Get round and stiff for mitosis

CRIPTO-based micro-heterogeneity of mouse muscle satellite cells enables adaptive response to regenerative microenvironment

Skeletal muscle repair relies on heterogeneous populations of satellite cells (SCs). The mechanisms that regulate SC homeostasis and state transition during activation are currently unknown. Here, we investigated the emerging role of non-genetic micro-heterogeneity, i.e., intrinsic cell-to-cell variability of a population, in this process. We demonstrate that micro-heterogeneity of the membrane protein CRIPTO in mouse-activated SCs (ASCs) identifies metastable cell states that allow a rapid response of the population to environmental changes. Mechanistically, CRIPTO micro-heterogeneity is generated and maintained through a process of intracellular trafficking coupled with active shedding of CRIPTO from the plasma membrane. Irreversible perturbation of CRIPTO micro-heterogeneity affects the balance of proliferation, self-renewal, and myogenic commitment in ASCs, resulting in increased self-renewal in vivo. Our findings demonstrate that CRIPTO micro-heterogeneity regulates the adaptative response of ASCs to microenvironmental changes, providing insights into the role of intrinsic heterogeneity in preserving stem cell population diversity during tissue repair.

Molecular mechanocytometry using tension-activated cell tagging

Flow cytometry is used routinely to measure single-cell gene expression by staining cells with fluorescent antibodies and nucleic acids. Here, we present tension-activated cell tagging (TaCT) to label cells fluorescently based on the magnitude of molecular force transmitted through cell adhesion receptors. As a proof-of-concept, we analyzed fibroblasts and mouse platelets after TaCT using conventional flow cytometry.

Mitotic outcomes and errors in fibrous environments

During mitosis, cells round up and utilize the interphase adhesion sites within the fibrous extracellular matrix (ECM) as guidance cues to orient the mitotic spindles. Here, using suspended ECM-mimicking nanofiber networks, we explore mitotic outcomes and error distribution for various interphase cell shapes. Elongated cells attached to single fibers through two focal adhesion clusters (FACs) at their extremities result in perfect spherical mitotic cell bodies that undergo significant 3-dimensional (3D) displacement while being held by retraction fibers (RFs). Increasing the number of parallel fibers increases FACs and retraction fiber-driven stability, leading to reduced 3D cell body movement, metaphase plate rotations, increased interkinetochore distances, and significantly faster division times. Interestingly, interphase kite shapes on a crosshatch pattern of four fibers undergo mitosis resembling single-fiber outcomes due to rounded bodies being primarily held in position by RFs from two perpendicular suspended fibers. We develop a cortex-astral microtubule analytical model to capture the retraction fiber dependence of the metaphase plate rotations. We observe that reduced orientational stability, on single fibers, results in increased monopolar mitotic defects, while multipolar defects become dominant as the number of adhered fibers increases. We use a stochastic Monte Carlo simulation of centrosome, chromosome, and membrane interactions to explain the relationship between the observed propensity of monopolar and multipolar defects and the geometry of RFs. Overall, we establish that while bipolar mitosis is robust in fibrous environments, the nature of division errors in fibrous microenvironments is governed by interphase cell shapes and adhesion geometries.

A high-throughput microfluidic device inspired by the Wheatstone bridge principle for characterizing the mechanical properties of single cells

The mechanical properties of single cells have been recognized as biomarkers for identifying individual cells and diagnosing human diseases. Microfluidic devices based on the flow cytometry principle, which are not limited by the vision field of a microscope and can achieve a very high throughput, have been extensively adopted to measure the mechanical properties of single cells. However, these kinds of microfluidic devices usually required pressure-driven pumps with a very low flow rate and high precision. In this study, we developed a high-throughput microfluidic device inspired by the Wheatstone bridge principle for characterizing the mechanical properties of single cells. The microfluidic analogue of the Wheatstone bridge not only took advantage of flow cytometry, but also allowed precise control of a very low flow rate through the constricted channel with a higher input flow rate generated by a commercially available pressure-driven pump. Under different input flow rates of the pump, the apparent elastic moduli and the fluidity of osteosarcoma (U-2OS) cells and cervical carcinoma (HeLa) cells were measured by monitoring their dynamic deformations passing through the bridge-channel with different sizes of rectangular constrictions. The results showed that the input flow rate had little effect on measuring the mechanical properties of the cells, while the ratio of cell radius to effective constriction radius was different, i.e., for U-2OS cells it was 1.20 and for HeLa cells it was 1.09. Under this condition compared with predecessors, our statistic results of cell mechanical properties exhibited minimal errors. Furthermore, the cell viability after measurements was kept above 90% that demonstrated the non-destructive property of our proposed method.

How do cells stiffen?

Cell stiffness is an important characteristic of cells and their response to external stimuli. In this review, we survey methods used to measure cell stiffness, summarize stimuli that alter cell stiffness, and discuss signaling pathways and mechanisms that control cell stiffness. Several pathological states are characterized by changes in cell stiffness, suggesting this property can serve as a potential diagnostic marker or therapeutic target. Therefore, we consider the effect of cell stiffness on signaling and growth processes required for homeostasis and dysfunction in healthy and pathological states. Specifically, the composition and structure of the cell membrane and cytoskeleton are major determinants of cell stiffness, and studies have identified signaling pathways that affect cytoskeletal dynamics both directly and by altered gene expression. We present the results of studies interrogating the effects of biophysical and biochemical stimuli on the cytoskeleton and other cellular components and how these factors determine the stiffness of both individual cells and multicellular structures. Overall, these studies represent an intersection of the fields of polymer physics, protein biochemistry, and mechanics, and identify specific mechanisms involved in mediating cell stiffness that can serve as therapeutic targets.

Fine-tuning cell organelle dynamics during mitosis by small GTPases

During mitosis, the allocation of genetic material concurs with organelle transformation and distribution. The coordination of genetic material inheritance with organelle dynamics directs accurate mitotic progression, cell fate determination, and organismal homeostasis. Small GTPases belonging to the Ras superfamily regulate various cell organelles during division. Being the key regulators of membrane dynamics, the dysregulation of small GTPases is widely associated with cell organelle disruption in neoplastic and non-neoplastic diseases, such as cancer and Alzheimer's disease. Recent discoveries shed light on the molecular properties of small GTPases as sophisticated modulators of a remarkably complex and perfect adaptors for rapid structure reformation. This review collects current knowledge on small GTPases in the regulation of cell organelles during mitosis and highlights the mediator role of small GTPase in transducing cell cycle signaling to organelle dynamics during mitosis.

Skin epithelial cells change their mechanics and proliferation upon snail-mediated EMT signalling

Skin cancer is the most commonly occurring cancer in the USA and Germany, and the fourth most common cancer worldwide. Snail-dependent epithelial-mesenchymal transition (EMT) was shown to initiate and promote skin cancer. Previous studies could show that EMT changes actin cortex regulation and cellular mechanics in epithelial cells of diverse tissue origin. However, in spite of its potentially high significance in the context of skin cancer, the effect of EMT on cellular mechanics, mitotic rounding and proliferation has not been studied in skin epithelial cells so far. In this work, we show that TGF-β-induced partial EMT results in a transformation of the mechanical phenotype of skin epithelial cells in a cell-cycle dependent manner. Concomitantly, we looked at EMT-induced changes of cell proliferation. While EMT decreases proliferation in 2D culture, we observed an EMT-induced boost of cellular proliferation when culturing cells as mechanically confined aggregates of skin epithelial cells. This proliferation boost was accompanied by enhanced mitotic rounding and composition changes of the actin cortex. We give evidence that observed EMT-induced changes depend on the EMT-upregulated transcription factor snail. Overall, our findings indicate that EMT-induced changes of cellular mechanics might play a currently unappreciated role in EMT-induced promotion of skin tumor proliferation.

Anti-migratory effects of Piper betle leaf aqueous extract on cancer cells and its microtubule targeting properties

Piper betle (PB), also known as "betel" in Malay language, is a tropical Asian vine. PB leaves are commonly chewed by Asians along with betel quid. It contains phenols such as eugenol and hydroxychavicol along with chlorophyll, β-carotene, and vitamin C (Salehi et al., 2019). Extracts from PB leaves have various medicinal properties including anticancer, antioxidant, anti-inflammatory, and antibacterial effects (Salehi et al., 2019). Previous research has shown that PB induces cell cycle arrest at late S or G2/M phase and causes apoptosis at higher doses (Wu et al., 2014; Guha Majumdar and Subramanian, 2019). A combination of PB leaf extract has also been shown to enhance the cytotoxicity of the anticancer drug, 5-fluorouracil (5-FU), in cancer cells (Ng et al., 2014).

Nucleus-Cytoskeleton Crosstalk During Mitotic Entry

In preparation for mitosis, cells undergo extensive reorganization of the cytoskeleton and nucleus, so that chromosomes can be efficiently segregated into two daughter cells. Coordination of these cytoskeletal and nuclear events occurs through biochemical regulatory pathways, orchestrated by Cyclin-CDK activity. However, recent studies provide evidence that physical forces are also involved in the early steps of spindle assembly. Here, we will review how the crosstalk of physical forces and biochemical signals coordinates nuclear and cytoplasmic events during the G2-M transition, to ensure efficient spindle assembly and faithful chromosome segregation.

Human mammary epithelial cells in a mature, stratified epithelial layer flatten and stiffen compared to single and confluent cells

BACKGROUND

The epithelium forms a protective barrier against external biological, chemical and physical insults. So far, AFM-based, micro-mechanical measurements have only been performed on single cells and confluent cells, but not yet on cells in mature layers.

METHODS

Using a combination of atomic force, fluorescence and confocal microscopy, we determined the changes in stiffness, morphology and actin distribution of human mammary epithelial cells (HMECs) as they transition from single cells to confluency to a mature layer.

RESULTS

Single HMECs have a tall, round (planoconvex) morphology, have actin stress fibers at the base, have diffuse cortical actin, and have a stiffness of 1 kPa. Confluent HMECs start to become flatter, basal actin stress fibers start to disappear, and actin accumulates laterally where cells abut. Overall stiffness is still 1 kPa with two-fold higher stiffness in the abutting regions. As HMECs mature and form multilayered structures, cells on apical surfaces become flatter (apically more level), wider, and seven times stiffer (mean, 7 kPa) than single and confluent cells. The main drivers of these changes are actin filaments, as cells show strong actin accumulation in the regions where cells adjoin, and in the apical regions.

CONCLUSIONS

HMECs stiffen, flatten and redistribute actin upon transiting from single cells to mature, confluent layers.

GENERAL SIGNIFICANCE

Our findings advance the understanding of breast ductal morphogenesis and mechanical homeostasis.

Mechanochemical control of epidermal stem cell divisions by B-plexins

The precise spatiotemporal control of cell proliferation is key to the morphogenesis of epithelial tissues. Epithelial cell divisions lead to tissue crowding and local changes in force distribution, which in turn suppress the rate of cell divisions. However, the molecular mechanisms underlying this mechanical feedback are largely unclear. Here, we identify a critical requirement of B-plexin transmembrane receptors in the response to crowding-induced mechanical forces during embryonic skin development. Epidermal stem cells lacking B-plexins fail to sense mechanical compression, resulting in disinhibition of the transcriptional coactivator YAP, hyperproliferation, and tissue overgrowth. Mechanistically, we show that B-plexins mediate mechanoresponses to crowding through stabilization of adhesive cell junctions and lowering of cortical stiffness. Finally, we provide evidence that the B-plexin-dependent mechanochemical feedback is also pathophysiologically relevant to limit tumor growth in basal cell carcinoma, the most common type of skin cancer. Our data define a central role of B-plexins in mechanosensation to couple cell density and cell division in development and disease.

Two mechanisms drive pronuclear migration in mouse zygotes

A new life begins with the unification of the maternal and paternal chromosomes upon fertilization. The parental chromosomes first become enclosed in two separate pronuclei near the surface of the fertilized egg. The mechanisms that then move the pronuclei inwards for their unification are only poorly understood in mammals. Here, we report two mechanisms that act in concert to unite the parental genomes in fertilized mouse eggs. The male pronucleus assembles within the fertilization cone and is rapidly moved inwards by the flattening cone. Rab11a recruits the actin nucleation factors Spire and Formin-2 into the fertilization cone, where they locally nucleate actin and further accelerate the pronucleus inwards. In parallel, a dynamic network of microtubules assembles that slowly moves the male and female pronuclei towards the cell centre in a dynein-dependent manner. Both mechanisms are partially redundant and act in concert to unite the parental pronuclei in the zygote's centre.

Cyclin-dependent Kinase 1 and Aurora Kinase choreograph mitotic storage and redistribution of a growth factor receptor

Endosomal trafficking of receptors and associated proteins plays a critical role in signal processing. Until recently, it was thought that trafficking was shut down during cell division. Thus, remarkably, the regulation of trafficking during division remains poorly characterized. Here we delineate the role of mitotic kinases in receptor trafficking during asymmetric division. Targeted perturbations reveal that Cyclin-dependent Kinase 1 (CDK1) and Aurora Kinase promote storage of Fibroblast Growth Factor Receptors (FGFRs) by suppressing endosomal degradation and recycling pathways. As cells progress through metaphase, loss of CDK1 activity permits differential degradation and targeted recycling of stored receptors, leading to asymmetric induction. Mitotic receptor storage, as delineated in this study, may facilitate rapid reestablishment of signaling competence in nascent daughter cells. However, mutations that limit or enhance the release of stored signaling components could alter daughter cell fate or behavior thereby promoting oncogenesis.

This study provides fundamental insights into the crosstalk between cell division and signaling, with implications for cancer. High-resolution in vivo analysis reveals that dividing cells sequester signal receptor proteins into internal compartments; stored receptors are then redistributed as cells complete division.

Physicochemical characterization and genotoxicity of the broad class of carbon nanotubes and nanofibers used or produced in U.S. facilities

BACKGROUND

Carbon nanotubes and nanofibers (CNT/F) have known toxicity but simultaneous comparative studies of the broad material class, especially those with a larger diameter, with computational analyses linking toxicity to their fundamental material characteristics was lacking. It was unclear if all CNT/F confer similar toxicity, in particular, genotoxicity. Nine CNT/F (MW #1-7 and CNF #1-2), commonly found in exposure assessment studies of U.S. facilities, were evaluated with reported diameters ranging from 6 to 150 nm. All materials were extensively characterized to include distributions of physical dimensions and prevalence of bundled agglomerates. Human bronchial epithelial cells were exposed to the nine CNT/F (0-24 μg/ml) to determine cell viability, inflammation, cellular oxidative stress, micronuclei formation, and DNA double-strand breakage. Computational modeling was used to understand various permutations of physicochemical characteristics and toxicity outcomes.

RESULTS

Analyses of the CNT/F physicochemical characteristics illustrate that using detailed distributions of physical dimensions provided a more consistent grouping of CNT/F compared to using particle dimension means alone. In fact, analysis of binning of nominal tube physical dimensions alone produced a similar grouping as all characterization parameters together. All materials induced epithelial cell toxicity and micronuclei formation within the dose range tested. Cellular oxidative stress, DNA double strand breaks, and micronuclei formation consistently clustered together and with larger physical CNT/F dimensions and agglomerate characteristics but were distinct from inflammatory protein changes. Larger nominal tube diameters, greater lengths, and bundled agglomerate characteristics were associated with greater severity of effect. The portion of tubes with greater nominal length and larger diameters within a sample was not the majority in number, meaning a smaller percentage of tubes with these characteristics was sufficient to increase toxicity. Many of the traditional physicochemical characteristics including surface area, density, impurities, and dustiness did not cluster with the toxicity outcomes.

CONCLUSION

Distributions of physical dimensions provided more consistent grouping of CNT/F with respect to toxicity outcomes compared to means only. All CNT/F induced some level of genotoxicity in human epithelial cells. The severity of toxicity was dependent on the sample containing a proportion of tubes with greater nominal lengths and diameters.

Detection of large extracellular silver nanoparticle rings observed during mitosis using darkfield microscopy

During studies on the absorption and interactions between silver nanoparticles and mammalian cells grown in vitro it was observed that large extracellular rings of silver nanoparticles were deposited on the microscope slide, many located near post-mitotic cells. Silver nanoparticles (AgNP, 80nm), coated with citrate, were incubated at concentrations of 0.3 to 30 μg/ml with a human-derived culture of retinal pigment epithelial cells (ARPE-19) and observed using darkfield and fluorescent microscopy, 24 h after treatment. Approximately cell-sized extracellular rings of deposited AgNP were observed on the slides among a field of dispersed individual AgNP. The mean diameter of 45 nanoparticles circles was 62.5 +/-12 microns. Ring structures were frequently observed near what appeared to be post-mitotic daughter cells, giving rise to the possibility that cell membrane fragments were deposited on the slide during mitosis, and those fragments selectively attracted and retained silver nanoparticles from suspension in the cell culture medium. These circular structures were observable for the following technical reasons: 1) darkfield microscope could observe single nanoparticles below 100 nm in size, 2) a large concentration (108 and 109) of nanoparticles was used in these experiments 3) negatively charged nanoparticles were attracted to adhesion membrane proteins remaining on the slide from mitosis. The observation of silver nanoparticles attracted to apparent remnants of cellular mitosis could be a useful tool for the study of normal and abnormal mitosis.

EMT-Induced Cell-Mechanical Changes Enhance Mitotic Rounding Strength

To undergo mitosis successfully, most animal cells need to acquire a round shape to provide space for the mitotic spindle. This mitotic rounding relies on mechanical deformation of surrounding tissue and is driven by forces emanating from actomyosin contractility. Cancer cells are able to maintain successful mitosis in mechanically challenging environments such as the increasingly crowded environment of a growing tumor, thus, suggesting an enhanced ability of mitotic rounding in cancer. Here, it is shown that the epithelial-mesenchymal transition (EMT), a hallmark of cancer progression and metastasis, gives rise to cell-mechanical changes in breast epithelial cells. These changes are opposite in interphase and mitosis and correspond to an enhanced mitotic rounding strength. Furthermore, it is shown that cell-mechanical changes correlate with a strong EMT-induced change in the activity of Rho GTPases RhoA and Rac1. Accordingly, it is found that Rac1 inhibition rescues the EMT-induced cortex-mechanical phenotype. The findings hint at a new role of EMT in successful mitotic rounding and division in mechanically confined environments such as a growing tumor.

Improved 3D cellular morphometry of Caenorhabditis elegans embryos using a refractive index matching medium

Cell shape change is one of the driving forces of animal morphogenesis, and the model organism Caenorhabditis elegans has played a significant role in analyzing the underlying mechanisms involved. The analysis of cell shape change requires quantification of cellular shape descriptors, a method known as cellular morphometry. However, standard C. elegans live imaging methods limit the capability of cellular morphometry in 3D, as spherical aberrations generated by samples and the surrounding medium misalign optical paths. Here, we report a 3D live imaging method for C. elegans embryos that minimized spherical aberrations caused by refractive index (RI) mismatch. We determined the composition of a refractive index matching medium (RIMM) for C. elegans live imaging. The 3D live imaging with the RIMM resulted in a higher signal intensity in the deeper cell layers. We also found that the obtained images improved the 3D cell segmentation quality. Furthermore, our 3D cellular morphometry and 2D cell shape simulation indicated that the germ cell precursor P4 had exceptionally high cortical tension. Our results demonstrate that the RIMM is a cost-effective solution to improve the 3D cellular morphometry of C. elegans. The application of this method should facilitate understanding of C. elegans morphogenesis from the perspective of cell shape changes.

Listeria monocytogenes Interferes with Host Cell Mitosis through Its Virulence Factors InlC and ActA

Listeria monocytogenes is among the best-characterized intracellular pathogens. Its virulence factors, and the way they interfere with host cells to hijack host functions and promote the establishment and dissemination of the infection, have been the focus of multiple studies over the last 30 years. During cellular infection, L. monocytogenes was shown to induce host DNA damage and delay the host cell cycle to its own benefit. However, whether the cell cycle stage would interfere with the capacity of Listeria to infect human cultured cell lines was never assessed. We found here that L. monocytogenes preferentially infects cultured cells in G2/M phases. Inside G2/M cells, the bacteria lead to an increase in the overall mitosis duration by delaying the mitotic exit. We showed that L. monocytogenes infection causes a sustained activation of the spindle assembly checkpoint, which we correlated with the increase in the percentage of misaligned chromosomes detected in infected cells. Moreover, we demonstrated that chromosome misalignment in Listeria-infected cells required the function of two Listeria virulence factors, ActA and InlC. Our findings show the pleiotropic role of Listeria virulence factors and their cooperative action in successfully establishing the cellular infection.

Adherens junction remodelling during mitotic rounding of pseudostratified epithelial cells

Epithelial cells undergo cortical rounding at the onset of mitosis to enable spindle orientation in the plane of the epithelium. In cuboidal epithelia in culture, the adherens junction protein E-cadherin recruits Pins/LGN/GPSM2 and Mud/NuMA to orient the mitotic spindle. In the pseudostratified columnar epithelial cells of Drosophila, septate junctions recruit Mud/NuMA to orient the spindle, while Pins/LGN/GPSM2 is surprisingly dispensable. We show that these pseudostratified epithelial cells downregulate E-cadherin as they round up for mitosis. Preventing cortical rounding by inhibiting Rho-kinase-mediated actomyosin contractility blocks downregulation of E-cadherin during mitosis. Mitotic activation of Rho-kinase depends on the RhoGEF ECT2/Pebble and its binding partners RacGAP1/MgcRacGAP/CYK4/Tum and MKLP1/KIF23/ZEN4/Pav. Cell cycle control of these Rho activators is mediated by the Aurora A and B kinases, which act redundantly during mitotic rounding. Thus, in Drosophila pseudostratified epithelia, disruption of adherens junctions during mitosis necessitates planar spindle orientation by septate junctions to maintain epithelial integrity.

Recognition of Apoptotic Cells by Viruses and Cytolytic Lymphocytes: Target Selection in the Fog of War

Viruses and cytolytic lymphocytes operate in an environment filled with dying and dead cells, and cell fragments. For viruses, irreversible fusion with doomed cells is suicide. For cytotoxic T lymphocyte and natural killer cells, time and limited lytic resources spent on apoptotic targets is wasteful and may result in death of the host. We make the case that the target membrane cytoskeleton is the best source of information regarding the suitability of potential targets for engagement for both viruses and lytic effector cells, and we present experimental evidence for detection of apoptotic cells by HIV, without loss of infectivity.

“If you please… draw me a cell”. Insights from immune cells

Studies in recent years have shed light on the particular features of cytoskeleton dynamics in immune cells, challenging the classical picture drawn from typical adherent cell lines. New mechanisms linking the dynamics of the membrane–cytoskeleton interface to the mechanical properties of immune cells have been uncovered and shown to be essential for immune surveillance functions. In this Essay, we discuss these features, and propose immune cells as a new playground for cell biologists who try to understand how cells adapt to different microenvironments to fulfil their functions efficiently.

Limitation in Controlling the Morphology of Mammalian Vero Cells Induced by Cell Division on Asymmetric Tungsten-Silicon Oxide Nanocomposite

Engineered nanomaterials are often used in tissue engineering applications to influence and manipulate the behavior of cells. Recently, a number of tungsten-silicon oxide nanocomposite devices containing equal width (symmetric) tungsten and silicon oxide parallel line comb structures were developed and used by our group. The devices induced over 90% of seeded cells (Vero) to align within ±20° of the axes of 10 µm wide tungsten lines. Furthermore, a mathematical model was successfully developed to predict this alignment behavior and forecast the minimum width of isolated tungsten lines required to induce such behavior. However, the mechanism by which the widths of the symmetrical tungsten and silicon oxide lines induce the alignment behavior is still unknown. Furthermore, the model was never tested on more complex asymmetrical structures. Herewith, experiments were conducted with mammalian cells on complex asymmetrical structures with unequal tungsten and silicon oxide line widths. Results showed that the model could be extended to more complex pattern structures. In addition, cell morphology on the patterned structures reset during cell division because of mitotic rounding, which reduced the population of cells that elongated and aligned on the tungsten lines. Ultimately, we concluded that it was impossible to achieve a 100% alignment with cells having unsynchronized cell cycles because cell rounding during mitosis took precedence over cell alignment; in other words, internal chemical cues had a stronger role in cell morphology than external cues.

TGF-β Signaling Promotes Tissue Formation during Cardiac Valve Regeneration in Adult Zebrafish

Cardiac valve disease can lead to severe cardiac dysfunction and is thus a frequent cause of morbidity and mortality. Its main treatment is valve replacement, which is currently greatly limited by the poor recellularization and tissue formation potential of the implanted valves. As we still lack suitable animal models to identify modulators of these processes, here we used adult zebrafish and found that, upon valve decellularization, they initiate a rapid regenerative program that leads to the formation of new functional valves. After injury, endothelial and kidney marrow-derived cells undergo cell cycle re-entry and differentiate into new extracellular matrix-secreting valve cells. The TGF-β signaling pathway promotes the regenerative process by enhancing progenitor cell proliferation as well as valve cell differentiation. These findings reveal a key role for TGF-β signaling in cardiac valve regeneration and establish the zebrafish as a model to identify and test factors promoting cardiac valve recellularization and growth.

High-Throughput, Time-Resolved Mechanical Phenotyping of Prostate Cancer Cells

Worldwide, prostate cancer sits only behind lung cancer as the most commonly diagnosed form of the disease in men. Even the best diagnostic standards lack precision, presenting issues with false positives and unneeded surgical intervention for patients. This lack of clear cut early diagnostic tools is a significant problem. We present a microfluidic platform, the Time-Resolved Hydrodynamic Stretcher (TR-HS), which allows the investigation of the dynamic mechanical response of thousands of cells per second to a non-destructive stress. The TR-HS integrates high-speed imaging and computer vision to automatically detect and track single cells suspended in a fluid and enables cell classification based on their mechanical properties. We demonstrate the discrimination of healthy and cancerous prostate cell lines based on the whole-cell, time-resolved mechanical response to a hydrodynamic load. Additionally, we implement a finite element method (FEM) model to characterise the forces responsible for the cell deformation in our device. Finally, we report the classification of the two different cell groups based on their time-resolved roundness using a decision tree classifier. This approach introduces a modality for high-throughput assessments of cellular suspensions and may represent a viable application for the development of innovative diagnostic devices.

Heparan sulfate proteoglycan (HSPG) can take part in cell division: inside and outside

Prior to the cytokinesis, the cell-matrix interactions should be disrupted, and the mitotic cells round up. Prerequisite of mitosis, the centrosomes duplicate, spindle fibers are generated and move away from each other to opposite sides of the cells marking the cell poles. Later, an invagination in the plasma membrane is formed a few minutes after anaphase. This furrow ingression is driven by a contractile actomyosin ring, whose assembly is regulated by RhoA GTPase. At the completion of cytokinesis, the two daughter cells are still connected by a thin intercellular bridge, which is subjected to abscission, as the terminal step of cytokinesis. Here, it is overviewed, how syndecan-4, a transmembrane, heparan sulfate proteoglycan, can contribute to these processes in a phosphorylation-dependent manner.

A Microfluidic Micropipette Aspiration Device to Study Single-Cell Mechanics Inspired by the Principle of Wheatstone Bridge

The biomechanical properties of single cells show great potential for early disease diagnosis and effective treatments. In this study, a microfluidic device was developed for quantifying the mechanical properties of a single cell. Micropipette aspiration was integrated into a microfluidic device that mimics a classical Wheatstone bridge circuit. This technique allows us not only to effectively alter the flow direction for single-cell trapping, but also to precisely control the pressure exerted on the aspirated cells, analogous to the feature of the Wheatstone bridge that can precisely control bridge voltage and current. By combining the micropipette aspiration technique into the microfluidic device, we can effectively trap the microparticles and Hela cells as well as measure the deformability of cells. The Young's modulus of Hela cells was evaluated to be 387 ± 77 Pa, which is consistent with previous micropipette aspiration studies. The simplicity, precision, and usability of our device show good potential for biomechanical trials in clinical diagnosis and cell biology research.

Fluidization-mediated tissue spreading by mitotic cell rounding and non-canonical Wnt signalling

Tissue morphogenesis is driven by mechanical forces that elicit changes in cell size, shape and motion. The extent by which forces deform tissues critically depends on the rheological properties of the recipient tissue. Yet, whether and how dynamic changes in tissue rheology affect tissue morphogenesis and how they are regulated within the developing organism remain unclear. Here, we show that blastoderm spreading at the onset of zebrafish morphogenesis relies on a rapid, pronounced and spatially patterned tissue fluidization. Blastoderm fluidization is temporally controlled by mitotic cell rounding-dependent cell-cell contact disassembly during the last rounds of cell cleavages. Moreover, fluidization is spatially restricted to the central blastoderm by local activation of non-canonical Wnt signalling within the blastoderm margin, increasing cell cohesion and thereby counteracting the effect of mitotic rounding on contact disassembly. Overall, our results identify a fluidity transition mediated by loss of cell cohesion as a critical regulator of embryo morphogenesis.

Merlin/ERM proteins regulate growth factor-induced macropinocytosis and receptor recycling by organizing the plasma membrane:cytoskeleton interface

The architectural and biochemical features of the plasma membrane are governed by its intimate association with the underlying cortical cytoskeleton. The neurofibromatosis type 2 (NF2) tumor suppressor merlin and closely related membrane:cytoskeleton-linking protein ezrin organize the membrane:cytoskeleton interface, a critical cellular compartment that both regulates and is regulated by growth factor receptors. An example of this poorly understood interrelationship is macropinocytosis, an ancient process of nutrient uptake and membrane remodeling that can both be triggered by growth factors and manage receptor availability. We show that merlin deficiency primes the membrane:cytoskeleton interface for epidermal growth factor (EGF)-induced macropinocytosis via a mechanism involving increased cortical ezrin, altered actomyosin, and stabilized cholesterol-rich membranes. These changes profoundly alter EGF receptor (EGFR) trafficking in merlin-deficient cells, favoring increased membrane levels of its heterodimerization partner, ErbB2; clathrin-independent internalization; and recycling. Our work suggests that, unlike Ras transformed cells, merlin-deficient cells do not depend on macropinocytic protein scavenging and instead exploit macropinocytosis for receptor recycling. Finally, we provide evidence that the macropinocytic proficiency of NF2-deficient cells can be used for therapeutic uptake. This work provides new insight into fundamental mechanisms of macropinocytic uptake and processing and suggests new ways to interfere with or exploit macropinocytosis in NF2 mutant and other tumors.

Cell polarity: having and making sense of direction-on the evolutionary significance of the primary cilium/centrosome organ in Metazoa

Cell-autonomous polarity in Metazoans is evolutionarily conserved. I assume that permanent polarity in unicellular eukaryotes is required for cell motion and sensory reception, integration of these two activities being an evolutionarily constrained function. Metazoans are unique in making cohesive multicellular organisms through complete cell divisions. They evolved a primary cilium/centrosome (PC/C) organ, ensuring similar functions to the basal body/flagellum of unicellular eukaryotes, but in different cells, or in the same cell at different moments. The possibility that this innovation contributed to the evolution of individuality, in being instrumental in the early specification of the germ line during development, is further discussed. Then, using the example of highly regenerative organisms like planarians, which have lost PC/C organ in dividing cells, I discuss the possibility that part of the remodelling necessary to reach a new higher-level unit of selection in multi-cellular organisms has been triggered by conflicts among individual cell polarities to reach an organismic polarity. Finally, I briefly consider organisms with a sensorimotor organ like the brain that requires exceedingly elongated polarized cells for its activity. I conclude that beyond critical consequences for embryo development, the conservation of cell-autonomous polarity in Metazoans had far-reaching implications for the evolution of individuality.

Cellular aspect ratio and cell division mechanics underlie the patterning of cell progeny in diverse mammalian epithelia

Cell division is essential to expand, shape, and replenish epithelia. In the adult small intestine, cells from a common progenitor intermix with other lineages, whereas cell progeny in many other epithelia form contiguous patches. The mechanisms that generate these distinct patterns of progeny are poorly understood. Using light sheet and confocal imaging of intestinal organoids, we show that lineages intersperse during cytokinesis, when elongated interphase cells insert between apically displaced daughters. Reducing the cellular aspect ratio to minimize the height difference between interphase and mitotic cells disrupts interspersion, producing contiguous patches. Cellular aspect ratio is similarly a key parameter for division-coupled interspersion in the early mouse embryo, suggesting that this physical mechanism for patterning progeny may pertain to many mammalian epithelia. Our results reveal that the process of cytokinesis in elongated mammalian epithelia allows lineages to intermix and that cellular aspect ratio is a critical modulator of the progeny pattern.

Polyisoprenylated cysteinyl amide inhibitors disrupt actin cytoskeleton organization, induce cell rounding and block migration of non-small cell lung cancer

The malignant potential of Non-Small Cell Lung Cancer (NSCLC) is dependent on cellular processes that promote metastasis. F-actin organization is central to cell migration, invasion, adhesion and angiogenesis, processes involved in metastasis. F-actin remodeling is enhanced by the overexpression and/or hyper-activation of some members of the Rho family of small GTPases. Therefore, agents that mitigate hyperactive Rho proteins may be relevant for controlling metastasis. We previously reported the role of polyisoprenylated cysteinyl amide inhibitors (PCAIs) as potential inhibitors of cancers with hyperactive small GTPases. In this report, we investigate the potential role of PCAIs against NSCLC cells and show that as low as 0.5 μM PCAIs significantly inhibit 2D and 3D NCI-H1299 cell migration by 48% and 45%, respectively. PCAIs at 1 μM inhibited 2D and 3D NCI-H1299 cell invasion through Matrigel by 50% and 85%, respectively. Additionally, exposure to 5 μM of the PCAIs for 24 h caused at least a 66% drop in the levels of Rac1, Cdc42, and RhoA and a 38% drop in F-actin intensity at the cell membrane. This drop in F-actin was accompanied by a 73% reduction in the number of filopodia per cell. Interestingly, the polyisoprenyl group of the PCAIs is essential for these effects, as NSL-100, a non-farnesylated analog, does not elicit similar effects on F-actin assembly and organization. Our findings indicate that PCAIs disrupt F-actin assembly and organization to suppress cell motility and invasion. The PCAIs may be an effective therapy option for NSCLC metastasis and invasion control.

How cells sense their own shape - mechanisms to probe cell geometry and their implications in cellular organization and function

Cells come in a variety of shapes that most often underlie their functions. Regulation of cell morphogenesis implies that there are mechanisms for shape sensing that still remain poorly appreciated. Global and local cell geometry features, such as aspect ratio, size or membrane curvature, may be probed by intracellular modules, such as the cytoskeleton, reaction-diffusion systems or molecular complexes. In multicellular tissues, cell shape emerges as an important means to transduce tissue-inherent chemical and mechanical cues into intracellular organization. One emergent paradigm is that cell-shape sensing is most often based upon mechanisms of self-organization, rather than determinism. Here, we review relevant work that has elucidated some of the core principles of how cellular geometry may be conveyed into spatial information to guide processes, such as polarity, signaling, morphogenesis and division-plane positioning.

RhoA-stimulated intra-capillary morphology switch facilitates the arrest of individual circulating tumor cells

Metastasis is the primary cause of death for most cancer patients. Hematogenous arrest of circulating tumor cells (CTCs) is an essential prerequisite for metastases formation. Using transparent transgenic zebrafish (kdrl:eGFP; Casper), together with resonant laser scanning confocal microscopy, we tracked the fate of CTCs in vivo in the blood circulation for days. We found the intra-capillary morphology-switch (ICMS) of individual CTCs from strip to sphere was necessary for their intravascular arrests. Further genetic and pharmacological inhibition experiments indicated that the RhoA signaling was necessary for ICMS and the arrest of CTCs. At last, we demonstrated that early treatment by a clinically approved RhoA/ROCK inhibitor, Fasudil, could efficiently inhibit the initial arrest of individual CTCs and reduce the incidence of tumor metastasis in both zebrafish and mouse models. These results together indicate that RhoA-stimulated ICMS represents a mechanism for the arrest of individual CTCs, providing a potential target for future treatments of hematogenous metastatic disease.

Mechanical Forces Program the Orientation of Cell Division during Airway Tube Morphogenesis

Oriented cell division plays a key role in controlling organogenesis. The mechanisms for regulating division orientation at the whole-organ level are only starting to become understood. By combining 3D time-lapse imaging, mouse genetics, and mathematical modeling, we find that global orientation of cell division is the result of a combination of two types of spindles with distinct spindle dynamic behaviors in the developing airway epithelium. Fixed spindles follow the classic long-axis rule and establish their division orientation before metaphase. In contrast, rotating spindles do not strictly follow the long-axis rule and determine their division orientation during metaphase. By using both a cell-based mechanical model and stretching-lung-explant experiments, we showed that mechanical force can function as a regulatory signal in maintaining the stable ratio between fixed spindles and rotating spindles. Our findings demonstrate that mechanical forces, cell geometry, and oriented cell division function together in a highly coordinated manner to ensure normal airway tube morphogenesis.

Genome-scale single-cell mechanical phenotyping reveals disease-related genes involved in mitotic rounding

To divide, most animal cells drastically change shape and round up against extracellular confinement. Mitotic cells facilitate this process by generating intracellular pressure, which the contractile actomyosin cortex directs into shape. Here, we introduce a genome-scale microcantilever- and RNAi-based approach to phenotype the contribution of > 1000 genes to the rounding of single mitotic cells against confinement. Our screen analyzes the rounding force, pressure and volume of mitotic cells and localizes selected proteins. We identify 49 genes relevant for mitotic rounding, a large portion of which have not previously been linked to mitosis or cell mechanics. Among these, depleting the endoplasmic reticulum-localized protein FAM134A impairs mitotic progression by affecting metaphase plate alignment and pressure generation by delocalizing cortical myosin II. Furthermore, silencing the DJ-1 gene uncovers a link between mitochondria-associated Parkinson's disease and mitotic pressure. We conclude that mechanical phenotyping is a powerful approach to study the mechanisms governing cell shape.

Evaluating a novel dimensional reduction approach for mechanical fractionation of cells using a tandem flexible micro spring array (tFMSA)

We present a novel methodology to establish experimental models for the rational design of cell fractionation based on physical properties of cells. Label-free microfluidic separation of cells based on size is a widely employed technique. However, close observation reveals that cell capture results cannot be explained by cell sizes alone. This is particularly apparent with viable cell fractionation, where cells retain their native deformability. We have developed a principal size cutoff (PSC) model based on the analysis of size distribution and size-based filtration efficiency for cell populations. The goal of this analysis is to use an unbiased approach to achieve dimensional reduction of deformability and other mechanical properties that affect cell capture. The PSC model provides a single calibrated principal size component that may be compared directly to device gap width, which is the critical dimension for cell filtration. The PSC model was evaluated experimentally using a tandem flexible micro spring array (tFMSA) device made of parylene filtration elements applied within micro-molded polydimethylsiloxane (PDMS) chambers. In the tFMSA device, a mixture of cells is sequentially passed through individual filters with decreasing gap widths to allow size-based selection. We applied this method to demonstrate viable separation of subgroups of cells with different mechanical properties from complex mixtures, including fractionation according to cancer cell type, cell cycle stage, cell viability status, and leukocyte nuclear phenotype. The PSC methodology and tFMSA device can advance a better understanding of complex factors affecting mechanical cell fractionation and provide a miniature platform for obtaining rationally designed cell fractions for biomedical applications.

The invariant cleavage pattern displayed by ascidian embryos depends on spindle positioning along the cell's longest axis in the apical plane and relies on asynchronous cell divisions

The ascidian embryo is an ideal system to investigate how cell position is determined during embryogenesis. Using 3D timelapse imaging and computational methods we analyzed the planar cell divisions in ascidian early embryos and found that spindles in every cell tend to align at metaphase in the long length of the apical surface except in cells undergoing unequal cleavage. Furthermore, the invariant and conserved cleavage pattern of ascidian embryos was found to consist in alternate planar cell divisions between ectoderm and endomesoderm. In order to test the importance of alternate cell divisions we manipulated zygotic transcription induced by β-catenin or downregulated wee1 activity, both of which abolish this cell cycle asynchrony. Crucially, abolishing cell cycle asynchrony consistently disrupted the spindle orienting mechanism underpinning the invariant cleavage pattern. Our results demonstrate how an evolutionary conserved cell cycle asynchrony maintains the invariant cleavage pattern driving morphogenesis of the ascidian blastula.

DOI:http://dx.doi.org/10.7554/eLife.19290.001

The position of cells within an embryo early in development determines what type of cells they will become in the fully formed embryo. The embryos of ascidians, commonly known as sea squirts, are ideal for studying what influences cell positioning. These embryos consist of a small number of cells that divide according to an “invariant cleavage pattern”, which means that the positioning and timing of the cell divisions is identical between different individuals of the same species. The pattern of cell division is also largely the same across different ascidian species, which raises questions about how it is controlled.

When a cell divides, a structure called the spindle forms inside it to distribute copies of the cell’s genetic material between the new cells. The orientation of the spindle determines the direction in which the cell will divide. Now, by combining 3D imaging of living ascidian embryos with computational modeling, Dumollard et al. show that the spindles in every equally dividing cell tend to all align in the long length of the cell’s “apical” surface. Such alignment allows the cells to be on the outside of the embryo and implements the ascidian invariant cleavage pattern.

The cells in the embryo do not all divide at the same time. Indeed, the shape of the cells (and especially their apical surface) depends on two layers of cells in the embryo not dividing at the same time; instead, periods of cell division alternate between the layers. A network of genes in the embryo regulates the timing of these cell divisions and makes it possible for the cells to divide according to an invariant cleavage pattern.

However, this network of genes is not the only control mechanism that shapes the early embryo. A structure found in egg cells (and hence produced by the embryo’s mother) causes cells at the rear of the embryo to divide unequally, and this influences the shape of all the cells in the embryo. Thus it appears that maternal mechanisms work alongside the embryo’s gene network to shape the early embryo.

The next step will be to determine how physical forces – for example, from the cells pressing against each other – influence the position of the embryo’s cells. How do gene networks relay the biomechanical properties of the embryo to help it take shape?

DOI:http://dx.doi.org/10.7554/eLife.19290.002

Local 3D matrix confinement determines division axis through cell shape

How the division axis is determined in mammalian cells embedded in three-dimensional (3D) matrices remains elusive, despite that many types of cells divide in 3D environments. Cells on two-dimensional (2D) substrates typically round up completely to divide. Here, we show that in 3D collagen matrices, mammalian cells such as HT1080 human fibrosarcoma and MDA-MB-231 breast cancer cells exhibit division modes distinct from their Counterparts on 2D substrates, with a markedly higher fraction of cells remaining highly elongated through mitosis in 3D matrices. The long axis of elongated mitotic cells accurately predicts the division axis, independently of matrix density and cell-matrix interactions. This 3D-specific elongated division mode is determined by the local confinement produced by the matrix and the ability of cells to protrude and locally remodel the matrix via β1 integrin. Elongated division is readily recapitulated using collagen-coated microfabricated channels. Cells depleted of β1 integrin still divide in the elongated mode in microchannels, suggesting that 3D confinement is sufficient to induce the elongated cell-division phenotype.

Gamma-actin is involved in regulating centrosome function and mitotic progression in cancer cells

Reorganization of the actin cytoskeleton during mitosis is crucial for regulating cell division. A functional role for γ-actin in mitotic arrest induced by the microtubule-targeted agent, paclitaxel, has recently been demonstrated. We hypothesized that γ-actin plays a role in mitosis. Herein, we investigated the effect of γ-actin in mitosis and demonstrated that γ-actin is important in the distribution of β-actin and formation of actin-rich retraction fibers during mitosis. The reduced ability of paclitaxel to induce mitotic arrest as a result of γ-actin depletion was replicated with a range of mitotic inhibitors, suggesting that γ-actin loss reduces the ability of broad classes of anti-mitotic agents to induce mitotic arrest. In addition, partial depletion of γ-actin enhanced centrosome amplification in cancer cells and caused a significant delay in prometaphase/metaphase. This prolonged prometaphase/metaphase arrest was due to mitotic defects such as uncongressed and missegregated chromosomes, and correlated with an increased presence of mitotic spindle abnormalities in the γ-actin depleted cells. Collectively, these results demonstrate a previously unknown role for γ-actin in regulating centrosome function, chromosome alignment and maintenance of mitotic spindle integrity.

Characterizing Cellular Biophysical Responses to Stress by Relating Density, Deformability, and Size

Cellular physical properties are important indicators of specific cell states. Although changes in individual biophysical parameters, such as cell size, density, and deformability, during cellular processes have been investigated in great detail, relatively little is known about how they are related. Here, we use a suspended microchannel resonator (SMR) to measure single-cell density, volume, and passage time through a narrow constriction of populations of cells subjected to a variety of environmental stresses. Osmotic stress significantly affects density and volume, as previously shown. In contrast to density and volume, the effect of an osmotic challenge on passage time is relatively small. Deformability, as determined by comparing passage times for cells with similar volume, exhibits a strong dependence on osmolarity, indicating that passage time alone does not always provide a meaningful proxy for deformability. Finally, we find that protein synthesis inhibition, cell-cycle arrest, protein kinase inhibition, and cytoskeletal disruption result in unexpected relationships among deformability, density, and volume. Taken together, our results suggest that by measuring multiple biophysical parameters, one can detect unique characteristics that more specifically reflect cellular behaviors.

Megakaryocyte Polyploidization and Proplatelet Formation in Low-Attachment Conditions

In vitro-derived platelets (PLTs), which could provide an alternative source of PLTs for patient transfusions, are formed from polyploid megakaryocytes (MKs) that extend long cytoplasmic projections, termed proplatelets (proPLTs). In this study, we compared polyploidization and proPLT formation (PPF) of MKs cultured on surfaces that either promote or inhibit protein adsorption and subsequent cell adhesion. A megakaryoblastic cell line exhibited increased polyploidization and arrested PPF on a low-attachment surface. Primary human MKs also showed low levels of PPF on the same surface, but no difference in ploidy. Importantly, both cell types exhibited accelerated PPF after transfer to a surface that supports attachment, suggesting that pre-culture on a non-adhesive surface may facilitate synchronization of PPF and PLT generation in culture.

Molecular Insights into Division of Single Human Cancer Cells in On-Chip Transparent Microtubes

In vivo, mammalian cells proliferate within 3D environments consisting of numerous microcavities and channels, which contain a variety of chemical and physical cues. External environments often differ between normal and pathological states, such as the unique spatial constraints that metastasizing cancer cells experience as they circulate the vasculature through arterioles and narrow capillaries, where they can divide and acquire elongated cylindrical shapes. While metastatic tumors cause most cancer deaths, factors impacting early cancer cell proliferation inside the vasculature and those that can promote the formation of secondary tumors remain largely unknown. Prior studies investigating confined mitosis have mainly used 2D cell culture systems. Here, we mimic aspects of metastasizing tumor cells dividing inside blood capillaries by investigating single-cell divisions of living human cancer cells, trapped inside 3D rolled-up, transparent nanomembranes. We assess the molecular effects of tubular confinement on key mitotic features, using optical high- and super-resolution microscopy. Our experiments show that tubular confinement affects the morphology and dynamics of the mitotic spindle, chromosome arrangements, and the organization of the cell cortex. Moreover, we reveal that membrane blebbing and/or associated processes act as a potential genome-safety mechanism, limiting the extent of genomic instability caused by mitosis in confined circumstances, especially in tubular 3D microenvironments. Collectively, our study demonstrates the potential of rolled-up nanomembranes for gaining molecular insights into key cellular events occurring in tubular 3D microenvironments in vivo.

Microconstriction arrays for high-throughput quantitative measurements of cell mechanical properties

We describe a method for quantifying the mechanical properties of cells in suspension with a microfluidic device consisting of a parallel array of micron-sized constrictions. Using a high-speed charge-coupled device camera, we measure the flow speed, cell deformation, and entry time into the constrictions of several hundred cells per minute during their passage through the device. From the flow speed and the occupation state of the microconstriction array with cells, the driving pressure across each constriction is continuously computed. Cell entry times into microconstrictions decrease with increased driving pressure and decreased cell size according to a power law. From this power-law relationship, the cell elasticity and fluidity can be estimated. When cells are treated with drugs that depolymerize or stabilize the cytoskeleton or the nucleus, elasticity and fluidity data from all treatments collapse onto a master curve. Power-law rheology and collapse onto a master curve are predicted by the theory of soft glassy materials and have been previously shown to describe the mechanical behavior of cells adhering to a substrate. Our finding that this theory also applies to cells in suspension provides the foundation for a quantitative high-throughput measurement of cell mechanical properties with microfluidic devices.

Coupling between apical tension and basal adhesion allow epithelia to collectively sense and respond to substrate topography over long distances

Epithelial sheets fold into complex topographies that contribute to their function in vivo. Cells can sense and respond to substrate topography in their immediate vicinity by modulating their interfacial mechanics, but the extent to which these mechanical properties contribute to their ability to sense substrate topography across length scales larger than a single cell has not been explored in detail. To study the relationship between the interfacial mechanics of single cells and their collective behavior as tissues, we grew cell-sheets on substrates engraved with surface features spanning macroscopic length-scales. We found that many epithelial cell-types sense and respond to substrate topography, even when it is locally nearly planar. Cells clear or detach from regions of local negative curvature, but not from regions with positive or no curvature. We investigated this phenomenon using a finite element model where substrate topography is coupled to epithelial response through a balance of tissue contractility and adhesive forces. The model correctly predicts the focal sites of cell-clearing and epithelial detachment. Furthermore, the model predicts that local tissue response to substrate curvature is a function of the surrounding topography of the substrate across long distances. Analysis of cell-cell and cell-substrate contact angles suggests a relationship between these single-cell interfacial properties, epithelial interfacial properties, and collective epithelial response to substrate topography. Finally, we show that contact angles change upon activation of oncogenes or inhibition of cell-contractility, and that these changes correlate with collective epithelial response. Our results demonstrate that in mechanically integrated epithelial sheets, cell contractility can be transmitted through multiple cells and focused by substrate topography to affect a behavioral response at distant sites.

Discovery of MINC1, a GTPase-activating protein small molecule inhibitor, targeting MgcRacGAP

The Rho family of Ras superfamily small GTPases regulates a broad range of biological processes such as migration, differentiation, cell growth and cell survival. Therefore, the availability of small molecule modulators as tool compounds could greatly enhance research on these proteins and their biological function. To this end, we designed a biochemical, high throughput screening assay with complementary follow-up assays to identify small molecule compounds inhibiting MgcRacGAP, a Rho family GTPase activating protein involved in cytokinesis and transcriptionally upregulated in many cancers. We first performed an in-house screen of 20,480 compounds, and later we tested the assay against 342,046 compounds from the NIH Molecular Libraries Small Molecule Repository. Primary screening hit rates were about 1% with the majority of those affecting the primary readout, an enzyme-coupled GDP detection assay. After orthogonal and counter screens, we identified two hits with high selectivity towards MgcRacGAP, compared with other RhoGAPs, and potencies in the low micromolar range. The most promising hit, termed MINC1, was then examined with cell-based testing where it was observed to induce an increased rate of cytokinetic failure and multinucleation in addition to other cell division defects, suggesting that it may act as an MgcRacGAP inhibitor also in cells.