Coupled transmembrane mechanisms control MCU-mediated mitochondrial Ca ²⁺ uptake

Abstract

Ca2+ uptake by mitochondria regulates bioenergetics, apoptosis, and Ca2+ signaling. The primary pathway for mitochondrial Ca2+ uptake is the mitochondrial calcium uniporter (MCU), a Ca2+-selective ion channel in the inner mitochondrial membrane. MCU-mediated Ca2+ uptake is driven by the sizable inner-membrane potential generated by the electron-transport chain. Despite the large thermodynamic driving force, mitochondrial Ca2+ uptake is tightly regulated to maintain low matrix [Ca2+] and prevent opening of the permeability transition pore and cell death, while meeting dynamic cellular energy demands. How this is accomplished is controversial. Here we define a regulatory mechanism of MCU-channel activity in which cytoplasmic Ca2+ regulation of intermembrane space-localized MICU1/2 is controlled by Ca2+-regulatory mechanisms localized across the membrane in the mitochondrial matrix. Ca2+ that permeates through the channel pore regulates Ca2+ affinities of coupled inhibitory and activating sensors in the matrix. Ca2+ binding to the inhibitory sensor within the MCU amino terminus closes the channel despite Ca2+ binding to MICU1/2. Conversely, disruption of the interaction of MICU1/2 with the MCU complex disables matrix Ca2+ regulation of channel activity. Our results demonstrate how Ca2+ influx into mitochondria is tuned by coupled Ca2+-regulatory mechanisms on both sides of the inner mitochondrial membrane.

Mitochondrial Metal Ion Transport in Cell Metabolism and Disease

“…In order to prevent detrimental Ca 2+ overload, the activity of MCU must be tightly regulated [ 91 ]. The MCU-mediated Ca 2+ uptake is driven by the inner membrane potential produced by the electron-transport chain (ETC) [ 92 ]. MCU channels are closed when Ca 2+ is in equilibrium and activated when the concentration of Ca 2+ in cytoplasm elevates [ 93 , 94 ].…”

“…In turn, the mPTP leads to the collapse of the mitochondrial transmembrane potential and induces Ca 2+ to release nonspecifically from the mitochondrial matrix to the cytoplasm, which is called “calcium-induced calcium release”. Thus, the mitochondrial Ca 2+ uptake should be strictly regulated to maintain a low matrix Ca 2+ concentration that meets the dynamic cellular energy requirements and prevents mPTP from opening [ 92 ].…”

Structural characterization of the mitochondrial Ca2+ uniporter provides insights into Ca2+ uptake and regulation

“…The switch between oligomeric and monomeric NTDs reveals a potential negative feedback modulation mechanism in which a high [Ca 2+ ] m inhibits MCU activity owing to the destabilization of self-association ( Lee et al., 2016 ; Yuan et al., 2020 ). Interestingly, the negatively charged patch was subsequently identified as a matrix Ca 2+ inhibition sensor that can even override the MICU1-MICU2-dependent activation of MCU under Ca 2+ -binding conditions when this sensor is occupied ( Vais et al., 2020 ). In contrast, matrix negative feedback modulation strictly depends on the association between MICU1-MICU2 and the channel, demonstrating the coupled Ca 2+ -regulated mechanisms inside and outside the IMM ( Vais et al., 2020 ).…”

“…Interestingly, the negatively charged patch was subsequently identified as a matrix Ca 2+ inhibition sensor that can even override the MICU1-MICU2-dependent activation of MCU under Ca 2+ -binding conditions when this sensor is occupied ( Vais et al., 2020 ). In contrast, matrix negative feedback modulation strictly depends on the association between MICU1-MICU2 and the channel, demonstrating the coupled Ca 2+ -regulated mechanisms inside and outside the IMM ( Vais et al., 2020 ). However, how the NTD contributes to Ca 2+ uptake remains controversial.…”

Mitochondria at Work: New Insights into Regulation and Dysregulation of Cellular Energy Supply and Metabolism

“…Ca 2+ is transported via the mitochondrial calcium uniporter (MCU) of the IMM from the cytoplasm into the mitochondrial matrix and this transport is driven by membrane potential generated by the ETC. Ca 2+ influx through the channel pore is tuned by coupled inhibitory and activating sensors on both sides of the IMM [ 27 ]. This allows fine-tuning of aerobic metabolism.…”