Sodium-activated potassium (KNa) channels are widely expressed throughout the central nervous system. Activation of these channels is known to protect cells from hypoxic injury. The molecular correlate of KNa currents, however, was unknown until the genes underlying this new family of K+ channels were cloned relatively recently. Slack (Sequence like a calcium-activated K channel) and Slick, which are also referred to as Slo2.2 (KCa4.1) and Slo2.1 (KCa4.2), currently have no pharmacological tools that allow for modulation of their function. With the help of this grant we are therefore proposing to design potent and brain-penetrant Slack channel activators that could be used to explore the therapeutic potential of these interesting channels. In normal neurons, KNa channels contribute to the slow afterhyperpolarizations that follows repetitive firing, regulate rates of bursting and enhance the accuracy with which action potentials lock to incoming stimuli. Evidence further indicates that KNa channels play a crucial role in protecting cells from injury under ischemic conditions, when inhibition of the plasma membrane Na+-K+-ATPase by the lack of oxygen leads to an increase in intracellular sodium levels. Activation of KNa channels under these circumstances is likely to prevent calcium entry by stabilizing the membrane potential and protecting neurons from overloading with calcium. In proof of this concept, mutation of the ortholog of Slack in the nematode C. elegans renders these animals hypersensitive to hypoxia indicating that KNa channels provide endogenous protection against hypoxia in this species. Compounds that increase the activity of KNa channel therefore should be therapeutically useful for the treatment of stroke and the prevention of the effects of global cerebral ischemia as occurs, for example, in cerebral palsy. By increasing the slow afterhyperpolarizations, KNa channel activators may also be useful for reducing neuronal excitability in epilepsy and ataxia. By screening various pharmacophores known to activate the related large-conductance Ca2+-activated K+ channel BK (Slo1, Maxi-K) it was recently discovered that biphenylthioles and 4-arylquinolinones activate Slack channels in the low micromolar range. Interestingly, two compounds in the 4-arylquinolone series were found to increase Slack activity without exerting effects on BK channels demonstrating that it is possible to separate the two activities. By combining i) classical medicinal chemistry, ii) a recently developed high- throughput assay measuring mass redistribution at the plasma membrane to determine Slack activation, iii) electrophysiology and iv) pharmacokinetic experiments in rats we here propose to improve the potency, selectivity and brain-penetration of our leads. Our overall goal is to provide the scientific community with a Slack channel activator that is suitable for in vivo use.
Based on their abundant expression in the brain sodium-activated potassium (KNa) channels potentially constitute novel drug targets for the treatment of stroke, cerebral palsy, epilepsy and ataxia. However, these important channels currently have no pharmacological modulators. With the help of this grant we will attempt to design small molecule KNa channel activators that could be used as scientific tool compounds to test whether KNa channels indeed constitute novel targets for neurological diseases.
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