Samp1 is functionally associated with the LINC complex and A-type lamina networks

Feeling the force from within - new tools and insights into nuclear mechanotransduction

The ability of cells to sense and respond to mechanical signals is essential for many biological processes that form the basis of cell identity, tissue development and maintenance. This process, known as mechanotransduction, involves crucial feedback between mechanical force and biochemical signals, including epigenomic modifications that establish transcriptional programs. These programs, in turn, reinforce the mechanical properties of the cell and its ability to withstand mechanical perturbation. The nucleus has long been hypothesized to play a key role in mechanotransduction due to its direct exposure to forces transmitted through the cytoskeleton, its role in receiving cytoplasmic signals and its central function in gene regulation. However, parsing out the specific contributions of the nucleus from those of the cell surface and cytoplasm in mechanotransduction remains a substantial challenge. In this Review, we examine the latest evidence on how the nucleus regulates mechanotransduction, both via the nuclear envelope (NE) and through epigenetic and transcriptional machinery elements within the nuclear interior. We also explore the role of nuclear mechanotransduction in establishing a mechanical memory, characterized by a mechanical, epigenetic and transcriptomic cell state that persists after mechanical stimuli cease. Finally, we discuss current challenges in the field of nuclear mechanotransduction and present technological advances that are poised to overcome them.

A cooperative network at the nuclear envelope counteracts LINC-mediated forces during oogenesis in C. elegans

Oogenesis involves transduction of mechanical forces from the cytoskeleton to the nuclear envelope (NE). In Caenorhabditis elegans, oocyte nuclei lacking the single lamin protein LMN-1 are vulnerable to collapse under forces mediated through LINC (linker of nucleoskeleton and cytoskeleton) complexes. Here, we use cytological analysis and in vivo imaging to investigate the balance of forces that drive this collapse and protect oocyte nuclei. We also use a mechano-node-pore sensing device to directly measure the effect of genetic mutations on oocyte nuclear stiffness. We find that nuclear collapse is not a consequence of apoptosis. It is promoted by dynein, which induces polarization of a LINC complex composed of Sad1 and UNC-84 homology 1 (SUN-1) and ZYGote defective 12 (ZYG-12). Lamins contribute to oocyte nuclear stiffness and cooperate with other inner nuclear membrane proteins to distribute LINC complexes and protect nuclei from collapse. We speculate that a similar network may protect oocyte integrity during extended oocyte arrest in mammals.

Nuclear envelope transmembrane proteins involved in genome organization are misregulated in myotonic dystrophy type 1 muscle

Myotonic dystrophy type 1 is a multisystemic disorder with predominant muscle and neurological involvement. Despite a well described pathomechanism, which is primarily a global missplicing due to sequestration of RNA-binding proteins, there are still many unsolved questions. One such question is the disease etiology in the different affected tissues. We observed alterations at the nuclear envelope in primary muscle cell cultures before. This led us to reanalyze a published RNA-sequencing dataset of DM1 and control muscle biopsies regarding the misregulation of NE proteins. We could identify several muscle NE protein encoding genes to be misregulated depending on the severity of the muscle phenotype. Among these misregulated genes were NE transmembrane proteins (NETs) involved in nuclear-cytoskeletal coupling as well as genome organization. For selected genes, we could confirm that observed gene-misregulation led to protein expression changes. Furthermore, we investigated if genes known to be under expression-regulation by genome organization NETs were also misregulated in DM1 biopsies, which revealed that misregulation of two NETs alone is likely responsible for differential expression of about 10% of all genes being differentially expressed in DM1. Notably, the majority of NETs identified here to be misregulated in DM1 muscle are mutated in Emery-Dreifuss muscular dystrophy or clinical similar muscular dystrophies, suggesting a broader similarity on the molecular level for muscular dystrophies than anticipated. This shows not only the importance of muscle NETs in muscle health and disease, but also highlights the importance of the NE in DM1 disease progression.

Mechanics and functional consequences of nuclear deformations

As the home of cellular genetic information, the nucleus has a critical role in determining cell fate and function in response to various signals and stimuli. In addition to biochemical inputs, the nucleus is constantly exposed to intrinsic and extrinsic mechanical forces that trigger dynamic changes in nuclear structure and morphology. Emerging data suggest that the physical deformation of the nucleus modulates many cellular and nuclear functions. These functions have long been considered to be downstream of cytoplasmic signalling pathways and dictated by gene expression. In this Review, we discuss an emerging perspective on the mechanoregulation of the nucleus that considers the physical connections from chromatin to nuclear lamina and cytoskeletal filaments as a single mechanical unit. We describe key mechanisms of nuclear deformations in time and space and provide a critical review of the structural and functional adaptive responses of the nucleus to deformations. We then consider the contribution of nuclear deformations to the regulation of important cellular functions, including muscle contraction, cell migration and human disease pathogenesis. Collectively, these emerging insights shed new light on the dynamics of nuclear deformations and their roles in cellular mechanobiology.

Inner nuclear membrane protein TMEM201 maintains endothelial cell migration and angiogenesis by interacting with the LINC complex

The nuclear envelope comprises the outer nuclear membrane (ONM), inner nuclear membrane (INM), and nucleopore. Although ∼60 INM proteins have been identified, only a few of them have been well characterized, revealing their crucial roles. Our group focused on the INM protein transmembrane protein 201 (TMEM201), whose role in cellular function remains to be defined. In this study, we investigated the role of TMEM201 in endothelial cell migration and angiogenesis. Depletion of TMEM201 expression by short hairpin RNA-mediated interference impeded human umbilical vein endothelial cell (HUVEC) angiogenic behavior in tube formation and fibrin gel bead sprouting assays. Meanwhile, TMEM201-deficient HUVEC exhibited impaired migration ability. We next explored the underlying mechanism and found that the N-terminal of TMEM201 interacted with the linker of nucleoskeleton and cytoskeleton (LINC) complex and was required for regulating endothelial cell migration and angiogenesis. The above in vitro findings were further confirmed by using in vivo models. In Tmem201-knockout mice, retinal vessel development was arrested and aortic ring sprouting was defective. In addition, loss of tmem201 impaired zebrafish intersegmental vessel development. In summary, TMEM201 was shown to regulate endothelial cell migration and control the process of angiogenesis. This study is the first to reveal the role of INM proteins in the vascular system and angiogenesis.

A Multisensory Network Drives Nuclear Mechanoadaptation

Cells have adapted to mechanical forces early in evolution and have developed multiple mechanisms ensuring sensing of, and adaptation to, the diversity of forces operating outside and within organisms. The nucleus must necessarily adapt to all types of mechanical signals, as its functions are essential for virtually all cell processes, many of which are tuned by mechanical cues. To sense forces, the nucleus is physically connected with the cytoskeleton, which senses and transmits forces generated outside and inside the cell. The nuclear LINC complex bridges the cytoskeleton and the nuclear lamina to transmit mechanical information up to the chromatin. This system creates a force-sensing macromolecular complex that, however, is not sufficient to regulate all nuclear mechanoadaptation processes. Within the nucleus, additional mechanosensitive structures, including the nuclear envelope and the nuclear pore complex, function to regulate nuclear mechanoadaptation. Similarly, extra nuclear mechanosensitive systems based on plasma membrane dynamics, mechanotransduce information to the nucleus. Thus, the nucleus has the intrinsic structural components needed to receive and interpret mechanical inputs, but also rely on extra nuclear mechano-sensors that activate nuclear regulators in response to force. Thus, a network of mechanosensitive cell structures ensures that the nucleus has a tunable response to mechanical cues.

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.

The Diverse Cellular Functions of Inner Nuclear Membrane Proteins

The nuclear compartment is delimited by a specialized expanded sheet of the endoplasmic reticulum (ER) known as the nuclear envelope (NE). Compared to the outer nuclear membrane and the contiguous peripheral ER, the inner nuclear membrane (INM) houses a unique set of transmembrane proteins that serve a staggering range of functions. Many of these functions reflect the exceptional position of INM proteins at the membrane-chromatin interface. Recent research revealed that numerous INM proteins perform crucial roles in chromatin organization, regulation of gene expression, genome stability, and mediation of signaling pathways into the nucleus. Other INM proteins establish mechanical links between chromatin and the cytoskeleton, help NE remodeling, or contribute to the surveillance of NE integrity and homeostasis. As INM proteins continue to gain prominence, we review these advancements and give an overview on the functional versatility of the INM proteome.

High-Throughput Identification of Nuclear Envelope Protein Interactions in Schizosaccharomyces pombe Using an Arrayed Membrane Yeast-Two Hybrid Library

The nuclear envelope (NE) contains a specialized set of integral membrane proteins that maintain nuclear shape and integrity and influence chromatin organization and gene expression. Advances in proteomics techniques and studies in model organisms have identified hundreds of proteins that localize to the NE. However, the function of many of these proteins at the NE remains unclear, in part due to a lack of understanding of the interactions that these proteins participate in at the NE membrane. To assist in the characterization of NE transmembrane protein interactions we developed an arrayed library of integral and peripheral membrane proteins from the fission yeast Schizosaccharomyces pombe for high-throughput screening using the split-ubiquitin based membrane yeast two -hybrid system. We used this approach to characterize protein interactions for three conserved proteins that localize to the inner nuclear membrane: Cut11/Ndc1, Lem2 and Ima1/Samp1/Net5. Additionally, we determined how the interaction network for Cut11 is altered in canonical temperature-sensitive cut11-ts mutants. This library and screening approach is readily applicable to characterizing the interactomes of integral membrane proteins localizing to various subcellular compartments.

Non-Canonical Functions of the Gamma-Tubulin Meshwork in the Regulation of the Nuclear Architecture

Simple Summary

The appearance of a cell is connected to its function. For example, the fusiform of smooth muscle cells is adapted to facilitate muscle contraction, the lobed nucleus in white blood cells assists with the migratory behavior of these immune cells, and the condensed nucleus in sperm aids in their swimming efficiency. Thus, changes in appearance have been used for decades by doctors as a diagnostic method for human cancers. Here, we summarize our knowledge of how a cell maintains the shape of the nuclear compartment. Specifically, we discuss the role of a novel protein meshwork, the gamma-tubulin meshwork, in the regulation of nuclear morphology and as a therapeutic target against cancer.

Abstract

The nuclear architecture describes the organization of the various compartments in the nucleus of eukaryotic cells, where a plethora of processes such as nucleocytoplasmic transport, gene expression, and assembly of ribosomal subunits occur in a dynamic manner. During the different phases of the cell cycle, in post-mitotic cells and after oncogenic transformation, rearrangements of the nuclear architecture take place, and, among other things, these alterations result in reorganization of the chromatin and changes in gene expression. A member of the tubulin family, γtubulin, was first identified as part of a multiprotein complex that allows nucleation of microtubules. However, more than a decade ago, γtubulin was also characterized as a nuclear protein that modulates several crucial processes that affect the architecture of the nucleus. This review presents the latest knowledge regarding changes that arise in the nuclear architecture of healthy cells and under pathological conditions and, more specifically, considers the particular involvement of γtubulin in the modulation of the biology of the nuclear compartment.

Monitoring of chromatin organization in live cells by FRIC. Effects of the inner nuclear membrane protein Samp1

In most cells, transcriptionally inactive heterochromatin is preferentially localized in the nuclear periphery and transcriptionally active euchromatin is localized in the nuclear interior. Different cell types display characteristic chromatin distribution patterns, which change dramatically during cell differentiation, proliferation, senescence and different pathological conditions. Chromatin organization has been extensively studied on a cell population level, but there is a need to understand dynamic reorganization of chromatin at the single cell level, especially in live cells. We have developed a novel image analysis tool that we term Fluorescence Ratiometric Imaging of Chromatin (FRIC) to quantitatively monitor dynamic spatiotemporal distribution of euchromatin and total chromatin in live cells. A vector (pTandemH) assures stoichiometrically constant expression of the histone variants Histone 3.3 and Histone 2B, fused to EGFP and mCherry, respectively. Quantitative ratiometric (H3.3/H2B) imaging displayed a concentrated distribution of heterochromatin in the periphery of U2OS cell nuclei. As proof of concept, peripheral heterochromatin responded to experimental manipulation of histone acetylation. We also found that peripheral heterochromatin depended on the levels of the inner nuclear membrane protein Samp1, suggesting an important role in promoting peripheral heterochromatin. Taken together, FRIC is a powerful and robust new tool to study dynamic chromatin redistribution in live cells.

Single molecule analysis of lamin dynamics

The nuclear envelope (NE) is an essential cellular structure that contributes to nuclear stability, organization, and function. Mutations in NE-associated proteins result in a myriad of pathologies with widely diverse clinical manifestations, ages of onsets, and affected tissues. Notably, several hundred disease-causing mutations have been mapped to the LMNA gene, which encodes the intermediate filament proteins lamin A and C, two of the major architectural components of the nuclear envelope. However, how NE dysfunction leads to the highly variable pathologies observed in patient cells and tissues remains poorly understood. One model suggests alterations in the dynamic properties of the nuclear lamina and its associated proteins contribute to disease phenotype. Here, we describe the application of single molecule tracking (SMT) methodology to characterize the behavior of nuclear envelope transmembrane proteins and nuclear lamins in their native cellular environment at the single molecule level. As proof-of-concept, we demonstrate by SMT that Halo-tagged lamin B1, Samp1, lamin A, and lamin AΔ50 have distinct binding and kinetic properties, and we identify several disease-relevant mutants which exhibit altered binding dynamics. SMT is also able to separately probe the dynamics of the peripheral and the nucleoplasmic populations of lamin A mutants. We suggest that SMT is a robust and sensitive method to investigate the relationship between pathogenic mutations or cellular processes and protein dynamics at the NE.

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

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

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.

An Overview of The Globozoospermia as A Multigenic Identified Syndrome

Acrosome plays an integral role during fertilization and its absence in individuals with globozoospermia leads to failure of in vitro fertilization (IVF) and oocyte activation post-intracytoplasmic sperm injection (ICSI). A variety of processes, organelles and structures are involved in acrosome biogenesis including, trans-golgi network (TGN), acroplaxome and cellular trafficking. This review aims to explain roles of related signals and molecules involved in this process and also describe how their absence in form of mutation, deletion and knockout model may lead to phe- nomenon referred to globozoospermia.

Mitotic spindle assembly and γ-tubulin localisation depend on the integral nuclear membrane protein Samp1

We have investigated a possible role for the inner nuclear membrane protein Samp1 (also known as TMEM201) in the mitotic machinery. Live-cell imaging showed that Samp1a–YFP (Samp1a is the short isoform of Samp1) distributed as filamentous structures in the mitotic spindle, partially colocalising with β-tubulin. Samp1 depletion resulted in an increased frequency of cells with signs of chromosomal mis-segregation and prolonged metaphase, indicating problems with spindle assembly and/or chromosomal alignment. Consistent with this, mitotic spindles in Samp1-depleted cells contained significantly lower levels of β-tubulin and γ-tubulin, phenotypes that were rescued by overexpression of Samp1a–YFP. We found that Samp1 can bind directly to γ-tubulin and that Samp1 co-precipitated with γ-tubulin and the HAUS6 subunit of the Augmin complex in live cells. The levels of HAUS6, in the mitotic spindle also decreased after Samp1 depletion. We show that Samp1 is involved in the recruitment of HAUS6 and γ-tubulin to the mitotic spindle. Samp1 is the first inner nuclear membrane protein shown to have a function in mitotic spindle assembly.

Highlighted Article: The transmembrane inner nuclear membrane protein Samp1 has a functional role in recruitment of γ-tubulin to the mitotic spindle and correct spindle assembly.

RanGTPase regulates the interaction between the inner nuclear membrane proteins, Samp1 and Emerin

Samp1, spindle associated membrane protein 1, is a type II integral membrane protein localized in the inner nuclear membrane. Recent studies have shown that the inner nuclear membrane protein, Emerin and the small monomeric GTPase, Ran are direct binding partners of Samp1. Here we addressed the question whether Ran could regulate the interaction between Samp1 and Emerin in the inner nuclear membrane. To investigate the interaction between Samp1 and Emerin in live cells, we performed FRAP experiments in cells overexpressing YFP-Emerin. We compared the mobility of YFP-Emerin in Samp1 knock out cells and cells overexpressing Samp1. The results showed that the mobility of YFP-Emerin was higher in Samp1 knock out cells and lower in cells overexpressing Samp1, suggesting that Samp1 significantly attenuates the mobility of Emerin in the nuclear envelope. FRAP experiments using tsBN2 cells showed that the mobility of Emerin depends on RanGTP. Consistently, in vitro binding experiments showed that the affinity between Samp1 and Emerin is decreased in the presence of Ran, suggesting that Ran attenuates the interaction between Samp1 and Emerin. This is the first demonstration that Ran can regulate the interaction between two proteins in the nuclear envelope.

LINC complexes as regulators of meiosis

Meiosis is a key processes of sexual reproduction in eukaryotes. By combining two cell division cycles with a single round of DNA replication meiosis provides a mechanism to generate haploid gametes. Coincidentally, processes involved in ensuring appropriate segregation of homologous chromosomes also result in genetic recombination and shuffling of genes between each generation. During the first meiotic prophase, rapid telomere-led chromosome movements facilitate alignment and pairing of homologous chromosomes. Forces that produce these movements are generated by the cytoskeleton. Force transmission across the nuclear envelope is dependent upon LINC complexes. These structures consist of SUN and KASH domain proteins that span the two nuclear membranes. Together they represent a pair of links in a molecular chain that couples telomeres to the cytoskeleton. In addition to their force transducing role, LINC complexes also have essential functions ensuring the fidelity of recombination between homologous chromosomes. In this way, LINC complexes are now seen as playing an active and integral role in meiotic progression.

Spindle associated membrane protein 1 (Samp1) is required for the differentiation of muscle cells

Muscles are developed and regenerated in a differentiation process called myogenesis, which involves components of the nuclear envelope. We have investigated Samp1 (Spindle Associated Membrane Protein 1), a transmembrane nuclear envelope protein, which interacts with emerin and lamin A, both of which are linked to Emery-Dreifuss muscular dystrophy (EDMD). We found that the levels of Samp1 increased seven-fold during differentiation of mouse C2C12 muscle progenitor cells. To test if Samp1 could have a role in myogenesis we developed stable C2C12 knockdown cell lines expressing short hairpin RNA targeting Samp1 expression. The Samp1 depleted C2C12 cells displayed normal mobility and normal distribution of emerin and lamin A. However, Samp1 depletion increased ERK signaling and completely blocked differentiation of C2C12 cells, which failed to express myogenic marker proteins and failed to form myotubes. The block in myogenesis in Samp1 depleted cells was completely rescued by ectopic expression of RNAi resistant human Samp1, showing that Samp1 is required for muscle differentiation.

LINC complexes and nuclear positioning

One of the characteristics of eukaryotic cells is their structural plasticity associated with the ability to carry out a broad range of complex functions, both autonomously and as components of tissues and organs. Major cellular rearrangements can be observed in various systems from meiosis in fission yeast, through dermal differentiation in nematodes, to muscle and neuronal development in vertebrates. Each of these processes involves oftentimes dramatic relocation of the nucleus within the cell. During the last decade it has become apparent that the nuclear periphery represents a nexus of cytoskeletal interactions that are involved not only in nuclear movement but also in the distribution and dissemination of mechanical forces throughout the cell. Nucleocytoskeletal coupling is mediated in large part by SUN- and KASH-domain proteins of the nuclear membranes, that together assemble to form LINC (Linker of the Nucleoskeleton and Cytoskeleton) complexes. In this review we will describe how the LINC complex repertoire contributes to nuclear positioning and chromosome dynamics in a variety of cellular contexts.

Integrity of the Linker of Nucleoskeleton and Cytoskeleton Is Required for Efficient Herpesvirus Nuclear Egress

Herpesvirus capsids assemble in the nucleus, while final virion maturation proceeds in the cytoplasm. This requires that newly formed nucleocapsids cross the nuclear envelope (NE), which occurs by budding at the inner nuclear membrane (INM), release of the primary enveloped virion into the perinuclear space (PNS), and subsequent rapid fusion with the outer nuclear membrane (ONM). During this process, the NE remains intact, even at late stages of infection. In addition, the spacing between the INM and ONM is maintained, as is that between the primary virion envelope and nuclear membranes. The linker of nucleoskeleton and cytoskeleton (LINC) complex consists of INM proteins with a luminal SUN (Sad1/UNC-84 homology) domain connected to ONM proteins with a KASH (Klarsicht, ANC-1, SYNE homology) domain and is thought to be responsible for spacing the nuclear membranes. To investigate the role of the LINC complex during herpesvirus infection, we generated cell lines constitutively expressing dominant negative (dn) forms of SUN1 and SUN2. Ultrastructural analyses revealed a significant expansion of the PNS and the contiguous intracytoplasmic lumen, most likely representing endoplasmic reticulum (ER), especially in cells expressing dn-SUN2. After infection, primary virions accumulated in these expanded luminal regions, also very distant from the nucleus. The importance of the LINC complex was also confirmed by reduced progeny virus titers in cells expressing dn-SUN2. These data show that the intact LINC complex is required for efficient nuclear egress of herpesviruses, likely acting to promote fusion of primary enveloped virions with the ONM.IMPORTANCE While the viral factors for primary envelopment of nucleocapsids at the inner nuclear membrane are known to the point of high-resolution structures, the roles of cellular components and regulators remain enigmatic. Furthermore, the machinery responsible for fusion with the outer nuclear membrane is unsolved. We show here that dominant negative SUN2 interferes with efficient herpesvirus nuclear egress, apparently by interfering with fusion between the primary virion envelope and outer nuclear membrane. This identifies a new cellular component important for viral egress and implicates LINC complex integrity in nonconventional nuclear membrane trafficking.

Samp1, a RanGTP binding transmembrane protein in the inner nuclear membrane

Samp1 is a transmembrane protein of the inner nuclear membrane (INM), which interacts with the nuclear lamina and the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex in interphase and during mitosis, it localizes to the mitotic spindle. Samp1 was recently found to coprecipitate a protein complex containing Ran, a GTPase with fundamental regulatory functions both in interphase and in mitosis. To investigate the interaction between Samp1 and Ran in further detail, we have designed and expressed recombinant fusion proteins of the Chaetomium thermophilum homolog of Samp1 (Ct.Samp1) and human Ran. Pulldown experiments show that Samp1 binds directly to Ran and that Samp1 binds better to RanGTP compared to RanGDP. Samp1 also preferred RanGTP over RanGDP in living tsBN2 cells. We also show that the Ran binding domain is located between amino acids 75–135 in the nucleoplasmically exposed N-terminal tail of Samp1. This domain is unique for Samp1, without homology in any other proteins in fungi or metazoa. Samp1 is the first known transmembrane protein that binds to Ran and could provide a unique local binding site for RanGTP in the INM. Samp1 overexpression resulted in increased Ran concentrations in the nuclear periphery supporting this idea.

Nuclear envelope localization of LEMD2 is developmentally dynamic and lamin A/C dependent yet insufficient for heterochromatin tethering

Peripheral heterochromatin in mammalian nuclei is tethered to the nuclear envelope by at least two mechanisms here referred to as the A- and B-tethers. The A-tether includes lamins A/C and additional unknown components presumably INM protein(s) interacting with both lamins A/C and chromatin. The B-tether includes the inner nuclear membrane (INM) protein Lamin B-receptor, which binds B-type lamins and chromatin. Generally, at least one of the tethers is always present in the nuclear envelope of mammalian cells. Deletion of both causes the loss of peripheral heterochromatin and consequently inversion of the entire nuclear architecture, with this occurring naturally in rod photoreceptors of nocturnal mammals. The tethers are differentially utilized during development, regulate gene expression in opposite manners, and play an important role during cell differentiation. Here we aimed to identify the unknown chromatin binding component(s) of the A-tether. We analyzed 10 mouse tissues by immunostaining with antibodies against 7 INM proteins and found that every cell type has specific, although differentially and developmentally regulated, sets of these proteins. In particular, we found that INM protein LEMD2 is concomitantly expressed with A-type lamins in various cell types but is lacking in inverted nuclei of rod cells. Truncation or deletion of Lmna resulted in the downregulation and mislocalization of LEMD2, suggesting that the two proteins interact and pointing at LEMD2 as a potential chromatin binding mediator of the A-tether. Using nuclei of mouse rods as an experimental model lacking peripheral heterochromatin, we expressed a LEMD2 transgene alone or in combination with lamin C in these cells and observed no restoration of peripheral heterochromatin in either case. We conclude that in contrary to the B-tether, the A-tether has a more intricate composition and consists of multiple components that presumably vary, at differing degrees of redundancy, between cell types and differentiation stages.

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

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

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

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

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

Control of heterochromatin localization and silencing by the nuclear membrane protein Lem2

Transcriptionally silent chromatin localizes to the nuclear periphery, which provides a special microenvironment for gene repression. A variety of nuclear membrane proteins interact with repressed chromatin, yet the functional role of these interactions remains poorly understood. Here, we show that, in Schizosaccharomyces pombe, the nuclear membrane protein Lem2 associates with chromatin and mediates silencing and heterochromatin localization. Unexpectedly, we found that these functions can be separated and assigned to different structural domains within Lem2, excluding a simple tethering mechanism. Chromatin association and tethering of centromeres to the periphery are mediated by the N-terminal LEM (LAP2-Emerin-MAN1) domain of Lem2, whereas telomere anchoring and heterochromatin silencing require exclusively its conserved C-terminal MSC (MAN1-Src1 C-terminal) domain. Particularly, silencing by Lem2 is epistatic with the Snf2/HDAC (histone deacetylase) repressor complex SHREC at telomeres, while its necessity can be bypassed by deleting Epe1, a JmjC protein with anti-silencing activity. Furthermore, we found that loss of Lem2 reduces heterochromatin association of SHREC, which is accompanied by increased binding of Epe1. This reveals a critical function of Lem2 in coordinating these antagonistic factors at heterochromatin. The distinct silencing and localization functions mediated by Lem2 suggest that these conserved LEM-containing proteins go beyond simple tethering to play active roles in perinuclear silencing.

Perinuclear Anchoring of H3K9-Methylated Chromatin Stabilizes Induced Cell Fate in C. elegans Embryos

Interphase chromatin is organized in distinct nuclear sub-compartments, reflecting its degree of compaction and transcriptional status. In Caenorhabditis elegans embryos, H3K9 methylation is necessary to silence and to anchor repeat-rich heterochromatin at the nuclear periphery. In a screen for perinuclear anchors of heterochromatin, we identified a previously uncharacterized C. elegans chromodomain protein, CEC-4. CEC-4 binds preferentially mono-, di-, or tri-methylated H3K9 and localizes at the nuclear envelope independently of H3K9 methylation and nuclear lamin. CEC-4 is necessary for endogenous heterochromatin anchoring, but not for transcriptional repression, in contrast to other known H3K9 methyl-binders in worms, which mediate gene repression but not perinuclear anchoring. When we ectopically induce a muscle differentiation program in embryos, cec-4 mutants fail to commit fully to muscle cell fate. This suggests that perinuclear sequestration of chromatin during development helps restrict cell differentiation programs by stabilizing commitment to a specific cell fate. PAPERCLIP.

Mitotic phosphorylation of SUN1 loosens its connection with the nuclear lamina while the LINC complex remains intact

At the onset mitosis in higher eukaryotes, the nuclear envelope (NE) undergoes dramatic deconstruction to allow separation of duplicated chromosomes. Studies have shown that during this process of nuclear envelope breakdown (NEBD), the extensive protein networks of the nuclear lamina are disassembled through phosphorylation of lamins and several inner nuclear membrane (INM) proteins. The LINC complex, composed of SUN and nesprin proteins, is involved in multiple interactions at the NE and plays vital roles in nuclear and cellular mechanics by connecting the nucleus to the cytoskeleton. Here, we show that SUN1, located in the INM, undergoes mitosis-specific phosphorylation on at least 3 sites within its nucleoplasmic N-terminus. We further identify Cdk1 as the kinase responsible for serine 48 and 333 phosphorylation, while serine 138 is phosphorylated by Plk1. In mitotic cells, SUN1 loses its interaction with N-terminal domain binding partners lamin A/C, emerin, and short nesprin-2 isoforms. Furthermore, a triple phosphomimetic SUN1 mutant displays increased solubility and reduced retention at the NE. In contrast, the central LINC complex interaction between the SUN1 C-terminus and the KASH domain of nesprin-2 is maintained during mitosis. Together, these data support a model whereby mitotic phosphorylation of SUN1 disrupts interactions with nucleoplasmic binding partners, promoting disassembly of the nuclear lamina and, potentially, its chromatin interactions. At the same time, our data add to an emerging picture that the core LINC complex plays an active role in NEBD.

LINC'ing form and function at the nuclear envelope

The nuclear envelope is an amazing piece of engineering. On one hand it is built like a mediaeval fortress with filament systems reinforcing its membrane walls and its double membrane structure forming a lumen like a castle moat. On the other hand its structure can adapt while maintaining its integrity like a reed bending in a river. Like a fortress it has guarded drawbridges in the nuclear pore complexes, but also has other mechanical means of communication. All this is enabled largely because of the LINC complex, a multi-protein structure that connects the intermediate filament nucleoskeleton across the lumen of the double membrane nuclear envelope to multiple cytoplasmic filament systems that themselves could act simultaneously both like mediaeval buttresses and like lines on a suspension bridge. Although many details of the greater LINC structure remain to be discerned, a number of recent findings are giving clues as to how its structural organization can yield such striking dynamic yet stable properties. Combining double- and triple-helical coiled-coils, intrinsic disorder and order, tissue-specific components, and intermediate filaments enables these unique properties.

Shaping of interphase chromosomes by the microtubule network

It is well established that microtubule dynamics play a major role in chromosome condensation and localization during mitosis. During interphase, however, it is assumed that the metazoan nuclear envelope presents a physical barrier, which inhibits interaction between the microtubules located in the cytoplasm and the chromatin fibers located in the nucleus. In recent years, it has become apparent that microtubule dynamics alter chromatin structure and function during interphase as well. Microtubule motor proteins transport several transcription factors and exogenous DNA (such as plasmid DNA) from the cytoplasm to the nucleus. Various soluble microtubule components are able to translocate into the nucleus, where they bind various chromatin elements leading to transcriptional alterations. In addition, microtubules may apply force on the nuclear envelope, which is transmitted into the nucleus, leading to changes in chromatin structure. Thus, microtubule dynamics during interphase may affect chromatin spatial organization, as well as transcription, replication and repair.

Nuclear membrane diversity: underlying tissue-specific pathologies in disease?

Human 'laminopathy' diseases result from mutations in genes encoding nuclear lamins or nuclear envelope (NE) transmembrane proteins (NETs). These diseases present a seeming paradox: the mutated proteins are widely expressed yet pathology is limited to specific tissues. New findings suggest tissue-specific pathologies arise because these widely expressed proteins act in various complexes that include tissue-specific components. Diverse mechanisms to achieve NE tissue-specificity include tissue-specific regulation of the expression, mRNA splicing, signaling, NE-localization and interactions of potentially hundreds of tissue-specific NETs. New findings suggest these NETs underlie tissue-specific NE roles in cytoskeletal mechanics, cell-cycle regulation, signaling, gene expression and genome organization. This view of the NE as 'specialized' in each cell type is important to understand the tissue-specific pathology of NE-linked diseases.

Accessorizing and anchoring the LINC complex for multifunctionality

The linker of nucleoskeleton and cytoskeleton (LINC) complex, composed of outer and inner nuclear membrane Klarsicht, ANC-1, and Syne homology (KASH) and Sad1 and UNC-84 (SUN) proteins, respectively, connects the nucleus to cytoskeletal filaments and performs diverse functions including nuclear positioning, mechanotransduction, and meiotic chromosome movements. Recent studies have shed light on the source of this diversity by identifying factors associated with the complex that endow specific functions as well as those that differentially anchor the complex within the nucleus. Additional diversity may be provided by accessory factors that reorganize the complex into higher-ordered arrays. As core components of the LINC complex are associated with several diseases, understanding the role of accessory and anchoring proteins could provide insights into pathogenic mechanisms.

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.

Connecting the nucleus to the cytoskeleton for nuclear positioning and cell migration

The position of the nucleus in the cytoplasm is a highly regulated process and is required for multiple cellular and developmental processes. Defects on different nuclear positioning events are associated with several pathologies such as muscle and nervous system disorders. In this chapter we describe the current knowledge on the mechanism of nuclear positioning. We discuss how the nucleus connects to the cytoskeleton by nesprins and SUN proteins, how this connection is regulated by Samp1, and how this connection is required for proper nuclear positioning. Furthermore, we discuss how nesprins, SUN, and Samp1 form transmembrane actin-associated nuclear (TAN) lines, novel nuclear envelope structures involved in force transduction during nuclear movement. Finally, we describe the recent evidences suggesting a role for the connection between the nucleus and the cytoskeleton in cancer.

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.

Nuclear mechanics in cancer

Despite decades of research, cancer metastasis remains an incompletely understood process that is as complex as it is devastating. In recent years, there has been an increasing push to investigate the biomechanical aspects of tumorigenesis, complementing the research on genetic and biochemical changes. In contrast to the high genetic variability encountered in cancer cells, almost all metastatic cells are subject to the same physical constraints as they leave the primary tumor, invade surrounding tissues, transit through the circulatory system, and finally infiltrate new tissues. Advances in live cell imaging and other biophysical techniques, including measurements of subcellular mechanics, have yielded stunning new insights into the physics of cancer cells. While much of this research has been focused on the mechanics of the cytoskeleton and the cellular microenvironment, it is now emerging that the mechanical properties of the cell nucleus and its connection to the cytoskeleton may play a major role in cancer metastasis, as deformation of the large and stiff nucleus presents a substantial obstacle during the passage through the dense interstitial space and narrow capillaries. Here, we present an overview of the molecular components that govern the mechanical properties of the nucleus, and we discuss how changes in nuclear structure and composition observed in many cancers can modulate nuclear mechanics and promote metastatic processes. Improved insights into this interplay between nuclear mechanics and metastatic progression may have powerful implications in cancer diagnostics and therapy and may reveal novel therapeutic targets for pharmacological inhibition of cancer cell invasion.

Broken nuclei--lamins, nuclear mechanics, and disease

Mutations in lamins, which are ubiquitous nuclear intermediate filaments, lead to a variety of disorders including muscular dystrophy and dilated cardiomyopathy. Lamins provide nuclear stability, help connect the nucleus to the cytoskeleton, and can modulate chromatin organization and gene expression. Nonetheless, the diverse functions of lamins remain incompletely understood. We focus here on the role of lamins on nuclear mechanics and their involvement in human diseases. Recent findings suggest that lamin mutations can decrease nuclear stability, increase nuclear fragility, and disturb mechanotransduction signaling, possibly explaining the muscle-specific defects in many laminopathies. At the same time, altered lamin expression has been reported in many cancers, where the resulting increased nuclear deformability could enhance the ability of cells to transit tight interstitial spaces, thereby promoting metastasis.

Tissue specificity in the nuclear envelope supports its functional complexity

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

Emerin organizes actin flow for nuclear movement and centrosome orientation in migrating fibroblasts

Emerin, a nuclear membrane protein, and myosin IIB contribute to nuclear movement by regulating the directionality of nuclear movement and dorsal actin cable flow. Emerin interacts with myosin IIB and is required for its perinuclear localization. The results show that the nuclear envelope actively organizes cytoplasmic polarity.

In migrating fibroblasts, rearward movement of the nucleus orients the centrosome toward the leading edge. Nuclear movement results from coupling rearward-moving, dorsal actin cables to the nucleus by linear arrays of nesprin-2G and SUN2, termed transmembrane actin-associated nuclear (TAN) lines. A-type lamins anchor TAN lines, prompting us to test whether emerin, a nuclear membrane protein that interacts with lamins and TAN line proteins, contributes to nuclear movement. In fibroblasts depleted of emerin, nuclei moved nondirectionally or completely failed to move. Consistent with these nuclear movement defects, dorsal actin cable flow was nondirectional in cells lacking emerin. TAN lines formed normally in cells lacking emerin and were coordinated with the erratic nuclear movements, although in 20% of the cases, TAN lines slipped over immobile nuclei. Myosin II drives actin flow, and depletion of myosin IIB, but not myosin IIA, showed similar nondirectional nuclear movement and actin flow as in emerin-depleted cells. Myosin IIB specifically coimmunoprecipitated with emerin, and emerin depletion prevented myosin IIB localization near nuclei. These results show that emerin functions with myosin IIB to polarize actin flow and nuclear movement in fibroblasts, suggesting a novel function for the nuclear envelope in organizing directional actin flow and cytoplasmic polarity.

Protein adaptation: mitotic functions for membrane trafficking proteins

Membrane trafficking and mitosis are two essential processes in eukaryotic cells. Surprisingly, many proteins best known for their role in membrane trafficking have additional 'moonlighting' functions in mitosis. Despite having proteins in common, there is insufficient evidence for a specific connection between these two processes. Instead, these phenomena demonstrate the adaptability of the membrane trafficking machinery that allows its repurposing for different cellular functions.

Specific nuclear envelope transmembrane proteins can promote the location of chromosomes to and from the nuclear periphery

Background

Different cell types have distinctive patterns of chromosome positioning in the nucleus. Although ectopic affinity-tethering of specific loci can be used to relocate chromosomes to the nuclear periphery, endogenous nuclear envelope proteins that control such a mechanism in mammalian cells have yet to be widely identified.

Results

To search for such proteins, 23 nuclear envelope transmembrane proteins were screened for their ability to promote peripheral localization of human chromosomes in HT1080 fibroblasts. Five of these proteins had strong effects on chromosome 5, but individual proteins affected different subsets of chromosomes. The repositioning effects were reversible and the proteins with effects all exhibited highly tissue-restricted patterns of expression. Depletion of two nuclear envelope transmembrane proteins that were preferentially expressed in liver each reduced the normal peripheral positioning of chromosome 5 in liver cells.

Conclusions

The discovery of nuclear envelope transmembrane proteins that can modulate chromosome position and have restricted patterns of expression may enable dissection of the functional relevance of tissue-specific patterns of radial chromosome positioning.

Absence of Dpy19l2, a new inner nuclear membrane protein, causes globozoospermia in mice by preventing the anchoring of the acrosome to the nucleus

Sperm-head elongation and acrosome formation, which take place during the last stages of spermatogenesis, are essential to produce competent spermatozoa that are able to cross the oocyte zona pellucida and to achieve fertilization. During acrosome biogenesis, acrosome attachment and spreading over the nucleus are still poorly understood and to date no proteins have been described to link the acrosome to the nucleus. We recently demonstrated that a deletion of DPY19L2, a gene coding for an uncharacterized protein, was responsible for a majority of cases of type I globozoospermia, a rare cause of male infertility that is characterized by the exclusive production of round-headed acrosomeless spermatozoa. Here, using Dpy19l2 knockout mice, we describe the cellular function of the Dpy19l2 protein. We demonstrate that the protein is expressed predominantly in spermatids with a very specific localization restricted to the inner nuclear membrane facing the acrosomal vesicle. We show that the absence of Dpy19l2 leads to the destabilization of both the nuclear dense lamina (NDL) and the junction between the acroplaxome and the nuclear envelope. Consequently, the acrosome and the manchette fail to be linked to the nucleus leading to the disruption of vesicular trafficking, failure of sperm nuclear shaping and eventually to the elimination of the unbound acrosomal vesicle. Finally, we show for the first time that Dpy19l3 proteins are also located in the inner nuclear envelope, therefore implying that the Dpy19 proteins constitute a new family of structural transmembrane proteins of the nuclear envelope.

Microtubule-associated nuclear envelope proteins in interphase and mitosis

The LINC (linker of nucleoskeleton and cytoskeleton) complex forms a transcisternal bridge across the NE (nuclear envelope) that connects the cytoskeleton with the nuclear interior. This enables some proteins of the NE to communicate with the centrosome and the microtubule cytoskeleton. The position of the centrosome relative to the NE is of vital importance for many cell functions, such as cell migration and division, and centrosomal dislocation is a frequent phenotype in laminopathic disorders. Also in mitosis, a small group of transmembrane NE proteins associate with microtubules when they concentrate in a specific membrane domain associated with the mitotic spindle. The present review discusses structural and functional aspects of microtubule association with NE proteins and how this association may be maintained over the cell cycle.

A promiscuous biotin ligase fusion protein identifies proximal and interacting proteins in mammalian cells

Proximity-dependent biotin identification (BioID) is a new approach making use of biotin ligase fusion proteins for the identification of both interacting and neighboring proteins in their native cellular environment.

We have developed a new technique for proximity-dependent labeling of proteins in eukaryotic cells. Named BioID for proximity-dependent biotin identification, this approach is based on fusion of a promiscuous Escherichia coli biotin protein ligase to a targeting protein. BioID features proximity-dependent biotinylation of proteins that are near-neighbors of the fusion protein. Biotinylated proteins may be isolated by affinity capture and identified by mass spectrometry. We apply BioID to lamin-A (LaA), a well-characterized intermediate filament protein that is a constituent of the nuclear lamina, an important structural element of the nuclear envelope (NE). We identify multiple proteins that associate with and/or are proximate to LaA in vivo. The most abundant of these include known interactors of LaA that are localized to the NE, as well as a new NE-associated protein named SLAP75. Our results suggest BioID is a useful and generally applicable method to screen for both interacting and neighboring proteins in their native cellular environment.

Samp1 is a component of TAN lines and is required for nuclear movement

The position of the nucleus is regulated in different developmental stages and cellular events. During polarization, the nucleus moves away from the future leading edge and this movement is required for proper cell migration. Nuclear movement requires the LINC complex components nesprin-2G and SUN2, which form transmembrane actin-associated nuclear (TAN) lines at the nuclear envelope. Here we show that the nuclear envelope protein Samp1 (NET5) is involved in nuclear movement during fibroblast polarization and migration. Moreover, we demonstrate that Samp1 is a component of TAN lines that contain nesprin-2G and SUN2. Finally, Samp1 associates with SUN2 and lamin A/C, and the presence of Samp1 at the nuclear envelope requires lamin A/C. These results support a role for Samp1 in the association between the LINC complex and lamins during nuclear movement.