Synthesis of the neurotoxin quinolinic acid in apoptotic tissue from Suberites domuncula: cell biological, molecular biological and chemical analyses

Sessile marine animals, such as sponges, are prone to infection by prokaryotic as well as by eukaryotic attacking organisms. In the present study we document for the first time that in tissue from sponges which underwent apoptosis, a toxic compound is produced which very likely controls the elimination of the dying tissue. The marine sponge Suberites domuncula develops in the field occasionally apoptotic tissue areas which are rapidly eliminated. In the present study apoptosis was induced in S. domuncula by exposing the specimens in aquaria to 5 µg/ml Dip or by maintaining the sponges for 3 - 5 days under non-aeration conditions. After that treatment only one eukaryotic epibiont, the mollusk Trunculariopsis trunculus could be detected which was found to graze on the zones of dying tissue. The extent of apoptosis was determined quantitatively by a photometric immunoassay. Cell proliferation assays demonstrated that aqueous extracts from sponge tissue displayed no cytotoxicity. However, addition of an extract from apoptotic tissue to neuronal cells from rat brain exerted strong toxicity. Since the antagonist of the NMDA receptor, MK-801 abolished this activity, it was indicative to screen for an endogenous ligand of the NMDA receptor. This was identified as quinolinic acid and quantitative determination showed that quinolinic acid is present only in apoptotic tissue (4.8 mg/g dry wet weight). The cDNA encoding the key enzyme of the quinolinic acid pathway, 3-hydroxyanthranilate 3,4-dioxygenase could be cloned and characterized from S. domuncula. With this cDNA as probe it could be proven that the expression of this enzyme is upregulated in apoptotic tissue. We conclude (i) that a complex molecular network controls apoptotic elimination of sponge tissue and (ii) that the bioactive compound quinolinic acid controls the elimination of the tissue via differential effects on grazing epibionts.