Interaction of Ca2+ with (Na+ + K+)-ATPase: properties of the Ca2+-stimulated phosphatase activity

Astrocytic transporters in Alzheimer's disease

Astrocytes play a fundamental role in maintaining the health and function of the central nervous system. Increasing evidence indicates that astrocytes undergo both cellular and molecular changes at an early stage in neurological diseases, including Alzheimer's disease (AD). These changes may reflect a change from a neuroprotective to a neurotoxic phenotype. Given the lack of current disease-modifying therapies for AD, astrocytes have become an interesting and viable target for therapeutic intervention. The astrocyte transport system covers a diverse array of proteins involved in metabolic support, neurotransmission and synaptic architecture. Therefore, specific targeting of individual transporter families has the potential to suppress neurodegeneration, a characteristic hallmark of AD. A small number of the 400 transporter superfamilies are expressed in astrocytes, with evidence highlighting a fraction of these are implicated in AD. Here, we review the current evidence for six astrocytic transporter subfamilies involved in AD, as reported in both animal and human studies. This review confirms that astrocytes are indeed a viable target, highlights the complexities of studying astrocytes and provides future directives to exploit the potential of astrocytes in tackling AD.

Divalent cation interactions with Na,K-ATPase cytoplasmic cation sites: implications for the para-nitrophenyl phosphatase reaction mechanism

The interactions of divalent cations with the adenosine triphosphatase (ATPase) and para-nitrophenyl phosphatase (pNPPase) activity of the purified dog kidney Na pump and the fluorescence of fluorescein isothiocyanate (FITC)-labeled pump were determined. Sr(2+) and Ba(2+) did not compete with K(+) for ATPase (an extracellular K(+) effect). Sr(2+) and Ba(2+) did compete with Na(+) for ATPase (an intracellular Na(+) effect) and with K(+) for pNPPase (an intracellular K(+) effect). These results suggest that Ba(2+) or Sr(2+) can bind to the intracellular transport site, yet neither Ba(2+) nor Sr(2+) was able to activate pNPPase activity; we confirmed that Ca(2+) and Mn(2+) did activate. As another measure of cation binding, we observed that Ca(2+) and Mn(2+), but not Ba(2+), decreased the fluorescence of the FITC-labeled pump; we confirmed that K(+) substantially decreased the fluorescence. Interestingly, Ba(2+) did shift the K(+) dose-response curve. Ethane diamine inhibited Mn(2+) stimulation of pNPPase (as well as K(+) and Mg(2+) stimulation) but did not shift the 50% inhibitory concentration (IC(50)) for the Mn(2+)-induced fluorescence change of FITC, though it did shift the IC(50) for the K(+)-induced change. These results suggest that the Mn(2+)-induced fluorescence change is not due to Mn(2+) binding at the transport site. The drawbacks of models in which Mn(2+) stimulates pNPPase by binding solely to the catalytic site vs. those in which Mn(2+) stimulates by binding to both the catalytic and transport sites are presented. Our results provide new insights into the pNPPase kinetic mechanism as well as how divalent cations interact with the Na pump.

Characterization of K(+)-dependent and K(+)-independent p-nitrophenylphosphatase activity of synaptosomes

These experiments examined effects of several ligands on the K+ p-nitrophenylphosphatase activity of the (Na+,K+)-ATPase in membranes of a rat brain cortex synaptosomal preparation. K(+)-independent hydrolysis of this substrate by the synaptosomal preparation was studied in parallel; the rate of hydrolysis in the absence of K+ was approximately 75% less than that observed when K+ was included in the incubation medium. The response to the H+ concentrations was different: K(+)-independent activity showed a pH optimum around 6.5-7.0, while the K(+)-dependent activity was relatively low at this pH range. Ouabain (0.1 mM) inhibited K(+)-dependent activity 50%; a concentration 10 times higher did not produce any appreciable effect on the K(+)-independent activity. Na+ did not affect K(+)-independent activity at all, while the same ligand concentration inhibited sharply the K(+)-dependent activity; this inhibition was not competitive with the substrate, p-nitrophenyl phosphate. K(+)-dependent activity was stimulated by Mg2+ with low affinity (millimolar range), and 3 mM Mg2+ produced a slight stimulation of the activity in absence of K+, which could be interpreted as Mg2+ occupying the K+ sites. Ca2+ had no appreciable effect on the activity in the absence of K+. However, in the presence of K+ a sharp inhibition was found with all Ca2+ concentrations studied. ATP (0.5 mM) did not affect the K(+)-independent activity, but this nucleotide behaved as a competitive inhibitor to p-nitrophenylphosphate. Pi inhibited activity in the presence of K+, competitively to the substrate, so it could be considered as the second product of the reaction sequence.

Effects of Ca2+ on the sodium pump observed in cardiac myocytes isolated from guinea pigs

It is presently unknown whether Ca2+ plays a role in the physiological control of Na+/K+-ATPase or sodium pump activity. Because the enzyme is exposed to markedly different intra- and extracellular Ca2+ concentrations, tissue homogenates or purified enzyme preparations may not provide pertinent information regarding this question. Therefore, the effects of Ca2+ on the sodium pump were examined with studies of [3H]ouabain binding and 86Rb+ uptake using viable myocytes isolated from guinea-pig heart and apparently maintaining ion gradients. In the presence of K+, a reduction of the extracellular Ca2+ increased specific [3H]ouabain binding observed at apparent binding equilibria: a half-maximal stimulation was observed when extracellular Ca2+ was lowered to about 50 microM. The change in [3H]ouabain binding was caused by a change in the number of binding sites accessible by ouabain instead of a change in their affinity for the glycoside. Ouabain-sensitive 86Rb+ uptake was increased by a reduction of extracellular Ca2+ concentration. Benzocaine in concentrations reported to reduce the rate of Na+ influx failed to influence the inhibitory effect of Ca2+ on glycoside binding. When [3H]ouabain binding was at equilibrium, the addition of Ca2+ decreased and that of EGTA increased the glycoside binding. Mn2+, which does not penetrate the cell membrane, had effects similar to Ca2+. In the absence of K+, cells lose their tolerance to Ca2+. Reducing Ca2+ concentration prevented the loss of rod-shaped cells but failed to affect specific [3H]ouabain binding observed in the absence of K+. These results indicate that a large change in extracellular Ca2+ directly affects the sodium pump in cardiac myocytes isolated from guinea pigs.

Effects of mono and divalent cations on total and partial reactions catalysed by pig kidney Na,K-ATPase

Interactions of Na, K, Ca and Mg ions with partially purified Na,K-ATPase from pig kidneys were investigated using as a tool simultaneous determinations of adenosine 5'-triphosphate (ATP)-adenosine 5'-diphosphate (ADP) exchange and ATPase activity catalysed by the enzyme. In the presence of 120 mM-NaCl and 0.5 mM-MgCl2, the effects 12 mM-KCl on ATP-ADP exchange depended on the ATP and ADP concentrations: stimulation (about 10-fold) with 3 mM-ATP-0.75 mM-ADP, no effect with 0.04 mM-ATP-0.01 mM-ADP and inhibition when ATP and ADP concentrations were 0.003 mM and 0.001 mM respectively. The apparent affinities (K0.5) for K stimulation of ATP-ADP exchange and Na,K-ATPase activity were indistinguishable from each other both at 20 mM- and 120 mM-Na. All Na and Na+K-dependent exchanges were completely abolished by 10(-4) M-ouabain. In the absence of K, intermediate Na concentrations were inhibitory of ouabain-sensitive ATP-ADP exchange and ATPase activity. This Na inhibition was more pronounced at low than at high ionic strength and was more noticeable in the exchange reaction. K ions antagonized these Na effects; this K-Na antagonism was more effective on ATPase than on the ATP-ADP exchange. Ca ions, in the absence of Mg, could sustain ATP-ADP exchange. Qualitatively the behaviour of the exchange reaction was similar to that observed with Mg. Quantitatively the rates were always lower with Ca. In the presence of Mg, at low ionic strength and at concentrations which stimulated phosphatase activity, Ca ions were not able to counteract Na inhibition of ATPase and ATP-ADP exchange or to stimulate ATPase activity in the presence of Na. This is another indication of the different reactivity of the K sites involved in phosphatase and ATPase activities as well as of those responsible for the release of Na inhibition of ATPase and exchange reaction. In K-free solutions containing 0.5 mM-Na and low ionic strength, Mg was a powerful inhibitor of ouabain-sensitive ATPase and ATP-ADP exchange; the curves relating remaining activities to Mg concentrations were identical for both reactions. When Na was increased to 5 mM, ATP-ADP exchange became more sensitive and ATPase more resistant to Mg inhibition. These data are consistent with (i) a Na-Mg antagonism at the phosphorylation step, and (ii) a Na-Mg synergism in blocking the E2PNa-E1PNa transformation.(ABSTRACT TRUNCATED AT 400 WORDS)

Phosphatase activity of (Na+ + K+)-ATPase. Ligand interactions and related enzyme forms

The prevailing conformations of partially purified pig kidney (Na+ + K+)-ATPase interacting with ligands related to its phosphatase activity were determined following time-dependent trypsin digestion and inactivation as well as the amounts of Rb+ or Ca2+ bound to the enzyme after passage through cation-exchange resin columns. In the presence of 150 mM choline chloride, alone or with 3 mM MgCl2, 3 mM MnCl2 or 1 mM CaCl2, the major enzyme conformation was E1. Similar forms were seen with 5 mM p-nitrophenyl phosphate with and without 3 mM MgCl2. KCl, at 0.5 mM or 150 mM, produced an E2 enzyme state; the effects of 0.5 mM KCl were completely counteracted by 5 mM p-nitrophenyl phosphate. Under optimal conditions for phosphatase activity (3 mM MgCL2/5 mM p-nitrophenyl phosphate/10 mM KCl) the (Na+ + K+)-ATPase was in the E2 state. At low ionic strength and 20 degrees C and under 85% of maximal RbCl-stimulated phosphatase turnover (1 mM RbCl/3 mM MgCl2/5 mM p-nitrophenyl phosphate) no Rb+ occlusion could be detected. Ca2+, at low ionic strength and in the presence of 3 mM MgCl2, stimulated an ouabain-sensitive phosphatase activity. The rates of hydrolysis obtained wit 1 mM CaCl2 were similar to those seen with 0.5 mM KCl; under both conditions, similar patterns of trypsin digestion and inactivation of the enzyme were obtained. On the other hand, Ca2+ could not mimic Rb+ in its ability to induce an E2-occluding state. These results suggest that during phosphatase activity of (Na+ + K+)-ATPase, the most abundant form is a non-occluding E2 and that at least one of the mechanisms of potassium stimulation of that activity it to take the enzyme into the E2 state.

Divalent cations and the phosphatase activity of the (Na + K)-dependent ATPase

Phosphatase activity of a kidney (Na + K)-ATPase preparation was optimally active with Mg2+ plus K+. Mn2+ was less effective and Ca2+ could not substitute for Mg2+. However, adding Ca2+ with Mg2+ or substituting Mn2+ for Mg2+ activated it appreciably in the absence of added K+, and all three divalent cations decreased apparent affinity for K+. Inhibition by Na+ decreased with higher Mg2+ concentrations, when Ca2+ was added, and when Mn2+ was substituted for Mg2+. Dimethyl sulfoxide, which favors E2 conformations of the enzyme, increased apparent affinity for K+, whereas oligomycin, which favors E1 conformations, decreased it. These observations are interpretable in terms of activation through two cases of cation sites. (i) At divalent cation sites, Mg2+ and Mn2+, favoring (under these conditions) E2 conformations, are effective, whereas Ca2+, favoring E1, is not, and monovalent cations complete. (ii) At monovalent cation sites divalent cations compete with K+, while Na+ at these sites favors E1 conformations. K+ increases the Km for substrate, but both Ca2+ and Mn2+ decrease it, perhaps by competing with K+. On the other hand, phosphatase activity in the presence of Na+ plus K+ is stimulated by dimethyl sulfoxide, by higher concentrations of Mg2+ and Mn2+, but not by adding Ca2+; this is consistent with stimulation occurring through facilitation of an E1 to E2 transition, perhaps an E1-P to E2-P step like that in the (Na + K)-ATPase reaction sequence. However, oligomycin stimulates phosphatase activity with Mg2+ plus Na+ alone or Mg2+ plus low K+: this effect of oligomycin may reflect acceleration, in the absence of adequate K+, of an alternative E2-P to E1 pathway bypassing the monovalent cation-activated steps in the hydrolytic sequence.