NA,K-ATPASE SUBUNIT BETA-1 KNOCK-IN PREVENTS LETHALITY OF BETA-2 DEFICIENCY IN MICE

The beta 2 subunit of the Na,K-ATPase displays functional properties o f both an integral constituent of an ion pump and an adhesion and neur ite outgrowth-promoting molecule in vitro. To investigate whether the beta 1 subunit of the Na,K-ATPase can functionally substitute for the beta 2 isoform in vivo, we have generated beta 2/beta 1 knock-in mice by homologous recombination in embryonic stem cells. In beta 2/beta 1 knock-in mice, expression of beta 2 was abolished, whereas beta 1 mRNA expression from the mutated gene amounted to similar to 15% of the no rmal expression of beta 2 in the adult mouse brain and prevented the j uvenile lethality observed for beta 2 null mutant mice. In contrast to beta 2 null mutant mice, the overall morphological structure of all a nalyzed brain regions was normal. By immunohistochemical analysis, bet a 1 expression was detected in photoreceptor cells in the retina of kn ock-in mice at an age when expression of beta 1 and beta 2, respective ly, is downregulated and persisting in the wild-type mice. Morphologic al analysis by light and electron microscopy revealed a progressive de generation of photoreceptor cells. Apoptotic death of photoreceptor ce lls determined quantitatively by terminal deoxynucleotidyl transferase -mediated dUTP nick end labeling analysis increased in beta 2/beta 1 k nock-in mice with age. These observations suggest that the beta 1 subu nit of the Na,K-ATPase can substitute sufficiently, at least in certai n cell types, for the role of the beta 2 subunit as a component of a: functional Na,K-ATPase, but they do not allow us to determine the poss ible role of the beta 2 subunit as an adhesion molecule in vivo.