A unified view of neighbour cell engagement during apoptotic cell extrusion.

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.