Maternal program of apoptosis activated shortly after midblastula transition by overexpression of S-adenosylmethionine decarboxylase in Xenopus early embryos

Effects of S-adenosylmethionine decarboxylase, polyamines, amino acids, and weak bases (amines and ammonia) on development and ribosomal RNA synthesis in Xenopus embryos

We have been studying control mechanisms of gene expression in early embryogenesis in a South African clawed toad Xenopus laevis, especially during the period of midblastula transition (MBT), or the transition from the phase of active cell division (cleavage stage) to the phase of extensive morphogenesis (post-blastular stages). We first found that ribosomal RNA synthesis is initiated shortly after MBT in Xenopus embryos and those weak bases, such as amines and ammonium ion, selectively inhibit the initiation and subsequent activation of rRNA synthesis. We then found that rapidly labeled heterogeneous mRNA-like RNA is synthesized in embryos at pre-MBT stage. We then performed cloning and expression studies of several genes, such as those for activin receptors, follistatin and aldolases, and then reached the studies of S-adenosylmethionine decarboxylase (SAMDC), a key enzyme in polyamine metabolism. Here, we cloned a Xenopus SAMDC cDNA and performed experiments to overexpress the in vitro-synthesized SAMDC mRNA in Xenopus early embryos, and found that the maternally preset program of apoptosis occurs in cleavage stage embryos, which is executed when embryos reach the stage of MBT. In the present article, we first summarize results on SAMDC and the maternal program of apoptosis, and then describe our studies on small-molecular-weight substances like polyamines, amino acids, and amines in Xenopus embryos. Finally, we summarize our studies on weak bases, especially on ammonium ion, as the specific inhibitor of ribosomal RNA synthesis in Xenopus embryonic cells.

Gene expression in Pre-MBT embryos and activation of maternally-inherited program of apoptosis to be executed at around MBT as a fail-safe mechanism in Xenopus early embryogenesis

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.

Occurrence of pre-MBT synthesis of caspase-8 mRNA and activation of caspase-8 prior to execution of SAMDC (S-adenosylmethionine decarboxylase)-induced, but not p53-induced, apoptosis in Xenopus late blastulae

Overexpression of S-adenosylmethionine decarboxylase (SAMDC) in Xenopus fertilized eggs activates caspase-9 and executes maternal program of apoptosis shortly after midblastula transition (MBT). We find that overexpression of caspase-8 and p53, like that of SAMDC, induces apoptosis in Xenopus late blastulae. The apoptosis induced by p53 was abolished by injection of mRNA for xdm-2, a negative regulator of p53, and by injection of a peptide inhibitor or a dominant-negative type mutant of caspase-9, but not caspase-8. The apoptosis induced by SAMDC was not abolished by injection of xdm-2 mRNA, but was abolished by injection of a peptide inhibitor or a dominant-negative type mutant mRNA of both caspase-9 and caspase-8. Unlike caspase-9 mRNA, caspase-8 mRNA did not occur as a maternal mRNA rather induced to be expressed during cleavage stage (pre-MBT stage) by overexpression of SAMDC but not p53. Furthermore, while activities to process procaspase-8 and procaspase-9 appeared in SAMDC-overexpressed apoptotic embryos, the activity to process procaspase-8 did not appear in p53-overexpressed apoptotic embryos. We conclude there are at least two pathways in the execution of the maternal program of apoptosis in Xenopus embryos; one being through do novo expression of caspase-8 gene during cleavage stage, and the other without involvement of caspase-8.

Involvement of caspase-9 in execution of the maternal program of apoptosis in Xenopus late blastulae overexpressed with S-adenosylmethionine decarboxylase

We previously demonstrated that overexpression of S-adenosylmethionine decarboxylase (SAMDC) in Xenopus early embryos induces execution of maternal program of apoptosis shortly after midblastula transition, which likely serves as a fail-safe mechanism of early development to eliminate physiologically damaged cells before they entering the gastrula stage. To determine how caspases are involved in this process, we microinjected peptide inhibitors and "dominant-negative forms" of caspase-9 and -1 into Xenopus fertilized eggs, and found that inhibitors of caspase-9, but not caspase-1, completely suppress SAMDC-induced apoptosis. The lysate of SAMDC-overexpressing late blastulae contained activity to cleave in vitro-synthesized [(35)S]procaspase-9, but not [(35)S]procaspase-1, and mRNA for caspase-9, but not caspase-1, occurred abundantly in the unfertilized egg as maternal mRNA. We also found that overexpression of caspase-9 and -1 equally executes the apoptosis, but the apoptosis executed by these mRNAs was only partially rescued by Bcl-2 and rescued embryos did not develop beyond neurula stage. These results indicate that activation of caspase-9 is a key step for execution of the maternally preset program of apoptosis in Xenopus early embryos.

Loss of XChk1 function triggers apoptosis after the midblastula transition in Xenopus laevis embryos

Prior to the midblastula transition (MBT), Xenopus laevis embryos do not engage cell cycle checkpoints, although overexpression of the kinase XChk1 arrests cell divisions. At the MBT, XChk1 transiently activates and promotes cell cycle lengthening. In this study, endogenous XChk1 was inhibited by the expression of dominant-negative XChk1 (DN-XChk1). Development appeared normal until the early gastrula stage, when cells lost attachments and chromatin condensed. TUNEL and caspase assays indicated these embryos died by apoptosis during gastrulation. Embryos with unreplicated DNA likewise died by apoptosis. Embryos expressing DN-XChk1 proceeded through additional rapid rounds of DNA replication but initiated zygotic transcription on schedule. Therefore, XChk1 is essential in the early Xenopus embryo for cell cycle remodeling and for survival after the MBT.