Regulation of Neuronal Excitability through Pumilio-Dependent Control of a Sodium Channel Gene

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Abstract
Dynamic changes in synaptic connectivity and strength, which occur during both embryonic development and learning, have the tendency to destabilize neural circuits. To overcome this, neurons have developed a diversity of homeostatic mechanisms to maintain firing within physiologically defined limits. In this study, we show that activity-dependent control of mRNA for a specific voltage-gated Na+channel [encoded byparalytic(para)] contributes to the regulation of membrane excitability inDrosophilamotoneurons. Quantification ofparamRNA, by real-time reverse-transcription PCR, shows that levels are significantly decreased in CNSs in which synaptic excitation is elevated, whereas, conversely, they are significantly increased when synaptic vesicle release is blocked. Quantification of mRNA encoding the translational repressorpumilio(pum) reveals a reciprocal regulation to that seen forpara. Pumilio is sufficient to influenceparamRNA. Thus,paramRNA is significantly elevated in a loss-of-function allele ofpum(pumbemused), whereas expression of a full-lengthpumtransgene is sufficient to reduceparamRNA. In the absence ofpum, increased synaptic excitation fails to reduceparamRNA, showing that Pum is also necessary for activity-dependent regulation ofparamRNA. Analysis of voltage-gated Na+current (INa) mediated byparain two identified motoneurons (termed aCC and RP2) reveals that removal ofpumis sufficient to increase one of two separableINacomponents (persistentINa), whereas overexpression of apumtransgene is sufficient to suppress both components (transient and persistent). We show, through use of anemone toxin (ATX II), that alteration in persistentINais sufficient to regulate membrane excitability in these two motoneurons.