Exosomal TNF-[alpha] mediates voltage-gated [Na.sup.+] channel 1.6 overexpression and contributes to brain tumor-induced neuronal hyperexcitability

Patients affected by glioma frequently experience epileptic discharges; however, the causes of brain tumor-related epilepsy (BTRE) are still not completely understood. We investigated the mechanisms underlying BTRE by analyzing the effects of exosomes released by U87 glioma cells and by patient-derived glioma cells. Rat hippocampal neurons incubated for 24 hours with these exosomes exhibited increased spontaneous firing, while their resting membrane potential shifted positively by 10-15 mV. Voltage clamp recordings demonstrated that the activation of the [Na.sup.+] current shifted toward more hyperpolarized voltages by 10-15 mV. To understand the factors inducing hyperexcitability, we focused on exosomal cytokines. Western blot and ELISAs showed that TNF-[alpha] was present inside glioma-derived exosomes. Remarkably, incubation with TNF-[alpha] fully mimicked the phenotype induced by exosomes, with neurons firing continuously, while their resting membrane potential shifted positively. Real-time PCR revealed that both exosomes and TNF-[alpha] induced overexpression of the voltage-gated [Na.sup.+] channel Nav1.6, a low-threshold [Na.sup.+] channel responsible for hyperexcitability. When neurons were preincubated with infliximab, a specific TNF-[alpha] inhibitor, the hyperexcitability induced by exosomes and TNF-[alpha] was drastically reduced. We propose that infliximab, an FDA-approved drug to treat rheumatoid arthritis, could ameliorate the conditions of glioma patients with BTRE.