Rearrangement and Reactivation of the Myelin Basic Protein Locus in Myelin-Deficient (mld) Mouse Brain

Abstract: Myelin-deficient ( mld ) is a complex mutation affecting the myelin basic protein (MBP) locus of the mouse. It consists of duplication and partial inversion of the MBP gene and results in a dysfunctional MBP locus. The mutant phenotype is reversed, both in vivo and in vitro, in ∼5% of mld oligodendrocytes. One possible mechanism for the somatic reversion is recombination between homologous sequences of the duplicated gene copies to reconstitute a functional MBP locus. There are several possible recombination events that could reconstitute a functional MBP locus by DNA rearrangement. Two of these would result in reinversion and circularization of specific MBP gene sequences, respectively. In this work polymerase chain reaction analysis was used to detect both reinverted and circularized MBP gene sequences in mld mouse tissues, indicating that DNA rearrangement at the MBP locus does occur. Analysis of individually harvested cells showed that in revertant MBP-positive mld oligodendrocytes DNA rearrangement at the MBP locus was correlated with reactivation of the MBP gene. Fluctuation analysis showed that reactivation of the MBP locus is a stochastic event occurring with a frequency of ∼1.4 × 10⁻⁶ per cell per cell cycle during oligodendrocyte development. The frequency of rearrangement and reactivation of the MBP locus was comparable in double mutant ( mld/mld , scid/scid ) and single mutant ( mld/mld , + ^scid /+ ^scid ) mice, indicating that the scid factor is not required for MBP gene reactivation in mld . The significance of DNA rearrangement in mammalian development is discussed.     

Gene organization and transcription of duplicated MBP genes of myelin deficient (shi(mld)) mutant mouse

A hereditary dysmyelinating mutation, named myelin deficient (shi(mld)), is characterized by reduced expression of myelin basic protein (MBP). In shi(mld), the MBP gene is duplicated and its reduced expression is mainly determined by the level of mRNA. We have characterized the structure and function of the promoter regions of the duplicated MBP genes in shi(mld). Among the lambda clones containing promoter regions of the duplicated MBP genes in shi(mld), one (gene 1) had the same restriction enzyme pattern as that in control mice, but another (gene 2) had a rearrangement on a distal part of the promoter. A 712-bp nucleotide sequence upstream of the first exons of both of the duplicated MBP genes of shi(mld) was completely consistent with that of the control. Promoter activities of 1.3-kb 5'-flanking regions from respective genes of shi(mld) measured by in vitro run-off assay using HeLa whole-cell extracts were indistinguishable from that of the control MPB gene. Chromosomal mapping by in situ hybridization suggested that the duplicated MBP genes were located closely to each other at the distal part of chromosome 18. A recombinational event including the inversion seemed to have occurred within gene 1 and its possible relationship to the reduced expression of MBP is discussed.     

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.     

Immunoreactive myelin basic proteins are not detected when shiverer mutant Schwann cells and fibroblasts are co-cultured with normal neurons

Shiverer (shi) is an autosomal recessive mutation in mice that results in hypomyelination in the central nervous system (CNS) but normal myelination in the peripheral nervous system (PNS). Myelin basic proteins (MBPs) are virtually absent in both PNS and CNS. It is not known whether the cellular target in the PNS is the myelin-forming Schwann cell or another cell type which secondarily affects the Schwann cell. To determine the cellular target of the shi gene, we have adapted tissue culture techniques that allow co-culture of pure populations of mouse sensory neurons of one genotype with Schwann cells and fibroblasts of another genotype under conditions that permit myelin formation. These cultures were stained immunocytochemically as whole mounts to determine whether MBPs were expressed under various in vitro conditions. In single-genotype cultures, presence or absence of MBPs was consistent with earlier in vivo results: +/+ cultures were MBP-positive and shi/shi cultures were MBP-negative. In mixed-genotype cultures, visualization of MBPs in myelin accorded with the genotype of the non-neuronal Schwann cells and fibroblasts and not with the neurons--those cultures that contained +/+ non-neuronal cells were MBP-positive and those with shi/shi non-neuronal cells were MBP-negative, independent of the neuronal genotype. These results rule out neurons or circulating substances as mediators of the influence of the shi genetic locus on MBP synthesis and deposition in peripheral myelin.     

Structure and Expression of Myelin Basic Protein Gene Sequences in the mld Mutant Mouse: Reiteration and Rearrangement of the Mbp Gene

The mld mutation on chromosome 18 in the mouse is a putative allele of the shiverer (shi) mutation. We have analyzed the structure of myelin basic protein (MBP) gene sequences in mld DNA by restriction mapping of genomic DNA. The results indicate that the mld chromosome carries two copies of the MBP structural gene, one of which is intact and one of which is interrupted. Genetic analysis indicates that the interrupted gene is close to the intact MBP structural gene and cosegregates with the mld mutation. We have also analyzed the levels of MBP polypeptides and MBP-specific mRNA in wild-type, homozygous and heterozygous shiverer and mld mice and in mice carrying both mutations. The results indicate that both shi and mld are cis-acting codominant mutations that cause severely reduced steady state levels of MBP-specific mRNA and MBP polypeptides in the brain. We have analyzed the total number of oligodendrocytes and the number of MBP-positive oligodendrocytes in mld and shi brain primary cultures. In shi cultures, none of the oligodendrocytes expresses MBP. However, in mld cultures, approximately 5% of the oligodendrocytes express MBP. The nature of the "revertant" mld oligodendrocytes is not known.     

Optic nerve tissue sections derived from myelin-deficient mutant mice as culture substrata for fibroblast adhesion and spreading

The substrate properties were compared between normal and myelin-deficient central nervous system (CNS) tissues by an in vitro assay of cell attachment and spreading. Fibroblasts (3T3) were plated onto culture substrata consisting of optic nerve tissue sections cut from normal or two myelin-deficient mutant mice, Shiverer and Quaking. Optic nerve sections from either of the mutant animals supported more 3T3 fibroblast spreading and adhesion than sections derived from animals with normal myelin. These results demonstrate that CNS myelin influences the ability of cells to attach and spread and that it is the actual presence of myelin which is inhibitory rather than the presence of optic nerve axons or oligodendrocytes.     

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. Special issue dedicated to Drs. Anthony and Celia Campagnoni.     

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.     

Antigen processing of myelin basic protein is required prior to recognition by T cells inducing EAE.

The processing and presentation of whole myelin basic protein (MBP) and a 12 amino acid encephalitogenic peptide were investigated using MBP-immune and peptide-immune murine T cell lines. Myelin basic protein is the major component of central nervous system (CNS) white matter capable of inciting an autoimmune response which leads to the disease, experimental allergic encephalomyelitis (EAE), in a number of animal species. MBP-immune T cell lines caused a form of adoptively transferred EAE when injected into naive, syngeneic recipients. It has been found that both whole MBP and peptide required processing in order to induce proliferation of the T cell lines. The proliferative response was greatest when MBP was processed under conditions in which proteolysis was prevented. The demonstration that activation of encephalitogenic MBP immune T cells requires a processed form of MBP may have relevance to the human inflammatory CNS demyelinating condition, multiple sclerosis, for which EAE is the EAE is the prime animal model.     

Identification of the New Isoforms of Mouse Myelin Basic Protein: The Existence of Exon 5a

Myelin basic protein (MBP) is a major constituent in the myelin of the CNS. In mice, five forms of MBPs (14 kDa, two types of 17 kDa, 18.5 kDa, and 21.5 kDa) encoded by separate mRNAs have been identified based on cDNA cloning studies. These mRNAs are considered to be produced by alternative splicing from a single gene composed of seven exons. Here we report the existence of two novel MBP mRNAs encoding 19.7-kDa and 21-kDa MBPs identified by cDNA cloning using the polymerase chain reaction. Both of these MBPs contain a sequence of a previously unidentified exon of 66 nucleotides, which was mapped to be just 5' of exon 5 in the MBP gene. MBP mRNAs containing this novel exon (exon 5a) belong to a minor population in the whole brain and PNS and are somewhat enriched in the spinal cord. Exon 5a encodes a very hydrophobic segment rich in valine residues, which presumably forms a beta-pleated sheet.