PEPCK as the mechanism of action for the anti-epileptic effect of 2-DG and the Ketogenic Diet.
Recent clinical and biochemical investigations on the mechanism of action for the ketogenic diet have focused on the role of low carbohydrates, instead of ketone bodies. This includes work demonstrating that the low-carbohydrate Atkins Diet can be anti-epileptic in humans, and work in rats correlating blood glucose levels with seizures. There is also work showing that chemically inhibiting glycolysis with iodoacetate or 2-deoxyglucose (2-DG) reduces epileptiform electrical activity in brain slices and that 2-DG can raise the threshold for stimulated seizures in rats. These studies have led to a new way of looking at the ketogenic diet, but still lack a mechanism to correlate cytoplasm metabolism with changes in the electrical excitability and seizure activity in the brain. Below I present a model of the ketogenic diet where the key step is alterations in enzymatic activity of PEPCK .
The main assumption for my model of the ketogenic is that it’s anti-epileptic activity comes from reduction of glycolysis in the brain. Once this happens, there is a reduction in the glycolytic intermediate phosphoenolpyruate (PEP). Reduction in the cellular concentration of PEP will increase the conversion of oxaloacetate to PEP by the enzyme PEPCK. Increasing the enzymatic activity (or flux) of PEPCK speeds the conversion of GTP to GDP and reduction of the neuronal GTP to GDP ratio. Since PEPCK is localized in synaptic terminals, alterations in nucleotide levels may be localized there. Lowering neuronal GTP levels probably has multiple cellular effects, including the reduction of high-frequency synaptic transmission (read more about GTP and synaptic transmission here) by slowing the re-filling of the readily releasable pool of synaptic vesicles.
Data for the role of the replenishment of synaptic vesicles in epilepsy has been published by Kevin Staley at the University of Colorado-Boulder. He used an experimental system very similar to the one I used (see data). Dr. Staley published a mathematical model for epileptiform electrical behavior in brain slices from rats. The output axons of the hippocampal region has many connections that form on cells in the same region. These are called recurrent connections and lead to a positive feedback loop in the hippocampus. Under the proper conditions, this positive feedback circuitry will cause the hippocampus to synchronize and all of the neurons will fire action potentials simultaneously. This behavior is analogous to the positive feedback during an action potential, where depolarization of a neuronal membrane opens sodium channels causing further depolarization. The way this cycle is broken is when the sodium channels are no longer able to open when the membrane is depolarized. This process is called voltage dependent inactivation, and it is sodium channel inactivation that terminates an action potential. Many current anti-epileptic drugs work by altering sodium channel inactivation and action potential trains. Similar to sodium channels, stimulation of the recurrent synaptic connections in the hippocampus excite more hippocampal neurons, increasing synaptic transmission. The cycle is broken when synaptic transmission fails as the synapses run out of vesicles to release. Neurons re-form synaptic vesicles via endocytosis, and get them ready to release in the readily releasable pool. Dr. Staley’s model predicts that this rate of refilling of the RRP may set the rate at which tissue reaches threshold for epileptic activity.
Slowing the refilling of the RRP would be expected to reduce the amount of time that neuronal tissue is above the threshold for epileptic activity. As stated above, PEPCK may reduce GTP levels at synapses. Recent work on GTP and synaptic transmission has demonstrated that reducing pre-synaptic GTP greatly slows the refilling of the RRP of synaptic vesicles. Thus my model can be summarized as such:
Ketogenic diet
¯ Glycolysis
¯ Cellular PEP
Flux through PEPCK
¯ Synaptic GTP concentration
↓ rate of endocytosis
¯ Size of RRP
Neuronal network kept below threshold for epileptic behavior
2-DEOXYGLUCOSE EXERTS ANTICONVULSANT AND ANTIEPILEPTIC ACTIONS IN EXPERIMENTAL EPILEPSY MODELS C. E. Stafstrom, S. M. Kriegler, M. T. Valley, J. C. Ockuly, A. S. Roopra, and T. P. Sutula Epilepsia Volume 46 Page 268 -October 2005
Efficacy of the Atkins diet as therapy for intractable epilepsy. Kossoff EH, Krauss GL, McGrogan JR, Freeman JM. Neurology. 2003 Dec 23;61(12):1789-91.
A modified Atkins diet is effective for the treatment of intractable pediatric epilepsy. Kossoff EH, McGrogan JR, Bluml RM, Pillas DJ, Rubenstein JE, Vining EP. Epilepsia. 2006 Feb;47(2):421-4.
Presynaptic modulation of CA3 network activity. Staley KJ, Longacher M, Bains JS, Yee A. Nat Neurosci. 1998 Jul;1(3):201-9.
Vesicle endocytosis requires dynamin-dependent GTP hydrolysis at a fast CNS synapse. Yamashita T, Hige T, Takahashi T. Science. 2005 Jan 7;307(5706):124-7.
Myristyl trimethyl ammonium bromide and octadecyl trimethyl ammonium bromide are surface-active small molecule dynamin inhibitors that block endocytosis mediated by dynamin I or dynamin II. Quan A, McGeachie AB, Keating DJ, van Dam EM, Rusak J, Chau N, Malladi CS, Chen C, McCluskey A, Cousin MA, Robinson PJ. Mol Pharmacol. 2007 Dec;72(6):1425-39.
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Copyright 2011 Steve Kriegler
