E-cadherin plays a pivotal role in epithelial morphogenesis. It controls the intercellular adhesion required for tissue cohesion and anchors the actomyosin-driven tension needed to change cell shape. In the early Drosophila embryo, Myosin-II (Myo-II) controls the planar polarized remodelling of cell junctions and tissue extension. The E-cadherin distribution is also planar polarized and complementary to the Myosin-II distribution. Here we show that E-cadherin polarity is controlled by the polarized regulation of clathrin- and dynamin-mediated endocytosis. Blocking E-cadherin endocytosis resulted in cell intercalation defects. We delineate a pathway that controls the initiation of E-cadherin endocytosis through the regulation of AP2 and clathrin coat recruitment by E-cadherin. This requires the concerted action of the formin Diaphanous (Dia) and Myosin-II. Their activity is controlled by the guanine exchange factor RhoGEF2, which is planar polarized and absent in non-intercalating regions. Finally, we provide evidence that Dia and Myo-II control the initiation of E-cadherin endocytosis by regulating the lateral clustering of E-cadherin.

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SummaryRegulation of tissue size requires fine tuning at the single-cell level of proliferation rate, cell volume, and cell death. Whereas the adjustment of proliferation and growth has been widely studied [1, 2, 3, 4, 5], the contribution of cell death and its adjustment to tissue-scale parameters have been so far much less explored. Recently, it was shown that epithelial cells could be eliminated by live-cell delamination in response to an increase of cell density [6]. Cell delamination was supposed to occur independently of caspase activation and was suggested to be based on a gradual and spontaneous disappearance of junctions in the delaminating cells [6]. Studying the elimination of cells in the midline region of the Drosophila pupal notum, we found that, contrary to what was suggested before, Caspase 3 activation precedes and is required for cell delamination. Yet, using particle image velocimetry, genetics, and laser-induced perturbations, we confirmed [6] that local tissue crowding is necessary and sufficient to drive cell elimination and that cell elimination is independent of known fitness-dependent competition pathways [7, 8, 9]. Accordingly, activation of the oncogene Ras in clones was sufficient to compress the neighboring tissue and eliminate cells up to several cell diameters away from the clones. Mechanical stress has been previously proposed to contribute to cell competition [10, 11]. These results provide the first experimental evidences that crowding-induced death could be an alternative mode of super-competition, namely mechanical super-competition, independent of known fitness markers [7, 8, 9], that could promote tumor growth.

Cell-cell intercalation is used in several developmental processes to shape the normal body plan. There is no clear evidence that intercalation is involved in pathologies. Here we use the proto-oncogene myc to study a process analogous to early phase of tumour expansion: myc-induced cell competition. Cell competition is a conserved mechanism driving the elimination of slow-proliferating cells (so-called 'losers') by faster-proliferating neighbours (so-called 'winners') through apoptosis and is important in preventing developmental malformations and maintain tissue fitness. Here we show, using long-term live imaging of myc-driven competition in the Drosophila pupal notum and in the wing imaginal disc, that the probability of elimination of loser cells correlates with the surface of contact shared with winners. As such, modifying loser-winner interface morphology can modulate the strength of competition. We further show that elimination of loser clones requires winner-loser cell mixing through cell-cell intercalation. Cell mixing is driven by differential growth and the high tension at winner-winner interfaces relative to winner-loser and loser-loser interfaces, which leads to a preferential stabilization of winner-loser contacts and reduction of clone compactness over time. Differences in tension are generated by a relative difference in F-actin levels between loser and winner junctions, induced by differential levels of the membrane lipid phosphatidylinositol (3,4,5)-trisphosphate. Our results establish the first link between cell-cell intercalation induced by a proto-oncogene and how it promotes invasiveness and destruction of healthy tissues.

Actomyosin flows are involved in a variety of cellular processes, including cytokinesis, cell migration, polarization, and morphogenesis. In epithelia, flow polarization orients cell deformations. It is unclear, however, how flows are polarized and how global patterns of junction remodeling emerge from flow polarization locally. We address this question during intercalation-driving extension of the Drosophila germband. Intercalation is associated with polarized junction remodeling, whereby actomyosin pulses flow anisotropically toward dorsal-ventral junctions and shrink them. Here, we show that planar polarization of flows emerges from polarized fluctuations in the levels of E-cadherin clusters that produce transient and oscillating asymmetries of coupling. These fluctuations are triggered by polarized E-cadherin endocytosis and are amplified by flow itself. This work suggests that fluctuations and mechanical instability are not the consequences of limited control over the systems key parameters, but rather that they define the axis of symmetry breaking.

SummaryThe plasticity of developing tissues relies on the adjustment of cell survival and growth rate to environmental cues. This includes the effect of mechanical cues on cell survival. Accordingly, compaction of an epithelium can lead to cell extrusion and cell death. This process was proposed to contribute to tissue homeostasis but also to facilitate the expansion of pretumoral cells through the compaction and elimination of the neighboring healthy cells. However, we know very little about the pathways that can trigger apoptosis upon tissue deformation, and the contribution of compaction-driven death to clone expansion has never been assessed in vivo. Using the Drosophila pupal notum and a new live sensor of ERK, we show first that tissue compaction induces cell elimination through the downregulation of epidermal growth factor receptor/extracellular signal regulated kinase (EGFR/ERK) pathway and the upregulation of the pro-apoptotic protein Hid. Those results suggest that the sensitivity of EGFR/ERK pathway to mechanics could play a more general role in the fine tuning of cell elimination during morphogenesis and tissue homeostasis. Second, we assessed in vivo the contribution of compaction-driven death to pretumoral cell expansion. We found that the activation of the oncogene Ras in clones can downregulate ERK and activate apoptosis in the neighboring cells through their compaction, which eventually contributes to Ras clone expansion. The mechanical modulation of EGFR/ERK during growth-mediated competition for space may contribute to tumor progression.

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Cell competition is the short-range elimination of slow-dividing cells through apoptosis when confronted with a faster growing population. It is based on the comparison of relative cell fitness between neighboring cells and is a striking example of tissue adaptability that could play a central role in developmental error correction and cancer progression in both Drosophila melanogaster and mammals. Cell competition has led to the discovery of multiple pathways that affect cell fitness and drive cell elimination. The diversity of these pathways could reflect unrelated phenomena, yet recent evidence suggests some common wiring and the existence of a bona fide fitness comparison pathway.