Cell free synthesis of rat brain myelin proteolipids and their identification by immunoprecipitation

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.

Human myelin DM-20 proteolipid protein deletion defined by cDNA sequence

Using a myelin proteolipid protein (PLP) cDNA as probe, a 1kb cDNA was isolated from a human retinal cDNA library. This clone, designated pDM-20, contained all of the coding sequence of PLP except for 105 base pairs which encode amino acids 117-151 of PLP. This deletion would result in a protein of the size of DM-20, the second major proteolipid of Central Nervous System (CNS) myelin. Homology between the DNA sequence at the 5' end of the deletion and the donor splicing consensus sequence suggests that PLP and DM-20 transcripts are derived from a single PLP gene by alternate splicing.

Structure and expression of the mouse myelin proteolipid protein gene

The gene for the mouse myelin proteolipid protein has been isolated and the seven exons have been sequenced. Since the sequence of a rat proteolipid protein cDNA and partial sequence of the human proteolipid protein gene have been determined, it was possible to demonstrate a very high degree of conservation for the proteolipid protein gene exons among species. While there are some nucleotide changes, the protein coding region of the mouse gene encodes protein that is totally conserved relative to both rat and human proteolipid proteins. The regulatory and noncoding regions of the proteolipid protein gene are also highly conserved. The upstream regulatory and 5'-noncoding region of the gene is 92% homologous to the comparable region of the human proteolipid protein gene, and the 3'-noncoding region of the mouse gene is approximately 90% homologous to a rat proteolipid protein cDNA through 2,200 nucleotides of 3'-noncoding DNA. S1 nuclease protection experiments indicated that the major 5'-end for proteolipid protein mRNAs from mouse, rat, human, or baboon is approximately 147-160 nucleotides upstream from the initial methionine codon of the protein coding region. Other S1 nuclease protection experiments indicated the possible existence of an alternative splice site within exon 3, which may produce mRNA for DM20. This mRNA is approximately 100 nucleotides shorter than that for the proteolipid protein, and it is missing the latter half of exon 3, that is, amino acids 116-150 of the proteolipid protein sequence.

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.

Expression of myelin basic protein genes in several dysmyelinating mouse mutants during early postnatal brain development

Northern blot and "dot" blot analyses using a myelin basic protein (MBP) specific cDNA probe and in vitro translation techniques were utilized to estimate the relative levels of myelin basic protein messenger RNA (mRNA) in the brains of C57BL/6J control mice, three dysmyelinating mutants (qk/qk, jp/Y, and shi/shi), and three heterozygote controls (qk/+, jp/+, and shi+) during early postnatal development. In general, the MBP mRNA levels measured directly by Northern blot and "dot" blot analyses correlated well with the indirect in vitro translation measurements. The Northern blots indicated that the size of MBP mRNAs in quaking and jimpy brain polysomes appeared to be similar to controls. Very low levels of MBP mRNAs were observed in shi/shi brain polyribosomes throughout early postnatal development. Compared to C57BL/6J controls, accumulation of MBP mRNAs in qk/qk and qk/+ brain polyribosomes was delayed by several days. That is, whereas MBP mRNA levels were below normal between 12 and 18 days, normal levels of message had accumulated in both qk/qk and qk/+ brain polyribosomes by 21 days. Furthermore, normal levels of MBP mRNAs were observed to be maintained until at least 27 days. MBP mRNA levels remained well below control levels in jp/Y brain polyribosomes throughout early postnatal development. The levels did, however, fluctuate slightly and peaked at 15 days in both jp/Y and jp/+ brains, 3 days earlier than in normal mice. Thus, it appears that jimpy and quaking mice exhibit developmental patterns of MBP expression different from each other and from C57BL/6J control mice.

In vitro translation of mRNA from the developing sciatic nerve

Poly(A+)RNA from sciatic nerves of embryos and chicks was translated in a rabbit reticulocyte cell-free system. The 30-K mol. wt. PO protein which is the major and specific protein constituent of peripheral nervous system myelin was used as a marker of myelin synthesis in the preparations. Immunoprecipitation of total translation products with rabbit anti-PO protein antibody showed the presence of a 29K protein band on the autoradiograms. The intensity of this specific immunoprecipitated band increased significantly from very low levels with RNA from the 15-day embryonic nerve to a maximum with extracts from 1-day-old chicks. This increase parallels that of the rapid deposition of the PO protein in the developing sciatic nerve in vivo. The experiments demonstrate for the first time, that it is possible to show synthesis of PO protein in vitro using exogenous RNA as the template.

Site-specific antibodies to rat myelin proteolipids directed against the C-terminal hexapeptide

A site-specific antiserum against the rat myelin proteolipids was produced in rabbits by injection of a synthetic polypeptide composed of the C-terminal amino acids of the proteolipid sequence. The immunogenic hexapeptide H-Gly-Arg-Gly-Thr-Lys-Phe-OH was coupled to chicken egg-albumin with dimethylsuberimidate. Antibodies specific for this peptide reacted with the 2 myelin proteolipid protein bands after SDS polyacrylamide gel electrophoresis and electrophoretic transfer onto nitrocellulose. Immunocytochemical investigations with this anti-peptide antiserum showed that the Golgi complexes of the oligodendrocytes were highly labeled as noted previously with multivalent antibodies. Labeling of vesicles and discontinuous staining of the plasmalemma were also observed in the most actively myelinating oligodendrocytes. In contrast to previous results, the major dense line was free of staining; this may indicate that at this site the C-terminal hexapeptide is inaccessible to these antibodies and perhaps buried in the lipid bilayer, in disagreement with the proposed organization of the myelin proteolipid in the myelin membrane.

In vitro synthesis of myelin proteolipid protein from rat brain using a total homogenate system

In vitro synthesis of myelin proteolipid protein (PLP) was explored at different ages using rat brain total homogenates, incubated for 30 min with [3H]glycine. Total proteolipids, extracted from the incubated samples, were separated by SDS-PAGE and the radioactivity was measured in the band corresponding to myelin PLP. The incorporation into PLP in relation to the incorporation into brain total proteins increased from 0.04% at 10 days of age to 0.63% at 20 days, and declined slowly thereafter. Time course experiments were carried out using brain homogenates obtained from rats of 20 days of age (i.e. at the period of maximal synthesis of PLP). Labeled PLP molecules were already found at 2.5 min of incubation and the incorporation of the label into this protein, relative to the incorporation into total proteins, did not vary throughout the entire incubation time (30 min). Pulse-chase experiments using a similar system and adding cycloheximide at different incubation times showed that the appearance of label into mature PLP was immediately blocked by the inhibitor of protein synthesis. These data suggest that PLP is synthesized as such and not as a pre-protein which is subsequently processed to render mature PLP.

Immunocytochemical demonstration of the transport of myelin proteolipids through the Golgi apparatus

Purified antibodies directed against myelin proteolipids were isolated by affinity chromatography of whole serum obtained from rabbits inoculated with myelin. These antibodies were specific for light, medium and dark oligodendrocytes. Astrocytes, neurons and their processes were not reactive. Immunocytochemical investigations showed that the membranes of the Golgi complex are highly labeled by these antibodies. Diffuse cytoplasmic labeling was only observed on the light and medium oligodendrocytes and was absent from the dark types. Vesicles possessing a punctate staining were detected in the vicinity of the Golgi complex and the oligodendroglial membrane. A discontinuous labeling of the plasmalemma appears to be characteristic of the actively myelinating light and medium oligodendrocytes. In compact myelin sheaths positive immunostaining was only detected at the dense line. The immunocytochemical localization of the myelin proteolipids in the oligodendrocytes is in accordance with previously published biochemical data.

Detection of Wolfgram W1 protein, myelin basic proteins and proteolipids in cultured oligodendrocytes by the electro-immunoblotting method

Oligodendrocytes, mechanically obtained from primary cultures of newborn rat brain, were investigated by a sensitive electro-immunoblotting method for the presence of the characteristic myelin proteins: Wolfgram W1 protein, basic proteins and proteolipids. These three major myelin protein types were detectable from the 20th to the 40th day in culture. The present biochemical findings are in accordance with previous immunohistochemical data and provide additional evidence that oligodendrocytes in culture are capable of synthesizing every myelin and oligodendrocyte marker found in vivo, in the absence of neurones.