Nuclear lamins: key regulators of nuclear structure and activities

Impact of near-infrared radiation in dermatology

Sunlight that reaches the human skin contains solar energy composed of 6.8% ultraviolet (UV), 38.9% visible light and 54.3% infrared radiation. In addition to natural near-infrared (NIR), human skin is increasingly exposed to artificial NIR from medical devices and electrical appliances. Thus, we are exposed to tremendous amounts of NIR. Many studies have proven the effects of UV exposure on human skin and skin cancers but have not investigated well the effects of NIR exposure. Furthermore, many of the previous NIR studies have used NIR resources without a water filter or a contact cooling. With these resources, a substantial amount of NIR energy is absorbed in the superficial layers and only limited NIR energy can be delivered to deeper tissues. Thus, they could not sufficiently evaluate the effects of incident solar NIR. In order to simulate solar NIR that reaches the skin, a water filter is essential because solar NIR is filtered by atmospheric water. In reality, NIR increases the surface temperature and induces thermal effects so a contact cooling is needed to pursue the properties of NIR. I clarify that NIR can penetrate the skin and non-thermally affect the subcutaneous tissues, including muscle and bone marrow, using a NIR resource with a water filter and a cooling system. I would like to emphasize the biological effects of NIR which have both merits and demerits. Appropriate NIR irradiation induces dermal heating thermally and non-thermally induces collagen and elastin stimulation, which results in skin tightening. NIR also induces non-thermal DNA damage of mitotic cells, which may have the potential application for treating cancer. However, as continuous NIR exposure may induce photoaging and potentially photocarcinogenesis, we should consider the effect of, not only UV, but also NIR and the necessity for protection against solar NIR. Here, this paper introduces the new aspects of the biological effects of NIR radiation.

LMNA knock-down affects differentiation and progression of human neuroblastoma cells

Background

Neuroblastoma (NB) is one of the most aggressive tumors that occur in childhood. Although genes, such as MYCN, have been shown to be involved in the aggressiveness of the disease, the identification of new biological markers is still desirable. The induction of differentiation is one of the strategies used in the treatment of neuroblastoma. A-type lamins are components of the nuclear lamina and are involved in differentiation. We studied the role of Lamin A/C in the differentiation and progression of neuroblastoma.

Methodology/Principal Findings

Knock-down of Lamin A/C (LMNA-KD) in neuroblastoma cells blocked retinoic acid-induced differentiation, preventing neurites outgrowth and the expression of neural markers. The genome-wide gene-expression profile and the proteomic analysis of LMNA-KD cells confirmed the inhibition of differentiation and demonstrated an increase of aggressiveness-related genes and molecules resulting in augmented migration/invasion, and increasing the drug resistance of the cells. The more aggressive phenotype acquired by LMNA-KD cells was also maintained in vivo after injection into nude mice. A preliminary immunohistochemistry analysis of Lamin A/C expression in nine primary stages human NB indicated that this protein is poorly expressed in most of these cases.

Conclusions/Significance

We demonstrated for the first time in neuroblastoma cells that Lamin A/C plays a central role in the differentiation, and that the loss of this protein gave rise to a more aggressive tumor phenotype.

GAGE cancer-germline antigens are recruited to the nuclear envelope by germ cell-less (GCL)

GAGE proteins are highly similar, primate-specific molecules with unique primary structure and undefined cellular roles. They are restricted to cells of the germ line in adult healthy individuals, but are broadly expressed in a wide range of cancers. In a yeast two-hybrid screen we identified the metazoan transcriptional regulator, Germ cell-less (GCL), as an interaction partner of GAGE12I. GCL directly binds LEM-domain proteins (LAP2β, emerin, MAN1) at the nuclear envelope, and we found that GAGE proteins were recruited to the nuclear envelope inner membrane by GCL. Based on yeast two-hybrid analysis and pull-down experiments of GCL polypeptides, GCL residues 209–320 (which includes the BACK domain) were deduced sufficient for association with GAGE proteins. GAGE mRNAs and GCL mRNA were demonstrated in human testis and most types of cancers, and at the protein level GAGE members and GCL were co-expressed in cancer cell lines. Structural studies of GAGE proteins revealed no distinct secondary or tertiary structure, suggesting they are intrinsically disordered. Interestingly GAGE proteins formed stable complexes with dsDNA in vitro at physiological concentrations, and GAGE12I bound several different dsDNA fragments, suggesting sequence-nonspecific binding. Dual association of GAGE family members with GCL at the nuclear envelope inner membrane in cells, and with dsDNA in vitro, implicate GAGE proteins in chromatin regulation in germ cells and cancer cells.

An inherited LMNA gene mutation in atypical Progeria syndrome

Hutchinson-Gilford Progeria syndrome (HGPS) is a rare genetic disorder, characterized by several clinical features that begin in early childhood, recalling an accelerated aging process. The diagnosis of HGPS is based on the recognition of common clinical features and detection of the recurrent heterozygous c.1824C>T (p.Gly608Gly) mutation within exon 11 in the Lamin A/C encoding gene (LMNA). Besides "typical HGPS," several "atypical progeria" syndromes (APS) have been described, in a clinical spectrum ranging from mandibuloacral dysplasia to atypical Werner syndrome. These patients's clinical features include progeroid manifestations, such as short stature, prominent nose, premature graying of hair, partial alopecia, skin atrophy, lipodystrophy, skeletal anomalies, such as mandibular hypoplasia and acroosteolyses, and in some cases severe atherosclerosis with metabolic complications. APS are due in several cases to de novo heterozygous LMNA mutations other than the p.Gly608Gly, or due to homozygous BAFN1 mutations in Nestor-Guillermo Progeria syndrome (NGPS). We report here and discuss the observation of a non-consanguineous Moroccan patient presenting with atypical progeria. The molecular studies showed the heterozygous mutation c.412G>A (p.Glu138Lys) of the LMNA gene. This mutation, previously reported as a de novo mutation, was inherited from the apparently healthy father who showed a somatic cell mosaicism.

Oxydative stress alters nuclear shape through lamins dysregulation: a route to senescence

Progeroid phenotypes are mainly encountered in 2 types of syndromes: in laminopathies, which are characterized by nuclear shape abnormalities due to lamin A alteration, and in DNA damage response defect syndromes. Because lamin A dysregulation leads to DNA damages, it has been proposed that senescence occurs in both types of syndromes through the accumulation of damages. We recently showed that elevated oxidative stress is responsible for lamin B1 accumulation, nuclear shape alteration and senescence in the DDR syndrome, ataxia telangiectasia (A-T). Interestingly, overexpression of lamin B1 in wild type cells is sufficient to induce senescence without the induction of DNA damages. Here, we will discuss the importance of controlling the lamins level in order for maintenance nuclear architecture and we will comment the relationships of lamins with other senescence mechanisms. Finally, we will describe emerging data reporting redox control by lamins, leading us to propose a general mechanism by which reactive oxygen species can induce senescence through lamin dysregulation and NSA.

S100A6 is transcriptionally regulated by β-catenin and interacts with a novel target, lamin A/C, in colorectal cancer cells

In this paper we document an increased expression of S100A6, a calcium binding protein of the S100 family, and its co-localization with β-catenin in colorectal cancer tissues and in metastatic, SW620, versus non-metastatic, SW480, human colorectal cancer cell lines. Moreover, we show up-regulation of the S100A6 protein level in non-metastatic SW480 cells due to overexpression of β-catenin as well as the activation of the S100A6 gene promoter upon cell transfection with β-catenin and the TCF-Lef1 transcription factor. Since we found a high level of S100A6 in metastatic SW620 cells we searched for its interacting partners in the protein extract prepared from these cells. Using several methods we found that S100A6 interacts with lamin A/C, a protein known to be implicated in colon carcinogenesis. Our results reveal a novel and important network of relations and interactions between proteins potentially involved in colorectal cancer development and progression.

The interferon stimulated gene 12 inactivates vasculoprotective functions of NR4A nuclear receptors

RATIONALE

Innate and adaptive immune responses alter numerous homeostatic processes that are controlled by nuclear hormone receptors. NR4A1 is a nuclear receptor that is induced in vascular pathologies, where it mediates protection.

OBJECTIVE

The underlying mechanisms that regulate the activity of NR4A1 during vascular injury are not clear. We therefore searched for modulators of NR4A1 function that are present during vascular inflammation.

METHODS AND RESULTS

We report that the protein encoded by interferon stimulated gene 12 (ISG12), is a novel interaction partner of NR4A1 that inhibits the transcriptional activities of NR4A1 by mediating its Crm1-dependent nuclear export. Using 2 models of vascular injury, we show that ISG12-deficient mice are protected from neointima formation. This effect is dependent on the presence of NR4A1, as mice deficient for both ISG12 and NR4A1 exhibit neointima formation similar to wild-type mice.

CONCLUSIONS

These findings identify a previously unrecognized feedback loop activated by interferons that inhibits the vasculoprotective functions of NR4A nuclear receptors, providing a potential new therapeutic target for interferon-driven pathologies.

Gene expression, chromosome position and lamin A/C mutations

The nuclear lamina is increasingly being appreciated for its epigenetic role in regulating gene expression. The nuclear lamina underlies the inner nuclear membrane and, in post mitotic cells, is composed of a latticework primarily formed by the intermediate filament protein, lamin A/C. Although not well defined, lamin-associated domains have been described, and these domains are determined by DNA sequence and chromatin conformation. Lamin-associated domains are positioned to mediate the interaction with the nuclear membrane, where they contribute to transcriptional regulation. Although lamin-associated domains are primarily considered to be repressive in nature, those nearer to nuclear pores may actually promote transcription. Mutations in LMNA, the gene encoding lamins A and C, are a relatively common cause of inherited cardiomyopathy. As substantial data supports a role for the lamina in its interaction with chromatin and gene regulation, we examined the role of a genetically disrupted lamina and the consequences thereof. A dominant LMNA mutation, E161K, that causes inherited cardiomyopathy was studied. Gene expression changes were profiled in a human cardiomyopathic E161K heart, and it was found that chromosome 13 had a high percentage of misexpressed genes. Chromosome 13 was also found to be less tightly associated with the nuclear membrane in E161K mutant cells, thereby linking abnormal gene expression and intranuclear position. These and other studies support a role for the nuclear membrane as an active regulator of gene expression and provide additional support that disrupting this regulation is a mechanism of human disease.

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.

Oxidative stress induces an ATM-independent senescence pathway through p38 MAPK-mediated lamin B1 accumulation

We report crosstalk between three senescence-inducing conditions, DNA damage response (DDR) defects, oxidative stress (OS) and nuclear shape alterations. The recessive autosomal genetic disorder Ataxia telangiectasia (A-T) is associated with DDR defects, endogenous OS and premature ageing. Here, we find frequent nuclear shape alterations in A-T cells, as well as accumulation of the key nuclear architecture component lamin B1. Lamin B1 overexpression is sufficient to induce nuclear shape alterations and senescence in wild-type cells, and normalizing lamin B1 levels in A-T cells reciprocally reduces both nuclear shape alterations and senescence. We further show that OS increases lamin B1 levels through p38 Mitogen Activated Protein kinase activation. Lamin B1 accumulation and nuclear shape alterations also occur during stress-induced senescence and oncogene-induced senescence (OIS), two canonical senescence situations. These data reveal lamin B1 as a general molecular mediator that controls OS-induced senescence, independent of established Ataxia Telangiectasia Mutated (ATM) roles in OIS.

Understanding and recognizing the Pelger-Huët anomaly

The Pelger-Huët anomaly (PHA) is a recognized morphologic variant affecting all granulocytes but is most evident in polymorphonuclear neutrophils (PMNs). PHA is caused by a decreased amount of the lamin B receptor (LBR). Recognition of PHA morphologic features serves as a marker for mutations in the LBR gene. This review summarizes the history of PHA and the current knowledge of the functions of the LBR. Guidance is given for distinguishing PHA from other hematologic disorders in which granulocytes may show similar changes. Recognition of PHA in the laboratory should prompt communication to the patient's physician about the possible clinical significance of this finding and the recommended screening for the anomaly in other family members by CBC and review of a peripheral blood smear.

Nuclear actin and lamins in viral infections

Lamins are the best characterized cytoskeletal components of the cell nucleus that help to maintain the nuclear shape and participate in diverse nuclear processes including replication or transcription. Nuclear actin is now widely accepted to be another cytoskeletal protein present in the nucleus that fulfills important functions in the gene expression. Some viruses replicating in the nucleus evolved the ability to interact with and probably utilize nuclear actin for their replication, e.g., for the assembly and transport of capsids or mRNA export. On the other hand, lamins play a role in the propagation of other viruses since nuclear lamina may represent a barrier for virions entering or escaping the nucleus. This review will summarize the current knowledge about the roles of nuclear actin and lamins in viral infections.

The nuclear envelope environment and its cancer connections

Because of the association between aberrant nuclear structure and tumour grade, nuclear morphology is an indispensible criterion in the current pathological assessment of cancer. Components of the nuclear envelope environment have central roles in many aspects of cell function that affect tumour development and progression. As the roles of the nuclear envelope components, including nuclear pore complexes and nuclear lamina, are being deciphered in molecular detail there are opportunities to harness this knowledge for cancer therapeutics and biomarker development. In this Review, we summarize the progress that has been made in our understanding of the nuclear envelope and the implications of changes in this environment for cancer biology.

Evolution: functional evolution of nuclear structure

The evolution of the nucleus, the defining feature of eukaryotic cells, was long shrouded in speculation and mystery. There is now strong evidence that nuclear pore complexes (NPCs) and nuclear membranes coevolved with the endomembrane system, and that the last eukaryotic common ancestor (LECA) had fully functional NPCs. Recent studies have identified many components of the nuclear envelope in living Opisthokonts, the eukaryotic supergroup that includes fungi and metazoan animals. These components include diverse chromatin-binding membrane proteins, and membrane proteins with adhesive lumenal domains that may have contributed to the evolution of nuclear membrane architecture. Further discoveries about the nucleoskeleton suggest that the evolution of nuclear structure was tightly coupled to genome partitioning during mitosis.

Lamin A/C protein is overexpressed in tissue-invading prostate cancer and promotes prostate cancer cell growth, migration and invasion through the PI3K/AKT/PTEN pathway

Prostate cancer (PC) remains the second most common cause of cancer-related death in Western countries. A previous proteomics study suggested that the nuclear membrane protein lamin A/C to be a maker to discriminate low- and high-Gleason score tumors and to identify high-risk cancers. To characterize its function in PC cells, we performed a detailed expression analysis in PC tissue and explored the consequences of down or upregulation of lamin A/C in PC cells. Our results confirm an increased lamin A/C protein expression in high-risk cancers and show association of expression with tumor cell formations at the invasion fronts of tumors and in invasion 'spearheading' tumor cell clusters. In the prostate tumor cell lines, LNCaP, DU145, and PC3 small hairpin RNA knockdown or overexpression of lamin A/C resulted in inhibition or stimulation, respectively, of cell growth, colony formation, migration and invasion. Further mechanism studies suggested that the lamin A/C-related malignant behavior is regulated through modulation of the phosphoinositide 3-kinase (PI3K)/AKT/PTEN signaling pathway. Western blot results indicated that knockdown or overexpression of lamin A/C decreased or increased, respectively, protein levels of the PI3K subunits p110 and p85 in all three cell lines; phosphor-AKT in the PTEN-negative cell lines LNCaP and PC3, and, increased or decreased, respectively, PTEN protein levels in PTEN-positive DU145 cells. Together, our data suggest that lamin A/C proteins are positively involved in malignant behavior of PC cells through the PI3K/AKT/PTEN pathway. Lamin A/C may represent a new oncogenic factor and a novel therapeutic target for PC.

Chromosomal regions associated with prostate cancer risk localize to lamin B-deficient microdomains and exhibit reduced gene transcription

The lamins are major determinants of nuclear shape and chromatin organization and these features are frequently altered in prostate cancer (CaP). Human CaP cell lines frequently have nuclear lobulations, which are enriched in A-type lamins but lack B-type lamins and have been defined as lamin B-deficient microdomains (LDMDs). LDMD frequency is correlated with CaP cell line aggressiveness and increased cell motility. In addition, LNCaP cells grown in the presence of dihydrotestosterone (DHT) show an increased frequency of LDMDs. The LDMDs are enriched in activated RNA polymerase II (Pol IIo) and androgen receptor (AR) and A-type lamins form an enlarged meshwork that appears to co-align with chromatin fibres and AR. Furthermore, fluorescence in situ hybridization and comparative genomic hybridization demonstrated that chromosomal regions associated with CaP susceptibility are preferentially localized to LDMDs. Surprisingly, these regions lack histone marks for transcript elongation and exhibit reduced BrU incorporation, suggesting that Pol II is stalled within LDMDs. Real-time PCR of genes near androgen response elements (AREs) was used to compare transcription between cells containing LDMDs and controls. Genes preferentially localized to LDMDs showed significantly decreased expression, while genes in the main nuclear body were largely unaffected. Furthermore, LDMDs were observed in human CaP tissue and the frequency was correlated with increased Gleason grade. These results imply that lamins are involved in chromatin organization and Pol II transcription, and provide insights into the development and progression of CaP.

The nuclear lamina and heterochromatin: a complex relationship

In metazoan cells, the heterochromatin is generally localized at the nuclear periphery, whereas active genes are preferentially found in the nuclear interior. In the present paper, we review current evidence showing that components of the nuclear lamina interact directly with heterochromatin, which implicates the nuclear lamina in a mechanism of specific gene retention at the nuclear periphery and release to the nuclear interior upon gene activation. We also discuss recent data showing that mutations in lamin proteins affect gene positioning and expression, providing a potential mechanism for how these mutations lead to tissue-specific diseases.

Barrier-to-Autointegration Factor influences specific histone modifications

Defects in the nuclear envelope or nuclear 'lamina' networks cause disease and can perturb histone posttranslational (epigenetic) regulation. Barrier-to-Autointegration Factor (BAF) is an essential but enigmatic lamina component that binds lamins, LEM-domain proteins, DNA and histone H3 directly. We report that BAF copurified with nuclease-digested mononucleosomes and associated with modified histones in vivo. BAF overexpression significantly reduced global histone H3 acetylation by 18%. In cells that stably overexpressed BAF 3-fold, silencing mark H3-K27-Me1/3 and active marks H4-K16-Ac and H4-Ac5 decreased significantly. Significant increases were also seen for silencing mark H3-K9-Me3, active marks H3-K4-Me2, H3-K9/K14-Ac and H4-K5-Ac and a mark (H3-K79-Me2) associated with both active and silent chromatin. Other increases (H3-S10-P, H3-S28-P and silencing mark H3-K9-Me2) did not reach statistical significance. BAF overexpression also significantly influenced cell cycle distribution. Moreover, BAF associated in vivo with SET/I2PP2A (protein phosphatase 2A inhibitor; blocks H3 dephosphorylation) and G9a (H3-K9 methyltransferase), but showed no detectable association with HDAC1 or HATs. These findings reveal BAF as a novel epigenetic regulator and are discussed in relation to BAF deficiency phenotypes, which include a hereditary progeria syndrome and loss of pluripotency in embryonic stem cells.

Regulating the levels of key factors in cell cycle and DNA repair: new pathways revealed by lamins

Spatial and temporal organization of the genome represents an additional step in the regulation of nuclear functions. The nuclear lamina, a polymeric meshwork formed by lamins (A/C and B type) and lamin-associated proteins, plays a key role in the maintenance of genome localization, structure and function. Specifically, mutations in the LMNA gene encoding lamins A/C or changes in its expression, either upregulation or silencing, are associated with defects in DNA replication, transcription and repair, as well as alterations in epigenetic modifications of chromatin. These data, together with the fact that defects in A-type lamins are associated with a whole variety of degenerative disorders, premature aging syndromes and cancer, support the notion that these proteins operate as caretakers of the genome. However, our understanding of their functions is limited due to the lack of well-defined mechanisms behind the genomic instability observed in lamin-related diseases. Here, we summarize our recent discovery of new pathways that are affected by the loss of A-type lamins. In particular, we found that A-type lamins control transcription and degradation of proteins with key roles in cell cycle regulation and DNA double-strand breaks (DSBs) repair by non-homologous end-joining (NHEJ) and homologous-recombination (HR). Importantly, the proteins regulated by A-type lamins--Rb family members, 53BP1, BRCA1 and RAD51--exert tumor suppressor functions, with their loss being associated with cancer susceptibility. Moreover, our studies revealed novel pathways that contribute to genomic instability and that can be activated in disease states independent of the status of A-type lamins.

A dual role for A-type lamins in DNA double-strand break repair

A-type lamins are emerging as regulators of nuclear organization and function. Changes in their expression are associated with cancer and mutations are linked to degenerative diseases -laminopathies-. Although a correlation exists between alterations in lamins and genomic instability, the molecular mechanisms remain largely unknown. We previously found that loss of A-type lamins leads to degradation of 53BP1 protein and defective long-range non-homologous end-joining (NHEJ) of dysfunctional telomeres. Here, we determined how loss of A-type lamins affects the repair of short-range DNA double-strand breaks (DSBs) induced by ionizing radiation (IR). We find that lamins deficiency allows activation of the DNA damage response, but compromises the accumulation of 53BP1 at IR-induced foci (IRIF), hindering the fast phase of repair corresponding to classical-NHEJ. Importantly, reconstitution of 53BP1 is sufficient to rescue long-range and short-range NHEJ. Moreover, we demonstrate an unprecedented role for A-type lamins in the maintenance of homologous recombination (HR). Depletion of lamins compromises HR by a mechanism involving transcriptional downregulation of BRCA1 and RAD51 by the repressor complex formed by the Rb family member p130 and E2F4. In line with the DNA repair defects, lamins-deficient cells exhibit increased radiosensitivity. This study demonstrates that A-type lamins promote genomic stability by maintaining the levels of proteins with key roles in DNA DSBs repair by NHEJ and HR. Our results suggest that silencing of A-type lamins by DNA methylation in some cancers could contribute to the genomic instability that drives malignancy. In addition, lamins-deficient tumor cells could represent a good target for radiation therapy.

A new pathway that regulates 53BP1 stability implicates cathepsin L and vitamin D in DNA repair

Genomic instability due to telomere dysfunction and defective repair of DNA double-strand breaks (DSBs) is an underlying cause of ageing-related diseases. 53BP1 is a key factor in DNA DSBs repair and its deficiency is associated with genomic instability and cancer progression. Here, we uncover a novel pathway regulating the stability of 53BP1. We demonstrate an unprecedented role for the cysteine protease Cathepsin L (CTSL) in the degradation of 53BP1. Overexpression of CTSL in wild-type fibroblasts leads to decreased 53BP1 protein levels and changes in its cellular distribution, resulting in defective repair of DNA DSBs. Importantly, we show that the defects in DNA repair associated with 53BP1 deficiency upon loss of A-type lamins are due to upregulation of CTSL. Furthermore, we demonstrate that treatment with vitamin D stabilizes 53BP1 and promotes DNA DSBs repair via inhibition of CTSL, providing an as yet unsuspected link between vitamin D action and DNA repair. Given that CTSL upregulation is a hallmark of cancer and progeria, regulation of this pathway could be of great therapeutic significance for these diseases.

Herpesviruses and intermediate filaments: close encounters with the third type

Intermediate filaments (IF) are essential to maintain cellular and nuclear integrity and shape, to manage organelle distribution and motility, to control the trafficking and pH of intracellular vesicles, to prevent stress-induced cell death, and to support the correct distribution of specific proteins. Because of this, IF are likely to be targeted by a variety of pathogens, and may act in favor or against infection progress. As many IF functions remain to be identified, however, little is currently known about these interactions. Herpesviruses can infect a wide variety of cell types, and are thus bound to encounter the different types of IF expressed in each tissue. The analysis of these interrelationships can yield precious insights into how IF proteins work, and into how viruses have evolved to exploit these functions. These interactions, either known or potential, will be the focus of this review.

Breach of the nuclear lamina during assembly of herpes simplex viruses

Beneath the inner nuclear membrane lies the dense meshwork of the nuclear lamina, which provides structural support for the nuclear envelope and serves as an important organizing center for a number of nuclear and cytoplasmic constituents and processes. Herpesviruses have a significant and wide-ranging impact on human health, and their capacity to replicate and cause disease includes events that occur in the host cell nucleus. Herpesviruses begin assembly of progeny virus in the nuclei of infected cells and their capsids must escape the confines of the nucleus by budding through the inner nuclear membrane (INM) to proceed with later stages of virion assembly and egress. Access of viral capsids to the INM thus necessitates disruption of the dense nuclear lamina layer. We review herpesvirus effects on the nuclear lamina and in particular the roles of the herpes simplex virus-encoded nuclear envelope complex and viral kinases on lamin phosphorylation, dissociation, and nucleocapsid envelopment at the INM.

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.

Loss of lamin A/C expression in stage II and III colon cancer is associated with disease recurrence

AIM OF THE STUDY

Loss of the nuclear lamina protein lamin A/C (LMNA) has been observed in several human malignancies. The present study aimed to investigate associations between LMNA expression and clinical outcome in colon cancer patients.

PATIENTS AND METHODS

Clinicopathological data and formalin-fixed paraffin embedded tissues were collected from 370 stage II and III colon cancer patients. Tissue microarrays were constructed, stained for lamin A/C and evaluated microscopically. Microsatellite instability status was determined for 318 tumours.

RESULTS

Low levels of LMNA expression were observed in 17.8% of colon tumours, with disease recurrence occurring in 45.5% of stage II and III colon cancer patients with LMNA-low expressing tumours compared to 29.6% of patients with LMNA-high expressing tumours (p=0.01). For stage II patients, disease recurrence was observed for 35.7% of LMNA-low compared to 20.3% of LMNA-high expressing tumours (p=0.03). Microsatellite stable (MSS) tumours exhibited more frequently low LMNA expression than microsatellite instable (MSI) tumours (21% versus 9.8%; p=0.05). Interestingly, disease recurrence among LMNA-low and LMNA-high expressing MSS tumours varied significantly for stage III patients who had not received adjuvant chemotherapy (100% versus 37.8%; p<0.01) while no such difference was observed for patients who received adjuvant chemotherapy (46.7% versus 46.0%; p=0.96).

CONCLUSION

These data indicate that low expression of LMNA is associated with an increased disease recurrence in stage II and III colon cancer patients, and suggest that these patients in particular may benefit from adjuvant chemotherapy.

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

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

Laminopathies and lamin-associated signaling pathways

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

Osteoblasts from a mandibuloacral dysplasia patient induce human blood precursors to differentiate into active osteoclasts

Mandibuloacral dysplasia type A (MADA) is a rare disease caused by mutations in the LMNA gene encoding A type lamins. Patients affected by mandibuloacral dysplasia type A suffer from partial lipodystrophy, skin abnormalities and accelerated aging. Typical of mandibuloacral dysplasia type A is also bone resorption at defined districts including terminal phalanges, mandible and clavicles. Little is known about the biological mechanism underlying osteolysis in mandibuloacral dysplasia type A. In the reported study, we analyzed an osteoblast primary culture derived from the cervical vertebrae of a mandibuloacral dysplasia type A patient bearing the homozygous R527H LMNA mutation. Mandibuloacral dysplasia type A osteoblasts showed nuclear abnormalities typical of laminopathic cells, but they proliferated in culture and underwent differentiation upon stimulation with dexamethasone and beta-glycerophosphate. Differentiated osteoblasts showed proper production of bone mineral matrix until passage 8 in culture, suggesting a good differentiation activity. In order to evaluate whether mandibuloacral dysplasia type A osteoblast-derived factors affected osteoclast differentiation or activity, we used a conditioned medium from mandibuloacral dysplasia type A or control cultures to treat normal human peripheral blood monocytes and investigated whether they were induced to differentiate into osteoclasts. A higher osteoclast differentiation and matrix digestion rate was obtained in the presence of mandibuloacral dysplasia type A osteoblast medium with respect to normal osteoblast medium. Further, TGFbeta 2 and osteoprotegerin expression were enhanced in mandibuloacral dysplasia type A osteoblasts while the RANKL/osteoprotegerin ratio was diminished. Importantly, inhibition of TGFbeta 2 by a neutralizing antibody abolished the effect of mandibuloacral dysplasia type A conditioned medium on osteoclast differentiation. These data argue in favor of an altered bone turnover in mandibuloacral dysplasia type A, caused by upregulation of bone-derived stimulatory cytokines, which activate non-canonical differentiation stimuli. In this context, TGFbeta 2 appears as a major player in the osteolytic process that affects mandibuloacral dysplasia type A patients.

Biology of the Mi-2/NuRD Complex in SLAC (Stemness, Longevity/Ageing, and Cancer)

The dynamic chromatin activities of Mi-2/Nucleosome Remodeling and Histone deacetylation (Mi-2/NuRD) complexes in mammals are at the basis of current research on stemness, longevity/ageing, and cancer (4-2-1/SLAC), and have been widely studied over the past decade in mammals and the elegant model organism, Caenorhabditis elegans. Interestingly, a common emergent theme from these studies is that of distinct coregulator-recruited Mi-2/NuRD complexes largely orchestrating the 4-2-1/SLAC within a unique paradigm by maintaining genome stability via DNA repair and controlling three types of transcriptional programs in concert in a number of cellular, tissue, and organism contexts. Thus, the core Mi-2/NuRD complex plays a central role in 4-2-1/SLAC. The plasticity and robustness of 4-2-1/SLAC can be interpreted as modulation of specific coregulator(s) within cell-specific, tissue-specific, stage-specific, or organism-specific niches during stress induction, ie, a functional module and its networking, thereby conferring differential responses to different environmental cues. According to “Occam’s razor”, a simple theory is preferable to a complex one, so this simplified notion might be useful for exploring 4-2-1/SLAC with a holistic view. This thought could also be valuable in forming strategies for future research, and could open up avenues for cancer prevention and antiageing strategies.

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.

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.

The insulator protein SU(HW) fine-tunes nuclear lamina interactions of the Drosophila genome

Specific interactions of the genome with the nuclear lamina (NL) are thought to assist chromosome folding inside the nucleus and to contribute to the regulation of gene expression. High-resolution mapping has recently identified hundreds of large, sharply defined lamina-associated domains (LADs) in the human genome, and suggested that the insulator protein CTCF may help to demarcate these domains. Here, we report the detailed structure of LADs in Drosophila cells, and investigate the putative roles of five insulator proteins in LAD organization. We found that the Drosophila genome is also organized in discrete LADs, which are about five times smaller than human LADs but contain on average a similar number of genes. Systematic comparison to new and published insulator binding maps shows that only SU(HW) binds preferentially at LAD borders and at specific positions inside LADs, while GAF, CTCF, BEAF-32 and DWG are mostly absent from these regions. By knockdown and overexpression studies we demonstrate that SU(HW) weakens genome - NL interactions through a local antagonistic effect, but we did not obtain evidence that it is essential for border formation. Our results provide insights into the evolution of LAD organization and identify SU(HW) as a fine-tuner of genome - NL interactions.

Interactions between nuclei and the cytoskeleton are mediated by SUN-KASH nuclear-envelope bridges

The nuclear envelope links the cytoskeleton to structural components of the nucleus. It functions to coordinate nuclear migration and anchorage, organize chromatin, and aid meiotic chromosome pairing. Forces generated by the cytoskeleton are transferred across the nuclear envelope to the nuclear lamina through a nuclear-envelope bridge consisting of SUN (Sad1 and UNC-84) and KASH (Klarsicht, ANC-1 and Syne/Nesprin homology) proteins. Some KASH-SUN combinations connect microtubules, centrosomes, actin filaments, or intermediate filaments to the surface of the nucleus. Other combinations are used in cell cycle control, nuclear import, or apoptosis. Interactions between the cytoskeleton and the nucleus also affect global cytoskeleton organization. SUN and KASH proteins were identified through genetic screens for mispositioned nuclei in model organisms. Knockouts of SUN or KASH proteins disrupt neurological and muscular development in mice. Defects in SUN and KASH proteins have been linked to human diseases including muscular dystrophy, ataxia, progeria, lissencephaly, and cancer.

Genomics tools for unraveling chromosome architecture

The spatial organization of chromosomes inside the cell nucleus is still poorly understood. This organization is guided by intra- and interchromosomal contacts and by interactions of specific chromosomal loci with relatively fixed nuclear 'landmarks' such as the nuclear envelope and the nucleolus. Researchers have begun to use new molecular genome-wide mapping techniques to uncover both types of molecular interactions, providing insights into the fundamental principles of interphase chromosome folding.

Escape of herpesviruses from the nucleus

The nuclear envelope of eukaryotic cells is composed of double lipid-bilayer membranes, the membrane-connected nuclear pore complexes and an underlying nuclear lamina network. The nuclear pore complexes serve as gates for regulating the transport of macromolecules between cytoplasm and nucleus. The nuclear lamina not only provides an intact meshwork for maintaining the nuclear stiffness but also presents a natural barrier against most DNA viruses. Herpesviruses are large DNA viruses associated with multiple human and animal diseases. The complex herpesviral virion contains more than 30 viral proteins. After viral DNA replication, the newly synthesised genome is packaged into the pre-assembled intranuclear capsid. The nucleocapsid must then transverse through the nuclear envelope to the cytoplasm for the subsequent maturation process. Information regarding how nucleocapsid breaches the rigid nuclear lamina barrier and accesses the inner nuclear membrane for primary envelopment has emerged recently. From the point of view of both viral components and nuclear structure, this review summarises recent advances in the complicated protein-protein interactions and the phosphorylation regulations involved in the nuclear egress of herpesviral nucleocapsids.

LINCing lamin B2 to neuronal migration: growing evidence for cell-specific roles of B-type lamins

Nuclear lamins are major components of the nuclear lamina, and play essential roles in supporting the nucleus and organizing nuclear structures. While a large number of clinically important mutations have been mapped to the LMNA gene in humans, very few mutations have been associated with the B-type lamins. We have shown that lamin B2-deficiency in mice results in severe brain abnormalities. While the early stages of forebrain development in lamin B2-deficient mice appear to be normal, cortical neurons fail to migrate and organize into proper layers within the cerebral cortex. The morphogenesis of the hippocampus and cerebellum is also severely impaired. These phenotypes are reminiscent of lissencephaly, a human brain developmental disorder characterized by an abnormal neuronal migration. Most mutations in lissencephaly patients affect cytoplasmic regulators of nuclear translocation, which is a crucial step in neuronal migration. The phenotypes of lamin B2-deficient mice suggest that lamin B2 may also play a key role in nuclear translocation. Potential mechanisms for lamin B2 involvement, which include mechanical and non-mechanical roles, and participation in LINC complexes in the nuclear envelope, are discussed along with evidence that lamins B1 and B2 play distinct, cell-specific functions.

Molecular maps of the reorganization of genome-nuclear lamina interactions during differentiation

The three-dimensional organization of chromosomes within the nucleus and its dynamics during differentiation are largely unknown. To visualize this process in molecular detail, we generated high-resolution maps of genome-nuclear lamina interactions during subsequent differentiation of mouse embryonic stem cells via lineage-committed neural precursor cells into terminally differentiated astrocytes. This reveals that a basal chromosome architecture present in embryonic stem cells is cumulatively altered at hundreds of sites during lineage commitment and subsequent terminal differentiation. This remodeling involves both individual transcription units and multigene regions and affects many genes that determine cellular identity. Often, genes that move away from the lamina are concomitantly activated; many others, however, remain inactive yet become unlocked for activation in a next differentiation step. These results suggest that lamina-genome interactions are widely involved in the control of gene expression programs during lineage commitment and terminal differentiation.

Genome-nuclear lamina interactions and gene regulation

The nuclear lamina, a filamentous protein network that coats the inner nuclear membrane, has long been thought to interact with specific genomic loci and regulate their expression. Molecular mapping studies have now identified large genomic domains that are in contact with the lamina. Genes in these domains are typically repressed, and artificial tethering experiments indicate that the lamina can actively contribute to this repression. Furthermore, the lamina indirectly controls gene expression in the nuclear interior by sequestration of certain transcription factors. A variety of DNA-binding and chromatin proteins may anchor specific loci to the lamina, while histone-modifying enzymes partly mediate the local repressive effect of the lamina. Experimental tools are now available to begin to unravel the underlying molecular mechanisms.

Intranuclear membranes induced by lipidated proteins are derived from the nuclear envelope

Association of nuclear lamins with the inner nuclear membrane (INM) is mediated by lipid modifications: either by C-terminal isoprenylation or N-terminal myristoylation. Overexpression of lamins or other lipidated nuclear proteins induces the formation of intranuclear membrane-like arrays. Lamin-induced intranuclear array formation has been observed in Xenopus oocytes as well as in mammalian tissue culture cells. With the use of a membrane-specific fluorescence dye we show here that these arrays are made up of typical lipid membranes. While continuity between these intranuclear membranes and the INM has not been observed so far the presence of integral as well as luminal marker proteins of the endoplasmic reticulum (ER) indicates that these membranes are derived from the nuclear membrane/ER compartment. Earlier studies demonstrated that overexpression of integral membrane proteins of the INM can induce formation of intranuclear membranes, which bud from the INM. Integral membrane proteins reach the INM via the pore membranes while lipidated proteins are imported into the nucleoplasm via the classical NLS pathway where they interact with the INM via their lipid moieties. Together with the previously published data our results show that the formation of intranuclear membranes follows similar routes irrespective of whether the proteins triggering membrane formation are integral membrane or lipidated proteins.

RASCAL is a new human cytomegalovirus-encoded protein that localizes to the nuclear lamina and in cytoplasmic vesicles at late times postinfection

The products of numerous open reading frames (ORFs) present in the genome of human cytomegalovirus (CMV) have not been characterized. Here, we describe the identification of a new CMV protein localizing to the nuclear envelope and in cytoplasmic vesicles at late times postinfection. Based on this distinctive localization pattern, we called this new protein nuclear rim-associated cytomegaloviral protein, or RASCAL. Two RASCAL isoforms exist, a short version of 97 amino acids encoded by the majority of CMV strains and a longer version of 176 amino acids encoded by the Towne, Toledo, HAN20, and HAN38 strains. Both isoforms colocalize with lamin B in deep intranuclear invaginations of the inner nuclear membrane (INM) and in novel cytoplasmic vesicular structures possibly derived from the nuclear envelope. INM infoldings have been previously described as sites of nucleocapsid egress, which is mediated by the localized disruption of the nuclear lamina, promoted by the activities of viral and cellular kinases recruited by the lamina-associated proteins UL50 and UL53. RASCAL accumulation at the nuclear membrane required the presence of UL50 but not of UL53. RASCAL and UL50 also appeared to specifically interact, suggesting that RASCAL is a new component of the nuclear egress complex (NEC) and possibly involved in mediating nucleocapsid egress from the nucleus. Finally, the presence of RASCAL within cytoplasmic vesicles raises the intriguing possibility that this protein might participate in additional steps of virion maturation occurring after capsid release from the nucleus.

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.

Lamins: 'structure goes cycling'

Nuclear intermediate filaments formed by A- and B-type lamins are central components of the nucleoskeleton and are required for the architecture and integrity of the nucleus. There is growing evidence that lamins are also involved in regulatory pathways controlling cell proliferation and differentiation. Lamins affect the activity of several transcription factors, such as retinoblastoma protein and c-Fos, and signalling pathways, such as the ERK1/2 (extracellular-signal-regulated kinase 1/2) and Notch pathways, which are key regulators of cell-cycle progression and differentiation. During mitosis, lamins are dynamically reorganized and play active roles in spindle matrix formation and in post-mitotic nuclear reassembly. Several of the cell-cycle-regulating functions of lamins may be impaired in the diseases linked to mutations in lamins and lamin-associated proteins, including striated muscle diseases, lipodystrophies and premature aging syndromes, and contribute to the tissue-specific disease pathologies.

Lamin A-linked progerias: is farnesylation the be all and end all?

HGPS (Hutchinson–Gilford progeria syndrome) is a severe childhood disorder that appears to mimic an accelerated aging process. The disease is most commonly caused by gene mutations that disrupt the normal post-translational processing of lamin A, a structural component of the nuclear envelope. Impaired processing results in aberrant retention of a farnesyl group at the C-terminus of lamin A, leading to altered membrane dynamics. It has been widely proposed that persistence of the farnesyl moiety is the major factor responsible for the disease, prompting clinical trials of farnesyltransferase inhibitors to prevent lamin A farnesylation in children afflicted with HGPS. Although there is evidence implicating farnesylation in causing some of the cellular defects of HGPS, results of several recent studies suggest that aberrant lamin A farnesylation is not the only determinant of the disease. These findings have important implications for the design of treatments for this devastating disease.