Developmental expression of the calcium release channels during early neurogenesis of the mouse cerebral cortex

The developmental changes of intracellular calcium release channels of mouse neocortex were studied at the onset of neurogenesis, which occurs between embryonic days E11 and E17. The three main isoforms of the two families of intracellular calcium release channels, namely the inositol trisphosphate receptors (IP3R) and the ryanodine receptors (RyR), were detected by their transcripts in the cerebral hemispheres, as early as stage E11. The major isoforms of each family, IP3R-1 and RyR-2, were found at the protein level by Western blot analysis. Expression of these proteins increases progressively throughout brain development. Their localization in coronal sections of cortex has been observed by immunodetection from E12, and compared to the TuJ1 (anti-class III beta-tubulin antibody) neuronal specific labelling. The expression of both channels is greatly enhanced after E12, and both were seen to be present in most of the proliferative and neuronal cells of the slice. Between E12 and E13, there is a striking transition in the pattern of calcium release elicited by specific agonists of these channels, thimerosal for IP3R and caffeine for RyR. The signals induced by thimerosal were not zone-specific, while the observed calcium release signals induced by caffeine were predominantly restricted out of the ventricular zone. This zone-specific caffeine sensitivity is consistent with the main RyR localization immunodetected at E13. Our results indicate that there is a time lag of several days between the molecular detection of calcium release channels and their functional expression, around the time of neuronal differentiation. Altogether, they provide a molecular basis for analyzing the developmental modulation of calcium signals useful for neurogenesis progression.

Calmodulin and immunophilin are required as functional partners of a ryanodine receptor in ascidian oocytes at fertilization

Fertilization of oocytes incites numerous changes relying on Ca(2+) signaling. In inseminated ascidian eggs, an increase in the egg surface membrane, monitored by a change in electrical capacitance, is recorded at the onset of meiosis resumption. This membrane addition to the cell surface is controlled by calcium release through a ryanodine receptor (RyR), sensitive to cyclic ADP-ribose. Using confocal microscopy analysis of ascidian oocytes immunostained with anti-RyR antibody, we show here that this calcium channel is asymmetrically located in the vegetal cortical zone. Interestingly, the increase in cell capacitance occurring at fertilization is correlated with a fluorescent signal, imaged by the marker of vesicle trafficking FM 1-43, located close to the RyR region. Two putative partners of RyR, namely an FKBP-like protein and a calmodulin, are identified in these oocyte extracts by detection of enzyme activity and PCR amplification. Both are necessary to sustain ryanodine receptor activity in these oocytes since the membrane insertion triggered by fertilization is inhibited by the FKBP ligand rapamycin and by a calmodulin antagonist peptide. These findings suggest that exocytosis in ascidian eggs is triggered at fertilization by a functional Ca(2+) release unit operating as a complex of several proteins, including a calmodulin and an immunophilin, around the intracellular calcium channel itself.

Autonomous folding of the recombinant large cytoplasmic loop of sarcoplasmic reticulum Ca2+-ATPase probed by affinity labeling and trypsin digestion

Recombinant large cytoplasmic loop (LCL, residues 329-740) of sarcoplasmic reticulum Ca2+-ATPase, expressed in and purified from Escherichia coli, comprises most of the active site and binds ATP [Moutin, M.-J., Cuillel, M., Rapin, C., Miras, R., Anger, M., Lompré, A.-M. & Dupont, Y. (1994) J. Biol. Chem. 269, 11147-11154]. In this study, we show that fluorescein-5' isothiocyanate (FITC) specifically labels the same lysine residue as in the native Ca2+-ATPase (Lys515), with similar kinetics and pH dependence. ATP blocks the reaction with the lysine residue, but at higher concentrations compared with those for the native pump, in agreement with the lower ATP-binding affinity found previously. Graded tryptic digestion of LCL shows that favored cleavage is at the T1 site and that the N-terminal 75% of LCL are resistant to trypsin, as is native Ca2+-ATPase. Other experiments reveal differences to the native pump. (a) FITC derivatizes some -SH groups of LCL. (b) The C-terminal 25% of the polypeptide is susceptible to end-clipping by trypsin. (c) 2',3'-O-(2,4,6-trinitrophenyl)-ATP fails to specifically label the LCL (on the equivalent of Lys492), although it binds tightly (KD = 1.3 microM) and (d) Glutaraldehyde does not specifically cross-link LCL (between the equivalent of Lys492 and Arg678). These results could be explained by a flexible and loose structure of the hinge region of LCL (C-terminal 25%). Anchoring this region in the membrane and/or interaction with the missing beta-strand domain may be required for its compact folding and proper interaction with the rest of LCL. The results suggest that the N-terminal 75% of LCL expressed in E. coli folds autonomously to a fairly stable unit and native-like structure, encompassing the phosphorylation and central ATP binding sections. The hinge region does not appear to be part of the FITC-binding site but constitutes portions of the 2',3'-O-(2,4,6-trinitrophenyl)-ATP and, probably, ATP-binding site.

Expression of the sarcoplasmic reticulum Ca(2+)-ATPase in yeast

We describe here an easy system for the production of mg amounts of the rabbit Ca(2+)-ATPase SERCA 1a in the yeast S. cerevisiae. The protein is present in several membranes, including the plasma membrane of the yeast, in a native conformation. It can be purified by immunoprecipitation and can be phosphorylated from ATP in a Ca(2+)-dependent manner. Using a temperature-sensitive secretion mutant strain, the fully active protein can also be obtained in secretory vesicles.

Measurements of ATP binding on the large cytoplasmic loop of the sarcoplasmic reticulum Ca(2+)-ATPase overexpressed in Escherichia coli

The large cytoplasmic loop of the sarcoplasmic reticulum Ca(2+)-ATPase (LCL), situated between Lys329 and Phe740, is believed to contain both its phosphorylation and ATP binding domains. A cDNA fragment coding for this amino acid sequence was generated in vitro and cloned in vector pQE8 which allowed the overexpression in Escherichia coli of this Ca(2+)-ATPase domain fused with a cluster of 6 histidines at its NH2 terminus. The fusion protein produced in an insoluble form within bacteria was solubilized in 4 M urea, purified on immobilized Ni2+, and then renatured by elimination of urea. More than 4 mg of purified renatured fusion protein was obtained from 500 ml of culture. ATP binding on the refolded protein was demonstrated by two methods: 1) detection of ATP-induced intrinsic fluorescence change and 2) binding of the fluorescent ATP analogue 2',3'-O-(2,4,6-trinitrophenyl)-adenosine-5'-triphosphate (TNP-ATP) and its chase by ATP. It is shown that the LCL protein has one single TNP-ATP binding site having a dissociation constant (Kd) of 1.6-1.9 microM. Both methods yielded a Kd for ATP around 200 microM. Binding of other nucleotides was detected with a sequence of Kd identical to that found for native Ca(2+)-ATPase: ATP < ADP < GTP < AMP < ITP. A Mg2+ binding site was also found on the LCL protein (Kd = 100 microM at pH 7.2). The fluorescence of bound TNP-ATP was found to be highly dependent on Mg2+ binding on this site.

Inward barium current and excitation-contraction coupling in frog twitch muscle fibres

The role of barium ions in excitation-contraction coupling was studied in single isolated frog semitendinosus fibres. Simultaneous recordings of membrane currents and contraction under voltage-clamp conditions in a sucrose-vaseline gap device show that barium ions have a reversible inhibiting effect on contraction. This inhibiting action was correlated to the entry of barium ions via the DHP-sensitive tubular calcium channel. Cytological observations and X-ray microanalysis performed on the fibres used in the electrophysiological experiments indicate that barium ions do not accumulate in the junctional sarcoplasmic reticulum; they can freely diffuse in the intermyofibrillar space and they accumulate in mitochondria. Calcium release experiments performed on isolated sarcoplasmic reticulum vesicles show that barium ions are not able to induce calcium release from calcium-loaded vesicles, they behave as calcium release inhibitors. These results are discussed in relation with the possible role of the slow Ca current in excitation-contraction coupling.

BisG10, a K+ channel blocker, affects the calcium release channel from skeletal muscle sarcoplasmic reticulum

The action of bisG10, a potent K+ channel inhibitor, was tested on the Ca2+ release from isolated sarcoplasmic reticulum vesicles of rabbit skeletal muscle. Using a rapid filtration technique, we found that the drug inhibited Ca(2+)-induced Ca2+ release elicited in the presence of extravesicular K+ as counter-ion. This inhibition was not reversed by the addition of valinomycin and still occurred when Cl- was used as co-ion, indicating that not only K+ channels are involved in the inhibiting effect. We found that bisG10 decreased the binding of ryanodine to sarcoplasmic reticulum vesicles, showing that bisG10 is able to block the sarcoplasmic reticulum Ca2+ release channel.

Ruthenium red affects the intrinsic fluorescence of the calcium-ATPase of skeletal sarcoplasmic reticulum

We have studied the effect of Ruthenium red on the sarcoplasmic reticulum Ca(2+)-ATPase. Ruthenium red does not modify the Ca2+ pumping activity of the enzyme, despite its interaction with cationic binding sites on sarcoplasmic reticulum vesicles. Two pools of binding sites were distinguished. One pool (10 nmol/mg) is dependent upon the presence of micromolar Ca2+ and may therefore represent the high-affinity Ca2+ transport sites of the Ca(2+)-ATPase. However, Ruthenium red only slightly competes with Ca2+ on these sites. The other pool (15-17 nmol/mg) is characterized as low-affinity cation binding sites of sarcoplasmic reticulum, distinct from the Mg2+ site involved in the ATP binding to the Ca(2+)-ATPase. The interaction of Ruthenium red with these low-affinity cation binding sites, which may be located either on the Ca(2+)-ATPase or on surrounding lipids, decreases tryptophan fluorescence level of the protein. As much as 25% of the tryptophan fluorescence of the Ca(2+)-ATPase is quenched by Ruthenium red (with a dissociation constant of 100 nM), tryptophan residues located near the bilayer being preferentially affected.

Interaction of potassium and magnesium with the high affinity calcium-binding sites of the sarcoplasmic reticulum calcium-ATPase

The sarcoplasmic reticulum Ca2(+)-ATPase of skeletal muscle has two high affinity calcium sites, one of fast access ("f" site) and one of slow access ("s" site). In addition to Ca2+ these sites are able to interact with other cations like Mg2+ or K+. We have studied with a stopped-flow method the modifications produced by Mg2+ and K+ on the kinetics of the intrinsic fluorescence changes produced by Ca2+ binding to and dissociation from the Ca2(+)-ATPase of sarcoplasmic reticulum. The presence of Mg2+ ions (K1/2 = 0.5 mM at pH 7.2) leads to the appearance of a rapid phase in the Ca2+ binding, which represents half of the signal amplitude at optimal Mg2+. The presence of K+ greatly accelerates both the Ca2+ binding and the Ca2+ dissociation reactions, giving, respectively, a 4- and 8-fold increase of the rate constant of the induced fluorescence change. K+ ions also increase the rate of the 45Ca/40Ca exchange reaction at the s site measured by rapid filtration. These results lead us to build up a model for the Ca2(+)-binding mechanism of the sarcoplasmic reticulum Ca2(+)-ATPase in which Mg2+ and K+ participate at particular steps of the reaction. Moreover, we propose that, in the absence of Ca2+, this enzyme may be the pathway for monovalent ion fluxes across the sarcoplasmic reticulum membrane.

Rapid Ag+-induced release of Ca2+ from sarcoplasmic reticulum vesicles of skeletal muscle: a rapid filtration study

Using a rapid filtration method, we show that Ag+ is able to trigger Ca2+ release from sarcoplasmic reticulum vesicles at a rate as fast as that induced by Ca2+ itself. The Ag+ concentration dependence of the rate constant of Ca2+ release presents a bell shape, similar to that of Ca2+-induced Ca2+ release, with a maximum at 30 microM free Ag+. The rapid phase of Ca2+-release induced by Ag+ is activated by millimolar ATP and inhibited by 5 microM ruthenium red. Moreover, micromolar Ca2+ produces a shift of the Ag+ concentration dependence of the Ca2+ release rate. All these results suggest that Ag+ acts on the same sites as Ca2+ to regulate the release of Ca2+.

Rapid filtration studies of Ca2+-induced Ca2+ release from skeletal sarcoplasmic reticulum. Role of monovalent ions

We have developed a rapid filtration technique for the measurement of Ca2+ release from isolated sarcoplasmic reticulum vesicles. Using this technique, we have studied the Ca2+-induced Ca2+ release of sarcoplasmic reticulum vesicles from rabbit skeletal muscle passively loaded with 5 mM Ca2+. The effect of known effectors (adenine nucleotides and caffeine) and inhibitors (Mg2+ and ruthenium red) of this release were investigated. In a medium composed of 100 mM KCl buffered at pH 6.8 with 20 mM K/3-(N-morpholino)propanesulfonic acid the Ca2+ release rate was maximal (500 nmol of Ca2+ released.(mg of protein)-1.s-1) at 1 micron external Ca2+ and 5 mM ATP. We also observed a rapid Ca2+ release induced by micromolar Ag+ in the presence of ATP (at 1 nM Ca2+). The Ag+-induced Ca2+ release was totally inhibited by 5 micron ruthenium red. We have also investigated the effect of monovalent ions on the Ca2+ release elicited by Ca2+ or Ag+. We show that the Ca2+ release rate: 1) was dependent upon the presence of K+ or Na+ in the release medium and 2) was influenced by a K+ gradient created across the sarcoplasmic reticulum membrane. These results directly support the idea of the involvement of an influx of K+ (through K+ channels) during the Ca2+ release and allow to reconsider a possible influence of the membrane potential of the sarcoplasmic reticulum on the Ca2+ release.

Modulation of murine erythroleukemic cell differentiation by inhibitors of polyamine biosynthesis

The differentiation of murine erythroleukemic cells induced by hexamethylene bisacetamide is shown to be differently affected by two inhibitors of polyamine biosynthesis. Methyl glyoxal bis(guanyl hydrazone) (inhibitor or S-adenosyl methionine decarboxylase) inhibited this differentiation process. By using a novel experiment protocol the inhibitory effect of this drug on the induced differentiation was dissociated from pleiotropic effects on cell growth. Methyl glyoxal bis(guanyl hydrazone) only inhibited the induced differentiation if present during the first 6 h of culture of the cells with the inducer. No effect on the induced differentiation was observed if the drug was added to the culture medium 6 h after the inducer. alpha-Difluoro methylornithine (inhibitor of ornithine decarboxylase) stimulated the differentiation of these cells. Polyamine analysis demonstrated that alpha-difluoro methylornithine increased the rapidity and the amplitude of the changes in intracellular polyamines associated with this induced differentiation. The presence of methyl glyoxal bis(guanyl hydrazone) during the first 3 h with the inducer was sufficient to produce opposing changes in the intracellular polyamines. These results suggest that changes in either intracellular polyamines or the activities of polyamine biosynthetic enzymes play a regulatory role in the differentiation process induced in murine erythroleukemic cells by hexamethylene bisacetamide.

Catabolites produced by the deacetylation of hexamethylenebisacetamide play a key role in murine erythroleukaemic-cell differentiation

N-Acetyl-1,6-diaminohexane and 1,6-diaminohexane, formed by deacetylation of the inducer hexamethylenebisacetamide (HMBA), are shown to accumulate rapidly inside murine erythroleukaemic cells. The appearance of these molecules preceded the differentiation-associated changes in intracellular polyamines. A quantitative relationship was observed between the accumulation of these molecules and the changes in intracellular polyamines. In the absence of HMBA, exogenous N-acetyl-1,6-diaminohexane was able not only to cause changes in polyamine biosynthesis, but also to induce the complete differentiation process. These results imply that these catabolites of HMBA are directly responsible for the changes in polyamine biosynthesis and probably also for initiating other events regulatory for the differentiation of these cells.

Relationship between ornithine decarboxylase activity and hexamethylene bisacetamide-induced differentiation of murine erythroleukemia cells

A transitory increase in ornithine decarboxylase (ODC) activity is shown not to be a prerequisite for the differentiation induced by hexamethylene bisacetamide (HMBA) in murine erythroleukemic (MEL) cells. On the contrary, conditions are described, where inhibition of the ODC activity with alpha-difluoromethyl ornithine (DFMO) stimulated the induced differentiation. Polyamine analysis demonstrated that a reduction in intracellular putrescine and spermidine occurred in MEL cells before commitment to erythrodifferentiation. The presence of DFMO increased the rapidity and the amplitude of these changes. No effect of dexamethasone on these changes in ODC activity or intracellular polyamines was observed.