Association of emerin with nuclear and cytoplasmic actin is regulated in differentiating myoblasts

Filamentous nuclear actin regulation of PML NBs during the DNA damage response is deregulated by prelamin A

Nuclear actin participates in a continuously expanding list of core processes within eukaryotic nuclei, including the maintenance of genomic integrity. In response to DNA damage, nuclear actin polymerises into filaments that are involved in the repair of damaged DNA through incompletely defined mechanisms. We present data to show that the formation of nuclear F-actin in response to genotoxic stress acts as a scaffold for PML NBs and that these filamentous networks are essential for PML NB fission and recruitment of microbodies to DNA lesions. Further to this, we demonstrate that the accumulation of the toxic lamin A precursor prelamin A induces mislocalisation of nuclear actin to the nuclear envelope and prevents the establishment of nucleoplasmic F-actin networks in response to stress. Consequently, PML NB dynamics and recruitment to DNA lesions is ablated, resulting in impaired DNA damage repair. Inhibition of nuclear export of formin mDia2 restores nuclear F-actin formation by augmenting polymerisation of nuclear actin in response to stress and rescues PML NB localisation to sites of DNA repair, leading to reduced levels of DNA damage.

Cellular Polarity Transmission to the Nucleus

Polarity is an intrinsic and fundamental property of unicellular organisms and, as well, of single cells in multicellular ones. It can be defined as asymmetric cell organization that is self-reinforced and maintained by appropriate signaling. While cellular polarity is widely studied at the membrane and cytoplasmic level, if and how it is transmitted to the nucleus is still a matter of research and discussion. However, there is growing evidence of polarity transmission from the cell to the nucleus. In this chapter, we discuss recent reports on nuclear polarity and involvement of potential molecular players including emerin, nesprins, and nuclear F-actin which may play a significant role in establishment of this phenomenon.

From the Matrix to the Nucleus and Back: Mechanobiology in the Light of Health, Pathologies, and Regeneration of Oral Periodontal Tissues

Among oral tissues, the periodontium is permanently subjected to mechanical forces resulting from chewing, mastication, or orthodontic appliances. Molecularly, these movements induce a series of subsequent signaling processes, which are embedded in the biological concept of cellular mechanotransduction (MT). Cell and tissue structures, ranging from the extracellular matrix (ECM) to the plasma membrane, the cytosol and the nucleus, are involved in MT. Dysregulation of the diverse, fine-tuned interaction of molecular players responsible for transmitting biophysical environmental information into the cell's inner milieu can lead to and promote serious diseases, such as periodontitis or oral squamous cell carcinoma (OSCC). Therefore, periodontal integrity and regeneration is highly dependent on the proper integration and regulation of mechanobiological signals in the context of cell behavior. Recent experimental findings have increased the understanding of classical cellular mechanosensing mechanisms by both integrating exogenic factors such as bacterial gingipain proteases and newly discovered cell-inherent functions of mechanoresponsive co-transcriptional regulators such as the Yes-associated protein 1 (YAP1) or the nuclear cytoskeleton. Regarding periodontal MT research, this review offers insights into the current trends and open aspects. Concerning oral regenerative medicine or weakening of periodontal tissue diseases, perspectives on future applications of mechanobiological principles are discussed.

Tailoring Cellular Function: The Contribution of the Nucleus in Mechanotransduction

Cells sense a variety of different mechanochemical stimuli and promptly react to such signals by reshaping their morphology and adapting their structural organization and tensional state. Cell reactions to mechanical stimuli arising from the local microenvironment, mechanotransduction, play a crucial role in many cellular functions in both physiological and pathological conditions. To decipher this complex process, several studies have been undertaken to develop engineered materials and devices as tools to properly control cell mechanical state and evaluate cellular responses. Recent reports highlight how the nucleus serves as an important mechanosensor organelle and governs cell mechanoresponse. In this review, we will introduce the basic mechanisms linking cytoskeleton organization to the nucleus and how this reacts to mechanical properties of the cell microenvironment. We will also discuss how perturbations of nucleus-cytoskeleton connections, affecting mechanotransduction, influence health and disease. Moreover, we will present some of the main technological tools used to characterize and perturb the nuclear mechanical state.

Lamin A involvement in ageing processes

Lamin A, a main constituent of the nuclear lamina, is the major splicing product of the LMNA gene, which also encodes lamin C, lamin A delta 10 and lamin C2. Involvement of lamin A in the ageing process became clear after the discovery that a group of progeroid syndromes, currently referred to as progeroid laminopathies, are caused by mutations in LMNA gene. Progeroid laminopathies include Hutchinson-Gilford Progeria, Mandibuloacral Dysplasia, Atypical Progeria and atypical-Werner syndrome, disabling and life-threatening diseases with accelerated ageing, bone resorption, lipodystrophy, skin abnormalities and cardiovascular disorders. Defects in lamin A post-translational maturation occur in progeroid syndromes and accumulated prelamin A affects ageing-related processes, such as mTOR signaling, epigenetic modifications, stress response, inflammation, microRNA activation and mechanosignaling. In this review, we briefly describe the role of these pathways in physiological ageing and go in deep into lamin A-dependent mechanisms that accelerate the ageing process. Finally, we propose that lamin A acts as a sensor of cell intrinsic and environmental stress through transient prelamin A accumulation, which triggers stress response mechanisms. Exacerbation of lamin A sensor activity due to stably elevated prelamin A levels contributes to the onset of a permanent stress response condition, which triggers accelerated ageing.

Role of the nuclear membrane protein Emerin in front-rear polarity of the nucleus

Cell polarity refers to the intrinsic asymmetry of cells, including the orientation of the cytoskeleton. It affects cell shape and structure as well as the distribution of proteins and organelles. In migratory cells, front-rear polarity is essential and dictates movement direction. While the link between the cytoskeleton and nucleus is well-studied, we aim to investigate if front-rear polarity can be transmitted to the nucleus. We show that the knock-down of emerin, an integral protein of the nuclear envelope, abolishes preferential localization of several nuclear proteins. We propose that the frontally biased localization of the endoplasmic reticulum, through which emerin reaches the nuclear envelope, is sufficient to generate its observed bias. In primary emerin-deficient myoblasts, its expression partially rescues the polarity of the nucleus. Our results demonstrate that front-rear cell polarity is transmitted to the nucleus and that emerin is an important determinant of nuclear polarity.

During cell migration, cells are polarized with distinct front vs. rear regions but whether and how polarity is transmitted to the nucleus is unclear. Here the authors show that frontally-biased endoplasmic reticulum and the nuclear membrane protein Emerin contribute to front-rear nuclear cell polarity.

Emery-Dreifuss muscular dystrophy

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

Clinical aspects of Emery-Dreifuss muscular dystrophy

Emery-Dreifuss muscular dystrophy (EDMD), clinically characterized by scapulo-humero-peroneal muscle atrophy and weakness, multi-joint contractures with spine rigidity and cardiomyopathy with conduction defects, is associated with structural/functional defect of genes that encode the proteins of nuclear envelope, including lamin A and several lamin-interacting proteins. This paper presents clinical aspects of EDMD in context to causative genes, genotype-phenotype correlation and its emplacement within phenotypic spectrum of skeletal muscle diseases associated with envelopathies.

The Driving Force: Nuclear Mechanotransduction in Cellular Function, Fate, and Disease

Cellular behavior is continuously affected by microenvironmental forces through the process of mechanotransduction, in which mechanical stimuli are rapidly converted to biochemical responses. Mounting evidence suggests that the nucleus itself is a mechanoresponsive element, reacting to cytoskeletal forces and mediating downstream biochemical responses. The nucleus responds through a host of mechanisms, including partial unfolding, conformational changes, and phosphorylation of nuclear envelope proteins; modulation of nuclear import/export; and altered chromatin organization, resulting in transcriptional changes. It is unclear which of these events present direct mechanotransduction processes and which are downstream of other mechanotransduction pathways. We critically review and discuss the current evidence for nuclear mechanotransduction, particularly in the context of stem cell fate, a largely unexplored topic, and in disease, where an improved understanding of nuclear mechanotransduction is beginning to open new treatment avenues. Finally, we discuss innovative technological developments that will allow outstanding questions in the rapidly growing field of nuclear mechanotransduction to be answered.

Nuclear Actin: From Discovery to Function

While actin was discovered in the nucleus over 50 years ago, research lagged for decades due to strong skepticism. The revitalization of research into nuclear actin occurred after it was found that cellular stresses induce the nuclear localization and alter the structure of actin. These studies provided the first hints that actin has a nuclear function. Subsequently, it was established that the nuclear import and export of actin is highly regulated. While the structures of nuclear actin remain unclear, it can function as monomers, polymers, and even rods. Furthermore, even within a given structure, distinct pools of nuclear actin that can be differentially labeled have been identified. Numerous mechanistic studies have uncovered an array of functions for nuclear actin. It regulates the activity of RNA polymerases, as well as specific transcription factors. Actin also modulates the activity of several chromatin remodeling complexes and histone deacetylases, to ultimately impinge on transcriptional programing and DNA damage repair. Further, nuclear actin mediates chromatin movement and organization. It has roles in meiosis and mitosis, and these functions may be functionally conserved from ancient bacterial actin homologs. The structure and integrity of the nuclear envelope and sub-nuclear compartments are also regulated by nuclear actin. Furthermore, nuclear actin contributes to human diseases like cancer, neurodegeneration, and myopathies. Here, we explore the early discovery of actin in the nucleus and discuss the forms and functions of nuclear actin in both normal and disease contexts. Anat Rec, 301:1999-2013, 2018. © 2018 Wiley Periodicals, Inc.

MAPK signaling pathways and HDAC3 activity are disrupted during differentiation of emerin-null myogenic progenitor cells

Mutations in the gene encoding emerin cause Emery–Dreifuss muscular dystrophy (EDMD). Emerin is an integral inner nuclear membrane protein and a component of the nuclear lamina. EDMD is characterized by skeletal muscle wasting, cardiac conduction defects and tendon contractures. The failure to regenerate skeletal muscle is predicted to contribute to the skeletal muscle pathology of EDMD. We hypothesize that muscle regeneration defects are caused by impaired muscle stem cell differentiation. Myogenic progenitors derived from emerin-null mice were used to confirm their impaired differentiation and analyze selected myogenic molecular pathways. Emerin-null progenitors were delayed in their cell cycle exit, had decreased myosin heavy chain (MyHC) expression and formed fewer myotubes. Emerin binds to and activates histone deacetylase 3 (HDAC3). Here, we show that theophylline, an HDAC3-specific activator, improved myotube formation in emerin-null cells. Addition of the HDAC3-specific inhibitor RGFP966 blocked myotube formation and MyHC expression in wild-type and emerin-null myogenic progenitors, but did not affect cell cycle exit. Downregulation of emerin was previously shown to affect the p38 MAPK and ERK/MAPK pathways in C2C12 myoblast differentiation. Using a pure population of myogenic progenitors completely lacking emerin expression, we show that these pathways are also disrupted. ERK inhibition improved MyHC expression in emerin-null cells, but failed to rescue myotube formation or cell cycle exit. Inhibition of p38 MAPK prevented differentiation in both wild-type and emerin-null progenitors. These results show that each of these molecular pathways specifically regulates a particular stage of myogenic differentiation in an emerin-dependent manner. Thus, pharmacological targeting of multiple pathways acting at specific differentiation stages may be a better therapeutic approach in the future to rescue muscle regeneration in vivo.

Editors' choice: HDAC3, p38 MAPK and ERK signaling are altered during differentiation of myogenic progenitors lacking emerin; pharmacological activation or inhibition of these signaling proteins rescues specific stages of myogenic differentiation.

Diseases of the Nucleoskeleton

The nucleus is separated from the cytosol by the nuclear envelope, which is a double lipid bilayer composed of the outer nuclear membrane and the inner nuclear membrane. The intermediate filament proteins lamin A, lamin B, and lamin C form a network underlying the inner nuclear membrane. This proteinaceous network provides the nucleus with its strength, rigidity, and elasticity. Positioned within the inner nuclear membrane are more than 150 inner nuclear membrane proteins, many of which interact directly with lamins and require lamins for their inner nuclear membrane localization. Inner nuclear membrane proteins and the nuclear lamins define the nuclear lamina. These inner nuclear membrane proteins have tissue-specific expression and diverse functions including regulating cytoskeletal organization, nuclear architecture, cell cycle dynamics, and genomic organization. Loss or mutations in lamins and inner nuclear membrane proteins cause a wide spectrum of diseases. Here, I will review the functions of the well-studied nuclear lamina proteins and the diseases associated with loss or mutations in these proteins. © 2016 American Physiological Society. Compr Physiol 6:1655-1674, 2016.

Actin, actin-binding proteins, and actin-related proteins in the nucleus

Extensive research in the past decade has significantly broadened our view about the role actin plays in the life of the cell and added novel aspects to actin research. One of these new aspects is the discovery of the existence of nuclear actin which became evident only recently. Nuclear activities including transcriptional activation in the case of all three RNA polymerases, editing and nuclear export of mRNAs, and chromatin remodeling all depend on actin. It also became clear that there is a fine-tuned equilibrium between cytoplasmic and nuclear actin pools and that this balance is ensured by an export-import system dedicated to actin. After over half a century of research on conventional actin and its organizing partners in the cytoplasm, it was also an unexpected finding that the nucleus contains more than 30 actin-binding proteins and new classes of actin-related proteins which are not able to form filaments but had evolved nuclear-specific functions. The actin-binding and actin-related proteins in the nucleus have been linked to RNA transcription and processing, nuclear transport, and chromatin remodeling. In this paper, we attempt to provide an overview of the wide range of information that is now available about actin, actin-binding, and actin-related proteins in the nucleus.

Muscular Dystrophy Mutations Impair the Nuclear Envelope Emerin Self-assembly Properties

More than 100 genetic mutations causing X-linked Emery-Dreifuss muscular dystrophy have been identified in the gene encoding the integral inner nuclear membrane protein emerin. Most mutations are nonsense or frameshift mutations that lead to the absence of emerin in cells. Only very few cases are due to missense or short in-frame deletions. Molecular mechanisms explaining the corresponding emerin variants' loss of function are particularly difficult to identify because of the mostly intrinsically disordered state of the emerin nucleoplasmic region. We now demonstrate that this EmN region can be produced as a disordered monomer, as revealed by nuclear magnetic resonance, but rapidly self-assembles in vitro. Increases in concentration and temperature favor the formation of long curvilinear filaments with diameters of approximately 10 nm, as observed by electron microscopy. Assembly of these filaments can be followed by fluorescence through Thioflavin-T binding and by Fourier-transform Infrared spectrometry through formation of β-structures. Analysis of the assembly properties of five EmN variants reveals that del95-99 and Q133H impact filament assembly capacities. In cells, these variants are located at the nuclear envelope, but the corresponding quantities of emerin-emerin and emerin-lamin proximities are decreased compared to wild-type protein. Furthermore, variant P183H favors EmN aggregation in vitro, and variant P183T provokes emerin accumulation in cytoplasmic foci in cells. Substitution of residue Pro183 might systematically favor oligomerization, leading to emerin aggregation and mislocalization in cells. Our results suggest that emerin self-assembly is necessary for its proper function and that a loss of either the protein itself or its ability to self-assemble causes muscular dystrophy.

Role of structural flexibility in the evolution of emerin

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

Diverse lamin-dependent mechanisms interact to control chromatin dynamics. Focus on laminopathies

Interconnected functional strategies govern chromatin dynamics in eukaryotic cells. In this context, A and B type lamins, the nuclear intermediate filaments, act on diverse platforms involved in tissue homeostasis. On the nuclear side, lamins elicit large scale or fine chromatin conformational changes, affect DNA damage response factors and transcription factor shuttling. On the cytoplasmic side, bridging-molecules, the LINC complex, associate with lamins to coordinate chromatin dynamics with cytoskeleton and extra-cellular signals.

 

Consistent with such a fine tuning, lamin mutations and/or defects in their expression or post-translational processing, as well as mutations in lamin partner genes, cause a heterogeneous group of diseases known as laminopathies. They include muscular dystrophies, cardiomyopathy, lipodystrophies, neuropathies, and progeroid syndromes. The study of chromatin dynamics under pathological conditions, which is summarized in this review, is shedding light on the complex and fascinating role of the nuclear lamina in chromatin regulation.

Emerin in health and disease

Emery-Dreifuss muscular dystrophy (EDMD) is caused by mutations in the genes encoding emerin, lamins A and C and FHL1. Additional EDMD-like syndromes are caused by mutations in nesprins and LUMA. This review will specifically focus on emerin function and the current thinking for how loss or mutations in emerin cause EDMD. Emerin is a well-conserved, ubiquitously expressed protein of the inner nuclear membrane. Emerin has been shown to have diverse functions, including the regulation of gene expression, cell signaling, nuclear structure and chromatin architecture. This review will focus on the relationships between these functions and the EDMD disease phenotype. Additionally it will highlight open questions concerning emerin's roles in cell and nuclear biology and disease.

A proteomic study of Hutchinson-Gilford progeria syndrome: Application of 2D-chromotography in a premature aging disease

The Hutchinson-Gilford progeria syndrome (HGPS) is a rare genetic disease characterized by segmental premature aging. Applying a two-dimensional chromatographic proteomic approach, the 2D Protein Fractionation System (PF2D), we identified 30 differentially expressed proteins in cultured HGPS fibroblasts. We categorized them into five groups: methylation, calcium ion binding, cytoskeleton, duplication, and regulation of apoptosis. Among these 30 proteins, 23 were down-regulated, while seven were up-regulated in HGPS fibroblasts as compared to normal fibroblasts. Three differentially expressed cytoskeleton proteins, vimentin, actin, and tubulin, were validated via Western blotting and characterized by immunostaining that revealed densely thickened bundles and irregular structures. Furthermore in the HGPS cells, the cell cycle G1 phase was elongated and the concentration of free cytosolic calcium was increased, suggesting intracellular retention of calcium. The results that we obtained have implications for understanding the aging process.

Identification of a novel muscle A-type lamin-interacting protein (MLIP)

Mutations in the A-type lamin (LMNA) gene are associated with age-associated degenerative disorders of mesenchymal tissues, such as dilated cardiomyopathy, Emery-Dreifuss muscular dystrophy, and limb-girdle muscular dystrophy. The molecular mechanisms that connect mutations in LMNA with different human diseases are poorly understood. Here, we report the identification of a Muscle-enriched A-type Lamin-interacting Protein, MLIP (C6orf142 and 2310046A06rik), a unique single copy gene that is an innovation of amniotes (reptiles, birds, and mammals). MLIP encodes alternatively spliced variants (23-57 kDa) and possesses several novel structural motifs not found in other proteins. MLIP is expressed ubiquitously and most abundantly in heart, skeletal, and smooth muscle. MLIP interacts directly and co-localizes with lamin A and C in the nuclear envelope. MLIP also co-localizes with promyelocytic leukemia (PML) bodies within the nucleus. PML, like MLIP, is only found in amniotes, suggesting that a functional link between the nuclear envelope and PML bodies may exist through MLIP. Down-regulation of lamin A/C expression by shRNA results in the up-regulation and mislocalization of MLIP. Given that MLIP is expressed most highly in striated and smooth muscle, it is likely to contribute to the mesenchymal phenotypes of laminopathies.

Laminopathies and lamin-associated signaling pathways

Laminopathies are genetic diseases due to mutations or altered post-translational processing of nuclear envelope/lamina proteins. The majority of laminopathies are caused by mutations in the LMNA gene, encoding lamin A/C, but manifest as diverse pathologies including muscular dystrophy, lipodystrophy, neuropathy, and progeroid syndromes. Lamin-binding proteins implicated in laminopathies include lamin B2, nuclear envelope proteins such as emerin, MAN1, LBR, and nesprins, the nuclear matrix protein matrin 3, the lamina-associated polypeptide, LAP2alpha and the transcriptional regulator FHL1. Thus, the altered functionality of a nuclear proteins network appears to be involved in the onset of laminopathic diseases. The functional interplay among different proteins involved in this network implies signaling partners. The signaling effectors may either modify nuclear envelope proteins and their binding properties, or use nuclear envelope/lamina proteins as platforms to regulate signal transduction. In this review, both aspects of lamin-linked signaling are presented and the major pathways so far implicated in laminopathies are summarized.

Prelamin A-mediated recruitment of SUN1 to the nuclear envelope directs nuclear positioning in human muscle

Lamin A is a nuclear lamina constituent expressed in differentiated cells. Mutations in the LMNA gene cause several diseases, including muscular dystrophy and cardiomyopathy. Among the nuclear envelope partners of lamin A are Sad1 and UNC84 domain-containing protein 1 (SUN1) and Sad1 and UNC84 domain-containing protein 2 (SUN2), which mediate nucleo-cytoskeleton interactions critical to the anchorage of nuclei. In this study, we show that differentiating human myoblasts accumulate farnesylated prelamin A, which elicits upregulation and recruitment of SUN1 to the nuclear envelope and favors SUN2 enrichment at the nuclear poles. Indeed, impairment of prelamin A farnesylation alters SUN1 recruitment and SUN2 localization. Moreover, nuclear positioning in myotubes is severely affected in the absence of farnesylated prelamin A. Importantly, reduced prelamin A and SUN1 levels are observed in Emery-Dreifuss muscular dystrophy (EDMD) myoblasts, concomitant with altered myonuclear positioning. These results demonstrate that the interplay between SUN1 and farnesylated prelamin A contributes to nuclear positioning in human myofibers and may be implicated in pathogenetic mechanisms.

Comparison of two dimensional electrophoresis mouse colon proteomes before and after knocking out Aquaporin 8

Aquaporin (AQP) family plays a fundamental role in transmembrane water and small solutes movement. Within this family, aquaporin 8 (AQP8), showed to be widely distributed in the digestive system especially colon. To investigate the possible protein alterations involved in AQP8 regulation and trafficking, we extensively compared between wild type and AQP8 knockout mouse colon using semi-quantitative fluorescence- stained two dimensional gel electrophoresis (2-DE) coupled with nano LC-Ms/Ms. Our analysis revealed identification and regulation of 21 proteins, most notably, actin-related family which suggests its possible involvement in regulating AQP8 secretory vesicles migration to be integrated as a cell membrane protein.

Mechanobiology and the microcirculation: cellular, nuclear and fluid mechanics

Endothelial cells are stimulated by shear stress throughout the vasculature and respond with changes in gene expression and by morphological reorganization. Mechanical sensors of the cell are varied and include cell surface sensors that activate intracellular chemical signaling pathways. Here, possible mechanical sensors of the cell including reorganization of the cytoskeleton and the nucleus are discussed in relation to shear flow. A mutation in the nuclear structural protein lamin A, related to Hutchinson-Gilford progeria syndrome, is reviewed specifically as the mutation results in altered nuclear structure and stiffer nuclei; animal models also suggest significantly altered vascular structure. Nuclear and cellular deformation of endothelial cells in response to shear stress provides partial understanding of possible mechanical regulation in the microcirculation. Increasing sophistication of fluid flow simulations inside the vessel is also an emerging area relevant to the microcirculation as visualization in situ is difficult. This integrated approach to study--including medicine, molecular and cell biology, biophysics and engineering--provides a unique understanding of multi-scale interactions in the microcirculation.

Identification of an emerin-beta-catenin complex in the heart important for intercalated disc architecture and beta-catenin localisation

How mutations in the protein emerin lead to the cardiomyopathy associated with X-linked Emery-Dreifuss muscular dystrophy (X-EDMD) is unclear. We identified emerin at the adherens junction of the intercalated disc, where it co-localised with the catenin family of proteins. Emerin bound to wild type beta-catenin both in vivo and in vitro. Mutating the GSK3beta phosphorylation sites on beta-catenin abolished this binding. Wild type but not mutant forms of emerin associated with X-EDMD were able to reduce beta-catenin protein levels. Cardiomyocytes from emerin-null mice hearts exhibited erroneous beta-catenin distribution and intercalated disc architecture. Treatment of wild type cardiomyocytes with phenylephrine, which inactivates GSK3beta, redistributed emerin and beta-catenin. Emerin was identified as a direct target of GSK3beta activity since exogenous expression of GSK3beta reduced emerin levels at the nuclear envelope. We propose that perturbation to or total loss of the emerin-beta-catenin complex compromises both intercalated disc function and beta-catenin signalling in cardiomyocytes.

A-type lamins and signaling: the PI 3-kinase/Akt pathway moves forward

Lamin A/C is a nuclear lamina constituent mutated in a number of human inherited disorders collectively referred to as laminopathies. The occurrence and significance of lamin A/C interplay with signaling molecules is an old question, suggested by pioneer studies performed in vitro. However, this relevant question has remained substantially unanswered, until data obtained in cellular and organismal models of laminopathies have indicated two main aspects of lamin A function. The first aspect is that lamins establish functional interactions with different protein platforms, the second aspect is that lamin A/C activity and altered function may elicit different effects in different cells and tissue types and even in different districts of the same tissue. Both these observations strongly suggest that signaling mechanisms targeting lamin A/C or its binding partners may regulate such a plastic behavior. A number of very recent data show involvement of kinases, as Akt and Erk, or phosphatases, as PP1 and PP2, in lamin A-linked cellular mechanisms. Moreover, altered activation of signaling in laminopathies and rescue of the pathological phenotype in animal models by inhibitors of signaling pathways, strongly suggest that signaling effectors related to lamin A/C may be implicated in the pathogenesis of laminopathies and may represent targets of therapeutic intervention. In face of such an open perspective of basic and applied research, we review current evidence of lamin A/C interplay with signaling molecules, with particular emphasis on the lamin A-Akt interaction and on the biological significance of their relationship.

In vitro nuclear interactome of the HIV-1 Tat protein

BACKGROUND

One facet of the complexity underlying the biology of HIV-1 resides not only in its limited number of viral proteins, but in the extensive repertoire of cellular proteins they interact with and their higher-order assembly. HIV-1 encodes the regulatory protein Tat (86-101aa), which is essential for HIV-1 replication and primarily orchestrates HIV-1 provirus transcriptional regulation. Previous studies have demonstrated that Tat function is highly dependent on specific interactions with a range of cellular proteins. However they can only partially account for the intricate molecular mechanisms underlying the dynamics of proviral gene expression. To obtain a comprehensive nuclear interaction map of Tat in T-cells, we have designed a proteomic strategy based on affinity chromatography coupled with mass spectrometry.

RESULTS

Our approach resulted in the identification of a total of 183 candidates as Tat nuclear partners, 90% of which have not been previously characterised. Subsequently we applied in silico analysis, to validate and characterise our dataset which revealed that the Tat nuclear interactome exhibits unique signature(s). First, motif composition analysis highlighted that our dataset is enriched for domains mediating protein, RNA and DNA interactions, and helicase and ATPase activities. Secondly, functional classification and network reconstruction clearly depicted Tat as a polyvalent protein adaptor and positioned Tat at the nexus of a densely interconnected interaction network involved in a range of biological processes which included gene expression regulation, RNA biogenesis, chromatin structure, chromosome organisation, DNA replication and nuclear architecture.

CONCLUSION

We have completed the in vitro Tat nuclear interactome and have highlighted its modular network properties and particularly those involved in the coordination of gene expression by Tat. Ultimately, the highly specialised set of molecular interactions identified will provide a framework to further advance our understanding of the mechanisms of HIV-1 proviral gene silencing and activation.

Autosomal-dominant distal myopathy associated with a recurrent missense mutation in the gene encoding the nuclear matrix protein, matrin 3

Distal myopathies represent a heterogeneous group of inherited skeletal muscle disorders. One type of adult-onset, progressive autosomal-dominant distal myopathy, frequently associated with dysphagia and dysphonia (vocal cord and pharyngeal weakness with distal myopathy [VCPDM]), has been mapped to chromosome 5q31 in a North American pedigree. Here, we report the identification of a second large VCPDM family of Bulgarian descent and fine mapping of the critical interval. Sequencing of positional candidate genes revealed precisely the same nonconservative S85C missense mutation affecting an interspecies conserved residue in the MATR3 gene in both families. MATR3 is expressed in skeletal muscle and encodes matrin 3, a component of the nuclear matrix, which is a proteinaceous network that extends throughout the nucleus. Different disease related haplotype signatures in the two families provided evidence that two independent mutational events at the same position in MATR3 cause VCPDM. Our data establish proof of principle that the nuclear matrix is crucial for normal skeletal muscle structure and function and put VCPDM on the growing list of monogenic disorders associated with the nuclear proteome.

Nuclear motors and nuclear structures containing A-type lamins and emerin: is there a functional link?

Rapid interphase chromosome territory repositioning appears to function through the action of nuclear myosin and actin, in a nuclear motor complex. We have found that chromosome repositioning when cells leave the cell cycle is not apparent in cells that have mutant lamin A or that are lacking emerin. We discuss the possibility that there is a functional intranuclear complex comprising four proteins: nuclear actin, lamin A, emerin and nuclear myosin. If any of the components are lacking or aberrant, then the nuclear motor complex involved in moving chromosomes or genes will be dysfunctional, leading to an inability to move chromosomes in response to signalling events.

Cell and molecular biology of nuclear actin

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

Proteomic characterization of protein phosphatase 1 complexes in ischemia-reperfusion and ischemic tolerance

Serine/threonine protein phosphatase 1 (PP1) regulates multiple cellular processes. Protein phosphorylation-dephosphorylation is largely altered during ischemia and subsequent reperfusion. The brain is particularly vulnerable to stress resulting from ischemia-reperfusion (IR), however, the acquisition of ischemic tolerance (IT) protects against IR stress. We studied PP1 complexes in response to IR stress and IT in brain using proteomic characterization of PP1 complexes in animal models of IR and IT. PP1alpha and PP1gamma were immunoprecipitated and resolved by 2-D. DIGE analysis detected 14 different PP1-interacting proteins that exhibited significant changes in their association with PP1alpha or PP1gamma. These proteins were identified by MALDI-TOF MS. Seven had the PP1-binding RVxF motif. IR altered the interaction of heat shock cognate 71 kDa-protein, creatine kinase B, and dopamine- and cAMP-regulated phosphoprotein 32 kDa (DARPP32) with both PP1alpha and PP1gamma, and the interaction of phosphodiesterase-6B, transitional ER ATPase, lamin-A, glucose-regulated 78 kDa-protein, dihydropyrimidinase-related protein-2, gamma-enolase, neurofilament-L, and ubiquitin ligase SIAH2 with PP1gamma. IT prevented most of the IR-induced effects. This study identifies novel PP1alpha- and PP1gamma-interacting proteins and reveals an in vivo modularity of PP1 holoenzymes in response to physiological ischemic stress. It supports a potential role of PP1 in IR stress and as a target of the endogenous protective mechanisms induced by IT.

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

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

A-type lamin networks in light of laminopathic diseases

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

Role of nuclear lamina-cytoskeleton interactions in the maintenance of cellular strength

The response of individual cells to cellular stress is vital for cellular functioning. A large network of physically interconnected cellular components, starting from the structural components of the cells' nucleus, via cytoskeleton filaments to adhesion molecules and the extracellular matrix, constitutes an integrated matrix that functions as a scaffold allowing the cell to cope with mechanical stress. Next to a role in mechanical properties, this network also has a mechanotransductional function in the response to mechanical stress. This signaling route does not only regulate a rapid reorganization of structural components such as actin filaments, but also stimulates for example gene activation via NFkappaB and other transcription factors. The importance of an intact mechano-signaling network is illustrated by the physiological consequences of several genetic defects of cellular network components e.g. actin, dystrophin, desmin and lamins. These give rise to an impaired response of the affected cells to mechanical stress and often result in dystrophy of the affected tissue. Recently, the importance of the cell nucleus in cellular strength has been established. Several new interconnecting proteins, such as the nesprins that link the nuclear lamina to the cytoskeleton, have been identified. Furthermore, the function of nuclear lamins in determining cellular strength and nuclear stability was illustrated in lamin-knock-out cells. Absence of the A-type lamins or mutations in these structural components of the nuclear lamina lead to an impaired cellular response to mechanical stress and disturbances in cytoskeletal organization. In addition, laminopathies show clinical phenotypes comparable to those seen for diseases resulting from genetic defects in cytoskeletal components, further indicating that lamins play a central role in maintaining the mechanical properties of the cell.

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

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

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

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

Multiple roles for emerin: implications for Emery-Dreifuss muscular dystrophy

X-linked Emery-Dreifuss muscular dystrophy (X-EDMD) is inherited through mutations in EMD, which encodes a nuclear membrane protein named emerin. Emerin is expressed in most cells, but EDMD strikes specific tissues. This review summarizes growing evidence that emerin has roles in both tissue-specific gene regulation and the mechanical integrity of the nucleus and discusses how these roles might impact EDMD.

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

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

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

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

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

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

Nuclear Lamins: Laminopathies and Their Role in Premature Ageing

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

Lamin A N-terminal phosphorylation is associated with myoblast activation: impairment in Emery-Dreifuss muscular dystrophy

BACKGROUND

Skeletal muscle disorders associated with mutations of lamin A/C gene include autosomal Emery-Dreifuss muscular dystrophy and limb girdle muscular dystrophy 1B. The pathogenic mechanism underlying these diseases is unknown. Recent data suggest an impairment of signalling mechanisms as a possible cause of muscle malfunction. A molecular complex in muscle cells formed by lamin A/C, emerin, and nuclear actin has been identified. The stability of this protein complex appears to be related to phosphorylation mechanisms.

OBJECTIVE

To analyse lamin A/C phosphorylation in control and laminopathic muscle cells.

METHODS

Lamin A/C N-terminal phosphorylation was determined in cultured mouse myoblasts using a specific antibody. Insulin treatment of serum starved myoblast cultures was carried out to evaluate involvement of insulin signalling in the phosphorylation pathway. Screening of four Emery-Dreifuss and one limb girdle muscular dystrophy 1B cases was undertaken to investigate lamin A/C phosphorylation in both cultured myoblasts and mature muscle fibres.

RESULTS

Phosphorylation of lamin A was observed during myoblast differentiation or proliferation, along with reduced lamin A/C phosphorylation in quiescent myoblasts. Lamin A N-terminus phosphorylation was induced by an insulin stimulus, which conversely did not affect lamin C phosphorylation. Lamin A/C was also hyperphosphorylated in mature muscle, mostly in regenerating fibres. Lamin A/C phosphorylation was strikingly reduced in laminopathic myoblasts and muscle fibres, while it was preserved in interstitial fibroblasts.

CONCLUSIONS

Altered lamin A/C interplay with a muscle specific phosphorylation partner might be involved in the pathogenic mechanism of Emery-Dreifuss muscular dystrophy and limb girdle muscular dystrophy 1B.