Mitochondrial Calcium Uniporter MCU Supports Cytoplasmic Ca ²⁺ Oscillations, Store-Operated Ca ²⁺ Entry and Ca ²⁺-Dependent Gene Expression in Response to Receptor Stimulation

Abstract

Ca ²⁺ flux into mitochondria is an important regulator of cytoplasmic Ca ²⁺ signals, energy production and cell death pathways. Ca ²⁺ uptake can occur through the recently discovered mitochondrial uniporter channel (MCU) but whether the MCU is involved in shaping Ca ²⁺ signals and downstream responses to physiological levels of receptor stimulation is unknown. Here, we show that modest stimulation of leukotriene receptors with the pro-inflammatory signal LTC ₄ evokes a series of cytoplasmic Ca ²⁺ oscillations that are rapidly and faithfully propagated into mitochondrial matrix. Knockdown of MCU or mitochondrial depolarisation, to reduce the driving force for Ca ²⁺ entry into the matrix, prevents the mitochondrial Ca ²⁺ rise and accelerates run down of the oscillations. The loss of cytoplasmic Ca ²⁺ oscillations appeared to be a consequence of enhanced Ca ²⁺-dependent inactivation of InsP ₃ receptors, which arose from the loss of mitochondrial Ca ²⁺ buffering. Ca ²⁺ dependent gene expression in response to leukotriene receptor activation was suppressed following knockdown of the MCU. In addition to buffering Ca ²⁺ release, mitochondria also sequestrated Ca ²⁺ entry through store-operated Ca ²⁺ channels and this too was prevented following loss of MCU. MCU is therefore an important regulator of physiological pulses of cytoplasmic Ca ²⁺.