Ion Channel Regulation By Signal Transduction Pathways

National Institute Of Environmental Health Sciences

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Abstract

We have focused on three specific ion channels: CaV1.2, Kv11.1, BKca. CaV1.2 channels show modal gating that is regulated by dihydropyridine, protein phosphorylatiion, and calcium. We have obtained new evidence from single channel recordings that dihydropyridines modulate channel activity by altering their susceptibility to phosphorylation. Furthermore, calcium inhibits channel activity through dephosphorylation of the channel protein. Polymorphisms in the human ether-a-go-go-related gene 1, hERG1, are associated with cardiac arrhythmias. The Kv11.1 channels encoded by hERG1 are also essential for rhythmic excitability of the pituitary, where they are regulated by thyroid hormone through a signal transduction cascade involving the phosphatidylinositol 3-kinase (PI3K) and the Ser/Thr-directed protein phosphatase, PP5. Here we show that the hERG1 polymorphism at codon 897, which is read as a Thr instead of a Lys, creates a new phosphorylation site for the Akt protein kinase on the Kv11.1 channel protein. Consequently hormonal signaling through the PI3K signaling cascade, which normally stimulates K897 channels through PP5-mediated dephosphorylation, inhibits T897 channels through Akt-mediated phosphorylation. Thus, hormonal regulation of Kv11.1 in humans with the T897 polymorphism is predicted to prolong the QT interval of cardiac myocytes. A systematic bioinformatics search for single nucleotide polymorphisms in human ion channel genes identified fifteen additional candidates for such "phosphorylopathies," which are predicted to create or destroy putative phosphorylation sites. Changes in protein phosphorylation might represent a general mechanism for the effects of genetic variation on human health and its interaction with the environment. The BKca potassium channels, which are encoded by the KCNMA1 gene (slo) and regulate action potential duration in endocrine cells and nerve terminals. Genetic studies of ethanol action on locomotor coordination in invertebrate model organisms have identified BKca channels as a prominent target of ethanol action. Other studies have identified the Gs-cAMP-PKA signaling system as a target of ethanol action. Therefore, we have studied the mechanism of BKca regulation by ethanol in a model genetic organism, C. elegans. We identified the PKA site on ceBKca;when it was mutated the worms were impervious to ethanol up t o 500 mM ! Inhibition of the Ser/Thr protein phosphatase PP1 by targeted expression of a mammalian inhibitor protein in ce neurons also prevented ethanol from disrupting locomotion. We have confirmed this dependence of ethanol action on PP1 activity in rat hippocampal slices, in which ethanol inhibits LTP on CA1 pyramidal dendrites. In collaboration with Dr. Birnbaumer's group, TRPC channels have been implicated in calcium entry in response to signaling through Gq coupled receptors.