Modulators of Kv3 Potassium Channels Rescue the Auditory Function of Fragile X Mice

Abstract

Fragile X syndrome (FXS) is characterized by hypersensitivity to sensory stimuli, including environmental sounds. We compared the auditory brainstem response (ABR) recorded in vivo in mice lacking the gene (Fmr1 ؊/y ) for fragile X mental retardation protein (FMRP) with that in wild-type animals. We found that ABR wave I, which represents input from the auditory nerve, is reduced in Fmr1 ؊/y animals, but only at high sound levels. In contrast, wave IV, which represents the activity of auditory brainstem nuclei is enhanced at all sound levels, suggesting that loss of FMRP alters the central processing of auditory signals. Current-clamp recordings of neurons in the medial nucleus of the trapezoid body in the auditory brainstem revealed that, in contrast to neurons from wild-type animals, sustained depolarization triggers repetitive firing rather than a single action potential. In voltage-clamp recordings, K ϩ currents that activate at positive potentials ("high-threshold" K ϩ currents), which are required for high-frequency firing and are carried primarily by Kv3.1 channels, are elevated in Fmr1 ؊/y mice, while K ϩ currents that activate near the resting potential and inhibit repetitive firing are reduced. We therefore tested the effects of AUT2 [((4-({5-[(4R)-4-ethyl-2,5-dioxo-1-imidazolidinyl]-2-pyridinyl}oxy)-2-(1-methylethyl) benzonitrile], a compound that modulates Kv3.1 channels. AUT2 reduced the high-threshold K ϩ current and increased the low-threshold K ϩ currents in neurons from Fmr1 ؊/y animals by shifting the activation of the high-threshold current to more negative potentials. This reduced the firing rate and, in vivo, restored wave IV of the ABR. Our results from animals of both sexes suggest that the modulation of the Kv3.1 channel may have potential for the treatment of sensory hypersensitivity in patients with FXS. Key words: auditory brainstem response; AUT2; fragile X; high-and low-threshold potassium channels; medial nucleus of the trapezoid body; potassium channelsSignificance Statement mRNA encoding the Kv3.1 potassium channel was one of the first described targets of the fragile X mental retardation protein (FMRP). Fragile X syndrome is caused by loss of FMRP and, in humans and mice, causes hypersensitivity to auditory stimuli. We found that components of the auditory brain response (ABR) corresponding to auditory brainstem activity are enhanced in mice lacking FMRP. This is accompanied by hyperexcitability and altered potassium currents in auditory brainstem neurons. Treatment with a drug that alters the voltage dependence of Kv3.1 channels normalizes the imbalance of potassium currents, as well as ABR responses in vivo, suggesting that such compounds may be effective in treating some symptoms of fragile X syndrome.

Mechanisms underlying auditory processing deficits in Fragile X syndrome

“…Several ABR alterations have been identified by comparing Fmr1 knockout mice and age‐matched wild types under anesthesia. Similar to human studies, peak latencies appear to be a subject of FMRP loss in FVB mice, but not in B6 mice . Interestingly, although there is no report of altered ABR wave amplitudes in human FXS, the amplitudes of several ABR peaks are altered in Fmr1 knockout mice, including decreased peak I and increased peak IV, suggesting impaired peripheral and central auditory systems.…”

“…In wild‐type mice, the normal response of MNTB neurons to a sustained depolarization is to fire only a single action potential at the onset of the depolarization. In MNTB neurons from Fmr1 knockout animals, however, the same depolarization results in repetitive firing throughout the current pulse (Figure A). Such changes in the intrinsic excitability of neurons produced by loss of FMRP must reflect changes in the numbers and types of ion channels expressed in neurons.…”

“…Kv3.1 is normally expressed in a gradient along the tonotopic axis of MNTB, with highest levels in neurons that respond optimally to high‐frequency sounds . This gradient is absent and overall levels of this Kv3.1 protein and Kv3.1 currents are significantly higher in mice lacking FMRP (Figure B,C) . Moreover, levels of Kv3.1 in VCN and MNTB are increased by physiological increases in auditory stimulation in vivo .…”

“…MNTB neurons from Fmr1 knockout mice are hyperexcitable. A, Current‐clamp recordings of action potentials in MNTB neurons from wild‐type and Fmr1 knockout mice in response to hyperpolarizing and depolarizing current pulses (modified from 64). B, Bar graphs showing quantification of Kv3.1 immunoreactivity in MNTB in silence or after 30 minutes of exposure to physiological sound stimulation.…”

“…In both cases, the direct binding of the channel subunit to FMRP promotes channel opening, increasing current amplitude . Accordingly, the amplitude of the low‐threshold potassium currents is reduced in MNTB neurons from Fmr1 knockout mice, promoting the abnormal firing patterns shown in Figure . Novel drugs that alter the voltage‐dependence of Kv3.1 channels have been shown to rectify these abnormal firing patterns by increasing low‐threshold currents and decreasing high‐threshold currents in Fmr1 knockout animals .…”