Pharmacological inhibition of c-Jun N-terminal kinase signaling prevents cardiomyopathy caused by mutation in LMNA gene

Targeting MAPK-ERK/JNK pathway: A potential intervention mechanism of myocardial fibrosis in heart failure

Myocardial fibrosis is a significant pathological basis of heart failure. Overactivation of the ERK1/2 and JNK1/2 signaling pathways of MAPK family members synergistically promotes the proliferation of myocardial fibroblasts and accelerates the development of myocardial fibrosis. In addition to some small molecule inhibitors and Western drugs, many Chinese medicines can also inhibit the activity of ERK1/2 and JNK1/2, thus slowing down the development of myocardial fibrosis, and are generally safe and effective. However, the specific biological mechanisms of ERK1/2 and JNK1/2 signaling pathways in myocardial fibrosis still need to be fully understood, and there is no systematic review of existing drugs and methods to inhibit them from improving myocardial fibrosis. This study aims to summarize the roles and cross-linking mechanisms of ERK1/2 and JNK1/2 signaling pathways in myocardial fibrosis and to systematically sort out the small-molecule inhibitors, Western drugs, traditional Chinese medicines, and non-pharmacological therapies that inhibit ERK1/2 and JNK1/2 to alleviate myocardial fibrosis. In the future, we hope to conduct more in-depth research from the perspective of precision-targeted therapy, using this as a basis for developing new drugs that provide new perspectives on the prevention and treatment of heart failure.

Emerin deficiency does not exacerbate cardiomyopathy in a murine model of Emery-Dreifuss muscular dystrophy caused by an LMNA gene mutation

Emery-Dreifuss muscular dystrophy (EDMD), caused by mutations in genes encoding nuclear envelope proteins, is clinically characterized by muscular dystrophy, early joint contracture, and life-threatening cardiac abnormalities. To elucidate the pathophysiological mechanisms underlying striated muscle involvement in EDMD, we previously established a murine model with mutations in Emd and Lmna (Emd-/-/LmnaH222P/H222P; EH), and reported exacerbated skeletal muscle phenotypes and no notable cardiac phenotypes at 12 weeks of age. We predicted that lack of emerin in LmnaH222P/H222P mice causes an earlier onset and more pronounced cardiac dysfunction at later stages. In this study, cardiac abnormalities of EDMD mice were compared at 18 and 30 weeks of age. Contrary to our expectations, physiological and histological analyses indicated that emerin deficiency causes no prominent differences of cardiac involvement in LmnaH222P/H222P mice. These results suggest that emerin does not contribute to cardiomyopathy progression in LmnaH222P/H222P mice.

Epigenetics in LMNA-Related Cardiomyopathy

Mutations in the gene for lamin A/C (LMNA) cause a diverse range of diseases known as laminopathies. LMNA-related cardiomyopathy is a common inherited heart disease and is highly penetrant with a poor prognosis. In the past years, numerous investigations using mouse models, stem cell technologies, and patient samples have characterized the phenotypic diversity caused by specific LMNA variants and contributed to understanding the molecular mechanisms underlying the pathogenesis of heart disease. As a component of the nuclear envelope, LMNA regulates nuclear mechanostability and function, chromatin organization, and gene transcription. This review will focus on the different cardiomyopathies caused by LMNA mutations, address the role of LMNA in chromatin organization and gene regulation, and discuss how these processes go awry in heart disease.

Preclinical Advances of Therapies for Laminopathies

Laminopathies are a group of rare disorders due to mutation in LMNA gene. Depending on the mutation, they may affect striated muscles, adipose tissues, nerves or are multisystemic with various accelerated ageing syndromes. Although the diverse pathomechanisms responsible for laminopathies are not fully understood, several therapeutic approaches have been evaluated in patient cells or animal models, ranging from gene therapies to cell and drug therapies. This review is focused on these therapies with a strong focus on striated muscle laminopathies and premature ageing syndromes.

A CRISPR/Cas9 zebrafish lamin A/C mutant model of muscular laminopathy

BACKGROUND

Lamin A/C gene (LMNA) mutations frequently cause cardiac and/or skeletal muscle diseases called striated muscle laminopathies. We created a zebrafish muscular laminopathy model using CRISPR/Cas9 technology to target the zebrafish lmna gene.

RESULTS

Heterozygous and homozygous lmna mutants present skeletal muscle damage at 1 day post-fertilization (dpf), and mobility impairment at 4 to 7 dpf. Cardiac structure and function analyses between 1 and 7 dpf show mild and transient defects in the lmna mutants compared to wild type (WT). Quantitative RT-PCR analysis of genes implicated in striated muscle laminopathies show a decrease in jun and nfκb2 expression in 7 dpf homozygous lmna mutants compared to WT. Homozygous lmna mutants have a 1.26-fold protein increase in activated Erk 1/2, kinases associated with striated muscle laminopathies, compared to WT at 7 dpf. Activated Protein Kinase C alpha (Pkc α), a kinase that interacts with lamin A/C and Erk 1/2, is also upregulated in 7 dpf homozygous lmna mutants compared to WT.

CONCLUSIONS

This study presents an animal model of skeletal muscle laminopathy where heterozygous and homozygous lmna mutants exhibit prominent skeletal muscle abnormalities during the first week of development. Furthermore, this is the first animal model that potentially implicates Pkc α in muscular laminopathies.

THE NUCLEUS: Mechanosensing in cardiac disease

The nucleus provides a physical and selective chemical boundary to segregate the genome from the cytoplasm. The contents of the nucleus are surrounded by the nuclear envelope, which acts as a hub of mechanosensation, transducing forces from the external cytoskeleton to the nucleus, thus impacting on nuclear morphology, genome organisation, gene transcription and signalling pathways. Muscle tissues such as the heart are unique in that they actively generate large contractile forces, resulting in a distinctive mechanical environment which impacts nuclear properties, function and mechanosensing. In light of this, mutations that affect the function of the nuclear envelope (collectively known as nuclear envelopathies and laminopathies) disproportionately result in striated muscle diseases, which include dilated and arrhythmogenic cardiomyopathies. Here we review the nucleus and its role in mechanotransduction, as well as associated defects that lead to cardiac dysfunction.

Role of Cdkn2a in the Emery-Dreifuss Muscular Dystrophy Cardiac Phenotype

The Cdkn2a locus is one of the most studied tumor suppressor loci in the context of several cancer types. However, in the last years, its expression has also been linked to terminal differentiation and the activation of the senescence program in different cellular subtypes. Knock-out (KO) of the entire locus enhances the capability of stem cells to proliferate in some tissues and respond to severe physiological and non-physiological damages in different organs, including the heart. Emery-Dreifuss muscular dystrophy (EDMD) is characterized by severe contractures and muscle loss at the level of skeletal muscles of the elbows, ankles and neck, and by dilated cardiomyopathy. We have recently demonstrated, using the LMNA Δ8-11 murine model of Emery-Dreifuss muscular dystrophy (EDMD), that dystrophic muscle stem cells prematurely express non-lineage-specific genes early on during postnatal growth, leading to rapid exhaustion of the muscle stem cell pool. Knock-out of the Cdkn2a locus in EDMD dystrophic mice partially restores muscle stem cell properties. In the present study, we describe the cardiac phenotype of the LMNA Δ8-11 mouse model and functionally characterize the effects of KO of the Cdkn2a locus on heart functions and life expectancy.

Computational model predicts paracrine and intracellular drivers of fibroblast phenotype after myocardial infarction

The fibroblast is a key mediator of wound healing in the heart and other organs, yet how it integrates multiple time-dependent paracrine signals to control extracellular matrix synthesis has been difficult to study in vivo. Here, we extended a computational model to simulate the dynamics of fibroblast signaling and fibrosis after myocardial infarction (MI) in response to time-dependent data for nine paracrine stimuli. This computational model was validated against dynamic collagen expression and collagen area fraction data from post-infarction rat hearts. The model predicted that while many features of the fibroblast phenotype at inflammatory or maturation phases of healing could be recapitulated by single static paracrine stimuli (interleukin-1 and angiotensin-II, respectively), mimicking the reparative phase required paired stimuli (e.g. TGFβ and endothelin-1). Virtual overexpression screens simulated with either static cytokine pairs or post-MI paracrine dynamic predicted phase-specific regulators of collagen expression. Several regulators increased (Smad3) or decreased (Smad7, protein kinase G) collagen expression specifically in the reparative phase. NADPH oxidase (NOX) overexpression sustained collagen expression from reparative to maturation phases, driven by TGFβ and endothelin positive feedback loops. Interleukin-1 overexpression had mixed effects, both enhancing collagen via the TGFβ positive feedback loop and suppressing collagen via NFκB and BAMBI (BMP and activin membrane-bound inhibitor) incoherent feed-forward loops. These model-based predictions reveal network mechanisms by which the dynamics of paracrine stimuli and interacting signaling pathways drive the progression of fibroblast phenotypes and fibrosis after myocardial infarction.

Genotype-phenotype analysis of LMNA-related diseases predicts phenotype-selective alterations in lamin phosphorylation

Laminopathies are rare diseases associated with mutations in LMNA, which encodes nuclear lamin A/C. LMNA variants lead to diverse tissue-specific phenotypes including cardiomyopathy, lipodystrophy, myopathy, neuropathy, progeria, bone/skin disorders, and overlap syndromes. The mechanisms underlying these heterogeneous phenotypes remain poorly understood, although post-translational modifications, including phosphorylation, are postulated as regulators of lamin function. We catalogued all known lamin A/C human mutations and their associated phenotypes, and systematically examined the putative role of phosphorylation in laminopathies. In silico prediction of specific LMNA mutant-driven changes to lamin A phosphorylation and protein structure was performed using machine learning methods. Some of the predictions we generated were validated via assessment of ectopically expressed wild-type and mutant LMNA. Our findings indicate phenotype- and mutant-specific alterations in lamin phosphorylation, and that some changes in phosphorylation may occur independently of predicted changes in lamin protein structure. Therefore, therapeutic targeting of phosphorylation in the context of laminopathies will likely require mutant- and kinase-specific approaches.

Molecular dissection of Chagas induced cardiomyopathy reveals central disease associated and druggable signaling pathways

Chagas disease, the clinical presentation of T. cruzi infection, is a major human health concern. While the acute phase of Chagas disease is typically asymptomatic and self-resolving, chronically infected individuals suffer numerous sequelae later in life. Cardiomyopathies in particular are the most severe consequence of chronic Chagas disease and cannot be reversed solely by parasite load reduction. To prioritize new therapeutic targets, we unbiasedly interrogated the host signaling events in heart tissues isolated from a Chagas disease mouse model using quantitative, multiplexed proteomics. We defined the host response to infection at both the proteome and phospho-proteome levels. The proteome showed an increase in the immune response and a strong repression of several mitochondrial proteins. Complementing the proteome studies, the phospho-proteomic survey found an abundance of phospho-site alterations in plasma membrane and cytoskeletal proteins. Bioinformatic analysis of kinase activity provided substantial evidence for the activation of NDRG2 and JNK/p38 kinases during Chagas disease. A significant activation of DYRK2 and AMPKA2 and the inhibition of casein family kinases were also predicted. We concluded our analyses by linking the diseased heart proteome profile to known therapeutic interventions, uncovering a potential to target mitochondrial proteins, secreted immune effectors and core kinases for the treatment of chronic Chagas disease. Together, this study provides molecular insight into host proteome and phospho-proteome responses to T. cruzi infection in the heart for the first time, highlighting pathways that can be further validated for functional contributions to disease and suitability as drug targets.

Mechanotransduction, nuclear architecture and epigenetics in Emery Dreifuss Muscular Dystrophy: tous pour un, un pour tous

The alteration of the several roles that Lamin A/C plays in the mammalian cell leads to a broad spectrum of pathologies that – all together – are named laminopathies. Among those, the Emery Dreifuss Muscular Dystrophy (EDMD) is of particular interest as, despite the several known mutations of Lamin A/C, the genotype–phenotype correlation still remains poorly understood; this suggests that the epigenetic background of patients might play an important role during the time course of the disease. Historically, both a mechanical role of Lamin A/C and a regulative one have been suggested as the driving force of laminopathies; however, those two hypotheses are not mutually exclusive. Recent scientific evidence shows that Lamin A/C sustains the correct gene expression at the epigenetic level thanks to the Lamina Associated Domains (LADs) reorganization and the crosstalk with the Polycomb Group of Proteins (PcG). Furthermore, the PcG-dependent histone mark H3K27me3 increases under mechanical stress, finally pointing out the link between the mechano-properties of the nuclear lamina and epigenetics. Here, we summarize the emerging mechanisms that could explain the high variability seen in Emery Dreifuss muscular dystrophy.

Deficiency of emerin contributes differently to the pathogenesis of skeletal and cardiac muscles in LmnaH222P/H222P mutant mice

Laminopathies are tissue-selective diseases that affect differently in organ systems. Mutations in nuclear envelopes, emerin (Emd) and lamin A/C (Lmna) genes, cause clinically indistinguishable myopathy called Emery-Dreifuss muscular dystrophy (EDMD) and limb-girdle muscular dystrophy. Several murine models for EDMD have been generated; however, emerin-null (Emd) mice do not show obvious skeletal and cardiac muscle phenotypes, and Lmna H222P/H222P mutant (H222P) mice show only a mild phenotype in skeletal muscle when they already have severe cardiomyopathy. Thus, the underlying molecular mechanism of muscle involvement due to nuclear abnormalities is still unclarified. We generated double mutant (Emd-/-/LmnaH222P/H222P; EH) mice to characterize dystrophic changes and to elucidate interactions between emerin and lamin A/C in skeletal and cardiac muscles. As H222P mice, EH mice grow normally and have breeding productivity. EH mice showed severer muscle involvement compared with that of H222P mice which was an independent of cardiac abnormality at 12 weeks of age. Nuclear abnormalities, reduced muscle fiber size and increased fibrosis were prominent in EH mice. Roles of emerin and lamin A/C in satellite cells function and regeneration of muscle fiber were also evaluated by cardiotoxin-induced muscle injury. Delayed increases in myog and myh3 expression were seen in both H222P and EH mice; however, the expression levels of those genes were similar with control and regenerated muscle fiber size was not different at day 7 after injury. These results indicate that EH mouse is a suitable model for studying skeletal muscle involvement, independent of cardiac function, in laminopathies and an interaction between emerin and lamin A/C in different tissues.

Cellular and Animal Models of Striated Muscle Laminopathies

The lamin A/C (LMNA) gene codes for nuclear intermediate filaments constitutive of the nuclear lamina. LMNA has 12 exons and alternative splicing of exon 10 results in two major isoforms—lamins A and C. Mutations found throughout the LMNA gene cause a group of diseases collectively known as laminopathies, of which the type, diversity, penetrance and severity of phenotypes can vary from one individual to the other, even between individuals carrying the same mutation. The majority of the laminopathies affect cardiac and/or skeletal muscles. The underlying molecular mechanisms contributing to such tissue-specific phenotypes caused by mutations in a ubiquitously expressed gene are not yet well elucidated. This review will explore the different phenotypes observed in established models of striated muscle laminopathies and their respective contributions to advancing our understanding of cardiac and skeletal muscle-related laminopathies. Potential future directions for developing effective treatments for patients with lamin A/C mutation-associated cardiac and/or skeletal muscle conditions will be discussed.

Targeting MRTF/SRF in CAP2-dependent dilated cardiomyopathy delays disease onset

About one-third of dilated cardiomyopathy (DCM) cases are caused by mutations in sarcomere or cytoskeletal proteins. However, treating the cytoskeleton directly is not possible because drugs that bind to actin are not well tolerated. Mutations in the actin binding protein CAP2 can cause DCM and KO mice, either whole body (CAP2-KO) or cardiomyocyte-specific KOs (CAP2-CKO) develop DCM with cardiac conduction disease. RNA sequencing analysis of CAP2-KO hearts and isolated cardiomyocytes revealed overactivation of fetal genes, including serum response factor-regulated (SRF-regulated) genes such as Myl9 and Acta2 prior to the emergence of cardiac disease. To test if we could treat CAP2-KO mice, we synthesized and tested the SRF inhibitor CCG-1423-8u. CCG-1423-8u reduced expression of the SRF targets Myl9 and Acta2, as well as the biomarker of heart failure, Nppa. The median survival of CAP2-CKO mice was 98 days, while CCG-1423-8u-treated CKO mice survived for 116 days and also maintained normal cardiac function longer. These results suggest that some forms of sudden cardiac death and cardiac conduction disease are under cytoskeletal stress and that inhibiting signaling through SRF may benefit DCM by reducing cytoskeletal stress.

Hutchinson-Gilford Progeria Syndrome-Current Status and Prospects for Gene Therapy Treatment

Hutchinson-Gilford progeria syndrome (HGPS) is one of the most severe disorders among laminopathies—a heterogeneous group of genetic diseases with a molecular background based on mutations in the LMNA gene and genes coding for interacting proteins. HGPS is characterized by the presence of aging-associated symptoms, including lack of subcutaneous fat, alopecia, swollen veins, growth retardation, age spots, joint contractures, osteoporosis, cardiovascular pathology, and death due to heart attacks and strokes in childhood. LMNA codes for two major, alternatively spliced transcripts, give rise to lamin A and lamin C proteins. Mutations in the LMNA gene alone, depending on the nature and location, may result in the expression of abnormal protein or loss of protein expression and cause at least 11 disease phenotypes, differing in severity and affected tissue. LMNA gene-related HGPS is caused by a single mutation in the LMNA gene in exon 11. The mutation c.1824C > T results in activation of the cryptic donor splice site, which leads to the synthesis of progerin protein lacking 50 amino acids. The accumulation of progerin is the reason for appearance of the phenotype. In this review, we discuss current knowledge on the molecular mechanisms underlying the development of HGPS and provide a critical analysis of current research trends in this field. We also discuss the mouse models available so far, the current status of treatment of the disease, and future prospects for the development of efficient therapies, including gene therapy for HGPS.

The Pathogenesis and Therapies of Striated Muscle Laminopathies

Emery-Dreifuss muscular dystrophy (EDMD) is a genetic condition characterized by early contractures, skeletal muscle weakness, and cardiomyopathy. During the last 20 years, various genetic approaches led to the identification of causal genes of EDMD and related disorders, all encoding nuclear envelope proteins. By their respective localization either at the inner nuclear membrane or the outer nuclear membrane, these proteins interact with each other and establish a connection between the nucleus and the cytoskeleton. Beside this physical link, these proteins are also involved in mechanotransduction, responding to environmental cues, such as increased tension of the cytoskeleton, by the activation or repression of specific sets of genes. This ability of cells to adapt to environmental conditions is altered in EDMD. Increased knowledge on the pathophysiology of EDMD has led to the development of drug or gene therapies that have been tested on mouse models. This review proposed an overview of the functions played by the different proteins involved in EDMD and related disorders and the current therapeutic approaches tested so far.

Genetic mutation of familial dilated cardiomyopathy based on next‑generation semiconductor sequencing

Dilated cardiomyopathy (DCM) is a complex myocardial disease of multifactorial etiologies, including enlarged cardiac chambers and contractile dysfunction. It has been suggested that the inheritance of DCM‑associated mutations predominates its onset. Therefore, the present study investigated the pathogenesis of DCM via pedigree analysis and genetic diagnosis by massive whole‑exome screening, and targeted exon capture. To study the familial gene‑phenotype association, the exon and splice sites of 325 hereditary disease‑associated genes in the proband with familial dilated cardiomyopathy (FDC), including 61 cardiac disease‑associated genes, such as the lamins A/C (LMNA), were analyzed by ultra‑high multiplex polymerase chain reaction and the Ion AmpliSeq™ Inherited Disease Panel. The present study also conducted Sanger DNA Sequencing for family members with global minor allele frequencies <1% to verify potential pathogenic mutation sites. A total of three rare missense mutations were detected, including heterozygous c.244G>A in LMNA, c.546C>G in potassium voltage‑gated channel subfamily KQT (KCNQ4) and c.1276G>A in EYA transcriptional coactivator and phosphatase 1 (EYA1), indicating a glutamic acid to lysine substitution at amino acid 82 (p.E82K) in LMNA, a p.F182L in KCNQ4 (a mutation associated with pathogenic deafness) and p.G426S in EYA1 (associated with Branchiootorenal syndrome 1 and Branchiootic syndrome 1 pathogenesis). In the present study, a carrier with slight hearing impairment was detected in the family analyzed; however, no patients with deafness or branchiootorenal syndrome were observed. LMNA p.E82K revealed SIFT and PolyPhen‑2 scores of 0 and 1, respectively. In the second generation, 3 patients with DCM underwent permanent pacemaker implantation due to sick sinus syndrome, atrioventricular block and unstable cardiac electrophysiology. The present study suggested that LMNA p.E82K may contribute to the pathogenesis of FDC and concomitant atrioventricular block. At present, only three families with DCM resulting from similar mutations have been reported. The present study demonstrated the strong pathogenic effects of LMNA p.E82K on DCM.

Intermediate filaments in cardiomyopathy

Intermediate filament (IF) proteins are critical regulators in health and disease. The discovery of hundreds of mutations in IF genes and posttranslational modifications has been linked to a plethora of human diseases, including, among others, cardiomyopathies, muscular dystrophies, progeria, blistering diseases of the epidermis, and neurodegenerative diseases. The major IF proteins that have been linked to cardiomyopathies and heart failure are the muscle-specific cytoskeletal IF protein desmin and the nuclear IF protein lamin, as a subgroup of the known desminopathies and laminopathies, respectively. The studies so far, both with healthy and diseased heart, have demonstrated the importance of these IF protein networks in intracellular and intercellular integration of structure and function, mechanotransduction and gene activation, cardiomyocyte differentiation and survival, mitochondrial homeostasis, and regulation of metabolism. The high coordination of all these processes is obviously of great importance for the maintenance of proper, life-lasting, and continuous contraction of this highly organized cardiac striated muscle and consequently a healthy heart. In this review, we will cover most known information on the role of IFs in the above processes and how their deficiency or disruption leads to cardiomyopathy and heart failure.

Linker of nucleoskeleton and cytoskeleton complex proteins in cardiomyopathy

The linker of nucleoskeleton and cytoskeleton (LINC) complex couples the nuclear lamina to the cytoskeleton. The LINC complex and its associated proteins play diverse roles in cells, ranging from genome organization, nuclear morphology, gene expression, to mechanical stability. The importance of a functional LINC complex is highlighted by the large number of mutations in genes encoding LINC complex proteins that lead to skeletal and cardiac myopathies. In this review, the structure, function, and interactions between components of the LINC complex will be described. Mutations that are known to cause cardiomyopathy in patients will be discussed alongside their respective mouse models. Furthermore, future challenges for the field and emerging technologies to investigate LINC complex function will be discussed.

Cell signaling abnormalities in cardiomyopathy caused by lamin A/C gene mutations

Mutations in the lamin A/C gene (LMNA) encoding intermediate filament proteins associated with the inner nuclear membrane cause diseases known as laminopathies. Most LMNA mutations cause dilated cardiomyopathy with variable skeletal muscular dystrophy. Cell signaling abnormalities have been discovered in hearts of mouse models of cardiomyopathy caused by LMNA mutations that contribute to pathogenesis. These include abnormally increased signaling by extracellular signal-regulated kinase 1 and kinase 2 and other mitogen-activated protein kinases, protein kinase B/mammalian target of rapamycin complex 1 and transforming growth factor-β. Preclinical research suggests that specific inhibitors of these abnormally activated cell signaling pathways may be useful in treating human patients with this disease.

Current insights into LMNA cardiomyopathies: Existing models and missing LINCs

The nuclear lamina is a critical structural domain for the maintenance of genomic stability and whole-cell mechanics. Mutations in the LMNA gene, which encodes nuclear A-type lamins lead to the disruption of these key cellular functions, resulting in a number of devastating diseases known as laminopathies. Cardiomyopathy is a common laminopathy and is highly penetrant with poor prognosis. To date, cell mechanical instability and dysregulation of gene expression have been proposed as the main mechanisms driving cardiac dysfunction, and indeed discoveries in these areas have provided some promising leads in terms of therapeutics. However, important questions remain unanswered regarding the role of lamin A dysfunction in the heart, including a potential role for the toxicity of lamin A precursors in LMNA cardiomyopathy, which has yet to be rigorously investigated.

Lamin A/C Cardiomyopathies: Current Understanding and Novel Treatment Strategies

OPINION STATEMENT

Dilated cardiomyopathy (DCM) is the third leading cause of heart failure in the USA. A major gene associated with DCM with cardiac conduction system disease is lamin A/C (LMNA) gene. Lamins are type V filaments that serve a variety of roles, including nuclear structure support, DNA repair, cell signaling pathway mediation, and chromatin organization. In 1999, LMNA was found responsible for Emery-Dreifuss muscular dystrophy (EDMD) and, since then, has been found in association with a wide spectrum of diseases termed laminopathies, including LMNA cardiomyopathy. Patients with LMNA mutations have a poor prognosis and a higher risk for sudden cardiac death, along with other cardiac effects like dysrhythmias, development of congestive heart failure, and potential need of a pacemaker or ICD. As of now, there is no specific treatment for laminopathies, including LMNA cardiomyopathy, because the mechanism of LMNA mutations in humans is still unclear. This review discusses LMNA mutations and how they relate to DCM, the necessity for further investigation to better understand LMNA mutations, and potential treatment options ranging from clinical and therapeutic to cellular and molecular techniques.

Decreased WNT/β-catenin signalling contributes to the pathogenesis of dilated cardiomyopathy caused by mutations in the lamin a/C gene

Cardiomyopathy caused by lamin A/C gene (LMNA) mutations (hereafter referred as LMNA cardiomyopathy) is characterized by cardiac conduction abnormalities and left ventricular systolic dysfunction predisposing to heart failure. Previous cardiac transcriptional profiling of LmnaH222P/H222P mouse, a small animal model of LMNA cardiomyopathy, suggested decreased WNT/β-catenin signalling. We confirmed decreased WNT/β-catenin signalling in the hearts of these mice by demonstrating decreased β-catenin and WNT proteins. This was correlated with increased expression of soluble Frizzled-related proteins that modulate the WNT/β-catenin signalling pathway. Hearts of LmnaH222P/H222P mice also demonstrated lowered expression of the gap junction connexin 43. Activation of WNT/β-catenin activity with 6-bromoindirubin-3'-oxime improved cardiac contractility and ameliorated intraventricular conduction defects in LmnaH222P/H222P mice, which was associated with increased expression of myocardial connexin 43. These results indicate that decreased WNT/β-catenin contributes to the pathophysiology of LMNA cardiomyopathy and that drugs activating β-catenin may be beneficial in affected individuals.

Recent advances in animal and human pluripotent stem cell modeling of cardiac laminopathy

Laminopathy is a disease closely related to deficiency of the nuclear matrix protein lamin A/C or failure in prelamin A processing, and leads to accumulation of the misfold protein causing progeria. The resultant disrupted lamin function is highly associated with abnormal nuclear architecture, cell senescence, apoptosis, and unstable genome integrity. To date, the effects of loss in nuclear integrity on the susceptible organ, striated muscle, have been commonly associated with muscular dystrophy, dilated cardiac myopathy (DCM), and conduction defeats, but have not been studied intensively. In this review, we aim to summarize recent breakthroughs in an in vivo laminopathy model and in vitro study using patient-specific human induced pluripotent stem cells (iPSCs) that reproduce the pathophysiological phenotype for further drug screening. We describe several in-vivo transgenic mouse models to elucidate the effects of Lmna H222P, N195K mutations, and LMNA knockout on cardiac function, in terms of hemodynamic and electrical signal propagation; certain strategies targeted on stress-related MAPK are mentioned. We will also discuss human iPSC cardiomyocytes serving as a platform to reveal the underlying mechanisms, such as the altered mechanical sensation in electrical coupling of the heart conduction system and ion channel alternation in relation to altered nuclear architecture, and furthermore to enable screening of drugs that can attenuate this cardiac premature aging phenotype by inhibition of prelamin misfolding and oxidative stress, and also enhancement of autophagy protein clearance and cardiac-protective microRNA.

What Should the Cardiologist know about Lamin Disease?

Lamins are intermediate filament proteins able to polymerise and form an organised meshwork underlying the inner nuclear membrane in most differentiated somatic cells. Mutations in the LMNA gene, which encodes the two major lamin A and C isoforms, cause a diverse range of diseases, called laminopathies, including dilated cardiomyopathy, associated with a poor prognosis and high rate of sudden death due to conduction defect and early ventricular arrhythmia. Identification of mutations in LMNA gene in clinical practice is rapidly increasing, as well as comprehensive cardiac and genetic family screening. As a consequence, cardiologists are more and more frequently faced to difficult questions regarding optimal management of patients and relatives, especially timing for prophylactic cardioverter defibrillator. This review focuses on recent data useful for the clinician, as well as therapeutic perspectives both in human and animal models.

PKC412 normalizes mutation-related keratin filament disruption and hepatic injury in mice by promoting keratin-myosin binding

Keratins, among other cytoskeletal intermediate filament proteins, are mutated at a highly conserved arginine with consequent severe disease phenotypes due to disruption of keratin filament organization. We screened a kinase inhibitor library, using A549 cells that are transduced with a lentivirus keratin 18 (K18) construct, to identify compounds that normalize filament disruption due to K18 Arg90Cys mutation at the conserved arginine. High-throughput screening showed that PKC412, a multikinase inhibitor, ameliorated K18 Arg90Cys-mediated keratin filament disruption in cells and in the livers of previously described transgenic mice that overexpress K18 Arg90Cys. Furthermore, PKC412 protected cultured A549 cells that express mutant or wild-type K18 and mouse livers of the K18 Arg90Cys-overexpressing transgenic mice from Fas-induced apoptosis. Proteomic analysis of proteins that associated with keratins after exposure of K18-expressing A549 cells to PKC412 showed that nonmuscle myosin heavy chain-IIA (NMHC-IIA) partitions with the keratin fraction. The nonmuscle myosin-IIA (NM-IIA) association with keratins was confirmed by immune staining and by coimmunoprecipitation. The keratin-myosin association is myosin dephosphorylation-dependent; occurs with K8, the obligate K18 partner; is enhanced by PKC412 in cells and mouse liver; and is blocked by hyperphosphorylation conditions in cultured cells and mouse liver. Furthermore, NMHC-IIA knockdown inhibits PKC412-mediated normalization of K18 R90C filaments.

CONCLUSION

The inhibitor PKC412 normalizes K18 Arg90Cys mutation-induced filament disruption and disorganization by enhancing keratin association with NM-IIA in a myosin dephosphorylation-regulated manner. Targeting of intermediate filament disorganization by compounds that alter keratin interaction with their associated proteins offers a potential novel therapeutic approach for keratin and possibly other intermediate filament protein-associated diseases.

High-Throughput Screening for Drugs that Modulate Intermediate Filament Proteins

Intermediate filament (IF) proteins have unique and complex cell and tissue distribution. Importantly, IF gene mutations cause or predispose to more than 80 human tissue-specific diseases (IF-pathies), with the most severe disease phenotypes being due to mutations at conserved residues that result in a disrupted IF network. A critical need for the entire IF-pathy field is the identification of drugs that can ameliorate or cure these diseases, particularly since all current therapies target the IF-pathy complication, such as diabetes or cardiovascular disease, rather than the mutant IF protein or gene. We describe a high-throughput approach to identify drugs that can normalize disrupted IF proteins. This approach utilizes transduction of lentivirus that expresses green fluorescent protein-tagged keratin 18 (K18) R90C in A549 cells. The readout is drug "hits" that convert the dot-like keratin filament distribution, due to the R90C mutation, to a wild-type-like filamentous array. A similar strategy can be used to screen thousands of compounds and can be utilized for practically any IF protein with a filament-disrupting mutation, and could therefore potentially target many IF-pathies. "Hits" of interest require validation in cell culture then using in vivo experimental models. Approaches to study the mechanism of mutant IF normalization by potential drugs of interest are also described. The ultimate goal of this drug screening approach is to identify effective and safe compounds that can potentially be tested for clinical efficacy in patients.

Targeting Mitogen-Activated Protein Kinase Signaling in Mouse Models of Cardiomyopathy Caused by Lamin A/C Gene Mutations

The most frequently occurring mutations in the gene encoding nuclear lamin A and nuclear lamin C cause striated muscle diseases virtually always involving the heart. In this review, we describe the approaches and methods used to discover that cardiomyopathy-causing lamin A/C gene mutations increase MAP kinase signaling in the heart and that this plays a role in disease pathogenesis. We review different mouse models of cardiomyopathy caused by lamin A/C gene mutations and how transcriptomic analysis of one model identified increased cardiac activity of the ERK1/2, JNK, and p38α MAP kinases. We describe methods used to measure the activity of these MAP kinases in mouse hearts and then discuss preclinical treatment protocols using pharmacological inhibitors to demonstrate their role in pathogenesis. Several of these kinase inhibitors are in clinical development and could potentially be used to treat human subjects with cardiomyopathy caused by lamin A/C gene mutations.

LMO7-null mice exhibit phenotypes consistent with emery-dreifuss muscular dystrophy

INTRODUCTION

Mutations in the inner nuclear envelope protein emerin cause Emery-Dreifuss muscular dystrophy (EDMD), which is characterized by progressive skeletal muscle wasting, cardiac conduction defects, and tendon contractures. We previously showed that emerin binds directly to the transcription regulator Lmo7 and attenuates its activity to regulate the proper temporal expression of important myogenic differentiation genes.

METHODS

The skeletal muscle and cardiac phenotypes were analyzed in a newly generated Lmo7-null mouse using histological analysis, echocardiography, and various neuromuscular tests to determine if Lmo7 was important for skeletal muscle and cardiac function.

RESULTS

Lmo7-null mice had growth retardation, decreased fiber size, and impaired skeletal muscle and cardiac function. Lmo7-null mice also had lower levels of phosphorylated retinoblastoma (Rb), extracellular signal-regulated kinase, and c-Jun N-terminal kinase, which is consistent with altered Rb and mitogen-activated protein kinase signaling.

CONCLUSIONS

These findings demonstrate that loss of Lmo7 in mice causes myopathic phenotypes similar to those seen in other EDMD mouse models.

Mitogen-activated protein kinase kinase 1/2 inhibition and angiotensin II converting inhibition in mice with cardiomyopathy caused by lamin A/C gene mutation

BACKGROUND

Mutations in the LMNA gene encoding A-type nuclear lamins can cause dilated cardiomyopathy with or without skeletal muscular dystrophy. Previous studies have shown abnormally increased extracellular signal-regulated kinase 1/2 activity in hearts of Lmna(H222P/H222P) mice, a small animal model. Inhibition of this abnormal signaling activity with a mitogen-activated protein kinase kinase 1/2 (MEK1/2) inhibitor has beneficial effects on heart function and survival in these mice. However, such treatment has not been examined relative to any standard of care intervention for dilated cardiomyopathy or heart failure. We therefore examined the effects of an angiotensin II converting enzyme (ACE) inhibitor on left ventricular function in Lmna(H222P/H222P) mice and assessed if adding a MEK1/2 inhibitor would provide added benefit.

METHODS

Male Lmna(H222P/H222P) mice were treated with the ACE inhibitor benazepril, the MEK1/2 inhibitor selumetinib or both. Transthoracic echocardiography was used to measure left ventricular diameters and fractional shortening was calculated.

RESULTS

Treatment of Lmna(H222P/H222P) mice with either benazepril or selumetinib started at 8weeks of age, before the onset of detectable left ventricular dysfunction, lead to statistically significantly increased fractional shortening compared to placebo at 16weeks of age. There was a trend towards a great value for fractional shortening in the selumetinib-treated mice. When treatment was started at 16weeks of age, after the onset of left ventricular dysfunction, the addition of selumetinib treatment to benazepril lead to a statistically significant increase in left ventricular fractional shortening at 20weeks of age.

CONCLUSIONS

Both ACE inhibition and MEK1/2 inhibition have beneficial effects on left ventricular function in Lmna(H222P/H222P) mice and both drugs together have a synergistic benefit when initiated after the onset of left ventricular dysfunction. These results provide further preclinical rationale for a clinical trial of a MEK1/2 inhibitor in addition to standard of care in patients with dilated cardiomyopathy caused by LMNA mutations.

Striated muscle laminopathies

Lamins A and C, encoded by LMNA, are constituent of the nuclear lamina, a meshwork of proteins underneath the nuclear envelope first described as scaffolding proteins of the nucleus. Since the discovery of LMNA mutations in highly heterogeneous human disorders (including cardiac and muscular dystrophies, lipodystrophies and progeria), the number of functions described for lamin A/C has expanded. Lamin A/C is notably involved in the regulation of chromatin structure and gene transcription, and in the resistance of cells to mechanical stress. This review focuses on studies performed on knock-out and knock-in Lmna mouse models, which have led to decipher some of the lamin A/C functions in striated muscles and to the first preclinical trials of pharmaceutical therapies.

Genetic and clinical characteristics of skeletal and cardiac muscle in patients with lamin A/C gene mutations

Alterations of the lamin A/C (LMNA) gene are associated with different clinical entities, including disorders that affect skeletal and cardiac muscle, peripheral nerves, metabolism, bones, and disorders that cause premature aging. In this article we review the clinical and genetic characteristics of cardiac and skeletal muscle diseases related to alterations in the LMNA gene. There is no single explanation of how LMNA gene alterations may cause these disorders; however, important goals have been achieved in understanding the pathogenic effects of LMNA gene mutations on cardiac and skeletal muscle.

When lamins go bad: nuclear structure and disease

Mutations in nuclear lamins or other proteins of the nuclear envelope are the root cause of a group of phenotypically diverse genetic disorders known as laminopathies, which have symptoms that range from muscular dystrophy to neuropathy to premature aging syndromes. Although precise disease mechanisms remain unclear, there has been substantial progress in our understanding of not only laminopathies, but also the biological roles of nuclear structure. Nuclear envelope dysfunction is associated with altered nuclear activity, impaired structural dynamics, and aberrant cell signaling. Building on these findings, small molecules are being discovered that may become effective therapeutic agents.

Muscle development, regeneration and laminopathies: how lamins or lamina-associated proteins can contribute to muscle development, regeneration and disease

The aim of this review article is to evaluate the current knowledge on associations between muscle formation and regeneration and components of the nuclear lamina. Lamins and their partners have become particularly intriguing objects of scientific interest since it has been observed that mutations in genes coding for these proteins lead to a wide range of diseases called laminopathies. For over the last 10 years, various laboratories worldwide have tried to explain the pathogenesis of these rare disorders. Analyses of the distinct aspects of laminopathies resulted in formulation of different hypotheses regarding the mechanisms of the development of these diseases. In the light of recent discoveries, A-type lamins—the main building blocks of the nuclear lamina—together with other key elements, such as emerin, LAP2α and nesprins, seem to be of great importance in the modulation of various signaling pathways responsible for cellular differentiation and proliferation.

Abnormal p38α mitogen-activated protein kinase signaling in dilated cardiomyopathy caused by lamin A/C gene mutation

We previously interrogated the transcriptome in heart tissue from Lmna(H222P/H222P) mice, a mouse model of cardiomyopathy caused by lamin A/C gene (LMNA) mutation, and found that the extracellular signal-regulated kinase 1/2 and Jun N-terminal kinase branches of the mitogen-activated protein (MAP) kinase signaling pathway were abnormally hyperactivated prior to the onset of significant cardiac impairment. We have now used an alternative gene expression analysis tool to reanalyze this transcriptome and identify hyperactivation of a third branch of the MAP kinase cascade, p38α signaling. Biochemical analysis of hearts from Lmna(H222P/H222P) mice showed enhanced p38α activation prior to and after the onset of heart disease as well as in hearts from human subjects with cardiomyopathy caused by LMNA mutations. Treatment of Lmna(H222P/H222P) mice with the p38α inhibitor ARRY-371797 prevented left ventricular dilatation and deterioration of fractional shortening compared with placebo-treated mice but did not block the expression of collagen genes involved in cardiac fibrosis. These results demonstrate that three different branches of the MAP kinase signaling pathway with overlapping consequences are involved in the pathogenesis of cardiomyopathy caused by LMNA mutations. They further suggest that pharmacological inhibition of p38α may be useful in the treatment of this disease.

Mitogen-activated protein kinase inhibitor regulation of heart function and fibrosis in cardiomyopathy caused by lamin A/C gene mutation

Mutations in the lamin A/C gene (LMNA) encoding A-type nuclear lamins cause dilated cardiomyopathy. We have uncovered a novel connection between these mutations and hyperactivation of the extracellular signal-regulated kinase 1/2 and c-jun N-terminal kinase branches of the mitogen-activated protein kinase signaling pathway in a mouse model of the disease. This discovery has identified targets that can be inhibited by drugs that improve heart function and prevent fibrosis.

Nuclear lamins and laminopathies

Nuclear lamins are intermediate filament proteins that polymerize to form the nuclear lamina on the inner aspect of the inner nuclear membrane. Long known to be essential for maintaining nuclear structure and disassembling/reassembling during mitosis in metazoans, research over the past dozen years has shown that mutations in genes encoding nuclear lamins, particularly LMNA encoding the A-type lamins, cause a broad range of diverse diseases, often referred to as laminopathies. Lamins are expressed in all mammalian somatic cells but mutations in their genes lead to relatively tissue-selective disease phenotypes in most cases. While mutations causing laminopathies have been shown to produce abnormalities in nuclear morphology, how these disease-causing mutations or resultant alterations in nuclear structure lead to pathology is only starting to be understood. Despite the incomplete understanding of pathogenic mechanisms underlying the laminopathies, basic research in cellular and small animal models has produced promising leads for treatments of these rare diseases.

Treatment with selumetinib preserves cardiac function and improves survival in cardiomyopathy caused by mutation in the lamin A/C gene

AIMS

Mutations in A-type nuclear lamins gene, LMNA, lead to a dilated cardiomyopathy. We have reported abnormal activation of the extracellular signal-regulated kinase1/2 (ERK1/2) signalling in hearts from Lmna(H222P/H222P) mice, which develop dilated cardiomyopathy. We therefore determined whether an inhibitor of ERK1/2 signalling that has been investigated in clinical trials for cancer has the potential to be translated to humans with LMNA cardiomyopathy.

METHODS AND RESULTS

To evaluate the relevance of this finding in mice to patients, we analysed the ERK1/2 signalling in heart tissue from human subjects with LMNA cardiomyopathy and showed that it was abnormally activated. To determine whether pharmacological inhibitors of the ERK1/2 signalling pathway could potentially be used to treat LMNA cardiomyopathy, we administered selumetinib to male Lmna(H222P/H222P) mice starting at 16 weeks of age, after they show signs of cardiac deterioration, up to 20 weeks of age. Selumetinib is an inhibitor of ERK1/2 signalling and has been given safely to human subjects in clinical trials for cancer. Systemic treatment with selumetinib inhibited cardiac ERK1/2 phosphorylation and blocked increased expression of RNAs encoding natriuretic peptide precursors and proteins involved in sarcomere architecture that occurred in placebo-treated mice. Echocardiography and histological analysis demonstrated that treatment increases cardiac fractional shortening, prevents myocardial fibrosis, and prolongs survival. Selumetinib treatment did not induce biochemical abnormalities suggestive of renal or hepatic toxicity.

CONCLUSION

Our results suggest that selumetinib or other related inhibitors that have been safely administered to humans in clinical trials could potentially be used to treat LMNA cardiomyopathy.

[Laminopathies: one gene, several diseases]

Lamins A and C, encoded by the LMNA gene, are nuclear proteins expressed in all post-mitotic cells. Together with B-type lamins, they form a meshwork of proteins beneath the inner nuclear membrane, the lamina, in connection with the cytoskeleton. Lamins A/C also interact with chromatin and numerous proteins, including transcription factors. Mutations in LMNA are responsible for more than ten different disorders, commonly called "laminopathies". These diseases affect tissues in a specific (striated muscle, adipose tissue, peripheral nerve) or in a systemic manner (premature ageing syndromes). This wide spectrum of phenotypes is associated to a wide variety of mutations. This large clinical and genetic heterogeneity, unique to the LMNA gene, makes genotype-phenotype relations particularly difficult to establish. However, correlations have been obtained in several cases. Hence, LMNA mutations identified in premature ageing syndromes lead to the accumulation of immature proteins with a toxic effect for cells. Mutations in laminopathies of the adipose tissue mainly localize in the Ig-like domain of the proteins, potentially affecting the interaction with the SREBP-1 transcription factor. In laminopathies of the striated muscles, the mutations are spread throughout the gene. These mutations are thought to induce structural modifications of the proteins, thereby affecting their polymerization into nuclear lamina. Such defect would lead to a mechanical weakness of the nuclear lamina and of the cells, particularly in striated muscles continuously stretching. The exploration of pathophysiological mechanisms of LMNA mutations largely benefits from the numerous mouse models created, which have been widely used to analyze affected molecular pathways and to test putative therapeutic treatments.

LMNA cardiomyopathy: cell biology and genetics meet clinical medicine

Mutations in the LMNA gene, which encodes A-type nuclear lamins (intermediate filament proteins expressed in most differentiated somatic cells), cause a diverse range of diseases, called laminopathies, that selectively affect different tissues and organ systems. The most prevalent laminopathy is cardiomyopathy with or without different types of skeletal muscular dystrophy. LMNA cardiomyopathy has an aggressive clinical course with higher rates of deadly arrhythmias and heart failure than most other heart diseases. As awareness among physicians increases, and advances in DNA sequencing methods make the genetic diagnosis of LMNA cardiomyopathy more common, cardiologists are being faced with difficult questions regarding patient management. These questions concern the optimal use of intracardiac cardioverter defibrillators to prevent sudden death from arrhythmias, and medical interventions to prevent heart damage and ameliorate heart failure symptoms. Data from a mouse model of LMNA cardiomyopathy suggest that inhibitors of mitogen-activated protein kinase (MAPK) signaling pathways are beneficial in preventing and treating cardiac dysfunction; this basic research discovery needs to be translated to human patients.

Role of TGF-β signaling in inherited and acquired myopathies

The transforming growth factor-beta (TGF-β) superfamily consists of a variety of cytokines expressed in many different cell types including skeletal muscle. Members of this superfamily that are of particular importance in skeletal muscle are TGF-β1, mitogen-activated protein kinases (MAPKs), and myostatin. These signaling molecules play important roles in skeletal muscle homeostasis and in a variety of inherited and acquired neuromuscular disorders. Expression of these molecules is linked to normal processes in skeletal muscle such as growth, differentiation, regeneration, and stress response. However, chronic elevation of TGF-β1, MAPKs, and myostatin is linked to various features of muscle pathology, including impaired regeneration and atrophy. In this review, we focus on the aberrant signaling of TGF-β in various disorders such as Marfan syndrome, muscular dystrophies, sarcopenia, and critical illness myopathy. We also discuss how the inhibition of several members of the TGF-β signaling pathway has been implicated in ameliorating disease phenotypes, opening up novel therapeutic avenues for a large group of neuromuscular disorders.

Laminopathies: the molecular background of the disease and the prospects for its treatment

Laminopathies are rare human degenerative disorders with a wide spectrum of clinical phenotypes, associated with defects in the main protein components of the nuclear envelope, mostly in the lamins. They include systemic disorders and tissue-restricted diseases. Scientists have been trying to explain the pathogenesis of laminopathies and find an efficient method for treatment for many years. In this review, we discuss the current state of knowledge about laminopathies, the molecular mechanisms behind the development of particular phenotypes, and the prospects for stem cell and/or gene therapy treatments.

Mitogen-activated protein kinase inhibitors improve heart function and prevent fibrosis in cardiomyopathy caused by mutation in lamin A/C gene

BACKGROUND

Mutations in the lamin A/C gene, LMNA, can cause dilated cardiomyopathy. We have shown abnormal activation of the extracellular signal-regulated kinase (ERK) and the c-jun N-terminal kinase (JNK) branches of the mitogen-activated protein kinase signaling cascade in hearts from Lmna(H222P/H222P) mice that develop dilated cardiomyopathy. We recently showed that partial inhibition of ERK and JNK signaling before the onset of cardiomyopathy in Lmna(H222P/H222P) mice prevented the development of left ventricle dilatation and decreased cardiac ejection fraction at a time when they occurred in untreated mice.

METHODS AND RESULTS

To determine whether pharmacological inhibitors of ERK and JNK signaling could be clinically useful to treat cardiomyopathy caused by LMNA mutation, we administered them to Lmna(H222P/H222P) mice after they developed left ventricular dilatation and decreased ejection fraction. Lmna(H222P/H222P) mice were treated with ERK and JNK signaling inhibitors from 16 to 20 or, in pilot experiments, 19 to 24 weeks of age. The inhibitors blocked increased expression of RNAs encoding natriuretic peptide precursors and proteins involved in sarcomere architecture that occurred in placebo-treated mice. Echocardiography and histological analysis demonstrated that treatment prevented left ventricular end-systolic dilatation, increased ejection fraction, and decreased myocardial fibrosis.

CONCLUSION

Inhibitors of ERK and JNK signaling could potentially be used to treat humans with cardiomyopathy caused by LMNA mutations.