title: The role of mitochondria for Ca2+ refilling of the endoplasmic reticulum.
abstract: Endoplasmic reticulum (ER) Ca2+ refilling is an active process to ensure an appropriate ER Ca2+ content under basal conditions and to maintain or restore ER Ca2+ concentration during/after cell stimulation. The mechanisms to achieve successful ER Ca2+ refilling are multiple and built on a concerted action of processes that provide a suitable reservoir for Ca2+ sequestration into the ER. Despite mitochondria having been found to play an essential role in the maintenance of capacitative Ca2+ entry by buffering subplasmalemmal Ca2+, their contribution to ER Ca2+ refilling was not subjected to detailed analysis so far. Thus, this study was designed to elucidate the involvement of mitochondria in Ca2+ store refilling during and after cell stimulation. ER Ca2+ refilling was found to be accomplished even during continuous inositol 1,4,5-trisphosphate (IP3)-triggered ER Ca2+ release by an agonist. Basically, ER Ca2+ refilling depended on the presence of extracellular Ca2+ as the source and sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) activity. Interestingly, in the presence of an IP3-generating agonist, ER Ca2+ refilling was prevented by the inhibition of trans-mitochondrial Ca2+ flux by CGP 37157 (7-chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one) that precludes the mitochondrial Na+/Ca2+ exchanger as well as by mitochondrial depolarization using a mixture of oligomycin and antimycin A. In contrast, after the removal of the agonist, ER refilling was found to be largely independent of trans-mitochondrial Ca2+ flux. Under these conditions, ER Ca2+ refilling took place even without an associated Ca2+ elevation in the deeper cytosol, thus, indicating that superficial ER domains mimic mitochondrial Ca2+ buffering and efficiently sequester subplasmalemmal Ca2+ and consequently facilitate capacitative Ca2+ entry. Hence, these data point to different contribution of mitochondria in the process of ER Ca2+ refilling based on the presence or absence of IP3, which represents the turning point for the dependence or autonomy of ER Ca2+ refilling from trans-mitochondrial Ca2+ flux.
authors.affiliation: Department of Molecular Biology and Biochemistry, Center of Molecular Medicine, Medical University Graz, 8010 Graz, Austria.
authors.forename: Rolland
authors.initials: R
authors.lastname: Malli
authors.forename: Maud
authors.initials: M
authors.lastname: Frieden
authors.forename: Michael
authors.initials: M
authors.lastname: Trenker
authors.forename: Wolfgang F
authors.initials: WF
authors.lastname: Graier
mesh_terms: D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002478:Cells, Cultured; D002998:Clonazepam / Q000031:analogs & derivatives* / Q000494:pharmacology; D003593:Cytoplasm / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D004533:Egtazic Acid / Q000737:chemistry; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D004730:Endothelium, Vascular / Q000166:cytology; D006632:Histamine / Q000378:metabolism; D006801:Humans; D007091:Image Processing, Computer-Assisted; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D007700:Kinetics; D008564:Membrane Potentials; D008928:Mitochondria / Q000378:metabolism*; D008954:Models, Biological; D010957:Plasmids / Q000378:metabolism; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D013841:Thiazepines / Q000494:pharmacology; D013997:Time Factors; D014162:Transfection; D014471:Umbilical Veins / Q000166:cytology
chemical_list: D013841:Thiazepines; D004533:Egtazic Acid; D002998:Clonazepam; C079032:CGP 37157; D006632:Histamine; D015544:Inositol 1,4,5-Trisphosphate; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D000252:Calcium-Transporting ATPases; D002118:Calcium