Research ArticlePosttranslational Modifications

Regulation of brain glutamate metabolism by nitric oxide and S-nitrosylation

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Sci. Signal.  07 Jul 2015:
Vol. 8, Issue 384, pp. ra68
DOI: 10.1126/scisignal.aaa4312

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Regulating neuronal glutamate status

The gasotransmitter nitric oxide (NO) is generated by nitric oxide synthase (NOS) and can affect protein function by modifying cysteine residues in a process called S-nitrosylation. Mice lacking the neuronal NOS (nNOS) isoform have a phenotype that could be explained by decreased availability of glutamate, an amino acid that is also an excitatory neurotransmitter. Glutamate is derived from and can be converted to glutamine, and oxidation of glutamate feeds into the energy-providing TCA (tricarboxylic acid) cycle. Raju et al. found that mice lacking nNOS showed decreased S-nitrosylation of proteins involved in glutamate metabolism and uptake. In extracts from these mice, conversion of glutamate to glutamine was decreased, oxidation of glutamate was decreased, and glutamate uptake was increased.

Abstract

Nitric oxide (NO) is a signaling intermediate during glutamatergic neurotransmission in the central nervous system (CNS). NO signaling is in part accomplished through cysteine S-nitrosylation, a posttranslational modification by which NO regulates protein function and signaling. In our investigation of the protein targets and functional impact of S-nitrosylation in the CNS under physiological conditions, we identified 269 S-nitrosocysteine residues in 136 proteins in the wild-type mouse brain. The number of sites was significantly reduced in the brains of mice lacking endothelial nitric oxide synthase (eNOS−/−) or neuronal nitric oxide synthase (nNOS−/−). In particular, nNOS−/− animals showed decreased S-nitrosylation of proteins that participate in the glutamate/glutamine cycle, a metabolic process by which synaptic glutamate is recycled or oxidized to provide energy. 15N-glutamine–based metabolomic profiling and enzymatic activity assays indicated that brain extracts from nNOS−/− mice converted less glutamate to glutamine and oxidized more glutamate than those from mice of the other genotypes. GLT1 [also known as EAAT2 (excitatory amino acid transporter 2)], a glutamate transporter in astrocytes, was S-nitrosylated at Cys373 and Cys561 in wild-type and eNOS−/− mice, but not in nNOS−/− mice. A form of rat GLT1 that could not be S-nitrosylated at the equivalent sites had increased glutamate uptake compared to wild-type GLT1 in cells exposed to an S-nitrosylating agent. Thus, NO modulates glutamatergic neurotransmission through the selective, nNOS-dependent S-nitrosylation of proteins that govern glutamate transport and metabolism.

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