Human glucose transporter type I-deficiency (G1D) leads to reduced brain glucose influx and neurological dysfunction principally manifested as epilepsy. Normally, most glucose is fully degraded into CO2 and water for brain energy generation via the tricarboxylic acid (TCA) cycle, which is also central to the synthesis and utilization of the neurotransmitters glutamate and GABA. Importantly, a fraction of glucose does not directly generate energy, but refills natural TCA cycle precursor loss through a reaction termed anaplerosis. Despite these long-established biochemical principles, it is unclear how most diseases that impair brain metabolism and cause seizures disrupt excitability within brain tissue (rather than in vitro), including G1D, which leads to spike-wave epilepsy. This knowledge gap about mechanisms critically limits treatment, as illustrated by anticonvulsant resistance in G1D, which is also the rule in many other neurometabolic disorders. Our laboratory and clinical long-term goal is to mechanistically understand these brain metabolism-excitability relationships in patients and mouse models to develop pharmacological and dietary therapies. The objectives of this application are to characterize hyperexcitability in a novel, robust G1D mouse model and to mitigate it by stimulating both anaplerosis and the TCA cycle with dietary substrates. Our human data and preliminary laboratory results, such as the finding of TCA cycle precursor depletion and of abnormal neocortical and thalamic excitability in G1D mice, justify investigating these mechanisms in more depth to understand epileptic hypersynchronization as a central feature of human and murine G1D. This leads to the main hypothesis that synaptic dysfunction is critical for disease pathophysiology. The proposal also includes the therapeutic consideration that even-carbon ketones, generated from common dietary fats or a ketogenic diet, can fuel the TCA cycle and ameliorate seizures in G1D, but are not anaplerotic. In contrast, our data open a new therapeutic opportunity because administered odd-carbon fat refills brain TCA cycle precursors efficiently in G1D; leading to the additional hypothesis that it restores neural functio more effectively than even-carbon fat via anaplerosis. The hypotheses will be tested in three aims: 1) Investigate the basis of cortical hyperexcitability in G1D; 2) Expand this mechanistic approach to the thalamus; 3) Restore brain metabolism and function via anaplerosis. The proposal is significant because its focus on metabolism-excitability relationships and anaplerosis in brain tissue using a very informative mouse model represents a shift in approach to neurometabolic diseases, where electrophysiology, 13C NMR and mass spectrometry offer a complementary understanding of mechanisms conducive to potential therapies. Particularly innovative is to combine an investigation of synaptic function in circuits or brain regions crucial for epilepsy, impaired behavior or mental retardation with the development of methodology sensitive to conscious mouse brain metabolism with broad applicability to other encephalopathies. In summary, we expect that this proposal will help define G1D as a synaptic disorder and render it amenable to excitable or metabolic target modification.
Most epilepsies caused by mutations in genes involved in energy metabolism are intractable and can only be alleviated by life-long palliative efforts, which constitutes an important health problem. Understanding how these disorders, which are part of an expanding group of disabling diseases, are associated with seizures fulfills the NIH mission by uncovering new fundamental aspects of brain function and by facilitating the development of potential therapies aimed to restore brain energy and excitation balance.
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