The ATP-Gated P2X7 Receptor as a Novel Drug Target for the Treatment of Drug Refractory Status Epilepticus

Epilepsy is a common and devastating neurological disorder characterised by increased and abnormal neuronal firing and recurrent, unprovoked seizures. Approximately 70 million people worldwide are affected by the disease, impacting lifestyle, health and work. Current drug treatments for epilepsy are primarily based around the promotion or inhibition of GABA (inhibitory) or glutamate (excitatory) neurotransmission. However, it is estimated that up to 30% of patients are drug refractory and new therapeutic approaches are being sought, particularly the identification of novel target genes which may provide new, and more effective, mechanisms of action for the treatment of drug refractory epilepsy.

Recent evidence has emerged showing how ATP-driven neuroinflammation contributes to both seizure severity and the epileptic phenotype. Under physiological conditions, ATP is present in low concentrations, maintaining CNS homeostasis as well as facilitating cellular communication. Under pathological conditions, extracellular ATP is increased, activating P2 receptors, regulating neuroinflammatory pathways and contributing to changes in glial function. Most notably, however, these insult-induced increases in ATP activates the P2X7 receptor, which has now been widely reported to contribute to prolonged and exacerbated seizure activity, as well as increased seizure-induced neuronal cell death and neuroinflammation.

The ionotropic ATP-gated P2X7 receptor has been repeatedly implicated in status epilepticus and epilepsy, and previous studies have reported increased P2X7 receptor expression in the hippocampus and cortex following seizures. Here, using a P2X7-overexpressing mouse, this study aimed to develop a greater understanding of the effect of the P2X7 receptor post-status epilepticus to characterise the precise role of the receptor, including where it is expressed under basal conditions, how and when it is activated post-SE and how it both influences down-stream signalling and contributes to neuroinflammation. Characterisation of a P2X7-EGFP reporter mouse revealed widespread expression of the P2X7 receptor and status epilepticus-induced increases in a number of brain regions including hippocampus, cortex, cerebellum, striatum and thalamus. Furthermore, fluorescent activated cell sorting and immunohistochemical analysis found glial-specific expression of the P2X7 receptor, with no conclusive evidence of the receptor in either neurons or astrocytes.

Anti-seizure medication resistance is a relatively common observation in epileptic patients and is an unmet clinical complication in the treatment of status epilepticus. Here, we attempted to elucidate the influence of the P2X7 receptor on seizure activity during status epilepticus. Although previous studies have hypothesised a role in exacerbating seizure severity and contributing to increased cell death, we showed that increased P2X7 receptor expression is, in fact, a possible contributor to the unresponsiveness to treatment. Using both a genetic and pharmacological model of P2X7 receptor overexpression, our data shows continued seizure activity following administration of several anti-convulsants. Our results suggest that by tempering the rapid escalation of P2X7 receptor activation with suitable pharmacological intervention during, and in the aftermath of, status epilepticus, P2rx7 may be used a suitable site for targeted treatments to alleviate status epilepticus and epilepsy.

Finally, prognostic tools are currently being developed to identify lesions, epileptic foci as well as structural abnormalities which may contribute to the development of epilepsy. Here, we tested a P2X7-specific radiotracer, 18F-JNJ64413739, using positron emission tomography (PET) imaging and computerized tomography (CT) scanning to determine whether it would be possible to identify increased P2X7 receptor expression following status epilepticus and whether it may be adapted as a novel prognostic tool for epilepsy. Results showed that increased seizure activity during status epilepticus correlated directly with radiotracer uptake observed at 48 hours following status epilepticus, suggesting it may be possible to use PET imaging of the P2X7 receptor to analyse and monitor seizure activity.

In conclusion, this thesis suggests the P2X7 receptor contributes to anticonvulsant resistance via inflammation following a period of status epilepticus. Thus, the development of P2X7-targeting as an adjunctive treatment to current anti-seizure therapies may be a promising therapeutic strategy to curtail seizures, particularly during drug refractory status epilepticus. Furthermore, identifying and using the P2X7 receptor as a criterion via PET imaging may also be an ideal prognostic tool for classification of severity of SE and subsequent risk of developing epilepsy.