Regulation of Neuronal Excitability by Interaction of Fragile X Mental Retardation Protein with Slack Potassium Channels

Abstract

Loss of the RNA-binding protein fragile X mental retardation protein (FMRP) represents the most common form of inherited intellectual disability. Studies with heterologous expression systems indicate that FMRP interacts directly with Slack Na+-activated K+channels (KNa), producing an enhancement of channel activity. We have now usedAplysiabag cell (BC) neurons, which regulate reproductive behaviors, to examine the effects of Slack and FMRP on excitability. FMRP and Slack immunoreactivity were colocalized at the periphery of isolated BC neurons, and the two proteins could be reciprocally coimmunoprecipitated. Intracellular injection of FMRP lacking its mRNA binding domain rapidly induced a biphasic outward current, with an early transient tetrodotoxin-sensitive component followed by a slowly activating sustained component. The properties of this current matched that of the native Slack potassium current, which was identified using an siRNA approach. Addition of FMRP to inside-out patches containing nativeAplysiaSlack channels increased channel opening and, in current-clamp recordings, produced narrowing of action potentials. Suppression of Slack expression did not alter the ability of BC neurons to undergo a characteristic prolonged discharge in response to synaptic stimulation, but prevented recovery from a prolonged inhibitory period that normally follows the discharge. Recovery from the inhibited period was also inhibited by the protein synthesis inhibitor anisomycin. Our studies indicate that, in BC neurons, Slack channels are required for prolonged changes in neuronal excitability that require new protein synthesis, and raise the possibility that channel–FMRP interactions may link changes in neuronal firing to changes in protein translation.

Regulation of synaptic development and function by the Drosophila PDZ protein Dyschronic

“…SLO channels contain a large intracellular Cterminal domain (Lee and Cui, 2010). Interestingly, recent studies have demonstrated that the C-terminal domain of SLACK channels, BK channel homologs that are potentiated by sodium rather than Ca 2+ , are bound to by the Fragile-X mental retardation protein in a manner that induces channel activation (Brown et al, 2010;Zhang et al, 2012). The C-terminal domain of SLO channels may similarly act as a scaffold to bind synaptic proteins that regulate AZ structure.…”

Therapeutic Strategies in Fragile X Syndrome: From Bench to Bedside and Back

“…Interestingly, apart from translational regulation, FMRP also directly binds to ion channels, which has revealed noncanonical functions. FMRP binds and activates Slack [105,106], and FMRP's association with BK channels alters their calcium sensitivity in a protein synthesis-independent manner [98,107]. A recent study also showed that FMRP interacts with the voltage-gated N-type calcium channel Ca V 2.2 and modulates its function and expression [108].…”

International Union of Basic and Clinical Pharmacology. C. Nomenclature and Properties of Calcium-Activated and Sodium-Activated Potassium Channels

“…The activity of K Na 1.1 channels can be also regulated by their interaction with other cellular proteins. These include the transmembrane protein TMEM16C (Huang et al, 2013) and the Fragile X Mental Retardation protein (Brown et al, 2010;Zhang et al, 2012b), both of which enhance K Na 1.1 channel activity. Interactions with several other cellular components, including the postsynaptic density protein PSD-95, have also been reported but their functional effects are not yet known (Uchino et al, 2003;Rizzi et al, 2015).…”