Glutamate toxicity mediated by mitochondria

Glutamate is an amino acid that is used in cells for the biosynthesis of proteins and for the generation of energy in the form of adenosine triphosphate (ATP). ATP is synthetized from glutamate in mitochondria, a subcellular organelle specialized for energy production using oxygen. In the nervous system, glutamate additionally plays an important role as an excitatory neurotransmitter necessary to conduct nerve impulses from one neuron to the other. Nerve impulses trigger the release of glutamate from the presynaptic cell into the synaptic cleft, then glutamate reacts with the glutamate receptors on the postsynaptic cell and activates it. After activation, glutamate must be quickly removed back into the cells. If its concentration remains high, this causes the overwhelmed activation of postsynaptic neurons, which exhausts these neurons causing their degradation. This process, called excitotoxicity, occurs in neurodegenerative diseases and after neuronal injury following traumatic brain injury. Preliminary data obtained in the Ludwig Boltzmann Institute for Experimental and Clinical Traumatology in the AUVA research center show that the elevated glutamate concentrations which are responsible for the excitotoxic effects are due to a defect in mitochondria. This defect prevents the consumption of glutamate for energetic purposes. The latter destabilizes glutamate metabolism causing excitotoxicity and neuronal death. The aim of this project is to identify the defect(s) in mitochondria destabilizing glutamate metabolism and to develop a pharmacological strategy able to ameliorate mitochondria-dependent glutamate toxicity and neuronal death.

Injured nerve cells release biologically active substances that trigger inflammation and stimulate the production of nitric oxide. This gaseous signaling molecule inhibits the essential mitochondrial enzyme OGDHC (2-oxoglutarate dehydrogenase complex), impairing mitochondrial energy metabolism and disrupting glutamate homeostasis. Elsewhere in the body, such as in muscles or the liver, elevated glutamate levels are generally harmless. In the brain, however, glutamate serves as the primary excitatory neurotransmitter, activating receptors that transmit signals from one neuron to another. Excessive glutamate accumulation leads to neuronal hyperexcitation. Neighboring cells become overactive and continue firing until their energy reserves are depleted. As a result, neurons die, leading to progressive loss of nervous tissue. In addition to excitotoxicity, glutamate can also induce ferroptotic cell death. Ferroptosis is a regulated form of cell death driven by iron-dependent lipid peroxidation. Under normal conditions, the enzyme glutathione peroxidase protects cells from ferroptosis by detoxifying lipid peroxides. However, high extracellular concentrations of glutamate inhibit the transport of cystine, a precursor required for glutathione synthesis, thereby reducing glutathione availability and impairing glutathione peroxidase activity. Our research demonstrates that both excitotoxic and ferroptotic cell death occur in the brain under specific pathological conditions. Importantly, we have shown that OGDHC function can be restored through the administration of high doses of vitamin B1 (thiamine), thereby preventing neuronal death. These findings have been validated in ex vivo models. We are now planning to translate this approach into clinical practice and establish this treatment strategy for patients with severe neurological disorders in collaboration with the Medical University of Vienna.