Fission of membrane nanotube induced by osmotic pressure

The final stage of endocytosis is fission of a thin membrane neck, or nanotube (NT), connecting cell membrane with a forming vesicle. We studied this process using a model system consisting of NT pulled out from a flat bilayer lipid membrane. Fission of NT was induced by an increase of osmotic pressure created by local application of a concentrated salt solution in the vicinity of NT. Superfusion of NT with distilled water instead of the concentrated salt solution led to the NT expansion. This observation demonstrates the reversibility of the NT expansion-compression process under the osmotic pressure. The overall picture of fission is similar to that described earlier for the NT fission with participation of dynamin GTPase. In both cases, in order for fission to occur, it is necessary to compress the NT to a critical radius. The critical radius estimated for the osmotic pressure-induced fission exceeds the value obtained for the fission occurring in the presence of the protein. Fission under osmotic pressure, akin the dynamin-promoted fission, proceeds without leaky defects.