βOHB Protective Pathways in Aralar-KO Neurons and Brain: An Alternative to Ketogenic Diet

Aralar/AGC1/Slc25a12, the mitochondrial aspartate-glutamate carrier expressed in neurons, is the regulatory component of the NADH malate-aspartate shuttle. AGC1 deficiency is a neuropediatric rare disease characterized by hypomyelination, hypotonia, developmental arrest, and epilepsy. We have investigated whether β-hydroxybutyrate (βOHB), the main ketone body (KB) produced in ketogenic diet (KD), is neuroprotective in aralar-knock-out (KO) neurons and mice. We report that βOHB efficiently recovers aralar-KO neurons from deficits in basal-stimulated and glutamate-stimulated respiration, effects requiring βOHB entry into the neuron, and protects from glutamate excitotoxicity. Aralar-deficient mice were fed a KD to investigate its therapeutic potential early in development, but this approach was unfeasible. Therefore, aralar-KO pups were treated without distinction of gender with daily intraperitoneal injections of βOHB during 5 d. This treatment resulted in a recovery of striatal markers of the dopaminergic system including dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC)/DA ratio, and vesicular monoamine transporter 2 (VMAT2) protein. Regarding postnatal myelination, myelin basic protein (MBP) and myelin-associated glycoprotein (MAG) myelin proteins were markedly increased in the cortices of βOHB-treated aralar-KO mice. Although brain Asp and NAA levels did not change by βOHB administration, a 4-d βOHB treatment to aralar-KO, but not to control, neurons led to a substantial increase in Asp (3-fold) and NAA (4-fold) levels. These results suggest that the lack of increase in brain Asp and NAA is possibly because of its active utilization by the aralar-KO brain and the likely involvement of neuronal NAA in postnatal myelination in these mice. The effectiveness of βOHB as a therapeutic treatment in AGC1 deficiency deserves further investigation.SIGNIFICANCE STATEMENT Aralar deficiency induces a fatal phenotype in humans and mice and is associated with impaired neurodevelopment, epilepsy, and hypomyelination. In neurons, highly expressing aralar, its deficiency causes a metabolic blockade hampering mitochondrial energetics and respiration. Here, we find that βOHB, the main metabolic product in KD, recovers defective mitochondrial respiration bypassing the metabolic failure in aralar-deficient neurons. βOHB oxidation in mitochondria boosts the synthesis of cytosolic aspartate (Asp) and NAA, which is impeded by aralar deficiency, presumably through citrate-malate shuttle. In aralar-knock-out (KO) mice, βOHB recovers from the drastic drop in specific dopaminergic and myelin markers. The βOHB-induced myelin synthesis occurring together with the marked increment in neuronal NAA synthesis supports the role of NAA as a lipid precursor during postnatal myelination.

ARALAR/AGC1 deficiency, a neurodevelopmental disorder with severe impairment of neuronal mitochondrial respiration, does not produce a primary increase in brain lactate

ARALAR/AGC1 (aspartate-glutamate mitochondrial carrier 1) is an important component of the NADH malate-aspartate shuttle (MAS). AGC1-deficiency is a rare disease causing global cerebral hypomyelination, developmental arrest, hypotonia, and epilepsy (OMIM ID #612949); the aralar-KO mouse recapitulates the major findings in humans. This study was aimed at understanding the impact of ARALAR-deficiency in brain lactate levels as a biomarker. We report that lactate was equally abundant in wild-type and aralar-KO mouse brain in vivo at postnatal day 17. We find that lactate production upon mitochondrial blockade depends on up-regulation of lactate formation in astrocytes rather than in neurons. However, ARALAR-deficiency decreased cell respiration in neurons, not astrocytes, which maintained unchanged respiration and lactate production. As the primary site of ARALAR-deficiency is neuronal, this explains the lack of accumulation of brain lactate in ARALAR-deficiency in humans and mice. On the other hand, we find that the cytosolic and mitochondrial components of the glycerol phosphate shuttle are present in astrocytes with similar activities. This suggests that glycerol phosphate shuttle is the main NADH shuttle in astrocytes and explains the absence of effects of ARALAR-deficiency in these cells.