Our data demonstrate that the trafficking of PLP in oligodendrocytes is under neuronal control. PLP is initially targeted to LEs/Ls by using a cholesterol-dependent and clathrin-independent endocytosis pathway. The situation changes dramatically upon receiving maturation signals from neurons. PLP is then redistributed from LEs/Ls to the plasma membrane. We provide evidence that the developmental regulation of PLP localization occurs by the down-regulation of endocytosis and by the transport from LEs/Ls to the cell surface.
The regulation of the transport of PLP is strikingly similar to the trafficking of MHC-II in dendritic cells (Mellman and Steinman, 2001
). Immature dendritic cells have a high rate of endocytosis and target MHC-II to lysosomes (Pierre et al., 1997
). After exposure to inflammatory mediators, the endocytosis of MHC-II is reduced, and the transport of MHC-II from lysosomes to the cell surface is triggered (Kleijmeer et al., 2001
; Boes et al., 2002
; Chow et al., 2002
). The transport pathway of PLP from LEs/Ls can also be related to the release of secretory lysosomes from hematopoietic cells. However, unlike the classic secretory lysosomes that are specialized to release luminal content, oligodendrocytes mainly transport membrane, and this may occur without significant extracellular release of lysosomal content, as is the case for dendritic cells (Kleijmeer et al., 2001
; Trombetta et al., 2003
). For oligodendrocytes, LE/L compartments may be particularly useful as storage compartments, as they are able to harbor large amounts of membrane in a multilamellar and multivesicular fashion for myelin biogenesis. In most cells, however, the majority of molecules that localizes to internal vesicles of the endosomal system are destined for lysosomal degradation. This raises the question of how PLP survives in an environment where protein degradation usually occurs. One possibility is that immature oligodendrocytes have specialized LEs/Ls with low proteolytic capacity. Our unpublished observation that the vesicular stomatitis virus glycoprotein accumulates undegraded in LEs/Ls of Oli-neu cells supports this notion. In dendritic cells, for example, lysosomal proteolysis is regulated in a developmentally linked fashion (Trombetta et al., 2003
; Delamarre et al., 2005
). Another possibility is that PLP is poorly degradable and, therefore, accumulates within LEs/Ls. A second issue is how PLP escapes from a compartment associated with the limited capacity for membrane recycling. Previous work has provided evidence that not all intralumenal membranes of LEs/Ls are destined for lysosomal degradation. It has been suggested that some vesicles may undergo back fusion with the limiting membrane, and, in some instances, this membrane is sorted via tubulovesicles to the plasma membrane (Kleijmeer et al., 2001
; Boes et al., 2002
; Chow et al., 2002
; Abrami et al., 2004
; Le Blanc et al., 2005
). Whether PLP is sorted by back fusion and tubules to the surface of oligodendrocytes are issues that have to be addressed in future studies. It is important to note that the accumulation of PLP at the surface of the plasma membrane after the receipt of maturation signals most likely reflects the contribution of multiple factors. Our finding that the endocytosis of PLP is reduced after receiving signals from neuronal cells suggests that the regulation of endocytosis may play an essential role in this process. It will be interesting to elucidate the molecular mechanisms of how neurons control the rate of endocytosis in oligodendrocytes. One attractive possibility is that the endocytic activity is controlled by the RhoA GTPase.
Importantly, not all myelin components were found to be internalized into LEs/Ls. Although PLP and cholesterol resided in LEs/Ls, myelin basic protein and galactosylceramide were mainly found in or at the plasma membrane (unpublished data). The differential compartmentalization of myelin components before the onset of myelination might be a mechanism to prevent premature and inappropriate assembly. Our results suggest that an external soluble factor regulates myelin membrane assembly by controlling the trafficking of PLP to and from the surface of the cell. Among the many potential candidates are soluble mediators such as neurotrophins, neuregulin, or adenosine that can now be tested with the described experimental system (Barres and Raff, 1999
; Fields and Stevens-Graham, 2002
Together, our findings reveal an unexpected and novel role of LEs/Ls in oligodendrocytes. It provides a striking example of how cell-to-cell communication regulates trafficking to and from a cellular compartment to guide the development of a multicellular tissue. The proposed role of LEs/Ls in myelin biogenesis may help to explain the cellular mechanisms of dysmyelination that is observed in many lysosomal storage diseases.