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

Mechanics of the nucleus

The nucleus is the distinguishing feature of eukaryotic cells. Until recently, it was often considered simply as a unique compartment containing the genetic information of the cell and associated machinery, without much attention to its structure and mechanical properties. This article provides compelling examples that illustrate how specific nuclear structures are associated with important cellular functions, and how defects in nuclear mechanics can cause a multitude of human diseases. During differentiation, embryonic stem cells modify their nuclear envelope composition and chromatin structure, resulting in stiffer nuclei that reflect decreased transcriptional plasticity. In contrast, neutrophils have evolved characteristic lobulated nuclei that increase their physical plasticity, enabling passage through narrow tissue spaces in their response to inflammation. Research on diverse cell types further demonstrates how induced nuclear deformations during cellular compression or stretch can modulate cellular function. Pathological examples of disturbed nuclear mechanics include the many diseases caused by mutations in the nuclear envelope proteins lamin A/C and associated proteins, as well as cancer cells that are often characterized by abnormal nuclear morphology. In this article, we will focus on determining the functional relationship between nuclear mechanics and cellular (dys-)function, describing the molecular changes associated with physiological and pathological examples, the resulting defects in nuclear mechanics, and the effects on cellular function. New insights into the close relationship between nuclear mechanics and cellular organization and function will yield a better understanding of normal biology and will offer new clues into therapeutic approaches to the various diseases associated with defective nuclear mechanics.

N-acetyl-L-cysteine prevents stress-induced desmin aggregation in cellular models of desminopathy

Mutations within the human desmin gene are responsible for a subcategory of myofibrillar myopathies called desminopathies. However, a single inherited mutation can produce different phenotypes within a family, suggesting that environmental factors influence disease states. Although several mouse models have been used to investigate organ-specific desminopathies, a more general mechanistic perspective is required to advance our knowledge toward patient treatment. To improve our understanding of disease pathology, we have developed cellular models to observe desmin behaviour in early stages of disease pathology, e.g., upon formation of cytoplasmic desmin aggregates, within an isogenic background. We cloned the wildtype and three mutant desmin cDNAs using a Tet-On Advanced® expression system in C2C12 cells. Mutations were selected based on positioning within desmin and capacity to form aggregates in transient experiments, as follows: DesS46Y (head domain; low aggregation), DesD399Y (central rod domain; high aggregation), and DesS460I (tail domain; moderate aggregation). Introduction of these proteins into a C2C12 background permitted us to compare between desmin variants as well as to determine the role of external stress on aggregation. Three different types of stress, likely encountered during muscle activity, were introduced to the cell models-thermal (heat shock), redox-associated (H2O2 and cadmium chloride), and mechanical (stretching) stresses-after which aggregation was measured. Cells containing variant DesD399Y were more sensitive to stress, leading to marked cytoplasmic perinuclear aggregations. We then evaluated the capacity of biochemical compounds to prevent this aggregation, applying dexamethasone (an inducer of heat shock proteins), fisetin or N-acetyl-L-cysteine (antioxidants) before stress induction. Interestingly, N-acetyl-L-cysteine pre-treatment prevented DesD399Y aggregation during most stress. N-acetyl-L-cysteine has recently been described as a promising antioxidant in myopathies linked to selenoprotein N or ryanodin receptor defects. Our findings indicate that this drug warrants further study in animal models to speed its potential development as a therapy for DesD399Y-linked desminopathies.

Mutations in LMNA modulate the lamin A--Nesprin-2 interaction and cause LINC complex alterations

Background

In eukaryotes the genetic material is enclosed by a continuous membrane system, the nuclear envelope (NE). Along the NE specific proteins assemble to form meshworks and mutations in these proteins have been described in a group of human diseases called laminopathies. Laminopathies include lipodystrophies, muscle and cardiac diseases as well as metabolic or progeroid syndromes. Most laminopathies are caused by mutations in the LMNAgene encoding lamins A/C. Together with Nesprins (Nuclear Envelope Spectrin Repeat Proteins) they are core components of the LINC complex (Linker of Nucleoskeleton and Cytoskeleton). The LINC complex connects the nucleoskeleton and the cytoskeleton and plays a role in the transfer of mechanically induced signals along the NE into the nucleus, and its components have been attributed functions in maintaining nuclear and cellular organization as well as signal transduction.

Results

Here we narrowed down the interaction sites between lamin A and Nesprin-2 to aa 403–425 in lamin A and aa 6146–6347 in Nesprin-2. Laminopathic mutations in and around the involved region of lamin A (R401C, G411D, G413C, V415I, R419C, L421P, R427G, Q432X) modulate the interaction with Nesprin-2 and this may contribute to the disease phenotype. The most notable mutation is the lamin A mutation Q432X that alters LINC complex protein assemblies and causes chromosomal and transcription factor rearrangements.

Conclusion

Mutations in Nesprin-2 and lamin A are characterised by complex genotype phenotype relations. Our data show that each mutation in LMNAanalysed here has a distinct impact on the interaction among both proteins that substantially explains how distinct mutations in widely expressed genes lead to the formation of phenotypically different diseases.

Partial lipodystrophy with severe insulin resistance and adult progeria Werner syndrome

Background

Laminopathies, due to mutations in LMNA, encoding A type-lamins, can lead to premature ageing and/or lipodystrophic syndromes, showing that these diseases could have close physiopathological relationships. We show here that lipodystrophy and extreme insulin resistance can also reveal the adult progeria Werner syndrome linked to mutations in WRN, encoding a RecQ DNA helicase.

Methods

We analysed the clinical and biological features of two women, aged 32 and 36, referred for partial lipodystrophic syndrome which led to the molecular diagnosis of Werner syndrome. Cultured skin fibroblasts from one patient were studied.

Results

Two normal-weighted women presented with a partial lipodystrophic syndrome with hypertriglyceridemia and liver steatosis. One of them had also diabetes. Both patients showed a peculiar, striking lipodystrophic phenotype with subcutaneous lipoatrophy of the four limbs contrasting with truncal and abdominal fat accumulation. Their oral glucose tolerance tests showed extremely high levels of insulinemia, revealing major insulin resistance. Low serum levels of sex-hormone binding globulin and adiponectin suggested a post-receptor insulin signalling defect. Other clinical features included bilateral cataracts, greying hair and distal skin atrophy. We observed biallelic WRN null mutations in both women (p.Q748X homozygous, and compound heterozygous p.Q1257X/p.M1329fs). Their fertility was decreased, with preserved menstrual cycles and normal follicle-stimulating hormone levels ruling out premature ovarian failure. However undetectable anti-müllerian hormone and inhibin B indicated diminished follicular ovarian reserve. Insulin-resistance linked ovarian hyperandrogenism could also contribute to decreased fertility, and the two patients became pregnant after initiation of insulin-sensitizers (metformin). Both pregnancies were complicated by severe cervical incompetence, leading to the preterm birth of a healthy newborn in one case, but to a second trimester-abortion in the other. WRN-mutated fibroblasts showed oxidative stress, increased lamin B1 expression, nuclear dysmorphies and premature senescence.

Conclusions

We show here for the first time that partial lipodystrophy with severe insulin resistance can reveal WRN-linked premature aging syndrome. Increased expression of lamin B1 with altered lamina architecture observed in WRN-mutated fibroblasts could contribute to premature cellular senescence. Primary alterations in DNA replication and/or repair should be considered as possible causes of lipodystrophic syndromes.

A novel lamin A/C mutation in a Dutch family with premature atherosclerosis

OBJECTIVE

We report a novel lamin A/C (LMNA) mutation, p.Glu223Lys, in a family with extensive atherosclerosis, diabetes mellitus and steatosis hepatis.

METHODS

Sequence analysis of LMNA (using Alamut version 2.2), co-segregation analysis, electron microscopy, extensive phenotypic evaluation of the mutation carriers and literature comparison were used to determine the loss of function of this mutation.

RESULTS

The father of three siblings died at the age of 45 years. The three siblings and the brother and sister of the father were referred to the cardiovascular genetics department, because of the premature atherosclerosis and dysmorphic characteristics observed in the father at autopsy. The novel LMNA mutation, p.Glu223Lys, was identified in the proband and his two sons. Clinical evaluation revealed atherosclerosis, insulin resistance and hypertension in the proband and dyslipidemia and hepatic steatosis in all the patients with the mutation.

CONCLUSION

Based on the facts that in silico analysis predicts a possibly pathogenic mutation, the mutation co-segregates with the disease, only fibroblasts from mutation carriers show nuclear blebbing and a similar phenotype was reported to be due to missense mutations in LMNA we conclude that we deal with a pathogenic mutation. We conclude that the phenotype is similar to Dunnigan-type familial partial lipodystrophy.

Mechanical model of blebbing in nuclear lamin meshworks

Much of the structural stability of the nucleus comes from meshworks of intermediate filament proteins known as lamins forming the inner layer of the nuclear envelope called the nuclear lamina. These lamin meshworks additionally play a role in gene expression. Abnormalities in nuclear shape are associated with a variety of pathologies, including some forms of cancer and Hutchinson-Gilford Progeria Syndrome, and often include protruding structures termed nuclear blebs. These nuclear blebs are thought to be related to pathological gene expression; however, little is known about how and why blebs form. We have developed a minimal continuum elastic model of a lamin meshwork that we use to investigate which aspects of the meshwork could be responsible for bleb formation. Mammalian lamin meshworks consist of two types of lamin proteins, A type and B type, and it has been reported that nuclear blebs are enriched in A-type lamins. Our model treats each lamin type separately and thus, can assign them different properties. Nuclear blebs have been reported to be located in regions where the fibers in the lamin meshwork have a greater separation, and we find that this greater separation of fibers is an essential characteristic for generating nuclear blebs. The model produces structures with comparable morphologies and distributions of lamin types as real pathological nuclei. Thus, preventing this opening of the meshwork could be a route to prevent bleb formation, which could be used as a potential therapy for the pathologies associated with nuclear blebs.

Soft substrates normalize nuclear morphology and prevent nuclear rupture in fibroblasts from a laminopathy patient with compound heterozygous LMNA mutations

Laminopathies, mainly caused by mutations in the LMNA gene, are a group of inherited diseases with a highly variable penetrance; i.e., the disease spectrum in persons with identical LMNA mutations range from symptom-free conditions to severe cardiomyopathy and progeria, leading to early death. LMNA mutations cause nuclear abnormalities and cellular fragility in response to cellular mechanical stress, but the genotype/phenotype correlations in these diseases remain unclear. Consequently, tools such as mutation analysis are not adequate for predicting the course of the disease.

 

Here, we employ growth substrate stiffness to probe nuclear fragility in cultured dermal fibroblasts from a laminopathy patient with compound progeroid syndrome. We show that culturing of these cells on substrates with stiffness higher than 10 kPa results in malformations and even rupture of the nuclei, while culture on a soft substrate (3 kPa) protects the nuclei from morphological alterations and ruptures. No malformations were seen in healthy control cells at any substrate stiffness. In addition, analysis of the actin cytoskeleton organization in this laminopathy cells demonstrates that the onset of nuclear abnormalities correlates to an increase in cytoskeletal tension.

Together, these data indicate that culturing of these LMNA mutated cells on substrates with a range of different stiffnesses can be used to probe the degree of nuclear fragility. This assay may be useful in predicting patient-specific phenotypic development and in investigations on the underlying mechanisms of nuclear and cellular fragility in laminopathies.

Protein farnesylation and disease

Prenylation consists of the addition of an isoprenoid group to a cysteine residue located near the carboxyl terminal of a protein. This enzymatic posttranslational modification is important for the maturation and processing of proteins. Both processes are necessary to mediate protein-protein and membrane-protein associations, in addition to regulating the localisation and function of proteins. The severe phenotype of animals deficient in enzymes involved in both prenylation and maturation highlights the significance of these processes. Moreover, alterations in the genes coding for isoprenylated proteins or enzymes that are involved in both prenylation and maturation processes have been found to be the basis of severe human diseases, such as cancer, neurodegenerative disorders, retinitis pigmentosa, and premature ageing syndromes. Recent studies on isoprenylation and postprenylation processing in pathological conditions have unveiled surprising aspects of these modifications and their roles in different cellular pathways. The identification of these enzymes as therapeutic targets has led researchers to validate their effects in vitro and in vivo as antitumour or antiageing agents. This review attempts to summarise the basic aspects of protein isoprenylation and postprenylation, integrating our data with that observed in other studies to provide a comprehensive scenario of progeroid syndromes and the therapeutic avenues.

Histological and molecular features of lipomatous and nonlipomatous adipose tissue in familial partial lipodystrophy caused by LMNA mutations

OBJECTIVES

Type 2 familial partial lipodystrophy (FPLD2) is a rare adipose tissue (AT) disease caused by mutations in LMNA, in which lipomas appear occasionally. In this study, we aimed to histologically characterize FPLD2-associated lipomatosis and study the expression of genes and proteins involved in cell cycle control, mitochondrial function, inflammation and adipogenesis.

DESIGN AND PATIENTS

One lipoma and perilipoma fat from each of four subjects with FPLD2 and 10 control subjects were analysed by optical microscopy. The presence of inflammatory cells was evaluated by immunohistochemistry. Real-time RT-PCR and Western blot were used to evaluate gene and protein levels.

RESULTS

Adipocytes from lipodystrophic patients were significantly larger than those of controls, in both the lipomas and perilipoma fat. Lipodystrophic AT exhibited CD68(+) macrophages and CD3(+) lymphocytes infiltration. TP53 expression was reduced in all types of lipomas. At protein level, C/EBPβ, p53 and pRb were severely disturbed in both lipodystrophic lipomas and perilipoma fat coming from lipoatrophic areas, whereas the expression of CEBPα was normal. Mitochondrial function genes were less expressed in lipoatrophic fat. In both lipomas and perilipoma fat from lipoatrophic areas, the expression of adipogenes was lower than controls.

CONCLUSIONS

Even in lipomas, the adipogenic machinery is impaired in lipodystrophic fat coming from lipoatrophic regions in FPLD2, although the histological phenotype is near-normal, exhibiting low-grade inflammatory features. Our results suggest that the p53 pathway and some adipogenic proteins, such as CEBPα, could contribute to the maintenance of this near normal phenotype in the remnant AT present in these patients.

LINC complex alterations in DMD and EDMD/CMT fibroblasts

Emery-Dreifuss muscular dystrophy (EDMD) is a late onset-disease characterized by skeletal muscle wasting and heart defects with associated risk of sudden death. The autosomal dominant form of the disease is caused by mutations in the LMNA gene encoding LaminA and C, the X-linked form results from mutations in the gene encoding the inner nuclear membrane protein Emerin (STA). Both Emerin and LaminA/C interact with the nuclear envelope proteins Nesprin-1 and -2 and mutations in genes encoding C-terminal isoforms of Nesprin-1 and -2 have also been implicated in EDMD. Here we analyse primary fibroblasts from patients affected by either Duchenne muscular dystrophy (DMD) or Emery-Dreifuss muscular dystrophy/Charcot-Marie-Tooth syndrome (EDMD/CMT) that in addition to the disease causing mutations harbour mutations in the Nesprin-1 gene and in the SUN1 and SUN2 gene, respectively. SUN proteins together with the Nesprins form the core of the LINC complex which connects the nucleus with the cytoskeleton. The mutations are accompanied by changes in cell adhesion, cell migration, senescence, and stress response, as well as in nuclear shape and nuclear envelope composition which are changes characteristic for laminopathies. Our results point to a potential influence of mutations in components of the LINC complex on the clinical outcome and the molecular pathology in the patients.

LMNA-linked lipodystrophies: from altered fat distribution to cellular alterations

Mutations in the LMNA gene, encoding the nuclear intermediate filaments the A-type lamins, result in a wide variety of diseases known as laminopathies. Some of them, such as familial partial lipodystrophy of Dunnigan and metabolic laminopathies, are characterized by lipodystrophic syndromes with altered fat distribution and severe metabolic alterations with insulin resistance and dyslipidaemia. Metabolic disturbances could be due either to the inability of adipose tissue to adequately store triacylglycerols or to other cellular alterations linked to A-type lamin mutations. Indeed, abnormal prelamin A accumulation and farnesylation, which are clearly involved in laminopathic premature aging syndromes, could play important roles in lipodystrophies. In addition, gene expression alterations, and signalling abnormalities affecting SREBP1 (sterol-regulatory-element-binding protein 1) and MAPK (mitogen-activated protein kinase) pathways, could participate in the pathophysiological mechanisms leading to LMNA (lamin A/C)-linked metabolic alterations and lipodystrophies. In the present review, we describe the clinical phenotype of LMNA-linked lipodystrophies and discuss the current physiological and biochemical hypotheses regarding the pathophysiology of these diseases.

The nuclear envelope and its involvement in cellular stress responses

The nuclear envelope is not only important for the structural integrity of the nucleus, but also involved in a number of cellular functions. It has been shown to be important for maintaining and controlling chromatin organization, sequestering transcription factors, replication, transcription and signalling. The nuclear envelope is thus important for development and differentiation, and some of its components are essential for cell viability. Among the many functions which are emerging for the nuclear envelope is its involvement in protecting the cell against different types of cellular stress. In the present paper, we review key findings which describe the roles of nuclear envelope components in responses to common types of stress conditions.

Protein profiling reveals energy metabolism and cytoskeletal protein alterations in LMNA mutation carriers

Nuclear envelope-related muscular dystrophies, in particular those referred to as laminopathies, are relatively novel and unclear diseases, also considering the increasing number of mutations identified so far in genes of the nuclear envelope. As regard LMNA gene, only tentative relations between phenotype, type and localization of the mutations have been established in striated muscle diseases, while laminopathies affecting adipose tissue, peripheral nerves or progerioid syndromes could be linked to specific genetic variants. This study describes the biochemical phenotype of neuromuscular laminopathies in samples derived from LMNA mutant patients. Since it has been reported that nuclear alterations, due to LMNA defects, are present also in fibroblasts from Emery-Dreifuss muscular dystrophy and familial partial lipodystrophy patients, we analyzed 2D-maps of skin fibroblasts of patients carrying 12 different LMNA mutations spread along the entire gene. To recognize distinctive proteins underlying affected biochemical pathways, we compared them with fibroblasts from healthy controls and, more importantly, fibroblasts from patients with non-lamin related neuromuscular disorders. We found less abundance of cytoskeletal/structural proteins, confirming a dominant role for Lamin A/C in structural support of nuclear architecture. Interestingly, we also established significant changes in the expression of proteins involved in cellular energy production and oxidative stress response. To our knowledge, this is the first report where proteomics was applied to characterize ex-vivo cells from LMNA patients, suggesting that this may represent a new approach to better understand the molecular mechanisms of these rare diseases and facilitate the development of novel therapeutic treatments.

Chromosomal regions associated with prostate cancer risk localize to lamin B-deficient microdomains and exhibit reduced gene transcription

The lamins are major determinants of nuclear shape and chromatin organization and these features are frequently altered in prostate cancer (CaP). Human CaP cell lines frequently have nuclear lobulations, which are enriched in A-type lamins but lack B-type lamins and have been defined as lamin B-deficient microdomains (LDMDs). LDMD frequency is correlated with CaP cell line aggressiveness and increased cell motility. In addition, LNCaP cells grown in the presence of dihydrotestosterone (DHT) show an increased frequency of LDMDs. The LDMDs are enriched in activated RNA polymerase II (Pol IIo) and androgen receptor (AR) and A-type lamins form an enlarged meshwork that appears to co-align with chromatin fibres and AR. Furthermore, fluorescence in situ hybridization and comparative genomic hybridization demonstrated that chromosomal regions associated with CaP susceptibility are preferentially localized to LDMDs. Surprisingly, these regions lack histone marks for transcript elongation and exhibit reduced BrU incorporation, suggesting that Pol II is stalled within LDMDs. Real-time PCR of genes near androgen response elements (AREs) was used to compare transcription between cells containing LDMDs and controls. Genes preferentially localized to LDMDs showed significantly decreased expression, while genes in the main nuclear body were largely unaffected. Furthermore, LDMDs were observed in human CaP tissue and the frequency was correlated with increased Gleason grade. These results imply that lamins are involved in chromatin organization and Pol II transcription, and provide insights into the development and progression of CaP.

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.

Beyond membrane channelopathies: alternative mechanisms underlying complex human disease

Over the past fifteen years, our understanding of the molecular mechanisms underlying human disease has flourished in large part due to the discovery of gene mutations linked with membrane ion channels and transporters. In fact, ion channel defects ("channelopathies" - the focus of this review series) have been associated with a spectrum of serious human disease phenotypes including cystic fibrosis, cardiac arrhythmia, diabetes, skeletal muscle defects, and neurological disorders. However, we now know that human disease, particularly excitable cell disease, may be caused by defects in non-ion channel polypeptides including in cellular components residing well beneath the plasma membrane. For example, over the past few years, a new class of potentially fatal cardiac arrhythmias has been linked with cytoplasmic proteins that include sub-membrane adapters such as ankyrin-B (ANK2), ankyrin-G (ANK3), and alpha-1 syntrophin, membrane coat proteins including caveolin-3 (CAV3), signaling platforms including yotiao (AKAP9), and cardiac enzymes (GPD1L). The focus of this review is to detail the exciting role of lamins, yet another class of gene products that have provided elegant new insight into human disease.

Protein isoprenylation regulates osteogenic differentiation of mesenchymal stem cells: effect of alendronate, and farnesyl and geranylgeranyl transferase inhibitors

BACKGROUND AND PURPOSE

Protein isoprenylation is an important step in the intracellular signalling pathway conducting cell growth and differentiation. In bone, protein isoprenylation is required for osteoclast differentiation and activation. However, its role in osteoblast differentiation and function remains unknown. In this study, we assessed the role of protein isoprenylation in osteoblastogenesis in a model of mesenchymal stem cells (MSC) differentiation.

EXPERIMENTAL APPROACH

We tested the effect of an inhibitor of farnesylation [farnesyl transferase inhibitor-277 (FTI-277)] and one of geranylgeranylation [geranylgeranyltransferase inhibitor-298 (GGTI-298)] on osteoblast differentiating MSC. In addition, we tested the effect of alendronate on protein isoprenylation in this model either alone or in combination with other inhibitors of isoprenylation.

KEY RESULTS

Initially, we found that levels of unfarnesylated proteins (prelamin A and HDJ-2) increased after treatment with FTI-277 concomitantly affecting osteoblastogenesis and increasing nuclear morphological changes without affecting cell survival. Furthermore, inhibition of geranylgeranylation by GGTI-298 alone increased osteoblastogenesis. This effect was enhanced by the combination of GGTI-298 and alendronate in the osteogenic media.

CONCLUSIONS AND IMPLICATIONS

Our data indicate that both farnesylation and geranylgeranylation play a role in osteoblastogenesis. In addition, a new mechanism of action for alendronate on protein isoprenylation in osteogenic differentiating MSC in vitro was found. In conclusion, protein isoprenylation is an important component of the osteoblast differentiation process that could constitute a new therapeutic target for osteoporosis in the future.

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.

Evaluation of micronucleus and erythrocytic nuclear abnormalities in Balkan whip snake Hierophis gemonensis

Over recent years, changes of erythrocytic nuclei have been increasingly used to evaluate genotoxic effects of different compounds such as polycyclic aromatic hydrocarbons (benzo-a-pyrene, naphthalene, β-naphthoflavone), heavy metals (cadmium, mercury), textile mill effluent especially in aquatic ecosystem. However, in fish, both micronuclei and erythrocytic nuclear abnormalities also appear spontaneously and their frequency can be seasonally dependent. The aim of the study was to evaluate the frequency of micronuclei (MN), nuclear abnormalities (NA) including vacuolated nuclei (VN) and cytoplasmic vacuoles (CV) in erythrocytes of Balkan whip snake Hierophis gemonensis and establish the level of spontaneous appearance during the annual cycle. Average frequency of NA was 10.89 ± 4.72% while the MN (0.03 ± 0.03%) and VN (0.04 ± 0.08%) were seldom detected. NA significantly positively correlated with MN (r = 0.319; P < 0.05) and VN (r = 0.363; P < 0.05). Appearance of CV did not correlate with other measured parameters and average frequency was 11.06 ± 8.33%. Significant seasonal variation was found in NA appearance with the lowest value in spring and the highest in winter. VN increase was observed in autumn. MN and CV levels varied between seasons but not significantly. Considering the biological cycle, frequency of NA, VN, MN and CV recorded in pre-hibernation/hibernation increased compared to the active phase, but only NA elevation was significant. Although the obtained results showed differences according to sex, statistical analysis of measured parameters showed the same pattern of seasonal variation in both sexes.

Cancer nucleus: morphology and beyond

There are many significant morphological alterations of a nucleus of cancer cell that are detectable by light microscopy on routine staining. These changes are often associated with deranged cellular functions of cancer cell. It is difficult to understand the exact relationship between nuclear morphology and alteration of nuclear structural organization in cancer. Herein, the salient visual and subvisual morphological changes of cancer nuclei and their possible etiology and significance have been reviewed.

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.

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.

Fibroblast phenotype in male carriers of FMR1 premutation alleles

Fragile X-associated tremor/ataxia syndrome (FXTAS) is an adult-onset neurodegenerative disorder among carriers of premutation expansions (55-200 CGG repeats) of the fragile X mental retardation 1 (FMR1) gene. The clinical features of FXTAS, as well as various forms of clinical involvement in carriers without FXTAS, are thought to arise through a direct toxic gain of function of high levels of FMR1 mRNA containing the expanded CGG repeat. Here we report a cellular endophenotype involving increased stress response (HSP27, HSP70 and CRYAB) and altered lamin A/C expression/organization in cultured skin fibroblasts from 11 male carriers of premutation alleles of the FMR1 gene, including six patients with FXTAS and five premutation carriers with no clinical evidence of FXTAS, compared with six controls. A similar abnormal cellular phenotype was found in CNS tissue from 10 patients with FXTAS. Finally, there is an analogous abnormal cellular distribution of lamin A/C isoforms in knock-in mice bearing the expanded CGG repeat in the murine Fmr1 gene. These alterations are evident even in mouse embryonic fibroblasts, raising the possibility that, in humans, the expanded-repeat mRNA triggers pathogenic mechanisms early in development, thus providing a molecular basis for the neurodevelopmental abnormalities observed in some children and clinical symptoms in some adults who are carriers of premutation FMR1 alleles. Cellular dysregulation in fibroblasts represents a novel and highly advantageous model for investigating disease pathogenesis in premutation carriers and for quantifying and monitoring disease progression. Fibroblast studies may also prove useful in screening and testing the efficacy of therapeutic interventions.

Mechanisms of muscle degeneration, regeneration, and repair in the muscular dystrophies

To withstand the rigors of contraction, muscle fibers have specialized protein complexes that buffer against mechanical stress and a multifaceted repair system that is rapidly activated after injury. Genetic studies first identified the mechanosensory signaling network that connects the structural elements of muscle and, more recently, have identified repair elements of muscle. Defects in the genes encoding the components of these systems lead to muscular dystrophy, a family of genetic disorders characterized by progressive muscle wasting. Although the age of onset, affected muscles, and severity vary considerably, all muscular dystrophies are characterized by muscle necrosis that overtakes the regenerative capacity of muscle. The resulting replacement of muscle by fatty and fibrous tissue leaves muscle increasingly weak and nonfunctional. This review discusses the cellular mechanisms that are primarily and secondarily disrupted in muscular dystrophy, focusing on membrane degeneration, muscle regeneration, and the repair of muscle.

Impaired nuclear functions lead to increased senescence and inefficient differentiation in human myoblasts with a dominant p.R545C mutation in the LMNA gene

We have studied myoblasts from a patient with a severe autosomal dominant Emery-Dreifuss muscular dystrophy (AD-EDMD) caused by an arginine 545 to cystein point mutation (p.R545C) in the carboxy-terminal domain of the lamin A/C gene. This mutation has pleiotropic cellular effects on these myoblasts as demonstrated by nuclear structural defects, exhibiting lobulations which increase with cell passages in culture. The organization of both lamin A/C and its inner nuclear membrane partner emerin are altered, eventually showing a honeycomb pattern upon immunofluorescence microscopy. In addition, the distribution of histone H3 trimethylated at lysine 27 and of phosphorylated RNA polymerase II, markers of inactive and active chromatin domains, respectively, are altered suggesting an impact on gene expression. Patient myoblasts also presented a high index of senescence in ex vivo culture. Moreover, our data show for the first time in an AD-EDMD context that the 20S core particle of the proteasome was inactivated. With cell passages, the 20S core protein progressively accumulated into discrete nuclear foci that largely colocalized with promyelocytic leukemia (PML) bodies while p21 accumulated throughout the nuclear compartment. Proteasome inactivation has been linked to normal cellular ageing. Our data indicate that it may also contribute to premature senescence in AD-EDMD patient myoblasts. Finally, when transferred to low-serum medium, patient myoblasts were deficient in ex vivo differentiation, as assessed by the absence of myotube formation and myogenin induction. Altogether, these data suggest that the LMNA mutation p.R545C impairs both proliferation and differentiation capacities of myoblasts as part of the pathogenesis of AD-EDMD.

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.

Seasonality of nuclear abnormalities in gilthead sea bream Sparus aurata (L.) erythrocytes

The frequencies of erythrocyte nuclear abnormalities, such as irregularly shaped (ISN), vacuolated nuclei (VN), binucleated cells (BN), including micronuclei (MN), were monitored in the peripheral blood of cultured gilthead sea bream (Sparus aurata L.) from the southern Adriatic Sea (Croatia) and assessed for a relation to seasonality. Fish were sampled in February, April, June, and September. The lowest frequencies of ISN and MN were measured in April, and the lowest frequencies of VN and BN were found in February. The highest values of ISN and BN were detected in September. The highest values of MN and the most frequent occurrence of VN occurred in June and April, respectively. The Spearman rank order correlation test revealed a significant positive correlation between the frequencies of ISN, BN and MN; VN did not correlate with any of the measured parameters. The ISN had significantly higher nuclear surface area values than normal nuclei (P < 0.001). Our results demonstrate that the measured parameters displayed a distinct seasonality. Consequently, we conclude that seasonal variations should be taken into account when nuclear abnormalities are used as biomarkers. This variation should be well understood in order to successfully separate true contamination effects from seasonal factors that have the potential to affect the frequency and occurrence of nuclear abnormalities.

The R439C mutation in LMNA causes lamin oligomerization and susceptibility to oxidative stress

Dunnigan-type familial partial lipodystrophy (FPLD) is a laminopathy characterized by an aberrant fat distribution and a metabolic syndrome for which oxidative stress has recently been suggested as one of the disease-causing mechanisms. In a family affected with FPLD, we identified a heterozygous missense mutation c.1315C>T in the LMNA gene leading to the p.R439C substitution. Cultured patient fibroblasts do not show any prelamin A accumulation and reveal honeycomb-like lamin A/C formations in a significant percentage of nuclei. The mutation affects a region in the C-terminal globular domain of lamins A and C, different from the FPLD-related hot spot. Here, the introduction of an extra cysteine allows for the formation of disulphide-mediated lamin A/C oligomers. This oligomerization affects the interaction properties of the C-terminal domain with DNA as shown by gel retardation assays and causes a DNA-interaction pattern that is distinct from the classical R482W FPLD mutant. Particularly, whereas the R482W mutation decreases the binding efficiency of the C-terminal domain to DNA, the R439C mutation increases it. Electron spin resonance spectroscopy studies show significantly higher levels of reactive oxygen species (ROS) upon induction of oxidative stress in R439C patient fibroblasts compared to healthy controls. This increased sensitivity to oxidative stress seems independent of the oligomerization and enhanced DNA binding typical for R439C, as both the R439C and R482W mutants show a similar and significant increase in ROS upon induction of oxidative stress by H2O2.

A novel phenotypic expression associated with a new mutation in LMNA gene, characterized by partial lipodystrophy, insulin resistance, aortic stenosis and hypertrophic cardiomyopathy

BACKGROUND

Lipodystrophies are a heterogeneous group of diseases characterized by abnormal fat distribution. Familial partial lipodystrophy 2 (FPLD2) is due to mutations in the LMNA gene. Previous studies have suggested that LMNA mutations 5' to the nuclear localization signal (NLS) are more likely to underlie laminopathies with cardiac or skeletal muscle involvement, while mutations 3' to the NLS are more likely to underlie lipodystrophy and progeroid syndromes.

OBJECTIVE

To study the clinical and molecular features of a subject with FPLD.

SUBJECTS AND METHODS

We carried out mutational analysis of LMNA gene in a woman with FPLD phenotype and in her relatives. Insulin resistance was evaluated by minimal model. Body composition was evaluated by dual-energy X-ray absorptiometry (DEXA). Echocardiography was done in affected subjects. 3T3-L1 preadipocytes were transfected with wild-type or mutant prelamin A constructs. In transfected cells, lamin A was detected using a Cy3-conjugated monoclonal anti-FLAG antibody.

RESULTS

The patient showed atypical fat distribution, insulin resistance, severe aortic stenosis and hypertrophic cardiomyopathy. She has an affected 11-year-old son, not yet lipodystrophic but with an incipient aortic disease. LMNA sequencing showed that mother and son were both heterozygous for a novel c.1772G > T missense mutation in exon 11, which causes the substitution of the cysteine at residue 591 by a phenylalanine (C591F). In mouse preadipocytes transfected with the mutant human LMNA gene, the mutant lamin A isoform was mislocated in the nucleus.

CONCLUSIONS

This patient shows a novel clinical form of FPLD2, due to a mutation affecting lamin A only, with cardiac involvement.

Drug-specific effect of nelfinavir and stavudine on primary culture of human preadipocytes

Lipodystrophic syndrome is a major side effect of antiviral therapy leading to profound disturbances in adipose tissue. Human preadipocyte primary culture represents a model to understand mechanisms by which antiretroviral drugs alter adipocyte biology. The aim of this study was to evaluate the effects of various protease and nucleoside reverse transcriptase inhibitors in this model. We tested the effect of drugs on triglyceride accumulation and expression of specific genes by real-time polymerase chain reaction. To determine differential mechanisms by which the efficient drugs operate, we studied mitochondrial effects by evaluating oxygen consumption rates and nuclear lamina alteration by immunocytology. Only stavudine and nelfinavir, both at 10 microM, altered human adipose cell differentiation, as shown by reduced triglyceride accumulation. Our studies revealed that stavudine increased expression of genes such as PGC1 and LPL and affected mitochondrial respiration. Cells treated with nelfinavir had a lower expression of PPARgamma, LPL, and ap2 and presented disorganization of lamin A/C. Our data suggest for the first time in a model of human adipocytes differentiated in vitro that stavudine and nelfinavir interfere with the process of differentiation by 2 distinct mechanisms. This may be particularly relevant in understanding the physiopathologic mechanisms underlying the lipodystrophic syndrome.

Towards an integrated understanding of the structure and mechanics of the cell nucleus

Changes in the shape and structural organization of the cell nucleus occur during many fundamental processes including development, differentiation and aging. In many of these processes, the cell responds to physical forces by altering gene expression within the nucleus. How the nucleus itself senses and responds to such mechanical cues is not well understood. In addition to these external forces, epigenetic modifications of chromatin structure inside the nucleus could also alter its physical properties. To achieve a better understanding, we need to elucidate the relationship between nuclear structure and material properties. Recently, new approaches have been developed to systematically investigate nuclear mechanical properties. These experiments provide important new insights into the disease mechanism of a growing class of tissue-specific disorders termed 'nuclear envelopathies'. Here we review our current understanding of what determines the shape and mechanical properties of the cell nucleus.

The nuclear envelope as an integrator of nuclear and cytoplasmic architecture

Initially perceived as little more than a container for the genome, our view of the nuclear envelope (NE) and its role in defining global nuclear architecture has evolved significantly in recent years. The recognition that certain human diseases arise from defects in NE components has provided new insight into its structural and regulatory functions. In particular, NE defects associated with striated muscle disease have been shown to cause structural perturbations not just of the nucleus itself but also of the cytoplasm. It is now becoming increasingly apparent that these two compartments display co-dependent mechanical properties. The identification of cytoskeletal binding complexes that localize to the NE now reveals a molecular framework that can seamlessly integrate nuclear and cytoplasmic architecture.

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.

HGPS and related premature aging disorders: from genomic identification to the first therapeutic approaches

Progeroid syndromes are heritable human disorders displaying features that recall premature ageing. In these syndromes, premature aging is defined as "segmental" since only some of its features are accelerated. A number of cellular biological pathways have been linked to aging, including regulation of the insulin/growth hormone axis, pathways involving ROS metabolism, caloric restriction, and DNA repair. The number of identified genes associated with progeroid syndromes has increased in recent years, possibly shedding light as well on mechanisms underlying ageing in general. Among these, premature aging syndromes related to alterations of the LMNA gene have recently been identified. This review focuses on Hutchinson-Gilford Progeria syndrome and Restrictive Dermopathy, two well-characterized Lamin-associated premature aging syndromes, pointing out the current knowledge concerning their pathophysiology and the development of possible therapeutic approaches.

Differentiation of C2C12 myoblasts expressing lamin A mutated at a site responsible for Emery-Dreifuss muscular dystrophy is improved by inhibition of the MEK-ERK pathway and stimulation of the PI3-kinase pathway

Mutation R453W in A-type lamins, that are major nuclear envelope proteins, generates Emery-Dreifuss muscular dystrophy. We previously showed that mouse myoblasts expressing R453W-lamin A incompletely exit the cell cycle and differentiate into myocytes with a low level of multinucleation. Here we attempted to improve differentiation by treating these cells with a mixture of PD98059, an extracellular-regulated kinase (ERK) kinase (also known as mitogen-activated kinase, MEK) inhibitor, and insulin-like growth factor-II, an activator of phosphoinositide 3-kinase. We show that mouse myoblasts expressing R453W-lamin A were sensitive to the drug treatment as shown by (i) an increase in multinucleation, (ii) downregulation of proliferation markers (cyclin D1, hyperphosphorylated Rb), (iii) upregulation of myogenin, and (iv) sustained activation of p21 and cyclin D3. However, nuclear matrix anchorage of p21 and cyclin D3 in a complex with hypophosphorylated Rb that is critical to trigger cell cycle arrest and myogenin induction was deficient and incompletely restored by drug treatment. As the turn-over of R453W-lamin A at the nuclear envelope was greatly enhanced, we propose that R453W-lamin A impairs the capacity of the nuclear lamina to serve as scaffold for substrates of the MEK-ERK pathway and for MyoD-induced proteins that play a role in the differentiation process.

New metabolic phenotypes in laminopathies: LMNA mutations in patients with severe metabolic syndrome

CONTEXT

Mutations in the LMNA gene are responsible for several laminopathies, including lipodystrophies, with complex genotype/phenotype relationships. OBJECTIVE, DESIGN, SETTING, AND PATIENTS: Sequencing of the LMNA coding regions in 277 unrelated adults investigated for lipodystrophy and/or insulin resistance revealed 17 patients with substitutions at codon 482 observed in typical Dunnigan's familial partial lipodystrophy and 10 patients with other mutations. We report here the phenotypes of the patients with non-codon 482 mutations and compare them with those of 11 patients with codon 482 mutations. We also studied skin fibroblasts or lymphocytes from seven patients.

RESULTS

LMNA mutations found in nine patients studied here affected the three protein domains. Eight of them were novel. The 10 patients with non-codon 482-associated mutations fulfilled the International Diabetes Federation diagnosis criteria for metabolic syndrome. Most of them lacked the typical lipoatrophy observed in Dunnigan's familial partial lipodystrophy. However, the severity of insulin resistance, altered glucose tolerance, and hypertriglyceridemia and the alterations of cell nuclei were similar in patients with codon 482- and non-codon 482-associated mutations. Calf hypertrophy, myalgia, and muscle cramps or weakness were present in nine patients and cardiac conduction disturbances in two patients with non-codon 482 LMNA mutations.

CONCLUSIONS

We describe here new phenotypes of metabolic laminopathy associated with non-codon 482 LMNA mutations and characterized, in the absence of obvious clinical lipoatrophy, by severe metabolic alterations and frequent muscle signs (muscular hypertrophy, myalgias, or weakness). Dual-energy x-ray absorptiometry and/or cross-sectional abdominal and thigh imaging can help diagnosis by revealing subclinical lipodystrophy. The prevalence and pathophysiology of metabolic laminopathies need to be studied further.

Nesprin-2 giant safeguards nuclear envelope architecture in LMNA S143F progeria cells

The S143F lamin A/C point mutation causes a phenotype combining features of myopathy and progeria. We demonstrate here that patient dermal fibroblast cells have dysmorphic nuclei containing numerous blebs and lobulations, which progressively accumulate as cells age in culture. The lamin A/C organization is altered, showing intranuclear and nuclear envelope (NE) aggregates and presenting often a honeycomb appearance. Immunofluorescence microscopy showed that nesprin-2 C-terminal isoforms and LAP2alpha were recovered in the cytoplasm, whereas LAP2beta and emerin were unevenly localized along the NE. In addition, the intranuclear organization of acetylated histones, histone H1 and the active form of RNA polymerase II were markedly different in patient cells. A subpopulation of mutant cells, however, expressing the 800 kDa nesprin-2 giant isoform, did not show an overt nuclear phenotype. Ectopic expression of p.S143F lamin A in fibroblasts recapitulates the patient cell phenotype, whereas no effects were observed in p.S143F LMNA keratinocytes, which highly express nesprin-2 giant. Overexpression of the mutant lamin A protein had a more severe impact on the NE of nesprin-2 giant deficient fibroblasts when compared with wild-type. In summary, our results suggest that the p.S143F lamin A mutation affects NE architecture and composition, chromatin organization, gene expression and transcription. Furthermore, our findings implicate a direct involvement of the nesprins in laminopathies and propose nesprin-2 giant as a structural reinforcer at the NE.

LITTLE NUCLEI genes affecting nuclear morphology in Arabidopsis thaliana

Efforts to understand nuclear organization in plant cells have received little assistance from the better-studied animal nuclei, because plant proteomes do not contain recognizable counterparts to the key animal proteins involved in nuclear organization, such as lamin nuclear intermediate filament proteins. Previous studies identified a plant-specific insoluble nuclear protein in carrot (Daucus carota), called Nuclear Matrix Constituent Protein1 (NMCP1), which contains extensive coiled-coil domains and localizes to the nuclear periphery. Here, we describe a genetic characterization of two NMCP1-related nuclear proteins in Arabidopsis thaliana, LITTLE NUCLEI1 (LINC1) and LINC2. Disruption of either gene caused a reduction in nuclear size and altered nuclear morphology. Moreover, combining linc1 and linc2 mutations had an additive effect on nuclear size and morphology but a synergistic effect on chromocenter number (reduction) and whole-plant morphology (dwarfing). The reduction in nuclear size in the linc1 linc2 double mutant was not accompanied by a corresponding change in endopolyploidy. Rather, the density of DNA packaging at all endopolyploid levels in the linc1 linc2 mutants was increased significantly. Our results indicate that the LINC coiled-coil proteins are important determinants of plant nuclear structure.

Human lipodystrophies linked to mutations in A-type lamins and to HIV protease inhibitor therapy are both associated with prelamin A accumulation, oxidative stress and premature cellular senescence

Lipodystrophic syndromes associated with mutations in LMNA, encoding A-type lamins, and with HIV antiretroviral treatments share several clinical characteristics. Nuclear alterations and prelamin A accumulation have been reported in fibroblasts from patients with LMNA mutations and adipocytes exposed to protease inhibitors (PI). As genetically altered lamin A maturation also results in premature ageing syndromes with lipodystrophy, we studied prelamin A expression and senescence markers in cultured human fibroblasts bearing six different LMNA mutations or treated with PIs. As compared to control cells, fibroblasts with LMNA mutations or treated with PIs had nuclear shape abnormalities and reduced proliferative activity that worsened with increasing cellular passages. They exhibited prelamin A accumulation, increased oxidative stress, decreased expression of mitochondrial respiratory chain proteins and premature cellular senescence. Inhibition of prelamin A farnesylation prevented cellular senescence and oxidative stress. Adipose tissue samples from patients with LMNA mutations or treated with PIs also showed retention of prelamin A, overexpression of the cell cycle checkpoint inhibitor p16 and altered mitochondrial markers. Thus, both LMNA mutations and PI treatment result in accumulation of farnesylated prelamin A and oxidative stress that trigger premature cellular senescence. These alterations could participate in the pathophysiology of lipodystrophic syndromes and lead to premature ageing complications.

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

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

"Laminopathies": a wide spectrum of human diseases

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

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.

Genetic modifiers of muscular dystrophy: Implications for therapy

The genetic understanding of the muscular dystrophies has advanced considerably in the last two decades. Over 25 different individual genes are now known to produce muscular dystrophy, and many different "private" mutations have been described for each individual muscular dystrophy gene. For the more common forms of muscular dystrophy, phenotypic variability can be explained by precise mutations. However, for many genetic mutations, the presence of the identical mutation is associated with marked phenotypic range that affects muscle function as well as cardiac function. The explanation for phenotype variability in the muscular dystrophies is only now being explored. The availability of genetically engineered animal models has allowed the generation of single mutations on the background of highly inbred strain. Phenotypic variation that is altered by genetic background argues for the presence of genetic modifier loci that can ameliorate or enhance aspects of the dystrophic phenotype. A number of individual genes have been implicated as modifiers of muscular dystrophy by studies in genetically engineered mouse models of muscular dystrophy. The value of these genes and products is that the pathways identified through these experiments may be exploited for therapy.