Colocalization of emerin and lamins in interphase nuclei and changes during mitosis

Expansion microscopy of zebrafish for neuroscience and developmental biology studies

Expansion microscopy (ExM) allows scalable imaging of preserved 3D biological specimens with nanoscale resolution on fast diffraction-limited microscopes. Here, we explore the utility of ExM in the larval and embryonic zebrafish, an important model organism for the study of neuroscience and development. Regarding neuroscience, we found that ExM enabled the tracing of fine processes of radial glia, which are not resolvable with diffraction-limited microscopy. ExM further resolved putative synaptic connections, as well as molecular differences between densely packed synapses. Finally, ExM could resolve subsynaptic protein organization, such as ring-like structures composed of glycine receptors. Regarding development, we used ExM to characterize the shapes of nuclear invaginations and channels, and to visualize cytoskeletal proteins nearby. We detected nuclear invagination channels at late prophase and telophase, potentially suggesting roles for such channels in cell division. Thus, ExM of the larval and embryonic zebrafish may enable systematic studies of how molecular components are configured in multiple contexts of interest to neuroscience and developmental biology.

Nanoscale invaginations of the nuclear envelope: Shedding new light on wormholes with elusive function

Recent advances in fluorescence microscopy have opened up new possibilities to investigate chromosomal and nuclear 3D organization on the nanoscale. We here discuss their potential for elucidating topographical details of the nuclear lamina. Single molecule localization microscopy (SMLM) in combination with immunostainings of lamina proteins readily reveals tube-like invaginations with a diameter of 100-500 nm. Although these invaginations have been established as a frequent and general feature of interphase nuclei across different cell types, their formation mechanism and function have remained largely elusive. We critically review the current state of research, propose possible connections to lamina associated domains (LADs), and revisit the discussion about the potential role of these invaginations for accelerating mRNA nuclear export. Illustrative studies using 3D super-resolution imaging are shown and will be instrumental to decipher the physiological role of these nanoscale invaginations.

Muscular Dystrophy Mutations Impair the Nuclear Envelope Emerin Self-assembly Properties

More than 100 genetic mutations causing X-linked Emery-Dreifuss muscular dystrophy have been identified in the gene encoding the integral inner nuclear membrane protein emerin. Most mutations are nonsense or frameshift mutations that lead to the absence of emerin in cells. Only very few cases are due to missense or short in-frame deletions. Molecular mechanisms explaining the corresponding emerin variants' loss of function are particularly difficult to identify because of the mostly intrinsically disordered state of the emerin nucleoplasmic region. We now demonstrate that this EmN region can be produced as a disordered monomer, as revealed by nuclear magnetic resonance, but rapidly self-assembles in vitro. Increases in concentration and temperature favor the formation of long curvilinear filaments with diameters of approximately 10 nm, as observed by electron microscopy. Assembly of these filaments can be followed by fluorescence through Thioflavin-T binding and by Fourier-transform Infrared spectrometry through formation of β-structures. Analysis of the assembly properties of five EmN variants reveals that del95-99 and Q133H impact filament assembly capacities. In cells, these variants are located at the nuclear envelope, but the corresponding quantities of emerin-emerin and emerin-lamin proximities are decreased compared to wild-type protein. Furthermore, variant P183H favors EmN aggregation in vitro, and variant P183T provokes emerin accumulation in cytoplasmic foci in cells. Substitution of residue Pro183 might systematically favor oligomerization, leading to emerin aggregation and mislocalization in cells. Our results suggest that emerin self-assembly is necessary for its proper function and that a loss of either the protein itself or its ability to self-assemble causes muscular dystrophy.

Emery-Dreifuss muscular dystrophy mutations impair TRC40-mediated targeting of emerin to the inner nuclear membrane

Emerin is a tail-anchored protein that is found predominantly at the inner nuclear membrane (INM), where it associates with components of the nuclear lamina. Mutations in the emerin gene cause Emery-Dreifuss muscular dystrophy (EDMD), an X-linked recessive disease. Here, we report that the TRC40/GET pathway for post-translational insertion of tail-anchored proteins into membranes is involved in emerin-trafficking. Using proximity ligation assays, we show that emerin interacts with TRC40 in situ. Emerin expressed in bacteria or in a cell-free lysate was inserted into microsomal membranes in an ATP- and TRC40-dependent manner. Dominant-negative fragments of the TRC40-receptor proteins WRB and CAML (also known as CAMLG) inhibited membrane insertion. A rapamycin-based dimerization assay revealed correct transport of wild-type emerin to the INM, whereas TRC40-binding, membrane integration and INM-targeting of emerin mutant proteins that occur in EDMD was disturbed. Our results suggest that the mode of membrane integration contributes to correct targeting of emerin to the INM.

Characterization of unfolding mechanism of human lamin A Ig fold by single-molecule force spectroscopy-implications in EDMD

A- and B-type lamins are intermediate filament proteins constituting the nuclear lamina underneath the nuclear envelope thereby conferring proper shape and mechanical rigidity to the nucleus. Lamin proteins are also shown to be related diversely to basic nuclear processes. More than 400 mutations in human lamin A protein alone have been reported to produce at least 11 different disease conditions jointly termed as laminopathies. These mutations in lamin A are scattered throughout its helical rod domain, as well as the C-terminal domain containing the conserved Ig-fold region. The commonality of phenotypes in all these diseases is characterized by misshapen nuclei of the affected tissues which might stem from altered rigidity of the supporting lamina hence lamins. Here we have focused on autosomal dominant Emery-Dreifuss Muscular Dystrophy, one such laminopathy where R453W is the causative mutation located in the Ig domain of lamin A. We have investigated by single-molecule force spectroscopy how a stretching mechanical perturbation senses the destabilizing effect of the mutation in the lamin A Ig domain and compared the mechanoelastic properties of the mutant R453W with that of the wild-type in conjunction with steered molecular dynamics. Furthermore, we have shown the interaction of Ig domain with emerin, another key player and interacting partner in the pathogenesis of EDMD, is disrupted in the R453W mutant. This altered mechanoresistance of Ig domain itself and consequent uncoupling of lamin A-emerin interaction might underlie the altered mechanotransduction properties of EDMD affected nuclei.

KSHV ORF67 encoded lytic protein localizes on the nuclear membrane and alters emerin distribution

p29, a newly identified Kaposi's sarcoma-associated herpesvirus (KSHV) protein, is the product of ORF67, the positional homolog of the conserved herpesvirus protein UL34. Like its homologues in other herpesviruses, p29 is expressed early during viral lytic cycle, and is localized on the nuclear rim. Upon chemical induction of viral replication in primary effusion lymphoma cells, p29 interacts with p33, encoded by ORF69, the positional homolog of the conserved herpesvirus protein UL31, and both proteins colocalize on the nuclear membrane. IFA and biochemical analysis of infected or transfected cells showed that p29 expression resulted in delocalization and hyperphosphorylation of emerin, whereas other nuclear lamin associated proteins, such as LUMA, LB1 and LBR were not affected. Mislocalization of emerin was robustly increased upon combined expression of p29 and p33, suggesting that emerin destabilization might represent the first step in nuclear lamina disassembling, a process necessary for nucleocapsid maturation.

LINC complexes in health and disease

The cell nucleus communicates with the rest of the cell via nucleo/cytoplasmic transport of proteins and RNA through the nuclear pores. Direct mechanical links between the nucleus and the cytoplasm have recently emerged in the form of LINC (Linkers of the nucleoskeleton to the cytoskeleton) protein complexes. A LINC complex consists of four components. At its core are an inner nuclear membrane (INM) transmembrane protein and an outer nuclear membrane (ONM) transmembrane protein which physically interact with each other in the lumen of the NE. The INM LINC component interacts on the nucleoplasmic side with either the lamina or with an INM-associated protein. The ONM LINC component on the other hand contacts on the cytoplasmatic side a component of the cytoskeleton. This review highlights the components of LINC complexes and their emerging roles in mechanotransduction, nuclear migration, chromosome positioning, signaling, meiosis, cytoskeletal organization and human disease.

SUN-domain and KASH-domain proteins during development, meiosis and disease

SUN-domain proteins interact directly with KASH-domain proteins to form protein complexes that connect the nucleus to every major cytoskeleton network. SUN-KASH protein complexes are also required for attaching centrosomes to the nuclear periphery and for alignment of homologous chromosomes, their pairing and recombination in meiosis. Other functions that require SUN-domain proteins include the regulation of apoptosis and maturation and survival of the germline. Laminopathic diseases affect the distribution of the SUN-KASH complexes, and mutations in KASH-domain proteins can cause Emery Dreifuss muscular dystrophy and recessive cerebellar ataxia. This review describes our current knowledge of the role of SUN-KASH domain protein complexes during development, meiosis and disease.

Dysfunctional connections between the nucleus and the actin and microtubule networks in laminopathic models

Laminopathies encompass a wide array of human diseases associated to scattered mutations along LMNA, a single gene encoding A-type lamins. How such genetic alterations translate to cellular defects and generate such diverse disease phenotypes remains enigmatic. Recent work has identified nuclear envelope proteins--emerin and the linker of the nucleoskeleton and cytoskeleton (LINC) complex--which connect the nuclear lamina to the cytoskeleton. Here we quantitatively examine the composition of the nuclear envelope, as well as the architecture and functions of the cytoskeleton in cells derived from two laminopathic mouse models, including Hutchinson-Gilford progeria syndrome (Lmna(L530P/L530P)) and Emery-Dreifuss muscular dystrophy (Lmna(-/-)). Cells derived from the overtly aphenotypical model of X-linked Emery-Dreifuss muscular dystrophy (Emd(-/y)) were also included. We find that the centrosome is detached from the nucleus, preventing centrosome polarization in cells under flow--defects that are mediated by the loss of emerin from the nuclear envelope. Moreover, while basal actin and focal adhesion structure are mildly affected, RhoA activation, cell-substratum adhesion, and cytoplasmic elasticity are greatly lowered, exclusively in laminopathic models in which the LINC complex is disrupted. These results indicate a new function for emerin in cell polarization and suggest that laminopathies are not directly associated with cells' inability to polarize, but rather with cytoplasmic softening and weakened adhesion mediated by the disruption of the LINC complex across the nuclear envelope.

A novel role for the nuclear membrane protein emerin in association of the centrosome to the outer nuclear membrane

The type II inner nuclear membrane protein emerin is a component of the LINC complex that connects the nuclear lamina to the actin cytoskeleton. In emerin-null or -deficient human dermal fibroblasts we find that the centrosome is detached from the nucleus. Moreover, following siRNA knockdown of emerin in wild-type fibroblasts, the centrosome also becomes detached from the nucleus. We show that emerin interacts with tubulin, and that nocadozole-treated wild-type cells phenocopy the detached centrosome characteristic of emerin-null/deficient cells. We also find that a significant fraction of emerin is located at the outer nuclear membrane and peripheral ER, where it interacts directly with the centrosome. Our data provide the first evidence in mammalian cells as to the nature of the linkage of the centrosome, and therefore the tubulin cytoskeleton, with the outer nuclear membrane.

The DUX4 gene at the FSHD1A locus encodes a pro-apoptotic protein

Facioscapulohumeral muscular dystrophy (FSHD) patients carry contractions of the D4Z4-tandem repeat array on chromosome 4q35. Decrease in D4Z4 copy number is thought to alter a chromatin structure and activate expression of neighboring genes. D4Z4 contains a putative double-homeobox gene called DUX4. We identified DUX4 mRNAs in cells transfected with genomic fragments containing the DUX4 gene. Using RT-PCR we also recognized expressed DUX4 mRNAs in primary FSHD myoblasts. Polyclonal antibodies raised against specific DUX4 peptides detected the DUX4 protein in cells transfected with D4Z4 elements. DUX4 localizes in the nucleus of cells transfected with CMV-DUX4 expression vectors. A DUX4-related protein is endogenously expressed in nuclei of adult and fetal human rhabdomyosarcoma cell lines. Overexpression of DUX4 induces cell death, induces caspase 3/7 activity and alters emerin distribution at the nuclear envelope. We propose that DUX4-mediated cell death contributes to the pathogenic pathway in FSHD.

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

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

Primary laminopathy fibroblasts display altered genome organization and apoptosis

A number of diseases associated with specific tissue degeneration and premature aging have mutations in the nuclear envelope proteins A-type lamins or emerin. Those diseases with A-type lamin mutation are inclusively termed laminopathies. Due to various hypothetical roles of nuclear envelope proteins in genome function we investigated whether alterations to normal genomic behaviour are apparent in cells with mutations in A-type lamins and emerin. Even though the distributions of these proteins in proliferating laminopathy fibroblasts appear normal, there is abnormal nuclear positioning of both chromosome 18 and 13 territories, from the nuclear periphery to the interior. This genomic organization mimics that found in normal nonproliferating quiescent or senescent cells. This finding is supported by distributions of modified pRb in the laminopathy cells. All laminopathy cell lines tested and an X-linked Emery-Dreifuss muscular dystrophy cell line also demonstrate increased incidences of apoptosis. The most extreme cases of apoptosis occur in cells derived from diseases with mutations in the tail region of the LMNA gene, such as Dunningan-type familial partial lipodystrophy and mandibuloacral dysplasia, and this correlates with a significant level of micronucleation in these cells.

Multiple roles for emerin: implications for Emery-Dreifuss muscular dystrophy

X-linked Emery-Dreifuss muscular dystrophy (X-EDMD) is inherited through mutations in EMD, which encodes a nuclear membrane protein named emerin. Emerin is expressed in most cells, but EDMD strikes specific tissues. This review summarizes growing evidence that emerin has roles in both tissue-specific gene regulation and the mechanical integrity of the nucleus and discusses how these roles might impact EDMD.

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.

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

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

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

Staurosporine treatment and serum starvation promote the cleavage of emerin in cultured mouse myoblasts: involvement of a caspase-dependent mechanism

Emerin is a nuclear membrane-anchored protein which is absent or mutated in patients affected by Emery-Dreifuss muscular dystrophy. In this study, we induced apoptosis in cultured mouse myoblasts to evaluate emerin fate during the nuclear destabilization involved in programmed cell death. Emerin proteolysis was observed in myocytes during the apoptotic process. Myoblast apoptosis and emerin degradation were associated with chromatin compaction and detachment from the nuclear lamina, as detected by electron microscopy. In vivo specific inhibition of caspase 3 or caspase 6 activity completely abolished emerin proteolysis. These results show that the process of programmed cell death in muscle cells leads to emerin proteolysis, which appears to be related to caspase 6 activation and to cleavage of other nuclear envelope proteins, that share sequence homologies or functional features with emerin.

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

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

How does a g993t mutation in the emerin gene cause Emery-Dreifuss muscular dystrophy?

X-linked Emery-Dreifuss muscular dystrophy is usually caused by absence of the nuclear membrane protein, emerin, due to nonsense mutations or deletions, but a few missense mutations also exist. A pathogenic g993t mutation causes a Q133H change in the nuclear targeting region of emerin, but it may also reduce emerin levels by affecting mRNA splicing. We have introduced the g993t mutation by in vitro mutagenesis and studied the effect of Q133H on nuclear targeting by transfection of COS-7 cells. No qualitative or quantitative differences in nuclear targeting were observed between normal and mutant emerin. Quantitative BIAcore analysis showed no significant change in lamin A binding to emerin when the mutation was present. We conclude that Q133 is not essential for nuclear targeting of emerin or its interaction with lamin A. Reduced emerin levels due to altered splicing or defective interaction with an unidentified binding partner remain possible pathogenic mechanisms.

Structural analysis of emerin, an inner nuclear membrane protein mutated in X-linked Emery-Dreifuss muscular dystrophy

Like Duchenne and Becker muscular dystrophies, Emery-Dreifuss muscular dystrophy (EDMD) is characterized by myopathic and cardiomyopathic abnormalities. EDMD has the particularity of being linked to mutations in nuclear proteins. The X-linked form of EDMD is caused by mutations in the emerin gene, whereas autosomal dominant EDMD is caused by mutations in the lamin A/C gene. Emerin colocalizes with lamin A/C in interphase cells, and binds in vitro to lamin A/C. Recent work suggests that lamin A/C might serve as a receptor for emerin. We have undertaken a structural analysis of emerin, and in particular of its N-terminal domain, which is comprised in the emerin segment critical for binding to lamin A/C. We show that region 2-54 of emerin adopts the LEM fold. This fold was originally described in the two N-terminal domains of another inner nuclear membrane protein called lamina-associated protein 2 (LAP2). The existence of a conserved solvent-exposed surface on the LEM domains of LAP2 and emerin is discussed, as well as the nature of a possible common target.

The R482Q lamin A/C mutation that causes lipodystrophy does not prevent nuclear targeting of lamin A in adipocytes or its interaction with emerin

Most pathogenic missense mutations in the lamin A/C gene identified so far cause autosomal-dominant dilated cardiomyopathy and/or Emery-Dreifuss muscular dystrophy. A few specific mutations, however, cause a disease with remarkably different clinical features: FPLD, or familial partial lipodystrophy (Dunnigan-type), which mainly affects adipose tissue. We have prepared lamin A with a known FPLD mutation (R482Q) by in vitro mutagenesis. Nuclear targeting of lamin A in transfected COS cells, human skeletal muscle cells or mouse adipocyte cell cultures (pre- and post-differentiation) was not detectably affected by the mutation. Quantitative in vitro measurements of lamin A interaction with emerin using a biosensor also showed no effect of the mutation. The results show that the loss of function of R482 in lamin A/C in FPLD does not involve loss of ability to form a nuclear lamina or to interact with the nuclear membrane protein, emerin.

Emerin expression at the early stages of myogenic differentiation

Emerin is an ubiquitous protein localized at the nuclear membrane of most cell types including muscle cells. The protein is absent in most patients affected by the X-linked form of Emery-Dreifuss muscular dystrophy, a disease characterized by slowly progressive muscle wasting and weakness, early contractures of the elbows, Achilles tendons, and post-cervical muscles, and cardiomyopathy. Besides the nuclear localization, emerin cytoplasmic distribution has been suggested in several cell types. We studied the expression and the subcellular distribution of emerin in mouse cultured C2C12 myoblasts and in primary cultures of human myoblasts induced to differentiate or spontaneously differentiating in the culture medium. In differentiating myoblasts transiently transfected with a cDNA encoding the complete emerin sequence, the protein localized at the nuclear rim of all transfected cells and also in the cytoplasm of some myoblasts and myotubes. Cytoplasmic emerin was also observed in detergent-treated myotubes, as determined by electron microscopy observation. Both immunofluorescence and biochemical analysis showed, that upon differentiation of C2C12 cells, emerin expression was decreased in the resting myoblasts but the protein was highly represented in the developing myotubes at the early stage of cell fusion. Labeling with specific markers of myogenesis such as troponin-T and myogenin permitted the correlation of increased emerin expression with the onset of muscle differentiation. These data suggest a role for emerin during proliferation of activated satellite cells and at the early stages of differentiation.

Nuclear proteins and cell death in inherited neuromuscular disease

X-linked Emery-Dreifuss muscular dystrophy is caused by mutations in emerin, a novel nuclear membrane protein. Other major inherited neuromuscular diseases have now also been shown to involve proteins which localize and function at least partly in the cell nucleus. These include lamin A/C in autosomal dominant Emery-Dreifuss muscular dystrophy, SMN in spinal muscular atrophy, SIX5 in myotonic dystrophy, calpain3 in type 2A limb-girdle muscular dystrophy, PABP2 in oculopharyngeal dystrophy, androgen receptor in spinal and bulbar muscular atrophy and the ataxins in hereditary ataxias. This review compares the molecular basis for these various disorders and considers the role of cell death, including apoptosis, in their pathogenesis.

Review: lamina-associated polypeptide 2 isoforms and related proteins in cell cycle-dependent nuclear structure dynamics

The lamina-associated polypeptide (LAP) 2 family comprises up to six alternatively spliced proteins in mammalian cells and three isoforms in Xenopus. LAP2beta is a type II integral protein of the inner nuclear membrane, which binds to lamin B and the chromosomal protein BAF, and may link the nuclear membrane to the underlying lamina and provide docking sites for chromatin. LAP2alpha shares only the N-terminus with the other isoforms and contains a unique C-terminus. It is a nonmembrane protein associated with the nucleoskeleton and may help to organize higher order chromatin structure by interacting with A-lamins and chromosomes. Recent studies using mutant proteins have just begun to unravel functions of LAP2 isoforms during postmitotic nuclear reassembly. LAP2alpha associates with chromosomes via an alpha-specific domain at early stages of assembly, possibly providing a structural framework for chromosome reorganization. The subsequent interaction of both LAP2alpha and LAP2beta with the chromosomal BAF may stabilize chromatin structure and target membranes to the chromosomes. At later stages LAP2 may regulate the assembly of lamins. LAP2 isoforms have been found to share a homologous approximately 40 amino acid long region, the LEM domain, with nuclear membrane proteins MAN1 and emerin, which has been implicated in Emery-Dreifuss muscular dystrophy.

Bcl-2 and Bax proteins are present in interphase nuclei of mammalian cells

The Bcl-2 family of proteins comprises both cell death inhibiting and cell death promoting members, generally believed to be cytoplasmic and predominantly membrane-associated. Like Bcl-2, many Bcl-2-related proteins contain a C-terminal membrane insertion domain and much research is aimed at evaluating the functional role of their localization to the outer membranes of mitochondria, the endoplasmic reticulum, and perinuclear membranes. However, confocal fluorescence microscopy of human breast cancer cells and rat colon cancer cells immunostained with commercial antibodies raised against different epitopes of the anti-apoptotic Bcl-2 and the pro-apoptotic Bax protein revealed that these proteins are not only present in the cellular cytoplasm, but also within interphase nuclei. This was confirmed by Western blot analysis of isolated nuclei. In human cells, certain epitopes of Bcl-2, but not of Bax, were also found to be associated with mitotic chromatin. Anti-estrogen treatment of human breast cancer cells or transfection with antisense bcl-2 led to a reduction in both cytoplasmic and nuclear Bcl-2. Transfection of human bcl-2 and bax into rat cells resulted in cytoplasmic and nuclear Bcl-2 and Bax. This data seems in line with increasing evidence that the role of the Bcl-2 family of proteins should be extended to activities inside the nuclear compartment.

Direct interaction between emerin and lamin A

Emerin is the protein of the inner nuclear membrane that is affected by mutation in X-linked Emery-Dreifuss muscular dystrophy. The autosomal dominant form of the disease is caused by mutations in the lamin A/C gene. Several lines of circumstantial evidence have suggested an interaction of emerin with lamins in the nuclear lamina but direct interaction between the two proteins has not yet been demonstrated. We now demonstrate direct interaction between recombinant emerin and lamin A molecules using biomolecular interaction analysis (BIA) and monoclonal antibodies. An emerin-lamin A interaction system may be related in function to the LAP2-lamin B system at the inner nuclear rim.

Two distal mutations in the gene encoding emerin have profoundly different effects on emerin protein expression

Emerin, the product of the gene responsible for X-linked Emery-Dreifuss muscular dystrophy (EDMD), has a ubiquitous tissue distribution and is localised to the nuclear envelope. We present here the relationship between emerin protein expression, nuclear localization and clinical phenotype for two distal mutations identified in unrelated EDMD patients. The first mutation predicts the replacement of the last eight amino acids of emerin with the addition of 101 amino acids, but no emerin expression is detected. The second mutation, 35 bp upstream from the first mutation, deletes six amino acids from the transmembrane region, but in this case emerin expression is seen. Emerin from this second patient is expressed at reduced levels, mistargeted and has altered biochemical properties compared to wild type emerin. In both cases the clinical phenotype was similar to patients with typical null mutations. We discuss these data in comparison with previous reports of other C-terminal mutations in the emerin gene and suggest that the efficiency of emerin's nuclear membrane localization is affected by the hydrophobicity (and possibly length) of its transmembrane region, and a longer C-terminal tail prevents nuclear localization.

Distribution of emerin during the cell cycle

Human emerin is a nuclear membrane protein that is lost or altered in patients with Emery-Dreifuss muscular dystrophy (EMD). While the protein is expressed in the majority of human tissues analyzed, the pathology predominates in cardiac and skeletal muscles of patients with EMD. Our results show that emerin can be detected by immunocytochemistry and immunoblotting in the nuclear envelope of all vertebrates studied from man to Xenopus. Immunolocalizations and nuclear envelope extraction experiments confirm that emerin possesses properties characteristic for integral membrane proteins of the inner nuclear membrane. Some nuclear envelope proteins are localized also in annulate lamellae (AL), i.e. cytoplasmic flattened membrane cisternae penetrated by pore complexes. To verify whether emerin is contained in these membrane stacks, we have induced the formation of AL by exposure of rat cells (line RV-SMC) to sublethal doses of the antimitotic drug vinblastine sulfate and found that emerin is present in the nuclear envelope, but is absent from AL. In contrast to the homogeneous distribution of emerin in the nuclear envelope of interphase cells, this protein shows a focal accumulation in the nuclear membranes of late telophase cells. During early reassembly of the nuclear envelope at this mitotic stage emerin colocalizes with lamin A/C but not with lamin B and LAP2 proteins. Confocal laser scanning microscopy after double-labeling experiments with emerin and tubulin shows that emerin is concentrated in areas of the mitotic spindle and in the midbody of mitotic cells suggesting a close interaction of these proteins. Our data suggest that emerin participates in the reorganisation of the nuclear envelope at the end of mitosis.

Emerin

Emerin encoded by the STA gene is the first nuclear protein linked with a muscular dystrophy. Emerin is a 34 kDa, predominantly hydrophilic protein with a single hydrophobic region supposed to serve as a transmembrane domain. It was classified as a type II integral membrane protein localized at the inner nuclear membrane/nuclear lamina with an ubiquitous tissue distribution. It is speculated that emerin is required for the stability and normal function of rigorously moving nuclei in skeletal muscle and heart. During mitosis, emerin is cell-cycle-dependent phosphorylated and shows stage-dependent changes in distribution and localization suggesting that it plays a role in re-assembly of nuclear membranes. Mutations of the emerin gene have been associated with X-linked Emery-Dreifuss muscular dystrophy clinically defined by early joint contractures, progressive muscle weakness, and cardiomyopathy. Hopefully, identification of the protein defect may promote new therapeutic strategies concerning muscle fiber development and stability.

The Emery-Dreifuss muscular dystrophy phenotype arises from aberrant targeting and binding of emerin at the inner nuclear membrane

The product of the X-linked Emery-Dreifuss muscular dystrophy gene is a single-membrane-spanning protein called emerin, which is localized to the inner nuclear membrane of all tissues studied. To examine whether a number of the mutant forms of emerin expressed in patients are mislocalized, we transfected GFP-emerin cDNA constructs reflecting these mutations into undifferentiated C2C12 myoblasts and showed that both wild type and all the mutant emerins are targeted to the nuclear membrane, but the mutants to a lesser extent. Mutant Del236-241 (deletion in transmembrane region) was mainly expressed as cytoplasmic aggregates, with only trace amounts at the nuclear envelope. Complete removal of the transmembrane region and C-terminal tail relocated emerin to the nucleoplasm. Mutations in emerin's N-terminal domain had a less severe effect on disrupting nuclear envelope targeting. This data suggests that emerin contains multiple non-overlapping nuclear-membrane-targeting determinants. Analysis of material immunoisolated using emerin antibodies, from either undifferentiated C2C12 myoblasts or purified hepatocyte nuclei, demonstrated that both A- and B-type lamins and nuclear actin interact with emerin. This is the first report of proteins interacting with emerin. The EDMD phenotype can thus arise by either the absence or a reduction in emerin at the nuclear envelope, and both of these disrupt its interactions with that of structural components of the nucleus. We propose that an emerin-nuclear protein complex exists at the nuclear envelope and that one of its primary roles is to stabilize the nuclear membrane against the mechanical stresses that are generated in muscle cells during contraction.

Velcro in the nuclear envelope: LBR and LAPs

The nuclear envelope is crucial for the functional organization of the nucleus. Lamin B receptor (LBR) and several lamina-associated proteins (LAPs), residing in the inner membrane, provide attachment sites for chromatin and the nuclear lamina. LAPs and LAP-related proteins are members of a growing family of proteins, whose genes are expressed in a tissue and development specific manner, opening the opportunity for a complex regulation of membrane-chromatin and membrane-lamina interactions. Post-translational modifications of LBR and LAPs are likely to modulate their binding to lamins and chromatin, interactions that need to be dynamic to accommodate nuclear growth in interphase and nuclear envelope disassembly in mitosis. Accumulation of proteins in the inner nuclear membrane is believed to depend on their retention mediated by the interaction with nuclear components such as chromatin and lamins.