A unified view of neighbour cell engagement during apoptotic cell extrusion

Abstract

Epithelia must eliminate apoptotic cells to preserve tissue barriers and prevent inflammation [1]. Several different mechanisms exist for apoptotic clearance, including efferocytosis [2, 3] and apical extrusion [4, 5]. We found that extrusion was the first-line response to apoptosis in cultured monolayers and in zebrafish epidermis. During extrusion, the apoptotic cell elicited active lamellipodial protrusions and assembly of a contractile extrusion ring in its neighbours. Depleting E-cadherin compromised both the contractile ring and extrusion, implying that a cadherin-dependent pathway allows apoptotic cells to engage their neighbours for extrusion. We identify RhoA as the cadherin-dependent signal in the neighbour cells and show that it is activated in response to contractile tension from the apoptotic cell. This mechanical stimulus is conveyed by a Myosin VI-dependent mechanotransduction pathway that is necessary both for extrusion and to preserve the epithelial barrier when apoptosis was stimulated. Earlier studies suggested that release of sphingosine-1-phosphate (S1P) from apoptotic cells might define where RhoA was activated. However, we found that although S1P is necessary for extrusion, its contribution does not require a localized source of S1P in the epithelium. We therefore propose a unified view of how RhoA is stimulated to engage neighbour cells for apoptotic extrusion. Here, tension-sensitive mechanotransduction is the proximate mechanism that activates RhoA specifically in the immediate neighbours of apoptotic cells, but this also must be primed by S1P in the tissue environment. Together, these elements provide a coincidence detection system that confers robustness on the extrusion response.

Collective effects in epithelial cell death and cell extrusion

“…Mechanical coupling through E-cad with direct neighbours was previously shown to reorganise actomyosin during extrusion through tensiondependent Coronin1B recruitment and actin reorganisation [19]. A new layer of coordination was recently revealed in MCF7 extruding cells, where E-cad dependent force transmission triggers the tension-dependent recruitment of MyoVI, p114/115 RhoGEF relocalisation and RhoA activation in the neighbours [20,21] (Figure 1 B,C). E-cad coupling not only regulates the actomyosin ring, but also the balance with other active contributors of extrusion.…”

“…For instance, a developmental switch which globally changes E-cad turnover and tissue tension is essential to modulate the rate of larval cell extrusion in the Drosophila pupal abdomen (albeit with contrasted contribution) [27,28]. Similarly, mammalian epithelium also needs to be primed by S1P from the serum to increase mechanosensitive recruitment of p114RhoGEF following initiation of extrusion [20,21],…”

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

“…UV irradiation or caspase activation), the apoptotic cells are extruded from the epithelial layer. [25][26][27][28] These different types of cell extrusion are currently regarded as distinct cellular processes, and it remains elusive whether and how common molecular mechanisms are involved in these homeostatic phenomena.…”

p115RhoGEF activates RhoA to support tight junction maintenance and remodeling

“…For example, during cell intercalation in the Drosophila ectoderm, dRhoGEF2 (an ortholog of p115RhoGEF) activates Rho1 at the medial-apical cell surface (Garcia De Las Bayonas et al, 2019). During cell extrusion, p115RhoGEF activates RhoA at AJs to maintain cell adhesion and direct the orientation of apoptotic cell extrusion (Slattum et al, 2009;Duszyc et al, 2021). Whether p115RhoGEF is required for local Rho flare activation during TJ remodeling is unknown.…”

“…In epithelial tissues, active RhoA localizes to apical cell-cell junctions at steady-state tension (Terry et al, 2011;Reyes et al, 2014;Priya et al, 2015). In high-tension environments, for example due to acute tensile stress or apoptosis, junctional active RhoA accumulates across epithelial monolayers to promote downstream signaling that preserves epithelial integrity (Acharya et al, 2018;Duszyc et al, 2021). RhoA also mediates local TJ remodeling in response to leaks in the TJ barrier associated with junction elongation, which were detected in Xenopus laevis embryos (Stephenson et al, 2019).…”