Biology subjects

Villars, A.

Publications and source records attributed to Villars, A..

5 recordsLinked to original sources

bioRxiv2025

Integration of past caspase activity biases cell elimination in vivo

The fine tuning of apoptosis in epithelia is essential for regulating tissue size, shape, homeostasis and the maintenance of sealing properties. Regulation of cell death is mostly orchestrated by the activation of Caspases, proteases which were long thought to trigger an irreversible engagement in cell death. However, recent data in vivo and in vitro outline numerous non-apoptotic functions of caspases as well as quite ubiquitous sublethal activation of effector caspases during development. Yet, it remains unclear in many instances what drives the bifurcation between cell death engagement and cell survival upon caspase activation. The existence of a caspase activity threshold was generally considered to underpin this binary decision, but this was never assessed quantitatively in vivo especially at the single cell level. Using quantitative live imaging combined with machine learning and optogenetics in the Drosophila pupal notum (a single layer epithelium), we reveal for the first time the existence of a large heterogeneity of caspase sensitivity between cells, as well as the existence of distinct spatial domains with low or high sensitivity to caspases. Using correlative and perturbative experiments, we outline the central role of past exposure to sublethal caspase activity which sensitises cells for apoptosis for several hours. Integrating information about past caspase activation is sufficient to explain most of the global pattern of caspase sensitivity and predict at the single cell level which cells will engage in apoptosis. Finally, we demonstrate that past sublethal caspase activation in a subset of cells is sufficient to bias cell elimination at the clonal and single cell level, thus revealing an alternative mechanism of physiological cell competition. Altogether, this work reveals for the first time the existence of a new layer of apoptosis regulation in vivo downstream of effector caspases which can be developmentally regulated and bias clonal selection and the spatial pattern of cell death.

developmental biology↗

bioRxiv2025

Interfacial tension and growth both contribute to mechanical cell competition

Tissue plasticity and homeostasis rely on the mutual interplay between cell behaviour and mechanical inputs1. Yet, mechanical stress can also contribute to the evolution of some pathologies, notably by accelerating pretumoural cell expansion through the process of mechanical cell competition2-5. Mechanical cell competition is a conserved process in which one cell population is preferentially eliminated when mixed with another cell population due to its higher sensitivity to mechanical stress2,3,5-8. Most of the recent theoretical and experimental explorations of mechanical cell competition focused so far on the contribution of growth and pressure to cell elimination and were limited to few genetic contexts, including the activation of Ras in vivo2,8, and the mutation of the polarity gene scribble in mammalian cell culture3,4,9. However, it remains unclear whether other oncogenes can trigger similar mechanisms and whether growth is generally the only central regulator of cell compaction and cell elimination. Using the Drosophila pupal notum (a single layer epithelium), quantitative live imaging and vertex modelling, we revisited the mechanisms contributing to cell compaction and cell elimination during mechanical cell competition. Doing so, we outlined the co-existence of two modes of wild type (WT) cell compaction near oncogenic cells, namely the compaction driven by growth versus local compaction driven by increased tension at tumoural/WT cell interfaces in zones of high curvature (similar to "Laplace pressure"). We highlighted distinctive signatures in cell deformation and cell elimination distribution that can delineate these two modes of compaction, and we recapitulated them in silico and in vivo using genetic backgrounds affecting growth and/or interfacial tension independently. Altogether, this study reveals for the first time the contribution of interfacial tension-driven compaction to mechanical cell competition and outlines the co-existence of various modes of compaction during cell elimination and pretumoural clone expansion.

cell biology↗

bioRxiv2024

Developmental delay ensures global tissue size robustness upon local induction of apoptosis

The capacity of our tissues to cope with external and internal stress relies on the tight coupling between cell proliferation, cell growth and cell death. This coupling is assumed to be based on compensatory proliferation, where local mitogenic signals and mechanical inputs generated by dying cells promote neighbouring cell proliferation. However, compensatory proliferation was mostly studied in the context of massive death induction, irradiation, surgical tissue ablation or upon genetic perturbation of apoptosis execution. It remains thus unclear whether the same mechanism operates during physiological programmed cell death or upon mild induction of apoptosis, especially in vivo. Here, we use the Drosophila prospective wing (the larval wing disc), to study the impact of local induction of apoptosis on tissue size and proliferation pattern. We first confirmed that the wing could recover its final size and compensate for mild induction of apoptosis. However, using spatial statistics we found surprisingly that local induction of death is not associated with any local increase of proliferation, could it be upon clonal or compartment induction of apoptosis. Compensation is driven instead by a JNK dependant delay of growth and lengthening of the larval stage which is required to reach the final tissue target size. These results suggest that compensation is here driven by a global response rather that a local proliferation induction. Accordingly, while total tissue size is maintained despite local induction of apoptosis, this mechanism fails to correct the local reduction of cell number, hence modulating wing shape and proportion. Overall, this study opens novel perspectives on tissue size regulation and outlines the context-dependency of compensatory mechanisms.

developmental biology↗

bioRxiv2023

DeXtrusion: Automatic recognition of epithelial cell extrusion through machine learning in vivo

Epithelial cell death is highly prevalent during development and in adult tissues. It plays an essential role in the regulation of tissue size, shape, and turnover. Cell elimination relies on the concerted remodelling of cell junctions, so-called cell extrusion, which allows the seamless expulsion of dying cells. The dissection of the regulatory mechanism giving rise to a certain number and pattern of cell death was so far limited by our capacity to generate high-throughput quantitative data on cell death/extrusion number and distribution in various perturbed backgrounds. Indeed, quantitative studies of cell death rely so far on manual detection of cell extrusion events or through tedious systematic error-free segmentation and cell tracking. Recently, deep learning was used to automatically detect cell death and cell division in cell culture mostly using transmission light microscopy. However, so far, no method was developed for fluorescent images and confocal microscopy, which constitute most datasets in embryonic epithelia. Here, we devised DeXtrusion, a pipeline for automatic detection of cell extrusion/cell death events in larges movies of epithelia marked with cell contour and based on recurrent neural networks. The pipeline, initially trained on large movies of the Drosophila pupal notum marked with fluorescent E-cadherin, is easily trainable, provides fast and accurate extrusion/cell death predictions in a large range of imaging conditions, and can also detect other cellular events such as cell division or cell differentiation. It also performs well on other epithelial tissues with markers of cell junctions with reasonable retraining.

developmental biology↗

bioRxiv2021

Microtubule disassembly by caspases is the rate-limiting step of cell extrusion

Epithelial cell death is essential for tissue homeostasis, robustness and morphogenesis. The expulsion of epithelial cells following caspase activation requires well-orchestrated remodeling steps leading to cell elimination without impairing tissue sealing. While numerous studies have provided insight about the process of cell extrusion, we still know very little about the relationship between caspase activation and the remodeling steps of cell extrusion. Moreover, most studies of cell extrusion focused on the regulation of actomyosin and steps leading to the formation of a supracellular contractile ring. However, the contribution of other cellular factors to cell extrusion has been poorly explored. Using the Drosophila pupal notum, a single layer epithelium where most extrusion events are caspase-dependent, we first showed that the initiation of cell extrusion and apical constriction are surprisingly not associated with the modulation of actomyosin concentration/dynamics. Instead, cell apical constriction is initiated by the disassembly of a medio-apical mesh of microtubules which is driven by effector caspases. We confirmed that local and rapid increase/decrease of microtubules is sufficient to respectively expand/constrict cell apical area. Importantly, the depletion of microtubules is sufficient to bypass the requirement of caspases for cell extrusion. This study shows that microtubules disassembly by caspases is a key rate-limiting steps of extrusion, and outlines a more general function of microtubules in epithelial cell shape stabilisation.

cell biology↗