Mitochondrial Ca(2+) Processing by a Unit of Mitochondrial Ca(2+) Uniporter and Na(+)/Ca(2+) Exchanger Supports the Neuronal Ca(2+) Influx via Activated Glutamate Receptors

The current study demonstrates that in hippocampal neurons mitochondrial Ca(2+) processing supports Ca(2+) influx via ionotropic glutamate (Glu) receptors. We define mitochondrial Ca(2+) processing as Ca(2+) uptake via mitochondrial Ca(2+) uniporter (MCU) combined with subsequent Ca(2+) release via mitochondrial Na(+)/Ca(2+) exchanger (NCX). Our tool is to measure the Ca(2+) influx rate in primary hippocampal co-cultures, i.e. neurons and astrocytes, by fluorescent digital microscopy, using a Fura-2-quenching method where we add small amounts of Mn(2+) in the superfusion medium. Thus, Ca(2+) influx is measured with Mn(2+) in the bath. Ru360 as inhibitor of mitochondrial Ca(2+) uptake through MCU strongly reduces the rate of Ca(2+) influx in Glu-stimulated primary hippocampal neurons. Similarly, the Ca(2+) influx rate in Glu-stimulated neurons declines after suppression of potential-dependent MCU, when we depolarize mitochondria with rotenone. With inhibition of Ca(2+) release from mitochondria via NCX using CGP-37157 the Ca(2+) influx via N-methyl-D-aspartate (NMDA)- and kainate-sensitive receptors is slowed down. Working jointly as mitochondrial Ca(2+) processing unit, MCU and NCX, apparently sustain the Ca(2+) throughput of activated Glu-sensitive receptors. Our results revise the role frequently attributed to mitochondria in neuronal Ca(2+) homeostasis, where mitochondria function mainly as Ca(2+) buffer, and prevent excessively high cytosolic Ca(2+) concentration increase during neuronal activity. The mechanism to control Ca(2+) influx in neurons, as discovered in this study, highlights mitochondrial Ca(2+) processing as a promising pharmacological target. We discuss this pathway in relation to the endoplasmic reticulum-related mechanisms of Ca(2+) processing.