Expression of myelin proteolipid protein in oligodendrocytes and transfected cells

Correlation of blood T cell and antibody reactivity to myelin proteins with HLA type and lesion localization in multiple sclerosis

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the CNS. The numbers of autoimmune T cells and Abs specific for proteins of CNS myelin are increased in the blood in some patients with MS. The aim of this study was to investigate whether there are correlations between the specificity of the autoimmune responses in the blood, the HLA molecules carried by the patient, and the clinical features of MS, because studies on experimental autoimmune encephalomyelitis, an animal model of MS, indicate that autoimmune responses targeting particular myelin proteins and the genetic background of the animal play a role in determining the pattern of lesion distribution. We tested blood T cell immunoreactivity to myelin proteins in 100 MS patients, 70 healthy controls, and 48 patients with other neurological disorders. Forty MS patients had strongly increased T cell reactivity to one or more myelin Ags. In these 40 patients, the most robust correlation was between CD4(+) T cell reactivity to myelin proteolipid protein residues 184-209 (PLP(184-209)) and development of lesions in the brainstem and cerebellum. Furthermore, carriage of HLA-DR4, -DR7, or -DR13 molecules by MS patients correlated with increased blood T cell immunoreactivity to PLP(184-209), as well as the development of lesions in the brainstem and cerebellum. Levels of PLP(190-209)-specific Abs in the blood also correlated with the presence of cerebellar lesions. These findings show that circulating T cells and Abs reactive against specific myelin Ags can correlate with lesion distribution in MS and suggest that they are of pathogenic relevance.

Different mutations in the same codon of the proteolipid protein gene, PLP, may help in correlating genotype with phenotype in Pelizaeus-Merzbacher disease/X-linked spastic paraplegia (PMD/SPG2)

Pelizaeus-Merzbacher disease/X-linked spastic paraplegia (PMD/SPG2) comprises a spectrum of diseases that range from severe to quite mild. The reasons for the variation in severity are not obvious, but suggested explanations include the extent of disruption of the transmembrane portion of the proteolipid protein caused by certain amino acid substitutions and interference with the trafficking of the PLP molecule in oligodendrocytes. Four codons in which substitution of more than one amino acid has occurred are available for examination of clinical and potential structural manifestations: Valine165 to either glutamate or glycine, leucine 045 to either proline or arginine, aspartate 202 to asparagine or histidine, and leucine 223 to isoleucine or proline. Three of these mutations, Val165Gly, Leu045Pro, and Leu223Ile have not been described previously in humans. The altered amino acids appear in the A-B loop, C helix, and C-D loop, respectively. We describe clinically patients with the mutations T494G (Val165Gly), T134C (Leu045Pro), and C667A (Leu223Ile). We discuss also the previously reported mutations Asp202Asn and Asp202His. We have calculated the changes in hydrophobicity of short sequences surrounding some of these amino acids and compared the probable results of the changes in transmembrane structure of the proteolipid protein for the various mutations with the clinical data available on the patients. While the Val165Glu mutation, which is expected to produce disruption of a transmembrane loop of the protein, produces more severe disease than does Val165Gly, no particular correlation with hydrophobicity is found for the other mutations. As these are not in transmembrane domains, other factors such as intracellular transport or interaction between protein chains during myelin formation are probably at work.

Overproduction and immuno-affinity purification of myelin proteolipid protein (PLP), an inositol hexakisphosphate-binding protein, in a baculovirus expression system

Myelin proteolipid protein (PLP) is a major integral membrane protein of central nervous system myelin and is considered to play a significant role in myelination. PLP has a four-transmembrane structure, judging from the hydropathy profile. In addition, it has InsP6 binding activity. Here, we have succeeded in producing PLP in large quantities of 3.9 pg/cell (6 mg/L) by using a baculovirus expression system and developing an efficient purification method, maintaining InsP6 binding activity. The recombinant PLP (rPLP) was purified by ion-exchange and immunoaffinity chromatography in a nonorganic solvent. The final yield of purified rPLP was 36%. The Kd and Bmax values for the InsP6-PLP binding were 55 nM and 33 pmol/microgram protein, respectively. The Kd value of purified rPLP is equal to that of mouse brain PLP. These results indicate that purified rPLP keeps its native conformation and binds InsP6 in an almost one-to-one ratio.

Myelin structure in proteolipid protein (PLP)-null mouse spinal cord

Fixed preparations of proteolipid protein (PLP)-null mouse spinal cord show myelin sheaths which in some regions consist of typical alternating major dense lines (MDLs) and intermediate lines (ILs) with a repeat period of 10.3 nm. More commonly, the lamellar structure consists of what appears to be a single population of dense lines, having a repeat period of 5.2 nm. These apparently equivalent lines are, however, sometimes distinguishable as MDLs or ILs based on continuity with cytoplasmic or extracellular regions. Focal separations of lamellae at the intermediate line are common. MDLs too may be replaced focally by cytoplasmic pockets, sometimes in the same quadrant over several lamellae, resembling Schmidt-Lanterman clefts. Occasional densities reminiscent of the "radial component" can be seen. Otherwise, this structure, which is prominent in wild-type myelin, is conspicuously absent. Redundant folding of some lamellae but not others may occur in the same sheath. These observations conform to those made previously on the isolated myelin segments that occur in the myelin-deficient rat central nervous system (CNS), which also lacks PLP. Thus, a compact lamellar structure can be seen in fixed PLP-null myelin, but defects in the apposition of both the extracellular and the cytoplasmic surfaces of the myelin membranes are common. The abnormalities seen suggest a lack of firm intermembrane bonding, resulting in structural instability. PLP-null myelin may therefore be more susceptible than normal myelin to disruption by mechanical or osmotic stresses. Although PLP is not essential for the formation of either major dense lines or intermediate lines, it may play a role in stabilizing the compact structure.