"Laminopathies": a wide spectrum of human diseases

A comparative study of Drosophila and human A-type lamins

Nuclear intermediate filament proteins, called lamins, form a meshwork that lines the inner surface of the nuclear envelope. Lamins contain three domains: an N-terminal head, a central rod and a C-terminal tail domain possessing an Ig-fold structural motif. Lamins are classified as either A- or B-type based on structure and expression pattern. The Drosophila genome possesses two genes encoding lamins, Lamin C and lamin Dm₀, which have been designated A- and B-type, respectively, based on their expression profile and structural features. In humans, mutations in the gene encoding A-type lamins are associated with a spectrum of predominantly tissue-specific diseases known as laminopathies. Linking the disease phenotypes to cellular functions of lamins has been a major challenge. Drosophila is being used as a model system to identify the roles of lamins in development. Towards this end, we performed a comparative study of Drosophila and human A-type lamins. Analysis of transgenic flies showed that human lamins localize predictably within the Drosophila nucleus. Consistent with this finding, yeast two-hybrid data demonstrated conservation of partner-protein interactions. Drosophila lacking A-type lamin show nuclear envelope defects similar to those observed with human laminopathies. Expression of mutant forms of the A-type Drosophila lamin modeled after human disease-causing amino acid substitutions revealed an essential role for the N-terminal head and the Ig-fold in larval muscle tissue. This tissue-restricted sensitivity suggests a conserved role for lamins in muscle biology. In conclusion, we show that (1) localization of A-type lamins and protein-partner interactions are conserved between Drosophila and humans, (2) loss of the Drosophila A-type lamin causes nuclear defects and (3) muscle tissue is sensitive to the expression of mutant forms of A-type lamin modeled after those causing disease in humans. These studies provide new insights on the role of lamins in nuclear biology and support Drosophila as a model for studies of human laminopathies involving muscle dysfunction.

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

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

The posttranslational processing of prelamin A and disease

Human geneticists have shown that some progeroid syndromes are caused by mutations that interfere with the conversion of farnesyl-prelamin A to mature lamin A. For example, Hutchinson-Gilford progeria syndrome is caused by LMNA mutations that lead to the accumulation of a farnesylated version of prelamin A. In this review, we discuss the posttranslational modifications of prelamin A and their relevance to the pathogenesis and treatment of progeroid syndromes.

Energetics, epigenetics, mitochondrial genetics

The epigenome has been hypothesized to provide the interface between the environment and the nuclear DNA (nDNA) genes. Key factors in the environment are the availability of calories and demands on the organism's energetic capacity. Energy is funneled through glycolysis and mitochondrial oxidative phosphorylation (OXPHOS), the cellular bioenergetic systems. Since there are thousands of bioenergetic genes dispersed across the chromosomes and mitochondrial DNA (mtDNA), both cis and trans regulation of the nDNA genes is required. The bioenergetic systems convert environmental calories into ATP, acetyl-Coenzyme A (acetyl-CoA), s-adenosyl-methionine (SAM), and reduced NAD(+). When calories are abundant, ATP and acetyl-CoA phosphorylate and acetylate chromatin, opening the nDNA for transcription and replication. When calories are limiting, chromatin phosphorylation and acetylation are lost and gene expression is suppressed. DNA methylation via SAM can also be modulated by mitochondrial function. Phosphorylation and acetylation are also pivotal to regulating cellular signal transduction pathways. Therefore, bioenergetics provides the interface between the environment and the epigenome. Consistent with this conclusion, the clinical phenotypes of bioenergetic diseases are strikingly similar to those observed in epigenetic diseases (Angelman, Rett, Fragile X Syndromes, the laminopathies, cancer, etc.), and an increasing number of epigenetic diseases are being associated with mitochondrial dysfunction. This bioenergetic-epigenomic hypothesis has broad implications for the etiology, pathophysiology, and treatment of a wide range of common diseases.

Barrier-to-autointegration factor proteome reveals chromatin-regulatory partners

Nuclear lamin filaments and associated proteins form a nucleoskeletal (“lamina”) network required for transcription, replication, chromatin organization and epigenetic regulation in metazoans. Lamina defects cause human disease (“laminopathies”) and are linked to aging. Barrier-to-autointegration factor (BAF) is a mobile and essential component of the nuclear lamina that binds directly to histones, lamins and LEM-domain proteins, including the inner nuclear membrane protein emerin, and has roles in chromatin structure, mitosis and gene regulation. To understand BAF's mechanisms of action, BAF associated proteins were affinity-purified from HeLa cell nuclear lysates using BAF-conjugated beads, and identified by tandem mass spectrometry or independently identified and quantified using the iTRAQ method. We recovered A- and B-type lamins and core histones, all known to bind BAF directly, plus four human transcription factors (Requiem, NonO, p15, LEDGF), disease-linked proteins (e.g., Huntingtin, Treacle) and several proteins and enzymes that regulate chromatin. Association with endogenous BAF was independently validated by co-immunoprecipitation from HeLa cells for seven candidates including Requiem, poly(ADP-ribose) polymerase 1 (PARP1), retinoblastoma binding protein 4 (RBBP4), damage-specific DNA binding protein 1 (DDB1) and DDB2. Interestingly, endogenous BAF and emerin each associated with DDB2 and CUL4A in a UV- and time-dependent manner, suggesting BAF and emerin have dynamic roles in genome integrity and might help couple DNA damage responses to the nuclear lamina network. We conclude this proteome is a rich source of candidate partners for BAF and potentially also A- and B-type lamins, which may reveal how chromatin regulation and genome integrity are linked to nuclear structure.

Emery-Dreifuss-Muskeldystrophie

Die Emery-Dreifuss-Muskeldystrophie (EDMD) ist eine seltene neuromuskuläre Erkrankung, die durch frühe Kontrakturen, eine langsam progrediente Muskelschwäche und lebensbedrohliche Herzrhythmusstörungen gekennzeichnet ist. Sowohl klinisch als auch genetisch findet man eine große intra- und interfamiliäre Variabilität. Genetisch lassen sich X-chromosomal rezessive (EMD1), autosomal-dominante (EMD2) und autosomal-rezessive (EMD3) Formen unterscheiden, die mit Mutationen in den Genen STA, LMNA, SYNE1, SYNE2 und FHL1 assoziiert sind. Nur bei etwa 46% nicht miteinander verwandter EDMD-Patienten findet man Mutationen in diesen Genen, sodass mit einer weiteren genetischen Heterogenität der EDMD zu rechnen ist.

Mutations of the FHL1 gene cause Emery-Dreifuss muscular dystrophy

Emery-Dreifuss muscular dystrophy (EDMD) is a rare disorder characterized by early joint contractures, muscular dystrophy, and cardiac involvement with conduction defects and arrhythmias. So far, only 35% of EDMD cases are genetically elucidated and associated with EMD or LMNA gene mutations, suggesting the existence of additional major genes. By whole-genome scan, we identified linkage to the Xq26.3 locus containing the FHL1 gene in three informative families belonging to our EMD- and LMNA-negative cohort. Analysis of the FHL1 gene identified seven mutations, in the distal exons of FHL1 in these families, three additional families, and one isolated case, which differently affect the three FHL1 protein isoforms: two missense mutations affecting highly conserved cysteines, one abolishing the termination codon, and four out-of-frame insertions or deletions. The predominant phenotype was characterized by myopathy with scapulo-peroneal and/or axial distribution, as well as joint contractures, and associated with a peculiar cardiac disease characterized by conduction defects, arrhythmias, and hypertrophic cardiomyopathy in all index cases of the seven families. Heterozygous female carriers were either asymptomatic or had cardiac disease and/or mild myopathy. Interestingly, four of the FHL1-mutated male relatives had isolated cardiac disease, and an overt hypertrophic cardiomyopathy was present in two. Expression and functional studies demonstrated that the FHL1 proteins were severely reduced in all tested patients and that this was associated with a severe delay in myotube formation in the two patients for whom myoblasts were available. In conclusion, FHL1 should be considered as a gene associated with the X-linked EDMD phenotype, as well as with hypertrophic cardiomyopathy.

Overlapping functions of nuclear envelope proteins NET25 (Lem2) and emerin in regulation of extracellular signal-regulated kinase signaling in myoblast differentiation

Mutations in certain nuclear envelope (NE) proteins cause muscular dystrophies and other disorders, but the disease mechanisms remain unclear. The nuclear envelope transmembrane protein NET25 (Lem2) is a truncated paralog of MAN1, an NE component linked to bone disorders. NET25 and MAN1 share an approximately 40-residue LEM homology domain with emerin, the protein mutated in X-linked Emery-Dreifuss muscular dystrophy. However, roles for NET25 and MAN1 in myogenesis have not yet been described. Using RNA interference in C2C12 myoblasts, we show for the first time that both NET25 and MAN1 are required for myogenic differentiation. NET25 depletion causes hyperactivation of extracellular signal-regulated kinase 1/2 at the onset of differentiation, and pharmacological inhibition of this transient overactivation rescues myogenesis. In contrast, pharmacological inhibition of both mitogen-activated protein kinase and transforming growth factor beta signaling is required to rescue differentiation after MAN1 depletion. Ectopic expression of silencing-resistant NET25 rescues myogenesis after depletion of emerin but not after MAN1 silencing. Thus, NET25 and emerin have at least partially overlapping functions during myogenic differentiation, which are distinct from those of MAN1. Our work supports the hypothesis that deregulation of cell signaling contributes to NE-linked disorders and suggests that mutations in NET25 and MAN1 may cause muscle diseases.

NET37, a nuclear envelope transmembrane protein with glycosidase homology, is involved in myoblast differentiation

The nuclear lamina and its associated proteins are important for nuclear structure and chromatin organization and also have been implicated in the regulation of cell signaling and gene expression. In this study we demonstrate that the lamina-associated nuclear envelope transmembrane protein NET37 is required for myogenic differentiation of C2C12 cells. NET37, a member of glycosidase family 31, is highly expressed in mouse skeletal muscle and is strongly up-regulated during C2C12 differentiation. By protease mapping we show that its glycosidase homology domain is located in the lumen of the nuclear envelope/endoplasmic reticulum. When NET37 is depleted from proliferating myoblasts by RNAi, myogenic differentiation is significantly impaired, and there is a concomitant delay in up-regulation of the late myogenic transcription factor myogenin. We expressed silencing-resistant NET37 mutated at a conserved residue in the glycosidase domain and found that this predicted catalytically inactive protein is unable to support myogenesis in cells depleted of wild type NET37. Therefore, the enzymatic function of NET37 appears to be important for myogenic differentiation. C2C12 cells depleted of NET37 have reduced activation of Akt after shifting to differentiation medium and are defective in insulin like growth factor-II (IGF-II) secretion, an autocrine/paracrine factor involved in Akt activation. We also observed that pro-IGF-II co-immunoprecipitates with NET37. Based on our results, we propose that NET37 has a role in IGF-II maturation in the secretory pathway during myoblast differentiation. The localization of NET37 at the nuclear envelope raises the possibility that it may coordinate myogenic events between the nuclear interior and the endoplasmic reticulum lumen via transmembrane communication.

Nuclear protein import is reduced in cells expressing nuclear envelopathy-causing lamin A mutants

Lamins, which form the nuclear lamina, not only constitute an important determinant of nuclear architecture, but additionally play essential roles in many nuclear functions. Mutations in A-type lamins cause a wide range of human genetic disorders (laminopathies). The importance of lamin A (LaA) in the spatial arrangement of nuclear pore complexes (NPCs) prompted us to study the role of LaA mutants in nuclear protein transport. Two mutants, causing prenatal skin disease restrictive dermopathy (RD) and the premature aging disease Hutchinson Gilford progeria syndrome, were used for expression in HeLa cells to investigate their impact on the subcellular localization of NPC-associated proteins and nuclear protein import. Furthermore, dynamics of the LaA mutants within the nuclear lamina were studied. We observed affected localization of NPC-associated proteins, diminished lamina dynamics for both LaA mutants and reduced nuclear import of representative cargo molecules. Intriguingly, both LaA mutants displayed similar effects on nuclear morphology and functions, despite their differences in disease severity. Reduced nuclear protein import was also seen in RD fibroblasts and impaired lamina dynamics for the nucleoporin Nup153. Our data thus represent the first study of a direct link between LaA mutant expression and reduced nuclear protein import.

ZMPSTE24, an integral membrane zinc metalloprotease with a connection to progeroid disorders

ZMPSTE24 is an integral membrane zinc metalloprotease originally discovered in yeast as an enzyme (called Ste24p) required for maturation of the mating pheromone a-factor. Surprisingly, ZMPSTE24 has recently emerged as a key protease involved in human progeroid disorders. ZMPSTE24 has only one identified mammalian substrate, the precursor of the nuclear scaffold protein lamin A. ZMPSTE24 performs a critical endoproteolytic cleavage step that removes the hydrophobic farnesyl-modified tail of prelamin A. Failure to do so has drastic consequences for human health and longevity. Here, we discuss the discovery of the yeast and mammalian ZMPSTE24 orthologs and review the unexpected connection between ZMPSTE24 and premature aging.

Accelerated features of age-related bone loss in zmpste24 metalloproteinase-deficient mice

Age-related bone loss is associated with changes in bone cellularity, which include marrow fat infiltration and decreasing levels of osteoblastogenesis. The mechanisms that explain these changes remain unclear. Although nuclear lamina alterations occur in premature aging syndromes that include changes in body fat and severe osteoporosis, the role of proteins of the nuclear lamina in age-related bone loss remains unknown. Using the Zmpste24-null progeroid mice (Zmpste24(-/-)), which exhibit nuclear lamina defects and accumulate unprocessed prelamin A, we identified several alterations in bone cellularity in vivo. We found that defective prelamin A processing induced accelerated features of age-related bone loss including lower osteoblast and osteocyte numbers and higher levels of marrow adipogenesis. In summary, processing of prelamin A could become a new approach to regulate osteoblastogenesis and bone turnover and thus for the prevention and treatment of senile osteoporosis.

Impaired nuclear functions lead to increased senescence and inefficient differentiation in human myoblasts with a dominant p.R545C mutation in the LMNA gene

We have studied myoblasts from a patient with a severe autosomal dominant Emery-Dreifuss muscular dystrophy (AD-EDMD) caused by an arginine 545 to cystein point mutation (p.R545C) in the carboxy-terminal domain of the lamin A/C gene. This mutation has pleiotropic cellular effects on these myoblasts as demonstrated by nuclear structural defects, exhibiting lobulations which increase with cell passages in culture. The organization of both lamin A/C and its inner nuclear membrane partner emerin are altered, eventually showing a honeycomb pattern upon immunofluorescence microscopy. In addition, the distribution of histone H3 trimethylated at lysine 27 and of phosphorylated RNA polymerase II, markers of inactive and active chromatin domains, respectively, are altered suggesting an impact on gene expression. Patient myoblasts also presented a high index of senescence in ex vivo culture. Moreover, our data show for the first time in an AD-EDMD context that the 20S core particle of the proteasome was inactivated. With cell passages, the 20S core protein progressively accumulated into discrete nuclear foci that largely colocalized with promyelocytic leukemia (PML) bodies while p21 accumulated throughout the nuclear compartment. Proteasome inactivation has been linked to normal cellular ageing. Our data indicate that it may also contribute to premature senescence in AD-EDMD patient myoblasts. Finally, when transferred to low-serum medium, patient myoblasts were deficient in ex vivo differentiation, as assessed by the absence of myotube formation and myogenin induction. Altogether, these data suggest that the LMNA mutation p.R545C impairs both proliferation and differentiation capacities of myoblasts as part of the pathogenesis of AD-EDMD.

Intermediate filaments: primary determinants of cell architecture and plasticity

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

Laminopathies and the long strange trip from basic cell biology to therapy

The main function of the nuclear lamina, an intermediate filament meshwork lying primarily beneath the inner nuclear membrane, is to provide structural scaffolding for the cell nucleus. However, the lamina also serves other functions, such as having a role in chromatin organization, connecting the nucleus to the cytoplasm, gene transcription, and mitosis. In somatic cells, the main protein constituents of the nuclear lamina are lamins A, C, B1, and B2. Interest in the nuclear lamins increased dramatically in recent years with the realization that mutations in LMNA, the gene encoding lamins A and C, cause a panoply of human diseases ("laminopathies"), including muscular dystrophy, cardiomyopathy, partial lipodystrophy, and progeroid syndromes. Here, we review the laminopathies and the long strange trip from basic cell biology to therapeutic approaches for these diseases.

The role of genetic variation in the lamin a/c gene in the etiology of polycystic ovary syndrome

OBJECTIVE

We performed this study to test the hypothesis that variation in the lamin a/c gene (LMNA) contributes to milder phenotypes of insulin resistance, hyperandrogenism, and/or metabolic syndrome associated with polycystic ovary syndrome (PCOS).

RESEARCH DESIGN AND METHODS

We resequenced the coding region, flanking intronic, and proximal promoter regions of the lamin a/c gene in 43 women with PCOS with evidence of upper-body obesity (waist circumference >88 cm) and identified 56 variants, two of which were nonsynonymous substitutions (lmna11 exon1 E98D; lmna24 exon 7 R455C). We genotyped 53 single-nucleotide polymorphisms (44 identified through resequencing and nine included to maximize informativeness of the entire gene) in 624 index (PCOS) cases and 544 controls of European ancestry. We tested for association between these variants and PCOS. In a subset of individuals, we also tested for association with metabolic syndrome and quantitative traits (body mass index, waist circumference, total testosterone, dehydroepiandrosterone sulfate, fasting glucose and insulin, low-density lipoprotein, and total triglycerides).

RESULTS

After correction for multiple testing, none of the variants showed significant evidence for association with PCOS, the metabolic syndrome, or any of the quantitative traits tested.

CONCLUSIONS

Whereas these studies cannot exclude the role of genetic variation in the lamin a/c gene in isolated cases of PCOS, we can conclude that common variation in the lamin a/c gene does not contribute to the etiology of PCOS in women of European ancestry.

Evidence for association between polycystic ovary syndrome (PCOS) and TCF7L2 and glucose intolerance in women with PCOS and TCF7L2

CONTEXT AND OBJECTIVE

Of the recently identified type 2 diabetes mellitus (T2D) susceptibility loci, transcription factor 7-like 2 (TCF7L2) confers the greatest relative risk for T2D and significantly predicts conversion to T2D in persons with impaired glucose tolerance. TCF7L2 is, therefore, also a strong candidate gene for polycystic ovary syndrome (PCOS), a common endocrine disorder characterized by androgen excess and menstrual irregularities and associated with insulin resistance and a 7-fold increased risk for T2D.

RESEARCH DESIGN AND METHODS

We tested for association between 58 single nucleotide polymorphisms mapping to TCF7L2 and PCOS in 624 index (PCOS) cases and 553 control women of European ancestry. Furthermore, in the women with PCOS, we tested for association with seven reproductive and metabolic quantitative traits.

RESULTS

Although we did not detect evidence for association between the previously described TCF7L2 T2D locus, the proinsulin:insulin molar ratio, a marker of pancreatic beta-cell dysfunction, was strongly associated with this locus (P = 2.1 x 10(-4)). We also observed evidence for association between PCOS and two single nucleotide polymorphisms, rs11196236 (P = 9.0 x 10(-4)) and rs11196229 (P = 0.0027) mapping more than 100 kb centromeric to the previously published T2D susceptibility loci.

CONCLUSIONS

We have observed evidence of association with two independent TCF7L2 loci in a PCOS cohort: 1) association between the proinsulin:insulin molar ratio and the T2D locus; and 2) association with reproductive PCOS phenotype and a novel locus. This study suggests that variation in different regions of a susceptibility gene contributes to distinct phenotypes.

Mechanism and a peptide motif for targeting peripheral proteins to the yeast inner nuclear membrane

Trm1 is a tRNA specific m(2)(2)G methyltransferase shared by nuclei and mitochondria in Saccharomyces cerevisiae. In nuclei, Trm1 is peripherally associated with the inner nuclear membrane (INM). We investigated the mechanism delivering/tethering Trm1 to the INM. Analyses of mutations of the Ran pathway and nuclear pore components showed that Trm1 accesses the nucleoplasm via the classical nuclear import pathway. We identified a Trm1 cis-acting sequence sufficient to target passenger proteins to the INM. Detailed mutagenesis of this region uncovered specific amino acids necessary for authentic Trm1 to locate at the INM. The INM information is contained within a sequence of less than 20 amino acids, defining the first motif for addressing a peripheral protein to this important subnuclear location. The combined studies provide a multi-step process to direct Trm1 to the INM: (i) translation in the cytoplasm; (ii) Ran-dependent import into the nucleoplasm; and (iii) redistribution from the nucleoplasm to the INM via the INM motif. Furthermore, we demonstrate that the Trm1 mitochondrial targeting and nuclear localization signals are in competition with each other, as Trm1 becomes mitochondrial if prevented from entering the nucleus.

Nuclear envelope-limited chromatin sheets (ELCS) and heterochromatin higher order structure

The interphase nucleus and nuclear envelope can acquire a myriad of shapes in normal or pathological cell states. There exist a wide variety of indentations and invaginations, of protrusions and evaginations. It has been difficult to classify and name all of these nuclear shapes and, consequently, a barrier to understanding the biochemical and biophysical causes. This review focuses upon one type of nuclear envelope shape change, named "nuclear envelope-limited chromatin sheets" (ELCS), which appears to involve exaggerated nuclear envelope growth, carrying with it one or more layers of approximately 30 nm diameter heterochromatin. A hypothesis on the formation of ELCS is proposed, relating higher order heterochromatin structure in an interphase nucleus, nuclear envelope growth, and nuclear envelope-heterochromatin interactions.

Nuclear lamins: key regulators of nuclear structure and activities

The nuclear lamina is a proteinaceous structure located underneath the inner nuclear membrane (INM), where it associates with the peripheral chromatin. It contains lamins and lamin-associated proteins, including many integral proteins of the INM, chromatin modifying proteins, transcriptional repressors and structural proteins. A fraction of lamins is also present in the nucleoplasm, where it forms stable complexes and is associated with specific nucleoplasmic proteins. The lamins and their associated proteins are required for most nuclear activities, mitosis and for linking the nucleoplasm to all major cytoskeletal networks in the cytoplasm. Mutations in nuclear lamins and their associated proteins cause about 20 different diseases that are collectively called laminopathies'. This review concentrates mainly on lamins, their structure and their roles in DNA replication, chromatin organization, adult stem cell differentiation, aging, tumorogenesis and the lamin mutations leading to laminopathic diseases.

A nuclear-envelope bridge positions nuclei and moves chromosomes

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

SUN-domain and KASH-domain proteins during development, meiosis and disease

SUN-domain proteins interact directly with KASH-domain proteins to form protein complexes that connect the nucleus to every major cytoskeleton network. SUN-KASH protein complexes are also required for attaching centrosomes to the nuclear periphery and for alignment of homologous chromosomes, their pairing and recombination in meiosis. Other functions that require SUN-domain proteins include the regulation of apoptosis and maturation and survival of the germline. Laminopathic diseases affect the distribution of the SUN-KASH complexes, and mutations in KASH-domain proteins can cause Emery Dreifuss muscular dystrophy and recessive cerebellar ataxia. This review describes our current knowledge of the role of SUN-KASH domain protein complexes during development, meiosis and disease.

Activating the synthesis of progerin, the mutant prelamin A in Hutchinson-Gilford progeria syndrome, with antisense oligonucleotides

Hutchinson-Gilford progeria syndrome (HGPS) is caused by point mutations that increase utilization of an alternate splice donor site in exon 11 of LMNA (the gene encoding lamin C and prelamin A). The alternate splicing reduces transcripts for wild-type prelamin A and increases transcripts for a truncated prelamin A (progerin). Here, we show that antisense oligonucleotides (ASOs) against exon 11 sequences downstream from the exon 11 splice donor site promote alternate splicing in both wild-type and HGPS fibroblasts, increasing the synthesis of progerin. Indeed, wild-type fibroblasts transfected with these ASOs exhibit progerin levels similar to (or greater than) those in fibroblasts from HGPS patients. This progerin was farnesylated, as judged by metabolic labeling studies. The synthesis of progerin in wild-type fibroblasts was accompanied by the same nuclear shape and gene-expression perturbations observed in HGPS fibroblasts. An ASO corresponding to the 5' portion of intron 11 also promoted alternate splicing. In contrast, an ASO against exon 11 sequences 5' to the alternate splice site reduced alternate splicing in HGPS cells and modestly lowered progerin levels. Thus, different ASOs can be used to increase or decrease 'HGPS splicing'. ASOs represent a new and powerful tool for recreating HGPS pathophysiology in wild-type cells.

Centrosome attachment to the C. elegans male pronucleus is dependent on the surface area of the nuclear envelope

A close association must be maintained between the male pronucleus and the centrosomes during pronuclear migration. In C. elegans, simultaneous depletion of inner nuclear membrane LEM proteins EMR-1 and LEM-2, depletion of the nuclear lamina proteins LMN-1 or BAF-1, or the depletion of nuclear import components leads to embryonic lethality with small pronuclei. Here, a novel centrosome detachment phenotype in C. elegans zygotes is described. Zygotes with defects in the nuclear envelope had small pronuclei with a single centrosome detached from the male pronucleus. ZYG-12, SUN-1, and LIS-1, which function at the nuclear envelope with dynein to attach centrosomes, were observed at normal concentrations on the nuclear envelope of pronuclei with detached centrosomes. Analysis of time-lapse images showed that as mutant pronuclei grew in surface area, they captured detached centrosomes. Larger tetraploid or smaller histone::mCherry pronuclei suppressed or enhanced the centrosome detachment phenotype respectively. In embryos fertilized with anucleated sperm, only one centrosome was captured by small female pronuclei, suggesting the mechanism of capture is dependent on the surface area of the outer nuclear membrane available to interact with aster microtubules. We propose that the limiting factor for centrosome attachment to the surface of abnormally small pronuclei is dynein.

CNV and nervous system diseases--what's new?

Several new genomic disorders caused by copy number variation (CNV) of genes whose dosage is critical for the physiological function of the nervous system have been recently identified. Dup(7)(q11.23) patients carry duplications of the genomic region deleted in Williams-Beuren syndrome, they are characterized by prominent speech delay. The phenotypes of Potocki-Lupski syndrome and MECP2 duplication syndrome were neuropsychologically examined in detail, which revealed autism as an endophenotype and a prominent behavioral feature of these disorders. Tandem duplication of LMNB1 was reported to cause adult-onset autosomal dominant leukodystrophy. PAFAH1B1/LIS1 and YWHAE, which were deleted in isolated lissencephaly (PAFAH1B1/LIS1 alone) and Miller-Dieker syndrome (both genes), were found to be duplicated in patients with developmental delay. Finally, two novel microdeletion syndromes affecting 17q21.31 and 15q13.3, as well as their reciprocal duplications, were also identified. In this review, we provide an overview of the phenotypic manifestation of these syndromes and the rearrangements causing them.

Congenital muscular dystrophy. Part I: a review of phenotypical and diagnostic aspects

The congenital muscular dystrophies (CMDs) are a group of genetically and clinically heterogeneous hereditary myopathies with preferentially autosomal recessive inheritance, that are characterized by congenital hypotonia, delayed motor development and early onset of progressive muscle weakness associated with dystrophic pattern on muscle biopsy. The clinical course is broadly variable and can comprise the involvement of the brain and eyes. From 1994, a great development in the knowledge of the molecular basis has occurred and the classification of CMDs has to be continuously up dated. We initially present the main clinical and diagnostic data concerning the CMDs related to changes in the complex dystrophin-associated glycoproteins-extracellular matrix: CMD with merosin deficiency (CMD1A), collagen VI related CMDs (Ullrich CMD and Bethlem myopathy), CMDs with abnormal glycosylation of alpha-dystroglycan (Fukuyama CMD, Muscle-eye-brain disease, Walker-Warburg syndrome, CMD1C, CMD1D), and the much rarer CMD with integrin deficiency. Finally, we present other forms of CMDs not related with the dystrophin/glycoproteins/extracellular matrix complex (rigid spine syndrome, CMD1B, CMD with lamin A/C deficiency), and some apparently specific clinical forms not yet associated with a known molecular mechanism. The second part of this review concerning the pathogenesis and therapeutic perspectives of the different subtypes of CMD will be described in a next number.

Microcephalia with mandibular and dental dysplasia in adult Zmpste24-deficient mice

ZMPSTE24 (also called FACE-1) is a zinc-metalloprotease involved in the post-translational processing of prelamin A to mature lamin A, a major component of the nuclear envelope. Mutations in the ZMPSTE24 gene or in that encoding its substrate prelamin A (LMNA) result in a series of human inherited diseases known collectively as laminopathies and showing regional or systemic manifestations (i.e. the Hutchinson-Gilford progeria syndrome). Typically, patients suffering some laminopathies show craniofacial or mandible anomalies, aberrant dentition or facial features characteristic of aged persons. To analyse whether Zmpste24(-/-) mice reproduce the cranial phenotype observed in humans due to mutations in ZMPSTE24 or LMNA, we conducted a craniometric study based on micro-computer tomography (microCT) images. Furthermore, using simple radiology, microCT, microCT-densitometry and scanning electron microscopy, we analysed the mandible and the teeth from Zmpste24(-/-) mice. Finally, the structure of the lower incisor was investigated using an H&E technique. The results demonstrate that Zmpste24(-/-) mice are microcephalic and show mandibular and dental dysplasia affecting only the mandible teeth. In all cases, the lower incisor of mice lacking Zmpste24 was smaller than in control animals, showed cylindrical morphology and a transverse fissure at the incisal edge, and the pulpal cavity was severely reduced. Structurally, the dental layers were normally arranged but cellular layers were disorganized. The inferior molars showed a reduced cusp size. Taken together, these data strongly suggest that Zmpste24(-/-) mice represent a good model to analyse the craniofacial and teeth malformations characteristic of lamin-related pathologies, and might contribute to a better understanding of the molecular events underlying these diseases.

Validation of high-resolution DNA melting analysis for mutation scanning of the LMNA gene

OBJECTIVES

LMNA mutations lead to a wide spectrum of disorders now called laminopathies. Due to large cohorts to investigate, mutational screening must be performed using an extremely sensitive and specific scanning method.

DESIGN AND METHODS

High Resolution Melting (HRM) analysis was developed for LMNA mutation detection. A cohort of 64 patients with dilated cardiomyopathy was prospectively screened using both HRM and DHPLC methodologies.

RESULTS

All gene variants detected by DHPLC or by direct sequencing were also readily identified as abnormal by HRM analysis. Mutations were identified in 7 patients (approximately 11%). Complete molecular LMNA investigation was completed two times faster and cheaper than using DHPLC strategy.

CONCLUSIONS

HRM analysis represents an inexpensive, highly sensitive and high-throughput method to identify LMNA genetic variants. The discovery of novel LMNA mutations will provide new insights into the pathophysiology of dilated cardiomyopathy and in all other laminopathies.

Functions of the nuclear envelope and lamina in development and disease

Recent findings that some 24 inherited diseases and anomalies are caused by defects in proteins of the NE (nuclear envelope) and lamina have resulted in a fundamental reassessment of the functions of the NE and underlying lamina. Instead of just regarding the NE and lamina as a molecular filtering device, regulating the transfer of macromolecules between the cytoplasm and nucleus, we now envisage the NE/lamina functioning as a key cellular 'hub' in integrating critical functions that include chromatin organization, transcriptional regulation, mechanical integrity of the cell and signalling pathways, as well as acting as a key component in the organization and function of the cytoskeleton.

Exploring the effects of a dysfunctional nuclear matrix

The nuclear matrix has remained a contentious structure for decades; many believe that it is an artefact of harsh non-physiological procedures. However, its visualization using milder experimental techniques is leading to its general acceptance by the scientific community. It is a permanent network of core filaments underlying thicker fibres which is proposed to be a platform for numerous important nuclear activities such as transcription and DNA repair. Interestingly, A- and B-type lamin proteins and emerin are components of this nuclear structure; however, they are often referred to only as nuclear envelope proteins. The present mini-review intends to provide an overview of the nuclear matrix, mentioning both its constituents and functional significance. The impact of disease-causing mutations in both emerin and lamin proteins on the structure's ability to regulate and mediate nuclear processes is then discussed.

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

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

Lamin A/C-mediated neuromuscular junction defects in Emery-Dreifuss muscular dystrophy

The LMNA gene encodes lamins A and C, two intermediate filament-type proteins that are important determinants of interphase nuclear architecture. Mutations in LMNA lead to a wide spectrum of human diseases including autosomal dominant Emery-Dreifuss muscular dystrophy (AD-EDMD), which affects skeletal and cardiac muscle. The cellular mechanisms by which mutations in LMNA cause disease have been elusive. Here, we demonstrate that defects in neuromuscular junctions (NMJs) are part of the disease mechanism in AD-EDMD. Two AD-EDMD mouse models show innervation defects including misexpression of electrical activity–dependent genes and altered epigenetic chromatin modifications. Synaptic nuclei are not properly recruited to the NMJ because of mislocalization of nuclear envelope components. AD-EDMD patients with LMNA mutations show the same cellular defects as the AD-EDMD mouse models. These results suggest that lamin A/C–mediated NMJ defects contribute to the AD-EDMD disease phenotype and provide insights into the cellular and molecular mechanisms for the muscle-specific phenotype of AD-EDMD.

Clinical utility of genetic tests for inherited hypertrophic and dilated cardiomyopathies

Genetic testing has become an increasingly important part of medical practice for heritable form of cardiomyopathies. Hypertrophic cardiomyopathy and about 50% of idiopathic dilatative cardiomyopathy are familial diseases, with an autosomal dominant pattern of inheritance.

Some genotype-phenotype correlations can provide important information to target DNA analyses in specific genes. Genetic testing may clarify diagnosis and help the optimal treatment strategies for more malignant phenotypes. In addition, genetic screening of first-degree relatives can help early identification and diagnosis of individuals at greatest risk for developing cardiomyopathy, allowing to focus clinical resources on high-risk family members.

This paper provides a concise overview of the genetic etiology as well as the clinical utilities and limitations of genetic testing for the heritable cardiomyopathies.

The supramolecular organization of the C. elegans nuclear lamin filament

Nuclear lamins are involved in most nuclear activities and are essential for retaining the mechano-elastic properties of the nucleus. They are nuclear intermediate filament (IF) proteins forming a distinct meshwork-like layer adhering to the inner nuclear membrane, called the nuclear lamina. Here, we present for the first time, the three-dimensional supramolecular organization of lamin 10 nm filaments and paracrystalline fibres. We show that Caenorhabditis elegans nuclear lamin forms 10 nm IF-like filaments, which are distinct from their cytoplasmic counterparts. The IF-like lamin filaments are composed of three and four tetrameric protofilaments, each of which contains two partially staggered anti-parallel head-to-tail polymers. The beaded appearance of the lamin filaments stems from paired globular tail domains, which are spaced regularly, alternating between 21 nm and 27 nm. A mutation in an evolutionarily conserved residue that causes Hutchison-Gilford progeria syndrome in humans alters the supramolecular structure of the lamin filaments. On the basis of our structural analysis, we propose an assembly pathway that yields the observed 10 nm IF-like lamin filaments and paracrystalline fibres. These results serve also as a platform for understanding the effect of laminopathic mutations on lamin supramolecular organization.

Fast regulation of AP-1 activity through interaction of lamin A/C, ERK1/2, and c-Fos at the nuclear envelope

Sequestration of c-Fos at the nuclear envelope (NE) through interaction with A-type lamins suppresses AP-1–dependent transcription. We show here that c-Fos accumulation within the extraction-resistant nuclear fraction (ERNF) and its interaction with lamin A are reduced and enhanced by gain-of and loss-of ERK1/2 activity, respectively. Moreover, hindering ERK1/2-dependent phosphorylation of c-Fos attenuates its release from the ERNF induced by serum and promotes its interaction with lamin A. Accordingly, serum stimulation rapidly releases preexisting c-Fos from the NE via ERK1/2-dependent phosphorylation, leading to a fast activation of AP-1 before de novo c-Fos synthesis. Moreover, lamin A–null cells exhibit increased AP-1 activity and reduced levels of c-Fos phosphorylation. We also find that active ERK1/2 interacts with lamin A and colocalizes with c-Fos and A-type lamins at the NE. Thus, NE-bound ERK1/2 functions as a molecular switch for rapid mitogen-dependent AP-1 activation through phosphorylation-induced release of preexisting c-Fos from its inhibitory interaction with lamin A/C.

Adult stem cell maintenance and tissue regeneration in the ageing context: the role for A-type lamins as intrinsic modulators of ageing in adult stem cells and their niches

Adult stem cells have been identified in most mammalian tissues of the adult body and are known to support the continuous repair and regeneration of tissues. A generalized decline in tissue regenerative responses associated with age is believed to result from a depletion and/or a loss of function of adult stem cells, which itself may be a driving cause of many age-related disease pathologies. Here we review the striking similarities between tissue phenotypes seen in many degenerative conditions associated with old age and those reported in age-related nuclear envelope disorders caused by mutations in the LMNA gene. The concept is beginning to emerge that nuclear filament proteins, A-type lamins, may act as signalling receptors in the nucleus required for receiving and/or transducing upstream cytosolic signals in a number of pathways central to adult stem cell maintenance as well as adaptive responses to stress. We propose that during ageing and in diseases caused by lamin A mutations, dysfunction of the A-type lamin stress-resistant signalling network in adult stem cells, their progenitors and/or stem cell niches leads to a loss of protection against growth-related stress. This in turn triggers an inappropriate activation or a complete failure of self-renewal pathways with the consequent initiation of stress-induced senescence. As such, A-type lamins should be regarded as intrinsic modulators of ageing within adult stem cells and their niches that are essential for survival to old age.

Epigenetic mechanisms of facioscapulohumeral muscular dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) seems to be caused by a complex epigenetic disease mechanism as a result of contraction of the polymorphic macrosatellite repeat D4Z4 on chromosome 4qter. Currently, the exact mechanism causing the FSHD phenotype is still not elucidated. In this review, we discuss the genetic and epigenetic changes observed in patients with FSHD and the possible disease mechanisms that may be associated with FSHD pathogenesis.

Does satellite cell dysfunction contribute to disease progression in Emery–Dreifuss muscular dystrophy?

Muscular dystrophies comprise at least 34 conditions, characterized by progressive skeletal muscle weakness and degeneration. The loci affected include mutations in both muscle-specific genes and genes that are more widely expressed such as LMNA and EMD, responsible for EDMD (Emery–Dreifuss muscular dystrophy). LMNA encodes A-type lamins, whereas EMD encodes emerin, both located in the nuclear envelope. Mutation or loss of A-type lamins or emerin in the terminally differentiated myonuclei of muscle fibres results in muscle damage. Importantly, since LMNA and EMD are also expressed by the resident skeletal muscle stem cells, the satellite cells, the mutations that cause muscle damage may also directly compromise the regenerative response. Thus EDMD is different from dystrophic conditions such as Duchenne muscular dystrophy, where the mutated gene is only expressed in the muscle fibres. In this brief review, we examine the evidence that myoblasts carrying EDMD-causing mutations are compromised, and discuss the possibility that such dysfunction results in reduced efficiency of muscle regeneration, so actively contributes to disease progression.

Lamin A: a putative colonic epithelial stem cell biomarker which identifies colorectal tumours with a more aggressive phenotype

Abnormalities in the expression, distribution and structural organization of A-type lamins are most commonly associated with a spectrum of inherited disorders which predominantly affect mesenchymal lineages, collectively known as laminopathies. However, a new role for lamin A has been discovered in the progression of a common epithelial cancer. CRC (colorectal cancer) patients expressing lamin A/C in their tumour tissue were found to have a 2-fold greater risk of CRC-related mortality compared with patients with lamin A/C-negative tumours. Consequently, lamin A/C is a prognostic biomarker in CRC. In vitro studies suggest that lamin A is an upstream regulator of a pathway linking actin dynamics to loss of cell adhesion, leading to enhanced cell motility and consequently increased invasive potential within a tumour. The finding that lamin A is a putative colonic epithelial stem cell biomarker suggests that the poor outcome associated with lamin A/C-positive tumours may be reflective of a more stem-cell-like phenotype. The present review discusses the link between lamin A expression and tumour progression in one of the commonest causes of cancer-related death in the Western world.

Inner nuclear membrane protein transport is mediated by multiple mechanisms

Work in the nuclear transport field has led to an incredibly detailed description of protein translocation through the central channel of the nuclear pore complex, yet the mechanism by which nuclear envelope transmembrane proteins reach the inner nuclear membrane after synthesis in the endoplasmic reticulum is still hotly debated. Three different translocation models have gained experimental support: (i) simple lateral diffusion through the nuclear envelope membrane system; (ii) translocation by vesicle fusion events; and (iii) a variation on classical transport mediated by the nuclear pore complex. Although these models appear to be mutually exclusive, in the present paper we argue that they probably all function for different inner nuclear membrane proteins according to their unique characteristics.

Progerin elicits disease phenotypes of progeria in mice whether or not it is farnesylated

Hutchinson-Gilford progeria syndrome (HGPS), a rare disease that results in what appears to be premature aging, is caused by the production of a mutant form of prelamin A known as progerin. Progerin retains a farnesyl lipid anchor at its carboxyl terminus, a modification that is thought to be important in disease pathogenesis. Inhibition of protein farnesylation improves the hallmark nuclear shape abnormalities in HGPS cells and ameliorates disease phenotypes in mice harboring a knockin HGPS mutation (LmnaHG/+). The amelioration of disease, however, is incomplete, leading us to hypothesize that nonfarnesylated progerin also might be capable of eliciting disease. To test this hypothesis, we created knockin mice expressing nonfarnesylated progerin (LmnanHG/+). LmnanHG/+ mice developed the same disease phenotypes observed in LmnaHG/+ mice, although the phenotypes were milder, and mouse embryonic fibroblasts (MEFs) derived from these mice contained fewer misshapen nuclei. The steady-state levels of progerin in LmnanHG/+ MEFs and tissues were lower, suggesting a possible explanation for the milder phenotypes. These data support the concept that inhibition of protein farnesylation in progeria could be therapeutically useful but also suggest that this approach may be limited, as progerin elicits disease phenotypes whether or not it is farnesylated.

Reduced expression of A-type lamins and emerin activates extracellular signal-regulated kinase in cultured cells

BACKGROUND

Mutations in genes encoding A-type lamins and emerin cause cardiomyopathy and muscular dystrophy. We previously showed activation of the extracellular signal-regulated kinase (ERK) branch of the mitogen-activated protein kinase (MAPK) cascade in hearts of mice with mutations in these genes. Here, we tested the hypothesis that reducing A-type lamins and emerin in cultured cells activate ERK signaling.

METHODS

We used siRNA to knockdown A-type lamins and emerin in HeLa and C2C12 cells. Activation of ERK was assessed by immunoblotting and immunofluorescence microscopy with antibodies against phosphorylated protein and by using real-time RT-PCR to measure RNAs encoded by genes for transcription factors stimulated by ERK.

RESULTS

Knockdown of A-type lamins and emerin in HeLa and C2C12 stimulated phosphorylation and nuclear translocation of ERK as well as activation of genes encoding downstream transcription factors. A MAPK/ERK kinase (MEK) inhibitor reduced ERK phosphorylation in cells with reduced expression of A-type lamins and emerin.

CONCLUSIONS

These results provide proof for the hypothesis that altered expression of emerin and A-type lamins activates ERK signaling, which in turn can cause cardiomyopathy.

GENERAL SIGNIFICANCE

ERK is a potential target for the pharmacological treatment of cardiomyopathy caused by mutations in the genes encoding emerin and A-type lamins.

Long-term outcome and risk stratification in dilated cardiolaminopathies

OBJECTIVES

The aim of this study was to analyze the long-term follow-up of dilated cardiolaminopathies.

BACKGROUND

Lamin A/C (LMNA) gene mutations cause a variety of phenotypes. In the cardiology setting, patients diagnosed with idiopathic dilated cardiomyopathy (DCM) plus atrioventricular block (AVB) constitute the majority of reported cases.

METHODS

Longitudinal retrospective observational studies were conducted with 27 consecutive families in which LMNA gene defects were identified in the probands, all sharing the DCM phenotype.

RESULTS

Of the 164 family members, 94 had LMNA gene mutations. Sixty of 94 (64%) were phenotypically affected whereas 34 were only genotypically affected, including 5 with pre-clinical signs. Of the 60 patients, 40 had DCM with AVB, 12 had DCM with ventricular tachycardia/fibrillation, 6 had DCM with AVB and Emery-Dreifuss muscular dystrophy type 2 (EDMD2), and 2 had AVB plus EDMD2. During a median of 57 months (interquartile range 36 to 107 months), we observed 49 events in 43 DCM patients (6 had a later event, excluded from the analysis). The events were related to heart failure (15 heart transplants, 1 death from end-stage heart failure) and ventricular arrhythmias (15 sudden cardiac deaths and 12 appropriate implantable cardioverter-defibrillator interventions). By multivariable analysis, New York Heart Association functional class III to IV and highly dynamic competitive sports for >or=10 years were independent predictors of total events. By a bivariable Cox model, splice site mutations and competitive sport predicted sudden cardiac death.

CONCLUSIONS

Dilated cardiomyopathies caused by LMNA gene defects are highly penetrant, adult onset, malignant diseases characterized by a high rate of heart failure and life-threatening arrhythmias, predicted by New York Heart Association functional class, competitive sport activity, and type of mutation.

Inhibition of extracellular signal-regulated kinase signaling to prevent cardiomyopathy caused by mutation in the gene encoding A-type lamins

Autosomal Emery-Dreifuss muscular dystrophy and related disorders with dilated cardiomyopathy and variable skeletal muscle involvement are caused by mutations in LMNA, which encodes A-type nuclear lamins. How alterations in A-type lamins, intermediate filament proteins of the nuclear envelope expressed in most differentiated somatic cells, cause cardiomyopathy is only poorly understood. We demonstrated previously abnormal activation of the extracellular signal-regulated kinase (ERK) branch of the mitogen-activated protein kinase (MAPK) signaling cascade in hearts of Lmna H222P 'knock in' mice, a model of autosomal Emery-Dreifuss muscular dystrophy. We therefore treated Lmna(H222P/H222P) mice that develop cardiomyopathy with PD98059, an inhibitor of ERK activation. Systemic treatment of Lmna(H222P/H222P) mice with PD98059 inhibited ERK phosphorylation and blocked the activation of downstream genes in heart. It also blocked increased expression of RNAs encoding natriuretic peptide precursors and proteins involved in sarcomere organization that occurred in placebo-treated mice. Histological analysis and echocardiography demonstrated that treatment with PD98059 delayed the development of left ventricular dilatation. PD98059-treated Lmna(H222P/H222P) mice had normal cardiac ejection fractions assessed by echocardiography when placebo-treated mice had a 30% decrease. These results emphasize the role of ERK activation in the development of cardiomyopathy caused by LMNA mutations. They further provide proof of principle for ERK inhibition as a therapeutic option to prevent or delay heart failure in humans with Emery-Dreifuss muscular dystrophy and related disorders caused by mutations in LMNA.

The lamin B receptor under transcriptional control of C/EBPepsilon is required for morphological but not functional maturation of neutrophils

The lamin B receptor (LBR) is an integral nuclear envelope protein that interacts with chromatin and has homology to sterol reductases. Mutations in LBR result in Pelger-Huët anomaly and HEM-Greenberg skeletal dysplasia, whereas in mice Lbr mutations result in ichthyosis. To further understand the function of the LBR and its role in disease, we derived a novel mouse model with a gene-trap insertion into the Lbr locus (Lbr(GT/GT)). Phenotypically, the Lbr(GT/GT) mice are similar to ichthyosis mice. The Lbr(GT/GT) granulocytes lack a mature segmented nucleus and have a block in late maturation. Despite these changes in nuclear morphology, the innate granulocyte immune function in the killing of Staphylococcus aureus bacteria appears to be intact. Granulocyte differentiation requires the transcription factor C/EBPepsilon. We identified C/EBPepsilon binding sites within the Lbr promoter and used EMSAs and luciferase assays to show that Lbr is transcriptionally regulated by C/EBPepsilon. Our findings indicate that the Lbr(GT/GT) mice are a model for Pelger-Huët anomaly and that Lbr, under transcriptional regulation of C/EBPepsilon, is necessary for morphological but not necessarily functional granulocyte maturation.

Comparative proteomic analyses of the nuclear envelope and pore complex suggests a wide range of heretofore unexpected functions

Since the discovery of several inherited diseases linked to the nuclear envelope the number of functions ascribed to this subcellular organelle has skyrocketed. However the molecular pathways underlying these functions are not clear in most cases, perhaps because of missing components. Several recent proteomic analyses of the nuclear envelope and nuclear pore complex proteomes have yielded not only enough missing components to potentially elucidate these pathways, but suggest an exponentially greater number of functions at the nuclear periphery than ever imagined. Many of these functions appear to derive from recapitulation of pathways utilized at the plasma membrane and from other membrane systems. Additionally, many proteins identified in the comparative nuclear envelope studies have sequence characteristics suggesting that they might also contribute to nuclear pore complex functions. In particular, the striking enrichment for proteins in the nuclear envelope fractions that carry phenylalanine-glycine (FG) repeats may be significant for the mechanism of nuclear transport. In retrospect, these findings are only surprising in context of the notion held for many years that the nuclear envelope was only a barrier protecting the genome. In fact, it is arguably the most complex membrane organelle in the cell.