Non-mitochondrial origin of ethacrynic acid high-sensitive Mg2+-ATPase activity in microsomal fractions from rabbit cortical gray matter

[Amyloid β hypothesis in Alzheimer's disease and Cl--ATPase-Neuronal cell death via PI4KIIα inhibition and recovery agents]

In the brains of patients with Alzheimer's disease, a decrease in phosphatidylinositol phosphate (PIP) requiring Cl^--ATPase activity was found. In cultured rat hippocampal neurons, pathophysiological concentrations of amyloid β proteins (Aβs≤10 nM) lowered PIP levels and Cl^--ATPase activity with an increase in intracellular Cl^- concentrations, resulting in Cl^--dependent enhancements in glutamate neurotoxicity and, ultimately, neuronal cell death. Pathophysiological concentrations of Aβs(0.1-10 nM) directly lowered phosphatidylinositol-4-kinase. Non-toxic peptide fragments of Aβ, such as Ile-Gly-Leu, recovered Aβ-induced inhibition of recombinant human phosphatidylinositol-4-kinase IIα (PI4KIIα) and the intrahippocampally administered Aβ-induced degeneration of hippocampal neurons and impairment of spatial memory in mice. Agents with the potential to block these neurotoxic mechanisms of Aβ were summarized herein as (1) Aβ antagonists, (2) substrates of PI4K, (3) PI4K product, (4) PI4K activators, and (5) GABAc receptor stimulants.

A Cl- pump in rat brain neurons

Cl(-)-stimulated and ethacrynic acid-sensitive ATPase (Cl(-)-ATPase) of plasma membrane origin in the rat brain is a candidate for an active outwardly directed Cl- translocating system. Biochemistry of Cl(-)-ATPase and ATP-dependent Cl- transport (Km values for ATP and Cl-, nucleotide specificity, pH dependency, and sensitivity to ethacrynic acid) suggested that Cl(-)-ATPase is an ATP-driven Cl- pump. Activity of the reconstituted Cl(-)-ATPase/pump increased in the presence of phosphatidylinositol-4-monophosphate, and this pump activity further increased at an inside-positive membrane potential or in the presence of a protonophore, suggesting that the Cl(-)-ATPase/pump is an electrogenic Cl- transporter, probably regulated by phosphoinositide turnover in vivo. In cultured hippocampal pyramidal cell-like neurons from embryonic rat brain, ethacrynic acid and ATP-consuming treatment increased, but furosemide, an inhibitor of Na+/K+/Cl- cotransporter, decreased, [Cl-]i when monitored using Cl(-)-sensitive fluorescent probes. The stationary levels of [Cl-]i were lower and the effects of ethacrynic acid were more prominent in perikarya than in dendrites, while the effects of furosemide were more obvious in dendrites than in perikarya. The lower perikaryonic [Cl-]i and the marked effects of ethacrynic acid were observed in the later stage rather than in the early stage of culture. Thus, region-specific localization and developmental changes in the activities of Cl- transporters probably result in uneven and age-dependent distribution of Cl- in the neurons.

Effects of phospholipases on Cl-ATPase in the rat brain

We investigated the effects of phospholipases on the activity of microsomal Cl-ATPase in the rat brain, in reference to those on the activities of Na,K-ATPase and anion-insensitive Mg-ATPase. In the presence of phospholipase A2 or phospholipase C, which almost completely hydrolyzed microsomal phosphoglycerides, the activities of Cl-ATPase and Na,K-ATPase were decreased to 8-50% of the control, but anion-insensitive Mg-ATPase activity was not altered. On the other hand, with sphingomyelinase treatment, only anion-insensitive Mg-ATPase was slightly inactivated. On the addition of phospholipids (phosphatidylserine (PS), phosphatidylinositol (PI) and microsomal phospholipid mixture), Cl-ATPase activity slightly recovered only with PI, while Na,K-ATPase activity partially recovered with either phospholipid. These data suggest that Cl-ATPase requires intact membrane lipid conformation and especially PI for its maximal activity.

ATP-dependent Cl- uptake by plasma membrane vesicles from the rat brain

Uptake of Cl- by plasma membrane vesicles from the rat brain was stimulated by ATP at 37 degrees C, but not by beta, gamma-methylene ATP or at 0 degrees C. The addition of Triton X-100 or sucrose to the incubation medium diminished the ATP-stimulated Cl- uptake, suggesting that Cl- was transported across the membranes into the intravesicular space. This ATP-stimulated Cl- uptake was not affected by 1 mM ouabain. 1 microM oligomycin, 0.1 mM gamma-aminobutyric acid or 0.1 mM picrotoxin. Thus, non-mitochondrial ATP-driven Cl- transport through a system other than Na, K-ATPase or Cl- channels occurs in neuronal plasma membrane vesicles.

Histochemical demonstration of Cl(-)-ATPase in rat spinal motoneurons

Using an enzyme-histochemical technique, rat spinal cords were stained for Cl(-)-ATPase, Na+,K+-ATPase and anion-insensitive Mg2+-ATPase. Cl(-)-ATPase activity was demonstrated in plasma membranes of spinal motoneurons, Na+,K+-ATPase activity and anion-insensitive Mg2+-ATPase activity were detected in neuronal plasma membranes and blood vessels, respectively.

Characteristics of ethacrynic acid highly sensitive Mg2+-ATPase in microsomal fractions of the rat brain: functional molecular size, inhibition by SITS and stimulation by Cl-

Studies were performed to characterize ethacrynic acid (EA) highly sensitive Mg2+-ATPase isolated from microsomal fractions of the rat brain. The functional molecular sizes of the EA highly sensitive and EA less sensitive Mg2+-ATPases, estimated by a radiation inactivation method, were 480 and 80 kDa, respectively. An anion transport inhibitor, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) inhibited the EA highly sensitive Mg2+-ATPase activity. The type of inhibition was uncompetitive with respect to ATP, and the inhibition was suppressed by anions such as Cl-, Br- and I-. Chloride ions stimulated enzyme activity with an increase in Vmax, but not in Km, for ATP. Anions tested also increased the enzyme activity in the following order of decreasing potency: Cl- greater than Br- greater than CH3COO- = I- greater than SO4(2-) = HCO3- greater than SO3(2-). These results suggest that EA highly sensitive Mg2+-ATPase is a relatively large molecule with anion-sensitive sites that affect the ATP hydrolyzing activity and the SITS binding capacity through anions, with Cl- being the most potent.

Novel microsomal anion-sensitive Mg2+-ATPase activity in rat brain

Ethacrynic acid (EA) highly sensitive Mg2+-ATPase activity was demonstrated in rat brain microsomes. Marker enzyme studies suggested that the EA highly sensitive Mg2+-ATPase activity originated mainly from plasma membranes, and possibly from synaptic vesicles. Oligomycin did not affect the EA highly sensitive Mg2+-ATPase activity. Sulfhydryl reagents, such as N-ethylmaleimide and 5,5'-dithiobis-(2-nitrobenzoic acid), and anion transport inhibitors, such as 4-acetamide-4'-isothiocyanostilbene-2,2'-disulfonic acid, 4,4'-diisothiocyano-stilbene-2,2'-disulfonic acid and 2,4-dinitro-1-fluorobenzene, completely inhibited the EA highly sensitive Mg2+-ATPase activity with apparent Ki values at 5, 5, 8, 8 and 10 microM respectively. Treatment of microsomes with ethylenediaminetetraacetic acid and ammonium sulfate increased the EA highly sensitive Mg2+ and Na+,K+-ATPase activities, but not EA less sensitive Mg2+- or HCO3-ATPase activity, 2- to 3-fold that in crude microsomes. Relative substrate specificities of ATP much greater than GTP greater than ITP greater than UTP, CTP, a Km for ATP at 0.77 mM, and an optimal pH at pH 7.4 were observed. Among the anions tested (Cl-, Br-, F-, HCO3-, I-, SCN-, NO3-), EA highly sensitive Mg2+-ATPase activity was stimulated significantly by Cl- and reduced by NO3-. These data suggest that a novel, plasma membrane-located and anion-sensitive Mg2+-ATPase activity exists in the brain.