Probing Endocytosis During the Cell Cycle with Minimal Experimental Perturbation

Endocytosis mediates the cellular uptake of nutrients, modulates signaling by regulating levels of cell surface receptors, and is usurped by pathogens during infection. Endocytosis activity is known to vary during the cell cycle, in particular during mitosis. Importantly, different experimental conditions can lead to opposite results and conclusions, thereby emphasizing the need for a careful design of protocols. For example, experiments using serum-starvation, ice-cold steps or using mitotic arrest produced by chemicals widely used to synchronize cells (nocodazole, RO-3306, or S-trityl-L-cysteine) induce a blockage of clathrin-mediated endocytosis during mitosis not observed in unperturbed, dividing cells. In addition, perturbations produced by mRNA interference or dominant-negative mutant overexpression affect endocytosis long before cells are being assayed. Here, we describe simple experimental procedures to assay endocytosis along the cell cycle with minimal perturbations.