Properties of lamin A mutants found in Emery-Dreifuss muscular dystrophy, cardiomyopathy and Dunnigan-type partial lipodystrophy

Dependence of diffusional mobility of integral inner nuclear membrane proteins on A-type lamins

Integral proteins of the nuclear envelope inner membrane have been proposed to reach their sites by diffusion after their co-translational insertion in the rough endoplasmic reticulum. They are then retained in the inner nuclear membrane by binding to nuclear structures. One such structure is the nuclear lamina, an intermediate filament meshwork composed of A-type and B-type lamin proteins. Emerin, MAN1, and LBR are three integral inner nuclear membrane proteins. We expressed these proteins fused to green fluorescent protein in embryonic fibroblasts from wild-type mice and Lmna -/- mice, which lack A-type lamins. We then studied the diffusional mobilities of emerin, MAN1, and LBR using fluorescence recovery after photobleaching. We show that emerin and MAN1, but not LBR, are more mobile in the inner nuclear membrane of cells from Lmna -/- mice than in cells from wild-type mice. In cells from Lmna -/- mice expressing exogenous lamin A, the protein mobilities were similar to those in cells from wild-type mice. This supports a model where emerin and MAN1 are at least partly retained in the inner nuclear membrane by binding to A-type lamins, while LBR depends on other binding partners for its retention.

Homozygous and compound heterozygous mutations in ZMPSTE24 cause the laminopathy restrictive dermopathy

Restrictive dermopathy (RD) is a lethal human genetic disorder characterized by very tight, thin, easily eroded skin, rocker bottom feet, and joint contractures. This disease was recently reported to be associated with a single heterozygous mutation in ZMPSTE24 and hypothesized to be a digenic disorder (Navarro et al, Lamin A and ZMPSTE24 (FACE-1) defects cause nuclear disorganization and identify restrictive dermopathy as a lethal neonatal laminopathy. Hum Mol Genet 13:2493-2503, 2004). ZMPSTE24 encodes an enzyme necessary for the correct processing and maturation of lamin A, an intermediate filament component of the nuclear envelope. Here we present four unrelated patients with homozygous mutations in ZMPSTE24 and a fifth patient with compound heterozygous mutations in ZMPSTE24. Two of the three different mutations we found are novel, and all are single base insertions that result in messenger RNA frameshifts. As a consequence of the presumed lack of ZMPSTE24 activity, prelamin A, the unprocessed toxic form of lamin A, was detected in the nuclei of both cultured cells and tissue from RD patients, but not in control nuclei. Abnormally aggregated lamin A/C was also observed. These results indicate that RD is an autosomal recessive laminopathy caused by inactivating ZMPSTE24 mutations that result in defective processing and nuclear accumulation of prelamin A.

Novel progerin-interactive partner proteins hnRNP E1, EGF, Mel 18, and UBC9 interact with lamin A/C

The Hutchinson-Gilford progeria syndrome (HGPS or progeria) is an apparent accelerated aging disorder of childhood. Recently, HGPS has been characterized as one of a growing group of disorders known as laminopathies, which result from genetic defects of the lamin A/C (LMNA) gene. The majority of HGPS mutant alleles involve a silent mutation, c.2063C>T resulting in G608G, that generates a cryptic splicing site in exon 11 of LMNA and consequently truncates 50 amino acids near the C-terminus of pre-lamin A/C. To explore possible mechanisms underlying the development of HGPS, we began a search for proteins that would uniquely interact with progerin (the truncated lamin A in HGPS) using a yeast two-hybrid system. Four new progerin interactive partner proteins were identified that had not been previously found to interact with lamin A/C: hnRNP E1, UBC9 (ubiquitin conjugating enzyme E2I), Mel-18, and EGF1. However, using control and progeria fibroblasts, co-immunoprecipitation studies of endogenous proteins did not show differential binding affinity compared to normal lamin A/C. Thus, we did not find evidence for uniquely interacting partner proteins using this approach, but did identify four new lamin A/C interactive partners.

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.

Functions and dysfunctions of the nuclear lamin Ig-fold domain in nuclear assembly, growth, and Emery-Dreifuss muscular dystrophy

The non-alpha-helical C terminus of Xenopus lamin B3 (LB3T) inhibits the polymerization of lamin B3 in vitro and prevents the assembly of nuclei in Xenopus egg interphase extracts. To more precisely define the functions of LB3T in nuclear assembly, we have expressed subdomains of LB3T and determined their effects on nuclear assembly in Xenopus extracts. The results demonstrate that the Ig-fold motif (LB3T-Ig) is sufficient to inhibit lamin polymerization in vitro. Addition of the LB3T-Ig to egg extracts before the introduction of chromatin prevents chromatin decondensation and the assembly of the lamina, membranes, and pore complexes comprising the nuclear envelope. When added to assembled nuclei, LB3T-Ig prevents the further incorporation of lamin B3 into the endogenous lamina and blocks nuclear growth. The introduction of a point mutation in LB3T-Ig (R454W; LB3T-IgRW), known to cause Emery-Dreifuss muscular dystrophy when present in lamin A, does not inhibit lamin polymerization, chromatin decondensation, or nuclear assembly and growth. These results shed light on the specific alterations in lamin functions attributable to a known muscular dystrophy mutation and provide an experimental framework for revealing the effects of other mutations causing a wide range of laminopathies.

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.

Remodelling of the nuclear lamina and nucleoskeleton is required for skeletal muscle differentiation in vitro

Changes in the expression and distribution of nuclear lamins were investigated during C2C12 myoblast differentiation. The expression of most lamins was unchanged during myogenesis. By contrast, lamin-B2 expression increased and LAP2alpha expression decreased twofold. These changes were correlated with reduced solubility and redistribution of A-type lamins. When C2C12 myoblasts were transfected with a lamin-A mutant that causes autosomal dominant Emery-Dreifuss muscular dystrophy (AD-EDMD), the mutant protein accumulated in the nucleoplasm and exerted dominant influences over endogenous lamins. Myoblasts transfected with wild-type lamins differentiated, albeit more slowly, whereas myoblasts transfected with mutant lamins failed to differentiate. Myoblast differentiation requires dephosphorylation of the retinoblastoma protein Rb. During myogenesis, Rb was rapidly and progressively dephosphorylated. Underphosphorylated Rb formed complexes with LAP2alpha in proliferating myoblasts and postmitotic myoblasts. In myoblasts transfected with the mutant lamins, this complex was disrupted. These data suggest that remodelling of the nucleoskeleton is necessary for skeletal-muscle differentiation and for correct regulation of Rb pathways.

Molecular genetic analysis of the nested Drosophila melanogaster lamin C gene

Lamins are intermediate filaments that line the inner surface of the nuclear envelope, providing structural support and making contacts with chromatin. There are two types of lamins, A- and B-types, which differ in structure and expression. Drosophila possesses both lamin types, encoded by the LamC (A-type) and lamin Dm0 (B-type) genes. LamC is nested within an intron of the essential gene ttv. We demonstrate that null mutations in LamC are lethal, and expression of a wild-type LamC transgene rescues lethality of LamC but not ttv mutants. Mutations in the human A-type lamin gene lead to diseases called laminopathies. To determine if Drosophila might serve as a useful model to study lamin biology and disease mechanisms, we generated transgenic flies expressing mutant LamC proteins modeled after human disease-causing lamins. These transgenic animals display a nuclear lamin aggregation phenotype remarkably similar to that observed when human mutant A-type lamins are expressed in mammalian cells. LamC aggregates also cause disorganization of lamin Dm0, indicating interdependence of both lamin types for proper lamina assembly. Taken together, these data provide the first detailed genetic analysis of the LamC gene and support using Drosophila as a model to study the role of lamins in disease.

A-type nuclear lamins, progerias and other degenerative disorders

Nuclear lamins were identified as core nuclear matrix constituents over 20 years ago. They have been ascribed structural roles such as maintaining nuclear integrity and assisting in nuclear envelope formation after mitosis, and have also been linked to nuclear activities including DNA replication and transcription. Recently, A-type lamin mutations have been linked to a variety of rare human diseases including muscular dystrophy, lipodystrophy, cardiomyopathy, neuropathy and progeroid syndromes (collectively termed laminopathies). Most diseases arise from dominant, missense mutations, leading to speculation as to how different mutations in the same gene can give rise to such a diverse set of diseases, some of which share little phenotypic overlap. Understanding the cellular dysfunctions that lead to laminopathies will almost certainly provide insight into specific roles of A-type lamins in nuclear organization. Here, we compare and contrast the LMNA mutations leading to laminopathies with emphasis on progerias, and discuss possible functional roles for A-type lamins in the maintenance of healthy tissues.

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.

Laminopathies: Involvement of structural nuclear proteins in the pathogenesis of an increasing number of human diseases

Just at the beginning of the millennium the neologism laminopathies has been introduced in the scientific vocabulary. An exponential increase of interest on the subject started concomitantly, so that a formerly quite neglected group of rare human diseases is now widely investigated. This review will cover the history of the identification of the molecular basis for fourteen (since now) hereditary diseases arising from defects in genes that encode nuclear envelope and nuclear lamina-associated proteins and will also consider the hypotheses that can account for the role of structural nuclear proteins in the pathogenesis of diseases affecting a wide spectrum of tissues.

Nuclear envelope, nuclear lamina, and inherited disease

The nuclear envelope is composed of the nuclear membranes, nuclear lamina, and nuclear pore complexes. In recent years, mutations in nuclear-envelope proteins have been shown to cause a surprisingly wide array of inherited diseases. While the mutant proteins are generally expressed in most or all differentiated somatic cells, many mutations cause fairly tissue-specific disorders. Perhaps the most dramatic case is that of mutations in A-type lamins, intermediate filament proteins associated with the inner nuclear membrane. Different mutations in the same lamin proteins have been shown to cause striated muscle diseases, partial lipodystrophy syndromes, a peripheral neuropathy, and disorders with features of severe premature aging. In this review, we summarize fundamental aspects of nuclear envelope structure and function, the inherited diseases caused by mutations in lamins and other nuclear envelope proteins, and possible pathogenic mechanisms.

Laminopathies

Nuclear lamins form a fibrous nucleoskeletal network of intermediate-sized filaments that underlies the inner nuclear membrane. It associates with this membrane through interactions with specific integral nuclear membrane proteins, while within this flattened lamin lattice the nuclear pore complexes are embedded. Next to this peripheral network, the lamins can form intranuclear structures. The lamins are the evolutionary progenitors of the cytoplasmic intermediate filament proteins and have profound influences on nuclear structure and function. These influences require that lamins have dynamic properties and dual identities as structural building blocks on the one hand, and transcription regulators on the other. Which of these identities underlies the laminopathies, a myriad of genetic diseases caused by mutations in lamins or lamin-associated proteins, is a topic of intense debate.

The nuclear envelope and human disease

Mutations in nuclear lamins A and C, intermediate filament proteins of the nuclear envelope, cause diseases affecting various tissues and the aging process. We review what is known about nuclear lamin function and the different diseases caused by mutations in lamins A and C and associated inner nuclear membrane proteins.

LMNA mutations in cardiac transplant recipients

Lamin A and C are components of the nuclear envelope, located at the nucleoplasmatic surface of the inner nuclear membrane within cells. Recently, mutations within LMNA encoding lamin A/C have been associated with various disease entities including cardiomyopathy. We screened heart transplant recipients suffering from dilated cardiomyopathy (DCM) with a positive family history of LMNA mutations. Four index patients and one relative belonging to four unrelated families carrying LMNA mutations were identified. The mutations p.Q355X and p.S22L have not been reported before, whereas p.R190W has already been reported in other studied DCM cohorts. In the patients of the present study, the mean age at manifestation of heart disease was 37.6 years (range 30-45 years), with progression to end-stage heart failure requiring transplantation at a mean age of 45.8 years (range 35-54 years). Three patients presented initially with atrial fibrillation. These data confirm the involvement of LMNA mutations in patients with DCM and extend the mutational spectrum of LMNA. The p.R190W mutation has been reported in different populations and may therefore be useful for analyzing the impact of a specific LMNA mutation on the phenotype of muscle disease.

A-type lamins: Guardians of the soma?

The gene LMNA encodes the proteins lamins A and C and is implicated in nine different laminopathies - inherited diseases that are linked to premature ageing. Recent evidence has demonstrated that lamins A and C have essential functions in protecting cells from physical damage, as well as in maintaining the function of transcription factors required for the differentiation of adult stem cells. Thus, the degenerative nature of laminopathies is explained because these lamins are essential for maintenance of somatic tissues in adulthood.

Nuclear envelope alterations in fibroblasts from patients with muscular dystrophy, cardiomyopathy, and partial lipodystrophy carrying lamin A/C gene mutations

Mutations in LMNA, the gene that encodes nuclear lamins A and C, cause up to eight different diseases collectively referred to as "laminopathies." These diseases affect striated muscle, adipose tissue, peripheral nerve, and bone, or cause features of premature aging. We investigated the consequences of LMNA mutations on nuclear architecture in skin fibroblasts from 13 patients with different laminopathies. Western-blotting showed that none of the mutations examined led to a decrease in cellular levels of lamin A or C. Regardless of the disease, we observed honeycomb nuclear structures and nuclear envelope blebs in cells examined by immunofluorescence microscopy. Concentrated foci of lamin A/C in the nucleoplasm were also observed. Only mutations in the head and tail domains of lamins A and C significantly altered the nuclear architecture of patient fibroblasts. These results confirm that mutations in lamins A and C may lead to a weakening of a structural support network in the nuclear envelope in fibroblasts and that nuclear architecture changes depend upon the location of the mutation in different domains of lamin A/C.

Lco1 is a novel widely expressed lamin-binding protein in the nuclear interior

A-type lamins are localized at the nuclear envelope and in the nucleoplasm, and are implicated in human diseases called laminopathies. In a yeast two-hybrid screen with lamin C, we identified a novel widely expressed 171-kDa protein that we named Lamin companion 1 (Lco1). Three independent biochemical assays showed direct binding of Lco1 to the C-terminal tail of A-type lamins with an affinity of 700 nM. Lco1 also bound the lamin B1 tail with lower affinity (2 microM). Ectopic Lco1 was found primarily in the nucleoplasm and colocalized with endogenous intranuclear A-type lamins in HeLa cells. Overexpression of prelamin A caused redistribution of ectopic Lco1 to the nuclear rim together with ectopic lamin A, confirming association of Lco1 with lamin A in vivo. Whereas the major C-terminal lamin-binding fragment of Lco1 was cytoplasmic, the N-terminal Lco1 fragment localized in the nucleoplasm upon expression in cells. Furthermore, full-length Lco1 was nuclear in cells lacking A-type lamins, showing that A-type lamins are not required for nuclear targeting of Lco1. We conclude that Lco1 is a novel intranuclear lamin-binding protein. We hypothesize that Lco1 is involved in organizing the internal lamin network and potentially relevant as a laminopathy disease gene or modifier.

Accumulation of mutant lamin A causes progressive changes in nuclear architecture in Hutchinson-Gilford progeria syndrome

Hutchinson-Gilford progeria syndrome (HGPS) is a premature aging disorder, commonly caused by a point mutation in the lamin A gene that results in a protein lacking 50 aa near the C terminus, denoted LADelta50. Here we show by light and electron microscopy that HGPS is associated with significant changes in nuclear shape, including lobulation of the nuclear envelope, thickening of the nuclear lamina, loss of peripheral heterochromatin, and clustering of nuclear pores. These structural defects worsen as HGPS cells age in culture, and their severity correlates with an apparent increase in LADelta50. Introduction of LADelta50 into normal cells by transfection or protein injection induces the same changes. We hypothesize that these alterations in nuclear structure are due to a concentration-dependent dominant-negative effect of LADelta50, leading to the disruption of lamin-related functions ranging from the maintenance of nuclear shape to regulation of gene expression and DNA replication.

How do mutations in lamins A and C cause disease?

Mutations in lamins A and C, nuclear intermediate-filament proteins in nearly all somatic cells, cause a variety of diseases that primarily affect striated muscle, adipocytes, or peripheral nerves or cause features of premature aging. Two new studies (see the related articles beginning on pages 357 and 370) use lamin A/C-deficient mice, which develop striated muscle disease, as a model to investigate pathogenic mechanisms. These reports provide evidence for a stepwise process in which mechanically stressed cells first develop chromatin and nuclear envelope damage and then develop secondary alterations in the transcriptional activation of genes in adaptive and protective pathways.

Expression of a mutant lamin A that causes Emery-Dreifuss muscular dystrophy inhibits in vitro differentiation of C2C12 myoblasts

Autosomal dominantly inherited missense mutations in lamins A and C cause several tissue-specific diseases, including Emery-Dreifuss muscular dystrophy (EDMD) and Dunnigan-type familial partial lipodystrophy (FPLD). Here we analyze myoblast-to-myotube differentiation in C2C12 clones overexpressing lamin A mutated at arginine 453 (R453W), one of the most frequent mutations in EDMD. In contrast with clones expressing wild-type lamin A, these clones differentiate poorly or not at all, do not exit the cell cycle properly, and are extensively committed to apoptosis. These disorders are correlated with low levels of expression of transcription factor myogenin and with the persistence of a large pool of hyperphosphorylated retinoblastoma protein. Since clones mutated at arginine 482 (a site responsible for FPLD) differentiate normally, we conclude that C2C12 clones expressing R453W-mutated lamin A represent a good cellular model to study the pathophysiology of EDMD. Our hypothesis is that lamin A mutated at arginine 453 fails to build a functional scaffold and/or to maintain the chromatin compartmentation required for differentiation of myoblasts into myocytes.

Multiple and surprising new functions for emerin, a nuclear membrane protein

Emerin is an integral protein of the nuclear inner membrane. Emerin is not essential, but its loss of function causes Emery-Dreifuss muscular dystrophy. We summarize significant recent progress in understanding emerin, which was previously known to interact with barrier-to-autointegration factor and lamins. New partners include transcription repressors, an mRNA splicing regulator, a nuclear membrane protein named nesprin, nuclear myosin I and F-actin. These interactors imply multiple roles for emerin in the nucleus, some of which overlap with related LEM-domain 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.

The laminopathies: nuclear structure meets disease

Most inherited diseases are associated with mutations in a specific gene. Sometimes, mutations in two or more different genes result in diseases with a similar phenotype. Rarely do different mutations in the same gene result in a multitude of seemingly different and unrelated diseases. In the past three years, different mutations in LMNA, the gene encoding the A-type lamins, have been shown to be associated with at least six different diseases. These diseases and at least two others caused by mutations in other proteins associated with the nuclear lamina are collectively called the laminopathies. How different tissue-specific diseases arise from unique mutations in the LMNA gene, encoding almost ubiquitously expressed nuclear proteins, are providing tantalizing insights into the structural organization of the nucleus, its relation to nuclear function in different tissues and the involvement of the nuclear envelope in the development of disease.

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.

The carboxyl-terminal region common to lamins A and C contains a DNA binding domain

Lamins A and C are intermediate filament proteins which polymerize into the nucleus to form the nuclear lamina network. The lamina is apposed to the inner nuclear membrane and functions in tethering chromatin to the nuclear envelope and in maintaining nuclear shape. We have recently characterized a globular domain that adopts an immunoglobulin fold in the carboxyl-terminal tail common to lamins A and C. Using an electrophoretic mobility shift assay (EMSA), we show that a peptide containing this domain interacts in vitro with DNA after dimerization through a disulfide bond, but does not interact with the core particle or the dinucleosome. The covalent dimer binds a 30-40 bp DNA fragment with a micromolar affinity and no sequence specificity. Using nuclear magnetic resonance (NMR) and an EMSA, we observed that two peptide regions participate in the DNA binding: the unstructured amino-terminal part containing the nuclear localization signal and a large positively charged region centered around amino acid R482 at the surface of the immunoglobulin-like domain. Mutations R482Q and -W, which are responsible for Dunnigan-type partial lipodystrophy, lower the affinity of the peptide for DNA. We conclude that the carboxyl-terminal end of lamins A and C binds DNA and suggest that alterations in lamin-DNA interactions may play a role in the pathophysiology of some lamin-linked diseases.

Functional consequences of an LMNA mutation associated with a new cardiac and non-cardiac phenotype

Heritable dilated cardiomyopathy is a genetically highly heterogeneous disease. To date 17 different chromosomal loci have been described for autosomal dominant forms of dilated cardiomyopathy with or without additional clinical manifestations. Among the 10 mutated genes associated with dilated cardiomyopathy, the lamin A/C (LMNA) gene has been reported in forms associated with conduction-system disease with or without skeletal muscle myopathy. For the first time, we report here a French family affected with a new phenotype composed of an autosomal dominant severe dilated cardiomyopathy with conduction defects or atrial/ventricular arrhythmias, and a specific quadriceps muscle myopathy. In all previously reported cases with both cardiac and neuromuscular involvement, neuromuscular disorders preceded cardiac abnormalities. The screening of the coding sequence of the LMNA gene on all family members was performed and we identified a missense mutation (R377H) in the lamin A/C gene that cosegregated with the disease in the family. Cell transfection experiments showed that the R377H mutation leads to mislocalization of both lamin and emerin. These results were obtained in both muscular (C2C12) and non-muscular cells (COS-7). This new phenotype points out the wide spectrum of neuromuscular and cardiac manifestations associated with lamin A/C mutations, with the functional consequence of this mutation seemingly associated with a disorganization of the lamina.

Nuclear envelope proteins and neuromuscular diseases

Several neuromuscular diseases are caused by mutations in emerin and A-type lamins, proteins of the nuclear envelope. Emery-Dreifuss muscular dystrophy is caused by mutations in emerin (X-linked) or A-type lamins (autosomal dominant). Mutations in A-type lamins also cause limb-girdle muscular dystrophy type 1B, dilated cardiomyopathy with conduction defect, and Charcot-Marie-Tooth disorder type 2B1. They also cause partial lipodystrophy syndromes. The functions of emerin and A-type lamins and the mechanisms of how mutations in these proteins cause tissue-specific diseases are not well understood. The mutated proteins may cause structural damage to cells but may also affect processes such as gene regulation. This review gives an overview of this topic and describes recent advances in identification of disease-causing mutations, studies of cells and tissues from subjects with these diseases, and animal and cell culture models.

Lamin A/C speckles mediate spatial organization of splicing factor compartments and RNA polymerase II transcription

The A-type lamins have been observed to colocalize with RNA splicing factors in speckles within the nucleus, in addition to their typical distribution at the nuclear periphery. To understand the functions of lamin speckles, the effects of transcriptional inhibitors known to modify RNA splicing factor compartments (SFCs) were examined. Treatment of HeLa cells with alpha-amanitin or 5,6-dichlorobenzimidazole riboside (DRB) inhibited RNA polymerase II (pol II) transcription and led to the enlargement of lamin speckles as well as SFCs. Removal of the reversible inhibitor DRB resulted in the reactivation of transcription and a rapid, synchronous redistribution of lamins and splicing factors to normal-sized speckles, indicating a close association between lamin speckles and SFCs. Conversely, the expression of NH2-terminally modified lamin A or C in HeLa cells brought about a loss of lamin speckles, depletion of SFCs, and down-regulation of pol II transcription without affecting the peripheral lamina. Our results suggest a unique role for lamin speckles in the spatial organization of RNA splicing factors and pol II transcription in the nucleus.

The nuclear lamina and inherited disease

Inherited disorders of the nuclear lamina present some of the most intriguing puzzles in cell biology. Mutations in lamin A and lamin C - nuclear intermediate filament proteins that are expressed in nearly all somatic cells - cause tissue-specific diseases that affect striated muscle, adipose tissue and peripheral nerve or skeletal development. Recent studies provide clues about how different mutations in these proteins cause either muscle disease or partial lipodystrophy. Although the precise pathogenic mechanisms are currently unknown, the involvement of lamins in several different disorders shows that research on the nuclear lamina will shed light on common human pathologies.

Lamins: building blocks or regulators of gene expression?

Intermediate filament (IF) proteins are the building blocks of cytoskeletal filaments, the main function of which is to maintain cell shape and integrity. The lamins are thought to be the evolutionary progenitors of IF proteins and they have profound influences on both nuclear structure and function. These influences require the lamins to have dynamic properties and dual identities--as building blocks and transcriptional regulators. Which one of these identities underlies a myriad of genetic diseases is a topic of intense debate.

Life at the edge: the nuclear envelope and human disease

A group of human diseases, known as 'laminopathies', are associated with defects in proteins of the nuclear envelope. Most laminopathy mutations have been mapped to the A-type lamin gene, which is expressed in most adult cell types. So, why should different mutations in a near-ubiquitously expressed gene be associated with various discrete tissue-restricted diseases? Attempts to resolve this paradox are uncovering new molecular interactions #151; both inside the nucleus and at its periphery -- which indicate that the nuclear envelope has functions that go beyond mere housekeeping.

The nuclear envelope, lamins and nuclear assembly

The nuclear lamina is composed of both A- and B-type lamins and lamin-binding proteins. Many lamin-binding proteins are integral proteins of the inner nuclear membrane. Lamins and inner nuclear membrane proteins are important for a variety of cell functions, including nuclear assembly, replication, transcription, and nuclear integrity. Recent advances in the field in the past year include the identification of a family of spectrin-repeat-containing inner nuclear membrane proteins and other novel inner-membrane proteins, and the discovery of a nuclear membrane fusion complex. There is also growing evidence that A- and B-type lamins and their binding partners have distinct roles during nuclear assembly and interphase.

The Ig-like structure of the C-terminal domain of lamin A/C, mutated in muscular dystrophies, cardiomyopathy, and partial lipodystrophy

Lamins are nuclear intermediate filaments that, together with lamin-associated proteins, maintain nuclear shape and provide a structural support for chromosomes and replicating DNA. We have determined the solution structure of the human lamin A/C C-terminal globular domain which contains specific mutations causing four different heritable diseases. This domain encompasses residues 430-545 and adopts an Ig-like fold of type s. We have also characterized by NMR and circular dichroism the structure and thermostability of three mutants, R453W and R482W/Q, corresponding to "hot spots" causing Emery-Dreifuss muscular dystrophy and Dunnigan-type lipodystrophy, respectively. Our structure determination and mutant analyses clearly show that the consequences of the mutations causing muscle-specific diseases or lipodystrophy are different at the molecular level.

Intracellular trafficking of MAN1, an integral protein of the nuclear envelope inner membrane

MAN1 is an integral protein of the inner nuclear membrane that shares the LEM domain, a conserved globular domain of approximately 40 amino acids, with lamina-associated polypeptide (LAP) 2 and emerin. Confocal immuofluorescence microscopy studies of the intracellular targeting of truncated forms of MAN1 showed that the nucleoplasmic, N-terminal domain is necessary for inner nuclear membrane retention. A protein containing the N-terminal domain with the first transmembrane segment of MAN1 is retained in the inner nuclear membrane, whereas the transmembrane segments with the C-terminal domain of MAN1 is not targeted to the inner nuclear membrane. The N-terminal domain of MAN1 is also sufficient for inner nuclear membrane targeting as it can target a chimeric type II integral protein to this subcellular location. Deletion mutants of the N-terminal of MAN1 are not efficiently retained in the inner nuclear membrane. When the N-terminal domain of MAN1 is increased in size from∼50 kDa to ∼100 kDa, the protein cannot reach the inner nuclear membrane. Fluorescence recovery after photobleaching experiments of MAN1 fused to green fluorescent protein show that the fusion protein is relatively immobile in the nuclear envelope compared with the endoplasmic reticulum of interphase cells, suggesting binding to a nuclear component. These results are in agreement with the `diffusion-retention' model for targeting integral proteins to the inner nuclear membrane.

Alteration of nuclear lamin organization inhibits RNA polymerase II-dependent transcription

Regulation of gene activity is mediated by alterations in chromatin organization. In addition, chromatin organization may be governed in part by interactions with structural components of the nucleus. The nuclear lamins comprise the lamina and a variety of nucleoplasmic assemblies that together are major structural components of the nucleus. Furthermore, lamins and lamin-associated proteins have been reported to bind chromatin. These observations suggest that the nuclear lamins may be involved in the regulation of gene activity. In this report, we test this possibility by disrupting the normal organization of nuclear lamins with a dominant negative lamin mutant lacking the NH2-terminal domain. We find that this disruption inhibits RNA polymerase II activity in both mammalian cells and transcriptionally active embryonic nuclei from Xenopus laevis. The inhibition appears to be specific for polymerase II as disruption of lamin organization does not detectably inhibit RNA polymerases I and III. Furthermore, immunofluorescence observations indicate that this selective inhibition of polymerase II-dependent transcription involves the TATA binding protein, a component of the basal transcription factor TFIID.

Nuclear envelope disorganization in fibroblasts from lipodystrophic patients with heterozygous R482Q/W mutations in the lamin A/C gene

Dunnigan-type familial partial lipodystrophy (FPLD), characterized by an abnormal body fat redistribution with insulin resistance, is caused by missense heterozygous mutations in A-type lamins (lamins A and C). A- and B-type lamins are ubiquitous intermediate filament proteins that polymerize at the inner face of the nuclear envelope. We have analyzed primary cultures of skin fibroblasts from three patients harboring R482Q or R482W mutations. These cells were euploid and able to cycle and divide. A subpopulation of these cells had abnormal blebbing nuclei with A-type lamins forming a peripheral meshwork, which was frequently disorganized. Inner nuclear membrane protein emerin, an A-type lamin-binding protein, strictly colocalized with this abnormal meshwork. Cells from lipodystrophic patients often had other nuclear envelope defects, mainly consisting of nuclear envelope herniations that were deficient in B-type lamins, nuclear pore complexes, lamina-associated protein 2 beta, and chromatin. The mechanical properties of nuclear envelopes were altered, as judged from the extensive deformations observed in nuclei from heat-shocked cells, and from the low stringency of extraction of their components. These structural nuclear alterations were caused by the lamins A/C mutations, as the same changes were introduced in human control fibroblasts by ectopic expression of R482W mutated lamin A.

Nuclear envelope defects associated withLMNAmutations cause dilated cardiomyopathy and Emery-Dreifuss muscular dystrophy

Nuclear lamin A and C alleles that are linked to three distinct human diseases have been expressed both in HeLa cells and in fibroblasts derived from Lmna null mice. Point mutations that cause dilated cardiomyopathy (L85R and N195K) and autosomal dominant Emery-Dreifuss muscular dystrophy (L530P) modify the assembly properties of lamins A and C and cause partial mislocalization of emerin, an inner nuclear membrane protein, in HeLa cells. At the same time, these mutant lamins interfere with the targeting and assembly of endogenous lamins and in this way may cause significant changes in the molecular organization of the nuclear periphery. By contrast, lamin A and C molecules harboring a point mutation (R482W), which gives rise to a dominant form of familial partial lipodystrophy, behave in a manner that is indistinguishable from wild-type lamins A and C, at least with respect to targeting and assembly within the nuclear lamina. Taken together, these results suggest that nuclear structural defects could contribute to the etiology of both dilated cardiomyopathy and autosomal dominant Emery-Dreifuss muscular dystrophy.