Dirk Claeys

The gamma subunit is a specific component of the Na,K-ATPase and modulates its transport function.

The role of small, hydrophobic peptides that are associated with ion pumps or channels is still poorly understood. By using the Xenopus oocyte as an expression system, we have characterized the structural and functional properties of the γ peptide which co‐purifies with Na,K‐ATPase. Immuno‐radiolabeling of epitope‐tagged γ subunits in intact oocytes and protease protection assays show that the γ peptide is a type I membrane protein lacking a signal sequence and exposing the N‐terminus to the extracytoplasmic side. Co‐expression of the rat or Xenopus γ subunit with various proteins in the oocyte reveals that it specifically associates only with isozymes of Na,K‐ATPase. The γ peptide does not influence the formation and cell surface expression of functional Na,K‐ATPase α–β complexes. On the other hand, the γ peptide itself needs association with Na,K‐ATPase in order to be stably expressed in the oocyte and to be transported efficiently to the plasma membrane. γ subunits do not associate with individual α or β subunits but only interact with assembled, transport‐competent α–β complexes. Finally, electrophysiological measurements indicate that the γ peptide modulates the K+ activation of Na,K pumps. These data document for the first time the membrane topology, the specificity of association and a potential functional role for the γ subunit of Na,K‐ATPase.

The gastric H+,K+-ATPase is a major autoantigen in chronic Helicobacter pylori gastritis with body mucosa atrophy ☆ ☆☆ ★ ★★

BACKGROUND & AIMS A subgroup of Helicobacter pylori-infected patients develops autoantibodies to gastric parietal cell canaliculi. The aim of this study was to define the unknown autoantigen. METHODS We screened 72 H. pylori-infected patients, 5 patients with autoimmune gastritis, and 36 healthy controls for immunoglobulin G autoantibodies to canaliculi by immunohistochemistry. The antigen specificity was determined by immunoprecipitation of the murine gastric H+,K+-adenosine triphosphatase (H+,K+-ATPase) expressed in oocytes and by immunoblotting on human gastric membranes from the body mucosa. RESULTS Autoantibodies specific for the conformational peptides of the H+,K+-ATPase were detected in 3% (1/36) of controls, in all patients with autoimmune gastritis (5/5), in 25% (18/72) of H. pylori-infected patients, and in 47% (15/32) of the infected patients with anticanalicular autoantibodies. No other major autoantigen was identified. Atrophy in the gastric body mucosa was found in 60% (9/15) of infected patients with both anticanalicular and anti-H+,K+-ATPase antibodies, but only in 13% (5/37) of infected patients lacking both autoantibodies (P < 0.01). CONCLUSIONS The gastric H+,K+-ATPase is a major autoantigen in H. pylori-associated antigastric autoimmunity. Thus, anti-H+,K+-ATPase autoantibodies, which are closely linked to classical autoimmune gastritis, are also significant indicators for body mucosa atrophy in chronic H. pylori gastritis.

Bugs on trial: the case of Helicobacter pylori and autoimmunity

Abstract Helicobacter pylori infection induces autoantibodies to gastric H + K + -ATPase similar to those found in autoimmune gastritis and pernicious anaemia (AIG/PA). Here, Ben Appelmelk and colleagues consider the possibility that H. pylori infection triggers an autoimmune process that leads to gastric atrophy and AIG/PA.

Mouse mammary tumor virus superantigens and murine autoimmune gastritis.

BACKGROUND/AIMS Neonatal thymectomy induces autoimmune gastritis in BALB/c (minor lymphocyte-stimulating antigen [Mls]-1b) mice, whereas DBA/2 (Mls-1a) mice are resistant. Resistance has been linked to the Mls-1a locus, which encodes a retroviral superantigen, and to superantigen reactive T cells that express V beta 6+ T-cell receptors. V beta 6+ T cells are known to be deleted in mice expressing Mls-1a superantigens. METHODS Neonatal thymectomized BALB/c and Mls-1a congenic BALB.D2.Mls-1a mice were analyzed to examine directly the role of Mls-1a self-superantigens and V beta 6+ T cells in autoimmune gastritis. RESULTS Autoimmune gastritis was detected in thymectomized BALB.D2.Mls-1a mice with high incidence. Autoantibodies to the gastric H+,K(+)-adenosine triphosphatase were present independent of the Mls phenotype in sera of gastritic mice. Severe gastritis had already appeared 1 month after thymectomy in BALB.D2.Mls-1a mice. V beta 6+ T cells were deleted in the stomach lymph nodes of 1-month-old gastritic BALB.D2.Mls-1a mice but could be detected by immunocytochemistry in the stomach lesions. CONCLUSIONS Endogenous Mls-1a self-superantigens and Mls-1a reactive V beta 6+ T cells are not involved in resistance to autoimmune gastritis in BALB.D2 mice.

Functional expression of the gastric H,K-ATPase in Xenopus oocytes

The gastric parietal cell H,K-ATPase mediates the secretion of acid by an energy dependent, electroneutral exchange of intracellular H+ against extracellular K+. Among the different known P-type ATPases, the α-subunits of the vertebrate H,K-ATPases and Na,K-ATPases share the greatest sequence identity (~65 % amino acid identity). Like the Na,K-ATPase, the functional complex of the H,K-ATPase is an α/β heterodimer. Although the β-subunits of the Na,K-ATPase and the H,K-ATPase are distinct proteins, it has recently been shown that the β-subunit of the H,K-ATPase can substitute for the β-subunit of the Na,K-ATPase (1). Despite these similarities, these two pumps are functionally distinct with characteristic sensitivities to inhibitors. For example, the Na,K-ATPase from most species is sensitive to inhibition by cardiac glycosides such as ouabain whereas the gastric H,K-ATPase is not. The converse case is also true: the gastric H,K-ATPase is sensitive to both the K+ competitive inhibitor SCH 28080 and to the covalent inhibitor omeprazole while the Na,K-ATPase is resistant.