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

Tail-anchored membrane protein SLMAP3 is essential for targeting centrosomal proteins to the nuclear envelope in skeletal myogenesis

The positioning and communication between the nucleus and centrosomes are essential in cell division, differentiation and tissue formation. During skeletal myogenesis, the nuclei become evenly spaced with the switch of the microtubule-organizing centre (MTOC) from the centrosome to the nuclear envelope (NE). We report that the tail-anchored sarcolemmal membrane associated protein 3 (SLMAP3), a component of the MTOC and NE, is crucial for myogenesis because its deletion in mice leads to a reduction in the NE-MTOC formation, mislocalization of the nuclei, dysregulation of the myogenic programme and abnormal embryonic myofibres. SLMAP3-/- myoblasts also displayed a similar disorganized distribution of nuclei with an aberrant NE-MTOC and defective myofibre formation and differentiation programming. We identified novel interactors of SLMAP3, including pericentrin, PCM1 (pericentriolar material 1), AKAP9 (A-kinase anchoring protein 9), kinesin-1 members Kif5B (kinesin family member 5B), KCL1 (kinesin light chain 1), KLC2 (kinesin light chain 2) and nuclear lamins, and observed that the distribution of centrosomal proteins at the NE together with Nesprin-1 was significantly altered by the loss of SLMAP3 in differentiating myoblasts. SLMAP3 is believed to negatively regulate Hippo signalling, but its loss was without impact on this pathway in developing muscle. These results reveal that SLMAP3 is essential for skeletal myogenesis through unique mechanisms involving the positioning of nuclei, NE-MTOC dynamics and gene programming.

Nesprin proteins: bridging nuclear envelope dynamics to muscular dysfunction

This review presents a comprehensive exploration of the pivotal role played by the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex, with a particular focus on Nesprin proteins, in cellular mechanics and the pathogenesis of muscular diseases. Distinguishing itself from prior works, the analysis delves deeply into the intricate interplay of the LINC complex, emphasizing its indispensable contribution to maintaining cellular structural integrity, especially in mechanically sensitive tissues such as cardiac and striated muscles. Additionally, the significant association between mutations in Nesprin proteins and the onset of Dilated Cardiomyopathy (DCM) and Emery-Dreifuss Muscular Dystrophy (EDMD) is highlighted, underscoring their pivotal role in disease pathogenesis. Through a comprehensive examination of DCM and EDMD cases, the review elucidates the disruptions in the LINC complex, nuclear morphology alterations, and muscular developmental disorders, thus emphasizing the essential function of an intact LINC complex in preserving muscle physiological functions. Moreover, the review provides novel insights into the implications of Nesprin mutations for cellular dynamics in the pathogenesis of muscular diseases, particularly in maintaining cardiac structural and functional integrity. Furthermore, advanced therapeutic strategies, including rectifying Nesprin gene mutations, controlling Nesprin protein expression, enhancing LINC complex functionality, and augmenting cardiac muscle cell function are proposed. By shedding light on the intricate molecular mechanisms underlying nuclear-cytoskeletal interactions, the review lays the groundwork for future research and therapeutic interventions aimed at addressing genetic muscle disorders.

Nesprin-1: novel regulator of striated muscle nuclear positioning and mechanotransduction

Nesprins (nuclear envelope spectrin repeat proteins) are multi-isomeric scaffolding proteins. Giant nesprin-1 and -2 localise to the outer nuclear membrane, interact with SUN (Sad1p/UNC-84) domain-containing proteins at the inner nuclear membrane to form the LInker of Nucleoskeleton and Cytoskeleton (LINC) complex, which, in association with lamin A/C and emerin, mechanically couples the nucleus to the cytoskeleton. Despite ubiquitous expression of nesprin giant isoforms, pathogenic mutations in nesprin-1 and -2 are associated with tissue-specific disorders, particularly related to striated muscle such as dilated cardiomyopathy and Emery-Dreifuss muscular dystrophy. Recent evidence suggests this muscle-specificity might be attributable in part, to the small muscle specific isoform, nesprin-1α2, which has a novel role in striated muscle function. Our current understanding of muscle-specific functions of nesprin-1 and its isoforms will be summarised in this review to provide insight into potential pathological mechanisms of nesprin-related muscle disease and may inform potential targets of therapeutic modulation.

Nuclear Envelope Alterations in Myotonic Dystrophy Type 1 Patient-Derived Fibroblasts

Myotonic dystrophy type 1 (DM1) is a hereditary and multisystemic disease characterized by myotonia, progressive distal muscle weakness and atrophy. The molecular mechanisms underlying this disease are still poorly characterized, although there are some hypotheses that envisage to explain the multisystemic features observed in DM1. An emergent hypothesis is that nuclear envelope (NE) dysfunction may contribute to muscular dystrophies, particularly to DM1. Therefore, the main objective of the present study was to evaluate the nuclear profile of DM1 patient-derived and control fibroblasts and to determine the protein levels and subcellular distribution of relevant NE proteins in these cell lines. Our results demonstrated that DM1 patient-derived fibroblasts exhibited altered intracellular protein levels of lamin A/C, LAP1, SUN1, nesprin-1 and nesprin-2 when compared with the control fibroblasts. In addition, the results showed an altered location of these NE proteins accompanied by the presence of nuclear deformations (blebs, lobes and/or invaginations) and an increased number of nuclear inclusions. Regarding the nuclear profile, DM1 patient-derived fibroblasts had a larger nuclear area and a higher number of deformed nuclei and micronuclei than control-derived fibroblasts. These results reinforce the evidence that NE dysfunction is a highly relevant pathological characteristic observed in DM1.

The LINC Complex Assists the Nuclear Import of Mechanosensitive Transcriptional Regulators

Mechanical forces play pivotal roles in directing cell functions and fate. To elicit gene expression, either intrinsic or extrinsic mechanical information are transmitted into the nucleus beyond the nuclear envelope via at least two distinct pathways, possibly more. The first and well-known pathway utilizes the canonical nuclear transport of mechanoresponsive transcriptional regulators through the nuclear pore complex, which is an exclusive route for macromolecular trafficking between the cytoplasm and nucleoplasm. The second pathway depends on the linker of the nucleoskeleton and cytoskeleton (LINC) complex, which is a molecular bridge traversing the nuclear envelope between the cytoskeleton and nucleoskeleton. This protein complex is a central component in mechanotransduction at the nuclear envelope that transmits mechanical information from the cytoskeleton into the nucleus to influence the nuclear structure, nuclear stiffness, chromatin organization, and gene expression. Besides the mechanical force transducing function, recent increasing evidence shows that the LINC complex plays a role in controlling nucleocytoplasmic transport of mechanoresponsive transcriptional regulators. Here we discuss recent findings regarding the contribution of the LINC complex to the regulation of intracellular localization of the most-notable mechanosensitive transcriptional regulators, β-catenin, YAP, and TAZ.

Evolution and diversification of the nuclear envelope

Eukaryotic cells arose ~1.5 billion years ago, with the endomembrane system a central feature, facilitating evolution of intracellular compartments. Endomembranes include the nuclear envelope (NE) dividing the cytoplasm and nucleoplasm. The NE possesses universal features: a double lipid bilayer membrane, nuclear pore complexes (NPCs), and continuity with the endoplasmic reticulum, indicating common evolutionary origin. However, levels of specialization between lineages remains unclear, despite distinct mechanisms underpinning various nuclear activities. Several distinct modes of molecular evolution facilitate organellar diversification and  to understand which apply to the NE, we exploited proteomic datasets of purified nuclear envelopes from model systems for comparative analysis. We find enrichment of core nuclear functions amongst the widely conserved proteins to be less numerous than lineage-specific cohorts, but enriched in core nuclear functions. This, together with consideration of additional evidence, suggests that, despite a common origin, the NE has evolved as a highly diverse organelle with significant lineage-specific functionality.

LINCing Nuclear Mechanobiology With Skeletal Muscle Mass and Function

Skeletal muscle demonstrates a high degree of adaptability in response to changes in mechanical input. The phenotypic transformation in response to mechanical cues includes changes in muscle mass and force generating capabilities, yet the molecular pathways that govern skeletal muscle adaptation are still incompletely understood. While there is strong evidence that mechanotransduction pathways that stimulate protein synthesis play a key role in regulation of muscle mass, there are likely additional mechano-sensitive mechanisms important for controlling functional muscle adaptation. There is emerging evidence that the cell nucleus can directly respond to mechanical signals (i.e., nuclear mechanotransduction), providing a potential additional level of cellular regulation for controlling skeletal muscle mass. The importance of nuclear mechanotransduction in cellular function is evident by the various genetic diseases that arise from mutations in proteins crucial to the transmission of force between the cytoskeleton and the nucleus. Intriguingly, these diseases preferentially affect cardiac and skeletal muscle, suggesting that nuclear mechanotransduction is critically important for striated muscle homeostasis. Here we discuss our current understanding for how the nucleus acts as a mechanosensor, describe the main cytoskeletal and nuclear proteins involved in the process, and propose how similar mechanoresponsive mechanisms could occur in the unique cellular environment of a myofiber. In addition, we examine how nuclear mechanotransduction fits into our current framework for how mechanical stimuli regulates skeletal muscle mass.

Nuclear Mechanotransduction in Skeletal Muscle

Skeletal muscle is composed of multinucleated, mature muscle cells (myofibers) responsible for contraction, and a resident pool of mononucleated muscle cell precursors (MCPs), that are maintained in a quiescent state in homeostatic conditions. Skeletal muscle is remarkable in its ability to adapt to mechanical constraints, a property referred as muscle plasticity and mediated by both MCPs and myofibers. An emerging body of literature supports the notion that muscle plasticity is critically dependent upon nuclear mechanotransduction, which is transduction of exterior physical forces into the nucleus to generate a biological response. Mechanical loading induces nuclear deformation, changes in the nuclear lamina organization, chromatin condensation state, and cell signaling, which ultimately impacts myogenic cell fate decisions. This review summarizes contemporary insights into the mechanisms underlying nuclear force transmission in MCPs and myofibers. We discuss how the cytoskeleton and nuclear reorganizations during myogenic differentiation may affect force transmission and nuclear mechanotransduction. We also discuss how to apply these findings in the context of muscular disorders. Finally, we highlight current gaps in knowledge and opportunities for further research in the field.

Actin on and around the Nucleus

Actin plays roles in many important cellular processes, including cell motility, organelle movement, and cell signaling. The discovery of transmembrane actin-binding proteins at the outer nuclear membrane (ONM) raises the exciting possibility that actin can play a role in direct force transmission to the nucleus and the genome at its interior. Actin-dependent nucleus displacement was first described a decade ago. We are now gaining a more detailed understanding of its mechanisms, as well as new roles for actin during mitosis and meiosis, for gene expression, and in the cell's response to mechanical stimuli. Here we review these recent developments, the actin-binding proteins involved, the tissue specificity of these mechanisms, and methods developed to reconstitute and study this interaction in vitro.

The Molecular Basis and Biologic Significance of the β-Dystroglycan-Emerin Interaction

β-dystroglycan (β-DG) assembles with lamins A/C and B1 and emerin at the nuclear envelope (NE) to maintain proper nuclear architecture and function. To provide insight into the nuclear function of β-DG, we characterized the interaction between β-DG and emerin at the molecular level. Emerin is a major NE protein that regulates multiple nuclear processes and whose deficiency results in Emery–Dreifuss muscular dystrophy (EDMD). Using truncated variants of β-DG and emerin, via a series of in vitro and in vivo binding experiments and a tailored computational analysis, we determined that the β-DG–emerin interaction is mediated at least in part by their respective transmembrane domains (TM). Using surface plasmon resonance assays we showed that emerin binds to β-DG with high affinity (KD in the nanomolar range). Remarkably, the analysis of cells in which DG was knocked out demonstrated that loss of β-DG resulted in a decreased emerin stability and impairment of emerin-mediated processes. β-DG and emerin are reciprocally required for their optimal targeting within the NE, as shown by immunofluorescence, western blotting and immunoprecipitation assays using emerin variants with mutations in the TM domain and B-lymphocytes of a patient with EDMD. In summary, we demonstrated that β-DG plays a role as an emerin interacting partner modulating its stability and function.

Lamin A/C Mechanotransduction in Laminopathies

Mechanotransduction translates forces into biological responses and regulates cell functionalities. It is implicated in several diseases, including laminopathies which are pathologies associated with mutations in lamins and lamin-associated proteins. These pathologies affect muscle, adipose, bone, nerve, and skin cells and range from muscular dystrophies to accelerated aging. Although the exact mechanisms governing laminopathies and gene expression are still not clear, a strong correlation has been found between cell functionality and nuclear behavior. New theories base on the direct effect of external force on the genome, which is indeed sensitive to the force transduced by the nuclear lamina. Nuclear lamina performs two essential functions in mechanotransduction pathway modulating the nuclear stiffness and governing the chromatin remodeling. Indeed, A-type lamin mutation and deregulation has been found to affect the nuclear response, altering several downstream cellular processes such as mitosis, chromatin organization, DNA replication-transcription, and nuclear structural integrity. In this review, we summarize the recent findings on the molecular composition and architecture of the nuclear lamina, its role in healthy cells and disease regulation. We focus on A-type lamins since this protein family is the most involved in mechanotransduction and laminopathies.

Emery-Dreifuss muscular dystrophy

Emery-Dreifuss muscular dystrophy (EDMD) is a rare muscular dystrophy, but is particularly important to diagnose due to frequent life-threatening cardiac complications. EDMD classically presents with muscle weakness, early contractures, cardiac conduction abnormalities and cardiomyopathy, although the presence and severity of these manifestations vary by subtype and individual. Associated genes include EMD, LMNA, SYNE1, SYNE2, FHL1, TMEM43, SUN1, SUN2, and TTN, encoding emerin, lamin A/C, nesprin-1, nesprin-2, FHL1, LUMA, SUN1, SUN2, and titin, respectively. The Online Mendelian Inheritance in Man database recognizes subtypes 1 through 7, which captures most but not all of the associated genes. Genetic diagnosis is essential whenever available, but traditional diagnostic tools can help steer the evaluation toward EDMD and assist with interpretation of equivocal genetic test results. Management is primarily supportive, but it is important to monitor patients closely, especially for potential cardiac complications. There is a high potential for progress in the treatment of EDMD in the coming years.

Drosophila emerins control LINC complex localization and transcription to regulate myonuclear position

Mispositioned nuclei are a hallmark of skeletal muscle disease. Many of the genes that are linked to Emery-Dreifuss muscular dystrophy (EDMD) encode proteins that are critical for nuclear movement in various cells, suggesting that disruptions in nuclear movement and position may contribute to disease progression. However, how these genes are coordinated to move nuclei is not known. Here, we focussed on two different emerin proteins in Drosophila, Bocksbeutel and Otefin, and their effects on nuclear movement. Although nuclear position was dependent on both, elimination of either Bocksbeutel or Otefin produced distinct phenotypes that were based in differential effects on the KASH-domain protein Klarsicht. Specifically, loss of Bocksbeutel reduced Klarsicht localization to the nucleus and resulted in a disruption in nuclear separation. Loss of Otefin increased the transcription of Klarsicht and led to premature separation of nuclei and their positioning closer to the edge of the muscle. Consistent with opposing functions, nuclear position is normal in otefin; bocksbeutel double mutants. These data indicate emerin-dependent regulation of Klarsicht levels in the nuclear envelope is a critical determinant of nuclear position.

Nesprin-1-alpha2 associates with kinesin at myotube outer nuclear membranes, but is restricted to neuromuscular junction nuclei in adult muscle

Nesprins, nuclear envelope spectrin-repeat proteins encoded by the SYNE1 and SYNE2 genes, are involved in localization of nuclei. The short isoform, nesprin-1-alpha2, is required for relocation of the microtubule organizer function from centromeres to the nuclear rim during myogenesis. Using specific antibodies, we now show that both nesprin-1-alpha2 and nesprin-1-giant co-localize with kinesin at the junctions of concatenated nuclei and at the outer poles of nuclear chains in human skeletal myotubes. In adult muscle, nesprin-1-alpha2 was found, together with kinesin, only on nuclei associated with neuromuscular junctions, whereas all adult cardiomyocyte nuclei expressed nesprin-1-alpha2. In a proteomics study, kinesin heavy and light chains were the only significant proteins in myotube extracts pulled down by nesprin-1-alpha2, but not by a mutant lacking the highly-conserved STAR domain (18 amino-acids, including the LEWD motif). The results support a function for nesprin-1-alpha2 in the specific localization of skeletal muscle nuclei mediated by kinesins and suggest that its primary role is at the outer nuclear membrane.

Rare BANF1 Alleles and Relatively Frequent EMD Alleles Including 'Healthy Lipid' Emerin p.D149H in the ExAC Cohort

Emerin (EMD) and barrier to autointegration factor 1 (BANF1) each bind A-type lamins (LMNA) as fundamental components of nuclear lamina structure. Mutations in LMNA, EMD and BANF1 are genetically linked to many tissue-specific disorders including Emery-Dreifuss muscular dystrophy and cardiomyopathy (LMNA, EMD), lipodystrophy, insulin resistance and type 2 diabetes (LMNA) and progeria (LMNA, BANF1). To explore human genetic variation in these genes, we analyzed EMD and BANF1 alleles in the Exome Aggregation Consortium (ExAC) cohort of 60,706 unrelated individuals. We identified 13 rare heterozygous BANF1 missense variants (p.T2S, p.H7Y, p.D9N, p.S22R, p.G25E, p.D55N, p.D57Y, p.L63P, p.N70T, p.K72R, p.R75W, p.R75Q, p.G79R), and one homozygous variant (p.D9H). Several variants are known (p.G25E) or predicted (e.g., p.D9H, p.D9N, p.L63P) to perturb BANF1 and warrant further study. Analysis of EMD revealed two previously identified variants associated with adult-onset cardiomyopathy (p.K37del, p.E35K) and one deemed 'benign' in an Emery-Dreifuss patient (p.D149H). Interestingly p.D149H was the most frequent emerin variant in ExAC, identified in 58 individuals (overall allele frequency 0.06645%), of whom 55 were East Asian (allele frequency 0.8297%). Furthermore, p.D149H associated with four 'healthy' traits: reduced triglycerides (-0.336; p = 0.0368), reduced waist circumference (-0.321; p = 0.0486), reduced cholesterol (-0.572; p = 0.000346) and reduced LDL cholesterol (-0.599; p = 0.000272). These traits are distinct from LMNA-associated metabolic disorders and provide the first insight that emerin influences metabolism. We also identified one novel in-frame deletion (p.F39del) and 62 novel emerin missense variants, many of which were relatively frequent and potentially disruptive including p.N91S and p.S143F (∼0.041% and ∼0.034% of non-Finnish Europeans, respectively), p.G156S (∼0.39% of Africans), p.R204G (∼0.18% of Latinx), p.R207P (∼0.08% of South Asians) and p.R221L (∼0.15% of Latinx). Many novel BANF1 variants are predicted to disrupt dimerization or binding to DNA, histones, emerin or A-type lamins. Many novel emerin variants are predicted to disrupt emerin filament dynamics or binding to BANF1, HDAC3, A-type lamins or other partners. These new human variants provide a foundational resource for future studies to test the molecular mechanisms of BANF1 and emerin function, and to understand the link between emerin variant p.D149H and a 'healthy' lipid profile.

Samp1 Mislocalization in Emery-Dreifuss Muscular Dystrophy

LMNA linked-Emery-Dreifuss muscular dystrophy (EDMD2) is a rare disease characterized by muscle weakness, muscle wasting, and cardiomyopathy with conduction defects. The mutated protein lamin A/C binds several nuclear envelope components including the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex and the inner nuclear membrane protein Samp1 (Spindle Associated Membrane Protein 1). Considering that Samp1 is upregulated during muscle cell differentiation and it is involved in nuclear movement, we hypothesized that it could be part of the protein platform formed by LINC proteins and prelamin A at the myotube nuclear envelope and, as previously demonstrated for those proteins, could be affected in EDMD2. Our results show that Samp1 is uniformly distributed at the nuclear periphery of normal human myotubes and committed myoblasts, but its anchorage at the nuclear poles is related to the presence of farnesylated prelamin A and it is disrupted by the loss of prelamin A farnesylation. Moreover, Samp1 is absent from the nuclear poles in EDMD2 myotubes, which shows that LMNA mutations associated with muscular dystrophy, due to reduced prelamin A levels in muscle cell nuclei, impair Samp1 anchorage. Conversely, SUN1 pathogenetic mutations do not alter Samp1 localization in myotubes, which suggests that Samp1 lies upstream of SUN1 in nuclear envelope protein complexes. The hypothesis that Samp1 is part of the protein platform that regulates microtubule nucleation from the myotube nuclear envelope in concert with pericentrin and LINC components warrants future investigation. As a whole, our data identify Samp1 as a new contributor to EDMD2 pathogenesis and our data are relevant to the understanding of nuclear clustering occurring in laminopathic muscle.

The increasing relevance of nuclear envelope myopathies

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Novel nesprin-1 mutations associated with dilated cardiomyopathy cause nuclear envelope disruption and defects in myogenesis

Nesprins-1 and -2 are highly expressed in skeletal and cardiac muscle and together with SUN (Sad1p/UNC84)-domain containing proteins and lamin A/C form the LInker of Nucleoskeleton-and-Cytoskeleton (LINC) bridging complex at the nuclear envelope (NE). Mutations in nesprin-1/2 have previously been found in patients with autosomal dominant Emery–Dreifuss muscular dystrophy (EDMD) as well as dilated cardiomyopathy (DCM). In this study, three novel rare variants (R8272Q, S8381C and N8406K) in the C-terminus of the SYNE1 gene (nesprin-1) were identified in seven DCM patients by mutation screening. Expression of these mutants caused nuclear morphology defects and reduced lamin A/C and SUN2 staining at the NE. GST pull-down indicated that nesprin-1/lamin/SUN interactions were disrupted. Nesprin-1 mutations were also associated with augmented activation of the ERK pathway in vitro and in hearts in vivo. During C2C12 muscle cell differentiation, nesprin-1 levels are increased concomitantly with kinesin light chain (KLC-1/2) and immunoprecipitation and GST pull-down showed that these proteins interacted via a recently identified LEWD domain in the C-terminus of nesprin-1. Expression of nesprin-1 mutants in C2C12 cells caused defects in myoblast differentiation and fusion associated with dysregulation of myogenic transcription factors and disruption of the nesprin-1 and KLC-1/2 interaction at the outer nuclear membrane. Expression of nesprin-1α₂ WT and mutants in zebrafish embryos caused heart developmental defects that varied in severity. These findings support a role for nesprin-1 in myogenesis and muscle disease, and uncover a novel mechanism whereby disruption of the LINC complex may contribute to the pathogenesis of DCM.

Nesprin-1α-Dependent Microtubule Nucleation from the Nuclear Envelope via Akap450 Is Necessary for Nuclear Positioning in Muscle Cells

The nucleus is the main microtubule-organizing center (MTOC) in muscle cells due to the accumulation of centrosomal proteins and microtubule (MT) nucleation activity at the nuclear envelope (NE) [1, 2, 3, 4]. The relocalization of centrosomal proteins, including Pericentrin, Pcm1, and γ-tubulin, depends on Nesprin-1, an outer nuclear membrane (ONM) protein that connects the nucleus to the cytoskeleton via its N-terminal region [5, 6, 7]. Nesprins are also involved in the recruitment of kinesin to the NE and play a role in nuclear positioning in skeletal muscle cells [8, 9, 10, 11, 12]. However, a function for MT nucleation from the NE in nuclear positioning has not been established. Using the proximity-dependent biotin identification (BioID) method [13, 14], we found several centrosomal proteins, including Akap450, Pcm1, and Pericentrin, whose association with Nesprin-1α is increased in differentiated myotubes. We show that Nesprin-1α recruits Akap450 to the NE independently of kinesin and that Akap450, but not other centrosomal proteins, is required for MT nucleation from the NE. Furthermore, we demonstrate that this mechanism is disrupted in congenital muscular dystrophy patient myotubes carrying a nonsense mutation within the SYNE1 gene (23560 G>T) encoding Nesprin-1 [15, 16]. Finally, using computer simulation and cell culture systems, we provide evidence for a role of MT nucleation from the NE on nuclear spreading in myotubes. Our data thus reveal a novel function for Nesprin-1α/Nesprin-1 in nuclear positioning through recruitment of Akap450-mediated MT nucleation activity to the NE.

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BioID of Nesprin-1α identifies centrosomal proteins at myotube nuclear envelope

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Nesprin-1α-containing LINC complexes recruit Akap450 to myotube nuclear envelope

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Akap450 is required for microtubule nucleation at the nuclear envelope

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Microtubule nucleation at the nuclear envelope is involved in nuclear positioning

Control of nuclear organization by F-actin binding proteins

The regulation of nuclear shape and deformability is a key factor in controlling diverse events from embryonic development to cancer cell metastasis, but the mechanisms governing this process are still unclear. Our recent study demonstrated an unexpected role for the F-actin bundling protein fascin in controlling nuclear plasticity through a direct interaction with Nesprin-2. Nesprin-2 is a component of the LINC complex that is known to couple the F-actin cytoskeleton to the nuclear envelope. We demonstrated that fascin, which is predominantly associated with peripheral F-actin rich filopodia, binds directly to Nesprin-2 at the nuclear envelope in a range of cell types. Depleting fascin or specifically blocking the fascin-Nesprin-2 complex leads to defects in nuclear polarization, movement and cell invasion. These studies reveal a novel role for an F-actin bundling protein in control of nuclear plasticity and underline the importance of defining nuclear-associated roles for F-actin binding proteins in future.

Moving and positioning the nucleus in skeletal muscle - one step at a time

Nuclear movement and positioning within cells has become an area of great interest in the past few years due to the identification of different molecular mechanisms and functions in distinct organisms and contexts. One extreme example occurs during skeletal muscle development and regeneration. Skeletal muscles are composed of individual multinucleated myofibers with nuclei positioned at their periphery. Myofibers are formed by fusion of mononucleated myoblasts and during their development, successive nuclear movements and positioning events have been described. The position of the nuclei in myofibers is important for muscle function. Interestingly, during muscle regeneration and in some muscular diseases, nuclei are positioned in the center of the myofiber. In this review, we discuss the multiple mechanisms of nuclear positioning that occur during myofiber formation and regeneration. We also discuss the role of nuclear positioning for skeletal muscle function.

Nuclear Mechanics and Stem Cell Differentiation

Stem cells are characterized by their self-renewal and multi-lineage differentiation potential. Stem cell differentiation is a prerequisite for the application of stem cells in regenerative medicine and clinical therapy. In addition to chemical stimulation, mechanical cues play a significant role in regulating stem cell differentiation. The integrity of mechanical sensors is necessary for the ability of cells to respond to mechanical signals. The nucleus, the largest and stiffest cellular organelle, interacts with the cytoskeleton as a key mediator of cell mechanics. Nuclear mechanics are involved in the complicated interactions of lamins, chromatin and nucleoskeleton-related proteins. Thus, stem cell differentiation is intimately associated with nuclear mechanics due to its indispensable role in mechanotransduction and mechanical response. This paper reviews several main contributions of nuclear mechanics, highlights the hallmarks of the nuclear mechanics of stem cells, and provides insight into the relationship between nuclear mechanics and stem cell differentiation, which may guide clinical applications in the future.

N-terminal nesprin-2 variants regulate β-catenin signalling

The spatial compartmentalisation of biochemical signalling pathways is essential for cell function. Nesprins are a multi-isomeric family of proteins that have emerged as signalling scaffolds, herein, we investigate the localisation and function of novel nesprin-2 N-terminal variants. We show that these nesprin-2 variants display cell specific distribution and reside in both the cytoplasm and nucleus. Immunofluorescence microscopy revealed that nesprin-2 N-terminal variants colocalised with β-catenin at cell-cell junctions in U2OS cells. Calcium switch assays demonstrated that nesprin-2 and β-catenin are lost from cell-cell junctions in low calcium conditions whereas emerin localisation at the NE remained unaltered, furthermore, an N-terminal fragment of nesprin-2 was sufficient for cell-cell junction localisation and interacted with β-catenin. Disruption of these N-terminal nesprin-2 variants, using siRNA depletion resulted in loss of β-catenin from cell-cell junctions, nuclear accumulation of active β-catenin and augmented β-catenin transcriptional activity. Importantly, we show that U2OS cells lack nesprin-2 giant, suggesting that the N-terminal nesprin-2 variants regulate β-catenin signalling independently of the NE. Together, these data identify N-terminal nesprin-2 variants as novel regulators of β-catenin signalling that tether β-catenin to cell-cell contacts to inhibit β-catenin transcriptional activity.

Mechanotransduction and nuclear function

Many signaling pathways converge on the nucleus to regulate crucial nuclear events such as transcription, DNA replication and cell cycle progression. Although the vast majority of research in this area has focused on signals generated in response to hormones or other soluble factors, the nucleus also responds to mechanical forces. During the past decade or so, much has been learned about how mechanical force can affect transcription, as well as the growth and differentiation of cells. Much has also been learned about how force is transmitted via the cytoskeleton to the nucleus and then across the nuclear envelope to the nuclear lamina and chromatin. In this brief review, we focus on some of the key proteins that transmit mechanical signals across the nuclear envelope.

Nuclear alignment in myotubes requires centrosome proteins recruited by nesprin-1

Myotubes are syncytial cells, generated by fusion of myoblasts. Among the numerous nuclei in myotubes of skeletal muscle fibres, the majority are equidistantly positioned at the periphery, except for clusters of multiple nuclei underneath the motor endplate. The correct positioning of nuclei is thought to be important for muscle function and requires nesprin-1, a protein of the nuclear envelope. Consistently, mice lacking functional nesprin-1 show defective nuclear positioning and mimic aspects of Emery-Dreifuss muscular dystrophy. In this study, we perform siRNA experiments in C2C12 myoblasts undergoing differentiation, demonstrating that the positioning of nuclei requires PCM-1, a protein of the centrosome that relocalizes to the nuclear envelope at the onset of differentiation, dependent on the presence of nesprin-1. PCM-1 itself is required for recruiting proteins of the dynein/dynactin complex and of kinesin motor complexes. This suggests that microtubule motors that are attached to the nuclear envelope support the movement of nuclei along microtubules, to ensure correct positioning in the myotube.

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

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

Making the LINC: SUN and KASH protein interactions

Cell nuclei are physically integrated with the cytoskeleton through the linker of nucleoskeleton and cytoskeleton (LINC) complex, a structure that spans the nuclear envelope to link the nucleoskeleton and cytoskeleton. Outer nuclear membrane KASH domain proteins and inner nuclear membrane SUN domain proteins interact to form the core of the LINC complex. In this review, we provide a comprehensive analysis of the reported protein-protein interactions for KASH and SUN domain proteins. This critical structure, directly connecting the genome with the rest of the cell, contributes to a myriad of cellular functions and, when perturbed, is associated with human disease.

Role of structural flexibility in the evolution of emerin

Emerin is a short inner nuclear membrane protein with an LEM-domain at the N-terminal end and a transmembrane domain at the C-terminal end. The middle region of human emerin contains multiple binding motifs. Since emerin is often found in evolutionarily newer species, the functional conservation of emerin becomes an interesting topic. In this study, we have demonstrated that most of the functional motifs of emerin are intrinsically disordered or highly flexible. Many post-translational modification sites and mutation sites are associated with these disordered regions. The averaged substitution rates of most functional motifs between species correlate positively with the averaged disorder scores of those functional motifs. Human emerin sequence may have acquired new functions on protein-protein interaction through the formation of hydrophobic motifs in the middle region, which is resulted from accumulated mutations during the evolution process.

Nesprins anchor kinesin-1 motors to the nucleus to drive nuclear distribution in muscle cells

During skeletal muscle development, nuclei move dynamically through myotubes in a microtubule-dependent manner, driven by the microtubule motor protein kinesin-1. Loss of kinesin-1 leads to improperly positioned nuclei in culture and in vivo. Two models have been proposed to explain how kinesin-1 functions to move nuclei in myotubes. In the cargo model, kinesin-1 acts directly from the surface of the nucleus, whereas in an alternative model, kinesin-1 moves nuclei indirectly by sliding anti-parallel microtubules. Here, we test the hypothesis that an ensemble of Kif5B motors acts from the nuclear envelope to distribute nuclei throughout the length of syncytial myotubes. First, using an inducible dimerization system, we show that controlled recruitment of truncated, constitutively active kinesin-1 motors to the nuclear envelope is sufficient to prevent the nuclear aggregation resulting from depletion of endogenous kinesin-1. Second, we identify a conserved kinesin light chain (KLC)-binding motif in the nuclear envelope proteins nesprin-1 and nesprin-2, and show that recruitment of the motor complex to the nucleus via this LEWD motif is essential for nuclear distribution. Together, our findings demonstrate that the nucleus is a kinesin-1 cargo in myotubes and that nesprins function as nuclear cargo adaptors. The importance of achieving and maintaining proper nuclear position is not restricted to muscle fibers, suggesting that the nesprin-dependent recruitment of kinesin-1 to the nuclear envelope through the interaction of a conserved LEWD motif with kinesin light chain might be a general mechanism for cell-type-specific nuclear positioning during development.

Muscular dystrophy-associated SUN1 and SUN2 variants disrupt nuclear-cytoskeletal connections and myonuclear organization

Proteins of the nuclear envelope (NE) are associated with a range of inherited disorders, most commonly involving muscular dystrophy and cardiomyopathy, as exemplified by Emery-Dreifuss muscular dystrophy (EDMD). EDMD is both genetically and phenotypically variable, and some evidence of modifier genes has been reported. Six genes have so far been linked to EDMD, four encoding proteins associated with the LINC complex that connects the nucleus to the cytoskeleton. However, 50% of patients have no identifiable mutations in these genes. Using a candidate approach, we have identified putative disease-causing variants in the SUN1 and SUN2 genes, also encoding LINC complex components, in patients with EDMD and related myopathies. Our data also suggest that SUN1 and SUN2 can act as disease modifier genes in individuals with co-segregating mutations in other EDMD genes. Five SUN1/SUN2 variants examined impaired rearward nuclear repositioning in fibroblasts, confirming defective LINC complex function in nuclear-cytoskeletal coupling. Furthermore, myotubes from a patient carrying compound heterozygous SUN1 mutations displayed gross defects in myonuclear organization. This was accompanied by loss of recruitment of centrosomal marker, pericentrin, to the NE and impaired microtubule nucleation at the NE, events that are required for correct myonuclear arrangement. These defects were recapitulated in C2C12 myotubes expressing exogenous SUN1 variants, demonstrating a direct link between SUN1 mutation and impairment of nuclear-microtubule coupling and myonuclear positioning. Our findings strongly support an important role for SUN1 and SUN2 in muscle disease pathogenesis and support the hypothesis that defects in the LINC complex contribute to disease pathology through disruption of nuclear-microtubule association, resulting in defective myonuclear positioning.

Emery-Dreifuss muscular dystrophy (EDMD) is an inherited disorder involving muscle wasting and weakness, accompanied by cardiac defects. The disease is variable in its severity and also in its genetic cause. So far, 6 genes have been linked to EDMD, most encoding proteins that form a structural network that supports the nucleus of the cell and connects it to structural elements of the cytoplasm. This network is particularly important in muscle cells, providing resistance to mechanical strain. Weakening of this network is thought to contribute to development of muscle disease in these patients. Despite rigorous screening, at least 50% of patients with EDMD have no detectable mutation in the 6 known genes. We therefore undertook screening and identified mutations in two additional genes that encode other components of the nuclear structural network, SUN1 and SUN2. Our findings add to the genetic complexity of this disease since some individuals carry mutations in more than one gene. We also show that the mutations disrupt connections between the nucleus and the structural elements of cytoplasm, leading to mis-positioning and clustering of nuclei in muscle cells. This nuclear mis-positioning is likely to be another factor contributing to pathogenesis of EDMD.

The molecular basis of emerin-emerin and emerin-BAF interactions

Emerin is a conserved membrane component of nuclear lamina structure. Here, we report an advance in understanding the molecular basis of emerin function: intermolecular emerin-emerin association. There were two modes: one mediated by association of residues 170-220 in one emerin molecule to residues 170-220 in another, and the second involving residues 170-220 and 1-132. Deletion analysis showed residues 187-220 contain a positive element essential for intermolecular association in cells. By contrast, deletion of residues 168-186 inactivated a proposed negative element, required to limit or control association. Association of GFP-emerin with nuclear BAF in cells required the LEM domain (residues 1-47) and the positive element. Emerin peptide arrays revealed direct binding of residues 170-220 to residues 206-225 (the proposed positive element), residues 147-174 (particularly P(153)MYGRDSAYQSITHYRP(169)) and the LEM domain. Emerin residues 1-132 and 159-220 were each sufficient to bind lamin A or B1 tails in vitro, identifying two independent regions of molecular contact with lamins. These results, and predicted emerin intrinsic disorder, support the hypothesis that there are multiple 'backbone' and LEM-domain configurations in a proposed intermolecular emerin network at the nuclear envelope.

Nesprin-1 role in DNA damage response

Nuclear envelope (NE) proteins have fundamental roles in maintaining nuclear structure, cell signaling, chromatin organization, and gene regulation, and mutations in genes encoding NE components were identified as primary cause of a number of age associated diseases and cancer. Nesprin-1 belongs to a family of multi-isomeric NE proteins that are characterized by spectrin repeats. We analyzed NE components in various tumor cell lines and found that Nesprin-1 levels were strongly reduced associated with alterations in further NE components. By reducing the amounts of Nesprin-1 by RNAi mediated knockdown, we could reproduce those alterations in mouse and human cell lines. In a search for novel Nesprin-1 binding proteins, we identified MSH2 and MSH6, proteins of the DNA damage response pathway, as interactors and found alterations in the corresponding pathways in cells with lower Nesprin-1 levels. We also noticed increased number of γH2AX foci in the absence of exogenous DNA damage as was seen in tumor cells. The levels of phosphorylated kinases Chk1 and 2 were altered in a manner resembling tumor cells and the levels of Ku70 were low and the protein was not recruited to the DNA after hydroxyurea (HU) treatment. Our findings indicate a role for Nesprin-1 in the DNA damage response pathway and propose Nesprin-1 as novel player in tumorigenesis and genome instability.

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.

The cellular mastermind(?)-mechanotransduction and the nucleus

Cells respond to mechanical stimulation by activation of specific signaling pathways and genes that allow the cell to adapt to its dynamic physical environment. How cells sense the various mechanical inputs and translate them into biochemical signals remains an area of active investigation. Recent reports suggest that the cell nucleus may be directly implicated in this cellular mechanotransduction process. Taken together, these findings paint a picture of the nucleus as a central hub in cellular mechanotransduction-both structurally and biochemically-with important implications in physiology and disease.

Protein interactions at the higher plant nuclear envelope: evidence for a linker of nucleoskeleton and cytoskeleton complex

Following the description of SAD1/UNC84 (SUN) domain proteins in higher plants, evidence has rapidly increased that plants contain a functional linker of nucleoskeleton and cytoskeleton (LINC) complex bridging the nuclear envelope (NE). While the SUN domain proteins appear to be highly conserved across kingdoms, other elements of the complex are not and some key components and interactions remain to be identified. This mini review examines components of the LINC complex, including proteins of the SUN domain family and recently identified plant Klarsicht/Anc/Syne-1 homology (KASH) domain proteins. First of these to be described were WIPs (WPP domain interacting proteins), which act as protein anchors in the outer NE. The plant KASH homologs are C-terminally anchored membrane proteins with the extreme C-terminus located in the nuclear periplasm; AtWIPs contain a highly conserved X-VPT motif at the C-terminus in contrast to PPPX in opisthokonts. The role of the LINC complex in organisms with a cell wall, and description of further LINC complex components will be considered, together with other potential plant-specific functions.

Nesprins in health and disease

LINC (Linker of Nucleoskeleton and Cytoskeleton) complex is an evolutionary conserved structure that spans the entire nuclear envelope (NE), and integrates the nuclear interior with the cytoskeleton, in order to support a diverse array of fundamental biological processes. Key components of the LINC complex are the nesprins (Nuclear Envelope SPectrin Repeat proteINS) that were initially described as large integral NE proteins. However, nesprin genes are complex and generate many variants, which occupy various sub-cellular compartments suggesting additional functions. Hence, the potential involvement of nesprins in disease has expanded immensely on what we already know. That is, nesprins are implicated in diseases such as cancer, myopathies, arthrogryposis, neurological disorders and hearing loss. Here we review nesprins by providing an in depth account of their structure, molecular interactions and cellular functions with relevance to their potential roles in disease. Specifically, we speculate about possible pathomechanisms underlying nesprin-associated diseases.

Nuclear positioning in muscle development and disease

Muscle disease as a group is characterized by muscle weakness, muscle loss, and impaired muscle function. Although the phenotype is the same, the underlying cellular pathologies, and the molecular causes of these pathologies, are diverse. One common feature of many muscle disorders is the mispositioning of myonuclei. In unaffected individuals, myonuclei are spaced throughout the periphery of the muscle fiber such that the distance between nuclei is maximized. However, in diseased muscles, the nuclei are often clustered within the center of the muscle cell. Although this phenotype has been acknowledged for several decades, it is often ignored as a contributor to muscle weakness. Rather, these nuclei are taken only as a sign of muscle repair. Here we review the evidence that mispositioned myonuclei are not merely a symptom of muscle disease but also a cause. Additionally, we review the working models for how myonuclei move from two different perspectives: from that of the nuclei and from that of the cytoskeleton. We further compare and contrast these mechanisms with the mechanisms of nuclear movement in other cell types both to draw general themes for nuclear movement and to identify muscle-specific considerations. Finally, we focus on factors that can be linked to muscle disease and find that genes that regulate myonuclear movement and positioning have been linked to muscular dystrophy. Although the cause-effect relationship is largely speculative, recent data indicate that the position of nuclei should no longer be considered only a means to diagnose muscle disease.

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

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

Nesprins: from the nuclear envelope and beyond

Nuclear envelope spectrin-repeat proteins (Nesprins), are a novel family of nuclear and cytoskeletal proteins with rapidly expanding roles as intracellular scaffolds and linkers. Originally described as proteins that localise to the nuclear envelope (NE) and establish nuclear-cytoskeletal connections, nesprins have now been found to comprise a diverse spectrum of tissue specific isoforms that localise to multiple sub-cellular compartments. Here, we describe how nesprins are necessary in maintaining cellular architecture by acting as essential scaffolds and linkers at both the NE and other sub-cellular domains. More importantly, we speculate how nesprin mutations may disrupt tissue specific nesprin scaffolds and explain the tissue specific nature of many nesprin-associated diseases, including laminopathies.

Large-scale modelling of the divergent spectrin repeats in nesprins: giant modular proteins

Nesprin-1 and nesprin-2 are nuclear envelope (NE) proteins characterized by a common structure of an SR (spectrin repeat) rod domain and a C-terminal transmembrane KASH [Klarsicht-ANC-Syne-homology] domain and display N-terminal actin-binding CH (calponin homology) domains. Mutations in these proteins have been described in Emery-Dreifuss muscular dystrophy and attributed to disruptions of interactions at the NE with nesprins binding partners, lamin A/C and emerin. Evolutionary analysis of the rod domains of the nesprins has shown that they are almost entirely composed of unbroken SR-like structures. We present a bioinformatical approach to accurate definition of the boundaries of each SR by comparison with canonical SR structures, allowing for a large-scale homology modelling of the 74 nesprin-1 and 56 nesprin-2 SRs. The exposed and evolutionary conserved residues identify important pbs for protein-protein interactions that can guide tailored binding experiments. Most importantly, the bioinformatics analyses and the 3D models have been central to the design of selected constructs for protein expression. 1D NMR and CD spectra have been performed of the expressed SRs, showing a folded, stable, high content α-helical structure, typical of SRs. Molecular Dynamics simulations have been performed to study the structural and elastic properties of consecutive SRs, revealing insights in the mechanical properties adopted by these modules in the cell.

Nuclear damages and oxidative stress: new perspectives for laminopathies

Mutations in genes encoding nuclear envelope proteins, particularly LMNA encoding the A-type lamins, cause a broad range of diverse diseases, referred to as laminopathies. The astonishing variety of diseased phenotypes suggests that different mechanisms could be involved in the pathogenesis of laminopathies. In this review we will focus mainly on two of these pathogenic mechanisms: the nuclear damages affecting the chromatin organization, and the oxidative stress causing un-repairable DNA damages. Alteration in the nuclear profile and in chromatin organization, which are particularly impressive in systemic laminopathies whose cells undergo premature senescence, are mainly due to accumulation of unprocessed prelamin A. The toxic effect of these molecular species, which interfere with chromatin-associated proteins, transcription factors, and signaling pathways, could be reduced by drugs which reduce their farnesylation and/or stability. In particular, inhibitors of farnesyl transferase (FTIs), have been proved to be active in rescuing the altered cellular phenotype, and statins, also in association with other drugs, have been included into pilot clinical trials. The identification of a mechanism that accounts for accumulation of un-repairable DNA damage due to reactive oxygen species (ROS) generation in laminopathic cells, similar to that found in other muscular dystrophies (MDs) caused by altered expression of extracellular matrix (ECM) components, suggests that anti-oxidant therapeutic strategies might prove beneficial to laminopathic patients.

LINC complexes in health and disease

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

The nucleoskeleton as a genome-associated dynamic 'network of networks'

In the cytosol, actin polymers, intermediate filaments and microtubules can anchor to cell surface adhesions and interlink to form intricate networks. This cytoskeleton is anchored to the nucleus through LINC (links the nucleoskeleton and cytoskeleton) complexes that span the nuclear envelope and in turn anchor to networks of filaments in the nucleus. The metazoan nucleoskeleton includes nuclear pore-linked filaments, A-type and B-type lamin intermediate filaments, nuclear mitotic apparatus (NuMA) networks, spectrins, titin, 'unconventional' polymers of actin and at least ten different myosin and kinesin motors. These elements constitute a poorly understood 'network of networks' that dynamically reorganizes during mitosis and is responsible for genome organization and integrity.

Nesprin-3 regulates endothelial cell morphology, perinuclear cytoskeletal architecture, and flow-induced polarization

Nesprin-3, a protein that links intermediate filaments to the nucleus, plays a role in vascular endothelial cell (EC) function. Nesprin-3 regulates EC morphology, perinuclear cytoskeletal organization, centrosome–nuclear connectivity, and flow-induced cell polarization and migration.

Changes in blood flow regulate gene expression and protein synthesis in vascular endothelial cells, and this regulation is involved in the development of atherosclerosis. How mechanical stimuli are transmitted from the endothelial luminal surface to the nucleus is incompletely understood. The linker of nucleus and cytoskeleton (LINC) complexes have been proposed as part of a continuous physical link between the plasma membrane and subnuclear structures. LINC proteins nesprin-1, -2, and -4 have been shown to mediate nuclear positioning via microtubule motors and actin. Although nesprin-3 connects intermediate filaments to the nucleus, no functional consequences of nesprin-3 mutations on cellular processes have been described. Here we show that nesprin-3 is robustly expressed in human aortic endothelial cells (HAECs) and localizes to the nuclear envelope. Nesprin-3 regulates HAEC morpho­logy, with nesprin-3 knockdown inducing prominent cellular elongation. Nesprin-3 also organizes perinuclear cytoskeletal organization and is required to attach the centrosome to the nuclear envelope. Finally, nesprin-3 is required for flow-induced polarization of the centrosome and flow-induced migration in HAECs. These results represent the most complete description to date of nesprin-3 function and suggest that nesprin-3 regulates vascular endothelial cell shape, perinuclear cytoskeletal architecture, and important aspects of flow-mediated mechanotransduction.

Structural protein 4.1R is integrally involved in nuclear envelope protein localization, centrosome-nucleus association and transcriptional signaling

The multifunctional structural protein 4.1R is required for assembly and maintenance of functional nuclei but its nuclear roles are unidentified. 4.1R localizes within nuclei, at the nuclear envelope, and in cytoplasm. Here we show that 4.1R, the nuclear envelope protein emerin and the intermediate filament protein lamin A/C co-immunoprecipitate, and that 4.1R-specific depletion in human cells by RNA interference produces nuclear dysmorphology and selective mislocalization of proteins from several nuclear subcompartments. Such 4.1R-deficiency causes emerin to partially redistribute into the cytoplasm, whereas lamin A/C is disorganized at nuclear rims and displaced from nucleoplasmic foci. The nuclear envelope protein MAN1, nuclear pore proteins Tpr and Nup62, and nucleoplasmic proteins NuMA and LAP2α also have aberrant distributions, but lamin B and LAP2β have normal localizations. 4.1R-deficient mouse embryonic fibroblasts show a similar phenotype. We determined the functional effects of 4.1R-deficiency that reflect disruption of the association of 4.1R with emerin and A-type lamin: increased nucleus-centrosome distances, increased β-catenin signaling, and relocalization of β-catenin from the plasma membrane to the nucleus. Furthermore, emerin- and lamin-A/C-null cells have decreased nuclear 4.1R. Our data provide evidence that 4.1R has important functional interactions with emerin and A-type lamin that impact upon nuclear architecture, centrosome-nuclear envelope association and the regulation of β-catenin transcriptional co-activator activity that is dependent on β-catenin nuclear export.

Nesprins, but not sun proteins, switch isoforms at the nuclear envelope during muscle development

Nesprins are a family of nuclear transmembrane proteins anchored via Sun proteins to the nuclear membrane. Analysis of nesprins during human muscle development revealed an increase in nesprin-1-giant during early myogenesis in vitro. During the transition from immature to mature muscle fibres in vivo, nesprin-2 partly replaced nesprin-1 at the nuclear envelope and short nesprin isoforms became dominant. Sun1 and Sun2 proteins remained unchanged during this fibre maturation. In emerin-negative skin fibroblasts, nesprin-2-giant was relocated from the nuclear envelope to the cytoplasm, not to the endoplasmic reticulum, while nesprin-1 remained at the nuclear envelope. In emerin-negative keratinocytes lacking nesprin-1, nesprin-2 remained at the nuclear envelope. HeLa cell nuclear envelopes lacked nesprin-1, which was the dominant form in myoblasts, while a novel 130-kD nesprin-2 isoform dominated Ntera-2 cells. The results suggest the possibility of isoform-specific and tissue-specific roles for nesprins in nuclear positioning.

Nuclear lamins

The nuclear lamins are type V intermediate filament proteins that are critically important for the structural properties of the nucleus. In addition, they are involved in the regulation of numerous nuclear processes, including DNA replication, transcription and chromatin organization. The developmentally regulated expression of lamins suggests that they are involved in cellular differentiation. Their assembly dynamic properties throughout the cell cycle, particularly in mitosis, are influenced by posttranslational modifications. Lamins may regulate nuclear functions by direct interactions with chromatin and determining the spatial organization of chromosomes within the nuclear space. They may also regulate chromatin functions by interacting with factors that epigenetically modify the chromatin or directly regulate replication or transcription.

Proteomics of dense core secretory vesicles reveal distinct protein categories for secretion of neuroeffectors for cell-cell communication

Regulated secretion of neurotransmitters and neurohumoral factors from dense core secretory vesicles provides essential neuroeffectors for cell-cell communication in the nervous and endocrine systems. This study provides comprehensive proteomic characterization of the categories of proteins in chromaffin dense core secretory vesicles that participate in cell-cell communication from the adrenal medulla. Proteomic studies were conducted by nano-HPLC Chip MS/MS tandem mass spectrometry. Results demonstrate that these secretory vesicles contain proteins of distinct functional categories consisting of neuropeptides and neurohumoral factors, protease systems, neurotransmitter enzymes and transporters, receptors, enzymes for biochemical processes, reduction/oxidation regulation, ATPases, protein folding, lipid biochemistry, signal transduction, exocytosis, calcium regulation, as well as structural and cell adhesion proteins. The secretory vesicle proteomic data identified 371 proteins in the soluble fraction and 384 membrane proteins, for a total of 686 distinct secretory vesicle proteins. Notably, these proteomic analyses illustrate the presence of several neurological disease-related proteins in these secretory vesicles, including huntingtin interacting protein, cystatin C, ataxin 7, and prion protein. Overall, these findings demonstrate that multiple protein categories participate in dense core secretory vesicles for production, storage, and secretion of bioactive neuroeffectors for cell-cell communication in health and disease.

Interactions between nuclei and the cytoskeleton are mediated by SUN-KASH nuclear-envelope bridges

The nuclear envelope links the cytoskeleton to structural components of the nucleus. It functions to coordinate nuclear migration and anchorage, organize chromatin, and aid meiotic chromosome pairing. Forces generated by the cytoskeleton are transferred across the nuclear envelope to the nuclear lamina through a nuclear-envelope bridge consisting of SUN (Sad1 and UNC-84) and KASH (Klarsicht, ANC-1 and Syne/Nesprin homology) proteins. Some KASH-SUN combinations connect microtubules, centrosomes, actin filaments, or intermediate filaments to the surface of the nucleus. Other combinations are used in cell cycle control, nuclear import, or apoptosis. Interactions between the cytoskeleton and the nucleus also affect global cytoskeleton organization. SUN and KASH proteins were identified through genetic screens for mispositioned nuclei in model organisms. Knockouts of SUN or KASH proteins disrupt neurological and muscular development in mice. Defects in SUN and KASH proteins have been linked to human diseases including muscular dystrophy, ataxia, progeria, lissencephaly, and cancer.

Nesprin-2 interacts with {alpha}-catenin and regulates Wnt signaling at the nuclear envelope

Nesprins and emerin are structural nuclear envelope proteins that tether nuclei to the cytoskeleton. In this work, we identified the cytoskeleton-associated α-N/E-catenins as novel nesprin-2-binding partners. The association involves the C termini of nesprin-2 giant and α-N/E-catenins. α-E/T/N-catenins are known primarily for their roles in cadherin-mediated cell-cell adhesion. Here, we show that, in addition, α-catenin forms complexes with nesprin-2 that include β-catenin and emerin. We demonstrate that the depletion of nesprin-2 reduces both the amount of active β-catenin inside the nucleus and T-cell factor/lymphoid-enhancing factor-dependent transcription. Taken together, these findings suggest novel nesprin-2 functions in cellular signaling. Moreover, we propose that, in contrast to emerin, nesprin-2 is a positive regulator of the Wnt signaling pathway.