Tertiary amines as antagonists of both the luminal and cytosolic K(+)-site of gastric H,K-ATPase

Trimethoprim inhibits renal H+-K+-ATPase in states of K+ depletion

There is growing consensus that under physiological conditions, collecting duct H+ secretion is independent of epithelial Na+ channel (ENaC) activity. We have recently shown that the direct ENaC inhibitor benzamil acutely impairs H+ excretion by blocking renal H+-K+-ATPase. However, the question remains whether inhibition of ENaC per se causes alterations in renal H+ excretion. To revisit this question, we studied the effect of the antibiotic trimethoprim (TMP), which is well known to cause K+ retention by direct ENaC inhibition. The acute effect of TMP (5 µg/g body wt) was assessed in bladder-catheterized mice, allowing real-time measurement of urinary pH, electrolyte, and acid excretion. Dietary K+ depletion was used to increase renal H+-K+-ATPase activity. In addition, the effect of TMP was investigated in vitro using pig gastric H+-K+-ATPase-enriched membrane vesicles. TMP acutely increased natriuresis and decreased kaliuresis, confirming its ENaC-inhibiting property. Under control diet conditions, TMP had no effect on urinary pH or acid excretion. Interestingly, K+ depletion unmasked an acute urine alkalizing effect of TMP. This finding was corroborated by in vitro experiments showing that TMP inhibits H+-K+-ATPase activity, albeit at much higher concentrations than benzamil. In conclusion, under control diet conditions, TMP inhibited ENaC function without changing urinary H+ excretion. This finding further supports the hypothesis that the inhibition of ENaC per se does not impair H+ excretion in the collecting duct. Moreover, TMP-induced urinary alkalization in animals fed a low-K+ diet highlights the importance of renal H+-K+-ATPase-mediated H+ secretion in states of K+ depletion.NEW & NOTEWORTHY The antibiotic trimethoprim (TMP) often mediates K+ retention and metabolic acidosis. We suggest a revision of the underlying mechanism that causes metabolic acidosis. Our results indicate that TMP-induced metabolic acidosis is secondary to epithelial Na+ channel-dependent K+ retention. Under control dietary conditions, TMP does not per se inhibit collecting duct H+ secretion. These findings add further argument against a physiologically relevant voltage-dependent mechanism of collecting duct H+ excretion.

Rab6 family proteins interact with the dynein light chain protein DYNLRB1

The small GTPase Rab6 is a key regulator in the retrograde transfer from endosomes via the Golgi to the ER. Three isoforms of Rab6 have been identified, the ubiquitously expressed Rab6A and Rab6A', and the brain specific Rab6B. Recent studies have shown that Rab6A' is the major isoform regulating this retrograde transport. Cytoplasmic dynein is the main motor protein complex for this transport. Dynein consists of two heavy chains, two intermediate chains, four light intermediate chains and several light chains, called roadblock/LC7 proteins or DYNLRB proteins. In mammalian cells two light chain isoforms have been identified, DYNLRB1 and DYNLRB2. We here show with yeast-two-hybrid, co-immunoprecipitation and pull down studies that DYNLRB1 specifically interacts with all three Rab6 isoforms and co-localises at the Golgi. This is the first example of a direct interaction between Rab6 isoforms and the dynein complex. Pull down experiments showed further preferred association of DYNLRB1 with GTP-bound Rab6A and interestingly GDP-bound Rab6A' and Rab6B. In addition DYNLRB1 was found in the Golgi apparatus where it co-localises with EYFP-Rab6 isoforms. DYNLRB is a putative modulator of the intrinsic GTPase activity of GTP-binding proteins. In vitro we were not able to reproduce this effect on Rab6 GTPase activity.

Osteocyte control of bone formation via sclerostin, a novel BMP antagonist

There is an unmet medical need for anabolic treatments to restore lost bone. Human genetic bone disorders provide insight into bone regulatory processes. Sclerosteosis is a disease typified by high bone mass due to the loss of SOST expression. Sclerostin, the SOST gene protein product, competed with the type I and type II bone morphogenetic protein (BMP) receptors for binding to BMPs, decreased BMP signaling and suppressed mineralization of osteoblastic cells. SOST expression was detected in cultured osteoblasts and in mineralizing areas of the skeleton, but not in osteoclasts. Strong expression in osteocytes suggested that sclerostin expressed by these central regulatory cells mediates bone homeostasis. Transgenic mice overexpressing SOST exhibited low bone mass and decreased bone strength as the result of a significant reduction in osteoblast activity and subsequently, bone formation. Modulation of this osteocyte-derived negative signal is therapeutically relevant for disorders associated with bone loss.

Large scale expression, purification and 2D crystallization of recombinant plant plasma membrane H+-ATPase

P-type ATPases convert chemical energy into electrochemical gradients that are used to energize secondary active transport. Analysis of the structure and function of P-type ATPases has been limited by the lack of active recombinant ATPases in quantities suitable for crystallographic studies aiming at solving their three-dimensional structure. We have expressed Arabidopsis thaliana plasma membrane H+-ATPase isoform AHA2, equipped with a His(6)-tag, in the yeast Saccharomyces cerevisiae. The H+-ATPase could be purified both in the presence and in the absence of regulatory 14-3-3 protein depending on the presence of the diterpene fusicoccin which specifically induces formation of the H+-ATPase/14-3-3 protein complex. Amino acid analysis of the purified complex suggested a stoichiometry of two 14-3-3 proteins per H+-ATPase polypeptide. The purified H(+)-ATPase readily formed two-dimensional crystals following reconstitution into lipid vesicles. Electron cryo-microscopy of the crystals yielded a projection map at approximately 8 A resolution, the p22(1)2(1) symmetry of which suggests a dimeric protein complex. Three distinct regions of density of approximately equal size are apparent and may reflect different domains in individual molecules of AHA2.

The beta-subunits of Na+,K+-ATPase and gastric H+,K+-ATPase have a high preference for their own alpha-subunit and affect the K+ affinity of these enzymes

The alpha- and beta-subunits of Na+,K+-ATPase and H+,K+-ATPase were expressed in Sf9 cells in different combinations. Immunoprecipitation of the alpha-subunits resulted in coprecipitation of the accompanying beta-subunit independent of the type of beta-subunit. This indicates cross-assembly of the subunits of the different ATPases. The hybrid ATPase with the catalytic subunit of Na+,K+-ATPase and the beta-subunit of H+,K+-ATPase (NaKalphaHKbeta) showed an ATPase activity, which was only 12 +/- 4% of the activity of the Na+,K+-ATPase with its own beta-subunit. Likewise, the complementary hybrid ATPase with the catalytic subunit of H+,K+-ATPase and the beta-subunit of Na+,K+-ATPase (HKalphaNaKbeta) showed an ATPase activity which was 9 +/- 2% of that of the recombinant H+,K+-ATPase. In addition, the apparent K+ affinity of hybrid NaKalphaHKbeta was decreased, while the apparent K+ affinity of the opposite hybrid HKalphaNaKbeta was increased. The hybrid NaKalphaHKbeta could be phosphorylated by ATP to a level of 21 +/- 7% of that of Na+,K+-ATPase. These values, together with the ATPase activity gave turnover numbers for NaKalphabeta and NaKalphaHKbeta of 8800 +/- 310 min-1 and 4800 +/- 160 min-1, respectively. Measurements of phosphorylation of the HKalphaNaKbeta and HKalphabeta enzymes are consistent with a higher turnover of the former. These findings suggest a role of the beta-subunit in the catalytic turnover. In conclusion, although both Na+,K+-ATPase and H+,K+-ATPase have a high preference for their own beta-subunit, they can function with the beta-subunit of the other enzyme, in which case the K+ affinity and turnover number are modified.

Constitutive activation of gastric H+,K+-ATPase by a single mutation

In the reaction cycle of P-type ATPases, an acid-stable phosphorylated intermediate is formed which is present in an intracellularly located domain of the membrane-bound enzymes. In some of these ATPases, such as Na+,K+-ATPase and gastric H+, K+-ATPase, extracellular K+ ions stimulate the rate of dephosphorylation of this phosphorylated intermediate and so stimulate the ATPase activity. The mechanism by which extracellular K+ ions stimulate the dephosphorylation process is unresolved. Here we show that three mutants of gastric H+,K+-ATPase lacking a negative charge on residue 820, located in transmembrane segment six of the alpha-subunit, have a high SCH 28080-sensitive, but K+-insensitive ATPase activity. This high activity is caused by an increased 'spontaneous' rate of dephosphorylation of the phosphorylated intermediate. A mutant with an aspartic acid instead of a glutamic acid residue in position 820 showed hardly any ATPase activity in the absence of K+, but K+ ions stimulated ATPase activity and the dephosphorylation process. These findings indicate that the negative charge normally present on residue 820 inhibits the dephosphorylation process. K+ ions do not stimulate dephosphorylation of the phosphorylated intermediate directly, but act by neutralizing the inhibitory effect of a negative charge in the membrane.

Purification of a histidine-tagged plant plasma membrane H+-ATPase expressed in yeast

In order to facilitate efficient purification of the plant plasma membrane H+-ATPase expressed in yeast, a recombinant H+-ATPase protein with an N-terminal affinity tag of six histidine residues was engineered. When expressed in yeast the recombinant protein accumulated in the endoplasmic reticulum in an active form and showed characteristics comparable with those of the wildtype plasma membrane H+ATPase (Km,ATP, 1.1 mM; pH optimum, 6.6). After solubilization of the membrane proteins from the endoplasmic reticulum with n-dodecyl-beta-d-maltoside, the recombinant protein was purified under nondenaturing conditions by chromatography on Ni2+-nitriloacetic acid-agarose. A fraction was obtained which contained 4.2% of the initial amount of the protein and 26.6% of the ATPase-activity present in the endoplasmic reticulum. The purified protein has a specific activity of 32.6 micromol min-1 mg protein-1 at pH 6.5 and 30 degrees C. This rate is equivalent to a molecular activity of 3400 min-1. The purified plasma membrane H+-ATPase could be reconstituted into liposomes and demonstrated in this configuration the ability to pump protons. The method proves to be a convenient and rapid method for the preparation of purified single isoforms and mutant proteins of the plant plasma membrane H+-ATPase in high and functional quantities. This method might also be useful for achieving purification of other P-type ATPases, normally expressed at very low levels in heterologous systems.

Sodium acts as a potassium analog on gastric H,K-ATPase

The effects of Na+ on gastric H,K-ATPase were investigated using leaky and ion-tight H,K-ATPase vesicles. Na+ activated the total ATPase activity in the absence of K+, reaching levels of 15% relative to those in the presence of K+. The Na+ activation, which takes place at the luminal side of the membrane, depended on the ATP concentration and the type of buffer used. The steady-state ATP phosphorylation level, studied with leaky vesicles, was reduced by Na+ due to both activation of the dephosphorylation reaction and a shift to E2 in the E1<==>E2 equilibrium. By studying this equilibrium in ion-tight H,K-ATPase vesicles, it was found that Na+ drives the enzyme via a cytosolic site to the nonphosphorylating E2 conformation. No H(+)-like properties of cytosolic Na+ could be detected. We therefore conclude that Na+ behaves like K+ rather than like H+ in the H,K-ATPase reaction.

Conductive cation transport in apical membrane vesicles prepared from fetal lung

In order to characterise the apically-located conductive cation pathway of the type II pneumocyte, apical plasma membranes were prepared from mature fetal guinea pig lung. The protocol yielded purified apical membranes that enriched 19-fold with the brush border enzyme marker alkaline phosphatase; there was no significant contamination with other cellular membranes. A technique for imposing an outwardly-directed electrochemical Na+ gradient was used to amplify conductive 22Na+ uptake into vesicles. Uptake of 22Na+ was time-dependent, proportional to the magnitude of the Na+ gradient, specific and sensitive to the amiloride analogues phenamil and EIPA (apparent minimum Ki values of 50 nM and 10 microM, respectively, with maximum uptake inhibition of 42% and 39% at 100 microM). Uptake experiments in which the outwardly-directed Na+ gradient was replaced by outwardly-directed gradients of small monovalent cations and molecular cations were performed. The Na+/K+ permeability ratio was 1.2:1, and over the extended range of small monovalent cations, a permeability sequence of Na+ > K+ > Li+ > Rb+ > Cs+ was observed, indicating the presence of fixed negative charge in or spatially close to the pore. The molecular cation permeability sequence of NH4+ > methylamine+ > dimethylamine+ > choline+ > N-methyl-D-glucamine+ > tetraethylammonium+ > tetramethylammonium+, after transformation, gives an estimate of 8 A for the conducting pore diameter. These data are consistent with the presence in the apical membrane of fetal type II pneumocytes of a cation specific channel with low Na+ selectivity and amiloride sensitivity.