Makropinocytose

Makropinocytose, or macropinocytosis, is a form of endocytosis in which cells ingest extracellular fluid and dissolved solutes through large, irregular vesicles called macropinosomes. The process is driven by actin-dependent plasma membrane ruffles that collapse back onto the cell surface, engulfing surrounding liquid in a non-selective manner. Macropinosomes are typically 0.2–5 micrometers in diameter and form independently of clathrin.

The mechanism often involves stimulation by growth factors, pathogens, or other signaling cues that trigger actin remodeling. Key players include small GTPases such as Rac1 and Cdc42, phosphoinositides, and kinases like PI3K and PAK. These signals promote membrane ruffling and vesicle formation. After internalization, macropinosomes may fuse with the endolysosomal system for degradation or processing, or recycle membrane components back to the surface. In immune cells, macropinocytosis contributes to sampling the extracellular environment and delivering antigens for presentation.

Biological significance and roles: In macrophages and dendritic cells, macropinocytosis supports surveillance and antigen uptake for MHC class II-mediated presentation. In other cell types, it can provide a route for nutrient acquisition or remodeling of the plasma membrane. Some pathogens hijack macropinocytosis to enter host cells, using induced ruffling as an entry pathway.

Relation to other endocytic Processes: Macropinocytosis is distinct from phagocytosis (which engulfs large particles) and from clathrin-mediated endocytosis (which relies on clathrin-coated pits). It is generally clathrin-independent and can be modulated by inhibitors of actin dynamics or Na+/H+ exchangers, such as amiloride.