Novel oligodendrocyte transmembrane signaling systems. Investigations utilizing antibodies as ligands

Role of galactosylceramide and sulfatide in oligodendrocytes and CNS myelin: formation of a glycosynapse

The two major glycosphingolipids of myelin, galactosylceramide (GalC) and sulfatide (SGC), interact with each other by trans carbohydrate-carbohydrate interactions in vitro. They face each other in the apposed extracellular surfaces of the multilayered myelin sheath produced by oligodendrocytes and could also contact each other between apposed oligodendrocyte processes. Multivalent galactose and sulfated galactose, in the form of GalC/SGC-containing liposomes or silica nanoparticles conjugated to galactose and galactose-3-sulfate, interact with GalC and SGC in the membrane sheets of oligodendrocytes in culture. This interaction causes transmembrane signaling, loss of the cytoskeleton and clustering of membrane domains, similar to the effects of cross-linking by anti-GalC and anti-SGC antibodies. These effects suggest that GalC and SGC could participate in glycosynapses, similar to neural synapses or the immunological synapse, between GSL-enriched membrane domains in apposed oligodendrocyte membranes or extracellular surfaces of mature myelin. Formation of such glycosynapses in vivo would be important for myelination and/or oligodendrocyte/myelin function.

Autoimmune T-cell reactivity to myelin proteolipids and glycolipids in multiple sclerosis

Central nervous system (CNS) myelin, the likely major target of autoimmune attack in multiple sclerosis (MS), contains a number of unique components that are potential targets of the attack. Two classes of molecules that are greatly enriched in CNS myelin compared to other parts of the body are certain types of proteolipids and glycolipids. Due to the hydrophobic nature of both of these classes of molecules, they present challenges for use in immunological assays and have therefore been somewhat neglected in studies of T-cell reactivity in MS compared to more soluble molecules such as the myelin basic proteins and the extracellular domain of myelin oligodendrocyte glycoprotein. This review firstly looks at the makeup of CNS myelin, with an emphasis on proteolipids and glycolipids. Next, a retrospective of what is known of T-cell reactivity directed against proteolipids and glycolipids in patients with MS is presented, and the implications of the findings are discussed. Finally, this review considers the question of what would be required to prove a definite role for autoreactivity against proteolipids and glycolipids in the pathogenesis of MS.

Cell-specific expression of neutral glycosphingolipids in vertebrate brain: immunochemical localization of 3-O-acetyl-sphingosine-series glycolipid(s) in myelin and oligodendrocytes

The tissue- and cell-specific expression of three neutral glycosphingolipids, gangliotetraosylceramide (GA1), gangliopentaosylceramide (GalNAc-GA1), and the novel 3-O-acetyl-sphingosine-series glycolipid (FMC-5), were examined with monospecific polyclonal antibodies. Immunohistochemical studies of rodent brain cross-sections indicated that both GA1 and FMC-5 antibodies stained myelin. In contrast, GalNAc-GA1 antibody distinctly stained neurons in cerebral cortex, but only partially delineated Purkinje cells and other neurons in cerebellum. Preliminary studies of mixed glial cultures suggested the following: 1) both FMC-5 and GA1 antibodies stained oligodendrocytes and oligo progenitors, and 2) GalNAc-GA1 antibody did not stain any cells in the culture. Because the GalNAc-GA1 was associated with neurons, we examined the immunoreactivity of GalNAc-GA1 antibody in primary neuronal cultures. Further studies using primary cultures of rat brain oligodendrocytes, and dissociated cerebellar neuronal cultures indicated that both GA1 and FMC-5 are specifically expressed by oligodendrocytes, whereas GalNAc-GA1 is primarily localized in interneurons and to some extent in Purkinje neurons.

Remyelination-promoting human IgMs: developing a therapeutic reagent for demyelinating disease

Promoting remyelination following injury to the central nervous system (CNS) promises to be an effective neuroprotective strategy to limit the loss of surviving axons and prevent disability. Studies confirm that multiple sclerosis (MS) and spinal cord injury lesions contain myelinating cells and their progenitors. Recruiting these endogenous cells to remyelinate may be of therapeutic value. This review addresses the use of antibodies reactive to CNS antigens to promote remyelination. Antibody-induced remyelination in a virus-mediated model of chronic spinal cord injury was initially observed in response to treatment with CNS reactive antisera. Monoclonal mouse and human IgMs, which bind to the surface of oligodendrocytes and myelin, were later identified that were functionally equivalent to antisera. A recombinant form of a human remyelination-promoting IgM (rHIgM22) targets areas of CNS injury and promotes maximal remyelination within 5 weeks after a single low dose (25 microg/kg). The IgM isoform of this reparative antibody is required for in vivo function. We hypothesize that the IgM clusters membrane domains and associated signaling molecules on the surface of target cells. Current therapies for MS are designed to modulate inflammation. In contrast, remyelination promoting IgMs are the first potential therapeutic molecules designed to induce tissue repair by acting within the CNS at sites of damage on the cells responsible for myelin synthesis.

A glycosynapse in myelin?

Myelin, the multilayered membrane which surrounds nerve axons, is the only example of a membranous structure where contact between extracellular surfaces of membrane from the same cell occurs. The two major glycosphingolipids (GSLs) of myelin, galactosylceramide (GalC) and its sulfated form, galactosylceramide I(3)-sulfate (SGC), can interact with each other by trans carbohydrate-carbohydrate interactions across apposed membranes. They occur in detergent-insoluble lipid rafts containing kinases and thus may be located in membrane signaling domains. These signaling domains may contact each other across apposed extracellular membranes, thus forming glycosynapses in myelin. Multivalent forms of these carbohydrates, GalC/SGC-containing liposomes, or galactose conjugated to albumin, have been added to cultured oligodendrocytes (OLs) to mimic interactions which might occur between these signaling domains when OL membranes or the extracellular surfaces of myelin come into contact. These interactions between multivalent carbohydrate and the OL membrane cause co-clustering or redistribution of myelin GSLs, GPI-linked proteins, several transmembrane proteins, and signaling proteins to the same membrane domains. They also cause depolymerization of the cytoskeleton, indicating that they cause transmission of a signal across the membrane. Their effects have similarities to those of anti-GSL antibodies on OLs, shown by others, suggesting that the multivalent carbohydrate interacts with GalC/SGC in the OL membrane. Communication between the myelin sheath and the axon regulates both axonal and myelin function and is necessary to prevent neurodegeneration. Participation of transient GalC and SGC interactions in glycosynapses between the apposed extracellular surfaces of mature compact internodal myelin might allow transmission of signals throughout the myelin sheath and thus facilitate myelin-axonal communication.

Transcriptional regulation of the human UDP-galactose:ceramide galactosyltransferase (hCGT) gene expression: functional role of GC-box and CRE

UDP-galactose:ceramide galactosyltransferase (CGT, EC 2.4.1.45) is a key enzyme in the biosynthetic pathway of galactocerebroside (GalC), the most abundant glycolipid in myelin. Using a GalC expressing cell line, human oligodendroglioma (HOG), one which does not express GalC, human neuroblastoma (LAN-5), we previously demonstrated that the human CGT (hCGT) gene promoter functions in a cell-specific manner. Because the proximal (-292/-256) and distal (-747/-688) positive domains were shown to be critically involved in regulating the expression of several myelin-specific genes, we further investigated the functional roles of these two motifs in hCGT expression. Mutation analysis confirmed that a GC-box (-267/-259) and a CRE (-697/-690) were critical for hCGT expression. Electrophoretic mobility shift assay (EMSA) demonstrated that these motifs specifically bound to nuclear extracts from both cell lines. Using antibodies to Sp1, Sp3, pCREB-1, and ATF-1, these proteins were shown to be components of the EMSA complexes. However, the only difference between the HOG and LAN-5 cells was found in the EMSA profile of the CRE complexes. This difference may account for the differential transcription of the hCGT gene in the two cell types. Furthermore, the expression levels of ATF-1 detected were much higher in HOG cells than in LAN-5 cells. Thus, our data suggest that the GC-box and CRE function cooperatively, and that the CRE regulates the cell-specific expression of the hCGT gene.

Antiapoptotic signaling by a remyelination-promoting human antimyelin antibody

Stabilizing the survival of oligodendrocytes and oligodendrocyte precursors within and near lesions in patients suffering from multiple sclerosis (MS) and other demyelinating diseases is an important therapeutic goal. Previous studies have identified a human-derived monoclonal IgM antibody designated rHIgM22 that induces remyelination in a mouse model of MS. We provide evidence that this antibody, directed against myelin, induces antiapoptotic signaling in premyelinating oligodendrocytes and reduces caspase-3 activation and caspase gene expression in mice undergoing antibody-induced remyelination. This effect was dependent on calcium entry via CNQX-sensitive channels and on lipid raft integrity, and was correlated with suppression of JNK signaling. We conclude that rHIgM22 may induce remyelination via rescue of oligodendrocytes, and suggest that such autoantibody-mediated signaling may have important therapeutic implications for a variety of neurological diseases, including stroke and Alzheimer's disease.

Remyelination-promoting antibodies activate distinct Ca2+ influx pathways in astrocytes and oligodendrocytes: relationship to the mechanism of myelin repair

Our laboratory has identified mouse and human monoclonal antibodies that promote myelin repair in multiple models of demyelinating disease. We have proposed that these antibodies promote remyelination by directly activating central nervous system glia. Intracellular calcium concentration was monitored using a Fura2 ratiometric assay. Repair-promoting antibodies induced distinct Ca2+ signals in both astrocytes and oligodendrocytes. Astrocyte Ca2+ signaling is mediated by a phospholipase C-dependent pathway while oligodendrocyte Ca2+ signaling is mediated via AMPA-sensitive glutamate receptors. An antibody's ability to induce Ca2+ signals is statistically correlated with promotion of myelin repair. These findings support the hypothesis that remyelination-promoting antibodies are acting directly at the surface of glial cells to induce calcium-dependent physiologic reparative function.

Galactolipids are molecular determinants of myelin development and axo-glial organization

Myelination is a developmentally regulated process whereby myelinating glial cells elaborate large quantities of a specialized plasma membrane that ensheaths axons. The myelin sheath contains an unusual lipid composition in that the glycolipid galactosylceramide (GalC) and its sulfated form sulfatide constitute a large proportion of the total lipid mass. These glycolipids have been implicated in a range of developmental processes such as cell differentiation and myelination initiation, but analyses of mice lacking UDP-galactose:ceramide galactosyltransferase (CGT), the enzyme required for myelin galactolipid synthesis, have more recently demonstrated that the galactolipids more subtly regulate myelin formation. The CGT mutants display a delay in myelin maturation and axo-glial interactions develop abnormally. By interbreeding the CGT mutants with mice that lack myelin-associated glycoprotein, it has been shown that these specialized myelin lipids and proteins act in concert to promote axo-glial adhesion during myelinogenesis. The analysis of the CGT mutants is helping to clarify the roles myelin galactolipids play in regulating the development, and ultimately the function of the myelin sheath.

The structure and function of myelin: from inert membrane to perfusion pump

The goal of this overview is to propose a novel structure/function model of central nervous system myelin. Although myelin is known to be a compact multilamellar structure that wraps around axons, the biologic role this structure plays in the nervous system remains an enigma. One means of ascertaining myelin's biologic role is by analyzing its structure. The recent discovery of tight junctions in myelin may be the key that unlocks the mysterious black box of myelin structure/function. Tight junctions in other cell types are invariably adjacent to adherens junctions, with both of these junctional plaques playing critical roles in paracellular barrier function, i.e., adhesion of cell membranes, signal transduction, and fluid movement between cells via aqueous pores and channels. The application of current knowledge about junctional plaques to myelin is an original concept. This knowledge, taken together with evidence from studies of normal and pathologic myelin, supports the possibility that a primary function of junctional plaques in myelin is to perfuse the periaxonal space.

Human antibodies accelerate the rate of remyelination following lysolecithin-induced demyelination in mice

Immunoglobulin-based therapies are becoming increasingly common for the treatment of neurologic and autoimmune diseases in humans. In this study, we demonstrate that systemic administration of either polyclonal human immunoglobulins or specific human monoclonal antibodies can accelerate the rate of CNS remyelination following toxin-induced demyelination. Injection of lysolecithin directly into the spinal cord results in focal demyelinated lesions. In contrast to other murine models of demyelinating disease, the mechanism of demyelination following lysolecithin injection is independent of immune system activation, and chronic inflammation at the site of the lesion is minimal. Administration of polyclonal human IgM (pHIgM) or a serum-derived human monoclonal antibody (sHIgM22) resulted in approximately a twofold increase in remyelinating axons when compared to animals treated with saline or with antibodies that do not promote repair. Both pHIgM and sHIgM22 show strong binding to CNS white matter and oligodendrocytes, while antibodies that did not accelerate remyelination do not. This differential staining pattern suggests that enhanced remyelination may result from direct stimulation of oligodendrocyte remyelination by binding to surface receptors on oligodendrocytes or glial progenitor cells. We propose the use of human polyclonal IgM or specific human monoclonal IgM antibodies as potential therapies to enhance myelin repair following CNS injury and disease.

Myelin-associated glycoprotein and myelin galactolipids stabilize developing axo-glial interactions

We have analyzed mice that lack both the myelin-associated glycoprotein (MAG) and the myelin galactolipids, two glial components implicated in mediating axo-glial interactions during the myelination process. The single-mutant mice produce abnormal myelin containing similar ultrastructural abnormalities, suggesting that these molecules may play an overlapping role in myelin formation. Furthermore, the absence of the galactolipids results in a disruption in paranodal axo-glial interactions, and we show here that similar, albeit less severe, abnormalities exist in the developing MAG mutant. In the double-mutant mice, maintenance of axo-glial adhesion is significantly more affected than in the single mutants, supporting the overlapping function hypothesis. We also show that independently of MAG, galactolipids, and paranodal junctional components, immature nodes of Ranvier form normally, but rapidly destabilize in their absence. These data indicate that distinct molecular mechanisms are responsible for the formation and maintenance of axo-glial interactions.

Humoral autoimmunity as a mediator of CNS repair

Autoimmune responses directed against the central nervous system (CNS) have generally been considered pathogenic in nature. Although there are several well understood conditions in which this is the case, there is also a growing body of experimental evidence to show that both the cellular and humoral immune responses can promote tissue repair following CNS injury and disease. Our laboratory has used a mouse model of chronic demyelinating disease to characterize a class of polyreactive IgM autoantibodies that react with oligodendrocyte surface antigens and promote myelin repair. By screening a large number of human monoclonal antibodies, we have found that IgM antibodies that react with CNS tissue are relatively common. Autoreactive IgM antibodies might constitute an endogenous system for tissue repair, and therefore these antibodies could be of value as therapeutic reagents.

Glial cells as targets for cytotoxic immune mediators

Oligodendrocytes and Schwann cells are the glia principally responsible for the synthesis and maintenance of myelin. Damage may occur to these cells in a number of conditions, but perhaps the most studied are the idiopathic inflammatory demyelinating diseases, multiple sclerosis in the CNS, and Guillain-Barré syndrome and its variants in the peripheral nervous system (PNS). This article explores the effects on these cells of cytotoxic immunological and inflammatory mediators: similarities are revealed, of which perhaps the most important is the sensitivity of both Schwann cells and oligodendrocytes to many such agents. This area of research is, however, characterised and complicated by numerous and often very substantial inter-observer discrepancies. Marked variability in cell culture techniques, and in assays of cell damage and death, provide artifactual explanations for some of this variability; true inter-species differences also contribute. Not the least important conclusion centres on the limited capacity of in vitro studies to reveal disease mechanisms: cell culture findings merely illustrate possibilities which must then be tested ex vivo using human tissue samples affected by the relevant disease.

Effect of liposomes containing cerebroside and cerebroside sulfate on cytoskeleton of cultured oligodendrocytes

Oligodendrocytes (OLs) and the myelin produced by them are enriched in two glycosphingolipids, galactosylceramide (GalC) and its sulfated form, cerebroside sulfate (CBS). We showed earlier that these two glycolipids in opposed liposomal membranes or in methanol solution can adhere to each other. Here we have examined the potential effect of an interaction between GalC/CBS in apposed membranes of oligodendrocytes (OLs) by incubating cultured OLs with GalC/CBS-containing liposomes and observing the effect on the membrane sheets produced by OLs and on the distribution of OL constituents using fluorescent antibodies and confocal microscopy. The GalC/CBS-containing liposomes caused redistribution or a decrease in the density of anti-GalC and anti-MBP staining but had no effect on the density or distribution of staining by anti-PI(4,5)P(2) that remained uniformly distributed in the membrane sheets. There was no apparent change in the area of the membrane sheets nor in the amount of MBP in OL membranes, as determined by slot blots. In addition, the GalC/CBS-containing liposomes caused depolymerization of microtubules and actin filaments suggesting that the interaction of GSL-containing liposomes with the extracellular surface of the OL caused transmission of a signal across the membrane. Because these two glycolipids can adhere to each other across apposed membranes, the liposomal glycolipids may be interacting with a GalC/CBS-enriched signaling domain in the OL plasma membrane.

Characterization of the human UDP-galactose:ceramide galactosyltransferase gene promoter

UDP-galactose:ceramide galactosyltransferase (CGT, EC 2.4.1.45) is a key enzyme in the biosynthesis of galactocerebroside, the most abundant glycosphingolipid in the myelin sheath. An 8 kb fragment upstream from the transcription initiation site of CGT gene was isolated from a human genomic DNA library. Primer extension analysis revealed a single transcription initiation site 329 bp upstream from the ATG start codon. Neither a consensus TATA nor a CCAAT box was identified in the proximity to the transcription start site; however, this region contains a high GC content and multiple putative regulatory elements. To investigate the transcriptional regulation of CGT, a series of 5' deletion constructs of the 5'-flanking region were generated and cloned upstream from the luciferase reporter gene. By comparing promoter activity in the human oligodendroglioma (HOG) and human neuroblastoma (LAN-5) cell lines, we found that the CGT promoter functions in a cell type-specific manner. Three positive cis-acting regulatory regions were identified, including a proximal region at -292/-256 which contains the potential binding sites for known transcription factors (TFs) such as Ets and SP1 (GC box), a distal region at -747/-688 comprising a number of binding sites such as the ERE half-site, NF1-like, TGGCA-BP, and CRE, and a third positive cis-acting region distally localized at -1325/-1083 consisting of binding sites for TFs such as nitrogen regulatory, TCF-1, TGGCA-BP, NF-IL6, CF1, bHLH, NF1-like, GATA, and gamma-IRE. A negative cis-acting domain localized in a far distal region at -1594/-1326 was also identified. Our results suggest the presence of both positive and negative cis-regulatory regions essential for the cell-specific expression in the TATA-less promoter of the human CGT gene.

Effects of galactolipid elimination on oligodendrocyte development and myelination

The galactolipids galactocerebroside and sulfatide, which require the enzyme UDP-galactose:ceramide galactosyltransferase (CGT) for their synthesis, are among the most prevalent molecules in the myelin sheath. Numerous studies, mainly using antibody perturbation methods in vitro, have suggested that these molecules are crucial mediators of oligodendrocyte differentiation and myelin formation. Although we have previously demonstrated that myelin formation occurs in CGT null mutant mice, which are incapable of synthesizing the myelin galactolipids, here we show that there are developmental alterations in the CNS of these animals. There is a significant decrease in the number of myelinated axon segments in the mutant spinal cord despite normal levels of myelin gene-specific mRNAs and proteins. Also, there is an increased cellularity in the mature mutant spinal cord and the distinctive morphology of the additional cells suggests that they are actively myelinating oligodendrocytes. Using in situ hybridization techniques, we show that there is a 50% increase in the number of oligodendrocytes in the mutant spinal cord. The data suggest that galactolipids play an important developmental role in regulating the maturation program and final number of oligodendrocytes.

Redistribution of cholesterol in oligodendrocyte membrane sheets after activation of distinct signal transduction pathways

Cultured oligodendrocytes produce extensive membrane sheets that contain an internal lacy network of vein-like structures composed of microtubules, actin filaments, and 2'3'-cyclic nucleotide 3'-phosphohydrolase (CNPase). These cytoplasmic vein-like structures surround domains of myelin basic protein (MBP). Using the antibiotic filipin, that binds to cholesterol, the relationship between plasma membrane cholesterol and cytoskeleton in membrane sheets was examined. Our results show that cholesterol was relatively uniformly distributed within the plasma membranes of prefixed control oligodendrocyte membrane sheets. When live cultures were extracted with Triton X-100, however, a subpopulation of cholesterol molecules remained colocalized with cytoskeleton in the membrane sheets. Activation of two well-characterized signaling pathways that differentially affect microtubule and actin filament stability in membrane sheets resulted in an apparent massive lateral movement of cholesterol molecules away from membrane regions overlying internal MBP domains to membrane tracts directly overlying cytoplasmic cytoskeletal veins. Depolymerization of microtubules by colchicine resulted in redistribution of cholesterol directly over actin filaments, whereas depolymerization of actin filaments by cytochalasin B resulted in redistribution of cholesterol directly over CNPase/microtubular veins. These data suggest that cholesterol forms an association with cytoskeletal components or proteins associated with cytoskeleton. These data also suggest that cholesterol, via interactions with cytoskeleton, plays a role in signaling pathways in oligodendrocyte membrane sheets.

GFAP-positive and myelin marker-positive glia in normal and pathologic environments

The data herein demonstrate that in addition to the well-characterized myelin marker-positive, glial fibrillary acidic protein (GFAP)-negative, membrane sheet-bearing oligodendrocytes, another type of myelin marker-positive, process-bearing glia exists in normal and pathologic conditions. This second type of myelin marker-positive glia expresses GFAP, and therefore these cells have been referred to as mixed phenotype glia. Although mixed phenotype glia have been documented previously, their identity and function have remained a mystery. The goal of this immunocytochemical study was to further characterize these cells. Using the MBPlacZ transgenic mouse in which beta-galactosidase is under the control of the myelin basic protein (MBP) gene promoter, GFAP-positive/beta-galactosidase-positive and myelin/oligodendrocyte-specific protein (MOSP)-positive/beta-galactosidase-positive cells were detected in subcortical white matter and in perivascular locations within cerebral white and gray matter. In cultures prepared from highly enriched myelin marker-positive immature glia, mixed phenotype glia were detected that were GFAP-positive and either MOSP-, MBP-, O1-, and O4-positive. The expression of multiple myelin markers by mixed phenotype glia may suggest that these cells are of oligodendrocyte origin. Increased numbers of MOSP-positive/GFAP-positive mixed phenotype glia were detected in sections from adult hypomyelinated brain from shiverer, quaking, and PKU mice compared to myelinated control adult mouse brain. Similarly, cultures from control brain exposed to elevated pH for 2-3 weeks showed dramatically increased numbers of mixed phenotype glia (80%) compared to control (<10%). Increased numbers of mixed phenotype glia also were detected in shiverer cultures (40%). Since increases in the number of mixed phenotype glia occur in shiverer, quaking, and PKU mouse brain, these data suggest that mixed phenotype glia contribute to gliosis in pathologic white matter.

Genetic analysis of myelin galactolipid function

The CGT enzyme is responsible for catalyzing the final step in GalC synthesis. The isolation of the CGT cDNA has allowed for the genetic analysis of galactolipid function by providing the opportunity to generate null mutants deficient in CGT enzymatic activity. The detailed analyses of CGT mutant mice demonstrate that the galactolipids are essential for the formation and maintenance of normal CNS myelin, but neither GalC or sulfatide appear to be required for the development of structurally normal PNS myelin. These studies also show that the differentiation of myelinating cells is not dependent on galactolipid function, in contrast to the conclusions drawn from prior antibody perturbation studies. The abnormal node of Ranvier formations present in the CNS likely explain the disrupted electrophysiological properties displayed by mutant spinal cord axons and the tremoring phenotype of these mice. The abnormal myelin structures present in the mutant animals are consistent with the possibility that the galactolipids play a role in regulating or mediating proper axo-glial interactions. The further detailed analysis of these animals should help refine our understanding of galactolipid function in the myelination process.

Myelin galactolipids: mediators of axon-glial interactions?

The precise alignment of myelin segments along the length of the axon is essential for the saltatory propagation of an electrical impulse. Furthermore, node of Ranvier formation and function are dependent on the proper interactions between myelinating glial cells and the axon. Nevertheless, the molecules that regulate the placement and association of myelinating cells with axons remain largely unidentified. Recently, however, the analysis of mutant mice incapable of synthesizing the galactolipids of myelin has revealed defects in these processes. The galactolipid-deficient mice display alterations in the spacing of internodal segments along the axon: large unmyelinated gaps are common and overlapping myelin segments are observed. Moreover, the normal tight association between the lateral loops of the myelinating cell and the axonal membrane at the paranode region is also disrupted in these animals. Strikingly, there is a complete absence of transverse bands at the axon-glial junction, with the lateral loops frequently turning away from the axon. These data indicate that the galactolipids play an essential role in axon-glial interactions and node of Ranvier formation.

Negative regulation of oligodendrocyte differentiation by galactosphingolipids

Galactocerebroside and sulfatide, major galactosphingolipid components of oligodendrocyte plasma membranes and myelin, are first expressed at a critical point, when progenitors cease to proliferate and commence terminal differentiation. We showed previously that an antibody to galactocerebroside/sulfatide arrested terminal differentiation, suggesting a role for these galactolipids in oligodendrocyte differentiation. We have now investigated the differentiation of oligodendrocytes (1) in response to other anti-galactolipid antibodies, showing that anti-sulfatide O4 but not anti-galactocerebroside O1 blocks terminal differentiation, perhaps by mimicking an endogenous ligand, and (2) in a transgenic mouse unable to synthesize these lipids because of mutation of the gene for ceramide galactosyltransferase, a key enzyme for galactosphingolipid synthesis. We find that galactosyltransferase mRNA expression begins at the late progenitor [pro-oligodendroblast (Pro-OL)] stage of the lineage and that the late progenitor marker pro-oligodendroblast antigen is not synthesized in the absence of galactosyltransferase. The principal outcome of the elimination of these galactolipids is a two- to threefold enhancement in the number of terminally differentiated oligodendrocytes both in culture and in vivo. Because the general pattern of differentiation and the level of progenitor proliferation and survival appear to be unaltered in the mutant cultures, we conclude that the increased number of oligodendrocytes is caused by an increased rate and probability of differentiation. In agreement with these two experimental approaches, we present a model in which galactosphingolipids (in particular galactocerebroside and/or sulfatide) act as sensors and/or transmitters of environmental information, interacting with endogenous ligands to function as negative regulators of oligodendrocyte differentiation, monitoring the timely progress of Pro-OLs into terminally differentiating, myelin-producing oligodendrocytes.

Insertion of a retrotransposon in Mbp disrupts mRNA splicing and myelination in a new mutant rat

Our understanding of myelination has been greatly enhanced via the study of spontaneous mutants that harbor a defect in a gene encoding one of the major myelin proteins (myelin mutants). In this study, we describe a unique genetic defect in a new myelin mutant called the Long Evans shaker (les) rat that causes severe dysmyelination of the CNS. Myelin deficits result from disruption of the myelin basic protein (Mbp) gene caused by the insertion of an endogenous retrotransposon [early transposons (ETn) element] into a noncoding region (intron 3) of the gene. The ETn element alters the normal splicing dynamics of MBP mRNA, leading to a dramatic reduction in the levels of full-length isoforms (<5% of normal) and the appearance of improperly spliced, chimeric transcripts. Although these aberrant transcripts contain proximal coding regions of the MBP gene (exons 1-3), they are unable to encode functional proteins required to maintain the structural integrity of the myelin sheath. These chimeric transcripts seem capable, however, of producing the necessary signal to initiate and coordinate myelin gene expression because normal numbers of mature oligodendrocytes synthesizing abundant levels of other myelin proteins are present in the mutant CNS. The les rat is thus an excellent model to study alternative functions of MBP beyond its well characterized role in myelin compaction.

The structure and function of myelin oligodendrocyte glycoprotein

Myelin oligodendrocyte glycoprotein (MOG) is a quantitatively minor component of CNS myelin whose function remains relatively unknown. As MOG is an autoantigen capable of producing a demyelinating multiple sclerosis-like disease in mice and rats, much of the research directed toward MOG has been immunological in nature. Although the function of MOG is yet to be elucidated, there is now a relatively large amount of biochemical and molecular data relating to MOG. Here we summarize this information and include our recent findings pertaining to the cloning of the marsupial MOG gene. On the basis of this knowledge we suggest three possible functions for MOG: (a) a cellular adhesive molecule, (b) a regulator of oligodendrocyte microtubule stability, and (c) a mediator of interactions between myelin and the immune system, in particular, the complement cascade. Given that antibodies to MOG and to the myelin-specific glycolipid galactocerebroside (Gal-C) both activate the same signaling pathway leading to MBP degradation, we propose that there is a direct interaction between the membrane-associated regions of MOG and Gal-C. Such an interaction may have important consequences regarding the membrane topology and function of both molecules. Finally, we examine how polymorphisms and/or mutations to the MOG gene could contribute to the pathogenesis of multiple sclerosis.

Demyelination and altered expression of myelin-associated glycoprotein isoforms in the central nervous system of galactolipid-deficient mice

Vertebrate myelin is enriched in the lipid galactocerebroside (GalC) and its sulfated derivated sulfatide. To understand the in vivo function of these lipids, we analyzed myelination in mice that contain a null mutation in the gene encoding UDP-galactose:ceramide galactosyltransferase, the enzyme responsible for catalyzing the final step in GalC synthesis. Galactolipid-deficient myelin is regionally unstable and progressively degenerates. At postnatal day 30, demyelination is restricted to the midbrain and hindbrain, but by postnatal day 90, it spreads throughout the central nervous system. Activated microglial cells and reactive astrocytes appear with the loss of myelin in older animals. Nonetheless, major myelin protein gene mRNA levels are normal throughout the life of these animals, suggesting that widespread oligodendrocyte death is not the primary cause of demyelination. The developmental switch in myelin-associated glycoprotein isoform expression, however, does not occur normally in these mice, suggesting an alteration in oligodendrocyte maturation. Taken together, these findings indicate that GalC and sulfatide are required for the long-term maintenance of myelin and that their absence may have subtle effects on the development of oligodendrocytes.

Galactolipids in the formation and function of the myelin sheath

Among the most abundant components of myelin are the galactolipids galactocerebroside (GalC) and sulfatide. In spite of this abundance, the roles that these molecules play in the myelin sheath are not well understood. Until recently, our concept of GalC and sulfatide functions had been principally defined by immunological and chemical perturbation studies that implicate these lipids in oligodendrocyte differentiation, myelin formation, and myelin stability. Recently, however, genetic studies have allowed us to re-analyze the functions of these lipids. Two laboratories have independently generated mice that are incapable of synthesizing either GalC or sulfatide by inactivating the gene encoding the enzyme UDP-galactose:ceramide galactosyltransferase (CGT), which is required for myelin galactolipid synthesis. These galactolipid-deficient animals exhibit a severe tremor, hindlimb paralysis, and display electrophysiological deficits in both the central and peripheral nervous systems. In addition, ultrastructural studies have revealed hypomyelinated white matter tracts with unstable myelin sheaths and a variety of myelin abnormalities including altered node length, reversed lateral loops, and compromised axo-oligodendrocytic junctions. Collectively, these observations indicate that cell-cell interactions, which are essential in the formation and maintenance of a properly functioning myelin sheath, are compromised in these galactolipid-deficient mice.

New perspectives on the function of myelin galactolipids

A defining feature of the vertebrate nervous system is the ensheathment of axons by myelin, a multilamellar membrane containing a small group of proteins and an abundance of the galactolipid galactocerebroside (GalC) and its sulfated derivative sulfatide. Several in vitro studies have suggested that these galactolipids transduce developmental signals, facilitate protein trafficking and stabilize membranes. In addition, mice lacking the ability to synthesize GalC or sulfatide form dysfunctional and unstable myelin. These findings suggest that the galactolipids are essential components of myelin, and that functional and structural properties of myelin result from the combined contributions of galactolipids and proteins.

Regulation of oligodendrocyte differentiation: protein kinase C activation prevents differentiation of O2A progenitor cells toward oligodendrocytes

Oligodendrocytes differentiate on a specific schedule in vivo in order to myelinate axons at the precise time and at the appropriate position. The current study was undertaken to obtain further insight as to how this timed appearance is regulated intracellularly. We observed that exposure of O2A progenitor cells in culture to phorbol 12-myristate 13-acetate (PMA; an activator of protein kinase C, PKC) inhibited their differentiation to oligodendrocytes by suppressing the expression of specific myelin markers at the O4-stage. To positively identify a role of PKC per se in differentiation, the use of a minimal medium with low serum content turned out to be essential. This was demonstrated by showing that the inhibitory effect of PMA on oligodendrocyte differentiation could be completely abolished by a combined action of insulin, triiodothyronine (T3), hydrocortisone and other components of a chemically defined medium (CDM). Furthermore, the PMA-mediated inhibition of oligodendrocyte differentiation could be partially restored by activation of the cAMP signal transduction pathway. The results indicate that PKC plays a crucial role in the differentiation of O2A progenitor cells toward oligodendrocytes: PKC activation prevents differentiation of O2A progenitor cells, whereas differentiation toward oligodendrocytes is dependent on other signaling compounds which may counteract the PKC signal transduction route.

Tenascin-R is an intrinsic autocrine factor for oligodendrocyte differentiation and promotes cell adhesion by a sulfatide-mediated mechanism

O4(+) oligodendrocyte (OL) progenitors in the mammalian CNS are committed fully to terminal differentiation into myelin-forming cells. In the absence of other cell types in vitro, OL differentiation reproduces the in vivo development with a correct timing, suggesting the existence of an intrinsic regulatory mechanism that presently is unknown. We have examined the effect of two isoforms of the extracellular matrix (ECM) molecule tenascin-R (TN-R), which is expressed by OLs during the process of myelination, on the adhesion and maturation of OLs in vitro. Here we show that the substrate-bound molecules supported the adhesion of O4(+) OLs independently of the CNS region or age from which they were derived. At the molecular level this process was mediated by protein binding to membrane surface sulfatides (Sulf), as indicated by the interference of O4 antibody and Sulf with the attachment of OLs or other Sulf+ cells, erythrocytes, to TN-R substrates and by direct protein-glycolipid binding studies. In the absence of platelet-derived growth factor (PDGF), exogenous TN-R induced myelin gene expression and the upregulation of its own synthesis by cultured cells, resulting in a rapid terminal differentiation of O4(+) progenitors. Our findings strongly suggest that TN-R represents an intrinsic regulatory molecule that controls the timed OL differentiation by an autocrine mechanism and imply the relevance of TN-R for CNS myelination and remyelination.

Cloning, characterization, and expression of human ceramide galactosyltransferase cDNA

Galactosylceramide (galactocerebroside, GalC) and its sulfated derivative, sulfatide, are major lipid components of the central and peripheral nervous system myelin sheath. The enzyme UDP-galactose:ceramide galactosyltransferase (CGT, EC 2.4.1.45) catalyzes the final step of galactosylceramide synthesis. In this report we describe isolation of the complete copy of human CGT cDNA. Total RNA from N-370 FG cells, a human fetal glioma cell line, was reverse-transcribed and dG-tailed. Degenerate primers synthesized based on rat CGT cDNA sequence were used in 5'- and 3'- rapid amplification of cDNA ends reaction (RACE). The obtained sequence was used to synthesize the primers for the complete coding region to be amplified and cloned into a pCR 3.1 expression vector. Following transfection of the CHOP cells with the resulting vector, the cell homogenate was assayed for the galactosyltransferase activity. Northern blot hybridization was used to determine the length of CGT mRNA and Southern blot hybridization was used to determine the number of homologous genes. Our results indicate that human CGT retains all conservative features of rat and mouse CGT. It is a single copy gene with mRNA transcript of about 4 kb.

Functional breakdown of the lipid bilayer of the myelin membrane in central and peripheral nervous system by disrupted galactocerebroside synthesis

The lipid bilayer of the myelin membrane of the central nervous system (CNS) and the peripheral nervous system (PNS) contains the oligodendrocyte- and Schwann cell-specific glycosphingolipids galactocerebrosides (GalC) and GalC-derived sulfatides (sGalC). We have generated a UDP-galactose ceramide galactosyltransferase (CGT) null mutant mouse (cgt-/-) with CNS and PNS myelin completely depleted of GalC and derived sGalC. Oligodendrocytes and Schwann cells are unable to restore the structure and function of these galactosphingolipids to maintain the insulator function of the membrane bilayer. The velocity of nerve conduction of homozygous cgt-/- mice is reduced to that of unmyelinated axons. This indicates a severely altered ion permeability of the lipid bilayer. GalC and sGalC are essential for the unperturbed lipid bilayer of the myelin membrane of CNS and PNS. The severe dysmyelinosis leads to death of the cgt-/- mouse at the end of the myelination period.

Myelination in the absence of galactocerebroside and sulfatide: normal structure with abnormal function and regional instability

The vertebrate nervous system is characterized by ensheathment of axons with myelin, a multilamellar membrane greatly enriched in the galactolipid galactocerebroside (GalC) and its sulfated derivative sulfatide. We have generated mice lacking the enzyme UDP-galactose:ceramide galactosyltransferase (CGT), which is required for GalC synthesis. CGT-deficient mice do not synthesize GalC or sulfatide but surprisingly form myelin containing glucocerebroside, a lipid not previously identified in myelin. Microscopic and morphometric analyses revealed myelin of normal ultrastructural appearance, except for slightly thinner sheaths in the ventral region of the spinal cord. Nevertheless, these mice exhibit severe generalized tremoring and mild ataxia, and electrophysiological analysis showed conduction deficits consistent with reduced insulative capacity of the myelin sheath. Moreover, with age, CGT-deficient mice develop progressive hindlimb paralysis and extensive vacuolation of the ventral region of the spinal cord. These results indicate that GalC and sulfatide play important roles in myelin function and stability.

Biochemistry of demyelination

The myelin sheath, a lipid-rich multilamellar membrane of relative stability, both insulates and enhances conduction in nerve axons. A notable feature of myelin-specific proteins, in particular myelin basic protein, is their susceptibility to proteolytic activity and their encephalitogenicity, which induces inflammatory demyelination in the CNS. The final common pathway of myelin breakdown in vivo is well documented and there is evidence that myelin disruption can be mediated directly by soluble (circulating) factors and for following receptor-driven phagocytosis by macrophages. However the exact mechanism(s) of demyelination in multiple sclerosis is still unresolved, both antigen-specific and--non-specific events having the potential to generate the myelinolytic process.

The oligodendroglial reaction to brain stab wounds: an immunohistochemical study

Myelin/oligodendrocyte specific protein was compared to glial fibrillary acidic protein and 2'3'-cyclic nucleotide 3'-phosphodiesterase expression in normal rat brains and following stab wounds to the cerebral cortex, corpus callosum and hippocampus. Animals with stab wounds were allowed to recover for 5, 15, 28, 45 and 70 days post-operation before fixation by perfusion. Sections were reacted with antibodies against myelin/oligodendrocyte specific protein, glial fibrillary acidic protein and 2'3'-cyclic nucleotide 3'-phosphodiesterase, and observed by light and electron microscopy. Normal cerebral cortex had very few myelin/oligodendrocyte specific protein-positive and 2'3'-cyclic nucleotide 3'-phosphodiesterase-positive cells, but some glial fibrillary acidic protein-positive cells. The myelinated fibres of the corpus callosum were heavily stained for myelin/oligodendrocyte specific protein but unstained by glial fibrillary acidic protein or 2'3'-cyclic nucleotide 3'-phosphodiesterase antibodies. Some immunopositive cells were present in the corpus callosum and hippocampus with all three antibodies. After stab wound myelin/oligodendrocyte specific protein-positive reactive cells had more and longer processes and stained more intensely than equivalent cells in normal brain. These cells were distributed along the wound track, including within the cerebral cortex. The numbers of these cells increased until 28 days post-operation and then decreased so that very few were found at 70 days post-operation except in the corpus callosum. Where demyelination occurred myelin/oligodendrocyte specific protein-staining was lost. Staining for 2'3-cyclic nucleotide 3'-phosphodiesterase revealed a similar pattern. Glial fibrillary acidic protein-positive reactive cells, which were also more robust than the normal cells, were more widely distributed. They increased in number throughout the time periods studied and gliosis was evident on the contralateral side. The glial fibrillary acidic protein-positive astrocytes were also different from the myelin/oligodendrocyte specific protein-positive and 2'3'-cyclic nucleotide 3'-phosphodiesterase-positive oligodendrocytes in terms of cell shape. With electron microscopy myelin/oligodendrocyte specific protein-positive cells showed features typical of immature oligodendrocytes. We conclude that the injury caused a numerical increase in oligodendrocytes and that myelin/oligodendrocyte specific protein is a good marker for the oligodendroglial response and demyelination in pathological conditions.

Glycolipid migration from the apical to the equatorial subdomains of the sperm head plasma membrane precedes the acrosome reaction. Evidence for a primary capacitation event in boar spermatozoa

In order to extend the static information of immunolabelling sulphogalactolipids in fixed boar spermatozoa, a fluorescent sulphogalactolipid analogue, galactose(3-sulphate)-beta 1–1′[(N-lissamine rhodaminyl)-12-aminodode-canoyl]-sphingosine, was incorporated into plasma membranes of living spermatozoa and its lateral distribution over the sperm head was studied. The fluorescent lipid was enriched in the apical ridge subdomain of freshly ejaculated sperm cells. After sperm binding to the zona pellucida the lipid redistributed to the equatorial segment of the sperm surface. A similar shift occurred during capacitation in vitro with 2 mM CaCl2 or with 4% (w/v) bovine serum albumin. The desulphated derivative galactose-beta 1–1′[(N-lissamine rhodaminyl)-12-aminododecanoyl]-sphingosine was also incorporated into the plasma membrane of freshly ejaculated sperm cells and clearly stained the apical ridge subdomain and the (pre)-equatorial subdomains of the sperm heads. The desulphogalactolipid analogue showed a slightly faster migration to the equatorial segment of the sperm plasma membrane than did its sulphated counterpart. The measured fluorescence intensity distributions correlated linearly with the spatial probe distribution, which was checked by fluorescence lifetime imaging microscopy. The observed migration of the incorporated glycolipids precedes the acrosome reaction and is one of the underlying molecular events likely to be important in the process of sperm capacitation. The results of this study suggest that lipid phase segregation is an important driving force for the organization of the sperm head plasma membrane into subdomains.

Myelin basic protein mediates extracellular signals that regulate microtubule stability in oligodendrocyte membrane sheets

Treatment of cultured oligodendrocytes with a monoclonal antibody to galactocerebroside (GalC) triggers a cascade of events including the redistribution of membrane surface GalC over internal domains of MBP and loss of microtubular structures within the sheets (Dyer and Benjamins: J Neurosci 8:4307-4318, 1988; Dyer and Benjamins: J Neurosci Res 24:212-221, 1989). In this report, wild type and myelin basic protein (MBP)-deficient shiverer oligodendrocytes were used to study the possible relationships between these events, and specifically to determine if MBP mediates signals which destabilize microtubular assemblies in cultured oligodendrocytes. We now show that MBP and GalC, which are both initially Triton X-100 soluble, become Triton X-100 insoluble following anti-GalC binding and anti-GalC:GalC complex redistribution, suggesting that the surface anti-GalC: GalC complexes become associated with cytoplasmic MBP. Mediation of the signaling event by MBP is further demonstrated by 1) a decreased phosphorylation of MBP in wild type oligodendrocytes after antibody binding, and 2) the absence of responses, such as GalC redistribution and microtubule loss, in MBP-deficient shiverer oligodendrocytes treated with anti-GalC. Continuous activation of the GalC/MBP pathway for 7 days in wild type oligodendrocytes results in enlarged cell bodies and production of numerous microprocesses, a morphology that is similar to MBP-deficient shiverer oligodendrocytes. A second signaling pathway which produces an opposite effect, i.e., the stabilization and apparent up-regulation of microtubular structures in cultured oligodendrocyte membrane sheets, remains functional in shiverer oligodendrocytes. Thus, MBP appears to be important for mediating extracellular signals that cause a loss of microtubular structures in oligodendrocyte membrane sheets and abnormal morphology.

Developmental expression of MOSP in cultured oligodendrocytes

Myelin/oligodendrocyte specific protein (MOSP) is a recently characterized 48 kDa surface membrane protein that is expressed exclusively by oligodendrocytes in the CNS. In this report, evidence is presented for the identification of the stage in the oligodendrocyte lineage when MOSP is first expressed. MOSP initially appears on immature oligodendrocytes about four to five days postnatal, which is about one to two days after the appearance of galactocerebroside and sulfatide. The initial expression of MOSP occurs at the stage in development when oligodendrocytes are elaborating processes and just beginning to form membrane sheets. Since 1) MOSP is capable of signaling increases in microtubular structures in oligodendrocytes and 2) microtubular structures may be essential for extension of growing processes and the formation of membrane sheaths, MOSP may play an important role in differentiation of oligodendrocytes and the formation of myelin.

Isolation, characterization, and expression of cDNA clones that encode rat UDP-galactose: ceramide galactosyltransferase

UDP-galactose:ceramide galactosyltransferase (CGT) (EC. 2.4.1.62) catalyzes the final step in the synthesis of galactocerebroside (GalC), a glycosphingolipid found in high amounts in the myelin sheath. Here, the isolation of rat CGT specific cDNA clones is reported. The CGT sequence contains an open reading frame of 1,623 bp which predicts a protein of M(r) 61,126 Da. In transfection experiments the cDNA was found to confer CGT activity to Chinese hamster ovary cells. In rat brain the developmental expression pattern of CGT mRNA was similar to the myelination profile, whereas the sciatic nerve contained high amounts of CGT message over a long developmental period. CGT mRNA expression in the sciatic nerve was found to drop substantially following nerve injury and recover slowly when compared to the expression of mRNAs specific for the predominant myelin-specific proteins. The absolute amounts of CGT message in sciatic nerve and brain were found to be comparable to those that encode the structural proteins of myelin. Except for low amounts in the kidney, the CGT mRNA was not detected in other tissues examined. Southern blot analysis revealed that the CGT protein is likely encoded by a single, relatively large gene.

Locus coeruleus neuron growth cones and spinal cord regeneration

One of the major impediments to successful recovery of function after a spinal cord injury is thought to be the reaction of the neuronal growth cone to inhibitory influences in the local environment in or around the site of the injury. The growth cones of locus coeruleus neurons studied in vitro collapsed upon contact with an extract of CNS myelin but did not collapse on contact with an extract of PNS myelin. Coincident with the collapse of the growth cone, was an increase in internal free calcium concentration that was predominantly the result of an influx of calcium through channels in the plasma membrane. Omega-conotoxin, which specifically blocks N-type voltage-gated calcium channels, blocked both the myelin-induced calcium influx and the coincident collapse of the growth cone.