Modulation of inflammation in epilepsy

Epilepsy is a disease characterized by the occurrence of epileptic seizures often without known cause. About 30% of patients do not respond adequately to antiepileptic drugs. Experimental and clinical studies during the last 15 years revealed that chronic inflammation in the brain might contribute to the occurrence of epileptic seizures. On the other hand, it has been shown that seizures themselves may cause brain inflammation. In this context, two proteins (Interleukin-1 beta and High mobility group box 1A, HMGB1) have been shown to be relevant. These proteins bind to their respective receptors (interleukin-1-receptor, toll-like receptor 4 TLR4) and trigger cellular processes that lead to increased excitability of neurons likely contributing to the generation of epileptic seizures. Some groups showed that the interaction of HMGB1 with TLR4 is important in epilepsy. We have also observed that a similar protein to TLR4, TLR2, is highly expressed in neurons and glia cells of the epileptic brain. Physical interaction between HMGB1 and TLR2 has been demonstrated. Thus, we think that this interaction may also contribute to the generation of seizures. We now developed several compounds that can activate or inhibit TLR2 and they are therefore valuable tools to investigate the role of TLR2 and HMGB1 in epilepsy. Consequently, TLR2 appears as a valid therapeutic target to develop new drugs for epilepsy.

In Europe, about 2.7 million people are affected by epilepsy. Unfortunately, 30 to 40% of patients do not respond well to anti-epilepsy medications and there is a need to develop new drugs that are also effective in these patients. The most common focal epilepsy is temporal lobe epilepsy. It is characterized by seizures that originate primarily in the hippocampus or amygdala. In these brain regions, inflammatory reactions in the tissue are often found in epilepsy patients. It is now known that inflammatory processes in the brain may contribute to the occurrence of spontaneous epileptic seizures. Best studied is the role of TLR4, a receptor from the Toll-like receptor family. These receptors are part of the innate immune system and normally recognize tissue-invaded germs (viruses, bacteria) and so-called DAMPs (damage-associated molecular patterns), molecules secreted by damaged or dying cells. In the present project, we investigated whether other Toll-like receptors (TLR1, TLR2, TLR6, and TLR8) also play a role in epilepsy and thus might represent potential targets for new therapeutic options. In the first series of experiments, we characterized the anti-inflammatory potential of eight previously developed inhibitors of TLR2. Four of the inhibitors showed clear anti-inflammatory effects and could serve as a starting point for the development of drugs against inflammatory diseases. In another experiment, we demonstrated that INH14, an inhibitor of intracellular signaling pathways of several TLRs, has potent anti-inflammatory properties, and we identified the IKK and IKK kinases as binding partners of INH14. In another experiment, we showed that TLR8 is present in mouse hippocampus mainly in parvalbumin-positive inhibitory interneurons, but not in glial cells or pyramidal cells. This may play a role in epilepsy, as many of these parvalbumin interneurons degenerate in epilepsy. Furthermore, in hippocampal neuronal cell cultures, we showed that activation of TLR8 by the specific agonist TL8-506 led to a marked increase in electrical activity and synchronization of firing. Inhibition of this receptor could likely dampen excitability within the hippocampus, but this will require further future experiments. We tested another inhibitor of TLRs, INH50, for its antiepileptic potential in a mouse model of chronic epilepsy and on nerve cell cultures. One-week treatment of epileptic mice with INH50 caused a marked decrease in spike trains recorded in the EEG, but failed to affect the number and duration of spontaneous epileptic seizures. In the cell culture model, epilepsy-like activity previously induced by picrotoxin was markedly suppressed by INH50. We also stimulated naturally occurring receptor combinations of TLR2 with TLR1/TLR6 in cell culture and observed increased synchronization of firing. Based on the data collected, it seems very likely that other TLRs besides TLR4 open up potential therapeutic options in epilepsy.