Nuclear Lamins: Laminopathies and Their Role in Premature Ageing

Lamin A/C deficiency is an independent risk factor for cervical cancer

BACKGROUND

In the past, cervical cancer has been linked to Human Papilloma Virus (HPV) infection. Previously, we found that pre-neoplastic breast and ovarian lesions may be associated with lamin A/C deficiency, resulting in abnormal nuclear morphologies and chromosomal instability. Ultimately, these phenomena are thought to lead to cancer. Here, we assessed lamin A/C deficiency as an indicator for the risk to develop cervical cancer.

METHODS

The expression of lamin A/C was assessed by Western blotting in cervical uterine smears (CUS) of 76 adult women from Benin concomitant with nuclear morphology assessment and HPV genotyping using microscopy and PCR-based assays, respectively. In vitro analyses were performed to uncover the mechanism underlying lamin A/C expression alterations observed in vivo. The presence of cervical intra-epithelial neoplasia (CIN) was assessed by colposcopy.

RESULTS

Normal lamin A/C expression (group A) was observed in 39% of the CUS, weak lamin A/C expression (group B) was observed in 28% of the CUS and no lamin A/C expression (group C) was observed in 33% of the CUS tested. Infection with oncogenic HPV was found to be significantly higher in group C (36%) than in groups A (17%) and B (14%). Two years after our first assessment, CIN was observed in 20% of the women in group C. The in vitro application of either a histone deacetylase inhibitor (trichostatin) or a protein kinase inhibitor (staurosporine) was found to restore lamin A/C expression in cervical cancer-derived cells.

CONCLUSION

Lamin A/C deficiency may serve as an independent risk factor for CIN development and as an indicator for preventive therapy in cervical cancer.

Heading in the Right Direction: Understanding Cellular Orientation Responses to Complex Biophysical Environments

The aim of cardiovascular regeneration is to mimic the biological and mechanical functioning of tissues. For this it is crucial to recapitulate the in vivo cellular organization, which is the result of controlled cellular orientation. Cellular orientation response stems from the interaction between the cell and its complex biophysical environment. Environmental biophysical cues are continuously detected and transduced to the nucleus through entwined mechanotransduction pathways. Next to the biochemical cascades invoked by the mechanical stimuli, the structural mechanotransduction pathway made of focal adhesions and the actin cytoskeleton can quickly transduce the biophysical signals directly to the nucleus. Observations linking cellular orientation response to biophysical cues have pointed out that the anisotropy and cyclic straining of the substrate influence cellular orientation. Yet, little is known about the mechanisms governing cellular orientation responses in case of cues applied separately and in combination. This review provides the state-of-the-art knowledge on the structural mechanotransduction pathway of adhesive cells, followed by an overview of the current understanding of cellular orientation responses to substrate anisotropy and uniaxial cyclic strain. Finally, we argue that comprehensive understanding of cellular orientation in complex biophysical environments requires systematic approaches based on the dissection of (sub)cellular responses to the individual cues composing the biophysical niche.

DAMPs, ageing, and cancer: The 'DAMP Hypothesis'

Ageing is a complex and multifactorial process characterized by the accumulation of many forms of damage at the molecular, cellular, and tissue level with advancing age. Ageing increases the risk of the onset of chronic inflammation-associated diseases such as cancer, diabetes, stroke, and neurodegenerative disease. In particular, ageing and cancer share some common origins and hallmarks such as genomic instability, epigenetic alteration, aberrant telomeres, inflammation and immune injury, reprogrammed metabolism, and degradation system impairment (including within the ubiquitin-proteasome system and the autophagic machinery). Recent advances indicate that damage-associated molecular pattern molecules (DAMPs) such as high mobility group box 1, histones, S100, and heat shock proteins play location-dependent roles inside and outside the cell. These provide interaction platforms at molecular levels linked to common hallmarks of ageing and cancer. They can act as inducers, sensors, and mediators of stress through individual plasma membrane receptors, intracellular recognition receptors (e.g., advanced glycosylation end product-specific receptors, AIM2-like receptors, RIG-I-like receptors, and NOD1-like receptors, and toll-like receptors), or following endocytic uptake. Thus, the DAMP Hypothesis is novel and complements other theories that explain the features of ageing. DAMPs represent ideal biomarkers of ageing and provide an attractive target for interventions in ageing and age-associated diseases.

BioID Identification of Lamin-Associated Proteins

A- and B-type lamins support the nuclear envelope, contribute to heterochromatin organization, and regulate a myriad of nuclear processes. The mechanisms by which lamins function in different cell types and the mechanisms by which lamin mutations cause over a dozen human diseases (laminopathies) remain unclear. The identification of proteins associated with lamins is likely to provide fundamental insight into these mechanisms. BioID (proximity-dependent biotin identification) is a unique and powerful method for identifying protein-protein and proximity-based interactions in living cells. BioID utilizes a mutant biotin ligase from bacteria that is fused to a protein of interest (bait). When expressed in living cells and stimulated with excess biotin, this BioID-fusion protein promiscuously biotinylates directly interacting and vicinal endogenous proteins. Following biotin-affinity capture, the biotinylated proteins can be identified using mass spectrometry. BioID thus enables screening for physiologically relevant protein associations that occur over time in living cells. BioID is applicable to insoluble proteins such as lamins that are often refractory to study by other methods and can identify weak and/or transient interactions. We discuss the use of BioID to elucidate novel lamin-interacting proteins and its applications in a broad range of biological systems, and provide detailed protocols to guide new applications.

Nuclear envelope lamin-A as a coordinator of T cell activation

Nuclear lamins A/C control several critical cellular functions, e.g., chromatin organization, gene transcription, DNA replication, DNA damage responses, cell cycle progression, cell differentiation, and cell polarization during migration. However, few studies have addressed the role of lamins A/C in the control of the functions of immune cells. Recently, we have demonstrated that lamins A/C are induced in T cells upon antigen recognition. Lamins A/C enhance T cell responses by coupling the plasma membrane to the nucleus via the linker of nucleoskeleton and cytoskeleton (LINC) complex and the actin cytoskeleton. Here, we discuss the possible physiological relevance and functional context of lamin A/C in T cell activation and propose a model in which lamins A/C are key modulators of immune cell functions.

Skin Disease in Laminopathy-Associated Premature Aging

The nuclear lamina, a protein network located under the nuclear membrane, has during the past decade found increasing interest due to its significant involvement in a range of genetic diseases, including the segmental premature aging syndromes Hutchinson-Gilford progeria syndrome, restrictive dermopathy, and atypical Werner syndrome. In this review we examine these diseases, some caused by mutations in the LMNA gene, and their skin disease features. Advances within this area might also provide novel insights into the biology of skin aging, as recent data suggest that low levels of progerin are expressed in unaffected individuals and these levels increase with aging.

Emerging roles of sumoylation in the regulation of actin, microtubules, intermediate filaments, and septins

Sumoylation is a powerful regulatory system that controls many of the critical processes in the cell, including DNA repair, transcriptional regulation, nuclear transport, and DNA replication. Recently, new functions for SUMO have begun to emerge. SUMO is covalently attached to components of each of the four major cytoskeletal networks, including microtubule-associated proteins, septins, and intermediate filaments, in addition to nuclear actin and actin-regulatory proteins. However, knowledge of the mechanisms by which this signal transduction system controls the cytoskeleton is still in its infancy. One story that is beginning to unfold is that SUMO may regulate the microtubule motor protein dynein by modification of its adaptor Lis1. In other instances, cytoskeletal elements can both bind to SUMO non-covalently and also be conjugated by it. The molecular mechanisms for many of these new functions are not yet clear, but are under active investigation. One emerging model links the function of MAP sumoylation to protein degradation through SUMO-targeted ubiquitin ligases, also known as STUbL enzymes. Other possible functions for cytoskeletal sumoylation are also discussed.

DNA repair defects and genome instability in Hutchinson-Gilford Progeria Syndrome

The integrity of the nuclear lamina has emerged as an important factor in the maintenance of genome stability. In particular, mutations in the LMNA gene, encoding A-type lamins (lamin A/C), alter nuclear morphology and function, and cause genomic instability. LMNA gene mutations are associated with a variety of degenerative diseases and devastating premature aging syndromes such as Hutchinson-Gilford Progeria Syndrome (HGPS) and Restrictive Dermopathy (RD). HGPS is a severe laminopathy, with patients dying in their teens from myocardial infarction or stroke. HGPS patient-derived cells exhibit nuclear shape abnormalities, changes in epigenetic regulation and gene expression, telomere shortening, genome instability, and premature senescence. This review highlights recent advances in identifying molecular mechanisms that contribute to the pathophysiology of HGPS, with a special emphasis on DNA repair defects and genome instability.

Regulation of adipogenesis and osteogenesis in mesenchymal stem cells by vascular endothelial growth factor A

Understanding the mechanisms by which bone marrow mesenchymal stem cells (BMSCs) differentiate into bone-forming osteoblasts and marrow adipocytes is crucial to develop strategies for the treatment of several bone diseases. Age-related bone loss resulting in osteopenia and osteoporosis has been associated with reduced numbers of osteoblasts and increased numbers of adipocytes, likely originating from differentiation defects in BMSCs. Although many factors involved in the complex regulation of osteoblast and adipocyte cell lineages have previously been identified, their functional interactions in the context of BMSC differentiation and maintenance of bone homeostasis during ageing are unknown. Recent discoveries have provided important new insights into the mechanisms by which the nuclear envelope protein lamin A and vascular endothelial growth factor A (VEGF) mutually control BMSC fate. Particularly interesting is the finding that VEGF in this context functions as an intracellular protein, unaffected by neutralizing antibodies, and not as a secreted growth factor. These insights may not only facilitate the identification of new targets for treating bone diseases but also lead to improved design of tissue engineering approaches aimed at stimulating bone regeneration and repair.

Mutant lamin A links prophase to a p53 independent senescence program

Expression of oncogenes or short telomeres can trigger an anticancer response known as cellular senescence activating the p53 and RB tumor suppressor pathways. This mechanism is switched off in most tumor cells by mutations in p53 and RB signaling pathways. Surprisingly, p53 disabled tumor cells could be forced into senescence by expression of a mutant allele of the nuclear envelope protein lamin A. The pro-senescence lamin A mutant contains a deletion in the sequence required for processing by the protease ZMPSTE24 leading to accumulation of farnesylated lamin A in the nuclear envelope. In addition, the serine at position 22, a target for CDK1-dependent phosphorylation, was mutated to alanine, preventing CDK1-catalyzed nuclear envelope disassembly. The accumulation of this mutant lamin A compromised prophase to prometaphase transition leading to invaginations of the nuclear lamina, nuclear fragmentation and impaired chromosome condensation. Cells exited this impaired mitosis without cytokinesis and re-replicated their DNA ultimately arresting in interphase as polyploid cells with features of cellular senescence including increased expression of inflammatory gene products and a significant reduction of tumorigenicity in vivo.

Sustained accumulation of prelamin A and depletion of lamin A/C both cause oxidative stress and mitochondrial dysfunction but induce different cell fates

The cell nucleus is structurally and functionally organized by lamins, intermediate filament proteins that form the nuclear lamina. Point mutations in genes that encode a specific subset of lamins, the A-type lamins, cause a spectrum of diseases termed laminopathies. Recent evidence points to a role for A-type lamins in intracellular redox homeostasis. To determine whether lamin A/C depletion and prelamin A accumulation differentially induce oxidative stress, we have performed a quantitative microscopy-based analysis of reactive oxygen species (ROS) levels and mitochondrial membrane potential (Δψ_m) in human fibroblasts subjected to sustained siRNA-mediated knockdown of LMNA and ZMPSTE24, respectively. We measured a highly significant increase in basal ROS levels and an even more prominent rise of induced ROS levels in lamin A/C depleted cells, eventually resulting in Δψ_m hyperpolarization and apoptosis. Depletion of ZMPSTE24 on the other hand, triggered a senescence pathway that was associated with moderately increased ROS levels and a transient Δψ_m depolarization. Both knockdowns were accompanied by an upregulation of several ROS detoxifying enzymes. Taken together, our data suggest that both persistent prelamin A accumulation and lamin A/C depletion elevate ROS levels, but to a different extent and with different effects on cell fate. This may contribute to the variety of disease phenotypes witnessed in laminopathies.

Identification of tissue-specific regulatory region in the zebrafish lamin A promoter

Lamins are major structural proteins present in the nuclei of metazoan cells and contribute significantly to nuclear organization and function. The expression of different types of lamins is developmentally regulated and lamin A is detectable in most differentiated tissues. Although the proximal promoter of the mammalian lamin A gene has been characterized, the tissue-specific regulatory elements of the gene have not been identified. In this study, we have cloned and functionally characterized a 2.99 kb segment upstream of exon 1 in the zebrafish lamin A gene. This fragment was able to drive GFP expression in several tissues of the developing embryo at 14-72 h post fertilization in stable transgenic lines. Deletion fragments of the 2.99 kb promoter were analyzed by microinjection into zebrafish embryos in transient assays as well as by luciferase reporter assays in cultured cells. A minimal promoter segment of 1.24 kb conferred tissue-specific expression of GFP in the zebrafish embryo as well as in a myoblast cell line. An 86 bp fragment of this 1.24 kb segment was able to activate a heterologous promoter in myoblasts. Mutational analysis revealed the importance of muscle-specific regulatory motifs in the promoter. Our results have important implications for understanding the tissue-specific regulation and functions of the lamin A gene.

Lamin A/C-dependent interaction with 53BP1 promotes cellular responses to DNA damage

Lamins A/C have been implicated in DNA damage response pathways. We show that the DNA repair protein 53BP1 is a lamin A/C binding protein. In undamaged human dermal fibroblasts (HDF), 53BP1 is a nucleoskeleton protein. 53BP1 binds to lamins A/C via its Tudor domain, and this is abrogated by DNA damage. Lamins A/C regulate 53BP1 levels and consequently lamin A/C-null HDF display a 53BP1 null-like phenotype. Our data favour a model in which lamins A/C maintain a nucleoplasmic pool of 53BP1 in order to facilitate its rapid recruitment to sites of DNA damage and could explain why an absence of lamin A/C accelerates aging.

Routine phasing of coiled-coil protein crystal structures with AMPLE

Coiled-coil protein folds are among the most abundant in nature. These folds consist of long wound α-helices and are architecturally simple, but paradoxically their crystallographic structures are notoriously difficult to solve with molecular-replacement techniques. The program AMPLE can solve crystal structures by molecular replacement using ab initio search models in the absence of an existent homologous protein structure. AMPLE has been benchmarked on a large and diverse test set of coiled-coil crystal structures and has been found to solve 80% of all cases. Successes included structures with chain lengths of up to 253 residues and resolutions down to 2.9 Å, considerably extending the limits on size and resolution that are typically tractable by ab initio methodologies. The structures of two macromolecular complexes, one including DNA, were also successfully solved using their coiled-coil components. It is demonstrated that both the ab initio modelling and the use of ensemble search models contribute to the success of AMPLE by comparison with phasing attempts using single structures or ideal polyalanine helices. These successes suggest that molecular replacement with AMPLE should be the method of choice for the crystallo-graphic elucidation of a coiled-coil structure. Furthermore, AMPLE may be able to exploit the presence of a coiled coil in a complex to provide a convenient route for phasing.

Tissue specific loss of A-type lamins in the gastrointestinal epithelium can enhance polyp size

The nuclear lamina, comprised of the A and B-type lamins, is important in maintaining nuclear shape and in regulating key nuclear functions such as chromatin organization and transcription. Deletion of the A-type lamins results in genome instability and many cancers show altered levels of A-type lamin expression. Loss of function mutations in the mouse Lmna gene result in early postnatal lethality, usually within 3-5 weeks of birth making an analysis of the role of lamins in carcinogenesis difficult. To circumvent early lethality, and determine the role of the A-type lamins in specific tissues in older mice we derived a conditional allele of Lmna(FL/FL) (floxed). Lmna(FL/FL) was specifically deleted in the gastrointestinal (GI) epithelium by crossing the Lmna(FL/FL) mice with Villin-Cre mice. Mice lacking Lmna in the GI are overtly normal with no effects on overall growth, longevity or GI morphology. On a GI specific sensitized (Apc(Min/+)) background, polyp numbers are unchanged, but polyp size is slightly increased, and only in the duodenum. Our findings reveal that although A-type lamins are dispensable in the postnatal GI epithelium, loss of Lmna under malignant conditions may, to a limited extent, enhance polyp size indicating that A-type lamins may regulate cell proliferation in the transformed GI epithelium.

The role of lamin B1 for the maintenance of nuclear structure and function

Lamins constitute an integral structural component of the nuclear lamina. However, their impact on the structure and stability of chromosome territories, and on the regulation of gene expression is explored to a lesser extent. By 3D-FISH, Camps and colleagues showed that lamin B1 (LMNB1) is required for proper chromosome condensation in interphase nuclei, and deficiency of LMNB1 triggers the relocation of the epigenetic mark of facultative heterochromatin, H3K27me3, toward the interior of the nucleus. Additionally, LMNB1 repression slowed cellular growth due to S-phase delays and increased genomic instability. Finally, silencing of LMNB1 resulted in enlarged nuclear speckles and in extensive changes in alternative splicing of multiple genes. Altogether, the data suggest a central role of LMNB1 for the condensation of chromosome territories, for the distribution of heterochromatin, and for the regulation of gene expression and splicing.

Evolutionary genetic bases of longevity and senescence

Senescence, as a time-dependent developmental process, affects all organisms at every stage in their development and growth. During this process, genetic, epigenetic and environmental factors are known to introduce a wide range of variation for longevity among individuals. As an important life-history trait, longevity shows ontogenetic relationships with other complex traits, and hence may be viewed as a composite trait. Factors that influence the origin and maintenance of diversity of life are ultimately governed by Darwinian processes. Here we review evolutionary genetic mechanisms underlying longevity and senescence in humans from a life-history and genotype-epigenetic-phenotype (G-E-P) map prospective. We suggest that synergistic and cascading effects of cis-ruptive mechanisms in the genome, and epigenetic disruptive processes in relation to environmental factors may lead to sequential slippage in the G-E-P space. These mechanisms accompany age, stage and individual specific senescent processes, influenced by positive pleiotropy of certain genes, superior genome integrity, negative-frequency dependent selection and other factors that universally regulate rarity in nature. Finally we interpret life span as an inherent property of self-organizing systems that, accordingly, maintain species-specific limits for the entire complex of fitness traits. We conclude that Darwinian approaches provide unique opportunities to discover the biological bases of longevity as well as devise individual specific medical or other interventions toward improving health span.

Sorting nexin 6 enhances lamin a synthesis and incorporation into the nuclear envelope

Nuclear lamins are important structural and functional proteins in mammalian cells, but little is known about the mechanisms and cofactors that regulate their traffic into the nucleus. Here, we demonstrate that trafficking of lamin A, but not lamin B1, and its assembly into the nuclear envelope are regulated by sorting nexin 6 (SNX6), a major component of the retromer that targets proteins and other molecules to specific subcellular locations. SNX6 interacts with lamin A in vitro and in vivo and links it to the outer surface of the endoplasmic reticulum in human and mouse cells. SNX6 transports its lamin A cargo to the nuclear envelope in a process that takes several hours. Lamin A protein levels in the nucleus augment or decrease, respectively, upon gain or loss of SNX6 function. We further show that SNX6-dependent lamin A nuclear import occurs across the nuclear pore complex via a RAN-GTP-dependent mechanism. These results identify SNX6 as a key regulator of lamin A synthesis and incorporation into the nuclear envelope.

Long-term expression of the lamin A mutant associated with dilated cardiomyopathy induces senescence

Mutation of the lamin A gene (LMNA) causes a diverse range of diseases referred to as laminopathies. Because most laminopathies have a dominant inheritance pattern and progress gradually, cultured cells stably expressing mutant lamin A at the same level as endogenous wild-type cells are required for chronological analysis. In this study, we showed that an expression system involving a lentiviral vector that carries the human metallothionein gene basal promoter ensures stable and basal-level expression of proteins and is thus suitable for investigating the properties of lamin A mutants. The small ubiquitin-related modifier (SUMO) modification (SUMOylation)-defective E203G mutant that is associated with familial dilated cardiomyopathy exhibited abnormal subnuclear distribution and inhibited normal localization of WT lamin A in a dominant-negative manner. Low-level and long-term expression of the E203G mutant resulted in multinucleated giant cells, aberrant lipid droplet accumulation in the cytoplasm and premature senescence. Expression of another SUMOylation-defective mutant (K201R) did not induce any phenotypes observed in cells expressing E203G. These results indicate that the E203G mutant may inhibit the normal functions of wild-type lamin A in a dominant-negative manner, but a defect in SUMOylation itself may not be involved in disease pathogenesis.

Progerin expression disrupts critical adult stem cell functions involved in tissue repair

Vascular disease is one of the leading causes of death worldwide. Vascular repair, essential for tissue maintenance, is critically reduced during vascular disease and aging. Efficient vascular repair requires functional adult stem cells unimpaired by aging or mutation. One protein candidate for reducing stem cell?mediated vascular repair is progerin, an alternative splice variant of lamin A. Progerin results from erroneous activation of cryptic splice sites within the LMNA gene, and significantly increases during aging. Mutations triggering progerin overexpression cause the premature aging disorder Hutchinson-Gilford Progeria Syndrome (HGPS), in which patients die at approximately 13-years of age due to atherosclerosis-induced disease. Progerin expression affects tissues rich in cells that can be derived from marrow stromal cells (MSCs. Studies using various MSC subpopulations and models have led to discrepant results. Using a well-defined, immature subpopulation of MSCs, Marrow Isolated Adult Multilineage Inducible (MIAMI) cells, we find progerin significantly disrupts expression and localization of self-renewal markers, proliferation, migration, and membrane elasticity. One potential treatment, farnesyltransferase inhibitor, ameliorates some of these effects. Our results confirm proposed progerin-induced mechanisms and suggest novel ways in which progerin disturbs critical stem cell functions collectively required for proper tissue repair, offering promising treatment targets for future therapies.

[Biochemical, hormonal and genetic evaluation of the families of two Brazilian patients with type 2 familial partial lipodystrophy]

OBJECTIVE

To evaluate clinical, biochemical, hormonal and genetic characteristics of relatives of two patients with familial partial lipodystrophy (FPLD) type 2.

MATERIALS AND METHODS

Fifty subjects, members of two non-related Brazilian families from two different probands with FPLD phenotype, were evaluated. A mutation in exon 8 of LMNA gene was confirmed in 18 of them, and a heterozygous substitution at codon 482 was identified, predicting a p.R482W mutation. Based on the presence or absence of the mutation, subjects were classified in affected and unaffected, and compared in terms of clinical, biochemical and hormonal parameters.

RESULTS

Affected subjects were 2.8 times more likely to manifest diabetes and PCOS, higher HOMA-IR, insulin and triglyceride levels, and lower levels of leptin. These changes preceded the onset of diabetes, because they were observed in diabetic and non-diabetic affected patients. A phenotypic heterogeneity was found among mutation carriers.

CONCLUSION

A mutation in the LMNA gene is a determinant of clinical, biochemical and hormonal changes that imply in metabolic deterioration in mutation carriers.

New ZMPSTE24 (FACE1) mutations in patients affected with restrictive dermopathy or related progeroid syndromes and mutation update

Restrictive dermopathy (RD) is a rare and extremely severe congenital genodermatosis, characterized by a tight rigid skin with erosions at flexure sites, multiple joint contractures, low bone density and pulmonary insufficiency generally leading to death in the perinatal period. RD is caused in most patients by compound heterozygous or homozygous ZMPSTE24 null mutations. This gene encodes a metalloprotease specifically involved in lamin A post-translational processing. Here, we report a total of 16 families for whom diagnosis and molecular defects were clearly established. Among them, we report seven new ZMPSTE24 mutations, identified in classical RD or Mandibulo-acral dysplasia (MAD) affected patients. We also report nine families with one or two affected children carrying the common, homozygous thymine insertion in exon 9 and demonstrate the lack of a founder effect. In addition, we describe several new ZMPSTE24 variants identified in unaffected controls or in patients affected with non-classical progeroid syndromes. In addition, this mutation update includes a comprehensive search of the literature on previously described ZMPSTE24 mutations and associated phenotypes. Our comprehensive analysis of the molecular pathology supported the general rule: complete loss-of-function of ZMPSTE24 leads to RD, whereas other less severe phenotypes are associated with at least one haploinsufficient allele.

Loss of lamin B1 results in prolongation of S phase and decondensation of chromosome territories

Nuclear lamin B1 (LMNB1) constitutes one of the major structural proteins in the lamina mesh. We silenced the expression of LMNB1 by RNA interference in the colon cancer cell line DLD-1 and showed a dramatic redistribution of H3K27me3 from the periphery to a more homogeneous nuclear dispersion. In addition, we observed telomere attrition and an increased frequency of micronuclei and nuclear blebs. By 3D-FISH analyses, we demonstrated that the volume and surface of chromosome territories were significantly larger in LMNB1-depleted cells, suggesting that LMNB1 is required to maintain chromatin condensation in interphase nuclei. These changes led to a prolonged S phase due to activation of Chk1. Finally, silencing of LMNB1 resulted in extensive changes in alternative splicing of multiple genes and in a higher number of enlarged nuclear speckles. Taken together, our results suggest a mechanistic role of the nuclear lamina in the organization of chromosome territories, maintenance of genome integrity and proper gene splicing.

Impact of farnesylation inhibitors on survival in Hutchinson-Gilford progeria syndrome

BACKGROUND

Hutchinson-Gilford progeria syndrome is an ultrarare segmental premature aging disease resulting in early death from heart attack or stroke. There is no approved treatment, but starting in 2007, several recent single-arm clinical trials administered inhibitors of protein farnesylation aimed at reducing toxicity of the disease-producing protein progerin. No study assessed whether treatments influence patient survival. The key elements necessary for this analysis are a robust natural history of survival and comparison with a sufficiently large patient population that has been treated for a sufficient time period with disease-targeting medications.

METHODS AND RESULTS

We generated Kaplan-Meier survival analyses for the largest untreated Hutchinson-Gilford progeria syndrome cohort to date. Mean survival was 14.6 years. Comparing survival for treated versus age- and sex-matched untreated cohorts, hazard ratio was 0.13 (95% confidence interval, 0.04-0.37; P<0.001) with median follow-up of 5.3 years from time of treatment initiation. There were 21 of 43 deaths in untreated versus 5 of 43 deaths among treated subjects. Treatment increased mean survival by 1.6 years.

CONCLUSIONS

This study provides a robust untreated disease survival profile that can be used for comparisons now and in the future to assess changes in survival with treatments for Hutchinson-Gilford progeria syndrome. The current comparisons estimating increased survival with protein farnesylation inhibitors provide the first evidence of treatments influencing survival for this fatal disease.

CLINICAL TRIAL REGISTRATION URL

http://www.clinicaltrials.gov. Unique Indentifiers: NCT00425607, NCT00879034, and NCT00916747.

Human adult stem cells as the target cells for the initiation of carcinogenesis and for the generation of "cancer stem cells"

The inference to stem cells has been found in ancient myths and the concept of stem cells has existed in the fields of plant biology, developmental biology and embryology for decades. In the field of cancer research, the stem cell theory was one of the earliest hypotheses on the origin of a cancer from a single cell. However, an opposing hypothesis had it that an adult differentiated somatic cell could "de-differentiate" to become a cancer cell. Only within the last decade, via the "cloning" of Dolly, the sheep, did the field of stem cell biology really trigger an exciting revolution in biological research. The isolation of human embryonic stem cells has created a true revolution in the life sciences that has led to the hope that these human stem cells could lead to (a) basic science understanding of gene regulation during differentiation and development; (b) stem cell therapy; (c) gene therapy via stem cells; (d) the use of stem cells for drug discovery; (e) screening for toxic effects of chemicals; and (f) understand the aging and diseases of aging processes.

Nuclear lamins and neurobiology

Much of the work on nuclear lamins during the past 15 years has focused on mutations in LMNA (the gene for prelamin A and lamin C) that cause particular muscular dystrophy, cardiomyopathy, partial lipodystrophy, and progeroid syndromes. These disorders, often called "laminopathies," mainly affect mesenchymal tissues (e.g., striated muscle, bone, and fibrous tissue). Recently, however, a series of papers have identified important roles for nuclear lamins in the central nervous system. Studies of knockout mice uncovered a key role for B-type lamins (lamins B1 and B2) in neuronal migration in the developing brain. Also, duplications of LMNB1 (the gene for lamin B1) have been shown to cause autosome-dominant leukodystrophy. Finally, recent studies have uncovered a peculiar pattern of nuclear lamin expression in the brain. Lamin C transcripts are present at high levels in the brain, but prelamin A expression levels are very low-due to regulation of prelamin A transcripts by microRNA 9. This form of prelamin A regulation likely explains why "prelamin A diseases" such as Hutchinson-Gilford progeria syndrome spare the central nervous system. In this review, we summarize recent progress in elucidating links between nuclear lamins and neurobiology.

Professional activity of Emery-Dreifuss muscular dystrophy patients in Poland

OBJECTIVES

Emery-Dreifuss muscular dystrophy (EDMD) is a very rare genetic disorder affecting skeletal and heart muscles. The aim of this study was to identify factors which might influence the ability to work in EDMD patients in Poland.

MATERIAL AND METHODS

The study included 24 patients suffering from either of the two EDMD forms: 17 with emerinopathy (EDMD1; EDMD caused by mutations in the emerin gene) and 7 with laminopathy (EDMD2; EDMD caused by the lamin A/C gene mutations). After clinical evaluation of EDMD course, study participants were questioned about their education, current and former employment, and disability certificates and pensions.

RESULTS

54% of the study participants were employed, and 90% of them had job position corresponding to their education. Undertaking work did not correlate with the level of physical performance or disease complication, but it revealed statistically significant correlation with a higher level of education (p = 0.015). Only 23% of professionally active patients were employed in a sheltered workplace. Disability certificate was granted to all EDMD2 and to 90% of EDMD1 patients. All EDMD2 and 50% of EDMD1 patients received a disability pension, which reflects more severe course of EDMD2.

CONCLUSIONS

Higher level of education increased the chance of employment, even if significant disability was present. Therefore, I hypothesize that advice on education and job counseling should be applied as early as possible after the diagnosis of EDMD.

B-type lamins in health and disease

For over two decades, B-type lamins were thought to have roles in fundamental processes including correct assembly of nuclear envelopes, DNA replication, transcription and cell survival. Recent studies have questioned these roles and have instead emphasised the role of these proteins in tissue building and tissue integrity, particularly in tissues devoid of A-type lamins. Other studies have suggested that the expression of B-type lamins in somatic cells influences the rate of entry into states of cellular senescence. In humans duplication of the LMNB1 gene (encoding lamin B1) causes an adult onset neurodegenerative disorder, termed autosomal dominant leukodystrophy, whilst very recently, LMNB1 has been implicated as a susceptibility gene in neural tube defects. This is consistent with studies in mice that reveal a critical role for B-type lamins in neuronal migration and brain development. In this review, I will consider how different model systems have contributed to our understanding of the functions of B-type lamins and which of those functions are critical for human health and disease.

Nuclear envelope lamin-A couples actin dynamics with immunological synapse architecture and T cell activation

In many cell types, nuclear A-type lamins regulate multiple cellular functions, including higher-order genome organization, DNA replication and repair, gene transcription, and signal transduction; however, their role in specialized immune cells remains largely unexplored. We showed that the abundance of A-type lamins was almost negligible in resting naïve T lymphocytes, but was increased upon activation of the T cell receptor (TCR). The increase in lamin-A was an early event that accelerated formation of the immunological synapse between T cells and antigen-presenting cells. Polymerization of F-actin in T cells is a critical step for immunological synapse formation, and lamin-A interacted with the linker of nucleoskeleton and cytoskeleton (LINC) complex to promote F-actin polymerization. We also showed that lamin-A expression accelerated TCR clustering and led to enhanced downstream signaling, including extracellular signal-regulated kinase 1/2 (ERK1/2) signaling, as well as increased target gene expression. Pharmacological inhibition of the ERK pathway reduced lamin-A-dependent T cell activation. Moreover, mice lacking lamin-A in immune cells exhibited impaired T cell responses in vivo. These findings underscore the importance of A-type lamins for TCR activation and identify lamin-A as a previously unappreciated regulator of the immune response.

Shear stress-induced mechanotransduction protein deregulation and vasculopathy in a mouse model of progeria

Introduction

A mouse model of progeria derived by insertion of the human mutant LMNA gene (mLMNA), producing mutant lamin A, shows loss of smooth muscle cells in the media of the ascending aorta. We hypothesized that high shear stress, in the presence of mutant lamin A, induces this vasculopathy and tried to define the molecular and cellular basis for aortic vasculopathy.

Methods

Ascending and descending aortas from wild type (WT) and mLMNA⁺ mice were compared using proteomics, Western blots, PCR and immunostaining. To determine whether high fluidic shear stress, known to occur in the ascending aorta, contributed to the vasculopathy, we exposed descending aortas of mLMNA⁺ mice, with no apparent vasculopathy, to 75 dynes/cm² shear stress for 30 minutes using a microfluidic system.

Results

When the mice were one year of age, expression of several mechanotransduction proteins in the ascending aorta, including vimentin, decreased in mLMNA⁺ mice but no decrease occurred in the descending aorta. High fluidic shear stress produced a significant reduction in vimentin of mLMNA⁺ mice but not in similarly treated WT mice.

Conclusions

The occurrence of mutant lamin A and high shear stress correlate with a reduction in the level of mechanotransduction proteins in smooth muscle cells of the media. Reduction of these proteins may contribute over time to development of vasculopathy in the ascending aorta in progeria syndrome.

Intermediate filament: structure, function, and applications in cytology

Intermediate filament (IF) constitutes an important cytoskeletal component in nearly all the vertebrate cells. IFs are present both in the cytoplasm and in the nucleus. They play an important role in providing mechanical strength of the cell and tissue, growth and regeneration, cell survival and apoptosis, and finally cell migration. IFs are also expressed differentially in different body tissues. Therefore, judicious use of IF may provide the diagnosis and confirmation of different malignancies. This is particularly helpful in the diagnosis of metastatic malignant tumor from an unknown primary. Expression of IFs particularly cytokeratin and vimentin is also related to prognosis of tumors. In this review, we have discussed the basic structure, dynamics, distribution of IF in cells, and its role in diagnosis of cytology. Possible prognostic roles of IF are also discussed.

Localization of Daucus carota NMCP1 to the nuclear periphery: the role of the N-terminal region and an NLS-linked sequence motif, RYNLRR, in the tail domain

Recent ultrastructural studies revealed that a structure similar to the vertebrate nuclear lamina exists in the nuclei of higher plants. However, plant genomes lack genes for lamins and intermediate-type filament proteins, and this suggests that plant-specific nuclear coiled-coil proteins make up the lamina-like structure in plants. NMCP1 is a protein, first identified in Daucus carota cells, that localizes exclusively to the nuclear periphery in interphase cells. It has a tripartite structure comprised of head, rod, and tail domains, and includes putative nuclear localization signal (NLS) motifs. We identified the functional NLS of DcNMCP1 (carrot NMCP1) and determined the protein regions required for localizing to the nuclear periphery using EGFP-fused constructs transiently expressed in Apium graveolens epidermal cells. Transcription was driven under a CaMV35S promoter, and the genes were introduced into the epidermal cells by a DNA-coated microprojectile delivery system. Of the NLS motifs, KRRRK and RRHK in the tail domain were highly functional for nuclear localization. Addition of the N-terminal 141 amino acids from DcNMCP1 shifted the localization of a region including these NLSs from the entire nucleus to the nuclear periphery. Using this same construct, the replacement of amino acids in RRHK or its preceding sequence, YNL, with alanine residues abolished localization to the nuclear periphery, while replacement of KRRRK did not affect localization. The sequence R/Q/HYNLRR/H, including YNL and the first part of the sequence of RRHK, is evolutionarily conserved in a subclass of NMCP1 sequences from many plant species. These results show that NMCP1 localizes to the nuclear periphery by a combined action of a sequence composed of R/Q/HYNLRR/H, NLS, and the N-terminal region including the head and a portion of the rod domain, suggesting that more than one binding site is implicated in localization of NMCP1.

Protective role of the mitochondrial Lon protease 1 in ochratoxin A-induced cytotoxicity in HEK293 cells

Ochratoxin A (OTA) is a common kind of mycotoxin and food contaminant, which has various toxicological effects, especially nephrotoxicity. Our previous work about OTA-induced renal cytotoxicity indicated that mitochondrial Lon Protease 1 (Lonp1) might play a protective role. Lonp1 is a multifunctional ATP-dependent protease which mainly participates in mitochondrial proteolysis and protein quality control. The study aimed at probing how Lonp1 functioned in OTA-induced renal cytotoxicity. By means of RNA interference, we down-regulated the expression of Lonp1 in HEK293 cells. Cell viability results revealed that cells with Lonp1 deficiency were more vulnerable to OTA. Then we identified differentially expressed proteins between Lonp1 knock-down cells and scrambled control both in the absence and presence of OTA, using iTRAQ-based quantitative proteomics approach. Thirty-four proteins were differentially expressed as a result of Lonp1 deficiency, while forty-four proteins were differentially expressed in response to both Lonp1 deficiency and OTA treatment. By function summary and pathway analysis, we presumed that Lonp1 realized its protective function in the resistance to OTA-induced renal cytotoxicity via 4 processes: defensing against OTA-induced oxidative stress in the mitochondria; regulating protein synthesis, modification and repair; maintaining the balance of carbohydrate metabolism; and assisting in mtDNA maintenance.

BIOLOGICAL SIGNIFICANCE

OTA is a kind of mycotoxin that seriously threatens human health and has various toxicological effects. However, the mechanisms of its toxicity have not been exactly elucidated yet. The method of combination of RNAi and iTRAQ-based quantitative proteomics paves the way to gain a better understanding of the toxicity mechanisms of OTA. The present study, for the first time, verified the protective role of Lonp1 in OTA-induced renal cytotoxicity and clarified the defensive mechanism. Proteomic changes in Lonp1 deficient cells induced by OTA added new knowledge to OTA cytotoxicity.

Specifying peripheral heterochromatin during nuclear lamina reassembly

A conserved organizational feature of eukaryotic nuclei is the peripheral heterochromatin compartment, which provides a protected area for epigenetically silent genes and gene-poor DNA. In metazoan cells this compartment is associated with the nuclear lamina, the protein meshwork at the inner edge of the nucleus. Heterochromatin-nuclear lamina interactions promote epigenetic gene silencing, which may drive many normal and diseased biological processes. We recently obtained evidence that a previously unstudied human protein, PRR14, participates in the tethering of heterochromatin to the inner nuclear periphery. PRR14 associates with the nuclear lamina and attaches to heterochromatin through its binding partner, heterochromatin protein 1 (HP1). After disassembly early in mitosis, PRR14 reassembles in two steps, first binding to anaphase chromosomes through HP1, followed by association with the nuclear lamina in telophase. PRR14 may thereby play a role in specifying HP1-bound heterochromatin for reattachment to the nuclear lamina at mitotic exit. Here we review the relevant literature, summarize our initial work, and provide additional comments and findings.

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.

The PPARγ locus makes long-range chromatin interactions with selected tissue-specific gene loci during adipocyte differentiation in a protein kinase A dependent manner

Differentiation signaling results in reprogramming of cellular gene expression that leads to morphological changes and functional specialization of a precursor cell. This global change in gene expression involves temporal regulation of differentiation-specific genes that are located throughout the genome, raising the idea that genome structure may also be re-organized during cell differentiation to facilitate regulated gene expression. Using in vitro adipocyte differentiation as a model, we explored whether gene organization within the nucleus is altered upon exposure of precursor cells to signaling molecules that induce adipogenesis. The peroxisome proliferator-activated receptor gamma (PPARγ) nuclear hormone receptor is a master determinant of adipogenesis and is required for adipose differentiation. We utilized the chromosome conformation capture (3C) assay to determine whether the position of the PPARγ locus relative to other adipogenic genes is changed during differentiation. We report that the PPARγ2 promoter is transiently positioned in proximity to the promoters of genes encoding adipokines and lipid droplet associated proteins at 6 hours post-differentiation, a time that precedes expression of any of these genes. In contrast, the PPARγ2 promoter was not in proximity to the EF1α promoter, which drives expression of a constitutively active, housekeeping gene that encodes a translation elongation factor, nor was the PPARγ2 promoter in proximity to the promoter driving the expression of the C/EBPα regulatory protein. The formation of the long-range, intergenic interactions involving the PPARγ2 promoter required the regulatory factor C/EBPβ, elevated cyclic AMP (cAMP) levels, and protein kinase A (PKA) signaling. We conclude that genome organization is dynamically remodeled in response to adipogenic signaling, and we speculate that these transient inter-genic interactions may be formed for the purposes of selecting some of the transcriptionally silent tissue-specific loci for subsequent transcriptional activation.

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.

Depletion of the protein kinase VRK1 disrupts nuclear envelope morphology and leads to BAF retention on mitotic chromosomes

The VRK1 protein kinase regulates the phosphorylation of BAF, which binds to dsDNA and LEM domain–containing proteins. VRK1 depletion increases the immobile fraction of BAF at the nuclear periphery and disturbs nuclear envelope architecture. It also leads to the retention of BAF on chromosomes as cells enter and progress through mitosis.

Barrier to autointegration factor (BAF), which is encoded by the BANF1 gene, binds with high-affinity to double-stranded DNA and LEM domain–containing proteins at the nuclear periphery. A BANF1 mutation has recently been associated with a novel human progeria syndrome, and cells from these patients have aberrant nuclear envelopes. The interactions of BAF with its DNA- and protein-binding partners are known to be regulated by phosphorylation, and previously we validated BAF as a highly efficient substrate for the VRK1 protein kinase. Here we show that depletion of VRK1 in MCF10a and MDA-MB-231 cells results in aberrant nuclear architecture. The immobile fraction of green fluorescent protein (GFP)–BAF at the nuclear envelope (NE) is elevated, suggesting that prolonged interactions of BAF with its binding partners is likely responsible for the aberrant NE architecture. Because detachment of BAF from its binding partners is associated with NE disassembly, we performed live-imaging analysis of control and VRK1-depleted cells to visualize GFP-BAF dynamics during mitosis. In the absence of VRK1, BAF does not disperse but instead remains chromosome bound from the onset of mitosis. VRK1 depletion also increases the number of anaphase bridges and multipolar spindles. Thus phosphorylation of BAF by VRK1 is essential both for normal NE architecture and proper dynamics of BAF–chromosome interactions during mitosis. These results are consistent with previous studies of the VRK/BAF signaling axis in Caenorhabditis elegans and Drosophila melanogaster and validate VRK1 as a key regulator of NE architecture and mitotic chromosome dynamics in mammalian cells.

DNA damage and lamins

The spatial and temporal organization of the genome has emerged as an additional level of regulation of nuclear functions. Structural proteins associated with the nuclear envelope play important roles in the organization of the genome. The nuclear lamina, a polymeric meshwork formed by lamins (A- and B-type) and lamin-associated proteins, is viewed as a scaffold for tethering chromatin and protein complexes regulating a variety of nuclear functions. Alterations in lamins function impact DNA transactions such as transcription, replication, and repair, as well as epigenetic modifications that change chromatin structure. These data, and the association of defective lamins with a whole variety of degenerative disorders, premature aging syndromes, and cancer, provide evidence for these proteins operating as caretakers of the genome. In this chapter, we summarize current knowledge about the function of lamins in the maintenance of genome integrity, with special emphasis on the role of A-type lamins in the maintenance of telomere homeostasis and mechanisms of DNA damage repair. These findings have begun to shed some light onto molecular mechanisms by which alterations in A-type lamins induce genomic instability and contribute to the pathophysiology of aging and aging-related diseases, especially cancer.

Nuclear envelope invaginations and cancer

The nuclear envelope (NE) surrounds the nucleus and separates it from the cytoplasm. The NE is not a passive structural component, but rather contributes to various cellular processes such as genome organization, transcription, signaling, and stress responses. Although the NE is mostly a smooth surface, it also forms invaginations that can reach deep into the nucleoplasm and may even traverse the nucleus completely. Cancer cells are generally characterized by irregularities and invaginations of the NE that are of diagnostic and prognostic significance. In the current chapter, we describe the link between nuclear invaginations and irregularities with cancer and explore possible mechanistic roles they might have in tumorigenesis.

Cellular stress induces Bax-regulated nuclear bubble budding and rupture followed by nuclear protein release

Cellular stress triggers many pathways including nuclear protein redistribution. We previously discovered that this process is regulated by Bax but the underlying mechanism has not yet been studied. Here we define this mechanism by showing that apoptotic stimuli cause Bax-regulated disturbances in lamin A/C and nuclear envelope (NE)-associated proteins which results in the generation and subsequent rupture of nuclear protein-containing bubbles. The bubbles do not contain DNA and are encapsulated by impaired nuclear pore-depleted NE. Stress-induced generation and rupture of nuclear bubbles ultimately leads to the discharge of nuclear proteins into the cytoplasm. This process precedes morphological changes of apoptosis and occurs independently of caspases. Rescue experiments revealed that this Bax effect is non-canonical, i.e. it requires the BH3 domain and α-helices 5 and 6 but it is not inhibited by Bcl(-)xL. Targeting Bax to the NE by the Klarsicht/ANC-1/Syne-1 homology (KASH) domain effectively triggers the generation and rupture of nuclear bubbles. Overall, our findings provide evidence for a novel stress-response, which is regulated by a non-canonical action of Bax on the NE.

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.

Deficient expression of aldehyde dehydrogenase 1A1 is consistent with increased sensitivity of Gorlin syndrome patients to radiation carcinogenesis

Human phenotypes that are highly susceptible to radiation carcinogenesis have been identified. Sensitive phenotypes often display robust regulation of molecular features that modify biological response, which can facilitate identification of the pathways/networks that contribute to pathophysiological outcomes. Here we interrogate primary dermal fibroblasts isolated from Gorlin syndrome patients (GDFs), who display a pronounced inducible tumorigenic response to radiation, in comparison to normal human dermal fibroblasts (NHDFs). Our approach exploits newly developed thiol reactive probes to define changes in protein thiol profiles in live cell studies, which minimizes artifacts associated with cell lysis. Redox probes revealed deficient expression of an apparent 55 kDa protein thiol in GDFs from independent Gorlin syndrome patients, compared with NHDFs. Proteomics tentatively identified this protein as aldehyde dehydrogenase 1A1 (ALDH1A1), a key enzyme regulating retinoic acid synthesis, and ALDH1A1 protein deficiency in GDFs was confirmed by Western blot. A number of additional protein thiol differences in GDFs were identified, including radiation responsive annexin family members and lamin A/C. Collectively, candidates identified in our study have plausible implications for radiation health effects and cancer susceptibility.

Lamin A Δexon9 mutation leads to telomere and chromatin defects but not genomic instability

Over 300 mutations in the LMNA gene, encoding A-type lamins, are associated with 15 human degenerative disorders and premature aging syndromes. Although genomic instability seems to contribute to the pathophysiology of some laminopathies, there is limited information about what mutations cause genomic instability and by which molecular mechanisms. Mouse embryonic fibroblasts depleted of A-type lamins or expressing mutants lacking exons 8-11 (Lmna(Δ8-11/Δ8-11)) exhibit alterations in telomere biology and DNA repair caused by cathepsin L-mediated degradation of 53BP1 and reduced expression of BRCA1 and RAD51. Thus, a region encompassing exons 8-11 seems essential for genome integrity. Given that deletion of lamin A exon 9 in the mouse (Lmna(Δ9/Δ9)) results in a progeria phenotype, we tested if this domain is important for genome integrity. Lmna(Δ9/Δ9) MEFs exhibit telomere shortening and heterochromatin alterations but do not activate cathepsin L-mediated degradation of 53BP1 and maintain expression of BRCA1 and RAD51. Accordingly, Lmna(Δ9/Δ9) MEFs do not present genomic instability, and expression of mutant lamin A Δexon9 in lamin-depleted cells restores DNA repair factors levels and partially rescues nuclear abnormalities. These data reveal that the domain encoded by exon 9 is important to maintain telomere homeostasis and heterochromatin structure but does not play a role in DNA repair, thus pointing to other exons in the lamin A tail as responsible for the genomic instability phenotype in Lmna(Δ8-11/Δ8-11) mice. Our study also suggests that the levels of DNA repair factors 53BP1, BRCA1 and RAD51 could potentially serve as biomarkers to identify laminopathies that present with genomic instability.

Autophagy drives epidermal deterioration in a Drosophila model of tissue aging

Organismal lifespan has been the primary readout in aging research. However, how longevity genes control tissue-specific aging remains an open question. To examine the crosstalk between longevity programs and specific tissues during aging, biomarkers of organ-specific aging are urgently needed. Since the earliest signs of aging occur in the skin, we sought to examine skin aging in a genetically tractable model. Here we introduce a Drosophila model of skin aging. The epidermis undergoes a dramatic morphological deterioration with age that includes membrane and nuclear loss. These changes were decelerated in a long-lived mutant and accelerated in a short-lived mutant. An increase in autophagy markers correlated with epidermal aging. Finally, the epidermis of Atg7 mutants retained younger characteristics, suggesting that autophagy is a critical driver of epidermal aging. This is surprising given that autophagy is generally viewed as protective during aging. Since Atg7 mutants are short-lived, the deceleration of epidermal aging in this mutant suggests that in the epidermis healthspan can be uncoupled from longevity. Because the aging readout we introduce here has an early onset and is easily visualized, genetic dissection using our model should identify other novel mechanisms by which lifespan genes feed into tissue-specific aging.