Na,K-ATPase plays a crucial role in cellular ion homeostasis and is the pharmacological receptor for digitalis in man. Nine different human Na,K-ATPase isozymes, composed of 3 α and β isoforms, were expressed in Xenopus oocytes and were analyzed for their transport and pharmacological properties. According to ouabain binding and K⁺-activated pump current measurements, all human isozymes are functional but differ in their turnover rates depending on the α isoform. On the other hand, variations in external K⁺ activation are determined by a cooperative interaction mechanism between α and β isoforms with α2-β2 complexes having the lowest apparent K⁺ affinity. α Isoforms influence the apparent internal Na⁺ affinity in the order α1 > α2 > α3 and the voltage dependence in the order α2 > α1 > α3. All human Na,K-ATPase isozymes have a similar, high affinity for ouabain. However, α2-β isozymes exhibit more rapid ouabain association as well as dissociation rate constants than α1-β and α3-β isozymes. Finally, isoform-specific differences exist in the K⁺/ouabain antagonism which may protect α1 but not α2 or α3 from digitalis inhibition at physiological K⁺ levels. In conclusion, our study reveals several new functional characteristics of human Na,K-ATPase isozymes which help to better understand their role in ion homeostasis in different tissues and in digitalis action and toxicity.

A family of small, single-span membrane proteins (the FXYD family) has recently been defined based on their sequence and structural homology. Some members of this family have already been identified as tissue-specific regulators of Na,K-ATPase (NKA). In the present study, we demonstrate that phospholemman (PLM) (FXYD1), so far considered to be a heart- and muscle-specific channel or channel-regulating protein, associates specifically and stably with six different α-β isozymes of NKA after coexpression in Xenopus oocytes, and with α1–β, and less efficiently with α2–β isozymes, in native cardiac and skeletal muscles. Stoichiometric association of PLM with NKA occurs posttranslationally either in the Golgi or the plasma membrane. Interaction of PLM with NKA induces a small decrease in the external K ⁺ affinity of α1–β1 and α2–β1 isozymes and a nearly 2-fold decrease in the internal Na ⁺ affinity. In conclusion, this study demonstrates that PLM is a tissue-specific regulator of NKA that may play an essential role in muscle contractility.

The biological role of small membrane proteins of the new FXYD family is largely unknown. The best characterized FXYD protein is the γ‐subunit of the Na,K‐ATPase (NKA) that modulates the Na,K‐pump function in the kidney. Here, we report that, similarly to γa and γb splice variants, the FXYD protein CHIF (corticosteroid‐induced factor) is a type I membrane protein which is associated with NKA in renal tissue, and modulates the Na,K‐pump transport when expressed in Xenopus oocytes. In contrast to γa and γb, which both decrease the apparent Na+ affinity of the Na,K‐pump, CHIF significantly increases the Na+ affinity and decreases the apparent K+ affinity due to an increased Na+ competition at external binding sites. The extracytoplasmic FXYD motif is required for stable γ‐subunit and CHIF interaction with NKA, while cytoplasmic, positively charged residues are necessary for the γ‐subunit's association efficiency and for CHIF's functional effects. These data document that CHIF is a new tissue‐specific regulator of NKA which probably plays a crucial role in aldosterone‐responsive tissues responsible for the maintenance of body Na+ and K+ homeostasis.

The β-subunit of Na,K-ATPase (βNK) interacts with the catalytic α-subunit (αNK) in the ectodomain, the transmembrane, and the cytoplasmic domain. The functional significance of these different interactions was studied by expressing αNK inXenopus oocytes along with N-terminally modified βNK or with chimeric βNK/βH,K-ATPase (βHK). Complete truncation of the βNK N terminus allows for cell surface-expressed, functional Na,K-pumps that exhibit, however, reduced apparent K⁺ and Na⁺ affinities as assessed by electrophysiological measurements. A mutational analysis suggests that these functional effects are not related to a direct interaction of the β N terminus with the αNK but rather that N-terminal truncation induces a conformational change in another functionally relevant β domain. Comparison of the functional properties of αNK·βNK, αNK·βHK, or αNK·βNK/βHK complexes shows that the effect of the βNK on K⁺ binding is mainly mediated by its ectodomain. Finally, βHK/NK containing the transmembrane domain of βHK produces stable but endoplasmic reticulum-retained αNK·β complexes, while αNK/βHK complexes can leave the ER but exhibit reduced ouabain binding capacity and transport function. Thus, interactions of both the transmembrane and the ectodomain of βNK with αNK are necessary to form correctly folded Na,K-ATPase complexes that can be targeted to the plasma membrane and/or become functionally competent. Furthermore, the β N terminus plays a role in the β-subunit’s folding necessary for correct interactions with the α-subunit.

Recently, corticosteroid hormone‐induced factor (CHIF) and the γ‐subunit, two members of the FXYD family of small proteins, have been identified as regulators of renal Na,K‐ATPase. In this study, we have investigated the tissue distribution and the structural and functional properties of FXYD7, another family member which has not yet been characterized. Expressed exclusively in the brain, FXYD7 is a type I membrane protein bearing N‐terminal, post‐translationally added modifications on threonine residues, most probably O ‐glycosylations that are important for protein stabilization. Expressed in Xenopus oocytes, FXYD7 can interact with Na,K‐ATPase α1–β1, α2–β1 and α3–β1 but not with α–β2 isozymes, whereas, in brain, it is only associated with α1–β isozymes. FXYD7 decreases the apparent K+ affinity of α1–β1 and α2–β1, but not of α3–β1 isozymes. These data suggest that FXYD7 is a novel, tissue‐ and isoform‐specific Na,K‐ATPase regulator which could play an important role in neuronal excitability.

Cystic fibrosis (CF) is caused by defective Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) protein. Morbidity is mainly due to early airway infection. We hypothesized that S. aureus clearance during the first hours of infection was impaired in CF human Airway Surface Liquid (ASL) because of a lowered pH. The ASL pH of human bronchial epithelial cell lines and primary respiratory cells from healthy controls (WT) and patients with CF was measured with a pH microelectrode. The antimicrobial capacity of airway cells was studied after S. aureus apical infection by counting surviving bacteria. ASL was significantly more acidic in CF than in WT respiratory cells. This was consistent with a defect in bicarbonate secretion involving CFTR and SLC26A4 (pendrin) and a persistent proton secretion by ATP12A. ASL demonstrated a defect in S. aureus clearance which was improved by pH normalization. Pendrin inhibition in WT airways recapitulated the CF airway defect and increased S. aureus proliferation. ATP12A inhibition by ouabain decreased bacterial proliferation. Antimicrobial peptides LL-37 and hBD1 demonstrated a pH-dependent activity. Normalizing ASL pH might improve innate airway defense in newborns with CF during onset of S. aureus infection. Pendrin activation and ATP12A inhibition could represent novel therapeutic strategies to normalize pH in CF airways.