Mitochondria Recycle Ca2+ to the Endoplasmic Reticulum and Prevent the Depletion of Neighboring Endoplasmic Reticulum Regions

Abstract

To study Ca2+ fluxes between mitochondria and the endoplasmic reticulum (ER), we used "cameleon" indicators targeted to the cytosol, the ER lumen, and the mitochondrial matrix. High affinity mitochondrial probes saturated in ∼20% of mitochondria during histamine stimulation of HeLa cells, whereas a low affinity probe reported averaged peak values of 106 ± 5 μm, indicating that Ca2+ transients reach high levels in a fraction of mitochondria. In concurrent ER measurements, [Ca 2+]ER averaged 371 ± 21 μm at rest and decreased to 133 ± 14 μm and 59 ± 5 μm upon stimulation with histamine and thapsigargin, respectively, indicating that substantial ER refilling occur during agonist stimulation. A larger ER depletion was observed when mitochondrial Ca2+ uptake was prevented by oligomycin and rotenone or when Ca2+ efflux from mitochondria was blocked by CGP 37157, indicating that some of the Ca2+ taken up by mitochondria is re-used for ER refilling. Accordingly, ER regions close to mitochondria released less Ca2+ than ER regions lacking mitochondria. The ER heterogeneity was abolished by thapsigargin, oligomycin/rotenone, or CGP 37157, indicating that mitochondrial Ca2+ uptake locally modulate ER refilling. These observations indicate that some mitochondria are very close to the sites of Ca2+ release and recycle a substantial portion of the captured Ca2+ back to vicinal ER domains. The distance between the two organelles thus determines both the amplitude of mitochondrial Ca 2+ signals and the filling state of neighboring ER regions.