An epithelial cell destined for apoptosis signals its neighbors to extrude it by an actin- and myosin-dependent mechanism.
BACKGROUND
Simple epithelia encase developing embryos and organs. Although these epithelia consist of only one or two layers of cells, they must provide tight barriers for the tissues that they envelop. Apoptosis occurring within these simple epithelia could compromise this barrier. How, then, does an epithelium remove apoptotic cells without disrupting its function as a barrier?


RESULTS
We show that apoptotic cells are extruded from a simple epithelium by the concerted contraction of their neighbors. A ring of actin and myosin forms both within the apoptotic cell and in the cells surrounding it, and contraction of the ring formed in the live neighbors is required for apoptotic cell extrusion, as injection of a Rho GTPase inhibitor into these cells completely blocks extrusion. Addition of apoptotic MDCK cells to an intact monolayer induces the formation of actin cables in the cells contacted, suggesting that the signal to form the cable comes from the dying cell. The signal is produced very early in the apoptotic process, before procaspase activation, cell shrinkage, or phosphatidylserine exposure. Remarkably, electrical resistance studies show that epithelial barrier function is maintained, even when large numbers of dying cells are being extruded.


CONCLUSIONS
We propose that apoptotic cell extrusion is important for the preservation of epithelial barrier function during cell death. Our results suggest that an early signal from the dying cell activates Rho in live neighbors to extrude the apoptotic cell out of the epithelium.
Rosenblatt
J
J
Medical Research Council-Laboratory for Molecular Cell Biology, University College London, WC1E 6BT, United Kingdom. jody.rosenblatt@ucl.ac.uk
Raff
M C
MC
Cramer
L P
LP
D000199:Actins / Q000378:metabolism*; D000818:Animals; D017209:Apoptosis; D020169:Caspases / Q000378:metabolism; D002478:Cells, Cultured; D002642:Chick Embryo; D046508:Culture Techniques; D004285:Dogs; D004789:Enzyme Activation; D004847:Epithelial Cells / Q000201:enzymology / Q000378:metabolism; D051379:Mice; D008856:Microscopy, Fluorescence; D009218:Myosins / Q000378:metabolism*; D015398:Signal Transduction
D000199:Actins; D020169:Caspases; D009218:Myosins