Topology of CNS Myelin Proteolipid Protein: Evidence for the Nonenzymic Glycosylation of Extracytoplasmic Domains in Normal and Diabetic Animals

Abstract

Myelin proteolipid protein (PLP), the main integral membrane protein in the central nervous system myelin, was labeled at the extracytoplasmic domains with the membrane impermeant reagents pyridoxal 5'-phosphate and tritiated borohydride. Lysine-217, located in the fourth hydrophilic domain of PLP, was found to be the major labeled residue, which defined this domain to be extracytoplasmic in agreement with our previously proposed topological model. The remarkably high reactivity in vitro of this residue as compared to all other lysines in PLP led us to investigate the possible modification of PLP in vivo by other carbonyl compounds. We demonstrate that PLP is the most highly nonenzymatically glycosylated membrane protein in murine and bovine brain. The degree of modification increases significantly under hyperglycemic conditions, as studied in diabetic mice. The majority of the glycosylation sites are also located at extracytoplasmic domains. The degree of nonenzymatic glycosylation of PLP may be related to late diabetic complications affecting the central nervous system.

Oligodendrocytes expressing exclusively the DM20 isoform of the proteolipid protein gene: Myelination and development

“…Chemical studies revealed a four-transmembrane topology with the N-and C-termini located on the cytosolic side of the myelin membrane (Weimbs and Stoffel, 1994) in accordance with one of the hypothetical models (Popot et al, 1991). The PLP polypeptide is thioacylated by six long-chain fatty acids (palmitic, stearic, and oleic acid) at cysteine residues on the cytosolic side.…”

“…This would further stabilize the adhesion of the adjacent cytosolic surfaces in the MDL. Figure 8 schematically depicts the suggested topology of tetraspan DM20 (Weimbs and Stoffel, 1994).…”

Molecular cloning and transfection studies of M6b-2, a novel splice variant of a member of the PLP-DM20/M6 gene family

“…Comparison of the most widely accepted current model for the topology of PLP at the plasma membrane of oligodendrocytes (Popot et al, 1991;Weimbs and Stoffel, 1994), and the hydrophobicity profiles for PLP, M6b-1 , and M6b-2 (this work) suggests that the novel insertion in M6b-2 is exposed to the cytoplasmic surface of the plasma membrane. Since M6b-2 has the potential to localise to the plasma membrane and the novel polypeptide is predominantly hydrophilic and probably exposed to the cytoplasm, this polypeptide may have the potential to interact with lipid head groups of the cytoplasmic face of the plasma membrane and/or with other cytoplasmic proteins.…”

New insights into the metabolic and molecular basis for diabetic neuropathy

“…Glycation of laminin, a major constituent of Schwann cell basal lamina, which is important in nerve sprouting, may contribute to impaired nerve fiber regeneration in diabetes [78]. Myelin components such as P0, myelin basic protein and proteolipid protein are subjected to non-enzymatic glycation [79,80] which may then be recognized and scavenged by macrophages via RAGE [81,82], thereby probably contributing to segmental demyelination. Increased production of fructose via activation of the polyol pathway enhances the generation of glycated proteins ( fig.…”