Differential involvement of Na+,K+-ATPase isozymes in preimplantation development of the mouse

Na+,K+-ATPase plays an essential role in mammalian blastocoel formation (ca vitation) by driving trans-epithelial sodium transport. Previously, the alp ha 1 and beta 1 subunit isoforms of this enzyme were identified in preimpla ntation mouse embryos and were assumed to be responsible for this function. Here we show that mRNAs encoding an additional alpha subunit isoform (alph a 3) and the remaining two beta subunit isoforms are also present in preimp lantation embryos, Whereas alpha 3 mRNA accumulates between the four-cell a nd the blastocyst stages and thus results from embryonic transcription, the same could not be demonstrated for beta 2 and beta 3 mRNAs. Immunoblot ana lyses confirmed that these subunits are present in cavitating embryos. Usin g confocal immunofluorescence microscopy we found that alpha 1 and beta 1 s ubunits are concentrated in the basolateral membranes of the trophectoderm while being equally distributed in plasma membranes of the inner cell mass. In contrast, alpha 3, beta 2, and beta 3 subunits were not detected in pla sma membranes. Our current assessment, therefore, is that as many as six is ozymes of Na+,K+-ATPase could be involved in preimplantation development al though it is primarily the alpha 1 beta 1 isozyme that is responsible for b lastocoel formation. Our findings imply that the regulation of sodium trans port within the preimplantation mouse embryo is more complex than had been appreciated. (C) 2000 Academic Press.