Summary Mitochondrial Ca2+ uptake via the uniporter is central to cell metabolism, signaling and survival. Recent studies identified MCU as the uniporter’s likely pore and MICU1, an EF-hand protein, as its critical regulator. How this complex decodes dynamic cytoplasmic [Ca2+] ([Ca2+]c) signals, to tune out small [Ca2+]c increases yet permit pulse transmission, remains unknown. We report that loss of MICU1 in mouse liver and cultured cells causes mitochondrial Ca2+ accumulation during small [Ca2+]c elevations, yet an attenuated response to agonist-induced [Ca2+]c pulses. The latter reflects loss of positive cooperativity, likely via the EF-hands. MICU1 faces the intermembrane space and responds to [Ca2+]c changes. Prolonged MICU1 loss leads to an adaptive increase in matrix Ca2+ binding, yet cells show impaired oxidative metabolism and sensitization to Ca2+ overload. Collectively, the data indicate that MICU1 senses the [Ca2+]c to establish the uniporter’s threshold and gain, thereby allowing mitochondria to properly decode different inputs.

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Exchanges of matrix contents are essential to the maintenance of mitochondria. Cardiac mitochondrial exchange matrix content in two ways: by direct contact with neighboring mitochondria and over longer distances. The latter mode is supported by thin tubular protrusions, called nanotunnels, that contact other mitochondria at relatively long distances. Here, we report that cardiac myocytes of heterozygous mice carrying a catecholaminergic polymorphic ventricular tachycardia-linked RyR2 mutation (A4860G) show a unique and unusual mitochondrial response: a significantly increased frequency of nanotunnel extensions. The mutation induces Ca 2+ imbalance by depressing RyR2 channel activity during excitation-contraction coupling, resulting in random bursts of Ca 2+ release probably due to Ca 2+ overload in the sarcoplasmic reticulum. We took advantage of the increased nanotunnel frequency in RyR2 A4860G+/− cardiomyocytes to investigate and accurately define the ultrastructure of these mitochondrial extensions and to reconstruct the overall 3D distribution of nanotunnels using electron tomography. Additionally, to define the effects of communication via nanotunnels, we evaluated the intermitochondrial exchanges of matrix-targeted soluble fluorescent proteins, mtDsRed and photoactivable mtPA-GFP, in isolated cardiomyocytes by confocal microscopy. A direct comparison between exchanges occurring at short and long distances directly demonstrates that communication via nanotunnels is slower. mitochondria | nanotunnels | CPVT | RyR2 | mitochondrial dynamics

Control of myocardial energetics by Ca2؉ signal propagation to the mitochondrial matrix includes local Ca 2؉ delivery from sarcoplasmic reticulum (SR) ryanodine receptors (RyR2) to the inner mitochondrial membrane (IMM) Ca 2؉ uniporter (mtCU). mtCU activity in cardiac mitochondria is relatively low, whereas the IMM surface is large, due to extensive cristae folding. Hence, stochastically distributed mtCU may not suffice to support local Ca 2؉ transfer. We hypothesized that mtCU concentrated at mitochondria-SR associations would promote the effective Ca 2؉ transfer. mtCU distribution was determined by tracking MCU and EMRE, the proteins essential for channel formation. Both proteins were enriched in the IMM-outer mitochondrial membrane (OMM) contact point submitochondrial fraction and, as super-resolution microscopy revealed, located more to the mitochondrial periphery (inner boundary membrane) than inside the cristae, indicating high accessibility to cytosol-derived Ca 2؉ inputs. Furthermore, MCU immunofluorescence distribution was biased toward the mitochondria-SR interface (RyR2), and this bias was promoted by Ca 2؉ signaling activity in intact cardiomyocytes. The SR fraction of heart homogenate contains mitochondria with extensive SR associations, and these mitochondria are highly enriched in EMRE. Size exclusion chromatography suggested for EMRE-and MCU-containing complexes a wide size range and also revealed MCU-containing complexes devoid of EMRE (thus disabled) in the mitochondrial but not the SR fraction. Functional measurements suggested more effective mtCU-mediated Ca 2؉ uptake activity by the mitochondria of the SR than of the mitochondrial fraction. Thus, mtCU "hot spots" can be formed at the cardiac muscle mitochondria-SR associations via localization and assembly bias, serving local Ca 2؉ signaling and the excitation-energetics coupling.