EMRE Is a Matrix Ca 2+ Sensor that Governs Gatekeeping of the Mitochondrial Ca 2+ Uniporter

Abstract

SUMMARY The mitochondrial uniporter (MCU) is an ion channel that mediates Ca2+ uptake into the matrix to regulate metabolism, cell death and cytoplasmic Ca2+ signaling. Matrix Ca2+ concentration is similar to that in cytoplasm, despite an enormous driving force for entry, but the mechanisms that prevent mitochondrial Ca2+ overload are unclear. Here, we show that MCU channel activity is governed by matrix Ca2+ concentration through EMRE. Deletion or charge neutralization of its matrix-localized acidic carboxyl terminus abolishes matrix Ca2+ inhibition of MCU Ca2+ currents, resulting in MCU channel activation, enhanced mitochondrial Ca2+ uptake and constitutively elevated matrix Ca2+ concentration. EMRE-dependent regulation of MCU channel activity requires intermembrane space-localized MICU1, MICU2 and cytoplasmic Ca2+. Thus, mitochondria are protected from Ca2+ depletion and Ca2+ overload by a unique molecular complex that involves Ca2+ sensors on both sides of the inner mitochondrial membrane, coupled through EMRE.

MICU2 Restricts Spatial Crosstalk between InsP 3 R and MCU Channels by Regulating Threshold and Gain of MICU1-Mediated Inhibition and Activation of MCU

“…It is possible that NCLX-mediated Ca 2+ -extrusion can compensate for uninhibited influx through MCU, or that MCU is inhibited by elevated [Ca 2+ ] m , limiting Ca 2+ uptake. Indeed, [Ca 2+ ] m regulates MCU Ca 2+ currents with a biphasic [Ca 2+ ] m dependence, with a peak-inhibition [Ca 2+ ] m at ~400 nM (Vais et al, 2016), similar to the resting [Ca 2+ ] m measured here in MICU1_KO cells. Measurements of [Ca 2+ ] m may not reflect total matrix Ca 2+ content.…”

“…Since mutation of EF hands did not affect hetero-dimer formation or association with MCU, MICU2 tunes the MICU1-mediated threshold for activation through distinct Ca 2+ -binding properties of MICU1 homo- vs. hetero-dimers. We previously observed loss of gatekeeping in cells with MICU2 deleted (Vais et al, 2016). However, those measurements used a low-affinity Ca 2+ indicator (Fura-2FF; K D = 5.5 μM) that provided inaccurate measurements below 1 μM.…”

Privileged crosstalk between TRPV1 channels and mitochondrial calcium shuttling machinery controls nociception

“…In addition, previous studies indicate that MCU is strongly regulated by Ca 2+ rise in the mitochondrial matrix [39][40][41]. We therefore reasoned that the profound inhibition of cytosolic Ca 2+ signals observed following the knockdown of NCLX is linked to the inhibition of MCU.…”

“…Previous studies have indicated that a sustained elevation of Ca 2+ concentration in the mitochondrial matrix strongly inhibits Ca 2+ influx through the mitochondrial Ca 2+ uniporter [39,40]. Recent studies show that such regulation is mediated by the interaction of MCU and EMRE [41]. Such a mechanism is required for preventing mitochondrial Ca 2+ overload and also attenuates mitochondrial Ca 2+ shuttling.…”

Calcium and ATP control multiple vital functions

“…The subsequent rise in [Ca 2þ ] i in the apical (secretory) region, where the IP 3 receptors are localized, causes immediate uptake of Ca 2þ into the peri-granular mitochondria via the mitochondrial Ca 2þ uniporter in the inner mitochondrial membrane. Recent work suggests that the gating of this pathway is controlled by a Ca 2þ sensor in the mitochondrial matrix [28]. The rise in the intra-mitochondrial [Ca 2þ ] i , in turn, activates the three Ca 2þ -sensitive dehydrogenases in the Krebs cycle, stimulating ATP production [26].…”