Mutually exclusive apicobasolateral sorting of two oligodendroglial membrane proteins, proteolipid protein and myelin/oligodendrocyte glycoprotein, in Madin‐Darby canine kidney cells

Abstract

Oligodendrocytes elaborate an extensive membrane network that ensheathes CNS axons in multilamellar wrappings. A compaction process excludes much of the cytoplasm in mature myelin membranes, giving rise to distinct lipid/protein compositions in two membrane compartments (compact myelin and membranes of the cell body and processes). Insofar as oligodendrocytes arise from neuroepithelial progenitors, it seems likely that some elements are shared for protein targeting by these two cell types. We hypothesized that certain membrane proteins targeting different oligodendroglial membrane compartments would preferentially sort to opposite domains when transfected into Madin-Darby canine kidney (MDCK) epithelial cells. Myelin/oligodendrocyte glycoprotein (MOG) is found in uncompacted membrane (cell body, processes), and it sorts exclusively to MDCK basolateral membrane. Proteolipid protein (PLP) is found in compact myelin, and it sorts exclusively to MDCK apical membrane. Myelin-associated glycoprotein (MAG) is primarily in the periaxonal inner loop of myelin; however, it fails to target preferentially within MDCK cells. This inability of MAG to sort within MDCK cells suggests a lack of required oligodendroglial-specific targeting components. In contrast, the sorting machinery in both oligodendrocytes and MDCK cells recognizes targeting signals for MOG and PLP, and we propose that these oligodendroglial membrane proteins delineate cognate basolateral and apical domains, respectively.

The Major Myelin-Resident Protein PLP Is Transported to Myelin Membranes via a Transcytotic Mechanism: Involvement of Sulfatide

“…Therefore, in line with the presence of polarity-based transport mechanisms in OLGs, newly synthesized PLP might reach its final destination via an indirect pathway. Consistent with this hypothesis is the observation that PLP, stably expressed in polarized Madin-Darby canine kidney cells, is transported to the apical rather than the basolateral surface (25), i.e., membrane domains reminiscent of the cell body plasma membrane and the myelin sheet in cultured OLGs, respectively (4,5).…”

Myelin sheaths: glycoproteins involved in their formation, maintenance and degeneration

“…In view of MAG's selective localization in periaxonal and other noncompacted glial membranes, one might expect specific signals affecting its intracellular transport could reside in the cytoplasmic domains. However, a recent study with Madin-Darby canine kidney (MDCK) epithelial cells, which are widely used to investigate protein targeting to apical or basolateral membranes, revealed little evidence for selective targeting of either MAG isoform [154] and challenged an earlier report showing some evidence for basolateral sorting [155]. It may be that because of the complex structure of myelinating cells, there are specialized sorting signals for MAG and some other glial proteins that are not recognized in MDCK cells.…”

“…However, topographical studies have indicated that the second hydrophobic domain does not transverse the membrane completely, so the carboxy terminus of MOG is in the oligodendroglial cytoplasm. It is of interest that the cytoplasmic domain of MOG contains targeting signals that direct it to the basolateral domain of MDCK cells and presumably account for its selective localization on the surface of oligodendrocytes and myelin sheaths [154,203]. Alternatively spliced forms of mRNA for MOG were not detected in rodents, but studies on humans have demonstrated several alternatively spliced mRNAs, some of which would encode isoforms of MOG truncated at various positions in the protein.…”

Polarity Development in Oligodendrocytes: Sorting and Trafficking of Myelin Components

“…The expression of myelin proteins in MDCK cells, the prototype of a polarized epithelial cell, has been particularly instructive. After transfection of MDCK cells, PLP is transported to the apical membrane, whereas MOG is transported exclusively to the basolateral membrane (Kroepfl and Gardinier 2001). Interestingly, the two isoforms of MAG, L-MAG and S-MAG, are distinctly transported in MDCK cells (Minuk and Braun 1996).…”