The nuclear lamins: flexibility in function

Insight into Mechanobiology: How Stem Cells Feel Mechanical Forces and Orchestrate Biological Functions

The cross-talk between stem cells and their microenvironment has been shown to have a direct impact on stem cells' decisions about proliferation, growth, migration, and differentiation. It is well known that stem cells, tissues, organs, and whole organisms change their internal architecture and composition in response to external physical stimuli, thanks to cells' ability to sense mechanical signals and elicit selected biological functions. Likewise, stem cells play an active role in governing the composition and the architecture of their microenvironment. Is now being documented that, thanks to this dynamic relationship, stemness identity and stem cell functions are maintained. In this work, we review the current knowledge in mechanobiology on stem cells. We start with the description of theoretical basis of mechanobiology, continue with the effects of mechanical cues on stem cells, development, pathology, and regenerative medicine, and emphasize the contribution in the field of the development of ex-vivo mechanobiology modelling and computational tools, which allow for evaluating the role of forces on stem cell biology.

Premature aging syndromes: From patients to mechanism

Aging is an inevitable consequence of human life resulting in a gradual deterioration of cell, tissue and organismal function and an increased risk to develop chronic ailments. Premature aging syndromes, also known as progeroid syndromes, recapitulate many clinical features of normal aging and offer a unique opportunity to elucidate fundamental mechanisms that contribute to human aging. Progeroid syndromes can be broadly classified into those caused by perturbations of the nuclear lamina, a meshwork of proteins located underneath the inner nuclear membrane (laminopathies); and a second group that is caused by mutations that directly impair DNA replication and repair. We will focus mainly on laminopathies caused by incorrect processing of lamin A, an intermediate filament protein that resides at the nuclear periphery. Hutchinson-Gilford Progeria (HGPS) is an accelerated aging syndrome caused by a mutation in lamin A and one of the best studied laminopathies. HGPS patients exhibit clinical characteristics of premature aging, including alopecia, aberrant pigmentation, loss of subcutaneous fat and die in their teens as a result of atherosclerosis and cardiovascular complications. Here we summarize how cell- and mouse-based disease models provided mechanistic insights into human aging and discuss experimental strategies under consideration for the treatment of these rare genetic disorders.

Electron tomography of mouse LINC complexes at meiotic telomere attachment sites with and without microtubules

Telomere movements during meiotic prophase I facilitate synapsis and recombination of homologous chromosomes. Hereby, chromosome movements depend on the dynamic attachment of meiotic telomeres to the nuclear envelope and generation of forces that actively move the telomeres. In most eukaryotes, forces that move telomeres are generated in the cytoplasm by microtubule-associated motor proteins and transduced into the nucleus through the LINC complexes of the nuclear envelope. Meiotic LINC complexes, in mouse comprised of SUN1/2 and KASH5, selectively localize to the attachment sites of meiotic telomeres. For a better understanding of meiotic telomere dynamics, here we provide quantitative information of telomere attachment sites that we have generated with the aid of electron microscope tomography (EM tomography). Our data on the number, length, width, distribution and relation with microtubules of the reconstructed structures indicate that an average number of 76 LINC complexes would be required to move a telomere attachment site.

The functional importance of lamins, actin, myosin, spectrin and the LINC complex in DNA repair

Three major proteins in the nucleoskeleton, lamins, actin, and spectrin, play essential roles in maintenance of nuclear architecture and the integrity of the nuclear envelope, in mechanotransduction and mechanical coupling between the nucleoskeleton and cytoskeleton, and in nuclear functions such as regulation of gene expression, transcription and DNA replication. Less well known, but critically important, are the role these proteins play in DNA repair. The A-type and B-type lamins, nuclear actin and myosin, spectrin and the LINC (linker of nucleoskeleton and cytoskeleton) complex each function in repair of DNA damage utilizing various repair pathways. The lamins play a role in repair of DNA double-strand breaks (DSBs) by nonhomologous end joining (NHEJ) or homologous recombination (HR). Actin is involved in repair of DNA DSBs and interacts with myosin in facilitating relocalization of these DSBs in heterochromatin for HR repair. Nonerythroid alpha spectrin (αSpII) plays a critical role in repair of DNA interstrand cross-links (ICLs) where it acts as a scaffold in recruitment of repair proteins to sites of damage and is important in the initial damage recognition and incision steps of the repair process. The LINC complex contributes to the repair of DNA DSBs and ICLs. This review will address the important functions of these proteins in the DNA repair process, their mechanism of action, and the profound impact a defect or deficiency in these proteins has on cellular function. The critical roles of these proteins in DNA repair will be further emphasized by discussing the human disorders and the pathophysiological changes that result from or are related to deficiencies in these proteins. The demonstrated function for each of these proteins in the DNA repair process clearly indicates that there is another level of complexity that must be considered when mechanistically examining factors crucial for DNA repair.

Impact statement

Proteins in the nucleoskeleton, lamins, actin, myosin, and spectrin, have been shown to play critical roles in DNA repair. Deficiencies in these proteins are associated with a number of disorders. This review highlights the role these proteins and their association with the LINC complex play in DNA repair processes, their mechanism of action and the impacts deficiencies in these proteins have on DNA repair and on disorders associated with a deficiency in these proteins. It will clarify how these proteins, which interact with “classic DNA repair proteins” (e.g., RAD51, XPF), represent another level of complexity in the DNA repair process, which must be taken into consideration when carrying out mechanistic studies on proteins involved in DNA repair and in developing models for DNA repair pathways. This knowledge is essential for determining how deficiencies in these proteins relate to disorders resulting from loss of functional activity of these proteins.

Nuclear Lamin B1 Interactions With Chromatin During the Circadian Cycle Are Uncoupled From Periodic Gene Expression

Many mammalian genes exhibit circadian expression patterns concordant with periodic binding of transcription factors, chromatin modifications, and chromosomal interactions. Here we investigate whether chromatin periodically associates with nuclear lamins. Entrainment of the circadian clock is accompanied, in mouse liver, by a net gain of lamin B1–chromatin interactions genome-wide, after which the majority of lamina-associated domains (LADs) are conserved during the circadian cycle. By tailoring a bioinformatics pipeline designed to identify periodic gene expression patterns, we also observe hundreds of variable lamin B1–chromatin interactions among which oscillations occur at 64 LADs, affecting one or both LAD extremities or entire LADs. Only a small subset of these oscillations however exhibit highly significant 12, 18, 24, or 30 h periodicity. These periodic LADs display oscillation asynchrony between their 5′ and 3′ borders, and are uncoupled from periodic gene expression within or in the vicinity of these LADs. Periodic gene expression is also unrelated to variations in gene-to-nearest LAD distances detected during the circadian cycle. Accordingly, periodic genes, including central clock-control genes, are located megabases away from LADs throughout circadian time, suggesting stable residence in a transcriptionally permissive chromatin environment. We conclude that periodic LADs are not a dominant feature of variable lamin B1–chromatin interactions during the circadian cycle in mouse liver. Our results also suggest that periodic hepatic gene expression is not regulated by rhythmic chromatin associations with the nuclear lamina.

The NSL complex maintains nuclear architecture stability via lamin A/C acetylation

While nuclear lamina abnormalities are hallmarks of human diseases, their interplay with epigenetic regulators and precise epigenetic landscape remain poorly understood. Here, we show that loss of the lysine acetyltransferase MOF or its associated NSL-complex members KANSL2 or KANSL3 leads to a stochastic accumulation of nuclear abnormalities with genomic instability patterns including chromothripsis. SILAC-based MOF and KANSL2 acetylomes identified lamin A/C as an acetylation target of MOF. HDAC inhibition or acetylation-mimicking lamin A derivatives rescue nuclear abnormalities observed in MOF-deficient cells. Mechanistically, loss of lamin A/C acetylation resulted in its increased solubility, defective phosphorylation dynamics and impaired nuclear mechanostability. We found that nuclear abnormalities include EZH2-dependent histone H3 Lys 27 trimethylation and loss of nascent transcription. We term this altered epigenetic landscape "heterochromatin enrichment in nuclear abnormalities" (HENA). Collectively, the NSL-complex-dependent lamin A/C acetylation provides a mechanism that maintains nuclear architecture and genome integrity.

Laminopathy-causing lamin A mutations reconfigure lamina-associated domains and local spatial chromatin conformation

The nuclear lamina contributes to the regulation of gene expression and to chromatin organization. Mutations in A-type nuclear lamins cause laminopathies, some of which are associated with a loss of heterochromatin at the nuclear periphery. Until recently however, little if any information has been provided on where and how lamin A interacts with the genome and on how disease-causing lamin A mutations may rearrange genome conformation. Here, we review aspects of nuclear lamin association with the genome. We highlight recent evidence of reorganization of lamin A-chromatin interactions in cellular models of laminopathies, and implications on the 3-dimensional rearrangement of chromatin in these models, including patient cells. We discuss how a hot-spot lipodystrophic lamin A mutation alters chromatin conformation and epigenetic patterns at an anti-adipogenic locus, and conclude with remarks on links between lamin A, Polycomb and the pathophysiology of laminopathies. The recent findings presented here collectively argue towards a deregulation of large-scale and local spatial genome organization by a subset of lamin A mutations causing laminopathies.

Causes and consequences of genomic instability in laminopathies: Replication stress and interferon response

Mammalian nuclei are equipped with a framework of intermediate filaments that function as a karyoskeleton. This nuclear scaffold, formed primarily by lamins (A-type and B-type), maintains the spatial and functional organization of the genome and of sub-nuclear compartments. Over the past decade, a body of evidence has highlighted the significance of these structural nuclear proteins in the maintenance of nuclear architecture and mechanical stability, as well as genome function and integrity. The importance of these structures is now unquestioned given the wide range of degenerative diseases that stem from LMNA gene mutations, including muscular dystrophy disorders, peripheral neuropathies, lipodystrophies, and premature aging syndromes. Here, we review our knowledge about how alterations in nuclear lamins, either by mutation or reduced expression, impact cellular mechanisms that maintain genome integrity. Despite the fact that DNA replication is the major source of DNA damage and genomic instability in dividing cells, how alterations in lamins function impact replication remains minimally explored. We summarize recent studies showing that lamins play a role in DNA replication, and that the DNA damage that accumulates upon lamins dysfunction is elicited in part by deprotection of replication forks. We also discuss the emerging model that DNA damage and replication stress are “sensed” at the cytoplasm by proteins that normally survey this space in search of foreign nucleic acids. In turn, these cytosolic sensors activate innate immune responses, which are materializing as important players in aging and cancer, as well as in the response to cancer immunotherapy.

Recent Advances on the Structure and Function of RNA Acetyltransferase Kre33/NAT10

Ribosome biogenesis is one of the most energy demanding processes in the cell. In eukaryotes, the main steps of this process occur in the nucleolus and include pre-ribosomal RNA (pre-rRNA) processing, post-transcriptional modifications, and assembly of many non-ribosomal factors and ribosomal proteins in order to form mature and functional ribosomes. In yeast and humans, the nucleolar RNA acetyltransferase Kre33/NAT10 participates in different maturation events, such as acetylation and processing of 18S rRNA, and assembly of the 40S ribosomal subunit. Here, we review the structural and functional features of Kre33/NAT10 RNA acetyltransferase, and we underscore the importance of this enzyme in ribosome biogenesis, as well as in acetylation of non-ribosomal targets. We also report on the role of human NAT10 in Hutchinson-Gilford progeria syndrome.

Structural basis for lamin assembly at the molecular level

Nuclear structure and function are governed by lamins, which are intermediate filaments that mostly consist of α-helices. Different lamin assembly models have been proposed based on low resolution and fragmented structures. However, their assembly mechanisms are still poorly understood at the molecular level. Here, we present the crystal structure of a long human lamin fragment at 3.2 Å resolution that allows the visualization of the features of the full-length protein. The structure shows an anti-parallel arrangement of the two coiled-coil dimers, which is important for the assembly process. We further discover an interaction between the lamin dimers by using chemical cross-linking and mass spectrometry analysis. Based on these two interactions, we propose a molecular mechanism for lamin assembly that is in agreement with a recent model representing the native state and could explain pathological mutations. Our findings also provide the molecular basis for assembly mechanisms of other intermediate filaments.

Lamins are intermediate filaments and the major component of the nuclear lamina. Here the authors determine the crystal structure of a construct comprising the N-terminal half of human lamin A/C and use their structure and cross-linking and biochemical experiments to discuss lamin assembly.

Phasor-assisted nanoscopy reveals differences in the spatial organization of major nuclear lamina proteins

Phasor-assisted Metal Induced Energy Transfer-Fluorescence Lifetime Imaging Microscopy (MIET-FLIM) nanoscopy is introduced as a powerful tool for functional cell biology research. Thin metal substrates can be used to obtain axial super-resolution via nanoscale distance-dependent MIET from fluorescent dyes towards a nearby metal layer, thereby creating fluorescence lifetime contrast between dyes located at different nanoscale distance from the metal. Such data can be used to achieve axially super-resolved microscopy images, a process known as MIET-FLIM nanoscopy. Suitability of the phasor approach in MIET-FLIM nanoscopy is first demonstrated using nanopatterned substrates, and furthermore applied to characterize the distance distribution of the epithelial basal membrane of a biological cell from the gold substrate. The phasor plot of an entire cell can be used to characterize the full Förster resonance energy transfer (FRET) trajectory as a large distance heterogeneity within the sensing range of about 100 nm from the metal surface is present due to the extended shape of cell with curvatures. In contrast, the different proteins of nuclear lamina show strong confinement close to the nuclear envelope in nanoscale. We find the lamin B layer resides in average at shorter distances from the gold surface compared to the lamin A/C layer located in more extended ranges. This and the observed heterogeneity of the protein layer thicknesses suggests that A- and B-type lamins form distinct networks in the nuclear lamina. Our results provide detailed insights for the study of the different roles of lamin proteins in chromatin tethering and nuclear mechanics.

Non-senescent Hydra tolerates severe disturbances in the nuclear lamina

The cnidarian Hydra is known for its unlimited lifespan and non-senescence, due to the indefinite self-renewal capacity of its stem cells. While proteins of the Lamin family are recognized as critical factors affecting senescence and longevity in human and mice, their putative role in the extreme longevity and non-senescence in long-living animals remains unknown. Here we analyze the role of a single lamin protein in non-senescence of Hydra. We demonstrate that proliferation of stem cells in Hydra is robust against the disturbance of Lamin expression and localization. While Lamin is indispensable for Hydra, the stem cells tolerate overexpression, downregulation and mislocalization of Lamin, and disturbances in the nuclear envelope structure. This extraordinary robustness may underlie the indefinite self-renewal capacity of stem cells and the non-senescence of Hydra. A relatively low complexity of the nuclear envelope architecture in basal Metazoa might allow for their extreme lifespans, while an increasing complexity of the nuclear architecture in bilaterians resulted in restricted lifespans.

Endothelial Nuclear Lamina in Mechanotransduction Under Shear Stress

Endothelial cells that line the lumen of blood vessels are at the interface between hemodynamic forces and vascular wall biology. Endothelial cells transduce mechanical and biological signals from blood flow into intracellular signaling cascades through a process called mechanotransduction. Mechanotransduction is an important part of normal cell functions, as well as endothelial dysfunction which leads to inflammation and pathological conditions. For example, atherosclerosis preferentially develops in regions of disturbed fluid flow and low shear stress. The nuclear lamina, which sits underneath the nuclear envelope, serves to maintain the nuclear structure while acting as a scaffold for heterochromatin and many transcriptional proteins. Defects in lamina and its associated proteins cause a variety of human diseases including accelerated aging diseases such as Hutchinson-Gilford Progeria syndrome. The role of nuclear lamina in endothelial mechanotransduction, specifically how nuclear mechanics impact gene regulation under shear stress, is not fully understood. In one study, lamin A/C was silenced in bovine aortic endothelial cells to determine its role in both glucocorticoid receptor (GR) nuclear translocation and glucocorticoid response element (GRE) transcriptional activation in response to its natural ligand dexamethasone as well as fluid shear stress. Results suggest that absence of lamin A/C does not hinder passage of GR into the nucleus but nuclear lamina is important to properly regulate GRE transcription. Ongoing research continues to investigate how nuclear lamins contribute to endothelial mechanotransduction and to better understand the role of Lamin A in vascular aging and in the progression of cardiovascular diseases.

Coaching from the sidelines: the nuclear periphery in genome regulation

The genome is packaged and organized nonrandomly within the 3D space of the nucleus to promote efficient gene expression and to faithfully maintain silencing of heterochromatin. The genome is enclosed within the nucleus by the nuclear envelope membrane, which contains a set of proteins that actively participate in chromatin organization and gene regulation. Technological advances are providing views of genome organization at unprecedented resolution and are beginning to reveal the ways that cells co-opt the structures of the nuclear periphery for nuclear organization and gene regulation. These genome regulatory roles of proteins of the nuclear periphery have important influences on development, disease and ageing.

Genomic instability and innate immune responses to self-DNA in progeria

In the last decade, we have seen increasing evidence of the importance of structural nuclear proteins such as lamins in nuclear architecture and compartmentalization of genome function and in the maintenance of mechanical stability and genome integrity. With over 400 mutations identified in the LMNA gene (encoding for A-type lamins) associated with more than ten distinct degenerative disorders, the role of lamins as genome caretakers and the contribution of lamins dysfunction to disease are unarguable. However, the molecular mechanisms whereby lamins mutations cause pathologies remain less understood. Here, we review pathways and mechanisms recently identified as playing a role in the pathophysiology of laminopathies, with special emphasis in Hutchinson Gilford Progeria Syndrome (HGPS). This devastating incurable accelerated aging disease is caused by a silent mutation in the LMNA gene that generates a truncated lamin A protein "progerin" that exerts profound cellular toxicity and organismal decline. Patients usually die in their teens due to cardiovascular complications such as myocardial infarction or stroke. To date, there are no efficient therapies that ameliorate disease progression, stressing the need to understand molecularly disease mechanisms that can be targeted therapeutically. We will summarize data supporting that replication stress is a major cause of genomic instability in laminopathies, which contributes to the activation of innate immune responses to self-DNA that in turn accelerate the aging process.

Fluoride exposure arrests the acrosome formation during spermatogenesis via down-regulated Zpbp1, Spaca1 and Dpy19l2 expression in rat testes

The exposure and health effects of fluoride are an ongoing topic that has attracted worldwide attention. Fluoride exposure disturbs the testicular development, sexual hormone levels and spermatogenesis. However, as to whether fluoride interferes with acrosome formation which is essential for production of capable spermatozoa during spermatogenesis still remains unclear. The objective was to determine the effects of fluoride on the acrosome formation and to further elucidate the potential mechanism of impaired reproductive function. For this, forty adult rats were assigned into four groups. The control group received distilled water, while the other three groups were treated with 25, 50 and 100 mg NaF/L via drinking water for 56 d, respectively. Testes were processed for total RNA extraction and western blot analysis. Three samples of each group were fixed in 2.5% glutaraldehyde solution for transmission electron microscopy analysis. From the results, we first found that fluoride decreased the expression of mRNA and protein levels of Zpbp1, Spaca1 and Dpy19l2 of seven markers during acrosome biogenesis in testes. Furthermore, fluoride damaged not only the acrosome structure, but also the structure of the nuclear lamina which was observed to be discontinuous and partially missing by transmission electron microscopy. Moreover, the results indicated that the altered structure in nuclear lamina maybe due to reduced LMNB2 expression in testis induced by fluoride. In a nutshell, fluoride exposure could restrain acrosome biogenesis during spermatogenesis and contribute to the elucidation of the underlying mechanisms of fluoride-induced male reproductive toxicity.

Nuclear Organization in Stress and Aging

The eukaryotic nucleus controls most cellular processes. It is isolated from the cytoplasm by the nuclear envelope, which plays a prominent role in the structural organization of the cell, including nucleocytoplasmic communication, chromatin positioning, and gene expression. Alterations in nuclear composition and function are eminently pronounced upon stress and during premature and physiological aging. These alterations are often accompanied by epigenetic changes in histone modifications. We review, here, the role of nuclear envelope proteins and histone modifiers in the 3-dimensional organization of the genome and the implications for gene expression. In particular, we focus on the nuclear lamins and the chromatin-associated protein BAF, which are linked to Hutchinson–Gilford and Nestor–Guillermo progeria syndromes, respectively. We also discuss alterations in nuclear organization and the epigenetic landscapes during normal aging and various stress conditions, ranging from yeast to humans.

Genetic and epigenetic regulation of human aging and longevity

Here we summarize the latest data on genetic and epigenetic contributions to human aging and longevity. Whereas environmental and lifestyle factors are important at younger ages, the contribution of genetics appears more important in reaching extreme old age. Genome-wide studies have implicated ~57 gene loci in lifespan. Epigenomic changes during aging profoundly affect cellular function and stress resistance. Dysregulation of transcriptional and chromatin networks is likely a crucial component of aging. Large-scale bioinformatic analyses have revealed involvement of numerous interaction networks. As the young well-differentiated cell replicates into eventual senescence there is drift in the highly regulated chromatin marks towards an entropic middle-ground between repressed and active, such that genes that were previously inactive "leak". There is a breakdown in chromatin connectivity such that topologically associated domains and their insulators weaken, and well-defined blocks of constitutive heterochromatin give way to generalized, senescence-associated heterochromatin, foci. Together, these phenomena contribute to aging.

Mechanosensing by the Lamina Protects against Nuclear Rupture, DNA Damage, and Cell-Cycle Arrest

Whether cell forces or extracellular matrix (ECM) can impact genome integrity is largely unclear. Here, acute perturbations (∼1 h) to actomyosin stress or ECM elasticity cause rapid and reversible changes in lamin-A, DNA damage, and cell cycle. The findings are especially relevant to organs such as the heart because DNA damage permanently arrests cardiomyocyte proliferation shortly after birth and thereby eliminates regeneration after injury including heart attack. Embryonic hearts, cardiac-differentiated iPS cells (induced pluripotent stem cells), and various nonmuscle cell types all show that actomyosin-driven nuclear rupture causes cytoplasmic mis-localization of DNA repair factors and excess DNA damage. Binucleation and micronuclei increase as telomeres shorten, which all favor cell-cycle arrest. Deficiencies in lamin-A and repair factors exacerbate these effects, but lamin-A-associated defects are rescued by repair factor overexpression and also by contractility modulators in clinical trials. Contractile cells on stiff ECM normally exhibit low phosphorylation and slow degradation of lamin-A by matrix-metalloprotease-2 (MMP2), and inhibition of this lamin-A turnover and also actomyosin contractility are seen to minimize DNA damage. Lamin-A is thus stress stabilized to mechano-protect the genome.

Constitutional abnormality of nuclear membrane proteins in small cell lung carcinoma

Nuclear membrane proteins reportedly play important roles in maintaining nuclear structures and coordinating cell activities. Studying profiles of nuclear membrane proteins may help us evaluate the biological and/or clinical nature of malignant tumors. Using immunohistochemistry with antibodies for emerin, lamin A/C, lamin B, and LAP2, we examined 105 lung cancer tissues from 33 small cell lung carcinomas (SCLCs) and 72 non-SCLCs (34 adenocarcinomas, 30 squamous cell carcinomas, and 8 large cell carcinomas). Emerin had negative or local/weak positivity in 79% of SCLCs and 1% of non-SCLCs, and lamin A/C had similar positivity in 91% of SCLCs and 3% of non-SCLCs. LAP2's expression was similar between SCLCs and non-SCLCs. RT-PCR analyses for these four nuclear membrane proteins over 7 cell lines showed that mRNA of emerin and lamin A/C were distinctly downregulated in the SCLC cell lines, supporting the immunohistochemical results. In conclusion, we suggest that downregulation of the nuclear membrane proteins emerin and lamin A/C is characteristic of SCLC cells, and this constitutional abnormality of the nuclear membrane may be related to the biological and/or clinical nature of SCLC. In addition, knowing the nuclear protein profile in SCLC cells may contribute to our understanding of nuclear fragility known as the crush artifact in pulmonary biopsy specimens.

Lamin B1 loss promotes lung cancer development and metastasis by epigenetic derepression of RET

Although abnormal nuclear structure is an important criterion for cancer diagnostics, remarkably little is known about its relationship to tumor development. Here we report that loss of lamin B1, a determinant of nuclear architecture, plays a key role in lung cancer. We found that lamin B1 levels were reduced in lung cancer patients. Lamin B1 silencing in lung epithelial cells promoted epithelial-mesenchymal transition, cell migration, tumor growth, and metastasis. Mechanistically, we show that lamin B1 recruits the polycomb repressive complex 2 (PRC2) to alter the H3K27me3 landscape and repress genes involved in cell migration and signaling. In particular, epigenetic derepression of the RET proto-oncogene by loss of PRC2 recruitment, and activation of the RET/p38 signaling axis, play a crucial role in mediating the malignant phenotype upon lamin B1 disruption. Importantly, loss of a single lamin B1 allele induced spontaneous lung tumor formation and RET activation. Thus, lamin B1 acts as a tumor suppressor in lung cancer, linking aberrant nuclear structure and epigenetic patterning with malignancy.

The Biology of the Nuclear Envelope and Its Implications in Cancer Biology

The formation of the nuclear envelope and the subsequent compartmentalization of the genome is a defining feature of eukaryotes. Traditionally, the nuclear envelope was purely viewed as a physical barrier to preserve genetic material in eukaryotic cells. However, in the last few decades, it has been revealed to be a critical cellular component in controlling gene expression and has been implicated in several human diseases. In cancer, the relevance of the cell nucleus was first reported in the mid-1800s when an altered nuclear morphology was observed in tumor cells. This review aims to give a current and comprehensive view of the role of the nuclear envelope on cancer first by recapitulating the changes of the nuclear envelope during cell division, second, by reviewing the role of the nuclear envelope in cell cycle regulation, signaling, and the regulation of the genome, and finally, by addressing the nuclear envelope link to cell migration and metastasis and its use in cancer prognosis.

Lamina Associated Domains and Gene Regulation in Development and Cancer

The nuclear lamina (NL) is a thin meshwork of filaments that lines the inner nuclear membrane, thereby providing a platform for chromatin binding and supporting genome organization. Genomic regions contacting the NL are lamina associated domains (LADs), which contain thousands of genes that are lowly transcribed, and enriched for repressive histone modifications. LADs are dynamic structures that shift spatial positioning in accordance with cell-type specific gene expression changes during differentiation and development. Furthermore, recent studies have linked the disruption of LADs and alterations in the epigenome with the onset of diseases such as cancer. Here we focus on the role of LADs and the NL in gene regulation during development and cancer.

Single molecule analysis of lamin dynamics

The nuclear envelope (NE) is an essential cellular structure that contributes to nuclear stability, organization, and function. Mutations in NE-associated proteins result in a myriad of pathologies with widely diverse clinical manifestations, ages of onsets, and affected tissues. Notably, several hundred disease-causing mutations have been mapped to the LMNA gene, which encodes the intermediate filament proteins lamin A and C, two of the major architectural components of the nuclear envelope. However, how NE dysfunction leads to the highly variable pathologies observed in patient cells and tissues remains poorly understood. One model suggests alterations in the dynamic properties of the nuclear lamina and its associated proteins contribute to disease phenotype. Here, we describe the application of single molecule tracking (SMT) methodology to characterize the behavior of nuclear envelope transmembrane proteins and nuclear lamins in their native cellular environment at the single molecule level. As proof-of-concept, we demonstrate by SMT that Halo-tagged lamin B1, Samp1, lamin A, and lamin AΔ50 have distinct binding and kinetic properties, and we identify several disease-relevant mutants which exhibit altered binding dynamics. SMT is also able to separately probe the dynamics of the peripheral and the nucleoplasmic populations of lamin A mutants. We suggest that SMT is a robust and sensitive method to investigate the relationship between pathogenic mutations or cellular processes and protein dynamics at the NE.

Identification of lamins as the molecular targets of LBL1 using a clickable photoaffinity probe

Phenotypic screening is a powerful approach to discover small molecules targeting pathways or disease biology with complex genetic causes. Following the initial discovery of these small molecules is their target identification, which is at the cornerstone in addressing their biological and clinical utility. Yet, finding the needle in the haystack remains a challenge. Nuclear lamins are type V intermediate filament proteins that form a filamentous structure underneath the inner nuclear envelope to support the mechanical stability of the mammalian cell nucleus. They also participate a myriad of other cellular signaling processes with incompletely understood molecular mechanisms. Small molecules that can directly bind to nuclear lamins will be incredible tools to address lamins' roles in different aspects of biology. However, these small molecules did not exist until recently. We previously discovered an acylpyrroloquinazoline called LBL1 that selectively killed breast cancer cells without harming normal human cells. To help understand the mechanism of action of LBL1, we recently took an unbiased chemical proteomics approach to identify its direct binding targets from the entire human cellular proteome. In this chapter, we describe our detailed methods to identify and validate lamins as the direct targets of LBL1. In this approach, we developed a clickable photoaffinity probe called LBL1-P that contains acylpyrroloquinazoline, trifluoromethyldiazirine and alkyne groups. Furthermore, we described a fluorescence microscopic method to validate that LBL1 directly targets lamin A in living cells. When properly designed, this approach should be broadly applicable to other bioactive small molecules.

KMT1 family methyltransferases regulate heterochromatin-nuclear periphery tethering via histone and non-histone protein methylation

Euchromatic histone methyltransferases (EHMTs), members of the KMT1 family, methylate histone and non-histone proteins. Here, we uncover a novel role for EHMTs in regulating heterochromatin anchorage to the nuclear periphery (NP) via non-histone methylation. We show that EHMTs methylate and stabilize LaminB1 (LMNB1), which associates with the H3K9me2-marked peripheral heterochromatin. Loss of LMNB1 methylation or EHMTs abrogates heterochromatin anchorage at the NP We further demonstrate that the loss of EHMTs induces many hallmarks of aging including global reduction of H3K27methyl marks and altered nuclear morphology. Consistent with this, we observe a gradual depletion of EHMTs, which correlates with loss of methylated LMNB1 and peripheral heterochromatin in aging human fibroblasts. Restoration of EHMT expression reverts peripheral heterochromatin defects in aged cells. Collectively, our work elucidates a new mechanism by which EHMTs regulate heterochromatin domain organization and reveals their impact on fundamental changes associated with the intrinsic aging process.

Deciphering Nuclear Mechanobiology in Laminopathy

Extracellular mechanical stimuli are translated into biochemical signals inside the cell via mechanotransduction. The nucleus plays a critical role in mechanoregulation, which encompasses mechanosensing and mechanotransduction. The nuclear lamina underlying the inner nuclear membrane not only maintains the structural integrity, but also connects the cytoskeleton to the nuclear envelope. Lamin mutations, therefore, dysregulate the nuclear response, resulting in abnormal mechanoregulations, and ultimately, disease progression. Impaired mechanoregulations even induce malfunction in nuclear positioning, cell migration, mechanosensation, as well as differentiation. To know how to overcome laminopathies, we need to understand the mechanisms of laminopathies in a mechanobiological way. Recently, emerging studies have demonstrated the varying defects from lamin mutation in cellular homeostasis within mechanical surroundings. Therefore, this review summarizes recent findings highlighting the role of lamins, the architecture of nuclear lamina, and their disease relevance in the context of nuclear mechanobiology. We will also provide an overview of the differentiation of cellular mechanics in laminopathy.

Concentric organization of A- and B-type lamins predicts their distinct roles in the spatial organization and stability of the nuclear lamina

The nuclear lamina is an intermediate filament meshwork adjacent to the inner nuclear membrane (INM) that plays a critical role in maintaining nuclear shape and regulating gene expression through chromatin interactions. Studies have demonstrated that A- and B-type lamins, the filamentous proteins that make up the nuclear lamina, form independent but interacting networks. However, whether these lamin subtypes exhibit a distinct spatial organization or whether their organization has any functional consequences is unknown. Using stochastic optical reconstruction microscopy (STORM) our studies reveal that lamin B1 and lamin A/C form concentric but overlapping networks, with lamin B1 forming the outer concentric ring located adjacent to the INM. The more peripheral localization of lamin B1 is mediated by its carboxyl-terminal farnesyl group. Lamin B1 localization is also curvature- and strain-dependent, while the localization of lamin A/C is not. We also show that lamin B1's outer-facing localization stabilizes nuclear shape by restraining outward protrusions of the lamin A/C network. These two findings, that lamin B1 forms an outer concentric ring and that its localization is energy-dependent, are significant as they suggest a distinct model for the nuclear lamina-one that is able to predict its behavior and clarifies the distinct roles of individual nuclear lamin proteins and the consequences of their perturbation.

The lipodystrophic hotspot lamin A p.R482W mutation deregulates the mesodermal inducer T/Brachyury and early vascular differentiation gene networks

The p.R482W hotspot mutation in A-type nuclear lamins causes familial partial lipodystrophy of Dunnigan-type (FPLD2), a lipodystrophic syndrome complicated by early onset atherosclerosis. Molecular mechanisms underlying endothelial cell dysfunction conferred by the lamin A mutation remain elusive. However, lamin A regulates epigenetic developmental pathways and mutations could perturb these functions. Here, we demonstrate that lamin A R482W elicits endothelial differentiation defects in a developmental model of FPLD2. Genome modeling in fibroblasts from patients with FPLD2 caused by the lamin A R482W mutation reveals repositioning of the mesodermal regulator T/Brachyury locus towards the nuclear center relative to normal fibroblasts, suggesting enhanced activation propensity of the locus in a developmental model of FPLD2. Addressing this issue, we report phenotypic and transcriptional alterations in mesodermal and endothelial differentiation of induced pluripotent stem cells we generated from a patient with R482W-associated FPLD2. Correction of the LMNA mutation ameliorates R482W-associated phenotypes and gene expression. Transcriptomics links endothelial differentiation defects to decreased Polycomb-mediated repression of the T/Brachyury locus and over-activation of T target genes. Binding of the Polycomb repressor complex 2 to T/Brachyury is impaired by the mutated lamin A network, which is unable to properly associate with the locus. This leads to a deregulation of vascular gene expression over time. By connecting a lipodystrophic hotspot lamin A mutation to a disruption of early mesodermal gene expression and defective endothelial differentiation, we propose that the mutation rewires the fate of several lineages, resulting in multi-tissue pathogenic phenotypes.

A nuclear lamina-chromatin-Ran GTPase axis modulates nuclear import and DNA damage signaling

The Ran GTPase regulates nuclear import and export by controlling the assembly state of transport complexes. This involves the direct action of RanGTP, which is generated in the nucleus by the chromatin‐associated nucleotide exchange factor, RCC1. Ran interactions with RCC1 contribute to formation of a nuclear:cytoplasmic (N:C) Ran protein gradient in interphase cells. In previous work, we showed that the Ran protein gradient is disrupted in fibroblasts from Hutchinson–Gilford progeria syndrome (HGPS) patients. The Ran gradient disruption in these cells is caused by nuclear membrane association of a mutant form of Lamin A, which induces a global reduction in heterochromatin marked with Histone H3K9me3 and Histone H3K27me3. Here, we have tested the hypothesis that heterochromatin controls the Ran gradient. Chemical inhibition and depletion of the histone methyltransferases (HMTs) G9a and GLP in normal human fibroblasts reduced heterochromatin levels and caused disruption of the Ran gradient, comparable to that observed previously in HGPS fibroblasts. HMT inhibition caused a defect in nuclear localization of TPR, a high molecular weight protein that, owing to its large size, displays a Ran‐dependent import defect in HGPS. We reasoned that pathways dependent on nuclear import of large proteins might be compromised in HGPS. We found that nuclear import of ATM requires the Ran gradient, and disruption of the Ran gradient in HGPS causes a defect in generating nuclear γ‐H2AX in response to ionizing radiation. Our data suggest a lamina–chromatin–Ran axis is important for nuclear transport regulation and contributes to the DNA damage response.

Chain reaction: LINC complexes and nuclear positioning

Nuclear positioning plays an essential role in defining cell architecture and behaviour in both development and disease, and nuclear location frequently adjusts according to internal and external cues. For instance, during periods of migration in many cell types, the nucleus may be actively repositioned behind the microtubule-organising centre. Nuclear movement, for the most part, is dependent upon coupling of the cytoskeleton to the nuclear periphery. This is accomplished largely through SUN and KASH domain proteins, which together assemble to form LINC (linker of the nucleoskeleton and cytoskeleton) complexes spanning the nuclear envelope. SUN proteins of the inner nuclear membrane provide a connection to nuclear structures while acting as a tether for outer nuclear membrane KASH proteins. The latter contain binding sites for diverse cytoskeletal components. Recent publications highlight new aspects of LINC complex regulation revealing that the interplay between SUN and KASH partners can strongly influence how the nucleus functionally engages with different branches of the cytoskeleton.

Nuclear envelope dynamics during mammalian spermatogenesis: new insights on male fertility

The production of highly specialized spermatozoa from undifferentiated spermatogonia is a strictly organized and programmed process requiring extensive restructuring of the entire cell. One of the most remarkable cellular transformations accompanying the various phases of spermatogenesis is the profound remodelling of the nuclear architecture, in which the nuclear envelope (NE) seems to be crucially involved. In recent years, several proteins from the distinct layers forming the NE (i.e. the inner and outer nuclear membranes as well as the nuclear lamina) have been associated with meiosis and/or spermiogenesis in different mammalian species. Among these are A- and B-type lamins, Dpy-19-like protein 2 (DPY19L2), lamin B receptor (LBR), lamina-associated polypeptide 1 (LAP1), LAP2/emerin/MAN1 (LEM) domain-containing proteins, spermatogenesis-associated 46 (SPATA46) and diverse elements of the linker of nucleoskeleton and cytoskeleton (LINC) complex, namely Sad-1/UNC-84 homology (SUN) and Klarsicht/ANC-1/Syne-1 homology (KASH) domain-containing proteins. Herein, we summarize the current state of the art on the cellular and subcellular distribution of NE proteins expressed during mammalian spermatogenesis, and discuss the latest research developments regarding their testis-specific functions. This review provides a comprehensive and innovative overview of the NE network as a regulatory platform and as an essential determinant of efficient meiotic chromosome recombination as well as spermiogenesis-associated nuclear remodelling and differentiation in mammalian male germline cells. Thus, this review provides important novel insights on the biological relevance of NE proteins for male fertility.

Microtubules Deform the Nuclear Membrane and Disrupt Nucleocytoplasmic Transport in Tau-Mediated Frontotemporal Dementia

The neuronal microtubule-associated protein tau, MAPT, is central to the pathogenesis of many dementias. Autosomal-dominant mutations in MAPT cause inherited frontotemporal dementia (FTD), but the underlying pathogenic mechanisms are unclear. Using human stem cell models of FTD due to MAPT mutations, we find that tau becomes hyperphosphorylated and mislocalizes to cell bodies and dendrites in cortical neurons, recapitulating a key early event in FTD. Mislocalized tau in the cell body leads to abnormal microtubule movements in FTD-MAPT neurons that grossly deform the nuclear membrane. This results in defective nucleocytoplasmic transport, which is corrected by microtubule depolymerization. Neurons in the post-mortem human FTD-MAPT cortex have a high incidence of nuclear invaginations, indicating that tau-mediated nuclear membrane dysfunction is an important pathogenic process in FTD. Defects in nucleocytoplasmic transport in FTD point to important commonalities in the pathogenic mechanisms of tau-mediated dementias and ALS-FTD due to TDP-43 and C9orf72 mutations.

Cytoskeletal control of nuclear morphology and stiffness are required for OPN-induced bone-marrow-derived mesenchymal stem cell migration

During cell migration, the movement of the nucleus must be coordinated with the cytoskeletal dynamics that influence the efficiency of cell migration. Our previous study demonstrated that osteopontin (OPN) significantly promotes the migration of bone-marrow-derived mesenchymal stem cells (BMSCs). However, the mechanism that regulates nuclear mechanics of the cytoskeleton during OPN-promoted BMSC migration remains unclear. In this study, we investigated how the actin cytoskeleton influences nuclear mechanics in BMSCs. We assessed the morphology and mechanics of the nuclei in the OPN-treated BMSCs subjected to disruption or polymerization of the actin cytoskeleton. We found that disruption of actin organization by cytochalasin D (Cyto D) resulted in a decrease in the nuclear projected area and nuclear stiffness. Stabilizing the actin assembly with jasplakinolide (JASP) resulted in an increase in the nuclear projected area and nuclear stiffness. SUN1 (Sad-1/UNC-84 1) is a component of the LINC (linker of nucleoskeleton and cytoskeleton) complex involved in the connections between the nucleus and the cytoskeleton. We found that SUN1 depletion by RNAi decreased the nuclear stiffness and OPN-promoted BMSC migration. Thus, the F-actin cytoskeleton plays an important role in determining the morphology and mechanical properties of the nucleus. We suggest that the cytoskeletal-nuclear interconnectivity through SUN1 proteins plays an important role in OPN-promoted BMSC migration.

Induced Pluripotent Stem Cells to Study Mechanisms of Laminopathies: Focus on Epigenetics

Laminopathies are a group of rare degenerative disorders that manifest with a wide spectrum of clinical phenotypes, including both systemic multi-organ disorders, such as the Hutchinson-Gilford Progeria Syndrome (HGPS), and tissue-restricted diseases, such as Emery-Dreifuss muscular dystrophy, dilated cardiomyopathy and lipodystrophies, often overlapping. Despite their clinical heterogeneity, which remains an open question, laminopathies are commonly caused by mutations in the LMNA gene, encoding the nuclear proteins Lamin A and C. These two proteins are main components of the nuclear lamina and are involved in several biological processes. Besides the well-known structural function in the nucleus, their role in regulating chromatin organization and transcription has emerged in the last decade, supporting the hypothesis that the disruption of this layer of regulation may be mechanism underlying the disease. Indeed, recent studies that show various epigenetic defects in cells carrying LMNA mutations, such as loss of heterochromatin, changes in gene expression and chromatin remodeling, strongly support this view. However, those findings are restricted to few cell types in humans, mainly because of the limited accessibility of primary cells and the difficulties to culture them ex-vivo. On the other hand, animal models might fail to recapitulate phenotypic hallmarks of the disease as of humans. To fill this gap, models based on induced pluripotent stem cell (iPSCs) technology have been recently generated that allowed investigations on diverse cells types, such as mesenchymal stem cells (MSCs), vascular and smooth muscle cells and cardiomyocytes, and provided a platform for investigating mechanisms underlying the pathogenesis of laminopathies in a cell-type specific human context. Nevertheless, studies on iPSC-based models of laminopathy have expanded only in the last few years and, with the advancement of reprogramming and differentiation protocols, their number is expecting to further increase over time. This review will give an overview of models developed thus far, with a focus on the novel insights on epigenetic mechanisms underlying the disease in different human cellular contexts. Perspectives and future directions of the field will be also given, highlighting the potential of those models for preclinical studies for identifying molecular targets and their translational impact on patients' cure.

Annexin A1 May Induce Pancreatic Cancer Progression as a Key Player of Extracellular Vesicles Effects as Evidenced in the In Vitro MIA PaCa-2 Model System

Pancreatic Cancer (PC) is one of the most aggressive malignancies worldwide. As annexin A1 (ANXA1) is implicated in the establishment of tumour metastasis, the role of the protein in PC progression as a component of extracellular vesicles (EVs) has been investigated. EVs were isolated from wild type (WT) and ANXA1 knock-out (KO) PC cells and then characterised by multiple approaches including Western blotting, Field Emission-Scanning Electron Microscopy, and Dynamic Light Scattering. The effects of ANXA1 on tumour aggressiveness were investigated by Wound-Healing and invasion assays and microscopic analysis of the Epithelial to Mesenchymal Transition (EMT). The role of ANXA1 on angiogenesis was also examined in endothelial cells, using similar approaches. We found that WT cells released more EVs enriched in exosomes than those from cells lacking ANXA1. Notably, ANXA1 KO cells recovered their metastatic potential only when treated by WT EVs as they underwent EMT and a significant increase of motility. Similarly, human umbilical vein endothelial cells (HUVEC) migrated and invaded more rapidly when treated by WT EVs whereas ANXA1 KO EVs weakly induced angiogenesis. This study suggests that EVs-related ANXA1 is able to promote cell migration, invasion, and angiogenesis, confirming the relevance of this protein in PC progression.

HiPLA: High-throughput imaging proximity ligation assay

Protein-protein interactions are essential for cellular structure and function. To delineate how the intricate assembly of protein interactions contribute to cellular processes in health and disease, new methodologies that are both highly sensitive and can be applied at large scale are needed. Here, we develop HiPLA (high-throughput imaging proximity ligation assay), a method that employs the well-established antibody-based proximity ligation assay in a high-throughput imaging screening format as a novel means to systematically visualize protein interactomes. Using HiPLA with a library of antibodies targeting nuclear proteins, we probe the interaction of 60 proteins and associated post-translational modifications (PTMs) with the nuclear lamina in a model of the premature aging disorder Hutchinson-Gilford progeria syndrome (HGPS). We identify a subset of proteins that differentially interact with the nuclear lamina in HGPS. Using HiPLA in combination with quantitative indirect immunofluorescence, we find that the majority of differential interactions are accompanied by corresponding changes in expression of the interacting protein. Taken together, HiPLA offers a novel approach to probe cellular protein-protein interaction at a large scale and reveals mechanistic insights into the assembly of protein complexes.

Differential expression of nuclear lamin subtypes in the neural cells of the adult rat cerebral cortex

Lamins are type V intermediate filament proteins that are located beneath the inner nuclear membrane. In mammalian somatic cells, LMNB1 and LMNB2 encode somatic lamins B1 and B2, respectively, and the LMNA gene is alternatively spliced to generate somatic lamins A and C. Mutations in lamin genes have been linked to many human hereditary diseases, including neurodegenerative disorders. Knowledge about lamins in the nervous system has been accumulated recently, but a precise analysis of lamin subtypes in glial cells has not yet been reported. In this study we investigated the composition of lamin subtypes in neurons, astrocytes, oligodendrocyte-lineage cells, and microglia in the adult rat cerebral cortex using an immunohistochemical staining method. Lamin A was not observed in neurons and glial cells. Lamin C was observed in astrocytes, mature oligodendrocytes and neurons, but not observed in oligodendrocyte progenitor cells. Microglia also did not stain positive for lamin C which differed from macrophages, with lamin C positive. Lamin B1 and B2 were observed in all glial cells and neurons. Lamin B1 was intensely positive in oligodendrocyte progenitor cells compared with other glial cells and neurons. Lamin B2 was weakly positive in all glial cells compared to neurons. Our current study might provide useful information to reveal how the onset mechanisms of human neurodegenerative diseases are associated with mutations in genes for nuclear lamin proteins.

The Pathogenesis and Therapies of Striated Muscle Laminopathies

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

Samp1 Mislocalization in Emery-Dreifuss Muscular Dystrophy

LMNA linked-Emery-Dreifuss muscular dystrophy (EDMD2) is a rare disease characterized by muscle weakness, muscle wasting, and cardiomyopathy with conduction defects. The mutated protein lamin A/C binds several nuclear envelope components including the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex and the inner nuclear membrane protein Samp1 (Spindle Associated Membrane Protein 1). Considering that Samp1 is upregulated during muscle cell differentiation and it is involved in nuclear movement, we hypothesized that it could be part of the protein platform formed by LINC proteins and prelamin A at the myotube nuclear envelope and, as previously demonstrated for those proteins, could be affected in EDMD2. Our results show that Samp1 is uniformly distributed at the nuclear periphery of normal human myotubes and committed myoblasts, but its anchorage at the nuclear poles is related to the presence of farnesylated prelamin A and it is disrupted by the loss of prelamin A farnesylation. Moreover, Samp1 is absent from the nuclear poles in EDMD2 myotubes, which shows that LMNA mutations associated with muscular dystrophy, due to reduced prelamin A levels in muscle cell nuclei, impair Samp1 anchorage. Conversely, SUN1 pathogenetic mutations do not alter Samp1 localization in myotubes, which suggests that Samp1 lies upstream of SUN1 in nuclear envelope protein complexes. The hypothesis that Samp1 is part of the protein platform that regulates microtubule nucleation from the myotube nuclear envelope in concert with pericentrin and LINC components warrants future investigation. As a whole, our data identify Samp1 as a new contributor to EDMD2 pathogenesis and our data are relevant to the understanding of nuclear clustering occurring in laminopathic muscle.

1H, 13C and 15N backbone resonance assignment of the lamin C-terminal region specific to prelamin A

Lamins are the main components of the nucleoskeleton. They form a protein meshwork that underlies the inner nuclear membrane. Mutations in the LMNA gene coding for A-type lamins (lamins A and C) cause a large panel of human diseases, referred to as laminopathies. These diseases include muscular dystrophies, lipodystrophies and premature aging diseases. Lamin A exhibits a C-terminal region that is different from lamin C and is post-translationally modified. It is produced as prelamin A and it is then farnesylated, cleaved, carboxymethylated and cleaved again in order to become mature lamin A. In patients with the severe Hutchinson-Gilford progeria syndrome, a specific single point mutation in LMNA leads to an aberrant splicing of the LMNA gene preventing the post-translational processing of prelamin A. This leads to the accumulation of a permanently farnesylated lamin A mutant lacking 50 amino acids named progerin. We here report the NMR ¹H, ¹⁵N, ¹³CO, ¹³Cα and ¹³Cβ chemical shift assignment of the C-terminal region that is specific to prelamin A, from amino acid 567 to amino acid 664. We also report the NMR ¹H, ¹⁵N, ¹³CO, ¹³Cα and ¹³Cβ chemical shift assignment of the C-terminal region of the progerin variant, from amino acid 567 to amino acid 614. Analysis of these chemical shift data confirms that both prelamin A and progerin C-terminal domains are largely disordered and identifies a common partially populated α-helix from amino acid 576 to amino acid 585. This helix is well conserved from fishes to mammals.

A Lamin-Binding Ligand Inhibits Homologous Recombination Repair of DNA Double-Strand Breaks

Nuclear lamins are type V intermediate filament proteins. Lamins, including LA, LB1, LB2, and LC, are the major protein components forming the nuclear lamina to support the mechanical stability of the mammalian cell nucleus. Increasing evidence has shown that LA participates in homologous recombination (HR) repair of DNA double-strand breaks (DSBs) . However, the mechanisms underlying this process are incompletely understood. We recently identified the first lamin-binding ligand 1 (LBL1) that directly binds LA and inhibited cancer cell growth. We provided here further mechanistic investigations of LBL1 and revealed that LA interacts with the HR recombinase Rad51 to protect Rad51 from degradation. LBL1 inhibits LA-Rad51 interaction leading to accelerated proteasome-mediated degradation of Rad51, culminating in inhibition of HR repair of DSBs. These results uncover a novel post-translational regulation of Rad51 by LA and suggest that targeting the LA-Rad51 axis may represent a promising strategy to develop cancer therapeutics.

Osteogenesis and aging: lessons from mesenchymal stem cells

Aging is a high risk factor for the development of osteoporosis, a multifactorial age-related progressive disease characterized by reduced bone mass and increased risk of fractures. At the cellular level, the mesenchymal stem cell pool in the bone marrow niche shows a biased differentiation into adipogenesis at the cost of osteogenesis. This differentiation shift leads to decreased bone formation, contributing to the etiology of osteoporosis. This review will focus on the most recent/relevant molecular findings driving this functional impairment of mesenchymal stem cells in the aging process.

Nuclear networking

Nuclear lamins are intermediate filament proteins that represent important structural components of metazoan nuclear envelopes (NEs). By combining proteomics and superresolution microscopy, we recently reported that both A- and B-type nuclear lamins form spatially distinct filament networks at the nuclear periphery of mouse fibroblasts. In particular, A-type lamins exhibit differential association with nuclear pore complexes (NPCs). Our studies reveal that the nuclear lamina network in mammalian somatic cells is less ordered and more complex than that of amphibian oocytes, the only other system in which the lamina has been visualized at high resolution. In addition, the NPC component Tpr likely links NPCs to the A-type lamin network, an association that appears to be regulated by C-terminal modification of various A-type lamin isoforms. Many questions remain, however, concerning the structure and assembly of lamin filaments, as well as with their mode of association with other nuclear components such as peripheral chromatin.

Fibroblasts lacking nuclear lamins do not have nuclear blebs or protrusions but nevertheless have frequent nuclear membrane ruptures

The nuclear lamina, an intermediate filament meshwork lining the inner nuclear membrane, is formed by the nuclear lamins (lamins A, C, B1, and B2). Defects or deficiencies in individual nuclear lamin proteins have been reported to elicit nuclear blebs (protrusions or outpouchings of the nuclear envelope) and increase susceptibility for nuclear membrane ruptures. It is unclear, however, how a complete absence of nuclear lamins would affect nuclear envelope morphology and nuclear membrane integrity (i.e., whether nuclear membrane blebs or protrusions would occur and, if not, whether cells would be susceptible to nuclear membrane ruptures). To address these issues, we generated mouse embryonic fibroblasts (MEFs) lacking all nuclear lamins. The nuclear lamin-deficient MEFs had irregular nuclear shapes but no nuclear blebs or protrusions. Despite a virtual absence of nuclear blebs, MEFs lacking nuclear lamins had frequent, prolonged, and occasionally nonhealing nuclear membrane ruptures. By transmission electron microscopy, the inner nuclear membrane in nuclear lamin-deficient MEFs have a "wavy" appearance, and there were discrete discontinuities in the inner and outer nuclear membranes. Nuclear membrane ruptures were accompanied by a large increase in DNA damage, as judged by γ-H2AX foci. Mechanical stress increased both nuclear membrane ruptures and DNA damage, whereas minimizing transmission of cytoskeletal forces to the nucleus had the opposite effects.