Force Triggers YAP Nuclear Entry by Regulating Transport across Nuclear Pores.

YAP is a mechanosensitive transcriptional activator with a critical role in cancer, regeneration, and organ size control. Here, we show that force applied to the nucleus directly drives YAP nuclear translocation by decreasing the mechanical restriction of nuclear pores to molecular transport. Exposure to a stiff environment leads cells to establish a mechanical connection between the nucleus and the cytoskeleton, allowing forces exerted through focal adhesions to reach the nucleus. Force transmission then leads to nuclear flattening, which stretches nuclear pores, reduces their mechanical resistance to molecular transport, and increases YAP nuclear import. The restriction to transport is further regulated by the mechanical stability of the transported protein, which determines both active nuclear transport of YAP and passive transport of small proteins. Our results unveil a mechanosensing mechanism mediated directly by nuclear pores, demonstrated for YAP but with potential general applicability in transcriptional regulation.

Matrix viscoelasticity controls spatiotemporal tissue organization.

Biomolecular and physical cues of the extracellular matrix environment regulate collective cell dynamics and tissue patterning. Nonetheless, how the viscoelastic properties of the matrix regulate collective cell spatial and temporal organization is not fully understood. Here we show that the passive viscoelastic properties of the matrix encapsulating a spheroidal tissue of breast epithelial cells guide tissue proliferation in space and in time. Matrix viscoelasticity prompts symmetry breaking of the spheroid, leading to the formation of invading finger-like protrusions, YAP nuclear translocation and epithelial-to-mesenchymal transition both in vitro and in vivo in a Arp2/3-complex-dependent manner. Computational modelling of these observations allows us to establish a phase diagram relating morphological stability with matrix viscoelasticity, tissue viscosity, cell motility and cell division rate, which is experimentally validated by biochemical assays and in vitro experiments with an intestinal organoid. Altogether, this work highlights the role of stress relaxation mechanisms in tissue growth dynamics, a fundamental process in morphogenesis and oncogenesis.

The laminin-keratin link shields the nucleus from mechanical deformation and signalling.

The mechanical properties of the extracellular matrix dictate tissue behaviour. In epithelial tissues, laminin is a very abundant extracellular matrix component and a key supporting element. Here we show that laminin hinders the mechanoresponses of breast epithelial cells by shielding the nucleus from mechanical deformation. Coating substrates with laminin-111-unlike fibronectin or collagen I-impairs cell response to substrate rigidity and YAP nuclear localization. Blocking the laminin-specific integrin ß4 increases nuclear YAP ratios in a rigidity-dependent manner without affecting the cell forces or focal adhesions. By combining mechanical perturbations and mathematical modelling, we show that ß4 integrins establish a mechanical linkage between the substrate and keratin cytoskeleton, which stiffens the network and shields the nucleus from actomyosin-mediated mechanical deformation. In turn, this affects the nuclear YAP mechanoresponses, chromatin methylation and cell invasion in three dimensions. Our results demonstrate a mechanism by which tissues can regulate their sensitivity to mechanical signals.

Force loading explains spatial sensing of ligands by cells.

Cells can sense the density and distribution of extracellular matrix (ECM) molecules by means of individual integrin proteins and larger, integrin-containing adhesion complexes within the cell membrane. This spatial sensing drives cellular activity in a variety of normal and pathological contexts. Previous studies of cells on rigid glass surfaces have shown that spatial sensing of ECM ligands takes place at the nanometre scale, with integrin clustering and subsequent formation of focal adhesions impaired when single integrin-ligand bonds are separated by more than a few tens of nanometres. It has thus been suggested that a crosslinking 'adaptor' protein of this size might connect integrins to the actin cytoskeleton, acting as a molecular ruler that senses ligand spacing directly. Here, we develop gels whose rigidity and nanometre-scale distribution of ECM ligands can be controlled and altered. We find that increasing the spacing between ligands promotes the growth of focal adhesions on low-rigidity substrates, but leads to adhesion collapse on more-rigid substrates. Furthermore, disordering the ligand distribution drastically increases adhesion growth, but reduces the rigidity threshold for adhesion collapse. The growth and collapse of focal adhesions are mirrored by, respectively, the nuclear or cytosolic localization of the transcriptional regulator protein YAP. We explain these findings not through direct sensing of ligand spacing, but by using an expanded computational molecular-clutch model, in which individual integrin-ECM bonds-the molecular clutches-respond to force loading by recruiting extra integrins, up to a maximum value. This generates more clutches, redistributing the overall force among them, and reducing the force loading per clutch. At high rigidity and high ligand spacing, maximum recruitment is reached, preventing further force redistribution and leading to adhesion collapse. Measurements of cellular traction forces and actin flow speeds support our model. Our results provide a general framework for how cells sense spatial and physical information at the nanoscale, precisely tuning the range of conditions at which they form adhesions and activate transcriptional regulation.

Traction forces at the cytokinetic ring regulate cell division and polyploidy in the migrating zebrafish epicardium.

Epithelial repair and regeneration are driven by collective cell migration and division. Both cellular functions involve tightly controlled mechanical events, but how physical forces regulate cell division in migrating epithelia is largely unknown. Here we show that cells dividing in the migrating zebrafish epicardium exert large cell-extracellular matrix (ECM) forces during cytokinesis. These forces point towards the division axis and are exerted through focal adhesions that connect the cytokinetic ring to the underlying ECM. When subjected to high loading rates, these cytokinetic focal adhesions prevent closure of the contractile ring, leading to multi-nucleation through cytokinetic failure. By combining a clutch model with experiments on substrates of different rigidity, ECM composition and ligand density, we show that failed cytokinesis is triggered by adhesion reinforcement downstream of increased myosin density. The mechanical interaction between the cytokinetic ring and the ECM thus provides a mechanism for the regulation of cell division and polyploidy that may have implications in regeneration and cancer.

Rigidity sensing and adaptation through regulation of integrin types.

Tissue rigidity regulates processes in development, cancer and wound healing. However, how cells detect rigidity, and thereby modulate their behaviour, remains unknown. Here, we show that sensing and adaptation to matrix rigidity in breast myoepithelial cells is determined by the bond dynamics of different integrin types. Cell binding to fibronectin through either α5ß1 integrins (constitutively expressed) or αvß6 integrins (selectively expressed in cancer and development) adapts force generation, actin flow and integrin recruitment to rigidities associated with healthy or malignant tissue, respectively. In vitro experiments and theoretical modelling further demonstrate that this behaviour is explained by the different binding and unbinding rates of both integrin types to fibronectin. Moreover, rigidity sensing through differences in integrin bond dynamics applies both when integrins bind separately and when they compete for binding to fibronectin.

N-terminal acetylome analyses and functional insights of the N-terminal acetyltransferase NatB.

Protein N-terminal acetylation (Nt-acetylation) is an important mediator of protein function, stability, sorting, and localization. Although the responsible enzymes are thought to be fairly well characterized, the lack of identified in vivo substrates, the occurrence of Nt-acetylation substrates displaying yet uncharacterized N-terminal acetyltransferase (NAT) specificities, and emerging evidence of posttranslational Nt-acetylation, necessitate the use of genetic models and quantitative proteomics. NatB, which targets Met-Glu-, Met-Asp-, and Met-Asn-starting protein N termini, is presumed to Nt-acetylate 15% of all yeast and 18% of all human proteins. We here report on the evolutionary traits of NatB from yeast to human and demonstrate that ectopically expressed hNatB in a yNatB-Δ yeast strain partially complements the natB-Δ phenotypes and partially restores the yNatB Nt-acetylome. Overall, combining quantitative N-terminomics with yeast studies and knockdown of hNatB in human cell lines, led to the unambiguous identification of 180 human and 110 yeast NatB substrates. Interestingly, these substrates included Met-Gln- N-termini, which are thus now classified as in vivo NatB substrates. We also demonstrate the requirement of hNatB activity for maintaining the structure and function of actomyosin fibers and for proper cellular migration. In addition, expression of tropomyosin-1 restored the altered focal adhesions and cellular migration defects observed in hNatB-depleted HeLa cells, indicative for the conserved link between NatB, tropomyosin, and actin cable function from yeast to human.

Cellular plasticity confers migratory and invasive advantages to a population of glioblastoma-initiating cells that infiltrate peritumoral tissue.

Glioblastoma (GBM) is associated with infiltration of peritumoral (PT) parenchyma by isolated tumor cells that leads to tumor regrowth. Recently, GBM stem-like or initiating cells (GICs) have been identified in the PT area, but whether these GICs have enhanced migratory and invasive capabilities compared with GICs from the tumor mass (TM) is presently unknown. We isolated GICs from the infiltrated PT tissue and the TM of three patients and found that PT cells have an advantage over TM cells in two-dimensional and three-dimensional migration and invasion assays. Interestingly, PT cells display a high plasticity in protrusion formation and cell shape and their migration is insensitive to substrate stiffness, which represent advantages to infiltrate microenvironments of different rigidity. Furthermore, mouse and chicken embryo xenografts revealed that only PT cells showed a dispersed distribution pattern, closely associated to blood vessels. Consistent with cellular plasticity, simultaneous Rac and RhoA activation are required for the enhanced invasive capacity of PT cells. Moreover, Rho GTPase signaling modulators αVß3 and p27 play key roles in GIC invasiveness. Of note, p27 is upregulated in TM cells and inhibits RhoA activity. Gene silencing of p27 increased the invasive capacity of TM GICs. Additionally, ß3 integrin is upregulated in PT cells. Blockade of dimeric integrin αVß3, a Rac activator, reduced the invasive capacity of PT GICs in vitro and abrogated the spreading of PT cells into chicken embryos. Thus, our results describe the invasive features acquired by a unique subpopulation of GICs that infiltrate neighboring tissue.

Differential stiffness between brain vasculature and parenchyma promotes metastatic infiltration through vessel co-option.

In brain metastasis, cancer cells remain in close contact with the existing vasculature and can use vessels as migratory paths-a process known as vessel co-option. However, the mechanisms regulating this form of migration are poorly understood. Here we use ex vivo brain slices and an organotypic in vitro model for vessel co-option to show that cancer cell invasion along brain vasculature is driven by the difference in stiffness between vessels and the brain parenchyma. Imaging analysis indicated that cells move along the basal surface of vessels by adhering to the basement membrane extracellular matrix. We further show that vessel co-option is enhanced by both the stiffness of brain vasculature, which reinforces focal adhesions through a talin-dependent mechanism, and the softness of the surrounding environment that permits cellular movement. Our work reveals a mechanosensing mechanism that guides cell migration in response to the tissue's intrinsic mechanical heterogeneity, with implications in cancer invasion and metastasis.

Durable lymph-node expansion is associated with the efficacy of therapeutic vaccination.

Following immunization, lymph nodes dynamically expand and contract. The mechanical and cellular changes enabling the early-stage expansion of lymph nodes have been characterized, yet the durability of such responses and their implications for adaptive immunity and vaccine efficacy are unknown. Here, by leveraging high-frequency ultrasound imaging of the lymph nodes of mice, we report more potent and persistent lymph-node expansion for animals immunized with a mesoporous silica vaccine incorporating a model antigen than for animals given bolus immunization or standard vaccine formulations such as alum, and that durable and robust lymph-node expansion was associated with vaccine efficacy and adaptive immunity for 100 days post-vaccination in a mouse model of melanoma. Immunization altered the mechanical and extracellular-matrix properties of the lymph nodes, drove antigen-dependent proliferation of immune and stromal cells, and altered the transcriptional features of dendritic cells and inflammatory monocytes. Strategies that robustly maintain lymph-node expansion may result in enhanced vaccination outcomes.

Basal delamination during mouse gastrulation primes pluripotent cells for differentiation.

The blueprint of the mammalian body plan is laid out during gastrulation, when a trilaminar embryo is formed. This process entails a burst of proliferation, the ingression of embryonic epiblast cells at the primitive streak, and their priming toward primitive streak fates. How these different events are coordinated remains unknown. Here, we developed and characterized a 3D culture of self-renewing mouse embryonic cells that captures the main transcriptional and architectural features of the early gastrulating mouse epiblast. Using this system in combination with microfabrication and in vivo experiments, we found that proliferation-induced crowding triggers delamination of cells that express high levels of the apical polarity protein aPKC. Upon delamination, cells become more sensitive to Wnt signaling and upregulate the expression of primitive streak markers such as Brachyury. This mechanistic coupling between ingression and differentiation ensures that the right cell types become specified at the right place during embryonic development.

Metabolic glycan labeling immobilizes dendritic cell membrane and enhances antitumor efficacy of dendritic cell vaccine.

Dendritic cell (DC) vaccine was among the first FDA-approved cancer immunotherapies, but has been limited by the modest cytotoxic T lymphocyte (CTL) response and therapeutic efficacy. Here we report a facile metabolic labeling approach that enables targeted modulation of adoptively transferred DCs for developing enhanced DC vaccines. We show that metabolic glycan labeling can reduce the membrane mobility of DCs, which activates DCs and improves the antigen presentation and subsequent T cell priming property of DCs. Metabolic glycan labeling itself can enhance the antitumor efficacy of DC vaccines. In addition, the cell-surface chemical tags (e.g., azido groups) introduced via metabolic glycan labeling also enable in vivo conjugation of cytokines onto adoptively transferred DCs, which further enhances CTL response and antitumor efficacy. Our DC labeling and targeting technology provides a strategy to improve the therapeutic efficacy of DC vaccines, with minimal interference upon the clinical manufacturing process.

Hydrogel viscoelasticity modulates migration and fusion of mesenchymal stem cell spheroids.

Multicellular spheroids made of stem cells can act as building blocks that fuse to capture complex aspects of native in vivo environments, but the effect of hydrogel viscoelasticity on cell migration from spheroids and their fusion remains largely unknown. Here, we investigated the effect of viscoelasticity on migration and fusion behavior of mesenchymal stem cell (MSC) spheroids using hydrogels with a similar elasticity but different stress relaxation profiles. Fast relaxing (FR) matrices were found to be significantly more permissive to cell migration and consequent fusion of MSC spheroids. Mechanistically, inhibition of ROCK and Rac1 pathways prevented cell migration. Moreover, the combination of biophysical and biochemical cues provided by fast relaxing hydrogels and platelet-derived growth factor (PDGF) supplementation, respectively, resulted in a synergistic enhancement of migration and fusion. Overall, these findings emphasize the important role of matrix viscoelasticity in tissue engineering and regenerative medicine strategies based on spheroids.

AMPK is a mechano-metabolic sensor linking cell adhesion and mitochondrial dynamics to Myosin-dependent cell migration.

Cell migration is crucial for cancer dissemination. We find that AMP-activated protein kinase (AMPK) controls cell migration by acting as an adhesion sensing molecular hub. In 3-dimensional matrices, fast-migrating amoeboid cancer cells exert low adhesion/low traction linked to low ATP/AMP, leading to AMPK activation. In turn, AMPK plays a dual role controlling mitochondrial dynamics and cytoskeletal remodelling. High AMPK activity in low adhering migratory cells, induces mitochondrial fission, resulting in lower oxidative phosphorylation and lower mitochondrial ATP. Concurrently, AMPK inactivates Myosin Phosphatase, increasing Myosin II-dependent amoeboid migration. Reducing adhesion or mitochondrial fusion or activating AMPK induces efficient rounded-amoeboid migration. AMPK inhibition suppresses metastatic potential of amoeboid cancer cells in vivo, while a mitochondrial/AMPK-driven switch is observed in regions of human tumours where amoeboid cells are disseminating. We unveil how mitochondrial dynamics control cell migration and suggest that AMPK is a mechano-metabolic sensor linking energetics and the cytoskeleton.

Tissue architecture in tumor initiation and progression.

The 3D architecture of tissues bearing tumors impacts on the mechanical microenvironment of cancer, the accessibility of stromal cells, and the routes of invasion. A myriad of intrinsic and extrinsic forces exerted by the cancer cells, the host tissue, and the molecular and cellular microenvironment modulate the morphology of the tumor and its malignant potential through mechanical, biochemical, genetic, and epigenetic cues. Recent studies have investigated how tissue architecture influences cancer biology from tumor initiation and progression to distant metastatic seeding and response to therapy. With a focus on carcinoma, the most common type of cancer, this review discusses the latest discoveries on how tumor architecture is built and how tissue morphology affects the biology and progression of cancer cells.

Correction: Actuated 3D microgels for single cell mechanobiology.

Correction for 'Actuated 3D microgels for single cell mechanobiology' by Berna Özkale et al., Lab Chip, 2022, 22, 1962-1970, https://doi.org/10.1039/D2LC00203E.

Actuated 3D microgels for single cell mechanobiology.

We present a new cell culture technology for large-scale mechanobiology studies capable of generating and applying optically controlled uniform compression on single cells in 3D. Mesenchymal stem cells (MSCs) are individually encapsulated inside an optically triggered nanoactuator-alginate hybrid biomaterial using microfluidics, and the encapsulating network isotropically compresses the cell upon activation by light. The favorable biomolecular properties of alginate allow cell culture in vitro up to a week. The mechanically active microgels are capable of generating up to 15% compressive strain and forces reaching 400 nN. As a proof of concept, we demonstrate the use of the mechanically active cell culture system in mechanobiology by subjecting singly encapsulated MSCs to optically generated isotropic compression and monitoring changes in intracellular calcium intensity.

Caveolae couple mechanical stress to integrin recycling and activation.

Cells are subjected to multiple mechanical inputs throughout their lives. Their ability to detect these environmental cues is called mechanosensing, a process in which integrins play an important role. During cellular mechanosensing, plasma membrane (PM) tension is adjusted to mechanical stress through the buffering action of caveolae; however, little is known about the role of caveolae in early integrin mechanosensing regulation. Here, we show that Cav1KO fibroblasts increase adhesion to FN-coated beads when pulled with magnetic tweezers, as compared to wild type fibroblasts. This phenotype is Rho-independent and mainly derived from increased active ß1-integrin content on the surface of Cav1KO fibroblasts. Florescence recovery after photobleaching analysis and endocytosis/recycling assays revealed that active ß1-integrin is mostly endocytosed through the clathrin independent carrier/glycosylphosphatidyl inositol (GPI)-enriched endocytic compartment pathway and is more rapidly recycled to the PM in Cav1KO fibroblasts, in a Rab4 and PM tension-dependent manner. Moreover, the threshold for PM tension-driven ß1-integrin activation is lower in Cav1KO mouse embryonic fibroblasts (MEFs) than in wild type MEFs, through a mechanism dependent on talin activity. Our findings suggest that caveolae couple mechanical stress to integrin cycling and activation, thereby regulating the early steps of the cellular mechanosensing response.

Cells can physically sense their immediate environment by pulling and pushing through integrins, a type of proteins which connects the inside and outside of a cell by being studded through the cellular membrane. This sensing role can only be performed when integrins are in an active state. Two main mechanisms regulate the relative amount of active integrins: one controls the activation of the proteins already at the cell surface; the other, known as recycling, impacts how many new integrins are delivered to the membrane. Both processes are affected by changes in cell membrane tension, which is itself controlled by dimples (or 'caveolae' ­ little caves in Latin) present in the cell surface. Caveolae limit acute changes in tension by taking in (pinching off the dimples) or releasing (dimples flattening) segments of the membrane. However, it is still unclear how integrins and caveolae mechanically interact to regulate the ability for a cell to read its environment. To understand this process, Lolo et al. focused on mouse cells genetically manipulated to not build caveolae on their surfaces, and which cannot properly sense mechanical changes in their surroundings. These were exposed to beads covered in an integrin-binding protein and manipulated using magnetic tweezers. The manipulation showed that mutated cells bound to the beads more strongly than non-modified cells, indicating that they had more active integrins on their surface. This change was due to both an accelerated recycling mechanism (which resulted in more integrin being brought at the surface) and an increase in integrin activation (which was triggered by a higher membrane tension). Caveolae therefore couple mechanical inputs to integrin recycling and activation. Healthy tissues rely on cells correctly sensing physical changes in their environment so they can mount an appropriate response. This ability, for example, is altered in cancerous cells which start to form tumours. The findings by Lolo et al. bring together physics and biology to provide new insights into the potential mechanisms causing such impairments.

Mechanical force application to the nucleus regulates nucleocytoplasmic transport.

Mechanical force controls fundamental cellular processes in health and disease, and increasing evidence shows that the nucleus both experiences and senses applied forces. Such forces can lead to the nuclear translocation of proteins, but whether force controls nucleocytoplasmic transport, and how, remains unknown. Here we show that nuclear forces differentially control passive and facilitated nucleocytoplasmic transport, setting the rules for the mechanosensitivity of shuttling proteins. We demonstrate that nuclear force increases permeability across nuclear pore complexes, with a dependence on molecular weight that is stronger for passive than for facilitated diffusion. Owing to this differential effect, force leads to the translocation of cargoes into or out of the nucleus within a given range of molecular weight and affinity for nuclear transport receptors. Further, we show that the mechanosensitivity of several transcriptional regulators can be both explained by this mechanism and engineered exogenously by introducing appropriate nuclear localization signals. Our work unveils a mechanism of mechanically induced signalling, probably operating in parallel with others, with potential applicability across signalling pathways.