The mitochondrial aspartate-glutamate carrier Aralar/AGC1 controls neuronal respiration and (beta)-Hydroxybutyrate rescues brain defects caused by AGC1 deficiency

  1. Pérez Liébana, Irene
Supervised by:
  1. Jorgina Satrústegui Gil-Delgado Director
  2. Beatriz Pardo Merino Director

Defence university: Universidad Autónoma de Madrid

Fecha de defensa: 30 June 2020

Committee:
  1. Mayte Montero Chair
  2. Alfredo Giménez-Cassina Sendón Secretary
  3. María Ángeles Almeida Parra Committee member

Type: Thesis

Abstract

Regulation of neuronal respiration has been traditionally attributed to mitochondrial Ca2+ uniporter (MCU), as it allows Ca2+ activation of matrix dehydrogenases and TCA cycle. However, Ca2+-regulated mitochondrial carriers (CaMCs) have revealed an important role in stimulating neuronal respiration in response to agents evoking of small cytosolic Ca2+ signals. Aralar/AGC1/Slc25a12 is the aspartate-glutamate carrier expressed in neurons, and the regulatory step in the malate-aspartate NADH shuttle (MAS). This study demonstrates that Ca2+ activation of Aralar, rather than MCU/mitochondrial Ca2+ (Ca2+- mit), controls the upregulation of respiration in response to NMDA in neurons using glucose. Ca2+ activation of Aralar-MAS is necessary to increase endogenous pyruvate synthesis, activating its transport into mitochondria and upregulating respiration and glycolysis, neither of which occurring in the absence of Ca2+. In addition, this study shows that following NMDA stimulation, neurons upregulate glucose uptake in a Ca2+-dependent and Aralar-independent way. Surprisingly, Mcu silencing not only fails to decrease NMDAstimulated respiration but in fact has a potentiator effect, probably due to suppression of the toxic effect of mitochondrial Ca2+. Aralar/AGC1 deficiency causes a neurodevelopmental rare disease characterized by growth retardation, epilepsy and secondary postnatal hypomyelination. Aralar-KO mice have reduced life expectancy and altered motor coordination with severe affectation of striatum. In human patients, ketogenic diet (KD) alleviates epilepsy and partially improves myelination. This study reveals that the main ketone body synthesized during treatment with KD, β-hydroxybutyrate (βOHB), reports beneficial effects in Aralar/AGC1 deficiency. βOHB rescues impaired mitochondrial respiration on glucose in aralar-KO primary cortical neurons, and also protects them from glutamate-induced excitotoxicity. In vivo, βOHB administration partially rescues striatal dopaminergic system in aralar-KO mice, increasing the levels of neuronal markers as dopamine, VMAT2 and DARPP-32. βOHB administration also increases the synthesis of myelin proteins, but not aspartate and NAA levels. However βOHB supply to aralar-KO neurons resulted in increased synthesis of aspartate and NAA, suggesting their increased use in the aralar-KO brain and reinforcing neuronal NAA as an essential precursor for myelin lipid synthesis in oligodendrocytes. In spite of the limitations imposed by the strong phenotype of aralar- KO mice, this study demonstrates that βOHB is an effective alternative substrate that bypasses Aralar rescuing the main hallmarks of Aralar/AGC1 deficiency. A deeper study of the neuroprotective roles of βOHB, also acting through signaling pathways, would be very interesting with the aim of finding an effective therapy that substitutes KD in human patients