Calcium-sequestering cell organelles: in situ localization, morphological and functional characterization

Caffeine- and ryanodine-sensitive Ca(2+)-induced Ca2+ release from the endoplasmic reticulum in honeybee photoreceptors

Light stimulation of invertebrate microvillar photoreceptors causes a large rapid elevation in Cai, shown previously to modulate the adaptational state of the cells. Cai rises, at least in part, as a result of Ins(1,4,5)P3-induced Ca2+ release from the submicrovillar endoplasmic reticulum (ER). Here, we provide evidence for Ca(2+)-induced Ca2+ release (CICR) in an insect photoreceptor. In situ microphotometric measurements of Ca2+ fluxes across the ER membrane in permeabilized slices of drone bee retina show that (a) caffeine induces Ca2+ release from the ER; (b) caffeine and Ins(1,4,5)P3 open distinct Ca2+ release pathways because only caffeine-induced Ca2+ release is ryanodine sensitive and heparin insensitive, and because caffeine and Ins(1,4,5)P3 have additive effects on the rate of Ca2+ release; (c) Ca2+ itself stimulates release of Ca2+ via a ryanodine-sensitive pathway; and (d) cADPR is ineffective in releasing Ca2+. Microfluorometric intracellular Ca2+ measurements with fluo-3 indicate that caffeine induces a persistent elevation in Cai. Electrophysiological recordings demonstrate that caffeine mimics all aspects of Ca(2+)-mediated facilitation and adaptation in drone photoreceptors. We conclude that the ER in drone photoreceptors contains, in addition to the Ins(1,4,5)P3-sensitive release pathway, a CICR pathway that meets key pharmacological criteria for a ryanodine receptor. Coexpression of both release mechanisms could be required for the production of rapid light-induced Ca2+ elevations, because Ca2+ amplifies its own release through both pathways by a positive feedback. CICR may also mediate the spatial spread of Ca2+ release from the submicrovillar ER toward more remote ER subregions, thereby activating Ca(2+)-sensitive cell processes that are not directly involved in phototransduction.

Release of Ca2+ from intracellular organelles by peptide analogues: evidence against involvement of metalloendoproteases in Ca2+ sequestration by the endoplasmic reticulum

N-Benzyloxycarbonyl-Gly-Phe-amide (Z-Gly-Phe-NH2), a competitive substrate for metalloendoproteases, mobilizes intracellular Ca2+ and suppresses protein synthesis and processing in a Ca(2+)-dependent, reversible manner. To ascertain whether Z-Gly-Phe-NH2 acts as Ca(2+)-storing organelles, effects of the dipeptide on Ca2+ sequestration by saponin-porated GH3 pituitary cells were examined. Porated preparations sequestered Ca2+ into two compartments with different Ca2+ affinities. Ca2+ accumulation at nM concentrations of free Ca2+ was inhibited by thapsigargin and inositol 1,4,5-triphosphate [Ins(1,4,5)P3], enhanced by oxalate and unaffected by oligomycin. Cation accumulation at microM concentrations of free Ca2+ was sensitive to oligomycin but not to thapsigargin. Z-Gly-Phe-NH2 reduced Ca2+ sequestration by both compartments. The dipeptide mobilized Ca2+ from the high-affinity compartment within 1-2 min without affecting Ca2+ uptake. Ca2+ was mobilized more rapidly by Z-Gly-Phe-NH2 and thapsigargin together than by either agent alone. The presence of a thiol-reducing agent was required for Ca2+ mobilization by Z-Gly-Phe-NH2 but not by thapsigargin or Ins(1,4,5)P3. Ca2+ mobilization by Z-Gly-Phe-NH2 could not be attributed to effects on anion-permeability or to actions at Ins(1,4,5)P3 or ryanodine receptors. Results with assorted peptide analogues did not favour suppression of metalloendoprotease activity in the Ca(2+)-mobilizing action of Z-Gly-Phe-NH2. The more hydrophobic analogue Z-L-Tyr-p-nitrophenyl ester was 60-80-fold more potent in mobilizing Ca2+ from intact and porated cells and perturbed the high-affinity Ca(2+)-sequestering compartment selectively. Z-Gly-Phe-NH2 and Z-L-Tyr-p-nitrophenyl ester are proposed to release Ca2+ from the endoplasmic reticulum through an ion pore with affinity for hydrophobic molecules containing internal peptide bonds.

Inhibition of protein synthesis and early protein processing by thapsigargin in cultured cells

Thapsigargin, a tumour-promoting sesquiterpene lactone, selectively inhibits the Ca(2+)-ATPase responsible for Ca2+ accumulation by the endoplasmic reticulum (ER). Mobilization of ER-sequestered Ca2+ to the cytosol and to the extracellular fluid subsequently ensues, with concomitant alteration of cellular functions. Thapsigargin was found to serve as a rapid, potent and efficacious inhibitor of amino acid incorporation in cultured mammalian cells. At concentrations mobilizing cell-associated Ca2+ to the extracellular fluid, thapsigargin provoked extensive inhibition of protein synthesis within 10 min. The inhibition in GH3 pituitary cells involved the synthesis of almost all polypeptides, was not associated with increased cytosolic free Ca2+ concentration ([Ca2+]i), and was not reversed at high extracellular Ca2+. The transient rise in [Ca2+]i triggered by ionomycin was diminished by thapsigargin. Polysomes failed to accumulate in the presence of the drug, indicative of impaired translational initiation. With longer (1-3 h) exposures to thapsigargin, recovery of translational activity was observed accompanied by increased synthesis of the ER protein glucose-regulated stress protein 78 or immunoglobulin heavy-chain binding protein ('GRP78/BiP') and its mRNA. Such inductions were comparable with those observed previously with Ca2+ ionophores which mobilize the cation from all intracellular sequestered sites. Actin mRNA concentrations declined significantly during such treatments. In HepG2 cells processing and secretion of the glycoprotein alpha 1-antitrypsin were rapidly suppressed by thapsigargin. Ca2+ sequestered specifically by the ER is concluded to be essential for optimal protein synthesis and processing. These rapid effects of thapsigargin on mRNA translation, protein processing and gene expression should be considered when evaluating potential mechanisms by which this tumour promoter influences cellular events.

Calreticulin

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Rat brain endoplasmic reticulum calcium pools are anatomically and functionally segregated

Autoradiographic imaging can localize 45Ca2+ selectively accumulated via the Ca2+, Mg2(+)-ATPase into endoplasmic reticulum stores in rat brain slices. 45Ca2+ accumulation is markedly stimulated by oxalate and displays a heterogeneous distribution which resembles the mRNA distribution for a sarcoendoplasmic reticulum Ca2+, Mg2(+)-ATPase. Inositol 1,4,5-triphosphate [I(1,4,5)P3] inhibits 45Ca2+ accumulation selectively into regions corresponding to those enriched in I(1,4,5)P3 receptor-binding sites and in Ca2+, -Mg2(+)-ATPase mRNA. Thus rat brain endoplasmic reticulum calcium stores are anatomically and functionally differentiated.