Allograft rejection in the mixed cell reaction system of the demosponge Suberites domuncula is controlled by differential expression of apoptotic genes

Until recently, the lack of molecular probes hampered the determination of the expression of pro-apoptotic and anti-apoptotic genes in sponge. In an approach to solve this problem, the present study describes a variety of cDNAs from the demosponge Suberites domuncula, coding for proteins that are characteristic for the initiation of apoptosis (caspase, MA3, ALG-2 protein), for the prevention of programmed cells death (2 Bcl-2 homology proteins, FAIM-related polypeptide, and DAD-1-related protein), and for morphogenetic processes (retinoid X receptor). They were used as probes to monitor the expression levels in vitro in the allogeneic mixed sponge cell reaction (MSCR) system. In the allogeneic MSCR, two-cell aggregates (primmorphs) from genetically different animals of the same species were positioned next to each other. After approximately 8 days in culture, one of the primmorphs underwent apoptotic death, while the second remained alive. The expression levels of the aforementioned genes were determined by Northern blotting and by in situ hybridization. These experiments revealed that in the apoptotic primmorph, the characteristic apoptotic genes were expressed, while in the non-apoptotic aggregates the cell-survival genes are highly upregulated. Interestingly, the transcript levels of retinoid X receptor were higher in apoptotic primmorphs than in the non-apoptotic aggregate in the assay. Our data show for the first time that in the in vitro MSCR system, allogeneic recognition led to apoptotic cell death in one partner, while the other one survived. We suggest that this process is controlled by a differential expression of the pro-apoptotic and pro-survival genes studied here.