Nesprin proteins: bridging nuclear envelope dynamics to muscular dysfunction

This review presents a comprehensive exploration of the pivotal role played by the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex, with a particular focus on Nesprin proteins, in cellular mechanics and the pathogenesis of muscular diseases. Distinguishing itself from prior works, the analysis delves deeply into the intricate interplay of the LINC complex, emphasizing its indispensable contribution to maintaining cellular structural integrity, especially in mechanically sensitive tissues such as cardiac and striated muscles. Additionally, the significant association between mutations in Nesprin proteins and the onset of Dilated Cardiomyopathy (DCM) and Emery-Dreifuss Muscular Dystrophy (EDMD) is highlighted, underscoring their pivotal role in disease pathogenesis. Through a comprehensive examination of DCM and EDMD cases, the review elucidates the disruptions in the LINC complex, nuclear morphology alterations, and muscular developmental disorders, thus emphasizing the essential function of an intact LINC complex in preserving muscle physiological functions. Moreover, the review provides novel insights into the implications of Nesprin mutations for cellular dynamics in the pathogenesis of muscular diseases, particularly in maintaining cardiac structural and functional integrity. Furthermore, advanced therapeutic strategies, including rectifying Nesprin gene mutations, controlling Nesprin protein expression, enhancing LINC complex functionality, and augmenting cardiac muscle cell function are proposed. By shedding light on the intricate molecular mechanisms underlying nuclear-cytoskeletal interactions, the review lays the groundwork for future research and therapeutic interventions aimed at addressing genetic muscle disorders.

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.

Deep clinicopathological phenotyping identifies a previously unrecognized pathogenic EMD splice variant

Exome sequencing (ES) has revolutionized rare disease management, yet only ~25%-30% of patients receive a molecular diagnosis. A limiting factor is the quality of available phenotypic data. Here, we describe how deep clinicopathological phenotyping yielded a molecular diagnosis for a 19-year-old proband with muscular dystrophy and negative clinical ES. Deep phenotypic analysis identified two critical data points: (1) the absence of emerin protein in muscle biopsy and (2) clinical features consistent with Emery-Dreifuss muscular dystrophy. Sequencing data analysis uncovered an ultra-rare, intronic variant in EMD, the gene encoding emerin. The variant, NM_000117.3: c.188-6A > G, is predicted to impact splicing by in silico tools. This case thus illustrates how better integration of clinicopathologic data into ES analysis can enhance diagnostic yield with implications for clinical practice.

THE ROLE OF LMNA MUTATIONS IN MYOGENIC DIFFERENTIATION OF PRIMARY SATELLITE CELLS AND C2C12 CELLS

Nuclear lamins form nuclear lamina localized under the inner nuclear membrane. It was previously considered that the nuclear lamina predominantly plays a structural role, however, its involvement have been recently described in the regulatory processes such as chromatin organization and gene transcription. It is known that mutations in the LMNA gene lead to the development of a large number of diseases, laminopathies, which mainly affect mesenchymal tissue. Nowadays, the mechanisms by which the lamina can regulate cell differentiation remain incompletely understood. In the present work, we have studied the effect of LMNA gene mutations on the process of muscle differentiation of primary satellite cells and Ñ2Ñ12 cell line. The genome of satellite cells and Ñ2Ñ12 cell line was modified by the introduction of lentiviral constructs encoding LMNA G232E associated with the development of muscular dystrophy Emery—Dreyfus and LMNA R571S associated with the development of dilated cardiomyopathy. The morphology of the cells was estimated using immunofluorescence, the expression level of myogenic genes were analyzed by qPCR. We have shown that the analyzed mutations reduce the ability of cells to differentiate, to fuse and to form myotubes. We have suggested that it is due to enhanced expression of markers at the early stages and to reduced expression markers at the late stages of myogenesis. Therefore, mutations in nuclear lamins can influence the process of muscle differentiation.

Partial deficiency of emerin caused by a splice site mutation in EMD

Emery-Dreifuss muscular dystrophy (EDMD) is caused by mutations in the EMD gene on the X chromosome, which codes for emerin, an inner nuclear membrane protein. Monoclonal antibodies against the N-terminus of emerin protein are used to screen for emerin deficiency in clinical practice. However, these tests may not accurately reflect the disease in some cases. We herein describe the identification of a splice site mutation in the EMD gene in a Japanese patient who suffered from complete atrioventricular conduction block, mild muscle weakness and joint contracture, and a persistently elevated serum creatine kinase level. We used multiple antibodies to confirm the presence of a novel truncating mutation in emerin without the transmembrane region and C-terminus in the skeletal muscle.

[NESPRINS--nuclear envelope proteins ensuring integrity]

This review describes the nesprins (nuclear envelope spectrin-repeat proteins), which are recently discovered family of nuclear envelope proteins. These proteins play an important role in maintaining the cellular architecture and establish the link between the nucleus and other sub-cellular compartments. Many tissue-specific diseases including lipodystrophies, hearing loss, cardiac and skeletal myopathies are associated with nesprins mutations. These proteins comprise of multiple tissue specific isoforms which contain spectrin repeats providing interaction of nesprins with other nuclear membrane proteins, cytoskeleton and intranuclear matrix. We summarize recent findings and suggestions about nesprins structural organization and function inside the cell. Human diseases caused by abnormal nesprins expression are also described.

Emery-Dreifuss muscular dystrophy, laminopathies, and other nuclear envelopathies

The nuclear envelopathies, more frequently known as laminopathies are a rapidly expanding group of human hereditary diseases caused by mutations of genes that encode proteins of the nuclear envelope. The most frequent and best known form is Emery-Dreifuss muscular dystrophy (EDMD), a skeletal myopathy characterized by progressive muscular weakness, joint contractures, and cardiac disease. EMD gene, encoding emerin, causes the X-linked form of EDMD, while LMNA gene encoding lamins A and C, is responsible for autosomal forms, usually with a dominant transmission. In the last years, the spectrum of conditions has been extraordinarily enlarged, from a congenital muscular dystrophy with severe paralytic or rapidly progressive picture due to de novo mutations in LMNA (L-CMD) to a limb-girdle muscular dystrophy with adult onset and much milder weakness (LGMD1B). LMNA has also been involved in a form of isolated cardiomyopathy associated with cardiac conduction disease and in an axonal form of hereditary neuropathy. Identification of this gene has been reported also in a number of non-neuromuscular disorders including lipodystrophy syndromes and a wide spectrum of premature aging syndromes ranging from mandibuloacral dysplasia to restrictive dermopathy. Mutations in other genes implicated in the processing or maturation of nuclear lamins have also been found. The extraordinary complexity of the molecular and pathophysiological mechanisms of these diseases is still not well known and the occurrence of modifying factors or genes is highly suspected. Identification of new genes and investigation of new therapeutic approaches are in progress.

Loss of emerin alters myogenic signaling and miRNA expression in mouse myogenic progenitors

Emerin is an integral membrane protein of the inner nuclear membrane. Mutations in emerin cause X-linked Emery-Dreifuss muscular dystrophy (EDMD), a disease characterized by skeletal muscle wasting and dilated cardiomyopathy. Current evidence suggests the muscle wasting phenotype of EDMD is caused by defective myogenic progenitor cell differentiation and impaired muscle regeneration. We obtained genome-wide expression data for both mRNA and micro-RNA (miRNA) in wildtype and emerin-null mouse myogenic progenitor cells. We report here that emerin-null myogenic progenitors exhibit differential expression of multiple signaling pathway components required for normal muscle development and regeneration. Components of the Wnt, IGF-1, TGF-β, and Notch signaling pathways are misexpressed in emerin-null myogenic progenitors at both the mRNA and protein levels. We also report significant perturbations in the expression and activation of p38/Mapk14 in emerin-null myogenic progenitors, showing that perturbed expression of Wnt, IGF-1, TGF-β, and Notch signaling components disrupts normal downstream myogenic signaling in these cells. Collectively, these data support the hypothesis that emerin is essential for proper myogenic signaling in myogenic progenitors, which is necessary for myogenic differentiation and muscle regeneration.

A laminopathic mutation disrupting lamin filament assembly causes disease-like phenotypes in Caenorhabditis elegans

Mutations in the human LMNA gene underlie many laminopathic diseases, including Emery-Dreifuss muscular dystrophy (EDMD); however, a mechanistic link between the effect of mutations on lamin filament assembly and disease phenotypes has not been established. We studied the ΔK46 Caenorhabditis elegans lamin mutant, corresponding to EDMD-linked ΔK32 in human lamins A and C. Cryo-electron tomography of lamin ΔK46 filaments in vitro revealed alterations in the lateral assembly of dimeric head-to-tail polymers, which causes abnormal organization of tetrameric protofilaments. Green fluorescent protein (GFP):ΔK46 lamin expressed in C. elegans was found in nuclear aggregates in postembryonic stages along with LEM-2. GFP:ΔK46 also caused mislocalization of emerin away from the nuclear periphery, consistent with a decreased ability of purified emerin to associate with lamin ΔK46 filaments in vitro. GFP:ΔK46 animals had motility defects and muscle structure abnormalities. These results show that changes in lamin filament structure can translate into disease-like phenotypes via altering the localization of nuclear lamina proteins, and suggest a model for how the ΔK32 lamin mutation may cause EDMD in humans.

Uncoordinated transcription and compromised muscle function in the lmna-null mouse model of Emery- Emery-Dreyfuss muscular dystrophy

LMNA encodes both lamin A and C: major components of the nuclear lamina. Mutations in LMNA underlie a range of tissue-specific degenerative diseases, including those that affect skeletal muscle, such as autosomal-Emery-Dreifuss muscular dystrophy (A-EDMD) and limb girdle muscular dystrophy 1B. Here, we examine the morphology and transcriptional activity of myonuclei, the structure of the myotendinous junction and the muscle contraction dynamics in the lmna-null mouse model of A-EDMD. We found that there were fewer myonuclei in lmna-null mice, of which ∼50% had morphological abnormalities. Assaying transcriptional activity by examining acetylated histone H3 and PABPN1 levels indicated that there was a lack of coordinated transcription between myonuclei lacking lamin A/C. Myonuclei with abnormal morphology and transcriptional activity were distributed along the length of the myofibre, but accumulated at the myotendinous junction. Indeed, in addition to the presence of abnormal myonuclei, the structure of the myotendinous junction was perturbed, with disorganised sarcomeres and reduced interdigitation with the tendon, together with lipid and collagen deposition. Functionally, muscle contraction became severely affected within weeks of birth, with specific force generation dropping as low as ∼65% and ∼27% of control values in the extensor digitorum longus and soleus muscles respectively. These observations illustrate the importance of lamin A/C for correct myonuclear function, which likely acts synergistically with myotendinous junction disorganisation in the development of A-EDMD, and the consequential reduction in force generation and muscle wasting.

Mammalian SUN protein interaction networks at the inner nuclear membrane and their role in laminopathy disease processes

The nuclear envelope (NE) LINC complex, in mammals comprised of SUN domain and nesprin proteins, provides a direct connection between the nuclear lamina and the cytoskeleton, which contributes to nuclear positioning and cellular rigidity. SUN1 and SUN2 interact with lamin A, but lamin A is only required for NE localization of SUN2, and it remains unclear how SUN1 is anchored. Here, we identify emerin and short nesprin-2 isoforms as novel nucleoplasmic binding partners of SUN1/2. These have overlapping binding sites distinct from the lamin A binding site. However, we demonstrate that tight association of SUN1 with the nuclear lamina depends upon a short motif within residues 209-228, a region that does not interact significantly with known SUN1 binding partners. Moreover, SUN1 localizes correctly in cells lacking emerin. Importantly then, the major determinant of SUN1 NE localization has yet to be identified. We further find that a subset of lamin A mutations, associated with laminopathies Emery-Dreifuss muscular dystrophy (EDMD) and Hutchinson-Gilford progeria syndrome (HGPS), disrupt lamin A interaction with SUN1 and SUN2. Despite this, NE localization of SUN1 and SUN2 is not impaired in cell lines from either class of patients. Intriguingly, SUN1 expression at the NE is instead enhanced in a significant proportion of HGPS but not EDMD cells and strongly correlates with pre-lamin A accumulation due to preferential interaction of SUN1 with pre-lamin A. We propose that these different perturbations in lamin A-SUN protein interactions may underlie the opposing effects of EDMD and HGPS mutations on nuclear and cellular mechanics.

A novel mutation in the central rod domain of lamin A/C producing a phenotype resembling the Emery-Dreifuss muscular dystrophy phenotype

Lamins are the principal components of the nuclear lamina, a network constituting the major structural framework of the nuclear envelope. Alterations in lamin A/C have been associated with a heterogeneous series of human disorders known as laminopathies. We report the finding of a novel deletion in the central rod domain of lamin A/C exon 3 gene in four members of the same family. This genetic alteration was likely responsible for the relatively homogeneous clinical phenotype observed in our three patients, represented by a prominent cardiac conduction-system disease necessitating permanent pacemaker implantation, and limited skeletal involvement manifested by spinal rigidity and contractures. The findings from these cases further expand the clinical spectrum associated with mutations in the LMNA gene.

Nesprin-1 and -2 are involved in the pathogenesis of Emery Dreifuss muscular dystrophy and are critical for nuclear envelope integrity

Emery-Dreifuss muscular dystrophy (EDMD) is a heterogeneous late-onset disease involving skeletal muscle wasting and heart defects caused, in a minority of cases, by mutations in either of two genes encoding the inner nuclear membrane (INM) proteins, emerin and lamins A/C. Nesprin-1 and -2 are multi-isomeric, spectrin-repeat proteins that bind both emerin and lamins A/C and form a network in muscle linking the nucleoskeleton to the INM, the outer nuclear membrane, membraneous organelles, the sarcomere and the actin cytoskeleton. Thus, disruptions in nesprin/lamin/emerin interactions might play a role in the muscle-specific pathogenesis of EDMD. Screening for DNA variations in the genes encoding nesprin-1 (SYNE1) and nesprin-2 (SYNE2) in 190 probands with EDMD or EDMD-like phenotypes identified four heterozygous missense mutations. Fibroblasts from these patients exhibited nuclear morphology defects and specific patterns of emerin and SUN2 mislocalization. In addition, diminished nuclear envelope localization of nesprins and impaired nesprin/emerin/lamin binding interactions were common features of all EDMD patient fibroblasts. siRNA knockdown of nesprin-1 or -2 in normal fibroblasts reproduced the nuclear morphological changes and mislocalization of emerin and SUN2 observed in patient fibroblasts. Taken together, these data suggest that EDMD may be caused, in part, by uncoupling of the nucleoskeleton and cytoskeleton because of perturbed nesprin/emerin/lamin interactions.

Barrier to autointegration factor blocks premature cell fusion and maintains adult muscle integrity in C. elegans

Barrier to autointegration factor (BAF) binds double-stranded DNA, selected histones, transcription regulators, lamins, and LAP2-emerin-MAN1 (LEM) domain proteins. During early Caenorhabditis elegans embryogenesis, BAF-1 is required to organize chromatin, capture segregated chromosomes within the nascent nuclear envelope, and assemble lamin and LEM domain proteins in reforming nuclei. In this study, we used C. elegans with a homozygous deletion of the baf-1 gene, which survives embryogenesis and larval stages, to report that BAF-1 regulates maturation and survival of the germline, cell migration, vulva formation, and the timing of seam cell fusion. In the seam cells, BAF-1 represses the expression of the EFF-1 fusogen protein, but fusion still occurs in C. elegans lacking both baf-1 and eff-1. This suggests the existence of an eff-1-independent mechanism for cell fusion. BAF-1 is also required to maintain the integrity of specific body wall muscles in adult animals, directly implicating BAF in the mechanism of human muscular dystrophies (laminopathies) caused by mutations in the BAF-binding proteins emerin and lamin A.

Distinct functional domains in nesprin-1α and nesprin-2β bind directly to emerin and both interactions are disrupted in X-linked Emery–Dreifuss muscular dystrophy

Emerin and specific isoforms of nesprin-1 and -2 are nuclear membrane proteins which are binding partners in multi-protein complexes spanning the nuclear envelope. We report here the characterisation of the residues both in emerin and in nesprin-1alpha and -2beta which are involved in their interaction and show that emerin requires nesprin-1 or -2 to retain it at the nuclear membrane. Using several protein-protein interaction methods, we show that residues 368 to 627 of nesprin-1alpha and residues 126 to 219 of nesprin-2beta, which show high homology to one another, both mediate binding to emerin residues 140-176. This region has previously been implicated in binding to F-actin, beta-catenin and lamin A/C suggesting that it is critical for emerin function. Confirmation that these protein domains interact in vivo was shown using GFP-dominant negative assays. Exogenous expression of either of these nesprin fragments in mouse myoblast C2C12 cells displaced endogenous emerin from the nuclear envelope and reduced the targeting of newly synthesised emerin. Furthermore, we are the first to report that emerin mutations which give rise to X-linked Emery-Dreifuss muscular dystrophy, disrupt binding to both nesprin-1alpha and -2beta isoforms, further indicating a role of nesprins in the pathology of Emery-Dreifuss muscular dystrophy.

Activation of MAPK in hearts of EMD null mice: similarities between mouse models of X-linked and autosomal dominant Emery Dreifuss muscular dystrophy

Emery-Dreifuss muscular dystrophy (EDMD) is an inherited disorder characterized by slowly progressive skeletal muscle weakness in a humero-peroneal distribution, early contractures and prominent cardiomyopathy with conduction block. Mutations in EMD, encoding emerin, and LMNA, encoding A-type lamins, respectively, cause X-linked and autosomal dominant EDMD. Emerin and A-type lamins are proteins of the inner membrane of the nuclear envelope. Whereas the genetic cause of EDMD has been described and the proteins well characterized, little is known on how abnormalities in nuclear envelope proteins cause striated muscle disease. In this study, we analyzed genome-wide expression profiles in hearts from Emd knockout mice, a model of X-linked EDMD, using Affymetrix GeneChips. This analysis showed a molecular signature similar to that we previously described in hearts from Lmna H222P knock-in mice, a model of autosomal dominant EDMD. There was a common activation of the ERK1/2 branch of the mitogen-activated protein kinase (MAPK) pathway in both murine models, as well as activation of downstream targets implicated in the pathogenesis of cardiomyopathy. Activation of MAPK signaling appears to be a cornerstone in the development of heart disease in both X-linked and autosomal dominant EDMD.

"Laminopathies": a wide spectrum of human diseases

Mutations in genes encoding the intermediate filament nuclear lamins and associated proteins cause a wide spectrum of diseases sometimes called "laminopathies." Diseases caused by mutations in LMNA encoding A-type lamins include autosomal dominant Emery-Dreifuss muscular dystrophy and related myopathies, Dunnigan-type familial partial lipodystrophy, Charcot-Marie-Tooth disease type 2B1 and developmental and accelerated aging disorders. Duplication in LMNB1 encoding lamin B1 causes autosomal dominant leukodystrophy and mutations in LMNB2 encoding lamin B2 are associated with acquired partial lipodystrophy. Disorders caused by mutations in genes encoding lamin-associated integral inner nuclear membrane proteins include X-linked Emery-Dreifuss muscular dystrophy, sclerosing bone dysplasias, HEM/Greenberg skeletal dysplasia and Pelger-Huet anomaly. While mutations and clinical phenotypes of "laminopathies" have been carefully described, data explaining pathogenic mechanisms are only emerging. Future investigations will likely identify new "laminopathies" and a combination of basic and clinical research will lead to a better understanding of pathophysiology and the development of therapies.

Desmin immunolocalisation in autosomal dominant Emery-Dreifuss muscular dystrophy

Autosomal dominant Emery-Dreifuss muscular dystrophy (AD-EDMD) is one of a number of allelic disorders caused by mutations in the nuclear lamina proteins, lamins A and C. The disorder is characterised by the early onset of skeletal muscle weakness and joint contractures and later, by dilated cardiomyopathy and cardiac arrythmias. Although the pathophysiology is not understood, one theory suggests that disordered structural organisation at weakened nuclei in contractile cells may underlie the disease. Previous work shows that mice deficient in lamin A/C develop similar skeletal and cardiac muscle signs to patients with AD-EDMD and ultrastructural examination of muscle from these mice shows abnormal localisation of desmin. We hypothesised therefore that desmin localisation may be abnormal in muscle or cells from patients with AD-EDMD and/or in cells expressing mutant lamins. In order to evaluate this, desmin immunolocalisation was determined in skeletal muscle biopsy sections from patients with AD-EDMD and cell lines including MyoD-transfected fibroblast-derived myotubes from AD-EDMD patients and murine embryonic stem cell-derived cardiomyocytes stably transfected with mutant human lamin A. Ultrastructural examination of patient muscle was also performed. Desmin was expressed and localised normally in patient muscle and cell lines and ultrastructural examination was similar to controls. These results fail to provide any evidence that dominant mutations in lamin A/C lead to a disorganisation of the desmin associated cytoskeleton.

Multiple roles for emerin: implications for Emery-Dreifuss muscular dystrophy

X-linked Emery-Dreifuss muscular dystrophy (X-EDMD) is inherited through mutations in EMD, which encodes a nuclear membrane protein named emerin. Emerin is expressed in most cells, but EDMD strikes specific tissues. This review summarizes growing evidence that emerin has roles in both tissue-specific gene regulation and the mechanical integrity of the nucleus and discusses how these roles might impact EDMD.

Lmo7 is an emerin-binding protein that regulates the transcription of emerin and many other muscle-relevant genes

X-linked Emery-Dreifuss muscular dystrophy (X-EDMD) is inherited through mutations in emerin, a nuclear membrane protein. Emerin has proposed roles in nuclear architecture and gene regulation, but direct molecular links to disease were unknown. We report that Lim-domain only 7 (Lmo7) binds emerin directly with 125 nM affinity; the C-terminal half of human Lmo7 (hLmo7C) was sufficient to bind emerin in vitro. Lmo7 appeared relevant to EDMD because a deletion that removes Lmo7 (plus eight exons of a neighboring gene) in mice causes dystrophic muscles [Semenova, E., Wang, X., Jablonski, M.M., Levorse, J. and Tilghman, S.M. (2003) An engineered 800 kilobase deletion of Uchl3 and Lmo7 on mouse chromosome 14 causes defects in viability, postnatal growth and degeneration of muscle and retina. Hum. Mol. Genet., 12, 1301-1312]. Lmo7 localizes in the nucleus, cytoplasm and cell surface, particularly adhesion junctions [Ooshio, T., Irie, K., Morimoto, K., Fukuhara, A., Imai, T. and Takai, Y. (2004) Involvement of LMO7 in the association of two cell-cell adhesion molecules, nectin and E-cadherin, through afadin and alpha-actinin in epithelial cells. J. Biol. Chem., 279, 31365-31373]. Our data suggest endogenous Lmo7 is a nucleocytoplasmic shuttling protein, and might also localize at focal adhesions in HeLa cells. Two key results show that Lmo7 regulates emerin gene expression: rat Lmo7 isoforms directly activated a luciferase reporter gene in vivo, and emerin mRNA expression decreased 93% in Lmo7-downregulated HeLa cells. Thus, Lmo7 not only binds emerin protein but is also required for emerin gene transcription. Microarray analysis of Lmo7-downregulated HeLa cells identified over 4200 misregulated genes, including 46 genes important for muscle or heart. Misregulation of 11 genes, including four (CREBBP, NAP1L1, LAP2, RBL2) known to be misregulated in X-EDMD patients and emerin-null mice [Bakay, M., Wang, Z., Melcon, G., Schiltz, L., Xuan, J., Zhao, P., Sartorelli, V., Seo, J., Pegoraro, E., Angelini, C. et al. (2006) Nuclear envelope dystrophies show a transcriptional fingerprint suggesting disruption of Rb-MyoD pathways in muscle regeneration. Brain, 129, 996-1013; Melcon, G., Kozlov, S., Cutler, D.A., Sullivan, T., Hernandez, L., Zhao, P., Mitchell, S., Nader, G., Bakay, M., Rottman, J.N. et al. (2006) Loss of emerin at the nuclear envelope disrupts the Rb1/E2F and MyoD pathways during muscle regeneration. Hum. Mol. Genet., 15, 637-651] was confirmed by real-time PCR. Overexpression of wild-type emerin, but not emerin mutant P183H (which causes EDMD and selectively disrupts binding to Lmo7), decreased the expression of CREBBP, NAP1L1 and LAP2, suggesting Lmo7 activity is both EDMD-relevant and inhibited by direct binding to emerin. We conclude that Lmo7 positively regulates many EDMD-relevant genes (including emerin), and is feedback-regulated by binding to emerin.

Emery-Dreifuss muscular dystrophy at the nuclear envelope: 10 years on

Emery-Dreifuss muscular dystrophy (EDMD) is a neuromuscular degenerative condition with an associated dilated cardiomyopathy and cardiac conduction defect. It can be inherited in either an X-linked or autosomal manner by mutations in the nuclear proteins emerin and lamin A/C, respectively. Traditionally muscular dystrophies were associated with defects in sarcolemma-associated proteins and, therefore, a nuclear connection suggested the existence of novel signalling pathways associated with this group of diseases. Subsequently, other mutations in the lamin A/C gene were attributed to a range of tissue-specific degenerative conditions, collectively known as the 'laminopathies'. Therefore, any proposed hypothesis underlying the molecular mechanism of EDMD needs to include this anomaly. As we celebrate the 10th anniversary of the identification of emerin as a component of the nuclear envelope, I discuss here the available evidence that currently implicates EDMD as arising from perturbations in myogenic regulatory pathways, causing temporal delays in both cell cycle progression and muscle regeneration.

The inner nuclear membrane protein emerin regulates beta-catenin activity by restricting its accumulation in the nucleus

Emerin is a type II inner nuclear membrane (INM) protein of unknown function. Emerin function is likely to be important because, when it is mutated, emerin promotes both skeletal muscle and heart defects. Here we show that one function of Emerin is to regulate the flux of beta-catenin, an important transcription coactivator, into the nucleus. Emerin interacts with beta-catenin through a conserved adenomatous polyposis coli (APC)-like domain. When GFP-emerin is expressed in HEK293 cells, beta-catenin is restricted to the cytoplasm and beta-catenin activity is inhibited. In contrast, expression of an emerin mutant, lacking its APC-like domain (GFP-emerinDelta), dominantly stimulates beta-catenin activity and increases nuclear accumulation of beta-catenin. Human fibroblasts that are null for emerin have an autostimulatory growth phenotype. This unusual growth phenotype arises through enhanced nuclear accumulation and activity of beta-catenin and can be replicated in wild-type fibroblasts by transfection with constitutively active beta-catenin. Our results support recent findings that suggest that INM proteins can influence signalling pathways by restricting access of transcription coactivators to the nucleus.

Lamin A/C assembly defects in Emery-Dreifuss muscular dystrophy can be regulated by culture medium composition

Emery-Dreifuss muscular dystrophy results from mutations in either emerin or lamin A/C and is caused by loss of some unknown function of emerin-lamin A/C complexes. This function must be of special importance in the skeletal and cardiac muscles that are affected by the disease. Some lamin A/C mutant proteins form 'nuclear foci' in the nucleoplasm when overexpressed by transient transfection and similar aggregates have been seen in cultured skin fibroblasts from patients with Emery-Dreifuss muscular dystrophy, suggesting that mis-assembly of the A-type lamina may be involved in the pathogenesis. Whereas an earlier study of cultured skin fibroblasts compared several different missense mutations in lamin A/C, we have chosen to study one particular Emery-Dreifuss mutation (R249Q) in greater detail. We found that the proportion of fibroblast nuclei containing abnormal lamin A/C aggregates can vary from 0.5 to 23.6% depending on the culture conditions. In particular, switching from a 'slow growth' medium to 'rapid growth' media increased both the number and size of nuclear aggregates. Similar results were obtained with fibroblasts from a second unrelated patient with the same mutation. In contrast to these aggregates of endogenous lamin A/C, 'nuclear foci' formed after transfection of mouse embryo fibroblasts by mutant lamin A/C were not affected by culture conditions. Faulty assembly of the nuclear lamina by mutated lamin A/C molecules could be partly responsible for the disease phenotype, though this has not been proven. The present study suggests that inappropriate lamin A/C assembly may be preventable by manipulation of cell growth conditions.

Lamin A N-terminal phosphorylation is associated with myoblast activation: impairment in Emery-Dreifuss muscular dystrophy

BACKGROUND

Skeletal muscle disorders associated with mutations of lamin A/C gene include autosomal Emery-Dreifuss muscular dystrophy and limb girdle muscular dystrophy 1B. The pathogenic mechanism underlying these diseases is unknown. Recent data suggest an impairment of signalling mechanisms as a possible cause of muscle malfunction. A molecular complex in muscle cells formed by lamin A/C, emerin, and nuclear actin has been identified. The stability of this protein complex appears to be related to phosphorylation mechanisms.

OBJECTIVE

To analyse lamin A/C phosphorylation in control and laminopathic muscle cells.

METHODS

Lamin A/C N-terminal phosphorylation was determined in cultured mouse myoblasts using a specific antibody. Insulin treatment of serum starved myoblast cultures was carried out to evaluate involvement of insulin signalling in the phosphorylation pathway. Screening of four Emery-Dreifuss and one limb girdle muscular dystrophy 1B cases was undertaken to investigate lamin A/C phosphorylation in both cultured myoblasts and mature muscle fibres.

RESULTS

Phosphorylation of lamin A was observed during myoblast differentiation or proliferation, along with reduced lamin A/C phosphorylation in quiescent myoblasts. Lamin A N-terminus phosphorylation was induced by an insulin stimulus, which conversely did not affect lamin C phosphorylation. Lamin A/C was also hyperphosphorylated in mature muscle, mostly in regenerating fibres. Lamin A/C phosphorylation was strikingly reduced in laminopathic myoblasts and muscle fibres, while it was preserved in interstitial fibroblasts.

CONCLUSIONS

Altered lamin A/C interplay with a muscle specific phosphorylation partner might be involved in the pathogenic mechanism of Emery-Dreifuss muscular dystrophy and limb girdle muscular dystrophy 1B.

Nuclear envelope dystrophies show a transcriptional fingerprint suggesting disruption of Rb–MyoD pathways in muscle regeneration

Mutations of lamin A/C (LMNA) cause a wide range of human disorders, including progeria, lipodystrophy, neuropathies and autosomal dominant Emery-Dreifuss muscular dystrophy (EDMD). EDMD is also caused by X-linked recessive loss-of-function mutations of emerin, another component of the inner nuclear lamina that directly interacts with LMNA. One model for disease pathogenesis of LMNA and emerin mutations is cell-specific perturbations of the mRNA transcriptome in terminally differentiated cells. To test this model, we studied 125 human muscle biopsies from 13 diagnostic groups (125 U133A, 125 U133B microarrays), including EDMD patients with LMNA and emerin mutations. A Visual and Statistical Data Analyzer (VISDA) algorithm was used to statistically model cluster hierarchy, resulting in a tree of phenotypic classifications. Validations of the diagnostic tree included permutations of U133A and U133B arrays, and use of two probe set algorithms (MAS5.0 and MBEI). This showed that the two nuclear envelope defects (EDMD LMNA, EDMD emerin) were highly related disorders and were also related to fascioscapulohumeral muscular dystrophy (FSHD). FSHD has recently been hypothesized to involve abnormal interactions of chromatin with the nuclear envelope. To identify disease-specific transcripts for EDMD, we applied a leave-one-out (LOO) cross-validation approach using LMNA patient muscle as a test data set, with reverse transcription-polymerase chain reaction (RT-PCR) validations in both LMNA and emerin patient muscle. A high proportion of top-ranked and validated transcripts were components of the same transcriptional regulatory pathway involving Rb1 and MyoD during muscle regeneration (CRI-1, CREBBP, Nap1L1, ECREBBP/p300), where each was specifically upregulated in EDMD. Using a muscle regeneration time series (27 time points) we develop a transcriptional model for downstream consequences of LMNA and emerin mutations. We propose that key interactions between the nuclear envelope and Rb and MyoD fail in EDMD at the point of myoblast exit from the cell cycle, leading to poorly coordinated phosphorylation and acetylation steps. Our data is consistent with mutations of nuclear lamina components leading to destabilization of the transcriptome in differentiated cells.

Nuclear lamin A inhibits adipocyte differentiation: implications for Dunnigan-type familial partial lipodystrophy

Mutations in the LMNA gene encoding A-type lamins cause several diseases, including Emery-Dreifuss muscular dystrophy and Dunnigan-type familial partial lipodystrophy (FPLD). We analyzed differentiation of 3T3-L1 preadipocytes to adipocytes in cells overexpressing wild-type lamin A as well as lamin A with amino acid substitutions at position 482 that cause FPLD. We also examined adipogenic conversion of mouse embryonic fibroblasts lacking A-type lamins. Overexpression of both wild-type and mutant lamin A inhibited lipid accumulation, triglyceride synthesis and expression of adipogenic markers. This was associated with inhibition of expression of peroxisome-proliferator-activated receptor gamma 2 (PPARgamma2) and Glut4. In contrast, embryonic fibroblasts lacking A-type lamins accumulated more intracellular lipid and exhibited elevated de novo triglyceride synthesis compared with wild-type fibroblasts. They also had increased basal phosphorylation of AKT1, a mediator of insulin signaling. We conclude that A-type lamins act as inhibitors of adipocyte differentiation, possibly by affecting PPARgamma2 and insulin signaling.

Loss of emerin at the nuclear envelope disrupts the Rb1/E2F and MyoD pathways during muscle regeneration

Emery-Dreifuss muscular dystrophy (EDMD1) is caused by mutations in either the X-linked gene emerin (EMD) or the autosomal lamin A/C (LMNA) gene. Here, we describe the derivation of mice lacking emerin in an attempt to derive a mouse model for EDMD1. Although mice lacking emerin show no overt pathology, muscle regeneration in these mice revealed defects. A bioinformatic array analysis of regenerating Emd null muscle revealed abnormalities in cell-cycle parameters and delayed myogenic differentiation, which were associated with perturbations to transcriptional pathways regulated by the retinoblastoma (Rb1) and MyoD genes. Temporal activation of MyoD transcriptional targets was significantly delayed, whereas targets of the Rb1/E2F transcriptional repressor complex remained inappropriately active. The inappropriate modulation of Rb1/MyoD transcriptional targets was associated with up-regulation of Rb1, MyoD and their co-activators/repressors transcripts, suggesting a compensatory effort to overcome a molecular block to differentiation at the myoblast/myotube transition during regeneration. This compensation appeared to be effective for MyoD transcriptional targets, although was less effective for Rb1 targets. Analysis of Rb1 phosphorylation states showed prolonged hyper-phosphorylation at key developmental stages in Emd null myogenic cells, both in vivo and in vitro. We also analyzed the same pathways in Lmna null muscle, which shows extensive dystrophy. Surprisingly, Lmna null muscle did not show the same perturbations to Rb- and MyoD-dependent pathways. We did observe increased transcriptional expression of Lap2alpha and delayed expression of Rb1, which may regulate alternative transcriptional pathways in the Lmna null myoblasts. We suggest that the dominant LMNA mutations seen in many clinically disparate laminopathies may similarly alter Rb function, with regard to either the timing of exit from the cell cycle or terminal differentiation programs or both.

Nuclear envelope proteins and chromatin arrangement: a pathogenic mechanism for laminopathies

The involvement of the nuclear envelope in the modulation of chromatin organization is strongly suggested by the increasing number of human diseases due to mutations of nuclear envelope proteins. A common feature of these diseases, named laminopathies, is the occurrence of major chromatin defects. We previously reported that cells from laminopathic patients show an altered nuclear profile, and loss or detachment of heterochromatin from the nuclear envelope. Recent evidence indicates that processing of the lamin A precursor is altered in laminopathies featuring pre-mature aging and/or lipodystrophy phenotype. In these cases, pre-lamin A is accumulated in the nucleus and heterochromatin is severely disorganized. Here we report evidence indicating that pre-lamin A is mis-localized in the nuclei of Emery-Dreifuss muscular dystrophy fibroblasts, either bearing lamin A/C or emerin mutations. Abnormal pre-lamin A-containing structures are formed following treatment with a farnesyl-transferase inhibitor, a drug that causes accumulation of pre-lamin A. Pre-lamin A-labeled structures co-localize with heterochromatin clumps. These data indicate that in almost all laminopathies the expression of the mutant lamin A precursor disrupts the organization of heterochromatin domains. Our results further show that the absence of emerin expression alters the distribution of pre-lamin A and of heterochromatin areas, suggesting a major involvement of emerin in pre-lamin A-mediated mechanisms of chromatin remodeling.

[Clinical, genealogical and molecular genetic study of Emery-Dreifuss muscular dystrophy]

A search for emerin and lamin A/C (LMNA) mutations was performed in a group of 63 unrelated patients with probable Emery-Dreifuss muscular dystrophy (EDMD) and other MD's with concomitant dilated cardiomyopathy (DCMP). Four different emerin mutations and 7 LMNA mutations were found in unrelated patients. One emerin mutation and 2 LMNA mutations, one of the latter being found twice, have been registered earlier; the rest of the mutations are novel. All the patients with emerin mutations and 3 patients with LMNA mutations represented single cases while 4 LMNA-related cases were familial. De novo origin was proved for one emerin and 3 LMNA mutations. Apart from EDMD phenotypes, varying also in age at onset and severity, 2 cases of limb girdle MD type 1B were diagnosed. One patient with LMNA mutation and severe DCMP had subclinical signs of skeletal myopathy only. There was an overlap between DCMP type 1A and MD's. Autosomal dominant EDMD seems to be more common than "classic" X-linked EDMD. We found neither emerin nor LMNA mutations in a subset of families with EDMD-like phenotypes that may imply an existence of other genes causing similar disorders. Taking into account clinical variability of MD's caused by emerin and LMNA mutations, DNA diagnosis should not confine to the "classic" phenotype. DNA diagnosis of EDMD is important boht for medical genetic counseling and for patients' management: timely diagnosis of the disease allows one to prevent fatal cardiologic complications.

Dissociation of Emerin from Barrier-to-autointegration Factor Is Regulated through Mitotic Phosphorylation of Emerin in a Xenopus Egg Cell-free System

Emerin is the gene product of STA whose mutations cause Emery-Dreifuss muscular dystrophy. It is an inner nuclear membrane protein and phosphorylated in a cell cycle-dependent manner. However, the means of phosphorylation of emerin are poorly understood. We investigated the regulation mechanism for the binding of emerin to chromatin, focusing on its cell cycle-dependent phosphorylation in a Xenopus egg cell-free system. It was shown that emerin dissociates from chromatin depending on mitotic phosphorylation of the former, and this plays a critical role in the dissociation of emerin from barrier-to-autointegration factor (BAF). Then, we analyzed the mitotic phosphorylation sites of emerin. Emerin was strongly phosphorylated in an M-phase Xenopus egg cell-free system, and five phosphorylated sites, Ser49, Ser66, Thr67, Ser120, and Ser175, were identified on analysis of chymotryptic and tryptic emerin peptides using a phosphopeptide-concentrating system coupled with a Titansphere column, which specifically binds phosphopeptides, and tandem mass spectrometry sequencing. An in vitro binding assay involving an emerin S175A point mutant protein suggested that phosphorylation at Ser175 regulates the dissociation of emerin from BAF.

Lamins A and C are differentially dysfunctional in autosomal dominant Emery-Dreifuss muscular dystrophy

Mutations in the LMNA gene, which encodes nuclear lamins A and C by alternative splicing, can give rise to Emery-Dreifuss muscular dystrophy. The mechanism by which lamins A and C separately contribute to this molecular phenotype is unknown. To address this question we examined ten LMNA mutations exogenously expressed as lamins A and C in COS-7 cells. Eight of the mutations when expressed in lamin A, exhibited a range of nuclear mislocalisation patterns. However, two mutations (T150P and delQ355) almost completely relocated exogenous lamin A from the nuclear envelope to the cytoplasm, disrupted nuclear envelope reassembly following cell division and altered the protein composition of the mid-body. In contrast, exogenously expressed DsRed2-tagged mutant lamin C constructs were only inserted into the nuclear lamina if co-expressed with any EGFP-tagged lamin A construct, except with one carrying the T150P mutation. The T150P, R527P and L530P mutations reduced the ability of lamin A, but not lamin C from binding to emerin. These data identify specific functional roles for the emerin-lamin C- and emerin-lamin A- containing protein complexes and is the first report to suggest that the A-type lamin mutations may be differentially dysfunctional for the same LMNA mutation.

Abnormal nuclear shape and impaired mechanotransduction in emerin-deficient cells

Emery-Dreifuss muscular dystrophy can be caused by mutations in the nuclear envelope proteins lamin A/C and emerin. We recently demonstrated that A-type lamin-deficient cells have impaired nuclear mechanics and altered mechanotransduction, suggesting two potential disease mechanisms (Lammerding, J., P.C. Schulze, T. Takahashi, S. Kozlov, T. Sullivan, R.D. Kamm, C.L. Stewart, and R.T. Lee. 2004. J. Clin. Invest. 113:370–378). Here, we examined the function of emerin on nuclear mechanics and strain-induced signaling. Emerin-deficient mouse embryo fibroblasts have abnormal nuclear shape, but in contrast to A-type lamin-deficient cells, exhibit nuclear deformations comparable to wild-type cells in cellular strain experiments, and the integrity of emerin-deficient nuclear envelopes appeared normal in a nuclear microinjection assay. Interestingly, expression of mechanosensitive genes in response to mechanical strain was impaired in emerin-deficient cells, and prolonged mechanical stimulation increased apoptosis in emerin-deficient cells. Thus, emerin-deficient mouse embryo fibroblasts have apparently normal nuclear mechanics but impaired expression of mechanosensitive genes in response to strain, suggesting that emerin mutations may act through altered transcriptional regulation and not by increasing nuclear fragility.

Nesprins: intracellular scaffolds that maintain cell architecture and coordinate cell function?

Nesprins are a recently discovered family of ubiquitously expressed intracellular proteins. Through alternative transcriptional initiation, termination and splicing, two genes - nesprin-1 and nesprin-2 (also known as syne-1 and syne-2) - give rise to many protein isoforms that vary markedly in size. The largest of these isoforms comprise a C-terminal transmembrane domain (the KLS domain) linked by a spectrin-repeat rod domain to an N-terminal paired, actin-binding, calponin-homology domain. This structure suggests that they are well suited to orchestrate signalling between cell membranes and the cytoskeleton. Other isoforms have variable lengths of this rod domain linked to either end of the protein. Smaller isoforms with the KLS domain are localised at the inner nuclear membrane, where they bind lamin A/C and emerin. Larger nesprin isoforms link the outer nuclear membrane with intracellular organelles and the actin cytoskeleton and are thought to regulate nuclear anchorage and organelle migration. Thus, nesprins might have a variety of fundamental roles in cells, particularly muscle cells where they are highly expressed. We speculate that nesprin mutations might contribute to a broad range of human disease syndromes, including laminopathies.

Both lamin A and lamin C mutations cause lamina instability as well as loss of internal nuclear lamin organization

We have applied the fluorescence loss of intensity after photobleaching (FLIP) technique to study the molecular dynamics and organization of nuclear lamin proteins in cell lines stably transfected with green fluorescent protein (GFP)-tagged A-type lamin cDNA. Normal lamin A and C proteins show abundant decoration of the inner layer of the nuclear membrane, the nuclear lamina, and a generally diffuse localization in the nuclear interior. Bleaching studies revealed that, while the GFP-tagged lamins in the lamina were virtually immobile, the intranuclear fraction of these molecules was partially mobile. Intranuclear lamin C was significantly more mobile than intranuclear lamina A. In search of a structural cause for the variety of inherited diseases caused by A-type lamin mutations, we have studied the molecular organization of GFP-tagged lamin A and lamin C mutants R453W and R386K, found in Emery-Dreifuss muscular dystrophy (EDMD), and lamin A and lamin C mutant R482W, found in patients with Dunnigan-type familial partial lipodystrophy (FPLD). In all mutants, a prominent increase in lamin mobility was observed, indicating loss of structural stability of lamin polymers, both at the perinuclear lamina and in the intranuclear lamin organization. While the lamin rod domain mutant showed overall increased mobility, the tail domain mutants showed mainly intranuclear destabilization, possibly as a result of loss of interaction with chromatin. Decreased stability of lamin mutant polymers was confirmed by flow cytometric analyses and immunoblotting of nuclear extracts. Our findings suggest a loss of function of A-type lamin mutant proteins in the organization of intranuclear chromatin and predict the loss of gene regulatory function in laminopathies.

A lamin-dependent pathway that regulates nuclear organization, cell cycle progression and germ cell development

The C. elegans genome encodes a single lamin protein (Ce-lamin), three LEM domain proteins (Ce-emerin, Ce-MAN1 and LEM-3) and a single BAF protein (Ce-BAF). Down-regulation of Ce-lamin causes embryonic lethality. Abnormalities include rapid changes in nuclear morphology during interphase, inability of cells to complete mitosis, abnormal condensation of chromatin, clustering of nuclear pore complexes (NPCs), and missing or abnormal germ cells. Ce-emerin and Ce-MAN1 are both embedded in the inner nuclear membrane, and both bind Ce-lamin and Ce-BAF; in addition, both require Ce-lamin for their localization. Mutations in human emerin cause X-linked recessive Emery-Dreifuss muscular dystrophy. In C. elegans, loss of Ce-emerin alone has no detectable phenotype, while loss of 90% Ce-MAN1 causes approximately 15% embryonic lethality. However in worms that lack Ce-emerin, a approximately 90% reduction of Ce-MAN1 is lethal to all embryos by the 100-cell stage, with a phenotype involving chromatin condensation and repeated cycles of anaphase chromosome bridging and cytokinesis. The anaphase-bridged chromatin retained a mitosis-specific phosphohistone H3 epitope, and failed to recruit detectable Ce-lamin or Ce-BAF. Down-regulation of Ce-BAF showed similar phenotypes. These findings suggest that lamin, LEM-domain proteins and BAF are part of a lamina network essential for chromatin organization and cell division, and that Ce-emerin and Ce-MAN1 share at least one and possibly multiple overlapping functions, which may be relevant to Emery-Dreifuss muscular dystrophy.

Emery-Dreiffus muscular dystrophy: MR imaging and spectroscopy in the brain and skeletal muscle

Emery-Dreifuss muscular dystrophy is a rare disorder characterized by childhood onset of contractures, humeroperoneal muscle atrophy, and cardiac conduction abnormalities. This report presents the cases of two brothers with this dystrophy in whom bilateral hypomyelination of the deep periatrial white matter was noted. In the hypomyelinated regions, a prominent peak centered at 1.5 parts per million was present on short-TE MR spectra likely representing prominence of proteolipids in the macromolecular region. Major peaks (N-acetyl-aspartate, creatine, choline, and myoinositol) were normal. With respect to muscle changes, atrophy of the medial head of the gastrocnemius muscle was noted at MR imaging, and phosphorus spectroscopy of this muscle revealed decreased phosphocreatine and inorganic phosphate peaks.

Emerin expression in tubular aggregates

Emerin is an inner nuclear membrane protein that is mutated or not expressed in patients with X-linked Emery-Dreifuss muscular dystrophy (X-EDMD/EMD). Cytoplasmic localization of emerin in cultured cells or tissues has been reported, although this remains a controversial issue. Tubular aggregates (TAs) are pathological structures seen in the sarcoplasm of human skeletal muscle fibers in various disorders. The TAs derive from the sarcoplasmic reticulum (SR) and represent, probably, an adaptive response of the SR to various insults to the muscle fibers. In the present study, we present immunohistochemical evidence of emerin expression in TAs. Muscle biopsies with tubular aggregates from four male, unrelated patients were studied. The percentage of muscle fibers containing TAs varied between 5 and 20%. Routine histochemistry revealed intense reaction of TAs with NADH-TR, AMPDA, and NSE, but not with COX, SDH, myosin ATPase (pH 9.4, 4.3, 4.6), PAS, and Oil red O staining. Immunohistochemical study revealed strong immunostaining of TAs with antibodies against emerin and 7 SERCA2-ATPase. Immunostaining of TAs was also seen with antibodies against heat shock protein and dysferlin, but not with antibodies to lamin A, dystrophin, adhalin, beta, gamma, delta sarcoglycans, and merosin. These results suggest that emerin, an inner nuclear membrane protein, is present at the TAs. The interpretation and significance of this finding is discussed in relation to experimental data suggesting that normal emerin localization at the inner nuclear membrane depends on lamin A and mutations in the N-terminal domain of emerin cause mislocalization of the protein to the sarcoplasmic membranes.

Expression and localization of nuclear proteins in autosomal-dominant Emery-Dreifuss muscular dystrophy with LMNA R377H mutation

BACKGROUND

The autosomal dominant form of Emery-Dreifuss muscular dystrophy (AD-EDMD) is caused by mutations in the gene encoding for the lamins A and C (LMNA). Lamins are intermediate filament proteins which form the nuclear lamina underlying the inner nuclear membrane. We have studied the expression and the localization of nuclear envelope proteins in three different cell types and muscle tissue of an AD-EDMD patient carrying a point mutation R377H in the lamin A/C gene.

RESULTS

Lymphoblastoid cells, skin fibroblasts, primary myoblasts and muscle thin sections were studied by immunocytochemistry and electron microscopy. Cellular levels of A-type lamins were reduced compared to control cells. In contrast, the amount of emerin and lamin B appeared unaltered. Cell synchronization experiments showed that the reduction of the cellular level of A-type lamin was due to instability of lamin A. By electron microscopy, we identified a proportion of nuclei with morphological alterations in lymphoblastoid cells, fibroblasts and mature muscle fibres. Immunofluorescence microscopy showed that a major population of the lamin B receptor (LBR), an inner nuclear membrane protein, was recovered in the cytoplasm in association with the ER. In addition, the intranuclear organization of the active form of RNA polymerase II was markedly different in cells of this AD-EDMD patient. This aberrant intranuclear distribution was specifically observed in muscle cells where the pathology of EDMD predominates.

CONCLUSIONS

From our results we conclude: Firstly, that structural alterations of the nuclei which are found only in a minor fraction of lymphoblastoid cells and mature muscle fibres are not sufficient to explain the clinical pathology of EDMD; Secondly, that wild type lamin A is required not only for the retention of LBR in the inner nuclear membrane but also for a correct localization of the transcriptionally active RNA pol II in muscle cells. We speculate that a rearrangement of the internal chromatin could lead to muscle-specific disease symptoms by interference with proper mRNA transcription.

Emerin binding to Btf, a death-promoting transcriptional repressor, is disrupted by a missense mutation that causes Emery-Dreifuss muscular dystrophy

Loss of functional emerin, a nuclear membrane protein, causes X-linked recessive Emery-Dreifuss muscular dystrophy. In a yeast two-hybrid screen, we found that emerin interacts with Btf, a death-promoting transcriptional repressor, which is expressed at high levels in skeletal muscle. Biochemical analysis showed that emerin binds Btf with an equilibrium affinity (KD) of 100 nm. Using a collection of 21 clustered alanine-substitution mutations in emerin, the residues required for binding to Btf mapped to two regions of emerin that flank its lamin-binding domain. Two disease-causing mutations in emerin, S54F and Delta95-99, disrupted binding to Btf. The Delta95-99 mutation was relatively uninformative, as this mutation also disrupts emerin binding to lamin A and a different transcription repressor named germ cell-less (GCL). In striking contrast, emerin mutant S54F, which binds normally to barrier-to-autointegration factor, lamin A and GCL, selectively disrupted emerin binding to Btf. We localized endogenous Btf in HeLa cells by indirect immunoflurorescence using affinity-purified antibodies against Btf. In nonapoptotic HeLa cells Btf was found in dot-like structures throughout the nuclear interior. However, within 3 h after treating cells with Fas antibody to induce apoptosis, the distribution of Btf changed, and Btf concentrated in a distinct zone near the nuclear envelope. These results suggest that Btf localization is regulated by apoptotic signals, and that loss of emerin binding to Btf may be relevant to muscle wasting in Emery-Dreifuss muscular dystrophy.

Increased resting energy expenditure in subjects with Emery-Dreifuss muscular dystrophy

We have studied changes in energy expenditure and body composition in adult males with Emery-Dreifuss muscular dystrophy, age-matched males with hyperCKemia and age-matched healthy controls. All participants were studied twice, 2-3 years apart. Resting energy expenditure was studied by indirect calorimetry, lean body mass and body fat by dual X-ray absorptiometry, and muscle mass was estimated based on 24-h urinary creatinine excretion. At baseline and 2-3 years later, body fat was significantly higher (P < 0.011 and P < 0.003, respectively) and lean body mass significantly lower (P < 0.024 and P < 0.012, respectively) in patients with Emery-Dreifuss muscular dystrophy as compared to subjects with hyperCKemia and healthy controls. Resting energy expenditure, over the study period, increased significantly in patients with Emery-Dreifuss muscular dystrophy (P < 0.031), but not in patients with hyperCKemia nor in healthy controls. Our study suggests that patients with Emery-Dreifuss muscular dystrophy may have increased energy expenditure relative to healthy subjects. If not met by increased caloric intake, this greater energy expenditure may partially contribute to a further deterioration in their muscle performance.

Immunocytochemistry of nuclear domains and Emery-Dreifuss muscular dystrophy pathophysiology

The present review summarizes recent cytochemical findings on the functional organization of the nuclear domains, with a particular emphasis on the relation between nuclear envelope-associated proteins and chromatin. Mutations in two nuclear envelope-associated proteins, emerin and lamin A/C cause the Emery-Dreifuss muscular dystrophy; the cellular pathology associated with the disease and the functional role of emerin and lamin A/C in muscle cells are not well established. On the other hand, a large body of evidence indicates that nuclear envelope-associated proteins are involved in tissue-specific gene regulation. Moreover, chromatin remodeling complexes trigger gene expression by utilizing the nuclear matrix-associated actin, which is known to interact with both emerin and lamin A/C. It is thus conceivable that altered expression of these nuclear envelope-associated proteins can account for an impairment of gene expression mainly during cell differentiation as suggested by recent experimental findings on the involvement of emerin in myogenesis. The possibility that Emery-Deifuss muscular dystrophy pathogenesis could involve alteration of the signaling pathway is considered.

Expression of emerin and lamins in muscle of patients with different forms of Emery-Dreifuss muscular dystrophy

Emerin and lamins are nuclear proteins, which are missing or defective in Emery-Dreifuss muscular dystrophy (EDMD). The aim of this study was to test the expression of these proteins in skeletal muscles in the X-linked (X-EDMD) and autosomal dominant (AD-EDMD) form. The study group consisted of 11 patients with X-EDMD, 11 patients of the AD-EDMD and 20 age-matched normal subjects. Expression of emerin and lamins in muscles were analyzed by Western blotting and the immunocytochemical technique. Using the Western blotting procedure emerin was detected in traces in the X-linked form. In the majority of these cases (6/11) it was connected with a decreased concentration of lamin A, in four patients a lowered concentration of lamin C was present. Lamin B2 was either normal (8/11), or decreased (3/11). Deficit of lamin A was a characteristic feature for AD-EDMD in the majority of these cases (9/11), while in two of these patients a decrease of lamin C, in four cases a lowered level of emerin was also present. In one AD-EDMD patient of a decrease of lamin C, but normal lamin A was present. Following the immunocytochemical examination the decrease of lamin A/C in X-EDMD and of emerin in AD-EDMD was also observed. The above mentioned data demonstrated that in X-EDMD and AD-EDMD the deficit of the appropriate proteins is not restricted either to emerin or lamins. The defect is more widespread and results in disruption of several nuclear proteins. This study also indicated that for the diagnostic EDMD purposes the immunocytochemical detection of emerin/lamins has to be accomplished by quantitative immunochemical analyses of the above mentioned proteins.

Effect of pathogenic mis-sense mutations in lamin A on its interaction with emerin in vivo

Mutations in lamin A/C can cause Emery-Dreifuss muscular dystrophy (EDMD) or a related cardiomyopathy (CMD1A). Using transfection of lamin-A/C-deficient fibroblasts, we have studied the effects of nine pathogenic mutations on the ability of lamin A to assemble normally and to localize emerin normally at the nuclear rim. Five mutations in the rod domain (L85R, N195K, E358K, M371K and R386K) affected the assembly of the lamina. With the exception of mutant L85R, all rod domain mutants induced the formation of large nucleoplasmic foci in about 10% of all nuclei. The presence of emerin in these foci suggests that the interaction of lamin A with emerin is not directly affected by the rod domain mutations. Three mutations in the tail region, R453W, W520S and R527P, might directly affect emerin binding by disrupting the structure of the putative emerin-binding site, because mutant lamin A localized normally to the nuclear rim but its ability to trap emerin was impaired. Nucleoplasmic foci rarely formed in these three cases (<2%) but, when they did so, emerin was absent, consistent with a direct effect of the mutations on emerin binding. The lipodystrophy mutation R482Q, which causes a different phenotype and is believed to act through an emerin-independent mechanism, was indistinguishable from wild-type in its localization and its ability to trap emerin at the nuclear rim. The novel hypothesis suggested by the data is that EDMD/CMD1A mutations in the tail domain of lamin A/C work by direct impairment of emerin interaction, whereas mutations in the rod region cause defective lamina assembly that might or might not impair emerin capture at the nuclear rim. Subtle effects on the function of the lamina-emerin complex in EDMD/CMD1A patients might be responsible for the skeletal and/or cardiac muscle phenotype.

Emerin interacts in vitro with the splicing-associated factor, YT521-B

Emerin is a nuclear membrane protein that interacts with lamin A/C at the nuclear envelope. Mutations in either emerin or lamin A/C cause Emery-Dreifuss muscular dystrophy (EDMD). The functions of emerin are poorly understood, but EDMD affects mainly skeletal and cardiac muscle. We used a high-stringency yeast two-hybrid method to screen a human heart cDNA library, with full-length emerin as bait. Four out of five candidate interactors identified were nuclear proteins: lamin A, splicing factor YT521-B, proteasome subunit PA28 gamma and transcription factor vav-1. Specific binding between emerin and the functional C-terminal domain of YT521-B was confirmed by pull-down assays and biomolecular interaction analysis (BIAcore). Inhibition by emerin of YT521-B-dependent splice site selection in vivo suggests that the interaction is physiologically significant. A 'bipartite' binding site for YT521-B in emerin was identified using alanine substitution or disease-associated mutations in emerin. The transcription factor GCL (germ cell-less) has previously been shown to bind to the same site. The results are consistent with an emerging view that lamins and lamina-associated proteins, like emerin, have a regulatory role, as well as a structural role in the nucleus. YT521-B joins a growing list of candidates for a role in a gene expression model of the pathogenesis of EDMD.

Effects of expressing lamin A mutant protein causing Emery-Dreifuss muscular dystrophy and familial partial lipodystrophy in HeLa cells

Patients with the autosomal dominant form of Emery-Dreifuss muscular dystrophy (EDMD) or familial partial lipodystrophy (FPLD) have specific mutations in the lamin A gene. Three such point mutations, G465D (FPLD), R482L, (FPLD), or R527P (EDMD), were introduced by site-specific mutagenesis in the C-terminal tail domain of a FLAG-tagged full-length lamin A construct. HeLa cells were transfected with mutant and wild-type constructs. Lamin A accumulated in nuclear aggregates and the number of cells with aggregates increased with time after transfection. At 72 h post transfection 60-80% of cells transfected with the mutant lamin A constructs had aggregates, while only 35% of the cells transfected with wild-type lamin A revealed aggregates. Mutant transfected cells expressed 10-24x, and wild-type transfected cells 20x, the normal levels of lamin A. Lamins C, B1 and B2, Nup153, LAP2, and emerin were recruited into aggregates, resulting in a decrease of these proteins at the nuclear rim. Aggregates were also characterized by electron microscopy and found to be preferentially associated with the inner nuclear membrane. Aggregates from mutant constructs were larger than those formed by the wild-type constructs, both in immunofluorescence and electron microscopy. The combined results suggest that aggregate formation is in part due to overexpression, but that there are also mutant-specific effects.

Expression of lamin A mutated in the carboxyl-terminal tail generates an aberrant nuclear phenotype similar to that observed in cells from patients with Dunnigan-type partial lipodystrophy and Emery-Dreifuss muscular dystrophy

Autosomal dominantly inherited missense mutations in lamins A and C cause familial partial lipodystrophy of the Dunnigan-type (FPLD), and myopathies including Emery-Dreifuss muscular dystrophy (EDMD). While mutations responsible for FPLD are restricted to the carboxyl-terminal tails, those responsible for EDMD are spread throughout the molecules. We observed here the same structural abnormalities in the nuclear envelope and chromatin of fibroblasts from patients with FPLD and EDMD, harboring missense mutations at codons 482 and 453, respectively. Similar nuclear alterations were generated in fibroblasts, myoblasts, and preadipocytes mouse cell lines overexpressing lamin A harboring either of these two mutations. A large variation in sensitivity to lamin A overexpression was observed among the three cell lines, which was correlated with their variable endogenous content in A-type lamins and emerin. The occurrence of nuclear abnormalities was reduced when lamin B1 was coexpressed with mutant lamin A, emphasizing the functional interaction of the two types of lamins. Transfected cells therefore develop similar phenotypes when expressing lamins mutated in the carboxyl-terminal tail at sites responsible for FPLD or EDMD.

Functional domains of the nucleus: implications for Emery-Dreifuss muscular dystrophy

Elucidation of the pathophysiology of Emery-Dreifuss muscular dystrophy, caused by mutations in emerin or lamin A/C, will require deciphering the role of these proteins in the functional organization of the nuclear envelope. This review focuses on nuclear envelope related mechanisms that modulate chromatin arrangement and control of gene transcription, both potential targets of the disease process in Emery-Dreifuss muscular dystrophy. Interactions of these proteins with chromatin- and nuclear matrix-associated proteins are now of particular interest, since chromatin alterations occur in cells in Emery-Dreifuss muscular dystrophy. Both emerin and lamin A/C interact with nuclear actin, a component of the chromatin remodeling complex associated with the nuclear matrix, suggesting that either chromatin arrangement, or gene transcription, or both, might be impaired in the disease.

Increased solubility of lamins and redistribution of lamin C in X-linked Emery-Dreifuss muscular dystrophy fibroblasts

Emery-Dreifuss muscular dystrophy (EDMD) is caused by mutations in the gene encoding the nuclear membrane protein emerin (X-linked EDMD) or in the gene encoding lamins A/C (autosomal dominant EDMD). One hypothesis explaining the disease suggests that the mutations lead to weakness of the nuclear lamina. To test this hypothesis we investigated lamin solubility and distribution in skin fibroblasts from X-EDMD patients. Using in situ extraction of cells and immunofluorescence microscopy or biochemical fractionation and immunoblotting, we found that all lamin subtypes displayed increased solubility properties in fibroblasts from X-EDMD patients compared to normal individuals. Lamin and emerin solubility was mildly increased in fibroblasts from an X-EDMD carrier. Biochemical fractionation and immunoblotting also indicated that lamin C but no other lamin became redistributed from the nuclear lamina to the nucleoplasm in X-EDMD fibroblasts. Indirect immunofluorescence and confocal microscopy studies using lamin A- and lamin C-specific antibodies confirmed that lamin C but not lamin A became redistributed to the nucleoplasm. Interestingly, the lamin A/C binding protein LAP2alpha was also mislocalized in X-EDMD fibroblasts.