Both emerin and lamin C depend on lamin A for localization at the nuclear envelope

Hutterite-type cataract maps to chromosome 6p21.32-p21.31, cosegregates with a homozygous mutation in LEMD2, and is associated with sudden cardiac death

BACKGROUND

Juvenile-onset cataracts are known among the Hutterites of North America. Despite being identified over 30 years ago, this autosomal recessive condition has not been mapped, and the disease gene is unknown.

METHODS

We performed whole exome sequencing of three Hutterite-type cataract trios and follow-up genotyping and mapping in four extended kindreds.

RESULTS

Trio exomes enabled genome-wide autozygosity mapping, which localized the disease gene to a 9.5-Mb region on chromosome 6p. This region contained two candidate variants, LEMD2 c.T38G and MUC21 c.665delC. Extended pedigrees recruited for variant genotyping revealed multiple additional relatives with juvenile-onset cataract, as well as six deceased relatives with both cataracts and sudden cardiac death. The candidate variants were genotyped in 84 family members, including 17 with cataracts; only the variant in LEMD2 cosegregated with cataracts (LOD = 9.62). SNP-based fine mapping within the 9.5 Mb linked region supported this finding by refining the cataract locus to a 0.5- to 2.9-Mb subregion (6p21.32-p21.31) containing LEMD2 but not MUC21. LEMD2 is expressed in mouse and human lenses and encodes a LEM domain-containing protein; the c.T38G missense mutation is predicted to mutate a highly conserved residue within this domain (p.Leu13Arg).

CONCLUSION

We performed a genetic and genomic study of Hutterite-type cataract and found evidence for an association of this phenotype with sudden cardiac death. Using combined genetic and genomic approaches, we mapped cataracts to a small portion of chromosome 6 and propose that they result from a homozygous missense mutation in LEMD2.

Lamins position the nuclear pores and centrosomes by modulating dynein

Nuclear lamins counterbalance dynein forces on nuclear pore complexes through BICD2 and ensure even nuclear pore complex distribution and proper centrosome separation at prophase.

Lamins, the type V nuclear intermediate filament proteins, are reported to function in both interphase and mitosis. For example, lamin deletion in various cell types can lead to an uneven distribution of the nuclear pore complexes (NPCs) in the interphase nuclear envelope, whereas deletion of B-type lamins results in spindle orientation defects in mitotic neural progenitor cells. How lamins regulate these functions is unknown. Using mouse cells deleted of different combinations or all lamins, we show that lamins are required to prevent the aggregation of NPCs in the nuclear envelope near centrosomes in late G2 and prophase. This asymmetric NPC distribution in the absence of lamins is caused by dynein forces acting on NPCs via the dynein adaptor BICD2. We further show that asymmetric NPC distribution upon lamin depletion disrupts the distribution of BICD2 and p150 dynactin on the nuclear envelope at prophase, which results in inefficient dynein-driven centrosome separation during prophase. Therefore lamins regulate microtubule-based motor forces in vivo to ensure proper NPC distribution in interphase and centrosome separation in the mitotic prophase.

Ameliorating pathogenesis by removing an exon containing a missense mutation: a potential exon-skipping therapy for laminopathies

Exon skipping, as a therapy to restore a reading frame or switch protein isoforms, is under clinical trial. We hypothesised that removing an in-frame exon containing a mutation could also improve pathogenic phenotypes. Our model is laminopathies: incurable tissue-specific degenerative diseases associated with LMNA mutations. LMNA encodes A-type lamins, that together with B-type lamins, form the nuclear lamina. Lamins contain an alpha-helical central rod domain composed of multiple heptad repeats. Eliminating LMNA exon 3 or 5 removes six heptad repeats, so shortens, but should not otherwise significantly alter, the alpha-helix. Human Lamin A or Lamin C with a deletion corresponding to amino acids encoded by exon 5 (Lamin A/C-Δ5) localised normally in murine lmna-null cells, rescuing both nuclear shape and endogenous Lamin B1/emerin distribution. However, Lamin A carrying pathogenic mutations in exon 3 or 5, or Lamin A/C-Δ3, did not. Furthermore, Lamin A/C-Δ5 was not deleterious to wild-type cells, unlike the other Lamin A mutants including Lamin A/C-Δ3. Thus Lamin A/C-Δ5 function as effectively as wild-type Lamin A/C and better than mutant versions. Antisense oligonucleotides skipped LMNA exon 5 in human cells, demonstrating the possibility of treating certain laminopathies with this approach. This proof-of-concept is the first to report the therapeutic potential of exon skipping for diseases arising from missense mutations.

Nuclear lamins are not required for lamina-associated domain organization in mouse embryonic stem cells

In mammals, the nuclear lamina interacts with hundreds of large genomic regions, termed lamina-associated domains (LADs) that are generally in a transcriptionally repressed state. Lamins form the major structural component of the lamina and have been reported to bind DNA and chromatin. Here, we systematically evaluate whether lamins are necessary for the LAD organization in murine embryonic stem cells. Surprisingly, removal of essentially all lamins does not have any detectable effect on the genome-wide interaction pattern of chromatin with emerin, a marker of the inner nuclear membrane. This suggests that other components of the lamina mediate these interactions.

Lamins at the crossroads of mechanosignaling

The intermediate filament proteins, A- and B-type lamins, form the nuclear lamina scaffold adjacent to the inner nuclear membrane. Lamins also contribute to chromatin regulation and various signaling pathways affecting gene expression. In this review, Osmanagic-Myers et al. focus on the role of nuclear lamins in mechanosensing and also discuss how disease-linked lamin mutants may impair the response of cells to mechanical stimuli and influence the properties of the extracellular matrix.

The intermediate filament proteins, A- and B-type lamins, form the nuclear lamina scaffold adjacent to the inner nuclear membrane. B-type lamins confer elasticity, while A-type lamins lend viscosity and stiffness to nuclei. Lamins also contribute to chromatin regulation and various signaling pathways affecting gene expression. The mechanical roles of lamins and their functions in gene regulation are often viewed as independent activities, but recent findings suggest a highly cross-linked and interdependent regulation of these different functions, particularly in mechanosignaling. In this newly emerging concept, lamins act as a “mechanostat” that senses forces from outside and responds to tension by reinforcing the cytoskeleton and the extracellular matrix. A-type lamins, emerin, and the linker of the nucleoskeleton and cytoskeleton (LINC) complex directly transmit forces from the extracellular matrix into the nucleus. These mechanical forces lead to changes in the molecular structure, modification, and assembly state of A-type lamins. This in turn activates a tension-induced “inside-out signaling” through which the nucleus feeds back to the cytoskeleton and the extracellular matrix to balance outside and inside forces. These functions regulate differentiation and may be impaired in lamin-linked diseases, leading to cellular phenotypes, particularly in mechanical load-bearing tissues.

Characterization of unfolding mechanism of human lamin A Ig fold by single-molecule force spectroscopy-implications in EDMD

A- and B-type lamins are intermediate filament proteins constituting the nuclear lamina underneath the nuclear envelope thereby conferring proper shape and mechanical rigidity to the nucleus. Lamin proteins are also shown to be related diversely to basic nuclear processes. More than 400 mutations in human lamin A protein alone have been reported to produce at least 11 different disease conditions jointly termed as laminopathies. These mutations in lamin A are scattered throughout its helical rod domain, as well as the C-terminal domain containing the conserved Ig-fold region. The commonality of phenotypes in all these diseases is characterized by misshapen nuclei of the affected tissues which might stem from altered rigidity of the supporting lamina hence lamins. Here we have focused on autosomal dominant Emery-Dreifuss Muscular Dystrophy, one such laminopathy where R453W is the causative mutation located in the Ig domain of lamin A. We have investigated by single-molecule force spectroscopy how a stretching mechanical perturbation senses the destabilizing effect of the mutation in the lamin A Ig domain and compared the mechanoelastic properties of the mutant R453W with that of the wild-type in conjunction with steered molecular dynamics. Furthermore, we have shown the interaction of Ig domain with emerin, another key player and interacting partner in the pathogenesis of EDMD, is disrupted in the R453W mutant. This altered mechanoresistance of Ig domain itself and consequent uncoupling of lamin A-emerin interaction might underlie the altered mechanotransduction properties of EDMD affected nuclei.

Do lamin A and lamin C have unique roles?

The A-type lamins, lamin A and lamin C, generated from a single gene, LMNA, are major structural components of the nuclear lamina. The two alternative splice products have mostly been studied together because they have been considered to be interchangeable. However, several lines of evidence indicate that in spite of being generated from the same gene and having high similarities in their primary sequences, the two isoforms are not equivalent in different biological aspects in both health and disease. The key question is whether they have both overlapping and unique functions and whether they are distinctly regulated. Based on the so far available experimental evidence, lamin A appears to be the most regulated A-type isoform during development, aging, and disease which indicates that lamin A is implicated in many different biological aspects and may have a greater repertoire of specialized functions than lamin C. The aim of this review is to point out differences between the two major LMNA splice variants and the consequences of these differences on their functions. This may guide further research and be of prime importance for the understanding of the pathogenesis of LMNA mutations.

Concentration-dependent lamin assembly and its roles in the localization of other nuclear proteins

The nuclear lamina (NL) consists of lamin polymers and proteins that bind to the polymers. Disruption of NL proteins such as lamin and emerin leads to developmental defects and human diseases. However, the expression of multiple lamins, including lamin-A/C, lamin-B1, and lamin-B2, in mammals has made it difficult to study the assembly and function of the NL. Consequently, it has been unclear whether different lamins depend on one another for proper NL assembly and which NL functions are shared by all lamins or are specific to one lamin. Using mouse cells deleted of all or different combinations of lamins, we demonstrate that the assembly of each lamin into the NL depends primarily on the lamin concentration present in the nucleus. When expressed at sufficiently high levels, each lamin alone can assemble into an evenly organized NL, which is in turn sufficient to ensure the even distribution of the nuclear pore complexes. By contrast, only lamin-A can ensure the localization of emerin within the NL. Thus, when investigating the role of the NL in development and disease, it is critical to determine the protein levels of relevant lamins and the intricate shared or specific lamin functions in the tissue of interest.

WITHDRAWN: Nuclear matrix, nuclear envelope and premature aging syndromes in a translational research perspective

The Publisher regrets that this article is an accidental duplication of an article that has already been published, http://dx.doi.org/10.1016/j.semcdb.2014.03.022. The duplicate article has therefore been withdrawn. The full Elsevier Policy on Article Withdrawal can be found at http://www.elsevier.com/locate/withdrawalpolicy.

Nuclear matrix, nuclear envelope and premature aging syndromes in a translational research perspective

Lamin A-related progeroid syndromes are genetically determined, extremely rare and severe. In the past ten years, our knowledge and perspectives for these diseases has widely progressed, through the progressive dissection of their pathophysiological mechanisms leading to precocious and accelerated aging, from the genes mutations discovery until therapeutic trials in affected children. A-type lamins are major actors in several structural and functional activities at the nuclear periphery, as they are major components of the nuclear lamina. However, while this is usually poorly considered, they also play a key role within the rest of the nucleoplasm, whose defects are related to cell senescence. Although nuclear shape and nuclear envelope deformities are obvious and visible events, nuclear matrix disorganization and abnormal composition certainly represent the most important causes of cell defects with dramatic pathological consequences. Therefore, lamin-associated diseases should be better referred as laminopathies instead of envelopathies, this later being too restrictive, considering neither the key structural and functional roles of soluble lamins in the entire nucleoplasm, nor the nuclear matrix contribution to the pathophysiology of lamin-associated disorders and in particular in defective lamin A processing-associated aging diseases. Based on both our understanding of pathophysiological mechanisms and the biological and clinical consequences of progeria and related diseases, therapeutic trials have been conducted in patients and were terminated less than 10 years after the gene discovery, a quite fast issue for a genetic disease. Pharmacological drugs have been repurposed and used to decrease the toxicity of the accumulated, unprocessed and truncated prelaminA in progeria. To date, none of them may be considered as a cure for progeria and these clinical strategies were essentially designed toward reducing a subset of the most dramatic and morbid features associated to progeria. New therapeutic strategies under study, in particular targeting the protein expression pathway at the mRNA level, have shown a remarkable efficacy both in vitro in cells and in vivo in mice models. Strategies intending to clear the toxic accumulated proteins from the nucleus are also under evaluation. However, although exceedingly rare, improving our knowledge of genetic progeroid syndromes and searching for innovative and efficient therapies in these syndromes is of paramount importance as, even before they can be used to save lives, they may significantly (i) expand the affected childrens' lifespan and preserve their quality of life; (ii) improve our understanding of aging-related disorders and other more common diseases; and (iii) expand our fundamental knowledge of physiological aging and its links with major physiological processes such as those involved in oncogenesis.

Nuclear localization signal deletion mutants of lamin A and progerin reveal insights into lamin A processing and emerin targeting

Lamin A is a major component of the lamina, which creates a dynamic network underneath the nuclear envelope. Mutations in the lamin A gene (LMNA) cause severe genetic disorders, one of which is Hutchinson-Gilford progeria syndrome (HGPS), a disease triggered by a dominant mutant named progerin. Unlike the wild-type lamin A, whose farnesylated C-terminus is excised during post-translational processing, progerin retains its farnesyl tail and accumulates on the nuclear membrane, resulting in abnormal nuclear morphology during interphase. In addition, membrane-associated progerin forms visible cytoplasmic aggregates in mitosis. To examine the potential effects of cytoplasmic progerin, nuclear localization signal (NLS) deleted progerin and lamin A (PGΔNLS and LAΔNLS, respectively) have been constructed. We find that both ΔNLS mutants are farnesylated in the cytosol and associate with a sub-domain of the ER via their farnesyl tails. While the farnesylation on LAΔNLS can be gradually removed, which leads to its subsequent release from the ER into the cytoplasm, PGΔNLS remains permanently farnesylated and membrane-bounded. Moreover, both ΔNLS mutants dominantly affect emerin’s nuclear localization. These results reveal new insights into lamin A biogenesis and lamin A-emerin interaction.

Striated muscle laminopathies

Lamins A and C, encoded by LMNA, are constituent of the nuclear lamina, a meshwork of proteins underneath the nuclear envelope first described as scaffolding proteins of the nucleus. Since the discovery of LMNA mutations in highly heterogeneous human disorders (including cardiac and muscular dystrophies, lipodystrophies and progeria), the number of functions described for lamin A/C has expanded. Lamin A/C is notably involved in the regulation of chromatin structure and gene transcription, and in the resistance of cells to mechanical stress. This review focuses on studies performed on knock-out and knock-in Lmna mouse models, which have led to decipher some of the lamin A/C functions in striated muscles and to the first preclinical trials of pharmaceutical therapies.

Nesprins in health and disease

LINC (Linker of Nucleoskeleton and Cytoskeleton) complex is an evolutionary conserved structure that spans the entire nuclear envelope (NE), and integrates the nuclear interior with the cytoskeleton, in order to support a diverse array of fundamental biological processes. Key components of the LINC complex are the nesprins (Nuclear Envelope SPectrin Repeat proteINS) that were initially described as large integral NE proteins. However, nesprin genes are complex and generate many variants, which occupy various sub-cellular compartments suggesting additional functions. Hence, the potential involvement of nesprins in disease has expanded immensely on what we already know. That is, nesprins are implicated in diseases such as cancer, myopathies, arthrogryposis, neurological disorders and hearing loss. Here we review nesprins by providing an in depth account of their structure, molecular interactions and cellular functions with relevance to their potential roles in disease. Specifically, we speculate about possible pathomechanisms underlying nesprin-associated diseases.

Novel insights into the disease etiology of laminopathies

Laminopathies are a heterogeneous group of diseases that are caused by mutations in the nuclear envelope proteins lamins A and C. Laminopathies include dilated cardiomyopathy, Emery-Dreifuss muscular dystrophy, and familial partial lipodystrophy. Despite their near-ubiquitous expression, most laminopathies involve highly tissue-specific phenotypes, often affecting skeletal and cardiac muscle. The underlying mechanism(s) remain incompletely understood. We recently reported that altered actin dynamics in lamin A/C-deficient and mutant cells disturb nuclear shuttling of the transcriptional co-activator MKL1, which is critical for cardiac function. Expression of the inner nuclear membrane protein emerin rescues MKL1 translocation through modulating actin dynamics. Here, we elaborate on these findings, discuss new insights into the role of nuclear actin in MKL1activity, and demonstrate that primary human skin fibroblasts from a patient with dilated cardiomyopathy have impaired MKL1 nuclear translocation. These findings further strengthen the relevance of impaired MKL1 signaling as a potential contributor to the disease mechanism in laminopathies.

Essential roles of LEM-domain protein MAN1 during organogenesis in Xenopus laevis and overlapping functions of emerin

Mutations in nuclear envelope proteins are linked to an increasing number of human diseases, called envelopathies. Mutations in the inner nuclear membrane protein emerin lead to X-linked Emery-Dreifuss muscular dystrophy, characterized by muscle weakness or wasting. Conversely, mutations in nuclear envelope protein MAN1 are linked to bone and skin disorders. Both proteins share a highly conserved domain, called LEM-domain. LEM proteins are known to interact with Barrier-to-autointegration factor and several transcription factors. Most envelopathies are tissue-specific, but knowledge on the physiological roles of related LEM proteins is still unclear. For this reason, we investigated the roles of MAN1 and emerin during Xenopus laevis organogenesis. Morpholino-mediated knockdown of MAN1 revealed that MAN1 is essential for the formation of eye, skeletal and cardiac muscle tissues. The MAN1 knockdown could be compensated by ectopic expression of emerin, leading to a proper organ development. Further investigations revealed that MAN1 is involved in regulation of genes essential for organ development and tissue homeostasis. Thereby our work supports that LEM proteins might be involved in signalling essential for organ development during early embryogenesis and suggests that loss of MAN1 may cause muscle and retina specific diseases.

When lamins go bad: nuclear structure and disease

Mutations in nuclear lamins or other proteins of the nuclear envelope are the root cause of a group of phenotypically diverse genetic disorders known as laminopathies, which have symptoms that range from muscular dystrophy to neuropathy to premature aging syndromes. Although precise disease mechanisms remain unclear, there has been substantial progress in our understanding of not only laminopathies, but also the biological roles of nuclear structure. Nuclear envelope dysfunction is associated with altered nuclear activity, impaired structural dynamics, and aberrant cell signaling. Building on these findings, small molecules are being discovered that may become effective therapeutic agents.

KSHV ORF67 encoded lytic protein localizes on the nuclear membrane and alters emerin distribution

p29, a newly identified Kaposi's sarcoma-associated herpesvirus (KSHV) protein, is the product of ORF67, the positional homolog of the conserved herpesvirus protein UL34. Like its homologues in other herpesviruses, p29 is expressed early during viral lytic cycle, and is localized on the nuclear rim. Upon chemical induction of viral replication in primary effusion lymphoma cells, p29 interacts with p33, encoded by ORF69, the positional homolog of the conserved herpesvirus protein UL31, and both proteins colocalize on the nuclear membrane. IFA and biochemical analysis of infected or transfected cells showed that p29 expression resulted in delocalization and hyperphosphorylation of emerin, whereas other nuclear lamin associated proteins, such as LUMA, LB1 and LBR were not affected. Mislocalization of emerin was robustly increased upon combined expression of p29 and p33, suggesting that emerin destabilization might represent the first step in nuclear lamina disassembling, a process necessary for nucleocapsid maturation.

Partners and post-translational modifications of nuclear lamins

Nuclear intermediate filament networks formed by A- and B-type lamins are major components of the nucleoskeleton that are required for nuclear structure and function, with many links to human physiology. Mutations in lamins cause diverse human diseases ('laminopathies'). At least 54 partners interact with human A-type lamins directly or indirectly. The less studied human lamins B1 and B2 have 23 and seven reported partners, respectively. These interactions are likely to be regulated at least in part by lamin post-translational modifications. This review summarizes the binding partners and post-translational modifications of human lamins and discusses their known or potential implications for lamin function.

A role for β-dystroglycan in the organization and structure of the nucleus in myoblasts

We recently characterized a nuclear import pathway for β-dystroglycan; however, its nuclear role remains unknown. In this study, we demonstrate for the first time, the interaction of β-dystroglycan with distinct proteins from different nuclear compartments, including the nuclear envelope (NE) (emerin and lamins A/C and B1), splicing speckles (SC35), Cajal bodies (p80-coilin), and nucleoli (Nopp140). Electron microscopy analysis revealed that β-dystroglycan localized in the inner nuclear membrane, nucleoplasm, and nucleoli. Interestingly, downregulation of β-dystroglycan resulted in both mislocalization and decreased expression of emerin and lamin B1, but not lamin A/C, as well in disorganization of nucleoli, Cajal bodies, and splicing speckles with the concomitant decrease in the levels of Nopp140, and p80-coilin, but not SC35. Quantitative reverse transcription PCR and cycloheximide-mediated protein arrest assays revealed that β-dystroglycan deficiency did not change mRNA expression of NE proteins emerin and lamin B1 bud did alter their stability, accelerating protein turnover. Furthermore, knockdown of β-dystroglycan disrupted NE-mediated processes including nuclear morphology and centrosome-nucleus linkage, which provides evidence that β-dystroglycan association with NE proteins is biologically relevant. Unexpectedly, β-dystroglycan-depleted cells exhibited multiple centrosomes, a characteristic of cancerous cells. Overall, these findings imply that β-dystroglycan is a nuclear scaffolding protein involved in nuclear organization and NE structure and function, and that might be a contributor to the biogenesis of nuclear envelopathies.

Reversal of laminopathies: the curious case of SUN1

Mutations in the LMNA gene are associated with a spectrum of human dystrophic diseases termed the "nuclear laminopathies." We recently found that the accumulation of the inner nuclear envelope proteins SUN1 is pathogenic in progeric and dystrophic laminopathies. This conclusion arose from the unexpected observation that the deletion of Sun1, instead of accelerating aging, actually ameliorated the progeric and dystrophic phenotypes in Lmna-deficient mice. In human cells, knocking down SUN1 corrected the nuclear aberrancies and the senescent tendencies of HGPS (Hutchinson-Gilford progeria syndrome) skin fibroblasts. Here we offer additional comments on the contributions of SUN1 and the process of normal protein turnover to cellular aging.

The overexpression of nuclear envelope protein Lap2β induces endoplasmic reticulum reorganisation via membrane stacking

Some nuclear envelope proteins are localised to both the nuclear envelope and the endoplasmic reticulum; therefore, it seems plausible that even small amounts of these proteins can influence the organisation of the endoplasmic reticulum. A simple method to study the possible effects of nuclear envelope proteins on endoplasmic reticulum organisation is to analyze nuclear envelope protein overexpression. Here, we demonstrate that Lap2β overexpression can induce the formation of cytoplasmic vesicular structures derived from endoplasmic reticulum membranes. Correlative light and electron microscopy demonstrated that these vesicular structures were composed of a series of closely apposed membranes that were frequently arranged in a circular fashion. Although stacked endoplasmic reticulum cisternae were highly ordered, Lap2β could readily diffuse into and out of these structures into the surrounding reticulum. It appears that low-affinity interactions between cytoplasmic domains of Lap2β can reorganise reticular endoplasmic reticulum into stacked cisternae. Although the effect of one protein may be insignificant at low concentrations, the cumulative effect of many non-specialised proteins may be significant.

The nuclear envelope protein emerin binds directly to histone deacetylase 3 (HDAC3) and activates HDAC3 activity

Organization of the genome is critical for maintaining cell-specific gene expression, ensuring proper cell function. It is well established that the nuclear lamina preferentially associates with repressed chromatin. However, the molecular mechanisms underlying repressive chromatin formation and maintenance at the nuclear lamina remain poorly understood. Here we show that emerin binds directly to HDAC3, the catalytic subunit of the nuclear co-repressor (NCoR) complex, and recruits HDAC3 to the nuclear periphery. Emerin binding stimulated the catalytic activity of HDAC3, and emerin-null cells exhibit increased H4K5 acetylation, which is the preferred target of the NCoR complex. Emerin-null cells exhibit an epigenetic signature similar to that seen in HDAC3-null cells. Emerin-null cells also had significantly less HDAC3 at the nuclear lamina. Collectively, these data support a model whereby emerin facilitates repressive chromatin formation at the nuclear periphery by increasing the catalytic activity of HDAC3.

Dialing down SUN1 for laminopathies

Laminopathies, caused by mutations in A-type nuclear lamins, encompass a range of diseases, including forms of progeria and muscular dystrophy. In this issue, Chen et al. provide evidence that elevated expression of the nuclear inner membrane protein SUN1 drives pathology in multiple laminopathies.

MUF1/leucine-rich repeat containing 41 (LRRC41), a substrate of RhoBTB-dependent cullin 3 ubiquitin ligase complexes, is a predominantly nuclear dimeric protein

RhoBTB (BTB stands for broad-complex, tramtrack, bric à brac) proteins are tumor suppressors involved in the formation of cullin 3 (Cul3)-dependent ubiquitin ligase complexes. However, no substrates of RhoBTB-Cul3 ubiquitin ligase complexes have been identified. We identified MUF1 (LRRC41, leucine-rich repeat containing 41) as a potential interaction partner of RhoBTB3 in a two-hybrid screening on a mouse brain cDNA library. MUF1 is a largely uncharacterized protein containing a leucine-rich repeat and, interestingly, a BC-box that serves as a linker in multicomponent, cullin 5 (Cul5)-based ubiquitin ligases. We confirmed the interaction of MUF1 with all three mammalian RhoBTB proteins using immunoprecipitation. We characterized MUF1 in terms of expression profile and subcellular localization, the latter also with respect to RhoBTB proteins. We found out that MUF1 is a ubiquitously expressed nuclear protein that, upon coexpression with RhoBTB, partially retains in the cytoplasm, where both proteins colocalize. We also show that MUF1 is able to dimerize similarly to other leucine-rich repeat-containing proteins. To explore the significance of MUF1-RhoBTB interaction within Cul-ligase complexes and the mechanism of MUF1 degradation, we performed a protein stability assay and found that MUF1 is degraded in the proteasome in a Cul5-independent manner by RhoBTB3-Cul3 ubiquitin ligase complex. Finally, we explored a possible heterodimerization of Cul3 and Cul5 and indeed discovered that these two cullins are capable of forming heterodimers. Thus, we have identified MUF1 as the first substrate for RhoBTB-Cul3 ubiquitin ligase complexes. Identification of substrates of these complexes will result in better understanding of the tumor suppressor function of RhoBTB.

LINC complex alterations in DMD and EDMD/CMT fibroblasts

Emery-Dreifuss muscular dystrophy (EDMD) is a late onset-disease characterized by skeletal muscle wasting and heart defects with associated risk of sudden death. The autosomal dominant form of the disease is caused by mutations in the LMNA gene encoding LaminA and C, the X-linked form results from mutations in the gene encoding the inner nuclear membrane protein Emerin (STA). Both Emerin and LaminA/C interact with the nuclear envelope proteins Nesprin-1 and -2 and mutations in genes encoding C-terminal isoforms of Nesprin-1 and -2 have also been implicated in EDMD. Here we analyse primary fibroblasts from patients affected by either Duchenne muscular dystrophy (DMD) or Emery-Dreifuss muscular dystrophy/Charcot-Marie-Tooth syndrome (EDMD/CMT) that in addition to the disease causing mutations harbour mutations in the Nesprin-1 gene and in the SUN1 and SUN2 gene, respectively. SUN proteins together with the Nesprins form the core of the LINC complex which connects the nucleus with the cytoskeleton. The mutations are accompanied by changes in cell adhesion, cell migration, senescence, and stress response, as well as in nuclear shape and nuclear envelope composition which are changes characteristic for laminopathies. Our results point to a potential influence of mutations in components of the LINC complex on the clinical outcome and the molecular pathology in the patients.

A mutation in VAPB that causes amyotrophic lateral sclerosis also causes a nuclear envelope defect

A proline to serine mutation (P56S) in vesicle-associated membrane protein-associated protein B and C (VAPB) causes an autosomal dominant form of amyotrophic lateral sclerosis (ALS). We show that the mutation also causes a nuclear envelope defect. Transport of nucleoporins (Nups) and emerin (EMD) to the nuclear envelope is blocked, resulting in their sequestration in dilated cytoplasmic membranes. Simultaneous overexpression of the FFAT motif (two phenylalanine residues in an acidic track) antagonizes the effect of mutant VAPB and restores transport to the nuclear envelope. VAPB function is required for transport to the nuclear envelope, with knockdown of endogenous VAPB recapitulating this phenotype. Moreover, we identified the compartment into which the Nups and EMD were sequestered as the endoplasmic reticulum (ER)-Golgi intermediate compartment (ERGIC), with nuclear envelope membrane proteins transiting to the ERGIC before VAPB-dependent retrograde transport to the nuclear envelope.

Nuclear mechanics in disease

Over the past two decades, the biomechanical properties of cells have emerged as key players in a broad range of cellular functions, including migration, proliferation, and differentiation. Although much of the attention has focused on the cytoskeletal networks and the cell's microenvironment, relatively little is known about the contribution of the cell nucleus. Here, we present an overview of the structural elements that determine the physical properties of the nucleus and discuss how changes in the expression of nuclear components or mutations in nuclear proteins can not only affect nuclear mechanics but also modulate cytoskeletal organization and diverse cellular functions. These findings illustrate that the nucleus is tightly integrated into the surrounding cellular structure. Consequently, changes in nuclear structure and composition are highly relevant to normal development and physiology and can contribute to many human diseases, such as muscular dystrophy, dilated cardiomyopathy, (premature) aging, and cancer.

[Laminopathies. Nuclear lamina diseases]

Laminopathies are a group of diseases that share wrong codification of lamins, building proteins of the nuclear lamina. Different tissues are affected in those disorders: striated muscle, adipose tissue, central or peripheral nervous system and aging process. Emery-Dreifuss muscular dystrophy and Hutchinson-Gildford Progery Syndrome are two examples of laminopathies. Other diseases, due to mutations in different genes, impair lamins function by a direct or an indirect way and they are frequently considered together. The last decade has seen an increasing interest and scientific advances on laminopathies that will allow us to answer key questions regarding metabolism, insulin resistance, sudden death and aging. Laminopathies are reviewed in this article from a molecular, pathogenic and clinical point of view.

The functions of the nuclear envelope in mediating the molecular crosstalk between the nucleus and the cytoplasm

Recent studies of the nuclear envelope (NE) have emphasized its role in linking the nuclear and cytoplasmic compartments of mammalian cells. The inner face of the NE is bound to chromatin and this interaction is involved in regulating DNA replication and transcription. The outer face of the NE binds to different components of the cytoskeleton, and these interactions are involved in nuclear positioning. Many disease causing mutations in genes encoding NE proteins cause significant changes in nuclear architecture and cytoskeletal interactions with the NE. These mutations are also providing important new insights into nuclear-cytoplasmic interactions.

Nuclear lamins are differentially expressed in retinal neurons of the adult rat retina

Lamins are type V intermediate filament proteins that support nuclear membranes. They are divided into A-type lamins, which include lamin A and C, and B-type lamins, which include lamin B1 and B2. In the rat brain, lamin A and C are expressed in relatively equal amounts, while the expressions of lamin B1 and B2 vary depending on the cell type. Lamins play important roles in normal morphogenesis and function. In the nervous system, their abnormal expression causes several neurodegenerative diseases such as peripheral neuropathy, leukodystrophy and lissencephaly. The retina belongs to the central nervous system (CNS) and has widely been used as a source of CNS neurons. We investigated the expression patterns of lamin subtypes in the adult rat retina by immunohistochemistry and found that the staining patterns differed when compared with the brain. All retinal neurons expressed lamin B1 and B2 in relatively equal amounts. In addition, horizontal cells and a subpopulation of retinal ganglion cells expressed lamin A and C, while photoreceptor cells expressed neither lamin A nor C, and all other retinal neurons expressed lamin C only. This differential expression pattern of lamins in retinal neurons suggests that they may be involved in cellular differentiation and expression of cell-specific genes in individual retinal neurons.

Beyond membrane channelopathies: alternative mechanisms underlying complex human disease

Over the past fifteen years, our understanding of the molecular mechanisms underlying human disease has flourished in large part due to the discovery of gene mutations linked with membrane ion channels and transporters. In fact, ion channel defects ("channelopathies" - the focus of this review series) have been associated with a spectrum of serious human disease phenotypes including cystic fibrosis, cardiac arrhythmia, diabetes, skeletal muscle defects, and neurological disorders. However, we now know that human disease, particularly excitable cell disease, may be caused by defects in non-ion channel polypeptides including in cellular components residing well beneath the plasma membrane. For example, over the past few years, a new class of potentially fatal cardiac arrhythmias has been linked with cytoplasmic proteins that include sub-membrane adapters such as ankyrin-B (ANK2), ankyrin-G (ANK3), and alpha-1 syntrophin, membrane coat proteins including caveolin-3 (CAV3), signaling platforms including yotiao (AKAP9), and cardiac enzymes (GPD1L). The focus of this review is to detail the exciting role of lamins, yet another class of gene products that have provided elegant new insight into human disease.

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

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

Structural protein 4.1R is integrally involved in nuclear envelope protein localization, centrosome-nucleus association and transcriptional signaling

The multifunctional structural protein 4.1R is required for assembly and maintenance of functional nuclei but its nuclear roles are unidentified. 4.1R localizes within nuclei, at the nuclear envelope, and in cytoplasm. Here we show that 4.1R, the nuclear envelope protein emerin and the intermediate filament protein lamin A/C co-immunoprecipitate, and that 4.1R-specific depletion in human cells by RNA interference produces nuclear dysmorphology and selective mislocalization of proteins from several nuclear subcompartments. Such 4.1R-deficiency causes emerin to partially redistribute into the cytoplasm, whereas lamin A/C is disorganized at nuclear rims and displaced from nucleoplasmic foci. The nuclear envelope protein MAN1, nuclear pore proteins Tpr and Nup62, and nucleoplasmic proteins NuMA and LAP2α also have aberrant distributions, but lamin B and LAP2β have normal localizations. 4.1R-deficient mouse embryonic fibroblasts show a similar phenotype. We determined the functional effects of 4.1R-deficiency that reflect disruption of the association of 4.1R with emerin and A-type lamin: increased nucleus-centrosome distances, increased β-catenin signaling, and relocalization of β-catenin from the plasma membrane to the nucleus. Furthermore, emerin- and lamin-A/C-null cells have decreased nuclear 4.1R. Our data provide evidence that 4.1R has important functional interactions with emerin and A-type lamin that impact upon nuclear architecture, centrosome-nuclear envelope association and the regulation of β-catenin transcriptional co-activator activity that is dependent on β-catenin nuclear export.

Nuclear envelope structural defects cause chromosomal numerical instability and aneuploidy in ovarian cancer

BACKGROUND

Despite our substantial understanding of molecular mechanisms and gene mutations involved in cancer, the technical approaches for diagnosis and prognosis of cancer are limited. In routine clinical diagnosis of cancer, the procedure is very basic: nuclear morphology is used as a common assessment of the degree of malignancy, and hence acts as a prognostic and predictive indicator of the disease. Furthermore, though the atypical nuclear morphology of cancer cells is believed to be a consequence of oncogenic signaling, the molecular basis remains unclear. Another common characteristic of human cancer is aneuploidy, but the causes and its role in carcinogenesis are not well established.

METHODS

We investigated the expression of the nuclear envelope proteins lamin A/C in ovarian cancer by immunohistochemistry and studied the consequence of lamin A/C suppression using siRNA in primary human ovarian surface epithelial cells in culture. We used immunofluorescence microscopy to analyze nuclear morphology, flow cytometry to analyze cellular DNA content, and fluorescence in situ hybridization to examine cell ploidy of the lamin A/C-suppressed cells.

RESULTS

We found that nuclear lamina proteins lamin A/C are often absent (47%) in ovarian cancer cells and tissues. Even in lamin A/C-positive ovarian cancer, the expression is heterogeneous within the population of tumor cells. In most cancer cell lines, a significant fraction of the lamin A/C-negative population was observed to intermix with the lamin A/C-positive cells. Down regulation of lamin A/C in non-cancerous primary ovarian surface epithelial cells led to morphological deformation and development of aneuploidy. The aneuploid cells became growth retarded due to a p53-dependent induction of the cell cycle inhibitor p21.

CONCLUSIONS

We conclude that the loss of nuclear envelope structural proteins, such as lamin A/C, may underlie two of the hallmarks of cancer--aberrations in nuclear morphology and aneuploidy.

AMP-activated protein kinase regulates beta-catenin transcription via histone deacetylase 5

AMP-activated protein kinase (AMPK) is a key regulator of energy metabolism; it is inhibited under obese conditions and is activated by exercise and by many anti-diabetic drugs. Emerging evidence also suggests that AMPK regulates cell differentiation, but the underlying mechanisms are unclear. We hypothesized that AMPK regulates cell differentiation via altering β-catenin expression, which involves phosphorylation of class IIa histone deacetylase 5 (HDAC5). In both C3H10T1/2 cells and mouse embryonic fibroblasts (MEFs), AMPK activity was positively correlated with β-catenin content. Chemical inhibition of HDAC5 increased β-catenin mRNA expression. HDAC5 overexpression reduced and HDAC5 knockdown increased H3K9 acetylation and cellular β-catenin content. HDAC5 formed a complex with myocyte enhancer factor-2 to down-regulate β-catenin mRNA expression. AMPK phosphorylated HDAC5, which promoted HDAC5 exportation from the nucleus; mutation of two phosphorylation sites in HDAC5, Ser-259 and -498, abolished the regulatory role of AMPK on β-catenin expression. In conclusion, AMPK promotes β-catenin expression through phosphorylation of HDAC5, which reduces HDAC5 interaction with the β-catenin promoter via myocyte enhancer factor-2. Thus, the data indicate that AMPK regulates cell differentiation and development via cross-talk with the wingless and Int (Wnt)/β-catenin signaling pathway.

Uncoordinated transcription and compromised muscle function in the lmna-null mouse model of Emery- Emery-Dreyfuss muscular dystrophy

LMNA encodes both lamin A and C: major components of the nuclear lamina. Mutations in LMNA underlie a range of tissue-specific degenerative diseases, including those that affect skeletal muscle, such as autosomal-Emery-Dreifuss muscular dystrophy (A-EDMD) and limb girdle muscular dystrophy 1B. Here, we examine the morphology and transcriptional activity of myonuclei, the structure of the myotendinous junction and the muscle contraction dynamics in the lmna-null mouse model of A-EDMD. We found that there were fewer myonuclei in lmna-null mice, of which ∼50% had morphological abnormalities. Assaying transcriptional activity by examining acetylated histone H3 and PABPN1 levels indicated that there was a lack of coordinated transcription between myonuclei lacking lamin A/C. Myonuclei with abnormal morphology and transcriptional activity were distributed along the length of the myofibre, but accumulated at the myotendinous junction. Indeed, in addition to the presence of abnormal myonuclei, the structure of the myotendinous junction was perturbed, with disorganised sarcomeres and reduced interdigitation with the tendon, together with lipid and collagen deposition. Functionally, muscle contraction became severely affected within weeks of birth, with specific force generation dropping as low as ∼65% and ∼27% of control values in the extensor digitorum longus and soleus muscles respectively. These observations illustrate the importance of lamin A/C for correct myonuclear function, which likely acts synergistically with myotendinous junction disorganisation in the development of A-EDMD, and the consequential reduction in force generation and muscle wasting.

Novel LMNA mutations in patients with Emery-Dreifuss muscular dystrophy and functional characterization of four LMNA mutations

Mutations in LMNA cause a variety of diseases affecting striated muscle including autosomal Emery-Dreifuss muscular dystrophy (EDMD), LMNA-associated congenital muscular dystrophy (L-CMD), and limb-girdle muscular dystrophy type 1B (LGMD1B). Here, we describe novel and recurrent LMNA mutations identified in 50 patients from the United States and Canada, which is the first report of the distribution of LMNA mutations from a large cohort outside Europe. This augments the number of LMNA mutations known to cause EDMD by 16.5%, equating to an increase of 5.9% in the total known LMNA mutations. Eight patients presented with either p.R249W/Q or p.E358K mutations and an early onset EDMD phenotype: two mutations recently associated with L-CMD. Importantly, 15 mutations are novel and include eight missense mutations (p.R189P, p.F206L, p.S268P, p.S295P, p.E361K, p.G449D, p.L454P, and p.W467R), three splice site mutations (c.IVS4 + 1G>A, c.IVS6 - 2A>G, and c.IVS8 + 1G>A), one duplication/in frame insertion (p.R190dup), one deletion (p.Q355del), and two silent mutations (p.R119R and p.K270K). Analysis of 4 of our lamin A mutations showed that some caused nuclear deformations and lamin B redistribution in a mutation specific manner. Together, this study significantly augments the number of EDMD patients on the database and describes 15 novel mutations that underlie EDMD, which will contribute to establishing genotype-phenotype correlations.

Emery-Dreifuss muscular dystrophy

Emery-Dreifuss muscular dystrophy (EDMD) is a progressive muscle-wasting disorder defined by early contractures of the Achilles tendon, spine, and elbows. EDMD is also distinctive for its association with defects of the cardiac conduction system that can result in sudden death. It can be inherited in an X-linked, autosomal dominant, or autosomal recessive fashion and is caused by mutations in proteins of the nuclear membrane. Mutations in the EMD gene, which encodes emerin, a transmembrane protein found at the inner nuclear membrane, are responsible for X-linked EDMD. The most common etiology of autosomal dominant EDMD is an LMNA gene mutation; LMNA encodes the intermediate filament protein lamins A and C, which constitute the major scaffolding protein of the inner nuclear membrane. Murine models of LMNA gene mutations have helped to identify different mechanisms of disease. Loss of LMNA function leads to nuclear fragility as well as other defects, such as abnormal nuclear function. Additional genes encoding nuclear membrane proteins such as SYNE1 and SYNE2 have also been implicated in EDMD, and in some cases their importance for cardiac and muscle function has been supported by animal modeling.

Laminopathies: the molecular background of the disease and the prospects for its treatment

Laminopathies are rare human degenerative disorders with a wide spectrum of clinical phenotypes, associated with defects in the main protein components of the nuclear envelope, mostly in the lamins. They include systemic disorders and tissue-restricted diseases. Scientists have been trying to explain the pathogenesis of laminopathies and find an efficient method for treatment for many years. In this review, we discuss the current state of knowledge about laminopathies, the molecular mechanisms behind the development of particular phenotypes, and the prospects for stem cell and/or gene therapy treatments.

Lamin A rod domain mutants target heterochromatin protein 1alpha and beta for proteasomal degradation by activation of F-box protein, FBXW10

BACKGROUND

Lamins are major structural proteins of the nucleus and contribute to the organization of various nuclear functions. Mutations in the human lamin A gene cause a number of highly degenerative diseases, collectively termed as laminopathies. Cells expressing lamin mutations exhibit abnormal nuclear morphology and altered heterochromatin organization; however, the mechanisms responsible for these defects are not well understood.

METHODOLOGY AND PRINCIPAL FINDINGS

The lamin A rod domain mutants G232E, Q294P and R386K are either diffusely distributed or form large aggregates in the nucleoplasm, resulting in aberrant nuclear morphology in various cell types. We examined the effects of these lamin mutants on the distribution of heterochromatin protein 1 (HP1) isoforms. HeLa cells expressing these mutants showed a heterogeneous pattern of HP1alpha and beta depletion but without altering HP1gamma levels. Changes in HP1alpha and beta were not observed in cells expressing wild-type lamin A or mutant R482L, which assembled normally at the nuclear rim. Treatment with proteasomal inhibitors led to restoration of levels of HP1 isoforms and also resulted in stable association of lamin mutants with the nuclear periphery, rim localization of the inner nuclear membrane lamin-binding protein emerin and partial improvement of nuclear morphology. A comparison of the stability of HP1 isoforms indicated that HP1alpha and beta displayed increased turnover and higher basal levels of ubiquitination than HP1gamma. Transcript analysis of components of the ubiquitination pathway showed that a specific F-box protein, FBXW10 was induced several-fold in cells expressing lamin mutants. Importantly, ectopic expression of FBXW10 in HeLa cells led to depletion of HP1alpha and beta without alteration of HP1gamma levels.

CONCLUSIONS

Mislocalized lamins can induce ubiquitin-mediated proteasomal degradation of certain HP1 isoforms by activation of FBXW10, a member of the F-box family of proteins that is involved in E3 ubiquitin ligase activity.

Lamins as cancer biomarkers

Lamins are multifunctional proteins that are often aberrantly expressed or localized in tumours. Here, we endeavour to assess their uses as cancer biomarkers: to diagnose tumours, analyse cancer characteristics and predict patient survival. It appears that the nature of lamin function in cancer is very complex. Lamin expression can be variable between and even within cancer subtypes, which limits their uses as diagnostic biomarkers. Expression of A-type lamins is a marker of differentiated tumour cells and has been shown to be a marker of good or poor patient survival depending on tumour subtype. Further research into the functions of lamins in cancer cells and the mechanisms that determine its patterns of expression may provide more potential uses of lamins as cancer biomarkers.

Role of A-type lamins in signaling, transcription, and chromatin organization

A-type lamins (lamins A and C), encoded by the LMNA gene, are major protein constituents of the mammalian nuclear lamina, a complex structure that acts as a scaffold for protein complexes that regulate nuclear structure and functions. Interest in these proteins has increased in recent years with the discovery that LMNA mutations cause a variety of human diseases termed laminopathies, including progeroid syndromes and disorders that primarily affect striated muscle, adipose, bone, and neuronal tissues. In this review, we discuss recent research supporting the concept that lamin A/C and associated nuclear envelope proteins regulate gene expression in health and disease through interplay with signal transduction pathways, transcription factors, and chromatin-associated proteins.

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.

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.

Actin dynamics and functions in the interphase nucleus: moving toward an understanding of nuclear polymeric actin

Actin exists as a dynamic equilibrium of monomers and polymers within the nucleus of living cells. It is utilized by the cell for many aspects of gene regulation, including mRNA processing, chromatin remodelling, and global gene expression. Polymeric actin is now specifically linked to transcription by RNA polymerase I, II, and III. An active process, requiring both actin polymers and myosin, appears to drive RNA polymerase I transcription, and is also implicated in long-range chromatin movement. This type of mechanism brings activated genes from separate chromosomal territories together, and then participates in their compartmentalization near nuclear speckles. Nuclear speckle formation requires polymeric actin, and factors promoting polymerization, such as profilin and PIP2, are concentrated there. A review of the literature shows that a functional population of G-actin cycles between the cytoplasm and the nucleoplasm. Its nuclear concentration is dependent on the cytoplasmic G-actin pool, as well as on the activity of import and export mechanisms and the availability of interactions that sequester it within the nucleus. The N-WASP-Arp2/3 actin polymer-nucleating mechanism functions in the nucleus, and its mediators, including NCK, PIP2, and Rac1, can be found in the nucleoplasm, where they likely influence the kinetics of polymer formation. The actin polymer species produced are tightly regulated, and may take on conformations not easily recognized by phalloidin. Many of the factors that cleave F-actin in the cytoplasm are present at high levels in the nucleoplasm, and are also likely to affect actin dynamics there. The absolute and relative G-actin content in the nucleoplasm and the cytoplasm of a cell contains information about the homeostatic state of that cell. We propose that the cycling of G-actin between the nucleus and cytoplasm represents a signal transduction mechanism that can function through both extremes of global cellular G-actin content. MAL signalling within the serum response factor pathway, when G-actin levels are low, represents a well-studied example of actin functioning in signal transduction. The translocation of NCK into the nucleus, along with G-actin, during dissolution of the cytoskeleton in response to DNA damage represents another instance of a unique signalling mechanism operating when G-actin levels are high.

Dynamic complexes of A-type lamins and emerin influence adipogenic capacity of the cell via nucleocytoplasmic distribution of beta-catenin

It is well documented that adipogenic differentiation of the cell is associated with downregulation of Wnt/beta-catenin signalling. Using preadipocytes and dermal fibroblasts, we have found that activation of the adipogenic program was associated with marked changes in the expression of nuclear beta-catenin-interacting partners, emerin and lamins A/C, to influence expression and activation of peroxisome proliferators-activated receptors gamma (PPARgamma). In addition, silencing of protein expression with siRNA revealed that beta-catenin and emerin influenced each other's levels of expression and the onset of adipogenesis, suggesting that changes in the expression of nuclear lamina proteins were intimately linked to the stability of beta-catenin. By contrast, dermal fibroblasts, which are emerin null, demonstrated increased nuclear accumulation of stable beta-catenin and constant lamin expression. This was also associated with an unusual adipogenic capacity of the cells, with adipogenesis occurring in the presence of activated beta-catenin but declining upon silencing of the protein expression with siRNA. We propose that the process of adipogenesis is affected by a dynamic link between complexes of emerin and lamins A/C at the nuclear envelope and nucleocytoplasmic distribution of beta-catenin, to influence cellular plasticity and differentiation.

Lamin A/C is a risk biomarker in colorectal cancer

Background

A-type lamins are type V intermediate filament proteins encoded by the gene LMNA. Mutations in LMNA give rise to diverse degenerative diseases related to premature ageing. A-type lamins also influence the activity of the Retinoblastoma protein (pRb) and oncogenes such a β-catenin. Consequently, it has been speculated that expression of A-type lamins may also influence tumour progression.

Methodology/Principal Findings

An archive of colorectal cancer (CRC) and normal colon tissue was screened for expression of A-type lamins. We used the Cox proportional hazard ratio (HR) method to investigate patient survival. Using CRC cell lines we investigated the effects of lamin A expression on other genes by RT-PCR; on cell growth by FACS analysis; and on invasiveness by cell migration assays and siRNA knockdown of targeted genes. We found that lamin A is expressed in colonic stem cells and that patients with A-type lamin-expressing tumours have significantly worse prognosis than patients with A-type lamin negative tumours (HR = 1.85, p = 0.005). To understand this finding, we established a model system based upon expression of GFP-lamin A in CRC cells. We found that expression of GFP-lamin A in these cells did not affect cell proliferation but did promote greatly increased cell motility and invasiveness. The reason for this increased invasiveness was that expression of lamin A promoted up-regulation of the actin bundling protein T-plastin, leading to down regulation of the cell adhesion molecule E-cadherin.

Conclusions

Expression of A-type lamins increases the risk of death from CRC because its presence gives rise to increased invasiveness and potentially a more stem cell-like phenotype. This report directly links A-type lamin expression to tumour progression and raises the profile of LMNA from one implicated in multiple but rare genetic conditions to a gene involved in one of the commonest diseases in the Western World.