New therapeutic strategies for ischemic stroke targeting TREK channels and NCX: identification of new drugs and optimization of their use

Ischemic stroke is a major cause of death and disability whose related costs pose a significant financial burden on social security system. The only approved drug for its treatment, rTPA, is far from being ideal because of its potential neurotoxicity and of its lack of efficacy when administered more than 3-6 hours beyond the beginning of symptoms. Therefore, the quest for better drugs is pressing. Emerging experimental evidence suggests that effective drugs could be developed targeting two families of plasmamembrane proteins, which control ionic homeostasis, the TREK family of two-pore potassium channels and the three isoforms of the sodium-calcium exchanger, NCX. TREK are open in a wide range of membrane potential hence sustaining a background K+ current which contributes to stabilize resting potential. In brain ischemia, TREK get activated because of the concurrent marked increase in poly-unsaturated fatty acids (PUFA) concentration and they halt ischemia-induced membrane depolarization. This accounts for their neuroprotective role which is underscored by the hypersusceptibility to brain ischemia of knock-out mice lacking TREK-1, one of the isoforms of these channels. Similarly, NCX contributes to keep low Ca2+ and Na+ cytosolic concentration and, its activation counteracts their pathological rise during brain ischemia. This accounts for NCX neuroprotective role as exemplified by the worsening of brain ischemia n rats treated with antisense oligonucleotides directed against NCX isoforms 1 and 3 (Pignataro et al, 2004). A neuro protective role for TREK and NCX is also suggested by the ischemia-induced compensatory changes in the transcription of their genes (Li et al., 2005; Boscia et al., in press). Despite these promising experimental data, selective NCX or TREK activators suitable for preclinical development as stroke therapy are still lacking. This project aims: 1- to develop selective NCX and/or TREK activators and to evaluate their effectiveness in brain ischemia; 2- to evaluate the benefits of adding a TREK activator to an NCX activator in the treatment of stroke and to determine strategies to optimize this combined therapeutic strategy. As far as the 1st point is concerned, it is worth to underline that no selective NCX activator is available yet whereas, besides gaseous anesthetics, the only TREK activator is the unselective agonist riluzole. Therefore, and given the potential interest of both TREK and NCX as drug targets in brain ischemia, the quest for TREK and NCX activators is of major pharmacological interest. The proposed collaboration with the Bejing Institute of Materia Medica is crucial to achieve the isolation of such compounds. This institution has, indeed, a long experience in isolating pharmacologically active molecules from natural sources, and, at this institution, a library of several thousands molecules obtained from plants of traditional Chinese medicine is already available for testing. Interestingly, a few of them already proved effective in experimental models of ischemia. Concerning the 2nd point, the idea of a potential synergism of drugs targeting NCX and TREK is supported by evidence that, during ischemia, NCX is the major factor controlling the release of PUFA (Pilitsis et al., 2001) which are potent TREK activators. As PUFA metabolism entails the generation of neurotoxic free radicals, we intend to explore whether the neuroprotective effect of NCX/TREK activators could be enhanced by the concomitant administration of drugs interfering with the generation of these toxic sub-products. To this aim, the effect of COX inhibition with FANS and of a number of free radical scavengers identified at the Bejing Institute of Materia Medica will be also studied.