Unfertilized frog eggs die by apoptosis following meiotic exit

Abstract

BackgroundA characteristic feature of frog reproduction is external fertilization accomplished outside the female's body. Mature fertilization-competent frog eggs are arrested at the meiotic metaphase II with high activity of the key meiotic regulators, maturation promoting factor (MPF) and cytostatic factor (CSF), awaiting fertilization. If the eggs are not fertilized within several hours of ovulation, they deteriorate and ultimately die by as yet unknown mechanism.ResultsHere, we report that the vast majority of naturally laid unfertilized eggs of the African clawed frog Xenopus laevis spontaneously exit metaphase arrest under various environmental conditions and degrade by a well-defined apoptotic process within 48 hours after ovulation. The main features of this process include cytochrome c release, caspase activation, ATP depletion, increase of ADP/ATP ratio, apoptotic nuclear morphology, progressive intracellular acidification, and egg swelling. Meiotic exit seems to be a prerequisite for execution of the apoptotic program, since (i) it precedes apoptosis, (ii) apoptotic events cannot be observed in the eggs maintaining high activity of MPF and CSF, and (iii) apoptosis in unfertilized frog eggs is accelerated upon early meiotic exit. The apoptotic features cannot be observed in the immature prophase-arrested oocytes, however, the maturation-inducing hormone progesterone renders oocytes susceptible to apoptosis.ConclusionsThe study reveals that naturally laid intact frog eggs die by apoptosis if they are not fertilized. A maternal apoptotic program is evoked in frog oocytes upon maturation and executed after meiotic exit in unfertilized eggs. The meiotic exit is required for execution of the apoptotic program in eggs. The emerging anti-apoptotic role of meiotic metaphase arrest needs further investigation.

Global decay of mRNA is a hallmark of apoptosis in agingXenopuseggs

“…Apoptosis-related and regulatory genes showed much higher expression variations. High expression variation of apoptosis-related genes, such as bid, bak, mcl1, casp3, casp7 , may help to explain a substantial heterogeneity of apoptotic response observed in unfertilized Xenopus eggs 9 . These results strongly suggested that monitoring transcript levels in single oocytes and eggs rather than in cell populations should be used for quantitative transcript profiling in these cells to circumvent intrinsically large gene expression heterogeneity.…”

“…They can be collected freshly from living frogs and treated after isolation like a primary cell culture. Recently, aging unfertilized Xenopus eggs were found to die by a classical apoptotic process after the spontaneous exit from meiotic arrest 9-11 . In the present study, to bypass large cell-to-cell variation of the transcript levels in the oocyte/egg population, single-cell gene expression analysis was implemented.…”

Unlaid Xenopus eggs degrade by apoptosis in the genital tract

“…Most recently, two research groups, including ours, reported that the unfertilized Xenopus eggs laid outside of the frog body die by a well-defined apoptotic process [20,21]. In this case too, meiotic exit was found to precede apoptosis [21].…”

The role of ion channels and intracellular metal ions in apoptosis of Xenopus oocytes

“…The size also allows injections to alter ionic composition or regulate the activity of different proteins (Dascal, 1987). Previous studies have shown that the X. laevis oocyte displays a normal apoptotic process, which includes activation of caspases, cytochrome c release from the mitochondria, nuclear condensation and ATP depletion (Braun et al, 2003;Johnson et al, 2010;Nutt et al, 2005;Tokmakov et al, 2011). The expression level of endogenous channels is low (Dascal, 1987) and this makes them suitable to explore the importance of ion channels and intracellular ion concentrations in the apoptotic process.…”

“…laevis oocytes display a normal apoptotic process compared to other cells including cytochrome c release and activation of caspase-3 (Braun et al, 2003;Johnson et al, 2010;Nutt et al, 2005;Tokmakov et al, 2011). In papers I-III, caspase-3 activity was measured by the fluorescence of 7-amino-4-methylcoumarin (AMC) resulting from the cleavage of acetyl Asp-Glu-Val-Asp 7-amido-4-methylcoumarin (Ac-DEVD-AMC) by activated caspase-3.…”