Nuclear positioning

Perinuclear organelle trauma at the nexus of cardiomyopathy pathogenesis arising from loss of function LMNA mutation

Over the past 25 years, nuclear envelope (NE) perturbations have been reported in various experimental models with mutations in the LMNA gene. Although the hypothesis that NE perturbations from LMNA mutations are a fundamental feature of striated muscle damage has garnered wide acceptance, the molecular sequalae provoked by the NE damage and how they underlie disease pathogenesis such as cardiomyopathy (LMNA cardiomyopathy) remain poorly understood. We recently shed light on one such consequence, by employing a cardiomyocyte-specific Lmna deletion in vivo in the adult heart. We observed extensive NE perturbations prior to cardiac function deterioration with collateral damage in the perinuclear space. The Golgi is particularly affected, leading to cytoprotective stress responses that are likely disrupted by the progressive deterioration of the Golgi itself. In this review, we discuss the etiology of LMNA cardiomyopathy with perinuclear 'organelle trauma' as the nexus between NE damage and disease pathogenesis.

Nuclear deformation by microtubule molecular motors

We present a model to calculate the displacement and extension of deformable cellular cargo pulled by molecular motors stepping along cytoskeletal filaments. We consider the case of a single type of molecular motor and cytoskeletal filaments oriented in one dimension in opposite directions on either side of a cargo. We model a deformable cargo as a simple elastic spring. We simulate this tug-of-war simple exclusion process model using a Monte Carlo Gillespie algorithm and calculate the displacement and extension of the cargo for different configurations of motors and filaments. We apply our model to kinesin-1 motors on microtubules to investigate whether they are strong enough to translocate and deform the largest cellular cargo, the nucleus. We show that the extension caused by motors on a single microtubule saturates for larger numbers of motors but that the extension and displacement scales linearly with the number of microtubules. We also show how the binding and unbinding behaviors of molecular motors on microtubule filaments affect the nuclear deformation. Our modelling results correspond to experiments on cells treated with the drug kinesore, which is thought to increase rescue events resulting in more stable microtubules and more active kinesin-1 molecular motors bound to them. Both the experiments and our simulations result in nuclear deformation.

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.

Emerging magic bullet: subcellular organelle-targeted cancer therapy

The therapeutic efficacy of anticancer drugs heavily relies on their concentration and retention at the corresponding target site. Hence, merely increasing the cellular concentration of drugs is insufficient to achieve satisfactory therapeutic outcomes, especially for the drugs that target specific intracellular sites. This necessitates the implementation of more precise targeting strategies to overcome the limitations posed by diffusion distribution and nonspecific interactions within cells. Consequently, subcellular organelle-targeted cancer therapy, characterized by its exceptional precision, have emerged as a promising approach to eradicate cancer cells through the specific disruption of subcellular organelles. Owing to several advantages including minimized dosage and side effect, optimized efficacy, and reversal of multidrug resistance, subcellular organelle-targeted therapies have garnered significant research interest in recent years. In this review, we comprehensively summarize the distribution of drug targets, targeted delivery strategies at various levels, and sophisticated strategies for targeting specific subcellular organelles. Additionally, we highlight the significance of subcellular targeting in cancer therapy and present essential considerations for its clinical translation.

The KASH protein UNC-83 differentially regulates kinesin-1 activity to control developmental stage-specific nuclear migration

Nuclear migration plays a fundamental role in development, requiring precise spatiotemporal control of bidirectional movement through dynein and kinesin motors. Here, we uncover a mechanism for developmental regulation of nuclear migration directionality. The nuclear envelope KASH protein UNC-83 in Caenorhabditis elegans exists in multiple isoforms that differentially control motor activity. The shorter UNC-83c isoform promotes kinesin-1-dependent nuclear movement in embryonic hyp7 precursors, while longer UNC-83a/b isoforms facilitate dynein-mediated nuclear migration in larval P cells. We demonstrate that UNC-83a's N-terminal domain functions as a kinesin-1 inhibitory module by directly binding kinesin heavy chain (UNC-116). This isoform-specific inhibition, combined with differential affinity for kinesin light chain (KLC-2), establishes a molecular switch for directional control. Together, these interdisciplinary studies reveal how alternative isoforms of cargo adaptors can generate developmental stage-specific regulation of motor activity during development.

Close cooperation between Semi1 and Semi2 proteins is essential for pronuclear positioning in Tetrahymena thermophila

During sexual reproduction in the ciliate Tetrahymena thermophila, meiosis occurs in the germline micronucleus, resulting in the formation of four haploid micronuclei. Of these, only one is selected to evade autophagy, and subsequently migrates to the membrane junction with the partner cell for reciprocal pronuclear exchange. We previously demonstrated that the transmembrane protein Semi1 is essential for this nuclear migration. Semi1 is specifically expressed in mating cells and localizes to the periphery of the selected nucleus. Loss of Semi1 disrupts nuclear attachment to the junction, leading to infertility. However, the mechanism by which Semi1 positions the nucleus at the junction remains unclear. Here, we report that the Semi1-interacting protein, Semi2, is also necessary for proper nuclear positioning. Deletion of Semi2 results in the same nuclear mislocalization phenotype and infertility observed in Semi1 mutant cells. Semi2 colocalizes with Semi1, but in the absence of Semi1, Semi2 fails to exhibit perinuclear localization. The selected nucleus anchors to microtubules prior to migration, a process dependent on both Semi1 and Semi2. We propose a model in which Semi1 recruits Semi2 to the selected nucleus, facilitating the interaction between the nucleus and microtubules required for proper nuclear positioning at the membrane junction.

Mechanical Forces, Nucleus, Chromosomes, and Chromatin

Individual cells and cells within the tissues and organs constantly face mechanical challenges, such as tension, compression, strain, shear stress, and the rigidity of cellular and extracellular surroundings. Besides the external mechanical forces, cells and their components are also subjected to intracellular mechanical forces, such as pulling, pushing, and stretching, created by the sophisticated force-generation machinery of the cytoskeleton and molecular motors. All these mechanical stressors switch on the mechanotransduction pathways, allowing cells and their components to respond and adapt. Mechanical force-induced changes at the cell membrane and cytoskeleton are also transmitted to the nucleus and its nucleoskeleton, affecting nucleocytoplasmic transport, chromatin conformation, transcriptional activity, replication, and genome, which, in turn, orchestrate cellular mechanical behavior. The memory of mechanoresponses is stored as epigenetic and chromatin structure modifications. The mechanical state of the cell in response to the acellular and cellular environment also determines cell identity, fate, and immune response to invading pathogens. Here, we give a short overview of the latest developments in understanding these processes, emphasizing their effects on cell nuclei, chromosomes, and chromatin.

Active microtubule-actin cross-talk mediated by a nesprin-2G-kinesin complex

Nesprin-2 Giant (N2G) is a large integral membrane protein that physically connects the nucleus to the cytoskeleton, but how N2G performs this activity to maintain nuclear positioning and drive nuclear movement is unclear. This study investigates N2G's role in nucleocytoskeletal coupling, a process critical for cellular function and development. We uncover multiple roles for N2G, including its activity as an F-actin bundler, an adapter that activates kinesin-1 motors, and a mediator of cytoskeletal cross-talk. Notably, N2G directly links kinesin-1 to F-actin, enabling the transport of actin filaments along microtubule tracks, establishing active cross-talk between the actin and microtubule cytoskeletons. These findings provide crucial insights into nuclear movement, advancing our understanding of fundamental cellular processes and their implications in development and disease.

Nuclear position controls the activity of cortical actomyosin networks powering simultaneous morphogenetic events

Tissue morphogenesis shapes epithelial sheets via cell remodelling to form functional living organisms. While the mechanisms underlying single morphogenetic events are well studied, how one tissue undergoes multiple concomitant shape changes remains largely unexplored. To tackle this, we study the process of simultaneous mesoderm folding and extension in the gastrulating Drosophila embryo. This composite transformation relies on a sharply timed reorganization of the cortical actomyosin network into two distinct subcellular tiers to drive concomitant cell apical constriction and lateral intercalation for tissue folding and convergence-extension, respectively. Here we elucidate the spatio-temporal control of the two-tiered actomyosin network. We show that, within the geometric constraints imposed by the columnar shape of mesoderm epithelial cells, the nucleus acts as a barrier shielding the lateral cortex from interactions with the microtubule network, thereby regulating the distribution of the key signalling molecule RhoGEF2. The relocation of the nucleus, driven by the contraction of the first actomyosin tier and the resulting cytoplasmic flow, unshields the lateral cortex for RhoGEF2 delivery to direct the stereotypic formation of the second tier. Thus, the nucleus and its position function as a spatio-temporal cytoskeleton compartmentalizer establishing a modular scaffold powering multiple simultaneous cell remodeling for composite morphogenesis.

NE-MTOC Formation in Skeletal Muscle Is Mbnl2-Dependent and Occurs in a Sequential and Gradual Manner

Non-centrosomal microtubule-organizing centers (ncMTOCs) are important for the function of differentiated cells. Yet, ncMTOCs are poorly understood. Previously, several components of the nuclear envelope (NE)-MTOC have been identified. However, the temporal localization of MTOC proteins and Golgi to the NE and factors controlling the switch from a centrosomal MTOC to a ncMTOC remain elusive. Here, we utilized the in vitro differentiation of C2C12 mouse myoblasts as a model system to study NE-MTOC formation. We find based on longitudinal co-immunofluorescence staining analyses that MTOC proteins are recruited in a sequential and gradual manner to the NE. AKAP9 localizes with the Golgi to the NE after the recruitment of MTOC proteins. Moreover, siRNA-mediated depletion experiments revealed that Mbnl2 is required for proper NE-MTOC formation by regulating the expression levels of AKAP6β. Finally, Mbnl2 depletion affects Pcnt isoform expression. Taken together, our results shed light on how mammals post-transcriptionally control the switch from a centrosomal MTOC to an NE-MTOC and identify Mbnl2 as a novel modulator of ncMTOCs in skeletal muscle cells.

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.

Claudin-4 Stabilizes the Genome via Nuclear and Cell-Cycle Remodeling to Support Ovarian Cancer Cell Survival

SIGNIFICANCE

High-grade serous ovarian carcinoma is marked by chromosomal instability, which can serve to promote disease progression and allow cancer to evade therapeutic insults. The report highlights the role of claudin-4 in regulating genomic instability and proposes a novel therapeutic approach to exploit claudin-4-mediated regulation.

Mechanical Remodeling of Nuclear Biomolecular Condensates

Organism health relies on cell proliferation, migration, and differentiation. These universal processes depend on cytoplasmic reorganization driven notably by the cytoskeleton and its force-generating motors. Their activity generates forces that mechanically agitate the cell nucleus and its interior. New evidence from reproductive cell biology revealed that these cytoskeletal forces can be tuned to remodel nuclear membraneless compartments, known as biomolecular condensates, and regulate their RNA processing function for the success of subsequent cell division that is critical for fertility. Both cytoskeletal and nuclear condensate reorganization are common to numerous physiological and pathological contexts, raising the possibility that mechanical remodeling of nuclear condensates may be a much broader mechanism regulating their function. Here, we review this newfound mechanism of condensate remodeling and venture into the contexts of health and disease where it may be relevant, with a focus on reproduction, cancer, and premature aging.

Targeted Conversion from Mitochondria to the Nucleus of Hydroxystyrylpyridinium by Introducing Only an Additional o-Hydroxyl Group

Aromatic cationic groups serve as crucial building blocks for the design of fluorescent probes targeting both the nucleus and mitochondria. Therefore, it is a significant challenge to develop aromatic cation-based probes that accurately image the nucleus without interference from mitochondria. However, this also presents an opportunity for rational design by modifying probes originally targeting mitochondria to redirect their targeting toward the nucleus. This study showcases the rapid development of a novel nucleus-targeting probe (DHSP) through a targeted conversion strategy based on structure modification of hydroxystyrylpyridinium (HSP), a well-established two-photon fluorescent probe that targets mitochondria. Importantly, DHSP, which is derived exclusively from introducing only an additional o-hydroxyl group into HSP, exhibits robust DNA-binding capability comparable to a commercially available nuclear dye 4',6-diamidino-2-phenylindole (DAPI). As a result, it rapidly enters the nucleus within 5 min and finds successful application in two-photon cellular and intravital imaging of the nucleus.

SIRT2 Inhibition by AGK2 Promotes Perinuclear Cytoskeletal Organisation and Reduces Invasiveness of MDA-MB-231 Triple-Negative Breast Cancer Cells in Confined In Vitro Models

Triple-negative breast cancer (TNBC) is a highly aggressive breast cancer subtype characterised by the absence of targetable hormone receptors and increased metastatic rates. As nuclear softening strongly contributes to TNBC's enhanced metastatic capacity, increasing the nuclear stiffness of TNBC cells may present a promising therapeutic avenue. Previous evidence has demonstrated the ability of Sirtuin 2 (SIRT2) inhibition to induce cytoskeletal reorganisation, a key factor in regulating nuclear mechanics. Thus, our study aimed to investigate the effect of SIRT2 inhibition on the nuclear mechanics and migratory behaviour of TNBC cells. To achieve this, SIRT2 was pharmacologically inhibited in MDA-MB-231 cells using AGK2, a SIRT2-specific inhibitor. Although SIRT2 inhibition had no effect on LINC complex composition, the AGK2-treated MDA-MB-231 cells displayed more prominent perinuclear organisations of acetylated α-tubulin, vimentin, and F-actin. Additionally, the nuclei of the AGK2-treated MDA-MB-231 cells exhibited greater resistance to collapse under osmotic shock. Scratch-wound assays also revealed that SIRT2 inhibition led to polarity defects in the MDA-MB-231 cells, while in vitro space-restrictive invasion assays highlighted their reduced migratory capacity upon AGK2 treatment. Taken together, our findings suggest that SIRT2 inhibition promotes a perinuclear cytoskeletal organisation in MDA-MB-231 cells, which enhances their nuclear rigidity and impedes their invasion through confined spaces in vitro.

Nuclear deformability facilitates apical nuclear migration in the developing zebrafish retina

Nuclear positioning is a crucial aspect of cell and developmental biology. One example is the apical movement of nuclei in neuroepithelia before mitosis, which is essential for proper tissue formation. While the cytoskeletal mechanisms that drive nuclei to the apical side have been explored, the influence of nuclear properties on apical nuclear migration is less understood. Nuclear properties, such as deformability, can be linked to lamin A/C expression levels, as shown in various in vitro studies. Interestingly, many nuclei in early development, including neuroepithelial nuclei, express only low levels of lamin A/C. Therefore, we investigated whether increased lamin A expression in the densely packed zebrafish retinal neuroepithelium affects nuclear deformability and, consequently, migration phenomena. We found that overexpressing lamin A in retinal nuclei increases nuclear stiffness, which in turn indeed impairs apical nuclear migration. Interestingly, nuclei that do not overexpress lamin A but are embedded in a stiffer lamin A-overexpressing environment also exhibit impaired apical nuclear migration, indicating that these effects can be cell non-autonomous. Additionally, in the less crowded hindbrain neuroepithelium, only minor effects on apical nuclear migration are observed. Together, this suggests that the material properties of the nucleus influence nuclear movements in a tissue-dependent manner.

Advances in mechanotransduction and sonobiology: effects of audible acoustic waves and low-vibration stimulations on mammalian cells

In recent decades, research on mechanotransduction has advanced considerably, focusing on the effects of audible acoustic waves (AAWs) and low-vibration stimulation (LVS), which has propelled the field of sonobiology forward. Taken together, the current evidence demonstrates the influence of these biosignals on key cellular processes, such as growth, differentiation and migration in mammalian cells, emphasizing the determining role of specific physical parameters during stimulation, such as frequency, sound pressure level/amplitude and exposure time. These mechanical waves interact with various cellular elements, including ion channels, primary cilia, cell-cell adhesion receptors, cell-matrix and extracellular matrix proteins, and focal adhesion complexes. These components connect with the cytoskeletal fibre network, enabling the transmission of mechanical stimuli towards the nucleus. The nucleus, in turn, linked to the cytoskeleton via the linkers of the nucleoskeleton and cytoskeleton complex, acts as a mechanosensitive centre, not only responding to changes in cytoskeletal stiffness and nuclear tension but also regulating gene expression through the transcriptional co-activator YAP/TAZ and interactions between chromatin and the nuclear envelope. This intricate chain of mechanisms highlights the potential of sonobiology in various fields, including dentistry, regenerative medicine, tissue engineering and cancer research. However, progress in these fields requires the establishment of standardized measurement methodologies and biocompatible experimental setups to ensure the reproducibility of results.

Chromatin compaction during confined cell migration induces and reshapes nuclear condensates

Cell migration through small constrictions during cancer metastasis requires significant deformation of the nucleus, with associated mechanical stress on the nuclear lamina and chromatin. However, how mechanical deformation impacts various subnuclear structures, including protein and nucleic acid-rich biomolecular condensates, is largely unknown. Here, we find that cell migration through confined spaces gives rise to mechanical deformations of the chromatin network, which cause embedded nuclear condensates, including nucleoli and nuclear speckles, to deform and coalesce. Chromatin deformations exhibit differential behavior in the advancing vs. trailing region of the nucleus, with the trailing half being more permissive for de novo condensate formation. We show that this results from increased chromatin heterogeneity, which gives rise to a shift in the binodal phase boundary. Taken together, our findings show how chromatin deformation impacts condensate assembly and properties, which can potentially contribute to cellular mechanosensing.

Nesprin-2 coordinates opposing microtubule motors during nuclear migration in neurons

Nuclear migration is critical for the proper positioning of neurons in the developing brain. It is known that bidirectional microtubule motors are required for nuclear transport, yet the mechanism of the coordination of opposing motors is still under debate. Using mouse cerebellar granule cells, we demonstrate that Nesprin-2 serves as a nucleus-motor adaptor, coordinating the interplay of kinesin-1 and dynein. Nesprin-2 recruits dynein-dynactin-BicD2 independently of the nearby kinesin-binding LEWD motif. Both motor binding sites are required to rescue nuclear migration defects caused by the loss of function of Nesprin-2. In an intracellular cargo transport assay, the Nesprin-2 fragment encompassing the motor binding sites generates persistent movements toward both microtubule minus and plus ends. Nesprin-2 drives bidirectional cargo movements over a prolonged period along perinuclear microtubules, which advance during the migration of neurons. We propose that Nesprin-2 keeps the nucleus mobile by coordinating opposing motors, enabling continuous nuclear transport along advancing microtubules in migrating cells.

Claudin-4 remodeling of nucleus-cell cycle crosstalk maintains ovarian tumor genome stability and drives resistance to genomic instability-inducing agents

During cancer development, the interplay between the nucleus and the cell cycle leads to a state of genomic instability, often accompanied by observable morphological aberrations. These aberrations can be controlled by tumor cells to evade cell death, either by preventing or eliminating genomic instability. In epithelial ovarian cancer (EOC), overexpression of the multifunctional protein claudin-4 is a key contributor to therapy resistance through mechanisms associated with genomic instability. However, the molecular mechanisms underlying claudin-4 overexpression in EOC remain poorly understood. Here, we altered claudin-4 expression and employed a unique claudin-4 targeting peptide (CMP) to manipulate the function of claudin-4. We found that claudin-4 facilitates genome maintenance by linking the nuclear envelope and cytoskeleton dynamics with cell cycle progression. Claudin-4 caused nuclei constriction by excluding lamin B1 and promoting perinuclear F-actin accumulation, associated with remodeling nuclear architecture, thus altering nuclear envelope dynamics. Consequently, cell cycle modifications due to claudin-4 overexpression resulted in fewer cells entering the S-phase and reduced genomic instability. Importantly, disrupting biological interactions of claudin-4 using CMP and forskolin altered oxidative stress cellular response and increased the efficacy of PARP inhibitor treatment. Our data indicate that claudin-4 protects tumor genome integrity by remodeling the crosstalk between the nuclei and the cell cycle, leading to resistance to genomic instability formation and the effects of genomic instability-inducing agents.

Life outside the LINC complex - Do SUN proteins have LINC-independent functions?

Sad1 and UNC84 (SUN) and Klarsicht, ANC-1, and Syne homology (KASH) proteins interact at the nuclear periphery to form the linker of nucleoskeleton and cytoskeleton (LINC) complex, spanning the nuclear envelope (NE) and connecting the cytoskeleton with the nuclear interior. It is now well-documented that several cellular functions depend on LINC complex formation, including cell differentiation and migration. Intriguingly, recent studies suggest that SUN proteins participate in cellular processes where their association with KASH proteins may not be required. Building on this recent research, we elaborate on the hypothesis that SUN proteins may perform LINC-independent functions and discuss the modalities that may allow SUN proteins to function at the INM when they are not forming LINC complex.

Natural convection in the cytoplasm: Theoretical predictions of buoyancy-driven flows inside a cell

The existence of temperature gradients within eukaryotic cells has been postulated as a source of natural convection in the cytoplasm, i.e. bulk fluid motion as a result of temperature-difference-induced density gradients. Recent computations have predicted that a temperature differential of ΔT ≈ 1 K between the cell nucleus and the cell membrane could be strong enough to drive significant intracellular material transport. We use numerical computations and theoretical calculations to revisit this problem in order to further understand the impact of temperature gradients on flow generation and advective transport within cells. Surprisingly, our computations yield flows that are an order of magnitude weaker than those obtained previously for the same relative size and position of the nucleus with respect to the cell membrane. To understand this discrepancy, we develop a semi-analytical solution of the convective flow inside a model cell using a bi-spherical coordinate framework, for the case of an axisymmetric cell geometry (i.e. when the displacement of the nucleus from the cell centre is aligned with gravity). We also calculate exact solutions for the flow when the nucleus is located concentrically inside the cell. The results from both theoretical analyses agree with our numerical results, thus providing a robust estimate of the strength of cytoplasmic natural convection and demonstrating that these are much weaker than previously predicted. Finally, we investigate the ability of the aforementioned flows to redistribute solute within a cell. Our calculations reveal that, in all but unrealistic cases, cytoplasmic convection has a negligible contribution toward enhancing the diffusion-dominated mass transfer of cellular material.

Engineering the Cellular Microenvironment: Integrating Three-Dimensional Nontopographical and Two-Dimensional Biochemical Cues for Precise Control of Cellular Behavior

The development of biomaterials capable of regulating cellular processes and guiding cell fate decisions has broad implications in tissue engineering, regenerative medicine, and cell-based assays for drug development and disease modeling. Recent studies have shown that three-dimensional (3D) nanoscale physical cues such as nanotopography can modulate various cellular processes like adhesion and endocytosis by inducing nanoscale curvature on the plasma and nuclear membranes. Two-dimensional (2D) biochemical cues such as protein micropatterns can also regulate cell function and fate by controlling cellular geometries. Development of biomaterials with precise control over nanoscale physical and biochemical cues can significantly influence programming cell function and fate. In this study, we utilized a laser-assisted micropatterning technique to manipulate the 2D architectures of cells on 3D nanopillar platforms. We performed a comprehensive analysis of cellular and nuclear morphology and deformation on both nanopillar and flat substrates. Our findings demonstrate the precise engineering of single cell architectures through 2D micropatterning on nanopillar platforms. We show that the coupling between the nuclear and cell shape is disrupted on nanopillar surfaces compared to flat surfaces. Furthermore, our results suggest that cell elongation on nanopillars enhances nanopillar-induced endocytosis. We believe our platform serves as a versatile tool for further explorations into programming cell function and fate through combined physical cues that create nanoscale curvature on cell membranes and biochemical cues that control the geometry of the cell.

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.

A novel MAP kinase-interacting protein MoSmi1 regulates development and pathogenicity in Magnaporthe oryzae

The cell wall is the first barrier against external adversity and plays roles in maintaining normal physiological functions of fungi. Previously, we reported a nucleosome assembly protein, MoNap1, in Magnaporthe oryzae that plays a role in cell wall integrity (CWI), stress response, and pathogenicity. Moreover, MoNap1 negatively regulates the expression of MoSMI1 encoded by MGG_03970. Here, we demonstrated that deletion of MoSMI1 resulted in a significant defect in appressorium function, CWI, cell morphology, and pathogenicity. Further investigation revealed that MoSmi1 interacted with MoOsm1 and MoMps1 and affected the phosphorylation levels of MoOsm1, MoMps1, and MoPmk1, suggesting that MoSmi1 regulates biological functions by mediating mitogen-activated protein kinase (MAPK) signalling pathway in M. oryzae. In addition, transcriptome data revealed that MoSmi1 regulates many infection-related processes in M. oryzae, such as membrane-related pathway and oxidation reduction process. In conclusion, our study demonstrated that MoSmi1 regulates CWI by mediating the MAPK pathway to affect development and pathogenicity of M. oryzae.

Exploring the maternal inheritance transmitted by the oocyte to its progeny

Fertility is declining worldwide and many couples are turning towards assisted reproductive technologies (ART) to conceive babies. Organisms that propagate via sexual reproduction often come from the fusion between two gametes, an oocyte and a sperm, whose qualities seem to be decreasing in the human species. Interestingly, while the sperm mostly transmits its haploid genome, the oocyte transmits not only its haploid set of chromosomes but also its huge cytoplasm to its progeny. This is what can be defined as the maternal inheritance composed of chromosomes, organelles, lipids, metabolites, proteins and RNAs. To decipher the decline in oocyte quality, it is essential to explore the nature of the maternal inheritance, and therefore study the last stages of murine oogenesis, namely the end of oocyte growth followed by the two meiotic divisions. These divisions are extremely asymmetric in terms of the size of the daughter cells, allowing to preserve the maternal inheritance accumulated during oocyte growth within these huge cells to support early embryo development. Studies performed in Marie-Hélène Verlhac's lab have allowed to discover the unprecedented impact of original acto-myosin based mechanisms in the constitution as well as the preservation of this maternal inheritance and the consequences when these processes go awry.

Pleomorphism in Biological Units of Life: Morphological Heterogeneity in Cells Does Not Translate Uniformly to Subcellular Components

The interplay of the three-dimensional (3D) distribution of various subcellular components and their interactions are expected to control overall cellular morphology in biology. In this study, we aimed to determine whether the pleomorphy observed at the whole-cell level is being reflected by the components constituting the cells by focusing on the 3D distribution of pixel intensities at the single-cell level of the whole (cell) and its parts (the seven subcellular components of the cells-self-assemblies of smaller units). We rigorously acquired and analyzed the image data of RAW264.7 cells at the single-cell level. We report asymmetries in the spatial distribution of pixel intensities at the whole-cell and subcellular component levels along with the occurrence of alterations when pleomorphism is reduced by synchronization of the cell cycle. From our repertoire of seven subcellular components, we report ER, mitochondria, and tubulin to be independent of whole-cell apico-basal heterogeneity of optical density while nuclear, plasma membrane, lysosomal, and actin fluorescence distributions are found to contribute to the apico-basal polarity of the whole cell. While doing so, we have also developed an image analysis algorithm utilizing 2D segmentation to analyze the single cells in 3D using confocal microscopy, a technique that allows us to analyze cellular states in their native hydrated state.

Active nuclear positioning and actomyosin contractility maintain leader cell integrity during gonadogenesis

Proper distribution of organelles can play an important role in a moving cell's performance. During C. elegans gonad morphogenesis, the nucleus of the leading distal tip cell (DTC) is always found at the front, yet the significance of this localization is unknown. Here, we identified the molecular mechanism that keeps the nucleus at the front, despite a frictional force that pushes it backward. The Klarsicht/ANC-1/Syne homology (KASH) domain protein UNC-83 links the nucleus to the motor protein kinesin-1 that moves along a polarized acentrosomal microtubule network. Interestingly, disrupting nuclear positioning on its own did not affect gonad morphogenesis. However, reducing actomyosin contractility on top of nuclear mispositioning led to a dramatic phenotype: DTC splitting and gonad bifurcation. Long-term live imaging of the double knockdown revealed that, while the gonad attempted to perform a planned U-turn, the DTC was stretched due to the lagging nucleus until it fragmented into a nucleated cell and an enucleated cytoplast, each leading an independent gonadal arm. Remarkably, the enucleated cytoplast had polarity and invaded, but it could only temporarily support germ cell proliferation. Based on a qualitative biophysical model, we conclude that the leader cell employs two complementary mechanical approaches to preserve its integrity and ensure proper organ morphogenesis while navigating through a complex 3D environment: active nuclear positioning by microtubule motors and actomyosin-driven cortical contractility.

Nuclear position and local acetyl-CoA production regulate chromatin state

Histone acetylation regulates gene expression, cell function and cell fate1. Here we study the pattern of histone acetylation in the epithelial tissue of the Drosophila wing disc. H3K18ac, H4K8ac and total lysine acetylation are increased in the outer rim of the disc. This acetylation pattern is controlled by nuclear position, whereby nuclei continuously move from apical to basal locations within the epithelium and exhibit high levels of H3K18ac when they are in proximity to the tissue surface. These surface nuclei have increased levels of acetyl-CoA synthase, which generates the acetyl-CoA for histone acetylation. The carbon source for histone acetylation in the rim is fatty acid β-oxidation, which is also increased in the rim. Inhibition of fatty acid β-oxidation causes H3K18ac levels to decrease in the genomic proximity of genes involved in disc development. In summary, there is a physical mark of the outer rim of the wing and other imaginal epithelia in Drosophila that affects gene expression.

Spatiotemporal regulation of plant cell division

Plant morphogenesis largely depends on the orientation and rate of cell division and elongation, and their coordination at all levels of organization. Despite recent progresses in the comprehension of pathways controlling division plane determination in plant cells, many pieces are missing to the puzzle. For example, we have a partial comprehension of formation, function and evolutionary significance of the preprophase band, a plant-specific cytoskeletal array involved in premitotic setup of the division plane, as well as the role of the nucleus and its connection to the preprophase band of microtubules. Likewise, several modeling studies point to a strong relationship between cell shape and division geometry, but the emergence of such geometric rules from the molecular and cellular pathways at play are still obscure. Yet, recent imaging technologies and genetic tools hold a lot of promise to tackle these challenges and to revisit old questions with unprecedented resolution in space and time.

Principles of organelle positioning in motile and non-motile cells

Cells are equipped with asymmetrically localised and functionally specialised components, including cytoskeletal structures and organelles. Positioning these components to specific intracellular locations in an asymmetric manner is critical for their functionality and affects processes like immune responses, tissue maintenance, muscle functionality, and neurobiology. Here, we provide an overview of strategies to actively move, position, and anchor organelles to specific locations. By conceptualizing the cytoskeletal forces and the organelle-to-cytoskeleton connectivity, we present a framework of active positioning of both membrane-enclosed and membrane-less organelles. Using this framework, we discuss how different principles of force generation and organelle anchorage are utilised by different cells, such as mesenchymal and amoeboid cells, and how the microenvironment influences the plasticity of organelle positioning. Given that motile cells face the challenge of coordinating the positioning of their content with cellular motion, we particularly focus on principles of organelle positioning during migration. In this context, we discuss novel findings on organelle positioning by anchorage-independent mechanisms and their advantages and disadvantages in motile as well as stationary cells.

Coherent light scattering from cellular dynamics in living tissues

This review examines the biological physics of intracellular transport probed by the coherent optics of dynamic light scattering from optically thick living tissues. Cells and their constituents are in constant motion, composed of a broad range of speeds spanning many orders of magnitude that reflect the wide array of functions and mechanisms that maintain cellular health. From the organelle scale of tens of nanometers and upward in size, the motion inside living tissue is actively driven rather than thermal, propelled by the hydrolysis of bioenergetic molecules and the forces of molecular motors. Active transport can mimic the random walks of thermal Brownian motion, but mean-squared displacements are far from thermal equilibrium and can display anomalous diffusion through Lévy or fractional Brownian walks. Despite the average isotropic three-dimensional environment of cells and tissues, active cellular or intracellular transport of single light-scattering objects is often pseudo-one-dimensional, for instance as organelle displacement persists along cytoskeletal tracks or as membranes displace along the normal to cell surfaces, albeit isotropically oriented in three dimensions. Coherent light scattering is a natural tool to characterize such tissue dynamics because persistent directed transport induces Doppler shifts in the scattered light. The many frequency-shifted partial waves from the complex and dynamic media interfere to produce dynamic speckle that reveals tissue-scale processes through speckle contrast imaging and fluctuation spectroscopy. Low-coherence interferometry, dynamic optical coherence tomography, diffusing-wave spectroscopy, diffuse-correlation spectroscopy, differential dynamic microscopy and digital holography offer coherent detection methods that shed light on intracellular processes. In health-care applications, altered states of cellular health and disease display altered cellular motions that imprint on the statistical fluctuations of the scattered light. For instance, the efficacy of medical therapeutics can be monitored by measuring the changes they induce in the Doppler spectra of livingex vivocancer biopsies.

Asymmetrical positioning of cell organelles reflects the cell chirality of mouse myoblast cells

Cell chirality is crucial for the chiral morphogenesis of biological tissues, yet its underlying mechanism remains unclear. Cell organelle polarization along multiple axes in a cell body, namely, apical-basal, front-rear, and left-right, is known to direct cell behavior such as orientation, rotation, and migration. Among these axes, the left-right bias holds significant sway in determining the chiral directionality of these behaviors. Normally, mouse myoblast (C2C12) cells exhibit a strong counterclockwise chirality on a ring-shaped micropattern, whereas they display a clockwise dominant chirality under Latrunculin A treatment. To investigate the relationship between multicellular chirality and organelle positioning in single cells, we studied the left-right positioning of cell organelles under distinct cell chirality in single cells via micropatterning technique, fluorescent microscopy, and imaging analysis. We found that on a "T"-shaped micropattern, a C2C12 cell adopts a triangular shape, with its nucleus-centrosome axis pointing toward the top-right direction of the "T." Several other organelles, including the Golgi apparatus, lysosomes, actin filaments, and microtubules, showed a preference to polarize on one side of the axis, indicating the universality of the left-right asymmetrical organelle positioning. Interestingly, upon reversing cell chirality with Latrunculin A, the organelles correspondingly reversed their left-right positioning bias, as suggested by the consistently biased metabolism and contractile properties at the leading edge. This left-right asymmetry in organelle positioning may help predict cell migration direction and serve as a potential marker for identifying cell chirality in biological models.

Matrix Nonlinear Viscoelasticity Regulates Skeletal Myogenesis through MRTF Nuclear Localization and Nuclear Mechanotransduction

Mechanically sensitive tissues (e.g., skeletal muscles) greatly need mechanical stimuli during the development and maturation. The extracellular matrix (ECM) mediates these signals through nonlinear viscoelasticity of collagen networks that are predominant components of the ECM. However, the interactions between cells and ECM form a feedback loop, and it has not yet been possible to determine the degree to which, if any, of the features of matrix nonlinear viscoelasticity affect skeletal muscle development and regeneration. In this study, a nonlinear viscoelastic feature (i.e., strain-enhanced stress relaxation (SESR)) in normal skeletal muscles is observed, which however is almost absent in diseased muscles from Duchenne muscular dystrophy mice. It is recapitulated such SESR feature in vitro and separated the effects of mechanical strain and ECM viscoelasticity on myoblast response by developing a collagen-based hydrogel platform. Both strain and stress relaxation induce myogenic differentiation and myotube formation by C2C12 myoblasts, and myogenesis is more promoted by applying SESR. This promotion can be explained by the effects of SESR on actin polymerization-mediated myocardin related transcription factor (MRTF) nuclear localization and nuclear mechanotransduction. This study represents the first attempt to investigate the SESR phenomenon in skeletal muscles and reveal underlying mechanobiology, which will provide new opportunities for the tissue injury treatments.

The nuclei of human adult stem cells can move within the cell and generate cellular protrusions to contact other cells

BACKGROUND

The neuronal transdifferentiation of adult bone marrow cells (BMCs) is still considered an artifact based on an alternative explanation of experimental results supporting this phenomenon obtained over decades. However, recent studies have shown that following neural induction, BMCs enter an intermediate cellular state before adopting neural-like morphologies by active neurite extension and that binucleated BMCs can be formed independent of any cell fusion events. These findings provide evidence to reject the idea that BMC neural transdifferentiation is merely an experimental artifact. Therefore, understanding the intermediate states that cells pass through during transdifferentiation is crucial given their potential application in regenerative medicine and disease modelling.

METHODS

In this study, we examined the functional significance of the variety of morphologies and positioning that cell nuclei of human bone marrow-derived mesenchymal stem cells (hBM-MSCs) can adopt during neural-like differentiation using live-cell nuclear fluorescence labelling, time-lapse microscopy, and confocal microscopy analysis.

RESULTS

Here, we showed that after neural induction, hBM-MSCs enter an intermediate cellular state in which the nuclei are able to move within the cells, switching shapes and positioning and even generating cellular protrusions as they attempt to contact the cells around them. These findings suggest that changes in nuclear positioning occur because human cell nuclei somehow sense their environment. In addition, we showed the process of direct interactions between cell nuclei, which opens the possibility of a new level of intercellular interaction.

CONCLUSIONS

The present study advances the understanding of the intermediate stage through which hBM-MSCs pass during neural transdifferentiation, which may be crucial to understanding the mechanisms of these cell conversion processes and eventually harness them for use in regenerative medicine. Importantly, our study provides for the first time evidence that the nuclei of hBM-MSC-derived intermediate cells somehow sense their environment, generating cellular protrusions to contact other cells. In summary, human mesenchymal stromal cells could not only help to increase our understanding of the mechanisms underlying cellular plasticity but also facilitate the exact significance of nuclear positioning in cellular function and in tissue physiology.

Claudin-4 modulates autophagy via SLC1A5/LAT1 as a tolerance mechanism for genomic instability in ovarian cancer

Genome instability is key for tumor heterogeneity and derives from defects in cell division and DNA damage repair. Tumors show tolerance for this characteristic, but its accumulation is regulated somehow to avoid catastrophic chromosomal alterations and cell death. Claudin-4 is upregulated and closely associated with genome instability and worse patient outcome in ovarian cancer. This protein is commonly described as a junctional protein participating in processes such as cell proliferation and DNA repair. However, its biological association with genomic instability is still poorly-understood. Here, we used CRISPRi and a claudin mimic peptide (CMP) to modulate the cladudin-4 expression and its function, respectively in in-vitro (high-grade serous carcinoma cells) and in-vivo (patient-derived xenograft in a humanized-mice model) systems. We found that claudin-4 promotes a protective cellular-mechanism that links cell-cell junctions to genome integrity. Disruption of this axis leads to irregular cellular connections and cell cycle that results in chromosomal alterations, a phenomenon associated with a novel functional link between claudin-4 and SLC1A5/LAT1 in regulating autophagy. Consequently, claudin-4's disruption increased autophagy and associated with engulfment of cytoplasm-localized DNA. Furthermore, the claudin-4/SLC1A5/LAT1 biological axis correlates with decrease ovarian cancer patient survival and targeting claudin-4 in-vivo with CMP resulted in increased niraparib (PARPi) efficacy, correlating with increased tumoral infiltration of T CD8+ lymphocytes. Our results show that the upregulation of claudin-4 enables a mechanism that promotes tolerance to genomic instability and immune evasion in ovarian cancer; thus, suggesting the potential of claudin-4 as a translational target for enhancing ovarian cancer treatment.

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.

Adaptive pathfinding by nucleokinesis during amoeboid migration

Motile cells encounter microenvironments with locally heterogeneous mechanochemical composition. Individual compositional parameters, such as chemokines and extracellular matrix pore sizes, are well known to provide guidance cues for pathfinding. However, motile cells face diverse cues at the same time, raising the question of how they respond to multiple and potentially competing signals on their paths. Here, we reveal that amoeboid cells require nuclear repositioning, termed nucleokinesis, for adaptive pathfinding in heterogeneous mechanochemical micro-environments. Using mammalian immune cells and the amoeba Dictyostelium discoideum, we discover that frequent, rapid and long-distance nucleokinesis is a basic component of amoeboid pathfinding, enabling cells to reorientate quickly between locally competing cues. Amoeboid nucleokinesis comprises a two-step polarity switch and is driven by myosin-II forces that readjust the nuclear to the cellular path. Impaired nucleokinesis distorts path adaptions and causes cellular arrest in the microenvironment. Our findings establish that nucleokinesis is required for amoeboid cell navigation. Given that many immune cells, amoebae, and some cancer cells utilize an amoeboid migration strategy, these results suggest that nucleokinesis underlies cellular navigation during unicellular biology, immunity, and disease.

Building and breaking mechanical bridges between the nucleus and cytoskeleton: Regulation of LINC complex assembly and disassembly

The nucleus is physically coupled to the cytoskeleton through LINC complexes, macromolecular bridges composed of SUN and KASH proteins that span the nuclear envelope. LINC complexes are involved in a wide variety of critical cellular processes. For these processes to occur, cells regulate the composition, assembly, and disassembly of LINC complexes. Here we discuss recent studies on the regulation of the SUN-KASH interaction that forms the core of the LINC complex. These new findings encompass the stages of LINC complex assembly, from the formation of SUN-KASH heterooligomers to higher-order assemblies of LINC complexes. There is also new work on how components of the LINC complex are selectively dismantled, particularly by proteasomal degradation. It is becoming increasingly clear that LINC complexes are subject to multiple layers of regulation.

SEM2: Introducing mechanics in cell and tissue modeling using coarse-grained homogeneous particle dynamics

Modeling multiscale mechanics in shape-shifting engineered tissues, such as organoids and organs-on-chip, is both important and challenging. In fact, it is difficult to model relevant tissue-level large non-linear deformations mediated by discrete cell-level behaviors, such as migration and proliferation. One approach to solve this problem is subcellular element modeling (SEM), where ensembles of coarse-grained particles interacting via empirically defined potentials are used to model individual cells while preserving cell rheology. However, an explicit treatment of multiscale mechanics in SEM was missing. Here, we incorporated analyses and visualizations of particle level stress and strain in the open-source software SEM++ to create a new framework that we call subcellular element modeling and mechanics or SEM2. To demonstrate SEM2, we provide a detailed mechanics treatment of classical SEM simulations including single-cell creep, migration, and proliferation. We also introduce an additional force to control nuclear positioning during migration and proliferation. Finally, we show how SEM2 can be used to model proliferation in engineered cell culture platforms such as organoids and organs-on-chip. For every scenario, we present the analysis of cell emergent behaviors as offered by SEM++ and examples of stress or strain distributions that are possible with SEM2. Throughout the study, we only used first-principles literature values or parametric studies, so we left to the Discussion a qualitative comparison of our insights with recently published results. The code for SEM2 is available on GitHub at https://github.com/Synthetic-Physiology-Lab/sem2.

Prelamin A and ZMPSTE24 in premature and physiological aging

As human longevity increases, understanding the molecular mechanisms that drive aging becomes ever more critical to promote health and prevent age-related disorders. Premature aging disorders or progeroid syndromes can provide critical insights into aspects of physiological aging. A major cause of progeroid syndromes which result from mutations in the genes LMNA and ZMPSTE24 is disruption of the final posttranslational processing step in the production of the nuclear scaffold protein lamin A. LMNA encodes the lamin A precursor, prelamin A and ZMPSTE24 encodes the prelamin A processing enzyme, the zinc metalloprotease ZMPSTE24. Progeroid syndromes resulting from mutations in these genes include the clinically related disorders Hutchinson-Gilford progeria syndrome (HGPS), mandibuloacral dysplasia-type B, and restrictive dermopathy. These diseases have features that overlap with one another and with some aspects of physiological aging, including bone defects resembling osteoporosis and atherosclerosis (the latter primarily in HGPS). The progeroid syndromes have ignited keen interest in the relationship between defective prelamin A processing and its accumulation in normal physiological aging. In this review, we examine the hypothesis that diminished processing of prelamin A by ZMPSTE24 is a driver of physiological aging. We review features a new mouse (LmnaL648R/L648R) that produces solely unprocessed prelamin A and provides an ideal model for examining the effects of its accumulation during aging. We also discuss existing data on the accumulation of prelamin A or its variants in human physiological aging, which call out for further validation and more rigorous experimental approaches to determine if prelamin A contributes to normal aging.

Centrosome clustering control in osteoclasts through CCR5-mediated signaling

Osteoclasts uniquely resorb calcified bone matrices. To exert their function, mature osteoclasts maintain the cellular polarity and directional vesicle trafficking to and from the resorbing bone surface. However, the regulatory mechanisms and pathophysiological relevance of these processes remain largely unexplored. Bone histomorphometric analyses in Ccr5-deficient mice showed abnormalities in the morphology and functional phenotype of their osteoclasts, compared to wild type mice. We observed disorganized clustering of nuclei, as well as centrosomes that organize the microtubule network, which was concomitant with impaired cathepsin K secretion in cultured Ccr5-deficient osteoclasts. Intriguingly, forced expression of constitutively active Rho or Rac restored these cytoskeletal phenotypes with recovery of cathepsin K secretion. Furthermore, a gene-disease enrichment analysis identified that PLEKHM1, a responsible gene for osteopetrosis, which regulates lysosomal trafficking in osteoclasts, was regulated by CCR5. These experimental results highlighted that CCR5-mediated signaling served as an intracellular organizer for centrosome clustering in osteoclasts, which was involved in the pathophysiology of bone metabolism.

The effect of the properties of cell nucleus and underlying substrate on the response of finite element models of astrocytes undergoing mechanical stimulations

Astrocyte cells play a critical role in the mechanical behaviour of the brain tissue; hence understanding the properties of Astrocytes is a big step toward understanding brain diseases and abnormalities. Conventionally, atomic force microscopy (AFM) has been used as one of the most powerful tools to characterize the mechanical properties of cells. However, due to the complexities of experimental work and the complex behaviour of living cells, the finite element method (FEM) is commonly used to estimate the cells' response to mechanical stimulations. In this study, we developed a finite element model of the Astrocyte cells to investigate the effect of two key parameters that could affect the response of the cell to mechanical loading; the properties of the underlying substrate and the nucleus. In this regard, the cells were placed on two different substrates in terms of thickness and stiffness (gel and glass) with varying properties of the nucleus. The main achievement of this study was to develop an insight to investigate the response of the Astrocytes to mechanical loading for future studies, both experimentally and computationally.

Physcomitrium patens SUN2 Mediates MTOC Association with the Nuclear Envelope and Facilitates Chromosome Alignment during Spindle Assembly

Plant cells lack centrosomes and instead utilize acentrosomal microtubule organizing centers (MTOCs) to rapidly increase the number of microtubules at the onset of spindle assembly. Although several proteins required for MTOC formation have been identified, how the MTOC is positioned at the right place is not known. Here, we show that the inner nuclear membrane protein SUN2 is required for MTOC association with the nuclear envelope (NE) during mitotic prophase in the moss Physcomitrium patens. In actively dividing protonemal cells, microtubules accumulate around the NE during prophase. In particular, regional MTOC is formed at the apical surface of the nucleus. However, microtubule accumulation around the NE was impaired and apical MTOCs were mislocalized in sun2 knockout cells. Upon NE breakdown, the mitotic spindle was assembled with mislocalized MTOCs. However, completion of chromosome alignment in the spindle was delayed; in severe cases, the chromosome was transiently detached from the spindle body. SUN2 tended to localize to the apical surface of the nucleus during prophase in a microtubule-dependent manner. Based on these results, we propose that SUN2 facilitates the attachment of microtubules to chromosomes during spindle assembly by localizing microtubules to the NE. MTOC mispositioning was also observed during the first division of the gametophore tissue. Thus, this study suggests that microtubule-nucleus linking, a well-known function of SUN in animals and yeast, is conserved in plants.

The Influence of Nucleus Mechanics in Modelling Adhesion-independent Cell Migration in Structured and Confined Environments

Recent biological experiments (Lämmermann et al. in Nature 453(7191):51-55, 2008; Reversat et al. in Nature 7813:582-585, 2020; Balzer et al. in ASEB J Off Publ Fed Am Soc Exp Biol 26(10):4045-4056, 2012) have shown that certain types of cells are able to move in structured and confined environments even without the activation of focal adhesion. Focusing on this particular phenomenon and based on previous works (Jankowiak et al. in Math Models Methods Appl Sci 30(03):513-537, 2020), we derive a novel two-dimensional mechanical model, which relies on the following physical ingredients: the asymmetrical renewal of the actin cortex supporting the membrane, resulting in a backward flow of material; the mechanical description of the nuclear membrane and the inner nuclear material; the microtubule network guiding nucleus location; the contact interactions between the cell and the external environment. The resulting fourth order system of partial differential equations is then solved numerically to conduct a study of the qualitative effects of the model parameters, mainly those governing the mechanical properties of the nucleus and the geometry of the confining structure. Coherently with biological observations, we find that cells characterized by a stiff nucleus are unable to migrate in channels that can be crossed by cells with a softer nucleus. Regarding the geometry, cell velocity and ability to migrate are influenced by the width of the channel and the wavelength of the external structure. Even though still preliminary, these results may be potentially useful in determining the physical limit of cell migration in confined environments and in designing scaffolds for tissue engineering.

Centrosome amplification fine tunes tubulin acetylation to differentially control intracellular organization

Intracellular organelle organization is conserved in eukaryotic cells and is primarily achieved through active transport by motor proteins along the microtubule cytoskeleton. Microtubule post-translational modifications (PTMs) can contribute to microtubule diversity and differentially regulate motor-mediated transport. Here, we show that centrosome amplification, commonly observed in cancer and shown to promote aneuploidy and invasion, induces a global change in organelle positioning towards the cell periphery and facilitates nuclear migration through confined spaces. This reorganization requires kinesin-1 and is analogous to the loss of dynein. Cells with amplified centrosomes display increased levels of acetylated tubulin, a PTM that could enhance kinesin-1-mediated transport. Depletion of α-tubulin acetyltransferase 1 (αTAT1) to block tubulin acetylation rescues the displacement of centrosomes, mitochondria, and vimentin but not Golgi or endosomes. Analyses of the distribution of total and acetylated microtubules indicate that the polarized distribution of modified microtubules, rather than levels alone, plays an important role in the positioning of specific organelles, such as the centrosome. We propose that increased tubulin acetylation differentially impacts kinesin-1-mediated organelle displacement to regulate intracellular organization.

EHBP1L1, an apicobasal polarity regulator, is critical for nuclear polarization during enucleation of erythroblasts

Cell polarity, the asymmetric distribution of proteins and organelles, is permanently or transiently established in various cell types and plays an important role in many physiological events. epidermal growth factor receptor substrate 15 homology domain-binding protein 1-like 1 (EHBP1L1) is an adapter protein that is localized on recycling endosomes and regulates apical-directed transport in polarized epithelial cells. However, the role of EHBP1L1 in nonepithelial cells, remains unknown. Here, Ehbp1l1-/- mice showed impaired erythroblast enucleation. Further analyses showed that nuclear polarization before enucleation was impaired in Ehbp1l1-/- erythroblasts. It was also revealed that EHBP1L1 interactors Rab10, Bin1, and dynamin were involved in erythroblast enucleation. In addition, Ehbp1l1-/- erythrocytes exhibited stomatocytic morphology and dehydration. These defects in erythroid cells culminated in early postnatal anemic lethality in Ehbp1l1-/- mice. Moreover, we found the mislocalization of nuclei and mitochondria in the skeletal muscle cells of Ehbp1l1-/- mice, as observed in patients with centronuclear myopathy with genetic mutations in Bin1 or dynamin 2. Taken together, our findings indicate that the Rab8/10-EHBP1L1-Bin1-dynamin axis plays an important role in multiple cell polarity systems in epithelial and nonepithelial cells.