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sodium-potassium-adenosinetriphosphatase isoforms; progesterone-induced oocyte maturation; Xenopus embryos; endocytosis
Abstract:
During progesterone-induced maturation of Xenopus oocytes, the transport and ouabain binding capacity of Na+-K+-ATPase at the plasma membrane is completely downregulated. To elucidate the mechanism and the physiological significance of this process, we have followed the fate of oocyte alpha-beta3-Na+-K+-ATPase complexes during meiotic maturation and early embryonic development. An immunocytochemical follow-up of the catalytic alpha-subunit, ouabain binding studies, cell surface iodination, and oocyte cell fractionation combined with immunochemical subunit detection provides evidence that following progesterone treatment Na+-K+-ATPase molecules are retrieved from the oocyte plasma membrane. The enzyme complexes are recovered in an active form in an intracellular compartment in both in vitro and in vivo matured eggs. Exogenous Xenopus alpha1- and beta1-complexes expressed in the oocyte from injected cRNAs are regulated by progesterone similar to endogenous Na+-K+-ATPase complexes. Finally, active Na+-K+ pumps internalized during oocyte maturation appear to be redistributed to plasma membrane fractions during blastula formation in Xenopus embryos. In conclusion, our data suggest that endocytosis of alpha1- and beta3-complexes during meiotic maturation of Xenopus oocytes is responsible for downregulation of Na+-K+-ATPase activity and results in an intracellular pool of functional enzymes, which might be reexpressed during early development in response to physiological needs.
