Gangliosides and Glycosphingolipids of Peripheral Nervous System Myelins—a Minireview

A summary is provided of the available data on the composition of gangliosides and glycosphingolipids in the peripheral nervous system (PNS) including myelins and their antigenic properties. The composition of gangliosides and glycosphingolipids in the PNS is very different from that in the central nervous system (CNS), both quantitatively and qualitatively. One major difference is the abundance of neolacto-series gangliosides in the PNS, with the backbone structure Gal 1-4GlcNAcl-3Gal l-4Glcl-lCer. Their abundance contrasts with the abundance of ganglio-series gangliosides in the CNS. The neolacto-series gangliosides are localized mainly in the myelins of the PNS. In addition to gangliosides, other acidic and neutral glycosphingolipids in the neolacto-series are also characteristic of the myelins of the PNS. The ceramide (fatty acid and sphingosine base) compositions of gangliosides in the PNS are different from those in the CNS gangliosides, having greater percentages of long-chain fatty acids and dehydrosphingosines than found in the CNS gangliosides.     

Wrapping it up: the cell biology of myelination

During nervous system development, oligodendroglia in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS) synthesise large amounts of specific proteins and lipids to generate myelin, a specialised membrane that spirally ensheathes axons and facilitates fast conduction of the action potential. Myelination is initiated after glial processes have attached to the axon and polarisation of the plasma membrane has been triggered. Myelin assembly is a multi-step process that occurs in spatially distinct regions of the cell. We propose that assembly of myelin proteins and lipids starts during their transport through the biosynthetic pathway and continues at the plasma membrane aided by myelin-basic protein (MBP). These sequential processes create the special lipid and protein composition necessary for myelin to perform its insulating function during nerve conduction.     

The lipid composition of axons from bovine brain

Abstract— Bovine axons derived from myelinated CNS axons were found to have 13.5% lipid. This lipid was composed of 20.1% cholesterol, 20.1% galactolipid, 14.6% ethanolarhine phosphatides (56.4% in the plasmalogen form), 18.3% choline phosphatides (10.0% in the plasmalogen form), 9.3% sphingomyelin, 5.6% phosphatidyl serine and 3.4% phosphatidyl inositol. The bovine axons had 0.33 μg ganglioside NeuNAc/mg dry weight. The axon gangliosides were found to contain the four major types of bovine gangliosides, as well as gangliosides G_M2 and G_D3. The latter two amounted to 20.9 and 15.8 mole per cent respectively, of total gangliosides. On a molar basis, about one half of the gangliosides were monosialogangliosides, with a decreased contribution by gangliosides G_T1 and GD_1b relative to ganglioside distributions which have been reported for most other CNS components. The relationship of the bovine axonal lipid composition to bovine white matter and its constituents, as well as to other CNS and PNS axonal preparations is discussed.     

Unique Expression of HRF20 (CD59) in Human Nervous Tissue

Damage to autologous tissue by complement is limited by several widely distributed membrane-associated glycoproteins which restrict the action of the complement in homologous species. These include decay accelerating factor (DAF), membrane cofactor protein (MCP) and 20 kDa homologous restriction factor (HRF20,CD59). Using immunohistochemical techniques, we examined the localization of these proteins in the centra] nervous system (CNS) and peripheral nervous system (PNS) using non-neurological human nervous tissue since some complement components have been demonstrated to be synthesized in the CNS. There was no evidence of parenchymal staining by anti-DAF or anti-MCP antibodies in either type of tissue except for the staining of the endothelium in capillaries. On the other hand, anti-HRF20 antibody clearly stained myelinated axons in the CNS as well as Schwann cells in the PNS. In addition, we detected positive staining by anti-DAF antibody in the PNS of a Paroxysmal nocturnal hemoglobinuria (PNH) patient who is genetically deficient in HRF20.     

Progesterone as a neurosteroid: synthesis and actions in rat glial cellsProceedings of Xth International Congress on Hormonal Steroids, Quebec, Canada, 17–21 June 1998.

The central nervous system (CNS) and the peripheral nervous system (PNS) are targets for steroid hormones where they regulate important neuronal functions. Some steroid hormones are synthesized within the nervous system, either de novo from cholesterol, or by the metabolism of precursors originating from the circulation, and they were termed ‘neurosteroids'. The sex steroid progesterone can also be considered as a neurosteroid since its synthesis was demonstrated in rat glial cell cultures of the CNS (oligodendrocytes and astrocytes) and of the PNS (Schwann cells). Both types of glial cells express steroid hormone receptors, ER, GR and PR. As in target tissue, e.g. the uterus, PR is estrogen-inducible in brain glial cell cultures. In the PNS, similar PR-induction could not be seen in pure Schwann cells derived from sciatic nerves. However, a significant PR-induction by estradiol was demonstrated in Schwann cells cocultured with dorsal root ganglia (DRG), and we will present evidence that neuronal signal(s) are required for this estrogen-mediated PR-induction. Progesterone has multiple effects on glial cells, it influences growth, differentiation and increases the expression of myelin-specific proteins in oligodendrocytes, and potentiates the formation of new myelin sheaths by Schwann cells in vivo. Progesterone and progesterone analogues also promotes myelination of DRG-Neurites in tissue culture, strongly suggesting a role for this neurosteroid in myelinating processes in the CNS and in the PNS.     

Membrane interactions in nerve myelin: II. Determination of surface charge from biochemical data.

In our accompanying paper (Inouye and Kirschner, 1988) we calculated the surface charge density at the extracellular surfaces in peripheral and central nervous system (PNS; CNS) myelins from observations on the dependency of the width of the extracellular space on pH and ionic strength. Here, we have determined the surface charge density of the membrane surfaces in myelin from its chemical composition and the localization of some of its molecular components. We then analyzed the attractive and repulsive forces between the apposed surfaces and calculated equilibrium periods for comparison with the measured values. The biochemical model accounts for the observed isoelectric range of the myelin period and, with the surface charge reduced (possibly by divalent cation binding or a space charge approximation), the model also accounts for the dependency of period on pH above the isoelectric range. At the extracellular (and cytoplasmic) surfaces the contribution of lipid (with pI approximately 2) to the net surface charge is about the same in both PNS and CNS myelin, whereas the contribution of protein depends on which ones are exposed at the two surfaces. The protein conformation and localization modulate the surface charge of the lipid, resulting in positively-charged cytoplasmic surfaces (pI approximately 9) and negatively-charged extracellular surfaces (pI approximately 2-4). The net negative charge at the extracellular surface is due in CNS myelin to lipid, and in PNS myelin to both lipid and (PO) glycoprotein. The net positive charge at the cytoplasmic surface is due in CNS myelin mostly to basic protein, and in PNS myelin to PO glycoprotein and basic protein. The invariance of the cytoplasmic packing may be due to specific short-range interactions. Our models demonstrate how the particular myelin proteins and their localization and conformation can account for the differences in inter-membrane interactions in CNS and PNS myelins.     

Ganglioside molecular species containing C18- and C20-sphingosine in mammalian nervous tissues and neuronal cell cultures

Gangliosides exist as a very complex mixture of species differing in both the hydrophilic and hydrophobic moieties. They are particularly abundant in the central nervous system (CNS), where they have been associated with development and maturation of the brain, neuritogenesis, synaptic transmission, memory formation and synaptic aging. Today, many data suggest that some of the effects exerted by gangliosides are due to interactions with proteins that participate in the transduction of signals through the membrane in membrane microdomains. A specific characteristic of CNS gangliosides is the structure of their long-chain base (LCB). In fact, considering all the mammalian cell sphingolipids, gangliosides, sulphatides, neutral glycosphingolipids, sphingomyelin and ceramides, it would seem that while the LCB with 18 carbons is the main component of all sphingolipids, only CNS gangliosides contain significant amounts of LCB with 20 carbons. C18-Sphingosine is always present in cell gangliosides; the individual ganglioside species containing C18-sphingosine increase during cell differentiation then remain constant during cell aging. Gangliosides containing C20-sphingosine are absent, or present only in traces, in undifferentiated cells but with the onset of cell differentiation they appear, their content slowly but continuously increasing throughout the life span. In this review we discuss the chemistry, physico-chemistry and metabolism of ganglioside species differing in LCB length and introduce the hypothesis that the varying ratio between C18- and C20-gangliosides during CNS development and aging can be instrumental in modulating membrane domain organisation and cell properties.     

DNA damage response in peripheral nervous system: Coping with cancer therapy-induced DNA lesions

In the absence of blood brain barrier (BBB) the DNA of peripheral nervous system (PNS) neurons is exposed to a broader spectrum of endogenous and exogenous threats compared to that of the central nervous system (CNS). Hence, while CNS and PNS neurons cope with many similar challenges inherent to their high oxygen consumption and vigorous metabolism, PNS neurons are also exposed to circulating toxins and inflammatory mediators due to relative permeability of PNS blood nerve barrier (BNB). Consequently, genomes of PNS neurons incur greater damage and the question awaiting investigation is whether specialized repair mechanisms for maintenance of DNA integrity have evolved to meet the additional needs of PNS neurons. Here, I review data showing how PNS neurons manage collateral DNA damage incurred in the course of different anti-cancer treatments designed to block DNA replication in proliferating tumor cells. Importantly, while PNS neurotoxicity and concomitant chemotherapy-induced peripheral neuropathy (CIPN) are among major dose limiting barriers in achieving therapy goals, CIPN is partially reversible during post-treatment nerve recovery. Clearly, cell recovery necessitates mobilization of the DNA damage response and underscores the need for systematic investigation of the scope of DNA repair capacities in the PNS to help predict post-treatment risks to recovering neurons.     

Occurrence and tissue distribution of c-series gangliosides in the common squid Todarodes pacificus

We have recently demonstrated that the common squid Todarodes pacificus express acidic lipids that were reactive with a monoclonal antibody A2B5. In the present study, two A2B5-reactive acidic lipids were isolated from squid hepatopancreatic tissue and characterized for their structures by methods including glycolipid overlay analysis, product analysis after sialidase treatment, and electrospray ionization-mass spectrometry (ESI-MS). Accordingly, the two acidic lipid were identified as GT3 and GQ1c, respectively. Another A2B5-reactive acidic lipid in the tissue was tentatively assigned to GT2 based upon its reactivity to A2B5 and chromatographic mobility on thin-layer chromatography. The composition and concentration of c-series gangliosides significantly differed among squid tissues (i.e. hepatopancreas, cerebral ganglion, eye lens, and mantle tissue). Interestingly, the percentages of c-series gangliosides within total gangliosides of hepatopancreas and cerebral ganglion were even higher than that of cod fish brain, which is known to be highly enriched with this ganglioside species. These findings strongly support the hypothesis that c-series gangliosides in squid tissues are not derived from ganglioside-containing food intake, but biosynthesized in a tissue-specific manner.     

Membrane lipids as signaling molecules

PURPOSE OF REVIEW: Membrane lipids play important roles in signaling reactions. They are involved in most if not all cellular signaling cascades and in a wide variety of tissue and cell types. The purpose of this review is to highlight major pathways of signaling originating in membrane lipids. Details of lipid metabolism, and its relation to protein function, will thus advance understanding of the role of lipids in health and disease. RECENT FINDINGS: Major classes of lipids including glycerophospholipids, their metabolites (eicosanoids, endocannabinoids), and sphingolipids have recently generated interest in the field of signal transduction. These lipids are tightly regulated and have an impact on various physiological functions. Importantly, aberrant lipid metabolism often leads to onset of pathology, and thus the precise balance of signaling lipids and their effectors can serve as biomarkers. SUMMARY: Membrane lipids form precursors for second messengers and functional assembly matrices on membrane domains during cellular stimulation. Many of these modifications are rapid reactions at lipid headgroups. Metabolism of the fatty acyl portion of membrane lipids leads to the generation of a bewildering complexity of lipid mediators with extended effects in space and time.     

Lateral neural borders as precursors of peripheral nervous systems: A comparative view across bilaterians

The nervous systems in most bilaterians are centralized, composed of central nervous systems (CNS) and peripheral nervous systems (PNS). Common molecular and cellular patterns of medial nerve cords have been observed in various distantly related bilaterians, suggesting deep homology of CNS. The development patterns of PNS, however, are more diverse than CNS across different phylogenetic lineages and the evolution of PNS so far has been thought to be polygenic. The molecular and cellular programs during the development of PNS among different bilaterian branches are drastically different. For example, vertebrate PNS is essentially derived from neural crest cells and placodes, which are largely vertebrate innovations and do not exist in invertebrates. On the other hand, the lack of common precursor cell types does not necessarily lead to the conclusion of different evolutionary origins. Homology needs to be examined with a deeper and broader scope. In this review, we examined the molecular, cellular and developmental characteristics of PNS in a broad range of bilaterians to summarize our current understanding of variation and potentially conserved themes. These comparisons demonstrate that there exist both migratory and non-migratory neuroblasts in the lateral border of CNS precursors in most model bilaterian animals. These lateral border neuroblasts are specified by conserved gene regulatory network and give rise to sensory neurons, suggesting that lateral border neuroblasts represent the progenitor of PNS and share deep homology among different branches of Bilateria. Future studies are needed to elucidate the evo-devo mechanisms of the lateral neural borders as PNS progenitors.     

Differential T cell response in central and peripheral nerve injury: connection with immune privilege.

The central nervous system (CNS), unlike the peripheral nervous system (PNS), is an immune-privileged site in which local immune responses are restricted. Whereas immune privilege in the intact CNS has been studied intensively, little is known about its effects after trauma. In this study, we examined the influence of CNS immune privilege on T cell response to central nerve injury. Immunocytochemistry revealed a significantly greater accumulation of endogenous T cells in the injured rat sciatic nerve than in the injured rat optic nerve (representing PNS and CNS white matter trauma, respectively). Use of the in situ terminal deoxytransferase-catalyzed DNA nick end labeling (TUNEL) procedure revealed extensive death of accumulating T cells in injured CNS nerves as well as in CNS nerves of rats with acute experimental autoimmune encephalomyelitis, but not in injured PNS nerves. Although Fas ligand (FasL) protein was expressed in white matter tissue of both systems, it was more pronounced in the CNS. Expression of major histocompatibility complex (MHC) class II antigens was found to be constitutive in the PNS, but in the CNS was induced only after injury. Our findings suggest that the T cell response to central nerve injury is restricted by the reduced expression of MHC class II antigens, the pronounced FasL expression, and the elimination of infiltrating lymphocytes through cell death.     

Conditional mutation of Rb causes cell cycle defects without apoptosis in the central nervous system

Targeted disruption of the retinoblastoma gene in mice leads to embryonic lethality in midgestation accompanied by defective erythropoiesis. Rb(-/-) embryos also exhibit inappropriate cell cycle activity and apoptosis in the central nervous system (CNS), peripheral nervous system (PNS), and ocular lens. Loss of p53 can prevent the apoptosis in the CNS and lens; however, the specific signals leading to p53 activation have not been determined. Here we test the hypothesis that hypoxia caused by defective erythropoiesis in Rb-null embryos contributes to p53-dependent apoptosis. We show evidence of hypoxia in CNS tissue from Rb(-/-) embryos. The Cre-loxP system was then used to generate embryos in which Rb was deleted in the CNS, PNS and lens, in the presence of normal erythropoiesis. In contrast to the massive CNS apoptosis in Rb-null embryos at embryonic day 13.5 (E13.5), conditional mutants did not have elevated apoptosis in this tissue. There was still significant apoptosis in the PNS and lens, however. Rb(-/-) cells in the CNS, PNS, and lens underwent inappropriate S-phase entry in the conditional mutants at E13.5. By E18.5, conditional mutants had increased brain size and weight as well as defects in skeletal muscle development. These data support a model in which hypoxia is a necessary cofactor in the death of CNS neurons in the developing Rb mutant embryo.     

Restricted inflammatory reaction in the CNS: a key impediment to axonal regeneration?

Axons in the central nervous system (CNS) of adult mammals do not regenerate after injury. Mammalian CNS differs in this respect from other mammalian tissues, including the peripheral nervous system (PNS), and from the CNS of lower vertebrates. In most parts of the body, including the nervous system, injury triggers an inflammatory reaction involving macrophages. This reaction is needed for tissue healing; when it is delayed or insufficient, healing is incomplete. The CNS, although needing an efficient inflammatory reaction resembling that in the periphery for tissue healing, appears to have lost the ability to supply it. We suggest that restricted CNS recruitment and activation of macrophages are linked to regeneration failure and might reflect the immune privilege that characterizes the mammalian CNS. As macrophages play a critical role in tissue restoration, and because their recruitment and activation are among the most upstream of the events leading to tissue healing, overcoming the deficiencies in these steps might trigger a self-repair process leading to recovery after CNS injury.     

Myelin lipids: A phylogenetic study

The lipid composition of CNS and PNS myelin was studied in rat,Xenopus, trout andTorpedo. The main difference lay in the proportion of cerebrosides, which decreased in the sequence rat > Xenopus > Torpedo > trout. In additionTorpedo CNS and PNS myelins were extremely rich in sulfatides. In some respects,Torpedo appeared closer to tetrapods than trout. Otherwise the proportion of the different lipid classes did not reveal any clear evolutionary trends.The presence of hydroxylated galactolipids in CNS myelin was investigated in several additional species. Considerable amounts were found inTorpedo, Polypterus, Protopterus, lizard, and chicken, with the highest values in rat and anurans. Only very small amounts of hydroxylated cerebrosides were detected in trout and in axolotl, while newt had none. This parameter appears therefore of doubtful usefulness for phylogenetic studies. In contrast to myelin proteins, myelin lipids are of limited value for establishing phylogenetic relationship among vertebrates.Abbreviations CH cholesterol - CNS central nervous system - FA fatty acids - GC cerebrosides - HPTLC high-performance thin-layer chromatography - PC phosphatidylcholine - PE phosphatidylethanolamine and ethanolamine plasmalogens - PI phosphatidylinositol - PLP proteolipid protein - PNS peripheral nervous system - PS phosphatidylserine - SM sphingomyelin - SU sulfatides     

Membrane interactions of tetanus and botulinum neurotoxins: a photolabelling study with photoactivatable phospholipids

Tetanus and botulinum neurotoxins (TeNT and BoNT) bind strongly and specifically to the nervous tissue, as it can be inferred from their potency and from their effects restricted to the nervous system. The molecular basis of these properties are presently unknown. As a first approach, we have investigated the interaction of TeNT and BoNT with model membranes by photolabelling with phospholipid analogues carrying the photoreceptor group at different positions of the lipid molecule in order to probe different membrane regions. We found that at neutral pH TeNT and BoNTs (type A, B and E) adsorb onto the surface of negatively charged liposomes. Polysialogangliosides increase this interaction only slightly thus suggesting that they provide a minor contribution to toxin lipid binding. On this basis we propose that clostridial neurotoxins bind to lipids via both a predominant unspecific interaction with negatively charged lipids (including gangliosides) and a specific, but weaker, interaction with polysialogangliosides. At acidic pH values both chains of these neurotoxins are labelled strongly by photogroups located in the hydrophobic milieu of the membrane with a pH dependence that overlaps the range of pH values reached in the endosomal lumen. This result is consistent with their insertion into the lipid bilayer in agreement with the idea that clostridial neurotoxins may penetrate into cells via intracellular low pH compartments.     

Immunocytochemical localization studies of myelin basic protein

The location of myelin encephalitogenic or basic protein (BP) in peripheral nervous system (PNS) and central nervous system (CNS) was investigated by immunofluorescence and horseradish peroxidase (HRP) immunocytochemistry. BP or cross-reacting material could be clearly localized to myelin by immunofluorescence and light microscope HRP immunocytochemistry. Fine structural studies proved to be much more difficult, especially in the CNS, due to problems in tissue fixation and penetration of reagents. Sequential fixation in aldehyde followed by ethanol or methanol provided the best conditions for ultrastructural indirect immunocytochemical studies. In PNS tissue, anti-BP was localized exclusively to the intraperiod line of myelin. Because of limitations in technique, the localization of BP in CNS myelin could not be unequivocally determined. In both PNS and CNS tissue, no anti-BP binding to nonmyelin cellular or membranous elements was detected.     

Two membrane protein fractions from rat central myelin with inhibitory properties for neurite growth and fibroblast spreading

Lack of neurite growth in optic nerve explants in vitro has been suggested to be due to nonpermissive substrate properties of higher vertebrate central nervous system (CNS) white matter. We have searched for surface components in CNS white matter, which would prevent neurite growth. CNS, but not peripheral nervous system (PNS) myelin fractions from rat and chick were highly nonpermissive substrates in vitro. We have used an in vitro spreading assay with 3T3 cells to quantify substrate qualities of membrane fractions and of isolated membrane proteins reconstituted in artificial lipid vesicles. CNS myelin nonpermissiveness was abolished by treatment with proteases and was not associated with myelin lipid. Nonpermissive proteins were found to be membrane bound and yielded highly nonpermissive substrates upon reconstitution into liposomes. Size fractionation of myelin protein by SDS-PAGE revealed two highly nonpermissive minor protein fractions of Mr 35 and 250-kD. Removal of 35- and of 250-kD protein fractions yielded a CNS myelin protein fraction with permissive substrate properties. Supplementation of permissive membrane protein fractions (PNS, liver) with low amounts of 35- or of 250-kD CNS myelin protein was sufficient to generate highly nonpermissive substrates. Inhibitory 35- and 250-kD proteins were found to be enriched in CNS white matter and were found in optic nerve cell cultures which contained highly nonpermissive, differentiated oligodendrocytes. The data presented demonstrate the existence of membrane proteins with potent nonpermissive substrate properties. Distribution and properties suggest that these proteins might play a crucial inhibitory role during development and regeneration in CNS white matter.     

Regional differences in brain gangliosides of a teleost fish following thermal acclimations

The ability of ectothermic vertebrates to adapt to thermal fluctuations in their environment is mainly based upon adaptive changes within the CNS. These changes are thought to be correlated with functional, metabolism changes in the central nervous system, especially in neuronal membranes. Gangliosides, being highly enriched in synaptic membranes (1)show characteristic perculiarities concerning concentration and molecular composition with regard to their sialylation status (2,3,4). In order to get further information concerning the biological function of gangliosides with respect to thermal adaptation, it was of interest to investigate possible correlation between the nervous ganglioside metabolism of different brain regions after various temperature adaptations.