A Ca2+-Dependent Mechanism Boosting Glycolysis and OXPHOS by Activating Aralar-Malate-Aspartate Shuttle, upon Neuronal Stimulation

Abstract

Calcium is an important second messenger regulating a bioenergetic response to the workloads triggered by neuronal activation. In embryonic mouse cortical neurons using glucose as only fuel, activation by NMDA elicits a strong workload (ATP demand)-dependent on Na⁺ and Ca²⁺ entry, and stimulates glucose uptake, glycolysis, pyruvate and lactate production, and oxidative phosphorylation (OXPHOS) in a Ca²⁺-dependent way. We find that Ca²⁺ upregulation of glycolysis, pyruvate levels, and respiration, but not glucose uptake, all depend on Aralar/AGC1/Slc25a12, the mitochondrial aspartate-glutamate carrier, component of the malate-aspartate shuttle (MAS). MAS activation increases glycolysis, pyruvate production, and respiration, a process inhibited in the presence of BAPTA-AM, suggesting that the Ca²⁺ binding motifs in Aralar may be involved in the activation. Mitochondrial calcium uniporter (MCU) silencing had no effect, indicating that none of these processes required MCU-dependent mitochondrial Ca²⁺ uptake. The neuronal respiratory response to carbachol was also dependent on Aralar, but not on MCU. We find that mouse cortical neurons are endowed with a constitutive ER-to-mitochondria Ca²⁺ flow maintaining basal cell bioenergetics in which ryanodine receptors, RyR2, rather than InsP₃R, are responsible for Ca²⁺ release, and in which MCU does not participate. The results reveal that, in neurons using glucose, MCU does not participate in OXPHOS regulation under basal or stimulated conditions, while Aralar-MAS appears as the major Ca²⁺-dependent pathway tuning simultaneously glycolysis and OXPHOS to neuronal activation.SIGNIFICANCE STATEMENT Neuronal activation increases cell workload to restore ion gradients altered by activation. Ca²⁺ is involved in matching increased workload with ATP production, but the mechanisms are still unknown. We find that glycolysis, pyruvate production, and neuronal respiration are stimulated on neuronal activation in a Ca²⁺-dependent way, independently of effects of Ca²⁺ as workload inducer. Mitochondrial calcium uniporter (MCU) does not play a relevant role in Ca²⁺ stimulated pyruvate production and oxygen consumption as both are unchanged in MCU silenced neurons. However, Ca²⁺ stimulation is blunt in the absence of Aralar, a Ca²⁺-binding mitochondrial carrier component of Malate-Aspartate Shuttle (MAS). The results suggest that Ca²⁺-regulated Aralar-MAS activation upregulates glycolysis and pyruvate production, which fuels mitochondrial respiration, through regulation of cytosolic NAD⁺/NADH ratio.

Keywords: Aralar/AGC1/Slc25a12; calcium regulation; glycolysis; malate aspartate shuttle; mitochondrial calcium uniporter; neuronal metabolism.