Myelin basic protein gene expression in quaking, jimpy, and myelin synthesis-deficient mice

Jimpy (jp), myelin synthesis-deficient (jpmsd), and quaking (qk) are mutations which affect myelination to different degrees in the mouse central nervous system (CNS). Total messenger RNA (mRNA) and myelin basic protein (MBP)-specific mRNA from brains of these three mutants have been analyzed by in vitro translation and immunoprecipitation with antibody to MBP. The results indicate that the three mutations do not affect the level of total MBP-specific mRNA in the CNS but do affect the relative proportions of the various MBP-related translation products encoded in vitro. In each case the proportions of 14K and 12K Mr MBP-related translation products are reduced and the proportions of 21.5K, 18.5K, and 17K Mr MBP-related translation products are increased relative to wild type. This effect is most pronounced in jp, less so in jpmsd, and least pronounced in qk animals. The MBP-related polypeptides that accumulate in vivo have also been analyzed in the three mutants by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) followed by immunoblotting with antibody to MBP. The levels of all the major MBP-related polypeptides that accumulate in vivo are reduced in all three mutations. The reduction is most pronounced in jp, less in jpmsd, and least pronounced in qk animals. These results indicate that the jp, jpmsd, and qk mutations exhibit qualitatively similar phenotypic effects on MBP gene expression but the magnitude of the effect is proportional to the extent of hypomyelination in each mutant.     

Myelin proteolipid protein is not esterified at the site of synthesis

Brain slices prepared from 20-day old rats were incubated with [³H]palmitic acid to study its incorporation into myelin proteins. After separation by SDS-PAGE, most of the label was found to be associated with the major proteolipid protein (PLP) and with the intermediate protein (I). The radioactivity measured in PLP at short incubation times was shown to be due to palmitic acid bound to the protein by ester linkages. Time-course incorporation of [³H]palmitic acid into PLP of fraction SN₄ (a myelin like membrane) and of purified myelin showed that the former was poorly labeled and no relationship of the type ‘precursor-product’ between these fractions could be detected. Incorporation of the fatty acid into PLP was not affected by inhibition of the synthesis or transport of myelin PLP with cycloheximide or colchicine, indicating that the pool of PLP that can be acylated must be larger than the extramyelin pool. Addition of unlabeled palmitic acid to the incubation medium, 30 min after the addition of [³H]palmitate, stopped the appearance of label in myelin PLP almost immediately, indicating that there is no significant extramyelin pool of PLP destined for transport into myelin. The results presented in this paper strongly suggest that esterification of PLP takes place in the myelin membrane or at a site very close to it.     

Characterization of myelin proteolipid mRNAs in normal and jimpy mice.

A clone specific for the rat myelin proteolipid protein (PLP) was isolated from a cDNA library made in pUC18 from 17-day-old rat brain stem mRNA. This clone corresponded to the carboxyl-terminal third of the PLP-coding region. The clone was used to identify PLP-specific mRNAs in mouse brain and to establish the time course of PLP mRNA expression during mouse brain development. Three PLP-specific mRNAs were seen, approximately 1,500, 2,400, and 3,200 bases in length, of which the largest was the most abundant. During brain development, the maximal period of PLP mRNA expression was from 14 to 25 days of age, and this was a similar time course to that for myelin basic protein mRNA expression. When the jimpy mouse, an X-linked dysmyelination mutant, was studied for PLP mRNA expression, low levels of PLP mRNA were seen which were approximately 5% of wild-type levels at 20 days of age. When jimpy brain RNA was analyzed by Northern blotting, the PLP-specific mRNA was shown to be 100 to 200 bases shorter than the wild-type PLP-specific mRNA. This size difference was seen in the two major PLP mRNAs, and it did not result from a loss of polyadenylation of these mRNAs.     

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.     

Cysteine-108 is an acylation site in myelin proteolipid protein.

Myelin proteolipid protein (PLP) contains covalently bound long-chain fatty acids. A large proportion of these acyl moieties are bound in thioester linkages, as demonstrated by alkylation of newly formed SH groups upon deacylation. To identify the Cys residue(s) involved in the thioester linkage(s), reduced and carboxyamidomethylated proteolipid protein was labeled with [14C]iodoacetamide upon deacylation with neutral hydroxylamine. The labeled protein was digested with trypsin or pepsin, and peptides analyzed by RP-HPLC. Identification of the isolated radioactive peptides by amino acid analysis, peptide sequencing and/or fast-atom bombardment-mass spectrometry revealed that Cys108 in the bovine PLP sequence is an acylated site. The sequence surrounding the palmitoylation site in the myelin PLP is strikingly similar to that found in rhodopsin. Furthermore, as in rhodopsin and other members of the G protein-coupled receptor family, this Cys residue is located within a hydrophilic, basic, and possibly cytoplasmic, domain.     

Cloned proteolipid protein and myelin basic protein cDNA. Transcription of the two genes during myelination

cDNA clones of rat brain proteolipid protein (PLP), also named lipophilin, the major integral myelin membrane protein, and of myelin basic protein (MBP), the major extrinsic myelin protein, have been isolated from a rat brain cDNA library cloned into the PstI site of pBR322. Poly(A)+ RNA from actively myelinating 18-day-old rats has been reversely transcribed. Oligonucleotides synthesized according to the established amino-acid sequence of lipophilin and the nucleotide sequence of the small myelin basic protein of the N-terminal, the central and C-terminal region of their sequences were used as hybridization probes for screening. The largest insert in one of several lipophilin clones was 2,585 base pairs (bp) in length (pLp 1). It contained 521 bp of the C-terminal coding sequence and the complete 2,064 bp long non-coding 3' sequence. The myelin basic protein cDNA insert of clones pMBP5 and pMBP6 is 2,530 bp long and that of clones pMBP2 and pMBP3 640 bp. These clones were also characterized. pMBP2 was sequenced and used together with the lipophilin cDNA clones as hybridization probes to estimate the lipophilin and myelin basic protein mRNA levels of rat brain during the myelination period. The expression of the lipophilin and myelin basic protein genes during development of the myelin sheath appears to be strictly coordinated.     

The fifth exon of the myelin proteolipid protein-coding gene is not utilized in the brain of jimpy mutant mice

The jimpy mouse, an X-linked recessive dysmyelinating and demyelinating mutant, has been shown to contain abnormal myelin proteolipid protein (PLP) mRNA. To understand the molecular basis of the mutation, we analyzed the structure of PLP mRNA by an RNase-mapping procedure, using the probes specific to each exon of the mouse PLP gene. We found that the fifth exon of the PLP gene is not utilized in the jimpy.     

Diphtheria toxin inhibits the synthesis of myelin proteolipid and basic proteins by peripheral nerve in vitro

Abstract— Diphtheria toxin (DT) did not produce measurable degradation of myelin proteins or sulphatide in sciatic nerves of chick embryos after incubation in vitro for 4 h. In contrast, DT inhibited the in vitro incorporation of L-[U-¹⁴C]leucine into myelin proteins by the nerves after a delay of 1 h. Separation of the myelin proteins by SDS-polyacrylamide gel electrophoresis indicated that the synthesis of Wolfgram proteins and proteins not entering the gel was inhibited by 21–22 per cent, whereas synthesis of myelin proteolipid and basic proteins was inhibited by 79–88 per cent. Incorporation of ³⁵SO₄ into myelin [³⁵S]sulphatide was also inhibited by DT after a delay of 2 h. The inhibition of [³⁵S]sulpha-tide incorporation into myelin caused by DT differed from that observed with puromycin in that it did not depend on depletion of an intracellular transport lipoprotein. Instead, the inhibition seemed to be secondary to the decreased synthesis of myelin proteolipid and basic proteins.     

Lipid-protein and protein-protein interactions in double recombinants of myelin proteolipid apoprotein and myelin basic protein with dimyristoylphosphatidylglycerol.

The integral proteolipid apoprotein (PLP) from bovine spinal cord has been reconstituted in dimyristoylphosphatidylglycerol (DMPG) bilayers, and the mutual interactions on binding the peripheral myelin basic protein (MBP) have been studied. Quantitation of protein and lipid contents in the MBP-PLP-DMPG double recombinants at different PLP:DMPG ratios led to the conclusion that MBP binds only to the DMPG lipid headgroups and is hindered from interaction with the first shell of lipids surrounding the PLP. No specific PLP-MBP association could be detected. Electron spin resonance (ESR) spectra of phosphatidylglycerol spin-labeled at position n = 5 in the sn-2 chain showed that complexation of MBP with the PLP-DMPG recombinants leads to a decrease in lipid chain mobility to an extent which correlates with the degree of MBP binding. At low DMPG:PLP ratios, the perturbations of lipid mobility by both proteins are mutually enhanced. In single recombinants of PLP with DMPG, the ESR spectra of phosphatidylglycerol spin-labeled at position n = 14 in the sn-2 chain indicated that approximately 10 lipids/protein are motionally restricted by direct contact with the intramembranous surface of the protein. This number is in agreement with earlier results for reconstitutions of PLP in dimyristoylphosphatidylcholine (DMPC) [Brophy, P. J., Horváth, L. I., & Marsh, D. (1984) Biochemistry 23, 860-865] and is consistent with a hexameric arrangement of the PLP molecules in DMPG bilayers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Spontaneous vesicularization of myelin lipids is counteracted by myelin basic protein

Hand-vortexed dispersions of several lipids (cerebrosides, sulfatides, PC, PE, PS and sphingomyelin), mixed in the ratios found for these categories of lipids in myelin, exhibit 31P-NMR spectra which have contributions from both isotropic and lamellar resonances. Investigation of this system by freeze-fracture electron microscopy and X-ray diffraction revealed that this lipid mixture has spontaneously formed small unilamellar vesicles (SUVs) (diam. approximately 400 A) and large highly convoluted unilamellar vesicles (LUVs) (diam. approximately 1000 A), the latter possibly resulting from aggregation and fusion of the SUV structures. This vesicularization of the myelin lipids was reversed by the addition of myelin basic protein: only large multilamellar aggregates were formed in the presence of protein, as shown by all three experimental methods. Although no rigorous physical-chemical explanation for these phenomena is yet available, the possibility is suggested that the high concentration of cerebrosides and/or phosphatidylethanolamine in this particular mixture of myelin lipids play pivotal roles in the formation of these unusual vesicles. Spontaneous vesicularization of myelin lipids is discussed as a potential pathway toward destabilization of the myelin sheath.     

Mechanisms of myelin basic protein and proteolipid protein targeting in oligodendrocytes (Review)

Summary

The segregation of proteins to specific cellular membranes is recognized as a common phenomenon. In oligodendrocytes of the central nervous system, localization of certain proteins to select regions of the plasma membrane gives rise to the myelin membrane. Whilst the fundamental structure and composition of myelin is well understood, less is known of the mechanisms by which the constituent proteins are specifically recruited to those regions of plasma membrane that are forming myelin. The two principal proteins of myelin, the myelin basic protein and proteolipid protein, differ greatly in character and sites of synthesis. The message for myelin basic protein is selectively translocated to the ends of the cell processes, where it is translated on free ribosomes and is incorporated directly into the membrane. Proteolipid protein synthesized at the rough endoplasmic reticulum, processed through the Golgi apparatus, and presumably transported via vesicles to the myelin membrane. This review examines the mechanisms by which these two proteins are targeted to the myelin membrane.     

Reactions of fluorescent probes with normal and chemically modified myelin basic protein and proteolipid. Comparisons with myelin.

Basic (encephalitogenic) protein and water-soluble proteolipid apoprotein isolated from bovine brain myelin bind 8-anilino-1-naphthalenesulfonate and 2-p-toluidinylnaphthalene-6-sulfonate with resulting enhancement of dye fluorescence and a blue-shift of the emission spectrum. The dyes had a higher affinity and quantum yield, when bound to the proteolipid (Kans=2.3x10--6,=0.67) than to the basic protein (Kans=3.3x10--5,=0.40). From the efficiency of radiationless energy transfer from trytophan to bound ANS the intramolecular distances were calculated to be 17 and 27 A for the proteolipid and basic protein, respectively. Unlike myelin, incubation with proteolytic enzymes (e.g., Pronase and trypsin) abolished fluorescence enhancement of ANS or TNS by the extracted proteins. In contrast to myelin, the fluorescence of solutions of fluorescent probes plus proteolipid was reduced by Ca-2+,not affected by La-3+, local anesthetics, or polymyxin B, and only slightly increased by low pH or blockade of free carboxyl groups. The reactions of the basic protein were similar under these conditions except for a two- to threefold increase in dye binding in the presence of La-3+, or after blockade of carboxyl groups. N-Bromosuccinimide oxidation of tryptophan groups nearly abolished native protein fluorescence, but did not affect dye binding. However, alkylation of tryptophan groups of both proteins by 2-hydroxy (or methoxy)-5-nitrobenzyl bromide reduced the of bound ANS (excited at 380 nm) to 0.15 normal. The same effect was observed with human serum albumin. The fluorescence emission of ANS bound to myelin was not affected by alkylation of membrane tryptophan groups with the Koshland reagents, except for abolition of energy transfer from tryptophan to bound dye molecules. This suggests that dye binding to protein is negligible in the intact membrane. Proteolipid incorporated into lipid vesicles containing phosphatidylserine did not bind ANS or TNS unless Ca-2+, La-3+, polymyxin B, or local anesthetics were added to reduce the net negative surface potential of the lipid membranes. However, binding to protein in the lipid-protein vesicles remained less than for soluble protein. Basic protein or bovine serum albumin dye binding sites remained accessible after equilibration of these proteins with the same lipid vesicles. It is proposed that in the intact myelin membrane the proteolipid is probably strongly associated with specific anionic membrane lipids (i.e., phosphatidylserine), and most likely deeply embedded within the lipid hydrocarbon matrix of the myelin membrane. Also, in the intact myelin membrane the fluorescent probes are associated primarily, if not solely with the membrane lipids as indicated by the binding data. This is particularly the case for TNS where the total number of myelin binding sites is three to four times the potential protein binding sites.     

Overexpression of the 3'-untranslated region of myelin proteolipid protein mRNA leads to reduced expression of endogenous proteolipid mRNA

The current studies focus on what mechanisms regulate the concentration of PLP mRNA in cells. The PLP mRNA is very stable and these studies suggest that its stability is regulated by a trans-acting factor specific to oligodendrocytes. In order to test whether the 3untranslated region (3UTR) of the PLP mRNA might regulate PLP RNA stability, C6 cells were transfected with cDNAs that expressed either luciferase or luciferase fused to the 3UTR of PLP. Although transgene expression was low, in cells transfected with the PLP 3UTR, there was a significant decrease in the endogenous PLP mRNA. These cells showed a distinct change in morphology and in adhesion properties. Thus, there may be a role for plp gene products in cell adhesion, which was downregulated in these cells, or an unknown function may be encoded by the PLP 3UTR. Transgenic mice that overexpress enhanced green fluorescent protein fused to the PLP 3UTR under control of PLP regulatory sequences were tested for the expression of the endogenous PLP mRNA. Three of four lines of transgenic mice had decreased endogenous PLP mRNA, relative to their non-transgenic littermates; the EGFP-PLP 3UTR mouse line that expressed the highest level of transgene mRNA had a 54% reduction in PLP mRNA. We hypothesize that the PLP mRNA is regulated by elements in the 3UTR and stabilizing proteins specific to oligodendrocytes, and that in cells that overexpress the PLP 3UTR, these stabilizing proteins may be insufficient to maintain the normal level of the endogenous PLP mRNA.     

An AG----GG transition at a splice site in the myelin proteolipid protein gene in jimpy mice results in the removal of an exon

The myelin proteolipid protein gene was characterized in jimpy mice to identify the specific mutation that produces dysmyelination, oligodendrocyte cell death, and death of the animal by 30 days of age. Exon 5 and flanking intron segments were isolated from jimpy proteolipid protein genomic clones and sequenced. A single nucleotide difference was noted between the normal and jimpy proteolipid protein genes: the conversion of an AG/GT to a GG/GT in the splice acceptor signal preceding exon 5, which apparently destroys the splice signal. Thus, exon 5 of the mouse myelin proteolipid protein gene is skipped during the processing of mRNA, producing a shortened proteolipid protein mRNA.     

Expression of recombinant forms of human 21.5 kDa myelin basic protein and proteolipid protein in CHO cells

MBP and PLP are major structural protein components of myelin. Both proteins play a functional role in formation of myelin sheath and in maintenance of its compaction. Immune responses to MBP and PLP have been implicated in the pathogenesis of multiple sclerosis (MS), an auto-immune disease of the central nervous system. Recombinant forms of both proteins isolated and purified from bacterial or insect cell systems are commonly used to study the specificity of auto-response in MS. We have prepared recombinant forms of MBP and PLP stably expressed in CHO cells. Several clones with proper cytoplasmic MBP or surface PLP localization were obtained and characterized by flow cytometry and indirect immunostaining. CHO cells expressing the recombinant forms of MBP and PLP can be very useful in studies on the autoimmune mechanism of MS.     

Myelin basic protein-dependent plasma membrane reorganization in the formation of myelin

During vertebrate development, oligodendrocytes wrap their plasma membrane around axons to produce myelin, a specialized membrane highly enriched in galactosylceramide (GalC) and cholesterol. Here, we studied the formation of myelin membrane sheets in a neuron-glia co-culture system. We applied different microscopy techniques to visualize lipid packing and dynamics in the oligodendroglial plasma membrane. We used the fluorescent dye Laurdan to examine the lipid order with two-photon microscopy and observed that neurons induce a dramatic lipid condensation of the oligodendroglial membrane. On a nanoscale resolution, using stimulated emission depletion and fluorescence resonance energy transfer microscopy, we demonstrated a neuronal-dependent clustering of GalC in oligodendrocytes. Most importantly these changes in lipid organization of the oligodendroglial plasma membrane were not observed in shiverer mice that do not express the myelin basic protein. Our data demonstrate that neurons induce the condensation of the myelin-forming bilayer in oligodendrocytes and that MBP is involved in this process of plasma membrane rearrangement. We propose that this mechanism is essential for myelin to perform its insulating function during nerve conduction.     

Developmental regulation of myelin basic protein expression in mouse brain

Developmental regulation of myelin basic protein expression in mouse brain has been examined by comparing the myelin basic protein coding potential of mRNA in vitro with the accumulation of myelin basic protein-related polypeptides in vivo. In vitro translation of mRNA isolated from mouse brain generated eight myelin basic protein-related polypeptides with apparent molecular weights of 34K, 30K, 29K, 26K, 21.5K, 18.5K, 17K, and 14K. A similar set of eight myelin basic protein-related polypeptides with corresponding molecular weights was identified in vivo when total brain proteins were analyzed by immunoblotting. Each of the myelin basic protein-related polypeptides shows a characteristic developmental profile in terms of mRNA level and rate of accumulation implying a complex developmental program of myelin basic protein gene expression with regulation and modulation at several different biosynthetic levels.