Pro-apoptotic activity and mono-/diubiquitylation of Xenopus Bid in egg extracts

Hyperosmotic Shock Engages Two Positive Feedback Loops through Caspase-3-dependent Proteolysis of JNK1-2 and Bid

Hyperosmotic shock induces early calpain activation, Smac/DIABLO release from the mitochondria, and p38/JNK activation in Xenopus oocytes. These pathways regulate late cytochrome c release and caspase-3 activation. Here, we show that JNK1-1 and JNK1-2 are activated early by osmostress, and sustained activation of both isoforms accelerates the apoptotic program. When caspase-3 is activated, JNK1-2 is proteolyzed at Asp-385 increasing the release of cytochrome c and caspase-3 activity, thereby creating a positive feedback loop. Expression of Bcl-xL markedly reduces hyperosmotic shock-induced apoptosis. In contrast, expression of Bid induces rapid caspase-3 activation, even in the absence of osmostress, which is blocked by Bcl-xL co-expression. In these conditions a significant amount of Bid in the cytosol is mono- and bi-ubiquitinated. Caspase-3 activation by hyperosmotic shock induces proteolysis of Bid and mono-ubiquitinated Bid at Asp-52 increasing the release of cytochrome c and caspase-3 activation, and thus creating a second positive feedback loop. Revealing the JNK isoforms and the loops activated by osmostress could help to design better treatments for human diseases caused by perturbations in fluid osmolarity.

Unfertilized Xenopus eggs die by Bad-dependent apoptosis under the control of Cdk1 and JNK

Ovulated eggs possess maternal apoptotic execution machinery that is inhibited for a limited time. The fertilized eggs switch off this time bomb whereas aged unfertilized eggs and parthenogenetically activated eggs fail to stop the timer and die. To investigate the nature of the molecular clock that triggers the egg decision of committing suicide, we introduce here Xenopus eggs as an in vivo system for studying the death of unfertilized eggs. We report that after ovulation, a number of eggs remains in the female body where they die by apoptosis. Similarly, ovulated unfertilized eggs recovered in the external medium die within 72 h. We showed that the death process depends on both cytochrome c release and caspase activation. The apoptotic machinery is turned on during meiotic maturation, before fertilization. The death pathway is independent of ERK but relies on activating Bad phosphorylation through the control of both kinases Cdk1 and JNK. In conclusion, the default fate of an unfertilized Xenopus egg is to die by a mitochondrial dependent apoptosis activated during meiotic maturation.

Anti-apoptotic activity and proteasome-mediated degradation of Xenopus Mcl-1 protein in egg extracts

Xenopus egg extracts execute spontaneous apoptosis without the requirement of transcription and translation, and this intrinsic mechanism is supposed to be involved in the physiological elimination of aged eggs. Although apoptosis in this system is carried out by maternally stockpiled materials, the endogenous apoptosis regulators present in egg extracts are still poorly characterized. Here we examined the mRNA expression profiles and apoptosis-regulating functions of 13 Xenopus Bcl-2 family proteins in egg extracts. Among these, we found that endogenous Xenopus Mcl-1 (xMcl-1) physiologically inhibited apoptosis by counteracting the pro-apoptotic activity of endogenous Xenopus Bid in egg extracts. Exogenously added recombinant xMcl-1 was rapidly degraded by proteasome in egg extracts, and we identified the destabilizing region in the N terminus of xMcl-1. Our results suggest that the proteolytic decay of xMcl-1 may change the functional balance between pro- and anti-apoptotic activities of Bcl-2 family proteins, thereby regulating the timing of cytochrome c release in egg extracts.