Distribution of mono-, di- and trisialo gangliosides in the brain of Actinopterygian fishes.

BACKGROUND: Mono-, di- and trisialo gangliosides are major glycosphingolipids in the brain of higher vertebrates involved in lipid raft assembly. In contrast, the fish brain is abundant in polisialo-gangliosides, whose function is implicated in the modulation of repulsive and attractive intercellular interactions during embryonic development and a temperature adaptation process. The histological distribution of gangliosides is usually studied in rodent and mammalian brains, but to date it has not been described in the case of fish brain. METHODS: Gangliosides were extracted from adult brains of trout, carp and zebrafish and separated by TLC. High-affinity anti-ganglioside (GM1, GD1a, GD1b, GT1b) IgG antibodies were used for immunohistochemistry. RESULTS: In trout and carp brains GM1 and GT1b are expressed in the same neuronal cell bodies from the telencephalon to the spinal cord. In zebrafish brain GM1 was not detected, whereas GT1b is a general neuropil staining. GD1a is specific for unmyelinated parallel fibers in carp and zebrafish brains as well as parallel fibers in the molecular layer of all cerebellar divisions. In trout brain GD1b is found in parallel fibers of the cerebellum, but not in the tectum mesencephali. GD1b is expressed in zebrafish neuronal cell bodies. CONCLUSIONS: Each studied species has a different expression of complex gangliosides. GT1b is widely present, whereas GD1a and GD1b appear in a specific group of unmyelinated fibers and could be used as their specific marker. GENERAL SIGNIFICANCE: This is the first report on mono-, di- and trisialo ganglioside (GM1, GD1a, GD1b and GT1b) distribution in the brain of adult Actinopterygian fishes. This article is part of a Special Issue entitled Glycoproteomics.

Biosynthesis of the major brain gangliosides GD1a and GT1b.

Gangliosides-sialylated glycosphingolipids-are the major glycoconjugates of nerve cells. The same four structures-GM1, GD1a, GD1b and GT1b-comprise the great majority of gangliosides in mammalian brains. They share a common tetrasaccharide core (Galß1-3GalNAcß1-4Galß1-4Glcß1-1'Cer) with one or two sialic acids on the internal galactose and zero (GM1 and GD1b) or one (GD1a and GT1b) α2-3-linked sialic acid on the terminal galactose. Whereas the genes responsible for the sialylation of the internal galactose are known, those responsible for terminal sialylation have not been established in vivo. We report that St3gal2 and St3gal3 are responsible for nearly all the terminal sialylation of brain gangliosides in the mouse. When brain ganglioside expression was analyzed in adult St3gal1-, St3gal2-, St3gal3- and St3gal4-null mice, only St3gal2-null mice differed significantly from wild type, expressing half the normal amount of GD1a and GT1b. St3gal1/2-double-null mice were no different than St3gal2-single-null mice; however, St3gal2/3-double-null mice were >95% depleted in gangliosides GD1a and GT1b. Total ganglioside expression (lipid-bound sialic acid) in the brains of St3gal2/3-double-null mice was equivalent to that in wild-type mice, whereas total protein sialylation was reduced by half. St3gal2/3-double-null mice were small, weak and short lived. They were half the weight of wild-type mice at weaning and displayed early hindlimb dysreflexia. We conclude that the St3gal2 and St3gal3 gene products (ST3Gal-II and ST3Gal-III sialyltransferases) are largely responsible for ganglioside terminal α2-3 sialylation in the brain, synthesizing the major brain gangliosides GD1a and GT1b.