Individuals with Fragile X syndrome commonly report an increased awareness of environmental sounds, abnormal loudness perception, and difficulty in filtering or “hearing out” the important auditory information in background noise. This manifests itself as paying too much attention to stimuli that most people ignore, or not responding to the most important messages that are confronting the person. These symptoms interfere with attention, learning, language development and social interactions and are likely due to changes in synaptic connections in sensory circuits; the breakdown of the normal flow of information along pathways in the brain.
The integrity of neuronal encoding and processing of sensory input is dependent on a precise complement of ion channels that shape neuronal action potentials and determine their firing patterns and frequency of firing. Specifically, Na+ – activated potassium channels (termed KNa channels) are critical for this function. We have found that the Slack KNa channel directly binds the Fragile X protein (FMRP), resulting in a several-fold activation of Slack channel activity.
The association of Slack channels with FMRP suggests that pharmacological manipulations of channel activity could prove to be a novel therapy for Fragile X syndrome. In particular, FMRP knockout mice lack a physiological activator of Slack channel (FMRP itself) and expression of a C-terminal Slack isoform is completely abolished in fmrl-/- mice, leading to significantly reduced Slack currents. Thus some of the neuronal abnormalities associated with Fragile X may result from the effect of altered K currents on neuronal excitability and timing. Moreover our findings suggest that the interaction of Slack channels with FMRP could regulate rates of translation of mRNAs bound to FMRP.
Our laboratory is developing a range of compounds that can activate Slack channels. We plan to determine whether these compounds enhance Slack channel activity and restore normal firing patterns in neurons from fmrl-/- mice. If we find potent and selective compounds, this will provide a strong impetus for the evaluation of Slack activators as novel therapeutic agents that could act either independently or in concert with agents that act at group1 metabotropic glutamate receptors, which have been found to regulate Slack channels.
Results published in Nature Neurosci. 2010
Fragile X mental retardation protein controls gating of the sodium-activated potassium channel Slack
by Leonard Kaczmarek, PhD and Jack Kronengold, PhD
Our laboratory has investigated how the excitability of neurons becomes modified in the absence of the FMRP protein. We have found that the levels of two potassium channels, termed Slack and Kv3.1 are altered in mice that lack this protein. We have made significant progress in identifying novel pharmacological activators of the Slack potassium channel for potential therapeutic intervention in FXS individuals.
The Slack potassium channel is widely expressed in the brain. Using neurons of the central auditory system, our laboratory has demonstrated that Slack is required for accurate timing of action potentials in response to synaptic stimuli. This channel is activated by the FMRP protein through a direct association of FMRP with the cytoplasmic domain of the channel itself. We have shown that levels of the Slack potassium current are reduced by about 50% in neurons of FMR1-/- mice. Treatment of neurons with pharmacological activators of Slack would be expected to restore levels of this current back to levels found in wild type animals. Using a combination of electrophysiological and biochemical assays, we have now identified five compounds that are effective activators of Slack channels and are now testing their specificity for this class of channels.
In addition, we have recently published work (see below) that has identified another potassium channel, Kv3.1, that is altered by loss of FMRP. The physiological role of this channel is to allow specific types of neurons to fire at very high rates. We have found that Kv3.1 mRNA is a binding partner for FMRP, and that in FMR1-/- mice, levels of this channel are elevated. Moreover, in the central auditory system Kv3.1 is normally expressed in an orderly tonotopic gradient, such that neurons responding to the highest frequency sounds have the highest levels of Kv3.1 channel. This orderly gradient is abolished in the absence of FMRP, causing all these neurons to have uniformly high levels of this channel. This finding suggests that auditory neurons are likely to be hyperexcitable in FXS individuals. In collaboration with a pharmaceutical company, we are therefore now investigating the actions of activators and inhibitors of the Kv3.1 potassium channel.