Abstract
S-adenosylmethionine decarboxylase (SAMDC) is an enzyme which converts S-adenosylmethione (SAM), a methyl donor, to decarboxylated SAM (dcSAM), an aminopropyl donor for polyamine biosynthesis. In our studies on gene expression control in Xenopus early embryogenesis, we cloned the mRNA for Xenopus SAMDC, and overexpressed the enzyme by microinjecting its mRNA into Xenopus fertilized eggs. In the mRNA-injected embryos, the level of SAMDC was enormously increased, the SAM was exhausted, and protein synthesis was greatly inhibited, but cellular polyamine content did not change appreciably. SAMDC-overexpressed embryos cleaved and developed normally up to the early blastula stage, but at the midblastula stage, or the stage of midblastula transition (MBT), all the embryos were dissociated into cells, and destroyed due to execution of apoptosis. During cleavage SAMDC-overexpressed embryos transcribed caspase-8 gene, and this was followed by activation of caspase-9. When we overexpressed p53 mRNA in fertilized eggs, similar apoptosis took place at MBT, but in this case, transcription of caspase-8 did not occur, however activation of caspase-9 took place. Apoptosis induced by SAMDC-overexpression was completely suppressed by Bcl-2, whereas apoptosis induced by p53 overexpression or treatments with other toxic agents was only partially rescued. When we injected SAMDC mRNA into only one blastomere of 8- to 32-celled embryos, descendant cells of the mRNA-injected blastomere were segregated into the blastocoel and underwent apoptosis within the blastocoel, although such embryos continued to develop and became tadpoles with various extents of anomaly, reflecting the developmental fate of the eliminated cells. Thus, embryonic cells appear to check themselves at MBT and if physiologically severely-damaged cells occur, they are eliminated from the embryo by activation and execution of the maternally-inherited program of apoptosis. We assume that the apoptosis executed at MBT is a "fail-safe" mechanism of early development to save the embryo from accidental damages that take place during cleavage.
Highlights
In Xenopus early embryogenesis, fertilized eggs go through the first cleavage following G1, S, G2, and M phases, but from the second cell cycle, cleavage proceeds rapidly without G1 and G2 phases until the end of the 12th cleavage (Heasman, 2006)
We summarize studies performed in our laboratory and in other laboratories in relation to the apoptosis program which is uniquely set in operation as early as the blastula stage in Xenopus embryogenesis
We found that treatments of cleavage stage embryos with polyamines, colchicine, bufalin, brefeldin A, and moderately-high temperatures all induce cell dissociation which is similar to that induced by S-adenosylmethionine decarboxylase (SAMDC) as in the following: p53 mRNA
Summary
In Xenopus early embryogenesis, fertilized eggs go through the first cleavage following G1, S, G2, and M phases, but from the second cell cycle, cleavage proceeds rapidly without G1 and G2 phases until the end of the 12th cleavage (Heasman, 2006). When we cultured Xenopus fertilized eggs in the medium that contained colchicine (1 mM), we observed cell dissociation at early blastula stage (Suwa M, Shiokawa K, unpublished results) (Fig. 4).
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