Overexpression of S-adenosylmethionine decarboxylase (SAMDC) activates thematernal program of apoptosis shortly after MBT in Xenopus embryos

Overexpression of S-adenosylmethionine decarboxylase (SAMDC) mRNA in 1- and 2-cell stage Xenopus embryos induces cell autonomous dissociation at the l ate blastula stage and developmental arrest at the early gastrula stage. Th e induction of cell dissociation took place "punctually" at the late blastu la stage in the SAMDC-overexpressing cells, irrespective of the stage of th e microinjection of SAMDC mRNA. When we examined the cells undergoing the d issociation, we found that they were TUNEL-positive and contained fragmente d nuclei with condensed chromatin and fragmented DNA. Furthermore, by injec ting Xenopus Bcl-2 mRNA together with SAMDC mRNA, we showed that SAMDC-over expressing embryos are rescued completely by Bcl-2 and become tadpoles. The se results indicate that cell dissociation induced by SAMDC overexpression is due to apoptotic cell death. Since the level of S-adenosylmethionine (SA M) is greatly reduced in SAMDC-overexpressing embryos and this induces inhi bition of protein synthesis accompanied by the inhibition of DNA and RNA sy ntheses, we conclude that deficiency in SAM induced by SAMDC overexpression activates the maternal program of apoptosis in Xenopus embryos at the late blastula stage, but not before. We propose that this mechanism serves as a surveillance mechanism to check and eliminate cells physiologically damage d during the cleavage stage.