Preclinical studies exploring the therapeutic potential of Kv7 potassium channels in CNS diseases

Many neuronal diseases are characterized by hyper-excitability. For instance, ischaemic stroke causes the activation of multiple processes, including ionic imbalance, increase in the extracellular concentration of glutamate and activation of the NMDA receptors, with following increase in intracellular calcium and cell death, peri-infarct depolarization (Doyle et al., 2008); epilepsy shows similar pathogenetic mechanisms (Costa et al., 2006). Excessive neurotransmitter release is a key events in the propagation of neuronal damage, and subsequent death, following ischaemia. Although glutamate release (and in particular on NMDA receptor over-stimulation) has been viewed as a main cause of neuronal damage, massive release of monoamine neurotransmitter such as dopamine, also occurs immediately following the onset of ischemia (Sarna et al., 1990). ). Voltage-gated Kv7 potassium channels family comprises five members (Kv7.1- Kv7.5) that have been demonstrated to regulate DA release in the caudate, a region rich in DA and frequently damaged in ischemic stroke (Martire et al., 2007). In this study we investigated the potential neuroprotective role of Kv7 channels in an in vitro model of ischemia, namely rat brain slices undergoing oxygen- and glucose- deprivation (OGD). In particular, we evaluated the effects of different Kv7-acting drugs on: 1) OGD-induced DA release, measured by fast cyclic voltammetry (Davidson et al., 2011); 2) OGD-induced damage, assessed by 2,3,5-triphenyltetrazolium chloride (TTC) staining. Kv7 activators and ICA27243 reduced dopamine release and brain damage induced by OGD. The Kv7 blocker linopiridine increased dopamine release and enhanced OGD-induced brain damage, and also prevented RT-induced neuroprotection. The results shown before suggest that changes in the activity of Kv7 channels might represent a relevant event in the pathological cascade leading to brain dysfunction and damage. This potential role of Kv7 channels, in particular Kv7.2, is underlined by the finding that mutations in Kv7.2 gene can lead not only to BFNS, but also to severe form of neuronal diseases, such as pharmacoresistant epilepsy or encephalopathy. Such heterogeneous phenotypes might be due to distinct mutations, differently affecting channel function. To better understand this phenomenon, we investigated the functional changes prompted by two mutations in the fifth arginine residue of the S4 transmembrane and responsible for epileptic encephalopathy (R6Q)(Weckhuysen et al., 2012), or for BFNS (R6W) (Sadewa et al., 2012). Macroscopic current analysis in transfected cells revealed that both mutations, in homomeric or heteromeric configuration with KV7.2 and/or KV7.3, did not modify the maximal current density, suggesting that they did not interfere with channel expression to plasma membrane. By contrast, both mutated channels showed a remarkable decrease in their voltage sensitivity, with a more dramatic effect for R6Q when compared with the R6W mutation. Such dramatic effects were observed both in homomeric conformation with Kv7.2, as well as in the heteromeric configuration mimicking the genetic balance of the affected individual. Computational results from a CA1 model of hippocampal neuron showed that incorporation of a single R6Q in heteromeric configuration with KV7.2 and KV7.3 subunits, dramatically increased neuronal firing, with an effect significantly higher than that produced by R6W mutation. Interestingly, retigabine, at clinically relevant was able to restore normal function in both R6Q and R6W channels, by shifting voltage-dependence of these channels to values similar to wild-type subunits.Taken together, these results suggest a main role for Kv7.2 also in the modulation of DA release induced by an ischemic insult and highlight pharmacological activation of Kv7 channels as a possible strategy for the treatment of brain ischemia. Taken together, these data suggest that Kv7 channels might play a key pathogenetic role in regulation of neuronal dysfunction characterized by hyper-excitability, and in the sequence of events that lead to severe brain damage and neurodegeneration, possibly by counteracting neuronal hyper-excitability and consequent massive dopamine (and other neurotransmitter) release occurring in this pathological condition. Thus, pharmacological modulation of Kv7 channels might represent a unique tool to counteract hyperexcitability dysfunction