Molecular and functional identification of a mitochondrial ryanodine receptor in neurons

Mitochondrial Ca²⁺ controls numerous cell functions, such as energy metabolism, reactive oxygen species generation, spatiotemporal dynamics of Ca²⁺ signaling, cell growth and death in various cell types including neurons. Mitochondrial Ca²⁺ accumulation is mainly mediated by the mitochondrial Ca²⁺ uniporter (MCU), but recent reports also indicate that mitochondrial Ca²⁺-influx mechanisms are regulated not only by MCU, but also by multiple channels/transporters. We previously reported that ryanodine receptor (RyR), which is a one of the main Ca²⁺-release channels at endoplasmic/sarcoplasmic reticulum (SR/ER) in excitable cells, is expressed at the mitochondrial inner membrane (IMM) and serves as a part of the Ca²⁺ uptake mechanism in cardiomyocytes. Although RyR is also expressed in neuronal cells and works as a Ca²⁺-release channel at ER, it has not been well investigated whether neuronal mitochondria possess RyR and, if so, whether this mitochondrial RyR has physiological functions in neuronal cells. Here we show that neuronal mitochondria express RyR at IMM and accumulate Ca²⁺ through this channel in response to cytosolic Ca²⁺ elevation, which is similar to what we observed in another excitable cell-type, cardiomyocytes. In addition, the RyR blockers dantrolene or ryanodine significantly inhibits mitochondrial Ca²⁺ uptake in permeabilized striatal neurons. Taken together, we identify RyR as an additional mitochondrial Ca²⁺ uptake mechanism in response to the elevation of [Ca²⁺]_c in neurons, suggesting that this channel may play a critical role in mitochondrial Ca²⁺-mediated functions such as energy metabolism.