Glycoproteins associated with myelin in the central nervous system

The endogenous lectin cerebellar soluble lectin and its ligands in central nervous system myelin of myelin-deficient (mld) mutant mice

The myelin-deficient (mld) mutation is autosomal recessive mutation in the murine CNS exhibiting severe hypomyelination. The primary defect results in a drastic reduction of myelin basic protein synthesis caused by a duplication of the myelin basic protein gene with partial inversion of the upstream gene copy. The severe deficit of myelin basic protein is responsible for the absence of the major dense line but cannot explain the heterogeneity of myelin compaction found in mld. We have tested the hypothesis that the endogenous cerebellar soluble lectin (CSL) and/or its endogenous glycoprotein ligands could be involved in myelin abnormalities in the dysmyelinating mutant, mld. Immunocytochemical and immunoblotting techniques showed that the CSL level was not reduced significantly in the mld mutant. Furthermore, two ligands of CSL, the myelin-associated glycoprotein and an axonal glycoprotein, with a relative molecular mass of 31 kDa, were not decreased in level in the purified myelin fraction isolated from mld mice. In contrast, three minor glycoprotein ligands of CSL of relative molecular mass of 23, 18, and 16 kDa were greatly reduced in content. The reduced concentration of these low-molecular-mass glycoproteins in mld myelin suggests that they are constituents of compact myelin. Furthermore, the observation that CSL is specifically localized in vivo in regions where mld myelin is more compact and absent from regions devoid of myelin compaction may suggest that the endogenous CSL lectin, as well as its minor glycoprotein ligands, plays a role in the stabilization of the myelin sheath.

Expression and localization in the developing cerebellum of the carbohydrate epitopes revealed by Elec-39, an IgM monoclonal antibody related to HNK-1

The immunochemical and immunocytochemical reactivity of an anti-carbohydrate monoclonal antibody (Elec-39), obtained against acetylcholinesterase from Electrophorus electricus electric organ, was followed during the postnatal development of the rat cerebellum. The specificity of this antibody resembles that of a family of anti-carbohydrate antibodies that includes HNK-1, L2, NC-1 and NSP-4, as well as IgMs that occur in some human neuropathies. As revealed by immunoblotting techniques, the reactivity of Elec-39 is maximum around postnatal days 10-12. At this age, the antibody reveals eight major proteins of mol. wt ranging between 14 and 150 kDa. Some of them (with mol. wts of 14, 18, 28 and 31 kDa) are transiently expressed. They correspond to previously identified glycoproteins binding to the plant lectin concanavalin A and binding also to the endogenous mannose-binding lectin CSL and endogenous membrane-bound mannose-binding lectin. In young animals, an important staining with the Elec-39 antibody can be observed on postmitotic precursors of granule cells, on astrocyte processes in the external granular layer, on newly formed parallel fibres and on unmyelinated axons of the white matter. In adult animals, the labelling is localized essentially in myelin and also in the cytoplasm of astrocytes. These results are discussed in relation to ontogenetic phenomena occurring during cerebellar development and the potential role of the carbohydrate epitope revealed with Elec-39 as a determinant in cell adhesion processes.

Regional differences in the distribution of endogenous receptors for carbohydrate constituents of cellular glycoconjugates, especially lectins, in cortex, hippocampus, basal ganglia and thalamus of adult human brain

Ten different types of labelled neoglycoproteins, exposing glycohistochemically pivotal carbohydrate moieties that mostly are constituents of naturally occurring glycoconjugates with an aromatic spacer, were synthesized. The panel was applied to fixed, paraffin-embedded sections of different cortical regions and white matter, of hippocampal gyrus, basal ganglia, thalamus nuclei and adjacent areas of adult human brain to comprehensively map the presence of respective binding sites in these parts. Compliance with accepted criteria for specificity of binding was routinely ascertained. Overall, not a uniform binding pattern, but a distinct distribution with regional differences on the level of specific cytoplasmic and nuclear staining in nerve cells was determined, fiber structures being generally labelled with medium or strong intensity. For example, among the neurons localized in the five cortical laminae the binding of N-acetyl-D-galactosamine varied from strong to undetectable. Biochemical analysis, employing carbohydrate residues as affinity ligands in chromatography, proved that the neuroanatomically different regions exhibited a pattern of receptors with notable similarities. These results on endogenous binding sites for glycoconjugates, especially lectins, are complementary to assessment of localization of cellular glycoconjugates by plant lectins and carbohydrate-specific monoclonal antibodies. They are thus a further obligatory step to substantiate the physiological roles of recognitive protein-carbohydrate interactions in the central nervous system.

Phylogenetic examination of vertebrate central nervous system myelin proteins by electro-immunoblotting

Central nervous system (CNS) myelin proteins from vertebrate classes were examined by immunoblotting with antisera against mammalian CNS myelin proteins. Higher vertebrates possessed proteolipid (PLP), DM-20 and Wolfgram (WP) proteins, except that DM-20 was missing in amphibia. Fish CNS myelins contained neither PLP nor WP; instead they bound antisera to mammalian peripheral nervous system P0 protein. All classes carried myelin basic protein, but only mammals exhibited a component equivalent to rat 21.5K (21,500 dalton). These phylogenetic data are consistent with major changes in CNS myelin protein composition at the transition from fishes to higher vertebrates.