GlycoWord / Fertilization / Role of Glycoconjugates in Species-specific Recognition between Egg and Sperm FS-A03

The ligand from the egg jelly binds to the sperm and induces the acrosome reaction in the eggs of the echinoderm sea urchin. The egg jelly sulfates the polysaccharides on the membrane of the sperm; this polysaccharides has been characterized, and it induces the acrosome reaction in a species-specific manner. In Strongylocentrotus purpuratus, the polysaccharide is a fucose sulfate polymer (FSP). The species specificity of the interaction is determined by the glycoside linkage of the polymer and the pattern of sulfation of the sugar residues.²⁾ S. franciscanus and S. purpuratus belong to the same genus. They differ in the position of the sulfate residue that is linked to their polysaccharides. In the sea urchin S. franciscanus, sulfation of the polysaccharide occurs at the 2-position of fucose, while in S. purpuratus, sulfation occurs at the 4- position.³⁾ Each sperm recognizes this difference and induces a species-specific acrosome reaction. An egg jelly receptor (suREJ1) to which FSP binds on the sperm membrane is present, and an anti-suREJ1 antibody can induce the acrosome reaction. The N-terminal end of SuREJ1 contains two carbohydrate recognition domains (CRDs)Ñá superfamily of immunoglobulin type C domains. It is assumed that CRD can contribute to the binding between suREJ1 and FSP, and it is known to be a domain that plays an important role in cell recognition, natural immunity, and cell signaling. Vacquier et al. analyzed the CRD of suREJ1-3 in six species of sea urchins in terms of evolution and found that this domain has an early evolution speed; they also showed that it a the gene which was controlled by positive selection (Darwinian selection).⁴⁾ The egg jelly of starfish contained three molecules that are involved in the acrosome reaction. In the case of Asterias amurensis, the Co-ARIS of steroidal saponin and peptide asterosap of 34 amino acid residues assist in the reactions of the acrosome reaction-inducing substance (ARIS), which is a large and highly sulfated glycoprotein conjugate. The activity of the sugar chain is maintained by pronase digestion but is lost on hyper-iodine acid oxidation. Ten repeats of five sugar chains

-Xylp(1

3)-

-Galp(1

3)-

-Fucp(4-SO3^-)(1

3)-

-Fucp(4-SO3^-)(1

4)-

-Fucp(1

form the smallest activity unit.⁵⁾ A. amurensis ARIS induces the acrosome reaction at the same level (intra-species level) for the sperm of a starfish belonging to Asteriidae (same genus as A. amurensis) but not for the sperm of a starfish from a different genus. A similar result was obtained with the egg jelly from four species of starfish belonging to Asteriidae, as well as with the ARIS of A. amurensis. The egg jelly from Asteriidae contains the sugar repeat, including the same sulfate as that in ARIS; however, this repeat is absent in the egg jelly from other starfish. It is believed that the sperm recognizes the structure of the sugar chain of a specific ARIS at the genus level. From these data, we can examine whether the sugar chain and sulfate residues of the sugar repeats contribute to species differentiation by reproductive isolation.

In the case of the egg and sperm binding, sugar chain-modifying enzymes, which usually act as lectins, bind the sperm to the egg. The binding with the fucose ends of the vitelline membrane and the fucosidase of the sperm surface is essential for fertilization in the Protochordata Ciona intestinalis and Halocynthia roretzi. Further, the binding of the GlcNAc end of the vitelline membrane and the hexosaminidase of the sperm surface is necessary for fertilization in Phallusia mammillata.^6-8)

As several millions of species inhabit the earth, their gametes should be able to recognize one other. The sugar chain is probably the best tool for ecognition between gametes in organisms.