Membrane expansion increases endocytosis rate during mitosis

Mitosis in mammalian cells is accompanied by a dramatic inhibition of endoc ytosis. We have found that the addition of amphyphilic compounds to metapha se cells increases the endocytosis rate even to interphase levels. Detergen ts and solvents all increased endocytosis rate, and the extent of increase was in direct proportion to the concentration added. Although the compounds could produce a variety of different effects, we have found a strong corre lation with a physical alteration in the membrane tension as measured by th e laser tweezers. Plasma membrane tethers formed by latex beads pull back o n the beads with a force that was related to the in-plane bilayer tension a nd membrane-cytoskeletal adhesion. We found that as cells enter mitosis, th e membrane tension rises as the endocytosis rate decreases; and as cells ex ited mitosis, the endocytosis rate increased as the membrane tension decrea sed. The addition of amphyphilic compounds decreased membrane tension and i ncreased the endocytosis rate. With the detergent, deoxycholate, the endocy tosis rate was restored to interphase levels when the membrane tension was restored to interphase levels. Although biochemical factors are clearly inv olved in the alterations in mitosis, we suggest that endocytosis is blocked primarily by the increase in apparent plasma membrane tension. Higher tens ions inhibit both the binding of the endocytic complex to the membrane and mechanical deformation of the membrane during invagination. We suggest that membrane tension is an important regulator of the endocytosis rate and alt eration of tension is sufficient to modify endocytosis rates during mitosis . Further, we postulate that the rise in membrane tension causes cell round ing and the inhibition of motility, characteristic of mitosis.