Biology of Demyelinating Diseases 545 We do not discuss the axonal forms of CMT, although they may involve secondary demyelination. Axonal loss is also observed in the demyelinating type 1 CMT (Bjartmar et al., 1999). There are some rare autosomal recessive forms called CMT4 which are also demyelinating. Acute autoimmune demyelinating polyneuropathies such as Guillain–Barré syn- drome and chronic polyneuropathies may involve some glycolipid antigens, as described later. 2.5.3 Ion Channels and Demyelination Voltage-gated ion channels of the Na v 1.6 type are normally localized at the Ranvier node. In experimental models of demyelination as well as in MS lesions, a diffuse distribution of Na v channels along the naked demyelinated axon has been reported. In addition, there is a reversion of the Na v channel from a mature Na v 1.6 expression to an immature Na v 1.2 isoform and this may limit axonal injury (Craner et al., 2004a, b). However, in addition to this diffuse distribution of Na v channels, loose clusters of Na v channels persist on some denuded axons. In demyelinated plaques, nodal, paranodal, and juxtanodal axonal molecules are diffusely distributed along the naked axons. Potassium channels Kv1.1 and Kv1.2 are normally confined to the juxtaparanodes. Demyelination can give rise to dispersion of these channels. During myelin repair, a thinner myelin sheath is produced with shorter intern- odes, but efficient nerve conduction is produced (Smith et al., 1979). The aggre- gation of nodal, paranodal, and juxtaparanodal axonal molecules recapitulates the pattern observed during development with the initial step being Na v channel clustering (Coman et al., 2005; Ogawa et al., 2006). 2.5.4 Astrocytes and Demyelination Astrocytes are situated in key positions among microvessels, neurons, and oligo- dendrocytes where they participate in a wide range of functions during brain construction and maintenance. A pattern of early and active astrocyte involve- ment in several neurodegenerative disorders is emerging. The detrimental role of activated astrocytes during neuroinflammation was recently demonstrated in vivo (Vesce et al., 2007). Thus, astrocytes are now considered as important regulators of many neurophysiological processes and neuropathological conditions (Baumann and Pham-Dinh, 2002; Giaume et al., 2007; Oberheim et al., 2006; Tian et al., 2005, 2007). Moreover, they have also been recognized as new players in CNS myelination via the secretion of molecules signaling oligodendrocytes to myelinate (Ishibashi et al., 2006). As a consequence, myelin could be the target of numerous types of afflictions, from inflammatory/autoimmune (multiple sclerosis) to genetic (leukodystrophies) diseases, which can involve astrocyte dysfunction. In further support of the important role of astrocytes in neurodegenerative dis- orders, the causative genes implicated in three leucodystrophies were recently characterized at the genetic level. Indeed the genes responsible for Alexander dis- ease, CACH/VWM syndrome, and “megalencephalic leukoencephalopathy with 546 D. Pham-Dinh and N. Baumann subcortical cysts” (MLC1), have been identified (GFAP gene, the 5 eIF2B (eukary- otic initiation factor 2B) genes and MLC1, respectively). These three diseases share some similarities: cavitated white matter lesions; sensitivity to different forms of stress, such as febrile episodes or head trauma, triggering episodes of rapid neu- rological deterioration; and astrocytes as primary target cells. GFAP expression is specific to nonmyelinating glial lineages, as is the MLC gene (Teijido et al., 2007), but EIF2B is ubiquitously expressed. However, the eIF2B mutations are specifically deleterious in astrocytes (Dietrich et al., 2005). 2.5.5 Experimental Models of Demyelination in the CNS Experimental autoimmune encephalomyelitis (EAE) is the most important animal model of MS. EAE is induced by injection (active EAE) in presence of immune stimulating adjuvants, of CNS tissue, whole spinal cord, purified myelin, or myelin proteins or peptides in susceptible animals (reviewed in Bradl and Linington, 1996; Lassmann, 2004). Specific myelin proteins have been used as immunogens (see below). The onset, severity, and nature of the disease (demyelinating or predomi- nantly inflammatory) are extremely variable and depend on the genetic background of the animal (species and strain differences) and on environmental factors such as the dose and nature of the sensitizing antigen and adjuvant. The initial model was generated to understand acute disseminated encephalomyelitis. Later versions of more chronic EAE have been developed with pathology including demyelina- tion, axonal damage, and clinical events such as relapsing and remitting episodes of paralysis, all of which are features common to MS (Steinman and Zamvil, 2006). EAE has led to therapies approved for use in MS. Cuprizone intoxication. The presence of the copper chelator cuprizone (bis- cyclohexanone oxalydihydrazone) in the diet of young adult mice produces a mas- sive demyelination of certain brain regions (Suzuki and Kikkawa, 1969;Blakemore, 1973a; Ludwin, 1978, 1994). Mice exhibit neurological symptoms in the late stages of exposure to cuprizone. Remyelination occurs if the metabolic insult is removed before the end of six weeks treatment (Blakemore, 1973b). After a period of recov- ery of a few weeks on a normal diet, animals appear normal neurologically and myelin is regained (Matsushima and Morell, 2001). The corpus callosum is one area that is preferentially affected by exposure to cuprizone as well as the cerebellar peduncles (Blakemore, 1973b). Although there is rapid regeneration of the oligo- dendrocyte population following an acute lesion, most of these newly regenerated cells undergo apoptosis if mice remain on a cuprizone diet. Interestingly, even if the mice are returned to a normal diet following 12 weeks of exposure to cuprizone, remyelination and oligodendrocyte regeneration do not occur (Mason et al., 2004). The mechanisms of the selective effect of cuprizone on oligodendrocytes in certain areas of the CNS need to be elucidated. Lysolecithin-induced demyelination. Injection of lysolecithin into the spinal cord causes a dramatic decrease in the levels of some myelin protein transcripts (MBP and PLP/DM20). Myelin protein gene expression associated with myelinogenesis Biology of Demyelinating Diseases 547 during remyelination f ollows a similar pattern to that of myelinogenesis during development (Woodruff and Franklin, 1999). One week after injection, very little extracellular myelin debris is detected and remyelination has begun. Remyelination progresses rapidly so that almost all axons are engulfed by myelin sheaths by the end of the third week. Remyelination is accompanied by a prominent astrocytosis. Ethidium bromide treatment. Intracisternal injection of ethidium bromide induces spongiosis with prominent degenerative changes in oligodendroglia in the sub- pial regions of the rat CNS. Chronologic investigation of the lesions has revealed that status spongiosus results in myelin degeneration, and by the sixth day postin- jection many axons are demyelinated. Vesicular transformation of myelin is the common degenerative change. In the demyelinated areas, oligodendroglial cells disappear completely. By the twelfth day postinjection, remyelination is apparent and numerous active oligodendroglia appear in association with thinly myelinated axons. Locally produced IGF could partly be involved in some of the mechanisms underlying remyelination in the mouse spinal cord (Fushimi and Shirabe, 2004). 3 Dys- and Demyelination in Relation to Specific Constituents Myelin disorders can have a genetic, toxic, or infectious origin and some such as MS even an immunological component. For most of these diseases, there are ani- mal models. Whatever the causes, the targets are structural or metabolic elements necessary for i ntact myelin. Diseases involving myelin are often related to specific myelin constituents whether common to CNS and PNS, or different. One can believe that these specialized compounds play significant roles in the pathology of myelin. Thus there are diseases or experimental models that involve only CNS or PNS, and others that involve both systems. To understand the role of the different constituents of oligodendrocytes, Schwann cells, and myelin, and the cellular interactions, it is necessary to consider each constituent and the abnormalities that occur in experimental models and in human pathology. Each specific compound of myelin, and many factors i nvolved in myeli- nation, can give rise to genetic diseases or autoimmune diseases. There are many animal models and human diseases that help in understanding the demyelination process. It is clear that specific components are particular targets for diseases, whether these are genetic or acquired. These components, although they may be present in other cell types or tissues, must be viewed as modular elements that, with other elements, generate complex and unique surface patterns in the nervous system. 3.1 CNS Myelin Proteins Major myelin proteolipid proteins, PLP and DM-20.In1951 Folch and Lees dis- covered that a substantial amount of proteins from brain white matter could be 548 D. Pham-Dinh and N. Baumann extracted by organic solvent techniques. They were given the generic name of pro- teolipids (PLP), as these proteins were lipid–protein complexes. The responsible gene is PLP1. The PLP1 gene encodes two major products, PLP itself, and DM-20 encoded by an alternatively spliced transcript that lacks 35 residues from the cyto- plasmic domain of PLP. PLP represents about half the protein mass of CNS myelin (reviewed in Garbern, 2005). PLP is also present in the PNS; however, it constitutes only a small fraction of myelin proteins in the periphery (Pham-Dinh et al., 1993). PLP is a highly hydrophobic transmembrane protein. In addition to a high content of hydrophobic amino acids, PLP is anchored to the lipid bilayer by fatty acylation of several cysteine residues. PLP and DM-20 are both acylated by covalent link- age of mainly palmitic, oleic, and stearic acids to cysteine residues in regions of the PLP/DM-20 proteins localized on the cytoplasmic side of the myelin membrane (Weimbs and Stoffel, 1992). DM-20 is present at about 10% of the level of PLP in the CNS whereas in the PNS they are present in approximately equivalent amounts. It is worth noting that there is a 100% sequence identity between rodent and human PLP proteins (Macklin et al., 1987), which is a rather striking conservation. The PLP1 gene is located on the X chromosome (Xq22 in humans) (Mattei et al., 1986). There are a great number of spontaneous myelin PLP1 gene mutants (Duncan, 2005). Myelin mutants are often named according to their phenotypes. Among the mutations are the jimpy, jimpy msd and jimpy 4 J mice. There is also a myelin- deficient rat (md rat). A long-lived md rat has been described (Duncan et al., 1995) that has recently allowed new therapeutic approaches (Espinosa-Jeffrey et al., 2006). A canine shaking pup has been also described. The rumpshaker mouse and the paralytic tremor rabbit (pt) have a normal life span. Null mutations are informative for inferring the normal biological functions of a gene product. Despite its abundance in normal CNS myelin, complete lack of PLP and DM20 results in a surprisingly mild phenotype in mice and humans; PLP appears not to be essential for oligodendrocyte differentiation and myelina- tion. The most striking pathology in both mice and humans with PLP null mutations is a relatively late and progressive degeneration of axons (Garbern et al., 2002). Interestingly, although PLP/DM20 represents less than 0.01% of peripheral nerve myelin, complete lack of these proteins causes a demyelinating peripheral neu- ropathy (Garbern et al., 1997). In the PLP-null CNS, axons large enough to be myelinated often lack myelin entirely or are surrounded by abnormally thin sheaths. Short stretches of cytoplasm persist in many myelin lamellae. In thinly myelinated fibers, there are interlamellar spaces across the full width of the sheaths. In thick myelin sheaths, the spaces appear irregular but diffuse. These spaces constitute a spiral pathway through which ions and other extracellular agents may penetrate gradually, possibly contributing to the axonal damage known to occur in this mutant, especially in thinly myelinated fibers, where the spiral path length is shortest. The “radial component” of myelin is distorted in the mutant (“diagonal component”), extending across the sheaths at 45 ◦ instead of 90 ◦ . These observations indicate a direct or indirect role for PLP in maintaining myelin compaction along the external surfaces of the lamellae and to a limited extent along the cytoplasmic surface as well, Biology of Demyelinating Diseases 549 and also in maintaining the normal alignment of the radial component (Rosenbluth et al., 2006). Absence of PLP may give rise to abnormal axonal transport (Edgar et al., 2004). In some mutants and transgenic mice overexpressing PLP1, there is an accu- mulation of PLP in the endoplasmic reticulum of the oligodendrocyte, which may eventually trigger apoptosis. The latter is related to an unfolded protein response, UPR (Gow and Sharma, 2003). In these forms, increasing the level of PLP may exacerbate pathology (Gow et al., 1998). Increased PLP1 gene dosage affects expression of other myelin proteins, partic- ularly MBP which is lower in homozygotes in both myelin and early myelinating oligodendrocytes (Karim et al., 2007). As shown in the jimpy mouse, there is a drastic decrease of the 14 kDa isoform of MBP (Campagnoni et al., 1984). Pelizaeus–Merzbacher disease (PMD) in an X-linked leukodystrophy primarily associated with the duplication, deletion, or mutation of the PLP1 gene. The princi- pal effect of many mutations in the coding region of PLP1 is to disrupt the highly ordered structure of the resulting protein isoforms DM20 and PLP. This leads to their accumulation in the endoplasmic reticulum of oligodendrocytes and ultimately to diminished biosynthetic capacity or survival of these cells. The UPR protein response modulates disease severity in Pelizaeus–Merzbacher disease (Southwood et al., 2002). PMD patients in which PLP1 is deleted or functionally null may well benefit from gene replacement (Gow et al., 1998). A spastic paraplegia type 2 (SPG- 2) phenotype is also caused by a PLP1 mutation. This milder form resembles that in the rumpshaker mutant mouse. As noted above, experimental autoimmune encephalomyelitis (EAE) has been studied for decades as an experimental model for MS (Steinman and Zamvil, 2006). A PLP peptide (139–151) induces a chronic relapsing disease; combinations of antigens are also used (Kuerten et al., 2007). The strain of mouse influences the expression of EAE (Kuerten et al., 2007). As gene knock-out and knock-in mice are also becoming increasingly indispensable for mechanism-oriented studies, animal models in the mouse have been increasingly useful. EAE has also been provoked using a proteolipid suppressor of cytokine signaling 1 (PLP/SOCS1) transgenic mouse line that displays suppressed oligodendrocyte responsiveness to interferon- gamma; mice under these conditions develop an accelerated onset and increased oligodendrocyte apoptosis (Balabanov et al., 2007). Myelin basic proteins (MBP). Basic proteins are abundant both in CNS and PNS myelin, where they are associated with negatively charged lipids. They are assumed to be involved in myelin compaction on the cytoplasmic side of the membrane bilayer. A spontaneous MBP mutant, the shiverer mouse, is devoid of the major dense line of myelin in the CNS (Dupouey et al., 1979). Another such mutant, the Long Evans Shaker (LES) rat, shows major changes in spinal cord white matter, with dispersed labeling of Kv1.1 and Kv1.2 K+ channel subunits as well as of Caspr, a molecule normally confined to paranodes along LES rat spinal axons (Eftekharpour et al., 2005). MBP constitutes about 30% of the protein content of myelin. In fact, the MBPs constitute a family of proteins comprising many isoforms (reviewed in Campagnoni 550 D. Pham-Dinh and N. Baumann and Skoff, 2001). The molecular weight of the major forms are 21.5, 20.2, 18.5, and 17.2 kDa in man, and 21.5, 18.5, 17, and 14 kDa in mouse. In the adult, two major isoforms constitute about 95% of the MBPs; they are the 18.5 and 17.2 kDa isoforms in humans, and the 18.5 and 14 kDa in mouse (Staugaitis et al., 1990). The MBP isoforms are coded by alternative transcripts generated from the MBP gene which consists of 7 exons (Roach et al., 1983). Subsequent studies have found that the classical MBP gene is contained within another and huge transcription unit called the Golli-MBP gene (golli for “gene expressed in the oligodendrocyte lineage”). It is 195 kb in mice and 179 kb in humans, and produces a number of alternative transcripts from three possible transcription start sites, most of them containing the MBP sequences. The Golli-MBP gene contains three additional specific exons located 5 to the seven constituting the classical MBP gene. The Golli-MBP gene is located on chromosome 18 in mouse and human (18q23). The MBP Golli transcripts and proteins are also found in immune system cells (reviewed in Feng, 2007): they have recently been found to directly regulate T- cell activation, thus modulating EAE induction. MBP mRNAs are transported to glial processes to be translated on free ribosomes. Posttranslational modifications can occur on the MBPs, including phosphorylation, methylation, citrullination, and N-terminal acetylation. Moreover, the presence of exon 6-containing MBPs in the nucleus suggests a regulatory role in myelination for these MBPs isoforms (reviewed in Baumann and Pham-Dinh, 2001; Campagnoni and Macklin, 1988). In the PNS, MBPs represent 5–20% of the total PNS myelin protein content and are located on the intracellular side of the myelin. Contrary to what occurs in the CNS, the absence of MBP isoforms does not alter the major dense line of myelin in the PNS. The exon-6 containing MBP isoforms of 17 and 21.5 kDa in the mouse, and 20.2 and 21.5 kDa in humans, are mainly expressed during myelinogenesis. They are re-expressed in chronic lesions of MS, and their re-expression correlates with remyelination (Capello et al., 1997). Some isoforms are modified as consequences of mutations on other myelin proteins, especially PLP (see above). MBP or MBP peptides are very commonly used to induce EAE. The MBP- induced disease in some strains of mice is often monophasic with inflammation and no demyelination. The mice recover completely after a single episode of a short and acute disease and become resistant to reinduction of EAE. Therefore, a combination of antigens involving proteins or peptides (Kuerten et al., 2006)or sphingoglycolipids (Raine et al., 1981) are often used to induce demyelination. A genetic disease, the 18q-syndrome is a rare leukodystrophy presenting a genomic deletion that includes the MBP gene. Proton magnetic resonance data indicate demyelination or increased myelin turnover rather than dysmyelination (Hausler et al., 2005). Recent evidence obtained via magnetic resonance imaging and spectroscopy techniques supports the view that the normal-appearing white matter (NAWM) in the MS brain is altered. Several biochemical changes in NAWM have been determined. These include the cationicity of myelin basic protein (MBP) as a Biology of Demyelinating Diseases 551 result of peptidyl arginine–deiminase (PAD) activity converting arginyl residues to citrulline. The accompanying loss of positive charges renders myelin suscepti- ble to vesiculation and MBP more susceptible to proteolytic activity. An increase of MBP autocatalysis in the MS brain might also contribute to the generation of immunodominant epitopes (Mastronardi and Moscarello, 2005). OSP/claudin-11 (Oligodendrocyte-specific protein). OSP/claudin-11, a 22-kDa protein, is the third most abundant protein in CNS myelin, after PLP/DM20 and MBP. It accounts for about 7% of the protein content. OSP is related to PMP-22 found in PNS myelin, with which it has 48% amino acid similarity and 21% identity. OSP was recognized as a previously known tight junction protein, claudin-11. The gene for OSP is located on chromosome 3 in the mouse and the 3q26.2-26.3 region of chromosome 3 in humans. Myelin compaction is not significantly disrupted in the knock-out mouse. This is not the case for a double knock-out OSP/Claudin 11 and PLP1/DM20, indicating that these proteins have essential structural functions in maintaining myelin compaction, but that there is redundancy in their functions (Chow et al., 2005). K+ channel Kv3.1 associates with OSP/claudin 11 and regulates oligodendrocyte development (Tiwari-Woodruff et al., 2006). MAL (myelin and lymphocyte protein) (formerly rMAL for rat MAL) was the first cloned member of a new myelin–oligodendrocyte proteolipid protein family (MAL family) including MVP17 (myelin vesicular protein of 17 kDa) and Plasmolipin. MAL is a tetraspan raft-associated proteolipid predominantly expressed by oligodendrocytes and Schwann cells. Genetic ablation of MAL leads to reverted paranodal loops away from the axon, with a marked reduction of contactin-associated protein/paranodin, neurofascin 155, and the potassium chan- nel Kv1.2, whereas nodal clusters of sodium channels remain unaltered. MAL has a critical role in the maintenance of CNS paranodes, likely by controlling the traffick- ing and/or sorting of NF155 and other membrane components in oligodendrocytes (Schaeren-Wiemers et al., 2004). MAL is modified in neurological mutants affect- ing myelination through a defect in the catabolism of sphingoglycolipids (Saravanan et al., 2004). Connexins (Cx) 32 and 47 are part of a family of gap junction proteins, which form channels, generally between adjacent cells. These specialized channels span two plasma membranes of adjacent cells and allow the passage of ions, amino acids, second messenger molecules, and small metabolites. In general, six connex- ins oligomerize to form a homomeric or heteromeric connexon (hemichannel), and a functional gap junction pathway between two cells is formed by homotypic or heterotypic interactions of two connexons. In myelin, Cx32 and 47 may form chan- nels between adjacent layers of the myelin sheath in regions where myelin is not compacted. Cx32 is a ubiquitous protein, also found in CNS myelin. Cx32 has a molecu- lar weight of 32 kDa and contains four transmembrane domains, two extracellular loops, and three intracellular domains. It is expressed widely in a number of tissues, particularly in the liver, and also in PNS and CNS myelin. Cx32 is found in non- compact regions (paranodes and Schmidt–Lantermann incisures) of the PNS myelin sheath (Scherer et al., 1995). The human Cx32 gene is located on chromosome 552 D. Pham-Dinh and N. Baumann Xq13.1. Its structure is similar to all connexin genes, that is, a large exon containing the coding sequence within one uninterrupted block, which is separated by an intron from a small noncoding exon located on the 5 -flanking region. Three alternative promoters, that appear to be activated in a cell-type manner, regulate the tissue- specific expression of Cx32. Its presence in Schwann cells was discovered when Cx32 mutations were associated with Charcot–Marie–Tooth disease of the CMTX type (see § PNS). This gap-junction molecule allows the direct passage of ions and small molecules through the myelin sheath in the paranodal regions. There are subtle abnormalities of the myelin sheath and of the Ranvier node (Hahn et al., 2001). In humans, Cx47 is expressed specifically in oligodendrocytes, where it is par- tially colocalized with Cx32. The gene encoding Cx47 is regulated in parallel with myelin genes. Mice lacking either Cx47 or Cx32 are viable. However, ani- mals lacking both connexins die by postnatal week 6 with profound anomalies in central myelin, characterized by thin or absent myelin sheaths, vacuolation, enlarged periaxonal collars, oligodendrocyte cell death, and axonal loss. Thus gap- junction communication is crucial for normal central myelination (Menichella et al., 2003). Connexin 47 (gap junction protein alpha 12) mutations cause a Pelizaeus– Merzbacher-like disease ( Orthmann-Murphy et al., 2007). Connexin 47 is also involved in peripheral myelination in humans (Uhlenberg et al., 2004). Tetraspanin 2 has recently been identified in cells of the oligodendrocyte lin- eage. Expressed after birth in rodents, tetraspanin may play a role in signaling in oligodendrocytes at the early stages of their terminal differentiation into myelin- forming glia; it is also hypothesized that it may function in stabilizing the mature sheath. Myelin-associated/oligodendrocyte basic protein (MOBP) is a small highly basic protein. Alternative splicing generates three isoforms of 8.2, 9.7, and 11.7 kDa. Like MBP, MOBPs are located in the major dense line of myelin where they could play a role similar to that played by MBP in myelin compaction. MOBP transcripts are less abundant than PLP1 but more than those for the CNP (cyclic nucleotide phosphodiesterase). The MOBP mRNA is initially located in the cell bodies of the oligodendrocytes, and moves distally into the processes when myelination occurs, as do MBP mRNAs. The gene for MOBP has been mapped to chromosome 9 in the mouse, in a region syntenic with the human chromosome 3 (3p22). Myelin-associated glycoprotein (MAG) has an apparent molecular weight of 100 kDa, of which 30% is carbohydrate; MAG bears t he L2/HNK1 epitope, a gly- cosylated epitope also present on glycolipids of the PNS. It is a minor constituent, representing 1% of the total protein content in CNS myelin and 0.1% in PNS myelin. It has been extensively reviewed recently (Quarles, 2007). Two MAG isoforms have been identified, large MAG (L-MAG) and small MAG (S-MAG), corresponding to polypeptides of 72 and 67 kDa, respectively, in the absence of glycosylation. MAG proteins have both a membrane-spanning domain and an extracellular region containing 5 immunoglobulin domains. The MAG gene includes 13 exons (from which exons 1, 2, and 3 are noncoding); the isoforms differ only in their cytoplasmic domains, resulting from alternative splicing. Exon 12, present in S-MAG, contains Biology of Demyelinating Diseases 553 an alternative stop codon. MAG is located on chromosome 7 in mice and 19 in humans (19q13.1). The 72 kDa L-MAG can be phosphorylated and acylated. MAG is found in the periaxonal space in CNS and PNS; it derives from the Schwann cell and oligodendroglial membrane. CNS findings suggest that the absence of MAG causes oligodendrocytes to form myelin less efficiently during development and to become dystrophic with aging. MAG, together with the proteins Nogo 66 and Omgp, inhibits axonal regeneration. MAG binds to the Nogo 66 receptor called NgR; this activates a P75 neurotrophin receptor (p75NTR) and the transduction of the resulting signal activates the small GTPase Rho leading to inhibition of axonal growth following injury (Spencer et al., 2003; Filbin, 2003). Although MAG binds to gangliosides, sialic acid binding is unnecessary for MAG to exert inhibition (Cao et al., 2007). MAG is also currently well known as an antigen for IgM mono- clonal antibodies that cause demyelinating peripheral neuropathies (reviewed also in Quarles, 2007). It comprises the HNK-1 epitope present on the sulfated glycolipid with glucuronic acid SGPG, that is, sulfated glucuronylparagloboside (see below). Interestingly the sensory-motor neuropathies with monoclonal IgM that react with both lipid and proteic antigens are demyelinating, and the neuropathies with IgM monoclonal antibodies that react with SGPG and not MAG are axonal (Chassande et al., 1998). Myelin-oligodendrocyte glycoprotein (MOG) was first identified as the anti- gen responsible for the demyelination observed in animals injected with whole CNS homogenate; it was later identified as a minor glycoprotein specific for CNS myelin. MOG was further characterized by immunological methods using a mouse monoclonal antibody obtained against glycoproteins of rat cerebellum. MOG is a minor protein of 25 kDa with some glycosylation resulting in doublets of 26– 28 kDa on SDS page, which can form dimers of 52–54 kDa. MOG is only present in mammalian species. In humans, MOG expresses the L2/HNK1 epitope. The amino-terminal, extracellular domain of MOG has characteristics of an Ig-variable domain and is 46% identical with the amino-terminus of bovine butyrophilin pro- tein expressed in the mammary gland, and chick histocompatibility BG antigens. Although MOG contains two highly hydrophobic regions, only one is a truly trans- membrane domain, thus MOG presents the same topology as other members of the Ig-superfamily. The human MOG gene is encoded by 11 exons that exhibit a complex pattern of alternative splicing (Pham-Dinh et al., 2004; Delarasse et al., 2006). Complex alternative splicing of MOG is unique to human and nonhuman primates (Delarasse et al., 2006). The MOG gene is located in the distal part of the major histocompatibility complex (MHC) in the class Ib region on chromosome 6p22-p21.3 in humans and 17 in rodents. MOG is specific to the CNS and localized on the outer surfaces of myelin sheaths and oligodendrocytes where it is accessi- ble to components in the external environment, such as complement and antibodies. MOG is a highly encephalitogenic autoantigen and a target for aggressive autoim- mune responses in CNS inflammatory demyelinating diseases (Delarasse et al., 2003). Autoantibody responses against conformational epitopes of MOG have the power to destroy myelin, as demonstrated in the marmoset model of human MS 554 D. Pham-Dinh and N. Baumann (von Budingen et al., 2004, 2006). Controversy exists regarding the pathogenic or predictive role of anti-MOG antibodies in patients with MS (Lalive et al., 2006; Pittock et al., 2007). 2 3 -Cyclic nucleotide-3 -phosphodiesterase (CNP) represents 4% of total CNS myelin proteins. In vitro, this protein hydrolyzes artificial substrates, 2 3 -cyclic nucleotides into their 2 derivatives. However, the biological role of this enzymatic activity is obscure because 2 3 nucleotides have not been detected in the brain. Overexpression of CNP in transgenic mice disturbs myelin formation and creates aberrant oligodendrocyte membrane expansion. CNP appears on SDS-page as a doublet of two peptides, with molecular weights of 48 and 46 kDa, referred as CNP2 and CNP1, respectively. The two CNP isoforms are produced by alternative use of two transcription start sites. The CNP gene is located on chromosome 17 (17q21) in human and chromosome 11 in mouse. CNP mRNAs are detected in mouse spinal cord during embryonic stages. CNP is present in the cytoplasm of noncompacted oligodendroglial ensheathment of axons and in the paranodal loops of myelin intern- odes. The protein is posttranslationally modified, acylated, and phosphorylated. CNP (mainly CNP1) is associated by isoprenylation to the cytoplasmic plasma membrane of the oligodendrocyte. In double knock-out mice, inactivation of both CNP1 and FGF-2 lead to hyperactivity, starting around two weeks of age. When hyperactive mice receive dopamine receptor antagonists or catecholamine synthe- sis inhibitors, their behavior reverts to normal, suggesting that their symptoms are caused by a dysregulation in the dopaminergic system. The molecular mechanisms leading to hyperactivity have not yet been elucidated. This mouse model supports the evidence cited above that oligodendrocytes and myelin may be involved in the genesis of neuropsychiatric disorders, and also that it is almost impossible to predict the impact of genetic interactions on the behavior of transgenic animals (Kaga et al., 2006). Nogo proteins, formerly named NI-35/250 proteins, are membrane-bound pro- teins highly enriched in mammalian CNS myelin and oligodendrocytes (reviewed in Bandtlow and Schwab, 2000; Goldberg and Barres, 2000). Nogo comprises three isoforms, Nogo-A, -B, and -C. It is predominantly associated with the endoplasmic reticulum of the oligodendrocyte. Following injury, Nogo would become exposed to the extracellular environment. MAG, Nogo, and OMgp share the same functional receptor (Spencer et al., 2003; Filbin, 2003). As mentioned previously, they inhibit axonal growth following injury. Antibodies against Nogo or Nogo-blocking pep- tides enhance sprouting of damaged axons after a partial spinal cord section in a primate, the marmoset, and this is associated with clinical improvement (Fouad et al., 2004). Nogo receptor-interacting protein (LRR and Ig domain-containing Nogo receptor-interacting protein, LINGO-1) is a negative regulator of oligoden- drocyte differentiation and myelination. Antagonism of LINGO-1 or its pathway is a promising approach for treatment of demyelinating diseases in the CNS (Mi et al., 2005, 2007). Enzymes. Many enzyme activities have been found in myelin: neuraminidase, cholesterol ester hydrolase, lipid synthesizing and catabolizing enzymes, proteases, protein kinases, and phosphatases. Two of them have been especially characterized. . elucidated. Lysolecithin-induced demyelination. Injection of lysolecithin into the spinal cord causes a dramatic decrease in the levels of some myelin protein transcripts (MBP and PLP/DM20). Myelin protein gene. a Biology of Demyelinating Diseases 551 result of peptidyl arginine–deiminase (PAD) activity converting arginyl residues to citrulline. The accompanying loss of positive charges renders myelin suscepti- ble. oligodendroglia appear in association with thinly myelinated axons. Locally produced IGF could partly be involved in some of the mechanisms underlying remyelination in the mouse spinal cord (Fushimi