Downregulation of surface sodium pumps by endocytosis during meiotic maturation of Xenopus laevis oocytes

Abstract

During meiotic maturation, plasma membranes of Xenopus laevis oocytes completely lose the capacity to transport Na and K and to bind ouabain. To explore whether the downregulation might be due to an internalization of the sodium pump molecules, the intracellular binding of ouabain was determined. Selective permeabilization of the plasma membrane of mature oocytes (eggs) by digitonin almost failed to disclose ouabain binding sites. However, when the eggs were additionally treated with 0.02% sodium dodecyl sulfate (SDS) to permeabilize inner membranes, all sodium pumps present before maturation were recovered. Phosphorylation by [gamma-32P]ATP combined with SDS-polyacrylamide gel electrophoresis (PAGE) and autoradiography showed that sodium pumps were greatly reduced in isolated plasma membranes of eggs. According to sucrose gradient fractionation, maturation induced a shift of sodium pumps from the plasma membrane fraction to membranes of lower buoyant density with a protein composition different from that of the plasma membrane. Endocytosed sodium pumps identified on the sucrose gradient from [3H]ouabain bound to the cell surface before maturation could be phosphorylated with inorganic [32P]phosphate. The findings suggest that downregulation of sodium pumps during maturation is brought about by translocation of surface sodium pumps to an intracellular compartment, presumably endosomes. This contrasts the mechanism of downregulation of Na-dependent cotransport systems, the activities of which are reduced as a consequence of a maturation-induced depolarization of the membrane without a removal of the corresponding transporter from the plasma membrane.

Store-operated Calcium Entry Inactivates at the Germinal Vesicle Breakdown Stage of Xenopus Meiosis

“…6A), which is responsible for the down-regulation of plasma membrane Na ϩ -K ϩ -ATPase (25,26). To determine whether SOCE channels are subject to the same regulation, we correlated I soce levels with the different phases of meiosis (see Figs.…”

“…Whereas membrane area decreases gradually after progesterone addition, SOCE inactivates acutely at GVBD. The Na ϩ -K ϩ -ATPase has also been shown to be down-regulated during meiotic maturation because of sequestration into intracellular vesicles (25,26). In this case Na ϩ -K ϩ -ATPase activity begins to decrease gradually after progesterone addition, even before GVBD, and continues after GVBD has occurred (28).…”

Regulation of store-operated Ca2+ entry during the cell cycle

“…This was the first report that addressed Orai1 trafficking; hence, it is not clear whether this mechanism is involved in SOCE inactivation during mitosis. However, internalization of membrane channels, receptors and transporters has been documented during meiosis of both Xenopus and mouse oocytes (El Jouni et al, 2008;Muller et al, 1993;Schmalzing et al, 1990;Zhou et al, 2009). These internalized membrane proteins incorporate into newly formed blastomeres during early embryogenesis and, as such, contribute to the formation of the polarized epithelium during the blastocoele stage of Xenopus embryogenesis (Angres et al, 1991; Gawantka et al, 1992;Muller, 2001).…”

Ca2+ signaling differentiation during oocyte maturation

“…Several proteins that localize to the cell membrane in oocytes are internalized during maturation, including bintegrin and the Na-K-ATPase (Schmalzing et al, 1990;Muller et al, 1993). The internalization of ionic transport pathways during oocyte maturation is thought to be critical for the formation of the fluid-filled blastocoel in the embryo (Muller and Hausen, 1995;Muller, 2001).…”