Lamins at the crossroads of mechanosignaling

Assembly studies of six intestinal intermediate filament (IF) proteins B2, C1, C2, D1, D2, and E1 in the nematode C. elegans

The dimerisation properties of six intestine-expressed intermediate filament (IF) proteins (B2, C1, C2, D1, D2, E1) were analysed in blot overlay assay on membranes containing all of the eleven recombinant C. elegans IF proteins (A1, A2, A3, A4, B1, B2, C1, C2, D1, D2, and E1). The interactions detected in the blot assays exclusively comprise intestine-expressed IF proteins and the protein A4, which is found in the dauer larva intestine. About 86% of these interactions are heterotypic, while the remaining interactions relate to C1, C2, and D2 homodimers. These multiple modes of interaction were also supported by calculations of the numbers of possible interchain ionic interactions derived from the individual rod sequences. The results predict that the six B2, C1, C2, D1, D2, and E1 IF proteins are able to form as many as eleven different heteropolymeric and three homopolymeric IFs in the C. elegans intestine. This simple model of the intestinal IF meshwork enables us to speculate that our previously reported triple RNAi worms arrested or decreased their growth because of feeding reduction due to morphological defects of the mechanically compromised intestine.

A-type lamins and cardiovascular disease in premature aging syndromes

Lamin A is a nuclear intermediate filament protein with important structural and regulatory roles in most differentiated mammalian cells. Excessive accumulation of its precursor prelamin A or the mutant form called 'progerin' causes premature aging syndromes. Progeroid 'laminopathies' are characterized by severe cardiovascular problems (cardiac electrical defects, vascular calcification and stiffening, atherosclerosis, myocardial infarction, and stroke) and premature death. Here, we review studies in cell and mouse models and patients that are unraveling how abnormal prelamin A and progerin accumulation accelerates cardiovascular disease and aging. This knowledge is essential for developing effective therapies to treat progeria and may help identify new mechanisms underlying normal aging.

Occurrence of Emery-Dreifuss muscular dystrophy in a rural setting of Cameroon: a case report and review of the literature

Background

Emery-Dreifuss muscular dystrophy is a rare genetic muscular disease, presenting mainly with contractures, weakness and cardiac conduction abnormalities. Its clinical and laboratory similarities to other muscular dystrophies, and rarity poses diagnostic challenges, requiring a high index of suspicion in resource limited settings.

Case presentation

An 8 year old sub-Saharan male presented with rigidity and deformity of both elbows and ankles, and weakness of the upper limbs and lower limbs for duration of 4 months. This progressed to inability to stand and walk. There was no mental impairment. Physical examination was remarkable for contractures of the elbows and ankles, and wasting of muscles of the limbs and trunk, with a scapulohumeroperoneal pattern, and tachycardia. After laboratory investigations, a diagnosis of Emery-Dreifuss muscular dystrophy was suspected. Physiotherapy was started, wheel chair was prescribed, and referral to a specialist center was done for appropriate management.

Conclusions

Emery-Dreifuss muscular dystrophy is a rare disabling muscular disease which poses a diagnostic challenge. High index of suspicion is paramount for its early diagnoses to prevent orthopedic and cardiac complications. Prompt diagnosis and management is essential to improve on the prognosis of this disease.

Emerin self-assembly mechanism: role of the LEM domain

At the nuclear envelope, the inner nuclear membrane protein emerin contributes to the interface between the nucleoskeleton and the chromatin. Emerin is an essential actor of the nuclear response to a mechanical signal. Genetic defects in emerin cause Emery-Dreifuss muscular dystrophy. It was proposed that emerin oligomerization regulates nucleoskeleton binding, and impaired oligomerization contributes to the loss of function of emerin disease-causing mutants. We here report the first structural characterization of emerin oligomers. We identified an N-terminal emerin region from amino acid 1 to amino acid 132 that is necessary and sufficient for formation of long curvilinear filaments. In emerin monomer, this region contains a globular LEM domain and a fragment that is intrinsically disordered. Solid-state nuclear magnetic resonance analysis identifies the LEM β-fragment as part of the oligomeric structural core. However, the LEM domain alone does not self-assemble into filaments. Additional residues forming a β-structure are observed within the filaments that could correspond to the unstructured region in emerin monomer. We show that the delK37 mutation causing muscular dystrophy triggers LEM domain unfolding and increases emerin self-assembly rate. Similarly, inserting a disulfide bridge that stabilizes the LEM folded state impairs emerin N-terminal region self-assembly, whereas reducing this disulfide bridge triggers self-assembly. We conclude that the LEM domain, responsible for binding to the chromatin protein BAF, undergoes a conformational change during self-assembly of emerin N-terminal region. The consequences of these structural rearrangement and self-assembly events on emerin binding properties are discussed.

The mammalian LINC complex regulates genome transcriptional responses to substrate rigidity

Mechanical integration of the nucleus with the extracellular matrix (ECM) is established by linkage between the cytoskeleton and the nucleus. This integration is hypothesized to mediate sensing of ECM rigidity, but parsing the function of nucleus-cytoskeleton linkage from other mechanisms has remained a central challenge. Here we took advantage of the fact that the LINC (linker of nucleoskeleton and cytoskeleton) complex is a known molecular linker of the nucleus to the cytoskeleton, and asked how it regulates the sensitivity of genome-wide transcription to substratum rigidity. We show that gene mechanosensitivity is preserved after LINC disruption, but reversed in direction. Combined with myosin inhibition studies, we identify genes that depend on nuclear tension for their regulation. We also show that LINC disruption does not attenuate nuclear shape sensitivity to substrate rigidity. Our results show for the first time that the LINC complex facilitates mechano-regulation of expression across the genome.

Scale Invariant Disordered Nanotopography Promotes Hippocampal Neuron Development and Maturation with Involvement of Mechanotransductive Pathways

The identification of biomaterials which promote neuronal maturation up to the generation of integrated neural circuits is fundamental for modern neuroscience. The development of neural circuits arises from complex maturative processes regulated by poorly understood signaling events, often guided by the extracellular matrix (ECM). Here we report that nanostructured zirconia surfaces, produced by supersonic cluster beam deposition of zirconia nanoparticles and characterized by ECM-like nanotopographical features, can direct the maturation of neural networks. Hippocampal neurons cultured on such cluster-assembled surfaces displayed enhanced differentiation paralleled by functional changes. The latter was demonstrated by single-cell electrophysiology showing earlier action potential generation and increased spontaneous postsynaptic currents compared to the neurons grown on the featureless unnaturally flat standard control surfaces. Label-free shotgun proteomics broadly confirmed the functional changes and suggests furthermore a vast impact of the neuron/nanotopography interaction on mechanotransductive machinery components, known to control physiological in vivo ECM-regulated axon guidance and synaptic plasticity. Our results indicate a potential of cluster-assembled zirconia nanotopography exploitable for the creation of efficient neural tissue interfaces and cell culture devices promoting neurogenic events, but also for unveiling mechanotransductive aspects of neuronal development and maturation.

Recent advances in understanding the role of lamins in health and disease

Lamins are major components of the nuclear lamina, a network of proteins that supports the nuclear envelope in metazoan cells. Over the past decade, biochemical studies have provided support for the view that lamins are not passive bystanders providing mechanical stability to the nucleus but play an active role in the organization of the genome and the function of fundamental nuclear processes. It has also become apparent that lamins are critical for human health, as a large number of mutations identified in the gene that encodes for A-type lamins are associated with tissue-specific and systemic genetic diseases, including the accelerated aging disorder known as Hutchinson-Gilford progeria syndrome. Recent years have witnessed great advances in our understanding of the role of lamins in the nucleus and the functional consequences of disease-associated A-type lamin mutations. Many of these findings have been presented in comprehensive reviews. In this mini-review, we discuss recent breakthroughs in the role of lamins in health and disease and what lies ahead in lamin research.

Modeling of the mechano-chemical behaviour of the nuclear pore complex: current research and perspectives

Recent evidence suggests that mechanical deformation of the cell nucleus regulates the nuclear import of the transcriptional activators of genes involved in primary physiological cell responses such as stem cell differentiation. In addition, this nuclear mechanosensing response is de-regulated in pathological states, such as cancer and neurodegeneration. One hypothesis that could greatly advance the field is that the deformation of the nuclear envelope activates nuclear pore complexes through a direct mechanical link. The understanding of this possible mechanism for nuclear pore complex stretch-activation entails studying the mechanical connection of this complex to the nuclear envelope at the nanoscale. The nanomechanics of the nuclear pore complex is thus emerging as a novel research field, bridging nanoscience with nanotechnology. This review examines the frontier of research methodologies that are potentially useful for building a computational model of this interaction. This includes, for example, electron tomography to assess the geometrical features of the nuclear pore complex and nanoindentation to estimate its mechanical properties and that of the nuclear envelope. In order to summarize the state-of-the-art and perspectives in the field of NPC nanomechanics, this review covers highly interdisciplinary experimental and theoretical research methodologies pertaining to the fields of physics, chemistry, biology, materials and mechanics.

Global transcriptional changes caused by an EDMD mutation correlate to tissue specific disease phenotypes in C. elegans

There are numerous heritable diseases associated with mutations in the LMNA gene. Most of these laminopathic diseases, including several muscular dystrophies, are autosomal dominant and have tissue-specific phenotypes. Our previous studies have shown that the globally expressed Emery-Dreifuss muscular dystrophy (EDMD)-linked lamin mutation, L535P, disrupts nuclear mechanical response specifically in muscle nuclei of C. elegans leading to atrophy of the body muscle cells and to reduced motility. Here we used RNA sequencing to analyze the global changes in gene expression caused by the L535P EDMD lamin mutation in order to gain better understanding of disease mechanisms and the correlation between transcription and phenotype. Our results show changes in key genes and biological pathways that can help explain the muscle specific phenotypes. In addition, the differential gene expression between wild-type and L535P mutant animals suggests that the pharynx function in the L535P mutant animals is affected by this lamin mutation. Moreover, these transcriptional changes were then correlated with reduced pharynx activity and abnormal pharynx muscle structure. Understanding disease mechanisms will potentially lead to new therapeutic approaches toward curing EDMD.

A potential role for genome structure in the translation of mechanical force during immune cell development

Immune cells react to a wide range of environments, both chemical and physical. While the former has been extensively studied, there is growing evidence that physical and in particular mechanical forces also affect immune cell behavior and development. In order to elicit a response that affects immune cell behavior or development, environmental signals must often reach the nucleus. Chemical and mechanical signals can initiate signal transduction pathways, but mechanical forces may also have a more direct route to the nucleus, altering nuclear shape via mechanotransduction. The three-dimensional organization of DNA allows for the possibility that altering nuclear shape directly remodels chromatin, redistributing critical regulatory elements and proteins, and resulting in wide-scale gene expression changes. As such, integrating mechanotransduction and genome architecture into the immunology toolkit will improve our understanding of immune development and disease.

Regulation of lamin properties and functions: does phosphorylation do it all?

The main functions of lamins are their mechanical and structural roles as major building blocks of the karyoskeleton. They are also involved in chromatin structure regulation, gene expression, intracellular signalling pathway modulation and development. All essential lamin functions seem to depend on their capacity for assembly or disassembly after the receipt of specific signals, and after specific, selective and precisely regulated interactions through their various domains. Reversible phosphorylation of lamins is crucial for their functions, so it is important to understand how lamin polymerization and interactions are modulated, and which sequences may undergo such modifications. This review combines experimental data with results of our in silico analyses focused on lamin phosphorylation in model organisms to show the presence of evolutionarily conserved sequences and to indicate specific in vivo phosphorylations that affect particular functions.

Skeletal Muscle Laminopathies: A Review of Clinical and Molecular Features

LMNA-related disorders are caused by mutations in the LMNA gene, which encodes for the nuclear envelope proteins, lamin A and C, via alternative splicing. Laminopathies are associated with a wide range of disease phenotypes, including neuromuscular, cardiac, metabolic disorders and premature aging syndromes. The most frequent diseases associated with mutations in the LMNA gene are characterized by skeletal and cardiac muscle involvement. This review will focus on genetics and clinical features of laminopathies affecting primarily skeletal muscle. Although only symptomatic treatment is available for these patients, many achievements have been made in clarifying the pathogenesis and improving the management of these diseases.

Cell-cell junctional mechanotransduction in endothelial remodeling

The vasculature is one of the most dynamic tissues that encounter numerous mechanical cues derived from pulsatile blood flow, blood pressure, activity of smooth muscle cells in the vessel wall, and transmigration of immune cells. The inner layer of blood and lymphatic vessels is covered by the endothelium, a monolayer of cells which separates blood from tissue, an important function that it fulfills even under the dynamic circumstances of the vascular microenvironment. In addition, remodeling of the endothelial barrier during angiogenesis and trafficking of immune cells is achieved by specific modulation of cell-cell adhesion structures between the endothelial cells. In recent years, there have been many new discoveries in the field of cellular mechanotransduction which controls the formation and destabilization of the vascular barrier. Force-induced adaptation at endothelial cell-cell adhesion structures is a crucial node in these processes that challenge the vascular barrier. One of the key examples of a force-induced molecular event is the recruitment of vinculin to the VE-cadherin complex upon pulling forces at cell-cell junctions. Here, we highlight recent advances in the current understanding of mechanotransduction responses at, and derived from, endothelial cell-cell junctions. We further discuss their importance for vascular barrier function and remodeling in development, inflammation, and vascular disease.

Normal stroma suppresses cancer cell proliferation via mechanosensitive regulation of JMJD1a-mediated transcription

Tissue homeostasis is dependent on the controlled localization of specific cell types and the correct composition of the extracellular stroma. While the role of the cancer stroma in tumour progression has been well characterized, the specific contribution of the matrix itself is unknown. Furthermore, the mechanisms enabling normal-not cancer-stroma to provide tumour-suppressive signals and act as an antitumorigenic barrier are poorly understood. Here we show that extracellular matrix (ECM) generated by normal fibroblasts (NFs) is softer than the CAF matrix, and its physical and structural features regulate cancer cell proliferation. We find that normal ECM triggers downregulation and nuclear exit of the histone demethylase JMJD1a resulting in the epigenetic growth restriction of carcinoma cells. Interestingly, JMJD1a positively regulates transcription of many target genes, including YAP/TAZ (WWTR1), and therefore gene expression in a stiffness-dependent manner. Thus, normal stromal restricts cancer cell proliferation through JMJD1a-dependent modulation of gene expression.

AIM2 inflammasome is activated by pharmacological disruption of nuclear envelope integrity

Inflammasomes are critical sensors that convey cellular stress and pathogen presence to the immune system by activating inflammatory caspases and cytokines such as IL-1β. The nature of endogenous stress signals that activate inflammasomes remains unclear. Here we show that an inhibitor of the HIV aspartyl protease, Nelfinavir, triggers inflammasome formation and elicits an IL-1R-dependent inflammation in mice. We found that Nelfinavir impaired the maturation of lamin A, a structural component of the nuclear envelope, thereby promoting the release of DNA in the cytosol. Moreover, deficiency of the cytosolic DNA-sensor AIM2 impaired Nelfinavir-mediated inflammasome activation. These findings identify a pharmacologic activator of inflammasome and demonstrate the role of AIM2 in detecting endogenous DNA release upon perturbation of nuclear envelope integrity.

Building up the nucleus: nuclear organization in the establishment of totipotency and pluripotency during mammalian development

In mammals, epigenetic reprogramming, the acquisition and loss of totipotency, and the first cell fate decision all occur within a 3-d window after fertilization from the one-cell zygote to the formation of the blastocyst. These processes are poorly understood in molecular detail, yet this is an essential prerequisite to uncover principles of stem cells, chromatin biology, and thus regenerative medicine. A unique feature of preimplantation development is the drastic genome-wide changes occurring to nuclear architecture. From studying somatic and in vitro cultured embryonic stem cells (ESCs) it is becoming increasingly established that the three-dimensional (3D) positions of genomic loci relative to each other and to specific compartments of the nucleus can act on the regulation of gene expression, potentially driving cell fate. However, the functionality, mechanisms, and molecular characteristics of the changes in nuclear organization during preimplantation development are only now beginning to be unraveled. Here, we discuss the peculiarities of nuclear compartments and chromatin organization during mammalian preimplantation development in the context of the transition from totipotency to pluripotency.

Under Pressure: Mechanical Stress Management in the Nucleus

Cells are constantly adjusting to the mechanical properties of their surroundings, operating a complex mechanochemical feedback, which hinges on mechanotransduction mechanisms. Whereas adhesion structures have been shown to play a central role in mechanotransduction, it now emerges that the nucleus may act as a mechanosensitive structure. Here, we review recent advances demonstrating that mechanical stress emanating from the cytoskeleton can activate pathways in the nucleus which eventually impact both its structure and the transcriptional machinery.

Nuclear lamins in cancer

Dysmorphic nuclei are commonly seen in cancers and provide strong motivation for studying the main structural proteins of nuclei, the lamins, in cancer. Past studies have also demonstrated the significance of microenvironment mechanics to cancer progression, which is extremely interesting because the lamina was recently shown to be mechanosensitive. Here, we review current knowledge relating cancer progression to lamina biophysics. Lamin levels can constrain cancer cell migration in 3D and thereby impede tumor growth, and lamins can also protect a cancer cell's genome. In addition, lamins can influence transcriptional regulators (RAR, SRF, YAP/TAZ) and chromosome conformation in lamina associated domains. Further investigation of the roles for lamins in cancer and even DNA damage may lead to new therapies or at least to a clearer understanding of lamins as bio-markers in cancer progression.

Probing cytoskeletal pre-stress and nuclear mechanics in endothelial cells with spatiotemporally controlled (de-)adhesion kinetics on micropatterned substrates

The mechanical properties of living cells reflect their propensity to migrate and respond to external forces. Both cellular and nuclear stiffnesses are strongly influenced by the rigidity of the extracellular matrix (ECM) through reorganization of the cyto- and nucleoskeletal protein connections. Changes in this architectural continuum affect cell mechanics and underlie many pathological conditions. In this context, an accurate and combined quantification of the mechanical properties of both cells and nuclei can contribute to a better understanding of cellular (dys-)function. To address this challenge, we have established a robust method for probing cellular and nuclear deformation during spreading and detachment from micropatterned substrates. We show that (de-)adhesion kinetics of endothelial cells are modulated by substrate stiffness and rely on the actomyosin network. We combined this approach with measurements of cell stiffness by magnetic tweezers to show that relaxation dynamics can be considered as a reliable parameter of cellular pre-stress in adherent cells. During the adhesion stage, large cellular and nuclear deformations occur over a long time span (>60 min). Conversely, nuclear deformation and condensed chromatin are relaxed in a few seconds after detachment. Finally, our results show that accumulation of farnesylated prelamin leads to modifications of the nuclear viscoelastic properties, as reflected by increased nuclear relaxation times. Our method offers an original and non-intrusive way of simultaneously gauging cellular and nuclear mechanics, which can be extended to high-throughput screens of pathological conditions and potential countermeasures.

Cross talk between the cytoplasm and nucleus during development and disease

Mechanotransduction is a process whereby mechanical stimuli outside the cell are sensed by components of the plasma membrane and transmitted as signals through the cytoplasm that terminate in the nucleus. The nucleus responds to these signals by altering gene expression. During mechanotransduction, complex networks of proteins are responsible for cross talk between the cytoplasm and the nucleus. These proteins include cell membrane receptors, cytoplasmic filaments, LINC complex members that bridge the nucleus and cytoplasm, and nuclear envelope proteins that connect to the chromatin. Mechanotransduction also plays a critical role in development. Furthermore, it is possible that disrupted mechanotransduction leads to changes in gene expression that underlie the pathogenic mechanisms of disease.

Molecular insights into the premature aging disease progeria

Hutchinson-Gilford progeria syndrome (HGPS) is an extremely rare premature aging disease presenting many features resembling the normal aging process. HGPS patients die before the age of 20 years due to cardiovascular problems and heart failure. HGPS is linked to mutations in the LMNA gene encoding the intermediate filament protein lamin A. Lamin A is a major component of the nuclear lamina, a scaffold structure at the nuclear envelope that defines mechanochemical properties of the nucleus and is involved in chromatin organization and epigenetic regulation. Lamin A is also present in the nuclear interior where it fulfills lamina-independent functions in cell signaling and gene regulation. The most common LMNA mutation linked to HGPS leads to mis-splicing of the LMNA mRNA and produces a mutant lamin A protein called progerin that tightly associates with the inner nuclear membrane and affects the dynamic properties of lamins. Progerin expression impairs many important cellular processes providing insight into potential disease mechanisms. These include changes in mechanosignaling, altered chromatin organization and impaired genome stability, and changes in signaling pathways, leading to impaired regulation of adult stem cells, defective extracellular matrix production and premature cell senescence. In this review, we discuss these pathways and their potential contribution to the disease pathologies as well as therapeutic approaches used in preclinical and clinical tests.

The nucleus is a conserved mechanosensation and mechanoresponse organelle

Cells in vivo exist in a dynamic environment where they experience variable mechanical influences. The precise mechanical environment influences cell-cell interactions, cell-extracellular matrix interactions, and in-turn, cell morphology and cell function. Therefore, the ability of each cell to constantly and rapidly alter their behavior in response to variations in their mechanical environment is essential for cell viability, development, and function. Mechanotransduction, the process by which mechanical force is translated into a biochemical signal to activate downstream cellular responses, is thus crucial to cell function during development and homeostasis. Although much research has focused on how protein complexes at the cell cortex respond to mechanical stress to initiate mechanotransduction, the nucleus has emerged as crucial to the ability of the cell to perceive and respond to changes in its mechanical environment. This additional method for mechanosensing allows for direct transmission of force through the cytoskeleton to the nucleus, which can increase the speed at which a cell changes its transcriptional profile. This review discusses recent work demonstrating the importance of the nucleus in mediating the cellular response to internal and external force, establishing the nucleus as an important mechanosensing organelle.

A Case for the Nuclear Membrane as a Mechanotransducer

The cell nucleus is becoming increasingly recognized as a mechanosensitive organelle. Most research on nuclear mechanosignaling focuses on the nuclear lamina and coupled actin structures. In this commentary, we discuss the possibility that the nuclear membrane senses and transduces mechanical signals similar to the plasma membrane. We briefly summarize possible (i) pathophysiological sources of nuclear membrane tension, (ii) features that render nuclear membranes particularly suited for mechanotransduction, and (iii) molecular sensing mechanisms.

The RanGTP Pathway: From Nucleo-Cytoplasmic Transport to Spindle Assembly and Beyond

The small GTPase Ran regulates the interaction of transport receptors with a number of cellular cargo proteins. The high affinity binding of the GTP-bound form of Ran to import receptors promotes cargo release, whereas its binding to export receptors stabilizes their interaction with the cargo. This basic mechanism linked to the asymmetric distribution of the two nucleotide-bound forms of Ran between the nucleus and the cytoplasm generates a switch like mechanism controlling nucleo-cytoplasmic transport. Since 1999, we have known that after nuclear envelope breakdown (NEBD) Ran and the above transport receptors also provide a local control over the activity of factors driving spindle assembly and regulating other aspects of cell division. The identification and functional characterization of RanGTP mitotic targets is providing novel insights into mechanisms essential for cell division. Here we review our current knowledge on the RanGTP system and its regulation and we focus on the recent advances made through the characterization of its mitotic targets. We then briefly review the novel functions of the pathway that were recently described. Altogether, the RanGTP system has moonlighting functions exerting a spatial control over protein interactions that drive specific functions depending on the cellular context.

Proliferation of progeria cells is enhanced by lamina-associated polypeptide 2α (LAP2α) through expression of extracellular matrix proteins

A single heterozygous mutation of LMNA generates the lamin A/C variant progerin and causes Hutchinson-Gilford progeria syndrome (HGPS). Vidak et al. show that this mutation leads to loss of LAP2α and nucleoplasmic lamins A/C, impaired proliferation, and down-regulation of extracellular matrix components. Ectopic expression of LAP2α in cells expressing progerin restores proliferation and extracellular matrix expression but not the levels of nucleoplasmic lamins A/C.

Lamina-associated polypeptide 2α (LAP2α) localizes throughout the nucleoplasm and interacts with the fraction of lamins A/C that is not associated with the peripheral nuclear lamina. The LAP2α–lamin A/C complex negatively affects cell proliferation. Lamins A/C are encoded by LMNA, a single heterozygous mutation of which causes Hutchinson-Gilford progeria syndrome (HGPS). This mutation generates the lamin A variant progerin, which we show here leads to loss of LAP2α and nucleoplasmic lamins A/C, impaired proliferation, and down-regulation of extracellular matrix components. Surprisingly, contrary to wild-type cells, ectopic expression of LAP2α in cells expressing progerin restores proliferation and extracellular matrix expression but not the levels of nucleoplasmic lamins A/C. We conclude that, in addition to its cell cycle-inhibiting function with lamins A/C, LAP2α can also regulate extracellular matrix components independently of lamins A/C, which may help explain the proliferation-promoting function of LAP2α in cells expressing progerin.

Identifying Protein-Protein Associations at the Nuclear Envelope with BioID

The nuclear envelope (NE) is a critical cellular structure whose constituents and roles in a myriad of cellular processes seem ever expanding. To determine the underlying mechanisms by which the NE constituents participate in various cellular events, it is necessary to understand the nature of their protein-protein associations. BioID (proximity-dependent biotin identification) is a recently established method to generate a history of protein-protein associations as they occur over time in living cells. BioID is based on fusion of a bait protein to a promiscuous biotin ligase. Expression of the BioID fusion protein in a relevant cellular environment enables biotinylation of vicinal and interacting proteins of the bait protein, permitting isolation and identification by conventional biotin-affinity capture and mass-spec analysis. In this way, BioID provides unique capabilities to identify protein-protein associations at the NE. In this chapter we provide a detailed protocol for the application of BioID to the study of NE proteins.

Nuclear deformability and telomere dynamics are regulated by cell geometric constraints

Forces generated by the cytoskeleton can be transmitted to the nucleus and chromatin via physical links on the nuclear envelope and the lamin meshwork. Although the role of these active forces in modulating prestressed nuclear morphology has been well studied, the effect on nuclear and chromatin dynamics remains to be explored. To understand the regulation of nuclear deformability by these active forces, we created different cytoskeletal states in mouse fibroblasts using micropatterned substrates. We observed that constrained and isotropic cells, which lack long actin stress fibers, have more deformable nuclei than elongated and polarized cells. This nuclear deformability altered in response to actin, myosin, formin perturbations, or a transcriptional down-regulation of lamin A/C levels in the constrained and isotropic geometry. Furthermore, to probe the effect of active cytoskeletal forces on chromatin dynamics, we tracked the spatiotemporal dynamics of heterochromatin foci and telomeres. We observed increased dynamics and decreased correlation of the heterochromatin foci and telomere trajectories in constrained and isotropic cell geometry. The observed enhanced dynamics upon treatment with actin depolymerizing reagents in elongated and polarized geometry were regained once the reagent was washed off, suggesting an inherent structural memory in chromatin organization. We conclude that active forces from the cytoskeleton and rigidity from lamin A/C nucleoskeleton can together regulate nuclear and chromatin dynamics. Because chromatin remodeling is a necessary step in transcription control and its memory, genome integrity, and cellular deformability during migration, our results highlight the importance of cell geometric constraints as critical regulators in cell behavior.

High-Throughput Screening for Drugs that Modulate Intermediate Filament Proteins

Intermediate filament (IF) proteins have unique and complex cell and tissue distribution. Importantly, IF gene mutations cause or predispose to more than 80 human tissue-specific diseases (IF-pathies), with the most severe disease phenotypes being due to mutations at conserved residues that result in a disrupted IF network. A critical need for the entire IF-pathy field is the identification of drugs that can ameliorate or cure these diseases, particularly since all current therapies target the IF-pathy complication, such as diabetes or cardiovascular disease, rather than the mutant IF protein or gene. We describe a high-throughput approach to identify drugs that can normalize disrupted IF proteins. This approach utilizes transduction of lentivirus that expresses green fluorescent protein-tagged keratin 18 (K18) R90C in A549 cells. The readout is drug "hits" that convert the dot-like keratin filament distribution, due to the R90C mutation, to a wild-type-like filamentous array. A similar strategy can be used to screen thousands of compounds and can be utilized for practically any IF protein with a filament-disrupting mutation, and could therefore potentially target many IF-pathies. "Hits" of interest require validation in cell culture then using in vivo experimental models. Approaches to study the mechanism of mutant IF normalization by potential drugs of interest are also described. The ultimate goal of this drug screening approach is to identify effective and safe compounds that can potentially be tested for clinical efficacy in patients.

Reorganization of Nuclear Pore Complexes and the Lamina in Late-Stage Parvovirus Infection

Canine parvovirus (CPV) infection induces reorganization of nuclear structures. Our studies indicated that late-stage infection induces accumulation of nuclear pore complexes (NPCs) and lamin B1 concomitantly with a decrease of lamin A/C levels on the apical side of the nucleus. Newly formed CPV capsids are located in close proximity to NPCs on the apical side. These results suggest that parvoviruses cause apical enrichment of NPCs and reorganization of nuclear lamina, presumably to facilitate the late-stage infection.

GM1 Ganglioside is Involved in Epigenetic Activation Loci of Neuronal Cells

Gangliosides are sialic acid-containing glycosphingolipids that are most abundant in the nerve tissues. The quantity and expression pattern of gangliosides in brain change drastically throughout development and are mainly regulated through stage-specific expression of glycosyltransferase (ganglioside synthase) genes. We previously demonstrated that acetylation of histones H3 and H4 on the N-acetylgalactosaminyltransferase I (GalNAcT, GA2/GM2/GD2/GT2-synthase) gene promoter resulted in recruitment of trans-activation factors. In addition, we reported that epigenetic activation of the GalNAcT gene was also detected as accompanied by an apparent induction of neuronal differentiation in neural stem cells responding to an exogenous supplement of ganglioside GM1. Here, we present evidence supporting the concept that nuclear GM1 is associated with gene regulation in neuronal cells. We found that nuclear GM1 binds acetylated histones on the promoters of the GalNAcT and NeuroD1 genes in differentiated neurons. Our study demonstrates for the first time that GM1 interacts with chromatin via acetylated histones at the nuclear periphery of neuronal cells.

Inner nuclear envelope protein SUN1 plays a prominent role in mammalian mRNA export

Nuclear export of messenger ribonucleoproteins (mRNPs) through the nuclear pore complex (NPC) can be roughly classified into two forms: bulk and specific export, involving an nuclear RNA export factor 1 (NXF1)-dependent pathway and chromosome region maintenance 1 (CRM1)-dependent pathway, respectively. SUN proteins constitute the inner nuclear envelope component of the linker of nucleoskeleton and cytoskeleton (LINC) complex. Here, we show that mammalian cells require SUN1 for efficient nuclear mRNP export. The results indicate that both SUN1 and SUN2 interact with heterogeneous nuclear ribonucleoprotein (hnRNP) F/H and hnRNP K/J. SUN1 depletion inhibits the mRNP export, with accumulations of both hnRNPs and poly(A)+RNA in the nucleus. Leptomycin B treatment indicates that SUN1 functions in mammalian mRNA export involving the NXF1-dependent pathway. SUN1 mediates mRNA export through its association with mRNP complexes via a direct interaction with NXF1. Additionally, SUN1 associates with the NPC through a direct interaction with Nup153, a nuclear pore component involved in mRNA export. Taken together, our results reveal that the inner nuclear envelope protein SUN1 has additional functions aside from being a central component of the LINC complex and that it is an integral component of the mammalian mRNA export pathway suggesting a model whereby SUN1 recruits NXF1-containing mRNP onto the nuclear envelope and hands it over to Nup153.

Structural organization of nuclear lamins A, C, B1, and B2 revealed by superresolution microscopy

Superresolution microscopy and computational image analysis demonstrate that the four nuclear lamin isoforms of mammalian cells are each organized into distinct meshwork structures sharing similar physical characteristics. Knockouts of single lamins alter the structure of the remaining lamins, suggesting interactions among the meshworks.

The nuclear lamina is a key structural element of the metazoan nucleus. However, the structural organization of the major proteins composing the lamina is poorly defined. Using three-dimensional structured illumination microscopy and computational image analysis, we characterized the supramolecular structures of lamin A, C, B1, and B2 in mouse embryo fibroblast nuclei. Each isoform forms a distinct fiber meshwork, with comparable physical characteristics with respect to mesh edge length, mesh face area and shape, and edge connectivity to form faces. Some differences were found in face areas among isoforms due to variation in the edge lengths and number of edges per face, suggesting that each meshwork has somewhat unique assembly characteristics. In fibroblasts null for the expression of either lamins A/C or lamin B1, the remaining lamin meshworks are altered compared with the lamin meshworks in wild-type nuclei or nuclei lacking lamin B2. Nuclei lacking LA/C exhibit slightly enlarged meshwork faces and some shape changes, whereas LB1-deficient nuclei exhibit primarily a substantial increase in face area. These studies demonstrate that individual lamin isoforms assemble into complex networks within the nuclear lamina and that A- and B-type lamins have distinct roles in maintaining the organization of the nuclear lamina.