Len Kaczmarek

Leonard K. Kaczmarek, PhD, Principal Investigator
Jack Kronengold, PhD, Postdoctoral Fellow 2006-8,2010
Vali Gazula, Ph.D., Postdoctoral Fellow 2009
Yale University School of Medicine

FRAXA Awards:
  $45,000 in 2010
  $60,000 in 2009
  $60,000 in 2008
  $40,000 in 2007
  $40,000 in 2006

The Slack Potassium Ion channel as a Therapeutic Target for Fragile X Syndrome

Individuals with Fragile X syndrome have a range of perceptual processing deficits related to auditory stimuli. They 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 Mental Retardation Protein (FMRP), resulting in a several-fold activation of Slack channel activity. This is an entirely novel finding because previous work has focused on the role of FMRP in binding messenger RNAs.

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 animals (fmr1-/-) lack a physiological activator of Slack channel (FMRP itself) and expression of a C-terminal Slack isoform is completely abolished in fmrl-/- animals, 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.

Paper published in Nature Neurosci. 2010 May 30. Fragile X mental retardation protein controls gating of the sodium-activated potassium channel Slack

Individuals with Fragile X Syndrome exhibit extreme sensitivity to auditory stimuli. The debilitating behavioral symptoms associated with "sensory overload" interfere with attention, learning, language development and social interactions and are likely due to changes in synaptic connections in the auditory circuitry.

Studies have shown that the Fragile X knockout mouse also exhibits abnormal sensitivity to auditory stimuli including hyper reactivity and the induction of audiogenic seizures by acoustic stimulation. While audiogenic seizures have not been reported in Fragile X patients, the onset and manifestation of autistic behavior in these individuals has been directly correlated with auditory hypersensitivity. The source of the audiogenic seizures is believed to be due to increased excitation in auditory nuclei and not to an overall increase in brain excitability.

A paper in Cell by Darnell et al. (2001) has identified the mRNA for the voltage gated potassium (K+) channel, Kv3.1, as a candidate "binding target" for the Fragile X Mental Retardation Protein (FMRP). The absence of FMRP in the Fragile X knockout mouse would be expected to result in the altered regulation of Kv3.1 which is critical for normal synaptic function. The goals of this project are to determine the changes in expression of Kv3.1 in auditory neurons of the Fragile X knockout mouse. The Kv3.1 channel, which belongs to the Kv3 family of voltage-dependent K+ channels, is found at particularly high levels in neurons of auditory nuclei. The biophysical properties of the Kv3.1 channel impart a "fast spiking" (FS) phenotype to neurons that need to fire repetitively at high frequencies, in response to high frequency stimuli. Moreover, the response to all sound frequencies is dependent on a precise tonotopic expression of Kv3.1 in neurons of the auditory nuclei. Disruption of this auditory space code, or map, by altered regulation of Kv3.1, would be expected to interfere with auditory processing in auditory nuclei of the brain stem and in the auditory cortex.

We have identified a new FMRP-interacting protein which is widely expressed in neurons in many parts of the brain.We are currently examining its function in the auditory brainstem and how this is likely to be misregulated in Fragile X and Autism Spectrum Disorder individuals.

Our project now includes not only studies of the Kv3.1 channel but of the newly discovered protein as well. Because of the exciting discoveries centered around the mGluR theory of Fragile X, we have added studies involving the inferior colliculus, a brain region which expresses high levels of mGluR5. Fortuitously, we can study all these components simultaneously in neurons of the auditory brainstem.

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