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

Shaping epithelial tissues by stem cell mechanics in development and cancer

Adult stem cells balance self-renewal and differentiation to build, maintain and repair tissues. The role of signalling pathways and transcriptional networks in controlling stem cell function has been extensively studied, but there is increasing appreciation that mechanical forces also have a crucial regulatory role. Mechanical forces, signalling pathways and transcriptional networks must be coordinated across diverse length and timescales to maintain tissue homeostasis and function. Such coordination between stem cells and neighbouring cells dictates when cells divide, migrate and differentiate. Recent advances in measuring and manipulating the mechanical forces that act upon and are produced by stem cells are providing new insights into development and disease. In this Review, we discuss the mechanical forces involved when epithelial stem cells construct their microenvironment and what happens in cancer when stem cell niche mechanics are disrupted or dysregulated. As the skin has evolved to withstand the harsh mechanical pressures from the outside environment, we often use the stem cells of mammalian skin epithelium as a paradigm for adult stem cells shaping their surrounding tissues.

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.

Selective inhibition of stromal mechanosensing suppresses cardiac fibrosis

Matrix-derived biophysical cues are known to regulate the activation of fibroblasts and their subsequent transdifferentiation into myofibroblasts1-6, but whether modulation of these signals can suppress fibrosis in intact tissues remains unclear, particularly in the cardiovascular system7-10. Here we demonstrate across multiple scales that inhibition of matrix mechanosensing in persistently activated cardiac fibroblasts potentiates-in concert with soluble regulators of the TGFβ pathway-a robust transcriptomic, morphological and metabolic shift towards quiescence. By conducting a meta-analysis of public human and mouse single-cell sequencing datasets, we identify the focal-adhesion-associated tyrosine kinase SRC as a fibroblast-enriched mechanosensor that can be targeted selectively in stromal cells to mimic the effects of matrix softening in vivo. Pharmacological inhibition of SRC by saracatinib, coupled with TGFβ suppression, induces synergistic repression of key profibrotic gene programs in fibroblasts, characterized by a marked inhibition of the MRTF-SRF pathway, which is not seen after treatment with either drug alone. Importantly, the dual treatment alleviates contractile dysfunction in fibrotic engineered heart tissues and in a mouse model of heart failure. Our findings point to joint inhibition of SRC-mediated stromal mechanosensing and TGFβ signalling as a potential mechanotherapeutic strategy for treating cardiovascular fibrosis.

The dilemma of nuclear mechanical forces in DNA damage and repair

Genomic stability, encompassing DNA damage and repair mechanisms, plays a pivotal role in the onset of diseases and the aging process. The stability of DNA is intricately linked to the chemical and mechanical forces exerted on chromatin, particularly within lamina-associated domains (LADs). Mechanical stress can induce DNA damage through the deformation and rupture of the nuclear envelope, leading to DNA bending and cleavage. However, DNA can evade such mechanical stress-induced damage by relocating away from the nuclear membrane, a process facilitated by the depletion of H3K9me3-marked heterochromatin and its cleavage from the lamina. When DNA double-stranded breaks occur, they prompt the rapid recruitment of Lamin B1 and the deposition of H3K9me3. Despite these insights, the precise mechanisms underlying DNA damage and repair under mechanical stress remain unclear. In this review, we explore the interplay between mechanical forces and the nuclear envelope in the context of DNA damage, elucidate the molecular pathways through which DNA escapes force-induced damage, and discuss the corresponding repair strategies involving the nuclear cytoskeleton. By summarizing the mechanisms of force-induced DNA damage and repair, we aim to underscore the potential for developing targeted therapeutic strategies to bolster genomic stability and alleviate the impacts of aging and disease.

Dynamic substrate topographies drive actin- and vimentin-mediated nuclear mechanoprotection events in human fibroblasts

BACKGROUND

Dynamic physical changes in the extracellular environment of living tissues present a mechanical challenge for resident cells that can lead to damage to the nucleus, genome, and DNA. Recent studies have started to uncover nuclear mechanoprotection mechanisms that prevent excessive mechanical deformations of the nucleus. Here, we hypothesized that dynamic topographical changes in the cellular environment can be mechanically transmitted to the nucleus and trigger nuclear mechanoprotection events. We tested this using a photoresponsive hydrogel whose surface topography can be reversibly changed on demand upon light illumination, allowing us to subject cells to recurring microscale topographical changes.

RESULTS

With each recurring topographical change, fibroblasts were found to increasingly compact and relocate their nuclei away from the dynamic regions of the hydrogel. These cell-scale reorganization events were accompanied by an increase of global histone acetylation and decreased methylation in cells on the dynamic topographies, resulting in a minimization of DNA strand breakage. We further found that these nuclear mechanoprotection events were mediated by both vimentin intermediate filaments and the actin cytoskeleton.

CONCLUSIONS

Together, these data reveal that fibroblasts actively protect their nuclei in the presence of dynamic topographical changes through cytoskeleton-mediated mechanisms. Broadly, these results stress the importance of gaining a deeper fundamental understanding of the cellular mechanoresponse under dynamically changing conditions.

Piezo1, F-Actin Remodeling, and Podocyte Survival and Regeneration

Key Points

Background

Podocytes and podocyte progenitors are interdependent components of the kidney's glomerular structure, with podocytes forming the glomerular filtration barrier and progenitors being key players in podocyte regeneration during pathophysiologic processes. Both cell types are subjected to constant mechanical forces, whose alterations can initiate podocytopathy and worsen glomerular injury. Despite this, the specific mechanosensors and mechanotransduction pathways involved in their response to mechanical cues remain only partially explored.

Methods

We used transcriptomics, immunofluorescence, and silencing experiments on human primary podocyte progenitor cell cultures to demonstrate the expression and function of Piezo1 channels. We generated inducible podocyte-specific and podocyte progenitor–specific Piezo1 knockout (KO) mice to evaluate the effects of Piezo1 loss in the context of adriamycin nephropathy and over 10 months of aging.

Results

Silencing of Piezo1 in progenitors triggered F-actin remodeling, which induced cell shape modification and nuclear envelope defects with accumulation of DNA damage that led to mitotic catastrophe in differentiated podocytes. Podocyte-specific KO of Piezo1 induced higher susceptibility to podocyte injury in adriamycin nephropathy and led to accumulation of DNA damage and mild albuminuria starting from adult age. Podocyte progenitor–specific KO of Piezo1 in mouse resulted in severe albuminuria during adriamycin nephropathy, leading to the generation of defective podocytes.

Conclusions

These results demonstrated that Piezo1, thanks to its role in F-actin cytoskeleton maintenance, is essential for the survival of podocytes exposed to mechanical stress conditions and for their correct regeneration.

Engineering aortic valves via transdifferentiating fibroblasts into valvular endothelial cells without using viruses or iPS cells

The technology of induced pluripotent stem cells (iPSCs) has enabled the conversion of somatic cells into primitive undifferentiated cells via reprogramming. This approach provides possibilities for cell replacement therapies and drug screening, but the potential risk of tumorigenesis hampers its further development and in vivo application. How to generate differentiated cells such as valvular endothelial cells (VECs) has remained a major challenge. Utilizing a combinatorial strategy of selective soluble chemicals, cytokines and substrate stiffness modulation, mouse embryonic fibroblasts are directly and efficiently transdifferentiated into induced aortic endothelial cell-like cells (iAECs), or human primary adult fibroblasts are transdifferentiated into induced valvular endothelial cell-like cells (hiVECs), without expressing pluripotency stem cell markers. These iAECs and hiVECs express VEC-associated genes and proteins and VEC-specific marker NFATC1 and are functional in culture and on decellularized porcine aortic valves, like mouse aortic endothelial cells or human primary aortic valvular endothelial cells. The iAECs and hiVECs seeded on decellularized porcine aortic valves stay intact and express VEC-associated proteins for 60 days after grafting into abdominal aorta of immune-compromised rats. In contrast, induced pluripotent stem cells (iPSCs) are less efficient in differentiating into VEC-like cells and pluripotency marker Nanog is expressed in a small subpopulation of iPSC-derived VEC-like cells that generate teratomas in SCID mice whereas hiVECs derived from transdifferentiation do not generate teratomas in vivo. Our findings highlight an approach to efficiently convert fibroblasts into iAECs and hiVECs and seed them onto decellularized aortic valves for safely generating autologous tissue-engineered aortic valves without using viruses or first reprogramming the cells into pluripotent stem cells.

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.

Mechanical Force-Induced cGAS Activation in Carcinoma Cells Facilitates Splenocytes into Liver to Drive Metastasis

Liver metastasis is the main cause of cancer-related mortality. During the metastasis process, circulating carcinoma cells hardly pass through narrow capillaries, leading to nuclear deformation. However, the effects of nuclear deformation and its underlying mechanisms on metastasis need further study. Here, it is shown that mechanical force-induced nuclear deformation exacerbates liver metastasis by activating the cGAS-STING pathway, which promotes splenocyte infiltration in the liver. Mechanical force results in nuclear deformation and rupture of the nuclear envelope with inevitable DNA leakage. Cytoplasmic DNA triggers the activation of cGAS-STING pathway, enhancing the production of IL6, TNFα, and CCL2. Additionally, splenocyte recruitment by the proinflammatory cytokines support carcinoma cell survival and colonization in the liver. Importantly, both intervening activity of cGAS and blocking of splenocyte migration to the liver efficiently ameliorate liver metastasis. Overall, these findings reveal a mechanism by which mechanical force-induced nuclear deformation exacerbates liver metastasis by regulating splenocyte infiltration into the liver and support targeting cGAS and blocking splenocyte recruitment as candidate therapeutic approaches for liver metastasis.

Deciphering mechanical cues in the microenvironment: from non-malignant settings to tumor progression

The tumor microenvironment functions as a dynamic and intricate ecosystem, comprising a diverse array of cellular and non-cellular components that precisely orchestrate pivotal tumor behaviors, including invasion, metastasis, and drug resistance. While unraveling the intricate interplay between the tumor microenvironment and tumor behaviors represents a tremendous challenge, recent research illuminates a crucial biological phenomenon known as cellular mechanotransduction. Within the microenvironment, mechanical cues like tensile stress, shear stress, and stiffness play a pivotal role by activating mechanosensitive effectors such as PIEZO proteins, integrins, and Yes-associated protein. This activation initiates cascades of intrinsic signaling pathways, effectively linking the physical properties of tissues to their physiological and pathophysiological processes like morphogenesis, regeneration, and immunity. This mechanistic insight offers a novel perspective on how the mechanical cues within the tumor microenvironment impact tumor behaviors. While the intricacies of the mechanical tumor microenvironment are yet to be fully elucidated, it exhibits distinct physical attributes from non-malignant tissues, including elevated solid stresses, interstitial hypertension, augmented matrix stiffness, and enhanced viscoelasticity. These traits exert notable influences on tumor progression and treatment responses, enriching our comprehension of the multifaceted nature of the microenvironment. Through this innovative review, we aim to provide a new lens to decipher the mechanical attributes within the tumor microenvironment from non-malignant contexts, broadening our knowledge on how these factors promote or inhibit tumor behaviors, and thus offering valuable insights to identify potential targets for anti-tumor strategies.

LMNA-related cardiomyopathy: From molecular pathology to cardiac gene therapy

BACKGROUND

The genetic variants of LMNA cause an array of diseases that often affect the heart. LMNA-related cardiomyopathy exhibits high-penetrance and early-onset phenotypes that lead to late-stage heart failure or lethal arrhythmia. As a subtype of dilated cardiomyopathy and arrhythmogenic cardiomyopathy, LMNA-related cardiac dysfunction is resistant to existing cardiac therapeutic strategies, leaving a major unmet clinical need in cardiomyopathy management.

AIM OF REVIEW

Here we comprehensively summarize current knowledge about the genetic basis, disease models and pathological mechanisms of LMNA-related cardiomyopathy. Recent translational studies were highlighted to indicate new therapeutic modalities such as gene supplementation, gene silencing and genome editing therapy, which offer potential opportunities to overcome the difficulties in the development of specific drugs for this disease.

KEY SCIENTIFIC CONCEPTS OF REVIEW

LMNA-related cardiomyopathy involves many diverse disease mechanisms that preclude small-molecule drugs that target only a small fraction of the mechanisms. Agreeing to this notion, the first-in-human clinical trial for this disease recently reported futility. By contrast, gene therapy offers the new hope to directly intervene LMNA variants and demonstrates a tremendous potential for breakthrough therapy for this disease. Concepts in this review are also applicable to studies of other genetic diseases that lack effective therapeutics.

Phosphorylation of Lamin A/C regulates the structural integrity of the nuclear envelope

Dynamic disassembly and reconstruction of the nuclear lamina during entry and exit of mitosis, respectively, are pivotal steps in the proliferation of higher eukaryotic cells. Although numerous post-translational modifications of lamin proteins have been identified, key factors driving the nuclear lamina dynamics remain elusive. Here we identified CDK1-elicited phosphorylation sites on endogenous Lamin A/C and characterized their functions in regulation of the nuclear lamina. Specifically, mass spectrometry revealed CDK1-mediated phosphorylation of Lamin A/C at the N-terminal Thr19/Ser22 and the C-terminal Ser390/Ser392 during mitosis. Importantly, the phospho-mimicking 4D mutant T19D/S22D/S390D/S392D completely disrupted Lamin A filamentous structure in interphase cells. Conversely, the non-phosphorylatable mutant T19A/S22A and especially the 4A mutant T19A/S22A/S390A/S392A protected Lamin A from depolymerization during mitosis. These results suggest that phosphorylation and dephosphorylation of both N- and C-terminal sites regulate the nuclear lamina dynamics. Engineering the non-phosphorylatable mutant T19A/S22A into the endogenous LMNA gene resulted in nuclear abnormalities and micronucleus formation during telophase. Perturbation of the Lamin A phosphorylation is shown to prevent proper nuclear envelope dynamics and impair nuclear integrity. These findings reveal a previously undefined link between the CDK1-elicited Lamin A phosphorylation dynamics, nuclear envelope plasticity, and genomic stability during the cell cycle.

Laminin-α2 chain deficiency in skeletal muscle causes dysregulation of multiple cellular mechanisms

LAMA2, coding for the laminin-α2 chain, is a crucial ECM component, particularly abundant in skeletal muscle. Mutations in LAMA2 trigger the often-lethal LAMA2-congenital muscular dystrophy (LAMA2-CMD). Various phenotypes have been linked to LAMA2-CMD; nevertheless, the precise mechanisms that malfunction during disease onset in utero remain unknown. We generated Lama2-deficient C2C12 cells and found that Lama2-deficient myoblasts display proliferation, differentiation, and fusion defects, DNA damage, oxidative stress, and mitochondrial dysfunction. Moreover, fetal myoblasts isolated from the dy W mouse model of LAMA2-CMD display impaired differentiation and fusion in vitro. We also showed that disease onset during fetal development is characterized by a significant down-regulation of gene expression in muscle fibers, causing pronounced effects on cytoskeletal organization, muscle differentiation, and altered DNA repair and oxidative stress responses. Together, our findings provide unique insights into the critical importance of the laminin-α2 chain for muscle differentiation and muscle cell homeostasis.

Regulation of TAR DNA binding protein 43 (TDP-43) homeostasis by cytosolic DNA accumulation

TAR DNA-binding protein 43 (TDP-43) is a DNA/RNA binding protein predominantly localized in the nucleus under physiological conditions. TDP-43 proteinopathy, characterized by cytoplasmic aggregation and nuclear loss, is associated with many neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). Thus it is crucial to understand the molecular mechanism regulating TDP-43 homeostasis. Here, we show that the uptake of oligodeoxynucleotides (ODNs) from the extracellular space induces reversible TDP-43 cytoplasmic puncta formation in both neurons and glia. ODNs facilitate the liquid-liquid phase separation of TDP-43 in vitro. Importantly, persistent accumulation of DNA in the cytoplasm leads to nuclear depletion of TDP-43 and enhanced production of a short isoform of TDP-43 (sTDP-43). In addition, in response to ODN uptake, the nuclear import receptor karyopherin subunit β1 (KPNB1) is sequestered in the cytosolic TDP-43 puncta. ALS-linked Q331K mutation decreases the dynamics of cytoplasmic TDP-43 puncta and increases the levels of sTDP-43. Moreover, the TDP-43 cytoplasmic puncta are induced by DNA damage and by impaired nuclear envelope integrity due to Lamin A/C deficiency. In summary, our data support that abnormal DNA accumulation in the cytoplasm may be one of the key mechanisms leading to TDP-43 proteinopathy and provides novel insights into molecular mechanisms of ALS caused by TDP-43 mutations.

Influence of Soft and Stiff Matrices on Cytotoxicity in Gingival Fibroblasts: Implications for Soft Tissue Biocompatibility

The biocompatibility of dental materials is critical for ensuring safety in clinical applications. However, standard in vitro cytotoxicity assays often rely on stiff tissue culture plastic (TCP), which does not accurately replicate the biomechanical properties of soft oral tissues. In this study, we compared human gingival fibroblasts (HGFs) cultured on soft, gel-based substrates mimicking gingival tissue stiffness (0.2 kPa) with those cultured on conventional TCP (3 GPa) to assess the influence of substrate stiffness on the cytotoxicity of methyl methacrylate (MMA), as well as other cytotoxic agents, including DMSO and H2O2. The results demonstrated that cells cultured on softer substrates exhibited enhanced resistance to cytotoxic stress, with increased viability and decreased apoptosis and DNA damage following exposure to MMA, DMSO, and H2O2. Notably, HGFs on soft substrates showed significantly greater resilience to MMA-induced cytotoxicity compared to those cultured on TCP. These findings emphasize the critical role of substrate stiffness in modulating cellular responses to toxic agents and highlight the necessity of using physiologically relevant models for cytotoxicity testing of dental materials. This study provides valuable insights for improving biocompatibility assessment protocols in clinical settings.

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.

Expansion in situ genome sequencing links nuclear abnormalities to hotspots of aberrant euchromatin repression

Microscopy and genomics are both used to characterize cell function, but approaches to connect the two types of information are lacking, particularly at subnuclear resolution. While emerging multiplexed imaging methods can simultaneously localize genomic regions and nuclear proteins, their ability to accurately measure DNA-protein interactions is constrained by the diffraction limit of optical microscopy. Here, we describe expansion in situ genome sequencing (ExIGS), a technology that enables sequencing of genomic DNA and superresolution localization of nuclear proteins in single cells. We applied ExIGS to fibroblast cells derived from an individual with Hutchinson-Gilford progeria syndrome to characterize how variation in nuclear morphology affects spatial chromatin organization. Using this data, we discovered that lamin abnormalities are linked to hotspots of aberrant euchromatin repression that may erode cell identity. Further, we show that lamin abnormalities heterogeneously increase the repressive environment of the nucleus in tissues and aged cells. These results demonstrate that ExIGS may serve as a generalizable platform for connecting nuclear abnormalities to changes in gene regulation across disease contexts.

Lamin A/C facilitates DNA damage response by modulating ATM signaling and homologous recombination pathways

Lamin A/C, a core component of the nuclear lamina, forms a mesh-like structure beneath the inner nuclear membrane. While its structural role is well-studied, its involvement in DNA metabolism remains unclear. We conducted sequential protein fractionation to determine the subcellular localization of early DNA damage response (DDR) proteins. Our findings indicate that most DDR proteins, including ATM and the MRE11-RAD50-NBS1 (MRN) complex, are present in the nuclease - and high salt-resistant pellet fraction. Notably, ATM and MRN remain stably associated with these structures throughout the cell cycle, independent of ionizing radiation (IR)-induced DNA damage. Although Lamin A/C interacts with ATM and MRN, its depletion does not disrupt their association with nuclease-resistant structures. However, it impairs the IR-enhanced association of ATM with the nuclear matrix and ATM-mediated DDR signaling, as well as the interaction between ATM and MRN. This disruption impedes the recruitment of MRE11 to damaged DNA and the association of damaged DNA with the nuclear matrix. Additionally, Lamin A/C depletion results in reduced protein levels of CtIP and RAD51, which is mediated by transcriptional regulation. This, in turn, impairs the efficiency of homologous recombination (HR). Our findings indicate that Lamin A/C plays a pivotal role in DNA damage repair (DDR) by orchestrating ATM-mediated signaling, maintaining HR protein levels, and ensuring efficient DNA repair processes.

N-terminal tags impair the ability of lamin A to provide structural support to the nucleus

Lamins are intermediate filament proteins that contribute to numerous cellular functions, including nuclear morphology and mechanical stability. The N-terminal head domain of lamin is crucial for higher order filament assembly and function, yet the effects of commonly used N-terminal tags on lamin function remain largely unexplored. Here, we systematically studied the effect of two differently sized tags on lamin A (LaA) function in a mammalian cell model engineered to allow for precise control of expression of tagged lamin proteins. Untagged, FLAG-tagged and GFP-tagged LaA completely rescued nuclear shape defects when expressed at similar levels in lamin A/C-deficient (Lmna-/-) MEFs, and all LaA constructs prevented increased nuclear envelope ruptures in these cells. N-terminal tags, however, altered the nuclear localization of LaA and impaired the ability of LaA to restore nuclear deformability and to recruit emerin to the nuclear membrane in Lmna-/- MEFs. Our finding that tags impede some LaA functions but not others might explain the partial loss of function phenotypes when tagged lamins are expressed in model organisms and should caution researchers using tagged lamins to study the nucleus.

Microstructure-based nuclear lamina constitutive model

The nuclear lamina is widely recognized as the most crucial component in providing mechanical stability to the nucleus. However, it is still a significant challenge to model the mechanics of this multilayered protein network. We developed a constitutive model of the nuclear lamina network based on its microstructure, which accounts for the deformation phases at the dimer level, as well as the orientational arrangement and density of lamin filaments. Instead of relying on homology modeling in the previous studies, we conducted molecular simulations to predict the force-extension response of a highly accurate lamin dimer structure obtained through X-ray diffraction crystallography experimentation. Furthermore, we devised a semiflexible worm-like chain extension-force model of lamin dimer as a substitute, incorporating phases of initial stretching, uncoiling of the dimer coiled-coil, and transition of secondary structures. Subsequently, we developed a 2D network continuum model for the nuclear lamina by using our extension-force lamin dimer model and derived stress resultants. By comparing with experimentally measured lamina modulus, we found that the lamina network has sharp initial strain-hardening behavior. This also enabled us to carry out finite element simulations of the entire nucleus with an accurate microstructure-based nuclear lamina model. Finally, we conducted simulations of transendothelial transmigration of a nucleus and investigated the impact of varying network density and uncoiling constants on the critical force required for successful transmigration. The model allows us to incorporate the microstructure characteristics of the nuclear lamina into the nucleus model, thereby gaining insights into how laminopathies and mutations affect nuclear mechanics.

Nuclear periphery and its mechanical regulation in cell fate transitions

Cell fate changes require rewiring of transcriptional programs to generate functionally specialized cell states. Reconfiguration of transcriptional networks requires overcoming epigenetic barriers imposed by silenced heterochromatin in order to activate lineage-specific genes. Further, cell fate decisions are made in a tissue-specific context, where cells are physically linked to each other as well as to the connective tissue environment. Here, cells are continuously exposed to a multitude of mechanical forces emanating from cellular dynamics in their local microenvironments, for example through cell movements, cell divisions, tissue contractions, or fluid flow. Through their ability to deform cellular structures and activate receptors, mechanical forces can be sensed at the plasma membrane, but also at the nuclear periphery through direct or cytoskeleton-mediated deformation of the nuclear envelope. This deformation and the associated signaling is capable of triggering changes in the mechanical state of the nuclear membranes, the organization and rigidity of the underlying nuclear lamina, compaction state of chromatin, and ultimately transcription. This review focuses on the role of nuclear architecture, particularly the nuclear lamina-chromatin interface, and its mechanical regulation in cell fate decisions as well as its physiological role in development and cellular reprogramming.

N-terminal tags impair the ability of Lamin A to provide structural support to the nucleus

Lamins are intermediate filament proteins that contribute to numerous cellular functions, including nuclear morphology and mechanical stability. The N-terminal head domain of lamin is critical for higher order filament assembly and function, yet the effects of commonly used N-terminal tags on lamin function remain largely unexplored. Here, we systematically studied the effect of two differently sized tags on Lamin A (LaA) function in a mammalian cell model engineered to allow for precise control of expression of tagged lamin proteins. Untagged, FLAG-tagged, and GFP-tagged LaA completely rescued nuclear shape defects when expressed at similar levels in lamin A/C-deficient ( Lmna -/- ) MEFs, and all LaA constructs prevented increased nuclear envelope (NE) ruptures in these cells. N-terminal tags, however, altered the nuclear localization of LaA and impaired the ability of LaA to restore nuclear deformability and to recruit Emerin to the nuclear membrane in Lmna -/- MEFs. Our finding that tags impede some LaA functions but not others may explain the partial loss of function phenotypes when tagged lamins are expressed in model organisms and should caution researchers using tagged lamins to study the nucleus.

Tau beyond Tangles: DNA Damage Response and Cytoskeletal Protein Crosstalk on Neurodegeneration

Neurons in the brain are continuously exposed to various sources of DNA damage. Although the mechanisms of DNA damage repair in mitotic cells have been extensively characterized, the repair pathways in post-mitotic neurons are still largely elusive. Moreover, inaccurate repair can result in deleterious mutations, including deletions, insertions, and chromosomal translocations, ultimately compromising genomic stability. Since neurons are terminally differentiated cells, they cannot employ homologous recombination (HR) for double-strand break (DSB) repair, suggesting the existence of neuron-specific repair mechanisms. Our research has centered on the microtubule-associated protein tau (MAPT), a crucial pathological protein implicated in neurodegenerative diseases, and its interplay with neurons' DNA damage response (DDR). This review aims to provide an updated synthesis of the current understanding of the complex interplay between DDR and cytoskeletal proteins in neurons, with a particular focus on the role of tau in neurodegenerative disorders.

Pervasive nuclear envelope ruptures precede ECM signaling and disease onset without activating cGAS-STING in Lamin-cardiomyopathy mice

Nuclear envelope (NE) ruptures are emerging observations in Lamin-related dilated cardiomyopathy, an adult-onset disease caused by loss-of-function mutations in Lamin A/C, a nuclear lamina component. Here, we test a prevailing hypothesis that NE ruptures trigger the pathological cGAS-STING cytosolic DNA-sensing pathway using a mouse model of Lamin cardiomyopathy. The reduction of Lamin A/C in cardio-myocyte of adult mice causes pervasive NE ruptures in cardiomyocytes, preceding inflammatory transcription, fibrosis, and fatal dilated cardiomyopathy. NE ruptures are followed by DNA damage accumulation without causing immediate cardiomyocyte death. However, cGAS-STING-dependent inflammatory signaling remains inactive. Deleting cGas or Sting does not rescue cardiomyopathy in the mouse model. The lack of cGAS-STING activation is likely due to the near absence of cGAS expression in adult cardiomyocytes at baseline. Instead, extracellular matrix (ECM) signaling is activated and predicted to initiate pro-inflammatory communication from Lamin-reduced cardiomyocytes to fibroblasts. Our work nominates ECM signaling, not cGAS-STING, as a potential inflammatory contributor in Lamin cardiomyopathy.

Proximal telomeric decompaction due to telomere shortening drives FOXC1-dependent myocardial senescence

Telomeres, TTAGGGn DNA repeat sequences located at the ends of eukaryotic chromosomes, play a pivotal role in aging and are targets of DNA damage response. Although we and others have demonstrated presence of short telomeres in genetic cardiomyopathic and heart failure cardiomyocytes, little is known about the role of telomere lengths in cardiomyocyte. Here, we demonstrate that in heart failure patient cardiomyocytes, telomeres are shortened compared to healthy controls. We generated isogenic human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs) with short telomeres (sTL-CMs) and normal telomeres (nTL-CMs) as model. Compared to nTL-CMs, short telomeres result in cardiac dysfunction and expression of senescent markers. Using Hi-C and RNASeq, we observe that short telomeres induced TAD insulation decrease near telomeric ends and this correlated with a transcription upregulation in sTL-CMs. FOXC1, a key transcription factor involved in early cardiogenesis, was upregulated in sTL-CMs and its protein levels were negatively correlated with telomere lengths in heart failure patients. Overexpression of FOXC1 induced hiPSC-CM aging, mitochondrial and contractile dysfunction; knockdown of FOXC1 rescued these phenotypes. Overall, the work presented demonstrate that increased chromatin accessibility due to telomere shortening resulted in the induction of FOXC1-dependent expression network responsible for contractile dysfunction and myocardial senescence.

Multi-level transcriptomic analysis of LMNA -related dilated cardiomyopathy identifies disease-driving processes

LMNA- related dilated cardiomyopathy ( LMNA -DCM) is one of the most severe forms of DCM. The incomplete understanding of the molecular disease mechanisms results in lacking treatment options, leading to high mortality amongst patients. Here, using an inducible, cardiomyocyte-specific lamin A/C depletion mouse model, we conducted a comprehensive transcriptomic study, combining both bulk and single nucleus RNA sequencing, and spanning LMNA -DCM disease progression, to identify potential disease drivers. Our refined analysis pipeline identified 496 genes already misregulated early in disease. The expression of these genes was largely driven by disease specific cardiomyocyte sub-populations and involved biological processes mediating cellular response to DNA damage, cytosolic pattern recognition, and innate immunity. Indeed, DNA damage in LMNA -DCM hearts was significantly increased early in disease and correlated with reduced cardiomyocyte lamin A levels. Activation of cytosolic pattern recognition in cardiomyocytes was independent of cGAS, which is rarely expressed in cardiomyocytes, but likely occurred downstream of other pattern recognition sensors such as IFI16. Altered gene expression in cardiac fibroblasts and immune cell infiltration further contributed to tissue-wide changes in gene expression. Our transcriptomic analysis further predicted significant alterations in cell-cell communication between cardiomyocytes, fibroblasts, and immune cells, mediated through early changes in the extracellular matrix (ECM) in the LMNA -DCM hearts. Taken together, our work suggests a model in which nuclear damage in cardiomyocytes leads to activation of DNA damage responses, cytosolic pattern recognition pathway, and other signaling pathways that activate inflammation, immune cell recruitment, and transcriptional changes in cardiac fibroblasts, which collectively drive LMNA -DCM pathogenesis.

YAP dysregulation triggers hypertrophy by CCN2 secretion and TGFβ uptake in human pluripotent stem cell-derived cardiomyocytes

Hypertrophy Cardiomyopathy (HCM) is the most prevalent hereditary cardiovascular disease - affecting >1:500 individuals. Advanced forms of HCM clinically present with hypercontractility, hypertrophy and fibrosis. Several single-point mutations in b-myosin heavy chain (MYH7) have been associated with HCM and increased contractility at the organ level. Different MYH7 mutations have resulted in increased, decreased, or unchanged force production at the molecular level. Yet, how these molecular kinetics link to cell and tissue pathogenesis remains unclear. The Hippo Pathway, specifically its effector molecule YAP, has been demonstrated to be reactivated in pathological hypertrophic growth. We hypothesized that changes in force production (intrinsically or extrinsically) directly alter the homeostatic mechano-signaling of the Hippo pathway through changes in stresses on the nucleus. Using human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), we asked whether homeostatic mechanical signaling through the canonical growth regulator, YAP, is altered 1) by changes in the biomechanics of HCM mutant cardiomyocytes and 2) by alterations in the mechanical environment. We use genetically edited hiPSC-CM with point mutations in MYH7 associated with HCM, and their matched controls, combined with micropatterned traction force microscopy substrates to confirm the hypercontractile phenotype in MYH7 mutants. We next modulate contractility in healthy and disease hiPSC-CMs by treatment with positive and negative inotropic drugs and demonstrate a correlative relationship between contractility and YAP activity. We further demonstrate the activation of YAP in both HCM mutants and healthy hiPSC-CMs treated with contractility modulators is through enhanced nuclear deformation. We conclude that the overactivation of YAP, possibly initiated and driven by hypercontractility, correlates with excessive CCN2 secretion (connective tissue growth factor), enhancing cardiac fibroblast/myofibroblast transition and production of known hypertrophic signaling molecule TGFβ. Our study suggests YAP being an indirect player in the initiation of hypertrophic growth and fibrosis in HCM. Our results provide new insights into HCM progression and bring forth a testbed for therapeutic options in treating HCM.

DNA repair deficiencies and neurodegeneration

Neurodegenerative diseases are the second most prevalent cause of death in industrialized countries. Alzheimer's Disease is the most widespread and also most acknowledged form of dementia today. Together with Parkinson's Disease they account for over 90 % cases of neurodegenerative disorders caused by proteopathies. Far less known are the neurodegenerative pathologies in DNA repair deficiency syndromes. Such diseases like Cockayne - or Werner Syndrome are described as progeroid syndromes - diseases that cause the premature ageing of the affected persons, and there are clear implications of such diseases in neurologic dysfunction and degeneration. In this review, we aim to draw the attention on commonalities between proteopathy-associated neurodegeneration and neurodegeneration caused by DNA repair defects and discuss how mitochondria are implicated in the development of both disorder classes. Furthermore, we highlight how nematodes are a valuable and indispensable model organism to study conserved neurodegenerative processes in a fast-forward manner.

The nexus of nuclear envelope dynamics, circular economy and cancer cell pathophysiology

The nuclear envelope (NE) is a critical component in maintaining the function and structure of the eukaryotic nucleus. The NE and lamina are disassembled during each cell cycle to enable an open mitosis. Nuclear architecture construction and deconstruction is a prime example of a circular economy, as it fulfills a highly efficient recycling program bound to continuous assessment of the quality and functionality of the building blocks. Alterations in the nuclear dynamics and lamina structure have emerged as important contributors to both oncogenic transformation and cancer progression. However, the knowledge of the NE breakdown and reassembly is still limited to a fraction of participating proteins and complexes. As cancer cells contain highly diverse nuclei in terms of DNA content, but also in terms of nuclear number, size, and shape, it is of great interest to understand the intricate relationship between these nuclear features in cancer cell pathophysiology. In this review, we provide insights into how those NE dynamics are regulated, and how lamina destabilization processes may alter the NE circular economy. Moreover, we expand the knowledge of the lamina-associated domain region by using strategic algorithms, including Artificial Intelligence, to infer protein associations, assess their function and location, and predict cancer-type specificity with implications for the future of cancer diagnosis, prognosis and treatment. Using this approach we identified NUP98 and MECP2 as potential proteins that exhibit upregulation in Acute Myeloid Leukemia (LAML) patients with implications for early diagnosis.

Mechanically guided cell fate determination in early development

Cell fate determination, a vital process in early development and adulthood, has been the focal point of intensive investigation over the past decades. Its importance lies in its critical role in shaping various and diverse cell types during embryonic development and beyond. Exploration of cell fate determination started with molecular and genetic investigations unveiling central signaling pathways and molecular regulatory networks. The molecular studies into cell fate determination yielded an overwhelming amount of information invoking the notion of the complexity of cell fate determination. However, recent advances in the framework of biomechanics have introduced a paradigm shift in our understanding of this intricate process. The physical forces and biochemical interplay, known as mechanotransduction, have been identified as a pivotal drive influencing cell fate decisions. Certainly, the integration of biomechanics into the process of cell fate pushed our understanding of the developmental process and potentially holds promise for therapeutic applications. This integration was achieved by identifying physical forces like hydrostatic pressure, fluid dynamics, tissue stiffness, and topography, among others, and examining their interplay with biochemical signals. This review focuses on recent advances investigating the relationship between physical cues and biochemical signals that control cell fate determination during early embryonic development.

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.

Perinuclear damage from nuclear envelope deterioration elicits stress responses that contribute to LMNA cardiomyopathy

Mutations in the LMNA gene encoding lamins A/C cause an array of tissue-selective diseases, with the heart being the most commonly affected organ. Despite progress in understanding the perturbations emanating from LMNA mutations, an integrative understanding of the pathogenesis underlying cardiac dysfunction remains elusive. Using a novel conditional deletion model capable of translatome profiling, we observed that cardiomyocyte-specific Lmna deletion in adult mice led to rapid cardiomyopathy with pathological remodeling. Before cardiac dysfunction, Lmna-deleted cardiomyocytes displayed nuclear abnormalities, Golgi dilation/fragmentation, and CREB3-mediated stress activation. Translatome profiling identified MED25 activation, a transcriptional cofactor that regulates Golgi stress. Autophagy is disrupted in the hearts of these mice, which can be recapitulated by disrupting the Golgi. Systemic administration of modulators of autophagy or ER stress significantly delayed cardiac dysfunction and prolonged survival. These studies support a hypothesis wherein stress responses emanating from the perinuclear space contribute to the LMNA cardiomyopathy development.

Nuclear Softness Promotes the Metastatic Potential of Large-Nucleated Colorectal Cancer Cells via the ErbB4-Akt1-Lamin A/C Signaling Pathway

Abnormal nuclear enlargement is a diagnostic and physical hallmark of malignant tumors. Large nuclei are positively associated with an increased risk of developing metastasis; however, a large nucleus is inevitably more resistant to cell migration due to its size. The present study demonstrated that the nuclear size of primary colorectal cancer (CRC) cells at an advanced stage was larger than cells at an early stage. In addition, the nuclei of CRC liver metastases were larger than those of the corresponding primary CRC tissues. CRC cells were sorted into large-nucleated cells (LNCs) and small-nucleated cells (SNCs). Purified LNCs exhibited greater constricted migratory and metastatic capacity than SNCs in vitro and in vivo. Mechanistically, ErbB4 was highly expressed in LNCs, which phosphorylated lamin A/C at serine 22 via the ErbB4-Akt1 signaling pathway. Furthermore, the level of phosphorylated lamin A/C was a negative determinant of nuclear stiffness. Taken together, CRC LNCs possessed greater constricted migratory and metastatic potential than SNCs due to ErbB4-Akt1-mediated lamin A/C phosphorylation and nuclear softening. These results may provide a potential treatment strategy for tumor metastasis by targeting nuclear stiffness in patients with cancer, particularly CRC.

Pervasive nuclear envelope ruptures precede ECM signaling and disease onset without activating cGAS-STING in Lamin-cardiomyopathy mice

Nuclear envelope (NE) ruptures are emerging observations in Lamin-related dilated cardiomyopathy, an adult-onset disease caused by loss-of-function mutations in Lamin A/C, a nuclear lamina component. Here, we tested a prevailing hypothesis that NE ruptures trigger pathological cGAS-STING cytosolic DNA-sensing pathway, using a mouse model of Lamin-cardiomyopathy. Reduction of Lamin A/C in cardiomyocytes of adult mice caused pervasive NE ruptures in cardiomyocytes, preceding inflammatory transcription, fibrosis, and fatal dilated cardiomyopathy. NE ruptures were followed by DNA damage accumulation without causing immediate cardiomyocyte death. However, cGAS-STING-dependent inflammatory signaling remained inactive. Deleting cGas or Sting did not rescue cardiomyopathy. The lack of cGAS-STING activation was likely due to the near absence of cGAS expression in adult cardiomyocytes at baseline. Instead, extracellular matrix (ECM) signaling was activated and predicted to initiate pro-inflammatory communication from Lamin-reduced cardiomyocytes to fibroblasts. Our work nominates ECM signaling, not cGAS-STING, as a potential inflammatory contributor in Lamin-cardiomyopathy.

Cytoskeletal rearrangement precedes nucleolar remodeling during adipogenesis

Differentiation of adipose progenitor cells into mature adipocytes entails a dramatic reorganization of the cellular architecture to accommodate lipid storage into cytoplasmic lipid droplets. Lipid droplets occupy most of the adipocyte volume, compressing the nucleus beneath the plasma membrane. How this cellular remodeling affects sub-nuclear structure, including size and number of nucleoli, remains unclear. We describe the morphological remodeling of the nucleus and the nucleolus during in vitro adipogenic differentiation of primary human adipose stem cells. We find that cell cycle arrest elicits a remodeling of nucleolar structure which correlates with a decrease in protein synthesis. Strikingly, triggering cytoskeletal rearrangements mimics the nucleolar remodeling observed during adipogenesis. Our results point to nucleolar remodeling as an active, mechano-regulated mechanism during adipogenic differentiation and demonstrate a key role of the actin cytoskeleton in defining nuclear and nucleolar architecture in differentiating human adipose stem cells.

Harnessing molecular mechanism for precision medicine in dilated cardiomyopathy caused by a mutation in troponin T

Familial dilated cardiomyopathy (DCM) is frequently caused by autosomal dominant point mutations in genes involved in diverse cellular processes, including sarcomeric contraction. While patient studies have defined the genetic landscape of DCM, genetics are not currently used in patient care, and patients receive similar treatments regardless of the underlying mutation. It has been suggested that a precision medicine approach based on the molecular mechanism of the underlying mutation could improve outcomes; however, realizing this approach has been challenging due to difficulties linking genotype and phenotype and then leveraging this information to identify therapeutic approaches. Here, we used multiscale experimental and computational approaches to test whether knowledge of molecular mechanism could be harnessed to connect genotype, phenotype, and drug response for a DCM mutation in troponin T, deletion of K210. Previously, we showed that at the molecular scale, the mutation reduces thin filament activation. Here, we used computational modeling of this molecular defect to predict that the mutant will reduce cellular and tissue contractility, and we validated this prediction in human cardiomyocytes and engineered heart tissues. We then used our knowledge of molecular mechanism to computationally model the effects of a small molecule that can activate the thin filament. We demonstrate experimentally that the modeling correctly predicts that the small molecule can partially rescue systolic dysfunction at the expense of diastolic function. Taken together, our results demonstrate how molecular mechanism can be harnessed to connect genotype and phenotype and inspire strategies to optimize mechanism-based therapeutics for DCM.

Targeting Nuclear Mechanics Mitigates the Fibroblast Invasiveness in Pathological Dermal Scars Induced by Matrix Stiffening

Pathological dermal scars such as keloids present significant clinical challenges lacking effective treatment options. Given the distinctive feature of highly stiffened scar tissues, deciphering how matrix mechanics regulate pathological progression can inform new therapeutic strategies. Here, it is shown that pathological dermal scar keloid fibroblasts display unique metamorphoses to stiffened matrix. Compared to normal fibroblasts, keloid fibroblasts show high sensitivity to stiffness rather than biochemical stimulation, activating cytoskeletal-to-nuclear mechanosensing molecules. Notably, keloid fibroblasts on stiff matrices exhibit nuclear softening, concomitant with reduced lamin A/C expression, and disrupted anchoring of lamina-associated chromatin. This nuclear softening, combined with weak adhesion and high contractility, facilitates the invasive migration of keloid fibroblasts through confining matrices. Inhibiting lamin A/C-driven nuclear softening, via lamin A/C overexpression or actin disruption, mitigates such invasiveness of keloid fibroblasts. These findings highlight the significance of the nuclear mechanics of keloid fibroblasts in scar pathogenesis and propose lamin A/C as a potential therapeutic target for managing pathological scars.

A high-content screen reveals new regulators of nuclear membrane stability

Nuclear membrane rupture is a physiological response to multiple in vivo processes, such as cell migration, that can cause extensive genome instability and upregulate invasive and inflammatory pathways. However, the underlying molecular mechanisms of rupture are unclear and few regulators have been identified. In this study, we developed a reporter that is size excluded from re-compartmentalization following nuclear rupture events. This allows for robust detection of factors influencing nuclear integrity in fixed cells. We combined this with an automated image analysis pipeline in a high-content siRNA screen to identify new proteins that both increase and decrease nuclear rupture frequency in cancer cells. Pathway analysis identified an enrichment of nuclear membrane and ER factors in our hits and we demonstrate that one of these, the protein phosphatase CTDNEP1, is required for nuclear stability. Analysis of known rupture determinants, including an automated quantitative analysis of nuclear lamina gaps, are consistent with CTDNEP1 acting independently of actin and nuclear lamina organization. Our findings provide new insights into the molecular mechanism of nuclear rupture and define a highly adaptable program for rupture analysis that removes a substantial barrier to new discoveries in the field.

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.

Actin contraction controls nuclear blebbing and rupture independent of actin confinement

The nucleus is a mechanically stable compartment of the cell that contains the genome and performs many essential functions. Nuclear mechanical components chromatin and lamins maintain nuclear shape, compartmentalization, and function by resisting antagonistic actin contraction and confinement. Studies have yet to compare chromatin and lamins perturbations side-by-side as well as modulated actin contraction while holding confinement constant. To accomplish this, we used nuclear localization signal green fluorescent protein to measure nuclear shape and rupture in live cells with chromatin and lamin perturbations. We then modulated actin contraction while maintaining actin confinement measured by nuclear height. Wild type, chromatin decompaction, and lamin B1 null present bleb-based nuclear deformations and ruptures dependent on actin contraction and independent of actin confinement. Actin contraction inhibition by Y27632 decreased nuclear blebbing and ruptures while activation by CN03 increased rupture frequency. Lamin A/C null results in overall abnormal shape also reliant on actin contraction, but similar blebs and ruptures as wild type. Increased DNA damage is caused by nuclear blebbing or abnormal shape which can be relieved by inhibition of actin contraction which rescues nuclear shape and decreases DNA damage levels in all perturbations. Thus, actin contraction drives nuclear blebbing, bleb-based ruptures, and abnormal shape independent of changes in actin confinement.

Nuclear growth and import can be uncoupled

What drives nuclear growth? Studying nuclei assembled in Xenopus egg extract and focusing on importin α/β-mediated nuclear import, we show that, while import is required for nuclear growth, nuclear growth and import can be uncoupled when chromatin structure is manipulated. Nuclei treated with micrococcal nuclease to fragment DNA grew slowly despite exhibiting little to no change in import rates. Nuclei assembled around axolotl chromatin with 20-fold more DNA than Xenopus grew larger but imported more slowly. Treating nuclei with reagents known to alter histone methylation or acetylation caused nuclei to grow less while still importing to a similar extent or to grow larger without significantly increasing import. Nuclear growth but not import was increased in live sea urchin embryos treated with the DNA methylator N-nitrosodimethylamine. These data suggest that nuclear import is not the primary driving force for nuclear growth. Instead, we observed that nuclear blebs expanded preferentially at sites of high chromatin density and lamin addition, whereas small Benzonase-treated nuclei lacking DNA exhibited reduced lamin incorporation into the nuclear envelope. In summary, we report experimental conditions where nuclear import is not sufficient to drive nuclear growth, hypothesizing that this uncoupling is a result of altered chromatin structure.

Genome maintenance meets mechanobiology

Genome stability is key for healthy cells in healthy organisms, and deregulated maintenance of genome integrity is a hallmark of aging and of age-associated diseases including cancer and neurodegeneration. To maintain a stable genome, genome surveillance and repair pathways are closely intertwined with cell cycle regulation and with DNA transactions that occur during transcription and DNA replication. Coordination of these processes across different time and length scales involves dynamic changes of chromatin topology, clustering of fragile genomic regions and repair factors into nuclear repair centers, mobilization of the nuclear cytoskeleton, and activation of cell cycle checkpoints. Here, we provide a general overview of cell cycle regulation and of the processes involved in genome duplication in human cells, followed by an introduction to replication stress and to the cellular responses elicited by perturbed DNA synthesis. We discuss fragile genomic regions that experience high levels of replication stress, with a particular focus on telomere fragility caused by replication stress at the ends of linear chromosomes. Using alternative lengthening of telomeres (ALT) in cancer cells and ALT-associated PML bodies (APBs) as examples of replication stress-associated clustered DNA damage, we discuss compartmentalization of DNA repair reactions and the role of protein properties implicated in phase separation. Finally, we highlight emerging connections between DNA repair and mechanobiology and discuss how biomolecular condensates, components of the nuclear cytoskeleton, and interfaces between membrane-bound organelles and membraneless macromolecular condensates may cooperate to coordinate genome maintenance in space and time.

Heterologous expression of Dictyostelium discoideum NE81 in mouse embryo fibroblasts reveals conserved mechanoprotective roles of lamins

Lamins are nuclear intermediate filament proteins that are ubiquitously found in metazoan cells, where they contribute to nuclear morphology, stability, and gene expression. Lamin-like sequences have recently been identified in distantly related eukaryotes, but it remains unclear whether these proteins share conserved functions with the lamins found in metazoans. Here, we investigate conserved features between metazoan and amoebozoan lamins using a genetic complementation system to express the Dictyostelium discoideum lamin-like protein NE81 in mammalian cells lacking either specific lamins or all endogenous lamins. We report that NE81 localizes to the nucleus in cells lacking Lamin A/C, and that NE81 expression improves nuclear circularity, reduces nuclear deformability, and prevents nuclear envelope rupture in these cells. However, NE81 did not completely rescue loss of Lamin A/C, and was unable to restore normal distribution of metazoan lamin interactors, such as emerin and nuclear pore complexes, which are frequently displaced in Lamin A/C deficient cells. Collectively, our results indicate that the ability of lamins to modulate the morphology and mechanical properties of nuclei may have been a feature present in the common ancestor of Dictyostelium and animals, whereas other, more specialized interactions may have evolved more recently in metazoan lineages.

Mechanisms by which barrier-to-autointegration factor regulates dynamics of nucleocytoplasmic leakage and membrane repair following nuclear envelope rupture

The nuclear envelope (NE) creates a barrier between the cytosol and nucleus during interphase that is key for cellular compartmentalization and protecting genomic DNA. NE rupture can expose genomic DNA to the cytosol and allow admixture of the nuclear and cytosolic constituents, a proposed mechanism of cancer and NE-associated diseases. Barrier-to-autointegration factor (BAF) is a DNA-binding protein that localizes to NE ruptures where it recruits LEM-domain proteins, A-type lamins, and participates in rupture repair. To further reveal the mechanisms by which BAF responds to and aids in repairing NE ruptures, we investigated known properties of BAF including LEM domain binding, lamin binding, compartmentalization, phosphoregulation of DNA binding, and BAF dimerization. We demonstrate that it is the cytosolic population of BAF that functionally repairs NE ruptures, and phosphoregulation of BAF's DNA-binding that enables its ability to facilitate that repair. Interestingly, BAF's LEM or lamin binding activity appears dispensable for its role in functional repair. Furthermore, we demonstrate that BAF functions to reduce the extent of leakage though NE ruptures, suggesting that BAF effectively forms a diffusion barrier prior to NE repair. Collectively, these results enhances our knowledge of the mechanisms by which BAF responds to NE ruptures and facilitates their repair.

Lamins as structural nuclear elements through evolution

Lamins are nuclear intermediate filament proteins with important, well-established roles in humans and other vertebrates. Lamins interact with DNA and numerous proteins at the nuclear envelope to determine the mechanical properties of the nucleus, coordinate chromatin organization, and modulate gene expression. Many of these functions are conserved in the lamin homologs found in basal metazoan organisms, including Drosophila and Caenorhabditis elegans. Lamin homologs have also been recently identified in non-metazoans, like the amoeba Dictyostelium discoideum, yet how these proteins compare functionally to the metazoan isoforms is only beginning to emerge. A better understanding of these distantly related lamins is not only valuable for a more complete picture of eukaryotic evolution, but may also provide new insights into the function of vertebrate lamins.

Intermediate filaments in the heart: The dynamic duo of desmin and lamins orchestrates mechanical force transmission

The intermediate filament (IF) cytoskeleton supports cellular structural integrity, particularly in response to mechanical stress. The most abundant IF proteins in mature cardiomyocytes are desmin and lamins. The desmin network tethers the contractile apparatus and organelles to the nuclear envelope and the sarcolemma, while lamins, as components of the nuclear lamina, provide structural stability to the nucleus and the genome. Mutations in desmin or A-type lamins typically result in cardiomyopathies and recent studies emphasized the synergistic roles of desmin and lamins in the maintenance of nuclear integrity in cardiac myocytes. Here we explore the emerging roles of the interdependent relationship between desmin and lamins in providing resilience to nuclear structure while transducing extracellular mechanical cues into the nucleus.

Naturally occurring canine laminopathy leading to a dilated and fibrosing cardiomyopathy in the Nova Scotia Duck Tolling Retriever

Dilated cardiomyopathy (DCM) is characterized by decreased systolic function and dilation of one or both ventricles, often leading to heart failure or sudden death. Two 10-month-old sibling Nova Scotia Duck Tolling Retrievers (NSDTR) died acutely with evidence of dilated cardiomyopathy with myocardial fibrosis. Association analysis using two cases and 35 controls identified three candidate regions homozygous in the two cases. Whole genome sequencing identified a frameshift deletion in the LMNA gene (NC_049228.1:g.41688530del, NP_001274080:p.(Asp576ThrfsTer124)). Three retrospectively identified NSDTRs with sudden death before 2 years of age and severe myocardial fibrosis were also homozygous for the deletion. One 5 year old with sudden death and myocardial fibrosis was heterozygous for the deletion. This variant was not identified in 722 dogs of other breeds, nor was it identified to be homozygous in 784 NSDTR. LMNA codes for lamin A/C proteins, which are type V intermediate filaments that provide structural support to the nuclear membrane. In humans, LMNA variants can cause DCM with sudden death as well as diseases of striated muscles, lipodystrophy, neuropathies, and accelerated aging disorders. This frameshift deletion is predicted to affect processing of prelamin A into lamin A. Pedigree analysis in the NSDTR and functional evaluation of heterozygotes is consistent with a predominantly recessive mode of inheritance and possibly low penetrance in heterozygotes in contrast to people, where most pathogenic LMNA variants are dominantly inherited.

Scaffold, mechanics and functions of nuclear lamins

Nuclear lamins are type-V intermediate filaments that are involved in many nuclear processes. In mammals, A- and B-type lamins assemble into separate physical meshwork underneath the inner nuclear membrane, the nuclear lamina, with some residual fraction localized within the nucleoplasm. Lamins are the major part of the nucleoskeleton, providing mechanical strength and flexibility to protect the genome and allow nuclear deformability, while also contributing to gene regulation via interactions with chromatin. While lamins are the evolutionary ancestors of all intermediate filament family proteins, their ultimate filamentous assembly is markedly different from their cytoplasmic counterparts. Interestingly, hundreds of genetic mutations in the lamina proteins have been causally linked with a broad range of human pathologies, termed laminopathies. These include muscular, neurological and metabolic disorders, as well as premature aging diseases. Recent technological advances have contributed to resolving the filamentous structure of lamins and the corresponding lamina organization. In this review, we revisit the multiscale lamin organization and discuss its implications on nuclear mechanics and chromatin organization within lamina-associated domains.

Heterologous expression of Dictyostelium discoideum NE81 in mouse embryo fibroblasts reveals conserved mechanoprotective roles of lamins

Lamins are nuclear intermediate filament proteins that are ubiquitously found in metazoan cells, where they contribute to nuclear morphology, stability, and gene expression. Lamin-like sequences have recently been identified in distantly related eukaryotes, but it remains unclear if these proteins share conserved functions with the lamins found in metazoans. Here, we investigate conserved features between metazoan and amoebozoan lamins using a genetic complementation system to express the Dictyostelium discoideum lamin-like protein NE81 in mammalian cells lacking either specific lamins or all endogenous lamins. We report that NE81 localizes to the nucleus in cells lacking Lamin A/C, and that NE81 expression improves nuclear circularity, reduces nuclear deformability, and prevents nuclear envelope rupture in these cells. However, NE81 did not completely rescue loss of Lamin A/C, and was unable to restore normal distribution of metazoan lamin interactors, such as emerin and nuclear pore complexes, which are frequently displaced in Lamin A/C deficient cells. Collectively, our results indicate that the ability of lamins to modulate the morphology and mechanical properties of nuclei may have been a feature present in the common ancestor of Dictyostelium and animals, whereas other, more specialized interactions may have evolved more recently in metazoan lineages.

Mechanical stress confers nuclear and functional changes in derived leukemia cells from persistent confined migration

Nuclear deformability plays a critical role in cell migration. During this process, the remodeling of internal components of the nucleus has a direct impact on DNA damage and cell behavior; however, how persistent migration promotes nuclear changes leading to phenotypical and functional consequences remains poorly understood. Here, we described that the persistent migration through physical barriers was sufficient to promote permanent modifications in migratory-altered cells. We found that derived cells from confined migration showed changes in lamin B1 localization, cell morphology and transcription. Further analysis confirmed that migratory-altered cells showed functional differences in DNA repair, cell response to chemotherapy and cell migration in vivo homing experiments. Experimental modulation of actin polymerization affected the redistribution of lamin B1, and the basal levels of DNA damage in migratory-altered cells. Finally, since major nuclear changes were present in migratory-altered cells, we applied a multidisciplinary biochemical and biophysical approach to identify that confined conditions promoted a different biomechanical response of the nucleus in migratory-altered cells. Our observations suggest that mechanical compression during persistent cell migration has a role in stable nuclear and genomic alterations that might handle the genetic instability and cellular heterogeneity in aging diseases and cancer.