BIOCHEMICAL AND GENETIC STUDIES OF THE MITOCHONDRIAL ASPARTATE/GLUTAMATE CARRIER AGC1

Background: the SLC25A12 gene encodes the brain isoform of the mitochondrial asp/glu carrier (AGC), a key molecule in the malate/aspartate NADH shuttle and in energy metabolism. SLC25A12 gene variants were found associated with autism (Ramoz et al., 2004).

Objectives: to correlate asp/glu transport rates and SLC25A12 gene variants in postmortem brains of autistic patients and controls.

Methods: temporocortical gray matter (BA 41/42) from six patient-control pairs was used to measure reconstituted AGC1 activity from tissue homogenates or isolated mitochondria; cDNAs were sequenced and a family-based association study was performed on 246 simplex and 9 multiplex families.

Results: AGC transport rates were significantly higher in tissue homogenates from all six autistic patients, including those with no history of seizures and with normal EEGs prior to death. This increase was consistently blunted by the Ca²⁺ chelator EGTA; no difference in AGC transport rates was found in isolated mitochondria from patients and controls; mitochondria from each control showed increased AGC activity when exposed to the post-mitochondrial supernatant of his/her matched patient than to his/her own supernatant. AGC1 expression and cytochrome c oxidase activity were both increased; oxidized mitochondrial proteins were markedly increased in four of the six patients. SLC25A12 gene variants were neither correlated with AGC activation nor associated with autism, while a protective gene variant may be present among unaffected siblings.

Conclusions: excessive Ca²⁺ entry or release from intracellular stores boosts AGC activity, mitochondrial metabolism and, to a more variable degree, oxidative stress in autistic brains. The modulation of AGC and/or Ca²⁺ homeostasis could provide new preventive and therapeutic strategies.