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

CIP, a cardiac Isl1-interacting protein, represses cardiomyocyte hypertrophy

RATIONALE

Mammalian heart has minimal regenerative capacity. In response to mechanical or pathological stress, the heart undergoes cardiac remodeling. Pressure and volume overload in the heart cause increased size (hypertrophic growth) of cardiomyocytes. Whereas the regulatory pathways that activate cardiac hypertrophy have been well-established, the molecular events that inhibit or repress cardiac hypertrophy are less known.

OBJECTIVE

To identify and investigate novel regulators that modulate cardiac hypertrophy.

METHODS AND RESULTS

Here, we report the identification, characterization, and functional examination of a novel cardiac Isl1-interacting protein (CIP). CIP was identified from a bioinformatic search for novel cardiac-expressed genes in mouse embryonic hearts. CIP encodes a nuclear protein without recognizable motifs. Northern blotting, in situ hybridization, and reporter gene tracing demonstrated that CIP is highly expressed in cardiomyocytes of developing and adult hearts. Yeast two-hybrid screening identified Isl1, a LIM/homeodomain transcription factor essential for the specification of cardiac progenitor cells in the second heart field, as a cofactor of CIP. CIP directly interacted with Isl1, and we mapped the domains of these two proteins, which mediate their interaction. We show that CIP represses the transcriptional activity of Isl1 in the activation of the myocyte enhancer factor 2C. The expression of CIP was dramatically reduced in hypertrophic cardiomyocytes. Most importantly, overexpression of CIP repressed agonist-induced cardiomyocyte hypertrophy.

CONCLUSIONS

Our studies therefore identify CIP as a novel regulator of cardiac hypertrophy.

Perturbation of nuclear lamin A causes cell death in chondrocytes

OBJECTIVE

Mutations in LMNA encoding the A-type lamins cause several diseases, including those with features of premature aging and skeletal abnormalities. The aim of this study was to examine the expression of lamin A in cartilage from patients with osteoarthritis (OA) and the effects of its overexpression on chondrocyte senescence and apoptosis.

METHODS

Human chondrocyte-like cells (SW-1353) were used. RNA isolated from human OA and non-OA cartilage was used for profiling messenger RNA expression, using Affymetrix microarray analysis. The effects of lamin A overexpression on mitochondrial function and apoptosis were examined by assessing mitochondrial membrane potential, ATP levels, and cytochrome c release, and with a TUNEL assay. Western blotting was performed to determine protein expression.

RESULTS

Lamin A expression was markedly elevated in OA cartilage samples compared with non-OA control samples. Western blot analysis confirmed increased expression of lamin A in OA compared with non-OA cartilage. Interleukin-1β treatment inhibited lamin A accumulation, whereas treatment with prostaglandin E(2) (PGE(2) ) caused a marked increase in lamin A accumulation. These effects of exogenous PGE(2) on lamin A expression were mediated via the EP(2) /EP(4) receptors. Transfected chondrocytes that expressed lamin A displayed markers of early senescence/apoptosis.

CONCLUSION

The results of this study suggest that lamin A is up-regulated in OA chondrocytes, and that increased nuclear accumulation of lamin A in response to catabolic stress may account for the premature aging phenotype and apoptosis of OA chondrocytes.

The functions of the nuclear envelope in mediating the molecular crosstalk between the nucleus and the cytoplasm

Recent studies of the nuclear envelope (NE) have emphasized its role in linking the nuclear and cytoplasmic compartments of mammalian cells. The inner face of the NE is bound to chromatin and this interaction is involved in regulating DNA replication and transcription. The outer face of the NE binds to different components of the cytoskeleton, and these interactions are involved in nuclear positioning. Many disease causing mutations in genes encoding NE proteins cause significant changes in nuclear architecture and cytoskeletal interactions with the NE. These mutations are also providing important new insights into nuclear-cytoplasmic interactions.

Subcellular localization of SREBP1 depends on its interaction with the C-terminal region of wild-type and disease related A-type lamins

Lamins A and C are nuclear intermediate filament proteins expressed in most differentiated somatic cells. Previous data suggested that prelamin A, the lamin A precursor, accumulates in some lipodystrophy syndromes caused by mutations in the lamin A/C gene, and binds and inactivates the sterol regulatory element binding protein 1 (SREBP1). Here we show that, in vitro, the tail regions of prelamin A, lamin A and lamin C bind a polypeptide of SREBP1. Such interactions also occur in HeLa cells, since expression of lamin tail regions impedes nucleolar accumulation of the SREBP1 polypeptide fused to a nucleolar localization signal sequence. In addition, the tail regions of A-type lamin variants that occur in Dunnigan-type familial partial lipodystrophy of (R482W) and Hutchison Gilford progeria syndrome (∆607-656) bind to the SREBP1 polypeptide in vitro, and the corresponding FLAG-tagged full-length lamin variants co-immunoprecipitate the SREBP1 polypeptide in cells. Overexpression of wild-type A-type lamins and variants favors SREBP1 polypeptide localization at the intranuclear periphery, suggesting its sequestration. Our data support the hypothesis that variation of A-type lamin protein level and spatial organization, in particular due to disease-linked mutations, influences the sequestration of SREBP1 at the nuclear envelope and thus contributes to the regulation of SREBP1 function.

An EDMD mutation in C. elegans lamin blocks muscle-specific gene relocation and compromises muscle integrity

BACKGROUND

In worms, as in other organisms, many tissue-specific promoters are sequestered at the nuclear periphery when repressed and shift inward when activated. It has remained unresolved, however, whether the association of facultative heterochromatin with the nuclear periphery, or its release, has functional relevance for cell or tissue integrity.

RESULTS

Using ablation of the unique lamin gene in C. elegans, we show that lamin is necessary for the perinuclear positioning of heterochromatin. We then express at low levels in otherwise wild-type worms a lamin carrying a point mutation, Y59C, which in humans is linked to an autosomal-dominant form of Emery-Dreifuss muscular dystrophy. Using embryos and differentiated tissues, we track the subnuclear position of integrated heterochromatic arrays and their expression. In LMN-1 Y59C-expressing worms, we see abnormal retention at the nuclear envelope of a gene array bearing a muscle-specific promoter. This correlates with impaired activation of the array-borne myo-3 promoter and altered expression of a number of muscle-specific genes. However, an equivalent array carrying the intestine-specific pha-4 promoter is expressed normally and shifts inward when activated in gut cells of LMN-1 Y59C worms. Remarkably, adult LMN-1 Y59C animals have selectively perturbed body muscle ultrastructure and reduced muscle function.

CONCLUSION

Lamin helps sequester heterochromatin at the nuclear envelope, and wild-type lamin permits promoter release following tissue-specific activation. A disease-linked point mutation in lamin impairs muscle-specific reorganization of a heterochromatic array during tissue-specific promoter activation in a dominant manner. This dominance and the correlated muscle dysfunction in LMN-1 Y59C worms phenocopies Emery-Dreifuss muscular dystrophy.

Quantitative nucleolar proteomics reveals nuclear re-organization during stress- induced senescence in mouse fibroblast

Background

Nucleolus is the most prominent mammalian organelle within the nucleus which is also the site for ribosomal biogenesis. There have been many reports indicating the involvement of nucleolus in the process of aging. Several proteins related to aging have been shown to localize in the nucleolus, which suggests the role of this organelle in senescence.

Results

In this study, we used quantitative mass spectrometry to map the flux of proteins into and out of the nucleolus during the induction of senescence in cultured mammalian cells. Changes in the abundance of 344 nucleolar proteins in sodium butyrate-induced senescence in NIH3T3 cells were studied by SILAC (stable isotope labeling by amino acids in cell culture)-based mass spectrometry. Biochemically, we have validated the proteomic results and confirmed that B23 (nucleophosmin) protein was down-regulated, while poly (ADP-ribose) polymerase (PARP) and nuclear DNA helicase II (NDH II/DHX9/RHA) were up-regulated in the nucleolus upon treatment with sodium butyrate. Accumulation of chromatin in the nucleolus was also observed, by both proteomics and microscopy, in sodium butyrate-treated cells. Similar observations were found in other models of senescence, namely, in mitoxantrone- (MTX) treated cells and primary fibroblasts from the Lamin A knockout mice.

Conclusion

Our data indicate an extensive nuclear organization during senescence and suggest that the redistribution of B23 protein and chromatin can be used as an important marker for senescence.

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.

Novel LMNA mutations in patients with Emery-Dreifuss muscular dystrophy and functional characterization of four LMNA mutations

Mutations in LMNA cause a variety of diseases affecting striated muscle including autosomal Emery-Dreifuss muscular dystrophy (EDMD), LMNA-associated congenital muscular dystrophy (L-CMD), and limb-girdle muscular dystrophy type 1B (LGMD1B). Here, we describe novel and recurrent LMNA mutations identified in 50 patients from the United States and Canada, which is the first report of the distribution of LMNA mutations from a large cohort outside Europe. This augments the number of LMNA mutations known to cause EDMD by 16.5%, equating to an increase of 5.9% in the total known LMNA mutations. Eight patients presented with either p.R249W/Q or p.E358K mutations and an early onset EDMD phenotype: two mutations recently associated with L-CMD. Importantly, 15 mutations are novel and include eight missense mutations (p.R189P, p.F206L, p.S268P, p.S295P, p.E361K, p.G449D, p.L454P, and p.W467R), three splice site mutations (c.IVS4 + 1G>A, c.IVS6 - 2A>G, and c.IVS8 + 1G>A), one duplication/in frame insertion (p.R190dup), one deletion (p.Q355del), and two silent mutations (p.R119R and p.K270K). Analysis of 4 of our lamin A mutations showed that some caused nuclear deformations and lamin B redistribution in a mutation specific manner. Together, this study significantly augments the number of EDMD patients on the database and describes 15 novel mutations that underlie EDMD, which will contribute to establishing genotype-phenotype correlations.

Nup358, a nucleoporin, functions as a key determinant of the nuclear pore complex structure remodeling during skeletal myogenesis

The nuclear pore complex (NPC) is the only gateway for molecular trafficking across the nuclear envelope. The NPC is not merely a static nuclear-cytoplasmic transport gate; the functional analysis of nucleoporins has revealed dynamic features of the NPC in various cellular functions, such as mitotic spindle formation and protein modification. However, it is not known whether the NPC undergoes dynamic changes during biological processes such as cell differentiation. In the present study, we evaluate changes in the expression levels of several nucleoporins and show that the amount of Nup358/RanBP2 within individual NPCs increases during muscle differentiation in C2C12 cells. Using atomic force microscopy, we demonstrate structural differences between the cytoplasmic surfaces of myoblast and myotube NPCs and a correlation between the copy number of Nup358 and the NPC structure. Furthermore, small interfering RNA-mediated depletion of Nup358 in myoblasts suppresses myotube formation without affecting cell viability, suggesting that NUP358 plays a role in myogenesis. These findings indicate that the NPC undergoes dynamic remodeling during muscle cell differentiation and that Nup358 is prominently involved in the remodeling process.

Emery-Dreifuss muscular dystrophy

Emery-Dreifuss muscular dystrophy (EDMD) is a progressive muscle-wasting disorder defined by early contractures of the Achilles tendon, spine, and elbows. EDMD is also distinctive for its association with defects of the cardiac conduction system that can result in sudden death. It can be inherited in an X-linked, autosomal dominant, or autosomal recessive fashion and is caused by mutations in proteins of the nuclear membrane. Mutations in the EMD gene, which encodes emerin, a transmembrane protein found at the inner nuclear membrane, are responsible for X-linked EDMD. The most common etiology of autosomal dominant EDMD is an LMNA gene mutation; LMNA encodes the intermediate filament protein lamins A and C, which constitute the major scaffolding protein of the inner nuclear membrane. Murine models of LMNA gene mutations have helped to identify different mechanisms of disease. Loss of LMNA function leads to nuclear fragility as well as other defects, such as abnormal nuclear function. Additional genes encoding nuclear membrane proteins such as SYNE1 and SYNE2 have also been implicated in EDMD, and in some cases their importance for cardiac and muscle function has been supported by animal modeling.

Lamin A variants that cause striated muscle disease are defective in anchoring transmembrane actin-associated nuclear lines for nuclear movement

Mutations in LMNA, which encodes A-type lamins, result in disparate diseases, known collectively as laminopathies, that affect distinct tissues, including striated muscle and adipose tissue. Lamins provide structural support for the nucleus and sites of attachment for chromatin, and defects in these functions may contribute to disease pathogenesis. Recent studies suggest that A-type lamins may facilitate connections between the nucleus and the cytoskeleton mediated by nuclear envelope nesprin and SUN proteins. In mammalian cells, however, interfering with A-type lamins does not affect the localization of these proteins. Here, we used centrosome orientation in fibroblasts, which requires separate nuclear and centrosome positioning pathways, as a model system to understand how LMNA mutations affect nucleus-cytoskeletal connections. We find that LMNA mutations causing striated muscle diseases block actin-dependent nuclear movement, whereas most that affect adipose tissue inhibit microtubule-dependent centrosome positioning. Genetic deletion or transient depletion of A-type lamins also blocked nuclear movement, showing that mutations affecting muscle exhibit the null phenotype. Lack of A-type lamins, or expression of variants that cause striated muscle disease, did not affect assembly of nesprin-2G and SUN2 into transmembrane actin-associated nuclear (TAN) lines that attach the nucleus to retrogradely moving actin cables. Nesprin-2G TAN lines were less stable, however, and slipped over the nucleus rather than moving with it, indicating that they were not anchored. Nesprin-2G TAN lines also slipped in SUN2-depleted cells. Our results establish A-type lamins as anchors for nesprin-2G-SUN2 TAN lines to allow productive movement and proper positioning of the nucleus by actin.

Lamin A rod domain mutants target heterochromatin protein 1alpha and beta for proteasomal degradation by activation of F-box protein, FBXW10

BACKGROUND

Lamins are major structural proteins of the nucleus and contribute to the organization of various nuclear functions. Mutations in the human lamin A gene cause a number of highly degenerative diseases, collectively termed as laminopathies. Cells expressing lamin mutations exhibit abnormal nuclear morphology and altered heterochromatin organization; however, the mechanisms responsible for these defects are not well understood.

METHODOLOGY AND PRINCIPAL FINDINGS

The lamin A rod domain mutants G232E, Q294P and R386K are either diffusely distributed or form large aggregates in the nucleoplasm, resulting in aberrant nuclear morphology in various cell types. We examined the effects of these lamin mutants on the distribution of heterochromatin protein 1 (HP1) isoforms. HeLa cells expressing these mutants showed a heterogeneous pattern of HP1alpha and beta depletion but without altering HP1gamma levels. Changes in HP1alpha and beta were not observed in cells expressing wild-type lamin A or mutant R482L, which assembled normally at the nuclear rim. Treatment with proteasomal inhibitors led to restoration of levels of HP1 isoforms and also resulted in stable association of lamin mutants with the nuclear periphery, rim localization of the inner nuclear membrane lamin-binding protein emerin and partial improvement of nuclear morphology. A comparison of the stability of HP1 isoforms indicated that HP1alpha and beta displayed increased turnover and higher basal levels of ubiquitination than HP1gamma. Transcript analysis of components of the ubiquitination pathway showed that a specific F-box protein, FBXW10 was induced several-fold in cells expressing lamin mutants. Importantly, ectopic expression of FBXW10 in HeLa cells led to depletion of HP1alpha and beta without alteration of HP1gamma levels.

CONCLUSIONS

Mislocalized lamins can induce ubiquitin-mediated proteasomal degradation of certain HP1 isoforms by activation of FBXW10, a member of the F-box family of proteins that is involved in E3 ubiquitin ligase activity.

Monoclonal antibodies specific for disease-associated point-mutants: lamin A/C R453W and R482W

BACKGROUND

Disease-linked missense mutations can alter a protein's function with fatal consequences for the affected individual. How a single amino acid substitution in a protein affects its properties, is difficult to study in the context of the cellular proteome, because mutant proteins can often not be traced in cells due to the lack of mutation-specific detection tools. Antibodies, however, with their exquisite epitope specificity permit the detection of single amino acid substitutions but are not available for the vast majority of disease-causing mutant proteins. One of the most frequently missense-mutated human genes is the LMNA gene coding for A-type lamins. Mutations in LMNA cause phenotypically heterogenous, mostly autosomal-dominant inherited diseases, termed laminopathies. The molecular mechanisms underlying the phenotypic heterogeneity of laminopathies, however, are not well understood. Hence, the goal of this study was the development of monoclonal antibodies specific for disease-linked point-mutant A-type lamins.

METHODOLOGY/PRINCIPAL FINDINGS

Using two different approaches of antigen presentation, namely KLH-coupled peptides and the display of a complete protein domain fused to the Hepatitis B virus capsid protein, we developed monoclonal antibodies against two disease-associated lamin A/C mutants. Both antibodies display exquisite specificity for the respective mutant proteins. We show that with the help of these novel antibodies it is now possible for the first time to study specifically the properties of the mutant proteins in primary patient cells in the background of wild-type protein.

CONCLUSIONS

We report here the development of two point-mutant specific antibodies against A-type lamins. While synthetic peptides may be the prime choice of antigen, our results show that a given target sequence may have to be presented in alternative ways to ensure the induction of a mutant-specific immune response. Point-mutant specific antibodies will represent valuable tools for basic and clinical research on a number of hereditary as well as acquired diseases caused by dominant missense mutations.

Morphological analysis of 13 LMNA variants identified in a cohort of 324 unrelated patients with idiopathic or familial dilated cardiomyopathy

BACKGROUND

Mutations in the LMNA gene, encoding lamins A/C, represent a significant cause of dilated cardiomyopathy. We recently identified 18 protein-altering LMNA variants in a cohort of 324 unrelated patients with dilated cardiomyopathy. However, at least one family member with dilated cardiomyopathy in each of 6 pedigrees lacked the LMNA mutation (nonsegregation), whereas small sizes of 5 additional families precluded definitive determinations of segregation, raising questions regarding contributions by those variants to disease.

METHODS AND RESULTS

We have consequently expressed, in COS7 cells, GFP-prelamin A (GFPLaA) fusion constructs incorporating the 6 variants in pedigrees with nonsegregation (R101P, A318T, R388H, R399C, S437Hfsx1, and R654X), the 4 variants in pedigrees with unknown segregation (R89L, R166P [in 2 families], I210S, R471H), and 3 additional missense variants (R190Q, E203K, and L215P) that segregated with disease. Confocal immunofluorescence microscopy was used to characterize GFP-lamin A localization and nuclear morphology. Abnormal phenotypes were observed for 10 of 13 (77%) variants (R89L, R101P, R166P, R190Q, E203K, I210S, L215P, R388H, S437Hfsx1, and R654X), including 4 of 6 showing nonsegregation and 3 of 4 with uncertain segregation. All 7 variants affecting coil 1B and the lamin A-only mutation, R654X, exhibited membrane-bound GFP-lamin A aggregates and nuclear shape abnormalities. Unexpectedly, R388H largely restricted GFP-lamin A to the cytoplasm. Equally unexpected were unique streaked aggregates with S437Hfsx1 and giant aggregates with both S437Hfsx1 and R654X.

CONCLUSIONS

This work expands the recognized spectrum of lamin A localization abnormalities in dilated cardiomyopathy. It also provides evidence supporting pathogenicity of 10 of 13 tested LMNA variants, including some with uncertain or nonsegregation.

The nuclear envelope as a signaling node in development and disease

The development of a membrane-bound structure separating DNA from other cellular components was the epochal evolutionary event that gave rise to eukaryotes, possibly occurring up to 2 billion years ago. Yet, this view of the nuclear envelope as a physical barrier greatly underestimates its fundamental impact on cellular organization and complexity, much of which is only beginning to be understood. Indeed, alterations of nuclear envelope structure and protein composition are essential to many aspects of metazoan development and cellular differentiation. Mutations in genes encoding nuclear envelope proteins cause a fascinating array of diseases referred to as "nuclear envelopathies" or "laminopathies" that affect different tissues and organ systems. We review recent work on the nuclear envelope, including insights derived from the study of nuclear envelopathies. These studies are uncovering new functions for nuclear envelope proteins and underlie an emerging view of the nuclear envelope as a critical signaling node in development and disease.

Attenuated hypertrophic response to pressure overload in a lamin A/C haploinsufficiency mouse

Inherited mutations cause approximately 30% of all dilated cardiomyopathy cases, with autosomal dominant mutations in the LMNA gene accounting for more than one third of these. The LMNA gene encodes the nuclear envelope proteins lamins A and C, which provide structural support to the nucleus and also play critical roles in transcriptional regulation. Functional deletion of a single allele is sufficient to trigger dilated cardiomyopathy in humans and mice. However, whereas Lmna(-/-) mice develop severe muscular dystrophy and dilated cardiomyopathy and die by 8 weeks of age, heterozygous Lmna(+/-) mice have a much milder phenotype, with changes in ventricular function and morphology only becoming apparent at 1 year of age. Here, we studied 8- to 20-week-old Lmna(+/-) mice and wild-type littermates in a pressure overload model to examine whether increased mechanical load can accelerate or exacerbate myocardial dysfunction in the heterozygotes. While overall survival was similar between genotypes, Lmna(+/-) animals had a significantly attenuated hypertrophic response to pressure overload as evidenced by reduced ventricular mass and myocyte size. Analysis of pressure overload-induced transcriptional changes suggested that the reduced hypertrophy in the Lmna(+/-) mice was accompanied by impaired activation of the mechanosensitive gene Egr-1. In conclusion, our findings provide further support for a critical role of lamins A and C in regulating the cellular response to mechanical stress in cardiomyocytes and demonstrate that haploinsufficiency of lamins A and C alone is sufficient to alter hypertrophic responses and cardiac function in the face of pressure overload in the heart.

Laminopathies and the long strange trip from basic cell biology to therapy

The main function of the nuclear lamina, an intermediate filament meshwork lying primarily beneath the inner nuclear membrane, is to provide structural scaffolding for the cell nucleus. However, the lamina also serves other functions, such as having a role in chromatin organization, connecting the nucleus to the cytoplasm, gene transcription, and mitosis. In somatic cells, the main protein constituents of the nuclear lamina are lamins A, C, B1, and B2. Interest in the nuclear lamins increased dramatically in recent years with the realization that mutations in LMNA, the gene encoding lamins A and C, cause a panoply of human diseases ("laminopathies"), including muscular dystrophy, cardiomyopathy, partial lipodystrophy, and progeroid syndromes. Here, we review the laminopathies and the long strange trip from basic cell biology to therapeutic approaches for these diseases.

miR-23 regulation of lamin B1 is crucial for oligodendrocyte development and myelination

Duplication of the gene encoding lamin B1 (LMNB1) with increased mRNA and protein levels has been shown to cause severe myelin loss in the brains of adult-onset autosomal dominant leukodystrophy patients. Similar to many neurodegenerative disorders, patients with adult-onset autosomal dominant leukodystrophy are phenotypically normal until adulthood and the defect is specific to the central nervous system despite the ubiquitous expression pattern of lamin B1. We set out to dissect the molecular mechanisms underlying this demyelinating phenotype. Increased lamin B1 expression results in disturbances of inner nuclear membrane proteins, chromatin organization and nuclear pore transport in vitro. It also leads to premature arrest of oligodendrocyte differentiation, which might be caused by reduced transcription of myelin genes and by mislocalization of myelin proteins. We identified the microRNA miR-23 as a negative regulator of lamin B1 that can ameliorate the consequences of excessive lamin B1 at the cellular level. Our results indicate that regulation of lamin B1 is important for myelin maintenance and that miR-23 contributes to this process, at least in part, by downregulating lamin B1, therefore establishing novel functions of lamin B1 and miR-23 in the regulation of oligodendroglia development and myelin formation in vitro.

Functions of the nuclear envelope and lamina in development and disease

Recent findings that some 24 inherited diseases and anomalies are caused by defects in proteins of the NE (nuclear envelope) and lamina have resulted in a fundamental reassessment of the functions of the NE and underlying lamina. Instead of just regarding the NE and lamina as a molecular filtering device, regulating the transfer of macromolecules between the cytoplasm and nucleus, we now envisage the NE/lamina functioning as a key cellular 'hub' in integrating critical functions that include chromatin organization, transcriptional regulation, mechanical integrity of the cell and signalling pathways, as well as acting as a key component in the organization and function of the cytoskeleton.

Reduced expression of A-type lamins and emerin activates extracellular signal-regulated kinase in cultured cells

BACKGROUND

Mutations in genes encoding A-type lamins and emerin cause cardiomyopathy and muscular dystrophy. We previously showed activation of the extracellular signal-regulated kinase (ERK) branch of the mitogen-activated protein kinase (MAPK) cascade in hearts of mice with mutations in these genes. Here, we tested the hypothesis that reducing A-type lamins and emerin in cultured cells activate ERK signaling.

METHODS

We used siRNA to knockdown A-type lamins and emerin in HeLa and C2C12 cells. Activation of ERK was assessed by immunoblotting and immunofluorescence microscopy with antibodies against phosphorylated protein and by using real-time RT-PCR to measure RNAs encoded by genes for transcription factors stimulated by ERK.

RESULTS

Knockdown of A-type lamins and emerin in HeLa and C2C12 stimulated phosphorylation and nuclear translocation of ERK as well as activation of genes encoding downstream transcription factors. A MAPK/ERK kinase (MEK) inhibitor reduced ERK phosphorylation in cells with reduced expression of A-type lamins and emerin.

CONCLUSIONS

These results provide proof for the hypothesis that altered expression of emerin and A-type lamins activates ERK signaling, which in turn can cause cardiomyopathy.

GENERAL SIGNIFICANCE

ERK is a potential target for the pharmacological treatment of cardiomyopathy caused by mutations in the genes encoding emerin and A-type lamins.

Epidermal expression of the truncated prelamin A causing Hutchinson-Gilford progeria syndrome: effects on keratinocytes, hair and skin

Hutchinson-Gilford progeria syndrome (HGPS) is an accelerated aging disorder caused by point mutation in LMNA encoding A-type nuclear lamins. The mutations in LMNA activate a cryptic splice donor site, resulting in expression of a truncated, prenylated prelamin A called progerin. Expression of progerin leads to alterations in nuclear morphology, which may underlie pathology in HGPS. We generated transgenic mice expressing progerin in epidermis under control of a keratin 14 promoter. The mice had severe abnormalities in morphology of skin keratinocyte nuclei, including nuclear envelope lobulation and decreased nuclear circularity not present in transgenic mice expressing wild-type human lamin A. Primary keratinocytes isolated from these mice had a higher frequency of nuclei with abnormal shape compared to those from transgenic mice expressing wild-type human lamin A. Treatment with a farnesyltransferase inhibitor significantly improved nuclear shape abnormalities and induced the formation of intranuclear foci in the primary keratinocytes expressing progerin. Similarly, spontaneous immortalization of progerin-expressing cultured keratinocytes selected for cells with normal nuclear morphology. Despite morphological alterations in keratinocyte nuclei, mice expressing progerin in epidermis had normal hair grown and wound healing. Hair and skin thickness were normal even after crossing to Lmna null mice to reduce or eliminate expression of normal A-type lamins. Although progerin induces significant alterations in keratinocyte nuclear morphology that are reversed by inhibition of farnesyltransferasae, epidermal expression does not lead to alopecia or other skin abnormalities typically seen in human subjects with HGPS.

The laminopathies: the functional architecture of the nucleus and its contribution to disease

Most inherited diseases are associated with mutations in a specific gene. Often, 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. Mutations in the Lamin A gene (LMNA), which encodes largely ubiquitously expressed nuclear proteins (A-type lamins), are associated with at least eight different diseases, collectively called the laminopathies. Studies examining how different tissue-specific diseases arise from unique LMNA mutations are providing unanticipated insights into the structural organization of the nucleus, and how disruption of this organization relates to novel mechanisms of disease.

Mislocalization of human transcription factor MOK2 in the presence of pathogenic mutations of lamin A/C

BACKGROUND INFORMATION

hsMOK2 (human MOK2) is a DNA-binding transcriptional repressor. For example, it represses the IRBP (interphotoreceptor retinoid-binding protein) gene by competing with the CRX (cone-rod homeobox protein) transcriptional activator for DNA binding. Previous studies have shown an interaction between hsMOK2 and nuclear lamin A/C. This interaction could be important to explain hsMOK2 ability to repress transcription.

RESULTS

In the present study, we have tested whether missense pathogenic mutations of lamin A/C, which are located in the hsMOK2-binding domain, could affect the interaction with hsMOK2. We find that none of the tested mutations is able to disrupt hsMOK2 binding in vitro or in vivo. However, we observe an aberrant cellular localization of hsMOK2 into nuclear aggregates when pathogenic lamin A/C mutant proteins are expressed.

CONCLUSIONS

These results indicate that pathogenic mutations in lamin A/C lead to sequestration of hsMOK2 into nuclear aggregates, which may deregulate MOK2 target genes.

Fraying at the edge mouse models of diseases resulting from defects at the nuclear periphery

Eukaryotic cells compartmentalize their genetic material within the nucleus. The boundary separating the genetic material from the cytoplasm is the nuclear envelope (NE) and lamina. Historically, the NE was perceived as functioning primarily as a barrier regulating the entry and exit of macromolecules between the nucleus and cytoplasm via the nuclear pore complexes (NPCs) that traverse the nuclear membranes. However, recent findings have caused a fundamental reassessment with regard to NE and lamina functions. Evidence now points to the NE and lamina functioning as a "hub" in regulating and perhaps integrating critical cellular functions that include chromatin organization, transcriptional regulation, mechanical integrity of the cell, signaling pathways, as well as acting as a key component of the cytoskeleton. Such an integral role for the nuclear boundary has emerged from increased interest into the functions of the NE/lamina, which has been largely stimulated by the discovery that some 24 different diseases and anomalies are caused by defects in proteins of the NE and lamina.

Mouse models of the laminopathies

The A and B type lamins are nuclear intermediate filament proteins that comprise the bulk of the nuclear lamina, a thin proteinaceous structure underlying the inner nuclear membrane. The A type lamins are encoded by the lamin A gene (LMNA). Mutations in this gene have been linked to at least nine diseases, including the progeroid diseases Hutchinson-Gilford progeria and atypical Werner's syndromes, striated muscle diseases including muscular dystrophies and dilated cardiomyopathies, lipodystrophies affecting adipose tissue deposition, diseases affecting skeletal development, and a peripheral neuropathy. To understand how different diseases arise from different mutations in the same gene, mouse lines carrying some of the same mutations found in the human diseases have been established. We, and others have generated mice with different mutations that result in progeria, muscular dystrophy, and dilated cardiomyopathy. To further our understanding of the functions of the lamins, we also created mice lacking lamin B1, as well as mice expressing only one of the A type lamins. These mouse lines are providing insights into the functions of the lamina and how changes to the lamina affect the mechanical integrity of the nucleus as well as signaling pathways that, when disrupted, may contribute to the disease.

"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.

Mechanotransduction from the ECM to the genome: Are the pieces now in place?

A multitude of biochemical signaling processes have been characterized that affect gene expression and cellular activity. However, living cells often need to integrate biochemical signals with mechanical information from their microenvironment as they respond. In fact, the signals received by shape alone can dictate cell fate. This mechanotrasduction of information is powerful, eliciting proliferation, differentiation, or apoptosis in a manner dependent upon the extent of physical deformation. The cells internal "prestressed" structure and its "hardwired" interaction with the extra-cellular matrix (ECM) appear to confer this ability to filter biochemical signals and decide between divergent cell functions influenced by the nature of signals from the mechanical environment. In some instances mechanical signaling through the tissue microenvironment has been shown to be dominant over genomic defects, imparting a normal phenotype on cells that otherwise have transforming genetic lesions. This mechanical control of phenotype is postulated to have a central role in embryogenesis, tissue physiology as well as the pathology of a wide variety of diseases, including cancer. We will briefly review studies showing physical continuity between the external cellular microenvironment and the interior of the cell nucleus. Newly characterized structures, termed nuclear envelope lamina spanning complexes (NELSC), and their interactions will be described as part of a model for mechanical transduction of extracellular cues from the ECM to the genome.

Phenomics and lamins: From disease to therapy

Systematic correlation of phenotype with genotype is a key goal of the emerging field of phenomics, which is expected to help define complex diseases. Careful evaluation of phenotype-genotype associations in monogenic disorders, such as laminopathies, might provide new hypotheses to be tested with molecular and cellular studies and might also suggest potential new intervention strategies. For instance, evaluation of the clinical features of carriers of mutant LMNA in kindreds with familial partial lipodystrophy suggests rational, staged intervention using established pharmaceutical agents to prevent cardiovascular complications not just for patients with lipodystrophy but by extension for patients with the common metabolic syndrome. Careful non-invasive imaging shows phenotypic differences between partial lipodystrophy due to mutant LMNA and not due to mutant LMNA. Furthermore, hierarchical cluster analysis detects systematic relationships between organ involvement in laminopathies and mutation position in the LMNA genomic sequence. However, sometimes the same LMNA mutation can underlie markedly different clinical phenotypes; cellular and molecular experiments can help to explain the mechanistic basis for such differences. Finally, promising novel treatment modalities for laminopathies, such as farnesyl transferase inhibition and gene-based therapies, might help not only to illuminate mechanisms that link genotype to phenotype, but also to provide hope for patients suffering with laminopathies, since these treatments are designed to modulate key early or proximal steps in the pathogenesis of these disorders.

A-type lamin networks in light of laminopathic diseases

Lamins are major structural components of the lamina providing mechanical support for the nuclear envelope in vertebrates. A subgroup of lamins, the A-type lamins, are only expressed in differentiated cells and serve important functions both at the nuclear envelope and in the nucleoplasm in higher order chromatin organization and gene regulation. Mutations in A-type lamins cause a variety of diseases from muscular dystrophy and lipodystrophy to systemic diseases such as premature ageing syndromes. The molecular basis of these diseases is still unknown. Here we summarize known interactions of A-type lamins with components of the nuclear envelope and the nucleoplasm and discuss their potential involvement in the etiology and molecular mechanisms of the diseases. Lamin binding partners involve chromatin proteins potentially involved in higher order chromatin organization, transcriptional regulators controlling gene expression during cell cycle progression, differentiation and senescence, and several enzymes involved in a multitude of functions.

Emery-Dreifuss muscular dystrophy

Emery-Dreifuss muscular dystrophy (EDMD) is inherited in an X-linked or autosomal manner. X-linked EDMD is caused by mutations in EMD, which encodes an integral protein of the nuclear envelope inner membrane called emerin. Autosomally inherited EDMD is caused by mutations in LMNA, which encodes A-type nuclear lamins, intermediate filament proteins associated with inner nuclear membrane. Although the causative mutations have been described and mouse models have been created, the pathogenic processes by which mutations in genes encoding nuclear envelope proteins cause striated muscle abnormalities in EDMD remain obscure. Working hypotheses include effects on nuclear structural integrity, increased cellular susceptibility to mechanical stress damage, alterations in gene expression in response to nuclear envelope changes, and effects on cell proliferation and differentiation.

Nuclear envelope defects in muscular dystrophy

Muscular dystrophies are a heterogeneous group of disorders linked to defects in 20-30 different genes. Mutations in the genes encoding a pair of nuclear envelope proteins, emerin and lamin A/C, have been shown to cause the X-linked and autosomal forms respectively of Emery-Dreifuss muscular dystrophy. A third form of muscular dystrophy, limb girdle muscular dystrophy 1b, has also been linked to mutations in the lamin A/C gene. Given that these two genes are ubiquitously expressed, a major goal is to determine how they can be associated with tissue specific diseases. Recent results suggest that lamin A/C and emerin contribute to the maintenance of nuclear envelope structure and at the same time may modulate the expression patterns of certain mechanosensitive and stress induced genes. Both emerin and lamin A/C may play an important role in the response of cells to mechanical stress and in this way may help to maintain muscle cell integrity.

LEM2 is a novel MAN1-related inner nuclear membrane protein associated with A-type lamins

The LEM (lamina-associated polypeptide-emerin-MAN1) domain is a motif shared by a group of lamin-interacting proteins in the inner nuclear membrane (INM) and in the nucleoplasm. The LEM domain mediates binding to a DNA-crosslinking protein, barrier-to-autointegration factor (BAF). We describe a novel, ubiquitously expressed LEM domain protein, LEM2, which is structurally related to MAN1. LEM2 contains an N-terminal LEM motif, two predicted transmembrane domains and a MAN1-Src1p C-terminal (MSC) domain highly homologous to MAN1, but lacks the MAN1-specific C-terminal RNA-recognition motif. Immunofluorescence microscopy of digitonin-treated cells and subcellular fractionation identified LEM2 as a lamina-associated protein residing in the INM. LEM2 binds to the lamin C tail in vitro. Targeting of LEM2 to the nuclear envelope requires A-type lamins and is mediated by the N-terminal and transmembrane domains. Highly overexpressed LEM2 accumulates in patches at the nuclear envelope and forms membrane bridges between nuclei of adjacent cells. LEM2 structures recruit A-type lamins, emerin, MAN1 and BAF, whereas lamin B and lamin B receptor are excluded. Our data identify LEM2 as a novel A-type-lamin-associated INM protein involved in nuclear structure organization.

Laminopathies: multiple disorders arising from defects in nuclear architecture

Lamins are the major structural proteins of the nucleus in an animal cell. In addition to being essential for nuclear integrity and assembly, lamins are involved in the organization of nuclear processes such as DNA replication, transcription and repair. Mutations in the human lamin A gene lead to highly debilitating genetic disorders that primarily affect muscle, adipose, bone or neuronal tissues and also cause premature ageing syndromes. Mutant lamins alter nuclear integrity and hinder signalling pathways involved in muscle differentiation and adipocyte differentiation, suggesting tissue-specific roles for lamins. Furthermore, cells expressing mutant lamins are impaired in their response to DNA damaging agents. Recent reports indicate that certain lamin mutations act in a dominant negative manner to cause nuclear defects and cellular toxicity, and suggest a possible role for aberrant lamins in normal ageing processes.

Guilt by Association

The discovery that many inherited diseases are linked to interacting nuclear envelope proteins has raised the possibility that human genetic studies could be assisted by a fusion with proteomics. Two principles could be applied. In the first, the proteome of an organelle associated with a genetically variable disease is determined. The chromosomal locations of the genes encoding the organellar proteins are then determined. If a related disease is linked to a large chromosomal region that includes a gene identified in the organelle, then that gene has an increased likelihood of causing the disease. Directly sequencing this allele from patient samples might speed identification compared with further genetic linkage studies as has been demonstrated for multiple diseases associated with the nuclear envelope. The second principle is that if an organelle has been implicated in the pathology of a particular disorder, then comparison of the organelle proteome from control and patient cells might highlight differences that could indicate the causative protein. The distinct, tissue-specific pathologies associated with nuclear envelope diseases suggest that many tissues will have a set of disorders linked to this organelle, and there are numerous as yet unmapped or partially mapped syndromes that could benefit from such an approach.

The Emery-Dreifuss muscular dystrophy associated-protein emerin is phosphorylated on serine 49 by protein kinase A

Emerin is a ubiquitously expressed inner nuclear membrane protein of unknown function. Mutations in its gene give rise to X-linked Emery-Dreifuss muscular dystrophy (X-EDMD), a neuromuscular condition with an associated life-threatening cardiomyopathy. We have previously reported that emerin is phosphorylated in a cell cycle-dependent manner in human lymphoblastoid cell lines [Ellis et al. (1998) Aberrant intracellular targeting and cell cycle-dependent phosphorylation of emerin contribute to the EDMD phenotype. J. Cell Sci. 111, 781-792]. Recently, five residues in human emerin were identified as undergoing cell cycle-dependent phosphorylation using a Xenopus egg mitotic cytosol model system (Hirano et al. (2005) Dissociation of emerin from BAF is regulated through mitotic phosphorylation of emerin in a Xenopus egg cell-free system. J. Biol. Chem.280, 39 925-39 933). In the present paper, recombinant human emerin was purified from a baculovirus-Sf9 heterogeneous expression system, analyzed by protein mass spectrometry and shown to exist in at least four different phosphorylated species, each of which could be dephosphorylated by treatment with alkaline phosphatase. Further analysis identified three phosphopeptides with m/z values of 2191.9 and 2271.7 corresponding to the singly and doubly phosphorylated peptide 158-DSAYQSITHYRPVSASRSS-176, and a m/z of 2396.9 corresponding to the phosphopeptide 47-RLSPPSSSAASSYSFSDLNSTR-68. Sequence analysis confirmed that residue S49 was phosphorylated and also demonstrated that this residue was phosphorylated in interphase. Using an in vitro protein kinase A assay, we observed two phospho-emerin species, one of which was phosphorylated at residue S49. Protein kinase A is thus the first kinase that has been identified to specifically phosphorylate emerin. These results improve our understanding of the molecular mechanisms underlying X-EDMD and point towards possible signalling pathways involved in regulating emerin's functions.

Quantitative analysis of localization and nuclear aggregate formation induced by GFP-lamin A mutant proteins in living HeLa cells

Although A-type lamins are ubiquitously expressed, their role in the tissue-specificity of human laminopathies remains enigmatic. In this study, we generate a series of transfection constructs encoding missense lamin A mutant proteins fused to green fluorescent protein and investigate their subnuclear localization using quantitative live cell imaging. The mutant constructs used included the laminopathy-inducing lamin A rod domain mutants N195K, E358K, M371K, R386K, the tail domain mutants G465D, R482L, and R527P, and the Hutchinson-Gilford progeria syndrome-causing deletion mutant, progerin (LaA delta50). All mutant derivatives induced nuclear aggregates, except for progerin, which caused a more lobulated phenotype of the nucleus. Quantitative analysis revealed that the frequency of nuclear aggregate formation was significantly higher (two to four times) for the mutants compared to the wild type, although the level of lamin fusion proteins within nuclear aggregates was not. The distribution of endogenous A-type lamins was altered by overexpression of the lamin A mutants, coexpression experiments revealing that aberrant localization of the N195K and R386K mutants had no effect on the subnuclear distribution of histones H2A or H2B, or on nuclear accumulation of H2A overexpressed as a DsRed2 fusion protein. The GFP-lamin fusion protein-expressing constructs will have important applications in the future, enabling live cell imaging of nuclear processes involving lamins and how this may relate to the pathogenesis of laminopathies.

Nuclear Lamins: Laminopathies and Their Role in Premature Ageing

It has been demonstrated that nuclear lamins are important proteins in maintaining cellular as well as nuclear integrity, and in maintaining chromatin organization in the nucleus. Moreover, there is growing evidence that lamins play a prominent role in transcriptional control. The family of laminopathies is a fast-growing group of diseases caused by abnormalities in the structure or processing of the lamin A/C ( LMNA) gene. Mutations or incorrect processing cause more than a dozen different inherited diseases, ranging from striated muscular diseases, via fat- and peripheral nerve cell diseases, to progeria. This broad spectrum of diseases can only be explained if the responsible A-type lamin proteins perform multiple functions in normal cells. This review gives an overview of current knowledge on lamin structure and function and all known diseases associated with LMNA abnormalities. Based on the knowledge of the different functions of A-type lamins and associated proteins, explanations for the observed phenotypes are postulated. It is concluded that lamins seem to be key players in, among others, controlling the process of cellular ageing, since disturbance in lamin protein structure gives rise to several forms of premature ageing.

Thymopoietin (lamina-associated polypeptide 2) gene mutation associated with dilated cardiomyopathy

Thymopoietin or TMPO (indicated by its alternative gene symbol, LAP2, in this work) has been proposed as a candidate disease gene for dilated cardiomyopathy (DCM), since a LAP2 product associates with nucleoplasmic lamins A/C, which are encoded by the DCM gene LMNA. We developed a study to screen for genetic mutations in LAP2 in a large collection of DCM patients and families. A total of 113 subjects from 88 families (56 with familial DCM (FDC) and 32 with sporadic DCM) were screened for LAP2 mutations using denaturing high-performance liquid chromatography and sequence analysis. We found a single putative mutation affecting the LAP2alpha isoform in one FDC pedigree. The mutation predicts an Arg690Cys substitution (c.2068C>T; p.R690C) located in the C-terminal domain of the LAP2alpha protein, a region that is known to interact with lamin A/C. RT-PCR, Western blot analyses, and immunolocalization revealed low-level LAP2alpha expression in adult cardiac muscle, and localization to a subset of nuclei. Mutated Arg690Cys LAP2alpha expressed in HeLa cells localized to the nucleoplasm like wild-type LAP2alpha, with no effect on peripheral and nucleoplasmic lamin A distribution. However, the in vitro interaction of mutated LAP2alpha with the pre-lamin A C-terminus was significantly compromised compared to the wild-type protein. LAP2 mutations may represent a rare cause of DCM. The Arg690Cys mutation altered the observed LAP2alpha interaction with A-type lamins. Our finding implicates a novel nuclear lamina-associated protein in the pathogenesis of genetic forms of dilated cardiomyopathy.

Expression of disease-causing lamin A mutants impairs the formation of DNA repair foci

A-type lamins are components of the nuclear lamina. Mutations in the gene encoding lamin A are associated with a range of highly degenerative diseases termed laminopathies. To evaluate sensitivity to DNA damage, GFP-tagged lamin A cDNAs with disease-causing mutations were expressed in HeLa cells. The inner nuclear membrane protein emerin was mislocalised upon expression of the muscular dystrophy mutants G232E, Q294P or R386K, which aberrantly assembled into nuclear aggregates, or upon expression of mutants causing progeria syndromes in vivo (lamin A del50, R471C, R527C and L530P). The ability of cells expressing these mutants to form DNA repair foci comprising phosphorylated H2AX in response to mild doses of cisplatin or UV irradiation was markedly diminished, unlike the nearly normal response of cells expressing wild-type GFP-lamin A or disease-causing H222P and R482L mutants. Interestingly, mutants that impaired the formation of DNA repair foci mislocalised ATR (for ;ataxia telangiectasia-mutated and Rad3-related') kinase, which is a key sensor in the response to DNA damage. Our results suggest that a subset of lamin A mutants might hinder the response of components of the DNA repair machinery to DNA damage by altering interactions with chromatin.

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.

Mutation Glu82Lys in lamin A/C gene is associated with cardiomyopathy and conduction defect

Dilated cardiomyopathy is a form of heart muscle disease characterized by impaired systolic function and ventricular dilation. The mutations in lamin A/C gene have been linked to dilated cardiomyopathy. We screened genetic mutations in a large Chinese family of 50 members including members with dilated cardiomyopathy and found a Glu82Lys substitution mutation in the rod domain of the lamin A/C protein in eight family members, three of them have been diagnosed as dilated cardiomyopathy, one presented with heart dilation. The pathogenic mechanism of lamin A/C gene defect is poorly understood. Glu82Lys mutated lamin A/C and wild type protein was transfected into HEK293 cells. The mutated protein was not properly localized at the inner nuclear membrane and the emerin protein, which interacts with lamin A/C, was also aberrantly distributed. The nuclear membrane structure was disrupted and heterochromatin was aggregated aberrantly in the nucleus of the HEK293 cells stably transfected with mutated lamin A/C gene as determined by transmission electron microscopy.

Lamin A/C and cardiac diseases

PURPOSE OF REVIEW

In this review, we will outline the most recent and significant findings on the role of the lamin A/C in cardiac diseases.

RECENT FINDINGS

Mutations in the lamin A/C gene (LMNA) are associated with numerous diseases involving the heart, skeletal muscles, bones, adipose and nervous tissues. LMNA is one of the most prevalent genes in dilated cardiomyopathy in which it is associated with a high risk of dysrhythmias, sudden death and heart failure. Lamins A and C interact with several proteins reflecting their multiple functions, some of which are likely still unknown. No abnormalities specific to dilated cardiomyopathy are emerging from investigations of striated muscles biopsies or fibroblasts from LMNA mutation carriers. An early diagnosis of the disease is difficult. Both animal and cellular models tend to confirm that lamins A and C play a key role in maintaining the nuclear architecture as well as in regulating transcription.

SUMMARY

The cardiac phenotype associated to LMNA mutations is now much clearer, but the molecular mechanisms underlying cellular and tissue specific phenotypes are still puzzling. Systematic mutation screenings and cardioverter-defibrillator implantation have been recommended in patients with cardiac symptoms.

Laminopathies: multisystem dystrophy syndromes

Laminopathies are a heterogeneous group of genetic disorders due to abnormalities in type A lamins and can manifest varied clinical features affecting many organs including the skeletal and cardiac muscle, adipose tissue, nervous system, cutaneous tissue, and bone. Mutations in the gene encoding lamins A and C (LMNA) cause primary laminopathies, including various types of lipodystrophies, muscular dystrophies and progeroid syndromes, mandibuloacral dysplasia, dilated cardiomyopathies, and restrictive dermopathy. The secondary laminopathies are due to mutations in ZMPSTE24 gene which encodes for a zinc metalloproteinase involved in processing of prelamin A into mature lamin A and cause mandibuloacral dysplasia and restrictive dermopathy. Skin fibroblast cells from many patients with laminopathies show a range of abnormal nuclear morphology including bleb formation, honeycombing, and presence of multi-lobulated nuclei. The mechanisms by which mutations in LMNA gene cause multisystem dystrophy are an active area of current investigation. Further studies are needed to understand the underlying mechanisms of marked pleiotropy in laminopathies.