Pyruvate dehydrogenase encephalopathy: mechanisms and therapy

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ABSTRACT This proposal constitutes one of the first mechanistic investigations of pyruvate dehydrogenase (PDH) deficiency encephalopathy (PDHD), an inborn developmental brain disorder. PDHD leads to impaired brain metabolism and neurological dysfunction that manifests as intellectual disability and intractable seizures in infants and children. The elementary biochemical framework has been elucidated primarily in vitro using cell cultures and homogenates. Normally PDH receives about 80% of the brain’s glucose metabolic flux, converting it into substrate for the tricarboxylic acid (TCA) cycle. The TCA cycle is responsible for brain energy generation and also for the synthesis of the neurotransmitters glutamate and GABA, which regulate excitability. The remainder 20% of brain glucose refills natural TCA cycle precursor loss, which is accomplished through a separate process known as anaplerosis. Yet, anaplerosis can also be secondarily downregulated in PDHD. Nevertheless, despite these long-established biochemical principles, it is unknown how PDHD causes encephalopathy. In particular, very little is known about metabolism or excitability within PDHD brain tissue and even less is known in an in vivo context. This knowledge gap drastically limits therapy, as illustrated by the drug-refractoriness of PDHD seizures. The objectives of this application are to characterize in vivo abnormal neural excitability and metabolism in a novel, robust PDHD mouse model and to mitigate them by stimulating both the TCA cycle and anaplerosis with alternative dietary substrates. Our preliminary results include abnormal neocortical excitability in PDHD in vivo and amelioration of brain TCA cycle precursor depletion, and thus justify investigating these mechanisms in greater depth. This leads to a first general hypothesis that metabolic fuel-dependent (rather than fixed) cortical dysfunction is a central feature of disease pathophysiology. The proposal also includes the therapeutic consideration that ketone bodies containing an even number of carbons, generated from common dietary fats or a ketogenic diet, can fuel the TCA cycle and ameliorate seizures in PDHD patients, but cannot correct anaplerotic deficits. In contrast, our data that exogenous anaplerotic fat metabolites containing an odd number of carbons can additionally refill brain TCA cycle precursors, lead to a second general hypothesis: That odd-carbon fat restores neural function in PDHD more effectively via anaplerosis than even-carbon fat. These two general hypotheses will be tested in three aims: 1) Investigate the electrophysiological bases of cortical hyperexcitability in PDHD; 2) Test key metabolic mechanisms relevant to synaptic function; 3) Restore brain metabolism and excitability via anaplerotic odd-carbon fat derivatives. In summary, we expect to help define PDHD as an excitability disorder and establish the therapeutic value of anaplerotic modulation, thus initiating the first step of a medical practice transformation by capitalizing on metabolic or excitable targets.