Danielle Pham-Dinh

Alexander disease: putative mechanisms of an astrocytic encephalopathy

Alexander disease (AXD) is the first primary astrocytic disorder. This encephalopathy is caused by dominant mutations in the glial fibrillary acidic protein (GFAP) gene, encoding the main intermediate filament of astrocyte. Pathologically, this neurodegenerative disease is characterised by dystrophic astrocytes containing intermediate filament aggregates associated with myelin abnormalities.More than 20 GFAP mutations have been reported. Many of them cluster in highly conserved regions between several intermediate filaments. Contrary to other intermediate filament-related diseases, AXD seems to be the consequence of a toxic gain of function induced by aggregates. This is supported by the phenotype of mice overexpressing human GFAP. Nevertheless, GFAP null mice display myelin abnormalities and blood-brain barrier dysfunction that are present in AXD.Given the pivotal role of astrocytes in brain physiology, there are many possibilities for astrocytes to dysfunction and to impair the functions of other cells. Physiopathological hypotheses are discussed in the frame of AXD.

Native myelin oligodendrocyte glycoprotein promotes severe chronic neurological disease and demyelination in Biozzi ABH mice

Myelin oligodendrocyte glycoprotein (MOG) is a powerful encephalitogen for experimental autoimmune demyelination. However, the use of MOG peptides or recombinant proteins representing part of the protein fails to fully address the possible pathogenic role of the full‐length myelin‐derived protein expressing post‐translational modifications. Immunization of mice with central nervous system tissues from wild‐type (WT) and MOG‐deficient (MOG–/–) mice demonstrates that MOG in myelin is necessary for the development of chronic demyelinating experimental autoimmune encephalomyelitis (EAE) in mice. While immunization with WT spinal cord homogenate (SCH) resulted in a progressive EAE phenotype, MOG–/– SCH induced a mild self‐limiting acute disease. Following acute EAE with MOG–/– SCH, mice developed Tu2004cell responses to recombinant mouse MOG (rmMOG), indicating that MOG released from myelin is antigenic; however, the lack of chronic disease indicates that such responses were not pathogenic. Chronic demyelinating EAE was observed when MOG–/– SCH was reconstituted with a dose of rmMOG comparable to MOG in myelin (2.5% of total white matter‐derived protein). These data reveal that while immunization with the full‐length post‐translational modified form of MOG in myelin promotes the development of a more chronic autoimmune demyelinating neurological disease, MOG (and/or other myelin proteins) released from myelin during ongoing disease do not induce destructive autoimmunity.

Persistence of autoreactive myelin oligodendrocyte glycoprotein (MOG)-specific T cell repertoires in MOG-expressing mice

Experimental autoimmune encephalomyelitis, an experimental murine model for multiple sclerosis, is induced by stimulation of myelin‐specific Tu2004lymphocytes. Myelin oligodendrocyte glycoprotein (MOG), a minor component of myelin proteins, is a potent autoantigen which contributes extensively to the anti‐myelin response. In the present work, immunoscope analyses and sequencing of the oligoclonal expansions revealed anti‐MOGu2004Vα and Vβ public repertoires in lymphocytes infiltrating the CNS of wild‐type (WT) mice. Moreover, a subset of CNS‐infiltrating CD4+ Tu2004lymphocytes bearing the public Vβ8.2 segment have an inflammatory phenotype strongly suggesting that it is encephalitogenic. We then observed that, in lymph node cells of MOG‐deficient and WT animals, the Vα and Vβ public repertoires expressed by MOG‐specific Tu2004cells are identical in both strains of mice and correspond to those found in the CNS of WT animals. These findings indicate that the MOG immunodominant determinant is unable to induce tolerance by deletion, and public anti‐MOG Tu2004cell repertoires are selected for, regardless of the presence of MOG in the thymus and peripheral organs.

Autoimmunity against myelin oligodendrocyte glycoprotein is dispensable for the initiation although essential for the progression of chronic encephalomyelitis in common marmosets.

To elucidate the pathogenetic significance of myelin/oligodendrocyte glycoprotein (MOG)-specific autoreactivity in a genetically and immunologically heterogeneous nonhuman primate model of multiple sclerosis, we analyzed experimental autoimmune encephalomyelitis (EAE) in the outbred common marmoset (Callithrix jacchus). One sibling each of 5 bone marrow chimeric marmoset twins was immunized with myelin derived from wild-type (WT) C57BL/6 mice (WT myelin); the other sibling was immunized with myelin from MOG-deficient C57BL/6 mice (MOG−/− myelin). One twin pair developed acute EAE simultaneously; the 4 remaining twin siblings immunized with WT myelin developed chronic progressive EAE, whereas siblings of these 4 monkeys remained free of clinical disease signs. Many EAE-related abnormalities were identified in the CNS of both groups by magnetic resonance imaging and histologic analysis, but mean percentages of spinal cord demyelination were lower in monkeys immunized with MOG−/− myelin (8.2%) than in WT myelin-immunized animals (40.5%). There was a strong correlation between the development of overt clinical EAE and seropositivity for anti-MOG antibodies, but blood and lymph node T-cell proliferative responses showed no relationship to disease. These results indicate that the initiation of CNS inflammation and demyelination can take place in the absence of detectable autoimmunity against MOG, but the clinical progression and histopathologic severity depends on the presence of antibodies against MOG in this multiple sclerosis model.

Myelin oligodendrocyte glycoprotein is expressed in the peripheral nervous system of rodents and primates

The myelin oligodendrocyte glycoprotein (MOG) is a minor CNS myelin-specific protein that is an important candidate autoantigen in multiple sclerosis. We now report that MOG mRNA transcripts are present in the peripheral nervous system of rodents and primates at levels approximately ten-fold lower than in brain as demonstrated by real time PCR. A major source of this signal are Schwann cells which are also shown to express MOG protein within their cytoplasm in vitro by immunohistochemistry. Expression of MOG by Schwann cells associated with tissue innervation may account for the widespread distribution of low levels of MOG mRNA transcripts, and potentially may provide a source of antigen that can influence the composition and function of the MOG-specific immune repertoire.

T Cell Repertoire Diversity Is Required for Relapses in Myelin Oligodendrocyte Glycoprotein-Induced Experimental Autoimmune Encephalomyelitis

Comparison of TCRαβ repertoires of myelin oligodendrocyte glycoprotein (MOG)-specific T lymphocytes in C57BL/6 and TdT-deficient littermates (TdT−/−) generated during experimental autoimmune encephalomyelitis (EAE) highlights a link between a diversified TCRαβ repertoire and EAE relapses. At the onset of the disease, the EAE-severity is identical in TdT+/− and TdT−/− mice and the neuropathologic public MOG-specific T cell repertoires express closely similar public Vα-Jα and Vβ-Jβ rearrangements in both strains. However, whereas TdT+/+ and TdT+/− mice undergo successive EAE relapses, TdT−/− mice recover definitively and the lack of relapses does not stem from dominant regulatory mechanisms. During the first relapse of the disease in TdT+/− mice, new public Vα-Jα and Vβ-Jβ rearrangements emerge that are distinct from those detected at the onset of the disease. Most of these rearrangements contain N additions and are found in CNS-infiltrating T lymphocytes. Furthermore, CD4+ T splenocytes bearing these rearrangements proliferate to the immunodominant epitope of MOG and not to other immunodominant epitopes of proteolipid protein and myelin basic protein autoantigens, excluding epitope spreading to these myelin proteins. Thus, in addition to epitope spreading, a novel mechanism involving TCRαβ repertoire diversification contributes to autoimmune progression.

Identification of a Val I 45 IIe substitution in the human myelin oligodendrocyte glycoprotein: lack of association with multiple sclerosis

Myelin/oligodendrocyte glycoprotein (MOG) is a major target antigen in experimental autoimmune encephalomyelitis and it has been suggested that it may as well play a key role in the demyelination process in multiple sclerosis (MS). As MOG variants could be pathogenic in autoimmune demyelinating diseases of the central nervous system, we analysed the coding sequence of MOG in MS patients and described a G→A transition occurring in exon 3 of the human MOG gene. The mutation predicts that isoleucine substitutes for a valine at codon I 45 (Val 145 lle) in the transmembrane region of the protein. This is the first aminoacid substitution reported in human MOG. The polymorphism can be detected by restriction enzyme digestion of genomic DNA or reverse-transcribed PCR amplified products, making it a simple tool to detect a potential implication of MOG alleles in susceptibility to MS by association study. The analysis of 83 unrelated MS patients and 82 unrelated healthy controls showed that the polymorphism is found in similar proportions in MS patients (18%) and controls (14.6%). It is therefore unlikely that the MOG Val 145 lle variant is responsible for genetic susceptibility to MS.

Chapter 18 – Myelin Oligodendrocyte Glycoprotein Gene

Publisher Summary Myelin oligodendrocyte glycoprotein (MOG) is a protein of great neuroimmunological interest, whose function is not known. This chapter examines the structural functional relationships of MOG in myelin. Two different strategies are used to clone MOG mRNAs. One uses a mixture of MOG mAbs to screen a rat ƛgt11 expression library, whereas other used degenerate primers developed from the N-terminal amino acid sequence of bovine MOG to screen bovine, rat, and mouse brain ƛgt10 phage libraries. Subsequently, the mouse gene is isolated and its organization characterized. Similar approaches are used to characterize the human MOG cDNA and the human gene. Sequence analyses indicate that the mature protein is highly conserved between species and consists of 218 amino acids comprising a putative signal peptide, an immunoglobulin (Ig)-like N-terminal domain, two highly hydrophobic segments separated by a short hydrophilic loop, and a short hydrophilic C-terminal domain. The presence of two putative membrane-spanning domains, if verified, would make MOG a unique member of the Ig super-gene family (IgSF).

Biology of Demyelinating Diseases

Demyelinating diseases are those in which myelin is the primary target of damage on the basis of neuroradiological, neuropatholgical, neurochemical, and genetic studies. This review describes the morphological aspects of the myelin sheath which is the most abundant membrane structure in the vertebrate nervous system. It is made of oligodendrocytes in the CNS and Schwann cells in the PNS. It comprises four distinct regions: the node of Ranvier which contains voltage-gated Na+ channels, paranodal loops which are major sites of myelin-axon adhesion, the juxtaparanode, and the internode which is the part of the axon which is ensheathed by a segment of myelin. Demyelination is segmental in the peripheral nervous system and focal in the central nervous system. Myelin is necessary for nerve conduction velocity. Dys- and demyelination can involve specific constituents of the CNS and the PNS both for genetic (leukodystrophies) or acquired diseases. Numerous components are different and differently involved in CNS and PNS myelin, both among proteins and lipids (sphingolipids). Outside of the abnormalities of specific myelin components leading to genetic diseases, experimental models of demyelination (experimental autoimmune encephalomyelitis, cuprizone intoxication, lysolecithin-induced demyelination, and ethidium bromide treatment are also described). During myelin repair, a thinner myelin sheath is produced with shorter internodes and efficient nerve conduction is produced. Dysfunction of astrocytes may be involved in some genetic diseases of myelin. There are many growth factors and transcription factors involved in the process of myelination and demyelination among which eukaryotic initiation factor 2B (elf2B) leading to vanishing white matter disease (CACH). The role of hormones and sexual dimorphism of oligodendrocytes and myelin anre also described. New areas of research are being developed showing the involvement of myelin deficiency in psychiatric diseases and cognition.