Activation of Slack Potassium Channels (KCNT1) Triggers an Increase in mRNA Translation

Abstract

The Slack ion channel is a member of a family of large conductance sodium-activated potassium channels. It is expressed predominantly in neurons of the central nervous system where it regulates neuronal excitability. FMRP, an important regulator of mRNA translation, binds both Slack mRNA and the Slack protein. The association of Slack with FMRP stimulates channel activity, raising the possibility that activation of Slack channels may also regulate the function of FMRP. Our laboratory has previously identified Slack as required for a translation-dependent recovery from an extended period of inhibition in Aplysia neurons following stimulation, further suggesting that Slack may play a role in the regulation of mRNA translation. Here we provide the initial evidence for such a role. We transfected cells with a fluorescent reporter for mRNA translation, which contains the 5’ and 3’ sequences of the mRNA for β-actin, but with the coding region replaced with that for the irreversibly-photoconvertible fluorescent protein dendra2. Using experiments that visualize real-time translation in a stable Slack-expressing HEK cell line and in mouse cortical neurons, together with pharmacological manipulation and silencing RNA knockdowns, we have found that Slack activation causes an increase in translation that is enhanced in the absence of FMRP. This increase in translation persists in the presence of the Slack channel blocker quinidine, indicating that it does not require ion flux through the channel. Puromycin incorporation experiments suggest Slack activation may produce a global increase in translation. Additional experiments in HEK cell culture suggest that Slack binding partners CYFIP1, another FMRP binding protein; and Phactr-1, a PP1 binding protein, may also modulate this channel-dependent translation. This mechanism of Slack-dependent translation potentially represents the first instance of the direct modulation of mRNA translation by activation of an ion channel.