Nesprin-2 is a multi-isomeric protein that binds lamin and emerin at the nuclear envelope and forms a subcellular network in skeletal muscle

Defining contributions of paternally methylated imprinted genes at the Igf2-H19 and Dlk1-Gtl2 domains to mouse placentation by transcriptomic analysis

Parental genome functions in ontogeny are determined by interactions among transcripts from the maternal and paternal genomes, which contain many genes whose expression is strictly dependent on their parental origin as a result of genomic imprinting. Comprehensive recognition of the interactions between parental genomes is important for understanding genomic imprinting in mammalian development. The placenta is a key organ for exploring the biological significance of genomic imprinting. To decipher the unknown roles of paternally methylated imprinted genes on chromosomes 7 and 12 in mouse placentation, we performed a transcriptomic analysis on placentae in three types of bimaternal conceptuses that contained genomes derived from both non-growing and fully grown oocytes. Furthermore, we used the Ingenuity pathway analysis software to predict key networks and identify functions specific to paternally methylated imprinted genes regulated by the Igf2-H19 imprinting control region and Dlk1-Dio3 imprinting control region. The data suggested that dynamic conversion of the gene expression profile by restoring the expression of paternally methylated imprinted genes resulted in phenotypic improvements in bimaternal placentae. These results provide a framework to further explore the role of epigenetic modifications in paternal genome during mouse placentation.

The LINC-less granulocyte nucleus

The major blood granulocyte (neutrophil) is rapidly recruited to sites of bacterial and fungal infections. It is a highly malleable cell, allowing it to squeeze out of blood vessels and migrate through tight tissue spaces. The human granulocyte nucleus is lobulated and exhibits a paucity of nuclear lamins, increasing its capability for deformation. The present study examined the existence of protein connections between the nuclear envelope and cytoskeletal elements (the LINC complex) in differentiated cell states (i.e. granulocytic, monocytic and macrophage) of the human leukemic cell line HL-60, as well as in human blood leukocytes. HL-60 granulocytes exhibited a deficiency of several LINC complex proteins (i.e. nesprin 1 giant, nesprin 2 giant, SUN1, plectin and vimentin); whereas, the macrophage state revealed nesprin 1 giant, plectin and vimentin. Both states possessed SUN2 in the nuclear envelope. Parallel differences were observed with some of the LINC complex proteins in isolated human blood leukocytes, including macrophage cells derived from blood monocytes. The present study documenting the paucity of LINC complex proteins in granulocytic forms, in combination with previous data on granulocyte nuclear shape and nuclear envelope composition, suggest the hypothesis that these adaptations evolved to facilitate granulocyte cellular malleability.

Patterns of evolutionary conservation in the nesprin genes highlight probable functionally important protein domains and isoforms

The nesprins [also known as SYNEs (synaptic nuclear envelope proteins)] are a family of type II transmembrane proteins implicated in the tethering of membrane-bound organelles and in the genetic aetiology of cerebellar ataxia and Emery-Dreifuss muscular dystrophy. They are characterized by a common structure of an SR (spectrin repeat) rod domain and a C-terminal transmembrane KLS (klarsicht)/KASH [klarsicht/ANC-1 (anchorage 1)/SYNE homology] domain which interacts with SUN [Sad1p/UNC (uncoordinated)-84] proteins in the nuclear envelope; most nesprins also have N-terminal actin-binding CH (calponin homology) domains. The genes encoding the three vertebrate nesprins (five in bony fish) and the small transmembrane actin-binding protein calmin are related to each other by ancient duplications and rearrangements. In the present paper, we collate sequence data for nesprins and calmins across the vertebrate clade and use these to study evolutionary constraints acting on their genes. We show that the rod domains of the larger nesprins are composed almost entirely of unbroken SR-like structures (74 in nesprin-1 and 56 in nesprin-2) and that these range from poorly conserved purely structural elements to highly conserved regions with a presumed protein-protein interaction function. The analysis suggests several interesting regions for future study. We also assess the evolutionary and EST (expressed sequence tag) expression support for nesprin isoforms, both known and novel; our findings suggest that substantial reassessment is required.

Nesprin 4 is an outer nuclear membrane protein that can induce kinesin-mediated cell polarization

Nucleocytoplasmic coupling is mediated by outer nuclear membrane (ONM) nesprin proteins and inner nuclear membrane Sun proteins. Interactions spanning the perinuclear space create nesprin-Sun complexes connecting the cytoskeleton to nuclear components. A search for proteins displaying a conserved C-terminal sequence present in nesprins 1-3 identified nesprin 4 (Nesp4), a new member of this family. Nesp4 is a kinesin-1-binding protein that displays Sun-dependent localization to the ONM. Expression of Nesp4 is associated with dramatic changes in cellular organization involving relocation of the centrosome and Golgi apparatus relative to the nucleus. These effects can be accounted for entirely by Nesp4's kinesin-binding function. The implication is that Nesp4 may contribute to microtubule-dependent nuclear positioning.

Cell and molecular biology of nuclear actin

Actin is a highly conserved protein and one of the major components of the cytoplasm and the nucleus in eukaryotic cells. In the nucleus, actin is involved in a variety of nuclear processes that include transcription and transcription regulation, RNA processing and export, intranuclear movement, and structure maintenance. Recent advances in the field of nuclear actin have established that functions of actin in the nucleus are versatile, complex, and interconnected. It also has become increasingly evident that the cytoplasmic and nuclear pools of actin are functionally linked. However, while the biological significance of nuclear actin has become clear, we are only beginning to understand the mechanisms that lie behind the regulation of nuclear actin. This review provides an overview of our current understanding of the functions of actin in the nucleus.

Comparative proteomic analyses of the nuclear envelope and pore complex suggests a wide range of heretofore unexpected functions

Since the discovery of several inherited diseases linked to the nuclear envelope the number of functions ascribed to this subcellular organelle has skyrocketed. However the molecular pathways underlying these functions are not clear in most cases, perhaps because of missing components. Several recent proteomic analyses of the nuclear envelope and nuclear pore complex proteomes have yielded not only enough missing components to potentially elucidate these pathways, but suggest an exponentially greater number of functions at the nuclear periphery than ever imagined. Many of these functions appear to derive from recapitulation of pathways utilized at the plasma membrane and from other membrane systems. Additionally, many proteins identified in the comparative nuclear envelope studies have sequence characteristics suggesting that they might also contribute to nuclear pore complex functions. In particular, the striking enrichment for proteins in the nuclear envelope fractions that carry phenylalanine-glycine (FG) repeats may be significant for the mechanism of nuclear transport. In retrospect, these findings are only surprising in context of the notion held for many years that the nuclear envelope was only a barrier protecting the genome. In fact, it is arguably the most complex membrane organelle in the cell.

Loss of Drop1 expression already at early tumor stages in a wide range of human carcinomas

In a study on gene deregulation in ovarian carcinoma we found a mRNA coding for a 350 kDa protein, Drop1, to be downregulated 20- to 180-fold in the majority of ovarian and mammary carcinomas. The mRNA is encoded by a set of exons in the 5' region of the SYNE1 gene. Immunohistochemical staining for Drop1 protein by a specific monoclonal antibody corresponds to the pattern seen for the mRNA. cDNA arrays of matched pairs of tumor and normal tissue and in situ hybridizations confirmed the drastic loss of Drop1 mRNA as a common feature in uterus, cervix, kidney, lung, thyroid and pancreas carcinomas, already at early tumor stages and in all metastases. Two-hybrid studies suggest a role of this deficiency in the malignant progression of epithelial tumors.

A network of nuclear envelope membrane proteins linking centromeres to microtubules

In the fission yeast S. pombe, nuclei are actively positioned at the cell center by microtubules. Here, we show that cytoplasmic microtubules are mechanically coupled to the nuclear heterochromatin through proteins embedded in the nuclear envelope. This includes an integral outer nuclear membrane protein of the KASH family (Kms2) and two integral inner nuclear membrane proteins, the SUN-domain protein Sad1 and the previously uncharacterized protein Ima1. Ima1 specifically binds to heterochromatic regions and promotes the tethering of centromeric DNA to the SUN-KASH complex. In the absence of Ima1, or in cells harboring mutations in the centromeric Ndc80 complex, inefficient coupling of centromeric heterochromatin to Sad1 leads to striking defects in the ability of the nucleus to tolerate microtubule-dependent forces, leading to changes in nuclear shape, loss of spindle pole body components from the nuclear envelope, and partial dissociation of SUN-KASH complexes. This work highlights a framework for communication between cytoplasmic microtubules and chromatin.

Emerin and the Nuclear Lamina in Muscle and Cardiac Disease

The human genome is contained within the nucleus and is separated from the cytoplasm by the nuclear envelope. Mutations in the nuclear envelope proteins emerin and lamin A cause a number of diseases including premature aging syndromes, muscular dystrophy, and cardiomyopathy. Emerin and lamin A are implicated in regulating muscle- and heart-specific gene expression and nuclear architecture. For example, lamin A regulates the expression and localization of gap junction and intercalated disc components. Additionally, emerin and lamin A are also required to maintain nuclear envelope integrity. Demonstrating the importance of maintaining nuclear integrity in heart disease, atrioventricular node cells lacking lamin A exhibit increased nuclear deformation and apoptosis. This review highlights the present understanding of lamin A and emerin function in regulating nuclear architecture, gene expression, and cell signaling and discusses putative mechanisms for how specific mutations in lamin A and emerin cause cardiac- or muscle-specific disease.

Adult stem cell maintenance and tissue regeneration in the ageing context: the role for A-type lamins as intrinsic modulators of ageing in adult stem cells and their niches

Adult stem cells have been identified in most mammalian tissues of the adult body and are known to support the continuous repair and regeneration of tissues. A generalized decline in tissue regenerative responses associated with age is believed to result from a depletion and/or a loss of function of adult stem cells, which itself may be a driving cause of many age-related disease pathologies. Here we review the striking similarities between tissue phenotypes seen in many degenerative conditions associated with old age and those reported in age-related nuclear envelope disorders caused by mutations in the LMNA gene. The concept is beginning to emerge that nuclear filament proteins, A-type lamins, may act as signalling receptors in the nucleus required for receiving and/or transducing upstream cytosolic signals in a number of pathways central to adult stem cell maintenance as well as adaptive responses to stress. We propose that during ageing and in diseases caused by lamin A mutations, dysfunction of the A-type lamin stress-resistant signalling network in adult stem cells, their progenitors and/or stem cell niches leads to a loss of protection against growth-related stress. This in turn triggers an inappropriate activation or a complete failure of self-renewal pathways with the consequent initiation of stress-induced senescence. As such, A-type lamins should be regarded as intrinsic modulators of ageing within adult stem cells and their niches that are essential for survival to old age.

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.

A novel locus for dilated cardiomyopathy maps to canine chromosome 8

Dilated cardiomyopathy (DCM), the most common form of cardiomyopathy, often leads to heart failure and sudden death. While a substantial proportion of DCMs are inherited, mutations responsible for the majority of DCMs remain unidentified. A genome-wide linkage study was performed to identify the locus responsible for an autosomal recessive inherited form of juvenile DCM (JDCM) in Portuguese water dogs using 16 families segregating the disease. Results link the JDCM locus to canine chromosome 8 with two-point and multipoint lod scores of 10.8 and 14, respectively. The locus maps to a 3.9-Mb region, with complete syntenic homology to human chromosome 14, that contains no genes or loci known to be involved in the development of any type of cardiomyopathy. This discovery of a DCM locus with a previously unknown etiology will provide a new gene to examine in human DCM patients and a model for testing therapeutic approaches for heart failure.

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

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

Nesprin-2 Giant (NUANCE) maintains nuclear envelope architecture and composition in skin

Giant isoforms, encoded by Nesprin-1 (Syne1) and Nesprin-2 (Syne2), are multifunctional actin-binding and nuclear-envelope-associated proteins belonging to the spectrin superfamily. Here, we investigate the function of Nesprin-2 Giant (NUANCE) in skin by generating mice lacking the actin-binding domain of Nesprin-2 (Nesprin-2DeltaABD). This loss results in a slight but significant thickening of the epidermis, which is a consequence of the increased epithelial nuclear size. Nonetheless, epidermal proliferation and differentiation appear normal in the knockout epidermis. Surprisingly, Nesprin-2 C-terminal-isoform expression and nuclear envelope localization were affected in certain tissues. Nuclei of primary dermal knockout fibroblasts and keratinocytes were heavily misshapen, displaying a striking similarity to nuclear deformations characteristic of laminopathies. Furthermore, emerin, the protein involved in the X-linked form of Emery-Dreifuss muscular dystrophy (EDMD), was unevenly distributed along the nuclear envelope in mutant fibroblasts, often forming aggregates in the deformed nuclear envelope areas. Thus, Nesprin-2 is an important scaffold protein implicated in the maintenance of nuclear envelope architecture. Aged knockout fibroblasts readily generated, by alternative splicing and alternative translation initiation, aberrant Nesprin-2 Giant isoforms that lacked an ABD but that were sufficient to restore nuclear shape and emerin localization; this suggests that other regions of Nesprin-2 Giant, potentially including its spectrin repeats, are crucial for these functions.

The nuclear envelope as an integrator of nuclear and cytoplasmic architecture

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

Structural requirements for the assembly of LINC complexes and their function in cellular mechanical stiffness

The evolutionary-conserved interactions between KASH and SUN domain-containing proteins within the perinuclear space establish physical connections, called LINC complexes, between the nucleus and the cytoskeleton. Here, we show that the KASH domains of Nesprins 1, 2 and 3 interact promiscuously with luminal domains of Sun1 and Sun2. These constructs disrupt endogenous LINC complexes as indicated by the displacement of endogenous Nesprins from the nuclear envelope. We also provide evidence that KASH domains most probably fit a pocket provided by SUN domains and that post-translational modifications are dispensable for that interaction. We demonstrate that the disruption of endogenous LINC complexes affect cellular mechanical stiffness to an extent that compares to the loss of mechanical stiffness previously reported in embryonic fibroblasts derived from mouse lacking A-type lamins, a mouse model of muscular dystrophies and cardiomyopathies. These findings support a model whereby physical connections between the nucleus and the cytoskeleton are mediated by interactions between diverse combinations of Sun proteins and Nesprins through their respective evolutionary-conserved domains. Furthermore, they emphasize, for the first time, the relevance of LINC complexes in cellular mechanical stiffness suggesting a possible involvement of their disruption in various laminopathies, a group of human diseases linked to mutations of A-type lamins.

The laminopathies: the functional architecture of the nucleus and its contribution to disease

Most inherited diseases are associated with mutations in a specific gene. Often, mutations in two or more different genes result in diseases with a similar phenotype. Rarely do different mutations in the same gene result in a multitude of seemingly different and unrelated diseases. Mutations in the Lamin A gene (LMNA), which encodes largely ubiquitously expressed nuclear proteins (A-type lamins), are associated with at least eight different diseases, collectively called the laminopathies. Studies examining how different tissue-specific diseases arise from unique LMNA mutations are providing unanticipated insights into the structural organization of the nucleus, and how disruption of this organization relates to novel mechanisms of disease.

Requirements for the localization of nesprin-3 at the nuclear envelope and its interaction with plectin

The outer nuclear membrane proteins nesprin-1 and nesprin-2 are retained at the nuclear envelope through an interaction of their klarsicht/ANC-1/syne homology (KASH) domain with Sun proteins present at the inner nuclear membrane. We investigated the requirements for the localization of nesprin-3alpha at the outer nuclear membrane and show that the mechanism by which its localization is mediated is similar to that reported for the localization of nesprin-1 and nesprin-2: the last four amino acids of the nesprin-3alpha KASH domain are essential for its interaction with Sun1 and Sun2. Moreover, deletion of these amino acids or knockdown of the Sun proteins results in a redistribution of nesprin-3alpha away from the nuclear envelope and into the endoplasmic reticulum (ER), where it becomes colocalized with the cytoskeletal crosslinker protein plectin. Both nesprin-3alpha and plectin can form dimers, and dimerization of plectin is required for its interaction with nesprin-3alpha at the nuclear envelope, which is mediated by its N-terminal actin-binding domain. Additionally, overexpression of the plectin actin-binding domain stabilizes the actin cytoskeleton and prevents the recruitment of endogenous plectin to the nuclear envelope. Our studies support a model in which the actin cytoskeleton influences the binding of plectin dimers to dimers of nesprin-3alpha, which in turn are retained at the nuclear envelope through an interaction with Sun proteins.

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

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

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.

Nesprin-1 and -2 are involved in the pathogenesis of Emery Dreifuss muscular dystrophy and are critical for nuclear envelope integrity

Emery-Dreifuss muscular dystrophy (EDMD) is a heterogeneous late-onset disease involving skeletal muscle wasting and heart defects caused, in a minority of cases, by mutations in either of two genes encoding the inner nuclear membrane (INM) proteins, emerin and lamins A/C. Nesprin-1 and -2 are multi-isomeric, spectrin-repeat proteins that bind both emerin and lamins A/C and form a network in muscle linking the nucleoskeleton to the INM, the outer nuclear membrane, membraneous organelles, the sarcomere and the actin cytoskeleton. Thus, disruptions in nesprin/lamin/emerin interactions might play a role in the muscle-specific pathogenesis of EDMD. Screening for DNA variations in the genes encoding nesprin-1 (SYNE1) and nesprin-2 (SYNE2) in 190 probands with EDMD or EDMD-like phenotypes identified four heterozygous missense mutations. Fibroblasts from these patients exhibited nuclear morphology defects and specific patterns of emerin and SUN2 mislocalization. In addition, diminished nuclear envelope localization of nesprins and impaired nesprin/emerin/lamin binding interactions were common features of all EDMD patient fibroblasts. siRNA knockdown of nesprin-1 or -2 in normal fibroblasts reproduced the nuclear morphological changes and mislocalization of emerin and SUN2 observed in patient fibroblasts. Taken together, these data suggest that EDMD may be caused, in part, by uncoupling of the nucleoskeleton and cytoskeleton because of perturbed nesprin/emerin/lamin interactions.

Distinct functional domains in nesprin-1α and nesprin-2β bind directly to emerin and both interactions are disrupted in X-linked Emery–Dreifuss muscular dystrophy

Emerin and specific isoforms of nesprin-1 and -2 are nuclear membrane proteins which are binding partners in multi-protein complexes spanning the nuclear envelope. We report here the characterisation of the residues both in emerin and in nesprin-1alpha and -2beta which are involved in their interaction and show that emerin requires nesprin-1 or -2 to retain it at the nuclear membrane. Using several protein-protein interaction methods, we show that residues 368 to 627 of nesprin-1alpha and residues 126 to 219 of nesprin-2beta, which show high homology to one another, both mediate binding to emerin residues 140-176. This region has previously been implicated in binding to F-actin, beta-catenin and lamin A/C suggesting that it is critical for emerin function. Confirmation that these protein domains interact in vivo was shown using GFP-dominant negative assays. Exogenous expression of either of these nesprin fragments in mouse myoblast C2C12 cells displaced endogenous emerin from the nuclear envelope and reduced the targeting of newly synthesised emerin. Furthermore, we are the first to report that emerin mutations which give rise to X-linked Emery-Dreifuss muscular dystrophy, disrupt binding to both nesprin-1alpha and -2beta isoforms, further indicating a role of nesprins in the pathology of Emery-Dreifuss muscular dystrophy.

SUN1 is required for telomere attachment to nuclear envelope and gametogenesis in mice

Prior to the pairing and recombination between homologous chromosomes during meiosis, telomeres attach to the nuclear envelope and form a transient cluster. However, the protein factors mediating meiotic telomere attachment to the nuclear envelope and the requirement of this attachment for homolog pairing and synapsis have not been determined in animals. Here we show that the inner nuclear membrane protein SUN1 specifically associates with telomeres between the leptotene and diplotene stages during meiotic prophase I. Disruption of Sun1 in mice prevents telomere attachment to the nuclear envelope, efficient homolog pairing, and synapsis formation in meiosis. Massive apoptotic events are induced in the mutant gonads, leading to the abolishment of both spermatogenesis and oogenesis. This study provides genetic evidence that SUN1-telomere interaction is essential for telomere dynamic movement and is required for efficient homologous chromosome pairing/synapsis during mammalian gametogenesis.

Another way to move chromosomes

A typical way of moving chromosomes is exemplified by mitotic segregation, in which the centromere is directly captured by spindle microtubules. In this study, we highlight another way of moving chromosomes remotely from outside the nucleus, which involves SUN and KASH domain nuclear envelope proteins. SUN and KASH domain protein families are known to connect the nucleus to cytoskeletal networks and play a role in migration and positioning of the nucleus. Recent studies in the fission yeast Schizossacharomyces pombe demonstrated an additional role for the SUN-KASH protein complex in chromosome movements. During meiotic prophase, telomeres are moved to rearrange chromosomes within the nucleus. The SUN-KASH protein complex located in the nuclear envelope is involved in this process. Telomeres are connected to the SUN protein on the nucleoplasmic side, and the dynein motor complex binds to the KASH protein on the cytoplasmic side. Telomeres are then moved along the nuclear envelope using cytoplasmic microtubules. These findings illustrate a general mechanism for transmitting a cytoskeletal driving force to chromosomes across the nuclear envelope.

Cell nuclei spin in the absence of lamin b1

Mutations of the nuclear lamins cause a wide range of human diseases, including Emery-Dreifuss muscular dystrophy and Hutchinson-Gilford progeria syndrome. Defects in A-type lamins reduce nuclear structural integrity and affect transcriptional regulation, but few data exist on the biological role of B-type lamins. To assess the functional importance of lamin B1, we examined nuclear dynamics in fibroblasts from Lmnb1(Delta/Delta) and wild-type littermate embryos by time-lapse videomicroscopy. Here, we report that Lmnb1(Delta/Delta) cells displayed striking nuclear rotation, with approximately 90% of Lmnb1(Delta/Delta) nuclei rotating at least 90 degrees during an 8-h period. The rotation involved the nuclear interior as well as the nuclear envelope. The rotation of nuclei required an intact cytoskeletal network and was eliminated by expressing lamin B1 in cells. Nuclear rotation could also be abolished by expressing larger nesprin isoforms with long spectrin repeats. These findings demonstrate that lamin B1 serves a fundamental role within the nuclear envelope: anchoring the nucleus to the cytoskeleton.

Proteins that associate with lamins: many faces, many functions

Lamin-associated polypeptides (LAPs) comprise inner nuclear membrane proteins tightly associated with the peripheral lamin scaffold as well as proteins forming stable complexes with lamins in the nucleoplasm. The involvement of LAPs in a wide range of human diseases may be linked to an equally bewildering range of their functions, including sterol reduction, histone modification, transcriptional repression, and Smad- and beta-catenin signaling. Many LAPs are likely to be at the center of large multi-protein complexes, components of which may dictate their functions, and a few LAPs have defined enzymatic activities. Here we discuss the definition of LAPs, review their many binding partners, elaborate their functions in nuclear architecture, chromatin organization, gene expression and signaling, and describe what is currently known about their links to human disease.

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.

A-type lamin networks in light of laminopathic diseases

Lamins are major structural components of the lamina providing mechanical support for the nuclear envelope in vertebrates. A subgroup of lamins, the A-type lamins, are only expressed in differentiated cells and serve important functions both at the nuclear envelope and in the nucleoplasm in higher order chromatin organization and gene regulation. Mutations in A-type lamins cause a variety of diseases from muscular dystrophy and lipodystrophy to systemic diseases such as premature ageing syndromes. The molecular basis of these diseases is still unknown. Here we summarize known interactions of A-type lamins with components of the nuclear envelope and the nucleoplasm and discuss their potential involvement in the etiology and molecular mechanisms of the diseases. Lamin binding partners involve chromatin proteins potentially involved in higher order chromatin organization, transcriptional regulators controlling gene expression during cell cycle progression, differentiation and senescence, and several enzymes involved in a multitude of functions.

Herpes simplex virus infection induces phosphorylation and delocalization of emerin, a key inner nuclear membrane protein

The inner nuclear membrane (INM) contains specialized membrane proteins that selectively interact with nuclear components including the lamina, chromatin, and DNA. Alterations in the organization of and interactions with INM and lamina components are likely to play important roles in herpesvirus replication and, in particular, exit from the nucleus. Emerin, a member of the LEM domain class of INM proteins, binds a number of nuclear components including lamins, the DNA-bridging protein BAF, and F-actin and is thought to be involved in maintaining nuclear integrity. Here we report that emerin is quantitatively modified during herpes simplex virus (HSV) infection. Modification begins early in infection, involves multiple steps, and is reversed by phosphatase treatment. Emerin phosphorylation during infection involves one or more cellular kinases but can also be influenced by the US3 viral kinase, a protein whose function is known to be involved in HSV nuclear egress. The results from biochemical extraction analyses and from immunofluorescence of the detergent-resistant population demonstrate that emerin association with the INM significantly reduced during infection. We propose that the induction of emerin phosphorylation in infected cells may be involved in nuclear egress and uncoupling interactions with targets such as the lamina, chromatin, or cytoskeletal components.

Highway to the inner nuclear membrane: rules for the road

To enter the nucleus a protein must be chaperoned by a transport factor through the nuclear pore complex or it must be small enough to pass through by diffusion. Although these principles have long described the nuclear import of soluble proteins, recent evidence indicates that they also apply to the import of integral inner nuclear membrane proteins. Here we develop a set of rules that might govern the transport of proteins to the inner nuclear membrane.

Syne-1 and Syne-2 play crucial roles in myonuclear anchorage and motor neuron innervation

Proper nuclear positioning is important to cell function in many biological processes during animal development. In certain cells, the KASH-domain-containing proteins have been shown to be associated with the nuclear envelope, and to be involved in both nuclear anchorage and migration. We investigated the mechanism and function of nuclear anchorage in skeletal muscle cells by generating mice with single and double-disruption of the KASH-domain-containing genes Syne1 (also known as Syne-1)and Syne2 (also known as Syne-2). We showed that the deletion of the KASH domain of Syne-1 abolished the formation of clusters of synaptic nuclei and disrupted the organization of non-synaptic nuclei in skeletal muscle. Further analysis indicated that the loss of synaptic nuclei in Syne-1 KASH-knockout mice significantly affected the innervation sites and caused longer motor nerve branches. Although disruption of neither Syne-1 nor Syne-2 affected viability or fertility, Syne-1; Syne-2 double-knockout mice died of respiratory failure within 20 minutes of birth. These results suggest that the KASH-domain-containing proteins Syne-1 and Syne-2 play crucial roles in anchoring both synaptic and non-synaptic myonuclei that are important for proper motor neuron innervation and respiration.

What MAN1 does to the Smads

The inner nuclear membrane protein MAN1 has been identified as an important factor in transforming growth factor beta/bone morphogenic protein (TGFbeta/BMP) signaling. Loss of MAN1 results in three autosomal dominant diseases in humans; all three characterized by increased bone density. Xenopus embryos lacking MAN1 develop severe morphological defects. Both in humans and in Xenopus embryos the defects originate from deregulation of TGFbeta/BMP signaling. Several independent studies have shown that MAN1 is antagonizing TGFbeta/BMP signaling through binding to regulatory Smads. Here, recent progress in understanding MAN1 functions is summarized and a model for MAN1-dependent regulation of TGFbeta/BMP signaling is proposed.

Nuclear Mechanics and Methods

The role of the nucleus in protecting and sequestering the genome is intrinsically mechanical, and disease-causing structural mutants in lamins and other components underscore this function. Various methods to measure nuclear mechanics, isolated or in situ, are outlined here in some detail.

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

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

KASH-domain proteins in nuclear migration, anchorage and other processes

The nucleus in eukaryotic cells can move within the cytoplasm, and its position is crucial for many cellular events, including migration and differentiation. Nuclear anchorage and movement can be achieved through association of outer nuclear membrane (ONM) proteins with the three cytoskeletal systems. Two decades ago studies described C. elegans mutants with defects in such events, but only recently has it been shown that the strategies for nuclear positioning are indeed conserved in C. elegans, Drosophila, mammals and potentially all eukaryotes. The integral ONM proteins implicated in these processes thus far all contain a conserved Klarsicht/ANC-1/Syne homology (KASH) domain at their C-terminus that can associate with Sad1p/UNC-84 (SUN)-domain proteins of the inner nuclear membrane within the periplasmic space of the nuclear envelope (NE). The complex thus formed is responsible not only for association with cytoplasmic elements but also for the integrity of the NE itself.

Laminopathies: multiple disorders arising from defects in nuclear architecture

Lamins are the major structural proteins of the nucleus in an animal cell. In addition to being essential for nuclear integrity and assembly, lamins are involved in the organization of nuclear processes such as DNA replication, transcription and repair. Mutations in the human lamin A gene lead to highly debilitating genetic disorders that primarily affect muscle, adipose, bone or neuronal tissues and also cause premature ageing syndromes. Mutant lamins alter nuclear integrity and hinder signalling pathways involved in muscle differentiation and adipocyte differentiation, suggesting tissue-specific roles for lamins. Furthermore, cells expressing mutant lamins are impaired in their response to DNA damaging agents. Recent reports indicate that certain lamin mutations act in a dominant negative manner to cause nuclear defects and cellular toxicity, and suggest a possible role for aberrant lamins in normal ageing processes.

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

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

SUN-domain proteins: 'Velcro' that links the nucleoskeleton to the cytoskeleton

The novel SUN-domain family of nuclear envelope proteins interacts with various KASH-domain partners to form SUN-domain-dependent 'bridges' across the inner and outer nuclear membranes. These bridges physically connect the nucleus to every major component of the cytoskeleton. SUN-domain proteins have diverse roles in nuclear positioning, centrosome localization, germ-cell development, telomere positioning and apoptosis. By serving both as mechanical adaptors and nuclear envelope receptors, we propose that SUN-domain proteins connect cytoplasmic and nucleoplasmic activities.

The inner nuclear membrane protein emerin regulates beta-catenin activity by restricting its accumulation in the nucleus

Emerin is a type II inner nuclear membrane (INM) protein of unknown function. Emerin function is likely to be important because, when it is mutated, emerin promotes both skeletal muscle and heart defects. Here we show that one function of Emerin is to regulate the flux of beta-catenin, an important transcription coactivator, into the nucleus. Emerin interacts with beta-catenin through a conserved adenomatous polyposis coli (APC)-like domain. When GFP-emerin is expressed in HEK293 cells, beta-catenin is restricted to the cytoplasm and beta-catenin activity is inhibited. In contrast, expression of an emerin mutant, lacking its APC-like domain (GFP-emerinDelta), dominantly stimulates beta-catenin activity and increases nuclear accumulation of beta-catenin. Human fibroblasts that are null for emerin have an autostimulatory growth phenotype. This unusual growth phenotype arises through enhanced nuclear accumulation and activity of beta-catenin and can be replicated in wild-type fibroblasts by transfection with constitutively active beta-catenin. Our results support recent findings that suggest that INM proteins can influence signalling pathways by restricting access of transcription coactivators to the nucleus.

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

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

Nuclear Lamins: Laminopathies and Their Role in Premature Ageing

It has been demonstrated that nuclear lamins are important proteins in maintaining cellular as well as nuclear integrity, and in maintaining chromatin organization in the nucleus. Moreover, there is growing evidence that lamins play a prominent role in transcriptional control. The family of laminopathies is a fast-growing group of diseases caused by abnormalities in the structure or processing of the lamin A/C ( LMNA) gene. Mutations or incorrect processing cause more than a dozen different inherited diseases, ranging from striated muscular diseases, via fat- and peripheral nerve cell diseases, to progeria. This broad spectrum of diseases can only be explained if the responsible A-type lamin proteins perform multiple functions in normal cells. This review gives an overview of current knowledge on lamin structure and function and all known diseases associated with LMNA abnormalities. Based on the knowledge of the different functions of A-type lamins and associated proteins, explanations for the observed phenotypes are postulated. It is concluded that lamins seem to be key players in, among others, controlling the process of cellular ageing, since disturbance in lamin protein structure gives rise to several forms of premature ageing.

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.

SUN1 interacts with nuclear lamin A and cytoplasmic nesprins to provide a physical connection between the nuclear lamina and the cytoskeleton

Nuclear migration and positioning within cells are critical for many developmental processes and are governed by the cytoskeletal network. Although mechanisms of nuclear-cytoskeletal attachment are unclear, growing evidence links a novel family of nuclear envelope (NE) proteins that share a conserved C-terminal SUN (Sad1/UNC-84 homology) domain. Analysis of Caenorhabditis elegans mutants has implicated UNC-84 in actin-mediated nuclear positioning by regulating NE anchoring of a giant actin-binding protein, ANC-1. Here, we report the identification of SUN1 as a lamin A-binding protein in a yeast two-hybrid screen. We demonstrate that SUN1 is an integral membrane protein located at the inner nuclear membrane. While the N-terminal domain of SUN1 is responsible for detergent-resistant association with the nuclear lamina and lamin A binding, lamin A/C expression is not required for SUN1 NE localization. Furthermore, SUN1 does not interact with type B lamins, suggesting that NE localization is ensured by binding to an additional nuclear component(s), most likely chromatin. Importantly, we find that the luminal C-terminal domain of SUN1 interacts with the mammalian ANC-1 homologs nesprins 1 and 2 via their conserved KASH domain. Our data provide evidence of a physical nuclear-cytoskeletal connection that is likely to be a key mechanism in nuclear-cytoplasmic communication and regulation of nuclear position.

Emerin-lacking mice show minimal motor and cardiac dysfunctions with nuclear-associated vacuoles

Emery-Dreifuss muscular dystrophy is an inherited muscular disorder clinically characterized by slowly progressive weakness affecting humero-peroneal muscles, early joint contractures, and cardiomyopathy with conduction block. The X-linked recessive form is caused by mutation in the EMD gene encoding an integral protein of the inner nuclear membrane, emerin. In this study, mutant mice lacking emerin were produced by insertion of a neomycin resistance gene into exon 6 of the coding gene. Tissues taken from mutant mice lacked emerin. The mutant mice displayed a normal growth rate indistinguishable from their littermates and were fertile. No marked muscle weakness or joint abnormalities were observed; however, rotarod test revealed altered motor coordination. Electrocardiography showed mild prolongation of atrioventricular conduction time in emerin-lacking male mice older than 40 weeks of age. Electron microscopic analysis of skeletal and cardiac muscles from emerin-lacking mice revealed small vacuoles, which mostly bordered the myonuclei. Our results suggest that emerin deficiency causes minimal motor and cardiac dysfunctions in mice with a structural fragility of myonuclei.