The study of the process of fluid-phase endocytosis in cervical squamous cells using fluorescent microspheres

Sucrose-inducible endocytosis as a mechanism for nutrient uptake in heterotrophic plant cells

The capacity of plant heterotrophic organs to transport and accumulate incoming nutrients (mostly in the form of sucrose) directly impacts their final size, crop productivity and nutritional value. Endocytosis as a mechanism for nutrient uptake in heterotrophic cells was investigated using suspension culture cells of sycamore (Acer pseudoplatanus L.) and the endocytic inhibitors wortmannin and LY294002. Time course analysis of sucrose uptake in intact walled cells revealed a two-phase process involving an initial 90 min wortmannin- and LY294002-insensitive sucrose uptake period, followed by a prolonged phase of rapid sucrose accumulation which was greatly inhibited by the two endocytic inhibitors. Walled cells were assessed for their capacity to incorporate the fluorescent endocytosis marker lucifer yellow-CH (LY) in the presence or absence of sucrose. Rates of sucrose and LY accumulation were virtually identical, as was their response to wortmannin. In addition, LY incorporation increased as a function of external sucrose concentration. When sucrose was substituted by other sugars or amino acids, uptake of LY greatly diminished, indicating that sucrose itself is the primary signal of endocytosis. Microscopic observations revealed the formation of vesicles containing LY and its eventual accumulation on the vacuole when sucrose was present in the incubation medium. These results demonstrate the existence of a sucrose-inducible endocytic process as a viable mechanism for solute transport into the vacuole of storage cells.

cAMP-sensitive endocytic trafficking in A6 epithelia

Blocker-induced noise analysis and laser scanning confocal microscopy were used to test the idea that cAMP-mediated vesicle exocytosis/endocytosis may be a mechanism for regulation of functional epithelial Na+ channels (ENaCs) at apical membranes of A6 epithelia. After forskolin stimulation of Na+ transport and labeling apical membranes with the fluorescent dye N-(3-triethylammoniumpropyl)4-(6-4 diethylaminophenyl) hexatrienyl pyridinium dibromide (FM 4-64), ENaC densities (N(T)) decreased exponentially (time constant approximately 20 min) from mean values of 320 to 98 channels/cell within 55 min during washout of forskolin. Two populations of apical membrane-labeled vesicles appeared in the cytosol within 55 min, reaching mean values near 18 vesicles/cell, compared with five vesicles per cell in control, unstimulated tissues. The majority of cAMP-dependent endocytosed vesicles remained within a few micrometers of the apical membranes for the duration of the experiments. A minority of vesicles migrated to >5 microm below the apical membrane. Because steady states require identical rates of endocytosis and exocytosis, and because forskolin increased endocytic rates by fivefold or more, cAMP/protein kinase A acts kinetically not only to increase rates of cycling of vesicles at the apical membranes, but also principally to increase exocytic rates. These observations are consistent with and support, but do not prove, that vesicle trafficking is a mechanism for cAMP-mediated regulation of apical membrane channel densities in A6 epithelia.