Caveolin-rich lipid rafts of the plasma membrane of mature cerebellar granule neurons are microcompartments for calcium/reactive oxygen and nitrogen species cross-talk signaling

In previous works, we have shown that L-type voltage-operated calcium channels, N-methyl-d-aspartate receptors (NMDAr), neuronal nitric oxide synthase (nNOS) and cytochrome b5 reductase (Cb5R) co-localize within the same lipid rafts-associated nanodomains in mature cerebellar granule neurons (CGN). In this work, we show that the calcium transport systems of the plasma membrane extruding calcium from the cytosol, plasma membrane calcium pumps (PMCA) and sodium-calcium exchangers (NCX), are also associated with these nanodomains. All these proteins were found to co-immunoprecipitate with caveolin-1 after treatment with 25mM methyl-β-cyclodextrin, a lipid rafts solubilizing agent. However, the treatment of CGN with methyl-β-cyclodextrin largely attenuated the rise of cytosolic calcium induced by l-glutamate through NMDAr. Fluorescence energy transfer imaging revealed that all of them are present in sub-microdomains of a size smaller than 200nm, with a peripheral distribution of the calcium extrusion systems PMCA and NCX. Fluorescence microscopy images analysis revealed high calcium dynamic sub-microcompartments near the plasma membrane in fura-2-loaded CGN at short times after addition of l-glutamate. In addition, the close proximity between sources of nitric oxide (nNOS) and superoxide anion (Cb5R) suggests that these nanodomains are involved in the fast and efficient cross-talk between calcium and redox signaling in neurons.

L-type voltage-operated calcium channels, N-methyl-D-aspartate receptors and neuronal nitric-oxide synthase form a calcium/redox nano-transducer within lipid rafts

Cytosolic calcium plays a leading role in the control of neuronal excitability, plasticity and survival. This work aims to experimentally assess the possibility that lipid rafts of the plasma membrane can provide a structural platform for a faster and tighter functional coupling between calcium and nitric-oxide signaling in neurons. Using primary cerebellar granule neurons (CGN) in culture this hypothesis has been experimentally assessed with fluorescence resonance energy transfer imaging, preparations of lipid rafts-enriched membrane fragments and western blotting. The results obtained in this work demonstrated that major calcium entry systems of the plasma membrane of CGN (L-type calcium channels and N-methyl-D-aspartate receptors) and nitric-oxide synthase are separated by less than 80 nm from each other within lipid rafts-associated sub-microdomains, suggesting a new role of lipid rafts as neuronal calcium/redox nano-transducers.

Potassium-induced apoptosis in rat cerebellar granule cells involves cell-cycle blockade at the G1/S transition

The role of regulators controlling the G1/S transition of the cell cycle was analyzed during neuronal apoptosis in post-mitotic cerebellar granule cells in an attempt to identify common mechanisms of control with transformed cells. Cyclin D1 and its associated kinase activity CDK4 (cyclin-dependent kinase 4) are major regulators of the G1/S transition. Whereas cyclin D1 is the regulatory subunit of the complex, CDK4 represents the catalytic domain that, once activated, will phosphorylate downstream targets such as the retinoblastoma protein, allowing cell-cycle progression. Apoptosis was induced in rat cerebellar granule cells by depleting potassium in presence of serum. Western-blot analyses were performed and protein kinase activities were measured. As apoptosis proceeded, loss in cell viability was coincident with a significant increase in cyclin D1 protein levels, whereas CDK4 expression remained essentially constant. Synchronized to cyclin D1 accumulation, cyclin-dependent kinase inhibitor p27Kip1 drastically dropped to 20% normal values. Cyclin D1/CDK4-dependent kinase activity increased early during apoptosis, reaching a maximum at 9-12 h and decreasing to very low levels by 48 h. Cyclin E, a major downstream target of cyclin D1, decreased concomitantly to the reduction in cyclin D1/CDK4-dependent kinase activity. We suggest that neuronal apoptosis takes place through functional alteration of proteins involved in the control of the G1/S transition of the cell cycle. Thus, apoptosis in post-mitotic neurons could result from a failed attempt to re-enter cell cycle in response to extracellular conditions affecting cell viability and it could involve mechanisms similar to those that promote proliferation in transformed cells.

Hypothalamic hypophyseal inhibitory factor (HHIF) increases intrasynaptosomal free calcium concentration

We have isolated from bovine hypothalamic and pituitary tissues a sodium pump inhibitor that is structurally different from ouabain. By mass spectrometric analysis, this purified factor revealed a single unique molecular ion with an accurate mass of 412.277 and a mass spectra different from that of ouabain. It has been previously shown that this factor inhibits the Ca2+, Mg(2+)-ATPase of the plasma membrane of synaptosomes. Because Ca2+ plays a major role in cellular excitability, we carried out a systematic study of the effects of this inhibitor on the Ca2+ transport processes across the plasma membrane of synaptosomes: We measured ATP-dependent calcium uptake, Na(+)-Ca2+ exchange, and passive permeability using 45Ca2+ and Millipore filtration, chlortetracycline fluorescence, and light-scattering, respectively. This factor inhibits the Na+, K(+)-ATPase activity of the synaptosomal plasma membrane vesicles in the same range of concentrations that produced an increase of intrasynaptosomal free calcium, with nearly the same K0.5 value. In addition, in this concentration range, this factor stimulated 10- to 11-fold the passive flux of Ca2+ and 2.5- to 3-fold the Ca2+ influx via the Na(+)-Ca2+ exchange in these membranes with respect to control values. Measurements of fluorescence anisotropy showed that in this concentration range, the inhibitor did not significantly change the order parameter (fluidity) of these membranes. These results suggest that besides its known inhibition of the sodium pump, this factor could play a role in the control of Ca2+ homeostasis by direct modulation of transport systems implicated in the control of intracellular calcium.

Effect of immobilization on the activity of rat hepatic microsomal cytochrome P450 enzymes

Cytochrome P450 enzyme preparations were obtained from phenobarbital-treated rats. On immobilization in Romicon PM 10 polysulfone anisotropic hollow fibers, 20-25% of the original activity remained. The immobilization procedure did not significantly alter the Km of the preparations when 7-ethoxycoumarin was the substrate. Coimmobilization of the preparations with glucose-6-phosphate dehydrogenase allowed cofactor recycling to take place but resulted in an apparent drop in the value for the Km. The Vmax of immobilized preparations with or without cofactor recycling was about 25% of that observed with the nonimmobilized preparations. The flow rate through the reactors greatly affected the time at which product was detected in the effluent from the reactor and the amount of 7-hydroxycoumarin produced. It is suggested that this reflects alternative fates of the NADPH cofactor when residence times in the reactor are lengthened. Increasing the amounts of protein immobilized increased the Km and decreased the Vmax values in comparison to the free enzyme. Immobilization increased the stability of preparations with respect to ionic strength and storage. Little effect was noted with regard to changes in temperature stability or pH optima of immobilized preparations.

Properties of the purified hypothalamic pituitary NA/K-ATPase inhibitor

Previously we described the isolation and final purification of an endogenous sodium-pump inhibitor from the CNS, mainly from bovine hypothalamus and pituitary. The purification protocol consisted of lipophilic chromatography, followed by lipid extraction, and semipreparative and analytical reverse-phase high-pressure liquid chromatography. The bioassays used were in vitro Na+/K(+)-ATPase inhibition, and 3H-ouabain displacement from its specific binding site in the enzyme structure, as well as inhibition of 86Rb uptake from human red blood cells. We have obtained, from both tissues, a low-molecular-weight, nonpeptidic, nonlipidic substance that elutes as a single peak highly pure according to criteria of coincidence of its spectra properties. When rechromatographed in two different chromatographic systems, the same homogeneous peak is obtained suggesting complete purity. This pure substance can be isolated from other bovine tissues as well as from human plasma and human placenta. It shows a very distinctive fluorescence spectrum and it behaves as a potent inhibitor of the Ca2+ pump of synaptosomal plasma membrane.

Differential scanning calorimetry study of glycogen phosphorylase b-detergent interactions

The overall thermal denaturation of glycogen phosphorylase b is irreversible and our results conform to the theoretical prediction of a reversible process followed by a slower irreversible process. The basic thermodynamic parameters of glycogen phosphorylase b denaturation have been worked out and found to be: critical temperature 57.0 +/- 0.5 degrees C, transition half-width 8 +/- 1 degrees C, and calorimetric enthalpy change and Van't Hoff enthalpy change of the denaturation process 450 +/- 50 and 105 +/- 15 kcal/mol of enzyme monomer, respectively, at pH 7.4. These parameters have been found to be largely altered by the detergents octylglucoside, cholate, and deoxycholate at or below their critical micelle concentration, but not by Triton X-100 nor by lecithin liposomes. Organic solvents, such as dimethyl sulfoxide and methanol, and the presence of sarcoplasmic reticulum membranes produces an alteration of the denaturation thermogram of glycogen phosphorylase b similar to that produced by the above-mentioned detergents. These results allow us to hypothesize that hydrophobic domains of glycogen phosphorylase b are involved in its association to sarcoplasmic reticulum membranes in the sarcoplasmic reticulum/glycogenolytic complex of mammalian skeletal muscle.

Modulation by phosphorylation of glycogen phosphorylase-sarcoplasmic reticulum interaction

Glycogen phosphorylase b at concentrations close to those found in skeletal muscle interacts with sarcoplasmic reticulum membranes, but not with liposomes made of lipids extracted from these membranes, and is inhibited upon binding to the membrane. The interaction of glycogen phosphorylase with the sarcoplasmic reticulum membrane is modulated by phosphorylation, for the a form of this enzyme shows a K0.5 of interaction about 10-fold lower than the b form. Upon association to the membrane the fluorescence properties of the coenzyme of glycogen phosphorylase, pyridoxal-5'-phosphate, are strongly altered, for the fluorescence at 535 nm is partially quenched and the fluorescence at 415-420 nm increases. Using fluorescein labeled sarcoplasmic reticulum membranes we have found that the average conformation of the Ca2+ + Mg(2+)-ATPase is also altered on binding of phosphorylase b. In conclusion, the results reported in this paper suggest that glycogen phosphorylase and Ca2+ + Mg(2+)-ATPase directly interact under experimental conditions similar to those found in the sarcoplasm, and that this interaction is modulated by phosphorylation of the phosphorylase.

Distances between functional sites of the Ca2+ + Mg2(+)-ATPase from sarcoplasmic reticulum using Co2+ as a spectroscopic ruler

Cobalt ion inhibits the Ca2+ + Mg2(+)-ATPase activity of sealed sarcoplasmic reticulum vesicles, of solubilized membranes and of the purified enzyme. To use Co2+ appropriately as a spectroscopic ruler to map functional sites of the Ca2+ + Mg2(+)-ATPase, we have carried out studies to obtain the kinetic parameters needed to define the experimental conditions to conduct the fluorimetric studies. 1. The apparent K0.5 values of inhibition of this ATPase are 1.4 mM, 4.8 mM and 9.5 mM total Co2+ at pH 8.0, 7.0 and 6.0, respectively. The inhibition by Co2+ is likely to be due to free Co2+ binding to the enzyme. Millimolar Ca2+ can fully reverse this inhibition, and also reverses the quenching of the fluorescence of fluorescein-labeled sarcoplasmic reticulum membranes due to Co2+ binding to the Ca2+ + Mg2(+)-ATPase. Therefore, we conclude that Co2+ interacts with Ca2+ binding sites. 2. Co2+.ATP can be used as a substrate by this enzyme with Vmax of 2.4 +/- 0.2 mumol ATP hydrolyzed min-1 (mg protein)-1 at 20-22 degrees C and pH 8.0, and with a K0.5 of 0.4-0.5 mM. 3. Co2+ partially quenches, about 10 +/- 2%, the fluorescence of fluorescein-labeled sarcoplasmic reticulum Ca2+ + Mg2(+)-ATPase upon binding to this enzyme at pH 8.0. From the fluorescence data we have estimated an average distance between Co2+ and fluorescein in the ATPase of 1.1-1.8 nm or 1.3-2.1 nm for one or two equidistant Co2+ binding sites, respectively. 4. Co2+.ATP quenches about 20-25% of the fluorescence of fluorescein-labeled Ca2+ + Mg2(+)-ATPase, from which we obtain a distance of 1.1-1.9 nm between Co2+ and fluorescein located at neighbouring catalytic sites.

Modulation of the sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase by pentobarbital

The dependence of the (Ca2+ + Mg2+)-ATPase activity of sarcoplasmic reticulum vesicles upon the concentration of pentobarbital shows a biphasic pattern. Concentrations of pentobarbital ranging from 2 to 8 mM produce a slight stimulation, approximately 20-30%, of the ATPase activity of sarcoplasmic reticulum vesicles made leaky to Ca2+, whereas pentobarbital concentrations above 10 mM strongly inhibit the activity. The purified ATPase shows a higher sensitivity to pentobarbital, namely 3-4-fold shift towards lower values of the K0.5 value of inhibition by this drug. These effects of pentobarbital are observed over a wide range of ATP concentrations. In addition, this drug shifts the Ca2+ dependence of the (Ca2+ + Mg2+)-ATPase activity towards higher values of free Ca2+ concentrations and increases several-fold the passive permeability to Ca2+ of the sarcoplasmic reticulum membranes. At the concentrations of pentobarbital that inhibit this enzyme in the sarcoplasmic reticulum membrane, pentobarbital does not significantly alter the order parameter of these membranes as monitored with diphenylhexatriene, whereas the temperature of denaturation of the (Ca2+ + Mg2+)-ATPase is decreased by 4-5 C degrees, thus, indicating that the conformation of the ATPase is altered. The effects of pentobarbital on the intensity of the fluorescence of fluorescein-labeled (Ca2+ + Mg2+)-ATPase in sarcoplasmic reticulum also support the hypothesis of a conformational change in the enzyme induced by millimolar concentrations of this drug. It is concluded that the inhibition of the sarcoplasmic reticulum ATPase by pentobarbital is a consequence of its binding to hydrophobic binding sites in this enzyme.

The position of the ATP binding site on the (Ca2+ + Mg2+)-ATPase

We present a convenient method to calculate the efficiency of fluorescence energy transfer in two-dimensional membrane systems. We apply it to the analysis of energy transfer between phospholipid molecules labelled with fluorescein and rhodamine groups, and of energy transfer in reconstituted membranes containing (Ca2+ + Mg2+)-ATPase purified from sarcoplasmic reticulum, with the ATPase labelled at the ATP binding site with fluorescein as donor, and rhodamine-labelled lipid as acceptor. The ATP binding site is found to be distant from the plane of the lipid/water interface of the membrane. It is suggested that the ATPase is present in the membrane as a dimer, with the two ATP binding sites in the dimer being close to the protein/protein interface. Addition of vanadate causes no change in quenching, suggesting that the ATP binding site does not move significantly with respect to the lipid/water interface in the E1-E2 conformational transition of the ATPase.

Quantitation of the Förster energy transfer for two-dimensional systems. I. Lateral phase separation in unilamellar vesicles formed by binary phospholipid mixtures

An analytical solution is presented for the rate of energy transfer in unilamellar vesicles formed by binary mixtures of phospholipids showing lateral phase separation. The analytical approach developed here is mainly based on geometrical considerations and, therefore, is formally different for lateral phase separation phenomena taking place in the gel and in the liquid crystalline states of the lipid system. The rate of energy transfer among donor and acceptor molecules attached to chemically different phospholipids is mathematically correlated to the average cluster size of the less-rich component of the binary mixture, thus allowing its calculation from experimental measurements. Moreover, the equations derived here permit the calculation of the average cluster size as a function of the concentration of each lipid component within certain ranges, and this can be used to improve our knowledge of the thermodynamics of these processes.

Quantitation of the Förster energy transfer for two-dimensional systems. II. Protein distribution and aggregation state in biological membranes

Analytical solutions are presented of the average rate of the Förster energy transfer for several processes affecting intrinsic membrane proteins within a phospholipid bilayer. The physiol phenomena considered here are: lateral phase separation of the protein, i.e., formation of eutectic mixtures, changes in the aggregation state of the protein and non-random distribution of protein molecules. It is shown that the average rate of energy transfer among protein and phospholipid molecules labelled with donor and acceptor molecules, respectively, allows differentiation between them and also that the average rate of energy transfer can be used to quantitate these phenomena.

Relationship between the activity of pancreatic phospholipase A2 and the physical state of the phospholipid substrate

When pancreatic phospholipase A2 is mixed with small unilamellar vesicles made from dipalmitoyl phosphatidylcholine at or above the phase transition temperature (tm), the time course of hydrolysis exhibits a distinct lag period. On the other hand, if the enzyme is preincubated with substrate for a short period of time below the transition temperature (during which time only limited hydrolysis occurs) and then assayed at high temperature, no lag period is observed. This phenomenon does not appear to be the result of product formation. Instead, the reported results lend substantial support to the hypothesis that a relatively slow substrate-enzyme organizational step is required as part of an activation process and this is most rapid when the substrate is in the gel state. However, the intrinsic activity (after the initial step of activation) is maximal when the substrate is in the liquid-crystalline state.