Emerin deficiency at the nuclear membrane in patients with Emery-Dreifuss muscular dystrophy

An interkinetic envelope surrounds chromosomes between meiosis I and II in C. elegans oocytes

At the end of cell division, the nuclear envelope reassembles around the decondensing chromosomes. Female meiosis culminates in two consecutive cell divisions of the oocyte, meiosis I and II, which are separated by a brief transition phase known as interkinesis. Due to the absence of chromosome decondensation and the suppression of genome replication during interkinesis, it has been widely assumed that the nuclear envelope does not reassemble between meiosis I and II. By analyzing interkinesis in C. elegans oocytes, we instead show that an atypical structure made of two lipid bilayers, which we termed the interkinetic envelope, surrounds the surface of the segregating chromosomes. The interkinetic envelope shares common features with the nuclear envelope but also exhibits specific characteristics that distinguish it, including its lack of continuity with the endoplasmic reticulum, unique protein composition, assembly mechanism, and function in chromosome segregation. These distinct attributes collectively define the interkinetic envelope as a unique and specialized structure that has been previously overlooked.

Optimized simple culture protocol for inducing mature myotubes from MYOD1-overexpressed human iPS cells

The forced expression system of MYOD1, a master gene for myogenic differentiation, can efficiently and rapidly reproduce muscle differentiation of human induced pluripotent stem cells (hiPSCs). Despite these advantages of the MYOD1 overexpression system, developed myotubes are relatively immature and do not recapitulate several aspects of striated muscle fibers. Here, we developed a simple optimized protocol using an alternative culture medium for maximizing the advantages of the MYOD1 overexpression system, and successfully improved the formation of multinucleated mature myotubes within 10 days. In this study, we generated hiPSCs derived from healthy donors and an individual with congenial muscular dystrophy caused by LMNA mutation (laminopathy), and compared disease-associated phenotypes in differentiated myotubes generated by the conventional method and by our new optimized culture method. Using our optimized method, abnormal myonuclear shape was pronounced in the patient-derived iPSCs. In addition, abnormal accumulation of the nuclear membrane protein emerin was observed in LMNA-mutant hiPSCs. Our new culture method is expected to be widely applicable as a MYOD1 overexpression model of hiPSC-derived skeletal muscle cells for the analysis of a variety of muscle diseases.

An interkinetic envelope surrounds chromosomes between meiosis I and II in C. elegans oocytes

At the end of cell division, the nuclear envelope reassembles around the decondensing chromosomes. Female meiosis culminates in two consecutive cell divisions of the oocyte, meiosis I and II, which are separated by a brief transition phase known as interkinesis. Due to the absence of chromosome decondensation and the suppression of genome replication during interkinesis, it has been widely assumed that the nuclear envelope does not reassemble between meiosis I and II. By analyzing interkinesis in C. elegans oocytes, we instead show that an atypical structure made of two lipid bilayers, which we termed the interkinetic envelope, surrounds the surface of the segregating chromosomes. The interkinetic envelope shares common features with the nuclear envelope but also exhibits specific characteristics that distinguish it, including its lack of continuity with the endoplasmic reticulum, unique protein composition, assembly mechanism, and function in chromosome segregation. These distinct attributes collectively define the interkinetic envelope as a unique and specialized structure that has been previously overlooked.

Pushing the envelope - How the genome interacts with the nuclear envelope in health and disease

The nuclear envelope has for long been considered more than just the physical border between the nucleoplasm and the cytoplasm, emerging as a crucial player in genome organisation and regulation within the 3D nucleus. Consequently, its study has become a valuable topic in the research of cancer, ageing and several other diseases where chromatin organisation is compromised. In this chapter, we will delve into its several sub-elements, such as the nuclear lamina, nuclear pore complexes and nuclear envelope proteins, and their diverse roles in nuclear function and maintenance. We will explore their functions beyond nuclear structure and transport focusing on their interactions with chromatin and their paramount influence in its organisation, regulation and expression at the nuclear periphery. Finally, we will outline how this chromatin organisation and regulation at the nuclear envelope is affected in diseases, including laminopathies, cancer, neurodegenerative diseases and during viral infections.

Emerin mislocalization during chromatin bridge resolution can drive prostate cancer cell invasiveness in a collagen-rich microenvironment

Micronuclei (MN) can form through many mechanisms, including the breakage of aberrant cytokinetic chromatin bridges. The frequent observation of MN in tumors suggests that they might not merely be passive elements but could instead play active roles in tumor progression. Here, we propose a mechanism through which the presence of micronuclei could induce specific phenotypic and functional changes in cells and increase the invasive potential of cancer cells. Through the integration of diverse in vitro imaging and molecular techniques supported by clinical samples from patients with prostate cancer (PCa) defined as high-risk by the D'Amico classification, we demonstrate that the resolution of chromosome bridges can result in the accumulation of Emerin and the formation of Emerin-rich MN. These structures are negative for Lamin A/C and positive for the Lamin-B receptor and Sec61β. MN can act as a protein sinks and result in the pauperization of Emerin from the nuclear envelope. The Emerin mislocalization phenotype is associated with a molecular signature that is correlated with a poor prognosis in PCa patients and is enriched in metastatic samples. Emerin mislocalization corresponds with increases in the migratory and invasive potential of tumor cells, especially in a collagen-rich microenvironment. Our study demonstrates that the mislocalization of Emerin to MN results in increased cell invasiveness, thereby worsening patient prognosis.

Characteristics of nuclear architectural abnormalities of myotubes differentiated from LmnaH222P/H222P skeletal muscle cells

The presence of nuclear architectural abnormalities is a hallmark of the nuclear envelopathies, which are a group of diseases caused by mutations in genes encoding nuclear envelope proteins. Mutations in the lamin A/C gene cause several diseases, named laminopathies, including muscular dystrophies, progeria syndromes, and lipodystrophy. A mouse model carrying with the LmnaH222P/H222P mutation (H222P) was shown to develop severe cardiomyopathy but only mild skeletal myopathy, although abnormal nuclei were observed in their striated muscle. In this report, we analyzed the abnormal-shaped nuclei in myoblasts and myotubes isolated from skeletal muscle of H222P mice, and evaluated the expression of nuclear envelope proteins in these abnormal myonuclei. Primary skeletal muscle cells from H222P mice proliferated and efficiently differentiated into myotubes in vitro, similarly to those from wild-type mice. During cell proliferation, few abnormal-shaped nuclei were detected; however, numerous markedly abnormal myonuclei were observed in myotubes from H222P mice on days 5 and 7 of differentiation. Time-lapse observation demonstrated that myonuclei with a normal shape maintained their normal shape, whereas abnormal-shaped myonuclei remained abnormal for at least 48 h during differentiation. Among the abnormal-shaped myonuclei, 65% had a bleb with a string structure, and 35% were severely deformed. The area and nuclear contents of the nuclear blebs were relatively stable, whereas the myocytes with nuclear blebs were actively fused within primary myotubes. Although myonuclei were markedly deformed, the deposition of DNA damage marker (γH2AX) or apoptotic marker staining was rarely observed. Localizations of lamin A/C and emerin were maintained within the blebs, strings, and severely deformed regions of myonuclei; however, lamin B1, nesprin-1, and a nuclear pore complex protein were absent in these abnormal regions. These results demonstrate that nuclear membranes from H222P skeletal muscle cells do not rupture and are resistant to DNA damage, despite these marked morphological changes.

Nuclear mechanosensing of the aortic endothelium in health and disease

The endothelium, the monolayer of endothelial cells that line blood vessels, is exposed to a number of mechanical forces, including frictional shear flow, pulsatile stretching and changes in stiffness influenced by extracellular matrix composition. These forces are sensed by mechanosensors that facilitate their transduction to drive appropriate adaptation of the endothelium to maintain vascular homeostasis. In the aorta, the unique architecture of the vessel gives rise to changes in the fluid dynamics, which, in turn, shape cellular morphology, nuclear architecture, chromatin dynamics and gene regulation. In this Review, we discuss recent work focusing on how differential mechanical forces exerted on endothelial cells are sensed and transduced to influence their form and function in giving rise to spatial variation to the endothelium of the aorta. We will also discuss recent developments in understanding how nuclear mechanosensing is implicated in diseases of the aorta.

TOR1AIP1-Associated Nuclear Envelopathies

Human TOR1AIP1 encodes LAP1, a nuclear envelope protein expressed in most human tissues, which has been linked to various biological processes and human diseases. The clinical spectrum of diseases related to mutations in TOR1AIP1 is broad, including muscular dystrophy, congenital myasthenic syndrome, cardiomyopathy, and multisystemic disease with or without progeroid features. Although rare, these recessively inherited disorders often lead to early death or considerable functional impairment. Developing a better understanding of the roles of LAP1 and mutant TOR1AIP1-associated phenotypes is paramount to allow therapeutic development. To facilitate further studies, this review provides an overview of the known interactions of LAP1 and summarizes the evidence for the function of this protein in human health. We then review the mutations in the TOR1AIP1 gene and the clinical and pathological characteristics of subjects with these mutations. Lastly, we discuss challenges to be addressed in the future.

Childhood muscular dystrophies

Infancy- and childhood-onset muscular dystrophies are associated with a characteristic distribution and progression of motor dysfunction. The underlying causes of progressive childhood muscular dystrophies are heterogeneous involving diverse genetic pathways and genes that encode proteins of the plasma membrane, extracellular matrix, sarcomere, and nuclear membrane components. The prototypical clinicopathological features in an affected child may be adequate to fully distinguish it from other likely diagnoses based on four common features: (1) weakness and wasting of pelvic-femoral and scapular muscles with involvement of heart muscle; (2) elevation of serum muscle enzymes in particular serum creatine kinase; (3) necrosis and regeneration of myofibers; and (4) molecular neurogenetic assessment particularly utilizing next-generation sequencing of the genome of the likeliest candidates genes in an index case or family proband. A number of different animal models of therapeutic strategies have been developed for gene transfer therapy, but so far these techniques have not yet entered clinical practice. Treatment remains for the most part symptomatic with the goal of ameliorating locomotor and cardiorespiratory manifestations of the disease.

Emerin regulation of nuclear stiffness is required for fast amoeboid migration in confined environments

When metastasizing, tumor cells must traverse environments with diverse physicochemical properties. Recently, the cell nucleus has emerged as a major regulator of the transition from mesenchymal to fast amoeboid (leader bleb-based) migration. Here, in melanoma cells, we demonstrate that increasing nuclear stiffness through elevating Lamin A, inhibits fast amoeboid migration. Importantly, nuclei may respond to force through stiffening. A key factor in this process is the inner nuclear membrane (INM) protein, emerin. Accordingly, we determined the role of emerin in regulating fast amoeboid migration. Strikingly, we found that both the up- and down-regulation of emerin results in an inhibition of fast amoeboid migration. However, when key Src phosphorylation sites were removed, up-regulation of emerin no longer inhibited fast amoeboid migration. Interestingly, in confined cells, Src activity was low, as measured by a Src biosensor. Thus, the fast amoeboid migration of melanoma cells depends on the precise calibration of emerin activity.

Molecular Pathology of Laminopathies

The nuclear envelope is composed of the nuclear membranes, nuclear lamina, and nuclear pore complexes. Laminopathies are diseases caused by mutations in genes encoding protein components of the lamina and these other nuclear envelope substructures. Mutations in the single gene encoding lamin A and C, which are expressed in most differentiated somatic cells, cause diseases affecting striated muscle, adipose tissue, peripheral nerve, and multiple systems with features of accelerated aging. Mutations in genes encoding other nuclear envelope proteins also cause an array of diseases that selectively affect different tissues or organs. In some instances, the molecular and cellular consequences of laminopathy-causing mutations are known. However, even when these are understood, mechanisms explaining specific tissue or organ pathology remain enigmatic. Current mechanistic hypotheses focus on how alterations in the nuclear envelope may affect gene expression, including via the regulation of signaling pathways, or cellular mechanics, including responses to mechanical stress.

Evolution and diversification of the nuclear envelope

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

Microtubule-based transport is essential to distribute RNA and nascent protein in skeletal muscle

While the importance of RNA localization in highly differentiated cells is well appreciated, basic principles of RNA localization in skeletal muscle remain poorly characterized. Here, we develop a method to detect and quantify single molecule RNA localization patterns in skeletal myofibers, and uncover a critical role for directed transport of RNPs in muscle. We find that RNAs localize and are translated along sarcomere Z-disks, dispersing tens of microns from progenitor nuclei, regardless of encoded protein function. We find that directed transport along the lattice-like microtubule network of myofibers becomes essential to achieve this localization pattern as muscle development progresses; disruption of this network leads to extreme accumulation of RNPs and nascent protein around myonuclei. Our observations suggest that global active RNP transport may be required to distribute RNAs in highly differentiated cells and reveal fundamental mechanisms of gene regulation, with consequences for myopathies caused by perturbations to RNPs or microtubules.

The Role of Emerin in Cancer Progression and Metastasis

It is commonly recognized in the field that cancer cells exhibit changes in the size and shape of their nuclei. These features often serve as important biomarkers in the diagnosis and prognosis of cancer patients. Nuclear size can significantly impact cell migration due to its incredibly large size. Nuclear structural changes are predicted to regulate cancer cell migration. Nuclear abnormalities are common across a vast spectrum of cancer types, regardless of tissue source, mutational spectrum, and signaling dependencies. The pervasiveness of nuclear alterations suggests that changes in nuclear structure may be crucially linked to the transformation process. The factors driving these nuclear abnormalities, and the functional consequences, are not completely understood. Nuclear envelope proteins play an important role in regulating nuclear size and structure in cancer. Altered expression of nuclear lamina proteins, including emerin, is found in many cancers and this expression is correlated with better clinical outcomes. A model is emerging whereby emerin, as well as other nuclear lamina proteins, binding to the nucleoskeleton regulates the nuclear structure to impact metastasis. In this model, emerin and lamins play a central role in metastatic transformation, since decreased emerin expression during transformation causes the nuclear structural defects required for increased cell migration, intravasation, and extravasation. Herein, we discuss the cellular functions of nuclear lamina proteins, with a particular focus on emerin, and how these functions impact cancer progression and metastasis.

Defects in Emerin-Nucleoskeleton Binding Disrupt Nuclear Structure and Promote Breast Cancer Cell Motility and Metastasis

Nuclear envelope proteins play an important role in regulating nuclear size and structure in cancer. Altered expression of nuclear lamins are found in many cancers and its expression is correlated with better clinical outcomes. The nucleus is the largest organelle in the cell with a diameter between 10 and 20 μm. Nuclear size significantly impacts cell migration. Nuclear structural changes are predicted to impact cancer metastasis by regulating cancer cell migration. Here we show emerin regulates nuclear structure in invasive breast cancer cells to impact cancer metastasis. Invasive breast cancer cells had 40% to 50% less emerin than control cells, which resulted in decreased nuclear size. Overexpression of GFP-emerin in invasive breast cancer cells rescued nuclear size and inhibited migration through 3.0 and 8.0 μm pores. Mutational analysis showed emerin binding to nucleoskeletal proteins was important for its regulation of nuclear structure, migration, and invasion. Importantly, emerin expression inhibited lung metastasis by 91% in orthotopic mouse models of breast cancer. Emerin nucleoskeleton-binding mutants failed to inhibit metastasis. These results support a model whereby emerin binding to the nucleoskeleton regulates nuclear structure to impact metastasis. In this model, emerin plays a central role in metastatic transformation, because decreased emerin expression during transformation causes the nuclear structural defects required for increased cell migration, intravasation, and extravasation. IMPLICATIONS: Modulating emerin expression and function represents new targets for therapeutic interventions of metastasis, because increased emerin expression rescued cancer metastasis.

Using nuclear envelope mutations to explore age-related skeletal muscle weakness

Skeletal muscle weakness is an important determinant of age-related declines in independence and quality of life but its causes remain unclear. Accelerated ageing syndromes such as Hutchinson-Gilford Progerin Syndrome, caused by mutations in genes encoding nuclear envelope proteins, have been extensively studied to aid our understanding of the normal biological ageing process. Like several other pathologies associated with genetic defects to nuclear envelope proteins including Emery-Dreifuss muscular dystrophy, Limb-Girdle muscular dystrophy and congenital muscular dystrophy, these disorders can lead to severe muscle dysfunction. Here, we first describe the structure and function of nuclear envelope proteins, and then review the mechanisms by which mutations in genes encoding nuclear envelope proteins induce premature ageing diseases and muscle pathologies. In doing so, we highlight the potential importance of such genes in processes leading to skeletal muscle weakness in old age.

Congenital myasthenic syndrome due to a TOR1AIP1 mutation: a new disease pathway for impaired synaptic transmission

Congenital myasthenic syndromes are inherited disorders characterized by fatiguable muscle weakness resulting from impaired signal transmission at the neuromuscular junction. Causative mutations have been identified in genes that can affect the synaptic function or structure. We identified a homozygous frameshift deletion c.127delC, p. Pro43fs in TOR1AIP1 in two siblings with limb-girdle weakness and impaired transmission at the neuromuscular synapse. TOR1AIP1 encodes the inner nuclear membrane protein lamin-associated protein 1. On muscle biopsy from the index case, lamin-associated protein 1 was absent from myonuclei. A mouse model with lamin-associated protein 1 conditionally knocked out in striated muscle was used to analyse the role of lamin-associated protein 1 in synaptic dysfunction. Model mice develop fatiguable muscle weakness as demonstrated by using an inverted screen hang test. Electromyography on the mice revealed a decrement on repetitive nerve stimulation. Ex vivo analysis of hemi-diaphragm preparations showed both miniature and evoked end-plate potential half-widths were prolonged which was associated with upregulation of the foetal acetylcholine receptor γ subunit. Neuromuscular junctions on extensor digitorum longus muscles were enlarged and fragmented, and the number of subsynaptic nuclei was significantly increased. Following these findings, electromyography was performed on cases of other nuclear envelopathies caused by mutations in LaminA/C or emerin, but decrement on repetitive nerve stimulation or other indications of defective neuromuscular transmission were not seen. Thus, this report highlights the first nuclear membrane protein in which defective function can lead to impaired synaptic transmission.

Muscular involvement and tendon contracture in limb-girdle muscular dystrophy 2Y: a mild adult phenotype and literature review

Background

Limb girdle muscular dystrophy type 2Y (LGMD2Y) is a rare subgroup of limb girdle muscular dystrophy featuring limb-girdle weakness, tendon contracture and cardiac involvement. It is caused by the mutation of TOR1AIP1, which encodes nuclear membrane protein LAP1 (lamina-associated polypeptide 1) and comprises heterogeneous phenotypes. The present study reported a patient with a novel homozygous TOR1AIP1 mutation that presented with selective muscle weakness, which further expanded the phenotype of LGMD2Y- and TOR1AIP1-associated nuclear envelopathies.

Case presentation

A 40-year-old male presented with Achilles tendon contracture and muscle weakness that bothered him from 8 years old. While the strength of his distal and proximal upper limbs was severely impaired, the function of his lower limbs was relatively spared. Muscle pathology showed dystrophic features, and electron microscopy showed ultrastructural abnormalities of disrupted muscle nuclei envelopes. Whole-exome sequencing showed a frameshift mutation in TOR1AIP1 (c.98dupC).

Conclusion

We reported a novel mild phenotype of LGMD2Y with relatively selective distal upper limb weakness and joint contracture and revealed the heterogeneity of LGDM2Y and the role of the LAP1 isoform by literature review.

EDMD-Causing Emerin Mutant Myogenic Progenitors Exhibit Impaired Differentiation Using Similar Mechanisms

Mutations in the gene encoding emerin (EMD) cause Emery–Dreifuss muscular dystrophy (EDMD1), an inherited disorder characterized by progressive skeletal muscle wasting, irregular heart rhythms and contractures of major tendons. The skeletal muscle defects seen in EDMD are caused by failure of muscle stem cells to differentiate and regenerate the damaged muscle. However, the underlying mechanisms remain poorly understood. Most EDMD1 patients harbor nonsense mutations and have no detectable emerin protein. There are three EDMD-causing emerin mutants (S54F, Q133H, and Δ95–99) that localize correctly to the nuclear envelope and are expressed at wildtype levels. We hypothesized these emerin mutants would share in the disruption of key molecular pathways involved in myogenic differentiation. We generated myogenic progenitors expressing wildtype emerin and each EDMD1-causing emerin mutation (S54F, Q133H, Δ95–99) in an emerin-null (EMD^−/y) background. S54F, Q133H, and Δ95–99 failed to rescue EMD^−/y myogenic differentiation, while wildtype emerin efficiently rescued differentiation. RNA sequencing was done to identify pathways and networks important for emerin regulation of myogenic differentiation. This analysis significantly reduced the number of pathways implicated in EDMD1 muscle pathogenesis.

Emery-Dreifuss muscular dystrophy: focal point nuclear envelope

PURPOSE OF REVIEW

Emery-Dreifuss muscular dystrophy (EDMD) is caused by mutations in EMD encoding emerin and LMNA encoding A-type lamins, proteins of the nuclear envelope. In the past decade, there has been an extraordinary burst of research on the nuclear envelope. Discoveries resulting from this basic research have implications for better understanding the pathogenesis and developing treatments for EDMD.

RECENT FINDINGS

Recent clinical research has confirmed that EDMD is one of several overlapping skeletal muscle phenotypes that can result from mutations in EMD and LMNA with dilated cardiomyopathy as a common feature. Basic research on the nuclear envelope has provided new insights into how A-type lamins and emerin function in force transmission throughout the cell, which may be particularly important in striated muscle. Much of the recent research has focused on the heart and LMNA mutations. Prevalence and outcome studies have confirmed the relative severity of cardiac disease. Robust mouse models of EDMD caused by LMNA mutations has allowed for further insight into pathogenic mechanisms and potentially beneficial therapeutic approaches.

SUMMARY

Recent clinical and basic research on EDMD is gradually being translated to clinical practice and possibly novel therapies.

Probing the Environment of Emerin by Enhanced Ascorbate Peroxidase 2 (APEX2)-Mediated Proximity Labeling

Emerin is one of the best characterized proteins of the inner nuclear membrane, but can also occur at the level of the endoplasmic reticulum. We now use enhanced ascorbate peroxidase 2 (APEX2) to probe the environment of emerin. APEX2 can be used as a genetic tag that produces short-lived yet highly reactive biotin species, allowing the modification of proteins that interact with or are in very close proximity to the tagged protein. Biotinylated proteins can be isolated using immobilized streptavidin and analyzed by mass spectrometry. As an alternative to the standard approach with a genetic fusion of APEX2 to emerin, we also used RAPIDS (rapamycin- and APEX-dependent identification of proteins by SILAC), a method with improved specificity, where the peroxidase interacts with the protein of interest (i.e., emerin) only upon addition of rapamycin to the cells. We compare these different approaches, which, together, identify well-known interaction partners of emerin like lamin A and the lamina associated polypeptide 1 (LAP1), as well as novel proximity partners.

Emery-Dreifuss muscular dystrophy

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

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

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

Deficiency of emerin contributes differently to the pathogenesis of skeletal and cardiac muscles in LmnaH222P/H222P mutant mice

Laminopathies are tissue-selective diseases that affect differently in organ systems. Mutations in nuclear envelopes, emerin (Emd) and lamin A/C (Lmna) genes, cause clinically indistinguishable myopathy called Emery-Dreifuss muscular dystrophy (EDMD) and limb-girdle muscular dystrophy. Several murine models for EDMD have been generated; however, emerin-null (Emd) mice do not show obvious skeletal and cardiac muscle phenotypes, and Lmna H222P/H222P mutant (H222P) mice show only a mild phenotype in skeletal muscle when they already have severe cardiomyopathy. Thus, the underlying molecular mechanism of muscle involvement due to nuclear abnormalities is still unclarified. We generated double mutant (Emd-/-/LmnaH222P/H222P; EH) mice to characterize dystrophic changes and to elucidate interactions between emerin and lamin A/C in skeletal and cardiac muscles. As H222P mice, EH mice grow normally and have breeding productivity. EH mice showed severer muscle involvement compared with that of H222P mice which was an independent of cardiac abnormality at 12 weeks of age. Nuclear abnormalities, reduced muscle fiber size and increased fibrosis were prominent in EH mice. Roles of emerin and lamin A/C in satellite cells function and regeneration of muscle fiber were also evaluated by cardiotoxin-induced muscle injury. Delayed increases in myog and myh3 expression were seen in both H222P and EH mice; however, the expression levels of those genes were similar with control and regenerated muscle fiber size was not different at day 7 after injury. These results indicate that EH mouse is a suitable model for studying skeletal muscle involvement, independent of cardiac function, in laminopathies and an interaction between emerin and lamin A/C in different tissues.

Postnatal development of mice with combined genetic depletions of lamin A/C, emerin and lamina-associated polypeptide 1

Mutations in LMNA encoding lamin A/C and EMD encoding emerin cause cardiomyopathy and muscular dystrophy. Lmna null mice develop these disorders and have a lifespan of 7-8 weeks. Emd null mice show no overt pathology and have normal skeletal muscle but with regeneration defects. We generated mice with germline deletions of both Lmna and Emd to determine the effects of combined loss of the encoded proteins. Mice without lamin A/C and emerin are born at the expected Mendelian ratio, are grossly normal at birth but have shorter lifespans than those lacking only lamin A/C. However, there are no major differences between these mice with regards to left ventricular function, heart ultrastructure or electrocardiographic parameters except for slower heart rates in the mice lacking both lamin A/C and emerin. Skeletal muscle is similarly affected in both of these mice. Lmna+/- mice also lacking emerin live to at least 1 year and have no significant differences in growth, heart or skeletal muscle compared to Lmna+/- mice. Deletion of the mouse gene encoding lamina-associated protein 1 leads to prenatal death; however, mice with heterozygous deletion of this gene lacking both lamin A/C and emerin are born at the expected Mendelian ratio but had a shorter lifespan than those only lacking lamin A/C and emerin. These results show that mice with combined deficiencies of three interacting nuclear envelope proteins have normal embryonic development and that early postnatal defects are primarily driven by loss of lamin A/C or lamina-associated polypeptide 1 rather than emerin.

Pathogenic mutations in genes encoding nuclear envelope proteins and defective nucleocytoplasmic connections

Mutations in genes encoding nuclear lamins and associated nuclear envelope proteins have been linked to a broad range of inherited diseases affecting different tissues and organs. These diseases are often referred to as laminopathies. Scientists have yet to elucidate exactly how pathogenic mutations leading to alteration of a nuclear envelope protein cause disease. Our relatively recent research has shown that pathogenic mutations in genes encoding nuclear envelope proteins lead to defective nucleocytoplasmic connections that disrupt proper functioning of the linker of nucleoskeleton and cytoskeleton complex in the establishment of cell polarity. These defects may explain, at least in part, pathogenic mechanisms underlying laminopathies.

Impact statement

Mutations in genes encoding nuclear lamins and associated nuclear envelope proteins have been linked to several diseases affecting different tissues and organs. The pathogenic mechanisms underlying these diseases, often called laminopathies, remain poorly understood. Increased knowledge of the functions of different nuclear envelope proteins and the interactions between them is crucial to elucidate these disease mechanisms. Our research has shown that pathogenic mutations in genes encoding nuclear envelope proteins lead to defective nucleocytoplasmic connections that disrupt proper functioning of the linker of nucleoskeleton and cytoskeleton (LINC) complex in the establishment of cell polarity. These defects may contribute to the pathogenesis of laminopathies and provide novel targets for therapeutics.

An Emerin LEM-Domain Mutation Impairs Cell Response to Mechanical Stress

Emerin is a nuclear envelope protein that contributes to genome organization and cell mechanics. Through its N-terminal LAP2-emerin-MAN1 (LEM)-domain, emerin interacts with the DNA-binding protein barrier-to-autointegration (BAF). Emerin also binds to members of the linker of the nucleoskeleton and cytoskeleton (LINC) complex. Mutations in the gene encoding emerin are responsible for the majority of cases of X-linked Emery-Dreifuss muscular dystrophy (X-EDMD). Most of these mutations lead to an absence of emerin. A few missense and short deletion mutations in the disordered region of emerin are also associated with X-EDMD. More recently, missense and short deletion mutations P22L, ∆K37 and T43I were discovered in emerin LEM-domain, associated with isolated atrial cardiac defects (ACD). Here we reveal which defects, at both the molecular and cellular levels, are elicited by these LEM-domain mutations. Whereas K37 mutation impaired the correct folding of the LEM-domain, P22L and T43I had no impact on the 3D structure of emerin. Surprisingly, all three mutants bound to BAF, albeit with a weaker affinity in the case of K37. In human myofibroblasts derived from a patient's fibroblasts, emerin ∆K37 was correctly localized at the inner nuclear membrane, but was present at a significantly lower level, indicating that this mutant is abnormally degraded. Moreover, SUN2 was reduced, and these cells were defective in producing actin stress fibers when grown on a stiff substrate and after cyclic stretches. Altogether, our data suggest that the main effect of mutation K37 is to perturb emerin function within the LINC complex in response to mechanical stress.

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

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

Emerin anchors Msx1 and its protein partners at the nuclear periphery to inhibit myogenesis

BACKGROUND

Previous studies have shown that in myogenic precursors, the homeoprotein Msx1 and its protein partners, histone methyltransferases and repressive histone marks, tend to be enriched on target myogenic regulatory genes at the nuclear periphery. The nuclear periphery localization of Msx1 and its protein partners is required for Msx1's function of preventing myogenic precursors from pre-maturation through repressing target myogenic regulatory genes. However, the mechanisms underlying the maintenance of Msx1 and its protein partners' nuclear periphery localization are unknown.

RESULTS

We show that an inner nuclear membrane protein, Emerin, performs as an anchor settled at the inner nuclear membrane to keep Msx1 and its protein partners Ezh2, H3K27me3 enriching at the nuclear periphery, and participates in inhibition of myogenesis mediated by Msx1. Msx1 interacts with Emerin both in C2C12 myoblasts and mouse developing limbs, which is the prerequisite for Emerin mediating the precise location of Msx1, Ezh2, and H3K27me3. The deficiency of Emerin in C2C12 myoblasts disturbs the nuclear periphery localization of Msx1, Ezh2, and H3K27me3, directly indicating Emerin functioning as an anchor. Furthermore, Emerin cooperates with Msx1 to repress target myogenic regulatory genes, and assists Msx1 with inhibition of myogenesis.

CONCLUSIONS

Emerin cooperates with Msx1 to inhibit myogenesis through maintaining the nuclear periphery localization of Msx1 and Msx1's protein partners.

Hop/STIP1 depletion alters nuclear structure via depletion of nuclear structural protein emerin

Hop/STIP1 is a co-chaperone of Hsp70 and Hsp90 that regulates a number of cell biology processes via interactions with cellular proteins. Here we report a new relationship between Hop and the nuclear structural protein emerin in maintenance of nuclear morphology. Depletion or overexpression of Hop resulted in the reduction of emerin protein levels via proteasomal and lysosomal pathways. Co-immunoprecipitation assays confirmed that Hop and emerin are in a common complex, which could accommodate Hsp70 but not Hsp90, and that TPR2AB is required for the association. Loss of Hop or emerin led to a deformation of nuclear structure, a statistically significant decrease in nuclear size, and was associated with changes in the levels of nuclear proteins, lamin A-C and fibrillarin. The nuclear defects upon Hop loss could be rescued by emerin overexpression. Taken together, these data suggest that Hop stabilises emerin and that loss of Hop alters nuclear structure via emerin degradation.

The Pathogenesis and Therapies of Striated Muscle Laminopathies

Emery-Dreifuss muscular dystrophy (EDMD) is a genetic condition characterized by early contractures, skeletal muscle weakness, and cardiomyopathy. During the last 20 years, various genetic approaches led to the identification of causal genes of EDMD and related disorders, all encoding nuclear envelope proteins. By their respective localization either at the inner nuclear membrane or the outer nuclear membrane, these proteins interact with each other and establish a connection between the nucleus and the cytoskeleton. Beside this physical link, these proteins are also involved in mechanotransduction, responding to environmental cues, such as increased tension of the cytoskeleton, by the activation or repression of specific sets of genes. This ability of cells to adapt to environmental conditions is altered in EDMD. Increased knowledge on the pathophysiology of EDMD has led to the development of drug or gene therapies that have been tested on mouse models. This review proposed an overview of the functions played by the different proteins involved in EDMD and related disorders and the current therapeutic approaches tested so far.

Evaluation of the diagnostic value of emerin and CD56 in papillary thyroid carcinoma - an immunohistochemical study

Papillary thyroid carcinoma (PTC) is diagnosed in both cytological and histological specimens on the basis of distinct nuclear morphology. These features may not be prominent in some PTC variants and may be seen in some benign conditions. It is necessary to differentiate PTC from other neoplastic and nonneoplastic lesions since it affects treatment strategy and patients' fate. Emerin is a type II integral membrane protein of the inner nuclear membrane that has a characteristic staining pattern in PTC. CD56 is a homophilic membrane glycoprotein that is expressed in thyroid follicular epithelial cells and adrenal glands. The aim of this study was to evaluate the diagnostic value of emerin (positivity, percentage, and highlighting nuclear features) and CD56 (positive versus negative) both singly and in combination for differentiation of PTC from other neoplastic and nonneoplastic mimics. This study was performed on 50 cases of PTC, 9 cases of follicular adenoma (FA), and 12 cases of nonneoplastic thyroid lesions using immunohistochemistry for detection of emerin and CD56. Positive emerin expression was seen in 82% of PTC and in 16.7% of nonneoplastic cases with an absence of expression in FA. CD56 was expressed in 88.9% of FA, 91.7% of nonneoplastic cases and in a minority of PTC cases (6%). Positive emerin revealed 82% sensitivity and 90% specificity, while emerin-highlighted nuclear changes was more specific (95%). Negative CD56 expression revealed 84% sensitivity and 90% specificity. Combined positive emerin (including highlighting nuclear changes) and negative CD56 showed 72% sensitivity and 100% specificity. Positive emerin expression (moderate/strong) and its highlighting nuclear changes combined with negative CD56 could be a very helpful procedure in difficult and overlapping cases with high diagnostic validity (high specificity and positive predictive value).

Linker of nucleoskeleton and cytoskeleton complex proteins in cardiomyopathy

The linker of nucleoskeleton and cytoskeleton (LINC) complex couples the nuclear lamina to the cytoskeleton. The LINC complex and its associated proteins play diverse roles in cells, ranging from genome organization, nuclear morphology, gene expression, to mechanical stability. The importance of a functional LINC complex is highlighted by the large number of mutations in genes encoding LINC complex proteins that lead to skeletal and cardiac myopathies. In this review, the structure, function, and interactions between components of the LINC complex will be described. Mutations that are known to cause cardiomyopathy in patients will be discussed alongside their respective mouse models. Furthermore, future challenges for the field and emerging technologies to investigate LINC complex function will be discussed.

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

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

Nesprin-1/2: roles in nuclear envelope organisation, myogenesis and muscle disease

Nesprins (nuclear envelope spectrin repeat proteins) are multi-isomeric scaffolding proteins. Nesprin-1 and -2 are highly expressed in skeletal and cardiac muscles and together with SUN (Sad1p/UNC84) domain-containing proteins form the LInker of Nucleoskeleton and Cytoskeleton (LINC) complex at the nuclear envelope in association with lamin A/C and emerin. Mutations in nesprin-1/2 have been found in patients with autosomal dominant Emery-Dreifuss muscular dystrophy (EDMD) as well as dilated cardiomyopathy (DCM). Several lines of evidence indicate that compromised LINC complex function is the critical step leading to muscle disease. Here, we review recent advances in our understanding of the functions of nesprin-1/2 in the LINC complex and mechanistic insights into how mutations in nesprin-1/2 lead to nesprin-related muscle diseases, in particular DCM and EDMD.

High-Resolution Imaging Methods to Analyze LINC Complex Function During Drosophila Muscle Development

Using Drosophila muscle development as a model system makes possible the identification of genetic pathways, temporal regulation of development, mechanisms of cellular development, and physiological impacts in a single system. Here we describe the basic techniques for the evaluation of the cellular development of muscle in Drosophila in both embryos and in larvae. These techniques are discussed within the context of how the LINC complex contributes to muscle development.

Expression Profiling of Differentiating Emerin-Null Myogenic Progenitor Identifies Molecular Pathways Implicated in Their Impaired Differentiation

Mutations in the gene encoding emerin cause Emery-Dreifuss muscular dystrophy (EDMD), a disorder causing progressive skeletal muscle wasting, irregular heart rhythms and contractures of major tendons. RNA sequencing was performed on differentiating wildtype and emerin-null myogenic progenitors to identify molecular pathways implicated in EDMD, 340 genes were uniquely differentially expressed during the transition from day 0 to day 1 in wildtype cells. 1605 genes were uniquely expressed in emerin-null cells; 1706 genes were shared among both wildtype and emerin-null cells. One thousand and forty-seven transcripts showed differential expression during the transition from day 1 to day 2. Four hundred and thirty-one transcripts showed altered expression in both wildtype and emerin-null cells. Two hundred and ninety-five transcripts were differentially expressed only in emerin-null cells and 321 transcripts were differentially expressed only in wildtype cells. DAVID, STRING and Ingenuity Pathway Analysis identified pathways implicated in impaired emerin-null differentiation, including cell signaling, cell cycle checkpoints, integrin signaling, YAP/TAZ signaling, stem cell differentiation, and multiple muscle development and myogenic differentiation pathways. Functional enrichment analysis showed biological functions associated with the growth of muscle tissue and myogenesis of skeletal muscle were inhibited. The large number of differentially expressed transcripts upon differentiation induction suggests emerin functions during transcriptional reprograming of progenitors to committed myoblasts.

Lamina-associated polypeptide 1 is dispensable for embryonic myogenesis but required for postnatal skeletal muscle growth

Lamina-associated polypeptide 1 (LAP1) is an integral protein of the inner nuclear membrane that has been implicated in striated muscle maintenance. Mutations in its gene have been linked to muscular dystrophy and cardiomyopathy. As germline deletion of the gene encoding LAP1 is perinatal lethal, we explored its potential role in myogenic differentiation and development by generating a conditional knockout mouse in which the protein is depleted from muscle progenitors at embryonic day 8.5 (Myf5-Lap1CKO mice). Although cultured myoblasts lacking LAP1 demonstrated defective terminal differentiation and altered expression of muscle regulatory factors, embryonic myogenesis and formation of skeletal muscle occurred in both mice with a Lap1 germline deletion and Myf5-Lap1CKO mice. However, skeletal muscle fibres were hypotrophic and their nuclei were morphologically abnormal with a wider perinuclear space than normal myonuclei. Myf5-Lap1CKO mouse skeletal muscle contained fewer satellite cells than normal and these cells had evidence of reduced myogenic potential. Abnormalities in signalling pathways required for postnatal hypertrophic growth were also observed in skeletal muscles of these mice. Our results demonstrate that early embryonic depletion of LAP1 does not impair myogenesis but that it is necessary for postnatal skeletal muscle growth.

Progressive Atrial Conduction Defects Associated With Bone Malformation Caused by a Connexin-45 Mutation

BACKGROUND

Inherited cardiac conduction disease is a rare bradyarrhythmia associated with mutations in various genes that affect action potential propagation. It is often characterized by isolated conduction disturbance of the His-Purkinje system, but it is rarely described as a syndromic form.

OBJECTIVES

The authors sought to identify the genetic defect in families with a novel bradyarrhythmia syndrome associated with bone malformation.

METHODS

The authors genetically screened 15 European cases with genotype-negative de novo atrioventricular (AV) block and their parents by trio whole-exome sequencing, plus 31 Japanese cases with genotype-negative familial AV block or sick sinus syndrome by targeted exon sequencing of 457 susceptibility genes. Functional consequences of the mutation were evaluated using an in vitro cell expression system and in vivo knockout mice.

RESULTS

The authors identified a connexin-45 (Cx45) mutation (p.R75H) in 2 unrelated families (a de novo French case and a 3-generation Japanese family) who presented with progressive AV block, which resulted in atrial standstill without ventricular conduction abnormalities. Affected individuals shared a common extracardiac phenotype: a brachyfacial pattern, finger deformity, and dental dysplasia. Mutant Cx45 expressed in Neuro-2a cells showed normal hemichannel assembly and plaque formation. However, Lucifer yellow dye transfer and gap junction conductance between cell pairs were severely impaired, which suggested that mutant Cx45 impedes gap junction communication in a dominant-negative manner. Tamoxifen-induced, cardiac-specific Cx45 knockout mice showed sinus node dysfunction and atrial arrhythmia, recapitulating the intra-atrial disturbance.

CONCLUSIONS

Altogether, the authors showed that Cx45 mutant p.R75H is responsible for a novel disease entity of progressive atrial conduction system defects associated with craniofacial and dentodigital malformation.

Emerin plays a crucial role in nuclear invagination and in the nuclear calcium transient

Alteration of the nuclear Ca²⁺ transient is an early event in cardiac remodeling. Regulation of the nuclear Ca²⁺ transient is partly independent of the cytosolic Ca²⁺ transient in cardiomyocytes. One nuclear membrane protein, emerin, is encoded by EMD, and an EMD mutation causes Emery-Dreifuss muscular dystrophy (EDMD). It remains unclear whether emerin is involved in nuclear Ca²⁺ homeostasis. The aim of this study is to elucidate the role of emerin in rat cardiomyocytes by means of hypertrophic stimuli and in EDMD induced pluripotent stem (iPS) cell-derived cardiomyocytes in terms of nuclear structure and the Ca²⁺ transient. The cardiac hypertrophic stimuli increased the nuclear area, decreased nuclear invagination, and increased the half-decay time of the nuclear Ca²⁺ transient in cardiomyocytes. Emd knockdown cardiomyocytes showed similar properties after hypertrophic stimuli. The EDMD-iPS cell-derived cardiomyocytes showed increased nuclear area, decreased nuclear invagination, and increased half-decay time of the nuclear Ca²⁺ transient. An autopsied heart from a patient with EDMD also showed increased nuclear area and decreased nuclear invagination. These data suggest that Emerin plays a crucial role in nuclear structure and in the nuclear Ca²⁺ transient. Thus, emerin and the nuclear Ca²⁺ transient are possible therapeutic targets in heart failure and EDMD.

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.

Centrosome positioning in non-dividing cells

Centrioles and centrosomes are found in almost all eukaryotic cells, where they are important for organising the microtubule cytoskeleton in both dividing and non-dividing cells. The spatial location of centrioles and centrosomes is tightly controlled and, in non-dividing cells, plays an important part in cell migration, ciliogenesis and immune cell functions. Here, we examine some of the ways that centrosomes are connected to other organelles and how this impacts on cilium formation, cell migration and immune cell function in metazoan cells.

Molecular characterization and expression patterns of emerin (EMD) gene in skeletal muscle between Meishan and Large White pigs

The emerin protein is a nuclear membrane protein and has important functions in muscle development, regeneration, and cell signal transduction. However, knowledge regarding emerin in the domestic animal is limited. In this study, we cloned and characterized the pig emerin (EMD) gene. Semi-quantitative RT-PCR analysis revealed that the EMD gene was expressed at the highest level in the heart and fat at 120d. However, the fetal skeletal muscles displayed a greater abundance of EMD mRNA than that in skeletal muscles at postnatal development stages. In addition, the expression level of EMD at 60 day was significantly higher (p<0.05) in Meishan than Large White pigs. Pig EMD protein displayed the sarcolemma and perinuclear distribution in skeletal muscle sections, and there was no distribution change of EMD in skeletal muscle sections between Large White and Meishan pigs. These studies provide useful information for further research on the functions of pig EMD gene in skeletal muscle.

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.

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

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

Altering lamina assembly reveals lamina-dependent and -independent functions for A-type lamins

Lamins are intermediate filament proteins that form a fibrous meshwork, called the nuclear lamina, between the inner nuclear membrane and peripheral heterochromatin of metazoan cells. The assembly and incorporation of lamin A/C into the lamina, as well as their various functions, are still not well understood. Here, we employed designed ankyrin repeat proteins (DARPins) as new experimental tools for lamin research. We screened for DARPins that specifically bound to lamin A/C, and interfered with lamin assembly in vitro and with incorporation of lamin A/C into the native lamina in living cells. The selected DARPins inhibited lamin assembly and delocalized A-type lamins to the nucleoplasm without modifying lamin expression levels or the amino acid sequence. Using these lamin binders, we demonstrate the importance of proper integration of lamin A/C into the lamina for nuclear mechanical properties and nuclear envelope integrity. Finally, our study provides evidence for cell-type-specific differences in lamin functions.

Reinforcing the LINC complex connection to actin filaments: the role of FHOD1 in TAN line formation and nuclear movement

Positioning the nucleus is critical for many cellular processes including cell division, migration and differentiation. The linker of nucleoskeleton and cytoskeleton (LINC) complex spans the inner and outer nuclear membranes and has emerged as a major factor in connecting the nucleus to the cytoskeleton for movement and positioning. Recently, we discovered that the diaphanous formin family member FHOD1 interacts with the LINC complex component nesprin-2 giant (nesprin-2G) and that this interaction plays essential roles in the formation of transmembrane actin-dependent nuclear (TAN) lines and nuclear movement during cell polarization in fibroblasts. We found that FHOD1 strengthens the connection between nesprin-2G and rearward moving dorsal actin cables by providing a second site of interaction between nesprin-2G and the actin cable. These results indicate that the LINC complex connection to the actin cytoskeleton can be enhanced by cytoplasmic factors and suggest a new model for TAN line formation. We discuss how the nesprin-2G-FHOD1 interaction may be regulated and its possible functional significance for development and disease.