Cytosolic Calcium Coordinates Mitochondrial Energy Metabolism with Presynaptic Activity

Abstract

Most neurons fire in bursts, imposing episodic energy demands, but how these demands are coordinated with oxidative phosphorylation is still unknown. Here, using fluorescence imaging techniques on presynaptic termini of Drosophila motor neurons (MNs), we show that mitochondrial matrix pH (pHm), inner membrane potential (Δψm), and NAD(P)H levels ([NAD(P)H]m) increase within seconds of nerve stimulation. The elevations of pHm, Δψm, and [NAD(P)H]m indicate an increased capacity for ATP production. Elevations in pHm were blocked by manipulations which blocked mitochondrial Ca2+ uptake, including replacement of extracellular Ca2+ with Sr2+, and application of either tetraphenylphosphonium chloride or KB-R7943, indicating that it is Ca2+ that stimulates presynaptic mitochondrial energy metabolism. To place this phenomenon within the context of endogenous neuronal activity, the firing rates of a number of individually identified MNs were determined during fictive locomotion. Surprisingly, although endogenous firing rates are significantly different, there was little difference in presynaptic cytosolic Ca2+ levels ([Ca2+]c) between MNs when each fires at its endogenous rate. The average [Ca2+]c level (329±11nM) was slightly above the average Ca2+ affinity of the mitochondria (281±13nM). In summary, we show that when MNs fire at endogenous rates [Ca2+]c is driven into a range where mitochondria rapidly acquire Ca2+. As we also show that Ca2+ stimulates presynaptic mitochondrial energy metabolism, we conclude that [Ca2+]c levels play an integral role in coordinating mitochondrial energy metabolism with presynaptic activity in Drosophila MNs.

Physiology-Based Kinetic Modeling of Neuronal Energy Metabolism Unravels the Molecular Basis of NAD(P)H Fluorescence Transients

“…In case of physiologic extracellular calcium concentration, the cytosolic calcium transient was calibrated to a peak value of 0.4 μmol/L according to measurements. 28,29 To account for a lack of calcium in the extracellular space, the peak value of the cytosolic calcium transient was lowered to a peak maximum of 0.2 μmol/L according to findings in Llinas et al 30 The mitochondrial NADH responses obtained with normal and low cytosolic calcium concentrations are shown in Figure 3. In both cases, the NADH response showed an initial dip, but the overshoot was significantly higher at normal extracellular calcium concentration.…”

Cell death disguised: The mitochondrial permeability transition pore as the c-subunit of the F1FO ATP synthase

“…The discovery of the molecular structure for the Ca 2+ uniporter ion channel (MCU) at the mitochondrial inner membrane has generated increasing interest in mechanisms of Ca 2+ management within the cell body of many types of cells and also in the presynaptic terminals of neurons [10–13]. Additional isoforms of MCU and its helper MICU that confer tissue specificity and other behaviors have added to our understanding of the mechanisms of activity dependent energy production by mitochondria [14,15].…”

Widespread depolarization during expiration: A source of respiratory drive?

“…Ca 2+ signaling coordinates mitochondrial ATP production with cellular work so that energy conversion and utilization are equal [54]. Rate of respiration is directly proportional to the level of work [55,56].…”