Switching Off Gene Prevents Epilepsy in Mice
DURHAM, N.C. -- Neurobiologists have found that switching off a single gene for a neuronal protein prevents epilepsy in a mouse model of human epilepsy. They said their research offers the hope of drugs that could prevent the alterations in the brain that cause the normal brain to become epileptic. Such drugs could prove far superior to current antiseizure drugs that only inhibit seizures in people who already have epilepsy, rather than preventing its development.
The researchers, led by professor and chair of neurobiology James McNamara, M.D., published their findings in the July 8, 2004, Neuron. Besides McNamara, other co-authors on the paper were Xiao-Ping He, M.D., and Robert Kotloski of Duke; and Serge Nef, Bryan Luikart and Luis Parada of the University of Texas Southwestern Medical Center. The research was sponsored by the National Institute of Neurological Disorders and Stroke of the National Institutes of Health.
In the paper, the researchers reported studies exploring the effects of knocking out in mice two genes in the brain -- called BDNF and TrkB. The protein that BDNF encodes is a neurotrophic factor -- a neuronal protein that regulates the structure and function of synapses, the sites at which neurons communicate with one another. BDNF produces its effects by activating its receptor, TrkB. Earlier studies in the McNamara laboratory and others had shown marked increases in BDNF expression, and in the activation of the BDNF receptor, TrkB, during development of epilepsy. And earlier studies in other laboratories revealed that eliminating one of two copies of the BDNF gene markedly reduced the tendency to develop epilepsy. Thus the authors expected that eliminating both copies of either the BDNF or TrkB genes would prevent epilepsy.
The mice used in the experiments were those whose brains had been induced to become epileptic by a series of mild electrical stimuli – a process called "kindling." Such stimuli initially cause only subtle focal seizures, but these seizures gradually influence the brain to progress to full-blown epilepsy. Such kindling is believed to be the same basic process by which the human brain becomes epileptic due to stroke, trauma or genetic predisposition.
Because total knockout of the BDNF or TrkB genes in mice is lethal, the researchers used a technique in which they could selectively knock out the genes only in subsets of brain neurons, particularly those neurons thought to be involved in triggering epilepsy.
Contrary to their expectations, the researchers found that eliminating the BDNF gene in the mice only modestly reduced the epileptic kindling process. "However, to our surprise, despite the virtual absence of BDNF, we found the TrkB receptor to be nonetheless activated," said McNamara. Thus, he said, a still-unknown compensatory mechanism still affects the neurons through the TrkB receptors.
The real surprise, however, came when the researchers knocked out the TrkB gene in the mice, which completely eliminated their development of epilepsy.
"I must confess, I never thought I would live to see the day that you could produce a genetic or pharmacological perturbation that would completely prevent kindling in these mice," said McNamara. Although the mice did show evidence of a low level of focal seizures, "there wasn't a shred of evidence of behavioral seizures in these mice, despite many stimulations," he said.
However, the neurobiologists demonstrated that, although the kindling process was eliminated, the mice were still capable of exhibiting seizures when they received a more intense shock. Thus, said McNamara, the elimination of the TrkB gene was, indeed, selectively affecting the process by which epilepsy develops.
"The ability of the animal to exhibit a seizure -- yet not become epileptic -- is really important, because one of our concerns was that this genetic perturbation may compromise the structure and function of the brain, such that the animals couldn't even exhibit a behavioral seizure," said McNamara. "However, given this finding, we believe that knocking out the TrkB gene selectively interferes with the plasticity necessary for the brain to become epileptic."
According to McNamara, the TrkB receptor could represent an excellent target for drugs to prevent epilepsy.
"Typically, in cases of epilepsy resulting from stroke or head trauma, there is a latent period of months or years before epilepsy develops," said McNamara. "We believe that during that latent period, the pathology of kindling may be occurring at a subclinical level. This latent period may also exist in cases of epilepsy due to genetic predisposition Thus, there appears to be a window of opportunity to prevent development of epilepsy, and the field is working to identify targets for drugs that can take advantage of that window.
"Also, while this study shows that the TrkB receptor itself could be an important target of such drugs, there also may be other targets in the molecular cascade that activation of the receptor triggers in the neuron." Thus, said McNamara, further studies in his laboratory will aim at understanding the signaling pathways within the neuron that are activated by TrkB. Also, he said, the researchers will explore whether knocking out TrkB in other animal models of epilepsy produces the same effect of preventing development of epilepsy.