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Exp Gerontol. 2019 May;119:157-167. doi: 10.1016/j.exger.2019.02.002. Epub 2019 Feb 12.

Activity and intracellular localization of senescence-associated β-galactosidase in aging Xenopus oocytes and eggs.

Author information

1
Department of Molecular Biosciences, Kyoto Sangyo University, Kamigamo-motoyama, Kita-ku, Kyoto 603-8555, Japan. Electronic address: tokmak@cc.kyoto-su.ac.jp.
2
Department of Molecular Biosciences, Kyoto Sangyo University, Kamigamo-motoyama, Kita-ku, Kyoto 603-8555, Japan. Electronic address: kksato@cc.kyoto-su.ac.jp.

Abstract

Senescence-associated β-galactosidase (SA-β-gal) serves as a marker of senescence in aging somatic cells. However, little is known about SA-β-gal dynamics in aging gamete cells. To address this issue, here we investigated activity and intracellular localization of SA-β-gal in freshly obtained and aging oocytes and eggs of the African clawed frog Xenopus laevis. Data base mining revealed the presence of several homologous β-galactosidase sequences in the annotated Xenopus genome. Some of them were predicted to contain an N-terminal signal peptide sequence, suggesting enzyme translocation to cellular organelles. Biochemical and microscopic analyses confirmed SA-β-gal localization in the particulate and cytosolic fractions of oocytes and eggs. SA-β-gal activity was found to reside predominantly within a fraction of dense cytoplasmic granules that were extensively stained with the lysosome-specific dye LysoTracker Green DND-26 and had an average size of 8.9 ± 5.6 μm. These features identify the SA-β-gal-containing granules as a subpopulation of yolk platelets, specialized late endosomes or lysosomes that accumulate and store processed protein in frog oocytes. Further analysis revealed an increase of SA-β-gal activity in Xenopus eggs, but not in oocytes, aged in vitro over 48 h. Our data suggest that endosomal acidification during egg aging may be responsible for this increase.

KEYWORDS:

Aging; Eggs; Oocytes; Senescence-associated β-galactosidase; Xenopus laevis

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