To begin dividing, mitotic cells round up into tight spheres, thus decreasing their surface area. But where does the extra membrane go, and where does it come from when it's time to expand again? Because the Golgi apparatus is disabled until the end of division, researchers assumed that cell trafficking—including endocytosis—grinds to a halt. EM studies suggested instead that accordion-like folds at the cell surface took up the membrane slack.
In their new report, the researchers measured membrane decrease and recovery during mitosis using live-cell fluorescence imaging. They found that endocytosis is not shut off during division. Clathrin-coated vesicles formed at a steady rate, and the uptake of transferrin remained normal. Previous studies missed the ongoing endocytosis, says Kirchhausen, because they did not correct for surface area changes.
When the duo blocked endocytosis, cells failed to round up and subsequently divided incorrectly. In turn, blocking endosomal fusion at anaphase prevented cell membrane recovery. But inhibiting the Golgi apparatus had no effect, suggesting that recovery relies not on new membrane but on membrane reservoirs that accumulate within the cell.
Recycling during mitosis may help cells stay symmetrical as they divide, says Kirchhausen. The group is now testing whether the mechanism exists in asymmetrically dividing cells. Also, he notes, their results implicate calcium as a regulator of the mitotic recycling, perhaps akin to its role in endosomal fusion in wound healing.