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Séminaire impromptu - Irene Llorente-FolchRole of Aralar, the brain mitochondrial aspartate-glutamate carrier, in Ca2+ regulation of mitochondrial respiration and glutamate excitotoxicity in neurons

Abstract :

 Calcium is thought to regulate respiration but it is unclear
whether this is dependent on the increase in ATP demand caused by any Ca2+ signal or to Ca2+ itself. [Na+]i, [Ca2+]i and [ATP]i dynamics in intact neurons using glucose and exposed to different workloads in the absence and presence of Ca2+ clearly showed that Ca2+-stimulation of coupled respiration is required to maintain [ATP]i levels. Ca2+ may regulate respiration by activating metabolite transport in mitochondria from outer face of the inner mitochondrial membrane, or after Ca2+ entry in mitochondria through the calcium uniporter (MCU). The Ca2+-regulated mitochondrial aspartate-glutamate exchanger Aralar/AGC1, a component of the malate-aspartate shuttle, is activated by Ca2+ with S0.5 of 300 nM. The lack of ARALAR reduced basal OCR (by 46%) and the Ca2+-dependent responses to all workloads, with a 65-70% reduction in the response to the high workload imposed by veratridine, and complete suppression of the OCR responses to moderate (K+-depolarization) and small (carbachol) workloads, effects reverted by pyruvate supply. For K+-depolarization, this occurs in spite of the presence of large [Ca2+]mit signals and increased reduction of mitochondrial NAD(P)H. These results show that ARALAR-MAS is a major contributor of Ca2+-stimulated respiration in neurons by providing increased pyruvate supply to mitochondria.

The role of ARALAR was also analyzed under glutamate stimulation.
Glutamate excitotoxicity is caused by sustained activation of neuronal NMDA receptors causing a large Ca2+ and Na+ influx, activation of poly(ADP ribose) polymerase-1 (PARP-1), and delayed Ca2+ deregulation. We found that ARALAR-MAS deficiency promotes lower cytosolic Ca2+ signals, limits stimulation of mitochondrial respiration and enhances cytosolic ATP/ADP ratio drop upon glutamate stimulation in cultured neurons using glucose. Moreover, in vivo experiments have shown an increased susceptibility to KA-induced seizures and neuronal damage in adult ARALAR-hemizygous mice. Paradoxically, Aralar-MAS activity is not required to survive against glutamate-mediated excitotoxicity, in vitro. These results agree with the hypothesis that glutamate exposure leads to reduced glucose utilization, a condition which leads to a similar energetic failure comparing wild-type and ARALAR-KO cultured neurons where glucose is the only substrate. However, in vivo, in the presence of alternative substrates like lactate, already described as an important neuronal fuel under ischaemic conditions, ARALAR-MAS activity is essential to overcome the glutamate-induced energy failure.

Selected publications

Mitochondrial ATP-Mg/Pi Carrier SCaMC-3/Slc25a23 Counteracts PARP-1-Dependent Fall in Mitochondrial ATP Caused by Excitotoxic Insults in Neurons.Rueda CB, Traba J, Amigo I, Llorente-Folch I, González-Sánchez P, Pardo B, Esteban JA, Del Arco A, Satrústegui J. J Neurosci. 2015 Feb 25;35(8):3566-81. doi: 10.1523/JNEUROSCI.2702-14.2015.

 Ca(2+) regulation of mitochondrial function in neurons. Rueda CB, Llorente-Folch I, Amigo I, Contreras L, González-Sánchez P, Martínez-Valero P, Juaristi I, Pardo B, del Arco A, Satrústegui J. Biochim Biophys Acta. 2014 Oct;1837(10):1617-24.

Calcium-regulation of mitochondrial respiration maintains ATP homeostasis and requires ARALAR/AGC1-malate aspartate shuttle in intact cortical neurons. Llorente-Folch I, Rueda CB, Amigo I, del Arco A, Saheki T, Pardo B, Satrústegui J. J Neurosci. 2013 Aug 28;33(35):13957-71, 13971a. doi: 10.1523/JNEUROSCI.0929-13.2013.

Irene Llorente-Folch <illorente(at)>