Na,K-ATPase Subunit b1 knock-in Prevents Lethality of b2 Deficiency in Mice

The b2 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 b1 subunit of the Na,K-ATPase can functionally substitute for the b2 isoform in vivo, we have generated b2/b1 knock-in mice by homologous recombination in embryonic stem cells. In b2/b1 knock-in mice, expression of b2 was abolished, whereas b1 mRNA expression from the mutated gene amounted to ;15% of the normal expression of b2 in the adult mouse brain and prevented the juvenile lethality observed for b2 null mutant mice. In contrast to b2 null mutant mice, the overall morphological structure of all analyzed brain regions was normal. By immunohistochemical analysis, b1 expression was detected in photoreceptor cells in the retina of knock-in mice at an age when expression of b1 and b2, 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 b2/b1 knock-in mice with age. These observations suggest that the b1 subunit of the Na,K-ATPase can substitute sufficiently, at least in certain cell types, for the role of the b2 subunit as a component of a functional Na,K-ATPase, but they do not allow us to determine the possible role of the b2 subunit as an adhesion molecule in vivo.