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Formation and maintenance of tissue barriers require the coordination of cell mechanics and cell-cell junction assembly. Here, we combined methods to modulate ECM stiffness and to measure mechanical forces on adhesion complexes to investigate how tight junctions regulate cell mechanics and epithelial morphogenesis. We found that depletion of the tight junction adaptor ZO-1 regulates cytoskeletal tension at cell-matrix and cell-cell interfaces in an ECM stiffness-regulated manner, possibly via differential organisation of the actin cytoskeleton. ZO-1 depletion inhibited junction assembly and disrupted morphogenesis in an ECM stiffness-dependent manner. Both processes were rescued by inhibition of cell contractility. Although ZO-1-deficient cells could assemble functional barriers at low tension, their tight junctions remained corrupted with strongly reduced and discontinuous recruitment of junctional components. Our results thus reveal that reciprocal regulation between ZO-1 and cell mechanics controls tight junction assembly and epithelial morphogenesis, and that tension-independent roles of ZO-1 control proper junction organisation.

Collective Cell Migration In article number 2100478 by Tabea A. Oswald, Andreas Janshoff, and co‐workers, it is shown that tight junction ZO proteins maintain epithelial cell sheet fluidity and thereby ensure efficient collective migration. Cell layers lacking ZO proteins lose viscoelastic integrity due to actomyosin remodeling, inducing a severe mechanical imbalance and increased proliferation. Extremely contractile, small cells emerge in a mechanism reminiscent of a tug of war and particularly impair migration. [Image: see text]

Tight junctions (TJs) are essential components of epithelial tissues connecting neighboring cells to provide protective barriers. While their general function to seal compartments is well understood, their role in collective cell migration is largely unexplored. Here, the importance of the TJ zonula occludens (ZO) proteins ZO1 and ZO2 for epithelial migration is investigated employing video microscopy in conjunction with velocimetry, segmentation, cell tracking, and atomic force microscopy/spectroscopy. The results indicate that ZO proteins are necessary for fast and coherent migration. In particular, ZO1 and 2 loss (dKD) induces actomyosin remodeling away from the central cortex towards the periphery of individual cells, resulting in altered viscoelastic properties. A tug‐of‐war emerges between two subpopulations of cells with distinct morphological and mechanical properties: 1) smaller and highly contractile cells with an outward bulging apical membrane, and 2) larger, flattened cells, which, due to tensile stress, display a higher proliferation rate. In response, the cell density increases, leading to crowding‐induced jamming and more small cells over time. Co‐cultures comprising wildtype and dKD cells migrate inefficiently due to phase separation based on differences in contractility rather than differential adhesion. This study shows that ZO proteins are necessary for efficient collective cell migration by maintaining tissue fluidity and controlling proliferation., In this article, it is shown that tight junction zonula occludens (ZO) proteins maintain epithelial cell sheet fluidity and thereby ensure efficient and coherent migration. Cells lacking these proteins lose viscoelastic tissue integrity due to actomyosin remodeling and increase proliferation, inducing cellular crowding. Particularly, upon ZO protein loss, overly contractile small cells at high cell densities eventually impair migration by causing jamming.

Imaging dynamics of cellular morphogenesis with high spatial-temporal resolution in 3D is challenging, due to the low spatial resolution along the optical axis and photo-toxicity. However, some cellular structures are planar and hence 2D imaging should be sufficient, provided that the structure of interest can be oriented with respect to the optical axis of the microscope. Here, we report a 3D microfabrication method which positions and orients cell divisions very close to the microscope coverglass. We use this approach to study cytokinesis in fission yeasts and polarization to lumen formation in mammalian epithelial cells. We show that this method improves spatial resolution on range of common microscopies, including super-resolution STED. Altogether, this method could shed new lights on self-organization phenomena in single cells and 3D cell culture systems.

Primary cilia are microtubule-based organelles involved in key signaling and sensing processes in eukaryotic cells. Unlike motile cilia, which have been thoroughly studied, the structure and the composition of primary cilia remain largely unexplored despite their fundamental role in development and homeostasis. They have for long been falsely regarded as simplified versions of motile cilia because they lack distinctive elements such as dynein arms, radial spokes, and central pair complex. However, revealing the detailed molecular composition and 3D structure of primary cilia is necessary in order to understand the mechanisms that govern their functions. Such structural investigations are so far being precluded by the challenging preparation of primary cilia for cryo-electron microscopy. Here, we developed an enabling method for investigating the structure of primary cilia at molecular resolution by cryo-electron tomography. We show that the well-known “9+0” arrangement of microtubule doublets is present only at the base of the primary cilium. A few microns away from the base the ciliary architecture changes into an unstructured bundle of EB1-decorated microtubule singlets and some actin filaments. Our results suggest the existence of a previously unobserved crosstalk between actin filaments and microtubules in the primary cilium. Our work provides unprecedented insights into the molecular structure of primary cilia and a general framework for uncovering their molecular composition and function in health and disease. This opens up new possibilities to study aspects of this important organelle that have so far been out of reach.

Quantifying molecular dynamics of cell membrane constituents is required to understand organization and function of biological membranes. Because of its complex structure unambiguous interpretation of molecular membrane dynamics requires high spatial and temporal resolution measurements. In this paper, we provide a comprehensive description of circle scanning fluorescence correlation spectroscopy and its combination with stimulated emission depletion microscopy (CS-STED-FCS). This method allows quantification of sub-diffusion processes and direct mapping of heterogeneities in membranes with high spatiotemporal resolution. We show how to use model membranes to calibrate and test the technique and how to apply it in the context of living cells to quantify membrane dynamics with high spatiotemporal resolution and good statistics.

Primary cilia are microtubule-based organelles that are important for signaling and sensing in eukaryotic cells. Unlike the thoroughly studied motile cilia, the three-dimensional architecture and molecular composition of primary cilia are largely unexplored. Yet, studying these aspects is necessary to understand how primary cilia function in health and disease. We developed an enabling method for investigating the structure of primary cilia isolated from MDCK-II cells at molecular resolution by cryo-electron tomography. We show that the textbook ‘9 + 0’ arrangement of microtubule doublets is only present at the primary cilium base. A few microns out, the architecture changes into an unstructured bundle of EB1-decorated microtubules and actin filaments, putting an end to a long debate on the presence or absence of actin filaments in primary cilia. Our work provides a plethora of insights into the molecular structure of primary cilia and offers a methodological framework to study these important organelles. Primary cilia are isolated from monolayers of mammalian cells in culture for cryo-ET analyses, revealing new features as well as how they differ from motile cilia.

Imaging dynamics of cellular morphogenesis with high spatial-temporal resolution in 3D is challenging, due to the low spatial resolution along the optical axis and photo-toxicity. However, some cellular structures are planar and hence 2D imaging should be sufficient, provided that the structure of interest can be oriented with respect to the optical axis of the microscope. Here, we report a 3D microfabrication method which positions and orients cell divisions very close to the microscope coverglass. We use this approach to study cytokinesis in fission yeasts and polarization to lumen formation in mammalian epithelial cells. We show that this method improves spatial resolution on range of common microscopies, including super-resolution STED. Altogether, this method could shed new lights on self-organization phenomena in single cells and 3D cell culture systems.

Plasticity is an essential condition for cancer cells to invade surrounding tissues. The nucleus is the most rigid cellular organelle and it undergoes substantial deformations to get through environmental constrictions. Nuclear stiffness mostly depends on the nuclear lamina and chromatin, which in turn might be affected by nuclear architectural proteins. Among these is the HMGA1 (High Mobility Group A1) protein, a factor that plays a causal role in neoplastic transformation and that is able to disentangle heterochromatic domains by H1 displacement. Here we made use of atomic force microscopy to analyze the stiffness of breast cancer cellular models in which we modulated HMGA1 expression to investigate its role in regulating nuclear plasticity. Since histone H1 is the main modulator of chromatin structure and HMGA1 is a well-established histone H1 competitor, we correlated HMGA1 expression and cellular stiffness with histone H1 expression level, post-translational modifications, and nuclear distribution. Our results showed that HMGA1 expression level correlates with nuclear stiffness, is associated to histone H1 phosphorylation status, and alters both histone H1 chromatin distribution and expression. These data suggest that HMGA1 might promote chromatin relaxation through a histone H1-mediated mechanism strongly impacting on the invasiveness of cancer cells.

Tight junctions are cell adhesion complexes that seal tissues and are involved in cell polarity and signalling. Supra-molecular assembly and positioning of tight junctions as continuous networks of adhesion strands is dependent on the two membrane associated scaffolding proteins ZO1 and ZO2. To understand how ZO proteins organize junction assembly, we performed quantitative cell biology andin vitroreconstitution experiments. We discovered that ZO proteins self-organize membrane attached compartments via phase separation. We identified the multivalent interactions of the conserved PDZ-SH3-GuK supra-domain as the driver of phase separation. These interactions are regulated by phosphorylation and intra-molecular binding. Formation of condensed ZO protein compartments is sufficient to specifically enrich and localize tight junction proteins including adhesion receptors, cytoskeletal adapters and transcription factors. Our results suggest that an active phase transition of ZO proteins into a condensed membrane bound compartment drives claudin polymerization and coalescence of a continuous tight junction belt.

Stimulated emission depletion (STED) microscopy is routinely used to resolve the ultra-structure of cells with a ∼10-fold higher resolution compared to diffraction limited imaging. While STED microscopy is based on preparing the excited state of fluorescent probes with light, the recently developed expansion microscopy (ExM) provides sub-diffraction resolution by physically enlarging the sample before microscopy. Expansion of fixed cells by crosslinking and swelling of hydrogels easily enlarges the sample ∼4-fold and hence increases the effective optical resolution by this factor. To overcome the current limits of these complimentary approaches, we here combined ExM with STED (ExSTED) and demonstrate an increase in resolution of up to 30-fold compared to conventional microscopy (

Epithelia cells assemble into sheets that compartmentalize organs and generate tissue barriers. This is achieved by forming polarized membrane domains, which are connected by junctional complexes. While much is known about the organization of the basal membrane due to its easy accessibility by high and super-resolution microscopy, the apical and lateral membrane domains remain poorly characterized. Here we describe our methods to study the molecular organization of apical and lateral membrane domains by combining 2D and 3D epithelial cell culture with super-resolution STED microscopy. We show that inverted cell monolayers enable live cell imaging of the apical membrane with a resolution sufficient to resolve the densely packed micro-villi of human enterocytes. Furthermore, 3D cell culture enables us to resolve adhesion complexes in the lateral domain of kidney derived cells. We envision that these methods will help to reveal the supra-molecular structure of lateral and apical membrane domains in epithelial cells.

Summary Actin plays a critical role during the early stages of pathogenic microbe internalization by immune cells. In this study, we identified a key mechanism of actin filament tethering and stabilization to the surface of phagosomes in human dendritic cells. We found that the actin-binding protein SWAP70 is specifically recruited to nascent phagosomes by binding to the lipid phosphatidylinositol (3,4)-bisphosphate. Multi-color super-resolution stimulated emission depletion (STED) microscopy revealed that the actin cage surrounding early phagosomes is formed by multiple concentric rings containing SWAP70. SWAP70 colocalized with and stimulated activation of RAC1, a known activator of actin polymerization, on phagosomes. Genetic ablation of SWAP70 impaired actin polymerization around phagosomes and resulted in a phagocytic defect. These data show a key role for SWAP70 as a scaffold for tethering the peripheral actin cage to phagosomes., Graphical Abstract, Highlights • SWAP70 is transiently recruited to the nascent cup of emerging phagosomes • Phagosomal recruitment of SWAP70 is driven by RAC1 and phosphoinositides • SWAP70 tethers concentric rings of F-actin on the phagosomal membrane • Absence of SWAP70 results in phagocytic defects, Baranov et al. find a key role for SWAP70 in tethering concentric rings of F-actin to emerging phagosomes. Specific phagosomal recruitment of SWAP70 is achieved by interactions with the GTPase RAC1 and phosphatidylinositol (3,4)-bisphosphate. Absence of SWAP70 leads to disruption of phagosomal actin and impaired phagocytosis.