l-Lactate-Mediated Neuroprotection against Glutamate-Induced Excitotoxicity Requires ARALAR/AGC1

Abstract

ARALAR/AGC1/Slc25a12, the aspartate-glutamate carrier from brain mitochondria, is the regulatory step in the malate-aspartate NADH shuttle, MAS. MAS is used to oxidize cytosolic NADH in mitochondria, a process required to maintain oxidative glucose utilization. The role of ARALAR was analyzed in two paradigms of glutamate-induced excitotoxicity in cortical neurons: glucose deprivation and acute glutamate stimulation. ARALAR deficiency did not aggravate glutamate-induced neuronal death in vitro, although glutamate-stimulated respiration was impaired. In contrast, the presence of L-lactate as an additional source protected against glutamate-induced neuronal death in control, but not ARALAR-deficient neurons. L-Lactate supplementation increased glutamate-stimulated respiration partially prevented the decrease in the cytosolic ATP/ADP ratio induced by glutamate and substantially diminished mitochondrial accumulation of 8-oxoguanosine, a marker of reactive oxygen species production, only in the presence, but not the absence, of ARALAR. In addition, L-lactate potentiated glutamate-induced increase in cytosolic Ca 2ϩ , in a way independent of the presence of ARALAR. Interestingly, in vivo, the loss of half-a-dose of ARALAR in aralar ϩ/Ϫ mice enhanced kainic acid-induced seizures and neuronal damage with respect to control animals, in a model of excitotoxicity in which increased L-lactate levels and L-lactate consumption have been previously proven. These results suggest that, in vivo, an inefficient operation of the shuttle in the aralar hemizygous mice prevents the protective role of L-lactate on glutamate excitotoxiciy and that the entry and oxidation of L-lactate through ARALAR-MAS pathway is required for its neuroprotective function.

Extracellular ATP and glutamate drive pyruvate production and energy demand to regulate mitochondrial respiration in astrocytes

“…As the increase in [Na + ] I entails a much larger workload than that of [Ca 2+ ] I (Llorente-Folch et al, 2013), these results suggest that a Ca 2+ -regulatory boost of mitochondrial respiration in the presence of calcium compensates for the decreased workload in these conditions as compared with a Ca 2+ -free medium. The players of such Ca 2+ -regulatory boost may be located in mitochondria, but the participation of the Ca 2+ regulated mitochondrial transporters of ATP-Mg/Pi SCaMC-3/Slc25a23 Rueda et al, 2015) or Aralar/AGC1/Slc2512 (Juaristi et al, 2017;Llorente-Folch et al, 2013;Llorente-Folch, Rueda, Pérez-Liébana, Satrústegui, & Pardo 2016) in this Ca 2+ -boost has been ruled out. Further work will be required to clarify the mitochondrial players in this boost.…”

Brain microvasculature defects and Glut1 deficiency syndrome averted by early repletion of the glucose transporter-1 protein

“…The OCR and ECAR was measured using a Seahorse XFe24 Extracellular Flux Analyzer (Seahorse Bioscience)34. After a baseline measurement, mitochondrial function and glycolytic flux was determined as previously described66. Oxygen consumption and glycolytic rates in astrocytes were determined through sequential addition of 6 μM oligomycin, 0.5 mM 2,4-dinitrophenol, and 1 μM antimycin per 1 μM rotenone.…”

Metabolic Vulnerability in the Neurodegenerative Disease Glaucoma

“…In mammals, more lactate could feed the production of glucose through gluconeogenesis, though this is an energy-expending process. L-lactate can protect cortical neurons from excitotoxic cell death in vitro (Llorente-Folch et al, 2016); this effect depends upon the aspartate-glutamate carrier in CNS mitochondria, ARALAR/AGC. This carrier is one half of the malate-aspartate shuttle that moves malate/aspartate and α-ketoglutarate/glutamate between the cytoplasm and the mitochondria in order to regenerate NAD+ from NADH in both compartments.…”

“…This carrier is one half of the malate-aspartate shuttle that moves malate/aspartate and α-ketoglutarate/glutamate between the cytoplasm and the mitochondria in order to regenerate NAD+ from NADH in both compartments. Neuronal utilization of L-lactate depends on the this pathway (Llorente-Folch et al, 2016), underscoring the importance of NAD+ availability to managing the effects of too much NMDA receptor activation. This is one potential mechanism of ketone body neural protection that is separate from HDAC inhibition.…”