Endosome recycling

Green fluorescent protein-RhoB GFP-RhoB is localized in endocytic vesicles and has been shown to highlight early endosomes, recycling endosomes, and multivesicular bodies, but is absent from lysosomes (123). Because RhoB is toxic when applied for long periods of time, the cells should be analyzed within 24 hours of transient transfection. 

Exosomes are released upon fusion of a multivesicular structure with the plasma membrane. Such multivesicular structures are generally described for endosomal carrier vesicles/late endosomes which allow sorting of transmembrane proteins for delivery into lysosomes for degradation. Proteins that are not sorted into the lumenal vesicles of late endosomes recycle back to the plasma membrane or remain on the limiting membrane, thereby escaping dowmegulation. 

After 1 hour of treatment, fluorescent transferrin can be localized in early endosomes and the endosomal-recycling compartment, whereas the cholera toxin B-subunit distinctly traffics to the trans-golgi network of living cells. In cells treated with holo-cholera toxin comprising the B-subunit and the catalytic A-subunit, the toxin would traffic further to the endoplasmic reticulum, which would enable the release of the toxic A-subunit into the cytoplasm. 

Most mammalian cells produce cell-surface receptors that specifically bind to apoB-100 and internalize LDL particles by receptor-mediated endocytosis. After endocytosis, the LDL particles are transported to lysosomes via the en-docytlc pathway and then are degraded by lysosomal hydrolases. LDL receptors, which dissociate from their ligands in the late endosome, recycle to the cell surface. 

A similar situation occurs in neurons, where axons and dendrites constitute the polarized domains (C.G. Dotti, 1991). GPI-anchored proteins are endocytosed slowly compared to transmembrane proteins bearing internalization motifs they are also slower to exit the endosomal recycling compartment (S. Chatterjee, 2001). The retention of GPI proteins in endosomes, and the opportunity for the protein to persist in an acidic endosomal... 

Regulation of NHE3 involves many hormonal and physical mechanisms. Acutely, NHE3 activity is promoted by an increase in intracellular pH and this response is rapid. Furthermore, the increase in activity is proportional to the duration of acidification. Chronic acidification promotes recycling of NHE3 from subapi-cal endosomes to the plasma membrane. 

NHE5. The distribution of this isoform is distinct, being in neuronal-rich areas of the central nervous system. Low levels have also been found in testis, spleen and skeletal muscle. Like the preceding isoforms, NHE5 is found in the plasma membrane and is internalised by clathrin-associated endocytosis into recycling endosomes. The normal role of NHE5 is unknown but its malfunction is speculated to contribute to the development of neurodegenerative disease. 

However, not all proteins proceed directly to their eventual destination. Some proteins relocate from one plasma membrane compartment to another by means of trans-cytosis. Transcytosis involves endocytosis of selected proteins in one membrane compartment, followed by subsequent transport through early endosomes to recycling endosomes and finally translocation to a different membrane compartment, for example from the apical to the basolateral surfaces. Sorting at the TGN and endo-some recycling steps appear to have a primary role in the steady state distribution of proteins in different plasma membrane domains [47], However, selective retention of proteins at the plasma membrane by scaffolding proteins or selective removal may also contribute to normal distributions. Finally, microtubule-motor regulatory mechanisms have been discovered that might explain the specific delivery of membrane proteins to discrete plasma membrane domains [48]. 

Ligand (LDL, transferrin, etc.) Recycling receptor Retrograde vesicle Early endosome... 

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