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Overexpression of CALNUC (Nucleobindin) Increases Agonist and Thapsigargin Releasable Ca2+ Storage in the Golgi

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      Abstract

      We previously demonstrated that CALNUC, a Ca2+-binding protein with two EF-hands, is the major Ca2+-binding protein in the Golgi by 45Ca2+ overlay (Lin, P., H. Le-Niculescu, R. Hofmeister, J.M. McCaffery, M. Jin, H. Henneman, T. McQuistan, L. De Vries, and M. Farquhar. 1998. J. Cell Biol. 141:1515–1527). In this study we investigated CALNUC's properties and the Golgi Ca2+ storage pool in vivo. CALNUC was found to be a highly abundant Golgi protein (3.8 μg CALNUC/mg Golgi protein, 2.5 × 105 CALNUC molecules/NRK cell) and to have a single high affinity, low capacity Ca2+-binding site (Kd = 6.6 μM, binding capacity = 1.1 μmol Ca2+/μmol CALNUC). 45Ca2+ storage was increased by 2.5- and 3-fold, respectively, in HeLa cells transiently overexpressing CALNUC-GFP and in EcR-CHO cells stably overexpressing CALNUC. Deletion of the first EF-hand α helix from CALNUC completely abolished its Ca2+-binding capability. CALNUC was correctly targeted to the Golgi in transfected cells as it colocalized and cosedimented with the Golgi marker, α-mannosidase II (Man II). Approximately 70% of the 45Ca2+ taken up by HeLa and CHO cells overexpressing CALNUC was released by treatment with thapsigargin, a sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) (Ca2+ pump) blocker. Stimulation of transfected cells with the agonist ATP or IP3 alone (permeabilized cells) also resulted in a significant increase in Ca2+ release from Golgi stores. By immunofluorescence, the IP3 receptor type 1 (IP3R-1) was distributed over the endoplasmic reticulum and codistributed with CALNUC in the Golgi. These results provide direct evidence that CALNUC binds Ca2+ in vivo and together with SERCA and IP3R is involved in establishment of the agonist-mobilizable Golgi Ca2+ store.

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      Thapsigargin, a tumor promoter, discharges intracellular Ca2+ stores by specific inhibition of the endoplasmic reticulum Ca2(+)-ATPase.

      Thapsigargin, a tumor-promoting sesquiterpene lactone, discharges intracellular Ca2+ in rat hepatocytes, as it does in many vertebrate cell types. It appears to act intracellularly, as incubation of isolated rat liver microsomes with thapsigargin induces a rapid, dose-dependent release of stored Ca2+. The thapsigargin-releasable pool of microsomal Ca2+ includes the pools sensitive to inositol 1,4,5-trisphosphate and GTP. Thapsigargin pretreatment of microsomes blocks subsequent loading with 45Ca2+, suggesting that its target is the ATP-dependent Ca2+ pump of endoplasmic reticulum. This hypothesis is strongly supported by the demonstration that thapsigargin causes a rapid inhibition of the Ca2(+)-activated ATPase activity of rat liver microsomes, with an identical dose dependence to that seen in whole cell or isolated microsome Ca2+ discharge. The inhibition of the endoplasmic reticulum isoform of the Ca2(+)-ATPase is highly selective, as thapsigargin has little or no effect on the Ca2(+)-ATPases of hepatocyte or erythrocyte plasma membrane or of cardiac or skeletal muscle sarcoplasmic reticulum. These results suggest that thapsigargin increases the concentration of cytosolic free Ca2+ in sensitive cells by an acute and highly specific arrest of the endoplasmic reticulum Ca2+ pump, followed by a rapid Ca2+ leak from at least two pharmacologically distinct Ca2+ stores. The implications of this mechanism of action for the application of thapsigargin in the analysis of Ca2+ homeostasis and possible forms of Ca2+ control are discussed.
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        Measurement of cytosolic, mitochondrial, and Golgi pH in single living cells with green fluorescent proteins.

        Many cellular events depend on a tightly compartmentalized distribution of H+ ions across membrane-bound organelles. However, measurements of organelle pH in living cells have been scarce. Several mutants of the Aequorea victoria green fluorescent protein (GFP) displayed a pH-dependent absorbance and fluorescent emission, with apparent pKa values ranging from 6.15 (mutations F64L/S65T/H231L) and 6.4 (K26R/F64L/S65T/Y66W/N146I/M153T/ V163A/N164H/H231L) to a remarkable 7.1 (S65G/S72A/T203Y/H231L). We have targeted these GFPs to the cytosol plus nucleus, the medial/trans-Golgi by fusion with galactosyltransferase, and the mitochondrial matrix by using the targeting signal from subunit IV of cytochrome c oxidase. Cells in culture transfected with these cDNAs displayed the expected subcellular localization by light and electron microscopy and reported local pH that was calibrated in situ with ionophores. We monitored cytosolic and nuclear pH of HeLa cells, and mitochondrial matrix pH in HeLa cells and in rat neonatal cardiomyocytes. The pH of the medial/trans-Golgi was measured at steady-state (calibrated to be 6.58 in HeLa cells) and after various manipulations. These demonstrated that the Golgi membrane in intact cells is relatively permeable to H+, and that Cl- serves as a counter-ion for H+ transport and likely helps to maintain electroneutrality. The amenability to engineer GFPs to specific subcellular locations or tissue targets using gene fusion and transfer techniques should allow us to examine pH at sites previously inaccessible.
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          Store depletion and calcium influx.

          Calcium influx in nonexcitable cells regulates such diverse processes as exocytosis, contraction, enzyme control, gene regulation, cell proliferation, and apoptosis. The dominant Ca2+ entry pathway in these cells is the store-operated one, in which Ca2+ entry is governed by the Ca2+ content of the agonist-sensitive intracellular Ca2+ stores. Only recently has a Ca2+ current been described that is activated by store depletion. The properties of this new current, called Ca2+ release-activated Ca2+ current (ICRAC), have been investigated in detail using the patch-clamp technique. Despite intense research, the nature of the signal that couples Ca2+ store content to the Ca2+ channels in the plasma membrane has remained elusive. Although ICRAC appears to be the most effective and widespread influx pathway, other store-operated currents have also been observed. Although the Ca2+ release-activated Ca2+ channel has not yet been cloned, evidence continues to accumulate that the Drosophila trp gene might encode a store-operated Ca2+ channel. In this review, we describe the historical development of the field of Ca2+ signaling and the discovery of store-operated Ca2+ currents. We focus on the electrophysiological properties of the prototype store-operated current ICRAC, discuss the regulatory mechanisms that control it, and finally consider recent advances toward the identification of molecular mechanisms involved in this ubiquitous and important Ca2+ entry pathway.
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            Author and article information

            Affiliations
            [* ]Division of Cellular and Molecular Medicine, []Department of Pathology, []Department of Pharmacology, and [§ ]Howard Hughes Medical Institute, University of California San Diego, La Jolla, California 92093-0651
            Author notes

            Address correspondence to Marilyn G. Farquhar, Ph.D., Division of Cellular and Molecular Medicine, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0651. Tel.: (619) 534-7711. Fax: (619) 534-8549. E-mail: mfarquhar@ 123456ucsd.edu

            Journal
            J Cell Biol
            The Journal of Cell Biology
            The Rockefeller University Press
            0021-9525
            1540-8140
            19 April 1999
            : 145
            : 2
            : 279-289
            2133108
            10209024
            Categories
            Regular Articles

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