Biology of Demyelinating Diseases 555 1. UDP-Galactose: ceramide galactosyltransferase (CGT) is found mainly in myelinating glia where it parallels expression of MBP and PLP1. The CGT gene spans about 70 kb, comprises 5 exons and has been mapped to mouse chromosome 3 bands E3-F1 and to the human chromosome 4 band q26. Rodent and human CGT sequences are strongly conserved. More details on CGT are in the paragraph related to myelin lipids, below. 2. Fyn kinase is a nonreceptor-type tyrosine kinase that has been proposed to act as a signaling molecule downstream of MAG. MAG and Fyn are coexpressed in oligodendrocytes, and can be coimmunoprecipited by biochemical methods. Fyn- null mice show an important reduction (about 50%) of CNS myelin, whereas myelination is quite normal in the MAG-null mice (see above). Double-deficient mice present a massive hypomyelination, associated with behavioral deficits. These data indicate the importance of both molecules in the initiation of myelination; how- ever, they could also mean that MAG and Fyn act in concert or independently in initiating myelination (Biffiger et al., 2000). 3.2 PNS Myelin Proteins PNS myelin proteins include two abundant constituents: glycoprotein zero (P0) and MBPs (already discussed in III.1), and a set of minor ones including PMP22, MAG, and connexin 32 (already discussed in III.1). Protein zero (P0) is the most abundant glycoprotein of peripheral myelin; it rep- resents 50–70% of the total myelin protein content. P0 is specific for the PNS. It is predominantly localized to compacted regions of myelin. P0 is expressed constitutively in neural crest and embryonic nerves; its expression is strongly upreg- ulated in myelinating Schwann cells. P0 has a molecular weight of 28 kDa, and is composed of an Ig-like extracellular domain by which P0 can form tetramers, a single highly hydrophobic transmembrane domain and an intracellular cytoplasmic domain that contains abundant positive-charged amino acids that could stabilize negative-charged lipid heads. The gene encoding the P0 protein, called MPZ for myelin protein zero, contains 6 exons and is on chromosome 1 in humans (1q22). Interestingly, P0 is essential for normal spacing of PNS compact myelin (reviewed in Trapp et al., 2004). CMT1B is an autosomal dominant demyelinating hereditary neuropathy involving the MPZ gene, that is, a CMT disease (Dubourg, 2004;Shy, 2004). Other mutations of this gene cause severe neuropathies of infancy (Dejerine– Sottas disease), and still others lead to disability with a late onset (Shy et al., 2004). MPZ mutations disrupt the tertiary structure of P0 protein, interfering with P0- mediated adhesion during myelination and with myelin compaction. In contrast, late onset neuropathies result from mutations that allow myelination but chronically dis- rupt Schwann cell–axonal interactions. A genotype/phenotype correlation is clear even though penetrance can vary within single families. Peripheral myelin protein 22 (PMP22) has a molecular weight of 22 kDa and represents 2–5% of the total myelin protein content. Despite its name, PMP22 is not specific to the PNS inasmuch as it is expressed, albeit at low levels, in other 556 D. Pham-Dinh and N. Baumann tissues such as lung, gut, heart, and some neurons. It contains four transmembrane domains with two extracellular loops and one short intracellular loop and is pref- erentially localized in compacted regions of myelin. The human PMP22 gene is about 40 kb long and is located on chromosome 17p12-p11.2. It contains 6 exons (4 coding exons and 2 untranslated exons in the 5 flanking region) and its expres- sion is regulated by two alternative promoters. Charcot–Marie–Tooth 1A (CMT1A), the most frequent genetic demyelinating neuropathy, is a clinical expression of an autosomal dominant mutation in this gene (Dubourg, 2004;Shy,2004). The most frequent mutation is a duplication of the PMP-22 gene. There is a spontaneous mouse model of PMP-22 mutation, the trembler-J mouse, which leads to failure of myelination and continuous Schwann cell proliferation; this implies trophic support by Schwann cells (Friedman et al., 1996). Therapies reducing PMP22 overexpres- sion in rodent models of CMT1A offer potential treatments. Progesterone is known to increase PMP22 messenger RNA expression in cultured Schwann cells. On this basis, Sereda and colleagues (Sereda et al., 2003) in Nave’s laboratory, using a progesterone receptor antagonist, onapristone, have been able to reduce the expres- sion of PMP22 in a transgenic rat, thus opening ways for symptomatic treatment of this form of the disease. In another mouse model overexpressing a human PMP 22 gene, ascorbic acid reduces PMP22 levels, improving the phenotype of this CMT1A model (Passage et al., 2004). Lipopolysaccharide-induced tumor necrosis factor (LITAF) is encoded on a gene located on 16p13.1-p12.3 in humans. It is a putative degradation protein also called SIMPLE (for small integral membrane protein of the lysosome/late endosome). Although SIMPLE is expressed in many cell types, when mutated it seems to cause only a demyelinating neuropathy, which suggests that the disease specificity may come from the impaired degradation of specific Schwann cell proteins (Shy, 2004). These mutations give rise to a rare form of CMT called CMT1C. Periaxin represents about 5% of the total PNS myelin protein content; it was given this name because of its specific localization in the periaxonal membranes of myelinated Schwann cells (Gillespie et al., 1994). Two periaxin isoforms have been identified: L-periaxin (147 kDa), localized to the plasma membrane of myelinat- ing Schwann cells, and S-periaxin (16 kDa). The latter occurs diffusely throughout Schwann cell cytoplasm as a cytoskeletal component, and also in the nucleus. The human periaxin gene is located on chromosome 19q13.13-q13.2. Mutations in the periaxin gene cause a form of demyelinating CMT, CMT4F (Dubourg, 2004; Meyer zu Horste et al., 2006;Shy,2004). These mutations alter myelin and also involve onion bulb formation with chronic processes of demyelination and remyelination. As noted in the preceding section, Cx32 is a ubiquitous protein that is also found in CNS myelin. Its presence in Schwann cells was discovered when Cx32 mutations were associated with Charcot–Marie–Tooth disease of the CMTX type. CMTX is an X-linked demyelinating neuropathy. The molecule is located in the paranodes. There are subtle anomalies of the myelin sheath and the Ranvier node (Hahn et al., 2001). Mutations in Cx32 gap junction protein compromise Schwann cell functions Biology of Demyelinating Diseases 557 and lead to impaired Schwann cell–axon interactions with subsequent pathology in both myelin and axons. Cx47 (description in § CNS) is another protein involved in peripheral myelina- tion in humans (Uhlenberg et al., 2004). Diseases associated with mutations of the gene encoding this protein have not been described, at least as yet. Mutations in several other genes are known to give rise to demyelinating CMT (reviewed in Dubourg, 2004; Meyer zu Horste et al., 2006). A gene, located on 10q21.1-q22.1, can cause early demyelination and CMT1D when mutated. The gene codes for EGR2 (early growth response 2 gene) which is a transcription factor also termed Krox-20. It allows the regulation of key genes coding for myelin proteins such as PMP22, P0, and Cx32, and gives rise to CMT4E when mutated. There are also mutations involving the gene NEFL (neurofilament light chain) which in some cases can give rise to demyelinating neuropathies. Mutations in the CMT4 genes lead to hypomyelination and onion bulb forma- tion. The gene for CMT4A is located on 8q13.q21.1 and codes for a protein called GDAP1, that is, ganglioside-induced differentiation-associated protein 1. Another mutated protein may be involved in CMT4B1: MTMR2 (myotubularin-related protein 2). MTMR2 is a dual specific phosphatase that participates in the dephos- phorylation of membrane phospholipids involved in the regulation of intracellular membrane trafficking. Mutations in CMT4B2 are related to mutations in MTMR13. MTMR13 is also known as set-binding factor (SBF2). Mutations in CMT4B1 and CMT4B2 lead to characteristic misfolding and redundant loops of myelin. CMT4C is related to a defect in the gene coding for protein KIAA and CMT4D with mutations coding for the gene NDRG1 (N-myc downstream regulated gene). All these muta- tions alter myelin and also involve onion bulb formation with chronic processes of demyelination and remyelination. Epithelial cadherin (E-cadherin) has a molecular weight of 130 kDa and is found in PNS myelin. E-cadherin is a protein of the superfamily of calcium-dependent cell adhesion molecules that can usually form adherent junctions. This protein has an N-terminal extracellular domain, a short transmembrane domain and a C-terminal intracellular domain. Basic protein P2 has a molecular weight of 15 kDa and is a member of a family of cytoplasmic lipid-binding proteins. Unlike other myelin proteins, its quantitative expression varies greatly according to species, ranging from less than 1% in rodent sciatic nerves up to 5–14% in human, bovine, and rabbit nerve. P2 is located on the cytoplasmic side of compacted regions of myelin. 3.3 Proteins and Specific Lipids of the Node of Ranvier and the Paranodal and Juxtaparanodal Areas The nodes of Ranvier are critical for the proper function of the CNS and PNS. The composition of the myelin node and the modifications observed in natural mutant 558 D. Pham-Dinh and N. Baumann mice, transgenic mice, or knock-out mice have aided understanding of the function of these specialized molecules. Voltage-gated sodium channels (see § Ion Channels and demyelination), ankyrin G, NrCAM (NgCAM-related CAM; i.e., neuron–glia- related CAM) are highly enriched at the node (Simons and Trajkovic, 2006). Other constituents are also present and detailed below because they are more involved in demyelination. Oligodendrocyte-myelin glycoprotein (OMgp) is clustered at the nodes of Ranvier in both CNS and PNS. It regulates nodal formation through an unidentified mechanism. In the CNS, its abundance is closely linked with axonal size, and OMgp is undetectable in a subset of smaller axons. In transgenic mice in which expression of OMgp is downregulated, myelin thickness diminishes, lateral oligodendrocyte loops at the node–paranode junction are less compacted, and there are shortened nodal gaps (Nie et al., 2006). Transgenic mice, in which OMgp is reduced by 50–70%, show a significant abnormality in the node–paranode junction. Disorganized lateral glial loops at the node–paranode junctions and shortening of nodal space in the OMgp mutants strongly indicate that OMgp has a role in attaching lateral loops and in demarcation of the node–paranode junctions. Thus, this oligo- dendrocyte protein is involved in regulation of both myelin development and nodal formation. Nodal location of OMgp does not occur along demyelinated axons of either the shiverer natural mutant mice or PLP transgenic mice. Omgp is also a myelin-associated inhibitor of axonal regeneration as a GPI-linked Omgp (Spencer et al., 2003). The 186 kDa neuron specific isoform of the adhesion molecule neurofascin (NF186) is required for the clustering of voltage-gated channels at the node (Howell et al., 2006). Its expression is disrupted following demyelination. The myelin protein cyclic nucleotide phosphodiesterase (CNP) is required for maintenance of axon–glia interactions at the node of Ranvier. It also maintains the integrity of the paranodes (Rasband et al., 2005). In the PNS, both laminin and Schwann cell dystroglycan are necessary for the proper clustering of sodium channels at nodes of Ranvier (Occhi et al., 2005). Gliomedin is also necessary (Eshed et al., 2005). At the paranodes, myelin loops are anchored to axons through septate-like junc- tions characterized by the enrichment of paranodin/Caspr (contactin-associated protein) and the GPI-anchored cell adhesion molecule contactin. The paranodin/ Caspr–contactin complex interacts with the 155 kDa isoform of neurofascin NF155 that is expressed on the oligodendroglial membrane. NF155 is essential for the tight interaction between myelin and axon. It is a member of the L1 family of cell adhesion molecules (L1-CAM; reviewed in Maier et al., 2006). Changes to NF155 expression accompany inflammation and demyelination and contribute to the destruction of the neurofascin 186/sodium channel complex vital to successful neurotransmission in the CNS (Howell et al., 2006). The tetraspanin protein CD9 is a novel paranodal component regulating paran- odal junctional formation ( Ishibashi et al., 2004). Myelin galactolipids are essential for the proper formation of axo–glial inter- actions. Disruption of these interactions results in profound abnormalities i n the Biology of Demyelinating Diseases 559 molecular organization of the paranodal axolemma (Dupree et al., 1999). Mice incapable of synthesizing the abundant galactolipids of myelin exhibit disrupted paranodal axo–glial interactions in both the CNS and PNS. Whereas the clustering of the nodal proteins, sodium channels, ankyrin, and neurofascin are only slightly affected, the distribution of potassium channels and paranodin proteins is dramat- ically altered. The potassium channels, which are normally concentrated in the juxtaparanode, are no longer restricted to this region but are detected throughout the internode in the mutant. The paranodin/contactin-associated protein Caspr, a para- nodal protein, is not concentrated in the paranodal region, but diffusely distributed along the internodal regions. The paranodal junction also contains specialized cytoskeletal components that may be important in stabilizing axon–glia interactions (Ogawa et al., 2006; Voas et al., 2007). A significant alteration in NF155 paranodal structures occurs within and adjacent to actively demyelinating white matter regions that are associated with damaged axons (Howell et al., 2006). The juxtaparanode is just under the compact myelin sheath beyond the inner- most paranodal junction and may therefore be considered a specialized portion of the internode (Peles and Salzer, 2000). Potassium channels aggregate in the juxta- paranode. Potassium channels Kv1.1 and Kv1.2 are normally confined to this area. Demyelination can lead to the dispersion of these channels. The juxtaparanodal region, just beyond the innermost paranodal junction is enriched in shaker-type potassium (K v ) channels, in association with Caspr2, a sec- ond member of the Caspr family, as well as in the cell adhesion molecule TAG-1 (for review see Coman et al., 2006). Kv channels are in the juxtaparanode area (Howell et al., 2006). In demyelinating white matter, shaker-type Kv1.2 channels move and precede alterations at the node itself, in relation with NF155 disruption. TAG1 and Caspr 2 are essential for the molecular organization of the juxtaparanodal region of myelinated fibers (Traka et al., 2003). During myelin repair, NF 155 is an early marker of myelin damage (Howell et al., 2006). During myelin repair a thinner myelin sheath is produced with shorter intern- odes, but efficient nerve conduction is nevertheless produced (Smith et al., 1979). The aggregation process of nodal, paranodal, and juxtaparanodal axonal molecules recapitulate development, with the initial step being Na v channel clustering (Coman et al., 2006; Ogawa et al., 2006). 3.4 Dys- or Demyelination in the CNS and/or the PNS Related to Myelin Lipid Compounds One of the major characteristics of myelin lipids is their richness in sphingogly- colipids (SGLs) particularly the galactosphingolipids galactosylceramides (GalC) and their sulfated derivatives sulfogalactosylceramides, that is, sulfatides (Baumann and Pham-Dinh, 2001; Colsch et al., 2004). SGLs are present on virtually all mam- malian cell plasma membranes. They are amphipathic molecules consisting of a ceramide lipid moiety embedded in the outer leaflet of the membrane, linked to an 560 D. Pham-Dinh and N. Baumann oligosaccharide structure oriented externally. Both the lipid moiety and the oligosac- charide structure show huge structural diversity. SGLs are very abundant in the nervous system, with different constituents in the CNS and the PNS and within different cell types in these tissues. It is not clear yet whether each SGL isoform has a specific location or function, although the extreme diversity of their constitution possibly contributes to the diversity of stereospecific recognition at the surface of the cellular membranes. SGLs in myelin contain very long chain fatty acids. The fact that they are on the external surface of the cell favors their involvement in the mod- ulation of protein receptors and favors their acting as signaling molecules. These galactolipids are important for the activity and maintenance of myelin and myelin- producing cells (oligodendrocytes) and for the constitution of the Ranvier node area (Dupree et al., 1999; Honke et al., 2002). They are essential to the proper formation of axo–glial interactions and a disruption of these interactions results in profound abnormalities in the molecular organization of the paranodal axolemma (Dupree et al., 1999). Galactosylceramide (Galactocerebroside) synthesis and degradation in CNS and PNS. The inactivation of the CGT (ceramide galactosyl transferase) gene has allowed the analysis of galactolipid function (Coetzee et al., 1998). The mutants cannot synthesize major myelin lipids, galactosylceramide (GalC), and sulfogalac- tosylceramide (sulfatide). As explained in Section 3.3 myelin galactolipids are essential for the proper formation of axo–glial interactions and demonstrate that a disruption of these interactions results in profound abnormalities in the molecular organization of the paranodal axolemma. Many brain abnormalities are related to a defect in the catabolism of galacto- sylceramide, caused by galactocerebrosidase deficiency. There are similarities in mouse and human diseases, for example, the twitcher mouse and Krabbe’s dis- ease (Suzuki, 2003). Krabbe’s disease is a recessive autosomal disease caused by a deficiency in galactosylceramidase (galactocerebrosidase). It leads to demyelina- tion in CNS and often in PNS. Globoid cells, of macrophagic origin, are typical of this disease, and contain the undegraded substrate, galactosylceramide. There is a very early disappearance of oligodendrocytes, due to the accumulation of galac- tosylsphingosine (psychosine), a cytotoxic metabolite. Krabbe’s disease generally presents clinically in early childhood (∼6 months of age). The most common clini- cal manifestation is the onset of a paralysis in the four limbs (tetraplegia), but other neurological signs may occur. Peripheral nerve conduction velocity is also reduced. Decerebration follows rapidly, with total degradation of mental capabilities. Death occurs related to brain stem alterations. However, variant forms of this disorder are known. Isolated demyelinating peripheral neuropathies with no CNS involvement for a long period have been described, as well as Krabbe’s disease starting in ado- lescence and adulthood (Baumann and Turpin, 2000); in the latter, MRI shows that white matter involvement predominates symmetrically in the periventricular parieto- occipital regions. Up to now, there is no explanation for this late onset form with reduced brain regional alterations. The mouse model, the twitcher mouse, when on a mixed genetic background, gives rise to myelin alterations and also to neuronal death, especially in the hippocampus. Thus, some sphingolipids may have functions Biology of Demyelinating Diseases 561 in the hippocampal neuronal organization and maintenance (Tominaga et al., 2004), possibly in relation with modulator genes. Sulfogalactosylceramide (Sulfatide) synthesis and degradation in CNS and PNS. Regional and cellular abnormalities may also be related to defects in the synthe- sis of sulfatides. Sulfatides, coded by the galactosylceramide sulfotransferase gene, are essential for maintenance of Na + ion channels on myelinated axons but are not required for initial cluster formation (Ishibashi et al., 2002). Mice deficient for this gene are unable to synthesize sulfatides. They display abnormal paranodal junc- tions in the CNS and PNS, whereas their compact myelin is preserved (Honke et al., 2002). Recent work has evidenced that although sulfatide appears to play a lim- ited role in myelin development in comparison to galactocerebrosides, this lipid is essential for myelin maintenance, as the prevalence of redundant, uncompacted, and degenerating myelin sheaths as well as deteriorating nodal/paranodal structures is increased significantly in aged sulfatide-null mice as compared with wild-type lit- termates. The role played by sulfatide in CNS is not limited to the myelin sheath as axonal caliber is significantly altered in aged sulfatide-null mice (Marcus et al., 2006). Metachromatic leukodystrophy (MLD) is a disease involving a defect in sulfatide catabolism. The cause is deficiency of the catabolic lysosomal enzyme arylsulfatase A There is essentially a myelin deficiency, with an excess of sulfatides in myeli- nating cells, and also in neurons and macrophages (Gieselmann et al., 2003). In an experimental model, an arylsulfatase A-deficient mouse, the lysosomal sulfatide storage disease affects the lipid composition of myelin itself and the amount and localization of specific myelin membrane-associated proteins, particularly the pro- tein MAL (Saravanan et al., 2004). Sulfatiduria is an important element in the diag- nosis of MLD. Sulfatides in urine can be identified by thin-layer chromatography with immunodetection (Colsch et al., 2008), and quantitated by mass spectrometry (Cui et al., 2008). Clinically MLD is usually a disease of childhood, with clinical abnormalities developing at about the age of walking. It can, however, first manifest itself clinically at a later age. In some cases it presents clinically in adult life. In these patients, behavioral abnormalities may constitute the first symptoms and the only symptoms to occur for many years. These patients can receive a diagnosis of schizophrenia (Rauschka et al., 2006). The role of s ulfatide in brain cognitive func- tions is certainly important as the first Alzheimer case may have been a metachro- matic leukodystrophy (Amaducci et al., 1991). Interestingly, advances in imaging techniques associated with genetic findings suggest that white matter abnormalities are present in schizophrenia (Kubicki et al., 2005; Stewart and Davis, 2004). Previous work, which has not been revisited, showed that sulfatides are nec- essary for the optimal function of enzymes such as sodium–potassium-dependent ATPase, and the sulfatide content seems to be directly related to the activity of the enzyme (Karlsson et al., 1974). Sulfatides may also be involved in the functioning of certain opiate receptors (Craves et al., 1980) and in chloride transport systems (Zalc et al., 1978). Implantation in the spinal cord of a hybridoma secreting spe- cific antisulfatide antibodies has been shown to cause demyelination of the CNS in the rat (Rosenbluth et al., 2003). Antisulfatide antibodies have been found in HIV 562 D. Pham-Dinh and N. Baumann patients with distal sensory neuropathies (Lopate et al., 2005), in MS (Ilyas et al., 2003; Kanter et al., 2006) and in diabetic neuropathies (Buschard et al., 2005). These observations suggest that autoimmune responses directed against sulfatides can contribute to the pathogenesis of some of these diseases. The mechanism of demyelination still remains obscure. Very long chain fatty acids (VLCFA) of cerebrosides and sulfatides. VLCFA are major constituents of the ceramide part of galactocerebrosides and sulfatides. These are unbranched fatty acids with a chain length of 24 or more carbon atoms. They accumulate in the peroxisomal X-linked genetic disease adrenoleukodystro- phy (ALD) because of an impaired beta-oxidation in peroxisomes. A variety of clinical presentations can occur in a single kindred with this disorder (Turpin et al., 1985). The cerebral demyelinating form of ALD mainly affects boys between 5 and 12 years of age (40% of ALD cases) and leads to a vegetative stage or death within 2–5 years. The adult form, adrenomyeloneuropathy (AMN), which represents 40% of ALD cases, mainly affects the spinal cord and leads to spastic paraplegia often complicated by cerebral demyelination (35%) (Aubourg and Dubois-Dalcq, 2000). The ALD gene encodes an ATP-binding cassette (ABC) half-transporter of 75 KDa (ALDP) that must dimerize with itself or a related partner to exert its function within the peroxisomal membrane (Mosser et al., 1993). The disease affects the CNS and PNS. Understanding the mechanism of demyelination in ALD remains a major chal- lenge. The disease shows wide phenotypical variation that is not predictable and is probably under the influence of both genetic and environmental factors (Aubourg and Dubois-Dalcq, 2000). Although the cloning of the ALD gene has allowed the generation of an ALD model, these mice do not show any neurological symptoms and therefore do not reveal how VLCFA accumulation can lead to demyelination. Specific SGLs of PNS and demyelination. The major SGLs in the PNS are different. The major SGLs of the PNS are the sialosylparagloboside LM1 (also called SPG), the sulfoglucuronylparagloboside (SGPG) and the sulfoglucuronyl- lactosaminylparagloboside (SGLPG). SGPG has a terminal trisaccharide sequence, N-acetylglucosaminyl-galactosyl-glucuronylsulfate. This sequence is the HNK-1 epitope, common to the SGPG and its derivatives with two lactosaminyl residues, and to several adhesion molecules and myelin proteins including P0, the major protein of PNS, and MAG (Baumann, 2000; Willison and Yuki, 2002). Chronic polyneuropathies associated with IgM gammapathies are mostly sen- sory demyelinating neuropathies. A small proportion of cases are nondemyelinating and have the characteristics of axonal neuropathies (Chassande et al., 1998). Interestingly, the typical sensory demyelinating neuropathies have anti-MAG and anti-SGPG antibodies whereas axonal neuropathies, often predominantly motor, present only monoclonal IgM anti-SGPG activity with no anti-MAG reactivity. Thus, the fine structure of the epitope recognized by the IgM may be involved. Gangliosides. Gangliosides are mainly present in neurons, except for ganglioside GM1 and GM4. The latter is a sialylated derivative of galactosylceramide that is present in the myelin of mice and primates including humans. Ganglioside GM1 can be a target for an autoimmune demyelination process or motor conduction blocks in the PNS (Willison and Yuki, 2002). Biology of Demyelinating Diseases 563 Molecular mimicry between microbial and self-components is postulated as the mechanism accounting for the antigen and tissue specificity of immune responses in postinfectious autoimmune diseases. Guillain–Barré syndrome, the most frequent cause of acute neuromuscular paralysis, can occur 1–2 weeks after various infec- tions, in particular Campylobacter jejuni enteritis (C. jejuni). Carbohydrate mimicry between the bacterial lipo-oligosaccharide and human GM1 ganglioside is relevant to the pathogenesis of Guillain–Barré syndrome, as documented by Yuki (Yuki et al., 2004). Upon sensitization with C. jejuni lipo-oligosaccharide, rabbits develop anti- GM1 IgG antibodies and flaccid limb weakness. Paralyzed rabbits have pathological changes in their peripheral nerves identical with those present in Guillain–Barré syndrome. Immunization of mice with the lipo-oligosaccharide generates a mAb that reacts with GM1 and binds to human peripheral nerves. The mAb and anti- GM1 IgG from patients with Guillain–Barré syndrome did not induce paralysis but blocked muscle action potentials in a muscle–spinal cord coculture, indicating that anti-GM1 antibody can cause muscle weakness. These findings show that carbo- hydrate mimicry is an important cause of autoimmune neuropathy that can involve demyelination. Cholesterol and phospholipids. Most lipids found in myelin are common to other cellular membranes. Cholesterol content is high and cholesterol esters are not present in normal myelin. Phospholipids are also common to other cellular membranes, except for the great quantity of ethanolamine phosphoglycerides in the plasmalogen form. The synthesis of plasmalogens is modified in Zellweger syn- drome which is a peroxisomal syndrome that also increases VLCFA. This syndrome and other peroxisomal diseases may cause demyelination (Powers, 2005). 4 Other Glial Cell Types and Factors Involved in Myelination and Demyelination in the CNS 4.1 Astrocytes and Mutations in GFAP Alexander disease (AxD) is a leukodystrophy caused by dominant mutations in GFAP, the main intermediate filament protein of astrocytes. This neurodegenerative disease is characterized by dystrophic astrocytes containing intermediate filament aggregates associated with myelin abnormalities. Overexpressing human GFAP in mice leads to a toxic gain of function induced by aggregates of GFAP and small heat shock proteins. However, GFAP-null mice also display some myelin abnor- malities and blood–brain barrier dysfunction (reviewed in Mignot et al., 2004). AxD was the first human disease to be described in which astrocyte dysfunction induces myelin destruction (Brenner et al., 2001; Rodriguez et al., 2001). A trans- genic mouse expressing the R239H mutation presented aggregates (Tanaka et al., 2007), as did KI mice expressing R239C or R79H (Hagemann et al., 2006). This abnormality was correlated in both cases to an overexpression of the mutated allele. Diverse hypotheses have been put forward on the impact of GFAP mutations on 564 D. Pham-Dinh and N. Baumann myelination (Mignot et al., 2004). Recent works have focused on abnormal forma- tion of the astrocytic intermediate network (Der Perng et al., 2006; Quinlan et al., 2007; Tanaka et al., 2007; Tian et al., 2006), or on the fate of pathogenic astrocytes: cell death versus survival (Mignot et al., 2007). Rare radiological and pathologi- cal tumor-like lesions have already been reported in AXD patients. Enlargement of the optic chiasm is a rare feature of AXD, possibly linked to abnormal astro- cytic proliferation (Mignot et al., 2009). Taken together, these data highlight pathological astrocytes as key players and valuable therapeutic targets in neuro- logical disorders, in particular myelin diseases. The mechanisms leading to myelin pathogenicity by astrocytes (2.5.4) are undefined, and so far, there is no cure for such diseases. 4.2 Oligodendrocyte Precursors Myelination requires sequential steps in the maturation of the oligodendrocyte lin- eage with a co-ordinated change in the expression of cell surface antigens; these antigens can be recognized by monoclonal antibodies. Dys- and demyelination may also act at an early stage of development of the oligodendrocyte lineage, although the roles of oligodendrocyte precursors in human pathology are often not clearly defined. Transcription factors, growth factors, neurotransmitters, and modifications of cell surface components can be involved. Myelination requires a tightly regu- lated balance between the disappearance of inhibitory signals and the induction of positive signals, some of which are mediated by neuronal electrical activity (Demerens et al., 1996). Cocultures of oligodendrocyte progenitor cells (OPCs) and neurons in the presence of highly specific neurotoxins, which can either block (tetrodotoxin) or increase (alpha-scorpion toxin) the firing of neurons, demonstrate that myelinogenesis is dependent on the electrical activity of neurons (Demerens et al., 1996). We only develop here what occurs in relation to CNS myelination and demyelination (Collarini et al., 1991). Oligodendrocytes descend from a progenitor cell (OPC) which originates in specialized regions of the subventricular zone. Early OPCs express the platelet- derived growth factor receptor alpha (PDGFαR) and the sulphated proteoglycan NG2 (Nishiyama et al., 1999). Early OPCs differentiate into a late OPC stage (or oligodendroblasts) that appears to be committed to oligodendrogenesis. Late OPCs express the tetraspan protein CD9 (Terada et al., 2002) and the POA antigen. Although uncharacterized, the POA antigen may be recognized by the monoclonal antibody O4 that targets sulfatides in mature cells (Bansal et al., 1992). Once oligo- dendrogenesis is complete, NG2/PDGFαR positive cells remain as a major glial component of the adult mammalian CNS, apparently providing a pool of quiescent progenitors that can be tapped later for repair of demyelinated axons (Menn et al., 2006; Wilson et al., 2006). Premyelinating oligodendrocytes extend multiple pro- cesses. They express many but not all myelin proteins. Present are DM20, MAG, CNP, MBP, and CD; PLP and MOG are not detected by current methods. As myeli- nation begins, the oligodendrocyte targets myelin proteins to specific membrane . Fyn act in concert or independently in initiating myelination (Biffiger et al., 2000). 3.2 PNS Myelin Proteins PNS myelin proteins include two abundant constituents: glycoprotein zero (P0) and MBPs. Factors Involved in Myelination and Demyelination in the CNS 4.1 Astrocytes and Mutations in GFAP Alexander disease (AxD) is a leukodystrophy caused by dominant mutations in GFAP, the main intermediate. lipid-binding proteins. Unlike other myelin proteins, its quantitative expression varies greatly according to species, ranging from less than 1% in rodent sciatic nerves up to 5–14% in human, bovine,