The preparation of antibodies reactive against citrulline-containing charge isomers of myelin basic protein but not against the arginine-containing charge isomers

A Tale of Two Citrullines—Structural and Functional Aspects of Myelin Basic Protein Deimination in Health and Disease

Myelin basic protein (MBP) binds to negatively charged lipids on the cytosolic surface of oligodendrocyte membranes and is responsible for adhesion of these surfaces in the multilayered myelin sheath. The pattern of extensive post-translational modifications of MBP is dynamic during normal central nervous system (CNS) development and during myelin degeneration in multiple sclerosis (MS), affecting its interactions with the myelin membranes and with other molecules. In particular, the degree of deimination (or citrullination) of MBP is correlated with the severity of MS, and may represent a primary defect that precedes neurodegeneration due to autoimmune attack. That the degree of MBP deimination is also high in early CNS development indicates that this modification plays major physiological roles in myelin assembly. In this review, we describe the structural and functional consequences of MBP deimination in healthy and diseased myelin.

Myelin basic protein-diverse conformational states of an intrinsically unstructured protein and its roles in myelin assembly and multiple sclerosis

The 18.5 kDa isoform of myelin basic protein (MBP) is a major component of the myelin sheath in the central nervous system of higher vertebrates, and a member of a larger family of proteins with a multiplicity of forms and post-translational modifications (PTMs). The 18.5 kDa protein is the exemplar of the family, being most abundant in adult myelin, and thus the most-studied. It is peripherally membrane-associated, but has generally been investigated in isolated form. MBP is an 'intrinsically unstructured' protein with a high proportion (approximately 75%) of random coil, but postulated to have core elements of beta-sheet and alpha-helix. We review here the properties of the MBP family, especially of the 18.5 kDa isoform, and discuss how its three-dimensional (3D) structure may be resolved by direct techniques available to us, viz., X-ray and electron crystallography, and solution and solid-state NMR spectrometry. In particular, we emphasise that creating an appropriate environment in which the protein can adopt a physiologically relevant fold is crucial to such endeavours. By solving the 3D structure of 18.5 kDa MBP and the effects of PTMs, we will attain a better understanding of myelin architecture, and of the molecular mechanisms that transpire in demyelinating diseases such as multiple sclerosis.

Immunohistochemical localization of citrullinated proteins in adult rat brain

By using hybridoma technology, an IgM monoclonal antibody (F95) against multiple citrullinated synthetic and natural peptides was recently developed and used to stain immunohistochemically subsets of astrocytes and myelin basic protein (MBP) from selected regions of human brain (Nicholas and Whitaker [2002] Glia 37:328-336). With this antibody, the present study provides a more detailed localization of citrullinated epitopes in the central nervous system (CNS) by examining immunohistochemical staining patterns for F95 in the normal adult rat brain. Thus, immunohistochemical labeling for citrullinated epitopes was seen in white matter areas consistent with myelin staining; however, in general, it was more prominent and uniform in the caudal CNS (spinal cord, medulla oblongata, pons, and cerebellum) than in more rostral areas. F95 staining was also seen in cells and fibers often intimately associated with blood vessels and/or ventricular surfaces. By using dual-color immunofluorescence, the vast majority of this latter staining was colocalized within a subset of astrocytes also immunoreactive for glial fibrillary acidic protein (GFAP). By using Western blot analysis of rat brain proteins, multiple GFAP- and MBP-immunoreactive proteins and peptide fragments were seen, and many of them were also reactive with the F95 antibody. Thus, the present study not only demonstrates that citrullinated epitopes in normal rat brain are most concentrated in subsets of myelin and astrocytes but also provides evidence that GFAP, like MBP, may be present as multiple citrullinated isoforms.

Localization and partial characterization of a 60 kDa citrulline-containing transport form of myelin basic protein from MO3-13 cells and human white matter

The localization of myelin basic proteins (MBPs) in an immortalized human-human hybrid cell line (MO3-13) formed by fusion of rhabdomyosarcoma TE671-TG6 with primary human oligodendrocytes, cultured from surgical specimens, demonstrated an intracellular localization in vesicles and vacuoles with an intricate internal membranous network and to the external surface of the cell by immunogold electron microscopy. The availability of antibodies to one of the components of MBP, i.e., the citrulline containing component ("C-8"), permitted us to localize this component of MBP to intracellular vacuoles and also on the external surface of the MO3-13 cells. Since the apposition of the external surfaces of the oligodendrocyte is responsible for the intraperiod line of the myelin sheath, localization of C-8 to the external surface of non-permeabilized cells by immunogold scanning electron microscopy is consistent with our observations that C-8 is localized to the intraperiod line of myelin (McLaurin et al.: J Neurosci Res 35:618-628, 1993). Western blots of isolated MBP from MO3-13 cells, probed with an antibody reactive with residues 130-137 of MBP, recognized a protein in the 60 kDa range. No immunoreactivity was found in the 18.5 kDa range. This 60 kDa protein also reacted with a monoclonal antibody raised with residues 70-84 of MBP, 2 different polyclonals raised with whole bovine MBP, an antibody to human MBP raised in monkeys, and the anti-citrulline antibody. These data strongly suggested that the 60 kDa protein contained MBP sequences within its primary structure. A similar protein has been isolated from human myelin-containing fractions but not from compact myelin demonstrating that the 60 kDa protein from MO3-13 cells was not an artefact related to fusion. Sequence determination of peptides obtained from enzymic and chemical cleavages revealed that the 60 kDa protein contained MBP sequences and peptides with 55-60% homology with dynamin, a protein involved in intracellular transport. These data suggest that the externalization of MBP in this cell involves transport by fusion of MBP with another protein. By sequestering MBP in a larger protein, the possibility of inducing autoimmune disease by MBP released, due to cell death, is minimized.

Deimination of human myelin basic protein by a peptidylarginine deiminase from bovine brain

A peptidylarginine deiminase (PAD; EC 3.5.3.15) has been isolated from bovine brain and some of its characteristics have been studied. The enzyme showed an absolute requirement for Ca2+, a temperature optimum at approximately 50 degrees C, and two Km values when benzoylarginine ethyl ester was used as substrate, 0.78 mM and 11.2 mM. The higher Km has not been reported previously. Protein substrates for the enzyme included polyarginine and myelin basic protein but not histones. Because one of the components of MBP contains six citrullinyl residues per mole, enzymic deimination appeared to be a likely mechanism. When the most cationic component (C-1) was subjected to PAD in solution, 17 of the 19 arginyl residues were modified. From sequence analyses we concluded that the nature of the amino acid residues adjacent to the deiminated arginine were not modifiers of the reaction as arginyl residues in a variety of environments were deiminated. This deimination was reflected in a large increase in random structure, as measured by [theta]200. At 5 degrees C, the [theta]200 of the deiminated protein was -70 x 10(3) compared with -30 x 10(3) deg.cm2/dmol for the native protein. When the temperature was increased to 70 degrees C, the [theta]200 was -44 x 10(3) for the deiminated protein and -20 x 10(7) deg.cm2/dmol for the native C-1. When plotted as a function of temperature, [theta]200 decreased linearly from 5 degrees C to 50 degrees C for both proteins and did not change from 50 degrees C to 70 degrees C. PAD provides a mechanism for deimination of arginyl residues of myelin basic protein. The selective deimination of the six arginyl residues that are consistently found deiminated in C-8 may be determined by the orientation of the protein in the membrane and/or the more complex lipid composition of myelin may affect the selectivity of the deimination.

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.

Studies on NG-methylarginine derivatives in myelin basic protein from developing and mutant mouse brain

The amounts of NG-methylarginine derivatives in myelin basic protein (MBP) purified from dysmyelinating mutant and different stages of normal myelinating mouse brains have been studied by using h.p.l.c. with a highly sensitive post-column o-phthaldialdehyde derivative-formation method. All three naturally occurring derivatives (NG-monomethylarginine (MeArg), NGN'G-dimethylarginine [Me2(sym)Arg] and NGNG-dimethylarginine [Me2(asym)Arg]) were found in MBP; however, their relative concentrations varied significantly with the age of the animal. The amounts of MeArg and Me2(sym)Arg in MBP increased as a function of the age of the brain, whereas that of Me2(asym)Arg decreased. MBP from early-myelinating mouse brain was shown to contain a high proportion of Me2(asym)Arg, which was hardly detectable in older brain MBP. This derivative, Me2(asym)Arg, was also absent from MBP embedded in the most compact multilamellar myelin, but was present in MBP in the least compact myelin (P3B). Comparing the extent of total methylation in vivo (sum of all three arginine derivatives), MBP extracted from less-compact myelin (P3A and P3B) showed a level approx. 40% higher than that from compact myelin. MBPs isolated from dysmyelinating mutant mouse brains, such as jimpy (jp/y) and quaking (qk/qk), contained a much higher level of Me2(asym)Arg relative to the other two methyl derivatives and also in comparison with those levels in the mother brain MBP. SDS/PAGE analysis of MBPs extracted from the mutant (both jp/y and qk/qk) as well as young normal (6-13 days old) mouse brains indicated the presence of a high-molecular-mass isoform of MBP (about 32 kDa), but this isoform was not found in adult brains. These results therefore indicate that structural integrity of myelin membrane in which MBP is embedded appears to play a pivotal role in determining the extent and the kind of Me2Arg formation in MBP at the post-translational level.

An antibody specific for component 8 of myelin basic protein from normal brain reacts strongly with component 8 from multiple sclerosis brain

Myelin basic protein (MBP) consists of several components or charge isomers (C-1 through C-8) generated by one or a combination of posttranslational modifications. One of these, C-8, has been shown to contain citrulline (Cit) at defined sites formed by deimination of six arginyl residues. This unusual modification has allowed us to raise antibodies specific for this charge isomer only. To do this, a synthetic peptide, Gly-Cit-Cit-Cit-Cit, was coupled to keyhole limpet hemocyanin and injected into rabbits. The antibodies so generated reacted only with C-8 and not with any of the other charge isomers. A second antibody fraction was raised against the synthetic peptide ACitHGFLPCitHR naturally occurring between residues 24 and 33 of C-8 (all other charge isomers contain R instead of Cit at positions 25 and 31). These antibodies preferred C-8 but reacted with the other charge isomers, to the extent of approximately 25-30% of the reactivity shown with C-8. In studies with C-8 from multiple sclerosis (MS) MBP, much greater reactivity was obtained with these antibodies when compared with their reactivity with C-8 from normal MBP. Because the total number of Cit residues in C-8 from MS and normal MBP is the same, the difference in reactivity may be related to structural factors. The antibodies raised with the tetra-Cit peptide were reacted with three pairs of synthetic peptides: 24ARHGFLPRHR33 and ACitHGFLPCitHR; 120GQRPGFGYGGRAS132 and GQCitPGFGYGGCitAS; and 157GGRDSRSGSPMARR170 and GGCitDSRSGSPMACitR. They reacted only with the Cit-containing peptides in the order 157-170 greater than 120-130 greater than 24-33.(ABSTRACT TRUNCATED AT 250 WORDS)

Reactivity of two anti-galactosyl ceramide antibodies towards myelin basic protein

Two anti-galactosyl ceramide antibodies (polyclonal Ab142 and a monoclonal antibody) were characterized in terms of their reactivity towards purified lipids and myelin basic protein. Polyclonal Ab142 is a rabbit anti-mouse galactosyl ceramide (Gal C) IgG. Antigenic recognition is dependent on both galactose and ceramide since neither could inhibit galactosyl ceramide binding by more than 10%. MAb-Gal C is a monoclonal antibody raised in mice against galactosyl ceramide. Binding of MAb-Gal C to Gal-C was equally inhibited by ceramide and galactose to approximately 50%, indicating that both groups are important for antibody recognition. MAb-Gal C was also shown to be reactive with the structurally related lipids, sphingomyelin and sulfatide. Polyclonal Ab142, although raised against Gal C, was shown to be 3-fold more reactive with component 8 (C-8) of myelin basic protein than Gal C. On the other hand, the MAb-Gal C which also reacted with C-8 was 2-fold less reactive with C-8 than with Gal C. Neither of these antibodies were reactive with component 1 (C-1) of myelin basic protein. An anti-MBP IgG was shown to be reactive with C-1 and C-8 but unreactive with Gal C. In competitive inhibition ELISA, C-8 was able to compete out 44% and 41% of Gal C binding to polyclonal Ab142 and MAb, respectively. The reverse competition demonstrated that Gal C could inhibit 75% of C-8 binding to both antibodies. D-galactose was unable to inhibit C-8 binding to either antibody, whereas ceramide was as efficient as Gal C.(ABSTRACT TRUNCATED AT 250 WORDS)