The Neuromediator Glutamate, through Specific Substrate Interactions, Enhances Mitochondrial ATP Production and Reactive Oxygen Species Generation in Nonsynaptic Brain Mitochondria

Abstract

The finding that upon neuronal activation glutamate is transported postsynaptically from synaptic clefts and increased lactate availability for neurons suggest that brain mitochondria (BM) utilize a mixture of substrates, namely pyruvate, glutamate, and the tricarboxylic acid cycle metabolites. We studied how glutamate affected oxidative phosphorylation and reactive oxygen species (ROS) production in rat BM oxidizing pyruvate ؉ malate or succinate. Simultaneous oxidation of glutamate ؉ pyruvate ؉ malate increased state 3 and uncoupled respiration by 52 and 71%, respectively. The state 4 ROS generation increased 100% over BM oxidizing pyruvate ؉ malate and 900% over that of BM oxidizing glutamate ؉ malate. Up to 70% of ROS generation was associated with reverse electron transport. These effects of pyruvate ؉ glutamate ؉ malate were observed only with BM and not with liver or heart mitochondria. The effects of glutamate ؉ pyruvate on succinate-supported respiration and ROS generation were not organ-specific and depended only on whether mitochondria were isolated with or without bovine serum albumin. With the non-bovine serum albumin brain and heart mitochondria oxidizing succinate, the addition of pyruvate and glutamate abrogated inhibition of Complex II by oxaloacetate. We conclude that (i) during neuronal activation, simultaneous oxidation of glutamate ؉ pyruvate temporarily enhances neuronal mitochondrial ATP production, and (ii) intrinsic inhibition of Complex II by oxaloacetate is an inherent mechanism that protects against ROS generation during reverse electron transport.Recently, it has emerged that mitochondrial dysfunctions play an important role in the pathogenesis of degenerative diseases of the central nervous system (1-3). The processes underlying neuronal degeneration are complex, and some authors suggest that several genetic alterations are involved (4). However, another level of complexity may be derived from the fact that virtually all cellular activities depend upon energy metabolism in the cell (5). Alterations in energy metabolism processes within cells may also contribute to pathogenic mechanisms underlying neurodegenerative disease.A large body of evidence suggests that increased oxidative stress is an important pathogenic mechanism that promotes neurodegeneration (6). Because neurons have a long life span, and most neurodegenerative diseases have a clear association with age (7), it is important to understand mechanisms underlying reactive oxygen species (ROS) 2 production in neurons. Recently, Kudin et al. (8) analyzed the contribution of mitochondria to the total ROS production in brain tissue. They concluded that mitochondria are the major source of ROS and that at least 50% of ROS generated by brain mitochondria was associated with succinate-supported reverse electron transport (RET). Under conditions of normoxia, about 1% of the respiratory chain electron flow was redirected to form superoxide (8).Recently, we suggested that the organization of the respiratory chain complexes into supe...

Mitochondrial H2O2 generated from electron transport chain complex I stimulates muscle differentiation

“…In general, RET is observed in isolated mitochondria respiring on succinate. However, it has been reported that RET and RET-associated ROS production occur through high activity of the TCA cycle and mitochondrial respiration in the presence of NADH-dependent substrates, such as pyruvate and malate, in brain mitochondria and brain synaptosomes [33,34]. We observed increases in ATP levels, citrate activity, mitochondrial respiration, and mETC complex activities during muscle differentiation, indicating that metabolism is enhanced during muscle differentiation.…”

Role of Neuronal Mitochondrial Metabolic Phenotype in Pathogenesis of ALS

“…Nevertheless, our comparative studies of metabolic properties of BM and SCM suggest that potentially the loss of morbidity could be explained by different patterns of substrates metabolism and associated ROS generation in BM and SCM studied in 2007 and 2010. In 2008 we found that BM and SCM isolated from the wild type Sprague Dawley rats began to show metabolic features that were different from those described earlier for the same strain of the wild type and tgSOD1 rats (Panov et al, 2009(Panov et al, , 2011a. In this work we present a comparison of metabolic phenotypes and the substrate-dependent ROS generation in the wild type and transgenic rats with mutant G93A Cu/Zn-superoxide dismutase gene isolated in 2007 and 2010.…”

“…Respiratory activities were also diminished in SCM of Wobbler mice (Dave et al, 2003, Xu et al, 2001) and mice with motor neuron degeneration disease (Bertamini et al, 2002). A comparative study of isolated normal and tgSOD1 brain and spinal cord mitochondria was published recently by Panov et al (2011a, b), and the unique metabolic features of BM and SCM were described (see also Panov et al, 2009). Normal SCM produced significantly more ROS than BM, and this was associated with the succinate-dependent reverse electron transport (Panov et al 2011a).…”

“…The major consequence of overexpression of UCP1 is inhibition of ATP production and suppression of functions, which depend on mitochondrial ATP levels. Because in neuronal tissue reverse electron transport is the major source of ROS production (Panov et al 2007(Panov et al , 2009, and increased ROS generation is one of the leading pathogenic pathways in ALS, the mechanism proposed by Dupius et al (2009), although interesting, probably has little to do with neurodegeneration in ALS. It was hypothesized that sporadic and familial forms of ALS may share some final pathogenic mechanisms (Bendotti & Carri, 2004;von Lewinski & Keller, 2005).…”

“…Thus in a narrow space of spines and dendrites several million glutamate molecules postsynaptically transported from synaptic boutons may create local cytosolic concentration of glutamate in a low mM range. Consequently, neuronal mitochondria, particularly those located at the axonal or dendritic synaptic junctions may temporarily metabolize, in addition to pyruvate, some amounts of glutamate (Yudkoff et al 1994;Panov et al 2009). Besides, GABA is also transported postsynaptically where it is catabolized in mitochondria to succinate (Tillakaratne et al, 1995).…”