Spatial Separation of Mitochondrial Calcium Uptake and Extrusion for Energy-Efficient Mitochondrial Calcium Signaling in the Heart

Mitochondrial Ca2+ elevations enhance ATP production, but uptake must be balanced by efflux to avoid overload. Uptake is mediated by the mitochondrial Ca2+ uniporter channel complex (MCUC), and extrusion is controlled largely by the Na+/Ca2+ exchanger (NCLX), both driven electrogenically by the inner membrane potential (ΔΨm). MCUC forms hotspots at the cardiac mitochondria-junctional SR (jSR) association to locally receive Ca2+ signals; however, the distribution of NCLX is unknown. Our fractionation-based assays reveal that extensively jSR-associated mitochondrial segments contain a minor portion of NCLX and lack Na+-dependent Ca2+ extrusion. This pattern is retained upon in vivo NCLX overexpression, suggesting extensive targeting to non-jSR-associated submitochondrial domains and functional relevance. In cells with non-polarized MCUC distribution, upon NCLX overexpression the same given increase in matrix Ca2+ expends more ΔΨm. Thus, cardiac mitochondrial Ca2+ uptake and extrusion are reciprocally polarized, likely to optimize the energy efficiency of local calcium signaling in the beating heart. [Display omitted] •MCU complex (MCUC) and NCLX are spatially separated in heart mitochondria•Mitochondrial contact with SR Ca2+ signal source is an MCUC hotspot but lacks NCLX•This distribution minimizes the energy cost of maintaining mitochondrial Ca2+ signals•It also optimizes mitochondrial Ca2+ signal generation efficacy from Ca2+ nanodomains Calcium signals control mitochondrial fuel generation. De La Fuente et al. report that in heart mitochondria, calcium uptake and extrusion are spatially separated; the most calcium-exposed area is an uptake hotspot, but it lacks extrusion to optimize signaling efficiency and minimize the energy expense of controlling mitochondrial function by calcium.