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

Proteomic characterization of human LMNA-related congenital muscular dystrophy muscle cells

LMNA-related congenital muscular dystrophy (L-CMD) is caused by mutations in the LMNA gene, encoding lamin A/C. To further understand the molecular mechanisms of L-CMD, proteomic profiling using DIA mass spectrometry was conducted on immortalized myoblasts and myotubes from controls and L-CMD donors each harbouring a different LMNA mutation (R249W, del.32 K and L380S). Compared to controls, 124 and 228 differentially abundant proteins were detected in L-CMD myoblasts and myotubes, respectively, and were associated with enriched canonical pathways including synaptogenesis and necroptosis in myoblasts, and Huntington's disease and insulin secretion in myotubes. Abnormal nuclear morphology and reduced lamin A/C and emerin abundance was evident in all L-CMD cell lines compared to controls, while nucleoplasmic aggregation of lamin A/C was restricted to del.32 K cells, and mislocalization of emerin was restricted to R249W cells. Abnormal nuclear morphology indicates loss of nuclear lamina integrity as a common feature of L-CMD, likely rendering muscle cells vulnerable to mechanically induced stress, while differences between L-CMD cell lines in emerin and lamin A localization suggests that some molecular alterations in L-CMD are mutation specific. Nonetheless, identifying common proteomic alterations and molecular pathways across all three L-CMD lines has highlighted potential targets for the development of non-mutation specific therapies.

LMO7 as an Unrecognized Factor Promoting Pancreatic Cancer Progression and Metastasis

Pancreatic cancer (PC) is one of the most lethal human malignancies without effective treatment. In an effort to discover key genes and molecular pathways underlying PC growth, we have identified LIM domain only 7 (LMO7) as an under-investigated molecule, which highly expresses in primary and metastatic human and mouse PC with the potential of impacting PC tumorigenesis and metastasis. Using genetic methods with siRNA, shRNA, and CRISPR-Cas9, we have successfully generated stable mouse PC cells with LMO7 knockdown or knockout. Using these cells with loss of LMO7 function, we have demonstrated that intrinsic LMO7 defect significantly suppresses PC cell proliferation, anchorage-free colony formation, and mobility in vitro and slows orthotopic PC tumor growth and metastasis in vivo. Mechanistic studies demonstrated that loss of LMO7 function causes PC cell-cycle arrest and apoptosis. These data indicate that LMO7 functions as an independent and unrecognized druggable factor significantly impacting PC growth and metastasis, which could be harnessed for developing a new targeted therapy for PC.

Mechanotransduction, nuclear architecture and epigenetics in Emery Dreifuss Muscular Dystrophy: tous pour un, un pour tous

The alteration of the several roles that Lamin A/C plays in the mammalian cell leads to a broad spectrum of pathologies that – all together – are named laminopathies. Among those, the Emery Dreifuss Muscular Dystrophy (EDMD) is of particular interest as, despite the several known mutations of Lamin A/C, the genotype–phenotype correlation still remains poorly understood; this suggests that the epigenetic background of patients might play an important role during the time course of the disease. Historically, both a mechanical role of Lamin A/C and a regulative one have been suggested as the driving force of laminopathies; however, those two hypotheses are not mutually exclusive. Recent scientific evidence shows that Lamin A/C sustains the correct gene expression at the epigenetic level thanks to the Lamina Associated Domains (LADs) reorganization and the crosstalk with the Polycomb Group of Proteins (PcG). Furthermore, the PcG-dependent histone mark H3K27me3 increases under mechanical stress, finally pointing out the link between the mechano-properties of the nuclear lamina and epigenetics. Here, we summarize the emerging mechanisms that could explain the high variability seen in Emery Dreifuss muscular dystrophy.

The nucleoporin ELYS regulates nuclear size by controlling NPC number and nuclear import capacity

How intracellular organelles acquire their characteristic sizes is a fundamental question in cell biology. Given stereotypical changes in nuclear size in cancer, it is important to understand the mechanisms that control nuclear size in human cells. Using a high-throughput imaging RNAi screen, we identify and mechanistically characterize ELYS, a nucleoporin required for post-mitotic nuclear pore complex (NPC) assembly, as a determinant of nuclear size in mammalian cells. ELYS knockdown results in small nuclei, reduced nuclear lamin B2 localization, lower NPC density, and decreased nuclear import. Increasing nuclear import by importin α overexpression rescues nuclear size and lamin B2 import, while inhibiting importin α/β-mediated nuclear import decreases nuclear size. Conversely, ELYS overexpression increases nuclear size, enriches nuclear lamin B2 at the nuclear periphery, and elevates NPC density and nuclear import. Consistent with these observations, knockdown or inhibition of exportin 1 increases nuclear size. Thus, we identify ELYS as a novel positive effector of mammalian nuclear size and propose that nuclear size is sensitive to NPC density and nuclear import capacity.

Emerin induces nuclear breakage in Xenopus extract and early embryos

Emerin is an inner nuclear membrane protein often mutated in Emery–Dreifuss muscular dystrophy. Because emerin has diverse roles in nuclear mechanics, cytoskeletal organization, and gene expression, it has been difficult to elucidate its contribution to nuclear structure and disease pathology. In this study, we investigated emerin’s impact on nuclei assembled in Xenopus laevis egg extract, a simplified biochemical system that lacks potentially confounding cellular factors and activities. Notably, these extracts are transcriptionally inert and lack endogenous emerin and filamentous actin. Strikingly, emerin caused rupture of egg extract nuclei, dependent on the application of shear force. In egg extract, emerin localized to nonnuclear cytoplasmic membranes, and nuclear rupture was rescued by targeting emerin to the nucleus, disrupting its membrane association, or assembling nuclei with lamin A. Furthermore, emerin induced breakage of nuclei in early-stage X. laevis embryo extracts, and embryos microinjected with emerin were inviable, with ruptured nuclei. We propose that cytoplasmic membrane localization of emerin leads to rupture of nuclei that are more sensitive to mechanical perturbation, findings that may be relevant to early development and certain laminopathies.

Emerin modulates spatial organization of chromosome territories in cells on softer matrices

Cells perceive and relay external mechanical forces into the nucleus through the nuclear envelope. Here we examined the effect of lowering substrate stiffness as a paradigm to address the impact of altered mechanical forces on nuclear structure-function relationships. RNA sequencing of cells on softer matrices revealed significant transcriptional imbalances, predominantly in chromatin associated processes and transcriptional deregulation of human Chromosome 1. Furthermore, 3-Dimensional fluorescence in situ hybridization (3D-FISH) analyses showed a significant mislocalization of Chromosome 1 and 19 Territories (CT) into the nuclear interior, consistent with their transcriptional deregulation. However, CT18 with relatively lower transcriptional dysregulation, also mislocalized into the nuclear interior. Furthermore, nuclear Lamins that regulate chromosome positioning, were mislocalized into the nuclear interior in response to lowered matrix stiffness. Notably, Lamin B2 overexpression retained CT18 near the nuclear periphery in cells on softer matrices. While, cells on softer matrices also activated emerin phosphorylation at a novel Tyr99 residue, the inhibition of which in a phospho-deficient mutant (emerinY99F), selectively retained chromosome 18 and 19 but not chromosome 1 territories at their conserved nuclear locations. Taken together, emerin functions as a key mechanosensor, that modulates the spatial organization of chromosome territories in the interphase nucleus.

The increasing relevance of nuclear envelope myopathies

PURPOSE OF REVIEW

Nuclear envelope links to a wide range of disorders, including several myopathies and neuropathies over the past 2 decades, has spurred research leading to a completely changed view of this important cellular structure and its functions. However, the many functions now assigned to the nuclear envelope make it increasingly hard to determine which functions underlie these disorders.

RECENT FINDINGS

New nuclear envelope functions in genome organization, regulation and repair, signaling, and nuclear and cellular mechanics have been added to its classical barrier function. Arguments can be made for any of these functions mediating abnormality in nuclear envelope disorders and data exist supporting many. Moreover, transient and/or distal nuclear envelope connections to other cellular proteins and structures may increase the complexity of these disorders.

SUMMARY

Although the increased understanding of nuclear envelope functions has made it harder to distinguish specific causes of nuclear envelope disorders, this is because it has greatly expanded the spectrum of possible mechanisms underlying them. This change in perspective applies well beyond the known nuclear envelope disorders, potentially implicating the nuclear envelope in a much wider range of myopathies and neuropathies.

Immunohistochemistry on a panel of Emery-Dreifuss muscular dystrophy samples reveals nuclear envelope proteins as inconsistent markers for pathology

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Altered distribution of EDMD-linked proteins is not a general characteristic of EDMD.

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Tissue-specific proteins exhibit altered distributions in some EDMD patients.

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Variation in redistributed proteins in EDMD may underlie its clinical variability.

Reports of aberrant distribution for some nuclear envelope proteins in cells expressing a few Emery–Dreifuss muscular dystrophy mutations raised the possibility that such protein redistribution could underlie pathology and/or be diagnostic. However, this disorder is linked to 8 different genes encoding nuclear envelope proteins, raising the question of whether a particular protein is most relevant. Therefore, myoblast/fibroblast cultures from biopsy and tissue sections from a panel of nine Emery–Dreifuss muscular dystrophy patients (4 male, 5 female) including those carrying emerin and FHL1 (X-linked) and several lamin A (autosomal dominant) mutations were stained for the proteins linked to the disorder. As tissue-specific nuclear envelope proteins have been postulated to mediate the tissue-specific pathologies of different nuclear envelopathies, patient samples were also stained for several muscle-specific nuclear membrane proteins. Although linked proteins nesprin 1 and SUN2 and muscle-specific proteins NET5/Samp1 and Tmem214 yielded aberrant distributions in individual patient cells, none exhibited defects through the larger patient panel. Muscle-specific Tmem38A normally appeared in both the nuclear envelope and sarcoplasmic reticulum, but most patient samples exhibited a moderate redistribution favouring the sarcoplasmic reticulum. The absence of striking uniform defects in nuclear envelope protein distribution indicates that such staining will be unavailing for general diagnostics, though it remains possible that specific mutations exhibiting protein distribution defects might reflect a particular clinical variant. These findings further argue that multiple pathways can lead to the generally similar pathologies of this disorder while at the same time the different cellular phenotypes observed possibly may help explain the considerable clinical variation of EDMD.

Abnormal proliferation and spontaneous differentiation of myoblasts from a symptomatic female carrier of X-linked Emery-Dreifuss muscular dystrophy

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X-linked female presenting with EDMD1 not explained by uneven X-inactivation.

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First EDMD blood phenotype with highly lobulated lymphocytes in EDMD1 patient.

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Found high incidence of spontaneous differentiation in cultured patient myoblasts.

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Faster proliferation of emerin-null than emerin-positive EDMD1 patient myoblasts.

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Loss of satellite cells from the above might explain EDMD pathology.

Emery–Dreifuss muscular dystrophy (EDMD) is a neuromuscular disease characterized by early contractures, slowly progressive muscular weakness and life-threatening cardiac arrhythmia that can develop into cardiomyopathy. In X-linked EDMD (EDMD1), female carriers are usually unaffected. Here we present a clinical description and in vitro characterization of a mildly affected EDMD1 female carrying the heterozygous EMD mutation c.174_175delTT; p.Y59* that yields loss of protein. Muscle tissue sections and cultured patient myoblasts exhibited a mixed population of emerin-positive and -negative cells; thus uneven X-inactivation was excluded as causative. Patient blood cells were predominantly emerin-positive, but considerable nuclear lobulation was observed in non-granulocyte cells – a novel phenotype in EDMD. Both emerin-positive and emerin-negative myoblasts exhibited spontaneous differentiation in tissue culture, though emerin-negative myoblasts were more proliferative than emerin-positive cells. The preferential proliferation of emerin-negative myoblasts together with the high rate of spontaneous differentiation in both populations suggests that loss of functional satellite cells might be one underlying mechanism for disease pathology. This could also account for the slowly developing muscle phenotype.

NETs and cell cycle regulation

There are many ways that the nuclear envelope can influence the cell cycle. In addition to roles of lamins in regulating the master cell cycle regulator pRb and nuclear envelope breakdown in mitosis, many other nuclear envelope proteins influence the cell cycle through regulatory or structural functions. Of particular note among these are the nuclear envelope transmembrane proteins (NETs) that appear to influence cell cycle regulation through multiple separate mechanisms. Some NETs and other nuclear envelope proteins accumulate on the mitotic spindle, suggesting functional or structural roles in the cell cycle. In interphase exogenous overexpression of some NETs promotes an increase in G1 populations, while others promote an increase in G2/M populations, sometimes associated with the induction of senescence. Intriguingly, most of the NETs linked to the cell cycle are highly restricted in their tissue expression; thus, their misregulation in cancer could contribute to the many tissue-specific types of cancer.

Destination: inner nuclear membrane

The inner nuclear membrane (INM) of eukaryotic cells is enriched in proteins that are required for nuclear structure, chromosome organization, DNA repair, and transcriptional control. Mislocalization of INM proteins is observed in a wide spectrum of human diseases; however, the mechanism by which INM proteins reach their final destination is poorly understood. In this review we discuss how investigating INM composition, dissecting targeting pathways of conserved INM proteins in multiple systems and analyzing the nuclear transport of viruses and signaling complexes have broadened our knowledge of INM transport to include both nuclear pore complex-dependent and -independent pathways. The study of these INM targeting pathways is important to understanding nuclear organization and in both normal and diseased cells.

Tissue specificity in the nuclear envelope supports its functional complexity

Nuclear envelope links to inherited disease gave the conundrum of how mutations in near-ubiquitous proteins can yield many distinct pathologies, each focused in different tissues. One conundrum-resolving hypothesis is that tissue-specific partner proteins mediate these pathologies. Such partner proteins may have now been identified with recent proteome studies determining nuclear envelope composition in different tissues. These studies revealed that the majority of the total nuclear envelope proteins are tissue restricted in their expression. Moreover, functions have been found for a number these tissue-restricted nuclear envelope proteins that fit with mechanisms proposed to explain how the nuclear envelope could mediate disease, including defects in mechanical stability, cell cycle regulation, signaling, genome organization, gene expression, nucleocytoplasmic transport, and differentiation. The wide range of functions to which these proteins contribute is consistent with not only their involvement in tissue-specific nuclear envelope disease pathologies, but also tissue evolution.

O-Linked β-N-acetylglucosamine (O-GlcNAc) regulates emerin binding to barrier to autointegration factor (BAF) in a chromatin- and lamin B-enriched "niche"

Emerin, a membrane component of nuclear "lamina" networks with lamins and barrier to autointegration factor (BAF), is highly O-GlcNAc-modified ("O-GlcNAcylated") in mammalian cells. Mass spectrometry analysis revealed eight sites of O-GlcNAcylation, including Ser-53, Ser-54, Ser-87, Ser-171, and Ser-173. Emerin O-GlcNAcylation was reduced ~50% by S53A or S54A mutation in vitro and in vivo. O-GlcNAcylation was reduced ~66% by the triple S52A/S53A/S54A mutant, and S173A reduced O-GlcNAcylation of the S52A/S53A/S54A mutant by ~30%, in vivo. We separated two populations of emerin, A-type lamins and BAF; one population solubilized easily, and the other required sonication and included histones and B-type lamins. Emerin and BAF associated only in histone- and lamin-B-containing fractions. The S173D mutation specifically and selectively reduced GFP-emerin association with BAF by 58% and also increased GFP-emerin hyper-phosphorylation. We conclude that β-N-acetylglucosaminyltransferase, an essential enzyme, controls two regions in emerin. The first region, defined by residues Ser-53 and Ser-54, flanks the LEM domain. O-GlcNAc modification at Ser-173, in the second region, is proposed to promote emerin association with BAF in the chromatin/lamin B "niche." These results reveal direct control of a conserved LEM domain nuclear lamina component by β-N-acetylglucosaminyltransferase, a nutrient sensor that regulates cell stress responses, mitosis, and epigenetics.

Nuclear envelope influences on cell-cycle progression

The nuclear envelope is a complex double membrane system that serves as a dynamic interface between the nuclear and cytoplasmic compartments. Among its many roles is to provide an anchor for gene regulatory proteins on its nucleoplasmic surface and for the cytoskeleton on its cytoplasmic surface. Both sets of anchors are proteins called NETs (nuclear envelope transmembrane proteins), embedded respectively in the inner or outer nuclear membranes. Several lines of evidence indicate that the nuclear envelope contributes to cell-cycle regulation. These contributions come from both inner and outer nuclear membrane NETs and appear to operate through several distinct mechanisms ranging from sequestration of gene-regulatory proteins to activating kinase cascades.

Cancer biology and the nuclear envelope: a convoluted relationship

Although its properties have long been used for both typing and prognosis of various tumors, the nuclear envelope (NE) itself and its potential roles in tumorigenesis are only beginning to be understood. Historically viewed as merely a protective barrier, the nuclear envelope is now linked to a wide range of functions. Nuclear membrane proteins connect the nucleus to the cytoskeleton on one side and to chromatin on the other. Several newly identified nuclear envelope functions associated with these connections intersect with cancer pathways. For example, the nuclear envelope could affect genome stability by tethering chromatin. Some nuclear envelope proteins affect cell cycle regulation by directly binding to the master regulator pRb, others by interacting with TGF-ß and Smad signaling cascades, and others by affecting the mitotic spindle. Finally, the NE directly affects cytoskeletal organization and can also influence cell migration in metastasis. In this review we discuss the link between the nuclear envelope and cellular defects that are common in cancer cells, and we show that NE proteins are often aberrantly expressed in tumors. The NE represents a potential reservoir of diagnostic and prognostic markers in cancer.

Sumoylated protein tyrosine phosphatase 1B localizes to the inner nuclear membrane and regulates the tyrosine phosphorylation of emerin

Protein tyrosine phosphatase (PTP)1B is an abundant non-transmembrane enzyme that plays a major role in regulating insulin and leptin signaling. Recently, we reported that PTP1B is inhibited by sumoylation, and that sumoylated PTP1B accumulates in a perinuclear distribution, consistent with its known localization in the endoplasmic reticulum (ER) and the contiguous outer nuclear membrane. Here, we report that, in addition to its localization at the ER, PTP1B also is found at the inner nuclear membrane, where it is heavily sumoylated. We also find that PTP1B interacts with emerin, an inner nuclear membrane protein that is known to be tyrosine phosphorylated, and that PTP1B expression levels are inversely correlated with tyrosine phosphorylation levels of emerin. PTP1B sumoylation greatly increases as cells approach mitosis, corresponding to the stage where tyrosine phosphorylation of emerin is maximal. In addition, expression of a non-sumoylatable mutant of PTP1B greatly reduced levels of emerin tyrosine phosphorylation. These results suggest that PTP1B regulates the tyrosine phosphorylation of a key inner nuclear membrane protein in a sumoylation- and cell-cycle-dependent manner.

A flow cytometry-based screen of nuclear envelope transmembrane proteins identifies NET4/Tmem53 as involved in stress-dependent cell cycle withdrawal

Disruption of cell cycle regulation is one mechanism proposed for how nuclear envelope protein mutation can cause disease. Thus far only a few nuclear envelope proteins have been tested/found to affect cell cycle progression: to identify others, 39 novel nuclear envelope transmembrane proteins were screened for their ability to alter flow cytometry cell cycle/DNA content profiles when exogenously expressed. Eight had notable effects with seven increasing and one decreasing the 4N∶2N ratio. We subsequently focused on NET4/Tmem53 that lost its effects in p53^−/− cells and retinoblastoma protein-deficient cells. NET4/TMEM53 knockdown by siRNA altered flow cytometry cell cycle/DNA content profiles in a similar way as overexpression. NET4/TMEM53 knockdown did not affect total retinoblastoma protein levels, unlike nuclear envelope-associated proteins Lamin A and LAP2α. However, a decrease in phosphorylated retinoblastoma protein was observed along with a doubling of p53 levels and a 7-fold increase in p21. Consequently cells withdrew from the cell cycle, which was confirmed in MRC5 cells by a drop in the percentage of cells expressing Ki-67 antigen and an increase in the number of cells stained for ß-galactosidase. The ß-galactosidase upregulation suggests that cells become prematurely senescent. Finally, the changes in retinoblastoma protein, p53, and p21 resulting from loss of NET4/Tmem53 were dependent upon active p38 MAP kinase. The finding that roughly a fifth of nuclear envelope transmembrane proteins screened yielded alterations in flow cytometry cell cycle/DNA content profiles suggests a much greater influence of the nuclear envelope on the cell cycle than is widely held.

The nuclear envelopathies and human diseases

The nuclear envelope (NE) consists of two membrane layers that segregate the nuclear from the cytoplasmic contents. Recent progress in our understanding of nuclear-lamina associated diseases has revealed intriguing connections between the envelope components and nuclear processes. Here, we review the functions of the nuclear envelope in chromosome organization, gene expression, DNA repair and cell cycle progression, and correlate deficiencies in envelope function with human pathologies.

Defects in cell spreading and ERK1/2 activation in fibroblasts with lamin A/C mutations

In-frame mutations in nuclear lamin A/C lead to a multitude of tissue-specific degenerative diseases known as the 'laminopathies'. Previous studies have demonstrated that lamin A/C-null mouse fibroblasts have defects in cell polarisation, suggesting a role for lamin A/C in nucleo-cytoskeletal-cell surface cross-talk. However, this has not been examined in patient fibroblasts expressing modified forms of lamin A/C. Here, we analysed skin fibroblasts from 3 patients with Emery-Dreifuss muscular dystrophy and from 1 with dilated cardiomyopathy. The emerin-lamin A/C interaction was impaired in each mutant cell line. Mutant cells exhibited enhanced cell proliferation, collagen-dependent adhesion, larger numbers of filopodia and smaller cell spread size, compared with control cells. Furthermore, cell migration, speed and polarization were elevated. Mutant cells also showed an enhanced ability to contract collagen gels at early time points, compared with control cells. Phosphotyrosine measurements during cell spreading indicated an initial temporal lag in ERK1/2 activation in our mutant cells, followed by hyper-activation of ERK1/2 at 2 h post cell attachment. Deregulated ERK1/2 activation is linked with cardiomyopathy, cell spreading and proliferation defects. We conclude that a functional emerin-lamin A/C complex is required for cell spreading and proliferation, possibly acting through ERK1/2 signalling.

Characterization and functional analysis of a slow cycling stem cell-like subpopulation in pancreas adenocarcinoma

Evidence suggests that multiple tumors, including pancreatic adenocarcinoma, display heterogeneity in parameters that are critical for tumor formation, progression and metastasis. Understanding heterogeneity in solid tumors is increasingly providing a plethora of new diagnostic and therapeutic approaches. In this study, a particular focus was put on identifying a subpopulation of stem cell-like, slow cycling tumor cells in a pancreas adenocarcinoma cell lines. Using a label retention technique a subpopulation of slow cycling cells (DiI+/SCC) was identified and further evaluated in the BxPC-3 and Panc03.27 cell lines. These slowly cycling cells managed to retain the lipophilic labeling dye DiI, while the bulk of the cells (>94%) did not. The DiI+/SCC population, showed only a partial overlap with the CSC markers CD24⁺/CD44⁺, CD133⁺ and ALDH but they survived chemotherapeutic treatment, and were able to recreate the initial heterogeneous tumor cell population. DiI+/SCCs exhibited an increased invasive potential as compared with their non-label retaining, faster cycling cells (DiI−/FCC). They also had increased tumorigenic potential and morphological changes resembling cells that have undergone an epithelial to mesenchymal transition (EMT). Analysis of DiI+/SCC cells by real time PCR revealed a selective up-regulation of tell tale components of the Hedgehog/TGFβ pathways, as well as a down-regulation of EGFR, combined with a shift in crucial components implied in EMT. The presented findings offer an expanded mechanistic understanding that associates tumor initiating potential with cycling speed and EMT in pancreatic cancer cell lines.

Nuclear motors and nuclear structures containing A-type lamins and emerin: is there a functional link?

Rapid interphase chromosome territory repositioning appears to function through the action of nuclear myosin and actin, in a nuclear motor complex. We have found that chromosome repositioning when cells leave the cell cycle is not apparent in cells that have mutant lamin A or that are lacking emerin. We discuss the possibility that there is a functional intranuclear complex comprising four proteins: nuclear actin, lamin A, emerin and nuclear myosin. If any of the components are lacking or aberrant, then the nuclear motor complex involved in moving chromosomes or genes will be dysfunctional, leading to an inability to move chromosomes in response to signalling events.

Does satellite cell dysfunction contribute to disease progression in Emery–Dreifuss muscular dystrophy?

Muscular dystrophies comprise at least 34 conditions, characterized by progressive skeletal muscle weakness and degeneration. The loci affected include mutations in both muscle-specific genes and genes that are more widely expressed such as LMNA and EMD, responsible for EDMD (Emery–Dreifuss muscular dystrophy). LMNA encodes A-type lamins, whereas EMD encodes emerin, both located in the nuclear envelope. Mutation or loss of A-type lamins or emerin in the terminally differentiated myonuclei of muscle fibres results in muscle damage. Importantly, since LMNA and EMD are also expressed by the resident skeletal muscle stem cells, the satellite cells, the mutations that cause muscle damage may also directly compromise the regenerative response. Thus EDMD is different from dystrophic conditions such as Duchenne muscular dystrophy, where the mutated gene is only expressed in the muscle fibres. In this brief review, we examine the evidence that myoblasts carrying EDMD-causing mutations are compromised, and discuss the possibility that such dysfunction results in reduced efficiency of muscle regeneration, so actively contributes to disease progression.

Identification of the genes specifically expressed in orally tolerized T cells

Oral tolerance is the systemic immunological unresponsiveness that occurs after feeding protein antigens. Its physiological role is thought to be the prevention of hypersensitivity to food antigens, and its therapeutic use to treat inflammatory diseases has been suggested. Although it has been shown that CD4(+) T cells mediate oral tolerance, the precise molecular mechanisms remain unclear. In the present study, we employed suppression subtractive hybridization and identified 10 genes specifically expressed in orally tolerized T cells. These included genes that were interesting in terms of their putative functions in the negative regulation of T cell activation, e.g. Culin 1, LAX, and Zfhx1b, as well as four genes that encoded unknown proteins. We further investigated the expression of these genes in hyporesponsive T cells induced in vitro (in vitro anergized T cells). We found that six of the 10 genes were highly expressed in these cells, and kinetic studies suggested that one was associated with the induction of anergy, while the other five were associated with the maintenance of anergy. The remaining 4 genes that were not expressed in in vitro anergized T cells are also of interest as they may play a specific role in in vivo T cell tolerance. Functional analysis of these genes should help to understand the complex mechanisms underlying the induction and maintenance of oral tolerance, and moreover, in vivo immune tolerance in general.

"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.

Enhanced expression of the nuclear envelope LAP2 transcriptional repressors in normal and malignant activated lymphocytes

Extensive research in recent years has broadened the functions of nuclear envelope proteins beyond simply stabilizing the nucleus architecture. Particularly, integral nuclear membrane proteins, such as the alternative spliced isoforms of lamina-associated polypeptide 2 (LAP2), have been shown to be important for the initiation of replication and repression of transcription. The latter is regulated by epigenetic changes, induced by the binding of LAP2beta to histone deacetylase-3 (HDAC3), resulting in histone H4 deacetylation. Involvement of nuclear envelope proteins in pathological proliferative conditions, mainly those involving abnormal recruitment and activation of HDACs, is still unknown. In this paper, we show that various nuclear envelope proteins are highly expressed in normal and malignant activated lymphocytes. Specifically, rapidly replicating cells of various hematological malignancies highly express LAP2beta, while slowly proliferating malignant cells of chronic malignant hematological diseases do not. Taking together the elevated expression of LAP2beta in highly proliferative malignant cells with its known ability to modify histones through binding with HDAC3 raises the possibility of its role in hematological malignancies involving aberrant activity of HDAC3. Based on our presented results, we believe that the LAP2-HDAC regulatory pathway should be studied as a new target for rational therapy.

LEM‐Domain Proteins: New Insights into Lamin‐Interacting Proteins

LEM-domain proteins present a growing family of nonrelated inner nuclear membrane and intranuclear proteins, including emerin, MAN1, LEM2, several alternatively spliced isoforms of LAP2, and various uncharacterized proteins in higher eukaryotes as well as the Drosophila-specific proteins otefin and Bocksbeutel. LEM-domain proteins are involved in diverse cellular processes including replication and cell cycle control, chromatin organization and nuclear assembly, the regulation of gene expression and signaling pathways, as well as retroviral infection. Genetic analyses in different model organisms reveal new insights into the various functions of LEM-domain proteins, lamins, and their involvement in laminopathic diseases. All these findings as well as previously proposed ideas and models have been summarized to broaden our view of this exciting protein family.

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

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

Identification of tyrosine-phosphorylation sites in the nuclear membrane protein emerin

Although several proteins undergo tyrosine phosphorylation at the nuclear envelope, we achieved, for the first time, the identification of tyrosine-phosphorylation sites of a nuclear-membrane protein, emerin, by applying two mass spectrometry-based techniques. With a multiprotease approach combined with highly specific phosphopeptide enrichment and nano liquid chromatography tandem mass spectrometry analysis, we identified three tyrosine-phosphorylation sites, Y-75, Y-95, and Y-106, in mouse emerin. Stable isotope labeling with amino acids in cell culture revealed phosphotyrosines at Y-59, Y-74, Y-86, Y-161, and Y-167 of human emerin. The phosphorylation sites Y-74/Y-75 (human/mouse emerin), Y-85/Y-86, Y-94/Y-95, and Y-105/Y-106 are located in regions previously shown to be critical for interactions of emerin with lamin A, actin or the transcriptional regulators GCL and Btf, while the residues Y-161 and Y-167 are in a region linked to binding lamin-A or actin. Tyrosine Y-94/Y-95 is located adjacent to a five-residue motif in human emerin, whose deletion has been associated with X-linked Emery-Dreifuss muscle dystrophy.

Disease severity in dominant Emery Dreifuss is increased by mutations in both emerin and desmin proteins

Individuals with the same genetic disorder often show remarkable differences in clinical severity, a finding generally attributed to the genetic background. We identified two patients with genetically proven Emery-Dreifuss muscular dystrophy (EDMD) who followed an unusual course and had uncommon clinicopathological findings. We hypothesized digenic inheritance and looked for additional molecular explanations. Mutations in additional separate genes were identified in both patients. The first patient was a member of a family with molecularly proven X-linked EDMD. However, the clinical features were unusually severe for this condition in the propositus: he presented at 2.5 years with severe proximal weakness and markedly elevated serum creatine kinase. Muscle weakness rapidly progressed, leading to loss of independent ambulation by the age of 12. In addition, the patient developed cardiac conduction system disease requiring pacing at the age of 11 and severe dilated cardiomyopathy in the early teens. Despite pacing, he had several syncopal episodes attributed to ventricular dysrhythmias. As these resemble the cardiac features of patients with the autosomal dominant variant of EDMD, we examined the lamin A/C gene, identifying a de-novo mutation in the propositus. The second patient had a cardioskeletal myopathy, similar to his mother who had died more than 20 years previously. Because of the dominant family history, a laminopathy was suspected and a mutation in exon 11 of the LMNA gene was identified. This mutation, however, was not present in his mother, but instead, surprisingly, was identified in his virtually asymptomatic father. Unusual accumulations of desmin found in the cardiac muscle of the propositus prompted us to examine the desmin gene in this patient, and in so doing, we identified a desmin mutation, in addition to the LMNA mutation in the propositus. These cases suggest that separate mutations in related proteins that are believed to interact, or that represent different parts of a presumed functional pathway, may synergistically contribute to disease severity in autosomal dominant EDMD. Furthermore, digenic inheritance may well contribute to the clinical severity of many other neuromuscular disorders.

Lamins A and C are differentially dysfunctional in autosomal dominant Emery-Dreifuss muscular dystrophy

Mutations in the LMNA gene, which encodes nuclear lamins A and C by alternative splicing, can give rise to Emery-Dreifuss muscular dystrophy. The mechanism by which lamins A and C separately contribute to this molecular phenotype is unknown. To address this question we examined ten LMNA mutations exogenously expressed as lamins A and C in COS-7 cells. Eight of the mutations when expressed in lamin A, exhibited a range of nuclear mislocalisation patterns. However, two mutations (T150P and delQ355) almost completely relocated exogenous lamin A from the nuclear envelope to the cytoplasm, disrupted nuclear envelope reassembly following cell division and altered the protein composition of the mid-body. In contrast, exogenously expressed DsRed2-tagged mutant lamin C constructs were only inserted into the nuclear lamina if co-expressed with any EGFP-tagged lamin A construct, except with one carrying the T150P mutation. The T150P, R527P and L530P mutations reduced the ability of lamin A, but not lamin C from binding to emerin. These data identify specific functional roles for the emerin-lamin C- and emerin-lamin A- containing protein complexes and is the first report to suggest that the A-type lamin mutations may be differentially dysfunctional for the same LMNA mutation.

Abnormal nuclear shape and impaired mechanotransduction in emerin-deficient cells

Emery-Dreifuss muscular dystrophy can be caused by mutations in the nuclear envelope proteins lamin A/C and emerin. We recently demonstrated that A-type lamin-deficient cells have impaired nuclear mechanics and altered mechanotransduction, suggesting two potential disease mechanisms (Lammerding, J., P.C. Schulze, T. Takahashi, S. Kozlov, T. Sullivan, R.D. Kamm, C.L. Stewart, and R.T. Lee. 2004. J. Clin. Invest. 113:370–378). Here, we examined the function of emerin on nuclear mechanics and strain-induced signaling. Emerin-deficient mouse embryo fibroblasts have abnormal nuclear shape, but in contrast to A-type lamin-deficient cells, exhibit nuclear deformations comparable to wild-type cells in cellular strain experiments, and the integrity of emerin-deficient nuclear envelopes appeared normal in a nuclear microinjection assay. Interestingly, expression of mechanosensitive genes in response to mechanical strain was impaired in emerin-deficient cells, and prolonged mechanical stimulation increased apoptosis in emerin-deficient cells. Thus, emerin-deficient mouse embryo fibroblasts have apparently normal nuclear mechanics but impaired expression of mechanosensitive genes in response to strain, suggesting that emerin mutations may act through altered transcriptional regulation and not by increasing nuclear fragility.

Diagnostic immunohistochemistry in neuromuscular disorders

Most neuromuscular disorders display only non-specific myopathological features in routine histological preparations. However, a number of proteins, including sarcolemmal, sarcomeric, and nuclear proteins as well as enzymes with defects responsible for neuromuscular disorders, have been identified during the past two decades, allowing a more specific and firm diagnosis of muscle diseases. Identification of protein defects relies predominantly on immunohistochemical preparations and on Western blot analysis. While immunohistochemistry is very useful in identifying abnormal expression of primary protein abnormalities in recessive conditions, it is less helpful in detecting primary defects in dominantly inherited disorders. Abnormal immunohistochemical expression patterns can be confirmed by Western blot analysis which may also be informative in dominant disorders, although its role has yet to be established. Besides identification of specific protein defects, immunohistochemistry is also helpful in the differentiation of inflammatory myopathies by subtyping cellular infiltrates and demonstrating up-regulation of subtle immunological parameters such as cell adhesion molecules. The role of immunohistochemistry in denervating disorders, however, remains controversial in the absence of a reliable marker of muscle fibre denervation. Nevertheless, as well as the diagnostic value of immunocytochemical analysis it may also widen understanding of muscle fibre pathology as well as help in the development of therapeutic strategies.

Nuclear envelopathies--raising the nuclear veil

The nuclear envelope separates the chromosomes from cytoplasm in eukaryotic cells and consists of three main domains: inner and outer nuclear membranes and nuclear pore complexes. The inner nuclear membrane maintains close associations with the underlying chromatin and nuclear lamina. For many years, the nuclear envelope was thought to function mainly as an architectural stabilizer of the nucleus, participating in assembly and disassembly processes during mitosis. However, recent findings demonstrate that nuclear envelope proteins are involved in fundamental nuclear functions, such as gene transcription and DNA replication, and that inherited or de novo mutated proteins cause human diseases, termed "nuclear envelopathies." These findings emphasize the importance of understanding the functions of this cellular domain, in both physiologic and pathologic states. To date, mutations in the genes encoding the nuclear envelope proteins emerin, MAN1, lamin A/C, and lamin B receptor were found to cause nuclear envelopathies. The diseases that are caused by mutations in LMNA gene are collectively called "laminopathies." Nuclear envelopathies have diverse clinical phenotypes, ranging from cardiac and skeletal myopathies to partial lipodystrophy, peripheral neuropathy, and premature aging. This raises the question of how do such ubiquitously expressed proteins give rise to tissue-specific disease phenotypes. One possible explanation is the involvement of nuclear envelope proteins in the regulation of gene transcription, a novel mechanism that has been the focus of research in our lab in recent years. In this review, we describe recent discoveries in the field of nuclear envelopathies and discuss current proposed pathophysiological mechanisms underlying these diseases.

Silencing lamin A/C in human endometrial stromal cells: a model to investigate endometrial gene function and regulation

Silencing of a target mRNA by small interfering RNA (siRNA) has emerged as a new and powerful tool to study gene function, and post-transcriptional gene silencing can now be accomplished with 21-23 nucleotide RNA that mediate sequence-specific mRNA degradation. In the current study we employed lamin A/C siRNA to silence lamin A/C expression in cultured human endometrial stromal cells and investigated downstream cellular markers for proof of concept. Human endometrial stromal cells from three subjects were transfected with lamin A/C siRNA or non-silencing fluorescein-labelled siRNA, and flow cytometric analysis revealed 95-98% transfection efficiency after 6 h of treatment. RT-PCR and quantitative RT-PCR were used to measure mRNA degradation of lamin A/C, and 75-88% silencing was observed 48 h post-transfection. Western blotting and immunocytochemistry confirmed corresponding decrease in lamin A/C protein within 48 h of gene silencing. The downstream effect of lamin A/C silencing was investigated by immunocytochemical analysis of the cellular localization of the protein, emerin, an important component of the nuclear lamina and known to be regulated by lamin expression. Marked displacement of emerin from the nuclear lamina to the cytoplasm was observed when lamin A/C was silenced in human endometrial stromal cells, confirming functional silencing of lamin A/C resulting in a nuclear lamina assembly defect. Silencing target mRNA by siRNA in human endometrial stromal cells can be more broadly applied to investigate the function and regulation of other genes in this cell type, and the methodology and data presented herein strongly support the more widespread use of this powerful tool in endometrial biology research.

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

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