Proteolipid protein: synthesis and assembly into quaking mouse myelin

In vitro insertion of the myelin proteolipid apoprotein into oligodendrocyte plasma membranes

Uncoated vesicles (UCV) loaded with the myelin proteolipid apoprotein covalently tagged with fluorescein (PLPF) were found to interact with isolated oligodendrocytes from bovine brain at 4 degrees C as well as at 37 degrees C. After 1.5 hours of incubation, the labeled protein was localized in the cell membranes. After 2.5 hours the fluorescence intensity associated with the oligodendrocytes decreased and completely disappeared at t = 3.5 hours. Addition of KCl or EDTA in the incubation medium significantly hindered the interaction with cells. In contrast, the elimination of membrane proteins from UCV did not perturb cell labeling. A specific role of PLP was suggested since UCV loaded with a soluble protein (BSAF) led to a weak cell labeling.

Comparative study of myelin proteolipid apoprotein solvation by multilayer membranes of synthetic DPPC and biological lipid extract from bovine brain. An FT-IR investigation

The interaction between the aqueous form of the myelin proteolipid apoprotein (PLA) and model membranes prepared with either synthetic dipalmitoylphosphatidyl choline (DPPC) or biological lipids extracted from bovine brain (BE) has been investigated by Fourier-Transform IR spectroscopy. IR spectra obtained with lyophilized samples of PLA demonstrated 2 main peaks (amide I and amide II) culminating at 1656 cm-1 and 1545 cm-1, which we assigned to helical conformation. When PLA was solvated in DPPC or BE membranes, both the amide I and amide II features remained located at 1655 cm-1 and 1545 cm-1, although their half-width significantly decreased, demonstrating that the lipid environment favoured alpha helix structures. However differences between both mixtures were detected by measuring the amide I and amide II half-widths as a function of the L:P molar ratio. Moreover, analysis of the 1545/1515 peak intensity ratio brought evidence of different localization and/or molecular arrangement of the protein segments containing tyrosine residues, depending on the lipid composition of the membrane. According to previously published models, these data suggest that recombinants prepared with PLA and BE multilayers better mimic the biological membrane than do DPPC-PLA mixtures.

Molecular biology of myelin proteins from the central nervous system

Within the past several years, several of the genes coding for the major myelin proteins have been isolated, characterized, and mapped to specific chromosomes. In all cases, it has been clearly established that these proteins exist as multiple isoforms, and their structures have been established through an analysis of the cDNA clones encoding them. In each case, the isoforms appear to arise through the translation of individual mRNAs produced by alternative splicing of the primary transcript of a single gene. In several cases, the expression of the individual isoforms appears to be developmentally and/or regionally regulated, probably at the level of the splicing of the primary transcript. In the case of the dysmyelinating mutants shiverer and jimpy, the molecular defects involve the MBP gene and PLP gene, respectively; most of the dysmyelinating mutants, including those in which the genetic defect is established, appear to exhibit pleiotropy with respect to the expression of other myelin protein genes.

Cellular and molecular aspects of myelin protein gene expression

The cellular and molecular aspects of myelin protein metabolism have recently been among the most intensively studied in neurobiology. Myelination is a developmentally regulated process involving the coordination of expression of genes encoding both myelin proteins and the enzymes involved in myelin lipid metabolism. In the central nervous system, the oligodendrocyte plasma membrane elaborates prodigious amounts of myelin over a relatively short developmental period. During development, myelin undergoes characteristic biochemical changes, presumably correlated with the morphological changes during its maturation from loosely-whorled bilayers to the thick multilamellar structure typical of the adult membrane. Genes encoding four myelin proteins have been isolated, and each of these specifies families of polypeptide isoforms synthesized from mRNAs derived through alternative splicing of the primary gene transcripts. In most cases, the production of the alternatively spliced transcripts is developmentally regulated, leading to the observed protein compositional changes in myelin. The chromosomal localizations of several of the myelin protein genes have been mapped in mice and humans, and abnormalities in two separate genes appear to be the genetic defects in the murine dysmyelinating mutants, shiverer and jimpy. Insertion of a normal myelin basic protein gene into the shiverer genome appears to correct many of the clinical and cell biological abnormalities associated with the defect. Most of the dysmyelinating mutants, including those in which the genetic defect is established, appear to exhibit pleiotropy with respect to the expression of other myelin genes. Post-translational events also appear to be important in myelin assembly and metabolism. The major myelin proteins are synthesized at different subcellular locations and follow different routes of assembly into the membrane. Prevention of certain post-translational modifications of some myelin proteins can result in the disruption of myelin structure, reminiscent of naturally occurring myelin disorders. Studies on the expression of myelin genes in tissue culture have shown the importance of epigenetic factors (e.g., hormones, growth factors, and cell-cell interactions) in modulating myelin protein gene expression. Thus, myelinogenesis has proven to be very useful system in which to examine cellular and molecular mechanisms regulating the activity of a nervous system-specific process.

Increased turnover of myelin proteolipid protein in quaking mouse brain

Homozygous quaking and normal control littermate mice were injected intracerebrally with [3H]leucine at 19 days of age. The animals were sacrificed after 1 h and after 6 days. The proteolipid protein (PLP) and intermediate protein (IP) were extracted from whole brain by chloroform-methanol (2:1) and resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). One hour postinjection the labeling of total protein in quaking brain was the same as the control and the radioactivity of PLP and IP in quaking was approximately 35% of the control. Six days after precursor administration the radioactivity of the total protein decreased significantly in both groups and to the same extent. However, the labeling of PLP and IP more than doubled in the control, while it decreased by half in the quaking brain. The results indicate that there is an increased turnover rate of PLP and IP in quaking brain.

Myelin-specific domain on the plasmalemma of oligodendroglia: differential expression in the rat and hypomyelinating mouse mutants jimpy and quaking

Monoclonal antibody, 1A9, prepared against bovine white matter, recognizes a proteinaceous, myelin-specific domain in the CNS that is restricted to the surface of oligodendroglia in primary dissociated cell cultures. The antigen is not detected in the PNS or non-neural tissues. Antibody binding is abolished by heating, exposure to SDS and delipidation, indicating that a conformationally sensitive epitope is recognized. The antigen is present in tracts of developing white matter in rat cerebellum beginning at 5 days postnatally. In developing cultures of fetal rat brain the period of rapid onset for the phenotypic expression of 1A9 antigen is similar to that of galactocerebroside, corresponding to 2-4 postnatal days of age. The 1A9 antigen is not observed in white matter or cultured oligodendroglia of the hypomyelinating jimpy mutant mouse, but its expression is qualitatively normal in the quaking mutant. The possibility is raised that 1A9 may be the primary target of the jimpy mutation.

Synthesis and acylation of myelin proteolipid protein in quaking mouse brain

The synthesis and acylation of proteolipid proteins where investigated in tissue slices prepared from 19-day-old quaking and normal littermate mouse brain. The mutant CNS had a normal rate of total protein synthesis but synthesis of the myelin-specific proteins, proteolipid protein (PLP) and intermediate protein (IP), was impaired to approximately 50% of control. The acylation of myelin proteins with labeled palmitate was reduced to only about 20% of control. The acylation of two nonmyelin proteolipid proteins was also significantly reduced. The incorporation of palmitate into phospholipids was identical in control and mutant. The results indicate impaired synthesis of PLP and IP and a general deficit in protein acylation in quaking brain.

Proteolysis in quaking mouse brain and spinal cord

Six proteolytic enzymes were assayed for activity in quaking CNS in examining the hypothesis that increased proteolytic activity contributes to the hypomyelination characteristics of this mutant. Cathepsin B-like enzyme, cathepsin D, neutral proteinase, calcium-activated neutral proteinase, prolyl endopeptidase, and diaminopeptidase II were assayed in whole homogenate of brain or spinal cord and each was found to have activity similar to that in normal mice. These results do not support a relationship between proteolysis and the genetic defect and suggest that other factors should be investigated to delineate the pathogenesis of this mutant.

Expression of myelin proteolipid protein and basic protein in normal and dysmyelinating mutant mice

Expression of myelin proteins was studied in the brains of 21-day-old normal mice and three dysmyelinating mutants-jimpy, quaking, and shiverer. Total brain polyribosomes and poly(A)+ mRNA were translated in two cell-free systems and the levels of synthesis of the myelin basic proteins (MBPs) and proteolipid protein (PLP) were determined. Synthesis of the MBPs in quaking homozygotes was at or above normal levels but PLP synthesis was significantly reduced to approximately 15% of control values, indicating independent effects on the expression of these proteins in this mutant. Immunoblot analysis of 21-day-old quaking brain homogenates showed a reduction in the steady-state levels of MBPs and PLP, suggesting a failure of newly synthesized MBPs to be incorporated into a stable membrane structure such as myelin. In the shiverer mutant very little synthesis of MBPs was observed, whereas greater synthesis of PLP occurred (approximately 50% of control). Almost no MBP, and low levels of PLP, were detected in the immunoblots, suggesting the possibility of a partial failure of PLP to be assembled into myelin in shiverer. In the jimpy mutant, low levels of MBP synthesis were observed in vitro (approximately 26% of controls) and very little synthesis of PLP was evident. The immunoblots of 21-day jimpy brain homogenates revealed no appreciable steady-state levels of PLP or MBP, again indicating that most newly synthesized MBPs were not incorporated into a stable membrane structure in this mutant. In sum, the data show that in the three cases examined, the mutation appears to affect the expression of the MBPs and PLP independently. Furthermore, regardless of their absolute levels of synthesis these proteins may or may not be assembled into myelin.

Development of membrane-bound carbonic anhydrase and glial fibrillary acidic protein in normal and quaking mice

The developmental patterns of membrane-bound carbonic anhydrase (MBCA) and glial fibrillary acid protein (GFA) were analyzed in high speed pellets from subcortical regions of quaking and control mice at ages ranging from 10 to 42 days. In control, MBCA was found to increase from approximately 1.5 unit/mg protein at 10 days to about 11 units/mg protein at 42 days. The percent increase was greatest between days 11 and 18 where the activity increased 2.5-fold while the largest absolute weekly increment was in the fourth postnatal week. By contrast, the activity in quaking mice was 1.0 units/mg protein at day 10 and increased to only 5 units/mg protein by 42 days. The activity, although reduced at 10 days, appeared to increase in parallel with the controls between day 11 and 18; however, the large increment in activity observed in the controls during the fourth week was greatly reduced in the mutant. The above findings were also observed when immune reactive material was determined by immuno-blotting using antisera raised against MBCA isolated from brain. GFA was also estimated using an immuno-blotting procedure. In contrast to previous studies on soluble GFA, we found GFA to increase about 5-fold in the third postnatal week and level off thereafter. The Q/C ratio was 1.5 at 10 days, but, although GFA increased over 5-fold between 11 and 42 days, the absolute difference between quaking and control increased very little. These data on GFA and MBCA suggest that a defect in the expression of oligodendroglial plasma membrane proteins may be expressed early in development which may correlate with a small but significant gliosis and/or an increase in GFA synthesis.

An electrophoretic analysis of proteolipids from different rat brain subcellular fractions

Proteolipid proteins were extracted from adult rat brain subcellular fractions and purified by chromatography on Sephadex LH-60. Polyacrylamide gel electrophoresis of the delipidized proteins, in the presence or absence of 8 M urea, was carried out with all fractions. The distribution of the various types of proteolipid proteins was studied and their molecular weight calculated by the Ferguson relationship. Several bands of proteolipid proteins were found in the five membrane fractions analyzed. Some of them, such as the 17.5 K and 37 K components were very prominent in mitochondria and synaptosomes. The 30 K component was found in myelin-derived membranes and in microsomes, while the 20 K and 25 K proteolipid proteins were present in all subcellular fractions. The 30 K component (proteolipid protein (PLP)), typical of the purified myelin membranes, showed a similar distribution to that of 2',3'-cyclic-nucleotide 3'-phosphohydrolase (EC 3.1.4.37) activity, while the other major proteolipid protein present in all subcellular fractions (25 K) did not show such parallelism, indicating that it might not be an exclusive component of myelin. The electrophoretic pattern of microsomal proteolipid proteins did not show the high molecular weight components (aggregates of PLP) which are found in myelin. Furthermore, the 30 K component showed a smaller Y0 value than that of the 30 K found in myelin. Thus the presence of 30 K proteolipid protein in microsomes should not be considered as being due to myelin contamination.

Developmental dissociation of myelin synthesis and "myelin-associated" enzyme activities in the shiverer mouse

Developmental changes in three enzymes associated with myelin lipids were studied in the shiverer mouse, a murine mutant showing a severe deficiency of CNS myelin. Age-related changes in cerebroside sulfotransferase (measured in brain) and arylsulfatase A and cerebroside B-galactosidase (measured in brain and liver) were the same for shiverer and control mice. The shiverer mouse, therefore, demonstrates a dissociation between the genetic mechanisms regulating myelination in the CNS and developmental changes in enzyme activities thought to be closely related to the synthesis of myelin. In addition, we found no defect in the shiverer mouse in the incorporation of glycine-labeled basic protein into CNS myelin, indicating an important metabolic difference between the morphologically similar shiverer and quaking mutants.

Synthesis of the myelin proteolipid protein in the developing mouse brain

Mice ranging in age from 16 to 44 days were injected intracerebrally with 3H-leucine, and incorporation into total brain proteolipids and the myelin proteolipid protein was measured. All proteolipids were isolated from whole brain by ether precipitation and separated into their individual components by SDS polyacrylamide gel electrophoresis. Two major proteolipids with apparent molecular weights of 20,700 and 25,400 were observed in these preparations, and their proportion increased over the developmental period examined. A Ferguson plot analysis comparing these proteins with those of isolated myelin showed that the 25,400-dalton proteolipid component from whole brain was the myelin proteolipid protein. Rates of incorporation of 3H-leucine into total brain proteolipids peaked at 22 days of age. Synthesis of the myelin proteolipid protein increased rapidly to a maximum value at 22 days and decreased rather slowly until at 44 days it was about 83% of its maximum rate of synthesis. The data indicate that the developmental pattern of synthesis of the myelin proteolipid protein is unlike that of the myelin basic proteins. Synthesis of the major myelin proteins is developmentally asynchronous in that peak synthesis of the myelin proteolipid appears to occur several days later than the basic proteins. In addition, it maintains its maximum rate of synthesis over a longer period of time than do the basic proteins.