Contribution of septins to human platelet structure and function

Although septins have been well-studied in nucleated cells, their role in anucleate blood platelets remains obscure. Here, we elucidate the contribution of septins to human platelet structure and functionality. We show that Septin-2 and Septin-9 are predominantly distributed at the periphery of resting platelets and co-localize strongly with microtubules. Activation of platelets by thrombin causes clustering of septins and impairs their association with microtubules. Inhibition of septin dynamics with forchlorfenuron (FCF) reduces thrombin-induced densification of septins and lessens their colocalization with microtubules in resting and activated platelets. Exposure to FCF alters platelet shape, suggesting that septins stabilize platelet cytoskeleton. FCF suppresses platelet integrin αIIbβ3 activation, promotes phosphatidylserine exposure on activated platelets, and induces P-selectin expression on resting platelets, suggesting septin involvement in these processes. Inhibition of septin dynamics substantially reduces platelet contractility and abrogates their spreading on fibrinogen-coated surfaces. Overall, septins strongly contribute to platelet structure, activation and biomechanics.

Maturation of the Na,K-ATPase in the Endoplasmic Reticulum in Health and Disease

Abstract

The Na,K-ATPase establishes the electrochemical gradient of cells by driving an active exchange of Na⁺ and K⁺ ions while consuming ATP. The minimal functional transporter consists of a catalytic α-subunit and a β-subunit with chaperon activity. The Na,K-ATPase also functions as a cell adhesion molecule and participates in various intracellular signaling pathways. The maturation and trafficking of the Na,K-ATPase include co- and post-translational processing of the enzyme in the endoplasmic reticulum (ER) and the Golgi apparatus and subsequent delivery to the plasma membrane (PM). The ER folding of the enzyme is considered as the rate-limiting step in the membrane delivery of the protein. It has been demonstrated that only assembled Na,K-ATPase α:β-complexes may exit the organelle, whereas unassembled, misfolded or unfolded subunits are retained in the ER and are subsequently degraded. Loss of function of the Na,K-ATPase has been associated with lung, heart, kidney and neurological disorders. Recently, it has been shown that ER dysfunction, in particular, alterations in the homeostasis of the organelle, as well as impaired ER-resident chaperone activity may impede folding of Na,K-ATPase subunits, thus decreasing the abundance and function of the enzyme at the PM. Here, we summarize our current understanding on maturation and subsequent processing of the Na,K-ATPase in the ER under physiological and pathophysiological conditions.

Graphic Abstract

Helicobacter pylori infection impairs chaperone-assisted maturation of Na-K-ATPase in gastric epithelium

Helicobacter pylori infection always induces gastritis, which may progress to ulcer disease or cancer. The mechanisms underlying mucosal injury by the bacteria are incompletely understood. Here, we identify a novel pathway for H. pylori-induced gastric injury, the impairment of maturation of the essential transport enzyme and cell adhesion molecule, Na-K-ATPase. Na-K-ATPase comprises α- and β-subunits that assemble in the endoplasmic reticulum (ER) before trafficking to the plasma membrane. Attachment of H. pylori to gastric epithelial cells increased Na-K-ATPase ubiquitylation, decreased its surface and total levels, and impaired ion balance. H. pylori did not alter degradation of plasmalemma-resident Na-K-ATPase subunits or their mRNA levels. Infection decreased association of α- and β-subunits with ER chaperone BiP and impaired assembly of α/β-heterodimers, as was revealed by quantitative mass spectrometry and immunoblotting of immunoprecipitated complexes. The total level of BiP was not altered, and the decrease in interaction with BiP was not observed for other BiP client proteins. The H. pylori-induced decrease in Na-K-ATPase was prevented by BiP overexpression, stopping protein synthesis, or inhibiting proteasomal, but not lysosomal, protein degradation. The results indicate that H. pylori impairs chaperone-assisted maturation of newly made Na-K-ATPase subunits in the ER independently of a generalized ER stress and induces their ubiquitylation and proteasomal degradation. The decrease in Na-K-ATPase levels is also seen in vivo in the stomachs of gerbils and chronically infected children. Further understanding of H. pylori-induced Na-K-ATPase degradation will provide insights for protection against advanced disease.NEW & NOTEWORTHY This work provides evidence that Helicobacter pylori decreases levels of Na-K-ATPase, a vital transport enzyme, in gastric epithelia, both in acutely infected cultured cells and in chronically infected patients and animals. The bacteria interfere with BiP-assisted folding of newly-made Na-K-ATPase subunits in the endoplasmic reticulum, accelerating their ubiquitylation and proteasomal degradation and decreasing efficiency of the assembly of native enzyme. Decreased Na-K-ATPase expression contributes to H. pylori-induced gastric injury.

Hypercapnia Impairs Na,K-ATPase Function by Inducing Endoplasmic Reticulum Retention of the β-Subunit of the Enzyme in Alveolar Epithelial Cells

Alveolar edema, impaired alveolar fluid clearance, and elevated CO₂ levels (hypercapnia) are hallmarks of the acute respiratory distress syndrome (ARDS). This study investigated how hypercapnia affects maturation of the Na,K-ATPase (NKA), a key membrane transporter, and a cell adhesion molecule involved in the resolution of alveolar edema in the endoplasmic reticulum (ER). Exposure of human alveolar epithelial cells to elevated CO₂ concentrations caused a significant retention of NKA-β in the ER and, thus, decreased levels of the transporter in the Golgi apparatus. These effects were associated with a marked reduction of the plasma membrane (PM) abundance of the NKA-α/β complex as well as a decreased total and ouabain-sensitive ATPase activity. Furthermore, our study revealed that the ER-retained NKA-β subunits were only partially assembled with NKA α-subunits, which suggests that hypercapnia modifies the ER folding environment. Moreover, we observed that elevated CO₂ levels decreased intracellular ATP production and increased ER protein and, particularly, NKA-β oxidation. Treatment with α-ketoglutaric acid (α-KG), which is a metabolite that has been shown to increase ATP levels and rescue mitochondrial function in hypercapnia-exposed cells, attenuated the deleterious effects of elevated CO₂ concentrations and restored NKA PM abundance and function. Taken together, our findings provide new insights into the regulation of NKA in alveolar epithelial cells by elevated CO₂ levels, which may lead to the development of new therapeutic approaches for patients with ARDS and hypercapnia.

A Functional Interaction Between Na,K-ATPase β2-Subunit/AMOG and NF2/Merlin Regulates Growth Factor Signaling in Cerebellar Granule Cells

The Na,K-ATPase, consisting of a catalytic α-subunit and a regulatory β-subunit, is a ubiquitously expressed ion pump that carries out the transport of Na⁺ and K⁺ across the plasma membranes of most animal cells. In addition to its pump function, Na,K-ATPase serves as a signaling scaffold and a cell adhesion molecule. Of the three β-subunit isoforms, β₁ is found in almost all tissues, while β₂ expression is mostly restricted to brain and muscle. In cerebellar granule cells, the β₂-subunit, also known as adhesion molecule on glia (AMOG), has been linked to neuron-astrocyte adhesion and granule cell migration, suggesting its role in cerebellar development. Nevertheless, little is known about molecular pathways that link the β₂-subunit to its cellular functions. Using cerebellar granule precursor cells, we found that the β₂-subunit, but not the β₁-subunit, negatively regulates the expression of a key activator of the Hippo/YAP signaling pathway, Merlin/neurofibromin-2 (NF2). The knockdown of the β₂-subunit resulted in increased Merlin/NF2 expression and affected downstream targets of Hippo signaling, i.e., increased YAP phosphorylation and decreased expression of N-Ras. Further, the β₂-subunit knockdown altered the kinetics of epidermal growth factor receptor (EGFR) signaling in a Merlin-dependent mode and impaired EGF-induced reorganization of the actin cytoskeleton. Therefore, our studies for the first time provide a functional link between the Na,K-ATPase β₂-subunit and Merlin/NF2 and suggest a role for the β₂-subunit in regulating cytoskeletal dynamics and Hippo/YAP signaling during neuronal differentiation.

Fatal dysfunction and disintegration of thrombin-stimulated platelets

Platelets play a key role in the formation of hemostatic clots and obstructive thrombi as well as in other biological processes. In response to physiological stimulants, including thrombin, platelets change shape, express adhesive molecules, aggregate, and secrete bioactive substances, but their subsequent fate is largely unknown. Here we examined late-stage structural, metabolic, and functional consequences of thrombin-induced platelet activation. Using a combination of confocal microscopy, scanning and transmission electron microscopy, flow cytometry, biochemical and biomechanical measurements, we showed that thrombin-induced activation is followed by time-dependent platelet dysfunction and disintegration. After ~30 minutes of incubation with thrombin, unlike with collagen or ADP, human platelets disintegrated into cellular fragments containing organelles, such as mitochondria, glycogen granules, and vacuoles. This platelet fragmentation was preceded by Ca²⁺ influx, integrin α_IIbβ₃ activation and phosphatidylserine exposure (activation phase), followed by mitochondrial depolarization, generation of reactive oxygen species, metabolic ATP depletion and impairment of platelet contractility along with dramatic cytoskeletal rearrangements, concomitant with platelet disintegration (death phase). Coincidentally with the platelet fragmentation, thrombin caused calpain activation but not activation of caspases 3 and 7. Our findings indicate that the late functional and structural damage of thrombin-activated platelets comprise a calpain-dependent platelet death pathway that shares some similarities with the programmed death of nucleated cells, but is unique to platelets, therefore representing a special form of cellular destruction. Fragmentation of activated platelets suggests that there is an underappreciated pathway of enhanced elimination of platelets from the circulation in (pro)thrombotic conditions once these cells have performed their functions.

Analysis of N- and O-Glycosylation of Lysosomal Glycoproteins

The vast majority of lysosomal proteins are heavily glycosylated. The present protocol describes the method of analyzing N- and O-linked glycans in lysosomal proteins of interest. The method is based on using deglycosylating enzymes, endoglycosidases, and exoglycosidases. Endoglycosidases catalyze the cleavage of an internal bond in an oligosaccharide, while exoglycosidases remove terminal carbohydrates from glycans. Different types of carbohydrate residues or chains can be removed by specific glycosidases. Removing oligosaccharides with glycosidases increases the electrophoretic mobility of a protein. This increase in mobility depends on the size and number of removed carbohydrate chains. Therefore, the treatment of lysosomal proteins with specific glycosidases followed by a western blot analysis of a protein of interest provides a way to determine which types of glycans are present in the protein by comparing the gel mobility before and after treatment.

The cryo-EM structure of gastric H+,K+-ATPase with bound BYK99, a high-affinity member of K+-competitive, imidazo[1,2-a]pyridine inhibitors

The gastric proton pump H⁺,K⁺-ATPase acidifies the gastric lumen, and thus its inhibitors, including the imidazo[1,2-a]pyridine class of K⁺-competitive acid blockers (P-CABs), have potential application as acid-suppressing drugs. We determined the electron crystallographic structure of H⁺,K⁺-ATPase at 6.5 Å resolution in the E2P state with bound BYK99, a potent P-CAB with a restricted ring structure. The BYK99 bound structure has an almost identical profile to that of a previously determined structure with bound SCH28080, the original P-CAB prototype, but is significantly different from the previously reported P-CAB-free form, illustrating a common conformational change is required for P-CAB binding. The shared conformational changes include a distinct movement of transmembrane helix 2 (M2), from its position in the previously reported P-CAB-free form, to a location proximal to the P-CAB binding site in the present BYK99-bound structure. Site-specific mutagenesis within M2 revealed that D137 and N138, which face the P-CAB binding site in our model, significantly affect the inhibition constant (K_i) of P-CABs. We also found that A335 is likely to be near the bridging nitrogen at the restricted ring structure of the BYK99 inhibitor. These provide clues to elucidate the binding site parameters and mechanism of P-CAB inhibition of gastric acid secretion.

FXYD5 Is an Essential Mediator of the Inflammatory Response during Lung Injury

The alveolar epithelium secretes cytokines and chemokines that recruit immune cells to the lungs, which is essential for fighting infections but in excess can promote lung injury. Overexpression of FXYD5, a tissue-specific regulator of the Na,K-ATPase, in mice, impairs the alveolo-epithelial barrier, and FXYD5 overexpression in renal cells increases C-C chemokine ligand-2 (CCL2) secretion in response to lipopolysaccharide (LPS). The aim of this study was to determine whether FXYD5 contributes to the lung inflammation and injury. Exposure of alveolar epithelial cells (AEC) to LPS increased FXYD5 levels at the plasma membrane, and FXYD5 silencing prevented both the activation of NF-κB and the secretion of cytokines in response to LPS. Intratracheal instillation of LPS into mice increased FXYD5 levels in the lung. FXYD5 overexpression increased the recruitment of interstitial macrophages and classical monocytes to the lung in response to LPS. FXYD5 silencing decreased CCL2 levels, number of cells, and protein concentration in bronchoalveolar lavage fluid (BALF) after LPS treatment, indicating that FXYD5 is required for the NF-κB-stimulated epithelial production of CCL2, the influx of immune cells, and the increase in alveolo-epithelial permeability in response to LPS. Silencing of FXYD5 also prevented the activation of NF-κB and cytokine secretion in response to interferon α and TNF-α, suggesting that pro-inflammatory effects of FXYD5 are not limited to the LPS-induced pathway. Furthermore, in the absence of other stimuli, FXYD5 overexpression in AEC activated NF-κB and increased cytokine production, while FXYD5 overexpression in mice increased cytokine levels in BALF, indicating that FXYD5 is sufficient to induce the NF-κB-stimulated cytokine secretion by the alveolar epithelium. The FXYD5 overexpression also increased cell counts in BALF, which was prevented by silencing the CCL2 receptor (CCR2), or by treating mice with a CCR2-blocking antibody, confirming that FXYD5-induced CCL2 production leads to the recruitment of monocytes to the lung. Taken together, the data demonstrate that FXYD5 is a key contributor to inflammatory lung injury.

Septins: Regulators of Protein Stability

Septins are small GTPases that play a role in several important cellular processes. In this review, we focus on the roles of septins in protein stabilization. Septins may regulate protein stability by: (1) interacting with proteins involved in degradation pathways, (2) regulating the interaction between transmembrane proteins and cytoskeletal proteins, (3) affecting the mobility of transmembrane proteins in lipid bilayers, and (4) modulating the interaction of proteins with their adaptor or signaling proteins. In this context, we discuss the role of septins in protecting four different proteins from degradation. First we consider botulinum neurotoxin serotype A (BoNT/A) and the contribution of septins to its extraordinarily long intracellular persistence. Next, we discuss the role of septins in stabilizing the receptor tyrosine kinases EGFR and ErbB2. Finally, we consider the contribution of septins in protecting hypoxia-inducible factor 1α (HIF-1α) from degradation.

Selective Assembly of Na,K-ATPase α2β2 Heterodimers in the Heart: DISTINCT FUNCTIONAL PROPERTIES AND ISOFORM-SELECTIVE INHIBITORS

The Na,K-ATPase α₂ subunit plays a key role in cardiac muscle contraction by regulating intracellular Ca²⁺, whereas α₁ has a more conventional role of maintaining ion homeostasis. The β subunit differentially regulates maturation, trafficking, and activity of α-β heterodimers. It is not known whether the distinct role of α₂ in the heart is related to selective assembly with a particular one of the three β isoforms. We show here by immunofluorescence and co-immunoprecipitation that α₂ is preferentially expressed with β₂ in T-tubules of cardiac myocytes, forming α₂β₂ heterodimers. We have expressed human α₁β₁, α₂β₁, α₂β₂, and α₂β₃ in Pichia pastoris, purified the complexes, and compared their functional properties. α₂β₂ and α₂β₃ differ significantly from both α₂β₁ and α₁β₁ in having a higher K_0.5K⁺ and lower K_0.5Na⁺ for activating Na,K-ATPase. These features are the result of a large reduction in binding affinity for extracellular K⁺ and shift of the E₁P-E₂P conformational equilibrium toward E₁P. A screen of perhydro-1,4-oxazepine derivatives of digoxin identified several derivatives (e.g. cyclobutyl) with strongly increased selectivity for inhibition of α₂β₂ and α₂β₃ over α₁β₁ (range 22-33-fold). Molecular modeling suggests a possible basis for isoform selectivity. The preferential assembly, specific T-tubular localization, and low K⁺ affinity of α₂β₂ could allow an acute response to raised ambient K⁺ concentrations in physiological conditions and explain the importance of α₂β₂ for cardiac muscle contractility. The high sensitivity of α₂β₂ to digoxin derivatives explains beneficial effects of cardiac glycosides for treatment of heart failure and potential of α₂β₂-selective digoxin derivatives for reducing cardiotoxicity.

The O-glycosylated ectodomain of FXYD5 impairs adhesion by disrupting cell-cell trans-dimerization of Na,K-ATPase β1 subunits

FXYD5 (also known as dysadherin), a regulatory subunit of the Na,K-ATPase, impairs intercellular adhesion by a poorly understood mechanism. Here, we determined whether FXYD5 disrupts the trans-dimerization of Na,K-ATPase molecules located in neighboring cells. Mutagenesis of the Na,K-ATPase β1 subunit identified four conserved residues, including Y199, that are crucial for the intercellular Na,K-ATPase trans-dimerization and adhesion. Modulation of expression of FXYD5 or of the β1 subunit with intact or mutated β1-β1 binding sites demonstrated that the anti-adhesive effect of FXYD5 depends on the presence of Y199 in the β1 subunit. Immunodetection of the plasma membrane FXYD5 was prevented by the presence of O-glycans. Partial FXYD5 deglycosylation enabled antibody binding and showed that the protein level and the degree of O-glycosylation were greater in cancer than in normal cells. FXYD5-induced impairment of adhesion was abolished by both genetic and pharmacological inhibition of FXYD5 O-glycosylation. Therefore, the extracellular O-glycosylated domain of FXYD5 impairs adhesion by interfering with intercellular β1-β1 interactions, suggesting that the ratio between FXYD5 and α1-β1 heterodimer determines whether the Na,K-ATPase acts as a positive or negative regulator of intercellular adhesion.

Lysosome associated membrane proteins maintain pancreatic acinar cell homeostasis: LAMP-2 deficient mice develop pancreatitis

BACKGROUND & AIMS

The pathogenic mechanism of pancreatitis is poorly understood. Recent evidence implicates defective autophagy in pancreatitis responses; however, the pathways mediating impaired autophagy in pancreas remain largely unknown. Here, we investigate the role of lysosome associated membrane proteins (LAMPs) in pancreatitis.

METHODS

We analyzed changes in LAMPs in experimental models and human pancreatitis, and the underlying mechanisms: LAMP de-glycosylation and degradation. LAMP cleavage by cathepsin B (CatB) was analyzed by mass spectrometry. We used mice deficient in LAMP-2 to assess its role in pancreatitis.

RESULTS

Pancreatic levels of LAMP-1 and LAMP-2 greatly decrease across various pancreatitis models and in human disease. Pancreatitis does not trigger LAMPs' bulk de-glycosylation, but induces their degradation via CatB-mediated cleavage of LAMP molecule close to the boundary between luminal and transmembrane domains. LAMP-2 null mice spontaneously develop pancreatitis that begins with acinar cell vacuolization due to impaired autophagic flux, and progresses to severe pancreas damage characterized by trypsinogen activation, macrophage-driven inflammation, and acinar cell death. LAMP-2 deficiency causes a decrease in pancreatic digestive enzymes content, stimulates the basal and inhibits CCK-induced amylase secretion by acinar cells. The effects of LAMP-2 knockout and acute cerulein pancreatitis overlap, which corroborates the pathogenic role of LAMP decrease in experimental pancreatitis models.

CONCLUSIONS

The results indicate a critical role for LAMPs, particularly LAMP-2, in maintaining pancreatic acinar cell homeostasis, and provide evidence that defective lysosomal function, resulting in impaired autophagy, leads to pancreatitis. Mice with LAMP-2 deficiency present a novel genetic model of human pancreatitis caused by lysosomal/autophagic dysfunction.

Septin dynamics are essential for exocytosis

Septins are a family of 14 cytoskeletal proteins that dynamically form hetero-oligomers and organize membrane microdomains for protein complexes. The previously reported interactions with SNARE proteins suggested the involvement of septins in exocytosis. However, the contradictory results of up- or down-regulation of septin-5 in various cells and mouse models or septin-4 in mice suggested either an inhibitory or a stimulatory role for these septins in exocytosis. The involvement of the ubiquitously expressed septin-2 or general septin polymerization in exocytosis has not been explored to date. Here, by nano-LC with tandem MS and immunoblot analyses of the septin-2 interactome in mouse brain, we identified not only SNARE proteins but also Munc-18-1 (stabilizes assembled SNARE complexes), N-ethylmaleimide-sensitive factor (NSF) (disassembles SNARE complexes after each membrane fusion event), and the chaperones Hsc70 and synucleins (maintain functional conformation of SNARE proteins after complex disassembly). Importantly, α-soluble NSF attachment protein (SNAP), the adaptor protein that mediates NSF binding to the SNARE complex, did not interact with septin-2, indicating that septins undergo reorganization during each exocytosis cycle. Partial depletion of septin-2 by siRNA or impairment of septin dynamics by forchlorfenuron inhibited constitutive and stimulated exocytosis of secreted and transmembrane proteins in various cell types. Forchlorfenuron impaired the interaction between SNAP-25 and its chaperone Hsc70, decreasing SNAP-25 levels in cultured neuroendocrine cells, and inhibited both spontaneous and stimulated acetylcholine secretion in mouse motor neurons. The results demonstrate a stimulatory role of septin-2 and the dynamic reorganization of septin oligomers in exocytosis.

Recruitment of septin cytoskeletal proteins by botulinum toxin A protease determines its remarkable stability

Proteolytic cleavage of synaptosomal-associated protein 25 by the light chain of botulinum neurotoxin type A (LCA) results in a blockade of neurotransmitter release that persists for several months in motor neurons. The L428A/L429A mutation in LCA is known to significantly shorten both the proteolytic and neuroparalytic effects of the neurotoxin in mice. To elucidate the cellular mechanism for LCA longevity, we studied the effects of L428A/L429A mutation on the interactome, localization and stability of LCA expressed in cultured neuronal cells. Mass spectrometry analysis of the LCA interactome showed that the mutation prevented the interaction of LCA with septins. The wild-type LCA was concentrated in plasma-membrane-associated clusters, colocalizing with septins-2 and septin-7, which accumulated in these clusters only in the presence of LCA. The L428A/L429A mutation decreased co-clustering of LCA and septins and accelerated proteasomal and non-proteasomal degradation of LCA. Similarly, the impairment of septin oligomerization by forchlorfenuron or silencing of septin-2 prevented LCA interaction and clustering with septins and increased LCA degradation. Therefore, the dileucine-mediated LCA-septin co-clustering is crucial for the long-lasting stabilization of LCA-related proteolytic and presumably neuroparalytic activity.

Helicobacter pylori impedes acid-induced tightening of gastric epithelial junctions

Gastric infection by Helicobacter pylori is the most common cause of ulcer disease and gastric cancer. The mechanism of progression from gastritis and inflammation to ulcers and cancer in a fraction of those infected is not definitively known. Significant acidity is unique to the gastric environment and is required for ulcer development. The interplay between gastric acidity and H. pylori pathogenesis is important in progression to advanced disease. The aim of this study was to characterize the impact of acid on gastric epithelial integrity and cytokine release and how H. pylori infection alters these responses. Human gastric epithelial (HGE-20) cells were grown on porous inserts, and survival, barrier function, and cytokine release were studied at various apical pH levels in the presence and absence of H. pylori. With apical acidity, gastric epithelial cells demonstrate increased barrier function, as evidenced by increased transepithelial electrical resistance (TEER) and decreased paracellular permeability. This effect is reduced in the presence of wild-type, but not urease knockout, H. pylori. The epithelial inflammatory response is also modulated by acidity and H. pylori infection. Without H. pylori, epithelial IL-8 release decreases in acid, while IL-6 release increases. In the presence of H. pylori, acidic pH diminishes the magnitude of the previously reported increase in IL-8 and IL-6 release. H. pylori interferes with the gastric epithelial response to acid, contributing to altered barrier function and inflammatory response. H. pylori diminishes acid-induced tightening of cell junctions in a urease-dependent manner, suggesting that local pH elevation promotes barrier compromise and progression to mucosal damage.

Subunit isoform selectivity in assembly of Na,K-ATPase α-β heterodimers

To catalyze ion transport, the Na,K-ATPase must contain one α and one β subunit. When expressed by transfection in various expression systems, each of the four α subunit isoforms can assemble with each of the three β subunit isoforms and form an active enzyme, suggesting the absence of selective α-β isoform assembly. However, it is unknown whether in vivo conditions the α-β assembly is random or isoform-specific. The α(2)-β(2) complex was selectively immunoprecipitated by both anti-α(2) and anti-β(2) antibodies from extracts of mouse brain, which contains cells co-expressing multiple Na,K-ATPase isoforms. Neither α(1)-β(2) nor α(2)-β(1) complexes were detected in the immunoprecipitates. Furthermore, in MDCK cells co-expressing α(1), β(1), and β(2) isoforms, a greater fraction of the β(2) subunits was unassembled with α(1) as compared with that of the β(1) subunits, indicating preferential association of the α(1) isoform with the β(1) isoform. In addition, the α(1)-β(2) complex was less resistant to various detergents than the α(1)-β(1) complex isolated from MDCK cells or the α(2)-β(2) complex isolated from mouse brain. Therefore, the diversity of the α-β Na,K-ATPase heterodimers in vivo is determined not only by cell-specific co-expression of particular isoforms, but also by selective association of the α and β subunit isoforms.

Identification of the amino acid region involved in the intercellular interaction between the β1 subunits of Na+/K+ -ATPase

Epithelial junctions depend on intercellular interactions between β(1) subunits of the Na(+)/K(+)-ATPase molecules of neighboring cells. The interaction between dog and rat subunits is less effective than the interaction between two dog β(1) subunits, indicating the importance of species-specific regions for β(1)-β(1) binding. To identify these regions, the species-specific amino acid residues were mapped on a high-resolution structure of the Na(+)/K(+)-ATPase β(1) subunit to select those exposed towards the β(1) subunit of the neighboring cell. These exposed residues were mutated in both dog and rat YFP-linked β(1) subunits (YFP-β(1)) and also in the secreted extracellular domain of the dog β(1) subunit. Five rat-like mutations in the amino acid region spanning residues 198-207 of the dog YFP-β(1) expressed in Madin-Darby canine kidney (MDCK) cells decreased co-precipitation of the endogenous dog β(1) subunit with YFP-β(1) to the level observed between dog β(1) and rat YFP-β(1). In parallel, these mutations impaired the recognition of YFP-β(1) by the dog-specific antibody that inhibits cell adhesion between MDCK cells. Accordingly, dog-like mutations in rat YFP-β(1) increased both the (YFP-β(1))-β(1) interaction in MDCK cells and recognition by the antibody. Conversely, rat-like mutations in the secreted extracellular domain of the dog β(1) subunit increased its interaction with rat YFP-β(1) in vitro. In addition, these mutations resulted in a reduction of intercellular adhesion between rat lung epithelial cells following addition of the secreted extracellular domain of the dog β(1) subunit to a cell suspension. Therefore, the amino acid region 198-207 is crucial for both trans-dimerization of the Na(+)/K(+)-ATPase β(1) subunits and cell-cell adhesion.

The Na-K-ATPase α₁β₁ heterodimer as a cell adhesion molecule in epithelia

The ion gradients generated by the Na-K-ATPase play a critical role in epithelia by driving transepithelial transport of various solutes. The efficiency of this Na-K-ATPase-driven vectorial transport depends on the integrity of epithelial junctions that maintain polar distribution of membrane transporters, including the basolateral sodium pump, and restrict paracellular diffusion of solutes. The review summarizes the data showing that, in addition to pumping ions, the Na-K-ATPase located at the sites of cell-cell junction acts as a cell adhesion molecule by interacting with the Na-K-ATPase of the adjacent cell in the intercellular space accompanied by anchoring to the cytoskeleton in the cytoplasm. The review also discusses the experimental evidence on the importance of a specific amino acid region in the extracellular domain of the Na-K-ATPase β(1) subunit for the Na-K-ATPase trans-dimerization and intercellular adhesion. Furthermore, a possible role of N-glycans linked to the Na-K-ATPase β(1) subunit in regulation of epithelial junctions by modulating β(1)-β(1) interactions is discussed.

Characterization of a novel potassium-competitive acid blocker of the gastric H,K-ATPase, 1-[5-(2-fluorophenyl)-1-(pyridin-3-ylsulfonyl)-1H-pyrrol-3-yl]-N-methylmethanamine monofumarate (TAK-438)

Inhibition of the gastric H,K-ATPase by the potassium-competitive acid blocker (P-CAB) 1-[5-(2-fluorophenyl)-1-(pyridin-3-ylsulfonyl)-1H-pyrrol-3-yl]-N-methylmethanamine (TAK-438), is strictly K(+)-competitive with a K(i) of 10 nM at pH 7. In contrast to previous P-CABs, this structure has a point positive charge (pK(a) 9.06) allowing for greater accumulation in parietal cells compared with previous P-CABs [e.g., (8-benzyloxy-2-methyl-imidazo(1,2-a)pyridin-3-yl)acetonitrile (SCH28080), pK(a) 5.6]. The dissociation rate of the compound from the isolated ATPase is slower than other P-CABs, with the t(1/2) being 7.5 h in 20 mM KCl at pH 7. The stoichiometry of binding of TAK-438 to the H,K-ATPase is 2.2 nmol/mg in the presence of Mg-ATP, vanadate, or MgP(i). However, TAK-438 also binds enzyme at 1.3 nmol/mg in the absence of Mg(2+). Modeling of the H,K-ATPase to the homologous Na,K-ATPase predicts a close approach and hydrogen bonding between the positively charged N-methylamino group and the negatively charged Glu795 in the K(+)-binding site in contrast to the planar diffuse positive charge of previous P-CABs. This probably accounts for the slow dissociation and high affinity. The model also predicts hydrogen bonding between the hydroxyl of Tyr799 and the oxygens of the sulfonyl group of TAK-438. A Tyr799Phe mutation resulted in a 3-fold increase of the dissociation rate, showing that this hydrogen bonding also contributes to the slow dissociation rate. Hence, this K(+)-competitive inhibitor of the gastric H,K-ATPase should provide longer-lasting inhibition of gastric acid secretion compared with previous drugs of this class.

Epithelial junctions depend on intercellular trans-interactions between the Na,K-ATPase β₁ subunits

N-Glycans of the Na,K-ATPase β₁ subunit are important for intercellular adhesion in epithelia, suggesting that epithelial junctions depend on N-glycan-mediated interactions between the β₁ subunits of neighboring cells. The level of co-immunoprecipitation of the endogenous β₁ subunit with various YFP-linked β₁ subunits expressed in Madin-Darby canine kidney cells was used to assess β₁-β₁ interactions. The amount of co-precipitated endogenous dog β₁ was greater with dog YFP-β₁ than with rat YFP-β₁, showing that amino acid-mediated interactions are important for β₁-β₁ binding. Co-precipitation of β₁ was also less with the unglycosylated YFP-β₁ than with glycosylated YFP-β₁, indicating a role for N-glycans. Mixing cells expressing dog YFP-β₁ with non-transfected cells increased the amount of co-precipitated β₁, confirming the presence of intercellular (YFP-β₁)-β₁ complexes. Accordingly, disruption of intercellular junctions decreased the amount of co-precipitated β₁ subunits. The decrease in β₁ co-precipitation both with rat YFP-β₁ and unglycosylated YFP-β₁ was associated with decreased detergent stability of junctional proteins and increased paracellular permeability. Reducing N-glycan branching by specific inhibitors increased (YFP-β₁)-β₁ co-precipitation and strengthened intercellular junctions. Therefore, interactions between the β₁ subunits of neighboring cells maintain integrity of intercellular junctions, and alterations in the β₁ subunit N-glycan structure can regulate stability and tightness of intercellular junctions.

Diverse pathways for maturation of the Na,K-ATPase β1 and β2 subunits in the endoplasmic reticulum of Madin-Darby canine kidney cells

Proper folding of the Na,K-ATPase β subunits followed by assembly with the α subunits is necessary for their export from the endoplasmic reticulum (ER). Here we examine roles of the ER lectin chaperone, calnexin, and non-lectin chaperone, BiP, in folding and quality control of the β(1) and β(2) subunits in Madin-Darby canine kidney cells. Short term prevention of glycan-calnexin interactions by castanospermine slightly increases ER retention of β(1), suggesting minor involvement of calnexin in subunit folding. However, both prolonged incubation with castanospermine and removal of N-glycosylation sites do not affect the α(1)-assembly or trafficking of β(1) but increase the amount of the β(1)-bound BiP, showing that BiP can compensate for calnexin in assisting β(1) folding. In contrast to β(1), prevention of either N-glycosylation or glycan-calnexin interactions abolishes the α(1)-assembly and export of β(2) from the ER despite increased β(2)-BiP binding. Mutations in the α(1)-interacting regions of β(1) and β(2) subunits impair α(1) assembly but do not affect folding of the β subunits tested by their sensitivity to trypsin. At the same time, these mutations increase the amount of β-bound BiP but not of β-bound calnexin and increase ER retention of both β-isoforms. BiP, therefore, prevents the ER export of folded but α(1)-unassembled β subunits. These α(1)-unassembled β subunits are degraded faster than α(1)-bound β subunits, preventing ER overload. In conclusion, folding of the β(1) and β(2) subunits is assisted predominantly by BiP and calnexin, respectively. Folded β(1) and β(2) either assemble with α(1) or bind BiP. The α(1)-bound β subunits traffic to the Golgi, whereas BiP-bound β subunits are retained and degraded in the ER.

N-glycan-dependent quality control of the Na,K-ATPase beta(2) subunit

Bulky hydrophilic N-glycans stabilize the proper tertiary structure of glycoproteins. In addition, N-glycans comprise the binding sites for the endoplasmic reticulum (ER)-resident lectins that assist correct folding of newly synthesized glycoproteins. To reveal the role of N-glycans in maturation of the Na,K-ATPase beta(2) subunit in the ER, the effects of preventing or modifying the beta(2) subunit N-glycosylation on trafficking of the subunit and its binding to the ER lectin chaperone, calnexin, were studied in MDCK cells. Preventing N-glycosylation abolishes binding of the beta(2) subunit to calnexin and results in the ER retention of the subunit. Furthermore, the fully N-glycosylated beta(2) subunit is retained in the ER when glycan-calnexin interactions are prevented by castanospermine, showing that N-glycan-mediated calnexin binding is required for correct subunit folding. Calnexin binding persists for several hours after translation is stopped with cycloheximide, suggesting that the beta(2) subunit undergoes repeated post-translational calnexin-assisted folding attempts. Homology modeling of the beta(2) subunit using the crystal structure of the alpha(1)-beta(1) Na,K-ATPase shows the presence of a relatively hydrophobic amino acid cluster proximal to N-glycosylation sites 2 and 7. Combined, but not separate, removal of sites 2 and 7 dramatically impairs calnexin binding and prevents the export of the beta(2) subunit from the ER. Similarly, hydrophilic substitution of two hydrophobic amino acids in this cluster disrupts both beta(2)-calnexin binding and trafficking of the subunit to the Golgi. Therefore, the hydrophobic residues in the proximity of N-glycans 2 and 7 are required for post-translational calnexin binding to these N-glycans in incompletely folded conformers, which, in turn, is necessary for maturation of the Na,K-ATPase beta(2) subunit.

Assembly with the Na,K-ATPase alpha(1) subunit is required for export of beta(1) and beta(2) subunits from the endoplasmic reticulum

The level of the heterodimeric Na,K-ATPase is tightly controlled in epithelia to maintain appropriate transport function. The catalytic Na,K-ATPase alpha subunit is not able to exit the ER or catalyze ion transport unless assembled with the beta subunit. However, requirements for the ER exit of the Na,K-ATPase beta subunit that plays an additional, ion-transport-independent, role in intercellular adhesion are not clear. Exogenous beta(1) or beta(2) subunits expressed in renal MDCK cells replace endogenous beta(1) subunits in the alpha-beta complexes in the ER, resulting in a decrease in the amount of the alpha(1)-bound endogenous beta(1) subunits by 47-61% with no change in the amount of alpha(1) subunits. Disruption of the alpha(1)-beta association by mutations in defined alpha(1)-interacting regions of either beta(1) or beta(2) subunits results in the ER retention and rapid degradation of unassembled mutants. Hence, the ER quality control system allows export only of assembled alpha-beta complexes to the Golgi, thereby maintaining an equimolar ratio of alpha and beta subunits in the plasma membrane, whereas the number of alpha(1) subunits in the ER determines the amount of the alpha-beta complexes.

Role of N-glycosylation in trafficking of apical membrane proteins in epithelia

Polarized distribution of plasma membrane transporters and receptors in epithelia is essential for vectorial functions of epithelia. This polarity is maintained by sorting of membrane proteins into apical or basolateral transport containers in the trans-Golgi network and/or endosomes followed by their delivery to the appropriate plasma membrane domains. Sorting depends on the recognition of sorting signals in proteins by specific sorting machinery. In the present review, we summarize experimental evidence for and against the hypothesis that N-glycans attached to the membrane proteins can act as apical sorting signals. Furthermore, we discuss the roles of N-glycans in the apical sorting event per se and their contribution to folding and quality control of glycoproteins in the endoplasmic reticulum or retention of glycoproteins in the plasma membrane. Finally, we review existing hypotheses on the mechanism of apical sorting and discuss the potential roles of the lectins, VIP36 and galectin-3, as putative apical sorting receptors.

The gastric HK-ATPase: structure, function, and inhibition

The gastric H,K-ATPase, a member of the P(2)-type ATPase family, is the integral membrane protein responsible for gastric acid secretion. It is an alpha,beta-heterodimeric enzyme that exchanges cytoplasmic hydronium with extracellular potassium. The catalytic alpha subunit has ten transmembrane segments with a cluster of intramembranal carboxylic amino acids located in the middle of the transmembrane segments TM4, TM5,TM6, and TM8. Comparison to the known structure of the SERCA pump, mutagenesis, and molecular modeling has identified these as constituents of the ion binding domain. The beta subunit has one transmembrane segment with N terminus in cytoplasmic region. The extracellular domain of the beta subunit contains six or seven N-linked glycosylation sites. N-glycosylation is important for the enzyme assembly, maturation, and sorting. The enzyme pumps acid by a series of conformational changes from an E(1) (ion site in) to an E(2) (ion site out) configuration following binding of MgATP and phosphorylation. Several experimental observations support the hypothesis that expulsion of the proton at 160 mM (pH 0.8) results from movement of lysine 791 into the ion binding site in the E(2)P configuration. Potassium access from the lumen depends on activation of a K and Cl conductance via a KCNQ1/KCNE2 complex and Clic6. K movement through the luminal channel in E(2)P is proposed to displace the lysine along with dephosphorylation to return the enzyme to the E(1) configuration. This enzyme is inhibited by the unique proton pump inhibitor class of drug, allowing therapy of acid-related diseases.

Polarized membrane distribution of potassium-dependent ion pumps in epithelial cells: different roles of the N-glycans of their beta subunits

The Na,K-ATPases and the H,K-ATPases are two potassium-dependent homologous heterodimeric P2-type pumps that catalyze active transport of Na+ in exchange for K+ (Na,K-ATPase) or H+ in exchange for K+ (H,K-ATPase). The ubiquitous Na,K-ATPase maintains intracellular ion balance and membrane potential. The gastric H,K-ATPase is responsible for acid secretion by the parietal cell of the stomach. Both pumps consist of a catalytic alpha-subunit and a glycosylated beta-subunit that is obligatory for normal pump maturation and trafficking. Individual N-glycans linked to the beta-subunits of the Na,K-ATPase and H,K-ATPase are important for stable membrane integration of their respective alpha subunits, folding, stability, subunit assembly, and enzymatic activity of the pumps. They are also essential for the quality control of unassembled beta-subunits that results in either the exit of the subunits from the ER or their ER retention and subsequent degradation. Overall, the importance of N-glycans for the maturation and quality control of the H,K-ATPase is greater than that of the Na,K-ATPase. The roles of individual N-glycans of the beta-subunits in the post-ER trafficking, membrane targeting and plasma membrane retention of the Na,K-ATPase and H,K-ATPase are different. The Na,K-ATPase beta1-subunit is the major beta-subunit isoform in cells with lateral location of the pump. All three N-glycans of the Na,K-ATPase beta1-subunit are important for the lateral membrane retention of the pump due to glycan-mediated interaction between the beta1-subunits of the two neighboring cells in the cell monolayer and cytosolic linkage of the alpha-subunit to the cytoskeleton. This intercellular beta1-beta1 interaction is also important for formation of cell-cell contacts. In contrast, the N-glycans unique to the Na,K-ATPase beta2-subunit,which has up to eight N-glycosylation sites, contain apical sorting information. This is consistent with the apical location of the Na,K-ATPase in normal and malignant epithelial cells with high abundance of the beta2-subunit. Similarly, all seven N-glycans of the gastric H,K-ATPase beta-subunit determine apical sorting of this subunit.

Inverse correlation between the extent of N-glycan branching and intercellular adhesion in epithelia. Contribution of the Na,K-ATPase beta1 subunit

The majority of cell adhesion molecules are N-glycosylated, but the role of N-glycans in intercellular adhesion in epithelia remains ill-defined. Reducing N-glycan branching of cellular glycoproteins by swainsonine, the inhibitor of N-glycan processing, tightens and stabilizes cell-cell junctions as detected by a 3-fold decrease in the paracellular permeability and a 2-3-fold increase in the resistance of the adherens junction proteins to extraction by non-ionic detergent. In addition, exposure of cells to swainsonine inhibits motility of MDCK cells. Mutagenic removal of N-glycosylation sites from the Na,K-ATPase beta(1) subunit impairs cell-cell adhesion and decreases the effect of swainsonine on the paracellular permeability of the cell monolayer and also on detergent resistance of adherens junction proteins, indicating that the extent of N-glycan branching of this subunit is important for intercellular adhesion. The N-glycans of the Na,K-ATPase beta(1) subunit and E-cadherin are less complex in tight renal epithelia than in the leakier intestinal epithelium. The complexity of the N-glycans linked to these proteins gradually decreases upon the formation of a tight monolayer from dispersed MDCK cells. This correlates with a cell-cell adhesion-induced increase in expression of GnT-III (stops N-glycan branching) and a decrease in expression of GnTs IVC and V (promote N-glycan branching) as detected by real-time quantitative PCR. Consistent with these results, partial silencing of the gene encoding GnT-III increases branching of N-glycans linked to the Na,K-ATPase beta(1) subunit and other glycoproteins and results in a 2-fold increase in the paracellular permeability of MDCK cell monolayers. These results suggest epithelial cells can regulate tightness of cell junctions via remodeling of N-glycans, including those linked to the Na,K-ATPase beta(1)-subunit.

The gastric H,K ATPase as a drug target: past, present, and future

The recent progress in therapy if acid disease has relied heavily on the performance of drugs targeted against the H,K ATPase of the stomach and the H2 receptor antagonists. It has become apparent in the last decade that the proton pump is the target that has the likelihood of being the most sustainable area of therapeutic application in the regulation of acid suppression. The process of activation of acid secretion requires a change in location of the ATPase from cytoplasmic tubules into the microvilli of the secretory canaliculus of the parietal cell. Stimulation of the resting parietal cell, with involvement of F-actin and ezrin does not use significant numbers of SNARE proteins, because their message is depleted in the pure parietal cell transcriptome. The cell morphology and gene expression suggest a tubule fusion-eversion event. As the active H,K ATPase requires efflux of KCl for activity we have, using the transcriptome derived from 99% pure parietal cells and immunocytochemistry, provided evidence that the KCl pathway is mediated by a KCQ1/KCNE2 complex for supplying K and CLIC6 for supplying the accompanying Cl. The pump has been modeled on the basis of the structures of different conformations of the sr Ca ATPase related to the catalytic cycle. These models use the effects of site directed mutations and identification of the binding domain of the K competitive acid pump antagonists or the defined site of binding for the covalent class of proton pump inhibitors. The pump undergoes conformational changes associated with phosphorylation to allow the ion binding site to change exposure from cytoplasmic to luminal exposure. We have been able to postulate that the very low gastric pH is achieved by lysine 791 motion extruding the hydronium ion bound to carboxylates in the middle of the membrane domain. These models also allow description of the K entry to form the K liganded form of the enzyme and the reformation of the ion site inward conformation thus relating the catalytic cycle of the pump to conformational models. The mechanism of action of the proton pump inhibitor class of drug is discussed along with the cysteines covalently bound with these inhibitors. The review concludes with a discussion of the mechanism of action and binding regions of a possible new class of drug for acid control, the K competitive acid pump antagonists.

The role of the beta1 subunit of the Na,K-ATPase and its glycosylation in cell-cell adhesion

Based on recent data showing that overexpression of the Na,K-ATPase beta(1) subunit increased cell-cell adhesion of nonpolarized cells, we hypothesized that the beta(1) subunit can also be involved in the formation of cell-cell contacts in highly polarized epithelial cells. In support of this hypothesis, in Madin-Darby canine kidney (MDCK) cells, the Na,K-ATPase alpha(1) and beta(1) subunits were detected as precisely co-localized with adherens junctions in all stages of the monolayer formation starting from the initiation of cell-cell contact. The Na,K-ATPase and adherens junction protein, beta-catenin, stayed partially co-localized even after their internalization upon disruption of intercellular contacts by Ca(2+) depletion of the medium. The Na,K-ATPase subunits remained co-localized with the adherens junctions after detergent treatment of the cells. In contrast, the heterodimer formed by expressed unglycosylated Na,K-ATPase beta(1) subunit and the endogenous alpha(1) subunit was easily dissociated from the adherens junctions and cytoskeleton by the detergent extraction. The MDCK cell line in which half of the endogenous beta(1) subunits in the lateral membrane were substituted by unglycosylated beta(1) subunits displayed a decreased ability to form cell-to-cell contacts. Incubation of surface-attached MDCK cells with an antibody against the extracellular domain of the Na,K-ATPase beta(1) subunit specifically inhibited cell-cell contact formation. We conclude that the Na,K-ATPase beta(1) subunit is involved in the process of intercellular adhesion and is necessary for association of the heterodimeric Na,K-ATPase with the adherens junctions. Further, normal glycosylation of the Na,K-ATPase beta(1) subunit is essential for the stable association of the pump with the adherens junctions and plays an important role in cell-cell contact formation.

Topography of the membrane domain of the liver Na+-dependent bile acid transporter

The ileal apical and liver basolateral bile acid transporters catalyze the Na+-dependent uptake of these amphipathic molecules in the intestine and liver. They contain nine predicted helical hydrophobic sequences (H1-H9) between the exoplasmic N-glycosylated N terminus and the cytoplasmic C terminus. Previous in vitro translation and in vivo alanine insertion scanning studies gave evidence for either nine or seven transmembrane segments, with H3 and H8 noninserted in the latter model. N-terminal GFP constructs containing either successive predicted segments or only the last two domains of the liver transporter following a membrane anchor signal were expressed in HEK-293 cells, and a C-terminal glycosylation flag allowed detection of membrane insertion. Western blot analysis with anti-GFP antibody after alkali and PNGase treatment showed that H1, H2, H3 behaved as competent transmembrane (TM) sequences. Results from longer constructs were difficult to interpret. H9, however, but not H8 was membrane-inserted. To analyze the intact transporter, a C-terminal YFP fusion protein was expressed as a functionally active protein in the plasma membrane of HEK-293 cells as seen by confocal microscopy. After limited tryptic digestion to ensure the accessibility of only exoplasmic lysine or arginine residues, molecular weight (MW) analysis of the five cleavage products on SDS-PAGE predicted the presence of seven transmembrane segments, H1, H2, H3, H4, H5, H6, and H9, with H7 and H8 exoplasmic. This new method provided evidence for seven membrane segments giving a new model of the membrane domain of this protein and probably the homologous ileal transporter, with H7/H8 as the transport region.

Recombinant addition of N-glycosylation sites to the basolateral Na,K-ATPase beta1 subunit results in its clustering in caveolae and apical sorting in HGT-1 cells

In most polarized cells, the Na,K-ATPase is localized on the basolateral plasma membrane. However, an unusual location of the Na,K-ATPase was detected in polarized HGT-1 cells (a human gastric adenocarcinoma cell line). The Na,K-ATPase alpha1 subunit was detected along with the beta2 subunit predominantly on the apical membrane, whereas the Na,K-ATPase beta1 subunit was not found in HGT-1 cells. However, when expressed in the same cell line, a yellow fluorescent protein-linked Na,K-ATPase beta1 subunit was localized exclusively to the basolateral surface and resulted in partial redistribution of the endogenous alpha1 subunit to the basolateral membrane. The human beta2 subunit has eight N-glycosylation sites, whereas the beta1 isoform has only three. Accordingly, up to five additional N-glycosylation sites homologous to the ones present in the beta2 subunit were successively introduced in the beta1 subunit by site-directed mutagenesis. The mutated beta1 subunits were detected on both apical and basolateral membranes. The fraction of a mutant beta1 subunit present on the apical membrane increased in proportion to the number of glycosylation sites inserted and reached 80% of the total surface amount for the beta1 mutant with five additional sites. Clustered distribution and co-localization with caveolin-1 was detected by confocal microscopy for the endogenous beta2 subunit and the beta1 mutant with additional glycosylation sites but not for the wild type beta1 subunit. Hence, the N-glycans linked to the beta2 subunit of the Na,K-ATPase contain apical sorting information, and the high abundance of the beta2 subunit isoform, which is rich in N-glycans, along with the absence of the beta1 subunit, is responsible for the unusual apical location of the Na,K-ATPase in HGT-1 cells.

Electrogenic partial reactions of the gastric H,K-ATPase

The fluorescent styryl dye RH421 was used to identify and investigate electrogenic reaction steps of the H,K-ATPase pump cycle. Equilibrium titration experiments were performed with membrane vesicles isolated from hog gastric mucosa, and cytoplasmic and luminal binding of K(+) and H(+) ions was studied. It was found that the binding and release steps of both ion species in both principal conformations of the ion pump, E(1) and P-E(2), are electrogenic, whereas the conformation transitions do not contribute significantly to a charge movement within the membrane dielectric. This behavior is in agreement with the transport mechanism found for the Na,K-ATPase and the sarcoplasmic reticulum Ca-ATPase. The data were analyzed on the basis of the Post-Albers reaction cycle. For proton binding, two pK values were found in both conformations: 6.7 and </=4.5 in the E(1) conformation; 6.7 and </=2 in the P-E(2) conformation. The equilibrium dissociation constants for K(+) binding on the cytoplasmic side were 11 and 16 mM. The respective equilibrium dissociation constants on the luminal side were obtained via K(+) concentration dependence of the enzyme activity and determined to be 0.11 mM for both luminal binding sites.

The H,K-ATPase beta subunit as a model to study the role of N-glycosylation in membrane trafficking and apical sorting

The role of N-glycosylation in trafficking of an apical membrane protein, the gastric H,K-ATPase beta subunit linked to yellow fluorescent protein, was analyzed in polarized LLC-PK1 cells by confocal microscopy and surface-specific biotinylation. Deletion of the N-glycosylation sites at N1, N3, N5, and N7 but not at N2, N4, and N6 significantly slowed endoplasmic reticulum-to-Golgi trafficking, impaired apical sorting, and enhanced endocytosis from the apical membrane, resulting in decreased apical expression. Golgi mannosidase inhibition to prevent carbohydrate chain branching and elongation resulted in faster internalization and degradation of the beta subunit, indicating that terminal glycosylation is important for stabilization of the protein in the apical membrane and protection of internalized protein from targeting to the degradation pathway. The decrease in the apical content of the beta subunit was less with mannosidase inhibition compared with that found in the N1, N3, N5, and N7 site mutants, suggesting that the core region sugars are more important than the terminal sugars for apical sorting.

Plasma membrane delivery of the gastric H,K-ATPase: the role of beta-subunit glycosylation

The factors determining trafficking of the gastric H,K-ATPase to the apical membrane remain elusive. To identify such determinants in the gastric H,K-ATPase, fusion proteins of yellow fluorescent protein (YFP) and the gastric H,K-ATPase beta-subunit (YFP-beta) and cyan fluorescent protein (CFP) and the gastric H,K-ATPase alpha-subunit (CFP-alpha) were expressed in HEK-293 cells. Then plasma membrane delivery of wild-type CFP-alpha, wild-type YFP-beta, and YFP-beta mutants lacking one or two of the seven beta-subunit glycosylation sites was determined using confocal microscopy and surface biotinylation. Expression of the wild-type YFP-beta resulted in the plasma membrane localization of the protein, whereas the expressed CFP-alpha was retained intracellularly. When coexpressed, both CFP-alpha and YFP-beta were delivered to the plasma membrane. Removing each of the seven glycosylation sites, except the second one, from the extracellular loop of YFP-beta prevented plasma membrane delivery of the protein. Only the mutant lacking the second glycosylation site (Asn103Gln) was localized both intracellularly and on the plasma membrane. A double mutant lacking the first (Asn99Gln) and the second (Asn103Gln) glycosylation sites displayed intracellular accumulation of the protein. Therefore, six of the seven glycosylation sites in the beta-subunit are essential for the plasma membrane delivery of the beta-subunit of the gastric H,K-ATPase, whereas the second glycosylation site (Asn103), which is not conserved among the beta-subunits from different species, is not critical for plasma delivery of the protein.

Inhibition kinetics of the gastric H,K-ATPase by K-competitive inhibitor SCH28080 as a tool for investigating the luminal ion pathway

The gastric H,K-ATPase and the Na,K-ATPase both are stimulated by luminal K(+), but differ in sensitivity to K(+)-competitive inhibitors (ouabain and SCH28080), which implies a difference in structure near the luminal ion pathways in these two pumps. Knowledge of the amino acids in the H,K-ATPase that affect the mode of inhibition by SCH28080 and inhibitor affinity should provide insight into the regions of the membrane domain influencing the inhibitor selectivity and the luminal route to the ion transport site. Mutational scans in M4, 5, 6, and 8 have shown that amino acid residues affecting ion affinity (E343, K791, E795, E820, D824, E936) with either no or a lesser effect on the inhibitor affinity are located in the middle of the membrane domain. The residues significantly reducing inhibitor affinity, but not ion affinity (L809, P810, L811, T813, I816, Y925, T929), are located in the exoplasmic 5-6 loop and the luminal ends of M6 and M8. This suggests that the binding domain for SCH28080 contains the surface between L809 in the 5-6 loop and C813 at the luminal end of M6, approximately two helical turns out from the ion binding region, where it blocks an ion access pathway. The mutations that change inhibitor kinetics are on the opposing faces of M6 and M8 and apparently modify the normal ion pathway or, perhaps, create an alternate ion pathway.

Molecular modeling of SCH28080 binding to the gastric H,K-ATPase and MgATP interactions with SERCA- and Na,K-ATPases

We have used homology molecular modeling based on the srCaATPase E(2) conformation, pdb1kju, to predict side chains involved in docking the K(+) competitive inhibitor, SCH28080, to the H,K-ATPase. A model for SCH28080 binding between residues L809 and A335 in the same space utilized by omeprazole is proposed. We also describe modeling MgATP binding to the E(1) structure of the srATPase, pdb1eul, as a paradigm for the Na,K- and H,K-ATPases. The resulting model, E(1).MgATP, visualizes a conformation not yet available by crystallization and successfully predicts a range of published results, including backbone cleavages near V440 (N domain) and V712 (P domain) mediated by FeATP in the Na,K-ATPase. A separate model for MgATP docked to E(2) (pdb1kju) shows that access of the gamma phosphate to D351 is blocked by the A domain. The E(2). MgATP model explains FeATP-mediated cleavages of the Na,K-ATPase near V440 and E214 (A domain) and homologous results in the H,K-ATPase.

Current trends in the treatment of upper gastrointestinal disease

The past 25 years have seen an amazing improvement in the treatment and understanding of acid-related disorders. In particular, the introduction of selective histamine receptor antagonists and proton pump inhibitors has made the medical control of acid secretion an effective means of therapy. The demonstration that infection with Helicobacter pylori is responsible for most cases of peptic ulcer disease resulted in another major improvement in therapy in these areas as a result of the eradication of the organism. Research continues in an attempt to find improved means of acid control and better methods for the eradication of H. pylori based on unique proteins expressed by the organism to resist gastric acidity.

SCH28080, a K+-competitive inhibitor of the gastric H,K-ATPase, binds near the M5-6 luminal loop, preventing K+ access to the ion binding domain

Inhibition of the gastric H,K-ATPase by the imidazo[1,2-alpha]pyridine, SCH28080, is strictly competitive with respect to K+ or its surrogate, NH4+. The inhibitory kinetics [V(max), K(m,app)(NH4+), K(i)(SCH28080), and competitive, mixed, or noncompetitive] of mutants can define the inhibitor binding domain and the route to the ion binding region within M4-6. While mutations Y799F, Y802F, I803L, S806N, V807I (M5), L811V (M5-6), Y928H (M8), and Q905N (M7-8) had no effect on inhibitor kinetics, mutations P798C, Y802L, P810A, P810G, C813A or -S, I814V or -F, F818C, T823V (M5, M5-6, and M6), E914Q, F917Y, G918E, T929L, and F932L (M7-8 and M8) reduced the affinity for SCH28080 up to 10-fold without affecting the nature of the kinetics. In contrast, the L809F substitution in the loop between M5 and M6 resulted in an approximately 100-fold decrease in inhibitor affinity, and substitutions L809V, I816L, Y925F, and M937V (M5-6, M6, and M8) reduced the inhibitor affinity by 10-fold, all resulting in noncompetitive kinetics. The mutants L811F, Y922I, and I940A also reduced the inhibitor affinity up to 10-fold but resulted in mixed inhibition. The mutations I819L, Q923V, and Y925A also gave mixed inhibition but without a change in inhibitor affinity. These data, and the 9-fold loss of SCH28080 affinity in the C813T mutant, suggest that the binding domain for SCH28080 contains the surface between L809 in the M5-6 loop and C813 at the luminal end of M6, approximately two helical turns down from the ion binding region, where it blocks the normal ion access pathway. On the basis of a model of the Ca-ATPase in the E2 conformation (PDB entry 1kju), the mutants that change the nature of the kinetics are arranged on one side of M8 and on the adjacent side of the M5-6 loop and M6 itself. This suggests that mutations in this region modify the enzyme structure so that K+ can access the ion binding domain even with SCH28080 bound.

Mutational analysis of the K+-competitive inhibitor site of gastric H,K-ATPase

The gastric H,K-ATPase is inhibited selectively and K(+)-competitively from its luminal surface by protonated imidazo[1,2alpha]pyridines (e.g., SCH28080). Identification of the amino acids in the membrane domain that affect SCH28080 inhibition should provide a template for modeling a luminally directed vestibule in this enzyme, based on the crystal structure of the sr Ca-ATPase. Five conserved carboxylic residues, Glu343, Glu795, Glu820, Asp824, Glu936, and unique Lys791 in the H,K-ATPase were mutated, and the effects of mutations on the K(i) for SCH28080, V(max), and K(m,app)[NH(4)(+)] were measured. A kinetic analysis of the ATP hydrolysis data indicated that all of these residues significantly affect the interaction of NH(4)(+) ions with the protein but only three of them, Glu795, Glu936, and Lys791, greatly affected SCH28080 inhibition. A Glu795Asp mutation increased the K(i) from 64 +/- 11 to 700 +/- 110 nM. Since, however, the mutation Glu795Gln did not change the K(i) (86 +/- 31 nM), this site has a significant spatial effect on inhibitor kinetics. A Glu936Asp mutation resulted in noncompetitive kinetics while Gln substitution had no effect either on inhibitor affinity or on the nature of the kinetics, suggesting that the length of the Glu936 side chain is critical for the exclusive binding of the ion and SCH28080. Mutation of Lys791 to Ser, the residue present in the SCH28080-insensitive Na,K-ATPase, resulted in a 20-fold decrease in SCH28080 affinity, suggesting an important role of this residue in SCH28080 selectivity of the H,K-ATPase versus Na,K-ATPase. Mutations of Asp824, Glu343, and Glu820 increased the K(i) 2-3-fold, implying a relatively minor role for these residues in SCH28080 inhibition. It appears that the imidazopyridine moiety of SCH28080 in the protonated state interacts with residues near the negatively charged residues of the empty ion site from the luminal side (TM4, -5, -6, and -8) while the hydrophobic phenyl ring interacts with TM1 or TM2 (the latter conclusion based on previous data from photoaffinity labeling). The integrity of the SCH28080 binding site depends on the presence of Lys791, Glu936, and Glu795 in H,K-ATPase. A computer-generated model of this region illustrates the possible involvement of the residues previously shown to affect SCH28080 inhibition (Cys813, Ile816, Thr823, Met334, Val337) and may predict other residues that line the SCH28080 binding vestibule in the E(2) conformation of the pump.