The nuclear envelope, lamins and nuclear assembly

The role of inner nuclear membrane protein emerin in myogenesis

Emerin, a ubiquitously expressed inner nuclear membrane protein, plays a central role in maintaining nuclear structure and genomic organization, and in regulating gene expression and cellular signaling pathways. These functions are critical for proper myogenic differentiation and are closely linked to the pathology of Emery-Dreifuss muscular dystrophy 1 (EDMD1), a laminopathy caused by mutations in the EMD gene. Emerin, along with other nuclear lamina proteins, modulates chromatin organization, cell signaling, gene expression, and cellular mechanotransduction, processes essential for muscle development and homeostasis. Loss of emerin function disrupts chromatin localization, causes dysregulated gene expression, and alters nucleoskeletal organization, resulting in impaired myogenic differentiation. Recent findings suggest that emerin tethers repressive chromatin at the nuclear envelope, a process essential for robust myogenesis. This review provides an in-depth discussion of emerin's multifaceted roles in nuclear organization, gene regulation, and cellular signaling, highlighting its importance in myogenic differentiation and disease progression.

Identifying G-quadruplex-interacting proteins in cancer-related gene promoters

G-quadruplexes (G4s) are noncanonical DNA or RNA secondary structures involved in numerous biological processes. Their recognition by G4-related proteins (G4RPs) is essential for modulating biological pathways, particularly those associated with transcription and cancer progression. Identifying G4RPs is crucial for understanding their role in diseases like cancer, as these proteins may represent promising therapeutic targets. In this study, a proteomic-based fishing-for-partners approach was employed to identify putative interactors of G4-forming DNA sequences from the promoter regions of cancer-related genes DAP, HIF-1α, JAZF-1, and PDGF-A. A total of eighty-six G4RPs were identified, including nineteen known RNA and/or DNA G4 interactors. Notably, fourteen proteins were identified as potential interactors of all four investigated G4-forming DNA, seven of which were novel G4RPs. Direct interactions with G4s were validated for five of these proteins (AHNAK, GAPDH, HNRNP M, LMNA, and PPIA) using surface plasmon resonance experiments, which showed nanomolar binding affinities. This study not only validated known G4RPs but also led to the discovery of new G4/protein interactions, providing the basis for further investigation into their biological significance and potential implications in disease-associated pathways.

Nuclear NAD+ synthase nicotinamide mononucleotide adenylyltransferase 1 contributes to nuclear atypia and promotes glioma growth

Background

Glioma is a malignant primary brain tumor with a poor prognosis and short survival. NAD+ is critical for cancer growth; however, clinical trials targeting NAD+ biosynthesis had limited success, indicating the need for mechanistic characterization. Nuclear atypia, aberrations in the size and shape of the nucleus, is widely observed in cancer and is often considered a distinctive feature in diagnosis; however, the molecular underpinnings are unclear.

Methods

We carried out high-resolution immunohistochemical analyses on glioma tissue samples from 19 patients to analyze the expression of NAD+ synthase nicotinamide mononucleotide adenylyltransferase 1 (NMNAT1), and its correlation with nuclear atypia in gliomas. Utilizing a Drosophila model of glial neoplasia, we investigated the genetic role of nuclear NMNAT in glioma growth in vivo, elucidating the cellular mechanisms of NMNAT1 in promoting nuclear atypia and glioma growth.

Results

In low-grade glioma and glioblastoma, a higher transcription level of NMNAT1 is correlated with poorer disease-free survival. Samples of high-grade gliomas contained a higher percentage of glial cells enriched with NMNAT1 protein. We identified a specific correlation between nuclear NMNAT1 protein level with nuclear atypia. Mechanistic studies in human glioma cell lines and in vivo Drosophila model suggest that NMNAT1 disrupts the integrity of the nuclear lamina by altering the distribution of lamin A/C and promotes glioma growth.

Conclusions

Our study uncovers a novel functional connection between the NAD+ metabolic pathway and glioma growth, reveals the contribution of the NAD+ biosynthetic enzyme NMNAT1 to nuclear atypia, and underscores the role of nuclear NMNAT1 in exacerbating glioma pathology.

The Pathogenic Mechanisms of and Novel Therapies for Lamin A/C-Related Dilated Cardiomyopathy Based on Patient-Specific Pluripotent Stem Cell Platforms and Animal Models

Variants (pathogenic) of the LMNA gene are a common cause of familial dilated cardiomyopathy (DCM), which is characterised by early-onset atrioventricular (AV) block, atrial fibrillation and ventricular tachyarrhythmias (VTs), and progressive heart failure. The unstable internal nuclear lamina observed in LMNA-related DCM is a consequence of the disassembly of lamins A and C. This suggests that LMNA variants produce truncated or alternative forms of protein that alter the nuclear structure and the signalling pathway related to cardiac muscle diseases. To date, the pathogenic mechanisms and phenotypes of LMNA-related DCM have been studied using different platforms, such as patient-specific induced pluripotent stem-cell-derived cardiomyocytes (iPSC-CMs) and transgenic mice. In this review, point variants in the LMNA gene that cause autosomal dominantly inherited forms of LMNA-related DCM are summarised. In addition, potential therapeutic targets based on preclinical studies of LMNA variants using transgenic mice and human iPSC-CMs are discussed. They include mitochondria deficiency, variants in nuclear deformation, chromatin remodelling, altered platelet-derived growth factor and ERK1/2-related pathways, and abnormal calcium handling.

Reconstitution of nuclear envelope subdomain formation on mitotic chromosomes in semi-intact cells

In metazoans, the nuclear envelope (NE) disassembles during the prophase and reassembles around segregated chromatids during the telophase. The process of NE formation has been extensively studied using live-cell imaging. At the early step of NE reassembly in human cells, specific pattern-like localization of inner nuclear membrane (INM) proteins, connected to the nuclear pore complex (NPC), was observed in the so-called "core" region and "noncore" region on telophase chromosomes, which corresponded to the "pore-free" region and the "pore-rich" region, respectively, in the early G1 interphase nucleus. We refer to these phenomena as NE subdomain formation. To biochemically investigate this process, we aimed to develop an in vitro NE reconstitution system using digitonin-permeabilized semi-intact mitotic human cells coexpressing two INM proteins, emerin and lamin B receptor, which were labeled with fluorescent proteins. The targeting and accumulation of INM proteins to chromosomes before and after anaphase onset in semi-intact cells were observed using time-lapse imaging. Our in vitro NE reconstitution system recapitulated the formation of the NE subdomain, as in living cells, although chromosome segregation and cytokinesis were not observed. This in vitro NE reconstitution required the addition of a mitotic cytosolic fraction supplemented with a cyclin-dependent kinase inhibitor and energy sources. The cytoplasmic soluble factor(s) dependency of INM protein targeting differed among the segregation states of chromosomes. Furthermore, the NE reconstituted on segregated chromosomes exhibited active nucleocytoplasmic transport competency. These results indicate that the chromosome status changes after anaphase onset for recruiting NPC components.

LMNB1 deletion in ovarian cancer inhibits the proliferation and metastasis of tumor cells through PI3K/Akt pathway

Ovarian cancer (OC) is a common malignant tumor in gynecology. LMNB1 is an important component of the nuclear skeleton. The expression of LMNB1 in ovarian cancer is significantly higher than that in normal tissues, but its role in tumor still needs comprehensive investigation. In this study, we overexpressed and knocked down LMNB1 in ovarian cancer cells and explore the effect of LMNB1 on the cell proliferation, migration and the underlying mechanism. We analyzed the expression levels of LMNB1 in ovarian cancer and their clinical relevance by using bioinformatics methods, qRT-PCR, Western blot and immunohistochemistry. To state the effect and mechanism of LMNB1 on OC in vitro and in vivo, we performed mouse xenograft studies, CCK8, cloning formation, Edu incorporation, wound healing, transwell and flow cytometry assay in stable LMNB1 knockdown OC cells, following by RNA-seq. Overexpression of LMNB1 indicates the progression of OC. LMNB1 knockdown inhibited the proliferation and migration of OC cells by suppressing the FGF1-mediated PI3K-Akt signaling pathway. Our study shows LMNB1 as a novel prognostic factor and therapeutic target in OC.

Cellular Biomechanic Impairment in Cardiomyocytes Carrying the Progeria Mutation: An Atomic Force Microscopy Investigation

Given the clinical effect of progeria syndrome, understanding the cell mechanical behavior of this pathology could benefit the patient's treatment. Progeria patients show a point mutation in the lamin A/C gene (LMNA), which could change the cell's biomechanical properties. This paper reports a mechano-dynamic analysis of a progeria mutation (c.1824 C > T, p.Gly608Gly) in neonatal rat ventricular myocytes (NRVMs) using cell indentation by atomic force microscopy to measure alterations in beating force, frequency, and contractile amplitude of selected cells within cell clusters. Furthermore, we examined the beating rate variability using a time-domain method that produces a Poincaré plot because beat-to-beat changes can shed light on the causes of arrhythmias. Our data have been further related to our cell phenotype findings, using immunofluorescence and calcium transient analysis, showing that mutant NRVMs display changes in both beating force and frequency. These changes were associated with a decreased gap junction localization (Connexin 43) in the mutant NRVMs even in the presence of a stable cytoskeletal structure (microtubules and actin filaments) when compared with controls (wild type and non-treated cells). These data emphasize the kindred between nucleoskeleton (LMNA), cytoskeleton, and the sarcolemmal structures in NRVM with the progeria Gly608Gly mutation, prompting future mechanistic and therapeutic investigations.

Actin-related protein 2/3 complex regulates neutrophil extracellular trap expulsion and lung damage in abdominal sepsis

Neutrophil extracellular trap (NET) formation is a key feature in sepsis. The aim of the present study was to examine the role of the actin cytoskeleton in regulating the expulsion of NETs. Actin-related protein 2/3 (Arp 2/3) complex is an important regulator of F-actin polymerization. Co-incubation with CK666, a specific Arp 2/3 inhibitor, decreased PMA-induced NET formation in vitro. CK666 not only abolished F-actin polymerization but also caused intracellular retention of NETs. Inhibition of Arp 2/3 reduced NET formation on circulating neutrophils and in the bronchoalveolar space in mice undergoing cecal ligation and puncture (CLP). Notably, treatment with CK666 attenuated CLP-induced neutrophil recruitment, edema formation and tissue damage in the lungs. Moreover, Arp 2/3 inhibition decreased levels of CXCL-1 and interleukin-6 in the lung and plasma of septic animals. Taken together, this study shows that expulsion of NETs is regulated by the actin cytoskeleton and that inhibition of Arp 2/3-dependent F-actin polymerization not only decrease NET formation but also protect against pathological inflammation and tissue damage in septic lung injury. Thus, we suggest that targeting NET release is a novel and useful way to ameliorate lung damage in abdominal sepsis.

Nuclear envelope mechanobiology: linking the nuclear structure and function

The nucleus, central to cellular activity, relies on both direct mechanical input as well as its molecular transducers to sense external stimuli and respond by regulating intra-nuclear chromatin organization that determines cell function and fate. In mesenchymal stem cells of musculoskeletal tissues, changes in nuclear structures are emerging as a key modulator of their differentiation and proliferation programs. In this review we will first introduce the structural elements of the nucleoskeleton and discuss the current literature on how nuclear structure and signaling are altered in relation to environmental and tissue level mechanical cues. We will focus on state-of-the-art techniques to apply mechanical force and methods to measure nuclear mechanics in conjunction with DNA, RNA, and protein visualization in living cells. Ultimately, combining real-time nuclear deformations and chromatin dynamics can be a powerful tool to study mechanisms of how forces affect the dynamics of genome function.

Modulation of muscle redox and protein aggregation rescues lethality caused by mutant lamins

Mutations in the human LMNA gene cause a collection of diseases called laminopathies, which includes muscular dystrophy and dilated cardiomyopathy. The LMNA gene encodes lamins, filamentous proteins that form a meshwork on the inner side of the nuclear envelope. How mutant lamins cause muscle disease is not well understood, and treatment options are currently limited. To understand the pathological functions of mutant lamins so that therapies can be developed, we generated new Drosophila models and human iPS cell-derived cardiomyocytes. In the Drosophila models, muscle-specific expression of the mutant lamins caused nuclear envelope defects, cytoplasmic protein aggregation, activation of the Nrf2/Keap1 redox pathway, and reductive stress. These defects reduced larval motility and caused death at the pupal stage. Patient-derived cardiomyocytes expressing mutant lamins showed nuclear envelope deformations. The Drosophila models allowed for genetic and pharmacological manipulations at the organismal level. Genetic interventions to increase autophagy, decrease Nrf2/Keap1 signaling, or lower reducing equivalents partially suppressed the lethality caused by mutant lamins. Moreover, treatment of flies with pamoic acid, a compound that inhibits the NADPH-producing malic enzyme, partially suppressed lethality. Taken together, these studies have identified multiple new factors as potential therapeutic targets for LMNA-associated muscular dystrophy.

The Role of Emerin in Cancer Progression and Metastasis

It is commonly recognized in the field that cancer cells exhibit changes in the size and shape of their nuclei. These features often serve as important biomarkers in the diagnosis and prognosis of cancer patients. Nuclear size can significantly impact cell migration due to its incredibly large size. Nuclear structural changes are predicted to regulate cancer cell migration. Nuclear abnormalities are common across a vast spectrum of cancer types, regardless of tissue source, mutational spectrum, and signaling dependencies. The pervasiveness of nuclear alterations suggests that changes in nuclear structure may be crucially linked to the transformation process. The factors driving these nuclear abnormalities, and the functional consequences, are not completely understood. Nuclear envelope proteins play an important role in regulating nuclear size and structure in cancer. Altered expression of nuclear lamina proteins, including emerin, is found in many cancers and this expression is correlated with better clinical outcomes. A model is emerging whereby emerin, as well as other nuclear lamina proteins, binding to the nucleoskeleton regulates the nuclear structure to impact metastasis. In this model, emerin and lamins play a central role in metastatic transformation, since decreased emerin expression during transformation causes the nuclear structural defects required for increased cell migration, intravasation, and extravasation. Herein, we discuss the cellular functions of nuclear lamina proteins, with a particular focus on emerin, and how these functions impact cancer progression and metastasis.

Emerin Represses STAT3 Signaling through Nuclear Membrane-Based Spatial Control

Emerin is the inner nuclear membrane protein involved in maintaining the mechanical integrity of the nuclear membrane. Mutations in EMD encoding emerin cause Emery–Dreifuss muscular dystrophy (EDMD). Evidence is accumulating that emerin regulation of specific gene expression is associated with this disease, but the exact function of emerin has not been fully elucidated. Here, we show that emerin downregulates Signal transducer and activators of transcription 3 (STAT3) signaling, activated exclusively by Janus kinase (JAK). Deletion mutation experiments show that the lamin-binding domain of emerin is essential for the inhibition of STAT3 signaling. Emerin interacts directly and co-localizes with STAT3 in the nuclear membrane. Emerin knockdown induces STAT3 target genes Bcl2 and Survivin to increase cell survival signals and suppress hydrogen peroxide-induced cell death in HeLa cells. Specifically, downregulation of BAF or lamin A/C increases STAT3 signaling, suggesting that correct-localized emerin, by assembling with BAF and lamin A/C, acts as an intrinsic inhibitor against STAT3 signaling. In C2C12 cells, emerin knockdown induces STAT3 target gene, Pax7, and activated abnormal myoblast proliferation associated with muscle wasting in skeletal muscle homeostasis. Our results indicate that emerin downregulates STAT3 signaling by inducing retention of STAT3 and delaying STAT3 signaling in the nuclear membrane. This mechanism provides clues to the etiology of emerin-related muscular dystrophy and may be a new therapeutic target for treatment.

Depletion of HP1α alters the mechanical properties of MCF7 nuclei

Within the nucleus of the eukaryotic cell, DNA is partitioned into domains of highly condensed, transcriptionally silent heterochromatin and less condensed, transcriptionally active euchromatin. Heterochromatin protein 1α (HP1α) is an architectural protein that establishes and maintains heterochromatin, ensuring genome fidelity and nuclear integrity. Although the mechanical effects of changes in the relative amount of euchromatin and heterochromatin brought about by inhibiting chromatin-modifying enzymes have been studied previously, here we measure how the material properties of the nuclei are modified after the knockdown of HP1α. These studies were inspired by the observation that poorly invasive MCF7 breast cancer cells become more invasive after knockdown of HP1α expression and that, indeed, in many solid tumors the loss of HP1α correlates with the onset of tumor cell invasion. Atomic force microscopy (AFM), optical tweezers (OT), and techniques based on micropipette aspiration (MA) were each used to characterize the mechanical properties of nuclei extracted from HP1α knockdown or matched control MCF7 cells. Using AFM or OT to locally indent nuclei, those extracted from MCF7 HP1α knockdown cells were found to have apparent Young's moduli that were significantly lower than nuclei from MCF7 control cells, consistent with previous studies that assert heterochromatin plays a major role in governing the mechanical response in such experiments. In contrast, results from pipette-based techniques in the spirit of MA, in which the whole nuclei were deformed and aspirated into a conical pipette, showed considerably less variation between HP1α knockdown and control, consistent with previous studies reporting that it is predominantly the lamins in the nuclear envelope that determine the mechanical response to large whole-cell deformations. The differences in chromatin organization observed by various microscopy techniques between the MCF7 control and HP1α knockdown nuclei correlate well with the results of our measured mechanical responses and our hypotheses regarding their origin.

Role of A- and B-type lamins in nuclear structure-function relationships

Nuclear lamins are type V intermediate filament proteins that form a filamentous meshwork beneath the inner nuclear membrane. Additionally, a sub-population of A- and B-type lamins localizes in the nuclear interior. The nuclear lamina protects the nucleus from mechanical stress and mediates nucleo-cytoskeletal coupling. Lamins form a scaffold that partially tethers chromatin at the nuclear envelope. The nuclear lamina also stabilises protein-protein interactions involved in gene regulation and DNA repair. The lamin-based protein sub-complexes are implicated in both nuclear and cytoskeletal organisation, the mechanical stability of the nucleus, genome organisation, transcriptional regulation, genome stability and cellular differentiation. Here, we review recent research on nuclear lamins and unique roles of A- and B-type lamins in modulating various nuclear processes and their impact on cell function.

Separation of Coiled-Coil Structures in Lamin A/C Is Required for the Elongation of the Filament

Intermediate filaments (IFs) commonly have structural elements of a central α-helical coiled-coil domain consisting of coil 1a, coil 1b, coil 2, and their flanking linkers. Recently, the crystal structure of a long lamin A/C fragment was determined and showed detailed features of a tetrameric unit. The structure further suggested a new binding mode between tetramers, designated eA22, where a parallel overlap of coil 1a and coil 2 is the critical interaction. This study investigated the biochemical effects of genetic mutations causing human diseases, focusing on the eA22 interaction. The mutant proteins exhibited either weakened or augmented interactions between coil 1a and coil 2. The ensuing biochemical results indicated that the interaction requires the separation of the coiled-coils in the N-terminal of coil 1a and the C-terminal of coil 2, coupled with the structural transition in the central α-helical rod domain. This study provides insight into the role of coil 1a as a molecular regulator in the elongation of IF proteins.

Transcriptomics Reveal Altered Metabolic and Signaling Pathways in Podocytes Exposed to C16 Ceramide-Enriched Lipoproteins

Sphingolipids are bioactive lipids associated with cellular membranes and plasma lipoproteins, and their synthesis and degradation are tightly regulated. We have previously determined that low plasma concentrations of certain ceramide species predict the development of nephropathy in diabetes patients with normal albumin excretion rates at baseline. Herein, we tested the hypothesis that altering the sphingolipid content of circulating lipoproteins can alter the metabolic and signaling pathways in podocytes, whose dysfunction leads to an impairment of glomerular filtration. Cultured human podocytes were treated with lipoproteins from healthy subjects enriched in vitro with C16 ceramide, or D-erythro 2-hydroxy C16 ceramide, a ceramide naturally found in skin. The RNA-Seq data demonstrated differential expression of genes regulating sphingolipid metabolism, sphingolipid signaling, and mTOR signaling pathways. A multiplex analysis of mTOR signaling pathway intermediates showed that the majority (eight) of the pathway phosphorylated proteins measured (eleven) were significantly downregulated in response to C16 ceramide-enriched HDL2 compared to HDL2 alone and hydroxy ceramide-enriched HDL2. In contrast, C16 ceramide-enriched HDL3 upregulated the phosphorylation of four intermediates in the mTOR pathway. These findings highlight a possible role for lipoprotein-associated sphingolipids in regulating metabolic and signaling pathways in podocytes and could lead to novel therapeutic targets in glomerular kidney diseases.

Structural basis for lamin assembly at the molecular level

Nuclear structure and function are governed by lamins, which are intermediate filaments that mostly consist of α-helices. Different lamin assembly models have been proposed based on low resolution and fragmented structures. However, their assembly mechanisms are still poorly understood at the molecular level. Here, we present the crystal structure of a long human lamin fragment at 3.2 Å resolution that allows the visualization of the features of the full-length protein. The structure shows an anti-parallel arrangement of the two coiled-coil dimers, which is important for the assembly process. We further discover an interaction between the lamin dimers by using chemical cross-linking and mass spectrometry analysis. Based on these two interactions, we propose a molecular mechanism for lamin assembly that is in agreement with a recent model representing the native state and could explain pathological mutations. Our findings also provide the molecular basis for assembly mechanisms of other intermediate filaments.

Lamins are intermediate filaments and the major component of the nuclear lamina. Here the authors determine the crystal structure of a construct comprising the N-terminal half of human lamin A/C and use their structure and cross-linking and biochemical experiments to discuss lamin assembly.

Emerin Is Required for Proper Nucleus Reassembly after Mitosis: Implications for New Pathogenetic Mechanisms for Laminopathies Detected in EDMD1 Patients

Emerin is an essential LEM (LAP2, Emerin, MAN1) domain protein in metazoans and an integral membrane protein associated with inner and outer nuclear membranes. Mutations in the human EMD gene coding for emerin result in the rare genetic disorder: Emery⁻Dreifuss muscular dystrophy type 1 (EDMD1). This disease belongs to a broader group called laminopathies-a heterogeneous group of rare genetic disorders affecting tissues of mesodermal origin. EDMD1 phenotype is characterized by progressive muscle wasting, contractures of the elbow and Achilles tendons, and cardiac conduction defects. Emerin is involved in many cellular and intranuclear processes through interactions with several partners: lamins; barrier-to-autointegration factor (BAF), β-catenin, actin, and tubulin. Our study demonstrates the presence of the emerin fraction which associates with mitotic spindle microtubules and centrosomes during mitosis and colocalizes during early mitosis with lamin A/C, BAF, and membranes at the mitotic spindle. Transfection studies with cells expressing EGFP-emerin protein demonstrate that the emerin fusion protein fraction also localizes to centrosomes and mitotic spindle microtubules during mitosis. Transient expression of emerin deletion mutants revealed that the resulting phenotypes vary and are mutant dependent. The most frequent phenotypes include aberrant nuclear shape, tubulin network mislocalization, aberrant mitosis, and mislocalization of centrosomes. Emerin deletion mutants demonstrated different chromatin binding capacities in an in vitro nuclear assembly assay and chromatin-binding properties correlated with the strength of phenotypic alteration in transfected cells. Aberrant tubulin staining and microtubule network phenotype appearance depended on the presence of the tubulin binding region in the expressed deletion mutants. We believe that the association with tubulin might help to "deliver" emerin and associated membranes to decondensing chromatin. Preliminary analyses of cells from Polish patients with EDMD1 revealed that for several mutations thought to be null for emerin protein, a truncated emerin protein was present. We infer that the EDMD1 phenotype may be strengthened by the toxicity of truncated emerin expressed in patients with certain nonsense mutations in EMD.

Linker of nucleoskeleton and cytoskeleton complex proteins in cardiomyopathy

The linker of nucleoskeleton and cytoskeleton (LINC) complex couples the nuclear lamina to the cytoskeleton. The LINC complex and its associated proteins play diverse roles in cells, ranging from genome organization, nuclear morphology, gene expression, to mechanical stability. The importance of a functional LINC complex is highlighted by the large number of mutations in genes encoding LINC complex proteins that lead to skeletal and cardiac myopathies. In this review, the structure, function, and interactions between components of the LINC complex will be described. Mutations that are known to cause cardiomyopathy in patients will be discussed alongside their respective mouse models. Furthermore, future challenges for the field and emerging technologies to investigate LINC complex function will be discussed.

Emerin suppresses Notch signaling by restricting the Notch intracellular domain to the nuclear membrane

Emerin is an inner nuclear membrane protein that is involved in maintaining the mechanical integrity of the nuclear membrane. Increasing evidence supports the involvement of emerin in the regulation of gene expression; however, its precise function remains to be elucidated. Here, we show that emerin downregulated genes downstream of Notch signaling, which are activated exclusively by the Notch intracellular domain (NICD). Deletion mutant experiments revealed that the transmembrane domain of emerin is important for the inhibition of Notch signaling. Emerin interacted directly and colocalized with the NICD at the nuclear membrane. Emerin knockdown induced the phosphorylation of ERK and AKT, increased endogenous Notch signaling, and inhibited hydrogen peroxide-induced apoptosis in HeLa cells. Notably, the downregulation of barrier-to-autointegration factor (BAF) or lamin A/C increased Notch signaling by inducing the release of emerin into the cytosol, implying that nuclear membrane-bound emerin acts as an endogenous inhibitor of Notch signaling. Taken together, our results indicate that emerin negatively regulates Notch signaling by promoting the retention of the NICD at the nuclear membrane. This mechanism could constitute a new therapeutic target for the treatment of emerin-related diseases.

Alterations in nuclear structure promote lupus autoimmunity in a mouse model

Systemic lupus erythematosus (SLE) is an autoimmune disorder characterized by the development of autoantibodies that recognize components of the cell nucleus. The vast majority of lupus research has focused on either the contributions of immune cell dysfunction or the genetics of the disease. Because granulocytes isolated from human SLE patients had alterations in neutrophil nuclear morphology that resembled the Pelger-Huet anomaly, and had prominent mis-splicing of mRNA encoding the nuclear membrane protein lamin B receptor (LBR), consistent with their Pelger-Huet-like nuclear morphology, we used a novel mouse model system to test the hypothesis that a disruption in the structure of the nucleus itself also contributes to the development of lupus autoimmunity. The lupus-prone mouse strain New Zealand White (NZW) was crossed with c57Bl/6 mice harboring a heterozygous autosomal dominant mutation in Lbr (B6.Lbr(ic/+)), and the (NZW×B6.Lbr(ic))F1 offspring were evaluated for induction of lupus autoimmunity. Only female (NZW×B6.Lbr(ic))F1 mice developed lupus autoimmunity, which included splenomegaly, kidney damage and autoantibodies. Kidney damage was accompanied by immune complex deposition, and perivascular and tubule infiltration of mononuclear cells. The titers of anti-chromatin antibodies exceeded those of aged female MRL-Fas(lpr) mice, and were predominantly of the IgG2 subclasses. The anti-nuclear antibody staining profile of female (NZW×B6.Lbr(ic))F1 sera was complex, and consisted of an anti-nuclear membrane reactivity that colocalized with the A-type lamina, in combination with a homogeneous pattern that was related to the recognition of histones with covalent modifications that are associated with gene activation. An anti-neutrophil IgM recognizing calreticulin, but not myeloperoxidase (MPO) or proteinase 3 (PR3), was also identified. Thus, alterations in nuclear structure contribute to lupus autoimmunity when expressed in the context of a lupus-prone genetic background, suggesting a mechanism for the development of lupus autoimmunity in genetically predisposed individuals that is induced by the disruption of nuclear architecture.

BioID Identification of Lamin-Associated Proteins

A- and B-type lamins support the nuclear envelope, contribute to heterochromatin organization, and regulate a myriad of nuclear processes. The mechanisms by which lamins function in different cell types and the mechanisms by which lamin mutations cause over a dozen human diseases (laminopathies) remain unclear. The identification of proteins associated with lamins is likely to provide fundamental insight into these mechanisms. BioID (proximity-dependent biotin identification) is a unique and powerful method for identifying protein-protein and proximity-based interactions in living cells. BioID utilizes a mutant biotin ligase from bacteria that is fused to a protein of interest (bait). When expressed in living cells and stimulated with excess biotin, this BioID-fusion protein promiscuously biotinylates directly interacting and vicinal endogenous proteins. Following biotin-affinity capture, the biotinylated proteins can be identified using mass spectrometry. BioID thus enables screening for physiologically relevant protein associations that occur over time in living cells. BioID is applicable to insoluble proteins such as lamins that are often refractory to study by other methods and can identify weak and/or transient interactions. We discuss the use of BioID to elucidate novel lamin-interacting proteins and its applications in a broad range of biological systems, and provide detailed protocols to guide new applications.

Selective Regulation of Oocyte Meiotic Events Enhances Progress in Fertility Preservation Methods

Following early embryonic germ cell migration, oocytes are surrounded by somatic cells and remain arrested at diplotene stage until luteinizing hormone (LH) surge. Strict regulation of both meiotic arrest and meiotic resumption during dormant stage are critical for future fertility. Inter-cellular signaling system between the somatic compartment and oocyte regulates these meiotic events and determines the follicle quality. As well as the collected number of eggs, their qualities are also important for in vitro fertilization (IVF) outcome. In spontaneous and IVF cycles, germinal vesicle (GV)–stage oocytes, premature GV breakdown, and persistence of first meiotic arrest limit the reproductive performance. Likewise, both women with premature ovarian aging and young cancer women are undergoing chemoradiotherapy under the risk of follicle loss because of unregulated meiotic events. Understanding of oocyte meiotic events is therefore critical for the prevention of functional ovarian reserve. High levels of cyclic guanosine monophophate (cGMP), cyclic adenosine monophophate (cAMP) and low phosphodiesterase (PDE) 3A enzyme activity inside the oocyte are responsible for maintaining of meiotic arrest before the LH surge. cGMP is produced in the somatic compartment, and natriuretic peptide precursor C (Nppc) and natriuretic peptide receptor 2 (Npr2) regulate its production. cGMP diffuses into the oocyte and reduces the PDE3A activity, which inhibits the conversion of cAMP to the 5′AMP, and cAMP levels are enhanced. In addition, oocyte itself has the ability to produce cAMP. Taken together, accumulation of cAMP inside the oocyte induces protein kinase activity, which leads to the inhibition of maturation-promoting factor and meiotic arrest also continues. By stimulating the expression of epidermal growth factor, LH inhibits the Nppc/Npr2 system, blocks cGMP synthesis, and initiates meiotic resumption. Oocytes lacking the functional of this pathway may lead to persistence of the GV oocyte, which reduces the number of good quality eggs. Selective regulation of somatic cell signals and oocyte meiotic events enhance progress in fertility preservation methods, which may give us the opportunity to prevent follicle loss in prematurely aging women and young women with cancer are undergoing chemoradiotherapy.

Linker of nucleoskeleton and cytoskeleton complex proteins in cardiac structure, function, and disease

The linker of nucleoskeleton and cytoskeleton (LINC) complex, composed of proteins within the inner and the outer nuclear membranes, connects the nuclear lamina to the cytoskeleton. The importance of this complex has been highlighted by the discovery of mutations in genes encoding LINC complex proteins, which cause skeletal or cardiac myopathies. Herein, this review summarizes structure, function, and interactions of major components of the LINC complex, highlights how mutations in these proteins may lead to cardiac disease, and outlines future challenges in the field.

Nuclear lamin stiffness is a barrier to 3D migration, but softness can limit survival

Lamins impede 3D migration but also promote survival against migration-induced stresses.

Cell migration through solid tissue often involves large contortions of the nucleus, but biological significance is largely unclear. The nucleoskeletal protein lamin-A varies both within and between cell types and was shown here to contribute to cell sorting and survival in migration through constraining micropores. Lamin-A proved rate-limiting in 3D migration of diverse human cells that ranged from glioma and adenocarcinoma lines to primary mesenchymal stem cells (MSCs). Stoichiometry of A- to B-type lamins established an activation barrier, with high lamin-A:B producing extruded nuclear shapes after migration. Because the juxtaposed A and B polymer assemblies respectively conferred viscous and elastic stiffness to the nucleus, subpopulations with different A:B levels sorted in 3D migration. However, net migration was also biphasic in lamin-A, as wild-type lamin-A levels protected against stress-induced death, whereas deep knockdown caused broad defects in stress resistance. In vivo xenografts proved consistent with A:B-based cell sorting, and intermediate A:B-enhanced tumor growth. Lamins thus impede 3D migration but also promote survival against migration-induced stresses.

The regulation of maturation promoting factor during prophase I arrest and meiotic entry in mammalian oocytes

Mammalian oocytes arrest at prophase of meiosis I at around birth and they remain arrested at this stage until puberty when the preovulatory surge of luteinizing hormone (LH) causes ovulation. Prophase I arrest in the immature oocyte results from the maintenance of low activity of maturation promoting factor (MPF), which consists of a catalytic subunit (CDK1) and regulatory subunit (cyclin B1). Phosphorylation-mediated inactivation of CDK1 and constant degradation of cyclin B1 keep MPF activity low during prophase I arrest. LH-mediated signaling manipulates a vast array of molecules to activate CDK1. Active CDK1 not only phosphorylates different meiotic phosphoproteins during the resumption of meiosis but also inhibits their rapid dephosphorylation by inhibiting the activities of CDK1 antagonizing protein phosphatases (PPs). In this way, CDK1 both phosphorylates its substrates and protects them from being dephosphorylated. Accumulating evidence suggests that the net MPF activity that drives the resumption of meiosis in oocytes depends on the activation status of CDK1 antagonizing PPs. This review aims to provide a summary of the current understanding of the signaling pathways involved in regulating MPF activity during prophase I arrest and reentry into meiosis of mammalian oocytes.

Emerin in health and disease

Emery-Dreifuss muscular dystrophy (EDMD) is caused by mutations in the genes encoding emerin, lamins A and C and FHL1. Additional EDMD-like syndromes are caused by mutations in nesprins and LUMA. This review will specifically focus on emerin function and the current thinking for how loss or mutations in emerin cause EDMD. Emerin is a well-conserved, ubiquitously expressed protein of the inner nuclear membrane. Emerin has been shown to have diverse functions, including the regulation of gene expression, cell signaling, nuclear structure and chromatin architecture. This review will focus on the relationships between these functions and the EDMD disease phenotype. Additionally it will highlight open questions concerning emerin's roles in cell and nuclear biology and disease.

The nuclear envelope LEM-domain protein emerin

Emerin, a conserved LEM-domain protein, is among the few nuclear membrane proteins for which extensive basic knowledge—biochemistry, partners, functions, localizations, posttranslational regulation, roles in development and links to human disease—is available. This review summarizes emerin and its emerging roles in nuclear “lamina” structure, chromatin tethering, gene regulation, mitosis, nuclear assembly, development, signaling and mechano-transduction. We also highlight many open questions, exploration of which will be critical to understand how this intriguing nuclear membrane protein and its “family” influence the genome.

Rab6-mediated retrograde transport regulates inner nuclear membrane targeting of caveolin-2 in response to insulin

Here, we have identified a retrograde transport pathway of caveolin-2 (cav-2) for its regulatory function in the nucleus. Confocal microscopy analysis, photoactivation experiments and subcellular fractionation revealed that cav-2 localized in the Golgi was transported to the inner nuclear membrane (INM) in response to insulin. Exogenous caveolin-1 (cav-1) and P132L-cav-1 expression did not affect the Golgi localization and insulin-induced INM targeting of cav-2. Cav-2(DKV) mutant in the endoplasmic reticulum (ER) was unable to translocate to the INM in response to insulin. The GTP-bound form of Rab6 promoted, but Rab6 siRNA and the GDP-bound form of Rab6 abrogated, retrograde trafficking of cav-2 from the Golgi to ER. Colchicine or nocodazole treatment abolished insulin-induced INM targeting of cav-2. Knock down of gp210 inhibited insulin-induced import of cav-2 from ER/outer nuclear membrane (ONM) to the INM. The INM-targeted cav-2 prevented heterochromatinization and promoted transcriptional activation of Elk-1 and signal transducer and activator of transcription 3 (STAT3). The results provide molecular mechanisms for insulin-induced INM translocation of cav-2 initiated (i) by Golgi-to-ER retrograde trafficking of cav-2 via microtubule-based Rab6-GTP-dependent transport and subsequently processed (ii) by gp210-mediated import of cav-2 from ER/ONM to INM.

Cdk1, but not Cdk2, is the sole Cdk that is essential and sufficient to drive resumption of meiosis in mouse oocytes

Mammalian oocytes are arrested at the prophase of meiosis I during fetal or postnatal development, and the meiosis is resumed by the preovulatory surge of luteinizing hormone. The in vivo functional roles of cyclin-dependent kinases (Cdks) during the resumption of meiosis in mammalian oocytes are largely unknown. Previous studies have shown that deletions of Cdk3, Cdk4 or Cdk6 in mice result in viable animals with normal oocyte maturation, indicating that these Cdks are not essential for the meiotic maturation of oocytes. In addition, conventional knockout of Cdk1 and Cdk2 leads to embryonic lethality and postnatal follicular depletion, respectively, making it impossible to study the functions of Cdk1 and Cdk2 in oocyte meiosis. In this study, we generated conditional knockout mice with oocyte-specific deletions of Cdk1 and Cdk2. We showed that the lack of Cdk1, but not of Cdk2, leads to female infertility due to a failure of the resumption of meiosis in the oocyte. Re-introduction of Cdk1 mRNA into Cdk1-null oocytes largely resumed meiosis. Thus, Cdk1 is the sole Cdk that is essential and sufficient to drive resumption of meiosis in mouse oocytes. We also found that Cdk1 maintains the phosphorylation status of protein phosphatase 1 and lamin A/C in oocytes in order for meiosis resumption to occur.

LMNA variants cause cytoplasmic distribution of nuclear pore proteins in Drosophila and human muscle

Mutations in the human LMNA gene, encoding A-type lamins, give rise to laminopathies, which include several types of muscular dystrophy. Here, heterozygous sequence variants in LMNA, which result in single amino-acid substitutions, were identified in patients exhibiting muscle weakness. To assess whether the substitutions altered lamin function, we performed in vivo analyses using a Drosophila model. Stocks were generated that expressed mutant forms of the Drosophila A-type lamin modeled after each variant. Larvae were used for motility assays and histochemical staining of the body-wall muscle. In parallel, immunohistochemical analyses were performed on human muscle biopsy samples from the patients. In control flies, muscle-specific expression of the wild-type A-type lamin had no apparent affect. In contrast, expression of the mutant A-type lamins caused dominant larval muscle defects and semi-lethality at the pupal stage. Histochemical staining of larval body wall muscle revealed that the mutant A-type lamin, B-type lamins, the Sad1p, UNC-84 domain protein Klaroid and nuclear pore complex proteins were mislocalized to the cytoplasm. In addition, cytoplasmic actin filaments were disorganized, suggesting links between the nuclear lamina and the cytoskeleton were disrupted. Muscle biopsies from the patients showed dystrophic histopathology and architectural abnormalities similar to the Drosophila larvae, including cytoplasmic distribution of nuclear envelope proteins. These data provide evidence that the Drosophila model can be used to assess the function of novel LMNA mutations and support the idea that loss of cellular compartmentalization of nuclear proteins contributes to muscle disease pathogenesis.

Modeling nuclear blebs in a nucleoskeleton of independent filament networks

Correlations between altered nuclear shape and disease are empirically observed, but the causes of nuclear dysmorphisms are poorly understood. The nucleoskeleton, which provides the majority of the mechanical stability of the nucleus, is composed primarily of intermediate filaments of lamin proteins. The nucleoskeleton forms a mostly-planar network between the inner nuclear membrane and chromatin. It is unclear if blebs and larger scale changes in nuclear morphology are consequences of reorganization of the nucleoskeleton alone or of other cellular processes. To test this, we computationally recapitulate the lamina network using a mechanical network model created as a network of Hookean springs. A- and B-type lamin filaments were distributed over a spherical surface into distinct networks linked to one another by lamin-associated proteins. Iterative force-based adjustment of the network structure, together with a stochastically modified Bell model of bond breakage and formation, simulates nucleoskeleton reorganization with blebs. The rate of bleb retraction into the nucleus depends on both initial size of the bleb and number of networks being deformed. Our results show that induced blebs are more stable when only one filament component is deformed or when the networks have no interconnections. Also, the kinetics of retraction is influenced by the composition of the bleb. These results match with our experiments and others.

Distinct effects of nuclear membrane localization on gene transcription silencing in Drosophila S2 cells and germ cells

Nuclear envelope proteins have important roles in chromatin organization and signal-dependent transcriptional regulation. A previous study reported that the inner nuclear membrane protein, Otefin (Ote), was essential for germline stem cell (GSC) maintenance via interaction with Smad complex. The interaction of Ote with the Smad complex recruits the bam locus to the nuclear periphery and subsequently results in bam transcriptional silencing, revealing that nuclear peripheral localization is essential for bam gene regulation. However, it remains unknown whether the nuclear peripheral localization is sufficient for bam silencing. To address this issue, we have established a tethering system, in which the Gal4 DNA binding domain (DBD) of the Flag:Gal4 DBD:Ote▵LEM fusion protein physically interacts with the Gal4 binding sites upstream of bamP-gfp to artificially recruit the reporter gene gfp to the nuclear membrane. Our data demonstrated that the nuclear peripheral localization seemed to affect the expression of the target naked gene in S2 cells. By contrast, in Drosophila germ cells, the nuclear membrane localization was not sufficient for gene silencing.

Exceptional structural and mechanical flexibility of the nuclear pore complex

Nuclear pore complexes (NPCs) mediate all transport between the cytosol and the nucleus and therefore take centre stage in physiology. While transport through NPCs has been extensively investigated little is known about their structural and barley anything about their mechanical flexibility. Structural and mechanical flexibility of NPCs, however, are presumably of key importance. Like the cell and the cell nucleus, NPCs themselves are regularly exposed to physiological mechanical forces. Besides, NPCs reveal striking transport properties which are likely to require fairly high structural flexibility. The NPC transports up to 1,000 molecules per second through a physically 9 nm wide channel which repeatedly opens to accommodate macromolecules significantly larger than its physical diameter. We hypothesised that NPCs possess remarkable structural and mechanical stability. Here, we tested this hypothesis at the single NPC level using the nano-imaging and probing approach atomic force microscopy (AFM). AFM presents the NPC as a highly flexible structure. The NPC channel dilates by striking 35% on exposure to trans-cyclohexane-1,2-diol (TCHD), which is known to transiently collapse the hydrophobic phase in the NPC channel like receptor-cargo complexes do in transit. It constricts again to its initial size after TCHD removal. AFM-based nano-indentation measurements show that the 50 nm long NPC basket can astonishingly be squeezed completely into the NPC channel on exposure to incremental mechanical loads but recovers its original vertical position within the nuclear envelope plane when relieved. We conclude that the NPC possesses exceptional structural and mechanical flexibility which is important to fulfilling its functions.

The perinuclear actin cap in health and disease

We recently demonstrated the existence of a previously uncharacterized subset of actomyosin fibers that form the perinuclear actin cap, a cytoskeletal structure that tightly wraps around the nucleus of a wide range of somatic cells. Fibers in the actin cap are distinct from well-characterized, conventional actin fibers at the basal and dorsal surfaces of adherent cells in their subcellular location, internal organization, dynamics, ability to generate contractile forces, response to cytoskeletal pharmacological treatments, response to biochemical stimuli, regulation by components of the linkers of nucleoskeleton and cytoskeleton (LINC) complexes, and response to disease-associated mutations in LMNA, the gene that encodes for the nuclear lamin component lamin A/C. The perinuclear actin cap precisely shapes the nucleus in interphase cells. The perinuclear actin cap may also be a mediator of microenvironment mechanosensing and mechanotransduction, as well as a regulator of cell motility, polarization and differentiation.

Differential nuclear envelope assembly at the end of mitosis in suspension-cultured Apium graveolens cells

NMCP1 is a plant protein that has a long coiled-coil domain within the molecule. Newly identified NMCP2 of Daucus carota and Apium graveolens showed similar peripheral localization in the interphase nucleus, and the sequence spanning the coiled-coil domain exhibited significant similarity with the corresponding region of NMCP1. To better understand disassembly and assembly of the nuclear envelope (NE) during mitosis, subcellular distribution of NMCP1 and NMCP2 was examined using A. graveolens cells. AgNMCP1 (NMCP1 in Apium) disassembled at prometaphase, dispersed mainly within the spindle, and accumulated on segregating chromosomes, while AgNMCP2 (NMCP2 in Apium), following disassembly at prometaphase with timing similar to that of AgNMCP1, dispersed throughout the mitotic cytoplasm at metaphase and anaphase. The protein accumulated at the periphery of reforming nuclei at telophase. A probe for the endomembrane indicated that the nuclear membrane (NM) disappears at prometaphase and begins to reappear at early telophase. Growth of the NM continued after mitosis was completed. NMCP2 in the mitotic cytoplasm localized in vesicular structures that could be distinguished from the bulk endomembrane system. These results suggest that NMCP1 and NMCP2 are recruited for NE assembly in different pathways in mitosis and that NMCP2 associates with NM-derived vesicles in the mitotic cytoplasm.

Nuclear fusion and genome encounter during yeast zygote formation

When haploid cells of Saccharomyces cerevisiae are crossed, parental nuclei congress and fuse with each other. To investigate underlying mechanisms, we have developed assays that evaluate the impact of drugs and mutations. Nuclear congression is inhibited by drugs that perturb the actin and tubulin cytoskeletons. Nuclear envelope (NE) fusion consists of at least five steps in which preliminary modifications are followed by controlled flux of first outer and then inner membrane proteins, all before visible dilation of the waist of the nucleus or coalescence of the parental spindle pole bodies. Flux of nuclear pore complexes occurs after dilation. Karyogamy requires both the Sec18p/NSF ATPase and ER/NE luminal homeostasis. After fusion, chromosome tethering keeps tagged parental genomes separate from each other. The process of NE fusion and evidence of genome independence in yeast provide a prototype for understanding related events in higher eukaryotes.

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.

Specific contribution of lamin A and lamin C in the development of laminopathies

Mutations in the lamin A/C gene are involved in multiple human disorders for which the pathophysiological mechanisms are partially understood. Conflicting results prevail regarding the organization of lamin A and C mutants within the nuclear envelope (NE) and on the interactions of each lamin to its counterpart. We over-expressed various lamin A and C mutants both independently and together in COS7 cells. When expressed alone, lamin A with cardiac/muscular disorder mutations forms abnormal aggregates inside the NE and not inside the nucleoplasm. Conversely, the equivalent lamin C organizes as intranucleoplasmic aggregates that never connect to the NE as opposed to wild type lamin C. Interestingly, the lamin C molecules present within these aggregates exhibit an abnormal increased mobility. When co-expressed, the complex formed by lamin A/C aggregates in the NE. Lamin A and C mutants for lipodystrophy behave similarly to the wild type. These findings reveal that lamins A and C may be differentially affected depending on the mutation. This results in multiple possible physiological consequences which likely contribute in the phenotypic variability of laminopathies. The inability of lamin C mutants to join the nuclear rim in the absence of lamin A is a potential pathophysiological mechanism for laminopathies.

Nuclear shape, mechanics, and mechanotransduction

In eukaryotic cells, the nucleus contains the genome and is the site of transcriptional regulation. The nucleus is the largest and stiffest organelle and is exposed to mechanical forces transmitted through the cytoskeleton from outside the cell and from force generation within the cell. Here, we discuss the effect of intra- and extracellular forces on nuclear shape and structure and how these force-induced changes could be implicated in nuclear mechanotransduction, ie, force-induced changes in cell signaling and gene transcription. We review mechanical studies of the nucleus and nuclear structural proteins, such as lamins. Dramatic changes in nuclear shape, organization, and stiffness are seen in cells where lamin proteins are mutated or absent, as in genetically engineered mice, RNA interference studies, or human disease. We examine the different mechanical pathways from the force-responsive cytoskeleton to the nucleus. We also highlight studies that link changes in nuclear shape with cell function during developmental, physiological, and pathological modifications. Together, these studies suggest that the nucleus itself may play an important role in the response of the cell to force.

Nucleocytoplasmic trafficking and transcription effects of huntingtin in Huntington's disease

There are nine genetic neurodegenerative diseases caused by a similar genetic defect, a CAG DNA triplet-repeat expansion in the disease gene's open reading frame resulting in a polyglutamine expansion in the disease proteins. Despite the commonality of polyglutamine expansion, each of the polyglutamine diseases manifest as unique diseases, with some similarities, but important differences. These differences suggest that the context of the polyglutamine expansion is important to the mechanism of pathology of the disease proteins. Therefore, it is becoming increasingly paramount to understand the normal functions of these polyglutamine disease proteins, which include huntingtin, the polyglutamine-expanded protein in Huntington's disease (HD). Transcriptional dysregulation is seen in HD. Here we discuss the role of normal huntingtin in transcriptional regulation and misregulation in Huntington's disease in relation to potentially analogous model systems, and to other polyglutamine disease proteins. Huntingtin has functional roles in both the cytoplasm and the nucleus. One commonality of activity of polyglutamine disease proteins is at the level of protein dynamics and ability to import and export to and from the nucleus. Knowing the temporal location of huntingtin protein in response to signaling and neuronal communication could lead to valuable insights into an important trigger of HD pathology.

Histone Acetyltransferase hALP and Nuclear Membrane Protein hsSUN1 Function in De-condensation of Mitotic Chromosomes

Replicated mammalian chromosomes condense to segregate during anaphase, and they de-condense at the conclusion of mitosis. Currently, it is not understood what the factors and events are that specify de-condensation. Here, we demonstrate that chromosome de-condensation needs the function of an inner nuclear membrane (INM) protein hsSUN1 and a membrane-associated histone acetyltransferase (HAT), hALP. We propose that nascently reforming nuclear envelope employs hsSUN1 and hALP to acetylate histones for de-compacting DNA at the end of mitosis.

An emerin "proteome": purification of distinct emerin-containing complexes from HeLa cells suggests molecular basis for diverse roles including gene regulation, mRNA splicing, signaling, mechanosensing, and nuclear architecture

Using recombinant bead-conjugated emerin, we affinity-purified seven proteins from HeLa cell nuclear lysates that bind emerin either directly or indirectly. These proteins were identified by mass spectrometry as nuclear alphaII-spectrin, nonmuscle myosin heavy chain alpha, Lmo7 (a predicted transcription regulator; reported separately), nuclear myosin I, beta-actin (reported separately), calponin 3, and SIKE. We now report that emerin binds nuclear myosin I (NMI, a molecular motor) directly in vitro. Furthermore, bead-conjugated emerin bound nuclear alphaII-spectrin and NMI equally well with or without ATP (which stimulates motor activity), whereas ATP decreased actin binding by 65%. Thus alphaII-spectrin and NMI interact stably with emerin. To investigate the physiological relevance of these interactions, we used antibodies against emerin to affinity-purify emerin-associated protein complexes from HeLa cells and then further purified by ion-exchange chromatography to resolve by net charge and by size exclusion chromatography yielding six distinct emerin-containing fractions (0.5-1.6 MDa). Western blotting suggested that each complex had distinct components involved in nuclear architecture (e.g., NMI, alphaII-spectrin, lamins) or gene or chromatin regulation (BAF, transcription regulators, HDACs). Additional constituents were identified by mass spectrometry. One putative gene-regulatory complex (complex 32) included core components of the nuclear corepressor (NCoR) complex, which mediates gene regulation by thyroid hormone and other nuclear receptors. When expressed in HeLa cells, FLAG-tagged NCoR subunits Gps2, HDAC3, TBLR1, and NCoR each co-immunoprecipitated emerin, validating one putative complex. These findings support the hypothesis that emerin scaffolds a variety of functionally distinct multiprotein complexes at the nuclear envelope in vivo. Notably included are nuclear myosin I-containing complexes that might sense and regulate mechanical tension at the nuclear envelope.

A role for Rab5 in structuring the endoplasmic reticulum

The endoplasmic reticulum (ER) is a contiguous network of interconnected membrane sheets and tubules. The ER is differentiated into distinct domains, including the peripheral ER and nuclear envelope. Inhibition of two ER proteins, Rtn4a and DP1/NogoA, was previously shown to inhibit the formation of ER tubules in vitro. We show that the formation of ER tubules in vitro also requires a Rab family GTPase. Characterization of the 29 Caenorhabditis elegans Rab GTPases reveals that depletion of RAB-5 phenocopies the defects in peripheral ER structure that result from depletion of RET-1 and YOP-1, the C. elegans homologues of Rtn4a and DP1/NogoA. Perturbation of endocytosis by other means did not affect ER structure; the role of RAB-5 in ER morphology is thus independent of its well-studied requirement for endocytosis. RAB-5 and YOP-1/RET-1 also control the kinetics of nuclear envelope disassembly, which suggests an important role for the morphology of the peripheral ER in this process.

The inner nuclear membrane protein Lem2 is critical for normal nuclear envelope morphology

The inner nuclear membrane (INM) of eukaryotic cells is characterized by a unique set of transmembrane proteins which interact with chromatin and/or the nuclear lamina. The number of identified INM proteins is steadily increasing, mainly as a result of proteomic and computational approaches. However, despite a link between mutation of several of these proteins and disease, the function of most transmembrane proteins of the INM remains unknown and depletion of many of these proteins from a variety of systems did not produce an obvious phenotype in the affected cells. Here, we report that depletion of the conserved INM protein Lem2 from human cell lines leads to abnormally shaped nuclei and severely reduces cell survival. We suggest that interactions of Lem2 with lamins or chromatin are critical for maintaining the integrity of the nuclear envelope.

Lmo7 is an emerin-binding protein that regulates the transcription of emerin and many other muscle-relevant genes

X-linked Emery-Dreifuss muscular dystrophy (X-EDMD) is inherited through mutations in emerin, a nuclear membrane protein. Emerin has proposed roles in nuclear architecture and gene regulation, but direct molecular links to disease were unknown. We report that Lim-domain only 7 (Lmo7) binds emerin directly with 125 nM affinity; the C-terminal half of human Lmo7 (hLmo7C) was sufficient to bind emerin in vitro. Lmo7 appeared relevant to EDMD because a deletion that removes Lmo7 (plus eight exons of a neighboring gene) in mice causes dystrophic muscles [Semenova, E., Wang, X., Jablonski, M.M., Levorse, J. and Tilghman, S.M. (2003) An engineered 800 kilobase deletion of Uchl3 and Lmo7 on mouse chromosome 14 causes defects in viability, postnatal growth and degeneration of muscle and retina. Hum. Mol. Genet., 12, 1301-1312]. Lmo7 localizes in the nucleus, cytoplasm and cell surface, particularly adhesion junctions [Ooshio, T., Irie, K., Morimoto, K., Fukuhara, A., Imai, T. and Takai, Y. (2004) Involvement of LMO7 in the association of two cell-cell adhesion molecules, nectin and E-cadherin, through afadin and alpha-actinin in epithelial cells. J. Biol. Chem., 279, 31365-31373]. Our data suggest endogenous Lmo7 is a nucleocytoplasmic shuttling protein, and might also localize at focal adhesions in HeLa cells. Two key results show that Lmo7 regulates emerin gene expression: rat Lmo7 isoforms directly activated a luciferase reporter gene in vivo, and emerin mRNA expression decreased 93% in Lmo7-downregulated HeLa cells. Thus, Lmo7 not only binds emerin protein but is also required for emerin gene transcription. Microarray analysis of Lmo7-downregulated HeLa cells identified over 4200 misregulated genes, including 46 genes important for muscle or heart. Misregulation of 11 genes, including four (CREBBP, NAP1L1, LAP2, RBL2) known to be misregulated in X-EDMD patients and emerin-null mice [Bakay, M., Wang, Z., Melcon, G., Schiltz, L., Xuan, J., Zhao, P., Sartorelli, V., Seo, J., Pegoraro, E., Angelini, C. et al. (2006) Nuclear envelope dystrophies show a transcriptional fingerprint suggesting disruption of Rb-MyoD pathways in muscle regeneration. Brain, 129, 996-1013; Melcon, G., Kozlov, S., Cutler, D.A., Sullivan, T., Hernandez, L., Zhao, P., Mitchell, S., Nader, G., Bakay, M., Rottman, J.N. et al. (2006) Loss of emerin at the nuclear envelope disrupts the Rb1/E2F and MyoD pathways during muscle regeneration. Hum. Mol. Genet., 15, 637-651] was confirmed by real-time PCR. Overexpression of wild-type emerin, but not emerin mutant P183H (which causes EDMD and selectively disrupts binding to Lmo7), decreased the expression of CREBBP, NAP1L1 and LAP2, suggesting Lmo7 activity is both EDMD-relevant and inhibited by direct binding to emerin. We conclude that Lmo7 positively regulates many EDMD-relevant genes (including emerin), and is feedback-regulated by binding to emerin.