Illustration by William Hogan
magine a doctor telling you that you have to change your
diet to one with few carbohydrates in favor of high-fat cheeses, butter-fried
steaks, bacon and eggs, and eggnog—all while you snack in between meals on
macadamia nuts. Sounds great initially, but most of us would tolerate only a
few days of eating this way. Yet many young children with epilepsy, who do not
respond to conventional medications, benefit from just such a diet. Strict
adherence to the so-called ketogenic diet (i.e., with minimal calories from
carbohydrates) can often reduce their seizures enough to allow them to attend
school and experience the joys of growing up. The diet was developed to mimic
the effects of fasting, which has been known since antiquity to afford some
Epilepsy and epileptic
seizures affect nearly three million Americans and 65 million people of all
ages around the world. According to the International League Against Epilepsy,
seizures and epilepsy are not the same: “An epileptic seizure
is a transient occurrence of signs and/or symptoms due to abnormal excessive or
synchronous nerve cell activity in the brain. Epilepsy is a disease
characterized by an enduring predisposition to generate epileptic seizures and
by the neurobiological, cognitive, psychological, and social consequences of
this condition. Translation: a seizure is an event, and epilepsy is the disease
involving recurrent unprovoked seizures.”
In fact, “epilepsies”
are a group of neurologic disorders. When one or more neural circuits in the
brain develop a chronically low seizure threshold, normally innocuous stimuli
(external to or within the brain) can trigger a group of nerve cells to fire at
once. This abnormal synchronicity is
The disease has
been known for ages. A very early reference is found on a Babylonian tablet in
London’s British Museum dating from approximately 1060 BC, which refers to “the
falling disease,” with the subjective aura (an ominous feeling) and the
subsequent seizures themselves ascribed to the work of childless demons who
viewed humans with envy and spite.1 Hippocrates argued around 400 BC
that epilepsy is a physical disorder of the brain, but he was widely
disbelieved.2 Over the next 2,000 years, seizures were treated by
bleeding, exorcism, trepanation (a hole is bored in the skull), and ingestion
of silver nitrate or bromides.
Over the past
several decades more than 30 anticonvulsant medications have been developed.
They pass via the bloodstream into the brain and dampen seizures by reducing
the excitability of brain cells. They act on a restricted number of molecular
targets in the brain. Some of the drugs act on ion “channels” that allow sodium,
calcium, and potassium ions to pass into and out of brain cells. Others
potentiate the major inhibitory system of the brain, which uses the
neurotransmitter GABA to dampen nerve cell excitability. Additionally, there
are drugs that act on the synaptic vesicle protein SV2A, and the AMPA subtype
of glutamate receptor.
ketogenic diet (KD) was introduced in the 1920s in between the first two modern
anti-seizure medications, phenobarbital in 1912 and phenytoin in 1938.
Even though some patients did not respond to
these drugs and improved with the ketogenic diet, it nonetheless fell into
obscurity as more anti-seizure medications were introduced. Then, as now,
physicians found it far easier to prescribe a pill than to teach their patients
that all that was required was to adhere to a rigid and restricted eating
regimen. But even with the plethora of anticonvulsant drugs that are now
available for people who live with epilepsy in the developed world, a full
one-third of epilepsy patients still do not respond to any medication. This
situation led to the creation of a childhood epilepsy center and seizure clinic
in the mid-1970s at the Johns Hopkins Department of Neurology/Neurosurgery Hospital
Clinic, where the ketogenic diet was resurrected as an option.
A drug that
works for everyone, though, remains the goal. Because no magic pill exists to
as such, the search for new anti-seizure medications has continued, especially studies
of drugs that have novel molecular targets in the brain.
Hope on the Horizon
Last year a
study reporting an unexpected molecular target that could spawn a new
generation of anticonvulsant drugs stirred great interest.3 Sada et
al reported the results from four experiments. The
researchers started by looking at the effect in brain slices of bathing nerve
cells in a solution that contained beta-hydroxybutyrate (BHB) rather than
glucose (sugar) as an energy source. BHB is made in the liver when the body
breaks down fat, rather than carbohydrates, for energy. This switch from
carbohydrate to fat metabolism is, in fact, what occurs in people when they
fast or when they are on the ketogenic diet. The study found that BHB hyperpolarized nerve cells, that is, rendered them less excitable
and more stable, and thus less prone to epileptic activity.
When on the
ketogenic diet, blood levels of sugar decrease while blood levels of BHB
increase to exert its stabilizing influence. Even so, blood glucose remains around
one-half of the usual level.4,5 When BHB was added to a bathing
solution that contained a little rather than no glucose, the cells did not
hyperpolarize. Rather, they remained active and prone to epileptic activity. This
finding suggests that glucose might offset the stabilizing effects of BHB.
to the nerve cells in Sada’s experiment, the key question is whether they
became more stable from the presence of BHB or the absence of glucose. This question
gets at the heart of how the ketogenic diet works, because it is still not clear
whether its antiepileptic effect is due to low blood levels of glucose or to
the high levels of BHB that occur when the body breaks down fat for energy.5,6
et al. investigated this issue by asking whether the glucose-to-BHB switch acts
by reducing the formation of pyruvate and lactate. These organic compounds are
produced when glucose is broken down, and each can be converted into the other
(interconverted) by the enzyme lactate dehydrogenase (LDH). Through a series of
experiments, the researchers found: 1) that lactate could indeed undo the
stabilizing effect of BHB by reversing its hyperpolarizing action; 2) that
lactate caused this effect after it had been converted into pyruvate by LDH,
because 3) inhibition of LDH by the small molecule, oxamate, eliminated the
anti-hyperpolarizing (depolarizing) effect of lactate, but not that of
pyruvate, which continued to depolarize nerve cells even when LDH was inhibited
by oxamate, 4) just exposing the nerve
cells to oxamate caused them to become hyperpolarized.
At a Crossroads
led to a crucial question: does pyruvate reverse the hyperpolarizing (stabilizing)
effect of oxamate by acting as a nutrient — or does it do something else?
is a “keto” acid, carrying a keto (C=O) group. Sada et al. showed that other
keto acids (alpha-ketobutyrate and oxaloacetate, see Figure 1) also reversed the
stabilizing effect of oxamate, but that ATP or other energy metabolites derived
from pyruvate were ineffective. Together, these findings suggest that the
depolarizing effect of pyruvate is related to its ability to scavenge the
coenzyme NADH (Figure 1) rather than its role as a nutrient. It is likely that
BHB, a hydroxyacid, becomes converted to the ketoacid “acetoacetate” with an accompanying
formation of NADH. This is especially likely to be the case when BHB is
delivered in large quantities, as in a solution that bathes the nerve cells.
||Figure 1. A simplified scheme of the glycolytic pathway, emphasizing the generation of NADH and the purpose of LDH in consuming excess NADH. In the cytosol NADH is formed in the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) reaction. GAPDH is strongly inhibited if NADH accumulates. GAPDH catalyzes the oxidation and phosphorylation of glyceraldehyde-3-phosphate by inorganic phosphate (Pi), producing 1,3-bis-P-glycerate. In the subsequent enzymatic step the phosphate group is donated to ADP to produce ATP. After another three enzymatic steps, pyruvate is formed. The conversion of pyruvate to lactate, catalyzed by LDH, consumes NADH and relieves GAPDH of its inhibition by NADH, allowing the cycle to continue. α-Ketobutyrate (in blue), which was used by Sada et al., may substitute for pyruvate in the LDH reaction. Oxaloacetate (purple) may also consume cytosolic NADH in the cytosolic malate dehydrogenase reaction (MDHc), in which oxaloacetate is converted to malate at the expense of NADH.
If this system is flooded with lactate (e.g. by lactate exported from astrocytes to nerve cells), then NADH would accumulate. If the system is flooded by pyruvate (e.g. by experimental injection of pyruvate into nerve cells), then NADH levels would be reduced.
Mechanisms to Consider
the stabilizing or hyperpolarizing effect of BHB appears not to be related to energy production in the nerve cells, but
instead could be caused by its promotion of an NADH-dependent process regulating
the excitability of nerve cells. Several candidate mechanisms can be considered
to explain how LDH inhibition hyperpolarizes nerve cells; much work remains to
pin down the exact series of events.
next question that Sada et al. asked was how pyruvate, converted from lactate,
ends up in nerve cells. They found that it probably is delivered to the nerve
cells by another cell type, astrocytes. When glucose enters the brain from the
circulation, it is partly taken up into astrocytes. Inside these cells, some of
the glucose will be converted into lactate, which leaves the astrocytes. Nerve
cells then take up the lactate from astrocytes through a specific transport
protein, completing an astrocyte-to-neuron lactate shuttle.7-9 The
Sada group concluded that lactate from astrocytes is an important source of “epileptogenic”
(seizure-promoting) pyruvate in nerve cells.
et al had found that bathing nerve cells in the LDH inhibitor oxamate caused
the cells to hyperpolarize and become electrically stable. They followed this
finding up by injecting oxamate into the hippocampus of mice that had been
rendered epileptic after treatment with kainate (a neuroexcitatory amino acid).
Oxamate reduced seizures in this mouse model of epilepsy, suggesting that LDH
inhibition could be antiepileptic in vivo
too. Now that LDH was identified as a potential target for antiepileptic
therapy, Sada et al. looked for antiepileptic drugs (AEDs) that might inhibit
LDH. The researchers screened 20 AEDs and found that a little-used AED,
stiripentol, did inhibit LDH. Stiripentol is currently used to treat a rare and
devastating epileptic condition known as Dravet syndrome or severe myoclonic
epilepsy of infancy. They then tested similar molecules for LDH inhibition and
found that the structurally simpler compound, isosafrole, was a more potent
inhibitor of LDH than stiripentol itself.
Pyruvate depolarizing nerve
picture emerging from the work of Sada et al. is that pyruvate facilitates
epileptic activity by depolarizing nerve cells. It had been shown earlier that
rapid injection of pyruvate could actually cause seizures.10 Sada
and colleagues showed that blocking the enzyme responsible for pyruvate
formation from lactate (LDH) has an anti-seizure effect.
remarkable study helps explain the well-known observation that eating a
sugar-laden cookie can quickly promote seizures in a child on the ketogenic
diet:11 when the sugar (glucose) from the cookie is metabolized, it
forms pyruvate; the pyruvate then
rapidly reverses the stabilizing effect of BHB. Further, the Sada group
identified the role of LDH in the glucose effect, and they showed that
pharmacological inhibition of LDH exerts marked anti-seizure effects in
epilepsy animal models —thus fingering LDH as the first new target for
discovering epilepsy drugs since 2004.12 Important for understanding their results is
the recognition that the seizure-promoting mechanism of the glucose
metabolites—pyruvate and oxaloacetate—is
unrelated to ATP generation but could involve NADH scavenging.
steps remain in the quest to develop novel anti-seizure drugs based on reduced formation
of pyruvate. First, we might identify additional anti-seizure targets by identifying
the mechanism involved in oxaloacetate-induced depolarization.
A good next step for this would be systematic testing of other Kreb’s cycle
metabolites, especially NADH, for their ability to reverse oxamate-induced
hyperpolarization. Second, there are multiple, cell-specific but functionally
similar forms of LDH; further drug screening might allow glycolytic inhibition that
is restricted to nerve cells, which could reduce side effects. Third, their
unexpected finding that inhibitory interneurons are not hyperpolarized
(stabilized) by LDH inhibitors, although fortunate for epilepsy therapy, is
worth following up.
inhibition of glucose metabolism to treat epilepsy has been reported
previously, using 2-deoxyglucose (2-DG),13 and it would be
interesting to determine if 2-DG and isosafrole block one another’s action or
are synergistic. The answer could inform whether reduced glycolysis or some
unrelated action is responsible for the anti-seizure effect of 2-DG. If the two
are synergistic, combination therapy could be warranted. While not quite
“epilepsy diet in a pill”, the study by Sada et al. points the way to a
“starvation in a pill” strategy— i.e. pharmacologically simulating fasting— for
seizure control. Anti-seizure drugs have been repeatedly repurposed for other
neuropsychiatric disorders such as bipolar disorder and neuropathic pain. If
chronic treatment with 2-DG and an LDH inhibitor prove safe in people, might we
add another repurposed clinical use to this list—weight control?
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