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

Mammalian sperm head formation involves different polarization of two novel LINC complexes

BACKGROUND

LINC complexes are nuclear envelope bridging protein structures formed by interaction of SUN and KASH proteins. They physically connect the nucleus with the peripheral cytoskeleton and are critically involved in a variety of dynamic processes, such as nuclear anchorage, movement and positioning and meiotic chromosome dynamics. Moreover, they are shown to be essential for maintaining nuclear shape.

FINDINGS

Based on detailed expression analysis and biochemical approaches, we show here that during mouse sperm development, a terminal cell differentiation process characterized by profound morphogenic restructuring, two novel distinctive LINC complexes are established. They consist either of spermiogenesis-specific Sun3 and Nesprin1 or Sun1eta, a novel non-nuclear Sun1 isoform, and Nesprin3. We could find that these two LINC complexes specifically polarize to opposite spermatid poles likely linking to sperm-specific cytoskeletal structures. Although, as shown in co-transfection/immunoprecipitation experiments, SUN proteins appear to arbitrarily interact with various KASH partners, our study demonstrates that they actually are able to confine their binding to form distinct LINC complexes.

CONCLUSIONS

Formation of the mammalian sperm head involves assembly and different polarization of two novel spermiogenesis-specific LINC complexes. Together, our findings suggest that theses LINC complexes connect the differentiating spermatid nucleus to surrounding cytoskeletal structures to enable its well-directed shaping and elongation, which in turn is a critical parameter for male fertility.

The Drosophila SUN protein Spag4 cooperates with the coiled-coil protein Yuri Gagarin to maintain association of the basal body and spermatid nucleus

Maintaining the proximity of centrosomes to nuclei is important in several cellular contexts, and LINC complexes formed by SUN and KASH proteins are crucial in this process. Here, we characterize the presumed Drosophila ortholog of the mammalian SUN protein, sperm-associated antigen 4 (Spag4, previously named Giacomo), and demonstrate that Spag4 is required for centriole and nuclear attachment during spermatogenesis. Production of spag4 mRNA is limited to the testis, and Spag4 protein shows a dynamic pattern of association with the germline nuclei, including a concentration of protein at the site of attachment of the single spermatid centriole. In the absence of Spag4, nuclei and centrioles or basal bodies (BBs) dissociate from each other after meiosis. This role of Spag4 in centriolar attachment does not involve either of the two KASH proteins of the Drosophila genome (Klarsicht and MSP-300), but does require the coiled-coil protein Yuri Gagarin. Yuri shows an identical pattern of localization at the nuclear surface to Spag4 during spermatogenesis, and epistasis studies show that the activities of Yuri and dynein-dynactin are downstream of spag4 in this centriole attachment pathway. The later defects in spermatogenesis seen for yuri and spag4 mutants are similar, suggesting they could be secondary to initial disruption of events at the nuclear surface.

A classical NLS and the SUN domain contribute to the targeting of SUN2 to the inner nuclear membrane

Integral membrane proteins of the inner nuclear membrane (INM) are inserted into the endoplasmic reticulum membrane during their biogenesis and are then targeted to their final destination. We have used human SUN2 to delineate features that are required for INM targeting and have identified multiple elements that collectively contribute to the efficient localization of SUN2 to the nuclear envelope (NE). One such targeting element is a classical nuclear localization signal (cNLS) present in the N-terminal, nucleoplasmic domain of SUN2. A second motif proximal to the cNLS is a cluster of arginines that serves coatomer-mediated retrieval of SUN2 from the Golgi. Unexpectedly, also the C-terminal, lumenal SUN domain of SUN2 supports NE localization, showing that targeting elements are not limited to cytoplasmic or transmembrane domains of INM proteins. Together, SUN2 represents the first mammalian INM protein relying on a functional cNLS, a Golgi retrieval signal and a perinuclear domain to mediate targeting to the INM.

The perinuclear actin cap in health and disease

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

Bicaudal D2, dynein, and kinesin-1 associate with nuclear pore complexes and regulate centrosome and nuclear positioning during mitotic entry

Mammalian Bicaudal D2 is the missing molecular link between cytoplasmic motor proteins and the nucleus during nuclear positioning prior to the onset of mitosis.

BICD2 is one of the two mammalian homologues of the Drosophila Bicaudal D, an evolutionarily conserved adaptor between microtubule motors and their cargo that was previously shown to link vesicles and mRNP complexes to the dynein motor. Here, we identified a G2-specific role for BICD2 in the relative positioning of the nucleus and centrosomes in dividing cells. By combining mass spectrometry, biochemical and cell biological approaches, we show that the nuclear pore complex (NPC) component RanBP2 directly binds to BICD2 and recruits it to NPCs specifically in G2 phase of the cell cycle. BICD2, in turn, recruits dynein-dynactin to NPCs and as such is needed to keep centrosomes closely tethered to the nucleus prior to mitotic entry. When dynein function is suppressed by RNA interference-mediated depletion or antibody microinjection, centrosomes and nuclei are actively pushed apart in late G2 and we show that this is due to the action of kinesin-1. Surprisingly, depletion of BICD2 inhibits both dynein and kinesin-1-dependent movements of the nucleus and cytoplasmic NPCs, demonstrating that BICD2 is needed not only for the dynein function at the nuclear pores but also for the antagonistic activity of kinesin-1. Our study demonstrates that the nucleus is subject to opposing activities of dynein and kinesin-1 motors and that BICD2 contributes to nuclear and centrosomal positioning prior to mitotic entry through regulation of both dynein and kinesin-1.

Bidirectional microtubule-based transport is responsible for the positioning of a large variety of cellular organelles, but the molecular mechanisms underlying the recruitment of microtubule-based motors to their cargoes and their activation remain poorly understood. In particular, the molecular players involved in the important processes of nuclear and centrosomal positioning prior to the onset of cell division are not known. In this study we focus on the function of one of the mammalian homologues of Drosophila Bicaudal D, an adaptor for the microtubule minus-end-directed dynein-dynactin motor complex. Previously, Drosophila Bicaudal D and its mammalian homologues were shown to act as linkers between the dynein motor and mRNP complexes or secretory vesicles. Here, we identify a new cargo for mammalian Bicaudal D2 (BICD2)–the nucleus. We show that BICD2 specifically binds to nuclear pore complexes in cells in G2 phase of the cell division cycle. We also show that this interaction is required for G2-specific recruitment of dynein to the nuclear envelope and thus for proper positioning of the nucleus relative to centrosomes prior to the onset of mitosis. Further, our findings demonstrate that the motor protein kinesin-1 opposes dynein's activity during this process and requires BICD2 for its activity. Our study therefore reveals BICD2 as the critical molecular adaptor that allows molecular motors to regulate nuclear and centrosomal positioning before cell division.

Dynamics and molecular interactions of linker of nucleoskeleton and cytoskeleton (LINC) complex proteins

The linker of nucleoskeleton and cytoskeleton (LINC) complex is situated in the nuclear envelope and forms a connection between the lamina and cytoskeletal elements. Sun1, Sun2 and nesprin-2 are important components of the LINC complex. We expressed these proteins fused to green fluorescent protein in embryonic fibroblasts and studied their diffusional mobilities using fluorescence recovery after photobleaching. We show that they all are more mobile in embryonic fibroblasts from mice lacking A-type lamins than in cells from wild-type mice. Knockdown of Sun2 also increased the mobility of a short, chimeric form of nesprin-2 giant (mini-nesprin-2G), whereas the lack of emerin did not affect the mobility of Sun1, Sun2 or mini-nesprin-2G. Fluorescence resonance energy transfer experiments showed Sun1 to be more closely associated with lamin A than is Sun2. Sun1 and Sun2 had similar affinity for the nesprin-2 KASH domain in plasmon surface resonance (Biacore) experiments. This affinity was ten times higher than that previously reported between nesprin-2 and actin. Deletion of the actin-binding domain had no effect on mini-nesprin-2G mobility. Our data support a model in which A-type lamins and Sun2 anchor nesprin-2 in the outer nuclear membrane, whereas emerin, Sun1 and actin are dispensable for this anchoring.

SUN-1 and ZYG-12, mediators of centrosome-nucleus attachment, are a functional SUN/KASH pair in Caenorhabditis elegans

Klarsicht/ANC-1/Syne/homology (KASH)/Sad-1/UNC-84 (SUN) protein pairs can act as connectors between cytoplasmic organelles and the nucleoskeleton. Caenorhabditis elegans ZYG-12 and SUN-1 are essential for centrosome-nucleus attachment. Although SUN-1 has a canonical SUN domain, ZYG-12 has a divergent KASH domain. Here, we establish that the ZYG-12 mini KASH domain is functional and, in combination with a portion of coiled-coil domain, is sufficient for nuclear envelope localization. ZYG-12 and SUN-1 are hypothesized to be outer and inner nuclear membrane proteins, respectively, and to interact, but neither their topologies nor their physical interaction has been directly investigated. We show that ZYG-12 is a type II outer nuclear membrane (ONM) protein and that SUN-1 is a type II inner nuclear membrane protein. The proteins interact in the luminal space of the nuclear envelope via the ZYG-12 mini KASH domain and a region of SUN-1 that does not include the SUN domain. SUN-1 is hypothesized to restrict ZYG-12 to the ONM, preventing diffusion through the endoplasmic reticulum. We establish that ZYG-12 is indeed immobile at the ONM by using fluorescence recovery after photobleaching and show that SUN-1 is sufficient to localize ZYG-12 in cells. This work supports current models of KASH/SUN pairs and highlights the diversity in sequence elements defining KASH domains.

Bringing KASH under the SUN: the many faces of nucleo-cytoskeletal connections

The nucleus is the most prominent cellular organelle, and its sharp boundaries suggest the compartmentalization of the nucleoplasm from the cytoplasm. However, the recent identification of evolutionarily conserved linkers of the nucleoskeleton to the cytoskeleton (LINC) complexes, a family of macromolecular assemblies that span the double membrane of the nuclear envelope, reveals tight physical connections between the two compartments. Here, we review the structure and evolutionary conservation of SUN and KASH domain–containing proteins, whose interaction within the perinuclear space forms the “nuts and bolts” of LINC complexes. Moreover, we discuss the function of these complexes in nuclear, centrosomal, and chromosome dynamics, and their connection to human disease.

KDP-1 is a nuclear envelope KASH protein required for cell-cycle progression

Klarsicht, ANC-1 and Syne homology (KASH) proteins localize to the outer nuclear membrane where they connect the nucleus to the cytoskeleton. KASH proteins interact with Sad1-UNC-84 (SUN) proteins to transfer forces across the nuclear envelope to position nuclei or move chromosomes. A new KASH protein, KDP-1, was identified in a membrane yeast two-hybrid screen of a Caenorhabditis elegans library using the SUN protein UNC-84 as bait. KDP-1 also interacted with SUN-1. KDP-1 was enriched at the nuclear envelope in a variety of tissues and required SUN-1 for nuclear envelope localization in the germline. Genetic analyses showed that kdp-1 was essential for embryonic viability, larval growth and germline development. kdp-1(RNAi) delayed the entry into mitosis in embryos, led to a small mitotic zone in the germline, and caused an endomitotic phenotype. Aspects of these phenotypes were similar to those seen in sun-1(RNAi), suggesting that KDP-1 functions with SUN-1 in the germline and early embryo. The data suggest that KDP-1 is a novel KASH protein that functions to ensure the timely progression of the cell cycle between the end of S phase and the entry into mitosis.

Recruitment of functionally distinct membrane proteins to chromatin mediates nuclear envelope formation in vivo

Formation of the nuclear envelope (NE) around segregated chromosomes occurs by the reshaping of the endoplasmic reticulum (ER), a reservoir for disassembled nuclear membrane components during mitosis. In this study, we show that inner nuclear membrane proteins such as lamin B receptor (LBR), MAN1, Lap2beta, and the trans-membrane nucleoporins Ndc1 and POM121 drive the spreading of ER membranes into the emerging NE via their capacity to bind chromatin in a collaborative manner. Despite their redundant functions, decreasing the levels of any of these trans-membrane proteins by RNAi-mediated knockdown delayed NE formation, whereas increasing the levels of any of them had the opposite effect. Furthermore, acceleration of NE formation interferes with chromosome separation during mitosis, indicating that the time frame over which chromatin becomes membrane enclosed is physiologically relevant and regulated. These data suggest that functionally distinct classes of chromatin-interacting membrane proteins, which are present at nonsaturating levels, collaborate to rapidly reestablish the nuclear compartment at the end of mitosis.

Intermediate filaments: primary determinants of cell architecture and plasticity

Intermediate filaments (IFs) are major constituents of the cytoskeleton and nuclear boundary in animal cells. They are of prime importance for the functional organization of structural elements. Depending on the cell type, morphologically similar but biochemically distinct proteins form highly viscoelastic filament networks with multiple nanomechanical functions. Besides their primary role in cell plasticity and their established function as cellular stress absorbers, recently discovered gene defects have elucidated that structural alterations of IFs can affect their involvement both in signaling and in controlling gene regulatory networks. Here, we highlight the basic structural and functional properties of IFs and derive a concept of how mutations may affect cellular architecture and thereby tissue construction and physiology.

Defects in cell spreading and ERK1/2 activation in fibroblasts with lamin A/C mutations

In-frame mutations in nuclear lamin A/C lead to a multitude of tissue-specific degenerative diseases known as the 'laminopathies'. Previous studies have demonstrated that lamin A/C-null mouse fibroblasts have defects in cell polarisation, suggesting a role for lamin A/C in nucleo-cytoskeletal-cell surface cross-talk. However, this has not been examined in patient fibroblasts expressing modified forms of lamin A/C. Here, we analysed skin fibroblasts from 3 patients with Emery-Dreifuss muscular dystrophy and from 1 with dilated cardiomyopathy. The emerin-lamin A/C interaction was impaired in each mutant cell line. Mutant cells exhibited enhanced cell proliferation, collagen-dependent adhesion, larger numbers of filopodia and smaller cell spread size, compared with control cells. Furthermore, cell migration, speed and polarization were elevated. Mutant cells also showed an enhanced ability to contract collagen gels at early time points, compared with control cells. Phosphotyrosine measurements during cell spreading indicated an initial temporal lag in ERK1/2 activation in our mutant cells, followed by hyper-activation of ERK1/2 at 2 h post cell attachment. Deregulated ERK1/2 activation is linked with cardiomyopathy, cell spreading and proliferation defects. We conclude that a functional emerin-lamin A/C complex is required for cell spreading and proliferation, possibly acting through ERK1/2 signalling.

A nuclear-envelope bridge positions nuclei and moves chromosomes

Positioning the nucleus is essential for the formation of polarized cells, pronuclear migration, cell division, cell migration and the organization of specialized syncytia such as mammalian skeletal muscles. Proteins that are required for nuclear positioning also function during chromosome movement and pairing in meiosis. Defects in these processes lead to human diseases including laminopathies. To properly position the nucleus or move chromosomes within the nucleus, the cell must specify the outer surface of the nucleus and transfer forces across both membranes of the nuclear envelope. KASH proteins are specifically recruited to the outer nuclear membrane by SUN proteins, which reside in the inner nuclear membrane. KASH and SUN proteins physically interact in the perinuclear space, forming a bridge across the two membranes of the nuclear envelope. The divergent N-terminal domains of KASH proteins extend from the surface of the nucleus into the cytoplasm and interact with the cytoskeleton, whereas the N-termini of SUN proteins extend into the nucleoplasm to interact with the lamina or chromatin. The bridge of SUN and KASH across the nuclear envelope functions to transfer forces that are generated in the cytoplasm into the nucleoplasm during nuclear migration, nuclear anchorage, centrosome attachment, intermediate-filament association and telomere clustering.

Plectin contributes to mechanical properties of living cells

Plectin is a 500-kDa cross-linking protein that plays important roles in a number of cell functions including migration and wound healing. We set out to characterize the role of plectin in mechanical properties of living cells. Plectin(-/-) cells were less stiff than plectin(+/+) cells, but the slopes of the two power laws in response to loading frequencies (0.002-1,000 Hz) were similar. Plectin(-/-) cells lost the capacity to propagate mechanical stresses to long distances in the cytoplasm; traction forces in plectin(-/-) cells were only half of those in plectin(+/+) cells, suggesting that plectin deficiency compromised prestress generation, which, in turn, resulted in the inhibition of long distance stress propagation. Both plectin(+/+) and plectin(-/-) cells exhibited nonlinear stress-strain relationships. However, plectin(+/+) cells, but not plectin(-/-) cells, further stiffened in response to lysophosphatidic acid (LPA). Dynamic fluorescence resonance energy transfer analysis revealed that RhoA GTPase proteins were activated in plectin(+/+) cells but not in plectin(-/-) cells after treatment with LPA. Expression in plectin(-/-) cells of constitutively active RhoA (RhoA-V14) but not a dominant negative mutant of RhoA (RhoA-N19) or an empty vector restored the long distance force propagation behavior, suggesting that plectin is important in normal functions of RhoA. Our findings underscore the importance of plectin for mechanical properties, stress propagation, and prestress of living cells, thereby influencing their biological functions.

LULL1 retargets TorsinA to the nuclear envelope revealing an activity that is impaired by the DYT1 dystonia mutation

TorsinA (TorA) is an AAA+ ATPase in the endoplasmic reticulum (ER) lumen that is mutated in early onset DYT1 dystonia. TorA is an essential protein in mice and is thought to function in the nuclear envelope (NE) despite localizing throughout the ER. Here, we report that transient interaction of TorA with the ER membrane protein LULL1 targets TorA to the NE. FRAP and Blue Native PAGE indicate that TorA is a stable, slowly diffusing oligomer in either the absence or presence of LULL1. Increasing LULL1 expression redistributes both wild-type and disease-mutant TorA to the NE, while decreasing LULL1 with shRNAs eliminates intrinsic enrichment of disease-mutant TorA in the NE. When concentrated in the NE, TorA displaces the nuclear membrane proteins Sun2, nesprin-2G, and nesprin-3 while leaving nuclear pores and Sun1 unchanged. Wild-type TorA also induces changes in NE membrane structure. Because SUN proteins interact with nesprins to connect nucleus and cytoskeleton, these effects suggest a new role for TorA in modulating complexes that traverse the NE. Importantly, once concentrated in the NE, disease-mutant TorA displaces Sun2 with reduced efficiency and does not change NE membrane structure. Together, our data suggest that LULL1 regulates the distribution and activity of TorA within the ER and NE lumen and reveal functional defects in the mutant protein responsible for DYT1 dystonia.

KASH-domain proteins and the cytoskeletal landscapes of the nuclear envelope

Over the last few years, several novel proteins have been identified that facilitate the physical integration of the nucleus with the cytoplasmic compartment. The majority belong to the evolutionarily conserved KASH [klarsicht/ANC-1 (anchorage 1)/SYNE (synaptic nuclear envelope protein) homology]-domain family, which function primarily as exclusive outer nuclear membrane scaffolds that associate with the cytoskeleton, the centrosome and the motor protein apparatus. In the present paper, we propose a novel model, which may explain why these proteins also determine nuclear architecture. Moreover, we discuss further nuclear membrane-tethering devices, which indicate collectively the presence of specific molecular mechanisms that organize the cytoplasmic-nuclear membrane interface in mammalian cells.

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

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

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

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

The LINC-less granulocyte nucleus

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

TorsinA binds the KASH domain of nesprins and participates in linkage between nuclear envelope and cytoskeleton

A specific mutation (DeltaE) in torsinA underlies most cases of the dominantly inherited movement disorder, early-onset torsion dystonia (DYT1). TorsinA, a member of the AAA+ ATPase superfamily, is located within the lumen of the nuclear envelope (NE) and endoplasmic reticulum (ER). We investigated an association between torsinA and nesprin-3, which spans the outer nuclear membrane (ONM) of the NE and links it to vimentin via plectin in fibroblasts. Mouse nesprin-3alpha co-immunoprecipitated with torsinA and this involved the C-terminal region of torsinA and the KASH domain of nesprin-3alpha. This association with human nesprin-3 appeared to be stronger for torsinADeltaE than for torsinA. TorsinA also associated with the KASH domains of nesprin-1 and -2 (SYNE1 and 2), which link to actin. In the absence of torsinA, in knockout mouse embryonic fibroblasts (MEFs), nesprin-3alpha was localized predominantly in the ER. Enrichment of yellow fluorescent protein (YFP)-nesprin-3 in the ER was also seen in the fibroblasts of DYT1 patients, with formation of YFP-positive globular structures enriched in torsinA, vimentin and actin. TorsinA-null MEFs had normal NE structure, but nuclear polarization and cell migration were delayed in a wound-healing assay, as compared with wild-type MEFs. These studies support a role for torsinA in dynamic interactions between the KASH domains of nesprins and their protein partners in the lumen of the NE, with torsinA influencing the localization of nesprins and associated cytoskeletal elements and affecting their role in nuclear and cell movement.

Dysfunctional connections between the nucleus and the actin and microtubule networks in laminopathic models

Laminopathies encompass a wide array of human diseases associated to scattered mutations along LMNA, a single gene encoding A-type lamins. How such genetic alterations translate to cellular defects and generate such diverse disease phenotypes remains enigmatic. Recent work has identified nuclear envelope proteins--emerin and the linker of the nucleoskeleton and cytoskeleton (LINC) complex--which connect the nuclear lamina to the cytoskeleton. Here we quantitatively examine the composition of the nuclear envelope, as well as the architecture and functions of the cytoskeleton in cells derived from two laminopathic mouse models, including Hutchinson-Gilford progeria syndrome (Lmna(L530P/L530P)) and Emery-Dreifuss muscular dystrophy (Lmna(-/-)). Cells derived from the overtly aphenotypical model of X-linked Emery-Dreifuss muscular dystrophy (Emd(-/y)) were also included. We find that the centrosome is detached from the nucleus, preventing centrosome polarization in cells under flow--defects that are mediated by the loss of emerin from the nuclear envelope. Moreover, while basal actin and focal adhesion structure are mildly affected, RhoA activation, cell-substratum adhesion, and cytoplasmic elasticity are greatly lowered, exclusively in laminopathic models in which the LINC complex is disrupted. These results indicate a new function for emerin in cell polarization and suggest that laminopathies are not directly associated with cells' inability to polarize, but rather with cytoplasmic softening and weakened adhesion mediated by the disruption of the LINC complex across the nuclear envelope.

Anchoring junctions as drug targets: role in contraceptive development

In multicellular organisms, cell-cell interactions are mediated in part by cell junctions, which underlie tissue architecture. Throughout spermatogenesis, for instance, preleptotene leptotene spermatocytes residing in the basal compartment of the seminiferous epithelium must traverse the blood-testis barrier to enter the adluminal compartment for continued development. At the same time, germ cells must also remain attached to Sertoli cells, and numerous studies have reported extensive restructuring at the Sertoli-Sertoli and Sertoli-germ cell interface during germ cell movement across the seminiferous epithelium. Furthermore, the proteins and signaling cascades that regulate adhesion between testicular cells have been largely delineated. These findings have unveiled a number of potential "druggable" targets that can be used to induce premature release of germ cells from the seminiferous epithelium, resulting in transient infertility. Herein, we discuss a novel approach with the aim of developing a nonhormonal male contraceptive for future human use, one that involves perturbing adhesion between Sertoli and germ cells in the testis.

The nuclear envelope as an integrator of nuclear and cytoplasmic architecture

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

Nuclear lamins: major factors in the structural organization and function of the nucleus and chromatin

Over the past few years it has become evident that the intermediate filament proteins, the types A and B nuclear lamins, not only provide a structural framework for the nucleus, but are also essential for many aspects of normal nuclear function. Insights into lamin-related functions have been derived from studies of the remarkably large number of disease-causing mutations in the human lamin A gene. This review provides an up-to-date overview of the functions of nuclear lamins, emphasizing their roles in epigenetics, chromatin organization, DNA replication, transcription, and DNA repair. In addition, we discuss recent evidence supporting the importance of lamins in viral infections.

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

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

Intermediate filaments in smooth muscle

The intermediate filament (IF) network is one of the three cytoskeletal systems in smooth muscle. The type III IF proteins vimentin and desmin are major constituents of the network in smooth muscle cells and tissues. Lack of vimentin or desmin impairs contractile ability of various smooth muscle preparations, implying their important role for smooth muscle force development. The IF framework has long been viewed as a fixed cytostructure that solely provides mechanical integrity for the cell. However, recent studies suggest that the IF cytoskeleton is dynamic in mammalian cells in response to various external stimulation. In this review, the structure and biological properties of IF proteins in smooth muscle are summarized. The role of IF proteins in the modulation of smooth muscle force development and redistribution/translocation of signaling partners (such as p130 Crk-associated substrate, CAS) is depicted. This review also summarizes our latest understanding on how the IF network may be regulated in smooth muscle.