Identification of a cell surface-associated protein involved in mouse neural cell aggregation by means of antibodies against the sponge aggregation factor.

Polyclonal antibodies were raised against the purified aggregation factor (AF) from the sponge Geodia cydonium to elucidate possible immunological relationships between adhesion molecules of lower multicellular eukaryotic systems (sponges) and those of vertebrates. This anti-AF recognized a series of polypeptides associated with the AF, among them also a polypeptide with a Mr of 47,000 (p47). The formation of the antibody-p47 immunocomplexes could be prevented by adsorbing the anti-AF with a brain extract from DBA/2J mice. Moreover, this brain polypeptide inhibited the AF-mediated aggregation of sponge cells. Interestingly, the anti-AF recognized a p37 molecule in the brains of 2- to 3-day-old mice; no reaction could be traced using brain extracts from animals older than 2 months. The anti-AF failed to interact with polypeptides from mouse liver or spleen. By indirect immunofluorescence staining the p37 was found to be localized on the plasma membranes of brain cells. Moreover, Fab' fragments of the anti-AF inhibited aggregation of mouse brain cells. These data indicate that the sponge anti-AF recognizes a p37 molecule in mouse brain cells which is either directly or indirectly involved in brain cell aggregation.