title: Macropinocytosis: regulated coordination of endocytic and exocytic membrane traffic events.
abstract: Macropinocytosis, a form of bulk uptake of fluid and solid cargo into cytoplasmic vacuoles, called macropinosomes, has been studied mostly in relation to antigen presentation. Early membrane traffic events occurring in this process are, however, largely unknown. Using human dendritic cells we show that a marked increase in the rate of macropinocytosis occurs a few minutes after application of two markers (small latex beads or dextran), depends on a slow intracellular Ca2+ concentration ([Ca2+](i)) rise that precedes the PI3K-dependent step, and is preceded and accompanied by exocytosis of enlargeosomes compensating in part for the macropinocytic plasma membrane internalization. Unexpectedly, macropinosomes themselves, which share markers with endosomes, undergo Ca2+ -dependent exocytosis so that, after approximately 20 minutes of continuous bead or dextran uptake, an equilibrium is reached preventing cells from overloading themselves with the organelles. Large [Ca2+](i) increases induced by ionomycin trigger rapid (<1 minute) exocytic regurgitation of all macropinosomes, whereas endosomes remain apparently unaffected. We conclude that, in dendritic cells, the rate of macropinocytosis is not constant but increases in a regulated fashion, as previously shown in other cell types. Moreover, macropinosomes are not simple containers that funnel cargo to an endocytic pathway, but unique organelles, distinct from endosomes by their competence for regulated exocytosis and other membrane properties.
authors.affiliation: University of Milan, Department of Preclinical Sciences, via GB Grassi 74, 20157 Milan, Italy.
authors.forename: Sestina
authors.initials: S
authors.lastname: Falcone
authors.forename: Emanuele
authors.initials: E
authors.lastname: Cocucci
authors.forename: Paola
authors.initials: P
authors.lastname: Podini
authors.forename: Tomas
authors.initials: T
authors.lastname: Kirchhausen
authors.forename: Emilio
authors.initials: E
authors.lastname: Clementi
authors.forename: Jacopo
authors.initials: J
authors.lastname: Meldolesi
mesh_terms: D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D003713:Dendritic Cells / Q000187:drug effects / Q000502:physiology / Q000648:ultrastructure; D003911:Dextrans; D004705:Endocytosis / Q000187:drug effects / Q000502:physiology*; D005089:Exocytosis / Q000502:physiology*; D006801:Humans; D007425:Intracellular Membranes / Q000187:drug effects / Q000502:physiology* / Q000648:ultrastructure; D015759:Ionomycin / Q000494:pharmacology; D007476:Ionophores / Q000494:pharmacology; D008565:Membrane Proteins / Q000378:metabolism; D046529:Microscopy, Electron, Transmission; D008856:Microscopy, Fluorescence; D008863:Microspheres; D019869:Phosphatidylinositol 3-Kinases / Q000502:physiology; D010873:Pinocytosis / Q000187:drug effects / Q000502:physiology; D033921:Vesicular Transport Proteins / Q000378:metabolism
chemical_list: D003911:Dextrans; D007476:Ionophores; D008565:Membrane Proteins; D033921:Vesicular Transport Proteins; C093846:early endosome antigen 1; D015759:Ionomycin; D019869:Phosphatidylinositol 3-Kinases; D002118:Calcium