Carbohydrate self-recognition mediates marine sponge cellular adhesion

Sponges (Porifera), the simplest and earliest multicellular organisms, are thought to have evolved from their unicellular ancestors about 1 billion ye ars ago by developing cell-recognition and adhesion mechanisms to discrimin ate against "non-self." Consequently, they are used as models for investiga ting recognition phenomena. Cellular adhesion of marine sponges is an event involving adherence of extracellular proteoglycan-like molecules, otherwis e known as aggregation factors (AFs). In a calcium-independent process the AFs adhere to the cell surface, and in a calcium-dependent process they exh ibit AF self-association. A mechanism which has been implied but not defini tely proven to play a role in the calcium-dependent event is self-recogniti on of defined carbohydrate epitopes. For the red beard sponge, Microciona p rolifera, two carbohydrate epitopes, a sulfated disaccharide and a pyruvyla ted trisaccharide, have been implicated in cellular adhesion. To investigat e this phenomenon a system has been designed, by using surface plasmon reso nance detection, to mimic the role of carbohydrates in cellular adhesion of M. prolifera. The results show self-recognition of the sulfated disacchari de to be a major force behind the calcium-dependent event. The interaction is not simply based on electrostatic interactions, as other sulfated carboh ydrates analyzed by using this procedure did not self-associate. Furthermor e, the interaction is completely eradicated on substitution of Ca2+ ions by either Mg2+ or Mn2+ ions. This physiologically relevant recognition mechan ism confirms the existence of true carbohydrate self-recognition, and may h ave significant implications for the role of carbohydrates in cellular reco gnition of higher organisms.