Spatiotemporal activation of Rac1 for engulfment of apoptotic cells.

The engulfment of apoptotic cells requires phagocytes to coordinately activate Rho family GTPases that regulate actin dynamics. Here, we used a FRET biosensor to visualize the spatiotemporal activation of Rac1 during engulfment of apoptotic cells. We report that apoptotic cells were usually engulfed by the phagocytes' lamellipodia, where Rac1 was activated. Often, apoptotic cells were engulfed successively at the same lamellipodial site, suggesting the presence of portals for apoptotic cells. At this location, the activated Rac1 was recruited to form phagocytic cups that were comprised of actin patches. When the phagocytic cup was closed, Rac1 was down-regulated, and the actin patches were abruptly broken down. The constitutively active Rac1 remained at phagocytic cup for a longer period than the wild-type Rac1, and the closure of the phagocytic cup was significantly delayed in cells expressing a constitutive active form of Rac1, resulting in inefficient engulfment. These results indicate that activated Rac1 is necessary to assemble F-actin, but closing the phagocytic cup requires Rac1 to be deactivated.