Glycolipids and transmembrane signaling in oligodendroglia

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.

Adult CST-null mice maintain an increased number of oligodendrocytes

The galactolipids galactocerebroside and sulfatide have been implicated in oligodendrocyte (OL) development and myelin formation. Much of the early evidence for myelin galactolipid function has been derived from antibody and chemical perturbation of OLs in vitro. To determine the role of these lipids in vivo, we previously characterized mice lacking galactocerebroside and sulfatide and observed abundant, unstable myelin and an increased number of OLs. We have also reported that mice incapable of synthesizing sulfatide (CST-null) while maintaining normal levels of galactocerebroside generate relatively stable myelin with unstable paranodes. Additionally, Hirahara et al. (2004; Glia 45:269-277) reported that these CST-null mice also contain an increased number of OLs in the forebrain, medulla, and cerebellum at 7 days of age. Here, we further the findings of Hirahara et al. by demonstrating that the number of OLs in the CST-null mice is also increased in the spinal cord and that this elevated OL population is maintained through, at least, 7 months of age. Moreover, we show that the enhanced OL population is accompanied by increased proliferation and decreased apoptosis of oligodendrocytic-lineage cells. Finally, through ultrastructural analysis, we show that the CST-null OLs exhibit decreased morphological complexity, a feature that may result in decreased OL competition and increased OL survival.

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.

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.

Co-clustering of galactosylceramide and membrane proteins in oligodendrocyte membranes on interaction with polyvalent carbohydrate and prevention by an intact cytoskeleton

We have shown previously that addition of liposomes containing the two major glycosphingolipids of myelin, galactosylceramide (GalC) and cerebroside sulfate (CBS), to cultured oligodendrocytes (OLs) caused clustering of GalC on the extracellular surface and myelin basic protein (MBP) on the cytosolic surface to the same membrane domains. It also caused depolymerization of actin microfilaments and microtubules, indicating that interaction of the liposomes with the OL surface induces transmembrane signal transmission. We show that a multivalent form of galactose conjugated to bovine serum albumin has a similar effect as the multivalent GalC/CBS-containing liposomes. Because GalC and CBS can interact with each other across apposed membranes and because anti-GalC and anti-CBS antibodies also cause redistribution of GalC/CBS and depolymerization of microtubules, we believe that the multivalent carbohydrate interacts with GalC and CBS in the OL membrane. Several myelin-specific transmembrane proteins could be involved in this transmembrane signal transmission from GalC/CBS. We looked at co-clustering of several myelin constituents by confocal microscopy to determine if they are located in or redistribute to GalC/MBP-containing domains. Myelin oligodendrocyte glycoprotein (MOG), proteolipid protein (PLP), MAPK, and some phosphotyrosine-containing proteins were found to co-cluster with GalC and MBP, but myelin-associated glycoprotein (MAG) and phosphatidylinositol-4,5-bisphosphate (PIP(2)) did not. These results suggest that MOG and PLP, but probably not MAG, are possible candidates for transmembrane transmission of the signal received by GalC/CBS. To determine if depolymerization of actin microfilaments was required for co-clustering, or was secondary to clustering, we stabilized F-actin with jasplakinolide. This also prevented depolymerization of the microtubules and prevented clustering of all constituents, including GalC. The prevention of clustering or redistribution of these glycolipids and proteins by an intact cytoskeleton is consistent with the picket fence model.

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.

Trans interactions between galactosylceramide and cerebroside sulfate across apposed bilayers

The two glycosphingolipids galactosylceramide (GalC) and its sulfated form, cerebroside sulfate (CBS), are present at high concentrations in the multilayered myelin sheath and are involved in carbohydrate-carbohydrate interactions between the lipid headgroups. In order to study the structure of the complex of these two glycolipids by Fourier transform infrared (FTIR) spectroscopy, GalC dispersions were combined with CBS dispersions in the presence and absence of Ca(2+). The FTIR spectra indicated that a strong interaction occurred between these glycolipids even in the absence of Ca(2+). The interaction resulted in dehydration of the sulfate, changes in the intermolecular hydrogen bonding interactions of the sugar and other oxygens, decreased intermolecular hydrogen bonding of the amide C==O of GalC and dehydration of the amide region of one or both of the lipids in the mixture, and disordering of the hydrocarbon chains of both lipids. The spectra also show that Ca(2+) interacts with the sulfate of CBS. Although they do not reveal which other groups of CBS and GalC interact with Ca(2+) or which groups participate in the interaction between the two lipids, they do show that the sulfate is not directly involved in interaction with GalC, since it can still bind to Ca(2+) in the mixture. The interaction between these two lipids could be either a lateral cis interaction in the same bilayer or a trans interaction between apposed bilayers. The type of interaction between the lipids, cis or trans, was investigated using fluorescent and spin-label probes and anti-glycolipid antibodies. The results confirmed a strong interaction between the GalC and the CBS microstructures. They suggested further that this interaction caused the CBS microstructures to be disrupted so that CBS formed a single bilayer around the GalC multilayered microstructures, thus sequestering GalC from the external aqueous phase. Thus the CBS and GalC interacted via a trans interaction across apposed bilayers, which resulted in dehydration of the headgroup and interface region of both lipid bilayers. The strong interaction between these lipids may be involved in stabilization of the myelin sheath.

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.

Myelin glycolipids and their functions

During myelination, oligodendrocytes in the CNS and Schwann cells in the PNS synthesise myelin-specific proteins and lipids for the assembly of the axon myelin sheath. A dominant class of lipids in the myelin bilayer are the glycolipids, which include galactocerebroside (GalC), galactosulfatide (sGalC) and galactodiglyceride (GalDG). A promising approach for unravelling the roles played by various lipids in the myelin membrane involves knocking out the genes encoding important enzymes in lipid biosynthesis. The recent ablation of the ceramide galactosyltransferase ( cgt) gene in mice is the first example. The cgt gene encodes a key enzyme in glycolipid biosynthesis. Its absence causes glycolipid deficiency in the lipid bilayer, breakdown of axon insulation and loss of saltatory conduction. Additional knock-out studies should provide important insights into the various functions of glycolipids in myelinogenesis and myelin structure.

Monoclonal remyelination-promoting natural autoantibody SCH 94.03: pharmacokinetics and in vivo targets within demyelinated spinal cord in a mouse model of multiple sclerosis

Chronic inflammatory demyelination of the central nervous system is usually incompletely repaired. However, we previously reported that in vivo treatment with monoclonal antibody SCH 94.03 (produced using spinal cord homogenate as an immunogen) increased myelin repair 4-fold in the Theiler's virus mouse model of chronic progressive multiple sclerosis (Miller et al., 1994; J. Neurosci. 14: 6230-6238). A major issue regarding site and mechanism of action of this antibody is whether SCH 94.03 enters demyelinated CNS lesions and reacts with oligodendrocytes and myelin. To address this question, we radiolabeled SCH 94.03 and studied its distribution into tissues, pharmacokinetics, and binding to cells within demyelinating spinal cord lesions in vivo. SCH 94.03 distributed widely into extracellular water following intraperitoneal injection and was eliminated with a terminal half-life of 3-4.5 days. Only a portion of the total dose (0.4%) entered brain and spinal cord. SCH 94.03 accumulated 1.5-2.0-fold in brain between 1 and 7 days after injection, but its pharmacokinetics were otherwise similar to those of an isotype control IgMkappa antibody. Oligodendrocytes, myelin sheaths and, less frequently, axons were labeled within demyelinating lesions as detected by light and electron microscopic autoradiography. These findings suggest that remyelination-promoting autoantibodies could act within the demyelinating lesion of the central nervous system by binding to the oligodendrocyte, myelin, or axon.

Promotion of endogenous remyelination in multiple sclerosis

Studies in both human and experimental models demonstrate that myelin repair occurs in the central nervous system and is a normal physiologic response to myelin injury. However, remyelination in MS is often incomplete and limited. The outcome of an actively demyelinating lesion depends on the balance between factors promoting myelin destruction and myelin repair. Experimental models of CNS demyelination provide an opportunity to investigate the morphologic, cellular and molecular mechanisms involved in remyelination. This review focuses on experiments using the Theiler's virus model of demylination which indicate that manipulation of the immune response has the potential to promote endogenous CNS remyelination and functional recovery in MS.

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.

Patterns of reactivity with anti-glycolipid antibodies in human primary brain tumors

Antibodies against carbohydrates of three glycolipids were used to determine patterns of immunohistochemical reactivity of histologically identifiable cell subpopulations in 101 human primary brain tumors. For all tumor types fibrillary cells, polar cells, and gemistocytes (commonly seen in astrocytomas and ependymomas) stained more frequently for galactosylcerebroside with mAbO1 than small tumor cells and macrophages. Frequency of staining for sulfatide with mAbO4 was fibrillary > polar > small cells = macrophages. Gemistocytes stained more frequently with mAbO4 than polar cells in all tumors except low grade astrocytomas. These data indicate that tumors classified on histological grounds as astrocytic are often stained with antibodies that recognize oligodendrocytes and their progenitors. Thus, anti-glycolipid antibodies used in the study of developmental lineage may offer useful tools for classification of human brain tumors. Staining of fibrillary cells, polar cells, and gemistocytes for paragloboside directly with mAb F1H11 was much less common than with mAbO1, but this increased by pretreatment of the tissues with neuraminidase (F1H11 + N). Of particular note was the finding that small tumor cells frequently stained with F1H11 + N. Evidence that these were not macrophages was obtained using double immunostaining with F1H11 + N and anti-macrophage antibodies. In astrocytomas the frequency of small tumor cells immunostained with F1H11 + N was high grade > anaplastic > low grade, demonstrating a correlation of this tumor cell population with more aggressive astrocytomas. Thus, immunostaining with F1H11 + N may be of value in identifying small, anaplastic tumor cells, especially in small biopsies or tissue taken adjacent to the main tumor mass.

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.

Neutral glycolipid composition of primary human brain tumors

Neutral glycolipids (NGL) were isolated and quantitated in 98 primary human brain tumors; 19 low grade astrocytomas (LGA), 12 anaplastic astrocytomas (AA), 37 high grade astrocytomas (HGA), 18 oligodendroglial tumors, and 12 primitive neuroectodermal tumors (PNET). In 38 of these, the nature of the hexose in the cerebroside was determined using immunothin-layer chromatographic techniques. Galactosylceramide (GalCer) was the major ceramide monohexoside (CMH), and glucosylcerebroside never comprised more than 6% of this fraction in any tumor type. Furthermore, there was no correlation between the proportion of glucosylcerebroside and histological diagnosis. AA had the most characteristic neutral glycolipid pattern, with high levels of total lipid, total neutral glycolipid, CMH, and ceramide dihexoside (CDH) but low water contents. Consistent with this glycolipid composition is the finding that AA usually had neither ceramide trihexoside (CTH) nor globoside. Oligodendrogliomas were somewhat similar to AA in having high levels of CMH and infrequently having CTH or globoside. However, oligodendrogliomas had low water and total lipid contents. PNET had low levels of total lipid, total NGL, and CMH, but frequently contained CTH and globoside. LGA had high water contents but low levels of total lipid and CMH. HGA tended to have intermediate levels of almost all constituents analyzed, probably reflecting the pronounced cellular heterogeneity of these tumors. The frequent presence of GalCer in astrocytomas raises the possibility that some of these contain a population of cells that are related to the oligodendroglial lineage. However, the low amounts of GalCer and infrequent presence of sulfatide in PNET is consistent with their lack of differentiation toward oligodendrocytes. It will be of interest to determine if the neutral glycolipid patterns reported here will correlate with patient survival and be of prognostic significance.

Immune promotion of central nervous system remyelination

Remyelination by oligodendrocytes is the normal response to injury of the central nervous system following experimental demyelination by toxins and viruses in rodents. By contrast, in immune-mediated myelin disorders such as human MS, Theiler's virus-induced demyelination or EAE, remyelination is incomplete. We have considered two hypotheses to explain why myelin repair is incomplete in these disorders. Hypothesis I is that myelin repair is the normal consequence of primary myelin injury but there are immune factors which prevent its full expression. To test hypothesis I, we depleted T cells in Theiler's virus infected mice with cyclophosphamide or with monoclonal antibodies to CD4, CD8, or immune response gene products (Ia). Enhanced remyelination and proliferation of glial cells was observed in mice depleted of CD4+ or CD8+ T cells. Hypothesis II is that there are immune factors within some demyelinated lesions which, when present, promote new myelin synthesis. We envision these factors to be present in those lesions showing remyelination but absent in those lesions that remain demyelinated. To test hypothesis II, we generated polyclonal immunoglobulins directed against normal CNS antigens. Transfer of immunoglobulins from mice immunized repeatedly with spinal cord homogenate resulted in 4-5-fold enhancement of remyelination in Theiler's virus infected mice. We have also generated a series of monoclonal antibodies directed against normal autoantigens which also promote CNS remyelination. These experiments support the concept that full CNS remyelination is possible in human demyelinating diseases such as MS. Manipulation of the immune response either by inhibiting the function of T cells or by treatment with immunoglobulins (possibly normal autoantibodies) appears to promote remyelination. These experiments provide hope for patients with fixed neurological deficits for whom there are currently no available therapies.

Glial calcium

This review summarizes current knowledge relating intracellular calcium and glial function. During steady state, glia maintain a low cytosolic calcium level by pumping calcium into intracellular stores and by extruding calcium across the plasma membrane. Glial Ca2+ increases in response to a variety of physiological stimuli. Some stimuli open membrane calcium channels, others release calcium from intracellular stores, and some do both. The temporal and spatial complexity of glial cytosolic calcium changes suggest that these responses may form the basis of an intracellular or intercellular signaling system. Cytosolic calcium rises effect changes in glial structure and function through protein kinases, phospholipases, and direct interaction with lipid and protein constituents. Ultimately, calcium signaling influence glial gene expression, development, metabolism, and regulation of the extracellular milieu. Disturbances in glial calcium homeostasis may have a role in certain pathological conditions. The discovery of complex calcium-based glial signaling systems, capable of sensing and influencing neural activity, suggest a more integrated neuro-glial model of information processing in the central nervous system.

Central nervous system demyelination and remyelination in multiple sclerosis and viral models of disease

The mechanisms of myelin injury and repair were studied in acute multiple sclerosis lesions and in a murine model of demyelination induced by a virus. Injury to oligodendrocytes resulting in degeneration of inner glial loops and inner myelin lamellae (dying-back oligodendrogliopathy) was observed by electron microscopy in brain biopsies of acute demyelinating lesions. Attempts at central nervous system remyelination as manifested by thinly myelinated axons and proliferation of oligodendrocytes were observed at the edge of many acute plaques. To develop therapeutic strategies to inhibit demyelination or promote remyelination, mice infected intracranially with Theiler's virus (a picornavirus) were studied. Experimental manipulation of Theiler's virus-infected mice by treatment during chronic demyelinating disease with immunoglobulins directed at normal spinal cord antigens or with monoclonal antibodies which deplete CD4 or CD8-positive T cells resulted in augmentation of new myelin synthesis. These observations suggest that disturbances in the myelinating function of oligodendrocytes, events not accompanied by death of these cells, may be among the earliest pathological events in multiple sclerosis. Experiments using the Theiler's virus model of demyelination indicate that manipulation of the immune response has the potential to promote central nervous system remyelination and functional recovery in multiple sclerosis.