Enzymatic and metabolic evidence for a region specific mitochondrial dysfunction in brains of murine succinic semialdehyde dehydrogenase deficiency (Aldh5a1−/− mice)

Succinic semialdehyde dehydrogenase deficiency, a rare inherited defect of γ-aminobutyrate (GABA) catabolism, presents with characteristic biochemical abnormalities in the central nervous system (CNS). These include elevated concentrations of GABA, γ-hydroxybutyrate (GHB), succinic semialdehyde (SSA), 4,5-dihydroxyhexanoic acid (DHHA) and alanine as well as decreased concentrations of glutamine. GABA degradation is coupled to Krebs cycle function in mammalian CNS (“GABA shunt”) through succinate and α-ketoglutarate. Accordingly, we hypothesized that disruption of Krebs cycle and respiratory chain function in the CNS is involved in the neuropathogenesis of this disease. For this purpose, we investigated cerebral activities of Krebs cycle and respiratory chain enzymes as well as the glutathione content in Aldh5a1−/− mice, a recently generated mouse model for this disease. In CNS tissue of Aldh5a1−/− mice, we found a significantly decreased glutathione content (hippocampus, cortex) and decreased activities of complexes I–IV (hippocampus) suggesting increased oxidative stress and mitochondrial dysfunction. However, specific activities of Krebs cycle and respiratory chain were not affected by GABA, GHB, SSA, or DHHA (up to 1 mmol/L). Although our results suggest hippocampal and cortical dysfunction in Aldh5a1−/− brain, we found no evidence that accumulating key metabolites of SSADH deficiency directly induce impairment of energy metabolism.