Filaments made from A- and B-type lamins differ in structure and organization

Current perspectives and trends of neutrophil extracellular traps in organ fibrosis: a bibliometric and visualization study

New insights into the role of immune responses in the fibrosis process provide valuable considerations for the treatment of organ fibrotic diseases. Neutrophil extracellular traps (NETs) represent a novel understanding of neutrophil functions, and their involvement in organ fibrotic diseases has garnered widespread attention in recent years. This study aims to conduct a bibliometric analysis and literature review focusing on the mechanisms by which NETs participate in fibrotic diseases. Specifically, we utilized a bibliometric dataset that includes 220 papers published in 139 journals, originating from 425 organizations across 39 countries, with a total citation count of 12,301. Keyword co-occurrence analysis indicates that the research focus on the mechanisms of NETs in organ fibrosis is likely to center on NETosis, immune responses, immune thrombosis, inflammation, and tissue damage associated with NET formation. In conclusion, our findings underscore the current status and emerging trends in NET research related to organ fibrosis, offering novel insights into the mechanisms by which NETs contribute to the pathogenesis of fibrotic diseases, as well as potential therapeutic strategies.

Advances in research on the relationship between the LMNA gene and human diseases (Review)

The LMNA gene, which is responsible for encoding lamin A/C proteins, is recognized as a primary constituent of the nuclear lamina. This protein serves crucial roles in various cellular physiological activities, including the maintenance of cellular structural stability, regulation of gene expression, mechanosensing and cellular motility. A significant association has been established between the LMNA gene and several major human diseases. Mutations, dysregulated expression of the LMNA gene, and improper processing of its encoded protein can result in a spectrum of pathological conditions. These diseases, collectively termed laminopathies, are directly attributed to LMNA gene dysfunction. The present review examines the recent advancements in research concerning the LMNA gene and its association with human diseases, while exploring its pathological roles. Particular emphasis is placed on the current status of LMNA gene research in the context of tumors. This includes an analysis of the abundance of LMNA alterations in cancer and its interplay with various signaling pathways. The aim of the present review was to provide novel perspectives for studying the development of LMNA‑related diseases and additional theoretical insights for basic and clinical translational research in this field.

Pathological mechanisms and crosstalk among various cell death pathways in cardiac involvement of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a prevalent autoimmune disease primarily characterized by the involvement of multiple systems and organs. Cardiovascular disease is the primary cause of mortality in patients with SLE, though the mechanisms underlying the increased cardiovascular risk in SLE patients remain unclear. Recent studies indicate that abnormal activation of programmed cell death (PCD) signaling and the crosstalk among various forms of cell death are critical in the immunopathogenesis of SLE. Furthermore, apoptosis, necroptosis, pyroptosis, NETosis, and ferroptosis are recognized as key cellular processes in the pathogenesis of SLE and are closely linked to cardiac involvement. This review uniquely explores the intricate crosstalk between apoptosis, necroptosis, and other cell death pathways, discussing their roles and interactions in the pathogenesis of cardiac involvement in SLE. Investigating the interplay between PCD signaling and cardiac involvement in SLE in understanding the disease's underlying mechanisms and offers opportunities for new therapeutic interventions. The integration of precision medicine and innovative strategies targeting these complex pathways holds promise for enhancing the treatment prospects of SLE with cardiac involvement.

Evolutionary, structural and functional insights in nuclear organisation and nucleocytoplasmic transport in trypanosomes

One of the remarkable features of eukaryotes is the nucleus, delimited by the nuclear envelope (NE), a complex structure and home to the nuclear lamina and nuclear pore complex (NPC). For decades, these structures were believed to be mainly architectural elements and, in the case of the NPC, simply facilitating nucleocytoplasmic trafficking. More recently, the critical roles of the lamina, NPC and other NE constituents in genome organisation, maintaining chromosomal domains and regulating gene expression have been recognised. Importantly, mutations in genes encoding lamina and NPC components lead to pathogenesis in humans, while pathogenic protozoa disrupt the progression of normal development and expression of pathogenesis-related genes. Here, we review features of the lamina and NPC across eukaryotes and discuss how these elements are structured in trypanosomes, protozoa of high medical and veterinary importance, highlighting lineage-specific and conserved aspects of nuclear organisation.

Depletion of intracellular Ca2+ induces FOXM1 SUMOylation and accumulation on the inner nuclear membrane and accelerates G2/M cell cycle transition

Intracellular calcium (Ca²⁺) has been reported to regulate transcription factor activity and cancer development, but how it affects the function of Forkhead box protein M1 (FOXM1), a crucial transcription factor and key oncogene participating in tumorigenesis, remains unclear. Here, we investigated the regulatory role of Ca²⁺ on FOXM1 and found that Ca²⁺ depletion caused the distribution of FOXM1 to aggregate on the nuclear envelope, which was also observed in many cell lines. Further experiments revealed that sequestrated FOXM1 colocalized with lamin B in the inner nuclear membrane (INM) and was affected by the activity of nuclear export protein exportin 1 (XPO1). To investigate how intracellular Ca²⁺ affects FOXM1, we found that among the posttranscriptional modifications, only SUMOylation of FOXM1 showed a pronounced increase under reduced Ca²⁺, and suppressed SUMOylation rescued FOXM1 sequestration. In addition, Ca²⁺-dependent SUMOylated FOXM1 appeared to enhance the G2/M transition of the cell cycle and decrease cell apoptosis. In conclusion, our findings provide a molecular basis for the relationship between Ca²⁺ signaling and FOXM1 regulation, and we look to elucidate Ca²⁺-dependent FOXM1 SUMOylation-related biological functions in the future.

The nuclear lamina couples mechanical forces to cell fate in the preimplantation embryo via actin organization

During preimplantation development, contractile forces generated at the apical cortex segregate cells into inner and outer positions of the embryo, establishing the inner cell mass (ICM) and trophectoderm. To which extent these forces influence ICM-trophectoderm fate remains unresolved. Here, we found that the nuclear lamina is coupled to the cortex via an F-actin meshwork in mouse and human embryos. Actomyosin contractility increases during development, upregulating Lamin-A levels, but upon internalization cells lose their apical cortex and downregulate Lamin-A. Low Lamin-A shifts the localization of actin nucleators from nucleus to cytoplasm increasing cytoplasmic F-actin abundance. This results in stabilization of Amot, Yap phosphorylation and acquisition of ICM over trophectoderm fate. By contrast, in outer cells, Lamin-A levels increase with contractility. This prevents Yap phosphorylation enabling Cdx2 to specify the trophectoderm. Thus, forces transmitted to the nuclear lamina control actin organization to differentially regulate the factors specifying lineage identity.

The role of Lamin B2 in human diseases

Lamin B2 (LMNB2), on the inner side of the nuclear envelope, constitutes the nuclear skeleton by connecting with other nuclear proteins. LMNB2 is involved in a wide range of nuclear functions, including DNA replication and stability, regulation of chromatin, and nuclear stiffness. Moreover, LMNB2 regulates several cellular processes, such as tissue development, cell cycle, cellular proliferation and apoptosis, chromatin localization and stability, and DNA methylation. Besides, the influence of abnormal expression and mutations of LMNB2 has been gradually discovered in cancers and laminopathies. Therefore, this review summarizes the recent advances of LMNB2-associated biological roles in physiological or pathological conditions, with a particular emphasis on cancers and laminopathies, as well as the potential mechanism of LMNB2 in related cancers.

Moonlighting chromatin: when DNA escapes nuclear control

Extracellular chromatin, for example in the form of neutrophil extracellular traps (NETs), is an important element that propels the pathological progression of a plethora of diseases. DNA drives the interferon system, serves as autoantigen, and forms the extracellular scaffold for proteins of the innate immune system. An insufficient clearance of extruded chromatin after the release of DNA from the nucleus into the extracellular milieu can perform a secret task of moonlighting in immune-inflammatory and occlusive disorders. Here, we discuss (I) the cellular events involved in the extracellular release of chromatin and NET formation, (II) the devastating consequence of a dysregulated NET formation, and (III) the imbalance between NET formation and clearance. We include the role of NET formation in the occlusion of vessels and ducts, in lung disease, in autoimmune diseases, in chronic oral disorders, in cancer, in the formation of adhesions, and in traumatic spinal cord injury. To develop effective therapies, it is of utmost importance to target pathways that cause decondensation of chromatin during exaggerated NET formation and aggregation. Alternatively, therapies that support the clearance of extracellular chromatin are conceivable.

DISTINCT ROLES OF VIRAL US3 AND UL13 PROTEIN KINASES IN HERPES VIRUS SIMPLEX TYPE 1 (HSV-1) NUCLEAR EGRESS

Herpesviruses transport nucleocapsids from the nucleus to the cytoplasm by capsid envelopment into the inner nuclear membrane and de-envelopment from the outer nuclear membrane, a process that is coordinated by nuclear egress complex (NEC) proteins, pUL34, and pUL31. Both pUL31 and pUL34 are phosphorylated by the virus-encoded protein kinase, pUS3, and phosphorylation of pUL31 regulates NEC localization at the nuclear rim. pUS3 also controls apoptosis and many other viral and cellular functions in addition to nuclear egress, and the regulation of these various activities in infected cells is not well understood. It has been previously proposed that pUS3 activity is selectively regulated by another viral protein kinase, pUL13 such that its activity in nuclear egress is pUL13-dependent, but apoptosis regulation is not, suggesting that pUL13 might regulate pUS3 activity on specific substrates. We compared HSV-1 UL13 kinase-dead and US3 kinase-dead mutant infections and found that pUL13 kinase activity does not regulate the substrate choice of pUS3 in any defined classes of pUS3 substrates and that pUL13 kinase activity is not important for promoting de-envelopment during nuclear egress. We also find that mutation of all pUL13 phosphorylation motifs in pUS3, individually or in aggregate, does not affect the localization of the NEC, suggesting that pUL13 regulates NEC localization independent of pUS3. Finally, we show that pUL13 co-localizes with pUL31 inside the nucleus in large aggregates, further suggesting a direct effect of pUL13 on the NEC and suggesting a novel mechanism for both UL31 and UL13 in the DNA damage response pathway. IMPORTANCE Herpes simplex virus infections are regulated by two virus-encoded protein kinases, pUS3 and pUL13, which each regulate multiple processes in the infected cell, including capsid transport from the nucleus to the cytoplasm. Regulation of the activity of these kinases on their various substrates is poorly understood, but importantly, kinases are attractive targets for the generation of inhibitors. It has been previously suggested that pUS3 activity on specific substrates is differentially regulated by pUL13 and, specifically, that pUL13 regulates capsid egress from the nucleus by phosphorylation of pUS3. In this study, we determined that pUL13 and pUS3 have different effects on nuclear egress and that pUL13 may interact directly with the nuclear egress apparatus with implications both for virus assembly and egress and, possibly, the host cell DNA- damage response.

Nucleus size and its effect on nucleosome stability in living cells

DNA architectural proteins play a major role in organization of chromosomal DNA in living cells by packaging it into chromatin, whose spatial conformation is determined by an intricate interplay between the DNA-binding properties of architectural proteins and physical constraints applied to the DNA by a tight nuclear space. Yet, the exact effects of the nucleus size on DNA-protein interactions and chromatin structure currently remain obscure. Furthermore, there is even no clear understanding of molecular mechanisms responsible for the nucleus size regulation in living cells. To find answers to these questions, we developed a general theoretical framework based on a combination of polymer field theory and transfer-matrix calculations, which showed that the nucleus size is mainly determined by the difference between the surface tensions of the nuclear envelope and the endoplasmic reticulum membrane as well as the osmotic pressure exerted by cytosolic macromolecules on the nucleus. In addition, the model demonstrated that the cell nucleus functions as a piezoelectric element, changing its electrostatic potential in a size-dependent manner. This effect has been found to have a profound impact on stability of nucleosomes, revealing a previously unknown link between the nucleus size and chromatin structure. Overall, our study provides new insights into the molecular mechanisms responsible for regulation of the nucleus size, as well as the potential role of nuclear organization in shaping the cell response to environmental cues.

Knockdown of Lamin B1 and the Corresponding Lamin B Receptor Leads to Changes in Heterochromatin State and Senescence Induction in Malignant Melanoma

Modifications in nuclear structures of cells are implicated in several diseases including cancer. They result in changes in nuclear activity, structural dynamics and cell signalling. However, the role of the nuclear lamina and related proteins in malignant melanoma is still unknown. Its molecular characterisation might lead to a deeper understanding and the development of new therapy approaches. In this study, we analysed the functional effects of dysregulated nuclear lamin B1 (LMNB1) and its nuclear receptor (LBR). According to their cellular localisation and function, we revealed that these genes are crucially involved in nuclear processes like chromatin organisation. RNA sequencing and differential gene expression analysis after knockdown of LMNB1 and LBR revealed their implication in important cellular processes driving ER stress leading to senescence and changes in chromatin state, which were also experimentally validated. We determined that melanoma cells need both molecules independently to prevent senescence. Hence, downregulation of both molecules in a BRAF^V600E melanocytic senescence model as well as in etoposide-treated melanoma cells indicates both as potential senescence markers in melanoma. Our findings suggest that LMNB1 and LBR influence senescence and affect nuclear processes like chromatin condensation and thus are functionally relevant for melanoma progression.

Recent progress in the mechanistic understanding of NET formation in neutrophils

Neutrophils are the most abundant circulating white blood cells and one of the major cell types of the innate immune system. Neutrophil extracellular traps (NETs) are a result of the extracellular release of nuclear chromatin from the ruptured nuclear envelope and plasma membrane. The externalized chromatin is an ancient defense weapon for animals to entrap and kill microorganisms in the extracellular milieu, thus protecting animals ranging from lower invertebrates to higher vertebrates. Although the externalized chromatin has the advantage of acting as anti-infective to protect against infections, extracellular chromatin might be problematic in higher vertebrate animals as they have an adaptive immune system that can trigger further immune or autoimmune responses. NETs and their associated nuclear and/or cytoplasmic components may induce sterile inflammation, immune, and autoimmune responses, leading to various human diseases. Though important in human pathophysiology, the cellular and molecular mechanisms of NET formation (also called NETosis) are not well understood. Given that nuclear chromatin forms the backbone of NETs, the nucleus is the root of the nuclear DNA extracellular traps. Thus, nuclear chromatin decondensation, along with the rupture of nuclear envelope and plasma membrane, is required for nuclear chromatin extracellular release and NET formation. So far, most of the literature focuses on certain signaling pathways, which are involved in NET formation but without explanation of cellular events and morphological changes described above. Here, we have summarized emerging evidence and discuss new mechanistic understanding, with our perspectives, in NET formation in neutrophils.

Fibrous stroma: Driver and passenger in cancer development

Cumulative evidence shows that fibrogenic stroma and stiff extracellular matrix (ECM) not only result from tumor growth but also play pivotal roles in cellular transformation and tumor initiation. This emerging concept may largely account for the increased cancer risk associated with environmental fibrogenic agents, such as asbestos and silica, and with chronic conditions that are fibrogenic, such as obesity and diabetes. It may also contribute to poor outcomes in patients treated with certain chemotherapeutics that can promote fibrosis, such as bleomycin and methotrexate. Although the mechanistic details of this phenomenon are still being unraveled, we provide an overview of the experimental evidence linking fibrogenic stroma and tumor initiation. In this Review, we will summarize the causes and consequences of fibrous stroma and how this stromal cue is transmitted to the nuclei of parenchymal cells through a physical continuum from the ECM to chromatin, as well as ECM-dependent biochemical signaling that contributes to cellular transformation.

Regulation of Lipid Metabolism by Lamin in Mutation-Related Diseases

Nuclear lamins, known as type 5 intermediate fibers, are composed of lamin A, lamin C, lamin B1, and lamin B2, which are encoded by LMNA and LMNB genes, respectively. Importantly, mutations in nuclear lamins not only participate in lipid disorders but also in the human diseases, such as lipodystrophy, metabolic-associated fatty liver disease, and dilated cardiomyopathy. Among those diseases, the mechanism of lamin has been widely discussed. Thereby, this review mainly focuses on the regulatory mechanism of the mutations in the lamin gene in lipid alterations and the human diseases. Considering the protean actions, targeting nuclear lamins may be a potent therapeutic avenue for lipid metabolic disorders and human diseases in the future.

Nuclear Lamins: Key Proteins for Embryonic Development

Simple Summary

The biology of a multicellular organism is extremely complex, leaving behind a realm of compound yet systematic mechanisms still to be unraveled. The nucleus is a vital cellular organelle adapted to storing and regulating the hereditary genetic information. Dysregulation of the nucleus can have profound effects on the physiology and viability of cells. This becomes extremely significant in the context of development, where the whole organism arises from a single cell, the zygote. Therefore, even a mild aberration at this stage can have profound effects on the whole organism. However, studying the function of individual nuclear components at this point is exceptionally complicated because this phase is inherently under the control of maternal factors stored in the female germ cell, the egg. Here, we focus on the lamins, as essential nuclear components, and summarize the current knowledge of their role in development. Although scientists encounter challenges working with these miniscule yet key proteins, the demand to know more is increasing gradually due to the mutations caused in lamins leading to irreversible phenotypic conditions in humans.

Abstract

Lamins are essential components of the nuclear envelope and have been studied for decades due to their involvement in several devastating human diseases, the laminopathies. Despite intensive research, the molecular basis behind the disease state remains mostly unclear with a number of conflicting results regarding the different cellular functions of nuclear lamins being published. The field of developmental biology is no exception. Across model organisms, the types of lamins present in early mammalian development have been contradictory over the years. Due to the long half-life of the lamin proteins, which is a maternal factor that gets carried over to the zygote after fertilization, investigators are posed with challenges to dive into the functional aspects and significance of lamins in development. Due to these technical limitations, the role of lamins in early mammalian embryos is virtually unexplored. This review aims in converging results that were obtained so far in addition to the complex functions that ceases if lamins are mutated.

Scanning Electron Microscopy (SEM) and Immuno-SEM of Nuclear Pore Complexes from Amphibian Oocytes, Mammalian Cell Cultures, Yeast, and Plants

Scanning electron microscopy (SEM) can be used to image nuclear pore complex (NPC) surface structure of from a number of organisms and model systems. With a field emission SEM , this is a medium resolution technique where details of the organization of various components can be directly imaged. Some components, such as the NPC baskets and cytoplasmic filaments, are difficult to visualize in any other way. Protein components can be identified by immunogold labeling. Any surface that can be exposed can potentially be studied by SEM . Several overlapping protocols for SEM sample preparation and immunogold labeling of NPCs are given here. Various parameters for sample preparation, fixation, immunogold labeling, drying, metal coating, and imaging are detailed which have been optimized for different types of specimens and desired endpoints.

Targeting the Microtubule-Network Rescues CTL Killing Efficiency in Dense 3D Matrices

Efficacy of cytotoxic T lymphocyte (CTL)-based immunotherapy is still unsatisfactory against solid tumors, which are frequently characterized by condensed extracellular matrix. Here, using a unique 3D killing assay, we identify that the killing efficiency of primary human CTLs is substantially impaired in dense collagen matrices. Although the expression of cytotoxic proteins in CTLs remained intact in dense collagen, CTL motility was largely compromised. Using light-sheet microscopy, we found that persistence and velocity of CTL migration was influenced by the stiffness and porosity of the 3D matrix. Notably, 3D CTL velocity was strongly correlated with their nuclear deformability, which was enhanced by disruption of the microtubule network especially in dense matrices. Concomitantly, CTL migration, search efficiency, and killing efficiency in dense collagen were significantly increased in microtubule-perturbed CTLs. In addition, the chemotherapeutically used microtubule inhibitor vinblastine drastically enhanced CTL killing efficiency in dense collagen. Together, our findings suggest targeting the microtubule network as a promising strategy to enhance efficacy of CTL-based immunotherapy against solid tumors, especially stiff solid tumors.

Bend, Push, Stretch: Remarkable Structure and Mechanics of Single Intermediate Filaments and Meshworks

The cytoskeleton of the eukaryotic cell provides a structural and functional scaffold enabling biochemical and cellular functions. While actin and microtubules form the main framework of the cell, intermediate filament networks provide unique mechanical properties that increase the resilience of both the cytoplasm and the nucleus, thereby maintaining cellular function while under mechanical pressure. Intermediate filaments (IFs) are imperative to a plethora of regulatory and signaling functions in mechanotransduction. Mutations in all types of IF proteins are known to affect the architectural integrity and function of cellular processes, leading to debilitating diseases. The basic building block of all IFs are elongated α-helical coiled-coils that assemble hierarchically into complex meshworks. A remarkable mechanical feature of IFs is the capability of coiled-coils to metamorphize into β-sheets under stress, making them one of the strongest and most resilient mechanical entities in nature. Here, we discuss structural and mechanical aspects of IFs with a focus on nuclear lamins and vimentin.

A hub-and-spoke nuclear lamina architecture in trypanosomes

The nuclear lamina supports many functions, including maintaining nuclear structure and gene expression control, and correct spatio-temporal assembly is vital to meet these activities. Recently, multiple lamina systems have been described that, despite independent evolutionary origins, share analogous functions. In trypanosomatids the two known lamina proteins, NUP-1 and NUP-2, have molecular masses of 450 and 170 kDa, respectively, which demands a distinct architecture from the ∼60 kDa lamin-based system of metazoa and other lineages. To uncover organizational principles for the trypanosome lamina we generated NUP-1 deletion mutants to identify domains and their arrangements responsible for oligomerization. We found that both the N- and C-termini act as interaction hubs, and that perturbation of these interactions impacts additional components of the lamina and nuclear envelope. Furthermore, the assembly of NUP-1 terminal domains suggests intrinsic organizational capacity. Remarkably, there is little impact on silencing of telomeric variant surface glycoprotein genes. We suggest that both terminal domains of NUP-1 have roles in assembling the trypanosome lamina and propose a novel architecture based on a hub-and-spoke configuration.

Cross-Talk among Polymorphonuclear Neutrophils, Immune, and Non-Immune Cells via Released Cytokines, Granule Proteins, Microvesicles, and Neutrophil Extracellular Trap Formation: A Novel Concept of Biology and Pathobiology for Neutrophils

Polymorphonuclear neutrophils (PMNs) are traditionally regarded as professional phagocytic and acute inflammatory cells that engulf the microbial pathogens. However, accumulating data have suggested that PMNs are multi-potential cells exhibiting many important biological functions in addition to phagocytosis. These newly found novel activities of PMN include production of different kinds of cytokines/chemokines/growth factors, release of neutrophil extracellular traps (NET)/ectosomes/exosomes and trogocytosis (membrane exchange) with neighboring cells for modulating innate, and adaptive immune responses. Besides, PMNs exhibit potential heterogeneity and plasticity in involving antibody-dependent cellular cytotoxicity (ADCC), cancer immunity, autoimmunity, inflammatory rheumatic diseases, and cardiovascular diseases. Interestingly, PMNs may also play a role in ameliorating inflammatory reaction and wound healing by a subset of PMN myeloid-derived suppressor cells (PMN-MDSC). Furthermore, PMNs can interact with other non-immune cells including platelets, epithelial and endothelial cells to link hemostasis, mucosal inflammation, and atherogenesis. The release of low-density granulocytes (LDG) from bone marrow initiates systemic autoimmune reaction in systemic lupus erythematosus (SLE). In clinical application, identification of certain PMN phenotypes may become prognostic factors for severe traumatic patients. In the present review, we will discuss these newly discovered biological and pathobiological functions of the PMNs.

Role of A- and B-type lamins in nuclear structure-function relationships

Nuclear lamins are type V intermediate filament proteins that form a filamentous meshwork beneath the inner nuclear membrane. Additionally, a sub-population of A- and B-type lamins localizes in the nuclear interior. The nuclear lamina protects the nucleus from mechanical stress and mediates nucleo-cytoskeletal coupling. Lamins form a scaffold that partially tethers chromatin at the nuclear envelope. The nuclear lamina also stabilises protein-protein interactions involved in gene regulation and DNA repair. The lamin-based protein sub-complexes are implicated in both nuclear and cytoskeletal organisation, the mechanical stability of the nucleus, genome organisation, transcriptional regulation, genome stability and cellular differentiation. Here, we review recent research on nuclear lamins and unique roles of A- and B-type lamins in modulating various nuclear processes and their impact on cell function.

Nuclear filaments: role in chromosomal positioning and gene expression

Nuclear lamins form an elastic meshwork underlying the inner nuclear membrane and provide mechanical rigidity to the nucleus and maintain shape. Lamins also maintain chromosome positioning and play important roles in several nuclear processes like replication, DNA damage repair, transcription, and epigenetic modifications. LMNA mutations affect cardiac tissue, muscle tissues, adipose tissues to precipitate several diseases collectively termed as laminopathies. However, the rationale behind LMNA mutations and laminopathies continues to elude scientists. During interphase, several chromosomes form inter/intrachromosomal contacts inside nucleoplasm and several chromosomal loops also stretch out to make a ‘loop-cluster’ which are key players to regulate gene expressions. In this perspective, we have proposed that the lamin network in tandem with nuclear actin and myosin provide mechanical rigidity to the chromosomal contacts and facilitate loop-clusters movements. LMNA mutations thus might perturb the landscape of chromosomal contacts or loop-clusters positioning which can impair gene expression profile.

Nonlinear mechanics of lamin filaments and the meshwork topology build an emergent nuclear lamina

The nuclear lamina—a meshwork of intermediate filaments termed lamins—is primarily responsible for the mechanical stability of the nucleus in multicellular organisms. However, structural-mechanical characterization of lamin filaments assembled in situ remains elusive. Here, we apply an integrative approach combining atomic force microscopy, cryo-electron tomography, network analysis, and molecular dynamics simulations to directly measure the mechanical response of single lamin filaments in three-dimensional meshwork. Endogenous lamin filaments portray non-Hookean behavior – they deform reversibly at a few hundred picoNewtons and stiffen at nanoNewton forces. The filaments are extensible, strong and tough similar to natural silk and superior to the synthetic polymer Kevlar^®. Graph theory analysis shows that the lamin meshwork is not a random arrangement of filaments but exhibits small-world properties. Our results suggest that lamin filaments arrange to form an emergent meshwork whose topology dictates the mechanical properties of individual filaments. The quantitative insights imply a role of meshwork topology in laminopathies.

Mechanical strength of in situ assembled nuclear lamin filaments arranged in a 3D meshwork is unclear. Here, using mechanical, structural and simulation tools, the authors report the hierarchical organization of the lamin meshwork that imparts strength and toughness to lamin filaments at par with silk and Kevlar^®

Coordination of NMCP1- and NMCP2-class proteins within the plant nucleoskeleton

Plants lack lamin proteins but contain a class of coiled-coil proteins that serve as analogues to form a laminal structure at the nuclear periphery. These nuclear matrix constituent proteins (NMCPs) play important roles in regulating nuclear morphology and are partitioned into two distinct groups. We investigated Arabidopsis NMCPs (called CRWNs) to study the interrelationship between the three NMCP1-type paralogues (CRWN1, 2, and 3) and the lone NMCP2-type paralogue, CRWN4. An examination of crwn mutants using protein immunoblots demonstrated that CRWN4 abundance depends on the presence of the NMCP1-type proteins, particularly CRWN1. The possibility that CRWN4 is coimported into the nucleus with nuclear localization signal (NLS)-bearing paralogues in the NMCP1-clade was discounted based on recovery of a crwn4-2 missense allele that disrupts a predicted NLS and lowers the abundance of CRWN4 in the nucleus. Further, a screen for mutations that suppress the effects of the crwn4-2 mutation led to the discovery of a missense allele, impa-1^G146E, in one of the nine importin-α genes in the Arabidopsis genome. Our results indicate that the CRWN4 carries a functional NLS that interacts with canonic nuclear import machinery. Once imported, the level of CRWN4 within the nucleus is modulated by the abundance of NMCP1 proteins.

Increased Lamin B1 Levels Promote Cell Migration by Altering Perinuclear Actin Organization

Cell migration requires reposition and reshaping of the cell nucleus. The nuclear lamina is highly important for migration of both primary and cancer cells. B-type lamins are important for proper migration of epicardial cells and neurons and increased lamin B to lamin A ratio accelerates cancer cell migration through confined spaces. Moreover, a positive association between lamin B1 levels and tumor formation and progression is found in various cancer types. Still, the molecular mechanism by which B-type lamins promote cell migration is not fully understood. To better understand this mechanism, we tested the effects of lamin B1 on perinuclear actin organization. Here we show that induction of melanoma cell migration leads to the formation of a cytosolic Linker of Nucleoskeleton and Cytoskeleton (LINC) complex-independent perinuclear actin rim, which has not been detected in migrating cells, yet. Significantly, increasing the levels of lamin B1 but not the levels of lamin A prevented perinuclear actin rim formation while accelerated the cellular migration rate. To interfere with the perinuclear actin rim, we generated a chimeric protein that is localized to the outer nuclear membrane and cleaves perinuclear actin filaments in a specific manner without disrupting other cytosolic actin filaments. Using this tool, we found that disruption of the perinuclear actin rim accelerated the cellular migration rate in a similar manner to lamin B1 over-expression. Taken together, our results suggest that increased lamin B1 levels can accelerate cell migration by inhibiting the association of the nuclear envelope with actin filaments that may reduce nuclear movement and deformability.

The Nuclear Lamina: Protein Accumulation and Disease

Cellular health is reliant on proteostasis-the maintenance of protein levels regulated through multiple pathways modulating protein synthesis, degradation and clearance. Loss of proteostasis results in serious disease and is associated with aging. One proteinaceous structure underlying the nuclear envelope-the nuclear lamina-coordinates essential processes including DNA repair, genome organization and epigenetic and transcriptional regulation. Loss of proteostasis within the nuclear lamina results in the accumulation of proteins, disrupting these essential functions, either via direct interactions of protein aggregates within the lamina or by altering systems that maintain lamina structure. Here we discuss the links between proteostasis and disease of the nuclear lamina, as well as how manipulating specific proteostatic pathways involved in protein clearance could improve cellular health and prevent/reverse disease.

Nuclear envelope rupture and NET formation is driven by PKCα-mediated lamin B disassembly

The nuclear lamina is essential for the structural integration of the nuclear envelope. Nuclear envelope rupture and chromatin externalization is a hallmark of the formation of neutrophil extracellular traps (NETs). NET release was described as a cellular lysis process; however, this notion has been questioned recently. Here, we report that during NET formation, nuclear lamin B is not fragmented by destructive proteolysis, but rather disassembled into intact full-length molecules. Furthermore, we demonstrate that nuclear translocation of PKCα, which serves as the kinase to induce lamin B phosphorylation and disassembly, results in nuclear envelope rupture. Decreasing lamin B phosphorylation by PKCα inhibition, genetic deletion, or by mutating the PKCα consensus sites on lamin B attenuates extracellular trap formation. In addition, strengthening the nuclear envelope by lamin B overexpression attenuates NET release in vivo and reduces levels of NET-associated inflammatory cytokines in UVB-irradiated skin of lamin B transgenic mice. Our findings advance the mechanistic understanding of NET formation by showing that PKCα-mediated lamin B phosphorylation drives nuclear envelope rupture for chromatin release in neutrophils.

Lamin A/C Mechanotransduction in Laminopathies

Mechanotransduction translates forces into biological responses and regulates cell functionalities. It is implicated in several diseases, including laminopathies which are pathologies associated with mutations in lamins and lamin-associated proteins. These pathologies affect muscle, adipose, bone, nerve, and skin cells and range from muscular dystrophies to accelerated aging. Although the exact mechanisms governing laminopathies and gene expression are still not clear, a strong correlation has been found between cell functionality and nuclear behavior. New theories base on the direct effect of external force on the genome, which is indeed sensitive to the force transduced by the nuclear lamina. Nuclear lamina performs two essential functions in mechanotransduction pathway modulating the nuclear stiffness and governing the chromatin remodeling. Indeed, A-type lamin mutation and deregulation has been found to affect the nuclear response, altering several downstream cellular processes such as mitosis, chromatin organization, DNA replication-transcription, and nuclear structural integrity. In this review, we summarize the recent findings on the molecular composition and architecture of the nuclear lamina, its role in healthy cells and disease regulation. We focus on A-type lamins since this protein family is the most involved in mechanotransduction and laminopathies.

Nuclear envelope dysfunction and its contribution to the aging process

The nuclear envelope (NE) is the central organizing unit of the eukaryotic cell serving as a genome protective barrier and mechanotransduction interface between the cytoplasm and the nucleus. The NE is mainly composed of a nuclear lamina and a double membrane connected at specific points where the nuclear pore complexes (NPCs) form. Physiological aging might be generically defined as a functional decline across lifespan observed from the cellular to organismal level. Therefore, during aging and premature aging, several cellular alterations occur, including nuclear‐specific changes, particularly, altered nuclear transport, increased genomic instability induced by DNA damage, and telomere attrition. Here, we highlight and discuss proteins associated with nuclear transport dysfunction induced by aging, particularly nucleoporins, nuclear transport factors, and lamins. Moreover, changes in the structure of chromatin and consequent heterochromatin rearrangement upon aging are discussed. These alterations correlate with NE dysfunction, particularly lamins’ alterations. Finally, telomere attrition is addressed and correlated with altered levels of nuclear lamins and nuclear lamina‐associated proteins. Overall, the identification of molecular mechanisms underlying NE dysfunction, including upstream and downstream events, which have yet to be unraveled, will be determinant not only to our understanding of several pathologies, but as here discussed, in the aging process.

Role of the Nuclear Lamina in Age-Associated Nuclear Reorganization and Inflammation

Aging is characterized by the gradual loss of tissue function and integrity. Activation of inflammatory responses accelerates the deterioration of cells and tissues. Many studies have shown that alteration of the components of the nuclear lamina is associated with inflammation, both in vivo and in vitro. However, the mechanism by which the nuclear lamina regulates inflammation is largely unknown. Recent studies have suggested that the nuclear lamina regulates both organization of the three-dimensional chromatin structure at the nuclear periphery and global gene expression, such as the expression of inflammatory response genes. Here, we discuss the current updates in the research on nuclear lamina alteration, activation of inflammation, and nuclear reorganization in models of cellular senescence and organismal aging.

Lamin A/C and Emerin depletion impacts chromatin organization and dynamics in the interphase nucleus

Background

Nuclear lamins are type V intermediate filament proteins that maintain nuclear structure and function. Furthermore, Emerin - an interactor of Lamin A/C, facilitates crosstalk between the cytoskeleton and the nucleus as it also interacts with actin and Nuclear Myosin 1 (NM1).

Results

Here we show that the depletion of Lamin A/C or Emerin, alters the localization of the nuclear motor protein - Nuclear Myosin 1 (NM1) that manifests as an increase in NM1 foci in the nucleus and are rescued to basal levels upon the combined knockdown of Lamin A/C and Emerin. Furthermore, Lamin A/C-Emerin co-depletion destabilizes cytoskeletal organization as it increases actin stress fibers. This further impinges on nuclear organization, as it enhances chromatin mobility more toward the nuclear interior in Lamin A/C-Emerin co-depleted cells. This enhanced chromatin mobility was restored to basal levels either upon inhibition of Nuclear Myosin 1 (NM1) activity or actin depolymerization. In addition, the combined loss of Lamin A/C and Emerin alters the otherwise highly conserved spatial positions of chromosome territories. Furthermore, knockdown of Lamin A/C or Lamin A/C-Emerin combined, deregulates expression levels of a candidate subset of genes. Amongst these genes, both KLK10 (Chr.19, Lamina Associated Domain (LAD+)) and MADH2 (Chr.18, LAD-) were significantly repressed, while BCL2L12 (Chr.19, LAD-) is de-repressed. These genes differentially reposition with respect to the nuclear envelope.

Conclusions

Taken together, these studies underscore a remarkable interplay between Lamin A/C and Emerin in modulating cytoskeletal organization of actin and NM1 that impinges on chromatin dynamics and function in the interphase nucleus.

Electronic supplementary material

The online version of this article (10.1186/s12860-019-0192-5) contains supplementary material, which is available to authorized users.

Evolutionary history and structure of nuclear matrix constituent proteins, the plant analogues of lamins

Nuclear matrix constituent proteins (NMCPs), the structural components of the plant lamina, are considered to be the analogues of lamins in plants based on numerous structural and functional similarities. Current phylogenetic knowledge suggests that, in contrast to lamins, which are widely distributed in eukaryotes, NMCPs are taxonomically restricted to Streptophyta. At present, most information about NMCPs comes from angiosperms, and virtually no data are available from more ancestral groups. In angiosperms, the NMCP family comprises two phylogenetic groups, NMCP1 and NMCP2, which evolved from the NMCP1 and NMCP2 progenitor genes. Based on sequence conservation and the presence of NMCP-specific domains, we determined the structure and number of NMCP genes present in different Streptophyta clades. We analysed 91 species of embryophytes and report additional NMCP sequences from mosses, liverworts, clubmosses, horsetail, ferns, gymnosperms, and Charophyta algae. Our results confirm an origin of NMCPs in Charophyta (the earliest diverging group of Streptophyta), resolve the number and structure of NMCPs in the different clades, and propose the emergence of additional NMCP homologues by whole-genome duplication events. Immunofluorescence microscopy demonstrated localization of a basal NMCP from the moss Physcomitrella patens at the nuclear envelope, suggesting a functional conservation for basal and more evolved NMCPs.

Presence and distribution of progerin in HGPS cells is ameliorated by drugs that impact on the mevalonate and mTOR pathways

Hutchinson-Gilford progeria syndrome (HGPS) is a rare, premature ageing syndrome in children. HGPS is normally caused by a mutation in the LMNA gene, encoding nuclear lamin A. The classical mutation in HGPS leads to the production of a toxic truncated version of lamin A, progerin, which retains a farnesyl group. Farnesyltransferase inhibitors (FTI), pravastatin and zoledronic acid have been used in clinical trials to target the mevalonate pathway in HGPS patients to inhibit farnesylation of progerin, in order to reduce its toxicity. Some other compounds that have been suggested as treatments include rapamycin, IGF1 and N-acetyl cysteine (NAC). We have analysed the distribution of prelamin A, lamin A, lamin A/C, progerin, lamin B1 and B2 in nuclei of HGPS cells before and after treatments with these drugs, an FTI and a geranylgeranyltransferase inhibitor (GGTI) and FTI with pravastatin and zoledronic acid in combination. Confirming other studies prelamin A, lamin A, progerin and lamin B2 staining was different between control and HGPS fibroblasts. The drugs that reduced progerin staining were FTI, pravastatin, zoledronic acid and rapamycin. However, drugs affecting the mevalonate pathway increased prelamin A, with only FTI reducing internal prelamin A foci. The distribution of lamin A in HGPS cells was improved with treatments of FTI, pravastatin and FTI + GGTI. All treatments reduced the number of cells displaying internal speckles of lamin A/C and lamin B2. Drugs targeting the mevalonate pathway worked best for progerin reduction, with zoledronic acid removing internal progerin speckles. Rapamycin and NAC, which impact on the MTOR pathway, both reduced both pools of progerin without increasing prelamin A in HGPS cell nuclei.

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.

Supramolecular Structures of the Dictyostelium Lamin NE81

Nuclear lamins are nucleus-specific intermediate filaments (IF) found at the inner nuclear membrane (INM) of the nuclear envelope (NE). Together with nuclear envelope transmembrane proteins, they form the nuclear lamina and are crucial for gene regulation and mechanical robustness of the nucleus and the whole cell. Recently, we characterized Dictyostelium NE81 as an evolutionarily conserved lamin-like protein, both on the sequence and functional level. Here, we show on the structural level that the Dictyostelium NE81 is also capable of assembling into filaments, just as metazoan lamin filament assemblies. Using field-emission scanning electron microscopy, we show that NE81 expressed in Xenopous oocytes forms filamentous structures with an overall appearance highly reminiscent of Xenopus lamin B2. The in vitro assembly properties of recombinant His-tagged NE81 purified from Dictyostelium extracts are very similar to those of metazoan lamins. Super-resolution stimulated emission depletion (STED) and expansion microscopy (ExM), as well as transmission electron microscopy of negatively stained purified NE81, demonstrated its capability of forming filamentous structures under low-ionic-strength conditions. These results recommend Dictyostelium as a non-mammalian model organism with a well-characterized nuclear envelope involving all relevant protein components known in animal cells.

The role of lamin A/C in mesenchymal stem cell differentiation

Lamin A/C is the major architectural protein of cell nucleus in charge of the nuclear mechanosensing. By integrating extracellular mechanical and biochemical signals, lamin A/C regulates multiple intracellular events including mesenchymal stem cell (MSC) fate determination. Herein, we review the recent findings about the effects and mechanisms of lamin A/C in governing MSC lineage commitment, with a special focus on osteogenesis and adipogenesis. Better understanding of MSC differentiation regulated by lamin A/C could provide insights into pathogenesis of age-related osteoporosis.

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.

Immunogold Labeling for Scanning Electron Microscopy

Scanning electron microscopes are useful biological tools that can be used to image the surface of whole organisms, tissues, cells, cellular components, and macromolecules. Processes and structures that exist at surfaces can be imaged in pseudo, or real 3D at magnifications ranging from about 10× to 1,000,000×. Therefore a whole multicellular organism, such as a fly, or a single protein embedded in one of its cell membranes can be visualized. In order to identify that protein at high resolution, or to see and quantify its distribution at lower magnifications, samples can be labeled with antibodies. Any surface that can be exposed can potentially be studied in this way. Presented here is a generic method for immunogold labeling for scanning electron microscopy, using two examples of specimens: isolated nuclear envelopes and the cytoskeleton of mammalian culture cells. Various parameters for sample preparation, fixation, immunogold labeling, drying, metal coating, and imaging are discussed so that the best immunogold scanning electron microscopy results can be obtained from different types of specimens.

Protein sequestration at the nuclear periphery as a potential regulatory mechanism in premature aging

Serebryannyy and Misteli provide a perspective on how protein sequestration at the inner nuclear membrane and nuclear lamina might influence aging.

Despite the extensive description of numerous molecular changes associated with aging, insights into the driver mechanisms of this fundamental biological process are limited. Based on observations in the premature aging syndrome Hutchinson–Gilford progeria, we explore the possibility that protein regulation at the inner nuclear membrane and the nuclear lamina contributes to the aging process. In support, sequestration of nucleoplasmic proteins to the periphery impacts cell stemness, the response to cytotoxicity, proliferation, changes in chromatin state, and telomere stability. These observations point to the nuclear periphery as a central regulator of the aging phenotype.

The structure of lamin filaments in somatic cells as revealed by cryo-electron tomography

Metazoan nuclei are equipped with nuclear lamina - a thin layer of intermediate filaments (IFs) mostly built of nuclear lamins facing the inner nuclear membrane (INM). The nuclear lamina serves as an interaction hub for INM-proteins, soluble nuclear factors and DNA. It confers structural and mechanical stability to the nucleus, transduces mechanical forces and biochemical signals across the nuclear envelope (NE) and regulates the organization of chromatin. By using cryo-electron tomography (cryo-ET), we recently provided an unprecedented view into the 3D organization of lamin filaments within the lamina meshwork in mammalian somatic cells. Through implementation of averaging procedures, we resolved the rod and globular Ig-fold domains of lamin filaments. The density maps suggested that they assemble into 3.5 nm thick filaments. Our analysis revealed interesting structural differences between nucleoplasmic and cytoplasmic intermediate filaments, raising the question of which molecular cues define their assembly modes inside the cell.

Lamins in the nuclear interior - life outside the lamina

Nuclear lamins are components of the peripheral lamina that define the mechanical properties of nuclei and tether heterochromatin to the periphery. A-type lamins localize also to the nuclear interior, but the regulation and specific functions of this nucleoplasmic lamin pool are poorly understood. In this Commentary, we summarize known pathways that are potentially involved in the localization and dynamic behavior of intranuclear lamins, including their post-translational modifications and interactions with nucleoplasmic proteins, such as lamina-associated polypeptide 2α (LAP2α; encoded by TMPO). In addition, new data suggest that lamins in the nuclear interior have an important role in chromatin regulation and gene expression through dynamic binding to both hetero- and euchromatic genomic regions and promoter subdomains, thereby affecting epigenetic pathways and chromatin accessibility. Nucleoplasmic lamins also have a role in spatial chromatin organization and may be involved in mechanosignaling. In view of this newly emerging concept, we propose that the previously reported cellular phenotypes in lamin-linked diseases are, at least in part, rooted in an impaired regulation and/or function of the nucleoplasmic lamin A/C pool.

Evolutionary changes in lamin expression in the vertebrate lineage

The nuclear lamina is involved in fundamental nuclear functions and provides mechanical stability to the nucleus. Lamin filaments form a meshwork closely apposed to the inner nuclear membrane and a small fraction of lamins exist in the nuclear interior. Mutations in lamin genes cause severe hereditary diseases, the laminopathies. During vertebrate evolution the lamin protein family has expanded. While most vertebrate genomes contain 4 lamin genes, encoding the lamins A, B1, B2, and LIII, the majority of non-vertebrate genomes harbor only a single lamin gene. We have collected lamin gene and cDNA sequence information for representatives of the major vertebrate lineages. With the help of RNA-seq data we have determined relative lamin expression levels for representative tissues for species of 9 different gnathostome lineages. Here we report that the level of lamin A expression is low in cartilaginous fishes and ancient fishes and increases toward the mammals. Lamin B1 expression shows an inverse tendency to that of lamin A. Possible implications for the change in the lamin A to B ratio is discussed in the light of its role in nuclear mechanics.

Lamin A/C Cardiomyopathies: Current Understanding and Novel Treatment Strategies

OPINION STATEMENT

Dilated cardiomyopathy (DCM) is the third leading cause of heart failure in the USA. A major gene associated with DCM with cardiac conduction system disease is lamin A/C (LMNA) gene. Lamins are type V filaments that serve a variety of roles, including nuclear structure support, DNA repair, cell signaling pathway mediation, and chromatin organization. In 1999, LMNA was found responsible for Emery-Dreifuss muscular dystrophy (EDMD) and, since then, has been found in association with a wide spectrum of diseases termed laminopathies, including LMNA cardiomyopathy. Patients with LMNA mutations have a poor prognosis and a higher risk for sudden cardiac death, along with other cardiac effects like dysrhythmias, development of congestive heart failure, and potential need of a pacemaker or ICD. As of now, there is no specific treatment for laminopathies, including LMNA cardiomyopathy, because the mechanism of LMNA mutations in humans is still unclear. This review discusses LMNA mutations and how they relate to DCM, the necessity for further investigation to better understand LMNA mutations, and potential treatment options ranging from clinical and therapeutic to cellular and molecular techniques.

Implications and Assessment of the Elastic Behavior of Lamins in Laminopathies

Lamins are mechanosensitive and elastic components of the nuclear lamina that respond to external mechanical cues by altering gene regulation in a feedback mechanism. Numerous mutations in A-type lamins cause a plethora of diverse diseases collectively termed as laminopathies, the majority of which are characterized by irregularly shaped, fragile, and plastic nuclei. These nuclei are challenged to normal mechanotransduction and lead to disease phenotypes. Here, we review our current understanding of the nucleocytoskeleton coupling in mechanotransduction mediated by lamins. We also present an up-to-date understanding of the methods used to determine laminar elasticity both at the bulk and single molecule level.

Mechanics in human fibroblasts and progeria: Lamin A mutation E145K results in stiffening of nuclei

The lamina is a filamentous meshwork beneath the inner nuclear membrane that confers mechanical stability to nuclei. The E145K mutation in lamin A causes Hutchinson-Gilford progeria syndrome (HGPS). It affects lamin filament assembly and induces profound changes in the nuclear architecture. Expression of wild-type and E145K lamin A in Xenopus oocytes followed by atomic force microscopy (AFM) probing of isolated oocyte nuclei has shown significant changes in the mechanical properties of the lamina. Nuclei of oocytes expressing E145K lamin A are stiffer than those expressing wild-type lamin A. Here we present mechanical measurements by AFM on dermal fibroblasts obtained from a 4-year-old progeria patient bearing the E145K lamin A mutation and compared it to fibroblasts obtained from 2 healthy donors of 10 and 61 years of age, respectively. The abnormal shape of nuclei expressing E145K lamin A was analyzed by fluorescence microscopy. Lamina thickness was measured using electron micrographs. Fluorescence microscopy showed alterations in the actin network of progeria cells. AFM probing of whole dermal fibroblasts did not demonstrate significant differences in the elastic moduli of nuclear and cytoplasmic cell regions. In contrast, AFM measurements of isolated nuclei showed that nuclei of progeria and old person's cells are significantly stiffer than those of the young person, indicating that the process of aging, be it natural or abnormal, increases nuclear stiffness. Our results corroborate AFM data obtained using Xenopus oocyte nuclei and prove that the presence of E145K lamin A abnormally increases nuclear stiffness.

A-type Lamins Form Distinct Filamentous Networks with Differential Nuclear Pore Complex Associations

The nuclear lamina is a universal feature of metazoan nuclear envelopes (NEs) [1]. In mammalian cells, it appears as a 10-30 nm filamentous layer at the nuclear face of the inner nuclear membrane (INM) and is composed primarily of A- and B-type lamins, members of the intermediate filament family [2]. While providing structural integrity to the NE, the lamina also represents an important signaling and regulatory platform [3]. Two A-type lamin isoforms, lamins A and C (LaA and LaC), are expressed in most adult human cells. Encoded by a single gene, these proteins are largely identical, diverging only in their C-terminal tail domains. By contrast with that of LaC, the unique LaA tail undergoes extensive processing, including farnesylation and endo-proteolysis [4, 5]. However, functional differences between LaA and LaC are still unclear. Compounding this uncertainty, the structure of the lamina remains ill defined. In this study, we used BioID, an in vivo proximity-labeling method to identify differential interactors of A-type lamins [6]. One of these, Tpr, a nuclear pore complex (NPC) protein, is highlighted by its selective association with LaC. By employing superresolution microscopy, we demonstrate that this Tpr association is mirrored in enhanced interaction of LaC with NPCs. Further superresolution studies visualizing both endogenous A- and B-type lamins have allowed us to construct a nanometer-scale model of the mammalian nuclear lamina. Our data indicate that different A- and B-type lamin species assemble into separate filament networks that together form an extended composite structure at the nuclear periphery providing attachment sites for NPCs, thereby regulating their distribution.

Lipids contribute to epigenetic control via chromatin structure and functions

Abstract Isolated cases of experimental evidence over the last few decades have shown that, where specifically tested, both prokaryotes and eukaryotes have specific lipid species bound to nucleoproteins of the genome. In vitro, some of these lipid species exhibit stoichiometric association with DNA polynucleotides with differential affinities toward certain secondary and tertiary structures. Hydrophobic interactions with inner nuclear membrane could provide attractive anchor points for lipid-modified nucleoproteins in organizing the dynamic genome and accordingly there are precedents for covalent bonds between lipids and core histones and, under certain conditions, even DNA. Advances in biophysics, functional genomics, and proteomics in recent years brought about the first sparks of light that promises to uncover some coherent new level of the epigenetic code governed by certain types of lipid–lipid, DNA–lipid, and DNA-protein–lipid interactions among other biochemical lipid transactions in the nucleus. Here, we review some of the older and more recent findings and speculate on how critical nuclear lipid transactions are for individual cells, tissues, and organisms.