The interaction between nesprins and sun proteins at the nuclear envelope is critical for force transmission between the nucleus and cytoskeleton

Nuclear envelope components in vascular mechanotransduction: emerging roles in vascular health and disease

The vascular network, uniquely sensitive to mechanical changes, translates biophysical forces into biochemical signals for vessel function. This process relies on the cell's architectural integrity, enabling uniform responses to physical stimuli. Recently, the nuclear envelope (NE) has emerged as a key regulator of vascular cell function. Studies implicate nucleoskeletal elements (e.g. nuclear lamina) and the linker of nucleoskeleton and cytoskeleton (LINC) complex in force transmission, emphasizing nucleo-cytoskeletal communication in mechanotransduction. The nuclear pore complex (NPC) and its component proteins (i.e. nucleoporins) also play roles in cardiovascular disease (CVD) progression. We herein summarize evidence on the roles of nuclear lamina proteins, LINC complex members, and nucleoporins in endothelial and vascular cell mechanotransduction. Numerous studies attribute NE components in cytoskeletal-related cellular behaviors to insinuate dysregulation of nucleocytoskeletal feedback and nucleocytoplasmic transport as a mechanism of endothelial and vascular dysfunction, and hence implications for aging and vascular pathophysiology.

Microtubule-driven cell shape changes and actomyosin flow synergize to position the centrosome

The regulation of centrosome position is critical to the alignment of intracellular structures with extracellular cues. The exact nature and spatial distribution of the mechanical forces that balance at the centrosome are unknown. Here, we used laser-based nanoablations in adherent cells and found that forces along microtubules were damped by their anchoring to the actin network, rendering them ineffective in moving the microtubule aster. In contrast, the actomyosin contractile network was responsible for the generation of a centripetal flow that robustly drives the centrosome toward the geometrical center of the cell, even in the absence of microtubules. Unexpectedly, we discovered that the remodeling of cell shape around the centrosome was instrumental in aster centering. The radial array of microtubules and cytoplasmic dyneins appeared to direct this reorganization. This revised view of the respective roles of actin and microtubules in centrosome positioning offers a new perspective for understanding the establishment of cell polarity.

Attenuated Nuclear Tension Regulates Progerin-Induced Mechanosensitive Nuclear Wrinkling and Chromatin Remodeling

Hutchinson-Gilford progeria syndrome, caused by a mutation in the LMNA gene, leads to increased levels of truncated prelamin A, progerin, in the nuclear membrane. The accumulation of progerin results in defective nuclear morphology and is associated with altered expression of linker of the nucleoskeleton and cytoskeleton complex proteins, which are critical for nuclear signal transduction via molecular coupling between the extranuclear cytoskeleton and lamin-associated nuclear envelope. However, the molecular mechanisms underlying progerin accumulation-induced nuclear deformation and its effects on intranuclear chromosomal organization remain unclear. Here, the spatiotemporal evolution of nuclear wrinkles is analyzed in response to variations in substrate stiffness using a doxycycline-inducible progerin expression system. It is found that cytoskeletal tension regulates the onset of progerin-induced nuclear envelope wrinkling and that the molecular interaction between SUN1 and LMNA controls the actomyosin-dependent attenuation of nuclear tension. Genome-wide analysis of chromatin accessibility and gene expression further suggests that an imbalance in force between the intra- and extranuclear spaces induces nuclear deformation, which specifically regulates progeria-associated gene expression via modification of mechanosensitive signaling pathways. The findings highlight the crucial role of nuclear lamin-cytoskeletal connectivity in bridging nuclear mechanotransduction and the biological aging process.

Nuclear envelope proteins, mechanotransduction, and their contribution to breast cancer progression

Breast cancer cells frequently exhibit changes in the expression of nuclear envelope (NE) proteins such as lamins and emerin that determine the physical properties of the nucleus and contribute to cellular mechanotransduction. This review explores the emerging interplay between NE proteins, the physical challenges incurred during metastatic progression, and mechanotransduction. Improved insights into the underlying mechanisms may ultimately lead to better prognostic tools and treatment strategies for metastatic breast cancer.

Nuclear pores safeguard the integrity of the nuclear envelope

Nuclear pore complexes (NPCs) mediate nucleocytoplasmic exchange, which is essential for eukaryotes. Mutations in the central scaffolding components of NPCs are associated with genetic diseases, but how they manifest only in specific tissues remains unclear. This is exemplified in Nup133-deficient mouse embryonic stem cells, which grow normally during pluripotency, but differentiate poorly into neurons. Here, using an innovative in situ structural biology approach, we show that Nup133-/- mouse embryonic stem cells have heterogeneous NPCs with non-canonical symmetries and missing subunits. During neuronal differentiation, Nup133-deficient NPCs frequently disintegrate, resulting in abnormally large nuclear envelope openings. We propose that the elasticity of the NPC scaffold has a protective function for the nuclear envelope and that its perturbation becomes critical under conditions that impose an increased mechanical load onto nuclei.

The establishment of nuclear organization in mouse embryos is orchestrated by multiple epigenetic pathways

The folding of the genome in the 3D nuclear space is fundamental for regulating all DNA-related processes. The association of the genome with the nuclear lamina into lamina-associated domains (LADs) represents the earliest feature of nuclear organization during development. Here, we performed a gain-of-function screen in mouse embryos to obtain mechanistic insights. We find that perturbations impacting histone H3 modifications, heterochromatin, and histone content are crucial for the establishment of nuclear architecture in zygotes and/or 2-cell-stage embryos. Notably, some perturbations exerted differential effects on zygotes versus 2-cell-stage embryos. Moreover, embryos with disrupted LADs can rebuild nuclear architecture at the 2-cell stage, indicating that the initial establishment of LADs in zygotes might be dispensable for early development. Our findings provide valuable insights into the functional interplay between chromatin and structural components of the nucleus that guide genome-lamina interactions during the earliest developmental stages.

Small Accelerations of the cell generate sufficient nuclear motion to modulate transcriptional activity, driving cellular response independent of matrix strain

The cell's mechanical environment is a fundamental determinant of its activity. Ostensibly, the cellular response is dependent on interactions between extracellular matrix deformations and the cell adhesome. Low-intensity vibration (LIV) induces sinusoidal mechanical accelerations that stimulate mesenchymal stem cell (MSC) anabolism despite generating minimal matrix strain. In this study, we tested the hypothesis that accelerations of less than 1g cause nuclear motions relative to the cell membrane in adherent cells, resulting in elevated stresses in the cytoskeleton that promote transcriptional activity. Coupling a piezoelectric vibration platform with real-time microscopy, we applied a 0.7g, 90Hz LIV signal that oscillates the cell with displacements of up to ±11 µm. Live-cell tracking revealed that the sinusoidal vibrations caused the nucleus to move ±1.27 µm (17% of total displacement) out of phase with the cell membrane. Disruption of the LINC complex, which mechanically couples the nucleoskeleton to the cytoskeleton, doubled the magnitude of this relative motion, indicating that the nucleo-cytoskeletal configuration plays a major role in regulating nuclear motion. Consistent with a previously reported increase in nuclear stiffness caused by LIV, machine-learning-based image segmentation of confocal micrographs showed that LIV increased both apical and basal F-actin fiber numbers, generating a denser, more branched actin network near the nucleus. Following six 20 min bouts of LIV applied to MSC, RNA sequencing identified 372 differentially expressed genes. Upregulated gene sets were linked to F-actin assembly and focal adhesion pathways. Finite element simulations showed that nuclear stresses increased by LIV up to 18% were associated with nuclei flattening and a 30-50% increase in actin-generated forces. These findings demonstrate that low-intensity accelerations, independent of matrix strain, can directly activate a response of the nucleus, leading to cytoskeletal reorganization and heightened nuclear stresses. Thus, even very small oscillatory mechanical signals can markedly influence cell outcomes, establishing a mechanosensing pathway independent of extracellular strains.

SUN-domain proteins of the malaria parasite Plasmodium falciparum are essential for proper nuclear division and DNA repair

The protozoan parasite Plasmodium falciparum, which is responsible for the deadliest form of human malaria, accounts for over half a million deaths a year. These parasites proliferate in human red blood cells by consecutive rounds of closed mitoses called schizogony. Their virulence is attributed to their ability to modify the infected red cells to adhere to the vascular endothelium and to evade immunity through antigenic switches. Spatial dynamics at the nuclear periphery were associated with the regulation of processes that enable the parasites to establish long-term infection. However, our knowledge of components of the nuclear envelope (NE) in Plasmodium remains limited. One of the major protein complexes at the NE is the linker of nucleoskeleton and cytoskeleton (LINC) complex that forms a connecting bridge between the cytoplasm and the nucleus through the interaction of SUN and KASH domain proteins. Here, we have identified two SUN-domain proteins as possible components of the LINC complex of P. falciparum and show that their proper expression is essential for the parasite's proliferation in human red blood cells, and their depletion leads to the formation of membranous whorls and morphological changes of the NE. In addition, their differential expression highlights different functions at the nuclear periphery as PfSUN2 is specifically associated with heterochromatin, while PfSUN1 expression is essential for activation of the DNA damage response. Our data provide indications for the involvement of the LINC complex in crucial biological processes in the intraerythrocytic development cycle of malaria parasites.

IMPORTANCE

Plasmodium falciparum, the parasite causing the deadliest form of malaria, is able to thrive in its human host by tight regulation of cellular processes, orchestrating nuclear dynamics with cytoplasmic machineries that are separated by the nuclear envelope. One of the major protein complexes that connect nuclear and cytoplasmic processes in eukaryotes is the linker of nucleoskeleton and cytoskeleton (LINC) complex. However, while the nuclear periphery of P. falciparum was implicated in several important functions, the role of the LINC complex in Plasmodium biology is unknown. Here, we identify two components of P. falciparum LINC complex and demonstrate that they are essential for the parasites' proliferation in human blood, and their depletion leads to the formation of morphological changes in the cell. In addition, the two components have different functions in activating the DNA damage response and in their association with heterochromatin. Our data provide evidence for their essential roles in the parasites' cell cycle.

The actin protrusion deforms the nucleus during invasion through basement membrane

Cell invasion through basement membrane (BM) extracellular matrix barriers is important during organ development, immune cell trafficking, and cancer metastasis. Here we study an invasion event, anchor cell (AC) invasion, which occurs during Caenorhabditis elegans development. The actin protrusion of the invading AC mechanically displaces the BM, but it is not known how forces are balanced to prevent the growing actin protrusion from pushing itself backward when confronted with a load. Here we observe that the distal end of the actin protrusion in the invading AC abuts the nucleus and deforms it. Further we show that there is a correlation between invasion efficiency and nuclear deformation: under mutant conditions where invasion is reduced, nuclear deformation is diminished. However, nuclear deformation and invasion are unaffected by interfering with the molecular connections between the actin and microtubule cytoskeletons and the nuclear envelope. Together these data suggest that the AC actin protrusion braces against the nucleus to apply forces during invasion, but that nucleus-cytoskeleton molecular connections are not necessary for this to occur.

SUMMARY STATEMENT

Actin-based membrane protrusions in invading cells apply force to basement membrane (BM) barriers to help break through them. In cell motility in 2D, the actin protrusion uses cell-substrate adhesions for leverage to push forward against obstacles in what is known as the molecular clutch. The situation is different in 3D invasion, where the adhesive substrate is being effaced by the invading cell. It is not clear, in this case, why the growing actin protrusion doesn't push itself backwards instead of extending forwards through the BM. The data presented here provide evidence that the distal end of the invasive actin protrusion is braced against the stiff, immobile nucleus, allowing growth of the proximal end to apply force on the BM.

Depletion of SUN1/2 induces heterochromatin accrual in mesenchymal stem cells during adipogenesis

Critical to the mechano-regulation of mesenchymal stem cells (MSC), Linker of the Nucleoskeleton and Cytoskeleton (LINC) complex transduces cytoskeletal forces to the nuclei. The LINC complex contains outer nuclear membrane Nesprin proteins that associate with the cytoskeleton and their inner nuclear membrane couplers, SUN proteins. Here we tested the hypothesis that severing of the LINC complex-mediated cytoskeletal connections may have different effects on chromatin organization and MSC differentiation than those due to ablation of SUN proteins. In cells cultured under adipogenic conditions, interrupting LINC complex function through dominant-negative KASH domain expression (dnKASH) increased adipogesis while heterochromatin H3K27 and H3K9 methylation was unaltered. In contrast, SUN1/2 depletion inhibited adipogenic gene expression and fat droplet formation; as well the anti-adipogenic effect of SUN1/2 depletion was accompanied by increased H3K9me3, which was enriched on Adipoq, silencing this fat locus. We conclude that releasing the nucleus from cytoskeletal constraints via dnKASH accelerates adipogenesis while depletion of SUN1/2 increases heterochromatin accrual on adipogenic genes in a fashion independent of LINC complex function. Therefore, while these two approaches both disable LINC complex functions, their divergent effects on the epigenetic landscape indicate they cannot be used interchangeably to study mechanical regulation of cell differentiation.

Endothelial cytoskeleton in mechanotransduction and vascular diseases

The cytoskeleton is an important structural component that regulates various aspects of cell morphology, movement, and intracellular signaling. It plays a pivotal role in the cellular response to biomechanical stimuli, particularly in endothelial cells, which are critical for vascular homeostasis and the pathogenesis of cardiovascular diseases. Mechanical forces, such as shear and tension, activate intracellular signaling cascades that regulate transcription, translation, and cellular behaviors. Despite extensive research into cytoskeletal functions, the precise mechanisms by which the cytoskeleton transduces mechanical signals remain incompletely understood. This review focuses on the role of cytoskeletal components in membrane, cytoplasm, and nucleus in mechanotransduction, with an emphasis on their structure, mechanical and biological behaviors, dynamic interactions, and response to mechanical forces. The collaboration between membrane cytoskeleton, cytoplasmic cytoskeleton, and nucleoskeleton is indispensable for endothelial cells to respond to mechanical stimuli. Understanding their mechanoresponsive mechanisms is essential for advancing therapeutic strategies for cardiovascular diseases.

Focus on mechano-immunology: new direction in cancer treatment

The immune response is modulated by a diverse array of signals within the tissue microenvironment, encompassing biochemical factors, mechanical forces, and pressures from adjacent tissues. Furthermore, the extracellular matrix and its constituents significantly influence the function of immune cells. In the case of carcinogenesis, changes in the biophysical properties of tissues can impact the mechanical signals received by immune cells, and these signals c1an be translated into biochemical signals through mechano-transduction pathways. These mechano-transduction pathways have a profound impact on cellular functions, influencing processes such as cell activation, metabolism, proliferation, and migration, etc. Tissue mechanics may undergo temporal changes during the process of carcinogenesis, offering the potential for novel dynamic levels of immune regulation. Here, we review advances in mechanoimmunology in malignancy studies, focusing on how mechanosignals modulate the behaviors of immune cells at the tissue level, thereby triggering an immune response that ultimately influences the development and progression of malignant tumors. Additionally, we have also focused on the development of mechano-immunoengineering systems, with the help of which could help to further understand the response of tumor cells or immune cells to alterations in the microenvironment and may provide new research directions for overcoming immunotherapeutic resistance of malignant tumors.

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.

Structural response of microtubule and actin cytoskeletons to direct intracellular load

Microtubule and actin are the two major cytoskeletal polymers that form organized functional structures in the interior of eukaryotic cells. Although the structural mechanics of the cytoskeleton has been extensively studied by direct manipulations in in vitro reconstitution systems, such unambiguous characterizations inside the living cell are sparse. Here, we report a comprehensive analysis of how the microtubule and actin cytoskeletons structurally respond to direct intracellular load. Ferrofluid-based intracellular magnetic tweezers reveal rheological properties of the microtubule complex primarily determined by filamentous actin. The strain fields of the microtubule complex and actin meshwork follow the same scaling, suggesting that the two cytoskeletal systems behave as an integrated elastic body. The structural responses of single microtubules to contact and remote forces further evidence that the individual microtubules are enclosed by the elastic medium of actin. These results, directly characterizing the microtubule and actin cytoskeletons as an interacting continuum throughout the cytoplasm, serve as a cornerstone for the physical understanding of intracellular organization.

The LINC complex in blood vessels: from physiology to pathological implications in arterioles

The LINC (linker of nucleoskeleton and cytoskeleton) complex is a critical component of the cellular architecture that bridges the nucleoskeleton and cytoskeleton and mediates mechanotransduction to and from the nucleus. Though it plays important roles in all blood vessels, it is in arterioles that this complex plays a pivotal role in maintaining endothelial cell integrity, regulating vascular tone, forming new microvessels and modulating responses to mechanical and biochemical stimuli. It is also important in vascular smooth muscle cells and fibroblasts, where it possibly plays a role in the contractile to secretory phenotypic transformation during atherosclerosis and vascular ageing, and in fibroblasts' migration and inflammatory responses in the adventitia. Physiologically, the LINC complex contributes to the stability of arteriolar structure, adaptations to changes in blood flow and injury repair mechanisms. Pathologically, dysregulation or mutations in LINC complex components can lead to compromised endothelial function, vascular remodelling and exacerbation of cardiovascular diseases such as atherosclerosis (arteriolosclerosis). This review summarizes our current understanding of the roles of the LINC complex in cells from arterioles, highlighting its most important physiological functions, exploring its implications for vascular pathology and emphasizing some of its functional characteristics in endothelial cells. By elucidating the LINC complex's role in health and disease, we aim to provide insights that could improve future therapeutic strategies targeting LINC complex-related vascular disorders.

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.

Physical principles and mechanisms of cell migration

Cell migration is critical in processes such as developmental biology, wound healing, immune response, and cancer invasion/metastasis. Understanding its regulation is essential for developing targeted therapies in regenerative medicine, cancer treatment and immune modulation. This review examines cell migration mechanisms, highlighting fundamental physical principles, key molecular components, and cellular behaviors, identifying existing gaps in current knowledge, and suggesting potential directions for future research.

Cellular and Nuclear Forces: An Overview

Biological cells sample their surrounding microenvironments using nanoscale force sensors on the cell surfaces. These surface-based force and stress sensors generate physical and chemical responses inside the cell. The inherently well-connected cytoskeleton and its physical contacts with the force elements on the nuclear membrane lead these physicochemical responses to cascade all the way inside the cell nucleus, physically altering the nuclear state. These physical alterations of the cell nucleus, through yet-unknown complex steps, elicit physical and functional responses from the chromatin in the form of altered gene expression profiles. This mechanism of force/stress sensing by the cell and then its nuclear response has been shown to play a vital role in maintaining robust cellular homeostasis, controlling gene expression profiles during developmental phases as well as cell differentiation. In the last few years, there has been appreciable progress toward the identification of the molecular players responsible for force sensing. However, the actual sensing mechanism of cell surface-bound force sensors and more importantly cascading of the signals, both physical (via cytosolic force sensing elements such as microtubule and actin framework) as well as chemical (cascade of biochemical signaling from cell surface to nuclear surface and further to the chromatin), inside the cell is poorly understood. In this chapter, we present a review of the currently known molecular players in cellular as well as nuclear force sensing repertoire and their possible mechanistic aspects. We also introduce various biophysical concepts and review some frequently used techniques that are used to describe the force/stress sensing and response of a cell. We hope that this will help in asking clearer questions and designing pointed experiments for better understanding of the force-dependent design principles of the cell surface, nuclear surface, and gene expression.

Amphipathic helices sense the inner nuclear membrane environment through lipid packing defects

Amphipathic helices (AHs) are ubiquitous protein motifs that modulate targeting to organellar membranes by sensing differences in bulk membrane properties. However, the adaptation between membrane-targeting AHs and the nuclear membrane environment that surrounds the genome is poorly understood. Here, we computationally screened for candidate AHs in a curated list of characterized and putative human inner nuclear membrane (INM) proteins. Cell biological and in vitro experimental assays combined with computational calculations demonstrated that AHs detect lipid packing defects over electrostatics to bind to the INM, indicating that the INM is loosely packed under basal conditions. Membrane tension resulting from hypotonic shock further promoted AH binding to the INM, whereas cell-substrate stretch did not enhance recruitment of membrane tension-sensitive AHs. Together, our work demonstrates the rules driving lipid-protein interactions at the INM, and its implications in the response of the nucleus to different stimuli.

Hypo-osmolarity promotes naive pluripotency by reshaping cytoskeleton and increasing chromatin accessibility

INTRODUCTION

Cell fate determination and transition are of paramount importance in biology and medicine. Naive pluripotency could be achieved by reprogramming differentiated cells. However, the mechanism is less clear. Osmolarity is an essential physical factor that acts on living cells, especially for pluripotent cells, but its significance in cell fate transition remains unexplored.

OBJECTIVES

To investigate the role of osmolarity in cell fate transition and its underlying mechanism.

METHODS

Flow cytometry, quantitative real-time PCR, teratoma and chimeric mice assays were performed to assess reprogramming efficiency and characterize iPSCs. TEM, immunofluorescence staining, western blot, chemical treatment and genetic modification were utilized to evaluate cell morphology, signaling pathways, cytoskeleton and nuclear structure. Multiomic sequencings were applied to unveil the transcriptome, histone markers and chromatin accessibility of EpiSCs in hypo-osmotic condition.

RESULTS

In hypo-osmotic condition, the reprogramming efficiency of hypo-osmotic EpiSCs increased over 60-fold than that of iso-osmotic cells (1100 vs 18 colonies per 3 × 105 cells), whereas no colony formed in hyper-osmotic cells. As expected, the converted cells displayed naive pluripotency. The hypo-osmotic EpiSCs exhibited larger cell size, nuclear area and less heterochromatin; ATAC-seq and ChIP-seq confirmed the increased accessibility of naive pluripotent gene loci with more H3K27ac. Mechanistically, hypo-osmolarity activated PI3K-AKT-SP1 signaling in EpiSCs, which reshaped cytoskeleton and nucleoskeleton, resulting in genome reorganization and pluripotent gene expression. In contrast, hypo-osmolarity delayed the ESCs' exit from naive pluripotency. Moreover, in MEFs reprograming, hypo-osmolarity promoted the conversion to naive pluripotency.

CONCLUSION

Hypo-osmolarity promotes cell fate transition by remodeling cytoskeleton, nucleoskeleton and genome via PI3K-AKT-SP1 pathway.

Dynamic strain and β-catenin mediated suppression of interferon responsive genes in quiescent mesenchymal stromal/stem cells

Multipotent bone marrow mesenchymal stromal/stem cells (MSCs) respond to mechanical forces. MSCs perceive static and dynamic forces through focal adhesions, as well as cytoskeletal and intranuclear actin. Dynamic strain stimulates nuclear β-catenin (Ctnnb1) that controls gene expression and suppresses osteogenesis. The sensitivity of MSCs to external mechanical forces may be altered by cessation of proliferation, when MSCs begin to express extracellular matrix (ECM) proteins and generate cell/cell contact. Therefore, we assessed whether and how gene expression of proliferating versus quiescent MSCs responds to mechanical stimuli. We used RNA-seq and RT-qPCR to evaluate transcriptomes at 3 h after dynamic strain (200 cycles × 2 % for 20 min) once daily during a two-day time course in naïve (uninduced) MSCs. Transcriptomes of untreated MSCs show that cells become quiescent at day 2 when proliferation markers are downregulated, and ECM related genes are upregulated. On both day 1 and day 2, dynamic strain stimulates expression of oxidative stress related genes (e.g., Nqo1, Prl2c2, Prl2c3). Strikingly, in quiescent MSCs, we observe that dynamic strain suppresses multiple interferon (IFN) responsive genes (e.g., Irf7, Oasl2 and Isg15). IFN responsive genes are activated in MSCs depleted of β-catenin using siRNAs, indicating that β-catenin normally suppresses these genes. Our data indicate that the functional effects of dynamic strain and β-catenin on IFN responsive genes in MSCs are mechanistically coupled. Because dynamic strain and β-catenin reduce the osteogenic potential of MSCs, our findings suggest that IFN responsive genes are novel biomarkers and potential regulators of mechanical responses at early stages of lineage-commitment in post-proliferative MSCs.

Sensing the squeeze: nuclear mechanotransduction in health and disease

The nucleus not only is a repository for DNA but also a center of cellular and nuclear mechanotransduction. From nuclear deformation to the interplay between mechanosensing components and genetic control, the nucleus is poised at the nexus of mechanical forces and cellular function. Understanding the stresses acting on the nucleus, its mechanical properties, and their effects on gene expression is therefore crucial to appreciate its mechanosensitive function. In this review, we examine many elements of nuclear mechanotransduction, and discuss the repercussions on the health of cells and states of illness. By describing the processes that underlie nuclear mechanosensation and analyzing its effects on gene regulation, the review endeavors to open new avenues for studying nuclear mechanics in physiology and diseases.

Mechanobiology of the nucleus during the G2-M transition

Cellular behavior is continuously influenced by mechanical forces. These forces span the cytoskeleton and reach the nucleus, where they trigger mechanotransduction pathways that regulate downstream biochemical events. Therefore, the nucleus has emerged as a regulator of cellular response to mechanical stimuli. Cell cycle progression is regulated by cyclin-CDK complexes. Recent studies demonstrated these biochemical pathways are influenced by mechanical signals, highlighting the interdependence of cellular mechanics and cell cycle regulation. In particular, the transition from G2 to mitosis (G2-M) shows significant changes in nuclear structure and organization, ranging from nuclear pore complex (NPC) and nuclear lamina disassembly to chromosome condensation. The remodeling of these mechanically active nuclear components indicates that mitotic entry is particularly sensitive to forces. Here, we address how mechanical forces crosstalk with the nucleus to determine the timing and efficiency of the G2-M transition. Finally, we discuss how the deregulation of nuclear mechanics has consequences for mitosis.

SUN2 mediates calcium-triggered nuclear actin polymerization to cluster active RNA polymerase II

The nucleoskeleton is essential for nuclear architecture as well as genome integrity and gene expression. In addition to lamins, titin or spectrins, dynamic actin filament polymerization has emerged as a potential intranuclear structural element but its functions are less well explored. Here we found that calcium elevations trigger rapid nuclear actin assembly requiring the nuclear membrane protein SUN2 independently of its function as a component of the LINC complex. Instead, SUN2 colocalized and associated with the formin and actin nucleator INF2 in the nuclear envelope in a calcium-regulated manner. Moreover, SUN2 is required for active RNA polymerase II (RNA Pol II) clustering in response to calcium elevations. Thus, our data uncover a SUN2-formin module linking the nuclear envelope to intranuclear actin assembly to promote signal-dependent spatial reorganization of active RNA Pol II.

Microtubule forces drive nuclear damage in LMNA cardiomyopathy

Nuclear homeostasis requires a balance of forces between the cytoskeleton and nucleus. Mutations in the LMNA gene, which encodes the nuclear envelope proteins lamin A/C, disrupt this balance by weakening the nuclear lamina. This results in nuclear damage in contractile tissues and ultimately muscle disease. Intriguingly, disrupting the LINC complex that connects the cytoskeleton to the nucleus has emerged as a promising strategy to ameliorate LMNA-associated cardiomyopathy. Yet how LINC complex disruption protects the cardiomyocyte nucleus remains unclear. To address this, we developed an assay to quantify the coupling of cardiomyocyte contraction to nuclear deformation and interrogated its dependence on the nuclear lamina and LINC complex. We found that, surprisingly, the LINC complex was mostly dispensable for transferring contractile strain to the nucleus, and that increased nuclear strain in lamin A/C-deficient cardiomyocytes was not rescued by LINC complex disruption. Instead, LINC complex disruption eliminated the cage of microtubules encircling the nucleus. Disrupting microtubules was sufficient to prevent nuclear damage and rescue cardiac function induced by lamin A/C deficiency. We computationally simulated the stress fields surrounding cardiomyocyte nuclei and show how microtubule forces generate local vulnerabilities that damage lamin A/C-deficient nuclei. Our work pinpoints localized, microtubule-dependent force transmission through the LINC complex as a pathological driver and therapeutic target for LMNA-cardiomyopathy.

Common SYNE2 Genetic Variant Associated With Atrial Fibrillation Lowers Expression of Nesprin-2α1 With Downstream Effects on Nuclear and Electrophysiological Traits

BACKGROUND

Atrial fibrillation GWAS (genome-wide association studies) identified significant associations for rs1152591 and linked variants in the SYNE2 gene encoding Nesprin-2, which connects the nuclear membrane with the cytoskeleton.

METHODS

Reporter gene vector transfection and CRISPR-Cas9 editing were used to identify the causal variant regulating the expression of SYNE2α1. After SYNE2 knockdown or SYNE2α1 overexpression in human stem cell-derived cardiomyocytes, nuclear phenotypes were assessed by imaging and atomic force microscopy. Gene expression was assessed by RNAseq and gene set enrichment analysis. Fura-2 AM staining assessed calcium transients. Optical mapping assessed action potential duration and conduction velocity.

RESULTS

The risk allele of rs1152591 had lower promoter and enhancer activity and was significantly associated with lower expression of the short SYNE2α1 isoform in human stem cell-derived cardiomyocytes, without an effect on the expression of the full-length SYNE2 mRNA. SYNE2α1 overexpression had dominant negative effects on the nucleus with its overexpression or SYNE2 knockdown leading to increased nuclear area and decreased nuclear stiffness. Gene expression results from SYNE2α1 overexpression demonstrated both concordant and nonconcordant effects with SYNE2 knockdown. SYNE2α1 overexpression had a gain of function on electrophysiology, leading to significantly faster calcium reuptake and decreased assessed action potential duration, while SYNE2 knockdown showed both shortened assessed action potential duration and decreased conduction velocity.

CONCLUSIONS

rs1152591 was identified as a causal atrial fibrillation variant, with the risk allele decreasing SYNE2α1 expression. Downstream effects of SYNE2α1 overexpression include changes in nuclear stiffness and electrophysiology, which may contribute to the mechanism for the risk allele's association with AF.

Confinement controls the directional cell responses to fluid forces

Our understanding of how fluid forces influence cell migration in confining environments remains limited. By integrating microfluidics with live-cell imaging, we demonstrate that cells in tightly-but not moderately-confined spaces reverse direction and move upstream upon exposure to fluid forces. This fluid force-induced directional change occurs less frequently when cells display diminished mechanosensitivity, experience elevated hydraulic resistance, or sense a chemical gradient. Cell reversal requires actin polymerization to the new cell front, as shown mathematically and experimentally. Actin polymerization is necessary for the fluid force-induced activation of NHE1, which cooperates with calcium to induce upstream migration. Calcium levels increase downstream, mirroring the subcellular distribution of myosin IIA, whose activation enhances upstream migration. Reduced lamin A/C levels promote downstream migration of metastatic tumor cells by preventing cell polarity establishment and intracellular calcium rise. This mechanism could allow cancer cells to evade high-pressure environments, such as the primary tumor.

The cytoskeleton dynamics-dependent LINC complex in periodontal ligament stem cells transmits mechanical stress to the nuclear envelope and promotes YAP nuclear translocation

BACKGROUND

Periodontal ligament stem cells (PDLSCs) are important seed cells in tissue engineering and clinical applications. They are the priority receptor cells for sensing various mechanical stresses. Yes-associated protein (YAP) is a recognized mechanically sensitive transcription factor. However, the role of YAP in regulating the fate of PDLSCs under tension stress (TS) and its underlying mechanism is still unclear.

METHODS

The effects of TS on the morphology and fate of PDLSCs were investigated using fluorescence staining, transmission electron microscopy, flow cytometry and quantitative real-time polymerase chain reaction (qRT-PCR). Then qRT-PCR, western blotting, immunofluorescence staining and gene knockdown experiments were performed to investigate the expression and distribution of YAP and its correlation with PDLSCs proliferation. The effects of cytoskeleton dynamics on YAP nuclear translocation were subsequently explored by adding cytoskeleton inhibitors. The effect of cytoskeleton dynamics on the expression of the LINC complex was proved through qRT-PCR and western blotting. After destroying the LINC complex by adenovirus, the effects of the LINC complex on YAP nuclear translocation and PDLSCs proliferation were investigated. Mitochondria-related detections were then performed to explore the role of mitochondria in YAP nuclear translocation. Finally, the in vitro results were verified by constructing orthodontic tooth movement models in Sprague-Dawley rats.

RESULTS

TS enhanced the polymerization and stretching of F-actin, which upregulated the expression of the LINC complex. This further strengthened the pull on the nuclear envelope, enlarged the nuclear pore, and facilitated YAP's nuclear entry, thus enhancing the expression of proliferation-related genes. In this process, mitochondria were transported to the periphery of the nucleus along the reconstructed microtubules. They generated ATP to aid YAP's nuclear translocation and drove F-actin polymerization to a certain degree. When the LINC complex was destroyed, the nuclear translocation of YAP was inhibited, which limited PDLSCs proliferation, impeded periodontal tissue remodeling, and hindered tooth movement.

CONCLUSIONS

Our study confirmed that appropriate TS could promote PDLSCs proliferation and periodontal tissue remodeling through the mechanically driven F-actin/LINC complex/YAP axis, which could provide theoretical guidance for seed cell expansion and for promoting healthy and effective tooth movement in clinical practice.

Emerin deficiency drives MCF7 cells to an invasive phenotype

During metastasis, cancer cells traverse the vasculature by squeezing through very small gaps in the endothelium. Thus, nuclei in metastatic cancer cells must become more malleable to move through these gaps. Our lab showed invasive breast cancer cells have 50% less emerin protein resulting in smaller, misshapen nuclei, and higher metastasis rates than non-cancerous controls. Thus, emerin deficiency was predicted to cause increased nuclear compliance, cell migration, and metastasis. We tested this hypothesis by downregulating emerin in noninvasive MCF7 cells and found emerin knockdown causes smaller, dysmorphic nuclei, resulting in increased impeded cell migration. Emerin reduction in invasive breast cancer cells showed similar results. Supporting the clinical relevance of emerin reduction in cancer progression, our analysis of 192 breast cancer patient samples showed emerin expression inversely correlates with cancer invasiveness. We conclude emerin loss is an important driver of invasive transformation and has utility as a biomarker for tumor progression.

Emerin deficiency drives MCF7 cells to an invasive phenotype

During metastasis, cancer cells traverse the vasculature by squeezing through very small gaps in the endothelium. Thus, nuclei in metastatic cancer cells must become more malleable to move through these gaps. Our lab showed invasive breast cancer cells have 50% less emerin protein resulting in smaller, misshapen nuclei, and higher metastasis rates than non-cancerous controls. Thus, emerin deficiency was predicted to cause increased nuclear compliance, cell migration, and metastasis. We tested this hypothesis by downregulating emerin in noninvasive MCF7 cells and found emerin knockdown causes smaller, dysmorphic nuclei, resulting in increased impeded cell migration. Emerin reduction in invasive breast cancer cells showed similar results. Supporting the clinical relevance of emerin reduction in cancer progression, our analysis of 192 breast cancer patient samples showed emerin expression inversely correlates with cancer invasiveness. We conclude emerin loss is an important driver of invasive transformation and has utility as a biomarker for tumor progression.

Preserving Genome Integrity: Unveiling the Roles of ESCRT Machinery

The endosomal sorting complex required for transport (ESCRT) machinery is composed of an articulated architecture of proteins that assemble at multiple cellular sites. The ESCRT machinery is involved in pathways that are pivotal for the physiology of the cell, including vesicle transport, cell division, and membrane repair. The subunits of the ESCRT I complex are mainly responsible for anchoring the machinery to the action site. The ESCRT II subunits function to bridge and recruit the ESCRT III subunits. The latter are responsible for finalizing operations that, independently of the action site, involve the repair and fusion of membrane edges. In this review, we report on the data related to the activity of the ESCRT machinery at two sites: the nuclear membrane and the midbody and the bridge linking cells in the final stages of cytokinesis. In these contexts, the machinery plays a significant role for the protection of genome integrity by contributing to the control of the abscission checkpoint and to nuclear envelope reorganization and correlated resilience. Consistently, several studies show how the dysfunction of the ESCRT machinery causes genome damage and is a codriver of pathologies, such as laminopathies and cancer.

Mechano-metabolism of metastatic breast cancer cells in 2D and 3D microenvironments

Cells regulate their shape and metabolic activity in response to the mechano-chemical properties of their microenvironment. To elucidate the impact of matrix stiffness and ligand density on the bioenergetics of mesenchymal cells, we developed a nonequilibrium, active chemo-mechanical model that accounts for the mechanical energy of the cell and matrix, chemical energy from ATP hydrolysis, interfacial energy, and mechano-sensitive regulation of stress fiber assembly through signaling. By integrating the kinetics and energetics of these processes, we define the cell "metabolic potential" that, when minimized, provides testable predictions of cell contractility, shape, and ATP consumption. Specifically, we show that the morphology of MDA-MB-231 breast cancer cells in 3D collagen changes from spherical to elongated to spherical with increasing matrix stiffness, which is consistent with experimental observations. On 2D hydrogels, our model predicts a hemispherical-to-spindle-to-disc shape transition with increasing gel stiffness. In both cases, we show that these shape transitions emerge from competition between the energy of ATP hydrolysis associated with increased contractility that drives cell elongation and the interfacial energy that favors a rounded shape. Furthermore, our model can predict how increased energy demand in stiffer microenvironments is met by AMPK activation, which is confirmed experimentally in both 2D and 3D microenvironments and found to correlate with the upregulation of mitochondrial potential, glucose uptake, and ATP levels, as well as provide estimates of changes in intracellular adenosine nucleotide concentrations with changing environmental stiffness. Overall, we present a framework for relating adherent cell energy levels and contractility through biochemical regulation of underlying physical processes.

Statement of Significance

Increasing evidence indicates that cellular metabolism is regulated by mechanical cues from the extracellular environment. Forces transmitted from the microenvironment activate mechanotransduction pathways in the cell, which trigger a cascade of biochemical events that impact cytoskeletal tension, cellular morphology and energy budget available to the cell. Using a nonequilibrium free energy-based theory, we can predict the ATP consumption, contractility, and shape of mesenchymal cancer cells, as well as how cells regulate energy levels dependent on the mechanosensitive metabolic regulator AMPK. The insights from our model can be used to understand the mechanosensitive regulation of metabolism during metastasis and tumor progression, during which cells experience dynamic changes in their microenvironment and metabolic state.

Characterization of latently infected EBV+ antibody-secreting B cells isolated from ovarian tumors and malignant ascites

Introduction

Intra-tumoral B cells mediate a plethora of immune effector mechanisms with key roles in anti-tumor immunity and serve as positive prognostic indicators in a variety of solid tumor types, including epithelial ovarian cancer (EOC). Several aspects of intra-tumoral B cells remain unclear, such as their state of activation, antigenic repertoires, and capacity to mature into plasma cells.

Methods

B lymphocytes were isolated from primary EOC tissue and malignant ascites and were maintained in cell culture medium. The stably maintained cell lines were profiled with flow cytometry and B cell receptor sequencing. Secreted antibodies were tested with a human proteome array comprising more than 21,000 proteins, followed by ELISA for validation. Originating tumor samples were used for spatial profiling with chip cytometry.

Results

Antibody-secreting B lymphocytes were isolated from the ovarian tumor microenvironment (TME) of four different EOC patients. The highly clonal cell populations underwent spontaneous immortalization in vitro, were stably maintained in an antibody-secreting state, and showed presence of Epstein-Barr viral (EBV) proteins. All originating tumors had high frequency of tumor-infiltrating B cells, present as lymphoid aggregates, or tertiary lymphoid structures. The antigens recognized by three of the four cell lines are coil-coil domain containing protein 155 (CCDC155), growth factor receptor-bound protein 2 (GRB2), and pyruvate dehydrogenase phosphatase2 (PDP2), respectively. Anti-CCDC155 circulating IgG antibodies were detected in 9 of 20 (45%) of EOC patients' sera. Tissue analyses with multiparameter chip cytometry shows that the antibodies secreted by these novel human B cell lines engage their cognate antigens on tumor cells.

Discussion

These studies demonstrate that within the tumor-infiltrating lymphocyte population in EOC resides a low frequency population of antibody-secreting B cells that have been naturally exposed to EBV. Once stably maintained, these novel cell lines offer unique opportunities for future studies on intratumor B cell biology and new target antigen recognition, and for studies on EBV latency and/or viral reactivation in the TME of non-EBV related solid tumors such as the EOC.

Nucleocytoplasmic transport rates are regulated by cellular processes that modulate GTP availability

Nucleocytoplasmic transport (NCT), the facilitated diffusion of cargo molecules between the nucleus and cytoplasm through nuclear pore complexes (NPCs), enables numerous fundamental eukaryotic cellular processes. Ran GTPase uses cellular energy in the direct form of GTP to create a gradient across the nuclear envelope (NE) that drives the majority of NCT. We report here that changes in GTP availability resulting from altered cellular physiology modulate the rate of NCT, as monitored using synthetic and natural cargo, and the dynamics of Ran itself. Cell migration, cell spreading, and/or modulation of the cytoskeleton or its connection to the nucleus alter GTP availability and thus rates of NCT, regulating RNA export and protein synthesis. These findings support a model in which changes in cellular physiology that alter GTP availability can regulate the rate of NCT, impacting fundamental cellular processes that extensively utilize NCT.

Rear cortex contraction aids in nuclear transit during confined migration by increasing pressure in the cell posterior

As cells migrate through biological tissues, they must frequently squeeze through micron-sized constrictions in the form of interstitial pores between extracellular matrix fibers and/or other cells. Although it is now well recognized that such confined migration is limited by the nucleus, which is the largest and stiffest organelle, it remains incompletely understood how cells apply sufficient force to move their nucleus through small constrictions. Here, we report a mechanism by which contraction of the cell rear cortex pushes the nucleus forward to mediate nuclear transit through constrictions. Laser ablation of the rear cortex reveals that pushing forces behind the nucleus are the result of increased intracellular pressure in the rear compartment of the cell. The pushing forces behind the nucleus depend on accumulation of actomyosin in the rear cortex and require Rho kinase (ROCK) activity. Collectively, our results suggest a mechanism by which cells generate elevated intracellular pressure in the posterior compartment to facilitate nuclear transit through three-dimensional (3D) constrictions. This mechanism might supplement or even substitute for other mechanisms supporting nuclear transit, ensuring robust cell migrations in confined 3D environments.

Impact of ionizing radiation on cell-ECM mechanical crosstalk in breast cancer

The stiffness of the extracellular matrix plays a crucial role in cell motility and spreading, influencing cell morphology through cytoskeleton organization and transmembrane proteins' expression. In this context, mechanical characterization of both cells and the extracellular matrix gains prominence for enhanced diagnostics and clinical decision-making. Here, we investigate the combined effect of mechanotransduction and ionizing radiations on altering cells' mechanical properties, analysing mammary cell lines (MCF10A and MDA-MB-231) after X-ray radiotherapy (2 and 10 Gy). We found that ionizing radiations sensitively affect adenocarcinoma cells cultured on substrates mimicking cancerous tissue stiffness (15 kPa), inducing an increased structuration of paxillin-rich focal adhesions and cytoskeleton: this process translates in the augmentation of tension at the actin filaments level, causing cellular stiffness and consequently affecting cytoplasmatic/nuclear morphologies. Deeper exploration of the intricate interplay between mechanical factors and radiation should provide novel strategies to orient clinical outcomes.

The role of the nucleus for cell mechanics: an elastic phase field approach

The nucleus of eukaryotic cells typically makes up around 30% of the cell volume and has significantly different mechanics, which can make it effectively up to ten times stiffer than the surrounding cytoplasm. Therefore it is an important element for cell mechanics, but a quantitative understanding of its mechanical role during whole cell dynamics is largely missing. Here we demonstrate that elastic phase fields can be used to describe dynamical cell processes in adhesive or confining environments in which the nucleus acts as a stiff inclusion in an elastic cytoplasm. We first introduce and verify our computational method and then study several prevalent cell-mechanical measurement methods. For cells on adhesive patterns, we find that nuclear stress is shielded by the adhesive pattern. For cell compression between two parallel plates, we obtain force-compression curves that allow us to extract an effective modulus for the cell-nucleus composite. For micropipette aspiration, the effect of the nucleus on the effective modulus is found to be much weaker, highlighting the complicated interplay between extracellular geometry and cell mechanics that is captured by our approach. We also show that our phase field approach can be used to investigate the effects of Kelvin-Voigt-type viscoelasticity and cortical tension.

Eliminating elevated p53 signaling fails to rescue skeletal muscle defects or extend survival in lamin A/C-deficient mice

Lamins A and C, encoded by the LMNA gene, are nuclear intermediate filaments that provide structural support to the nucleus and contribute to chromatin organization and transcriptional regulation. LMNA mutations cause muscular dystrophies, dilated cardiomyopathy, and other diseases. The mechanisms by which many LMNA mutations result in muscle-specific diseases have remained elusive, presenting a major hurdle in the development of effective treatments. Previous studies using striated muscle laminopathy mouse models found that cytoskeletal forces acting on mechanically fragile Lmna-mutant nuclei led to transient nuclear envelope rupture, extensive DNA damage, and activation of DNA damage response (DDR) pathways in skeletal muscle cells in vitro and in vivo. Furthermore, hearts of Lmna mutant mice have elevated activation of the tumor suppressor protein p53, a central regulator of DDR signaling. We hypothesized that elevated p53 activation could present a pathogenic mechanism in striated muscle laminopathies, and that eliminating p53 activation could improve muscle function and survival in laminopathy mouse models. Supporting a pathogenic function of p53 activation in muscle, stabilization of p53 was sufficient to reduce contractility and viability in wild-type muscle cells in vitro. Using three laminopathy models, we found that increased p53 activity in Lmna-mutant muscle cells primarily resulted from mechanically induced damage to the myonuclei, and not from altered transcriptional regulation due to loss of lamin A/C expression. However, global deletion of p53 in a severe muscle laminopathy model did not reduce the disease phenotype or increase survival, indicating that additional drivers of disease must contribute to the disease pathogenesis.

Life at the crossroads: the nuclear LINC complex and vascular mechanotransduction

Vascular endothelial cells line the inner surface of all blood vessels, where they are exposed to polarized mechanical forces throughout their lifespan. Both basal substrate interactions and apical blood flow-induced shear stress regulate blood vessel development, remodeling, and maintenance of vascular homeostasis. Disruption of these interactions leads to dysfunction and vascular pathologies, although how forces are sensed and integrated to affect endothelial cell behaviors is incompletely understood. Recently the endothelial cell nucleus has emerged as a prominent force-transducing organelle that participates in vascular mechanotransduction, via communication to and from cell-cell and cell-matrix junctions. The LINC complex, composed of SUN and nesprin proteins, spans the nuclear membranes and connects the nuclear lamina, the nuclear envelope, and the cytoskeleton. Here we review LINC complex involvement in endothelial cell mechanotransduction, describe unique and overlapping functions of each LINC complex component, and consider emerging evidence that two major SUN proteins, SUN1 and SUN2, orchestrate a complex interplay that extends outward to cell-cell and cell-matrix junctions and inward to interactions within the nucleus and chromatin. We discuss these findings in relation to vascular pathologies such as Hutchinson-Gilford progeria syndrome, a premature aging disorder with cardiovascular impairment. More knowledge of LINC complex regulation and function will help to understand how the nucleus participates in endothelial cell force sensing and how dysfunction leads to cardiovascular disease.

Nesprin proteins: bridging nuclear envelope dynamics to muscular dysfunction

This review presents a comprehensive exploration of the pivotal role played by the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex, with a particular focus on Nesprin proteins, in cellular mechanics and the pathogenesis of muscular diseases. Distinguishing itself from prior works, the analysis delves deeply into the intricate interplay of the LINC complex, emphasizing its indispensable contribution to maintaining cellular structural integrity, especially in mechanically sensitive tissues such as cardiac and striated muscles. Additionally, the significant association between mutations in Nesprin proteins and the onset of Dilated Cardiomyopathy (DCM) and Emery-Dreifuss Muscular Dystrophy (EDMD) is highlighted, underscoring their pivotal role in disease pathogenesis. Through a comprehensive examination of DCM and EDMD cases, the review elucidates the disruptions in the LINC complex, nuclear morphology alterations, and muscular developmental disorders, thus emphasizing the essential function of an intact LINC complex in preserving muscle physiological functions. Moreover, the review provides novel insights into the implications of Nesprin mutations for cellular dynamics in the pathogenesis of muscular diseases, particularly in maintaining cardiac structural and functional integrity. Furthermore, advanced therapeutic strategies, including rectifying Nesprin gene mutations, controlling Nesprin protein expression, enhancing LINC complex functionality, and augmenting cardiac muscle cell function are proposed. By shedding light on the intricate molecular mechanisms underlying nuclear-cytoskeletal interactions, the review lays the groundwork for future research and therapeutic interventions aimed at addressing genetic muscle disorders.

The LINC complex ensures accurate centrosome positioning during prophase

Accurate centrosome separation and positioning during early mitosis relies on force-generating mechanisms regulated by a combination of extracellular, cytoplasmic, and nuclear cues. The identity of the nuclear cues involved in this process remains largely unknown. Here, we investigate how the prophase nucleus contributes to centrosome positioning during the initial stages of mitosis, using a combination of cell micropatterning, high-resolution live-cell imaging, and quantitative 3D cellular reconstruction. We show that in untransformed RPE-1 cells, centrosome positioning is regulated by a nuclear signal, independently of external cues. This nuclear mechanism relies on the linker of nucleoskeleton and cytoskeleton complex that controls the timely loading of dynein on the nuclear envelope (NE), providing spatial cues for robust centrosome positioning on the shortest nuclear axis, before nuclear envelope permeabilization. Our results demonstrate how nuclear-cytoskeletal coupling maintains a robust centrosome positioning mechanism to ensure efficient mitotic spindle assembly.

A Hierarchical Mechanotransduction System: From Macro to Micro

Mechanotransduction is a strictly regulated process whereby mechanical stimuli, including mechanical forces and properties, are sensed and translated into biochemical signals. Increasing data demonstrate that mechanotransduction is crucial for regulating macroscopic and microscopic dynamics and functionalities. However, the actions and mechanisms of mechanotransduction across multiple hierarchies, from molecules, subcellular structures, cells, tissues/organs, to the whole-body level, have not been yet comprehensively documented. Herein, the biological roles and operational mechanisms of mechanotransduction from macro to micro are revisited, with a focus on the orchestrations across diverse hierarchies. The implications, applications, and challenges of mechanotransduction in human diseases are also summarized and discussed. Together, this knowledge from a hierarchical perspective has the potential to refresh insights into mechanotransduction regulation and disease pathogenesis and therapy, and ultimately revolutionize the prevention, diagnosis, and treatment of human diseases.

The Role of Mechanotransduction in Contact Inhibition of Locomotion and Proliferation

Contact inhibition (CI) represents a crucial tumor-suppressive mechanism responsible for controlling the unbridled growth of cells, thus preventing the formation of cancerous tissues. CI can be further categorized into two distinct yet interrelated components: CI of locomotion (CIL) and CI of proliferation (CIP). These two components of CI have historically been viewed as separate processes, but emerging research suggests that they may be regulated by both distinct and shared pathways. Specifically, recent studies have indicated that both CIP and CIL utilize mechanotransduction pathways, a process that involves cells sensing and responding to mechanical forces. This review article describes the role of mechanotransduction in CI, shedding light on how mechanical forces regulate CIL and CIP. Emphasis is placed on filamin A (FLNA)-mediated mechanotransduction, elucidating how FLNA senses mechanical forces and translates them into crucial biochemical signals that regulate cell locomotion and proliferation. In addition to FLNA, trans-acting factors (TAFs), which are proteins or regulatory RNAs capable of directly or indirectly binding to specific DNA sequences in distant genes to regulate gene expression, emerge as sensitive players in both the mechanotransduction and signaling pathways of CI. This article presents methods for identifying these TAF proteins and profiling the associated changes in chromatin structure, offering valuable insights into CI and other biological functions mediated by mechanotransduction. Finally, it addresses unanswered research questions in these fields and delineates their possible future directions.

Altered nuclear envelope homeostasis is a key pathogenic event in C9ORF72-linked ALS/FTD

ALS and FTD are complex neurodegenerative disorders that primarily affects motor neurons in the brain and spinal cord, and cortical neurons in the frontal lobe. Although the pathogenesis of ALS/FTD is unclear, recent research spotlights nucleocytoplasmic transport impairment, DNA damage, and nuclear abnormalities as drivers of neuronal death. In this study, we show that loss of nuclear envelope (NE) integrity is a key pathology associated with nuclear pore complex (NPC) injury in C9ORF72 mutant neurons. Importantly, we show that mechanical stresses generated by cytoskeletal forces on the NE can lead to NPC injury, loss of nuclear integrity, and accumulation of DNA damage. Importantly, we demonstrate that restoring NE tensional homeostasis, by disconnecting the nucleus from the cytoskeleton, can rescue NPC injury and reduce DNA damage in C9ORF72 mutant cells. Together, our data suggest that modulation of NE homeostasis and repair may represent a novel and promising therapeutic target for ALS/FTD.

Molecular and Structural Alterations of Skeletal Muscle Tissue Nuclei during Aging

Aging is accompanied by a progressive loss of skeletal muscle mass and strength. The mechanisms underlying this phenomenon are certainly multifactorial and still remain to be fully elucidated. Changes in the cell nucleus structure and function have been considered among the possible contributing causes. This review offers an overview of the current knowledge on skeletal muscle nuclei in aging, focusing on the impairment of nuclear pathways potentially involved in age-related muscle decline. In skeletal muscle two types of cells are present: fiber cells, constituting the contractile muscle mass and containing hundreds of myonuclei, and the satellite cells, i.e., the myogenic mononuclear stem cells occurring at the periphery of the fibers and responsible for muscle growth and repair. Research conducted on different experimental models and with different methodological approaches demonstrated that both the myonuclei and satellite cell nuclei of aged skeletal muscles undergo several structural and molecular alterations, affecting chromatin organization, gene expression, and transcriptional and post-transcriptional activities. These alterations play a key role in the impairment of muscle fiber homeostasis and regeneration, thus contributing to the age-related decrease in skeletal muscle mass and function.

Desmin and Plectin Recruitment to the Nucleus and Nuclei Orientation Are Lost in Emery-Dreifuss Muscular Dystrophy Myoblasts Subjected to Mechanical Stimulation

In muscle cells subjected to mechanical stimulation, LINC complex and cytoskeletal proteins are basic to preserve cellular architecture and maintain nuclei orientation and positioning. In this context, the role of lamin A/C remains mostly elusive. This study demonstrates that in human myoblasts subjected to mechanical stretching, lamin A/C recruits desmin and plectin to the nuclear periphery, allowing a proper spatial orientation of the nuclei. Interestingly, in Emery-Dreifuss Muscular Dystrophy (EDMD2) myoblasts exposed to mechanical stretching, the recruitment of desmin and plectin to the nucleus and nuclear orientation were impaired, suggesting that a functional lamin A/C is crucial for the response to mechanical strain. While describing a new mechanism of action headed by lamin A/C, these findings show a structural alteration that could be involved in the onset of the muscle defects observed in muscular laminopathies.

Nuclear membrane protein SUN2 promotes replication of flaviviruses through modulating cytoskeleton reorganization mediated by NS1

Cytoskeleton is extensively recruited by flaviviruses for their infection. In this study, we uncovered an essential role of a nuclear membrane protein, SAD1/UNC84 domain protein 2 (SUN2) linking cytoskeleton and nucleoskeleton in the flavivirus replication. CRISPR/Cas9-mediated knockout of SUN2, but not SUN1, significantly reduces the replication of Zika virus (ZIKV), dengue virus (DENV), and Japanese encephalitis virus (JEV). In contrast, SUN2 does not affect the infection of non-flaviviridae RNA viruses. All three regions of SUN2 are required for its proviral effect. Mechanistically, SUN2 facilitates rearrangement of cytoskeleton and formation of replication organelles induced by viral infection, and hence promotes viral RNA synthesis. SUN2 is required for the interaction between cytoskeleton actin and ZIKV nonstructural protein 1 (NS1). Expression of dominant negative Nesprin-1 and Nesprin-2, which connect SUN2 to cytoskeleton proteins, alleviates the interaction between actin and NS1 and reduces viral replication levels. In a neonatal mouse infection model, SUN2 knockout dramatically alleviates the in vivo ZIKV replication and development of neuropathology. This work elucidates that recruitment of cytoskeleton proteins by flavivirus is coordinated by nuclear membrane proteins SUN2 and Nesprins, providing evidence for a link between nuclear membrane proteins and flavivirus infection.

Nucleocytoplasmic transport rates are regulated by cellular processes that modulate GTP availability

Nucleocytoplasmic transport (NCT), the facilitated diffusion of cargo molecules between the nucleus and cytoplasm through nuclear pore complexes (NPCs), enables numerous fundamental eukaryotic cellular processes. Ran GTPase uses cellular energy in the direct form of GTP to create a gradient across the nuclear envelope (NE) that drives the majority of NCT. We report here that changes in GTP availability resulting from altered cellular physiology modulate the rate of NCT, as monitored using synthetic and natural cargo, and the dynamics of Ran itself. Cell migration, cell spreading and/or modulation of the cytoskeleton or its connection to the nucleus alter GTP availability and thus rates of NCT, regulating RNA export and protein synthesis. These findings support a model in which changes in cellular physiology that alter GTP availability can regulate the rate of NCT, impacting fundamental cellular processes that extensively utilize NCT.

3D matrix adhesion feedback controls nuclear force coupling to drive invasive cell migration

Cell invasion is a multi-step process, initiated by the acquisition of a migratory phenotype and the ability to move through complex 3D extracellular environments. We determine the composition of cell-matrix adhesion complexes of invasive breast cancer cells in 3D matrices and identify an interaction complex required for invasive migration. βPix and myosin18A (Myo18A) drive polarized recruitment of non-muscle myosin 2A (NM2A) to adhesion complexes at the tips of protrusions. Actomyosin force engagement then displaces the Git1-βPix complex from paxillin, establishing a feedback loop for adhesion maturation. We observe active force transmission to the nucleus during invasive migration that is needed to pull the nucleus forward. The recruitment of NM2A to adhesions creates a non-muscle myosin isoform gradient, which extends from the protrusion to the nucleus. We postulate that this gradient facilitates coupling of cell-matrix interactions at the protrusive cell front with nuclear movement, enabling effective invasive migration and front-rear cell polarity.

Force transmission and SUN-KASH higher-order assembly in the LINC complex models

The linkers of the nucleoskeleton and cytoskeleton (LINC) complex comprises Sad-1 and UNC-84 (SUN) and Klarsicht, ANC-1, SYNE homology (KASH) domain proteins, whose conserved interactions provide a physical coupling between the cytoskeleton and the nucleoskeleton, thereby mediating the transfer of physical forces across the nuclear envelope. The LINC complex can perform distinct cellular functions by pairing various KASH domain proteins with the same SUN domain protein. Recent studies have suggested a higher-order assembly of SUN and KASH instead of a more widely accepted linear trimer model for the LINC complex. In the present study, we use molecular dynamics simulations to investigate the mechanism of force transfer across the two proposed models of LINC complex assembly, namely the 3:3 linear trimer model and the 6:6 higher-order model. Employing steered molecular dynamics simulations with various structures using forces at different rates and directions, we examine the structural stability of the two models under various biologically relevant conditions. Our results suggest that both models can withstand and transfer significant levels of force while retaining their structural integrity. However, the force response of various SUN/KASH assemblies depend on the force direction and pulling rates. Slower pulling rates result in higher mean square fluctuations of the 3:3 assembly compared to the fast pulling. Interestingly, the 6:6 assembly tends to provide an additional range of motion flexibility and might be more advantageous to the structural rigidity and pliability of the nuclear envelope. These findings offer insights into how the SUN and KASH proteins maintain the structural integrity of the nuclear membrane.