Isotropic actomyosin dynamics promote organization of the apical cell cortex in epithelial cells

Distal renal tubular system-on-a-chip for studying the pathogenesis of influenza A virus-induced kidney injury

Influenza A viruses typically cause acute respiratory infections in humans. However, virus-induced acute kidney injury (AKI) has dramatically increased mortality. The pathogenesis remains poorly understood due to limited disease models. Here, a distal renal tubular system-on-a-chip (dRTSC) was constructed to explore the pathogenesis. The renal tubule-vascular reabsorption interface was recapitulated by co-culturing the distal renal tubule and peritubular vessel with a collagen-coated porous membrane. To study the pathways of influenza virus entry into the kidney, dynamic tracking of fluorescence-labeled virus-infected blood vessels was performed. For the first time, the virus was shown to enter the kidney rapidly by cell-free transmission without disrupting the vascular barrier. Direct virus infection of renal tubules in dRTSC reveals disruption of tight junctions, microvilli formation, polar distribution of ion transporters, and sodium reabsorption function. This robust platform allows for a straightforward investigation of virus-induced AKI pathogenesis. The combination with single-virus tracking technology provides new insights into understanding influenza virus-induced extra-respiratory disease.

Intestinal brush border formation requires a TMIGD1-based intermicrovillar adhesion complex

Intestinal epithelial cells absorb nutrients through the brush border, composed of dense arrays of highly ordered microvilli at their apical membranes. A protocadherin-based intermicrovillar adhesion complex localized at microvilli tips mediates microvilli packing and organization. Here, we identified a second adhesion complex localized at the proximal base region of microvilli. This complex contained the immunoglobulin superfamily member TMIGD1, which directly interacted with the microvillar scaffolding proteins EBP50 and E3KARP. Complex formation with EBP50 required the activation of EBP50 by the actin-binding protein ezrin and was enhanced by the dephosphorylation of Ser¹⁶² in the PDZ2 domain of EBP50 by the phosphatase PP1α. Binding of the EBP50-ezrin complex to TMIGD1 enhanced the dynamic turnover of EBP50 at microvilli. Enterocyte-specific inactivation of Tmigd1 in mice resulted in microvillar blebbing, loss of intermicrovillar adhesion, and perturbed brush border formation. Thus, we identified a second adhesion complex in microvilli and propose a mechanism that promotes microvillar formation and dynamics.

In Situ Visualization of Dynamic Cellular Effects of Phospholipid Nanoparticles via High-Speed Scanning Ion Conductance Microscopy

Phospholipid nanoparticles have been actively employed for numerous biomedical applications. A key factor in ensuring effective and safe applications of these nanomaterials is the regulation of their interactions with target cells, which is significantly dependent on an in-depth understanding of the nanoparticle-cell interactions. To date, most studies investigating these nano-bio interactions have been performed under static conditions and may lack crucial real-time information. It is, however, noteworthy that the nanoparticle-cell interactions are highly dynamic. Consequently, to gain a deeper insight into the cellular effects of phospholipid nanoparticles, real-time observation of cellular dynamics after nanoparticle introduction is necessary. Herein, a proof-of-concept in situ visualization of the dynamic cellular effects of sub-100 nm phospholipid nanoparticles using high-speed scanning ion conductance microscopy (HS-SICM) is reported. It is revealed that upon introduction into the cellular environment, within a short timescale of hundreds of seconds, phospholipid nanoparticles can selectively modulate the edge motility and surface roughness of healthy fibroblast and cancerous epithelial cells. Furthermore, the dynamic deformation profiles of these cells can be selectively altered in the presence of phospholipid nanoparticles. This work is anticipated to further shed light on the real-time nanoparticle-cell interactions for improved formulation of phospholipid nanoparticles for numerous bioapplications.

Roles of the actin cytoskeleton in ciliogenesis

Primary cilia play a key role in the ability of cells to respond to extracellular stimuli, such as signaling molecules and environmental cues. These sensory organelles are crucial to the development of many organ systems, and defects in primary ciliogenesis lead to multisystemic genetic disorders, known as ciliopathies. Here, we review recent advances in the understanding of several key aspects of the regulation of ciliogenesis. Primary ciliogenesis is thought to take different pathways depending on cell type, and some recent studies shed new light on the cell-type-specific mechanisms regulating ciliogenesis at the apical surface in polarized epithelial cells, which are particularly relevant for many ciliopathies. Furthermore, recent findings have demonstrated the importance of actin cytoskeleton dynamics in positively and negatively regulating multiple stages of ciliogenesis, including the vesicular trafficking of ciliary components and the positioning and docking of the basal body. Finally, studies on the formation of motile cilia in multiciliated epithelial cells have revealed requirements for actin remodeling in this process too, as well as showing evidence of an additional alternative ciliogenesis pathway.

Smoothelin-like 2 Inhibits Coronin-1B to Stabilize the Apical Actin Cortex during Epithelial Morphogenesis

The actin cortex is involved in many biological processes and needs to be significantly remodeled during cell differentiation. Developing epithelial cells construct a dense apical actin cortex to carry out their barrier and exchange functions. The apical cortex assembles in response to three-dimensional (3D) extracellular cues, but the regulation of this process during epithelial morphogenesis remains unknown. Here, we describe the function of Smoothelin-like 2 (SMTNL2), a member of the smooth-muscle-related Smoothelin protein family, in apical cortex maturation. SMTNL2 is induced during development in multiple epithelial tissues and localizes to the apical and junctional actin cortex in intestinal and kidney epithelial cells. SMTNL2 deficiency leads to membrane herniations in the apical domain of epithelial cells, indicative of cortex abnormalities. We find that SMTNL2 binds to actin filaments and is required to slow down the turnover of apical actin. We also characterize the SMTNL2 proximal interactome and find that SMTNL2 executes its functions partly through inhibition of coronin-1B. Although coronin-1B-mediated actin dynamics are required for early morphogenesis, its sustained activity is detrimental for the mature apical shape. SMTNL2 binds to coronin-1B through its N-terminal coiled-coil region and negates its function to stabilize the apical cortex. In sum, our results unveil a mechanism for regulating actin dynamics during epithelial morphogenesis, providing critical insights on the developmental control of the cellular cortex.

Epithelial colonies in vitro elongate through collective effects

Epithelial tissues of the developing embryos elongate by different mechanisms, such as neighbor exchange, cell elongation, and oriented cell division. Since autonomous tissue self-organization is influenced by external cues such as morphogen gradients or neighboring tissues, it is difficult to distinguish intrinsic from directed tissue behavior. The mesoscopic processes leading to the different mechanisms remain elusive. Here, we study the spontaneous elongation behavior of spreading circular epithelial colonies in vitro. By quantifying deformation kinematics at multiple scales, we report that global elongation happens primarily due to cell elongations, and its direction correlates with the anisotropy of the average cell elongation. By imposing an external time-periodic stretch, the axis of this global symmetry breaking can be modified and elongation occurs primarily due to orientated neighbor exchange. These different behaviors are confirmed using a vertex model for collective cell behavior, providing a framework for understanding autonomous tissue elongation and its origins.

Advances in fluorescence microscopy techniques to study kidney function

Fluorescence microscopy, in particular immunofluorescence microscopy, has been used extensively for the assessment of kidney function and pathology for both research and diagnostic purposes. The development of confocal microscopy in the 1950s enabled imaging of live cells and intravital imaging of the kidney; however, confocal microscopy is limited by its maximal spatial resolution and depth. More recent advances in fluorescence microscopy techniques have enabled increasingly detailed assessment of kidney structure and provided extraordinary insights into kidney function. For example, nanoscale precise imaging by rapid beam oscillation (nSPIRO) is a super-resolution microscopy technique that was originally developed for functional imaging of kidney microvilli and enables detection of dynamic physiological events in the kidney. A variety of techniques such as fluorescence recovery after photobleaching (FRAP), fluorescence correlation spectroscopy (FCS) and Förster resonance energy transfer (FRET) enable assessment of interaction between proteins. The emergence of other super-resolution techniques, including super-resolution stimulated emission depletion (STED), photoactivated localization microscopy (PALM), stochastic optical reconstruction microscopy (STORM) and structured illumination microscopy (SIM), has enabled functional imaging of cellular and subcellular organelles at ≤50 nm resolution. The deep imaging via emission recovery (DIVER) detector allows deep, label-free and high-sensitivity imaging of second harmonics, enabling assessment of processes such as fibrosis, whereas fluorescence lifetime imaging microscopy (FLIM) enables assessment of metabolic processes.

Effect of allosteric inhibition of non-muscle myosin 2 on its intracellular diffusion

Subcellular dynamics of non-muscle myosin 2 (NM2) is crucial for a broad-array of cellular functions. To unveil mechanisms of NM2 pharmacological control, we determined how the dynamics of NM2 diffusion is affected by NM2′s allosteric inhibitors, i.e. blebbistatin derivatives, as compared to Y-27632 inhibiting ROCK, NM2′s upstream regulator. We found that NM2 diffusion is markedly faster in central fibers than in peripheral stress fibers. Y-27632 accelerated NM2 diffusion in both peripheral and central fibers, whereas in peripheral fibers blebbistatin derivatives slightly accelerated NM2 diffusion at low, but markedly slowed it at high inhibitor concentrations. In contrast, rapid NM2 diffusion in central fibers was unaffected by direct NM2 inhibition. Using our optopharmacological tool, Molecular Tattoo, sub-effective concentrations of a photo-crosslinkable blebbistatin derivative were increased to effective levels in a small, irradiated area of peripheral fibers. These findings suggest that direct allosteric inhibition affects the diffusion profile of NM2 in a markedly different manner compared to the disruption of the upstream control of NM2. The pharmacological action of myosin inhibitors is channeled through autonomous molecular processes and might be affected by the load acting on the NM2 proteins.

Chlorpromazine Induces Basolateral Aquaporin-2 Accumulation via F-Actin Depolymerization and Blockade of Endocytosis in Renal Epithelial Cells

We previously showed that in polarized Madin-Darby canine kidney (MDCK) cells, aquaporin-2 (AQP2) is continuously targeted to the basolateral plasma membrane from which it is rapidly retrieved by clathrin-mediated endocytosis. It then undertakes microtubule-dependent transcytosis toward the apical plasma membrane. In this study, we found that treatment with chlorpromazine (CPZ, an inhibitor of clathrin-mediated endocytosis) results in AQP2 accumulation in the basolateral, but not the apical plasma membrane of epithelial cells. In MDCK cells, both AQP2 and clathrin were concentrated in the basolateral plasma membrane after CPZ treatment (100 µM for 15 min), and endocytosis was reduced. Then, using rhodamine phalloidin staining, we found that basolateral, but not apical, F-actin was selectively reduced by CPZ treatment. After incubation of rat kidney slices in situ with CPZ (200 µM for 15 min), basolateral AQP2 and clathrin were increased in principal cells, which simultaneously showed a significant decrease of basolateral compared to apical F-actin staining. These results indicate that clathrin-dependent transcytosis of AQP2 is an essential part of its trafficking pathway in renal epithelial cells and that this process can be inhibited by selectively depolymerizing the basolateral actin pool using CPZ.

Endothelial Nanomechanics in the Context of Endothelial (Dys)function and Inflammation

SIGNIFICANCE

Stiffness of endothelial cells is closely linked to the function of the vasculature as it regulates the release of vasoactive substances such as nitric oxide (NO) and reactive oxygen species. The outer layer of endothelial cells, consisting of the glycocalyx above and the cortical zone beneath the plasma membrane, is a vulnerable compartment able to adapt its nanomechanical properties to any changes of forces exerted by the adjacent blood stream. Sustained stiffening of this layer contributes to the development of endothelial dysfunction and vascular pathologies. Recent Advances: The development of specific techniques to quantify the mechanical properties of cells enables the detailed investigation of the mechanistic link between structure and function of cells.

CRITICAL ISSUES

Challenging the mechanical stiffness of cells, for instance, by inflammatory mediators can lead to the development of endothelial dysfunction. Prevention of sustained stiffening of the outer layer of endothelial cells in turn improves endothelial function.

FUTURE DIRECTIONS

The mechanical properties of cells can be used as critical marker and test system for the proper function of the vascular system. Pharmacological substances, which are able to improve endothelial nanomechanics and function, could take a new importance in the prevention and treatment of vascular diseases. Thus, detailed knowledge acquisition about the structure/function relationship of endothelial cells and the underlying signaling pathways should be promoted.

Spatial Fluctuations at Vertices of Epithelial Layers: Quantification of Regulation by Rho Pathway

In living matter, shape fluctuations induced by acto-myosin are usually studied in vitro via reconstituted gels, whose properties are controlled by changing the concentrations of actin, myosin, and cross-linkers. Such an approach deliberately avoids consideration of the complexity of biochemical signaling inherent to living systems. Acto-myosin activity inside living cells is mainly regulated by the Rho signaling pathway, which is composed of multiple layers of coupled activators and inhibitors. Here, we investigate how such a pathway controls the dynamics of confluent epithelial tissues by tracking the displacements of the junction points between cells. Using a phenomenological model to analyze the vertex fluctuations, we rationalize the effects of different Rho signaling targets on the emergent tissue activity by quantifying the effective diffusion coefficient, and the persistence time and length of the fluctuations. Our results reveal an unanticipated correlation between layers of activation/inhibition and spatial fluctuations within tissues. Overall, this work connects regulation via biochemical signaling with mesoscopic spatial fluctuations, with potential application to the study of structural rearrangements in epithelial tissues.

A two-phase response of endothelial cells to hydrostatic pressure

The vascular endothelium is exposed to three types of mechanical forces: blood flow-mediated shear stress, vessel-diameter dependent wall tension and hydrostatic pressure. Despite considerable variations of blood pressure in normal and pathological physiology, little is known about the acute molecular and cellular effects of hydrostatic pressure on endothelial cells. Here, we used a combination of quantitative fluorescence microscopy, atomic force microscopy and molecular perturbations to characterize the specific response of endothelial cells to pressure application. We identified a two-phase response of endothelial cells to acute (1 h) vs. chronic (24 h) pressure application (100 mmHg). While both regimes induce cortical stiffening, the acute response is linked to calcium-mediated myosin activation, whereas the chronic cell response is dominated by increased cortical actin density and a loss in endothelial barrier function. GsMTx-4 and amiloride inhibit the acute pressure response, which suggest the sodium channel ENaC as key player in endothelial pressure sensing. The described two-phase pressure response may participate in the differential effects of transient changes in blood pressure and hypertension.

Self-assembled nanofiber coatings for controlling cell responses

Nanofibers are thought to enhance cell adhesion, growth, and function. We demonstrate that the choice of building blocks in self-assembling nanofiber systems can be used to control cell behavior. The use of 2 D-coated, self-assembled nanofibers in controlling lens epithelial cells, fibroblasts, and mesenchymal stem cells was investigated, focusing on gene and protein expression related to the fibrotic response. To this end, three nanofibers with different characteristics (morphology, topography, and wettability) were compared with two standard materials frequently used in culturing cells, TCPS, and a collagen type I coating. Cell metabolic activity, cell morphology, and gene and protein expression were analyzed. The most hydrophilic nanofiber with more compact network consisting of small fibers proved to provide a beneficial 2 D environment for cell proliferation and matrix formation while decreasing the fibrotic/stress behavior in all cell lines when compared with TCPS and the collagen type I coating. This nanofiber demonstrates the potential to be used as a biomimetic coating to study the development of fibrosis through epithelial-to-mesenchymal transition. This study also shows that nanofiber structures do not enhance cell function by definition, because the physico-chemical characteristics of the nanofibers influence cell behavior as well and actually can be used to regulate cell behavior toward suboptimal performance. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 2252-2265, 2017.

Dissection and characterization of microtubule bundles in the mitotic spindle using femtosecond laser ablation

The mitotic spindle is a highly organized and dynamic structure required for segregation of the genetic material into two daughter cells. Although most of the individual players involved in building the spindle have been characterized in vitro, a general understanding of how all of the spindle players act together in vivo is still missing. Hence, in recent years, experiments have focused on introducing mechanical perturbations of the spindle on a micron scale, thereby providing insight into its function and organization, as well as into forces acting in the spindle. Among different types of mechanical perturbations, optical ones are more flexible, less invasive, and more precise than other approaches. In this chapter, we describe a detailed protocol for cutting the microtubule bundles in human cells using a near-infrared femtosecond laser. This type of laser microsurgery provides the ability to precisely sever a single microtubule bundle while preserving spindle integrity and dynamics. Furthermore, we describe quantitative measurements obtained from the response of a severed microtubule bundle to laser ablation, which reveal the structure and function of individual parts of the spindle, such as the bridging fiber connecting sister k-fibers. Finally, the method described here can be easily combined with other quantitative techniques to address the complexity of the spindle.

Calcium-mediated actin reset (CaAR) mediates acute cell adaptations

Actin has well established functions in cellular morphogenesis. However, it is not well understood how the various actin assemblies in a cell are kept in a dynamic equilibrium, in particular when cells have to respond to acute signals. Here, we characterize a rapid and transient actin reset in response to increased intracellular calcium levels. Within seconds of calcium influx, the formin INF2 stimulates filament polymerization at the endoplasmic reticulum (ER), while cortical actin is disassembled. The reaction is then reversed within a few minutes. This Calcium-mediated actin reset (CaAR) occurs in a wide range of mammalian cell types and in response to many physiological cues. CaAR leads to transient immobilization of organelles, drives reorganization of actin during cell cortex repair, cell spreading and wound healing, and induces long-lasting changes in gene expression. Our findings suggest that CaAR acts as fundamental facilitator of cellular adaptations in response to acute signals and stress.

DOI:http://dx.doi.org/10.7554/eLife.19850.001

Our skeleton plays a vital role in giving shape and structure to our body, it also allows us to make coordinated movements. Similarly, each cell contains a microscopic network of structures and supports called the cytoskeleton that helps cells to adopt specific shapes and is crucial for them to move around. Unlike our skeleton, which is relatively unchanging, the cytoskeleton of each cell constantly changes and adapts to the specific needs of the cell.

One part of the cytoskeleton is a dense, flexible meshwork of fibers called the cortex that lies just beneath the surface of the cell. The cortex is constructed using a protein called actin, and many of these proteins join together to form each fiber. When cells need to adapt rapidly to an injury or other sudden changes in their environment they activate a so-called stress response. This response often begins with a rapid increase in the amount of calcium ions inside a cell, which can then trigger changes in actin organization. However, it is not clear how cells under stress are able to globally remodel their actin cytoskeleton without compromising stability and integrity of the cortex.

Wales, Schuberth, Aufschnaiter et al. used a range of mammalian cells to investigate how actin responds to stress signals. All cells responded to the resulting influx of calcium ions by deconstructing large parts of the actin cortex and simultaneously forming actin filaments near the center of the cell. Wales, Schuberth, Aufschnaiter et al. termed this response calcium-mediated actin reset (CaAR), as it lasted for only a few minutes before the actin cortex reformed. The experiments show that a protein called INF2 controls CaAR by rapidly removing actin from the cortex and forming new filaments near a cell compartment called the endoplasmic reticulum.

CaAR allows cells to rapidly and drastically alter the cortex in response to stress. The experiments also show that this sudden shift in actin can change the activity of certain genes, leading to longer-term effects on the cell. The findings of Wales, Schuberth, Aufschnaiter et al. suggest that calcium ions globally regulate the actin cytoskeleton and hence cell shape and movement under stress. This could be relevant for many important processes and conditions such as wound healing, inflammation and cancer. A future challenge will be to understand the role of CaAR in these processes.

DOI:http://dx.doi.org/10.7554/eLife.19850.002

PBRM1 Regulates the Expression of Genes Involved in Metabolism and Cell Adhesion in Renal Clear Cell Carcinoma

Polybromo-1 (PBRM1) is a component of the PBAF (Polybromo-associated-BRG1- or BRM-associated factors) chromatin remodeling complex and is the second most frequently mutated gene in clear-cell renal cell Carcinoma (ccRCC). Mutation of PBRM1 is believed to be an early event in carcinogenesis, however its function as a tumor suppressor is not understood. In this study, we have employed Next Generation Sequencing to profile the differentially expressed genes upon PBRM1 re-expression in a cellular model of ccRCC. PBRM1 re-expression led to upregulation of genes involved in cellular adhesion, carbohydrate metabolism, apoptotic process and response to hypoxia, and a downregulation of genes involved in different stages of cell division. The decrease in cellular proliferation upon PBRM1 re-expression was confirmed, validating the functional role of PBRM1 as a tumor suppressor in a cell-based model. In addition, we identified a role for PBRM1 in regulating metabolic pathways known to be important for driving ccRCC, including the regulation of hypoxia response genes, PI3K signaling, glucose uptake, and cholesterol homeostasis. Of particular novelty is the identification of cell adhesion as a major downstream process uniquely regulated by PBRM1 expression. Cytoskeletal reorganization was induced upon PBRM1 reexpression as evidenced from the increase in the number of cells displaying cortical actin, a hallmark of epithelial cells. Genes involved in cell adhesion featured prominently in our transcriptional dataset and overlapped with genes uniquely regulated by PBRM1 in clinical specimens of ccRCC. Genes involved in cell adhesion serve as tumor suppressor and maybe involved in inhibiting cell migration. Here we report for the first time genes linked to cell adhesion serve as downstream targets of PBRM1, and hope to lay the foundation of future studies focusing on the role of chromatin remodelers in bringing about these alterations during malignancies.

Plasticity of the brush border - the yin and yang of intestinal homeostasis

The brush border on the apical surface of enterocytes is a highly specialized structure well-adapted for efficient digestion and nutrient transport, whilst at the same time providing a protective barrier for the intestinal mucosa. The brush border is constituted of a densely ordered array of microvilli, protrusions of the plasma membrane, which are supported by actin-based microfilaments and interacting proteins and anchored in an apical network of actomyosin and intermediate filaments, the so-called terminal web. The highly dynamic, specialized apical domain is both an essential partner for the gut microbiota and an efficient signalling platform that enables adaptation to physiological stimuli from the external and internal milieu. Nevertheless, genetic alterations or various pathological stresses, such as infection, inflammation, and mechanical or nutritional alterations, can jeopardize this equilibrium and compromise intestinal functions. Long-time neglected, the intestinal brush-border shall be enlightening again as the central actor of the complex but essential intestinal homeostasis. Here, we review the processes and components involved in brush border organization and discuss pathological mechanisms that can induce brush border defects and their physiological consequences.

Computer simulations of the mechanical response of brushes on the surface of cancerous epithelial cells

We report a model for atomic force microscopy by means of computer simulations of molecular brushes on surfaces of biological interest such as normal and cancerous cervical epithelial cells. Our model predicts that the force needed to produce a given indentation on brushes that can move on the surface of the cell (called “liquid” brushes) is the same as that required for brushes whose ends are fixed on the cell’s surface (called “solid” brushes), as long as the tip of the microscope covers the entire area of the brush. Additionally, we find that cancerous cells are softer than normal ones, in agreement with various experiments. Moreover, soft brushes are found to display larger resistance to compression than stiff ones. This phenomenon is the consequence of the larger equilibrium length of the soft brushes and the cooperative association of solvent molecules trapped within the brushes, which leads to an increase in the osmotic pressure. Our results show that a careful characterization of the brushes on epithelial cells is indispensable when determining the mechanical response of cancerous cells.

Move your microvilli

Polarized epithelial cells create tightly packed arrays of microvilli in their apical membrane, but the fate of these microvilli is relatively unknown when epithelial cell polarity is lost during wound healing. In this issue, Klingner et al. (2014. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201402037) show that, when epithelial cells become subconfluent, actomyosin contractions locally within the apical cortex cause their microvilli to become motile over the dorsal/apical surface. Their unexpected observations may have implications for epithelial responses in wound healing and disease.