Biogenesis of lysosomes by endocytic flow of plasma membrane

The objective of this review is to examine the biogenesis of lysosomes during the endocytic flow of plasma membrane. Two models have been proposed to explain the formation of lysosomes by this process: the "maturational" and the "stationary" models. According to the former, pinocytotic vesicles fuse among themselves to yield endosomes, which in turn, transform first into multivesicular bodies (MVB) and then into mature lysosomes. Therefore, endosomes and lysosomes would be transient organelles. On the other hand, the "stationary" model proposes that the endocytic pathway is formed by functionally and physically distinct compartments. Cultured cells exposed to ammonium chloride (NH4Cl) and leupeptin after a pulse of cationic ferritin were recently used to freeze endosomes and lysosomes. NH4Cl produced a retention of cationic ferritin in endosomes, indicating that this agent interfered with the endosomal/lysosomal progression. Leupeptin did not affect this process. The number of lysosomes increased in cells treated with both lysosomotropic agents. Thus, NH4Cl affected the endosomal and lysosomal compartments, whereas leupeptin had a preferential effect on lysosomes. Mice mutants with defects of plasma membrane degradation, including a Tay-Sachs model, a Sandhoff disease model, as well as, mice with the inactivated prosaposin gene were used to analyze the biogenesis of lysosomes. Thin sections of mutant cells were examined under the electron microscope, and the analysis revealed a selective accumulation of MVBs and the disappearance of lysosomes, suggesting that the formation of MVBs is a required step in lysosomal maturation and that the intravesicular content of MVBs is necessary for the digestion of plasma membrane components. Taken together, these data indicate that endosomes and MVBs are preceding steps in lysosomal biogenesis and that endosomes, MVBs, and lysosomes are transient organelles.