Cell extrusion - a novel mechanism driving neural crest cell delamination

Neural crest cells (NCC) comprise a heterogeneous population of cells with variable potency, that contribute to nearly every tissue and organ system throughout the body. Considered unique to vertebrates, NCC are transiently generated within the dorsolateral region of the neural plate or neural tube, during neurulation. Their delamination and migration are crucial events in embryo development as the differentiation of NCC is heavily influenced by their final resting locations. Previous work in avian and aquatic species has shown that NCC delaminate via an epithelial-mesenchymal transition (EMT), which transforms these stem and progenitor cells from static polarized epithelial cells into migratory mesenchymal cells with fluid front and back polarity. However, the cellular and molecular drivers facilitating NCC delamination in mammals are poorly understood. We performed live timelapse imaging of NCC delamination in mouse embryos and discovered a group of cells that exit the neuroepithelium as isolated round cells, which then halt for a short period prior to acquiring the mesenchymal migratory morphology classically associated with most delaminating NCC. High magnification imaging and protein localization analyses of the cytoskeleton, together with measurements of pressure and tension of delaminating NCC and neighboring neuroepithelial cells, revealed these round NCC are extruded from the neuroepithelium prior to completion of EMT. Furthermore, we demonstrate that cranial NCC are extruded through activation of the mechanosensitive ion channel, PIEZO1, a key regulator of the live cell extrusion pathway, revealing a new role for PIEZO1 in neural crest cell development. Our results elucidating the cellular and molecular dynamics orchestrating NCC delamination support a model in which high pressure and tension in the neuroepithelium results in activation of the live cell extrusion pathway and delamination of a subpopulation of NCC in parallel with EMT. This model has broad implications for our understanding of cell delamination in development and disease.

Apoptotic extracellular vesicle formation via local phosphatidylserine exposure drives efficient cell extrusion

Cell extrusion is a universal mode of cell removal from tissues, and it plays an important role in regulating cell numbers and eliminating unwanted cells. However, the underlying mechanisms of cell delamination from the cell layer are unclear. Here, we report a conserved execution mechanism of apoptotic cell extrusion. We found extracellular vesicle (EV) formation in extruding mammalian and Drosophila cells at a site opposite to the extrusion direction. Lipid-scramblase-mediated local exposure of phosphatidylserine is responsible for EV formation and is crucial for executing cell extrusion. Inhibition of this process disrupts prompt cell delamination and tissue homeostasis. Although the EV has hallmarks of an apoptotic body, its formation is governed by the mechanism of microvesicle formation. Experimental and mathematical modeling analysis illustrated that EV formation promotes neighboring cells' invasion. This study showed that membrane dynamics play a crucial role in cell exit by connecting the actions of the extruding cell and neighboring cells.

Mechanical basis and topological routes to cell elimination

Cell layers eliminate unwanted cells through the extrusion process, which underlines healthy versus flawed tissue behaviors. Although several biochemical pathways have been identified, the underlying mechanical basis including the forces involved in cellular extrusion remains largely unexplored. Utilizing a phase-field model of a three-dimensional cell layer, we study the interplay of cell extrusion with cell-cell and cell-substrate interactions in a flat monolayer. Independent tuning of cell-cell versus cell-substrate adhesion forces reveals that extrusion events can be distinctly linked to defects in nematic and hexatic orders associated with cellular arrangements. Specifically, we show that by increasing relative cell-cell adhesion forces the cell monolayer can switch between the collective tendency towards fivefold, hexatic, disclinations relative to half-integer, nematic, defects for extruding a cell. We unify our findings by accessing three-dimensional mechanical stress fields to show that an extrusion event acts as a mechanism to relieve localized stress concentration.

Bidirectional multiciliated cell extrusion is controlled by Notch driven basal extrusion and Piezo 1 driven apical extrusion

Xenopus embryos are covered with a complex epithelium containing numerous multiciliated cells (MCCs). During late stage development there is a dramatic remodeling of the epithelium that involves the complete loss of MCCs. Cell extrusion is a well-characterized process for driving cell loss while maintaining epithelial barrier function. Normal cell extrusion is typically unidirectional whereas bidirectional extrusion is often associated with disease (e.g. cancer). We describe two distinct mechanisms for MCC extrusion, a basal extrusion driven by Notch signaling and an apical extrusion driven by Piezo1. Early in the process there is a strong bias towards basal extrusion, but as development continues there is a shift towards apical extrusion. Importantly, receptivity to the Notch signal is age-dependent and governed by the maintenance of the MCC transcriptional program such that extension of this program is protective against cell loss. In contrast, later apical extrusion is regulated by Piezo 1 such that premature activation of Piezo 1 leads to early extrusion while blocking Piezo 1 leads to MCC maintenance. Distinct mechansms for MCC loss underlie the importance of their removal during epithelial remodeling.

Summay Statement

Cell extrusion typically occurs unidirectionally. We have identified a single population of multiciliated cells that extrudes bidirectionally: Notch-driven basal extrusion and Piezo 1-mediated apical extrusion.

Extracellular ATP facilitates cell extrusion from epithelial layers mediated by cell competition or apoptosis

For the maintenance of epithelial homeostasis, various aberrant or dysfunctional cells are actively eliminated from epithelial layers. This cell extrusion process mainly falls into two modes: cell-competition-mediated extrusion and apoptotic extrusion. However, it is not clearly understood whether and how these processes are governed by common molecular mechanisms. In this study, we demonstrate that the reactive oxygen species (ROS) levels are elevated within a wide range of epithelial layers around extruding transformed or apoptotic cells. The downregulation of ROS suppresses the extrusion process. Furthermore, ATP is extracellularly secreted from extruding cells, which promotes the ROS level and cell extrusion. Moreover, the extracellular ATP and ROS pathways positively regulate the polarized movements of surrounding cells toward extruding cells in both cell-competition-mediated and apoptotic extrusion. Hence, extracellular ATP acts as an "extrude me" signal and plays a prevalent role in cell extrusion, thereby sustaining epithelial homeostasis and preventing pathological conditions or disorders.

Yersinia pseudotuberculosis YopE prevents uptake by M cells and instigates M cell extrusion in human ileal enteroid-derived monolayers

Many pathogens use M cells to access the underlying Peyer's patches and spread to systemic sites via the lymph as demonstrated by ligated loop murine intestinal models. However, the study of interactions between M cells and microbial pathogens has stalled due to the lack of cell culture systems. To overcome this obstacle, we use human ileal enteroid-derived monolayers containing five intestinal cell types including M cells to study the interactions between the enteric pathogen, Yersinia pseudotuberculosis (Yptb), and M cells. The Yptb type three secretion system (T3SS) effector Yops inhibit host defenses including phagocytosis and are critical for colonization of the intestine and Peyer's patches. Therefore, it is not understood how Yptb traverses through M cells to breach the epithelium. By growing Yptb under two physiological conditions that mimic the early infectious stage (low T3SS-expression) or host-adapted stage (high T3SS-expression), we found that large numbers of Yptb specifically associated with M cells, recapitulating murine studies. Transcytosis through M cells was significantly higher by Yptb expressing low levels of T3SS, because YopE and YopH prevented Yptb uptake. YopE also caused M cells to extrude from the epithelium without inducing cell-death or disrupting monolayer integrity. Sequential infection with early infectious stage Yptb reduced host-adapted Yptb association with M cells. These data underscore the strength of enteroids as a model by discovering that Yops impede M cell function, indicating that early infectious stage Yptb more effectively penetrates M cells while the host may defend against M cell penetration of host-adapted Yptb.

Rapid lamellipodial responses by neighbor cells drive epithelial sealing in response to pyroptotic cell death

Cell injury poses a substantial challenge for epithelia homeostasis. Several cellular processes preserve epithelial barriers in response to apoptosis, but less is known about other forms of cell death, such as pyroptosis. Here we use an inducible caspase-1 system to analyze how colon epithelial monolayers respond to pyroptosis. We confirm that sporadic pyroptotic cells are physically eliminated from confluent monolayers by apical extrusion. This is accompanied by a transient defect in barrier function at the site of the pyroptotic cells. By visualizing cell shape changes and traction patterns in combination with cytoskeletal inhibitors, we show that rapid lamellipodial responses in the neighbor cells are responsible for correcting the leakage and resealing the barrier. Cell contractility is not required for this resealing response, in contrast to the response to apoptosis. Therefore, pyroptosis elicits a distinct homeostatic response from the epithelium that is driven by the stimulation of lamellipodia in neighbor cells.

Mechanical competition triggered by innate immune signaling drives the collective extrusion of bacterially infected epithelial cells

Intracellular pathogens alter their host cells' mechanics to promote dissemination through tissues. Conversely, host cells may respond to the presence of pathogens by altering their mechanics to limit infection. Here, we monitored epithelial cell monolayers infected with intracellular bacterial pathogens, Listeria monocytogenes or Rickettsia parkeri, over days. Under conditions in which these pathogens trigger innate immune signaling through NF-κB and use actin-based motility to spread non-lytically intercellularly, we found that infected cell domains formed three-dimensional mounds. These mounds resulted from uninfected cells moving toward the infection site, collectively squeezing the softer and less contractile infected cells upward and ejecting them from the monolayer. Bacteria in mounds were less able to spread laterally in the monolayer, limiting the growth of the infection focus, while extruded infected cells underwent cell death. Thus, the coordinated forceful action of uninfected cells actively eliminates large domains of infected cells, consistent with this collective cell response representing an innate immunity-driven process.

The EMT transcription factor Snai1 maintains myocardial wall integrity by repressing intermediate filament gene expression

The transcription factor Snai1, a well-known regulator of epithelial-to-mesenchymal transition, has been implicated in early cardiac morphogenesis as well as in cardiac valve formation. However, a role for Snai1 in regulating other aspects of cardiac morphogenesis has not been reported. Using genetic, transcriptomic, and chimeric analyses in zebrafish, we find that Snai1b is required in cardiomyocytes for myocardial wall integrity. Loss of snai1b increases the frequency of cardiomyocyte extrusion away from the cardiac lumen. Extruding cardiomyocytes exhibit increased actomyosin contractility basally as revealed by enrichment of p-myosin and α-catenin epitope α-18, as well as disrupted intercellular junctions. Transcriptomic analysis of wild-type and snai1b mutant hearts revealed the dysregulation of intermediate filament genes, including desmin b (desmb) upregulation. Cardiomyocyte-specific desmb overexpression caused increased cardiomyocyte extrusion, recapitulating the snai1b mutant phenotype. Altogether, these results indicate that Snai1 maintains the integrity of the myocardial epithelium, at least in part by repressing desmb expression.

Robustness of epithelial sealing is an emerging property of local ERK feedback driven by cell elimination

What regulates the spatiotemporal distribution of cell elimination in tissues remains largely unknown. This is particularly relevant for epithelia with high rates of cell elimination where simultaneous death of neighboring cells could impair epithelial sealing. Here, using the Drosophila pupal notum (a single-layer epithelium) and a new optogenetic tool to trigger caspase activation and cell extrusion, we first showed that death of clusters of at least three cells impaired epithelial sealing; yet, such clusters were almost never observed in vivo. Accordingly, statistical analysis and simulations of cell death distribution highlighted a transient and local protective phase occurring near every cell death. This protection is driven by a transient activation of ERK in cells neighboring extruding cells, which inhibits caspase activation and prevents elimination of cells in clusters. This suggests that the robustness of epithelia with high rates of cell elimination is an emerging property of local ERK feedback.

The CD44/COL17A1 pathway promotes the formation of multilayered, transformed epithelia

At the early stage of cancer development, oncogenic mutations often cause multilayered epithelial structures. However, the underlying molecular mechanism still remains enigmatic. By performing a series of screenings targeting plasma membrane proteins, we have found that collagen XVII (COL17A1) and CD44 accumulate in RasV12-, Src-, or ErbB2-transformed epithelial cells. In addition, the expression of COL17A1 and CD44 is also regulated by cell density and upon apical cell extrusion. We further demonstrate that the expression of COL17A1 and CD44 is profoundly upregulated at the upper layers of multilayered, transformed epithelia in vitro and in vivo. The accumulated COL17A1 and CD44 suppress mitochondrial membrane potential and reactive oxygen species (ROS) production. The diminished intracellular ROS level then promotes resistance against ferroptosis-mediated cell death upon cell extrusion, thereby positively regulating the formation of multilayered structures. To further understand the functional role of COL17A1, we performed comprehensive metabolome analysis and compared intracellular metabolites between RasV12 and COL17A1-knockout RasV12 cells. The data imply that COL17A1 regulates the metabolic pathway from the GABA shunt to mitochondrial complex I through succinate, thereby suppressing the ROS production. Moreover, we demonstrate that CD44 regulates membrane accumulation of COL17A1 in multilayered structures. These results suggest that CD44 and COL17A1 are crucial regulators for the clonal expansion of transformed cells within multilayered epithelia, thus being potential targets for early diagnosis and preventive treatment for precancerous lesions.

Blood Flow Limits Endothelial Cell Extrusion in the Zebrafish Dorsal Aorta

Blood flow modulates endothelial cell (EC) response during angiogenesis. Shear stress is known to control gene expression related to the endothelial-mesenchymal transition and endothelial-hematopoietic transition. However, the impact of blood flow on the cellular processes associated with EC extrusion is less well understood. To address this question, we dynamically record EC movements and use 3D quantitative methods to segregate the contributions of various cellular processes to the cellular trajectories in the zebrafish dorsal aorta. We find that ECs spread toward the cell extrusion area following the tissue deformation direction dictated by flow-derived mechanical forces. Cell extrusion increases when blood flow is impaired. Similarly, the mechanosensor polycystic kidney disease 2 (pkd2) limits cell extrusion, suggesting that ECs actively sense mechanical forces in the process. These findings identify pkd2 and flow as critical regulators of EC extrusion and suggest that mechanical forces coordinate this process by maintaining ECs within the endothelium.

EGF-mediated suppression of cell extrusion during mucosal damage attenuates opportunistic fungal invasion

Severe and often fatal opportunistic fungal infections arise frequently following mucosal damage caused by trauma or cytotoxic chemotherapy. Interaction of fungal pathogens with epithelial cells that comprise mucosae is a key early event associated with invasion, and, therefore, enhancing epithelial defense mechanisms may mitigate infection. Here, we establish a model of mold and yeast infection mediated by inducible epithelial cell loss in larval zebrafish. Epithelial cell loss by extrusion promotes exposure of laminin associated with increased fungal attachment, invasion, and larval lethality, whereas fungi defective in adherence or filamentation have reduced virulence. Transcriptional profiling identifies significant upregulation of the epidermal growth factor receptor ligand epigen (EPGN) upon mucosal damage. Treatment with recombinant human EPGN suppresses epithelial cell extrusion, leading to reduced fungal invasion and significantly enhanced survival. These data support the concept of augmenting epithelial restorative capacity to attenuate pathogenic invasion of fungi associated with human disease.

Wurster et al. show that extrusion of numerous epithelial cells from tissue can expose underlying extracellular matrix components to promote increased attachment and invasion of fungi associated with human disease. Treatment with recombinant human EPGN suppressed epithelial cell extrusion, leading to significantly reduced opportunistic fungal invasion.

Pulsatile contractions promote apoptotic cell extrusion in epithelial tissues

Extrusion is a mechanism used to eliminate unfit, excess, or dying cells from epithelial tissues. The initial events guiding which cells will be selectively extruded from the epithelium are not well understood. Here, we induced damage in a subset of epithelial cells in the developing zebrafish and used time-lapse imaging to examine cell and cytoskeletal dynamics leading to extrusion. We show that cell extrusion is preceded by actomyosin contractions that are pulsatile. Our data show that pulsatile contractions are induced by a junctional to medial re-localization of myosin. Analysis of cell area during contractions revealed that cells pulsing with the longest duration and highest amplitude undergo progressive area loss and extrude. Although pulses were driven by local increases in tension, damage to many cells promoted an overall decrease in the tensile state of the epithelium. We demonstrate that caspase activation leads to sphingosine-1-phosphate enrichment that controls both tissue tension and pulses to dictate areas of extrusion. These data suggest that the kinetics of pulsatile contractions define a key behavioral difference between extruding and non-extruding cells and are predictive of extrusion. Altogether, our study provides mechanistic insight into how localized changes in physical forces are coordinated to remove defective cells for homeostatic maintenance of living epithelial tissues.

aPKCi triggers basal extrusion of luminal mammary epithelial cells by tuning contractility and vinculin localization at cell junctions

This study shows that an oncogenic mammary epithelial cell surrounded by normal cells can extrude basally in vivo and invade surrounding tissues without formation of a primary tumor. Here, we show that overexpression of the key polarity protein atypical protein kinase C ι (aPKCi) is sufficient for triggering basally oriented epithelial cell extrusion and early cell invasion into the mammary gland stroma. Moreover, we highlight the importance of the difference between the mechanical properties of aPKCi-overexpressing cells and those of the normal surrounding cells associated with the decrease of vinculin at the cell junction, which triggers cell segregation, the first step toward promoting and controlling the direction of cell extrusion.

Metastasis is the main cause of cancer-related deaths. How a single oncogenic cell evolves within highly organized epithelium is still unknown. Here, we found that the overexpression of the protein kinase atypical protein kinase C ι (aPKCi), an oncogene, triggers basally oriented epithelial cell extrusion in vivo as a potential mechanism for early breast tumor cell invasion. We found that cell segregation is the first step required for basal extrusion of luminal cells and identify aPKCi and vinculin as regulators of cell segregation. We propose that asymmetric vinculin levels at the junction between normal and aPKCi⁺ cells trigger an increase in tension at these cell junctions. Moreover, we show that aPKCi⁺ cells acquire promigratory features, including increased vinculin levels and vinculin dynamics at the cell–substratum contacts. Overall, this study shows that a balance between cell contractility and cell–cell adhesion is crucial for promoting basally oriented cell extrusion, a mechanism for early breast cancer cell invasion.

The flow responsive transcription factor Klf2 is required for myocardial wall integrity by modulating Fgf signaling

Complex interplay between cardiac tissues is crucial for their integrity. The flow responsive transcription factor KLF2, which is expressed in the endocardium, is vital for cardiovascular development but its exact role remains to be defined. To this end, we mutated both klf2 paralogues in zebrafish, and while single mutants exhibit no obvious phenotype, double mutants display a novel phenotype of cardiomyocyte extrusion towards the abluminal side. This extrusion requires cardiac contractility and correlates with the mislocalization of N-cadherin from the lateral to the apical side of cardiomyocytes. Transgenic rescue data show that klf2 expression in endothelium, but not myocardium, prevents this cardiomyocyte extrusion phenotype. Transcriptome analysis of klf2 mutant hearts reveals that Fgf signaling is affected, and accordingly, we find that inhibition of Fgf signaling in wild-type animals can lead to abluminal cardiomyocyte extrusion. These studies provide new insights into how Klf2 regulates cardiovascular development and specifically myocardial wall integrity.

MARCKS phosphorylation by PKC strongly impairs cell polarity in the chick neural plate

Neurulation involves a complex coordination of cellular movements that are in great part based on the modulation of the actin cytoskeleton. MARCKS, an F-actin-binding protein and the major substrate for PKC, is necessary for gastrulation and neurulation morphogenetic movements in mice, frogs, and fish. We previously showed that this protein accumulates at the apical region of the closing neural plate in chick embryos, and here further explore its role in this process and how it is regulated by PKC phosphorylation. PKC activation by PMA caused extensive neural tube closure defects in cultured chick embryos, together with MARCKS phosphorylation and redistribution to the cytoplasm. This was concomitant with an evident disruption of neural plate cell polarity and extensive apical cell extrusion. This effect was not due to actomyosin hypercontractility, but it was reproduced upon MARCKS knockdown. Interestingly, the overexpression of a nonphosphorylatable form of MARCKS was able to revert the cellular defects observed in the neural plate after PKC activation. Altogether, these results suggest that MARCKS function during neurulation would be to maintain neuroepithelial polarity through the stabilization of subapical F-actin, a function that appears to be counteracted by PKC activation.

Anisotropic organization of circumferential actomyosin characterizes hematopoietic stem cells emergence in the zebrafish

Hematopoiesis leads to the formation of blood and immune cells. Hematopoietic stem cells emerge during development, from vascular components, via a process called the endothelial-to-hematopoietic transition (EHT). Here, we reveal essential biomechanical features of the EHT, using the zebrafish embryo imaged at unprecedented spatio-temporal resolution and an algorithm to unwrap the aorta into 2D-cartography. We show that the transition involves anisotropic contraction along the antero-posterior axis, with heterogenous organization of contractile circumferential actomyosin. The biomechanics of the contraction is oscillatory, with unusually long periods in comparison to other apical constriction mechanisms described so far in morphogenesis, and is supported by the anisotropic reinforcement of junctional contacts. Finally, we show that abrogation of blood flow impairs the actin cytoskeleton, the morphodynamics of EHT cells, and the orientation of the emergence. Overall, our results underline the peculiarities of the EHT biomechanics and the influence of the mechanical forces exerted by blood flow.

As humans, we have two major types of blood cell: our red blood cells transport oxygen around the body, while our white blood cells fight disease. Both types of cell come from the same stem cells, which first appear early in embryonic development. These stem cells emerge from the walls of major blood vessels, including the aorta – which carries blood from the heart.

Stem cells have not yet decided which adult cell to become. Given the right signals, blood stem cells can form red blood cells or any of the different types of white blood cell. Understanding this process could allow scientists to recreate it in the laboratory, making blood stem cells that can give rise to all blood cells found in the body. But, this is not yet possible because we do not know all the conditions needed to make the cells and ensure they survive. One crucial gap in our understanding concerns the importance of blood flow. As the main blood vessel leaving the heart, the aorta experiences mechanical stress every time the heart beats. Lancino et al. wanted to find out whether this influences the development of the blood stem cells.

Zebrafish embryos are transparent, making it easy to see their bodies developing under a microscope. Like humans, they also produce both red blood cells and white blood cells meaning Lancino et al. could watch the birth of blood stem cells in these embryos from a part of the aorta called the aortic floor. A new software tool unwrapped pictures of the tube-shaped blood vessel into flat, two-dimensional maps, making it possible to see how the aorta changed over time. This revealed that, as blood stem cells leave the aortic floor, they bend and contract with the direction of the blood flow. Rings of actin and myosin proteins that formed around the stem cells as they are born helped the process along, while stopping the heartbeat changed the way the blood cells emerged. Without any blood flow, the actin proteins did not assemble properly; the stem cells also emerged in the wrong direction and some of them even died.

These findings show that physical forces play a role in the formation of blood stem cells. Understanding this process brings scientists a step closer to recreating the conditions for making different kinds of blood cells outside of the body.

Cell Extrusion: A Stress-Responsive Force for Good or Evil in Epithelial Homeostasis

Epithelial tissues robustly respond to internal and external stressors via dynamic cellular rearrangements. Cell extrusion acts as a key regulator of epithelial homeostasis by removing apoptotic cells, orchestrating morphogenesis, and mediating competitive cellular battles during tumorigenesis. Here, we delineate the diverse functions of cell extrusion during development and disease. We emphasize the expanding role for apoptotic cell extrusion in exerting morphogenetic forces, as well as the strong intersection of cell extrusion with cell competition, a homeostatic mechanism that eliminates aberrant or unfit cells. While cell competition and extrusion can exert potent, tumor-suppressive effects, dysregulation of either critical homeostatic program can fuel cancer progression.

Slit-Robo Repulsive Signaling Extrudes Tumorigenic Cells from Epithelia

Cells dynamically interact throughout animal development to coordinate growth and deter disease. For example, cell-cell competition weeds out aberrant cells to enforce homeostasis. In Drosophila, tumorigenic cells mutant for the cell polarity gene scribble (scrib) are actively eliminated from epithelia when surrounded by wild-type cells. While scrib cell elimination depends critically on JNK signaling, JNK-dependent cell death cannot sufficiently explain scrib cell extirpation. Thus, how JNK executed cell elimination remained elusive. Here, we show that repulsive Slit-Robo2-Ena signaling exerts an extrusive force downstream of JNK to eliminate scrib cells from epithelia by disrupting E-cadherin. While loss of Slit-Robo2-Ena in scrib cells potentiates scrib tumor formation within the epithelium, Robo2-Ena hyperactivation surprisingly triggers luminal scrib tumor growth following excess extrusion. This extrusive signaling is amplified by a positive feedback loop between Slit-Robo2-Ena and JNK. Our observations provide a potential causal mechanism for Slit-Robo dysregulation in numerous human cancers.

Epithelial Cell Packing Induces Distinct Modes of Cell Extrusions

The control of tissue growth, which is a key to maintain the protective barrier function of the epithelium, depends on the balance between cell division and cell extrusion rates [1, 2]. Cells within confluent epithelial layers undergo cell extrusion, which relies on cell-cell interactions [3] and actomyosin contractility [4, 5]. Although it has been reported that cell extrusion is also dependent on cell density [6, 7], the contribution of tissue mechanics, which is tightly regulated by cell density [8-12], to cell extrusion is still poorly understood. By measuring the multicellular dynamics and traction forces, we show that changes in epithelial packing density lead to the emergence of distinct modes of cell extrusion. In confluent epithelia with low cell density, cell extrusion is mainly driven by the lamellipodia-based crawling mechanism in the neighbor non-dying cells in connection with large-scale collective movements. As cell density increases, cell motion is shown to slow down, and the role of a supracellular actomyosin cable formation and its contraction in the neighboring cells becomes the preponderant mechanism to locally promote cell extrusion. We propose that these two distinct mechanisms complement each other to ensure proper cell extrusion depending on the cellular environment. Our study provides a quantitative and robust framework to explain how cell density can influence tissue mechanics and in turn regulate cell extrusion mechanisms.

Active contractility at E-cadherin junctions and its implications for cell extrusion in cancer

Cellular contractility regulates tissue cohesion and morphogenesis. In epithelia, E-cadherin adhesion couples the contractile cortices of neighboring cells together to produce tension at junctions that can be transmitted across the epithelium in a planar fashion. We have recently demonstrated that contractility is also patterned in the apical-lateral axis within epithelial junctions. Our findings highlight the role that cytoskeletal regulation plays in controlling the levels of intra-junctional tension. Of note, dysregulation of this apicolateral pattern of tension can drive oncogenic cell extrusion. In this article, we provide a detailed description of the actomyosin cytoskeleton organization during oncogenic extrusion and discuss the implications of cell extrusion in cancer.

The exocyst gene Sec10 regulates renal epithelial monolayer homeostasis and apoptotic sensitivity

The highly conserved exocyst protein complex regulates polarized exocytosis of subsets of secretory vesicles. A previous study reported that shRNA knockdown of an exocyst central subunit, Sec10 (Sec10-KD) in Madin-Darby canine kidney (MDCK) cells disrupted primary cilia assembly and 3D cyst formation. We used three-dimensional collagen cultures of MDCK cells to further investigate the mechanisms by which Sec10 and the exocyst regulate epithelial polarity, morphogenesis, and homeostasis. Sec10-KD cysts initially demonstrated undisturbed lumen formation although later displayed significantly fewer and shorter primary cilia than controls. Later in cystogenesis, control cells maintained normal homeostasis, while Sec10-KD cysts displayed numerous apoptotic cells extruded basally into the collagen matrix. Sec10-KD MDCK cells were also more sensitive to apoptotic triggers than controls. These phenotypes were reversed by restoring Sec10 expression with shRNA-resistant human Sec10. Apico-basal polarity appeared normal in Sec10-KD cysts, whereas mitotic spindle angles differed significantly from controls, suggesting a planar cell polarity defect. In addition, analysis of renal tubules in a newly generated kidney-specific Sec10-knockout mouse model revealed significant defects in primary cilia assembly and in the targeted renal tubules; abnormal epithelial cell extrusion was also observed, supporting our in vitro results. We hypothesize that, in Sec10-KD cells, the disrupted exocyst activity results in increased apoptotic sensitivity through defective primary cilia signaling and that, in combination with an increased basal cell extrusion rate, it affects epithelial barrier integrity and homeostasis.