Quasi-synaptic calcium signal transmission between endoplasmic reticulum and mitochondria.

Transmission of cytosolic Ca2+] (Ca2+]c) oscillations into the mitochondrial matrix is thought to be supported by local calcium control between IP3 receptor Ca2+ channels (IP3R) and mitochondria, but study of the coupling mechanisms has been difficult. We established a permeabilized cell model in which the Ca2+ coupling between endoplasmic reticulum (ER) and mitochondria is retained, and mitochondrial Ca2+] (Ca2+]m) can be monitored by fluorescence imaging. We demonstrate that maximal activation of mitochondrial Ca2+ uptake is evoked by IP3-induced perimitochondrial Ca2+] elevations, which appear to reach values >20-fold higher than the global increases of Ca2+]c. Incremental doses of IP3 elicited Ca2+]m elevations that followed the quantal pattern of Ca2+ mobilization, even at the level of individual mitochondria. In contrast, gradual increases of IP3 evoked relatively small Ca2+]m responses despite eliciting similar Ca2+]c increases. We conclude that each mitochondrial Ca2+ uptake site faces multiple IP3R, a concurrent activation of which is required for optimal activation of mitochondrial Ca2+ uptake. This architecture explains why calcium oscillations evoked by synchronized periodic activation of IP3R are particularly effective in establishing dynamic control over mitochondrial metabolism. Furthermore, our data reveal fundamental functional similarities between ER-mitochondrial Ca2+ coupling and synaptic transmission.