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???displayArticle.abstract??? Egg overactivation occurs with a low frequency in the populations of naturally ovulated frog eggs. At present, its natural inducers, molecular mechanisms, and intracellular events remain unknown. Using microscopic and biochemical analyses, we demonstrate here that high levels of hydrogen peroxide-induced oxidative stress can cause time- and dose-dependent overactivation of Xenopus eggs. Lipofuscin accumulation, decrease of soluble cytoplasmic protein content, and depletion of intracellular ATP were found to take place in the overactivated eggs. Progressive development of these processes suggests that egg overactivation unfolds in a sequential and ordered fashion.
Figure 1. Morphological types of eggs observed in populations of naturally ovulated Xenopus eggs. Panels (a), (b), (c), and (d) present mature fertilization-competent, apoptotic, overactivated, and cortically contracted eggs, respectively. Panel (e) shows population of eggs matured in vitro in the presence of progesterone and aged by overnight incubation on bench. Arrows point to overactivated eggs.
Figure 2. Time and dose dependencies of egg overactivation by hydrogen peroxide. Panel (a) shows dynamics of morphological changes in the eggs overactivated by 100 mM hydrogen peroxide. Panel (b) presents time course of egg cortical contraction and overactivation after addition of 100 mM hydrogen peroxide. The untreated control (-H202) in panel (b) refers to overactivated phenotype. Panel (c) shows dose dependency of egg overactivation. Overactivated phenotype in panel (c) was counted in 240 min after addition of peroxide.
Figure 3. Oxidative stress-induced accumulation of lipofuscin in Xenopus eggs. Spot assay of lipofuscin autofluorescence in the insoluble particulate fractions of the peroxide-treated eggs and its quantification are presented in panels (a) and (b), respectively. The items labeled as 480∗ refer to control eggs incubated for 480 min in the absence of peroxide. Stars in panel (b) indicate statistical significance from the untreated control (p<0.05). Panel (c) shows SBB staining of the egg endosomal compartment. Hydrogen peroxide (f.c. 100 mM) was added to the eggs at 0 time.
Figure 4. Protein and ATP contents in overactivated Xenopus eggs. The contents of soluble cytosolic protein and intracellular ATP in the eggs overactivated by addition of 100 mM hydrogen peroxide are shown in panels (a) and (b), respectively. Control (Ctrl) in the panels refers to the eggs incubated for 480 min in the absence of hydrogen peroxide.
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