Lamins in disease: why do ubiquitously expressed nuclear envelope proteins give rise to tissue-specific disease phenotypes?

Distinct functions of the unique C terminus of LAP2alpha in cell proliferation and nuclear assembly

The non-membrane-bound lamina-associated polypeptide 2 isoform, LAP2alpha, forms nucleoskeletal structures with A-type lamins and interacts with chromosomes in a cell cycle-dependent manner. LAP2alpha contains a LEM (LAP2, emerin, and MAN1) domain in the constant N terminus that binds to chromosomal barrier-to-autointegration factor, and a C-terminal unique region that is essential for chromosome binding. Here we show that C-terminal LAP2alpha fragment efficiently bound to mitotic chromosomes and inhibited assembly of endogenous LAP2alpha, nuclear membranes, and lamins A/C in in vitro nuclear assembly assays. Full-length recombinant LAP2alpha, which bound to chromosomes, and N-terminal fragment, which did not bind, had no effect on assembly. This suggested an essential role for the LAP2alpha C terminus in chromosome association and for the N-terminal LEM domain in subsequent assembly stages. In vivo analysis upon transient expression of GFP-tagged LAP2alpha fragments confirmed that, unlike the N-terminal fragment, the C-terminal fragment was able to bind to chromosomes during mitosis, if expressed weakly. At higher expression levels, C-terminal LAP2alpha fragment and full-length protein led to cell cycle arrest in interphase and apoptosis, as shown by fluorescence-activated cell sorter analysis, time lapse microscopy, and BrdUrd incorporation assays. These data indicated distinct functions of LAP2alpha in cell cycle progression during interphase and in nuclear reassembly during mitosis.

Multisystem dystrophy syndrome due to novel missense mutations in the amino-terminal head and alpha-helical rod domains of the lamin A/C gene

Mutations in different domains of the LMNA (lamin A/C) gene encoding nuclear envelope proteins lamin A and lamin C cause familial partial lipodystrophy (Dunnigan variety), dilated cardiomyopathy, and autosomal dominant forms of Emery-Dreifuss and limb-girdle muscular dystrophies. The objective of this study was to evaluate LMNA variants in two families with familial partial lipodystrophy (Dunnigan variety) who also had cardiac conduction system defects and other manifestations related to cardiomyopathy. We performed mutational analysis of the lamin A/C gene in affected and unaffected subjects by deoxyribonucleic acid sequencing of the exons. Two novel missense mutations were identified in exon 1 of the lamin A/C gene. One mutation, R28W (CGG-->TGG), affected the amino-terminal head domain, and the other, R62G (CGC-->GGC), affected the alpha-helical rod domain. Affected subjects from both families had an increased prevalence of cardiac manifestations, such as atrioventricular conduction defects, atrial fibrillation, and heart failure due to ventricular dilatation, as well as pacemaker implantation. The proband from one of the families also had proximal muscle weakness. Novel genetic defects in the LMNA gene in two families with the Dunnigan variety of familial partial lipodystrophy, cardiac conduction system defects, and other manifestations related to cardiomyopathy suggest the occurrence of a multisystem dystrophy syndrome due to LMNA mutations.

Four signature motifs define the first class of structurally related large coiled-coil proteins in plants

BACKGROUND

Animal and yeast proteins containing long coiled-coil domains are involved in attaching other proteins to the large, solid-state components of the cell. One subgroup of long coiled-coil proteins are the nuclear lamins, which are involved in attaching chromatin to the nuclear envelope and have recently been implicated in inherited human diseases. In contrast to other eukaryotes, long coiled-coil proteins have been barely investigated in plants.

RESULTS

We have searched the completed Arabidopsis genome and have identified a family of structurally related long coiled-coil proteins. Filament-like plant proteins (FPP) were identified by sequence similarity to a tomato cDNA that encodes a coiled-coil protein which interacts with the nuclear envelope-associated protein, MAF1. The FPP family is defined by four novel unique sequence motifs and by two clusters of long coiled-coil domains separated by a non-coiled-coil linker. All family members are expressed in a variety of Arabidopsis tissues. A homolog sharing the structural features was identified in the monocot rice, indicating conservation among angiosperms.

CONCLUSION

Except for myosins, this is the first characterization of a family of long coiled-coil proteins in plants. The tomato homolog of the FPP family binds in a yeast two-hybrid assay to a nuclear envelope-associated protein. This might suggest that FPP family members function in nuclear envelope biology. Because the full Arabidopsis genome does not appear to contain genes for lamins, it is of interest to investigate other long coiled-coil proteins, which might functionally replace lamins in the plant kingdom.

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.

Autosomal dominant dilated cardiomyopathy with atrioventricular block: a lamin A/C defect-related disease

OBJECTIVES

We investigated the prevalence of lamin A/C (LMNA) gene defects in familial and sporadic dilated cardiomyopathies (DCM) associated with atrioventricular block (AVB) or increased serum creatine-phosphokinase (sCPK), and the corresponding changes in myocardial and protein expression.

BACKGROUND

It has been reported that familial DCM, associated with conduction disturbances or variable myopathies, is causally linked to LMNA gene defects.

METHODS

The LMNA gene and myocardial ultrastructural and immunochemical changes were analyzed in 73 cases of DCM (49 pure, 15 with AVB [seven familial, eight sporadic], 9 with increased sCPK), four cases of familial AVB and 19 non-DCM heart diseases. The normal controls included eight heart donor biopsies for tissue studies and 107 subjects for LMNA gene studies.

RESULTS

Five novel LMNA mutations (K97E, E111X, R190W, E317K, four base pair insertion at 1,713 cDNA) were identified in five cases of familial autosomal dominant DCM with AVB (5/15: 33%). The LMNA expression of the myocyte nuclei was reduced or absent. Western blot protein analyses of three hearts with different mutations showed an additional 30-kDa band, suggesting a degrading effect of mutated on wild-type protein. Focal disruptions, bleb formation and nuclear pore clustering were documented by electron microscopy of the myocyte nuclear membranes. None of these changes and no mutations were found in the nine patients with DCM and increased sCPK or in the disease and normal controls.

CONCLUSIONS

The LMNA gene mutations account for 33% of the DCMs with AVB, all familial autosomal dominant. Increased sCPK in patients with DCM without AVB is not a useful predictor of LMNA mutation.

Conservation of the gene structure and membrane-targeting signals of germ cell-specific lamin LIII in amphibians and fish

Targeting of nuclear lamins to the inner nuclear membrane requires CaaX motif-dependent posttranslational isoprenylation and carboxyl methylation. We previously have shown that two variants of lamin LIII (i.e., LIII and LIIIb) in amphibian oocytes are generated by alternative splicing and differ greatly in their membrane association. An extra cysteine residue (as a potential palmitoylation site) and a basic cluster in conjunction with the CaaX motif function as secondary targeting signals responsible for stable membrane association of lamin LIIIb. cDNA sequencing and genomic analysis of the zebrafish Danio rerio lamin LIII uncovers a remarkable conservation of the genomic organization and of the two secondary membrane-targeting signals in amphibians and fish. The expression pattern of lamin LIII genes is also conserved between amphibians and fish. Danio lamin LIII is expressed in diplotene oocytes. It is absent from male germ cells but is expressed in Sertoli cells of the testis. In addition, we provide sequence information of the entire coding sequence of zebrafish lamin A, which allows comparison of all major lamins from representatives of the four classes of vertebrates.

Mutations of TTN, encoding the giant muscle filament titin, cause familial dilated cardiomyopathy

Congestive heart failure (CHF) can result from various disease states with inadequate cardiac output. CHF due to dilated cardiomyopathy (DCM) is a familial disease in 20-30% of cases and is associated with mutations in genes encoding cytoskeletal, contractile or inner-nuclear membrane proteins. We show that mutations in the gene encoding giant-muscle filament titin (TTN) cause autosomal dominant DCM linked to chromosome 2q31 (CMD1G; MIM 604145). Titin molecules extend from sarcomeric Z-discs to M-lines, provide an extensible scaffold for the contractile machinery and are crucial for myofibrillar elasticity and integrity. In a large DCM kindred, a segregating 2-bp insertion mutation in TTN exon 326 causes a frameshift, truncating A-band titin. The truncated protein of approximately 2 mD is expressed in skeletal muscle, but western blot studies with epitope-specific anti-titin antibodies suggest that the mutant protein is truncated to a 1.14-mD subfragment by site-specific cleavage. In another large family with DCM linked to CMD1G, a TTN missense mutation (Trp930Arg) is predicted to disrupt a highly conserved hydrophobic core sequence of an immunoglobulin fold located in the Z-disc-I-band transition zone. The identification of TTN mutations in individuals with CMD1G should provide further insights into the pathogenesis of familial forms of CHF and myofibrillar titin turnover.

The cell cycle dependent mislocalisation of emerin may contribute to the Emery-Dreifuss muscular dystrophy phenotype

Emerin is the nuclear membrane protein defective in X-linked Emery-Dreifuss muscular dystrophy (X-EDMD). The majority of X-EDMD patients have no detectable emerin. However, there are cases that produce mutant forms of emerin, which can be used to study its function. Our previous studies have shown that the emerin mutants S54F, P183T, P183H, Del95-99, Del236-241 (identified in X-EDMD patients) are targeted to the nuclear membrane but to a lesser extent than wild-type emerin. In this paper, we have studied how the mislocalisation of these mutant emerins may affect nuclear functions associated with the cell cycle using flow cytometry and immunofluorescence microscopy. We have established that cells expressing the emerin mutant Del236-241 (a deletion in the transmembrane domain), which was mainly localised in the cytoplasm, exhibited an aberrant cell cycle length. Thereafter, by examining the intracellular localisation of endogenously expressed lamin A/C and exogenously expressed wild-type and mutant forms of emerin after a number of cell divisions, we determined that the mutant forms of emerin redistributed endogenous lamin A/C. The extent of lamin A/C redistribution correlated with the amount of EGFP-emerin that was mislocalised. The amount of EGFP-emerin mislocalized, in turn, was associated with alterations in the nuclear envelope morphology. The nuclear morphology and redistribution of lamin A/C was most severely affected in the cells expressing the emerin mutant Del236-241.

It is believed that emerin is part of a novel nuclear protein complex consisting of the barrier-to-autointegration factor (BAF), the nuclear lamina, nuclear actin and other associated proteins. The data presented here show that lamin A/C localisation is dominantly directed by its interaction with certain emerin mutants and perhaps wild-type emerin as well. These results suggest that emerin links A-type lamins to the nuclear envelope and that the correct localisation of these nuclear proteins is important for maintaining cell cycle timing.

Identification of essential genes in cultured mammalian cells using small interfering RNAs

We report the first RNAi-induced phenotypes in mammalian cultured cells using RNA interference mediated by duplexes of 21-nt RNAs. The 21 gene products studied have different functions and subcellular localizations. Knockdown experiments monitored by immunofluorescence and immunoblotting show that even major cellular proteins such as actin and vimentin can be silenced efficiently. Genes were classified as essential or nonessential depending on impaired cell growth after RNA silencing. Phenotypes also involved altered cell morphology and aberrant mitotic arrest. Among the essential genes identified by RNAi for which such information was previously not available are lamin B1, lamin B2, NUP153, GAS41, ARC21, cytoplasmic dynein, the protein kinase cdk1 and both β- and γ-actin. Newly defined nonessential genes are emerin and zyxin. Several genes previously characterized by other methods such as knockout of murine genes are included as internal controls and gave identical results when RNAi was used. In the case of two nonessential genes (lamin A/C and zyxin) RNAi provides a recognizable phenotype.

Our results complete the characterization of the mammalian nuclear lamins. While lamins A/C appear as nonessential proteins in the mouse embryo and in RNAi treated cultured cells, the two other lamins, B1 and B2, are now identified as essential proteins. Interestingly the inner nuclear membrane protein emerin, thought to be a ligand of lamin A/C, is also a nonessential protein in tissue culture cells.

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.

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

Autosomal dominant Emery-Dreifuss muscular dystrophy is caused by mutations in the LMNA gene, which encodes lamin A and lamin C. Mutations in this gene also give rise to limb girdle muscular dystrophy type 1B, dilated cardiomyopathy with atrioventricular conduction defect and Dunnigan-type partial lipodystrophy. The properties of the mutant lamins that cause muscular dystrophy, lipodystrophy and dilated cardiomyopathy are not known. We transfected C2C12 myoblasts with cDNA encoding wild-type lamin A and 15 mutant forms found in patients affected by these diseases. Immunofluorescence microscopy showed that four mutants, N195K, E358K, M371K and R386K, could have a dramatically aberrant localization, with decreased nuclear rim staining and formation of intranuclear foci. The distributions of endogenous lamin A/C, lamin B1 and lamin B2 were also altered in cells expressing these four mutants and three of them caused a loss of emerin from the nuclear envelope. In the yeast two-hybrid assay, the 15 lamin A mutants studied interacted with themselves and with wild-type lamin A and lamin B1. Pulse-chase experiments showed no decrease in the stability of several representative lamin A mutants compared with wild-type. These results indicate that some lamin A mutants causing disease can be aberrantly localized, partially disrupt the endogenous lamina and alter emerin localization, whereas others localize normally in transfected cells.

The role of the nuclear envelope in Emery-Dreifuss muscular dystrophy

The X-linked form of Emery-Dreifuss muscular dystrophy (X-EDMD) is caused by absence, or greatly reduced amounts, of the inner nuclear-membrane protein, emerin. The autosomal dominant form (AD-EDMD) is caused by missense mutations in lamins A and C, two components of the nuclear lamina that interact directly with emerin. Lamin A/C mutations also cause one form of dilated cardiomyopathy (CMD1A) and one form of limb-girdle muscular dystrophy (LGMD1B), both of which have clinical features in common with EDMD, as well as a rare, unrelated form of lipodystrophy (FPLD). Evidence is now emerging that defective assembly of the nuclear lamina is a feature of all these diseases, although not necessarily the direct cause. Why only heart and skeletal muscle, and possibly connective tissue, are affected in EDMD and why expression of the disease is so extremely variable between individuals remains to be explained.

Distinct changes in intranuclear lamin A/C organization during myoblast differentiation

Intranuclear lamin foci or speckles have been observed in various cell types. In order to explore the possibility of changes in internal lamin organization during muscle differentiation, we have examined the appearance of A-type lamin speckles that associate with RNA splicing factor speckles in C2C12 myoblasts and myotubes. Lamin speckles were observed in dividing myoblasts but disappeared early during the course of differentiation in postmitotic myocytes, and were absent in myotubes and muscle fibers. However, no changes were seen in the typical peripheral organization of lamins A/C or B1 or in RNA splicing factor speckles. Lamin speckles were also absent in quiescent myoblasts but reappeared as cells were reactivated to enter the cell cycle. These changes were not observed in other quiescent cell types. Immunoblot analysis indicated that the abundance and migration of lamins A and C was not altered in differentiated myoblasts. When myotube or quiescent myoblast nuclei were extracted with nucleases and detergent, a uniformly stained internal lamina was revealed, indicating that lamins A/C were antigenically masked in these cells, probably owing to structural reorganization of the lamina during differentiation or quiescence. Our results suggest that muscle cell differentiation is accompanied by regulated rearrangements in the organization of the A-type lamins.

The nuclear envelope in muscular dystrophy and cardiovascular diseases

Considerable interest has been focused on the nuclear envelope in recent years following the realization that several human diseases are linked to defects in genes encoding nuclear envelope specific proteins, most notably A-type lamins and emerin. These disorders, described as laminopathies or nuclear envelopathies, include both X-linked and autosomal dominant forms of Emery-Dreifuss muscular dystrophy, dilated cardiomyopathy with conduction system defects, limb girdle muscular dystrophy 1B with atrioventricular conduction disturbances, and Dunnigan-type familial partial lipodystrophy. Certain of these diseases are associated with nuclear structural abnormalities that can be seen in a variety of cells and tissues. These observations clearly demonstrate that A-type lamins in particular play a central role, not only in the maintenance of nuclear envelope integrity but also in the large-scale organization of nuclear architecture. What is not obvious, however, is why defects in nuclear envelope proteins that are found in most adult cell types should give rise to pathologies associated predominantly with skeletal and cardiac muscle and adipocytes. The recognition of these various disorders now raises the novel possibility that the nuclear envelope may have functions that go beyond housekeeping and which impact upon cell-type specific nuclear processes.

Novel lamin A/C mutations in two families with dilated cardiomyopathy and conduction system disease

BACKGROUND

The LMNA gene, one of 6 autosomal disease genes implicated in familial dilated cardiomyopathy, encodes lamins A and C, alternatively spliced nuclear envelope proteins. Mutations in lamin A/C cause 4 diseases: Emery-Dreifuss muscular dystrophy, limb girdle muscular dystrophy type 1B, Dunnigan-type familial partial lipodystrophy, and dilated cardiomyopathy.

METHODS AND RESULTS

Two 4-generation white families with autosomal dominant familial dilated cardiomyopathy and conduction system disease were found to have novel mutations in the rod segment of lamin A/C. In family A a missense mutation (nucleotide G607A, amino acid E203K) was identified in 14 adult subjects; disease was manifest as progressive conduction disease in the fourth and fifth decades. Death was caused by heart failure. In family B a nonsense mutation (nucleotide C673T, amino acid R225X) was identified in 10 adult subjects; disease was also manifest as progressive conduction disease but with earlier onset (third and fourth decades), ventricular dysrhythmias, left ventricular enlargement, and systolic dysfunction. Death was caused by heart failure and sudden cardiac death. Skeletal muscle disease was not observed in either family.

CONCLUSIONS

Novel rod segment mutations in lamin A/C cause variable conduction system disease and dilated cardiomyopathy without skeletal myopathy.

Genes for intermediate filament proteins and the draft sequence of the human genome

We screened the draft sequence of the human genome for genes that encode intermediate filament (IF) proteins in general, and keratins in particular. The draft covers nearly all previously established IF genes including the recent cDNA and gene additions, such as pancreatic keratin 23, synemin and the novel muscle protein syncoilin. In the draft, seven novel type II keratins were identified, presumably expressed in the hair follicle/epidermal appendages. In summary, 65 IF genes were detected, placing IF among the 100 largest gene families in humans. All functional keratin genes map to the two known keratin clusters on chromosomes 12 (type II plus keratin 18) and 17 (type I), whereas other IF genes are not clustered. Of the 208 keratin-related DNA sequences, only 49 reflect true keratin genes, whereas the majority describe inactive gene fragments and processed pseudogenes. Surprisingly, nearly 90% of these inactive genes relate specifically to the genes of keratins 8 and 18. Other keratin genes, as well as those that encode non-keratin IF proteins, lack either gene fragments/pseudogenes or have only a few derivatives. As parasitic derivatives of mature mRNAs, the processed pseudogenes of keratins 8 and 18 have invaded most chromosomes, often at several positions. We describe the limits of our analysis and discuss the striking unevenness of pseudogene derivation in the IF multigene family. Finally, we propose to extend the nomenclature of Moll and colleagues to any novel keratin.

Both emerin and lamin C depend on lamin A for localization at the nuclear envelope

Physical interactions between lamins and emerin were investigated by co-immunoprecipitation of in vitro translated proteins. Emerin interacted with in vitro translated lamins A, B1 and C in co-immunprecipitation reactions. Competition reactions revealed a clear preference for interactions between emerin and lamin C. Structural associations between lamins and emerin were investigated in four human cell lines displaying abnormal expression and/or localisation of lamins A and C. In each cell line absence of lamins A and C from the nuclear envelope (NE) was correlated with mis-localisation of endogenous and exogenous emerin to the ER. In two cell lines that did not express lamin A but did express lamin C, lamin C as well as emerin was mis-localised. When GFP-lamin A was expressed in SW13 cells (which normally express only very low levels of endogenous lamin A and mis-localise endogenous emerin and lamin C), all three proteins became associated with the NE. When GFP-lamin C was expressed in SW13 cells neither the endogenous nor the exogenous lamin C was localised to the NE and emerin remained in the ER. Finally, lamins A and C were selectively eliminated from the NE of HeLa cells using a dominant negative mutant of lamin B1. Elimination of these lamins from the lamina led to the accumulation of emerin as aggregates within the ER. Our data suggest that lamin A is essential for anchorage of emerin to the inner nuclear membrane and of lamin C to the lamina.

HIV-associated lipodystrophy

The HIV associated lipodystrophy syndrome is characterized by fat loss from the periphery, fat accumulation in the abdominal, dorsocervical regions and breasts, and hyperlipidaemia, insulin resistance and lactic acidaemia. Although several mechanisms have been proposed to explain these abnormalities, the exact aetiology of the condition remains unclear and will likely prove to be complex. The principal clinical concerns that arise from this disorder are possible increased risks of premature atherosclerosis and cardiovascular disease. A variety of therapeutic interventions, designed to limit these risks, are under evaluation.

Nuclear pores and nuclear assembly

Communication between the nucleus and cytoplasm occurs through large macromolecular structures, the nuclear pores. Quantitative scanning transmission electron microscopy has estimated the mass of a nuclear pore to be 60 million Daltons in yeast and 120 million Daltons in vertebrates. The past two years were noteworthy in that they saw: 1) the purification of both the yeast and vertebrate nuclear pores, 2) the initial description of routes through the pore for specific transport receptors, 3) glimpses of intranuclear organization imposed by the nuclear pores and envelope and 4) the revelation of new and pivotal roles for the small GTPase Ran not only in nuclear import but in spindle assembly and nuclear membrane fusion.