Epithelia remove excess cells through extrusion, and prevent accumulation of unnecessary or pathological cells. The extrusion process can be triggered by apoptotic signaling1, oncogenic transformation2,3, and overcrowding of cells4–6. Despite the important links of cell extrusion to developmental7, homeostatic5 and pathological processes2,8,9 such as cancer metastasis, its underlying mechanism and connections to the intrinsic mechanics of the epithelium are largely unexplored. Here, we show that apoptotic cell extrusion is provoked by singularities in cell alignments9,10 in the form of comet-like topological defects. We find a universal correlation between the extrusion sites and positions of nematic defects in the cell orientation field in different epithelium types. We model the epithelium as an active nematic liquid crystal and compare numerical simulations to strain rate and stress measurements within cell monolayers. The results confirm the active nematic nature of epithelia for the first time, and demonstrate that defect-induced isotropic stresses are the primary precursor of mechanotransductive responses in cells such as YAP (Yes-associated protein) transcription factor activity11, caspase-3 mediated cell death, and extrusions. Importantly, the defect-driven extrusion mechanism depends on intercellular junctions, since the weakening of cell-cell interactions in α-catenin knockdown (α-catKD) monolayer reduces the defect size and increases both the number of defects and extrusion rates, as also predicted by our model. We further demonstrate the ability to control extrusion hotspots by geometrically inducing defects through microcontact-printing of patterned monolayers. Together we propose a novel mechanism for apoptotic cell extrusion: spontaneously formed topological defects in epithelia govern cell fate. This new finding has important implications in predicting extrusion hotspots and dynamics in vivo, with potential applications to tissue regeneration and metastasis suppression. Moreover, we anticipate that the analogy between the epithelium and active nematic liquid crystals will trigger further investigations of the link between cellular processes and the material properties of epithelia.

Singapore# These authors contributed equally to this work. SummaryThe 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][9][10][11][12], to cell extrusion is still poorly understood. By measuring the multi-cellular 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 nondying cells in connection with large-scale collective movements. As cell density increases, cell motion is shown to slow down and the role of a supra-cellular actomyosin cable formation and its contraction in the neighboring cells becomes the preponderant mechanism to locally promote cell

Porphyromonas gingivalis is a major etiological agent of chronic periodontal diseases, the virulence of which has been attributed to different factors, including lipopolysaccharide (LPS). We investigated the differential responses induced by P. gingivalis LPS stimulation of human umbilical vein endothelial cells and human oral epithelial cells. RT-PCR analysis showed that P. gingivalis LPS used Toll-like receptor 2 (TLR2) to activate epithelial cells and Toll-like receptor 4 (TLR4) to activate endothelial cells. Both cell types were stimulated by P. gingivalis LPS to produce pro-inflammatory cytokines. Cytokine Array assay showed that although patterns of cytokine expression were similar in both cell types, some cytokines were specifically secreted by the endothelial cells, and others were specific to epithelial cells. These results support the idea that the same LPS preparation can act as a TLR2 or TLR4 agonist, depending on TLR expression of the host cell, inducing cytokine profiles that differ according to the cell type.

The presence of periodontal pathogens in atherosclerotic plaques and in apparently healthy vessels appeared to reflect a higher level of bacteremia rather than infection of endothelial cells.