Glycolipids and transmembrane signaling: antibodies to galactocerebroside cause an influx of calcium in oligodendrocytes

Nervonic acid and its sphingolipids: Biological functions and potential food applications

Nervonic acid, a 24-carbon fatty acid with only one double bond at the 9th carbon (C24:1n-9), is abundant in the human brain, liver, and kidney. It not only functions in free form but also serves as a critical component of sphingolipids which participate in many biological processes such as cell membrane formation, apoptosis, and neurotransmission. Recent studies show that nervonic acid supplementation is not only beneficial to human health but also can improve the many medical conditions such as neurological diseases, cancers, diabetes, obesity, and their complications. Nervonic acid and its sphingomyelins serve as a special material for myelination in infants and remyelination patients with multiple sclerosis. Besides, the administration of nervonic acid is reported to reduce motor disorder in mice with Parkinson's disease and limit weight gain. Perturbations of nervonic acid and its sphingolipids might lead to the pathogenesis of many diseases and understanding these mechanisms is critical for investigating potential therapeutic approaches for such diseases. However, available studies about this aspect are limited. In this review, relevant findings about functional mechanisms of nervonic acid have been comprehensively and systematically described, focusing on four interconnected functions: cellular structure, signaling, anti-inflammation, lipid mobilization, and their related diseases.

Wrapping axons in mammals and Drosophila: Different lipids, same principle

Plasma membranes of axon-wrapping glial cells develop specific cylindrical bilayer membranes that surround thin individual axons or axon bundles. Axons are wrapped with single layered glial cells in lower organisms whereas in the mammalian nervous system, axons are surrounded with a characteristic complex multilamellar myelin structure. The high content of lipids in myelin suggests that lipids play crucial roles in the structure and function of myelin. The most striking feature of myelin lipids is the high content of galactosylceramide (GalCer). Serological and genetic studies indicate that GalCer plays a key role in the formation and function of the myelin sheath in mammals. In contrast to mammals, Drosophila lacks GalCer. Instead of GalCer, ceramide phosphoethanolamine (CPE) has an important role to ensheath axons with glial cells in Drosophila. GalCer and CPE share similar physical properties: both lipids have a high phase transition temperature and high packing, are immiscible with cholesterol and form helical liposomes. These properties are caused by both the strong headgroup interactions and the tight packing resulting from the small size of the headgroup and the hydrogen bonds between lipid molecules. These results suggest that mammals and Drosophila wrap axons using different lipids but the same conserved principle.

Oligodendroglial membrane dynamics in relation to myelin biogenesis

In the central nervous system, oligodendrocytes synthesize a specialized membrane, the myelin membrane, which enwraps the axons in a multilamellar fashion to provide fast action potential conduction and to ensure axonal integrity. When compared to other membranes, the composition of myelin membranes is unique with its relatively high lipid to protein ratio. Their biogenesis is quite complex and requires a tight regulation of sequential events, which are deregulated in demyelinating diseases such as multiple sclerosis. To devise strategies for remedying such defects, it is crucial to understand molecular mechanisms that underlie myelin assembly and dynamics, including the ability of specific lipids to organize proteins and/or mediate protein-protein interactions in healthy versus diseased myelin membranes. The tight regulation of myelin membrane formation has been widely investigated with classical biochemical and cell biological techniques, both in vitro and in vivo. However, our knowledge about myelin membrane dynamics, such as membrane fluidity in conjunction with the movement/diffusion of proteins and lipids in the membrane and the specificity and role of distinct lipid-protein and protein-protein interactions, is limited. Here, we provide an overview of recent findings about the myelin structure in terms of myelin lipids, proteins and membrane microdomains. To give insight into myelin membrane dynamics, we will particularly highlight the application of model membranes and advanced biophysical techniques, i.e., approaches which clearly provide an added value to insight obtained by classical biochemical techniques.

Intact lipid imaging of mouse brain samples: MALDI, nanoparticle-laser desorption ionization, and 40 keV argon cluster secondary ion mass spectrometry

We have investigated the capability of nanoparticle-assisted laser desorption ionization mass spectrometry (NP-LDI MS), matrix-assisted laser desorption ionization (MALDI) MS, and gas cluster ion beam secondary ion mass spectrometry (GCIB SIMS) to provide maximum information available in lipid analysis and imaging of mouse brain tissue. The use of Au nanoparticles deposited as a matrix for NP-LDI MS is compared to MALDI and SIMS analysis of mouse brain tissue and allows selective detection and imaging of groups of lipid molecular ion species localizing in the white matter differently from those observed using conventional MALDI with improved imaging potential. We demonstrate that high-energy (40 keV) GCIB SIMS can act as a semi-soft ionization method to extend the useful mass range of SIMS imaging to analyze and image intact lipids in biological samples, closing the gap between conventional SIMS and MALDI techniques. The GCIB SIMS allowed the detection of more intact lipid compounds in the mouse brain compared to MALDI with regular organic matrices. The 40 keV GCIB SIMS also produced peaks observed in the NP-LDI analysis, and these peaks were strongly enhanced in intensity by exposure of the sample to trifluororacetic acid (TFA) vapor prior to analysis. These MS techniques for imaging of different types of lipids create a potential overlap and cross point that can enhance the information for imaging lipids in biological tissue sections.

Graphical abstract

Cross-Linking Mast Cell Specific Gangliosides Stimulates the Release of Newly Formed Lipid Mediators and Newly Synthesized Cytokines

Mast cells are immunoregulatory cells that participate in inflammatory processes. Cross-linking mast cell specific GD1b derived gangliosides by mAbAA4 results in partial activation of mast cells without the release of preformed mediators. The present study examines the release of newly formed and newly synthesized mediators following ganglioside cross-linking. Cross-linking the gangliosides with mAbAA4 released the newly formed lipid mediators, prostaglandins D2 and E2, without release of leukotrienes B4 and C4. The effect of cross-linking these gangliosides on the activation of enzymes in the arachidonate cascade was then investigated. Ganglioside cross-linking resulted in phosphorylation of cytosolic phospholipase A2 and increased expression of cyclooxygenase-2. Translocation of 5-lipoxygenase from the cytosol to the nucleus was not induced by ganglioside cross-linking. Cross-linking of GD1b derived gangliosides also resulted in the release of the newly synthesized mediators, interleukin-4, interleukin-6, and TNF-α. The effect of cross-linking the gangliosides on the MAP kinase pathway was then investigated. Cross-linking the gangliosides induced the phosphorylation of ERK1/2, JNK1/2, and p38 as well as activating both NFκB and NFAT in a Syk-dependent manner. Therefore, cross-linking the mast cell specific GD1b derived gangliosides results in the activation of signaling pathways that culminate with the release of newly formed and newly synthesized mediators.

[A case of pure-sensory-type Guillain-Barré syndrome with galactocerebroside antibody]

A 67-year-old man noticed paresthesia in both legs in July 2011. Three days later, he was found on a street where he was unable to stand up. On admission, the deep sensation in both legs was severely disturbed, but their muscle strength remained normal. Cranial nerves and autonomic functions were normal. The deep tendon reflexes were diminished in both legs. Magnetic resonance imaging of the spine was normal. Motor nerve conduction studies revealed normal conduction velocity, amplitude, and F-wave latency. However, sensory nerve conduction studies revealed severe reduction of amplitude in the upper and lower extremities. Cerebrospinal fluid analysis showed normal cell counts but elevated protein levels. Screening for glycolipid antibodies showed a selective increase of galactocerebroside (Gal-C) IgG antibody. We diagnosed him with pure-sensory-type Guillain-Barré syndrome (GBS). We administered intravenous immunoglobulin (IVIG) for 5 days. After IVIG therapy, his gait disturbance improved slightly but the disturbance of deep sensation remained severe and he was transferred to a rehabilitation ward 53 days after admission. To the best of our knowledge, this is the first report of a case of pure-sensory-type GBS with Gal-C antibody alone. This case suggests a close relationship between Gal-C antibody and sensory nerve disturbance.

ACTH protects mature oligodendroglia from excitotoxic and inflammation-related damage in vitro

Corticosteroids (CS) are widely employed to treat relapses in multiple sclerosis (MS). Endogenous ACTH is a 39-amino acid peptide that, among other functions, stimulates CS production. Exogenous ACTH 1-39 is used to treat MS relapses, presumably by stimulating endogenous CS production. However, unlike CS, ACTH binds to melanocortin receptors, found in the central nervous system (CNS) as well as on inflammatory cells. Since glia are implicated in MS and other neurodegenerative diseases, and oligodendroglia (OL) are more sensitive to injury than other glia, we characterized the protective effects of ACTH on OL in vitro without the confounding effects of CS. Rat brain cultures containing OL, astrocytes (AS), and microglia (MG) were incubated for 1 day with potentially cytotoxic agents with or without preincubation with ACTH 1-39. The cytotoxic agents killed 55-70% of mature OL, but caused little or no death of AS or MG at the concentrations used. ACTH protected OL from death induced by staurosporine, AMPA, NMDA, kainate, quinolinic acid, or reactive oxygen species, but did not protect against kynurenic acid or nitric oxide. The protective effects of ACTH were dose dependent, and decreased OL death induced by the different agents by 30-60% at 200 nM ACTH. We show for the first time that melanocortin 4 receptor is expressed on OL in addition to MG and AS. In summary, ACTH 1-39 protects OL in vitro from several excitotoxic and inflammation-related insults. ACTH may be activating melanocortin receptors on OL or alternately on AS or MG to prevent OL death.

A case with anti-galactocerebroside antibody-positive Mycoplasma pneumoniae meningoencephalitis presenting secondary hypersomnia

Galactocerebroside (Gal-C) is a major myelin component in the central nervous system. The anti-Gal-C antibody induced by mycoplasma infection may therefore be involved in the pathogenic mechanisms of mycoplasma-associated encephalitis. Here we report an adult case of mycoplasma encephalitis developing excessive daytime sleepiness. Brain MRI suggested that hypothalamic involvement was compatible with hypersomnia. This finding was corroborated by decreased hypocretin-1 in cerebrospinal fluid (CSF) and the manifestation of diabetes insipidus. Screening for anti-glycolipid antibody profiles showed the selective increase of serum anti-Gal-C antibody. After treatment with minocyclin, the patient's daytime sleepiness was markedly improved and the CSF hypocretin-1 level became almost normal, as well. It is known that CSF hypocretin-1 is decreased in Guillain-Barré syndrome mediated by anti-glycolipid antibody, suggesting a possible mechanistic link between anti-glycolipid antibodies and hypothalamic involvement. The present case further emphasizes the broad spectrum of neurological complications after mycoplasma infection.

Ceramide galactosyltransferase (UGT8) is a molecular marker of breast cancer malignancy and lung metastases

BACKGROUND

It was shown recently on the level of gene expression that UGT8, coding UDP-galactose:ceramide galactosyltransferase, is one of six genes whose elevated expression correlated with a significantly increased the risk of lung metastases in breast cancer patients. In this study primary tumours and their lung metastases as well as breast cancer cell lines were analysed for UGT8 expression at the protein level.

METHODS

Expression of UGT8 in breast cancer tissue specimens and breast cancer cell lines was analysed using IHC, real-time PCR and Western blotting.

RESULTS

Comparison of the average values of the reaction intensities (IRS scale) showed a significant difference in UGT8 expression between (1) primary and metastatic tumours (Mann-Whitney U, P<0.05), (2) tumours of malignancy grades G3 and G2 (Mann-Whitney U, P<0.01) as well as G3 and G1 (Mann-Whitney U, P<0.001) and (3) node-positive and node-negative tumours (Mann-Whitney U, P<0.001). The predictive ability of increased expression of UGT8 was validated at the mRNA level in three independent cohorts of breast cancer patients (721). Similarly, breast cancer cell lines with the 'luminal epithelial-like' phenotype did not express or weakly expressed UGT8, in contrast to malignant, 'mesenchymal-like,' cells forming metastases in nude mice.

CONCLUSION

Our data suggest that UGT8 is a significant index of tumour aggressiveness and a potential marker for the prognostic evaluation of lung metastases in breast cancer.

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.

Oligodendrocyte development and myelin biogenesis: parsing out the roles of glycosphingolipids

The myelin sheath is an extension of the oligoddendrocyte (OL) plasma membrane enriched in lipids that ensheaths the axons of the central and peripheral nervous system. Here, we review the involvement of glycosphingolipids in myelin/OL functions, including the regulation of OL differentiation, lipid raft-mediated trafficking and signaling, and neuron-glia interactions.

Immunostaining for oligodendrocyte-specific galactosphingolipids in fixed brain sections using the cholesterol-selective detergent digitonin

Galactocerebroside (GalC) and its sulfated derivative sulfatide (SUL) are galactosphingolipids abundantly expressed in oligodendrocytes (OLs). Despite their biological importance in OL development and function, attempts to visualize GalC/SUL in tissue sections have met with limited success. This is at least in part because permeabilization of tissue sections with detergents such as Triton X-100 results in significant degradation of GalC/SUL immunoreactivity. Here we establish a novel method that enables visualization of endogenous GalC/SUL in OLs and myelin throughout the entire depth of brain sections. We show that treating brain sections with the cholesterol-specific detergent digitonin instead of Triton X-100 or methanol leads to efficient antibody penetration into tissue sections without disrupting GalC/SUL immunoreactivity. We also determine the optimal concentrations of digitonin using confocal microscopy. With our method, the morphology and the number of GalC/SUL-expressing OLs can be visualized three-dimensionally. Furthermore, our method is applicable to double immunostaining with anti-GalC/SUL antibody and other antibodies which recognize intracellular antigens. Our simple method using digitonin should prove to be useful in enabling detailed examination of GalC/SUL expression in the brain in both physiological and pathological conditions.

Probing the influence of myelin and glia on the tensile properties of the spinal cord

Although glia have been historically classified as the structurally supporting cells of the central nervous system, their role in tissue mechanics is still largely unstudied. The influence of myelin and glia on the mechanical properties of spinal cord tissue was examined by testing embryonic day 18 chick embryo spinal cords in uniaxial tension following disruption of the glial matrix using either ethidium bromide (EB) or an antibody against galactocerebroside (alphaGalC) in the presence of complement. Demyelination was confirmed by myelin basic protein immunoreactivity and quantified using osmium tetroxide staining. A substantial loss of astrocytes and oligodendrocytes concurrent with demyelination was observed following EB injection but not alphaGalC injection. No morphological changes were observed following injection of saline or IgG with complement as controls for EB and alphaGalC. Demyelinated spinal cords demonstrated significantly lower stiffness and ultimate tensile stress than myelinated spinal cords. No significant differences were observed in the tensile response between the two demyelinating protocols. The results demonstrate that the glial matrix provides significant mechanical support to the spinal cord, and suggests that myelin and cellular coupling of axons via the glial matrix in large part dictates the tensile response of the tissue.

Molecular "negativity" may underlie multiple sclerosis: role of the myelin basic protein family in the pathogenesis of MS

Myelin basic protein (MBP) binds to negatively charged lipids on the cytosolic surface of oligodendrocyte membranes and is responsible for adhesion of these surfaces in the multilayered myelin sheath. The pattern of extensive posttranslational modifications of MBP is dynamic during normal central nervous system development and during myelin degeneration in multiple sclerosis (MS), affecting its interactions with the myelin membranes and other proteins. In particular, the degree of deimination (or citrullination) of MBP is correlated with the severity of MS, and may represent a primary defect that precedes neurodegeneration due to autoimmune attack. That MBP deimination also affects topological accessibility of an otherwise partially buried immunodominant epitope of the protein indicates that this modification may play a major role in the autoimmune pathogenesis of the disease. In this chapter, we describe the structural and functional consequences of MBP deimination in healthy and diseased myelin.

Cyclic GMP-dependent pathways protect differentiated oligodendrocytes from multiple types of injury

The cyclic GMP analog 8-bromo-cyclic GMP (8-Br-cGMP) protects differentiated murine oligodendrocytes (OLs) from caspase-mediated death initiated by staurosporine, thapsigargin or kainate. Caspase-independent death caused by high levels of NO is also partially prevented by 8-Br-cGMP. Inhibitors of protein kinase G (cGMP-dependent protein kinase, cGK) reversed protection, supporting involvement of cGK. Since NO stimulates soluble guanylate cyclase, increasing cGMP, we treated OLs with low levels of NO and observed partial protection against thapsigargin, staurosporine and kainate. Two inhibitors of mitochondrial pore transition (MPT), cyclosporin A and bongkrekic acid, were poorly protective, indicating that cGMP is not acting primarily by blocking MPT. 8Br-cGMP was more effective than 8Br-cAMP in protecting against staurosporine or release of intracellular Ca(++) by thapsigargin. The cAMP analog exhibited little or no protection against kainate or high levels of NO. Thus cGK signaling is more effective than protein kinase A or phosphodiesterase 3 signaling in preventing OL death.

Partitioning of myelin basic protein into membrane microdomains in a spontaneously demyelinating mouse model for multiple sclerosisThis paper is one of a selection of papers published in this Special Issue, entitled CSBMCB — Membrane Proteins in Health and Disease

We have characterized the lipid rafts in myelin from a spontaneously demyelinating mouse line (ND4), and from control mice (CD1 background), as a function of age and severity of disease. Myelin was isolated from the brains of CD1 and ND4 mice at various ages, and cold lysed with 1.5% CHAPS (3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulphonate). The lysate was separated by low-speed centrifugation into supernatant and pellet fractions, which were characterized by Western blotting for myelin basic protein (MBP) isoforms and their post-translationally modified variants. We found that, with maturation and with disease progression, there was a specific redistribution of the 14–21.5 kDa MBP isoforms (classic exon-II-containing vs exon-II-lacking) and phosphorylated forms into the supernatant and pellet. Further fractionation of the supernatant to yield detergent-resistant membranes (DRMs), representing coalesced lipid rafts, showed these to be highly enriched in exon-II-lacking MBP isoforms, and deficient in methylated MBP variants, in mice of both genotypes. The DRMs from the ND4 mice appeared to be enriched in MBP phosphorylated by MAP kinase at Thr95 (murine 18.5 kDa numbering). These studies indicate that different splice isoforms and post-translationally modified charge variants of MBP are targeted to different microdomains in the myelin membrane, implying multifunctionality of this protein family in myelin maintenance.

Secretory products of central nervous system glial cells induce Schwann cell proliferation and protect from cytokine-mediated death

There continues to be interest in Schwann cells (SC) as a possible source of myelinating cells for transplantation into the central nervous system (CNS) of patients with multiple sclerosis (MS) and spinal cord injury. It has been suggested that CNS glial cells interfere with SC migration, survival, maturation, and clinically significant remyelination in the CNS. To investigate the effects of CNS glial cells on SC, we examined the effects of serum-free supernatants obtained from rat mixed CNS glial cultures on rat neonatal SC cultures. Supernatants from 1-, 3-, and 5-day CNS glial cultures induced proliferation of SC assayed at 5 days in vitro but did not induce SC differentiation as measured by induction of surface expression of galactolipids (GalL). High concentrations of cAMP simulate many of the effects of axolemma on SC; CNS glial cell supernatants did not inhibit cAMP induction of SC differentiation. CNS glial cell supernatants had no apparent effect on SC viability at 48 hr as measured by trypan blue exclusion. We have previously demonstrated that incubation of SC with transforming growth factor-beta1 (TGF-beta1) + tumor necrosis factor-alpha (TNF-alpha) induces SC death via apoptosis. We now show that CNS glial supernatants inhibits TGF-beta1/TNF-alpha-induced SC death. Our data show that soluble products of CNS glial cells do not induce or inhibit SC differentiation or increase cell death but have the potential to increase proliferation of SC and their resistance to cytokine-mediated death, and thus may affect the outcome of SC transplantation into the CNS.

The myelin-axolemmal complex: biochemical dissection and the role of galactosphingolipids

Myelin-axolemmal interactions regulate many cellular and molecular events, including gene expression, oligodendrocyte survival and ion channel clustering. Here we report the biochemical fractionation and enrichment of distinct subcellular domains from myelinated nerve fibers. Using antibodies against proteins found in compact myelin, non-compact myelin and axolemma, we show that a rigorous procedure designed to purify myelin also results in the isolation of the myelin-axolemmal complex, a high-affinity protein complex consisting of axonal and oligodendroglial components. Further, the isolation of distinct subcellular domains from galactolipid-deficient mice with disrupted axoglial junctions is altered in a manner consistent with the delocalization of axolemmal proteins observed in these animals. These results suggest a paradigm for identification of proteins involved in neuroglial signaling.

Interactions between glucosylceramide and galactosylceramide I(3) sulfate and microstructures formed

The monohexoside glycosphingolipids (GSLs), galactosylceramide (GalC), glucosylceramide (GluC), and their sulfated forms are abundant in cell membranes from a number of tissues. Carbohydrate-carbohydrate interactions between the head groups of some GSLs can occur across apposed membranes and may be involved in cell-cell interactions. In the present study, the ability of GluC to participate in trans interactions with galactosylceramide I(3) sulfate (CBS) was investigated by transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy. Gaucher's spleen GluC had polymorphic phase behavior; in its metastable state, it formed large wrinkled vesicles. It transformed to a stable state via an intermediate state in which the surface of the vesicles consisted of narrow ribbons. In the stable state, the narrow ribbons split off from the surface to form membrane fragments and flat and helical ribbons. The strength of the intermolecular hydrogen bonding interactions between the carbonyls increased in the order metastable<intermediate<stable state. Aqueous dispersions of GluC and CBS were combined to allow trans carbohydrate-carbohydrate interaction across apposed bilayers, or the lipids were premixed in a solvent before hydration to allow lateral cis interactions. Premixed dispersions of NFA-GluC and CBS remained in the metastable state even when incubated under stable state conditions. When NFA-GluC dispersions were combined with CBS dispersions, they had a small effect on each other's amide groups in the metastable state. Furthermore, conversion of NFA-GluC to the stable state was inhibited, although it reached the intermediate state, suggesting that some degree of trans interaction between these two lipids occurred.

Protection of mature oligodendrocytes by inhibitors of caspases and calpains

Mature mouse oligodendrocytes (OLs) are susceptible to death in demyelinating diseases such as multiple sclerosis and in brain injury following neurotrauma, ischemia, or stroke. To understand mechanisms leading to death of mature OLs and develop strategies for protection, we utilized cultures of mature mouse OLs to investigate the role of caspases and calpains in OL cell death mediated by different mechanisms. The agents used were (i) staurosporine, which induces apoptotic death via inhibition of protein kinases; (ii) kainate, which activates non-NMDA glutamate receptors; (iii) thapsigargin, which releases intracellular calcium stores; and (iv) SNAP, which releases active NO species and causes necrotic cell death. Inhibitors blocking primary effector caspases (including caspase 3), the FAS (death receptor)-mediated initiator caspases (including caspase 8), and stress-induced caspases (including caspase 9), were tested for their protective effects. Inhibition of caspases 3, 8, and 9 each robustly protected OLs following insult with staurosporine, thapsigargin, or kainate when added at optimal times. The time of addition of the inhibitors for maximal protection varied with the agent, from 1 h of preincubation before addition of staurosporine to 6 h after addition of kainate. Much less protection was seen for the NO generator SNAP under any condition. The role of calcium in OL death in each model was investigated by chelating extracellular Ca++ with EGTA, and by inhibiting the Ca++-activated calpain proteases. Calcium chelation did not protect against staurosporine, but decreased OL death initiated by kainate, thapsigargin, or NO. The calpain inhibitors PD150606 and calpain inhibitor I protected from cell death initiated by staurosporine, kainate, and thapsigargin, but not from cell death initiated by the NO donor SNAP.

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.

Glycosyltransferases in different brain regions during chick embryo development

Glycosphingolipids, in particular gangliosides, play a crucial role in neuronal development and are known to change dramatically in total content and distribution in different brain areas during embryogenesis. In the present work we analyzed the activity of enzymes involved in the metabolism of gangliosides, at different periods of functional maturation in different regions of chick embryo brain. Our data demonstrate differences in the enzymatic activities in the examined areas; these differences might be correlated with the functional lateralization occurring in the brain during development. Significative differences were found in glycosphingolipid composition between controlateral cerebral hemispheres and optic lobes; these results together with previous data we found, contribute to reinforce our hypothesis on the occurrence of biochemical lateralization during early brain development.

Detergent-insoluble glycosphingolipid/cholesterol microdomains of the myelin membrane

Glycosphingolipids and cholesterol form lateral assemblies, or lipid 'rafts', within biological membranes. Lipid rafts are routinely studied biochemically as low-density, detergent-insoluble complexes (in non-ionic detergents at 4 degrees C; DIGs, detergent-insoluble glycosphingolipid/cholesterol microdomains). Recent discrepancies recommended a re-evaluation of the conditions used for the biochemical analysis of lipid rafts. We have investigated the detergent insolubility of several known proteins present in the glycosphingolipid/cholesterol-rich myelin membrane, using four detergents representing different chemical classes (TX-100, CHAPS, Brij 96 and TX-102), under four conditions: detergent extraction of myelin either at (i) 4 degrees C or (ii) 37 degrees C, or at 4 degrees C after pre-extraction with (iii) saponin or (iv) methyl-beta-cyclodextrin (MbetaCD). Each detergent was different in its ability to solubilize myelin proteins and in the density of the DIGs produced. Brij 96 DIGs floated to a lower density than other detergents tested, possibly representing a subpopulation of DIGs in myelin. DIGs pre-extracted with saponin were denser than DIGs pre-extracted with MbetaCD. Furthermore, pre-extraction with MbetaCD solubilized proteolipid protein (known to associate with cholesterol), whereas pre-extraction with saponin did not, suggesting that saponin is less effective as a cholesterol-perturbing agent than is MbetaCD. These results demonstrate that DIGs isolated by different detergents are not necessarily comparable, and that these detergent-specific DIGs may represent distinct biochemical, and possibly physiological, entities based on the solubilities of specific lipids/proteins in each type of detergent.

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.

Structure of hydroxylated galactocerebrosides from myelin at the air-water interface

Hydroxy-galactocerebrosides (mixed chain length, constituent of myelin membranes) from bovine brain are investigated as monolayers at the air-water interface with isotherms, fluorescence microscopy, x-ray reflectivity and grazing incidence diffraction. With grazing incidence diffraction a monoclinic tilted chain lattice is found in the condensed phase. According to x-ray reflectivity, the longest chains protrude above the chain lattice and roughen the lipid/air interface. On compressing the chain lattice, the correlation length increases by approximately 65%; obviously, the sugar headgroups are flexible enough to allow for lattice deformation. With fluorescence experiments, small coexisting fluid and ordered domains are observed, and there is lipid dissolution into the subphase as well. The dissolved hydroxy-galactocerebroside molecules reenter on monolayer expansion. The electron density profiles derived from x-ray reflectometry (coherent superposition) show that the chain-ordering transition causes the molecules to grow into the subphase.

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.

Involvement of cellular cytoskeleton components in antibody-induced internalization of viral glycoproteins in pseudorabies virus-infected monocytes

Addition of pseudorabies virus (PrV)-specific polyclonal immunoglobulins to PrV-infected monocytes induces internalization of plasma membrane-anchored viral glycoproteins and this may interfere with antibody-dependent cell lysis. We investigated the role of actin, microtubules, clathrin, and dynein, the major cellular components involved in physiological endocytosis during this virological internalization. Porcine monocytes were infected in vitro for 13 h and afterward treated with different concentrations of colchicine, cytochalasin D, latrunculin B, and amantadine-HCl, which inhibit polymerization of microtubules, actin/clathrin, actin, and clathrin, respectively. This resulted in a significant reduction of internalization compared to the nontreated control, indicating that these components are involved in the process. A double labeling was performed during the internalization process and a clear colocalization of actin, microtubules, clathrin, and dynein with the viral glycoproteins was observed at different stages during the internalization process. We conclude that these cellular components are used by PrV to generate the antibody-induced internalization of viral glycoproteins.

Calcium signalling in cells of oligodendroglial lineage

Intracellular Ca2+ is the key signal that regulates the efficacy of neurotransmitter release and synaptic plasticity in neurons but is also an important second messenger involved in the signal transduction and modulation of gene expression in both excitable and non-excitable cells. Glial cells, including cells of oligodendroglial (OLG) lineage, are capable of responding to extracellular stimuli via changes in the intracellular Ca2+. This review article focuses on the mechanisms of Ca2+ signalling in cells of OLG lineage with the goal of providing the basis for understanding the relevance of receptor- and non-receptor-mediated signalling to oligodendroglial development, myelination, and demyelination. Conclusions to date indicate that cells of OLG lineage exhibit remarkable plasticity with regard to the expression of ion channels and receptors linked to Ca2+ signalling and that perturbation of [Ca2](i) homeostasis contributes to the pathogenesis of demyelinating diseases.

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.

Role of calcium in nitric oxide-induced cytotoxicity: EGTA protects mouse oligodendrocytes

Active nitrogen species are overproduced in inflammatory brain lesions in multiple sclerosis (MS) and experimental allergic encephalomyelitis (EAE). NO has been shown to mediate the death of oligodendrocytes (OLs), a primary target of damage in MS. To develop strategies to protect OLs, we examined the mechanisms of cytotoxicity of two NO donors, S-nitroso-N-acetyl-penicillamine (SNAP) and sodium nitroprusside (SNP) on mature mouse OLs. Nitrosonium ion (NO+) rather than NO. mediates damage with both SNAP and SNP, as shown by significant protection with hemoglobin (HbO2), but not with the NO. scavenger PTIO. SNAP and SNP differ in time course and mechanisms of killing OLs. With SNAP, OL death is delayed for at least 6 hr, but with SNP, OL death is continuous over 18 hr with no delay. Relative to NO release, SNP is more toxic than SNAP, due to synergism of NO with cyanide released by SNP. SNAP elicits a Ca2+ influx in over half of the OLs within min. Further, OL death due to NO release from SNAP is Ca2+-dependent, because the Ca2+ chelator EGTA protects OLs from killing by SNAP, and also from killing by the NONOates NOC-9 and NOC-18, which spontaneously release NO. SNP does not elicit a Ca2+ influx, and EGTA is not protective. In comparison to the N20.1 OL cell line (Boullerne et al., [1999] J. Neurochem. 72:1050-1060), mature OLs are (1) more sensitive to SNAP, (2) much more resistant to SNP, (3) sensitive to cyanide, but not iron, and (4) exhibit a Ca2+ influx and EGTA protection in response to NO generated by SNAP.

Human monoclonal antibodies reactive to oligodendrocytes promote remyelination in a model of multiple sclerosis

Promoting remyelination, a major goal of an effective treatment for demyelinating diseases, has the potential to protect vulnerable axons, increase conduction velocity, and improve neurologic deficits. Strategies to promote remyelination have focused on transplanting oligodendrocytes (OLs) or recruiting endogenous myelinating cells with trophic factors. Ig-based therapies, routinely used to treat a variety of neurological and autoimmune diseases, underlie our approach to enhance remyelination. We isolated two human mAbs directed against OL surface antigens that promoted significant remyelination in a virus-mediated model of multiple sclerosis. Four additional OL-binding human mAbs did not promote remyelination. Both human mAbs were as effective as human i.v. Ig, a treatment shown to have efficacy in multiple sclerosis, and bound to the surface of human OLs suggesting a direct effect of the mAbs on the cells responsible for myelination. Alternatively, targeting human mAbs to areas of central nervous system (CNS) pathology may facilitate the opsonization of myelin debris, allowing repair to proceed. Human mAbs were isolated from the sera of individuals with a form of monoclonal gammopathy. These individuals carry a high level of monoclonal protein in their blood without detriment, lending support to the belief that administration of these mAbs as a therapy would be safe. Our results are (i) consistent with the hypothesis that CNS-reactive mAbs, part of the normal Ig repertoire in humans, may help repair and protect the CNS from pathogenic immune injury, and (ii) further challenge the premise that Abs that bind OLs are necessarily pathogenic.

Cross-reactivity of anti-galactocerebroside autoantibodies with a Trypanosoma brucei proteolipidic epitope

Pathogenic mechanisms of the demyelinating encephalopathy featuring the nervous phase of human African trypanosomiasis (HAT) are largely unknown. They might include autoimmune disorders. A variety of autoantibodies is detected during the disease and we have previously evidenced anti-galactocerebroside (GalC) antibodies in the serum and cerebrospinal fluid (CSF) from patients in the nervous stage (stage II) of HAT. We now show that anti-GalC antibodies recognize an antigen located on the parasite membrane and common to different strains of trypanosomes. By using affinity chromatography with a rabbit anti-GalC antiserum, a 52-kD proteolipid was isolated from the membrane of Trypanosoma brucei (T. b.) brucei AnTat 1.9, AnTat 1. 1E, and T. b. rhodesiense Etat 1.2/R and Etat 1.2/S. Antibodies directed against this antigen were found in the CSF from patients with nervous stage HAT. These CSF also contained anti-GalC antibodies and adsorption with the proteolipid decreased anti-GalC reactivity. Immunization of mice with this antigen induced the production of antibodies which cross-reacted with GalC but no protection from experimental infection with T. b. brucei. These data support the hypothesis that anti-GalC antibodies detected in the CSF from HAT patients might be induced by molecular mimicry with a parasite antigen.

Functional roles of glycosphingolipids in signal transduction via lipid rafts

The formation of glycosphingolipid (GSL)-cholesterol microdomains in cell membranes has been proposed to function as platforms for the attachment of lipid-modified proteins, such as glycosylphosphatidylinositol (GPI)-anchored proteins and src-family tyrosine kinases. The microdomains are postulated to be involved in GPI-anchored protein signaling via src-family kinase. Here, the functional roles of GSLs in signal transduction mediated by the microdomains are discussed. Antibodies against GSLs co-precipitate GPI-anchored proteins, src-family kinases and several components of the microdomains. Antibody-mediated crosslinking of GSLs, as well as that of GPI-anchored proteins, induces a rapid activation of src-family kinases and a transient increase in the tyrosine phosphorylation of several substrates. Enzymatic degradation of GSLs reduces the activation of src-family kinase and tyrosine phosphorylation by antibody-mediated crosslinking of GPI-anchored protein. Furthermore, GSLs can also modulate signal transduction of immunoreceptors and growth factor receptors in the microdomains. Thus, GSLs have important roles in signal transduction mediated by the microdomains.

The oligodendroglia cytoskeleton in health and disease

Oligodendrocytes have a high rate of synthetic activity and produce vast amounts of myelin. The membrane production requires specific sorting and transport processes and structural support. In culture, oligodendrocytes extend flat membranous sheets containing an extensive cytoskeletal network of microtubules (MTs) and microfilaments (MFs). The microtubules participate in the elaboration and stabilization of the myelin-containing cellular processes and have an impact not only on the complex oligodendroglia architecture but also influence their functions. They participate in intracellular sorting processes and the translocation of myelin basic protein (MBP) mRNAs to the forming myelin sheath. The two major groups of neuronal microtubule-associated proteins (MAPs), MAP2 and tau are expressed in oligodendrocytes and might be involved in the regulation of MT stability and organization. Myelin-specific proteins, such as MBP and 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP), interact with the cytoskeleton. Glial changes occur in a variety of neurodegenerative diseases, and glial fibrillary tangles and glial cytoplasmic inclusions (GCls), containing abnormal microtubular structures which stain positively for stress proteins and microtubule-associated proteins, are found in oligodendrocytes of the affected brains. The role of MTs and their associated proteins in oligodendrocytes during normal development and pathological situations is specifically emphasized in this review.

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.

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.

Enhanced axonal regeneration following combined demyelination plus schwann cell transplantation therapy in the injured adult spinal cord

We have treated spinal cord injured rats with demyelination plus Schwann cell transplantation and assessed neurite outgrowth in a quantifiable model of axonal regeneration. Axonal injuries of differing severity were induced in the dorsal funiculus of adult rats using a micromanipulator-controlled Scouten knife. Demyelinated regions were produced so as to overlap with the injury site by the injection of galactocerebroside antibodies plus complement one segment cranial to the axonal injury site. Schwann cells were isolated from the sciatic nerve, expanded in vitro, and transplanted into the injury site 1 day later. Animals were killed after an additional 7 days. Schwann cells were evenly distributed throughout the region of demyelination, which extended 6-7 mm cranial to the axonal injury site. The severity of axonal injury was quantified by counting degenerate axons in transverse resin sections. The degree of axonal regeneration was assessed by an electron microscopic analysis of growth cone frequency and distribution relative to the site of axonal injury. Quantification of growth cones at a distance from the site of axonal injury indicated a strong linear relationship (P < 0.001) between the number of growth cones and the number of severed axons; the ratio of growth cones to severed axons was increased by 26.5% in demyelinated plus transplanted animals compared to demyelinated animals without a transplant. Furthermore, only the demyelinated plus transplanted animals contained growth cones associated with myelin in white matter immediately outside of the region of complete demyelination. Growth cones were absent in transplanted-only animals at a distance from the site of axonal injury. These findings indicate that combined demyelination plus Schwann cell transplantation therapy enhances axonal regeneration following injury and suggests that growth cones are able to overcome myelin-associated inhibitors of neurite outgrowth in the presence of trophic support.

Synergism of nitric oxide and iron in killing the transformed murine oligodendrocyte cell line N20.1

Nitric oxide (NO) produced in inflammatory lesions may play a major role in the destruction of oligodendrocytes in multiple sclerosis and experimental allergic encephalomyelitis. The transformed murine oligodendroglial line N20.1 is much more resistant than primary oligodendrocytes to killing by the NO generator S-nitroso-N-acetyl-DL-penicillamine (SNAP). This observation prompted investigation of the mechanisms leading to cell death in the N20.1 cells and comparison of SNAP with another NO donor, sodium nitroprusside (SNP). We observed that N20.1 cells were 30 times more sensitive to SNP than to SNAP. The specific NO scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) protected against SNP only, not against SNAP. However, dithiothreitol protected against both SNAP and SNP, indicating that S-nitrosylation of cysteines plays a major role in the cytotoxicity of both NO donors. We did not observe any formation of peroxynitrite or increase of Ca2+ concentration with either SNAP or SNP, thus excluding their involvement in the mechanisms leading to N20.1 cell death. Based on two observations, (a) potentiation of the cytotoxic effect of SNP when coincubated with ferricyanide or ferrocyanide, but not sodium cyanide, and (b) protection by deferoxamine, an iron cyanide chelator, we conclude that the greater sensitivity of N20.1 cells to SNP compared with SNAP is due to synergism between NO released and the iron cyanide portion of SNP, with the cyanide accounting for very little of the cytotoxicity. Finally, SNP but not SNAP induces some apoptosis, as shown by DNA laddering and protection by a caspase-3 inhibitor. These results suggest that low levels of NO in combination with increased iron content lead to apoptotic cell death rather than the necrotic cell death seen with higher levels of NO generated by SNAP.