Lamin A precursor is localized to intranuclear foci

Progerin impairs 3D genome organization and induces fragile telomeres by limiting the dNTP pools

Chromatin organization within the nuclear volume is essential to regulate many aspects of its function and to safeguard its integrity. A key player in this spatial scattering of chromosomes is the nuclear envelope (NE). The NE tethers large chromatin domains through interaction with the nuclear lamina and other associated proteins. This organization is perturbed in cells from Hutchinson–Gilford progeria syndrome (HGPS), a genetic disorder characterized by premature aging features. Here, we show that HGPS-related lamina defects trigger an altered 3D telomere organization with increased contact sites between telomeres and the nuclear lamina, and an altered telomeric chromatin state. The genome-wide replication timing signature of these cells is perturbed, with a shift to earlier replication for regions that normally replicate late. As a consequence, we detected a higher density of replication forks traveling simultaneously on DNA fibers, which relies on limiting cellular dNTP pools to support processive DNA synthesis. Remarkably, increasing dNTP levels in HGPS cells rescued fragile telomeres, and improved the replicative capacity of the cells. Our work highlights a functional connection between NE dysfunction and telomere homeostasis in the context of premature aging.

Presence and distribution of progerin in HGPS cells is ameliorated by drugs that impact on the mevalonate and mTOR pathways

Hutchinson-Gilford progeria syndrome (HGPS) is a rare, premature ageing syndrome in children. HGPS is normally caused by a mutation in the LMNA gene, encoding nuclear lamin A. The classical mutation in HGPS leads to the production of a toxic truncated version of lamin A, progerin, which retains a farnesyl group. Farnesyltransferase inhibitors (FTI), pravastatin and zoledronic acid have been used in clinical trials to target the mevalonate pathway in HGPS patients to inhibit farnesylation of progerin, in order to reduce its toxicity. Some other compounds that have been suggested as treatments include rapamycin, IGF1 and N-acetyl cysteine (NAC). We have analysed the distribution of prelamin A, lamin A, lamin A/C, progerin, lamin B1 and B2 in nuclei of HGPS cells before and after treatments with these drugs, an FTI and a geranylgeranyltransferase inhibitor (GGTI) and FTI with pravastatin and zoledronic acid in combination. Confirming other studies prelamin A, lamin A, progerin and lamin B2 staining was different between control and HGPS fibroblasts. The drugs that reduced progerin staining were FTI, pravastatin, zoledronic acid and rapamycin. However, drugs affecting the mevalonate pathway increased prelamin A, with only FTI reducing internal prelamin A foci. The distribution of lamin A in HGPS cells was improved with treatments of FTI, pravastatin and FTI + GGTI. All treatments reduced the number of cells displaying internal speckles of lamin A/C and lamin B2. Drugs targeting the mevalonate pathway worked best for progerin reduction, with zoledronic acid removing internal progerin speckles. Rapamycin and NAC, which impact on the MTOR pathway, both reduced both pools of progerin without increasing prelamin A in HGPS cell nuclei.

AFM single-cell force spectroscopy links altered nuclear and cytoskeletal mechanics to defective cell adhesion in cardiac myocytes with a nuclear lamin mutation

Previous investigations suggested that lamin A/C gene (LMNA) mutations, which cause a variety of human diseases including muscular dystrophies and cardiomyopathies, alter the nuclear mechanical properties. We hypothesized that biomechanical changes may extend beyond the nucleus.

Nuclear myosin I regulates cell membrane tension

Plasma membrane tension is an important feature that determines the cell shape and influences processes such as cell motility, spreading, endocytosis and exocytosis. Unconventional class 1 myosins are potent regulators of plasma membrane tension because they physically link the plasma membrane with adjacent cytoskeleton. We identified nuclear myosin 1 (NM1) - a putative nuclear isoform of myosin 1c (Myo1c) - as a new player in the field. Although having specific nuclear functions, NM1 localizes predominantly to the plasma membrane. Deletion of NM1 causes more than a 50% increase in the elasticity of the plasma membrane around the actin cytoskeleton as measured by atomic force microscopy. This higher elasticity of NM1 knock-out cells leads to 25% higher resistance to short-term hypotonic environment and rapid cell swelling. In contrast, overexpression of NM1 in wild type cells leads to an additional 30% reduction of their survival. We have shown that NM1 has a direct functional role in the cytoplasm as a dynamic linker between the cell membrane and the underlying cytoskeleton, regulating the degree of effective plasma membrane tension.

Non-farnesylated B-type lamin can tether chromatin inside the nucleus and its chromatin interaction requires the Ig-fold region

Lamins are thought to direct heterochromatin to the nuclear lamina (NL); however, this function of lamin has not been clearly demonstrated in vivo. To address this, we analyzed polytene chromosome morphology when artificial lamin variants were expressed in Drosophila endoreplicating cells. We found that the CaaX-motif-deleted B-type lamin Dm₀, but not A-type lamin C, was able to form a nuclear envelope-independent layer that was closely associated with chromatin. Other nuclear envelope proteins were not detected in this "ectopic lamina," and the associated chromatin showed a repressive histone modification maker but not a permissive histone modification marker nor RNA polymerase II proteins. Furthermore, deletion of the C-terminal lamin-Ig-fold domain prevents chromatin association with this ectopic lamina. Thus, non-farnesylated B-type lamin Dm₀ can form an ectopic lamina and induce changes to chromatin structure and status inside the interphase nucleus.

The role of LMNA in adipose: a novel mouse model of lipodystrophy based on the Dunnigan-type familial partial lipodystrophy mutation

We investigated the role of LMNA in adipose tissue by developing a novel mouse model of lipodystrophy. Transgenic mice were generated that express the LMNA mutation that causes familial partial lipodystrophy of the Dunnigan type (FPLD2). The phenotype observed in FPLD-transgenic mice resembles many of the features of human FPLD2, including lack of fat accumulation, insulin resistance, and enlarged, fatty liver. Similar to the human disease, FPLD-transgenic mice appear to develop normally, but after several weeks they are unable to accumulate fat to the same extent as their wild-type littermates. One poorly understood aspect of lipodystrophies is the mechanism of fat loss. To this end, we have examined the effects of the FPLD2 mutation on fat cell function. Contrary to the current literature, which suggests FPLD2 results in a loss of fat, we found that the key mechanism contributing to the lack of fat accumulation involves not a loss, but an apparent inability of the adipose tissue to renew itself. Specifically, preadipocytes are unable to differentiate into mature and fully functional adipocytes. These findings provide insights not only for the treatment of lipodystrophies, but also for the study of adipogenesis, obesity, and insulin resistance.

Analysis of prelamin A biogenesis reveals the nucleus to be a CaaX processing compartment

Proteins establish and maintain a distinct intracellular localization by means of targeting, retention, and retrieval signals, ensuring most proteins reside predominantly in one cellular location. The enzymes involved in the maturation of lamin A present a challenge to this paradigm. Lamin A is first synthesized as a 74-kDa precursor, prelamin A, with a C-terminal CaaX motif and undergoes a series of posttranslational modifications including CaaX processing (farnesylation, aaX cleavage and carboxylmethylation), followed by endoproteolytic cleavage by Zmpste24. Failure to cleave prelamin A results in progeria and related premature aging disorders. Evidence suggests prelamin A is imported directly into the nucleus where it is processed. Paradoxically, the processing enzymes have been shown to reside in the cytosol (farnesyltransferase), or are ER membrane proteins (Zmpste24, Rce1, and Icmt) with their active sites facing the cytosol. Here we have reexamined the cellular site of prelamin A processing, and show that the mammalian and yeast processing enzymes Zmpste24 and Icmt exhibit a dual localization to the inner nuclear membrane, as well as the ER membrane. Our findings reveal the nucleus to be a physiologically relevant location for CaaX processing, and provide insight into the biology of a protein at the center of devastating progeroid diseases.

Targeted transgenic expression of the mutation causing Hutchinson-Gilford progeria syndrome leads to proliferative and degenerative epidermal disease

Hutchinson-Gilford progeria syndrome (HGPS) is a rare human genetic disorder characterized by striking progeroid features. Clinical findings in the skin include scleroderma, alopecia and loss of subcutaneous fat. HGPS is usually caused by a dominant-negative mutation in LMNA, a gene that encodes two major proteins of the inner nuclear lamina: lamin A and lamin C. We have generated tetracycline-inducible transgenic lines that carry a minigene of human LMNA under the control of a tet-operon. Two mouse lines were created: one carrying the wild-type sequence of LMNA and the other carrying the most common HGPS mutation. Targeted expression of the HGPS mutation in keratin-5-expressing tissues led to abnormalities in the skin and teeth, including fibrosis, loss of hypodermal adipocytes, structural defects in the hair follicles and sebaceous glands, and abnormal incisors. The severity of the defects was related to the level of expression of the transgene in different mouse lines. These transgenic mice appear to be good models for studies of the molecular mechanisms of skin abnormalities in HGPS and other related disorders.

Nuclear lamins: major factors in the structural organization and function of the nucleus and chromatin

Over the past few years it has become evident that the intermediate filament proteins, the types A and B nuclear lamins, not only provide a structural framework for the nucleus, but are also essential for many aspects of normal nuclear function. Insights into lamin-related functions have been derived from studies of the remarkably large number of disease-causing mutations in the human lamin A gene. This review provides an up-to-date overview of the functions of nuclear lamins, emphasizing their roles in epigenetics, chromatin organization, DNA replication, transcription, and DNA repair. In addition, we discuss recent evidence supporting the importance of lamins in viral infections.

Nucleoplasmic lamins and their interaction partners, LAP2alpha, Rb, and BAF, in transcriptional regulation

Lamins are major structural components of the nuclear envelope in multicellular eukaryotes. Particularly A-type lamins are also located in the nucleoplasm, likely involving a specific binding partner, lamina-associated polypeptide 2alpha (LAP2alpha). LAP2alpha-lamins A/C complexes in the nucleoplasm have been implicated in the regulation of gene expression by various means. They bind chromatin proteins and chromatin modifying enzymes, and can thus participate in epigenetic control pathways. Furthermore, binding of lamins A/C complexes to specific transcription factors and repressors may directly affect their transcriptional activity. LAP2alpha-lamins A/C also regulate retinoblastoma protein and influence cell cycle progression and differentiation, which could have important implications for molecular mechanisms of laminopathic diseases, linked to lamins A/C mutations.

Pre-Lamin A processing is linked to heterochromatin organization

Pre-lamin A undergoes subsequent steps of post-translational modification at its C-terminus, including farnesylation, methylation, and cleavage by ZMPSTE24 metalloprotease. Here, we show that accumulation of different intermediates of pre-lamin A processing in nuclei, induced by expression of mutated pre-lamin A, differentially affected chromatin organization in human fibroblasts. Unprocessed (non-farnesylated) pre-lamin A accumulated in intranuclear foci, caused the redistribution of LAP2alpha and of the heterochromatin markers HP1alpha and trimethyl-K9-histone 3, and triggered heterochromatin localization in the nuclear interior. In contrast, the farnesylated and carboxymethylated lamin A precursor accumulated at the nuclear periphery and caused loss of heterochromatin markers and Lap2alpha in enlarged nuclei. Interestingly, pre-lamin A bound both HP1alpha and LAP2alpha in vivo, but the farnesylated form showed reduced affinity for HP1alpha. Our data show a link between pre-lamin A processing and heterochromatin remodeling and have major implications for understanding molecular mechanisms of human diseases linked to mutations in lamins.

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.

Farnesylated lamins, progeroid syndromes and farnesyl transferase inhibitors

Three mammalian nuclear lamin proteins, lamin B1, lamin B2 and the lamin A precursor, prelamin A, undergo canonical farnesylation and processing at CAAX motifs. In the case of prelamin A, there is an additional farnesylation-dependent endoproteolysis, which is defective in two congenital diseases: Hutchinson-Gilford progeria (HGPS) and restrictive dermopathy (RD). These two diseases arise respectively from defects in the prelamin A substrate and the enzyme (ZmpSte24) that processes it. Recent work has shed light on the roles of the lamin proteins and the enzymes involved in their farnesylation-dependent maturation. Other experimental work, including mouse model studies, have examined the possibility that farnesyl transferase inhibitors can represent effective treatment for HGPS. However, there are concerns about their use for this purpose given the potential for alternative prenylation pathways.

Rescue of heterochromatin organization in Hutchinson-Gilford progeria by drug treatment

Hutchinson-Gilford progeria (HGPS) is a premature aging syndrome associated with LMNA mutations. Progeria cells bearing the G608G LMNA mutation are characterized by accumulation of a mutated lamin A precursor (progerin), nuclear dysmorphism and chromatin disorganization. In cultured HGPS fibroblasts, we found worsening of the cellular phenotype with patient age, mainly consisting of increased nuclear-shape abnormalities, progerin accumulation and heterochromatin loss. Moreover, transcript distribution was altered in HGPS nuclei, as determined by different techniques. In the attempt to improve the cellular phenotype, we applied treatment with drugs either affecting protein farnesylation or chromatin arrangement. Our results show that the combined treatment with mevinolin and the histone deacetylase inhibitor trichostatin A dramatically lowers progerin levels, leading to rescue of heterochromatin organization and reorganization of transcripts in HGPS fibroblasts. These results suggest that morpho-functional defects of HGPS nuclei are directly related to progerin accumulation and can be rectified by drug treatment.

Hutchinson-Gilford progeria mutant lamin A primarily targets human vascular cells as detected by an anti-Lamin A G608G antibody

Hutchinson-Gilford progeria syndrome (HGPS; Online Mendelian Inheritance in Man accession no. 176670) is a rare disorder that is characterized by segmental premature aging and death between 7 and 20 years of age from severe premature atherosclerosis. Mutations in the LMNA gene are responsible for this syndrome. Approximately 80% of HGPS cases are caused by a G608 (GGC-->GGT) mutation within exon 11 of LMNA, which elicits a deletion of 50 aa near the C terminus of prelamin A. In this article, we present evidence that the mutant lamin A (progerin) accumulates in the nucleus in a cellular age-dependent manner. In human HGPS fibroblast cultures, we observed, concomitantly to nuclear progerin accumulation, severe nuclear envelope deformations and invaginations preventable by farnesyltransferase inhibition. Nuclear alterations affect cell-cycle progression and cell migration and elicit premature senescence. Strikingly, skin biopsy sections from a subject with HGPS showed that the truncated lamin A accumulates primarily in the nuclei of vascular cells. This finding suggests that accumulation of progerin is directly involved in vascular disease in progeria.

Thematic review series: Lipid Posttranslational Modifications. Farnesyl transferase inhibitors

Some proteins undergo posttranslational modification by the addition of an isoprenyl lipid (farnesyl- or geranylgeranyl-isoprenoid) to a cysteine residue proximal to the C terminus. Protein isoprenylation promotes membrane association and contributes to protein-protein interactions. Farnesylated proteins include small GTPases, tyrosine phosphatases, nuclear lamina, cochaperones, and centromere-associated proteins. Prenylation is required for the transforming activity of Ras. Because of the high frequency of Ras mutations in cancer, farnesyl transferase inhibitors (FTIs) were investigated as a means to antagonize Ras function. Evaluation of FTIs led to the finding that both K- and N-Ras are alternatively modified by geranylgeranyl prenyltransferase-1 in FTI-treated cells. Geranylgeranylated forms of Ras retain the ability to associate with the plasma membrane and activate substrates. Despite this, FTIs are effective at inhibiting the growth of human tumor cells in vitro, suggesting that activity is dependent on blocking the farnesylation of other proteins. FTIs also inhibit the in vivo growth of human tumor xenografts and sensitize these models to chemotherapeutics, most notably taxanes. Several FTIs have entered clinical trials for various cancer indications. In some clinical settings, primarily hematologic malignancies, FTIs have displayed evidence of single-agent activity. Clinical studies in progress are exploring the antitumor activity of FTIs as single agents and in combination. This review will summarize the basic biology of FTIs, their antitumor activity in preclinical models, and the current status of clinical studies with these agents.

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.

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.

High affinity for farnesyltransferase and alternative prenylation contribute individually to K-Ras4B resistance to farnesyltransferase inhibitors

Farnesyltransferase inhibitors (FTIs) block Ras farnesylation, subcellular localization and activity, and inhibit the growth of Ras-transformed cells. Although FTIs are ineffective against K-Ras4B, the Ras isoform most commonly mutated in human cancers, they can inhibit the growth of tumors containing oncogenic K-Ras4B, implicating other farnesylated proteins or suggesting distinct functions for farnesylated and for geranylgeranylated K-Ras, which is generated when farnesyltransferase is inhibited. In addition to bypassing FTI blockade through geranylgeranylation, K-Ras4B resistance to FTIs may also result from its higher affinity for farnesyltransferase. Using chimeric Ras proteins containing all combinations of Ras background, CAAX motif, and K-Ras polybasic domain, we show that either a polybasic domain or an alternatively prenylated CAAX renders Ras prenylation, Ras-induced Elk-1 activation, and anchorage-independent cell growth FTI-resistant. The polybasic domain alone increases the affinity of Ras for farnesyltransferase, implying independent roles for each K-Ras4B sequence element in FTI resistance. Using microarray analysis and colony formation assays, we confirm that K-Ras function is independent of the identity of the prenyl group and, therefore, that FTI inhibition of K-Ras transformed cells is likely to be independent of K-Ras inhibition. Our results imply that relevant FTI targets will lack both polybasic and potentially geranylgeranylated methionine-CAAX motifs.

Lamin expression in normal human skin, actinic keratosis, squamous cell carcinoma and basal cell carcinoma

BACKGROUND

Aberrant expression patterns of nuclear lamins have been described in various types of cancer depending on the subtype of cancer, its aggressiveness, proliferative capacity and degree of differentiation. In general, the expression of A-type lamins (lamins A and C) has been correlated with a non-proliferating, differentiated state of cells and tissues.

OBJECTIVES

To establish and compare the expression patterns of lamins in normal human skin, actinic keratosis (AK), squamous cell carcinoma (SCC) and basal cell carcinoma (BCC).

METHODS

Expression patterns of the individual lamin subtypes were studied immunohistochemically. The proliferation capacity of the tumour cells was detected using a specific antibody to Ki-67, and was related to the A-type lamin expression patterns.

RESULTS

In normal skin, lamin A was expressed in the suprabasal cell compartment of the epidermis, whereas the basal cells were mostly unstained. BCCs and SCCs stained positive in most cells, while the epidermis overlying BCC and SCC and the epidermis in AK stained homogeneously and strongly in the basal cells in addition to the suprabasal cells. Lamin C was expressed in some basal cells of normal epidermis while the suprabasal cells stained strongly positive. Both BCCs and SCCs stained strongly positive for lamin C, with the difference that in BCC the staining was predominantly present in nucleolar structures with occasional staining of the nuclear envelope. The epidermis overlying SCC showed strong positivity in the lamina of virtually all cells. The expression of lamin C in the basal cells of AK resembled the expression pattern seen in the epidermis overlying BCC, i.e. a nucleolar staining next to nuclear envelope staining. Lamin B1 and B2 were found in virtually all cells in normal epidermis, AK, BCC, SCC and the epidermis overlying cancer. The percentage of Ki-67-expressing cells was highest in BCC (45%), and gradually decreased via epidermis overlying BCC, AK, SCC, and epidermis overlying SCC, to normal skin (11%). Simultaneous expression of A-type lamins and Ki-67 occurred in approximately 50% of the proliferating (Ki-67 positive) cells in BCC and SCC.

CONCLUSIONS

Significant changes occur in the expression patterns of A-type lamins in both premalignant and malignant lesions of the skin. The profound overlap of lamin A and Ki-67 staining patterns indicates that the proliferating tumour cells may obtain a certain degree of differentiation. Finally, lamin A expression in the basal cell layer of the apparently normal epidermis overlying BCC may suggest its involvement in the primary process.

Investigation of nuclear architecture with a domain-presenting expression system

We have investigated the topogenic properties of the nucleus by ectopic expression of chimeric proteins consisting of a NLS-modified cytoplasmic filament-forming protein, Xenopus laevis vimentin, and domains of inner nuclear membrane proteins. Whereas the "carrier" without cargo, the NLS-vimentin alone, is deposited in a few nuclear body-type structures (J.M. Bridger, H. Herrmann, C. Münkel, P. Lichter, J. Cell Sci., 111, 1241-1253), the distribution is entirely changed upon coupling with the evolutionarily conserved domain of the lamin B tail, the entire lamin B tail, the amino-terminal nucleoplasmic segment of the lamin B receptor (LBR), and the LEM domain of emerin, respectively. Remarkably, every individual chimeric protein exhibits a completely different distribution. Therefore, we assume that the chimeric parts are specifically recognized by factors engaged in nucleus-specific topogenesis. Thus, the conserved domain of the lamin B tail results in the formation of many small accumulations spread all over the nucleus. The chimera with the complete lamin B tail is deposited in short fibrillar aggregates within the nucleus. It does not mediate the integration of the chimeric protein into the nuclear membrane in cultured cells, indicating that the lamin tail alone is not sufficient to direct the integration of a protein into the lamina in vivo. In contrast, in the nuclear assembly system of Xenopus laevis the recombinant NLS-vimentin-lamin tail protein is concentrated at the nuclear membrane. The LBR chimera is arranged in a "beaded-chain"-type fashion, quite different from the more random deposition of NLS-vimentin alone. To our surprise, the LEM domain of emerin induces the retention of most of the chimeric proteins within the cytoplasm. Hence, it appears to be engaged in a strong cytoplasmic interaction that overrides the nuclear localization signal. Finally, the lamin chimera with the conserved part of the lamin B tail is shown to recruit LBR to the nuclear vimentin bodies and, vice versa, the LBR chimera attracts lamin B in transfected cells, thereby demonstrating their bona fide interaction in vivo.

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

The nuclear lamina is a filamentous structure composed of lamins that supports the inner nuclear membrane. Several integral membrane proteins including emerin, LBR, LAP1 and LAP2 bind to nuclear lamins in vitro and can influence lamin function and dynamics in vivo. Results from various studies suggest that lamins function in DNA replication and nuclear envelope assembly and determine the size and shape of the nuclear envelope. In addition, lamins also bind chromatin and certain DNA sequences, and might influence chromosome position. Recent evidence has revealed that mutations in A-type lamins give rise to a range of rare, but dominant, genetic disorders, including Emery-Dreifuss muscular dystrophy, dilated cardiomyopathy with conduction-system disease and Dunnigan-type familial partial lipodystrophy. An examination of how lamins A/C, emerin and other integral membrane proteins interact at the INM provides the basis for a novel model for how mutations that promote disease phenotypes are likely to influence these interactions and therefore cause cellular pathology through a combination of weakness of the lamina or altered gene expression.

Nuclear lamins A and B1: different pathways of assembly during nuclear envelope formation in living cells

At the end of mitosis, the nuclear lamins assemble to form the nuclear lamina during nuclear envelope formation in daughter cells. We have fused A- and B-type nuclear lamins to the green fluorescent protein to study this process in living cells. The results reveal that the A- and B-type lamins exhibit different pathways of assembly. In the early stages of mitosis, both lamins are distributed throughout the cytoplasm in a diffusible (nonpolymerized) state, as demonstrated by fluorescence recovery after photobleaching (FRAP). During the anaphase-telophase transition, lamin B1 begins to become concentrated at the surface of the chromosomes. As the chromosomes reach the spindle poles, virtually all of the detectable lamin B1 has accumulated at their surfaces. Subsequently, this lamin rapidly encloses the entire perimeter of the region containing decondensing chromosomes in each daughter cell. By this time, lamin B1 has assembled into a relatively stable polymer, as indicated by FRAP analyses and insolubility in detergent/high ionic strength solutions. In contrast, the association of lamin A with the nucleus begins only after the major components of the nuclear envelope including pore complexes are assembled in daughter cells. Initially, lamin A is found in an unpolymerized state throughout the nucleoplasm of daughter cell nuclei in early G1 and only gradually becomes incorporated into the peripheral lamina during the first few hours of this stage of the cell cycle. In later stages of G1, FRAP analyses suggest that both green fluorescent protein lamins A and B1 form higher order polymers throughout interphase nuclei.

Lamina-associated polypeptide 2alpha binds intranuclear A-type lamins

The nucleoskeletal protein lamina-associated polypeptide 2(α) (LAP2*) contains a large, unique C terminus and differs significantly from its alternatively spliced, mostly membrane-integrated isoforms, such as LAP2beta. Unlike lamin B-binding LAP2beta, LAP2alpha was found by confocal immunofluorescence microscopy to colocalize preferentially with A-type lamins in the newly formed nuclei assembled after mitosis. While only a subfraction of lamins A and C (lamin A/C) was associated with the predominantly nuclear LAP2alpha in telophase, the majority of lamin A/C colocalized with LAP2alpha in G(1)-phase nuclei. Furthermore, selective disruption of A-type lamin structures by overexpression of lamin mutants in HeLa cells caused a redistribution of LAP2alpha. Coimmunoprecipitation experiments revealed that a fraction of lamin A/C formed a stable, SDS-resistant complex with LAP2alpha in interphase cells and in postmetaphase cell extracts. Blot overlay binding studies revealed a direct binding of LAP2alpha to exclusively A-type lamins and located the interaction domains to the C-terminal 78 amino acids of LAP2alpha and to residues 319–566 in lamin A/C, which include the C terminus of the rod and the entire tail common to lamin A/C. These findings suggest that LAP2alpha and A-type lamins cooperate in the organization of internal nuclear structures.

Review: the dynamics of the nuclear lamins during the cell cycle-- relationship between structure and function

The nuclear lamins are members of the intermediate filament (IF) family of proteins. The lamins have an essential role in maintaining nuclear integrity, as do the other IF family members in the cytoplasm. Also like cytoplasmic IFs, the organization of lamins is dynamic. The lamins are found not only at the nuclear periphery but also in the interior of the nucleus, as distinct nucleoplasmic foci and possibly as a network throughout the nucleus. Nuclear processes such as DNA replication may be organized around these structures. In this review, we discuss changes in the structure and organization of the nuclear lamins during the cell cycle and during cell differentiation. These changes are correlated with changes in nuclear structure and function. For example, the interactions of lamins with chromatin and nuclear envelope components occur very early during nuclear assembly following mitosis. During S-phase, the lamins colocalize with markers of DNA replication, and proper lamin organization must be maintained for replication to proceed. When cells differentiate, the expression pattern of lamin isotypes changes. In addition, changes in lamin organization and expression patterns accompany the nuclear alterations observed in transformed cells. These lamin structures may modulate nuclear function in each of these processes.

Prenylated prelamin A interacts with Narf, a novel nuclear protein

Prelamin A is farnesylated and methylated on the cysteine residue of a carboxyl-terminal CaaX motif. In the nucleus, prelamin A is processed to lamin A by endoproteolytic removal of the final 18 amino acids, including the farnesylated cysteine residue. Using the yeast two-hybrid assay, we isolated a novel human protein, Narf, that binds the carboxyl-terminal tail of prelamin A. Narf has limited homology to iron-only bacterial hydrogenases and eukaryotic proteins of unknown function. Narf is encoded by a 2-kilobase mRNA expressed in all human cell lines and tissues examined. The protein is detected in the nuclear fraction of HeLa cell lysates on Western blots and can be extracted from nuclear envelopes with 0.5 M NaCl. When a FLAG epitope-tagged Narf is expressed in HeLa cells, it is exclusively nuclear and partially co-localizes with the nuclear lamina. The farnesylation status of prelamin A determines its ability to bind to Narf. Inhibition of farnesyltransferase and mutation or deletion of the CaaX motif from the prelamin A tail domain inhibits Narf binding in yeast two-hybrid and in vitro binding assays. The prenyl-dependent binding of Narf to prelamin A is an important first step in understanding the functional significance of the lamin A precursor.

Dynamics of the nuclear lamina as monitored by GFP-tagged A-type lamins

The behavior of chimeric proteins consisting of A-type lamins and green fluorescent protein (GFP) was studied to investigate the localization and dynamics of nuclear lamins in living cells. Cell line CHO-K1 was transfected with cDNA constructs encoding fusion proteins of lamin A-GFP, lamin Adelta10-GFP, or lamin C-GFP. In the interphase nucleus lamin-GFP fluorescence showed a perinuclear localization and incorporation into the lamina for all three constructs. Our findings show for the first time that the newly discovered lamin A 10 protein is localized to the nuclear membrane. The GFP-tagged lamins were processed and behaved similarly to the endogenous lamin molecules, at least in cells that expressed physiological levels of the GFP-lamins. In addition to the typical perinuclear localization, in the majority of transfected cells each individual A-type lamin-GFP revealed an extensive collection of branching intra- and trans-nuclear tubular structures, which showed a clear preference for a vertical orientation. Time-lapse studies of 3-D reconstructed interphase cells showed a remarkable stability in both number and location of these structures over time, while the lamina showed considerable dynamic movements, consisting of folding and indentation of large parts of the lamina. Fluorescence recovery after bleaching studies revealed a low protein turnover of both tubular and lamina-associated lamins. Repetitive bleaching of intranuclear areas revealed the presence of an insoluble intranuclear fraction of A-type lamins. Time-lapse studies of mitotic cells showed that reformation of the lamina and the tubular structures consisting of A-type lamins did not occur until after cytokinesis was completed.

Unique properties of the large antigen of hepatitis delta virus

The large form of the hepatitis delta virus (HDV) protein (L) can be isoprenylated near its C terminus, and this modification is considered essential for particle assembly. Using gel electrophoresis, we separated L into two species of similar mobilities. The slower species could be labeled by the incorporation of [(14)C]mevalonolactone and is interpreted to be isoprenylated L (L(i)). In serum particles, infected liver, transfected cells, and assembled particles, 25 to 85% of L was isoprenylated. Isoprenylation was also demonstrated by (14)C incorporation in vitro with a rabbit reticulocyte coupled transcription-translation system. However, the species obtained migrated even slower than that detected by labeling in vivo. Next, in studies of HDV particle assembly in the presence of the surface proteins of human hepatitis B virus, we observed the following. (i) Relative to L, L(i) was preferentially assembled into virus-like particles. (ii) L(i) could coassemble the unmodified L and the small delta protein, S. (iii) In contrast, a form of L with a deletion in the dimerization domain was both isoprenylated and assembled, but it could not support the coassembly of S. Finally, to test the expectation that the isoprenylation of L would increase its hydrophobicity, we applied a phase separation strategy based on micelle formation with the nonionic detergent Triton X-114. We showed the following. (i) The unique C-terminal 19 amino acids present on L relative to S caused a significant increase in the hydrophobicity. (ii) This increase was independent of isoprenylation. (iii) In contrast, other, artificial modifications at either the N or C terminus of S did not increase the hydrophobicity. (iv) The increased hydrophobicity was not sufficient for particle assembly; nevertheless, we speculate that it might facilitate virion assembly.

The role of sequences unique to nuclear intermediate filaments in the targeting and assembly of human lamin B: evidence for lack of interaction of lamin B with its putative receptor

The mechanism by which human nuclear lamin B is targeted and assembled has been studied by transfecting into mammalian cells lamin mutants deleted of three sequences unique to lamins. Nuclear lamins contain an extra 42 amino acids (aa) in their rod domains and NLS and CAAX motifs in their tail domains, which distinguishes them from cytoplasmic IF proteins. These three sequences act in concert to ensure correct temporal and spatial assembly of lamin B. Deletion of any one of these three sequences from lamin B did not significantly disrupt nuclear lamina targeting, but when two or more of these sequences were deleted, targeting was severely compromised. The CAAX motif is necessary for the efficient integration of lamin B into an already formed nuclear lamina, since lamin B CAAX- mutants had reduced targeting to the lamina when arrested in S phase of the cell cycle. CAAX-deficient mutant lamin B proteins were soluble and not associated with membranes at mitosis, proving that the CAAX motif is responsible for association of human lamin B with membranes. In addition, CAAX- mutant lamin B proteins fractionated independently of the lamin B-receptor (LBR), indicating that these two proteins do not bind directly to each other.

In vitro interaction of the carboxy-terminal domain of lamin A with actin

The nuclear lamina formed by lamins is localized between the inner nuclear membrane and chromatin. Lamins are thought to be implicated in the higher order chromatin structure. Interactions of lamins with chromatin have been described but the molecular components directly involved in these interactions remain to be identified. Using a GST-C-terminal domain of lamin A fusion protein to probe cellular extracts for interacting proteins, we have found that this domain of lamin A binds to nuclear actin. This result suggests that an actin-based molecular motor linked to the lamina could be involved in the movement of chromatin domains.

Dynamic organisation of intermediate filaments and associated proteins during the cell cycle

Intermediate filaments, which form the structural framework of both the cytoskeleton and the nuclear lamina in most eukaryotic cells, have been found to be highly dynamic structures. A continuous exchange of subunit proteins at the filament surface and a stabilisation of soluble subunits by chaperone-type proteins may modulate filament structure and plasticity. Recent studies on the cell cycle-dependent interaction of intermediate filaments with associated proteins, and a detailed analysis of intermediate filament phosphorylation in defined subcellular locations at various stages of mitosis, have brought new insights into the molecular mechanisms involved in the mitotic reorganisation of intermediate filaments. Some of these studies have allowed new speculations about the possible cellular functions of cytoplasmic intermediate filaments, and increased our understanding of the specific functions of the lamins and the lamina-associated membrane proteins in the post-mitotic reassembly of the nucleus.

Disruption of nuclear lamin organization alters the distribution of replication factors and inhibits DNA synthesis

The nuclear lamina is a fibrous structure that lies at the interface between the nuclear envelope and the nucleoplasm. The major proteins comprising the lamina, the nuclear lamins, are also found in foci in the nucleoplasm, distinct from the peripheral lamina. The nuclear lamins have been associated with a number of processes in the nucleus, including DNA replication. To further characterize the specific role of lamins in DNA replication, we have used a truncated human lamin as a dominant negative mutant to perturb lamin organization. This protein disrupts the lamin organization of nuclei when microinjected into mammalian cells and also disrupts the lamin organization of in vitro assembled nuclei when added to Xenopus laevis interphase egg extracts. In both cases, the lamina appears to be completely absent, and instead the endogenous lamins and the mutant lamin protein are found in nucleoplasmic aggregates. Coincident with the disruption of lamin organization, there is a dramatic reduction in DNA replication. As a consequence of this disruption, the distributions of PCNA and the large subunit of the RFC complex, proteins required for the elongation phase of DNA replication, are altered such that they are found within the intranucleoplasmic lamin aggregates. In contrast, the distribution of XMCM3, XORC2, and DNA polymerase alpha, proteins required for the initiation stage of DNA replication, remains unaltered. The data presented demonstrate that the nuclear lamins may be required for the elongation phase of DNA replication.

Distinct regions specify the targeting of otefin to the nucleoplasmic side of the nuclear envelope

Otefin is a 45-kDa nuclear envelope protein with no apparent homology to other known proteins. It includes a large hydrophilic domain, a single carboxyl-terminal hydrophobic sequence of 17 amino acids, and a high content of serine and threonine residues. Cytological labeling located otefin on the nucleoplasmic side of the nuclear envelope. Chemical extraction of nuclei from Drosophila embryos revealed that otefin is a peripheral protein whose association with the nuclear envelope is stronger than that of lamin. Deletion mutants of otefin were expressed in order to identify regions that direct otefin to the nuclear envelope. These experiments revealed that the hydrophobic sequence at the carboxyl terminus is essential for correct targeting to the nuclear envelope, whereas additional regions in the hydrophilic domain of otefin are required for its efficient targeting and stabilization in the nuclear envelope.

Remodelling of the nuclear periphery during muscle cell differentiation in vitro

We have examined the composition and ultrastructure of the nuclear periphery during in vitro myogenesis of the rat myoblast cell line, L6E9. Immunofluorescence labelling and immunoblotting showed that lamins A/C and B were all present in undifferentiated cells, but that they increased significantly before extensive cell fusion had occurred, with lamins A/C increasing proportionately more. Electron microscopic observations were consistent with these results, showing an increase in the prominence of the lamina during differentiation. On the other hand, immunofluorescence labelling suggested that the P1 antigen began to disappear from the nuclear periphery as the cells were fusing, after the increase in lamin quantity, and was no longer detectable in multinucleated cells. Unexpectedly, however, P1 was readily detected in isolated nuclei, whether prepared from myoblast or differentiated cultures, as well as in both myoblast and myotube nuclear matrices. It appears probable, therefore, that the fading of P1 labelling is due to masking of the epitope by a soluble factor recruited to the nuclear periphery as cell differentiate. These data, together with evidence that the genome is substantially rearranged during L6E9 myogenesis [Chaly and Munro, 1996], suggest that L6E9 cells are a useful model system in which to study the interrelationship of nuclear envelope organization, chromatin spatial order, and nuclear function.

Cytoskeleton-membrane interactions

Associations between the cytoskeleton and cellular membranes, both within the cell and at points of cell contact, play a central role in determining cell shape and tissue integrity. During the past few years, it has become clear that many of these cytoskeleton-membrane interactions go far beyond simple mechanical linkages. For example, proteins that act as linker molecules at the adherens junctions and desmosomes in the plasma membrane have newly recognized functions in signal transduction pathways. These functions have profound effects on cell behaviour during development. In addition, within the nucleus, the lamin branch of the intermediate filament protein family appears to have a key role in defining the protein composition of the inner nuclear membrane by means of extensive interactions with integral membrane proteins. The identities of these integral membrane proteins are only now coming to light.

The dynamic properties and possible functions of nuclear lamins

The nuclear lamins are thought to form a thin fibrous layer called the nuclear lamina, underlying the inner nuclear envelope membrane. In this review, we summarize data on the dynamic properties of nuclear lamins during the cell cycle and during development. We discuss the implications of dynamics for lamin functions. The lamins may be involved in DNA replication, chromatin organization, differentiation, nuclear structural support, and nuclear envelope reassembly. Emphasis is placed on recent data that indicate that the lamina, contrary to previous views, is not a static structure. For example, the lamins form nucleoplasmic foci, distinct from the peripheral lamina, which vary in their patterns of distribution as well as their composition in a cell cycle-dependent manner. During the S phase, these foci colocalize with chromatin and sites of DNA replication. At other points during the cell cycle, they may represent sites of lamin post-translation processing that take place prior to incorporation into the lamina. Secondary modifications of the lamins such as isoprenylation and phosphorylation are involved in the regulation of the dynamic properties and the assembly of lamins. In addition, a number of lamin-associated proteins have been recently identified and these are described along with their potential functions.