Abstract
Tau Reduction Prevents Disease in a Mouse Model of Dravet Syndrome
Gheyara AL, Ponnusamy R, Djukic B, Craft RJ, Ho K, Guo W, Finucane MM, Sanchez PE, Mucke L. Ann Neurol 2014;76:443–456.
OBJECTIVE: Reducing levels of the microtubule-associated protein tau has shown promise as a potential treatment strategy for diseases with secondary epileptic features such as Alzheimer disease. We wanted to determine whether tau reduction may also be of benefit in intractable genetic epilepsies. METHODS: We studied a mouse model of Dravet syndrome, a severe childhood epilepsy caused by mutations in the human SCN1A gene encoding the voltage-gated sodium channel subunit Nav1.1. We genetically deleted 1 or 2 Tau alleles in mice carrying an Nav1.1 truncation mutation (R1407X) that causes Dravet syndrome in humans, and examined their survival, epileptic activity, related hippocampal alterations, and behavioral abnormalities using observation, electroencephalographic recordings, acute slice electrophysiology, immunohistochemistry, and behavioral assays. RESULTS: Tau ablation prevented the high mortality of Dravet mice and reduced the frequency of spontaneous and febrile seizures. It reduced interictal epileptic spikes in vivo and drug-induced epileptic activity in brain slices ex vivo. Tau ablation also prevented biochemical changes in the hippocampus indicative of epileptic activity and ameliorated abnormalities in learning and memory, nest building, and open field behaviors in Dravet mice. Deletion of only 1 Tau allele was sufficient to suppress epileptic activity and improve survival and nesting performance. INTERPRETATION: Tau reduction may be of therapeutic benefit in Dravet syndrome and other intractable genetic epilepsies.
Commentary
Epileptogenesis involves the formation of neural circuits that produce excessive and synchronized action potential activity that leads to increased cellular and network excitability, epileptiform EEG activity, and spontaneous seizures. Comorbidities can include psychiatric abnormalities, cognitive dysfunction and sudden unexpected death in epilepsy (SUDEP). Whereas acquired epilepsies are often triggered by an event (e.g., brain injury) that stimulates epileptogenesis, in many epilepsies of genetic etiology, epileptogenic neuronal circuitry is thought to be established by altered neuronal properties conferred by genetic mutations, frequently involving neuronal ion channels. Genetic epilepsies are often resistant to drug therapies and can be particularly devastating when they lead to seizures and cognitive deficits in children. One of the most severe and intractable forms of childhood epilepsy is Dravet syndrome. Relevant animal models of human epilepsies, including Dravet syndrome, have been developed, with the aim of understanding the mechanisms underlying the causes of seizures and the triggers of epileptogenesis in order to develop novel disease-modifying treatments and, ultimately, prevent or cure the disorder.
Among the key molecular components contributing to normal neuronal structure and function during and after brain development is the microtubule-associated protein, tau. Tau normally functions in microtubule stability and therefore helps organize the neuronal cytoskeleton and participates in intra-cellular protein trafficking (1). Accordingly, tau participates in signal transduction and synaptic signaling processes, and all of these functions can be altered in brain disorders, including epilepsy. Previous studies have implicated tau's involvement in increased neuronal excitability and seizures associated with other genetic epilepsies, and tau suppression reduces chemical convulsant-induced seizure severity in normal mice (2–4). Suppression of tau can also reduce neuronal hyperexcitability and seizures in a mouse model of Alzheimer disease (5). The tau protein may therefore provide a mechanistic link between neuronal excitability and the formation of hyperexcitable circuits that increase seizure likelihood.
Genetic mutations affecting ion channels can lead to cellular excitability changes and are often presumed to underlie seizures in certain types of genetic epilepsy. In the case of Dravet syndrome, a type of genetic epilepsy characterized by seizure onset in the first year of life, cognitive decline, and often, premature death, it is most often a mutation in the SCN1A gene that leads to reduced expression of the NaV1.1 sodium channel and a consequent reduction in channel function in affected neurons. Transgenic mice with a truncation mutation of the SCN1A gene, referred to as Dravet mice, display similarities in phenotype to patients and constitute a model for the disease. These mice show increased susceptibility to hyperthermia-induced seizures and develop spontaneous seizures at a relatively early age, and epileptogenesis is accompanied by cellular changes and increased network excitability in the neocortex and hippocampus. Cognitive behavior is negatively affected, and sudden death often occurs within the first few months of life. Gheyara and colleagues in the Mucke lab crossed Dravet mice with transgenic mice containing a single or double-allele Tau deletion in order to determine the effect of reduced tau expression on the outcomes associated with epilepsy in the Dravet mouse model. Similar to what has been observed in mouse models of Alzheimer's disease (5) and an epilepsy model in which a potassium channel mutation (i.e., Kcna1) results in spontaneous seizures (3), outcomes associated with epileptogenesis were abrogated in Dravet mice in which one or both Tau alleles were depleted, supporting the hypothesis that tau is involved in the process of epileptogenesis in this highly relevant murine model of childhood epilepsy.
Deletion of tau in the Dravet mouse did not appear to alter Nav1.1 expression but resulted in fewer mice expressing seizures, reduced interictal EEG activity, reduced signs of hippocampal abnormality, improved cognitive behavioral outcomes, and a lower incidence of SUDEP. The results establish a clear link between tau expression and development of epilepsy and associated comorbidities that were separate from the sodium channel mutation itself. Since tau functions in many cellular processes associated with cellular growth and protein processing in neurons, it makes sense that deleting the protein could interfere with the process of epileptogenesis in genetic epilepsy models like the Dravet mouse. In addition, tau suppression could also prevent some of the cellular excitability changes stemming directly from the channelopathy. A major unresolved question, however, is whether suppression of tau activity would affect seizures or comorbidities in other types of epilepsy or after the disorder has already developed.
The link between sodium channelopathy and epilepsy development is unclear, as is the mechanism for how reduced sodium channel function is proconvulsant in the Dravet mouse (although reasonable hypotheses have been proposed based on Nav1.1 deletion in inhibitory neurons or genetic overcompensation by other sodium channels). Whether the resulting epilepsy is due to the channelopathy causing seizures directly or whether channel dysfunction initiates neural network reorganization that underlies epileptogenesis is also unknown. In other words, it is possible that local excitability change resulting from channel dysfunction induces or contributes to development of more widely expressed network changes reminiscent of the synaptic reorganization that characterizes many types of acquired epilepsy (e.g., axon sprouting, synaptic reorganization). These changes are evident in Dravet mice in the present study and other genetic epilepsy models (3) once spontaneous seizures are expressed; genetic Tau deletion reduces markers of these biochemical and functional correlates of epileptogenesis. Tau may thus participate in epileptogenesis indirectly by supporting or exacerbating functional synaptic reorganization. And yet, tau might participate more directly by modifying excitability of individual neurons. Conceivably, direct effects of tau on cellular excitability in neurons expressing the mutated channel could ultimately affect epilepsy-related outcomes.
It seems reasonable to hypothesize that the sodium channel dysfunction initiates a cascade of cellular and network events that eventually lead to epilepsy, and these events are mediated by tau, as suggested by the work of Gheyara and colleagues. More work on the link between channelopathies and epilepsy development is needed to clarify the nature of this association.
The manner in which excess tau protein participates in the formation of an epileptogenic circuit and how deletion of tau prevents the formation of key components of that circuit remain unknown. Perhaps of greater relevance, the degree to which tau participates in stabilizing the reorganized, epileptogenic circuitry, or whether tau inhibition might prevent seizures and epilepsy comorbidities after epileptogenesis has already occurred also remains unknown. Gheyara and colleagues have demonstrated antiepileptogenic effects of Tau deletion in a relevant model of human genetic epilepsy. Understanding the mechanisms by which tau contributes to or participates in epileptogenesis, seizure suppression, or both seems likely to improve our knowledge of epilepsy as a disease and, ultimately, lead to improved therapies for Dravet syndrome and other epilepsies, based on that knowledge.