Carbohydrate-Based Gamete Recognition During the Sperm Acrosome Reaction in Sea Urchins

Abstract

Sea urchins have been excellent model systems to study the molecular mechanisms of fertilization. The sperm acrosome reaction (AR), a necessary process for penetrating the extracellular matrix of the egg, binding to the egg surface, and fusing with the egg plasma membrane, has been a central issue in research into sperm–egg interactions. Unambiguously, sea urchin spermatozoa trigger the AR upon contact with the thick gelatinous layer (egg jelly) surrounding the egg. The inducer of the sperm AR is a pure polysaccharide composed of fucose and sulfate groups with no other detectable sugars or amino acids. Structural analysis of this compound has revealed an unbranched chain of α-l-fucopyranose (FSP) molecules with an α1-3 glycosidic link and O-sulfates at the C-2 and/or C-4 position, giving a tandem repeat of sulfated saccharides with an apparent molecular mass of over one million daltons. Studies have revealed species-unique structural, but limited, diversification within the monosaccharide composition, glycosidic links, and modification by sulfates. Species-specific or species-preferential induction of the sperm AR is well explained by differences or similarities in the arrangements of sulfated polysaccharides. Another polysaccharide found in egg jelly is a sialic acid composed of poly N-glycolylneuraminic acid (polyNeuGc). PolyNeuGc itself does not induce the AR; however, if FSP is present, it greatly potentiates biological activity independent of FSP signaling. The receptor for polyNeuGc on sea urchin spermatozoa has never been identified. There are many unanswered questions regarding carbohydrate-based sperm–egg recognition in sea urchins that – if answered – could offer insights into the fields of glycobiology, reproduction, and evolution.