Introne W, Boissy RE, Gahl WA. ubiquitous trafficking mechanisms with cell-specific machinery that targets cargo to a particular compartment. Whereas classic secretory E260 granules form directly from the trans-Golgi network (TGN), some or all LRO contents derive from the endosomal system. This group of individual compartments relies on sorting and trafficking of membrane structures, assisted by a network of filaments, tubules, motor proteins, Rabs and other small GTPases, and numerous other components such as BLOCs, SNAREs, syntaxins, and VPS proteins. A. The endosomal system Lysosome-related organelles maintain their structure, composition, and function by means of a continuous flow of proteins and membranes/vesicles among endosomal compartments (Figure 1). The endocytic pathway takes up molecules from outside of the cell and internalizes membrane receptors, while the exocytic E260 pathway sorts newly synthesized proteins from the endoplasmic reticulum toward the endosomal system (9). The two pathways connect at the early endosome, a tubulo-vesicular network with a pH of 5.9C6.0 that contains distinct resident proteins, including Early Endosome Associated Protein (EEA)-1, Rab5, and Rabaptin-5. Material is sorted from the early endosome to the cell surface, recycling endosomes, the biosynthetic pathway (of LROs) Gdf6 or late endosomes/lysosomes (9, 36, 99). Components destined for lysosomes are sorted to late endosomes, which have an acidic pH (5C6.0), contain whorls of membranes and vesicles, include multivesicular bodies, and recycle mannose-6-phosphate receptors (MPRs) back to the trans-Golgi membranes. Other late endosome-specific markers are Rab7 and lysobisphosphatidic acid (LBPA). Late endosomes morph into lysosomes, which lack multivesicular structures and MPRs and have a pH of 5.0C5.5. Components destined for specific LROs possess unique, cell-type and LRO-specific sorting and trafficking pathways (99). To support the endosomal system, cell-type specific chaperones exist for vesicle targeting, transport, and fusion events. These include a cytoskeletal system of tubules and filaments, coat associated proteins, SNAREs, syntaxins, rabs, motor proteins, and specific membrane lipids (9, 36, 95, 105). Open in a separate window Figure 1 The endosomal system and LRO biogenesisThe endosomal system is a collection of highly dynamic compartments defined by their morphology, contingent of marker proteins, function and accessibility to endocytic tracers. Rather than focus on a specific cell type, this model shows the basic endosomal elements involved in generic LRO biogenesis. Solid arrows depict maturation/genesis of a compartment while dashed arrows represent E260 the movement of cargo. The early (sorting) endosome is the major sorting center of the cell and is defined by the presence of EEA1 and Rab5 and by rapid accessibility to endocytic tracers (5C15 min). Sorting of Golgi derived biosynthetic cargo destined for LROs or late endosomes/lysosomes as well as receptors and other molecules internalized from the cell membrane occurs in the early endosome. Some receptors return to the cell membrane by way of recycling endosomes, while others continue on to late endosomes and lysosomes for degradation. As the early endosome matures, intralumenal vesicles containing proteins targeted for lysosomal degradation accumulate and the multivesicular body (MVB)/late endosome is formed. The MVB/late endosome is the last site of sorting and is defined by its morphology and the presence of MPRs and LBPA. MPRs deliver lysosomal hydrolases and are then recycled from the MVB/late endosome back to the TGN. MVBs/late endosomes fuse with (or mature into) lysosomes to deliver their contents for degradation. Lysosomes lack both intralumenal vesicles and MPRs and are considered end-stage degradative compartments. Specialized LROs co-exist in the same cell as non-specialized lysosomes and contents destined for each compartment must be sorted appropriately. In some LRO containing cells, LRO formation involves sequential delivery of LRO-specific proteins. A subset of proteins destined for LROs (e.g. PMEL17 in melanocytes) are sorted from a post-Golgi compartment to an endosomal intermediate and drive the formation of a pre-LRO (e.g. stage I/II melanosome). A second set of LRO specific proteins (e.g. TYR and TYRP1) are sorted from the early endosome to the pre-LRO compartment, resulting in a mature LRO. The mature LRO then acquires specific accessory proteins (Rabs, motor.
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