Glutamate excitotoxicity and Ca2+-regulation of respiration: Role of the Ca2+ activated mitochondrial transporters (CaMCs)

Abstract

Glutamate elicits Ca2+ signals and workloads that regulate neuronal fate both in physiological and pathological circumstances. Oxidative phosphorylation is required in order to respond to the metabolic challenge caused by glutamate. In response to physiological glutamate signals, cytosolic Ca2+ activates respiration by stimulation of the NADH malate–aspartate shuttle through Ca2+-binding to the mitochondrial aspartate/glutamate carrier (Aralar/AGC1/Slc25a12), and by stimulation of adenine nucleotide uptake through Ca2+ binding to the mitochondrial ATP-Mg/Pi carrier (SCaMC-3/Slc25a23). In addition, after Ca2+ entry into the matrix through the mitochondrial Ca2+ uniporter (MCU), it activates mitochondrial dehydrogenases. In response to pathological glutamate stimulation during excitotoxicity, Ca2+ overload, reactive oxygen species (ROS), mitochondrial dysfunction and delayed Ca2+ deregulation (DCD) lead to neuronal death. Glutamate-induced respiratory stimulation is rapidly inactivated through a mechanism involving Poly (ADP-ribose) Polymerase-1 (PARP-1) activation, consumption of cytosolic NAD+, a decrease in matrix ATP and restricted substrate supply. Glutamate-induced Ca2+-activation of SCaMC-3 imports adenine nucleotides into mitochondria, counteracting the depletion of matrix ATP and the impaired respiration, while Aralar-dependent lactate metabolism prevents substrate exhaustion. A second mechanism induced by excitotoxic glutamate is permeability transition pore (PTP) opening, which critically depends on ROS production and matrix Ca2+ entry through the MCU. By increasing matrix content of adenine nucleotides, SCaMC-3 activity protects against glutamate-induced PTP opening and lowers matrix free Ca2+, resulting in protracted appearance of DCD and protection against excitotoxicity in vitro and in vivo, while the lack of lactate protection during in vivo excitotoxicity explains increased vulnerability to kainite-induced toxicity in Aralar +/− mice. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.