Na,K-ATPase subunit β1 knock-in prevents lethality of β2 deficiency in mice

The β2 subunit of the Na,K-ATPase displays functional properties of both an integral constituent of an ion pump and an adhesion and neurite outgrowth-promoting molecule in vitro. To investigate whether the β1 subunit of the Na,K-ATPase can functionally substitute for the β2 isoform in vivo, we have generated β2/β1 knock-in mice by homologous recombination in embryonic stem cells. In β2/β1 knock-in mice, expression of β2 was abolished, whereas β1 mRNA expression from the mutated gene amounted to ~15% of the normal expression of β2 in the adult mouse brain and prevented the juvenile lethality observed for β2 null mutant mice. In contrast to β2 null mutant mice, the overall morphological structure of all analyzed brain regions was normal. By immunohistochemical analysis, β1 expression was detected in photoreceptor cells in the retina of knock-in mice at an age when expression of β1 and β2, respectively, is downregulated and persisting in the wild-type mice. Morphological analysis by light and electron microscopy revealed a progressive degeneration of photoreceptor cells. Apoptotic death of photoreceptor cells determined quantitatively by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling analysis increased in β2/β1 knock-in mice with age. These observations suggest that the β1 subunit of the Na,K-ATPase can substitute sufficiently, at least in certain cell types, for the role of the β2 subunit as a component of a functional Na,K-ATPase, but they do not allow us to determine the possible role of the β2 subunit as an adhesion molecule in vivo.