Low levels of citrin (SLC25A13) expression in adult mouse brain restricted to neuronal clusters

Abstract

The mitochondrial aspartate-glutamate carriers (AGC) aralar (SLC25A12) and citrin (SLC25A13) are components of the malate aspartate shuttle (MAS), a major intracellular pathway to transfer reducing equivalents from NADH to the mitochondrial matrix. Aralar is the main AGC isoform present in the adult brain, and it is expressed mainly in neurons. To search for the other AGC isoform, citrin, in brain glial cells, we used a citrin knockout mouse in which the lacZ gene was inserted into the citrin locus as reporter gene. In agreement with the low citrin levels known to be present in the adult mouse brain, beta-galactosidase expression was very low. Surprisingly, unlike the case with astroglial cultures that express citrin, no beta-galactosidase was found in brain glial cells. It was confined to neuronal cells within discrete neuronal clusters. Double-immunolabelling experiments showed that beta-galactosidase colocalized not with glial cell markers but with the pan-neuronal marker NeuN. The deep cerebellar nuclei and a few midbrain nuclei (reticular tegmental pontine nuclei; magnocellular red nuclei) were the regions where beta-galactosidase expression was highest, and it was up-regulated in fasted mice, as was also the case for liver beta-galactosidase. The results support the notion that glial cells have much lower AGC levels and MAS activity than neurons.

Fatty Acids in Energy Metabolism of the Central Nervous System

“…Based on a series of studies, Dienel and Hertz suggested that (i) the activity of oxidative pathways increases in working astrocytes in vivo and in vitro , (ii) oxidative pathways produce two to three times more ATP than glycolytic lactate production during exposure of cultured astrocytes to 100 mM glutamate, and (iii) glutamate accumulated in the astrocyte can enter the TCA cycle and serve as a substrate for mitochondrial ATP production [28]. The latter hypothesis was quickly abandoned because it was discovered that, in astrocytic mitochondria, the content of the glutamate-aspartate transporter (GAT) is very low [45, 46]. Dienel and Cruz have noted that the energetic demands of activated astrocytes were higher and more complex than recognized [43].…”

“…The “contamination” with the astroglial mitochondria is, more likely, negligible because they showed high rates of respiration with glutamate and pyruvate. As we have mentioned above, astrocytic mitochondria have low expression of glutamate-aspartate transporter [46, 118] and low activity of PDHC [103]. Electron microscopic study has shown that, purified in the Percoll gradient, brain and spinal cord mitochondria were not contaminated by other organelles [16].…”

The Mitochondrial Aspartate/Glutamate Carrier AGC1 and Calcium Homeostasis: Physiological Links and Abnormalities in Autism

“…AGC1 is the main AGC isoform present in the adult brain, and it is expressed mainly in neurons. [14,15]. The N-terminal portion of its predicted 678-amino acid sequence contains four EF-hand Ca 2+ -binding domains, which were conclusively shown to bind Ca 2+ in vitro and in vivo [16,17].…”

Fluctuations in Cytosolic Calcium Regulate the Neuronal Malate–Aspartate NADH Shuttle: Implications for Neuronal Energy Metabolism

“…It is composed of two sets of enzymes, malate dehydrogenase and aspartate aminotransferase each with mitochondrial and cytosolic localization, and two mitochondrial carriers, that of oxoglutarate/malate, OGC/Slc25a11 and those of aspartate/ glutamate, the AGCs. The AGCs have two isoforms in mammals, AGC1/aralar/Slc25a12 and AGC2/citrin/Slc25a13 [1], and AGC1 is the main brain isoform [2][3][4] with only marginal expression of AGC2 in a few brain nuclei [5]. The AGCs belong to a family of Ca 2?…”