Endosome - Types

Types

Endosomes comprise three different compartments: early endosomes, late endosomes, and recycling endosomes. They are distinguished by the time it takes for endocytosed material to reach them, and by markers such as rabs. They also have different morphology. Once endocytic vesicles have uncoated, they fuse with early endosomes. Early endosomes then mature into late endosomes before fusing with lysosomes.

Early endosomes mature in several ways to form late endosomes. They become increasingly acidic mainly through the activity of the V-ATPase. Many molecules that are recycled are removed by concentration in the tubular regions of early endosomes. Loss of these tubules to recycling pathways means that late endosomes mostly lack tubules. They also increase in size due to the homotypic fusion of early endosomes into larger vesicles. Molecules are also sorted into smaller vesicles that bud from the perimeter membrane into the endosome lumen, forming lumenal vesicles; this leads to the multivesicular appearance of late endosomes and so they are also known as multivesicular bodies (MVBs). Removal of recycling molecules such as transferrin receptors and mannose 6-phosphate receptors continues during this period, probably via budding of vesicles out of endosomes. Finally, the endosomes lose RAB5 and acquire RAB7, making them competent for fusion with lysosomes.

Fusion of late endosomes with lysosomes has been shown to result in the formation of a 'hybrid' compartment, with characteristics intermediate of the two source compartments. For example, lysosomes are more dense than late endosomes, and the hybrids have an intermediate density. Lysosomes reform by recondensation to their normal, higher density. However, before this happens, more late endosomes may fuse with the hybrid.

Some material recycles to the plasma membrane directly from early endosomes, but most traffics via recycling endosomes.

• Early endosomes consist of a dynamic tubular-vesicular network (vesicles up to 1 µm in diameter with connected tubules of approx. 50 nm diameter). Markers include RAB5 and RAB4, Transferrin and its receptor and EEA1.

• Late endosomes, also known as MVBs, are mainly spherical, lack tubules, and contain many close-packed lumenal vesicles. Markers include RAB7, RAB9, and mannose 6-phosphate receptors.

• Recycling endosomes are concentrated at the microtubule organizing center and consist of a mainly tubular network. Marker; RAB11.

More subtypes exist in specialized cells such as polarized cells and macrophages.

Phagosomes, macropinosomes and autophagosomes mature in a manner similar to endosomes, and may require fusion with normal endosomes for their maturation. Some intracellular pathogens subvert this process, for example, by preventing RAB7 acquisition.

Late endosomes/MVBs are sometimes called endocytic carrier vesicles, but this term was used to describe vesicles that bud from early endosomes and fuse with late endosomes. However, several observations (described above) have now demonstrated that it is more likely that transport between these two compartments occurs by a maturation process, rather than vesicle transport.