Microautophagy in mammalian cells: revisiting a 40-year-old conundrum

The term microautophagy was first used in 1966 by de Duve and Wattiaux and subsequently applied, over the following two decades, to processes described in mammalian cells and involving the presence of lysosome-like organelles having multiple vesicles trapped in their lumen ("multivesicular lysosomes"). Concurrently, many studies suggested a view of microautophagy where the lysosomal membrane was either invaginated or projected arm-like protrusions to sequester cytosolic constituents into intralysosomal vesicles. Although microautophagy in mammalian cells has been traditionally considered as a form of autophagy constitutively active in the turnover of long-lived proteins, little is known about the mechanism and regulation of cargo selection. The lack of specific approaches to directly detect microautophagy in mammalian systems, aside from electron microscopy, is the major current limitation to addressing its physiological role(s) and possible contribution to particular disease states. In this review we consider the current state of knowledge about microautophagic processes. We examine some of the main characteristics of microautophagy in yeast with a view to assessing their relevance for our understanding of microautophagy in mammalian cells.