Stabilizing and destabilizing effects on plasma membrane Ca(2+)-ATPase activity

Glutamate Deregulation in Ketamine-Induced Psychosis-A Potential Role of PSD95, NMDA Receptor and PMCA Interaction

Ketamine causes psychotic episodes and is often used as pharmacological model of psychotic-like behavior in animals. There is increasing evidence that molecular mechanism of its action is more complicated than just N-methyl-D-aspartic acid (NMDA) receptor antagonism and involves interaction with the components of calcium homeostatic machinery, in particular plasma membrane calcium pump (PMCA). Therefore, in this study we aimed to characterize brain region-specific effects of ketamine on PMCA activity, interaction with NMDA receptor through postsynaptic density protein 95 (PSD95) scaffolding proteins and glutamate release from nerve endings. In our study, ketamine induced behavioral changes in healthy male rats consistent with psychotic effects. In the same animals, we were able to demonstrate significant inhibition of plasma membrane calcium ATPase (PMCA) activity in cerebellum, hippocampus and striatum. The expression level and isoform composition of PMCAs were also affected in some of these brain compartments, with possible compensatory effects of PMCA1 substituting for decreased expression of PMCA3. Expression of the PDZ domain-containing scaffold protein PSD95 was induced and its association with PMCA4 was higher in most brain compartments upon ketamine treatment. Moreover, increased PSD95/NMDA receptor direct interaction was also reported, strongly suggesting the formation of multiprotein complexes potentially mediating the effect of ketamine on calcium signaling. We further support this molecular mechanism by showing brain region-specific changes in PSD95/PMCA4 spatial colocalization. We also show that ketamine significantly increases synaptic glutamate release in cortex and striatum (without affecting total tissue glutamate content), inducing the expression of vesicular glutamate transporters and decreasing the expression of membrane glutamate reuptake pump excitatory amino acid transporters 2 (EAAT2). Thus, ketamine-mediated PMCA inhibition, by decreasing total Ca²⁺ clearing potency, may locally raise cytosolic Ca²⁺ promoting excessive glutamate release. Regional alterations in glutamate secretion can be further driven by PSD95-mediated spatial recruitment of signaling complexes including glutamate receptors and calcium pumps, representing a novel mechanism of psychogenic action of ketamine.

Possible role of vitamin E, coenzyme Q10 and rutin in protection against cerebral ischemia/reperfusion injury in irradiated rats

PURPOSE

To investigate the possible role of vitamin E, coenzyme Q10 and rutin in ameliorating the biochemical changes in brain and serum induced by cerebral ischemia/reperfusion (I/R) in whole body γ-irradiated rats.

MATERIALS AND METHODS

Cerebral ischemia was induced in male Wistar rats (either irradiated or non-irradiated) followed by reperfusion.

RESULTS

I/R increased brain content of malondialdehyde (MDA) and depleted its glutathione (GSH) content with a compensatory elevation in cytosolic activities of glutathione peroxidase (GPx) and glutathione reductase (GR) enzymes. It also raised brain cytosolic lactate dehydrogenase (LDH) activity and calcium (Ca(2+)) level. Furthermore, I/R provoked an inflammatory response reflected by an increment in serum levels of the proinflammatory cytokines tumour necrosis factor-α (TNF-α) and interlukin-1β (IL-1β). Moreover, induction of I/R in irradiated rats resulted in a further increase in brain oxidative stress and cytosolic LDH activity, disturbed brain Ca(2+) homeostasis and exaggerated the inflammatory reaction. During irradiation, administration of each of vitamin E, coenzyme Q10 (CoQ10) and rutin to irradiated rats before induction of I/R, alleviated the brain oxidative stress. Moreover, these antioxidants caused attenuation of the rise of the cytosolic activities of GPx and GR. A lowering effect of the cytosolic LDH activity and Ca(2+) level were caused by treatment with antioxidants. Each of vitamin E and rutin revealed an anti-inflammatory action of these antioxidants, while CoQ10 had no effect on serum levels of TNF-α and IL-1β.

CONCLUSION

These findings indicate that supplementation with either vitamin E, CoQ10 or rutin ameliorated most of the biochemical changes induced by I/R in irradiated rat brain and serum.

Indicators of erythrocyte damage after microwave warming of packed red blood cells

BACKGROUND

Localized overheating of packed red blood cells (PRBCs) after microwave warming with consequent damage to erythrocytes has been reported. We therefore compared possible cellular markers of erythrocyte damage, as measured by flow cytometry, with laboratory indicators of hemolysis to evaluate the effects of microwave warming on PRBCs.

METHODS

PRBC samples were warmed to room temperature or to 37, 42, 47, 52, or 57 degrees C in a water bath. Flow cytometry was performed after fluorescein labeling using antibodies to spectrin, Ca(2+)-ATPase, and Na(+)-K(+)-ATPase. The forward-to-sideward scatter (FSC/SSC) ratio and antibody binding were evaluated. Plasma free hemoglobin (FHb) and alpha-hydroxybutyrate dehydrogenase (HBDH) were measured immediately after heating and after 48 h. In addition, all measurements were made before and after the heating of PRBCs to 35 degrees C by a microwave blood warmer.

RESULTS

Analysis of 15000 erythrocytes showed a decrease in the FSC/SSC ratio and antibody binding above 47 degrees C [at 37 degrees C, median (SD) of 94.2 (7.4) with 0.07 (0.05)% fluorescein-positive; at 52 degrees C, median (SD) of 177.0 (19.0) with 18.5 (6.4)% positively gated; P <0.001]. FHb [room temperature, 0.3 (0.2) g/L] was increased 2-fold at 37 and 42 degrees C, 4-fold at 47 degrees C, and 25-fold at 52 degrees C. HBDH increased in parallel. Hemolysis markers showed an additional twofold increase 48 h after heating to 42 and 47 degrees C. Microwave heating to 35 degrees C did not produce significant changes of any marker.

CONCLUSIONS

All markers of cellular damage were altered after heating to >47 degrees C, and a substantial part of hemolysis was delayed. The methodology can be used for future testing of other blood warming devices.

Spectroscopic analysis of halothane binding to the plasma membrane Ca2+-ATPase

The intrinsic tryptophan (Trp) fluorescence of the plasma membrane Ca2+-ATPase (PMCA) is significantly quenched by halothane, a volatile anesthetic common in clinical practice. It has been proposed that halothane inhibition of the Ca2+-ATPase activity results from conformational changes following anesthetic binding in the enzyme. We have investigated whether the observed quenching reflects halothane binding to PMCA. We have shown that the quenching is dose dependent and saturable and can be fitted to a binding curve with an equilibrium constant K(Hal) = 2.1 mM, a concentration at which the anesthetic approximately half-maximally inhibits the Ca2+-ATPase activity. The relatively low sensitivity of halothane quenching of Trp fluorescence to the concentration of phosphatidylcholine and detergent in the PMCA preparation concurs with the quenching resulting from anesthetic binding in the PMCA molecule. Analysis of the Trp fluorescence quenching by acrylamide indicates that the Trp residues are not considerably exposed to the solvent (Stern-Volmer quenching constant of 2.9 M(-1)) and do not differ significantly in their accessibility to halothane. Other volatile anesthetics, diethyl ether and diisopropyl ether, reduce the quenching caused by halothane in a dose-dependent manner, suggesting halothane displacement from its binding site(s). These observations indicate that halothane quenching of intrinsic Trp fluorescence of PMCA results from anesthetic binding to the protein. The analysis, used as a complementary approach, provides new information to the still rudimentary understanding of the process of anesthetic interaction with membrane proteins.

Entropy-driven interactions of anesthetics with membrane proteins

Thermodynamic analysis of anesthetic effects on Ca2+-ATPase activity was performed to evaluate the feasibility of anesthetic binding and gain insight into the molecular events underlying the anesthetic-enzyme interactions. The Ca2+-ATPases, integral membrane proteins vital in cellular Ca2+ regulation, are suitable models for investigation of the mechanism of anesthetic action on membrane proteins that are targeted by the anesthetics. Ca2+-ATPase of plasma membrane, PMCA, and SERCA1 in the intracellular sarcoplasmic reticulum membrane were used to study two general anesthetics: halothane, a halogenated two-carbon alkane; and propofol, an intravenous, strongly lipophilic-substituted phenol. Interactions of both anesthetics result in a negative Gibbs free energy change, which in both enzymes is more favorable for the more lipophilic propofol than halothane. Temperature dependence (more negative change in Gibbs free energy at increased temperature) is in agreement with predominantly nonpolar interactions. The interactions are entropy-driven, characterized by positive enthalpy which is overcompensated by positive entropy changes. This is in contrast to the reported in literature enthalpy-driven anesthetic binding to soluble proteins. The possible contributions to the observed positive entropy change are discussed including displacement of ordered water molecules by anesthetic binding in nonpolar cavities in the membrane proteins and subtle structural rearrangements.

Mechanism of inhibition of the plasma membrane Ca(2+)-ATPase by barbiturates

We have demonstrated that sodium pentobarbital inhibited the activation of the human red blood cell plasma membrane Ca(2+)-ATPase produced by dimerization of enzyme monomers or by calmodulin binding to enzyme monomers. The effects of the barbiturate were dose-dependent. Both Vmax and Ca2+ affinity were reduced. The Ca(2+)-ATPase activity of the dimeric enzyme was distinctly less sensitive with respect to the effective inhibitory concentrations of pentobarbital and to the rate of onset of inhibition than was the calmodulin-dependent activation of enzyme monomers. Temperature dependence of the inhibition was in agreement with direct, nonpolar interactions of pentobarbital with a water-exposed nonpolar patch on the surface of this transmembrane protein. The barbiturate prevented the increase of intrinsic tryptophan fluorescence associated with substrate Ca2+ binding to the enzyme dimer. On the basis of the barbiturate effects we propose a model for the action of detergent-like compounds on the enzyme. They inhibit Ca(2+)-ATPase activity by binding to a nonpolar patch on the water-exposed dimerization surface of the enzyme monomer, part of which is also the binding site for calmodulin. The model assumes that their binding to the nonpolar patch on the monomer interferes with dimerization and weakens but does not prohibit calmodulin binding, whose activation of the enzyme is then submaximal. The model should be applicable to other proteins as the two activation pathways studied have been demonstrated for various enzymes.

Neutral organic solute effects on the activity of the plasma membrane Ca(2+)-ATPase

We have compared effects of dimethylsulfoxide (Me2SO) and two polyols on the Ca(2+)-ATPase purified from human erythrocytes. As studied under steady-state conditions over a broad solute concentration range and temperature, Me2SO, glycerol, and xylitol do not inhibit the Ca(2+)-ATPase activity; this is in contrast to numerous other organic solutes that we have investigated. Under specific experimental conditions, Me2SO (but not glycerol) substantially increases Ca(2+)-ATPase activity, suggesting a possible facilitation of enzyme oligomerization. The activation is more pronounced at low Ca2+ concentrations. In contrast to glycerol, Me2SO shows no protective effect on enzyme structure as assessed by determining residual Ca(2+)-ATPase activity after exposing the enzyme to thermal denaturation at 45 degrees C. Under these conditions several other organic solutes strongly enhance the denaturating effect of temperature. Because of the temperature dependence of its effect on the Ca(2+)-ATPase activity we believe that Me2SO activates the Ca(2+)-ATPase by indirect water-mediated interactions.