A Network of Nuclear Envelope Membrane Proteins Linking Centromeres to Microtubules

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Mutations in LMNA, which encodes A-type lamins, result in disparate diseases, known collectively as laminopathies, that affect distinct tissues, including striated muscle and adipose tissue. Lamins provide structural support for the nucleus and sites of attachment for chromatin, and defects in these functions may contribute to disease pathogenesis. Recent studies suggest that A-type lamins may facilitate connections between the nucleus and the cytoskeleton mediated by nuclear envelope nesprin and SUN proteins. In mammalian cells, however, interfering with A-type lamins does not affect the localization of these proteins. Here, we used centrosome orientation in fibroblasts, which requires separate nuclear and centrosome positioning pathways, as a model system to understand how LMNA mutations affect nucleus-cytoskeletal connections. We find that LMNA mutations causing striated muscle diseases block actin-dependent nuclear movement, whereas most that affect adipose tissue inhibit microtubule-dependent centrosome positioning. Genetic deletion or transient depletion of A-type lamins also blocked nuclear movement, showing that mutations affecting muscle exhibit the null phenotype. Lack of A-type lamins, or expression of variants that cause striated muscle disease, did not affect assembly of nesprin-2G and SUN2 into transmembrane actinassociated nuclear (TAN) lines that attach the nucleus to retrogradely moving actin cables. Nesprin-2G TAN lines were less stable, however, and slipped over the nucleus rather than moving with it, indicating that they were not anchored. Nesprin-2G TAN lines also slipped in SUN2-depleted cells. Our results establish A-type lamins as anchors for nesprin-2G-SUN2 TAN lines to allow productive movement and proper positioning of the nucleus by actin.nesprin SUN | linker of nucleoskeleton and cytoskeleton complex | muscular dystrophy L amin A and lamin C, the predominant A-type lamins, are expressed in most differentiated somatic cells. Yet, different mutations in the LMNA gene encoding these proteins result in a variety of diseases that affect specific tissues. LMNA mutations cause autosomal dominant Emery-Dreifuss muscular dystrophy (EDMD) and related diseases with dilated cardiomyopathy (DCM) that affect cardiac muscle and skeletal muscle to variable degrees (1). Other LMNA mutations cause Dunnigan-type familial partial lipodystrophy (FPLD) that affects adipose tissue (2), Charcot-Marie-Tooth type 2B1 disease that affects peripheral neurons (3), and progeroid syndromes with features of accelerated aging (4, 5). The mechanism whereby mutations in a single gene that is widely expressed cause such diverse diseases remains a puzzle.There are two prevailing hypotheses to explain the pathogenesis of laminopathies. The mechanical stress hypothesis proposes that alterations in A-type lamins compromise nuclear integrity in tissues susceptible to stress, such as striated muscle. This model is supported by findings that A-type lamin deficiency disrupts nuclear integrity in model systems (6, 7). The secon...

Section: Discussionmentioning

confidence: 99%

Lamin A variants that cause striated muscle disease are defective in anchoring transmembrane actin-associated nuclear lines for nuclear movement

Folker

Östlund

Luxton

et al. 2010

166

202

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confidence: 99%

“…Similarly, in flies, the SUN protein Klaroid is required for the localization of KASH protein Klarsicht to the NE and for nuclear migration in the eye (15). Genetic studies in the worm and yeast also have indicated roles of SUN proteins in mediating the interactions between the NE and centrosome, centromeres, and telomeres at various stages of mitotic and meiotic cells (16)(17)(18)(19)(20)(21)(22).Mammalian SUN1 and SUN2 proteins were first identified as homologues of C. elegans UNC-84 (10). SUN1 and SUN2 are broadly distributed in mouse tissues, whereas 2 other, more recently identified SUN proteins, SUN3 and SPAG4, appear to be expressed in a limited amount of tissues (4,(23)(24)(25).…”

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confidence: 99%

SUN1 and SUN2 play critical but partially redundant roles in anchoring nuclei in skeletal muscle cells in mice

Lei

Zhang

Ding

et al. 2009

222

236

How the nuclei in mammalian skeletal muscle fibers properly position themselves relative to the cell body is an interesting and important cell biology question. In the syncytial skeletal muscle cells, more than 100 nuclei are evenly distributed at the periphery of each cell, with 3-8 nuclei anchored beneath the neuromuscular junction (NMJ). Our previous studies revealed that the KASH domain-containing Syne-1/Nesprin-1 protein plays an essential role in anchoring both synaptic and nonsynaptic myonuclei in mice. SUN domain-containing proteins (SUN proteins) have been shown to interact with KASH domain-containing proteins (KASH proteins) at the nuclear envelope (NE), but their roles in nuclear positioning in mice are unknown. Here we show that the synaptic nuclear anchorage is partially perturbed in Sun1, but not in Sun2, knockout mice. Disruption of 3 or all 4 Sun1/2 wild-type alleles revealed a gene dosage effect on synaptic nuclear anchorage. The organization of nonsynaptic nuclei is disrupted in Sun1/2 doubleknockout (DKO) mice as well. We further show that the localization of Syne-1 to the NE of muscle cells is disrupted in Sun1/2 DKO mice. These results clearly indicate that SUN1 and SUN2 function critically in skeletal muscle cells for Syne-1 localization at the NE, which is essential for proper myonuclear positioning.KASH domain ͉ Nesprin ͉ neuromuscular junction ͉ nuclear envelope protein ͉ Syne-1 P roper nuclear positioning relative to the cell body is important for many cellular processes during animal development. Mammalian skeletal muscle fibers are giant syncytial cells, each containing more than 100 nuclei. Each muscle fiber contains a neuromuscular junction (NMJ) that starts to form at approximately embryonic day 14 (E14) in mice (1). Following a multistep process involving the function of the agrin-MuSK-rapsynAChR pathway, the motor nerve terminal and the postsynaptic membrane become highly differentiated, ensuring reliable terminal transmission. The nuclei in the muscle fibers are positioned in a nonrandom manner, with 3-8 nuclei (synaptic nuclei) clustered beneath each NMJ and other nuclei (nonsynaptic nuclei) distributed evenly along the cell membrane (1, 2). How synaptic and nonsynaptic nuclei properly position themselves within each muscle fiber and the consequences of disrupting the nuclear positioning are attractive problems to investigate. Recent studies have indicated that KASH (Klarsicht/ANC-1/Syne homologue) domain-containing and SUN domain-containing proteins (KASH/SUN proteins) form a complex at the nuclear envelope (NE) for various cellular functions (3-6). Whereas a mammalian KASH protein has been found to have a critical function in myonuclear anchorage (7-9), the roles of SUN proteins in this process remain unclear.The SUN domain was first defined as a domain of shared homology between Sad1 in Schizosaccharomyces pombe and UNC-84 in Caenorhabditis elegans (10). SUN proteins are conserved inner nuclear membrane proteins containing at least 1 transmembrane domain and a C-terminal SUN...

“…Although most likely reside in the INM, several ONM proteins have been identified (7). These include members of the mammalian nesprin (or syne) family (8,9), Klarsicht (10) and Msp-300 (11,12) in Drosophila melanogaster, Anc-1 (13), Zyg-12 (14) and Unc-83 (15,16) in Caenorhabditis elegans, and Kms2 in the fission yeast Schizosaccharomyces pombe (17). A property that each of these has in common is that they interact with cytoskeletal components.…”

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confidence: 99%

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

Roux

Crisp

Liu

et al. 2009

321

348

Nucleocytoplasmic coupling is mediated by outer nuclear membrane (ONM) nesprin proteins and inner nuclear membrane Sun proteins. Interactions spanning the perinuclear space create nesprin-Sun complexes connecting the cytoskeleton to nuclear components. A search for proteins displaying a conserved C-terminal sequence present in nesprins 1-3 identified nesprin 4 (Nesp4), a new member of this family. Nesp4 is a kinesin-1-binding protein that displays Sun-dependent localization to the ONM. Expression of Nesp4 is associated with dramatic changes in cellular organization involving relocation of the centrosome and Golgi apparatus relative to the nucleus. These effects can be accounted for entirely by Nesp4's kinesin-binding function. The implication is that Nesp4 may contribute to microtubule-dependent nuclear positioning.centrosome ͉ LINC complex ͉ nuclear envelope T he nuclear envelope (NE) forms the interface between the nucleus and cytoplasm acting as a selective barrier that regulates nucleo-cytoplasmic traffic (1). In addition to this partition function, accumulating evidence reveals an essential role for the NE as a determinant of higher-order nuclear and chromatin organization (2). More surprising are findings that the NE directly impacts cytoskeletal architecture and in this way may help define the mechanical properties of the cell as a whole (3). The molecular bases for these effects are now emerging, uniting aspects of cellular physiology from mechanotransduction to nuclear positioning during differentiation and development (4).The NE is assembled from several elements, the most prominent being inner nuclear membranes (INMs) and outer nuclear membranes (ONMs) separated by a Ϸ40-nm gap or perinuclear space (PNS). The INM and ONM are joined where they are spanned by nuclear pore complexes (NPCs), the mediators of trafficking across the NE. The ONM also displays connections to the peripheral endoplasmic reticulum (ER). Accordingly, the INM, ONM, and ER represent a single membrane system with the PNS forming an extension of the ER lumen.The final feature of the NE is the nuclear lamina, a protein meshwork composed primarily of A-and B-type lamins that lines the nuclear face of the INM (2). The lamina is required for NE integrity and provides chromatin-anchoring sites at the nuclear periphery. Remarkably, aberrant A-type lamin expression, which is linked to several human diseases (5), is associated with altered cytoskeletal mechanics (3). The mechanisms underlying this phenomenon represent an intriguing biological problem.At least 60 NE membrane proteins are known (6). Although most likely reside in the INM, several ONM proteins have been identified (7). These include members of the mammalian nesprin (or syne) family (8, 9), Klarsicht (10) and 12) in Drosophila melanogaster, Anc-1 (13), and 16) in Caenorhabditis elegans, and Kms2 in the fission yeast Schizosaccharomyces pombe (17). A property that each of these has in common is that they interact with cytoskeletal components. They are also united in possessing a...