Localization of the basic protein and lipophilin in the myelin membrane with a non-penetrating reagent

Localization of basic proteins in human myelin

The myelin basic protein (MBPs) represent a family of proteins (charge isomers) which account for 35% of the total myelin protein. Localization studies have been inconclusive because MBP is not a single protein. Antibodies obtained by injection of MBP into animals recognized all members of the MBP family. In the studies reported here, we have fractionated the MBPs into specific components or charge isomers. One of these which contains citrulline accounts for about 20% of the total MBP. We report the localization of this single MBP to the intraperiod line of myelin by immunoelectron microscopy. For these studies several specific antibodies were used including antibodies raised against total MBP, specific MBP peptides, and against a tetracitrulline peptide. This latter antibody was specific for component 8 (C-8) of MBP. Since C-8 is the only MBP which contains citrulline it was used to localize this particular form of MBP principally to the intraperiod line by immunogold electron microscopy, while antibody against total MBP (consisting of all charge isomers C-1-->C-8) labelled both the major dense line and the intraperiod line. When the anti-citrulline antibody was used with a 3 nm gold conjugated Fab fragments prepared from the secondary antibody, 66.5% of the gold particles were localized to the intraperiod line, while 11.2% of gold particles were localized to the major dense line. On the other hand, with the monoclonal anti-MBP antibodies reactive with residues 69-74, 59.4% of the gold particles were localized to the major dense line and 23.6% of gold particles at the intraperiod line. Other supporting evidence includes increased labelling of myelin by 125I labelled anti-citrulline IgG when isolated myelin was swollen, a process known to take place at the intraperiod line. Gold particles were demonstrated at the intraperiod line in swollen and recompacted myelin. C-8 was shown to associate preferentially with lipids asymmetrically localized to the intraperiod line.

Membrane interactions are altered in myelin isolated from central and peripheral nervous system tissues

Isolated myelin has been used for determinations of membrane surface charge density and topographical mapping of components in the membrane. To determine how similar such myelin is to myelin of intact tissue, we have used x-ray diffraction to compare their intermembrane interactions. The interactions were monitored by measuring the myelin period in samples treated with distilled water, buffered saline at pH 4-9 and ionic strength 0.06-0.18, and saline containing HgCl2 or triethyl tin sulfate. Myelin was isolated from whole brains and sciatic nerves of mice by conventional methods involving sucrose gradient centrifugation and osmotic shock. Consistent with previous findings, electron microscopy showed that the multilamellar morphology, staining, and repeat periods of isolated myelin were essentially like those of intact myelin; however, the membrane stacks were less extensive than those in whole tissue. X-ray diffraction revealed that isolated CNS myelin was like intact myelin in showing reversible compaction in acidic media and in distilled water. However, unlike the myelin in whole tissue, isolated CNS myelin did not swell in hypotonic or alkaline media, or in the presence of HgCl2-saline or triethyl tin. The altered membrane interactions could result from an increase in adhesiveness of the apposed membrane surfaces. Reorganization of proteolipid protein and/or a reduction of surface charge could account for the change in surface properties of isolated CNS myelin. Isolated PNS myelin, like the membranes in whole tissue, showed both compaction and swelling; however, the membrane pairs were disordered in the swollen structure. This irregular membrane swelling could result from charge variation in the extracellular surfaces.

The secondary structure of a membrane-embedded peptide from the carboxy terminus of lipophilin as revealed by circular dichroism

Several intramembranous peptides have been isolated from the major myelin proteolipid protein (lipophilin) isolated from normal human myelin membrane after labelling the protein with a membrane-permeable photolabel, 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine. Peptide T-3, comprising residues 205-268, represents the C-terminal portion of the protein. Reconstitution of peptide T-3 into lipid vesicles prepared from egg phosphatidylcholine (PC) or into lysoPC micelles yielded visually transparent preparations, free of scattering artifacts, which were used for circular dichroism studies to assess the extent of secondary structure in the peptide. Peptide T-3 had a high degree of alpha-helix in various environments. In aqueous environment, the secondary structure was 45% alpha-helix, 33% beta-structure and 9% beta-turns. Transfer of the peptide to PC vesicles or lysoPC micelles increased the proportion of alpha-helix and decreased that of beta-structure. In PC vesicles, the alpha-helical content was 80% with little or no beta-structure. Small amounts of other structures such as beta-turns and unordered structures were also present. The partitioning of this C-terminal section of lipophilin into membranes may have an important role initiating and/or stabilizing the native conformation of lipophilin in the myelin membrane.

Susceptibility of the Wolfgram proteins and stability of 2',3'-cyclic nucleotide 3'-phosphodiesterase of rat brain myelin to limited proteolytic digestion

The susceptibility of proteins in the myelin membrane to proteases was studied. Lyophilized rat brain myelin suspended in water was subjected to controlled proteolytic digestion with pure trypsin (N-tosyl-L-phenylalanine chloromethyl ketone treated, 5 units/mg of myelin), and proteins remaining in the pellet were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Under these conditions, large basic protein (LBP) was completely hydrolyzed in 5-10 min, proteolipid proteins remained largely intact until 60 min, whereas Wolfgram protein (WP) was progressively degraded from 10 min onward with the simultaneous appearance of a new protein band with a molecular weight of 35K. A similar pattern was obtained on treatment with chymotrypsin or subtilisin. The 35K protein band was shown to be derived from WP by its immunological cross-reactivity with WP antibodies. Western blot analysis showed that 35K protein is the only major breakdown product of WP under these conditions. Treatment with higher concentrations of trypsin (greater than 20 units/mg of myelin) resulted in the degradation of all myelin proteins. Essentially all the 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) activity was observed in the myelin pellet after controlled or drastic digestion with trypsin. It is concluded that the major fragment of WP (35K) is located in the hydrophobic milieu of the bilayer, relatively inaccessible to trypsin, whereas a portion (20K) of the WP is exposed to the cytoplasmic side (major dense line), like LBP, and that peptide fragments (less than 14K) that remained in the myelin membrane lipid bilayer after trypsin digestion could exhibit CNP activity.

Identification of membrane-embedded domains of lipophilin from human myelin

The organization of lipophilin in the intact human myelin membrane has been studied by labeling with the carbene photogenerated from 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine ([125I]TID). This hydrophobic probe labels mostly lipophilin (the main intrinsic protein of myelin) and the lipids within the bilayer. The domains of lipophilin which are embedded within the membrane have been identified by proteolytic fragmentation of the [125I]TID-labeled myelin, extraction with organic solvents, and separation by chromatography. Four labeled peptides were purified in this way. Polyacrylamide gel electrophoresis, amino acid compositions, automated sequencing, and carboxy-terminal analyses identified a 15K molecular weight peptide, T1 (residues 1-143), as representing the amino-terminal fragment, a 10K peptide, T2 (residues 1-97), representing a smaller amino-terminal fragment, a 5K peptide, T4 (residues 53-97), which represented the COOH-terminal half of peptide T2, and a 7K peptide, T3 (residues 205-268), which represented a sequence near the COOH terminus of lipophilin. The specific radioactivities of the peptides were determined; peptides T1 and T2 had similar specific activities, which were twice the specific activities of peptides T3 and T4. The data provide direct chemical evidence that human lipophilin has membrane-embedded domains between residues 1-97, 53-97, and 205-268, in agreement with some of the predictions of other investigators based on the sequence of bovine myelin lipophilin (proteolipid apoprotein) and a hydrophobicity diagram.

Lipid/myelin basic protein multilayers. A model for the cytoplasmic space in central nervous system myelin

A multilayered complex forms when a solution of myelin basic protein is added to single-bilayer vesicles formed by sonicating myelin lipids. Vesicles and multilayers have been studied by electron microscopy, biochemical analysis, and X-ray diffraction. Freeze-fracture electron microscopy shows well-separated vesicles before myelin basic protein is added, but afterward there are aggregated, possibly multilayered, vesicles and extensive planar multilayers. The vesicles aggregate and fuse within seconds after the protein is added, and the multilayers form within minutes. No intra-bilayer particles are seen, with or without the protein. Some myelin basic protein, but no lipid, remains in the supernatant after the protein is added and the complex sedimented for X-ray diffraction. A rather variable proportion of the protein is bound. X-ray diffraction patterns show that the vesicles are stable in the absence of myelin basic protein, even under high g-forces. After the protein is added, however, lipid/myelin basic protein multilayers predominate over single-bilayer vesicles. The protein is in every space between lipid bilayers. Thus the vesicles are torn open by strong interaction with myelin basic protein. The inter-bilayer spaces in the multilayers are comparable to the cytoplasmic spaces in central nervous system myelins . The diffraction indicates the same lipid bilayer thickness in vesicles and multilayers, to within 1 A. By comparing electron-density profiles of vesicles and multilayers, most of the myelin basic protein is located in the inter-bilayer space while up to one-third may be inserted between lipid headgroups. When cytochrome c is added in place of myelin basic protein, multilayers also form. In this case the protein is located entirely outside the unchanged bilayer. Comparison of the various profiles emphasizes the close and extensive apposition of myelin basic protein to the lipid bilayer. Numerous bonds may form between myelin basic protein and lipids. Cholesterol may enhance binding by opening gaps between diacyl-lipid headgroups.

X-ray scattering studies of a model complex of lipid and basic protein of myelin

Low-angle and wide-angle X-ray scattering data from phosphatidylglycerol complexed with myelin basic protein, poly(L-lysine) and calcium ions are analyzed. The results confirm our earlier report (Brady, G.W., Murthy, N.S., Fein, D.B., Wood, D.D. and Moscarello, M.A. (1981) Biophys. J. 34, 345-350) that the basic protein interacts primarily with the polar headgroups of the lipid; and that at high protein concentrations (greater than 35%) the bilayers aggregate to form multilayers with a repeat period of 68 A, the single bilayer to multilayer transition being a cooperative process. Polylysine and Ca2+ produce multilayers with a smaller repeat of approx. 55 A. Basic protein and polylysine do not change the fluid-like arrangement of the hydrocarbon chains (diffuse 4.6 A peak in the wide-angle pattern), whereas Ca2+ probably induces a two-dimensional order (4.3 A and 3.9 A peak in the wide-angle pattern). Electron density profiles of the lipid and lipid-basic protein vesicles indicate that the basic protein penetrates into the bilayer.

Studies with antisera against peripheral nervous system myelin and myelin basic proteins. II. Immunohistochemical studies in cultures of rat dorsal root ganglion neurons and Schwann cells

Antiserum against rat peripheral nervous system (PNS) myelin contained immunoglobulins which bound preferentially to the extracellular surfaces of myelin-related Schwann cells in intact cultures of dorsal root ganglion (DRG) neurons and Schwann cells, while antiserum against basic protein (BP) from central nervous system myelin or the PNS basic protein P2 did not. We demonstrate the presence of PNS myelin proteins P1 (identical to BP) and P2 by immunoperoxidase techniques in DRG cultures that had been treated to disrupt cellular membranes. These observations suggest that P1 and P2 are not exposed on the extracellular surfaces of myelin-related Schwann cells in culture. The results also support the hypothesis concerning the possible mechanisms by which anti-PNS myelin serum demyelinates DRG cultures, while anti-BP serum and anti-P2 serum do not.

The polyadenylated RNA directing the synthesis of the rat myelin basic proteins is present in both free and membrane-bound forebrain polyribosomes

Free and membrane-bound polyribosomes were isolated from the forebrain of actively myelinating 24-day-old rats. The poly(A)+ RNA (polyadenylated RNA) extracted from both fractions was translated in vitro in reticulocyte lysates [Hall & Lim (1981) Biochem. J. 196. 327-336] in the presence or absence of a heterologous microsomal membrane fraction from dog pancreas. The rat myelin basic proteins synthesized in vitro were isolated by CM-cellulose chromatography and by immunoprecipitation with purified anti-(myelin basic protein) antibody. The large (mol.wt. 18 500) and small (mol.wt. 16 000) myelin basic proteins were translational products of poly(A)+ RNA from both free and membrane-bound polyribosomes. The identity of the myelin basic proteins was verified by analysis of peptides generated by the cathepsin D digestion of the immunoprecipitated proteins synthesized in vitro, in comparison with authentic rat myelin basic proteins. Although several other translational products of membrane-bound polyribosomal poly(A)+ RNA were modified when microsomal membranes were present during translation, molecular weights of the myelin basic proteins themselves were unchanged. The myelin basic proteins synthesized in vitro also did not differ significantly in size from the authentic myelin basic proteins, indicating that these membrane proteins are unlikely to be synthesized as substantially larger precursor molecules. The presence of the specific mRNA species on both free and membrane-bound polyribosomes is compatible with the extrinsic location of the myelin basic proteins on the cytoplasmic surface of the myelin membrane.

Tentative identification of a second central nervous system myelin membrane autoantigen (M2) by a biochemical comparison with the basic protein (BP)

Two central nervous system myelin autoantigens, M2 and basic protein (BP), were examined, using complement-fixing antibodies against each autoantigen as markers on myelin. M2 activity was very labile and very insoluble, PB activity was very resistant. Trypsin reduced both activities an this reduction was greater after phospholipase treatment. Both activities were slightly solubilized in 8 M urea. It is known that BP is not present on the surface of myelin and is considered a peripheral membrane protein. M2 appears to be a surface and integral membrane protein, and as such resembles Folch Pi proteolipid protein. The relationship between M2 and BP requires further study.

A marker for oligodendrocytes and its relation to myelinogenesis: an immunocytochemical study with experimental allergic encephalomyelitis serum and C.N.S. cultures

To investigate a possible marker for oligodendrocytes and its relation to myelinogenesis, experimental allergic encephalomyelitis (EAE) serum has been used to study C.N.S. cultures from the time of explantation to maturity at 26 days in vitro (DIV). Cultures of foetal mouse spinal cord were exposed for 1 h to heated (complement-inactivated), rabbit anti-bovine white matter (WM-EAE) or control serum, fixed and processed by an immunoperoxidase technique for demonstrating bound immunoglobulin (Ig) by light and electron microscopy. From 5 to 26 DIV, cells morphologically identical to oligodendrocytes displayed binding of Ig to the plasmalemma of the cell body and its processes. At 5 DIV, immunoreactive oligodendrocytes had a large nucleus and nucleolus, prominent Golgi apparatus, and microtubules but no filaments. Occasionally a centriole was present, suggesting an early stage of differentiation. In myelinated cultures (from 11-12 DIV onwards), reaction product was present on the oligodendroglial outer plasmalemma apposed to myelin and along the outer loop. Sometimes it extended into the external mesaxon, outer layer of myelin, inner mesaxon and periaxonal space. No other structures were reactive, and oligodendroglia did not bind control Ig. These findings indicate that WM-EAE serum can be used as a marker for oligodendrocytes in cultures from 5 DIV onwards. The findings that oligodendrocytes acquire the antigen(s) prior to myelination and that the antigen(s) is localized on the plasmalemma of the inner and outer loops of actively myelinating oligodendroglial processes suggest that the antigen(s) may have a role in oligodendrocyte maturation and myelinogenesis. The antigen(s) involved is not yet established, but it is probably not myelin basic protein. This marker should prove useful in studies of C.N.S. development and the demyelinating diseases.

The amino acid conjugate formed by the interaction of the anion transport inhibitor 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS) with band 3 protein from human red blood cell membranes

The specific anion transport inhibitor 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS) and its reduced analog (H2DIDS), when irreversibly bound to band 3 protein of the red blood cell membrane, form amino acid conjugates through interaction with the epsilon-amino group of a particular lysine residue. The specific residue is located in a transmembrane segment of band 3 protein and appears to be a close neighbor of the transport site.

Transmembrane orientation of lipophilin in phosphatidylcholine vesicles

Lipophilin, a hydrophobic myelin protein, was incorporated into phosphatidylcholine vesicles by dialysis from 2-chloroethanol which has been shown to produce single-layered lipid-protein vesicles. These vesicles were labeled with a nonpenetrating surface-labeling reagent, 4,4'-diisothiocyano-2,2'-ditritiostilbene disulfonic acid, ([3H]DIDS), in order to determine if the protein completely spans the bilayer. After labeling the vesicles, lipophilin was isolated. At least 88% of the protein ws labeled with [3H]DIDS. Dextran (mol wt 250,000-275,000) was converted to the dialdehyde form and reacted with lipophilin-PC vesicles. In this case greater than 90% of the protein was complexed to the dextran. The high degree of labeling obtained with both compounds was consistent with a model in which lipophilin was considered to span the bilayer completely.

Myelin-specific proteins and glycolipids in rat Schwann cells and oligodendrocytes in culture

We have used antibodies to identify Schwann cells and oligodendrocytes and to study the expression of myelin-specific glycolipids and proteins in these cells isolated from perinatal rats. Our findings suggest that only Schwann cells which have been induced to myelinate make detectable amounts of galactocerebroside (GC), sulfatide, myelin basic protein (BP), or the major peripheral myelin glycoprotein (P0). When rat Schwann cells were cultured, they stopped making detectable amounts of these myelin molecules, even when the cells were associated with neurites in short-term explant cultures of dorsal root ganglion. In contrast, oligodendrocytes in dissociated cell cultures of neonatal optic nerve, corpus callosum, or cerebellum continued to make GC, sulfatide and BP for many weeks, even in the absence of neurons. These findings suggest that while rat Schwann cells require a continuing signal from appropriate axons to make detectable amounts of myelin-specific glycolipids and proteins, oligodendrocytes do not. Schwann cells and oligodendrocytes also displayed very different morphologies in vitro which appeared to reflect their known differences in myelinating properties in vivo. Since these characteristic morphologies are maintained when Schwann cells and oligodendrocytes were grown together in mixed cultures and in the absence of neurons, we concluded that they are intrinsic properties of these two different myelin-forming cells.

Preparation of membrane vesicles from isolated myelin: studies on functional and structural properties

Myelin membranes purified from bovine brain are shown to form membrane vesicles when incubated in hypotonic buffer. Following restoration of isotonicity a resealing of the membrane occurs as judged by a significant decrease in 22Na+ permeability. Electron spin resonance measurements using stearic acid spin label I indicate a small decrease in membrane fluidity with increasing ionic strength between 50 and 80 mM NaCl. Iodination of myelin membrane vesicles by lactoperoxidase shows a four-fold increase in the amount of iodine incorporation into the myeline basic protein from 0--150 mM NaCl, while the iodination of the proteolipid protein remains essentially unaffected by the change in ionic strength. This dependence of the iodination of the myelin basic protein on the ionic strength can be explained by the electrostatic interactions of this protein with membrane lipids. In view of striking analogies with studies on model membranes correlating protein binding with membrane permeability changes, we suggest a similar structure-function relationship for the myelin basic protein.

Ultrastructural localization study of two Wolfgram proteins in rat brain tissue

The ultrastructural immunohistochemical localization of Wolfgram proteins W1 and W2 is described in young rat brain tissue. The labelling by the antiserum to W1 is restricted to oligodendroglial cells and myelin sheaths. The plasma membrane of the cells as well as the polysomes are positively stained whereas the mitochondria and the nuclei are always free of labelling. Glial cell processes with definite organelles, which are involved in the myelination of neighbouring axons, are also positive to the antiserum. In the myelin sheaths, the positive staining occurs predominantly at the dense period line of the innermost and outermost lamellae. The present results add further evidence for a specific local synthesis of these Wolfgram proteins in oligodendroglial cells during myelination.

The molecular architecture of myelin: identification of the external surface membrane components

Basic information concerning the molecular organization of the myelin membrane is an intrinsic requirement for understanding the neurochemical events leading to myelination, as well as the potential mechanism of demyelination that might exist at the molecular level for a variety of neurological diseases. The application of chemical, enzymatic, fluorescent, and immunological membrane probes has contributed significantly to this end, although the diverse structural complexity of the myelin sheath has permitted only a rudimentary understanding of its molecular organization. Nevertheless, compelling evidence is accumulating which suggests that components of myelin are asymmetrically distributed in the membrane. Such membrane asymmetry should not only provide important clues to the mechanisms of membrane assembly in the process of myelination, but should also serve as a paradigm for potential functional asymmetry of the individual components at the molecular level. One particularly useful membrane probe is galactose oxidase which has the capacity for identifying surface galactose residues in both glycoproteins and glycolipids on the external surface of the myelin sheath. The identification of these surface components on the myelin sheath is of primary importance since such components might be more readily susceptible to immunological damage or act as a viral receptor which ultimately might lead to demyelination.

The action of trypsin on central and peripheral nerve myelin

In contrast to other studies, our results demonstrate that low concentration of trypsin degrades a high proportion of proteolipid from CNS myelin. The Wolfgram protein and BP are vulnerable and completely lost on trypsinolysis, perhaps accounting for some of the peptides retained by the myelin. In PNS myelin, the major PO protein, a hydrophobic glycoprotein, is readily degraded to a stable 18,000--19,000 molecular weight unit, referred to as TPO protein, still retaining the carbohydrate unit which probably exists as a nonasaccharide grouping. Production of the TPO glycoprotein results from cleavage of a lysinyl-methionine or arginyl-methionine linkage probably found approximately 80--100 residues from the NH2-terminal isoleucine of the PO molecule. This linkage must be especially accessible to trypsin since the TPO protein is also generated in high yield when isolated PO protein is treated with trypsin in solution for 0.5 hours. Further incubation for 24 hours fully degrades the TPO protein to over 20 tryptic peptides, shown by peptide mapping, unlike the situation in myelin where the TPO unit is stable and resists further proteolysis. The TPO unit is also produced when PO protein is treated with BrCN. The PO protein contains 3 methionine residues but presumably the methionine residue in the trypsin-sensitive region is crucial; cleavage leads to the same TPO unit minus NH2-terminal methionine. Another methionine residue also exists in the TPO protein but it may be resistant to BrCN cleavage or else occupy a near-end position. Other proteins were also identified on PAGE of trypsinized PNS myelin: albumin, P2 protein, and PO protein. Albumin and P2 protein were identified in the acidic extract by reaction with specific antibody. The PO protein was isolated; it moved similarly to standard protein on SDS-PAGE and gave the appropriate amino acid analysis. However, it cannot be determined at this time whether a portion of these proteins remains because they are partially inaccessible to trypsin, or else are slightly attacked and thus represent early stages of trypsinolysis. The P2 protein of trypsinized myelin appears to migrate slightly faster than standard P2 protein on PAGE. Further work should clarify this point. Amino acid analysis and sequence data show that the PO protein is particularly hydrophobic, very likely existing in PNS myelin as an amphipathic molecule which penetrates the bilayer but which has a hydrophilic portion exposed. It is this hydrophilic region that contains much lysine, particularly the crucial lysinyl-methionine linkage, that is so trypsin-sensitive. Determination of the amino acid sequence of terminal portions of the isolated PO and TPO proteins serves to firmly establish the PO protein as a unique entity probably exclusive to PNS myelin. It can be concluded that the study of trypsin activity toward PNS myelin has made possible a new understanding of how proteins are positioned in the membrane, and provided valuable insight into the PO protein.

Covalent probe investigations with isolated central nerve myelin preparations

The interaction of the covalently reacting probes dansyl chloride, fluorodinitrobenzene and trinitrobenzene sulphonic acid with isolated central nerve myelin sheath preparations has been studied. The three probes interact preferentially with accessible amino groups on lipid and protein in the membrane. With isolated myelin some 13% of the total phosphatidyl ethanolamine is labelled with dansyl chloride while the figure is 66% with fluorodinitro benzene and 47% with trinitrobenzene sulphonic acid. Lower levels of phosphatidyl serine are labelled. Phosphatidyl ethanolamine seems to be more accessible to probes in the myelin sheath than is phosphatidyl serine perhaps because the ethanolamine-containing lipid class is localised partially at the external apposition surfaces of the membrane which are most accessible to the probes. The serine phospholipids may not react so well because they are preferentially distributed at the cytoplasmic surface of the system. Analysis of protein labelling patterns after reaction of intact myelin with dansyl chloride indicates that the high molecular weight proteins and the proteolipid protein is accessible to the probe while the basic protein is not, even though this latter component is readily labelled with dansyl choride in purified form. It is suggested that the inability of the basic protein to react with myelin is perhaps due to the fact that it is occluded from interaction with the probe at the cytoplasmic apposition surfaces of the lamellae.