Echinoderms are among the most primitive deuterostomes and have been used as model organisms to understand chordate biology because of their close evolutionary relationship to this phylogenetic group. However, there are almost no data available regarding the N-glycomic capacity of echinoderms, which are otherwise known to produce a diverse set of species-specific glycoconjugates, including ones heavily modified by fucose, sulfate, and sialic acid residues. To increase the knowledge of diversity of carbohydrate structures within this phylum, here we conducted an in-depth analysis of N-glycans from a brittle star (Ophiactis savignyi) as an example member of the class Ophiuroidea. To this end, we performed a multi-step N-glycan analysis by HPLC and various exoglyosidase and chemical treatments in combination with MALDI-TOF MS and MS/MS. Using this approach, we found a wealth of hybrid and complex oligosaccharide structures reminiscent of those in higher vertebrates as well as some classical invertebrate glycan structures. 70% of these N-glycans were anionic, carrying either sialic acid, sulfate, or phosphate residues. In terms of glycophylogeny, our data position the brittle star between invertebrates and vertebrates and confirm the high diversity of N-glycosylation in lower organisms. Figure 4. RP HPLC fractionation of anionic N-glycans. A-K, for the first dimension, anionic enriched N-glycans were separated on a Kinetex C18 RP HPLC (pH 6; calibrated in terms of glucose units). The color code and letters indicate pooled peaks whose second-dimension HIAX-HPLC profiles are shown in Fig. S5, A-K. Fractions are annotated with example glycans according to the symbol nomenclature for glycans (where Me and S represent methyl and sulfate, respectively). The illustrated structures are based on a combination of MS/MS analysis, RP and HIAX-HPLC elution times, as well as enzymatic and/or chemical treatments. Because of the separation properties of the RP column used as the first dimension, multiple sulfated structures are shown in A, modified hybrid structures in B, multiple sialylated biantennary structures in C-E, and those with methylated N-glycolylneuraminic acid and multiantennary structures in G-K. The 4 g.u. RP HPLC peak not subject to 2D-HPLC contains a nondigestible glycan not related to other analyzed structures.
Brittle star N-glycome