Targeted deletion of the muscular dystrophy gene myotilin does not perturb muscle structure or function in mice

Biological sex, sex steroids and sex chromosomes contribute to mouse cardiac aging

After menopause, the incidence of cardiovascular disease rapidly rises in women. The disappearing protection provided by sex steroids is a consequence of the development of many risk factors. Preclinical studies are necessary to understand better the effects of ovarian hormones loss cardiac aging. To mimic menopause in mice and study its consequences, we delayed ovariectomy at 12 months and followed animals for 12 months. Using RNA sequencing, we investigated changes in the myocardial exome with aging. In addition, with four-core genotypes (FCG) transgenic mice, we studied sex chromosome effects on cardiac aging. Heart weight increased from 3 to 24 months (males + 35%, females + 29%). In males, 75% of this increase had occurred at 12 months; in females, only 30%. Gonadectomy of mice at 12 months blocked cardiac hypertrophy in both sexes during the second year of life. The dosage of the X chromosomes did not influence cardiac growth in young and older mice. We performed an RNA sequencing study in young and old mice. We identified new highly expressed genes modulated during aging (Bdh, Myot, Cpxm2, and Slc38a1). The myocardial exome in older animals displayed few differences related to the animal's sex or the presence or absence of sex steroids for a year. We show that the morphological evolution of the heart depends on the biological sex via gonadal sex hormone actions. The myocardial exome of old male and female mice is relatively similar. Our study emphasizes the need to consider sex steroid effects in studying cardiac aging.

Droplet digital PCR or quantitative PCR for in-depth genomic and functional validation of genetically altered rodents

Gene targeting and additive (random) transgenesis have proven to be powerful technologies with which to decipher the mammalian genome. With the advent of CRISPR/Cas9 genome editing, the ability to inactivate or modify the function of a gene has become even more accessible. However, the impact of each generated modification may be different from what was initially desired. Minimal validation of mutant alleles from genetically altered (GA) rodents remains essential to guarantee the interpretation of experimental results. The protocol described here combines design strategies for genomic and functional validation of genetically modified alleles with droplet digital PCR (ddPCR) or quantitative PCR (qPCR) for target DNA or mRNA quantification. In-depth analysis of the results obtained with GA models through the analysis of target DNA and mRNA quantification is also provided, to evaluate which pitfalls can be detected using these two methods, and we propose recommendations for the characterization of different type of mutant allele (knock-out, knock-in, conditional knock-out, FLEx, IKMC model or transgenic). Our results also highlight the possibility that mRNA expression of any mutated allele can be different from what might be expected in theory or according to common assumptions. For example, mRNA analyses on knock-out lines showed that nonsense-mediated mRNA decay is generally not achieved with a critical-exon approach. Likewise, comparison of multiple conditional lines crossed with the same CreER^T2 deleter showed that the inactivation outcome was very different for each conditional model. DNA quantification by ddPCR of G0 to G2 generations of transgenic rodents generated by pronuclear injection showed an unexpected variability, demonstrating that G1 generation rodents cannot be considered as established lines.

Molecular basis of F-actin regulation and sarcomere assembly via myotilin

Sarcomeres, the basic contractile units of striated muscle cells, contain arrays of thin (actin) and thick (myosin) filaments that slide past each other during contraction. The Ig-like domain-containing protein myotilin provides structural integrity to Z-discs-the boundaries between adjacent sarcomeres. Myotilin binds to Z-disc components, including F-actin and α-actinin-2, but the molecular mechanism of binding and implications of these interactions on Z-disc integrity are still elusive. To illuminate them, we used a combination of small-angle X-ray scattering, cross-linking mass spectrometry, and biochemical and molecular biophysics approaches. We discovered that myotilin displays conformational ensembles in solution. We generated a structural model of the F-actin:myotilin complex that revealed how myotilin interacts with and stabilizes F-actin via its Ig-like domains and flanking regions. Mutant myotilin designed with impaired F-actin binding showed increased dynamics in cells. Structural analyses and competition assays uncovered that myotilin displaces tropomyosin from F-actin. Our findings suggest a novel role of myotilin as a co-organizer of Z-disc assembly and advance our mechanistic understanding of myotilin's structural role in Z-discs.

The Role of Z-disc Proteins in Myopathy and Cardiomyopathy

The Z-disc acts as a protein-rich structure to tether thin filament in the contractile units, the sarcomeres, of striated muscle cells. Proteins found in the Z-disc are integral for maintaining the architecture of the sarcomere. They also enable it to function as a (bio-mechanical) signalling hub. Numerous proteins interact in the Z-disc to facilitate force transduction and intracellular signalling in both cardiac and skeletal muscle. This review will focus on six key Z-disc proteins: α-actinin 2, filamin C, myopalladin, myotilin, telethonin and Z-disc alternatively spliced PDZ-motif (ZASP), which have all been linked to myopathies and cardiomyopathies. We will summarise pathogenic variants identified in the six genes coding for these proteins and look at their involvement in myopathy and cardiomyopathy. Listing the Minor Allele Frequency (MAF) of these variants in the Genome Aggregation Database (GnomAD) version 3.1 will help to critically re-evaluate pathogenicity based on variant frequency in normal population cohorts.

Assembly and Maintenance of Sarcomere Thin Filaments and Associated Diseases

Sarcomere assembly and maintenance are essential physiological processes required for cardiac and skeletal muscle function and organism mobility. Over decades of research, components of the sarcomere and factors involved in the formation and maintenance of this contractile unit have been identified. Although we have a general understanding of sarcomere assembly and maintenance, much less is known about the development of the thin filaments and associated factors within the sarcomere. In the last decade, advancements in medical intervention and genome sequencing have uncovered patients with novel mutations in sarcomere thin filaments. Pairing this sequencing with reverse genetics and the ability to generate patient avatars in model organisms has begun to deepen our understanding of sarcomere thin filament development. In this review, we provide a summary of recent findings regarding sarcomere assembly, maintenance, and disease with respect to thin filaments, building on the previous knowledge in the field. We highlight debated and unknown areas within these processes to clearly define open research questions.

Conformational plasticity and evolutionary analysis of the myotilin tandem Ig domains

Myotilin is a component of the sarcomere where it plays an important role in organisation and maintenance of Z-disk integrity. This involves direct binding to F-actin and filamin C, a function mediated by its Ig domain pair. While the structures of these two individual domains are known, information about their relative orientation and flexibility remains limited. We set on to characterise the Ig domain pair of myotilin with emphasis on its molecular structure, dynamics and phylogeny. First, sequence conservation analysis of myotilin shed light on the molecular basis of myotilinopathies and revealed several motifs in Ig domains found also in I-band proteins. In particular, a highly conserved Glu344 mapping to Ig domain linker, was identified as a critical component of the inter-domain hinge mechanism. Next, SAXS and molecular dynamics revealed that Ig domain pair exists as a multi-conformation species with dynamic exchange between extended and compact orientations. Mutation of AKE motif to AAA further confirmed its impact on inter-domain flexibility. We hypothesise that the conformational plasticity of the Ig domain pair in its unbound form is part of the binding partner recognition mechanism.

Analysis of compound heterozygotes reveals that the mouse floxed Pax6 (tm1Ued) allele produces abnormal eye phenotypes

Analysis of abnormal phenotypes produced by different types of mutations has been crucial for our understanding of gene function. Some floxed alleles that retain a neomycin-resistance selection cassette (neo cassette) are not equivalent to wild-type alleles and provide useful experimental resources. Pax6 is an important developmental gene and the aim of this study was to determine whether the floxed Pax6^tm1Ued (Pax6^fl) allele, which has a retained neo cassette, produced any abnormal eye phenotypes that would imply that it differs from the wild-type allele. Homozygous Pax6^fl/fl and heterozygous Pax6^fl/+ mice had no overt qualitative eye abnormalities but morphometric analysis showed that Pax6^fl/fl corneas tended be thicker and smaller in diameter. To aid identification of weak effects, we produced compound heterozygotes with the Pax6^Sey-Neu (Pax6⁻) null allele. Pax6^fl/− compound heterozygotes had more severe eye abnormalities than Pax6^+/− heterozygotes, implying that Pax6^fl differs from the wild-type Pax6⁺ allele. Immunohistochemistry showed that the Pax6^fl/− corneal epithelium was positive for keratin 19 and negative for keratin 12, indicating that it was abnormally differentiated. This Pax6^fl allele provides a useful addition to the existing Pax6 allelic series and this study demonstrates the utility of using compound heterozygotes with null alleles to unmask cryptic effects of floxed alleles.

Neuromuscular Disease Models and Analysis

Neuromuscular diseases can affect the survival of peripheral neurons, their axons extending to peripheral targets, their synaptic connections onto those targets, or the targets themselves. Examples include motor neuron diseases such as Amyotrophic Lateral Sclerosis, peripheral neuropathies such as Charcot-Marie-Tooth diseases, myasthenias, and muscular dystrophies. Characterizing these phenotypes in mouse models requires an integrated approach, examining both the nerve and muscle histologically, anatomically, and functionally by electrophysiology. Defects observed at these levels can be related back to onset, severity, and progression, as assessed by "Quality of life measures" including tests of gross motor performance such as gait or grip strength. This chapter describes methods for assessing neuromuscular disease models in mice, and how interpretation of these tests can be complicated by the inter-relatedness of the phenotypes.

Roles of Nebulin Family Members in the Heart

The members of the nebulin protein family, including nebulin, nebulette, LASP-1, LASP-2, and N-RAP, contain various numbers of nebulin repeats and bind to actin, but are otherwise heterogeneous with regard to size, expression pattern, and function. This review focuses on the roles of nebulin family members in the heart. Nebulin is the largest member predominantly expressed in skeletal muscle, where it stretches along the thin filament. In heart, nebulin is detectable only at low levels and its absence has no apparent effects. Nebulette is similar in structure to the nebulin C-terminal Z-line region and specifically expressed in heart. Nebulette gene mutations have been identified in dilated cardiomyopathy patients and transgenic mice overexpressing nebulette mutants partially recapitulate the human pathology. In contrast, nebulette knockout mice show no functional phenotype, but exhibit Z-line widening. LASP-2 is an isoform of nebulette expressed in multiple tissues, including the heart. It is present in the Z-line and intercalated disc and able to bind and cross-link filamentous actin. LASP-1 is similar in structure to LASP-2, but expressed only in non-muscle tissue. N-RAP is present in myofibril precursors during myofibrillogenesis and thought to be involved in myofibril assembly, while it is localized at the intercalated disc in adult heart. Additional in vivo models are required to provide further insights into the functions of nebulin family members in the heart.

Nebulette knockout mice have normal cardiac function, but show Z-line widening and up-regulation of cardiac stress markers

AIMS

Nebulette is a 109 kDa modular protein localized in the sarcomeric Z-line of the heart. In vitro studies have suggested a role of nebulette in stabilizing the thin filament, and missense mutations in the nebulette gene were recently shown to be causative for dilated cardiomyopathy and endocardial fibroelastosis in human and mice. However, the role of nebulette in vivo has remained elusive. To provide insights into the function of nebulette in vivo, we generated and studied nebulette-deficient (nebl(-) (/-)) mice.

METHODS AND RESULTS

Nebl(-) (/-) mice were generated by replacement of exon 1 by Cre under the control of the endogenous nebulette promoter, allowing for lineage analysis using the ROSA26 Cre reporter strain. This revealed specific expression of nebulette in the heart, consistent with in situ hybridization results. Nebl(-) (/-) mice exhibited normal cardiac function both under basal conditions and in response to transaortic constriction as assessed by echocardiography and haemodynamic analyses. Furthermore, histological, IF, and western blot analysis showed no cardiac abnormalities in nebl(-) (/-) mice up to 8 months of age. In contrast, transmission electron microscopy showed Z-line widening starting from 5 months of age, suggesting that nebulette is important for the integrity of the Z-line. Furthermore, up-regulation of cardiac stress responsive genes suggests the presence of chronic cardiac stress in nebl(-) (/-) mice.

CONCLUSION

Nebulette is dispensable for normal cardiac function, although Z-line widening and up-regulation of cardiac stress markers were found in nebl(-) (/-) heart. These results suggest that the nebulette disease causing mutations have dominant gain-of-function effects.

Expression of myotilin during chicken development

Several missense mutations in the Z-band protein, myotilin, have been implicated in human muscle diseases such as myofibrillar myopathy, spheroid body myopathy, and distal myopathy. Recently, we have reported the cloning of chicken myotilin cDNA. In this study, we have investigated the expression of myotilin in cross-striated muscles from developing chicken by qRT-PCR and in situ hybridizations. In situ hybridization of embryonic stages shows myotilin gene expression in heart, somites, neural tissue, eyes and otocysts. RT-PCR and qRT-PCR data, together with in situ hybridization results point to a biphasic transcriptional pattern for MYOT gene during early heart development with maximum expression level in the adult. In skeletal muscle, the expression level starts decreasing after embryonic day 20 and declines in the adult skeletal muscles. Western blot assays of myotilin in adult skeletal muscle reveal a decrease in myotilin protein compared with levels in embryonic skeletal muscle. Our results suggest that MYOT gene may undergo transcriptional activation and repression that varies between tissues in developing chicken. We believe this is the first report of the developmental regulation on myotilin expression in non-mammalian species.

RNAi-mediated Gene Silencing of Mutant Myotilin Improves Myopathy in LGMD1A Mice

Recent progress suggests gene therapy may one day be an option for treating some forms of limb girdle muscular dystrophy (LGMD). Nevertheless, approaches targeting LGMD have so far focused on gene replacement strategies for recessive forms of the disease. In contrast, no attempts have been made to develop molecular therapies for any of the eight dominantly inherited forms of LGMD. Importantly, the emergence of RNA interference (RNAi) therapeutics in the last decade provided new tools to combat dominantly inherited LGMDs with molecular therapy. In this study, we describe the first RNAi-based, preclinical gene therapy approach for silencing a gene associated with dominant LGMD. To do this, we developed adeno-associated viral vectors (AAV6) carrying designed therapeutic microRNAs targeting mutant myotilin (MYOT), which is the underlying cause of LGMD type 1A (LGMD1A). Our best MYOT-targeted microRNA vector (called miMYOT) significantly reduced mutant myotilin mRNA and soluble protein expression in muscles of LGMD1A mice (the TgT57I model) both 3 and 9 months after delivery, demonstrating short- and long-term silencing effects. This MYOT gene silencing subsequently decreased deposition of MYOT-seeded intramuscular protein aggregates, which is the hallmark feature of LGMD1A. Histological improvements were accompanied by significant functional correction, as miMYOT-treated animals showed increased muscle weight and improved specific force in the gastrocnemius, which is one of the most severely affected muscles in TgT57I mice and patients with dominant myotilin mutations. These promising results in a preclinical model of LGMD1A support the further development of RNAi-based molecular therapy as a prospective treatment for LGMD1A. Furthermore, this study sets a foundation that may be refined and adapted to treat other dominant LGMD and related disorders.

The muscle ankyrin repeat proteins CARP, Ankrd2, and DARP are not essential for normal cardiac development and function at basal conditions and in response to pressure overload

Ankrd1/CARP, Ankrd2/Arpp, and Ankrd23/DARP belong to a family of stress inducible ankyrin repeat proteins expressed in striated muscle (MARPs). The MARPs are homologous in structure and localized in the nucleus where they negatively regulate gene expression as well as in the sarcomeric I-band, where they are thought to be involved in mechanosensing. Together with their strong induction during cardiac disease and the identification of causative Ankrd1 gene mutations in cardiomyopathy patients, this suggests their important roles in cardiac development, function, and disease. To determine the functional role of MARPs in vivo, we studied knockout (KO) mice of each of the three family members. Single KO mice were viable and had no apparent cardiac phenotype. We therefore hypothesized that the three highly homologous MARP proteins may have redundant functions in the heart and studied double and triple MARP KO mice. Unexpectedly, MARP triple KO mice were viable and had normal cardiac function both at basal levels and in response to mechanical pressure overload induced by transverse aortic constriction as assessed by echocardiography and hemodynamic studies. Thus, CARP, Ankrd2, and DARP are not essential for normal cardiac development and function at basal conditions and in response to mechanical pressure overload.

Limb-girdle muscular dystrophies: where next after six decades from the first proposal (Review)

Limb-girdle muscular dystrophies (LGMD) are a heterogeneous group of disorders, which has led to certain investigators disputing its rationality. The mutual feature of LGMD is limb-girdle affection. Magnetic resonance imaging (MRI), perioral skin biopsies, blood-based assays, reverse-protein arrays, proteomic analyses, gene chips and next generation sequencing are the leading diagnostic techniques for LGMD and gene, cell and pharmaceutical treatments are the mainstay therapies for these genetic disorders. Recently, more highlights have been shed on disease biomarkers to follow up disease progression and to monitor therapeutic responsiveness in future trials. In this study, we review LGMD from a variety of aspects, paying specific attention to newly evolving research, with the purpose of bringing this information into the clinical setting to aid the development of novel therapeutic strategies for this hereditary disease. In conclusion, substantial progress in our ability to diagnose and treat LGMD has been made in recent decades, however enhancing our understanding of the detailed pathophysiology of LGMD may enhance our ability to improve disease outcome in subsequent years.

The physiological role of cardiac cytoskeleton and its alterations in heart failure

Cardiac muscle cells are equipped with specialized biochemical machineries for the rapid generation of force and movement central to the work generated by the heart. During each heart beat cardiac muscle cells perceive and experience changes in length and load, which reflect one of the fundamental principles of physiology known as the Frank-Starling law of the heart. Cardiac muscle cells are unique mechanical stretch sensors that allow the heart to increase cardiac output, and adjust it to new physiological and pathological situations. In the present review we discuss the mechano-sensory role of the cytoskeletal proteins with respect to their tight interaction with the sarcolemma and extracellular matrix. The role of contractile thick and thin filament proteins, the elastic protein titin, and their anchorage at the Z-disc and M-band, with associated proteins are reviewed in physiologic and pathologic conditions leading to heart failure. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé

Analysis of myotilin turnover provides mechanistic insight into the role of myotilinopathy-causing mutations

MFM (myofibrillar myopathies) are caused by mutations in several sarcomeric components, including the Z-disc protein myotilin. The morphological changes typical of MFM include Z-disc alterations and aggregation of dense filamentous sarcomeric material. The causes and mechanisms of protein aggregation in myotilinopathies and other forms of MFM remain unknown, although impaired degradation may explain, in part, the abnormal protein accumulation. In the present paper we have studied the mechanisms regulating myotilin turnover, analysed the consequences of defective myotilin degradation and tested whether disease-causing myotilin mutations result in altered protein turnover. The results indicate that myotilin is a substrate for the Ca(2+)-dependent protease calpain and identify two calpain cleavage sites in myotilin by MS. We further show that myotilin is degraded by the proteasome system in transfected COS7 cells and in myotubes, and that disease-causing myotilinopathy mutations result in reduced degradation. Finally, we show that proteolysis-inhibitor-induced reduction in myotilin turnover results in formation of intracellular myotilin and actin-containing aggregates, which resemble those seen in diseased muscle cells. These findings identify for the first time biological differences between wt (wild-type) and mutant myotilin. The present study provides novel information on the pathways controlling myotilin turnover and on the molecular defects associated with MFM.

Myotilin dynamics in cardiac and skeletal muscle cells

Myotilin cDNA has been cloned for the first time from chicken muscles and sequenced. Ectopically expressed chicken and human YFP-myotilin fusion proteins localized in avian muscle cells in the Z-bodies of premyofibrils and the Z-bands of mature myofibrils. Fluorescence recovery after photobleaching experiments demonstrated that chicken and human myotilin were equally dynamic with 100% mobile fraction in premyofibrils and Z-bands of mature myofibrils. Seven myotilin mutants cDNAs (S55F, S55I, T57I, S60C, S60F, S95I, R405K) with known muscular dystrophy association localized in mature myofibrils in the same way as normal myotilin without affecting the formation and maintenance of myofibrils. N- and C-terminal halves of human myotilin were cloned and expressed as YFP fusions in myotubes and cardiomyocytes. N-terminal myotilin (aa 1-250) localized weakly in Z-bands with a high level of unincorporated protein and no adverse effect on myofibril structure. C-terminal myotilin (aa 251-498) localized in Z-bands and in aggregates. Formation of aggregated C-terminal myotilin was accompanied by the loss of Z-band localization of C-terminal myotilin and partial or complete loss of alpha-actinin from the Z-bands. In regions of myotubes with high concentrations of myotilin aggregates there were no alpha-actinin positive Z-bands or organized F-actin. The dynamics of the C-terminal-myotilin and N-terminal myotilin fragments differed significantly from each other and from full-length myotilin. In contrast, no significant changes in dynamics were detected after expression in myotubes of myotilin mutants with single amino acid changes known to be associated with myopathies.

Myofibrillar myopathies

Myofibrillar myopathies (MFMs) represent a group of muscular dystrophies with a similar morphological phenotype. The diagnosis is established by muscle biopsy. The MFMs are characterized by a distinct pathological pattern of myofibrillar dissolution associated with disintegration of the Z-disk, accumulation of myofibrillar degradation products, and ectopic expression of multiple proteins that include desmin, αB-crystallin, dystrophin, and sometimes congophilic material. The clinical features of MFMs are more variable. These include progressive muscle weakness that often involves or begins in distal muscles, but limb-girdle or scapuloperoneal distributions can also occur. Cardiomyopathy and peripheral neuropathy are frequent associated features. Electromyography of the affected muscles reveals myopathic motor unit potentials and abnormal irritability, often with myotonic discharges. Rarely, neurogenic motor unit potentials or slowing of nerve conduction velocities are present. To date, all MFM mutations have appeared in Z-disk-associated proteins: namely, desmin, αB-crystallin, myotilin, ZASP, filamin C, and Bag3. However, in the majority of patients with MFM, the disease gene awaits discovery.

Loss of enigma homolog protein results in dilated cardiomyopathy

RATIONALE

The Z-line, alternatively termed the Z-band or Z-disc, is a highly ordered structure at the border between 2 sarcomeres. Enigma subfamily proteins (Enigma, Enigma homolog protein, and Cypher) of the PDZ-LIM domain protein family are Z-line proteins. Among the Enigma subfamily, Cypher has been demonstrated to play a pivotal role in the structure and function of striated muscle, whereas the role of Enigma homolog protein (ENH) in muscle remains largely unknown.

OBJECTIVE

We studied the role of Enigma homolog protein in the heart using global and cardiac-specific ENH knockout mouse models.

METHODS AND RESULTS

We identified new exons and splice isoforms for ENH in the mouse heart. Impaired cardiac contraction and dilated cardiomyopathy were observed in ENH null mice. Mice with cardiac specific ENH deletion developed a similar dilated cardiomyopathy. Like Cypher, ENH interacted with Calsarcin-1, another Z-line protein. Moreover, biochemical studies showed that ENH, Cypher short isoform and Calsarcin-1 are within the same protein complex at the Z-line. Cypher short isoform and Calsarcin-1 proteins are specifically downregulated in ENH null hearts.

CONCLUSIONS

We have identified an ENH-CypherS-Calsarcin protein complex at the Z-line. Ablation of ENH leads to destabilization of this protein complex and dilated cardiomyopathy.

Are biological sensors modulated by their structural scaffolds? The role of the structural muscle proteins alpha-actinin-2 and alpha-actinin-3 as modulators of biological sensors

Biological sensors and their ability to detect and respond to change in the cellular environment can be modulated by protein scaffolds acting within their interaction network. The skeletal muscle alpha-actinins have been considered as primarily structural scaffold proteins. However, deficiency of alpha-actinin-3 due to a common null polymorphism results in predominantly metabolic changes in skeletal muscle function. In this review, we explore the range of phenotypes associated with alpha-actinin-3 deficiency, and draw supporting evidence from known interaction partners for its role as a scaffold which acts to modulate biological sensors that result in changes in muscle metabolism and structure.

Protective action of tetramethylpyrazine phosphate against dilated cardiomyopathy in cTnT(R141W) transgenic mice

AIM

Dilated cardiomyopathy (DCM) is the most common cause of heart failure, and pharmacological intervention is not currently available. Here we investigate the effect of tetramethylpyrazine phosphate (TMPP) on the progression of DCM in the cTnT(R141W) transgenic mouse model.

METHODS

The cTnT(R141W) transgenic mice aged 2 months were divided into model group and TMPP group, whereas age-matched nontransgenic mice were used as wild-type control. TMPP 45 mg.kg(-1).d(-1) was administered for 7 months. Following assessment of cardiac function by echocardiography, cardiac tissues were prepared for histology and electron microscopy. Levels of molecular markers for cardiomyocyte hypertrophy and fibrosis were detected by RT-PCR. Expression of structural proteins of the sarcomere and intercalated disc was determined by Western blot.

RESULTS

TMPP significantly prevented cardiac dilatation and dysfunction with the development of DCM, and decreased mortality by 54%. TMPP decreased HW/BW ratios and expression of hypertrophic markers BNP and ACTA1, as well as reduced interstitial collagen deposition and expression of profibrotic markers Col1a1 and Col3a1. TMPP attenuated ultrastructural disruption caused by cTnT(R141W) expression and decreased expression of structural proteins myotilin and E-cadherin which were up-regulated in the cTnT(R141W) heart. Moreover, TMPP reduced the mRNA expression of Calm1 and Camk2b in the cTnT(R141W) heart.

CONCLUSION

Our results suggest that TMPP could be a promising drug for prevention and treatment of DCM.

Neuromuscular disease models and analysis

Neuromuscular diseases can affect the survival of peripheral neurons, their axons extending to peripheral targets, their synaptic connections onto those targets, or the targets themselves. Examples include motor neuron diseases such as amyotrophic lateral sclerosis, peripheral neuropathies, such as Charcot-Marie-Tooth diseases, myasthenias, and muscular dystrophies. Characterizing these phenotypes in mouse models requires an integrated approach, examining both the nerve and the muscle histologically, anatomically, and functionally by electrophysiology. Defects observed at these levels can be related back to onset, severity, and progression, as assessed by "quality-of-life measures" including tests of gross motor performance such as gait or grip strength. This chapter describes methods for assessing neuromuscular disease models in mice, and how interpretation of these tests can be complicated by the inter-relatedness of the phenotypes.

Genetic defects in muscular dystrophy

The muscular dystrophies are a group of neuromuscular disorders associated with muscle weakness and wasting, which in many forms can lead to loss of ambulation and premature death. A number of muscular dystrophies are associated with loss of proteins required for the maintenance of muscle membrane integrity, in particular with proteins that comprise the dystrophin-associated glycoprotein (DAG) complex. Proper glycosylation of O-linked mannose chains on alpha-dystroglycan, a DAG member, is required for the binding of the extracellular matrix to dystroglycan and for proper DAG function. A number of congenital disorders of glycosylation have now been described where alpha-dystroglycan glycosylation is altered and where muscular dystrophy is a predominant phenotype. Glycosylation is also increasingly being appreciated as a genetic modifier of disease phenotypes in many forms of muscular dystrophy and as a target for the development of new therapies. Here we will review the mouse models available for the study of this group of diseases and outline the methodologies required to describe disease phenotypes.

Cytoplasmic Ig-domain proteins: cytoskeletal regulators with a role in human disease

Immunoglobulin domains are found in a wide variety of functionally diverse transmembrane proteins, and also in a smaller number of cytoplasmic proteins. Members of this latter group are usually associated with the actin cytoskeleton, and most of them bind directly to either actin or myosin, or both. Recently, studies of inherited human disorders have identified disease-causing mutations in five cytoplasmic Ig-domain proteins: myosin-binding protein C, titin, myotilin, palladin, and myopalladin. Together with results obtained from cultured cells and mouse models, these clinical studies have yielded novel insights into the unexpected roles of Ig domain proteins in mechanotransduction and signaling to the nucleus. An emerging theme in this field is that cytoskeleton-associated Ig domain proteins are more than structural elements of the cell, and may have evolved to fill different needs in different cellular compartments. Cell Motil. Cytoskeleton 2009. (c) 2009 Wiley-Liss, Inc.

Solution structure of the first immunoglobulin domain of human myotilin

Myotilin is a 57 kDa actin-binding and -bundling protein that consists of a unique serine-rich amino-terminus, two Ig-domains and a short carboxy-terminus with a PDZ-binding motif. Myotilin localizes in sarcomeric Z-discs, where it interacts with several sarcomeric proteins. Point mutations in myotilin cause muscle disorders morphologically highlighted by sarcomeric disarray and aggregation. The actin-binding and dimerization propensity of myotilin has been mapped to the Ig-domains. Here we present high-resolution structure of the first Ig-domain of myotilin (MyoIg1) determined with solution state NMR spectroscopy. Nearly complete chemical shift assignments of MyoIg1 were achieved despite several missing backbone 1H-15N-HSQC signals. The structure derived from distance and dihedral angle restraints using torsion angle dynamics was further refined using molecular dynamics. The structure of MyoIg1 exhibits I-type Ig-fold. The absence of several backbone 1H-15N-HSQC signals can be explained by conformational exchange taking place at the hydrophobic core of the protein.

Maintenance of muscle mass, fiber size, and contractile function in mice lacking the Z-disc protein myotilin

BACKGROUND

Myofibrillar myopathies constitute a rare group of congenital neuromuscular disorders, frequently associated with mutations in Z-disc proteins such as myotilin. Myotilin location and interactions with other Z-disc proteins are clearly defined, but its role in the regulation of muscle structure and function remains unknown. The present study aims at investigating this specific role of myotilin.

METHODS

Skeletal and cardiac muscles were collected from adult mice with a targeted deletion of myotilin (myo(-/-)) and wild-type animals (myo(+/+)).

RESULTS AND CONCLUSION

Similar skeletal and cardiac muscle weights were observed in myo(-/-) and myo(+/+) mice. At the muscle cell level, the size and force production of single membrane permeabilized fibers were identical between myo(-/-) and myo(+/+) rodents. Thus, myotilin does not have a significant influence on muscle mass, muscle fiber size, or regulation of muscle contraction. Alternatively, compensatory over-expressions of other elements including proteins from the same subfamily, or Z-disc proteins such as telethonin, or intermediate filaments may compensate for the lack of myotilin.

A class III PDZ binding motif in the myotilin and FATZ families binds enigma family proteins: a common link for Z-disc myopathies

Interactions between Z-disc proteins regulate muscle functions and disruption of these interactions results in muscle disorders. Mutations in Z-disc components myotilin, ZASP/Cypher, and FATZ-2 (calsarcin-1/myozenin-2) are associated with myopathies. We report here that the myotilin and the FATZ (calsarcin/myozenin) families share high homology at their final C-terminal five amino acids. This C-terminal E[ST][DE][DE]L motif is present almost exclusively in these families and is evolutionary conserved. We show by in vitro and in vivo studies that proteins from the myotilin and FATZ (calsarcin/myozenin) families interact via this novel type of class III PDZ binding motif with the PDZ domains of ZASP/Cypher and other Enigma family members: ALP, CLP-36, and RIL. We show that the interactions can be modulated by phosphorylation. Calmodulin-dependent kinase II phosphorylates the C terminus of FATZ-3 (calsarcin-3/myozenin-3) and myotilin, whereas PKA phosphorylates that of FATZ-1 (calsarcin-2/myozenin-1) and FATZ-2 (calsarcin-1/myozenin-1). This is the first report of a binding motif common to both the myotilin and the FATZ (calsarcin/myozenin) families that is specific for interactions with Enigma family members.

Cardiac-specific ablation of Cypher leads to a severe form of dilated cardiomyopathy with premature death

Accumulating data suggest a link between alterations/deficiencies in cytoskeletal proteins and the progression of cardiomyopathy and heart failure, although the molecular basis for this link remains unclear. Cypher/ZASP is a cytoskeletal protein localized in the sarcomeric Z-line. Mutations in its encoding gene have been identified in patients with isolated non-compaction of the left ventricular myocardium, dilated cardiomyopathy (DCM) and hypertrophic cardiomyopathy. To explore the role of Cypher in myocardium and to better understand molecular mechanisms by which mutations in cypher cause cardiomyopathy, we utilized a conditional approach to knockout Cypher, specially in either developing or adult myocardium. Cardiac-specific Cypher knockout (CKO) mice developed a severe form of DCM with disrupted cardiomyocyte ultrastructure and decreased cardiac function, which eventually led to death before 23 weeks of age. A similar phenotype was observed in inducible cardiac-specific CKO mice in which Cypher was specifically ablated in adult myocardium. In both cardiac-specific CKO models, ERK and Stat3 signaling pathways were augmented. Finally, we demonstrate the specific binding of Cypher's PDZ domain to the C-terminal region of both calsarcin-1 and myotilin within the Z-line. In conclusion, our studies suggest that (i) Cypher plays a pivotal role in maintaining adult cardiac structure and cardiac function through protein-protein interactions with other Z-line proteins, (ii) myocardial ablation of Cypher results in DCM with premature death and (iii) specific signaling pathways participate in Cypher mutant-mediated dysfunction of the heart, and may in concert facilitate the progression to heart failure.

Myofibrillar myopathies

PURPOSE OF REVIEW

The aim of this communication is to provide an up-to-date overview of myofibrillar myopathies.

RECENT FINDINGS

The most important recent advance in the myofibrillar myopathies has been the discovery that mutations in Z band alternatively spliced PDZ-containing protein and filamin C, as well as in desmin, alphaB-crystallin and myotilin, result in similar pathologic alterations in skeletal muscle that are typical of myofibrillar myopathy. Despite the increasing genetic heterogeneity, the clinical and morphologic phenotypes are remarkably homogeneous. The typical clinical manifestation is slowly progressive proximal, distal or both proximal and distal limb muscle weakness. Cardiomyopathy can be associated and is sometimes the presenting finding. Peripheral neuropathy also occurs in some patients. In every myofibrillar myopathy, there is abnormal accumulation of an array of proteins at ectopic sites as well as accumulation of degraded myofibrillar proteins forming large aggregates. The key issue now is to analyze the molecular mechanisms underlying the cascade of events that destroy the myofibrillar architecture and trigger the aberrant expression of multiple proteins.

SUMMARY

Several disease genes have recently been recognized in myofibrillar myopathies. So far, the disease proteins identified are components of or chaperone for the Z-disk. In each case, the molecular defect leads to a stereotyped cascade of structural events in the muscle fiber.

Molecular pathology of myofibrillar myopathies

Myofibrillar myopathies (MFMs) are clinically and genetically heterogeneous muscle disorders that are defined morphologically by the presence of foci of myofibril dissolution, accumulation of myofibrillar degradation products, and ectopic expression of multiple proteins. MFMs are the paradigm of conformational protein diseases of the skeletal (and cardiac) muscles characterised by intracellular protein accumulation in muscle cells. Understanding of this group of disorders has advanced in recent years through the identification of causative mutations in various genes, most of which encode proteins of the sarcomeric Z-disc, including desmin, alphaB-crystallin, myotilin, ZASP and filamin C. This review focuses on the MFMs arising from defects in these proteins, summarising genetic and clinical features of the disorders and then discussing emerging understanding of the molecular pathogenic mechanisms leading to muscle fibre degeneration. Defective extralysosomal degradation of proteins is now recognised as an important element in this process. Several factors--including mutant proteins, a defective ubiquitin-proteasome system, aggresome formation, mutant ubiquitin, p62, oxidative stress and abnormal regulation of some transcription factors--are thought to participate in the cascade of events occurring in muscle fibres in MFMs.

A broken heart: a stretch too far: an overview of mouse models with mutations in stretch-sensor components

With every heartbeat the heart must contract and relax. This seemingly trivial process critically needs tight control of contraction and relaxation phases, and extremely efficient coordination between these two phases to control blood flow and maintain cardiac homeostasis. To achieve this, specialized sensors are required to detect the inherent repeatedly changing environment and needs. One sensor is a stretch-sensor that monitors the filling of the ventricles. Its molecular identity and localization are only partly understood. Here we give a synopsis of the genetic models that leap into our understanding of stretch-sensors. We focus on the widely acknowledged sarcomeric sensor at the Z-disc and the costamere sensor at the sarcolemma. Recently, several novel components of both sensors were discovered. Given that these two sensors seem physically connected, it is likely that these two models are not mutually exclusive and might even communicate. We describe briefly how candidate and known proteins within these sensors receive and transduce mechanical signals in the cardiomyocyte that lead to changes in gene expression underlying homeostasis and its restoration in the heart. Emphasis is placed on the putative link between altered stretch-sensor function and heart failure observed in different genetic mouse models of stretch-sensor components.

Myotilin overexpression enhances myopathology in the LGMD1A mouse model

Missense mutations in the myotilin gene cause limb-girdle muscular dystrophy type 1A (LGMD1A). We set out to examine the effect of overexpression of wild-type myotilin in an LGMD1A mouse model by crossing wild-type and mutant transgenic mice. Compared to single-transgenic mutant mice, double-transgenic mice overexpressing myotilin showed more severe muscle degeneration, enhanced myofibrillar aggregation, and earlier onset of aggregation. These data suggest that strategies aimed at lowering total myotilin levels in LGMD1A patients may be an effective therapeutic approach.

The sarcomere and sarcomerogenesis

Striated muscle owes its name to the microscopic appearance, caused by the longitudinal alignment of thousands of highly ordered contractile units, the sarcomeres. The assembly (and disassembly) of these multiprotein complexes (sarcomere assembly or sarcomerogenesis) follows ordered pathways, which are regulated on the transcriptional, translational and posttranslational level. Furthermore, myofibril assembly involves the participation of transient scaffolds and adaptors, notably the microtubule network. Studies in cell culture and developing embryos have revealed common pathways of sarcomere assembly in heart and skeletal muscle. Disruptions in these pathways are implicated in muscle diseases.

The Z-disk diseases

Recent studies have identified disease-causing mutations in four genes that encode Z-disk proteins. Mutations in myotilin (MYOT), ZASP and filamin C (FLNC) encoding genes cause autosomal dominant myopathy that manifests in adulthood. The clinical features and morphological changes in myopathies caused by mutations in all three genes are highly similar. The disease typically manifests as distal myopathy, but may also affect proximal muscles and the heart. The morphological findings are typical of myofibrillar myopathy (MFM) and include Z-disk alterations and aggregation of dense filamentous material visible in trichrome staining. The disease mechanism is still unclear, but may involve structural alterations of the Z-disk caused by dysfunctional proteins or their abnormal accumulation due to defective degradation. Although the fourth gene product, telethonin, is also involved in the Z-disk organization, its mutations cause a different phenotype. Telethonin mutations result in recessive muscular dystrophy, which manifests in childhood as proximal weakness. The morphologic alterations caused by telethonin mutations are not well characterized, but may share common features of MFM. Future work aims at understanding the pathophysiology of Z-disk related disorders and identification of novel genetic defects in patients with morphological features of MFM.

UNC-87, a calponin-related protein in C. elegans, antagonizes ADF/cofilin-mediated actin filament dynamics

Stabilization of actin filaments is critical for supporting actomyosin-based contractility and for maintaining stable cellular structures. Tropomyosin is a well-characterized ubiquitous actin stabilizer that inhibits ADF/cofilin-dependent actin depolymerization. Here, we show that UNC-87, a calponin-related Caenorhabditis elegans protein with seven calponin-like repeats, competes with ADF/cofilin for binding to actin filaments and inhibits ADF/cofilin-dependent filament severing and depolymerization in vitro. Mutations in the unc-87 gene suppress the disorganized actin phenotype in an ADF/cofilin mutant in the C. elegans body wall muscle, supporting their antagonistic roles in regulating actin stability in vivo. UNC-87 and tropomyosin exhibit synergistic effects in stabilizing actin filaments against ADF/cofilin, and direct comparison reveals that UNC-87 effectively stabilizes actin filaments at much lower concentrations than tropomyosin. However, the in vivo functions of UNC-87 and tropomyosin appear different, suggesting their distinct roles in the regulation of actomyosin assembly and cellular contractility. Our results demonstrate that actin binding via calponin-like repeats competes with ADF/cofilin-driven cytoskeletal turnover, and is critical for providing the spatiotemporal regulation of actin filament stability.