Megapinocytosis: a novel endocytic pathway
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M2 macrophages showed large endocytotic structures, very different from classical macropinosomes that we named megapinosomes. As observed in the scanning electron microscope, megapinosome formation started with a large (diameter of several micrometers) invagination of the plasma membrane. When the invagination was almost completed, the remaining opening was closed by an actinomorphous centripetal arrangement of many (about 50–100) microvilli-like structures. In transmission electron microscopy using high-pressure freezing, we observed that the megapinosome was filled with a trabecular meshwork that originated from the highly structured plasma membrane. The trabecular meshwork was topologically part of the cytosol and separated from the extracellular fluid by a lipid bilayer. According to ultrastructural features, we could define different phases of megapinosome formation and decay. Megapinosomes became more frequent when M2 macrophages were inoculated with human cytomegalovirus. We did not find megapinosome formation in M1 macrophages.
KeywordsMacrophages Macropinocytosis Endocytosis Electron microscopy High-pressure freezing TEM SEM
We thank Renate Kunz, Anke Lüske, Eberhard Schmid, and Reinhard Weih for technical assistance, and Dr. Rudolph Reimer, HPI Hamburg, and Gregor Neusser, Ulm University, for their help with the analysis of the stereo pairs and the height profiles.
Animation of stereo images of a M1 macrophage without a surface depression, a M2 macrophage with an early phase megapinosome formation, a M2 macrophage with a later stage megapinosome. A M2 macrophage with a putative late stage megapinosome formation. 25 images were recorded with a tilt increment of 2°. Finally the megapinosome was closed by an actinomorphous ring of microvilli-like structures. Supplementary material 2 (M4 V 46585 kb)
- Alberts B, Johnson A, Lewis J, Morgan D, Raff M, Roberts K, Walter P (2014) Molecular biology of the cell, 6th edn. Garland Publishing, New YorkGoogle Scholar
- Boyde A (1970) Practical problems and methods in the three dimensional analysis of scanning electron microscope images. In: Johari O (ed) Scanning electron microscopy. IITRI, Chicago, pp 105–112Google Scholar
- Haspot F, Lavault A, Sinzger C, Laib Sampaio K, Stierhof YD, Pilet P, Bressolette-Bodin C, Halary F (2012) Human cytomegalovirus entry into dendritic cells occurs via a macropinocytosis-like pathway in a ph-independent and cholesterol-dependent manner. PLoS ONE 7:e34795CrossRefPubMedPubMedCentralGoogle Scholar
- Lewis WH (1931) Pinocytosis. Johns Hopkins Hosp Bull 49:17–27Google Scholar