The non-gastric H,K-ATPase as a tool to study the ouabain-binding site in Na,K-ATPase

Expression of Na,K-ATPase and H,K-ATPase Isoforms with the Baculovirus Expression System

P-type ATPases can be expressed in several cell systems. The baculovirus expressions system uses an insect virus to enter and express proteins in Sf9 insect cells. This expression system is a lytic system in which the cells will die a few days after viral infection. Subsequently, the expressed proteins can be isolated. Insect cells are a perfect system to study P-type ATPases as they have little or no endogenous Na,K-ATPase activity and other ATPase activities can be inhibited easily. Here we describe in detail the expression and isolation of Na,K-ATPase and H,K-ATPase isoforms with the baculovirus expression system.

A missense variant of the ATP1A2 gene is associated with a novel phenotype of progressive sensorineural hearing loss associated with migraine

Hereditary sensorineural hearing loss is an extremely clinical and genetic heterogeneous disorder in humans. Especially, syndromic hearing loss is subdivided by combinations of various phenotypes, and each subtype is related to different genes. We present a new form of progressive hearing loss with migraine found to be associated with a variant in the ATP1A2 gene. The ATP1A2 gene has been reported as the major genetic cause of familial migraine by several previous studies. A Korean family presenting progressive hearing loss with migraine was ascertained. The affected members did not show any aura or other neurologic symptoms during migraine attacks, indicating on a novel phenotype of syndromic hearing loss. To identify the causative gene, linkage analysis and whole-exome sequencing were performed. A novel missense variant, c.571G>A (p.(Val191Met)), was identified in the ATP1A2 gene that showed co-segregation with the phenotype in the family. In silico studies suggest that this variant causes a change in hydrophobic interactions and thereby slightly destabilize the A-domain of Na(+)/K(+)-ATPase. However, functional studies failed to show any effect of the p.(Val191Met) substitution on the catalytic rate of this enzyme. We describe a new phenotype of progressive hearing loss with migraine associated with a variant in the ATP1A2 gene. This study suggests that a variant in Na(+)/K(+)-ATPase can be involved in both migraine and hearing loss.

Gastric H+,K+-ATPase

The gastric H(+),K(+)-ATPase is responsible for gastric acid secretion. This ATPase is composed of two subunits, the catalytic α subunit and the structural β subunit. The α subunit with molecular mass of about 100 kDa has 10 transmembrane domains and is strongly associated with the β subunit with a single transmembrane segment and a peptide mass of 35 kDa. Its three-dimensional structure is based on homology modeling and site-directed mutagenesis resulting in a proton extrusion and K(+) reabsorption model. There are three conserved H3O(+)-binding sites in the middle of the membrane domain and H3O(+) secretion depends on a conformational change involving Lys(791) insertion into the second H3O(+) site enclosed by E795, E820, and D824 that allows export of protons at a concentration of 160 mM. K(+) countertransport involves binding to this site after the release of protons with retrograde displacement of Lys(791) and then K(+) transfer to E343 and exit to the cytoplasm. This ATPase is the major therapeutic target in treatment of acid-related diseases and there are several known luminal inhibitors allowing analysis of the luminal vestibule. One class contains the acid-activated covalent, thiophilic proton pump inhibitors, the most effective of current acid-suppressive drugs. Their binding sites and trypsinolysis allowed identification of all ten transmembrane segments of the ATPase. In addition, various K(+)-competitive inhibitors of the ATPase are being developed, with the advantage of complete and rapid inhibition of acid secretion independent of pump activity and allowing further refinement of the structure of the luminal vestibule of the E2 form of this ATPase.

Amino acid substitutions of Na,K-ATPase conferring decreased sensitivity to cardenolides in insects compared to mammals

Mutagenesis analyses and a recent crystal structure of the mammalian Na,K-ATPase have identified amino acids which are responsible for high affinity binding of cardenolides (such as ouabain) which at higher doses block the enzyme in the phosphorylated state. Genetic analysis of the Na,K-ATPase of insects adapted to cardenolides in their food plants revealed that some species possess substitutions which confer strongly increased resistance to ouabain in the mammalian enzyme such as the substitution T797A or combined substitutions at positions 111 and 122. To test for the effect of these mutations against the background of insect Na,K-ATPase, we here expressed the ouabain sensitive Na,K-ATPase α-subunit of Drosophila melanogaster together with the β-subunit Nrv3 in baculovirus-infected Sf9 cells and introduced the substitutions N122H, T797A, Q111T-N122H, Q111V-N122H, all of which have been observed in cardenolide-adapted insects. While all constructs showed similar expression levels, ouabain affinity of mutated Na,K-ATPases was reduced compared to the wild-type fly enzyme. Ouabain sensitivity of the ATPase activity in inhibition assays was significantly decreased by all mutations, yet whereas the IC₅₀ for the single mutations of N122H (61.0 μM) or T797A (63.3 μM) was increased roughly 250-fold relative to the wild-type (0.24 μM), the double mutations of Q111V-N122H (IC₅₀ 550 μM) and Q111T-N122H (IC₅₀ 583 μM) proved to be still more effective yielding a 2.250-fold increased resistance to ouabain. The double mutations identified in cardenolide-adapted insects are more effective in reducing ouabain sensitivity of the enzyme than those found naturally in the rat Na,K-ATPase (Q111R-N122D) or in mutagenesis screens of the mammalian enzyme. Obviously, the intense selection pressure on cardenolide exposed insects has resulted in very efficient substitutions that decrease cardenolide sensitivity extremely.

Ion pumps as biological targets for decavanadate

The putative applications of poly-, oligo- and mono-oxometalates in biochemistry, biology, pharmacology and medicine are rapidly attracting interest. In particular, these compounds may act as potent ion pump inhibitors and have the potential to play a role in the treatment of e.g. ulcers, cancer and ischemic heart disease. However, the mechanism of action is not completely understood in most cases, and even remains largely unknown in other cases. In the present review we discuss the most recent insights into the interaction between mono- and polyoxometalate ions with ion pumps, with particular focus on the interaction of decavanadate with Ca(2+)-ATPase. We also compare the proposed mode of action with those of established ion pump inhibitors which are currently in therapeutic use. Of the 18 classes of compounds which are known to act as ion pump inhibitors, the complete mechanism of inhibition is only known for a handful. It has, however, been established that most ion pump inhibitors bind mainly to the E2 ion pump conformation within the membrane domain from the extracellular side and block the cation release. Polyoxometalates such as decavanadate, in contrast, interact with Ca(2+)-ATPase near the nucleotide binding site domain or at a pocket involving several cytoplasmic domains, and therefore need to cross through the membrane bilayer. In contrast to monomeric vanadate, which only binds to the E2 conformation, decavanadate binds to all protein conformations, i.e. E1, E1P, E2 and E2P. Moreover, the specific interaction of decavanadate with sarcoplasmic reticulum Ca(2+)-ATPase has been shown to be non-competitive with respect to ATP and induces protein cysteine oxidation with concomitant vanadium reduction which might explain the high inhibitory capacity of V10 (IC50 = 15 μM) which is quite similar to the majority of the established therapeutic drugs.

Familial hemiplegic migraine mutations affect Na,K-ATPase domain interactions

Familial hemiplegic migraine (FHM) is a monogenic variant of migraine with aura. One of the three known causative genes, ATP1A2, which encodes the α2 isoform of Na,K-ATPase, causes FHM type 2 (FHM2). Over 50 FHM2 mutations have been reported, but most have not been characterized functionally. Here we study the molecular mechanism of Na,K-ATPase α2 missense mutations. Mutants E700K and P786L inactivate or strongly reduce enzyme activity. Glutamic acid 700 is located in the phosphorylation (P) domain and the mutation most likely disrupts the salt bridge with Lysine 35, thereby destabilizing the interaction with the actuator (A) domain. Mutants G900R and E902K are present in the extracellular loop at the interface of the α and β subunit. Both mutants likely hamper the interaction between these subunits and thereby decrease enzyme activity. Mutants E174K, R548C and R548H reduce the Na(+) and increase the K(+) affinity. Glutamic acid 174 is present in the A domain and might form a salt bridge with Lysine 432 in the nucleotide binding (N) domain, whereas Arginine 548, which is located in the N domain, forms a salt bridge with Glutamine 219 in the A domain. In the catalytic cycle, the interactions of the A and N domains affect the K(+) and Na(+) affinities, as observed with these mutants. Functional consequences were not observed for ATP1A2 mutations found in two sporadic hemiplegic migraine cases (Y9N and R879Q) and in migraine without aura (R51H and C702Y).

Parallel molecular evolution in an herbivore community

Numerous insects have independently evolved the ability to feed on plants that produce toxic secondary compounds called cardenolides and can sequester these compounds for use in their defense. We surveyed the protein target for cardenolides, the alpha subunit of the sodium pump, Na(+),K(+)-ATPase (ATPα), in 14 species that feed on cardenolide-producing plants and 15 outgroups spanning three insect orders. Despite the large number of potential targets for modulating cardenolide sensitivity, amino acid substitutions associated with host-plant specialization are highly clustered, with many parallel substitutions. Additionally, we document four independent duplications of ATPα with convergent tissue-specific expression patterns. We find that unique substitutions are disproportionately associated with recent duplications relative to parallel substitutions. Together, these findings support the hypothesis that adaptation tends to take evolutionary paths that minimize negative pleiotropy.

First crystal structures of Na+,K+-ATPase: new light on the oldest ion pump

Na(+),K(+)-adenosine triphosphatase (NKA) is the first P-type ion translocating adenosine triphosphatase (ATPase) ever identified, and the significance of this class of proteins was highlighted by the 1997 Nobel Prize in Chemistry awarded to Jens C. Skou for the discovery in 1957. More than half a century passed between the initial identification and the publication of a high-resolution crystal structure of NKA. Although the new crystal structures provided many surprises and insights, structural biology on this system remains challenging, as NKA is a very difficult protein to crystallize. Here we explain the reasons behind the challenges, introduce a mechanism that governs the function, and summarize current knowledge of NKA structure in comparison with another member of the P-type ATPase family, Ca(2+)-ATPase.

Toxic cardenolides: chemical ecology and coevolution of specialized plant-herbivore interactions

Cardenolides are remarkable steroidal toxins that have become model systems, critical in the development of theories for chemical ecology and coevolution. Because cardenolides inhibit the ubiquitous and essential animal enzyme Na⁺/K⁺-ATPase, most insects that feed on cardenolide-containing plants are highly specialized. With a huge diversity of chemical forms, these secondary metabolites are sporadically distributed across 12 botanical families, but dominate the Apocynaceae where they are found in > 30 genera. Studies over the past decade have demonstrated patterns in the distribution of cardenolides among plant organs, including all tissue types, and across broad geographic gradients within and across species. Cardenolide production has a genetic basis and is subject to natural selection by herbivores. In addition, there is strong evidence for phenotypic plasticity, with the biotic and abiotic environment predictably impacting cardenolide production. Mounting evidence indicates a high degree of specificity in herbivore-induced cardenolides in Asclepias. While herbivores of cardenolide-containing plants often sequester the toxins, are aposematic, and possess several physiological adaptations (including target site insensitivity), there is strong evidence that these specialists are nonetheless negatively impacted by cardenolides. While reviewing both the mechanisms and evolutionary ecology of cardenolide-mediated interactions, we advance novel hypotheses and suggest directions for future work.

Na,K-ATPase activity modulates Src activation: a role for ATP/ADP ratio

Digitalis-like compounds (DLCs), specific inhibitors of Na,K-ATPase, are implicated in cellular signaling. Exposure of cell cultures to ouabain, a well-known DLC, leads to up- or down regulation of various processes and involves activation of Src kinase. Since Na,K-ATPase is the only known target for DLC binding an in vitro experimental setup using highly purified Na,K-ATPase from pig kidney and commercially available recombinant Src was used to investigate the mechanism of coupling between the Na,K-ATPase and Src. Digoxin was used as a representative DLC for inhibition of Na,K-ATPase. The activation of Src kinase was measured as the degree of its autophosphorylation. It was observed that in addition to digoxin, Src activation was dependent on concentrations of other specific ligands of Na,K-ATPase: Na(+), K(+), vanadate, ATP and ADP. The magnitude of the steady-state ATPase activity therefore seemed to affect Src activation. Further experiments with an ATP regenerating system showed that the ATP/ADP ratio determined the extent of Src activation. Thus, our model system which represents the proposed very proximal part of the Na,K-ATPase-Src signaling cascade, shows that Src kinase activity is regulated by both ATP and ADP concentrations and provides no evidence for a direct interaction between Na,K-ATPase and Src.

Ouabain binding site in a functioning Na+/K+ ATPase

The Na(+)/K(+) ATPase is an almost ubiquitous integral membrane protein within the animal kingdom. It is also the selective target for cardiotonic derivatives, widely prescribed inhibitors for patients with heart failure. Functional studies revealed that ouabain-sensitive residues distributed widely throughout the primary sequence of the protein. Recently, structural work has brought some consensus to the functional observations. Here, we use a spectroscopic approach to estimate distances between a fluorescent ouabain and a lanthanide binding tag (LBT), which was introduced at five different positions in the Na(+)/K(+) ATPase sequence. These five normally functional LBT-Na(+)/K(+) ATPase constructs were expressed in the cell membrane of Xenopus laevis oocytes, operating under physiological internal and external ion conditions. The spectroscopic data suggest two mutually exclusive distances between the LBT and the fluorescent ouabain. From the estimated distances and using homology models of the LBT-Na(+)/K(+) ATPase constructs, approximate ouabain positions could be determined. Our results suggest that ouabain binds at two sites along the ion permeation pathway of the Na(+)/K(+) ATPase. The external site (low apparent affinity) occupies the same region as previous structural findings. The high apparent affinity site is, however, slightly deeper toward the intracellular end of the protein. Interestingly, in both cases the lactone ring faces outward. We propose a sequential ouabain binding mechanism that is consistent with all functional and structural studies.

Coping with toxic plant compounds--the insect's perspective on iridoid glycosides and cardenolides

Specializing on host plants with toxic secondary compounds enforces specific adaptation in insect herbivores. In this review, we focus on two compound classes, iridoid glycosides and cardenolides, which can be found in the food plants of a large number of insect species that display various degrees of adaptation to them. These secondary compounds have very different modes of action: Iridoid glycosides are usually activated in the gut of the herbivores by β-glucosidases that may either stem from the food plant or be present in the gut as standard digestive enzymes. Upon cleaving, the unstable aglycone is released that unspecifically acts by crosslinking proteins and inhibiting enzymes. Cardenolides, on the other hand, are highly specific inhibitors of an essential ion carrier, the sodium pump. In insects exposed to both kinds of toxins, carriers either enabling the safe storage of the compounds away from the activating enzymes or excluding the toxins from sensitive tissues, play an important role that deserves further analysis. To avoid toxicity of iridoid glycosides, repression of activating enzymes emerges as a possible alternative strategy. Cardenolides, on the other hand, may lose their toxicity if their target site is modified and this strategy has evolved multiple times independently in cardenolide-adapted insects.