Micropinocytosis

Micropinocytosis is a form of endocytosis in which cells take up extracellular fluid and dissolved solutes through small vesicles formed at the plasma membrane. It is typically clathrin-independent and distinct from macropinocytosis, which yields larger vesicles and often involves robust membrane ruffling. Micropinocytosis is generally actin-dependent and can be constitutive or enhanced by signaling events that reorganize the cytoskeleton and membrane lipids.

The process is driven by cytoskeletal remodeling and membrane dynamics that promote vesicle scission and internalization. It can be regulated by signaling pathways that activate small GTPases such as Rac1 and Cdc42 and by phosphoinositide metabolism. Vesicle formation and uptake may involve various accessory proteins, and the exact molecular machinery can differ among cell types and contexts.

Physiological roles include sampling of extracellular fluid and solutes by immune cells such as dendritic cells and macrophages, where it contributes to antigen uptake and presentation. Some non-immune cells also use micropinocytosis to internalize soluble nutrients, proteins, or other extracellular materials. Because the process is relatively non-selective, it can be exploited by pathogens to gain cellular entry, and alterations in micropinocytosis can influence disease progression and cellular metabolism.

In experimental settings, micropinocytosis can be modulated or inhibited by agents that disrupt actin dynamics or signaling pathways, and by amiloride derivatives that reduce membrane turnover. Overall, micropinocytosis represents a distinct, regulated pathway for non-receptor–mediated uptake of extracellular fluid and solutes.