges and titin. Recent studies demonstrate that titin stiffness increases in active skeletal muscles, suggesting that titin contributions to negative work may have been underestimated in previous studies. The muscular dystrophy with myositis (mdm) mutation in mice results in a deletion in titin that leads to reduced titin stiffness in active muscle, providing an opportunity to investigate the contribution of titin to negative work in stretch-shortening cycles. Using the work loop technique, extensor digitorum longus and soleus muscles from mdm and wild type mice were stimulated during the stretch phase of stretch-shortening cycles to investigate negative work. The results demonstrate that, compared to wild type muscles, negative work is reduced in muscles from mdm mice. We suggest that changes in the viscoelastic properties of mdm titin reduce energy storage by muscles during stretch and energy dissipation during shortening. Maximum isometric stress is also reduced in muscles from mdm mice, possibly due to impaired transmission of cross bridge force, impaired cross bridge function, or both. Functionally, the reduction in negative work could lead to increased muscle damage during eccentric contractions that occur during braking movements.

The effects of a skeletal muscle titin mutation on walking in mice

Titin contributes to sarcomere assembly, muscle signaling, and mechanical properties of muscle. The mdm mouse exhibits a small deletion in the titin gene resulting in dystrophic mutants and phenotypically normal heterozygotes. We examined the effects of this mutation on locomotion to assess how, and if, changes to muscle phenotype explain observed locomotor differences. Mutant mice are much smaller in size than their siblings and gait abnormalities may be driven by differences in limb proportions and/or by changes to muscle phenotype caused by the titin mutation. We quantified differences in walking gait among mdm genotypes and also determined whether genotypes vary in limb morphometrics. Mice were filmed walking, and kinematic and morphological variables were measured. Mutant mice had a smaller range of motion at the ankle, shorter stride lengths, and shorter stance duration, but walked at the same relative speeds as the other genotypes. Although phenotypically similar to wildtype mice, heterozygous mice frequently exhibited intermediate gait mechanics. Morphological differences among genotypes in hindlimb proportions were small and do not explain the locomotor differences. We suggest that differences in locomotion among mdm genotypes are due to changes in muscle phenotype caused by the titin mutation.

CARP interacts with titin at a unique helical N2A sequence and at the domain Ig81 to form a structured complex

The cardiac ankyrin repeat protein (CARP) is up-regulated in the myocardium during cardiovascular disease and in response to mechanical or toxic stress. Stress-induced CARP interacts with the N2A spring region of the titin filament to modulate muscle compliance. We characterize the interaction between CARP and titin-N2A and show that the binding site in titin spans the dual domain UN2A-Ig81. We find that the unique sequence UN2A is not structurally disordered, but that it has a stable, elongated α-helical fold that possibly acts as a constant force spring. Our findings portray CARP/titin-N2A as a structured node and help to rationalize the molecular basis of CARP mechanosensing in the sarcomeric I-band.

A Review of the Giant Protein Titin in Clinical Molecular Diagnostics of Cardiomyopathies

Titin (TTN) is known as the largest sarcomeric protein that resides within the heart muscle. Due to alternative splicing of TTN, the heart expresses two major isoforms (N2B and N2BA) that incorporate four distinct regions termed the Z-line, I-band, A-band, and M-line. Next-generation sequencing allows a large number of genes to be sequenced simultaneously and provides the opportunity to easily analyze giant genes such as TTN. Mutations in the TTN gene can cause cardiomyopathies, in particular dilated cardiomyopathy (DCM). DCM is the most common form of cardiomyopathy, and it is characterized by systolic dysfunction and dilation of the left ventricle. TTN truncating variants have been described as the most common cause of DCM, while the real impact of TTN missense variants in the pathogenesis of DCM is still unclear. In a recent population screening study, rare missense variants potentially pathogenic based on bioinformatic filtering represented only 12.6% of the several hundred rare TTN missense variants found, suggesting that missense variants are very common in TTN and are frequently benign. The aim of this review is to understand the clinical role of TTN mutations in DCM and in other cardiomyopathies. Whereas TTN truncations are common in DCM, there is evidence that TTN truncations are rare in the hypertrophic cardiomyopathy (HCM) phenotype. Furthermore, TTN mutations can also cause arrhythmogenic right ventricular cardiomyopathy (ARVC) with distinct clinical features and outcomes. Finally, the identification of a rare TTN missense variant cosegregating with the restrictive cardiomyopathy (RCM) phenotype suggests that TTN is a novel disease-causing gene in this disease. Clinical diagnostic testing is currently able to analyze over 100 cardiomyopathy genes, including TTN; however, the size and presence of extensive genetic variation in TTN presents clinical challenges in determining significant disease-causing mutations. This review discusses the current knowledge of TTN genetic variations in cardiomyopathies and the impact of the diagnosis of TTN pathogenic mutations in the clinical setting.

Decreased force enhancement in skeletal muscle sarcomeres with a deletion in titin

In the cross-bridge theory, contractile force is produced by cross-bridges that form between actin and myosin filaments. However, when a contracting muscle is stretched, its active force vastly exceeds the force that can be attributed to cross-bridges. This unexplained, enhanced force has been thought to originate in the giant protein titin, which becomes stiffer in actively compared to passively stretched sarcomeres by an unknown mechanism. We investigated this mechanism using a genetic mutation (mdm) with a small but crucial deletion in the titin protein. Myofibrils from normal and mdm mice were stretched from sarcomere lengths of 2.5 to 6.0 μm. Actively stretched myofibrils from normal mice were stiffer and generated more force than passive myofibrils at all sarcomere lengths. No increase in stiffness, and just a small increase in force, was observed in actively compared to passively stretched mdm myofibrils. These results are in agreement with the idea that titin force enhancement stiffens and stabilizes the sarcomere during contraction and that this mechanism is lost with the mdm mutation.

Effects of activation on the elastic properties of intact soleus muscles with a deletion in titin

Titin has long been known to contribute to muscle passive tension. Recently, it was also demonstrated that titin-based stiffness increases upon Ca2+-activation of wildtype mouse psoas myofibrils stretched beyond overlap of the thick and thin filaments. In addition, this increase in titin-based stiffness upon activation was impaired in single psoas myofibrils from mdm mice with a deletion in titin. Here, we investigate the effects of muscle activation on elastic properties of intact soleus muscles from wildtype and mdm mice to determine whether titin may contribute to active muscle stiffness. Using load-clamp experiments, we compared the stress-strain relationships of elastic elements in active and passive muscles during unloading, and quantified the change in stiffness upon activation. We used the mdm mutation, characterized by a deletion in the N2A region of the Ttn gene, to test the hypothesis that titin contributes to active muscle stiffness. Results show that the elastic modulus of wildtype muscles increases upon activation. Elastic elements began to develop force at lengths that were 15% shorter in active than in passive soleus, and there was a 2.9-fold increase in the slope of the stress - strain relationship. In contrast, mdm soleus showed no effect of activation on the slope or intercept of the stress - strain relationship. These results from intact soleus muscles are qualitatively and quantitatively similar to results from single wildtype psoas myofibrils stretched beyond overlap of the thick and thin filaments. Therefore, it is likely that titin plays a role in the increase of stiffness during rapid unloading that we observed in intact soleus muscles upon activation. The results from intact mdm soleus muscles are also consistent with impaired titin activation observed in single mdm psoas myofibrils stretched beyond filament overlap, further suggesting that the mechanism of titin activation is impaired in skeletal muscles from mdm mice. These results are consistent with the idea that, in addition to the thin filaments, titin is activated upon Ca2+-influx in skeletal muscle.

Probing muscle ankyrin-repeat protein (MARP) structure and function

Muscle ankyrin-repeat proteins (MARPs) have been shown to serve diverse functions within cardiac and skeletal muscle cells. Apart from their interactions with sarcomeric proteins like titin or myopalladin that locate them along myofilaments, MARPs are able to shuttle to the nucleus where they act as modulators for a variety of transcription factors. The deregulation of MARPs in many cardiac and skeletal myopathies contributes to their use as biomarkers for these diseases. Many of their functions are attributed to their domain composition. MARPs consist of an N-terminal coiled-coil domain responsible for their dimerization. The C-terminus contains a series of ankyrin repeats, whose best-characterized function is to bind to the N2A region of the giant sarcomeric protein titin. Here we investigate the nature of their dimerization and their interaction with titin more closely. We demonstrate that the coiled-coil domain in all MARPs enables their homo- and hetero-dimerization in antiparallel fashion. Protein complementation experiments indicate further antiparallel binding of the ankyrin repeats to titin's N2A region. Binding of MARP to titin also affects its PKA mediated phosphorylation. We demonstrate further that MARPs themselves are phosphorylated by PKA and PKC, potentially altering their structure or function. These studies elucidate structural relationships within the stretch-responsive MARP/titin complex in cross-striated muscle cells, and may relate to disease relevant posttranslational modifications of MARPs and titin that alter muscle compliance.

Shiver me titin! Elucidating titin's role in shivering thermogenesis

Shivering frequency scales predictably with body mass and is 10 times higher in a mouse than a moose. The link between shivering frequency and body mass may lie in the tuning of muscle elastic properties. Titin functions as a muscle ‘spring’, so shivering frequency may be linked to titin's structure. The muscular dystrophy with myositis (mdm) mouse is characterized by a deletion in titin's N2A region. Mice that are homozygous for the mdm mutation have a lower body mass, stiffer gait and reduced lifespan compared with their wild-type and heterozygous siblings. We characterized thermoregulation in these mice by measuring metabolic rate and tremor frequency during shivering. Mutants were heterothermic at ambient temperatures of 20–37°C while wild-type and heterozygous mice were homeothermic. Metabolic rate increased at smaller temperature differentials (i.e. the difference between body and ambient temperatures) in mutants than in non-mutants. The difference between observed tremor frequencies and shivering frequencies predicted by body mass was significantly larger for mutant mice than for wild-type or heterozygous mice, even after accounting for differences in body temperature. Together, the heterothermy in mutants, the increase in metabolic rate at low temperature differentials and the decreased tremor frequency demonstrate the thermoregulatory challenges faced by mice with the mdm mutation. Oscillatory frequency is proportional to the square root of stiffness, and we observed that mutants had lower active muscle stiffness in vitro. The lower tremor frequencies in mutants are consistent with reduced active muscle stiffness and suggest that titin affects the tuning of shivering frequency.

Removal of immunoglobulin-like domains from titin's spring segment alters titin splicing in mouse skeletal muscle and causes myopathy

Changes in titin splicing resulting in decreased size and increased stiffness lead to pathological changes in skeletal muscle.

Titin is a molecular spring that determines the passive stiffness of muscle cells. Changes in titin’s stiffness occur in various myopathies, but whether these are a cause or an effect of the disease is unknown. We studied a novel mouse model in which titin’s stiffness was slightly increased by deleting nine immunoglobulin (Ig)-like domains from titin’s constitutively expressed proximal tandem Ig segment (IG KO). KO mice displayed mild kyphosis, a phenotype commonly associated with skeletal muscle myopathy. Slow muscles were atrophic with alterations in myosin isoform expression; functional studies in soleus muscle revealed a reduced specific twitch force. Exon expression analysis showed that KO mice underwent additional changes in titin splicing to yield smaller than expected titin isoforms that were much stiffer than expected. Additionally, splicing occurred in the PEVK region of titin, a finding confirmed at the protein level. The titin-binding protein Ankrd1 was highly increased in the IG KO, but this did not play a role in generating small titin isoforms because titin expression was unaltered in IG KO mice crossed with Ankrd1-deficient mice. In contrast, the splicing factor RBM20 (RNA-binding motif 20) was also significantly increased in IG KO mice, and additional differential splicing was reversed in IG KO mice crossed with a mouse with reduced RBM20 activity. Thus, increasing titin’s stiffness triggers pathological changes in skeletal muscle, with an important role played by RBM20.

Cloning, expression, and bioinformatics analysis of the sheep CARP gene

The cardiac ankyrin repeat protein (CARP) is a multifunctional protein that is expressed specifically in mammalian cardiac muscle and plays important roles in stress responses, transcriptional regulation, myofibrillar assembly, and the development of cardiac and skeletal muscle. In this study, the sheep homolog of the CARP gene was cloned and characterized. The coding region of the gene consists of 960 bp and encodes 319 amino acids with molecular weight 36.2 KD. Bioinformatics analysis demonstrated that the 3' untranslated region (3'-UTR) of the gene contains many AU-rich elements that are associated with mRNA stability and a potential regulatory site for miRNA binding. The protein was predicted to contain 14 potential phosphorylation sites and an O-GlcNAc glycosylation site and to be expressed in both the nucleus and cytoplasm. The evolutionary analysis revealed that the sheep CARP exhibited a high level of homology with the mammalian counterparts; however, the protein exhibited an increased evolutionary distance from the chicken, frog, and fish homologs. RT-PCR revealed that in addition to its high mRNA expression level in cardiac muscle, trace amounts of the sheep CARP mRNA were expressed in the skeletal muscle, stomach, and small intestine. However, western blot analysis demonstrated that the CARP protein was expressed only in cardiac muscle. The coding sequence was cloned into the pET30a-TEV-LIC vector, and the soluble CARP-MBP (maltose-binding protein) fusion protein was expressed in a prokaryotic host and purified by affinity chromatography. Our data provide the basis for future studies of the structure and function of sheep CARP.

Formation, contraction, and mechanotransduction of myofribrils in cardiac development: clues from genetics

Congenital heart disease (CHD) is the most common birth defect in humans. It is a leading infant mortality factor worldwide, caused by defective cardiac development. Mutations in transcription factors, signalling and structural molecules have been shown to contribute to the genetic component of CHD. Recently, mutations in genes encoding myofibrillar proteins expressed in the embryonic heart have also emerged as an important genetic causative factor of the disease, which implies that the contraction of the early heart primordium contributes to its morphogenesis. This notion is supported by increasing evidence suggesting that not only contraction but also formation, mechanosensing, and mechanotransduction of the cardiac myofibrillar proteins influence heart development. In this paper, we summarize the genetic clues supporting this idea.

Regulation and physiological roles of the calpain system in muscular disorders

Calpains, a family of Ca(2+)-dependent cytosolic cysteine proteases, can modulate their substrates' structure and function through limited proteolytic activity. In the human genome, there are 15 calpain genes. The most-studied calpains, referred to as conventional calpains, are ubiquitous. While genetic studies in mice have improved our understanding about the conventional calpains' physiological functions, especially those essential for mammalian life as in embryogenesis, many reports have pointed to overactivated conventional calpains as an exacerbating factor in pathophysiological conditions such as cardiovascular diseases and muscular dystrophies. For treatment of these diseases, calpain inhibitors have always been considered as drug targets. Recent studies have introduced another aspect of calpains that calpain activity is required to protect the heart and skeletal muscle against stress. This review summarizes the functions and regulation of calpains, focusing on the relevance of calpains to cardiovascular disease.

Mechanics on myocardium deficient in the N2B region of titin: the cardiac-unique spring element improves efficiency of the cardiac cycle

Titin (also known as connectin) is an intrasarcomeric muscle protein that functions as a molecular spring and generates passive tension upon muscle stretch. The N2B element is a cardiac-specific spring element within titin's extensible region. Our goal was to study the contribution of the N2B element to the mechanical properties of titin, particularly its hypothesized role in limiting energy loss during repeated stretch (diastole)-shortening (systole) cycles of the heart. We studied energy loss by measuring hysteresis from the area between the stretch and release passive force-sarcomere length curves and used both wild-type (WT) mice and N2B knockout (KO) mice in which the N2B element has been deleted. A range of protocols was used, including those that mimic physiological loading conditions. KO mice showed significant increases in hysteresis. Most prominently, in tissue that had been preconditioned with a physiological stretch-release protocol, hysteresis increased significantly from 320 ± 46 pJ/mm(2)/sarcomere in WT to 650 ± 94 pJ/mm(2)/sarcomere in N2B KO myocardium. These results are supported by experiments in which oxidative stress was used to mechanically inactivate portions of the N2B-Us of WT titin through cysteine cross-linking. Studies on muscle from which the thin filaments had been extracted (using the actin severing protein gelsolin) showed that the difference in hysteresis between WT and KO tissue cannot be explained by filament sliding-based viscosity. Instead the results suggest that hysteresis arises from within titin and most likely involves unfolding of immunoglobulin-like domains. These studies support that the mechanical function of the N2B element of titin includes reducing hysteresis and increasing the efficiency of the heart.

Genetic variation in titin in arrhythmogenic right ventricular cardiomyopathy-overlap syndromes

BACKGROUND

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited genetic myocardial disease characterized by fibrofatty replacement of the myocardium and a predisposition to cardiac arrhythmias and sudden death. We evaluated the cardiomyopathy gene titin (TTN) as a candidate ARVC gene because of its proximity to an ARVC locus at position 2q32 and the connection of the titin protein to the transitional junction at intercalated disks.

METHODS AND RESULTS

All 312 titin exons known to be expressed in human cardiac titin and the complete 3' untranslated region were sequenced in 38 ARVC families. Eight unique TTN variants were detected in 7 families, including a prominent Thr2896Ile mutation that showed complete segregation with the ARVC phenotype in 1 large family. The Thr2896IIe mutation maps within a highly conserved immunoglobulin-like fold (Ig10 domain) located in the spring region of titin. Native gel electrophoresis, nuclear magnetic resonance, intrinsic fluorescence, and proteolysis assays of wild-type and mutant Ig10 domains revealed that the Thr2896IIe exchange reduces the structural stability and increases the propensity for degradation of the Ig10 domain. The phenotype of TTN variant carriers was characterized by a history of sudden death (5 of 7 families), progressive myocardial dysfunction causing death or heart transplantation (8 of 14 cases), frequent conduction disease (11 of 14), and incomplete penetrance (86%).

CONCLUSIONS

Our data provide evidence that titin mutations can cause ARVC, a finding that further expands the origin of the disease beyond desmosomal proteins. Structural impairment of the titin spring is a likely cause of ARVC and constitutes a novel mechanism underlying myocardial remodeling and sudden cardiac death.

Anchoring skeletal muscle development and disease: the role of ankyrin repeat domain containing proteins in muscle physiology

The ankyrin repeat is a protein module with high affinity for other ankyrin repeats based on strong Van der Waals forces. The resulting dimerization is unusually resistant to both mechanical forces and alkanization, making this module exceedingly useful for meeting the extraordinary demands of muscle physiology. Many aspects of muscle function are controlled by the superfamily ankyrin repeat domain containing proteins, including structural fixation of the contractile apparatus to the muscle membrane by ankyrins, the archetypical member of the family. Additionally, other ankyrin repeat domain containing proteins critically control the various differentiation steps during muscle development, with Notch and developmental stage-specific expression of the members of the Ankyrin repeat and SOCS box (ASB) containing family of proteins controlling compartment size and guiding the various steps of muscle specification. Also, adaptive responses in fully formed muscle require ankyrin repeat containing proteins, with Myotrophin/V-1 ankyrin repeat containing proteins controlling the induction of hypertrophic responses following excessive mechanical load, and muscle ankyrin repeat proteins (MARPs) acting as protective mechanisms of last resort following extreme demands on muscle tissue. Knowledge on mechanisms governing the ordered expression of the various members of superfamily of ankyrin repeat domain containing proteins may prove exceedingly useful for developing novel rational therapy for cardiac disease and muscle dystrophies.

Dynamic distribution of muscle-specific calpain in mice has a key role in physical-stress adaptation and is impaired in muscular dystrophy

Limb-girdle muscular dystrophy type 2A (LGMD2A) is a genetic disease that is caused by mutations in the calpain 3 gene (CAPN3), which encodes the skeletal muscle-specific calpain, calpain 3 (also known as p94). However, the precise mechanism by which p94 functions in the pathogenesis of this disease remains unclear. Here, using p94 knockin mice (termed herein p94KI mice) in which endogenous p94 was replaced with a proteolytically inactive but structurally intact p94:C129S mutant protein, we have demonstrated that stretch-dependent p94 distribution in sarcomeres plays a crucial role in the pathogenesis of LGMD2A. The p94KI mice developed a progressive muscular dystrophy, which was exacerbated by exercise. The exercise-induced muscle degeneration in p94KI mice was associated with an inefficient redistribution of p94:C129S in stretched sarcomeres. Furthermore, the p94KI mice showed impaired adaptation to physical stress, which was accompanied by compromised upregulation of muscle ankyrin-repeat protein-2 and hsp upon exercise. These findings indicate that the stretch-induced dynamic redistribution of p94 is dependent on its protease activity and essential to protect muscle from degeneration, particularly under conditions of physical stress. Furthermore, our data provide direct evidence that loss of p94 protease activity can result in LGMD2A and molecular insight into how this could occur.

Identification of a Z-band associated protein complex involving KY, FLNC and IGFN1

The KY protein underlies a form of muscular dystrophy in the mouse but its role in muscle remains elusive. Immunodetection of endogenous KY protein in C2C12-derived myotubes and expression of a recombinant form in neonatal cardiomyocytes indicated that KY is a Z-band associated protein. Moreover, characterization of a KY interacting protein fragment led to the identification of Igfn1 (Immunoglobulin-like and fibronectin type 3 domain containing 1). Igfn1 is a transcriptionally complex locus encoding many protein variants. A yeast two-hybrid screen identified the Z-band protein filamin C (FLNC) as an interacting partner. Consistent with this, expression of an IGFN1 recombinant fragment showed that the three N-terminal globular domains, common to at least five IGFN1 variants, are sufficient to provide Z-band targeting. Taken together, the yeast two-hybrid, biochemical and immunofluorescence data support the notion that KY, IGFN1 and FLNC are part of a Z-band associated protein complex likely to provide structural support to the skeletal muscle sarcomere.

Role of titin in skeletal muscle function and disease

This review covers recent developments in the titin field. Most recent reviews have discussed titin's role in cardiac function: here we will mainly focus on skeletal muscle, and discuss recent advances in the understanding of titin's role in skeletal muscle function and disease.

Muscle giants: molecular scaffolds in sarcomerogenesis

Myofibrillogenesis in striated muscles is a highly complex process that depends on the coordinated assembly and integration of a large number of contractile, cytoskeletal, and signaling proteins into regular arrays, the sarcomeres. It is also associated with the stereotypical assembly of the sarcoplasmic reticulum and the transverse tubules around each sarcomere. Three giant, muscle-specific proteins, titin (3-4 MDa), nebulin (600-800 kDa), and obscurin (approximately 720-900 kDa), have been proposed to play important roles in the assembly and stabilization of sarcomeres. There is a large amount of data showing that each of these molecules interacts with several to many different protein ligands, regulating their activity and localizing them to particular sites within or surrounding sarcomeres. Consistent with this, mutations in each of these proteins have been linked to skeletal and cardiac myopathies or to muscular dystrophies. The evidence that any of them plays a role as a "molecular template," "molecular blueprint," or "molecular ruler" is less definitive, however. Here we review the structure and function of titin, nebulin, and obscurin, with the literature supporting a role for them as scaffolding molecules and the contradictory evidence regarding their roles as molecular guides in sarcomerogenesis.

Muscle LIM protein interacts with cofilin 2 and regulates F-actin dynamics in cardiac and skeletal muscle

The muscle LIM protein (MLP) and cofilin 2 (CFL2) are important regulators of striated myocyte function. Mutations in the corresponding genes have been directly associated with severe human cardiac and skeletal myopathies, and aberrant expression patterns have often been observed in affected muscles. Herein, we have investigated whether MLP and CFL2 are involved in common molecular mechanisms, which would promote our understanding of disease pathogenesis. We have shown for the first time, using a range of biochemical and immunohistochemical methods, that MLP binds directly to CFL2 in human cardiac and skeletal muscles. The interaction involves the inter-LIM domain, amino acids 94 to 105, of MLP and the amino-terminal domain, amino acids 1 to 105, of CFL2, which includes part of the actin depolymerization domain. The MLP/CFL2 complex is stronger in moderately acidic (pH 6.8) environments and upon CFL2 phosphorylation, while it is independent of Ca(2+) levels. This interaction has direct implications in actin cytoskeleton dynamics in regulating CFL2-dependent F-actin depolymerization, with maximal depolymerization enhancement at an MLP/CFL2 molecular ratio of 2:1. Deregulation of this interaction by intracellular pH variations, CFL2 phosphorylation, MLP or CFL2 gene mutations, or expression changes, as observed in a range of cardiac and skeletal myopathies, could impair F-actin depolymerization, leading to sarcomere dysfunction and disease.

Cardiac ankyrin repeat protein is a marker of skeletal muscle pathological remodelling

In an attempt to identify potential therapeutic targets for the correction of muscle wasting, the gene expression of several pivotal proteins involved in protein metabolism was investigated in experimental atrophy induced by transient or definitive denervation, as well as in four animal models of muscular dystrophies (deficient for calpain 3, dysferlin, alpha-sarcoglycan and dystrophin, respectively). The results showed that: (a) the components of the ubiquitin-proteasome pathway are upregulated during the very early phases of atrophy but do not greatly increase in the muscular dystrophy models; (b) forkhead box protein O1 mRNA expression is augmented in the muscles of a limb girdle muscular dystrophy 2A murine model; and (c) the expression of cardiac ankyrin repeat protein (CARP), a regulator of transcription factors, appears to be persistently upregulated in every condition, suggesting that CARP could be a hub protein participating in common pathological molecular pathway(s). Interestingly, the mRNA level of a cell cycle inhibitor known to be upregulated by CARP in other tissues, p21(WAF1/CIP1), is consistently increased whenever CARP is upregulated. CARP overexpression in muscle fibres fails to affect their calibre, indicating that CARP per se cannot initiate atrophy. However, a switch towards fast-twitch fibres is observed, suggesting that CARP plays a role in skeletal muscle plasticity. The observation that p21(WAF1/CIP1) is upregulated, put in perspective with the effects of CARP on the fibre type, fits well with the idea that the mechanisms at stake might be required to oppose muscle remodelling in skeletal muscle.

Early mechanical dysfunction of the diaphragm in the muscular dystrophy with myositis (Ttnmdm) model

A complex rearrangement mutation in the mouse titin gene leads to an in-frame 83-amino acid deletion in the N2A region of titin. Autosomal recessive inheritance of the titin muscular dystrophy with myositis (Ttn(mdm/mdm)) mutation leads to a severe early-onset muscular dystrophy and premature death. We hypothesized that the N2A deletion would negatively impact the force-generating capacity and passive mechanical properties of the mdm diaphragm. We measured in vitro active isometric contractile and passive length-tension properties to assess muscle function at 2 and 6 wk of age. Micro-CT, myosin heavy chain Western blotting, and histology were used to assess diaphragm structure. Marked chest wall distortions began at 2 wk and progressively worsened until 5 wk. The percentage of myofibers with centrally located nuclei in mdm mice was significantly (P < 0.01) increased at 2 and 6 wk by 4% and 17%, respectively, compared with controls. At 6 wk, mdm diaphragm twitch stress was significantly (P < 0.01) reduced by 71%, time to peak twitch was significantly (P < 0.05) reduced by 52%, and half-relaxation time was significantly (P < 0.05) reduced by 57%. Isometric tetanic stress was significantly (P < 0.05) depressed in 2- and 6-wk mdm diaphragms by as much as 64%. Length-tension relationships of the 2- and 6-wk mdm diaphragms showed significantly (P < 0.05) decreased extensibility and increased stiffness. Slow myosin heavy chain expression was aberrantly favored in the mdm diaphragm at 6 wk. Our data strongly support early contractile and passive mechanical aberrations of the respiratory pump in mdm mice.

Expression of cardiac ankyrin repeat protein, CARP, in malignant tumors: diagnostic use of CARP protein immunostaining in rhabdomyosarcoma

Cardiac ankyrin repeat protein (CARP) is highly expressed in cardiac muscles and detectable in normal skeletal muscles. Arpp, a close homolog of CARP, has been demonstrated to be useful for distinguishing rhabdomyosarcoma from other malignant tumors. However, the CARP distributions among malignant tumors have been poorly investigated. Here, we analyzed the comprehensive expression of CARP in malignant tumors and evaluated its potential use for rhabdomyosarcoma diagnosis. A total of 159 malignant tumors, including 34 rhabdomyosarcomas, 85 non-rhabdomyosarcomas, and 40 carcinomas, were immunohistochemically analyzed for CARP expression. Cytoplasmic expression of CARP was detected in 29 (85%) of 34 rhabdomyosarcomas. The immunoreactivity was observed in both small cells with little differentiation and differentiated tumor cells with abundant eosinophilic cytoplasm. In contrast, focal immunoreactivity for CARP was only observed in 5 (4%) of 125 non-rhabdomyosarcomas, comprising 2 malignant fibrous histiocytomas, 1 angiosarcoma, 1 epithelioid sarcoma, and 1 squamous cell carcinoma of the lung. Comparative analysis of the CARP expression profiles with those of myogenic markers in rhabdomyosarcomas revealed that myogenin (88%) and desmin (88%) exhibited the best sensitivity, followed by CARP (85%), MyoD (82%), muscle-specific actin (79%), and myoglobin (65%). MyoD (96%) and myoglobin (96%) had the best specificity, followed by CARP (95%), myogenin (95%), desmin (89%), and muscle-specific actin (86%). Our results indicate that CARP is a sensitive and specific marker for rhabdomyosarcoma and that it will be useful for the differential diagnosis of rhabdomyosarcoma.