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 basic protein inhibits histone-specific protein methylase I

Bovine brain myelin basic protein, free of associated proteolytic activity, was found to be a specific inhibitor of histone-specific protein methylase I (S-adenosyl-L-methionine:protein-L-arginine N-methyltransferase, EC 2.1.1.23) purified from bovine brain. 50% of the methyl group incorporation into the histone substrate catalyzed by the methylase I was inhibited by myelin basic protein at a concentration of 0.326 mM. However, neither of the peptide fragments (residues 1-116 and residues 117-170) generated by the chemical cleavage of myelin basic protein at the tryptophan residue retained the inhibitory activity for histone-specific protein methylase I. Proteins such as gamma-globulin, bovine serum albumin, bovine pancreatic ribonuclease and polyarginine did not exhibit significant inhibitory activity toward the enzyme. The Ki value for myelin basic protein was estimated to be 3.42 X 10(-5) M for histone-specific protein methylase I and the nature of the inhibition was uncompetitive toward histone substrate.     

A mitogen for Schwann cells is derived from myelin basic protein

Evidence is presented that a mitogen can be produced from myelin basic protein (MBP) which may be related to the Schwann cell proliferation characteristic of Wallerian degeneration. Myelin derived from the shiverer mutant which is devoid of MBP is also devoid of mitogenic activity. Absorption of the mitogen with a polyclonal antisera to MBP abolishes the mitogenic effect. In addition, only liposomes containing MBP are mitogenic to cultured Schwann cells; liposomes containing other myelin-specific proteins do not stimulate Schwann cell division. These results directly demonstrate the MBP origin of a mitogen for Schwann cells.     

Myelin basic protein inhibits formyl-peptide induced chemotaxis in human neutrophils

Myelin basic protein, one of the major membrane protein component of the central nervous system, was used to probe the molecular mechanism of cellular activation. Pre-treatment of human neutrophils with myelin basic protein selectively inhibits the formyl-peptide-induced chemotaxis, without affecting chemotaxis evoked by casein and activated serum. Furthermore, both the degranulation and superoxide anion production stimulated by the chemotactic peptide are not modified by the prior treatment of the neutrophils with myelin basic protein.     

Myelin instability and oligodendrocyte metabolism in myelin-deficient mutant mice

During the active phase of myelination in myelin-deficient mutant mice (mld), myelin basic protein (MBP) synthesis is defective and the myelin lamellae are uncompacted. In these mutants, we found a fast metabolism of the myelin-associated glycoprotein (MAG) and of sulfatides, and the presence of cholesterol esters and a degradation product of MAG, dMAG, indicating that mld myelin was unstable. The increased synthesis of MAG and Wolfgram protein, two proteins present in uncompacted myelin sheath and paranodal loops, was demonstrated by high levels of messengers. Simultaneously, we found an accumulation of inclusion bodies, vacuoles, and rough endoplasmic reticulum in mld oligodendrocytes. This material was heavily immunostained for MAG. Furthermore, the developmental change between the two molecular forms of MAG (p72MAG/p67MAG) was delayed in mld mice. In 85-d-old mld mice, the MBP content increased and myelin lamellae became better compacted. In these mutants, dMAG was absent and MAG mRNAs were found in normal amounts. Furthermore, the fine structure of mld oligodendrocytes was normal and the MAG immunostaining was similar to age-matched controls. These results support a functional role for MBP in maintaining the metabolic stability and the compact structure of myelin. Furthermore, in the absence of MBP and myelin compaction, the regulation of the synthesis of at least two membrane proteins related to myelin cannot proceed.     

Myelin basic protein as a binding partner and calmodulin adaptor for the BKCa channel

The activity and localization of large-conductance Ca2+ -activated K+ (BKCa) channels are known to be modulated by several different proteins. Although many binding partners have been identified via yeast two-hybrid screening, this method may not detect certain classes of interacting proteins such as low affinity binding proteins or multi-component protein complexes. In this study, we employed mass spectrometry to identify proteins that interact with BKCa channels. We expressed and purified the 'tail domain' of the rat BKCa channel alpha-subunit, a 54-kDa region that is crucial for expression and functional activity of the channel. Using rat brain lysate and purified 'tail domain', we identified several novel proteins that interact with the BKCa channel. These included the myelin basic protein (MBP), upon which we performed subsequent biochemical and electrophysiological studies. Interaction between the BKCa channel and MBP was confirmed in vivo and in vitro. MBP co-expression affected the Ca2+ -dependent activation of the BKCa channel by increasing its Ca2+ sensitivity. Moreover, we showed that calmodulin (CaM) interacts with the BKCa channel via MBP. Since CaM is a key regulator of many Ca2+ -dependent processes, it may be recruited by MBP to the vicinity of the BKCa channel, modulating its functional activity.     

Myelin basic protein and proteolipid protein reactivity of brain- and cerebrospinal fluid-derived T cell clones in multiple sclerosis and postinfectious encephalomyelitis

T cells were directly cloned from autopsied MS brain plaque tissue and reactivity was measured with the major encephalitogenic neuroantigens, myelin basic protein (MBP), and proteolipid protein (PLP). Control clones were simultaneously derived from the blood. The proportion of T4+ and T8+ T cell clones from the brain tissue differed from that of peripheral blood T cell clones derived at the same time, suggesting that the clones were not derived from the peripheral blood. None of 57 brain-derived T cell clones proliferated to either MBP or PLP, although they responded well to PHA and IL 2. An additional 235 clones derived from the cerebrospinal fluid and 126 clones from the peripheral blood of other subjects with multiple sclerosis also did not proliferate to MBP or PLP. In contrast, five of nine T4+ clones from the CSF of a subject with postinfectious encephalomyelitis exhibited low but clear reactivity to human MBP, supporting the possible role of MBP as the target antigen in this disease. These studies, the first to clone T cells directly from MS plaque tissue, suggest that the lack of consistent T cell reactivity to MBP or PLP in the peripheral blood of MS patients does not appear to be secondary to the sequestration of a large number of these cells in the brain.     

Myelin deficient mice: expression of myelin basic protein and generation of mice with varying levels of myelin

Mice homozygous for the mutation myelin deficient (mld), an allele of shiverer, exhibit decreased CNS myelination, tremors, and convulsions of progressively increasing severity leading to an early death. In this report we demonstrate in mld mice that the gene encoding myelin basic protein (MBP) is expressed at decreased levels and on an abnormal temporal schedule relative to the wild-type gene. Southern blot analyses, field-inversion gel electrophoresis studies, and analyses of mld MBP cosmid clones indicate that there are multiple linked copies of the MBP gene in mld mice. We have introduced an MBP transgene into mld mice and found that myelination increases and tremors and convulsions decrease. Mld and shiverer mice with zero, one, or two copies of the MBP transgene express distinct levels of MBP mRNA and myelin. The availability of a range of mice expressing graded levels of myelin should facilitate quantitative analysis of the roles of MBP in the myelination process and of myelin in nerve function.     

N-WASp is required for Schwann cell cytoskeletal dynamics, normal myelin gene expression and peripheral nerve myelination

Schwann cells elaborate myelin sheaths around axons by spirally wrapping and compacting their plasma membranes. Although actin remodeling plays a crucial role in this process, the effectors that modulate the Schwann cell cytoskeleton are poorly defined. Here, we show that the actin cytoskeletal regulator, neural Wiskott-Aldrich syndrome protein (N-WASp), is upregulated in myelinating Schwann cells coincident with myelin elaboration. When N-WASp is conditionally deleted in Schwann cells at the onset of myelination, the cells continue to ensheath axons but fail to extend processes circumferentially to elaborate myelin. Myelin-related gene expression is also severely reduced in the N-WASp-deficient cells and in vitro process and lamellipodia formation are disrupted. Although affected mice demonstrate obvious motor deficits these do not appear to progress, the mutant animals achieving normal body weights and living to advanced age. Our observations demonstrate that N-WASp plays an essential role in Schwann cell maturation and myelin formation.     

Myelin basic protein induces hexagonal phase formation in dispersions of diacylphosphatidic acid

31P nuclear magnetic resonance and low-angle X-ray diffraction measurements have shown that the basic protein of myelin caused diacylphosphatidic acid dispersions to change from a lamellar to a hexagonal lipid organisation. Several other basic proteins failed to effect a similar phase change, and had little influence on phospholipid headgroup structure and motion.