Substrate stiffness-dependent regulation of the SRF-Mkl1 co-activator complex requires the inner nuclear membrane protein Emerin

Feeling the force from within - new tools and insights into nuclear mechanotransduction

The ability of cells to sense and respond to mechanical signals is essential for many biological processes that form the basis of cell identity, tissue development and maintenance. This process, known as mechanotransduction, involves crucial feedback between mechanical force and biochemical signals, including epigenomic modifications that establish transcriptional programs. These programs, in turn, reinforce the mechanical properties of the cell and its ability to withstand mechanical perturbation. The nucleus has long been hypothesized to play a key role in mechanotransduction due to its direct exposure to forces transmitted through the cytoskeleton, its role in receiving cytoplasmic signals and its central function in gene regulation. However, parsing out the specific contributions of the nucleus from those of the cell surface and cytoplasm in mechanotransduction remains a substantial challenge. In this Review, we examine the latest evidence on how the nucleus regulates mechanotransduction, both via the nuclear envelope (NE) and through epigenetic and transcriptional machinery elements within the nuclear interior. We also explore the role of nuclear mechanotransduction in establishing a mechanical memory, characterized by a mechanical, epigenetic and transcriptomic cell state that persists after mechanical stimuli cease. Finally, we discuss current challenges in the field of nuclear mechanotransduction and present technological advances that are poised to overcome them.

At the nucleus of cancer: how the nuclear envelope controls tumor progression

Historically considered downstream effects of tumorigenesis-arising from changes in DNA content or chromatin organization-nuclear alterations have long been seen as mere prognostic markers within a genome-centric model of cancer. However, recent findings have placed the nuclear envelope (NE) at the forefront of tumor progression, highlighting its active role in mediating cellular responses to mechanical forces. Despite significant progress, the precise interplay between NE components and cancer progression remains under debate. In this review, we provide a comprehensive and up-to-date overview of how changes in NE composition affect nuclear mechanics and facilitate malignant transformation, grounded in the latest molecular and functional studies. We also review recent research that uses advanced technologies, including artificial intelligence, to predict malignancy risk and treatment outcomes by analyzing nuclear morphology. Finally, we discuss how progress in understanding nuclear mechanics has paved the way for mechanotherapy-a promising cancer treatment approach that exploits the mechanical differences between cancerous and healthy cells. Shifting the perspective on NE alterations from mere diagnostic markers to potential therapeutic targets, this review calls for further investigation into the evolving role of the NE in cancer, highlighting the potential for innovative strategies to transform conventional cancer therapies.

Role of Emerin in regulating fibroblast differentiation and migration at the substrate of stiffness coupled topology

In hypertrophic scars, the differentiation and migration of fibroblasts are influenced by the extracellular matrix microenvironment, which includes factors such as stiffness, restraint, and tensile force. These mechanical stresses incite alterations in cell behavior, accompanied by cytoskeletal protein reorganization. However, the role of nucleo-skeletal proteins in this context remains underexplored. In this study, we use a polyacrylamide hydrogel (PAA) to simulate the mechanical stress experienced by cells in scar tissue and investigate the impact of Emerin on cell behavior. We utilize atomic force microscopy (AFM) and RNA interference technology to analyze cell differentiation, migration, and stiffness. Our findings reveal that rigid substrates and cellular restriction elevate Emerin expression and diminish differentiation. Conversely, reducing Emerin expression leads to attenuated cell differentiation, where stiffness and constraining factors exert no notable influence. Furthermore, a softening of cells and an enhanced migration rate are also markedly observed. These observations indicate that variations in nuclear skeletal proteins, prompted by diverse matrix microenvironments, play a pivotal role in the pathogenesis of hypertrophic scars (HSs). This research offers novel insights and a reference point for understanding scar fibrosis formation mechanisms and preventing fibrosis.

Over-expression of Dyrk1A affects bleeding by modulating plasma fibronectin and fibrinogen level in mice

Down syndrome is the most common chromosomal abnormality in humans. Patients with Down syndrome have hematologic disorders, including mild to moderate thrombocytopenia. In case of Down syndrome, thrombocytopenia is not associated with bleeding, and it remains poorly characterized regarding molecular mechanisms. We investigated the effects of overexpression of Dyrk1A, an important factor contributing to some major Down syndrome phenotypes, on platelet number and bleeding in mice. Mice overexpressing Dyrk1A have a decrease in platelet number by 20%. However, bleeding time was found to be reduced by 50%. The thrombocytopenia and the decreased bleeding time observed were not associated to an abnormal platelet receptors expression, to a defect of platelet activation by ADP, thrombin or convulxin, to the presence of activated platelets in the circulation or to an abnormal half-life of the platelets. To propose molecular mechanisms explaining this discrepancy, we performed a network analysis of Dyrk1A interactome and demonstrated that Dyrk1A, fibronectin and fibrinogen interact indirectly through two distinct clusters of proteins. Moreover, in mice overexpressing Dyrk1A, increased plasma fibronectin and fibrinogen levels were found, linked to an increase of the hepatic fibrinogen production. Our results indicate that overexpression of Dyrk1A in mice induces decreased bleeding consistent with increased plasma fibronectin and fibrinogen levels, revealing a new role of Dyrk1A depending on its indirect interaction with these two proteins.

Glia maturation factor beta deficiency protects against diabetic osteoporosis by suppressing osteoclast hyperactivity

Excessive osteoclast activation, which depends on dramatic changes in actin dynamics, causes osteoporosis (OP). The molecular mechanism of osteoclast activation in OP related to type 1 diabetes (T1D) remains unclear. Glia maturation factor beta (GMFB) is considered a growth and differentiation factor for both glia and neurons. Here, we demonstrated that Gmfb deficiency effectively ameliorated the phenotype of T1D-OP in rats by inhibiting osteoclast hyperactivity. In vitro assays showed that GMFB participated in osteoclast activation rather than proliferation. Gmfb deficiency did not affect osteoclast sealing zone (SZ) formation but effectively decreased the SZ area by decreasing actin depolymerization. When GMFB was overexpressed in Gmfb-deficient osteoclasts, the size of the SZ area was enlarged in a dose-dependent manner. Moreover, decreased actin depolymerization led to a decrease in nuclear G-actin, which activated MKL1/SRF-dependent gene transcription. We found that pro-osteoclastogenic factors (Mmp9 and Mmp14) were downregulated, while anti-osteoclastogenic factors (Cftr and Fhl2) were upregulated in Gmfb KO osteoclasts. A GMFB inhibitor, DS-30, targeting the binding site of GMFB and Arp2/3, was obtained. Biocore analysis revealed a high affinity between DS-30 and GMFB in a dose-dependent manner. As expected, DS-30 strongly suppressed osteoclast hyperactivity in vivo and in vitro. In conclusion, our work identified a new therapeutic strategy for T1D-OP treatment. The discovery of GMFB inhibitors will contribute to translational research on T1D-OP.

Single-molecule tracking (SMT) and localization of SRF and MRTF transcription factors during neuronal stimulation and differentiation

In cells, proteins encoded by the same gene do not all behave uniformly but engage in functional subpopulations induced by spatial or temporal segregation. While conventional microscopy has limitations in revealing such spatial and temporal diversity, single-molecule tracking (SMT) microscopy circumvented this problem and allows for high-resolution imaging and quantification of dynamic single-molecule properties. Particularly in the nucleus, SMT has identified specific DNA residence times of transcription factors (TFs), DNA-bound TF fractions and positions of transcriptional hot-spots upon cell stimulation. By contrast to cell stimulation, SMT has not been employed to follow dynamic TF changes along stages of cell differentiation. Herein, we analysed the serum response factor (SRF), a TF involved in the differentiation of many cell types to study nuclear single-molecule dynamics in neuronal differentiation. Our data in living mouse hippocampal neurons show dynamic changes in SRF DNA residence time and SRF DNA-bound fraction between the stages of adhesion, neurite growth and neurite differentiation in axon and dendrites. Using TALM (tracking and localization microscopy), we identified nuclear positions of SRF clusters and observed changes in their numbers and size during differentiation. Furthermore, we show that the SRF cofactor MRTF-A (myocardin-related TF or MKL1) responds to cell activation by enhancing the long-bound DNA fraction. Finally, a first SMT colocalization study of two proteins was performed in living cells showing enhanced SRF/MRTF-A colocalization upon stimulation. In summary, SMT revealed modulation of dynamic TF properties during cell stimulation and differentiation.

Lamina-associated polypeptide 2α is required for intranuclear MRTF-A activity

Myocardin-related transcription factor A (MRTF-A), a coactivator of serum response factor (SRF), regulates the expression of many cytoskeletal genes in response to cytoplasmic and nuclear actin dynamics. Here we describe a novel mechanism to regulate MRTF-A activity within the nucleus by showing that lamina-associated polypeptide 2α (Lap2α), the nucleoplasmic isoform of Lap2, is a direct binding partner of MRTF-A, and required for the efficient expression of MRTF-A/SRF target genes. Mechanistically, Lap2α is not required for MRTF-A nuclear localization, unlike most other MRTF-A regulators, but is required for efficient recruitment of MRTF-A to its target genes. This regulatory step takes place prior to MRTF-A chromatin binding, because Lap2α neither interacts with, nor specifically influences active histone marks on MRTF-A/SRF target genes. Phenotypically, Lap2α is required for serum-induced cell migration, and deregulated MRTF-A activity may also contribute to muscle and proliferation phenotypes associated with loss of Lap2α. Our studies therefore add another regulatory layer to the control of MRTF-A-SRF-mediated gene expression, and broaden the role of Lap2α in transcriptional regulation.

Fibroblast Memory in Development, Homeostasis and Disease

Fibroblasts are the major cell population in the connective tissue of most organs, where they are essential for their structural integrity. They are best known for their role in remodelling the extracellular matrix, however more recently they have been recognised as a functionally highly diverse cell population that constantly responds and adapts to their environment. Biological memory is the process of a sustained altered cellular state and functions in response to a transient or persistent environmental stimulus. While it is well established that fibroblasts retain a memory of their anatomical location, how other environmental stimuli influence fibroblast behaviour and function is less clear. The ability of fibroblasts to respond and memorise different environmental stimuli is essential for tissue development and homeostasis and may become dysregulated in chronic disease conditions such as fibrosis and cancer. Here we summarise the four emerging key areas of fibroblast adaptation: positional, mechanical, inflammatory, and metabolic memory and highlight the underlying mechanisms and their implications in tissue homeostasis and disease.

The Diverse Cellular Functions of Inner Nuclear Membrane Proteins

The nuclear compartment is delimited by a specialized expanded sheet of the endoplasmic reticulum (ER) known as the nuclear envelope (NE). Compared to the outer nuclear membrane and the contiguous peripheral ER, the inner nuclear membrane (INM) houses a unique set of transmembrane proteins that serve a staggering range of functions. Many of these functions reflect the exceptional position of INM proteins at the membrane-chromatin interface. Recent research revealed that numerous INM proteins perform crucial roles in chromatin organization, regulation of gene expression, genome stability, and mediation of signaling pathways into the nucleus. Other INM proteins establish mechanical links between chromatin and the cytoskeleton, help NE remodeling, or contribute to the surveillance of NE integrity and homeostasis. As INM proteins continue to gain prominence, we review these advancements and give an overview on the functional versatility of the INM proteome.

Journey to the Center of the Cell: Cytoplasmic and Nuclear Actin in Immune Cell Functions

Actin cytoskeletal dynamics drive cellular shape changes, linking numerous cell functions to physiological and pathological cues. Mutations in actin regulators that are differentially expressed or enriched in immune cells cause severe human diseases known as primary immunodeficiencies underscoring the importance of efficienct actin remodeling in immune cell homeostasis. Here we discuss recent findings on how immune cells sense the mechanical properties of their environement. Moreover, while the organization and biochemical regulation of cytoplasmic actin have been extensively studied, nuclear actin reorganization is a rapidly emerging field that has only begun to be explored in immune cells. Based on the critical and multifaceted contributions of cytoplasmic actin in immune cell functionality, nuclear actin regulation is anticipated to have a large impact on our understanding of immune cell development and functionality.

The role of physical cues in the development of stem cell-derived organoids

Organoids are a novel three-dimensional stem cells’ culture system that allows the in vitro recapitulation of organs/tissues structure complexity. Pluripotent and adult stem cells are included in a peculiar microenvironment consisting of a supporting structure (an extracellular matrix (ECM)-like component) and a cocktail of soluble bioactive molecules that, together, mimic the stem cell niche organization. It is noteworthy that the balance of all microenvironmental components is the most critical step for obtaining the successful development of an accurate organoid instead of an organoid with heterogeneous morphology, size, and cellular composition. Within this system, mechanical forces exerted on stem cells are collected by cellular proteins and transduced via mechanosensing—mechanotransduction mechanisms in biochemical signaling that dictate the stem cell specification process toward the formation of organoids. This review discusses the role of the environment in organoids formation and focuses on the effect of physical components on the developmental system. The work starts with a biological description of organoids and continues with the relevance of physical forces in the organoid environment formation. In this context, the methods used to generate organoids and some relevant published reports are discussed as examples showing the key role of mechanosensing–mechanotransduction mechanisms in stem cell-derived organoids.

High Glucose Causes Distinct Expression Patterns of Primary Human Skin Cells by RNA Sequencing

Diabetes-related skin problems represent the most common long-term complications in diabetes mellitus patients. These complications, which include diabetic dermopathy, diabetic blisters, necrobiosis lipoidica diabeticorum, and eruptive xanthomatosis, may dramatically impair patients' quality of life and cause long-lasting disability. However, the cellular and molecular mechanisms linking diabetes-related hyperglycemia and skin complications are still incompletely understood. To assess the role of the various skin-cell types in hyperglycemia-induced skin disorders, we performed RNA sequencing-based transcriptome analysis, measuring gene expression patterns in biological replicates in normal- and high glucose-stimulated skin cells. Three primary human skin-cell types were examined, i.e., epidermal keratinocytes, dermal fibroblasts, and dermal microvascular endothelial cells. For each separate cell type, we identified gene expression. Comparing gene abundances and expression levels revealed that transcription profiles exhibit distinct patterns in the three skin-cell types exposed to normal (i.e., physiological) glucose treatment and high (i.e., supraphysiological) glucose treatment. The obtained data indicate that high glucose induced differential gene expression and distinct activity patterns in signaling pathways in each skin-cell type. We are adding these data to the public database in the hope that they will facilitate future studies to develop novel targeted interventions for diabetic skin complications.

Molecular Regulators of Cellular Mechanoadaptation at Cell-Material Interfaces

Diverse essential cellular behaviors are determined by extracellular physical cues that are detected by highly orchestrated subcellular interactions with the extracellular microenvironment. To maintain the reciprocity of cellular responses and mechanical properties of the extracellular matrix, cells utilize a variety of signaling pathways that transduce biophysical stimuli to biochemical reactions. Recent advances in the micromanipulation of individual cells have shown that cellular responses to distinct physical and chemical features of the material are fundamental determinants of cellular mechanosensation and mechanotransduction. In the process of outside-in signal transduction, transmembrane protein integrins facilitate the formation of focal adhesion protein clusters that are connected to the cytoskeletal architecture and anchor the cell to the substrate. The linkers of nucleoskeleton and cytoskeleton molecular complexes, collectively termed LINC, are critical signal transducers that relay biophysical signals between the extranuclear cytoplasmic region and intranuclear nucleoplasmic region. Mechanical signals that involve cytoskeletal remodeling ultimately propagate into the nuclear envelope comprising the nuclear lamina in assistance with various nuclear membrane proteins, where nuclear mechanics play a key role in the subsequent alteration of gene expression and epigenetic modification. These intracellular mechanical signaling cues adjust cellular behaviors directly associated with mechanohomeostasis. Diverse strategies to modulate cell-material interfaces, including alteration of surface rigidity, confinement of cell adhesive region, and changes in surface topology, have been proposed to identify cellular signal transduction at the cellular and subcellular levels. In this review, we will discuss how a diversity of alterations in the physical properties of materials induce distinct cellular responses such as adhesion, migration, proliferation, differentiation, and chromosomal organization. Furthermore, the pathological relevance of misregulated cellular mechanosensation and mechanotransduction in the progression of devastating human diseases, including cardiovascular diseases, cancer, and aging, will be extensively reviewed. Understanding cellular responses to various extracellular forces is expected to provide new insights into how cellular mechanoadaptation is modulated by manipulating the mechanics of extracellular matrix and the application of these materials in clinical aspects.

The Sky's the LEMit: New insights into nuclear structure regulation of transcription factor activity

The nucleoskeleton has been associated with partitioning the genome into active and inactive compartments that dictate local transcription factor (TF) activity. However, recent data indicate that the nucleoskeleton and TFs reciprocally influence each other in dynamic TF trafficking pathways through the functions of LEM proteins. While the conserved peripheral recruitment of TFs by LEM proteins has been viewed as a mechanism of repressing transcription, a diversity of release mechanisms from the lamina suggest this compartment serves as a refuge for nuclear TF accumulation for rapid mobilization and signal stability. Detailed mechanisms suggest that TFs toggle between nuclear lamina refuge and nuclear matrix lamin-LEM protein complexes at sites of active transcription. In this review we will highlight emerging LEM functions acting at the interface of chromatin and nucleoskeleton to create TF trafficking networks.

Lamin A/C deficiency enables increased myosin-II bipolar filament ensembles that promote divergent actomyosin network anomalies through self-organization

Nuclear envelope proteins influence cell cytoarchitecure by poorly understood mechanisms. Here we show that small interfering RNA-mediated silencing of lamin A/C (LMNA) promotes contrasting stress fiber assembly and disassembly in individual cells and within cell populations. We show that LMNA-deficient cells have elevated myosin-II bipolar filament accumulations, irregular formation of actin comet tails and podosome-like adhesions, increased steady state nuclear localization of the mechanosensitive transcription factors MKL1 and YAP, and induced expression of some MKL1/serum response factor-regulated genes such as that encoding myosin-IIA (MYH9). Our studies utilizing live cell imaging and pharmacological inhibition of myosin-II support a mechanism of deregulated myosin-II self-organizing activity at the nexus of divergent actin cytoskeletal aberrations resulting from LMNA loss. In light of our results, we propose a model of how the nucleus, via linkage to the cytoplasmic actomyosin network, may act to control myosin-II contractile behavior through both mechanical and transcriptional feedback mechanisms.

Collagen microarchitecture mechanically controls myofibroblast differentiation

Altered microarchitecture of collagen type I is a hallmark of wound healing and cancer that is commonly attributed to myofibroblasts. However, it remains unknown which effect collagen microarchitecture has on myofibroblast differentiation. Here, we combined experimental and computational approaches to investigate the hypothesis that the microarchitecture of fibrillar collagen networks mechanically regulates myofibroblast differentiation of adipose stromal cells (ASCs) independent of bulk stiffness. Collagen gels with controlled fiber thickness and pore size were microfabricated by adjusting the gelation temperature while keeping their concentration constant. Rheological characterization and simulation data indicated that networks with thicker fibers and larger pores exhibited increased strain-stiffening relative to networks with thinner fibers and smaller pores. Accordingly, ASCs cultured in scaffolds with thicker fibers were more contractile, expressed myofibroblast markers, and deposited more extended fibronectin fibers. Consistent with elevated myofibroblast differentiation, ASCs in scaffolds with thicker fibers exhibited a more proangiogenic phenotype that promoted endothelial sprouting in a contractility-dependent manner. Our findings suggest that changes of collagen microarchitecture regulate myofibroblast differentiation and fibrosis independent of collagen quantity and bulk stiffness by locally modulating cellular mechanosignaling. These findings have implications for regenerative medicine and anticancer treatments.

Nuclear Lamins and Emerin Are Differentially Expressed in Osteosarcoma Cells and Scale with Tumor Aggressiveness

The nuclear lamina is essential for the maintenance of nuclear shape and mechanics. Mutations in lamin genes have been identified in a heterogeneous spectrum of human diseases known as “laminopathies” associated with nuclear envelope defects and deregulation of cellular functions. Interestingly, osteosarcoma is the only neoplasm described in the literature in association with laminopathies. This study aims characterized the expression of A-type and B-type lamins and emerin in osteosarcoma, revealing a higher percentage of dysmorphic nuclei in osteosarcoma cells in comparison to normal osteoblasts and all the hallmarks of laminopathic features. Both lamins and emerin were differentially expressed in osteosarcoma cell lines in comparison to normal osteoblasts and correlated with tumor aggressiveness. We analysed lamin A/C expression in a tissue-microarray including osteosarcoma samples with different prognosis, finding a positive correlation between lamin A/C expression and the overall survival of osteosarcoma patients. An inefficient MKL1 nuclear shuttling and actin depolymerization, as well as a reduced expression of pRb and a decreased YAP nuclear content were observed in A-type lamin deficient 143B cells. In conclusion, we described for the first time laminopathic nuclear phenotypes in osteosarcoma cells, providing evidence for an altered lamins and emerin expression and a deregulated nucleoskeleton architecture of this tumor.

Nucleoskeletal regulation of transcription: Actin on MRTF

Myocardin-related transcription factor A (MRTF-A) and serum response factor (SRF) form an essential transcriptional complex that regulates the expression of many cytoskeletal genes in response to dynamic changes in the actin cytoskeleton. The nucleoskeleton, a “dynamic network of networks,” consists of numerous proteins that contribute to nuclear shape and to its various functions, including gene expression. In this review, we will discuss recent work that has identified many nucleoskeletal proteins, such as nuclear lamina and lamina-associated proteins, nuclear actin, and the linker of the cytoskeleton and nucleoskeleton complex as important regulators of MRTF-A/SRF transcriptional activity, especially in the context of mechanical control of transcription.

Impact statement

Regulation of gene expression is a fundamental cellular process that ensures the appropriate response of a cell to its surroundings. Alongside biochemical signals, mechanical cues, such as substrate rigidity, have been recognized as key regulators of gene expression. Nucleoskeletal components play an important role in mechanoresponsive transcription, particularly in controlling the activity of MRTF-A/SRF transcription factors. This ensures that the cell can balance the internal and external mechanical forces by fine-tuning the expression of cytoskeletal genes.

Rare BANF1 Alleles and Relatively Frequent EMD Alleles Including 'Healthy Lipid' Emerin p.D149H in the ExAC Cohort

Emerin (EMD) and barrier to autointegration factor 1 (BANF1) each bind A-type lamins (LMNA) as fundamental components of nuclear lamina structure. Mutations in LMNA, EMD and BANF1 are genetically linked to many tissue-specific disorders including Emery-Dreifuss muscular dystrophy and cardiomyopathy (LMNA, EMD), lipodystrophy, insulin resistance and type 2 diabetes (LMNA) and progeria (LMNA, BANF1). To explore human genetic variation in these genes, we analyzed EMD and BANF1 alleles in the Exome Aggregation Consortium (ExAC) cohort of 60,706 unrelated individuals. We identified 13 rare heterozygous BANF1 missense variants (p.T2S, p.H7Y, p.D9N, p.S22R, p.G25E, p.D55N, p.D57Y, p.L63P, p.N70T, p.K72R, p.R75W, p.R75Q, p.G79R), and one homozygous variant (p.D9H). Several variants are known (p.G25E) or predicted (e.g., p.D9H, p.D9N, p.L63P) to perturb BANF1 and warrant further study. Analysis of EMD revealed two previously identified variants associated with adult-onset cardiomyopathy (p.K37del, p.E35K) and one deemed 'benign' in an Emery-Dreifuss patient (p.D149H). Interestingly p.D149H was the most frequent emerin variant in ExAC, identified in 58 individuals (overall allele frequency 0.06645%), of whom 55 were East Asian (allele frequency 0.8297%). Furthermore, p.D149H associated with four 'healthy' traits: reduced triglycerides (-0.336; p = 0.0368), reduced waist circumference (-0.321; p = 0.0486), reduced cholesterol (-0.572; p = 0.000346) and reduced LDL cholesterol (-0.599; p = 0.000272). These traits are distinct from LMNA-associated metabolic disorders and provide the first insight that emerin influences metabolism. We also identified one novel in-frame deletion (p.F39del) and 62 novel emerin missense variants, many of which were relatively frequent and potentially disruptive including p.N91S and p.S143F (∼0.041% and ∼0.034% of non-Finnish Europeans, respectively), p.G156S (∼0.39% of Africans), p.R204G (∼0.18% of Latinx), p.R207P (∼0.08% of South Asians) and p.R221L (∼0.15% of Latinx). Many novel BANF1 variants are predicted to disrupt dimerization or binding to DNA, histones, emerin or A-type lamins. Many novel emerin variants are predicted to disrupt emerin filament dynamics or binding to BANF1, HDAC3, A-type lamins or other partners. These new human variants provide a foundational resource for future studies to test the molecular mechanisms of BANF1 and emerin function, and to understand the link between emerin variant p.D149H and a 'healthy' lipid profile.

Emerin induces nuclear breakage in Xenopus extract and early embryos

Emerin is an inner nuclear membrane protein often mutated in Emery–Dreifuss muscular dystrophy. Because emerin has diverse roles in nuclear mechanics, cytoskeletal organization, and gene expression, it has been difficult to elucidate its contribution to nuclear structure and disease pathology. In this study, we investigated emerin’s impact on nuclei assembled in Xenopus laevis egg extract, a simplified biochemical system that lacks potentially confounding cellular factors and activities. Notably, these extracts are transcriptionally inert and lack endogenous emerin and filamentous actin. Strikingly, emerin caused rupture of egg extract nuclei, dependent on the application of shear force. In egg extract, emerin localized to nonnuclear cytoplasmic membranes, and nuclear rupture was rescued by targeting emerin to the nucleus, disrupting its membrane association, or assembling nuclei with lamin A. Furthermore, emerin induced breakage of nuclei in early-stage X. laevis embryo extracts, and embryos microinjected with emerin were inviable, with ruptured nuclei. We propose that cytoplasmic membrane localization of emerin leads to rupture of nuclei that are more sensitive to mechanical perturbation, findings that may be relevant to early development and certain laminopathies.

Differential incorporation of SUN-domain proteins into LINC complexes is coupled to gene expression

LInkers of Nucleoskeleton and Cytoskeleton (LINC) complexes, composed of SUN and KASH-domain proteins, span the nuclear envelope and physically connect the nuclear interior to cytoskeletal elements. Most human cells contain two SUN proteins, Sun1 and Sun2, and several KASH-proteins suggesting that multiple functionally distinct LINC complexes co-exist in the nuclear envelope. We show here, however, that while Sun1 and Sun2 in HeLa cells are each able to bind KASH-domains, Sun1 is more efficiently incorporated into LINC complexes under normal growth conditions. Furthermore, the balance of Sun1 and Sun2 incorporated into LINC complexes is cell type-specific and is correlated with SRF/Mkl1-dependent gene expression. In addition, we found that Sun1 has a LINC complex-independent role in transcriptional control, possibly by regulating the SRF/Mkl1 pathway. Together, these data reveal novel insights into the mechanisms of LINC complex regulation and demonstrate that Sun1 modulates gene expression independently of its incorporation into LINC complexes.

Balancing forces in migration

The integrated molecular interactions of proteins can create active biological networks whose material properties and actions can impact a variety of physiological processes. Chief among these is the ability to generate and respond to physical forces. The cytoskeleton plays a key role in this behavior, characterized by active self-reorganization to control a cell's shape and mediate its physical interactions. This review discusses our current understanding of how the material properties of the cytoskeleton and its physical interactions with the extracellular environment impact cell migration.