The currently accepted view of membrane structure is that of a lipid bilayer with some integral membrane proteins embedded and other extrinsic proteins attached to one surface or the other by weaker linkages. Both proteins and lipids are distributed asymmetrically with the asymmetry of lipids being partial. The molecular architecture of the layered membranes of compact myelin appears to be determined by similar principles. Molecular models of compact myelin are hypothesized based on data from electron microscopy, immunostaining, X-ray diffraction, surface probe studies, structural abnormalities in mutant mice, correlations between structure and composition in various species and predictions of protein structure from sequencing information [5].

Presumably, the glycolipids in myelin, as in other membranes, are preferentially at the extracellular surfaces in the intraperiod line. Based on a finding referred to earlier in mice unable to synthesize galactocerebroside and sulfatide [10], one or both of these major glycolipids may contribute to the stability of the intraperiod line. Diffraction studies demonstrate that cholesterol also is enriched in the extracellular face of the myelin membrane, whereas ethanolamine plasmalogen is localized asymmetrically to the cytoplasmic half of the bilayer.

A diagrammatic representation of current ideas about the molecular organization of proteins in compact myelin of the CNS and PNS is shown in Figure 4-13. Although several models for the orientation of PLP in the membrane have been proposed, it is now widely believed that all four hydrophobic domains pass entirely through the membrane with both the N- and C-termini on the cytoplasmic side, as shown in Figure 4-13. By contrast, MBP is an extrinsic protein localized exclusively at the cytoplasmic surface in the major dense line, a conclusion based on its amino acid sequence, inaccessibility to surface probes and direct localization at the electron-microscope level by immunocytochemistry. MBP may form dimers and may be the principal protein stabilizing the major dense line of CNS myelin, possibly by interacting with negatively charged lipids. Failure of compaction of the major dense line in MBP-deficient mutants supports this hypothesis (see Chap. 39). An important role for PLP in stabilizing the intraperiod line generally has been assumed, based largely on the extracellular loops of this protein being present at this location. The intraperiod line is condensed abnormally both in the PLP/DM-20 knockout mouse [16] and in spontaneously occurring PLP-deficient mutants (see Chap. 39), confirming a structural role for PLP in determining the architecture of the intraperiod line. Because myelin formation is relatively normal in PLP knockout mice this major protein is not required for spiraling of the membrane or compaction of extracellular surfaces. This suggests that other proteins or lipids of myelin may contribute to the adherence of extracellular faces during compaction. However, myelin in the PLP-null mutant is extrasensitive to osmotic shock during fixation, suggesting that PLP does enhance the stability of myelin, possibly by forming a zipper-like structure after it is compacted.

Figure 4-13

Diagrammatic representation of current concepts of the molecular organization of compact CNS and PNS myelin. Apposition of the extracellular (Ext.) surfaces of the oligodendrocyte or Schwann cell membranes to form the intraperiod (IP) line is shown in (more...)

In the PNS, the major P₀ protein transverses the bilayer once and is believed to stabilize the intraperiod line by homophilic binding between extracellular domains on adjacent layers (Fig. 4-13). The relatively large, glycosylated, extracellular, immunoglobulin-like domain of P₀ probably accounts for the greater separation of extracellular surfaces in PNS myelin in comparison to CNS myelin, where closer apposition of these surfaces is possible in the presence of the smaller extracellular domains of PLP. This hypothesis is supported by findings in fish, the species in which compact myelin first appeared during evolution, which have P₀ in both PNS and CNS myelin and no PLP in CNS myelin [5]. The spacing of the intraperiod line in fish CNS myelin is greater than that in higher vertebrates and comparable to that of P₀-containing PNS myelin in other species. Homophilic interactions between P₀ molecules may involve both protein—protein and protein—carbohydrate interactions [26]. Furthermore, the crystal structure of the extracellular domain of P₀ [35] suggests that P₀ molecules emanate from each apposing membrane surface as tetramers. A tryptophan residue at the apex of its extracellular domain may interact directly with the lipid bilayer of the apposing membrane. A more detailed description of the function of P₀ as an adhesion molecule is provided in Chapter 7. P₀ protein also has a relatively large positively charged domain on the cytoplasmic side of the membrane, which contributes significantly to stabilization of the major dense line in the PNS. As a result, MBP is not as important for stability of the major dense line as in the CNS, where there is more of it. Comparison of animals doubly deficient in the genes for P₀ and MBP to animals deficient in one or the other indicates that both contribute to compaction of the cytoplasmic surfaces in PNS myelin [46]. The P₂ protein also may contribute to the stability of the major dense line, although its amount varies drastically from species to species. Interestingly, larger amounts of P₂ protein in myelin of various species correlate with larger widths of the major dense lines as determined by X-ray diffraction [5]. Although the tetraspan PMP-22 is localized in compact PNS myelin, as shown in Figure 4-13, it is not known if its extracellular or cytoplasmic domains play an important structural role. The relatively small amount of PMP-22 suggests a dynamic function.

Although the above model is a static representation, the relatively rapid metabolism of certain myelin components suggests that there may be some dynamic aspect of myelin structure, such as occasional separation of the cytoplasmic faces of the membranes. More detailed analysis of both the static structural aspect and the dynamic properties of the myelin sheath awaits conceptual and analytical advances.