Vesicle Transport in Endosomal and Lysosomal Systems

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Eukaryotic cells have developed an array of endomembrane systems that have differentiated to carry out various functions. They are involved in the pathways of endocytosis and exocytosis. V-ATPase is a multi-subunit enzyme responsible for acidification of the endomembrane systems. We found multiple isoforms of mouse Vo subunit a (a1, a2, a3 and a4), whereas c and c" subunits are encoded by single genes. a1 is predominantly localized to secretory vesicles, and a2 is associated with Golgi, whereas a3 is a late endosomal/lysosomal resident. These results demonstrated that the multiple isoforms are, in part, responsible for the deiverse localization and function of the acidic organelles.The lysosome functions are ensured by accurate membrane. We found that mouse syntaxin 7 is localized to late endosomes and that its dominant negative mutant blocked endocytic transport from early to late endosomes. Thus, syntaxin 7 mediates the endocytic trafficking from early endosomes to late endosomes and lysosomes.We were also interested in the molecular mechanisms how the cells acquire their specific functions. Osteoclasts generate an extracellular acidic milleu by V-ATPase localized to the plasma membrane. In the osteoclast precursor, the V-ATPase with the a3 isoforms was localized to lysosomes. Upon stimulation, the a3 isoform was transported to the plasma membrane, indicating that V-ATPases with the a3 isoform localized in late-endosomes/lysosomes were transported to the cell periphery during differentiation, and finally assembled into the plasma membrane of mature osteoclasts.Our studies demonstrated that the membrane dynamics along the endosome/lysosome and plasma membranes are involved in the regulation and expression of cell-specific functions like bone-resorption.