Modulation of Na,K-ATPase expression during early development of Xenopus laevis

In amphibian and mammalian systems, regulation of Na+ transport via the Na,K-ATPase plays an important role in distinct developmental processes such as blastocoele formation and neurulation. In this study, we have followed the Na,K-ATPase activity, the biosynthesis, and the cellular accumulation of catalytic alpha-subunits after fertilization of Xenopus laevis eggs up to neurula formation. Our data show that Na,K-ATPase activity increases significantly between stages 4 and 6 and again between stages 13 and 24. The four-fold rise in Na,K-ATPase activity during blastocoele formation is not mediated by an increased cellular pool of alpha-subunits. On the other hand, a five-fold increase of the biosynthesis rate around midblastula precedes a progressive accumulation up to neurula stage mainly of alpha 1-subunits and to a lesser extent of a second alpha-immunoreactive species. In contrast, newly synthesized glycoproteinic beta 1-subunits of Na,K-ATPase cannot be detected up to late neurula. These data indicate that (1) upregulation of Na,K-ATPase activity during blastocoele and neurula formation are mediated by different regulation mechanisms and (2) alpha- and possibly beta-isoforms are expressed in a developmentally regulated fashion during early Xenopus development

Regulation of alpha 1-beta 3-NA(+)-K(+)-ATPase isozyme during meiotic maturation of Xenopus laevis oocytes

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-beta 3-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 alpha 1- and beta 1-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 alpha 1- and beta 3-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

A novel vasopressin-induced transcript promotes MAP kinase activation and ENaC downregulation

In the principal cell of the renal collecting duct, vasopressin regulates the expression of a gene network responsible for sodium and water reabsorption through the regulation of the water channel and the epithelial sodium channel (ENaC). We have recently identified a novel vasopressin-induced transcript (VIT32) that encodes for a 142 amino acid vasopressin-induced protein (VIP32), which has no homology with any protein of known function. The Xenopus oocyte expression system revealed two functions: (i) when injected alone, VIT32 cRNA rapidly induces oocyte meiotic maturation through the activation of the maturation promoting factor, the amphibian homolog of the universal M phase trigger Cdc2/cyclin; and (ii) when co-injected with the ENaC, VIT32 cRNA selectively downregulates channel activity, but not channel cell surface expression. In the kidney principal cell, VIP32 may be involved in the downregulation of transepithelial sodium transport observed within a few hours after vasopressin treatment. VIP32 belongs to a novel gene family ubiquitously expressed in oocyte and somatic cells that may be involved in G to M transition and cell cycling