NITRATION AND INACTIVATION OF MANGANESE SUPEROXIDE DISMUTASE PLAYS A CRITICAL ROLE IN METABOLIC SWITCH

Alzheimer’s disease (AD) is a multifactorial, progressive, age-related neurodegenerative disease. Oxidative stress hypothesis is most prevalent and is gaining significant support. Inspite of the progress achieved on oxidative stress related damages in AD brain; the modification occurring on the various cellular antioxidant enzymes antioxidant has not been identified. Tyrosine nitration, a marker for peroxynitrite induced oxidative damage to protein is widespread in AD brain and Manganese superoxide dismutase (MnSOD), primary mitochondrial antioxidant enzyme is prone to peroxynitrite induced nitration and inactivation. Nitration of proteins involved in energy metabolism has been demonstrated in AD brain, which may explain the altered glucose metabolic status existing in AD brain. In the present study, we investigated the effect of tyrosine nitration of MnSOD on energy metabolism by the use of AD mouse model and cultured neuronal cells. The AD mouse model was generated from a double homozygous knock-in mouse, designated as APP/PS-1 mice, by incorporating the Swedish familial AD mutations in APP and P264L familial AD mutation in PS – 1. These animals develop age dependent increase in Aβ deposition beginning at 6 months along with an increase in insoluble Aβ1-40/Aβ1-42 levels. Genotype and age associated increase in nitration of MnSOD without any change in protein levels was also observed. MnSOD activity and mitochondrial respiration was decreased in APP/PS-1 mice. There was also concomitant increase in levels of lactate, an index of glycolytic activity in APP/PS-1 mice. To directly investigate the role of MnSOD inactivation in mitochondrial function and subsequent alteration in glycolytic activity, SH-SY5Y neuroblastoma cells line was used and treated with peroxynitrite. Enhanced nitration and reduction in the activity of MnSOD was observed upon peroxynitrite treatment. Peroxynitrite treatment also induced mitochondrial dysfunction, but MnSOD was inactivated at a concentration of peroxynitrite 10 times lower than that required to inhibit mitochondrial respiration. Mitochondrial dysfunction was alleviated by SOD mimetic and reproduced by MnSOD siRNA. The decline in mitochondrial function did not result in decreased ATP levels but was accompanied by an up-regulated glycolysis signified by high levels of lactate and lactate dehydrogenase activity but decreased activity of pyruvate dehydrogenase. These changes were prevented by SOD mimetic and were promoted by MnSOD siRNA. Specific reduction of MnSOD in MnSOD heterozygous knock-out mice resulted in decreased RCR and complex I activity with increased lactate levels. Taken together, these data demonstrate a critical role of MnSOD in influencing the mitochondrial function and thereby the switch in the energy metabolism switch that might occur under the pathological condition of MnSOD deficiency