Sweets Are BAD for Seizures

Commentary

BAD-Dependent Regulation of Fuel Metabolism and K_ATP Channel Activity Confers Resistance to Epileptic Seizures.

Giménez-Cassina A, Martínez-François JR, Fisher JK, Szlyk B, Polak K, Wiwczar J, Tanner GR, Lutas A, Yellen G, Danial NN. Neuron 2012;74(4):719–730

Neuronal excitation can be substantially modulated by alterations in metabolism, as evident from the anticonvulsant effect of diets that reduce glucose utilization and promote ketone body metabolism. We provide genetic evidence that BAD, a protein with dual functions in apoptosis and glucose metabolism, imparts reciprocal effects on metabolism of glucose and ketone bodies in brain cells. These effects involve phosphoregulation of BAD and are independent of its apoptotic function. BAD modifications that reduce glucose metabolism produce a marked increase in the activity of metabolically sensitive K_ATP channels in neurons, as well as resistance to behavioral and electrographic seizures in vivo. Seizure resistance is reversed by genetic ablation of the K_ATP channel, implicating the BAD-K_ATP axis in metabolic control of neuronal excitation and seizure responses.

Mitochondria are implicitly linked to neuronal excitability owing to their primary role in cellular ATP generation, maintenance of calcium homeostasis, reactive oxygen species production, control of apoptosis, and biosynthesis of amino acid neurotransmitters (1). It is becoming increasingly apparent that mitochondrial dysfunction is both a cause and a consequence of epileptic seizures (2, 3). Notwithstanding such evidence, the vast majority of studies involving mitochondria have focused on their involvement in cell death processes, and in particular, the consequences of epileptic seizure activity. Few have addressed the role of mitochondria in direct modulation or control of seizure activity (2, 3).

It is well known that mitochondria are dynamic organelles, not only in structure and motility but also in their ability to catabolize different metabolic fuels based on substrate availability and bioenergetic need (4). The importance of metabolic fuels in controlling seizures is underscored by both clinical and experimental observations attesting to the broad-spectrum efficacy of the ketogenic diet (KD). The KD's high-fat/low-carbohydrate composition induces a critical switch in brain utilization of ketones over glucose as a metabolic fuel. While KD is clinically efficacious in controlling diverse types of intractable epilepsies in children and adolescents, its mechanism of action remains obscure (5). Despite a relative lack of mechanistic data, there is renewed interest in the research community to explore ways to modify neuronal excitability by altering metabolic flux, such as through the nonoxidizable glucose analog, 2-deoxgyglucose (6).

In this light, Giménez-Cassina and colleagues (2012) provide an intriguing mechanistic link between alternative mitochondrial fuel utilization, apoptotic machinery, and neuronal excitability. These investigators demonstrate that the Bcl-2–associated agonist of cell death (BAD) controls mitochondrial fuel utilization and neuronal excitability via plasmalemmal K_ATP channels. The primary rationale of this novel study arose from the recognition that BAD exerts dual effects in cellular function: promoting apoptosis and regulating glucose metabolism (7).

A member of the BH3-only proteins of the Bcl-2 family, BAD exists in either phosphorylated or dephosphorylated states, with resultant opposing effects on cell death. BAD's apoptotic and metabolic roles require (de)phosphorylation of a specific serine 155. Giménez-Cassina and colleagues employed complementary technical approaches to examine the effects of BAD on seizure susceptibility, including BAD mutant mice, mitochondrial functional analysis, cellular electrophysiology, and video-EEG monitoring. Measurement of oxygen consumption rates (OCR) in cultured neurons or astrocytes from BAD knock-out (Bad^−/−) or phosphodeficient BAD mice engineered with the mutation at serine 155 revealed decreased glucose-driven OCR. Further, neurons or astrocytes from BAD-deficient or BadS155A mutant mice were able to switch to utilization of the primary ketone body β-hydroxybutyrate, but not alternative substrates such as L-glutamine or L-lactate—findings that closely mirror the actions of the KD. However, it is unclear whether the metabolic status of glycolysis in Bad^−/− and Bad^S155A mice reflected a global decrease in cellular metabolism or whether glycolytic rates were in fact elevated to compensate for increased OCR. Of importance, Bad^−/− and Bad^S155A phosphomutant mice were resistant to kainic acid– or pentylenetetrazol-induced seizures, suggesting that the switch in metabolic fuel from glucose to ketones may underlie their resistance to seizure provocation. Further, the authors carefully tease apart the dual apoptotic and metabolic roles of BAD in controlling seizure activity by demonstrating that mice deficient in Bid (BH3-interacting domain death agonist)—another pro-apoptotic gene—do not show seizure resistance.

To establish a causal relationship between fuel regulation by BAD and neuronal hyperexcitability, Giménez-Cassina and colleagues hypothesized that ATP-sensitive potassium (K_ATP) channels might be involved. These investigators had appreciated much earlier that K_ATP channels represent a compelling mechanistic link between changes in cellular metabolism and neuronal membrane excitability, based on their sensitivity to ATP levels and their ability to dampen excitability during excessive neuronal activation. Specifically, their group had provided strong evidence that ketone bodies increased K_ATP channel activity in neurons of both substantia nigra pars reticulata and hippocampus (8, 9).

Using a similar approach, Giménez-Cassina and colleagues found an increased open probability of single K_ATP channels in electrophysiological recordings of dentate granule cells in hippocampal slices from Bad^−/− mice. They also observed increased whole-cell K_ATP currents in Bad^−/− and Bad^S155A mice. Tolbutamide, a pharmacologic blocker of K_ATP channels, reversibly inhibited the increases in both single-channel and whole-cell K_ATP currents in Bad^−/− mice. To establish more directly whether K_ATP channels were indeed controlling seizure resistance in BAD-null mice, the authors created Bad^−/−: Kir6.2^−/− double mutant mice (Kir6.2 encodes an inwardly rectifying potassium channel subunit comprising K_ATP channels). They found that mutant mice lacking both Kir6.2 and BAD were resistant to kainic acid–induced seizures, whereas single deletion of Kir6.2 did not sensitize mice to seizures, confirming the role of K_ATP channels in the resistance to seizures exhibited by Bad^−/− mice.

In their elegant study, Giménez-Cassina et al. piece together mitochondrial fuel regulation by BAD with control of neuronal excitability by K_ATP channels and in so doing, help to fill a persistent gap in understanding how a metabolic switch from glycolysis to ketosis can confer an anticonvulsant effect. The renewed quest for elucidating the mechanisms through which the KD controls seizures has led to several intriguing metabolic substrates and enzymes as targets for therapeutic intervention (5). Their study reveals a critical molecular pathway that brings us one step closer towards achieving the lofty goal of replacing global nonpharmacologic interventions such as diet and exercise with a pharmacologic agent in a “pill” (10). Identification of a phosphorylation-dependent control of fuel choice and seizure control by BAD in mice fed a normal diet suggests a number of pharmacologic approaches, including manipulation of BAD expression/phosphorylation, control of K_ATP channels, and utilization of alternative mitochondrial fuels. The latter has recently emerged as a “hot topic” in fields such as aging, cancer, and hormesis (11, 12).