Subunit specific mechanisms by which potassium channels mediate intrinsic plasticity and neuronal integration in the auditory pathway

The brain receives signals from sense organs (such as the ear) and processes it to extract information about the world. The incoming signals are in the form of electric pulses called 'action potentials' (APs). They are around 0.1 Volts in amplitude and 1 millisecond in duration. The APs propagate along nerves to release chemical messengers between brain cells (neurons) at specialized connections known as synapses. Much of this signalling is done by proteins called ion channels. These proteins are built from subunits each specified by a gene. Neurons must assemble all these proteins into molecular machines for signalling. My lab focusses on one class of voltage-gated ion channels called "potassium channels", for which there are over 80 genes. These potassium channels are the foundation on which all other forms of excitability are built, and they help to dampen the excitability after one signal, so that neurone is ready for the next.

The mechanism(s) by which the neurons control their potassium channels are fundamental for brain function and consciousness: too little activity of potassium channels and the brain goes epileptic, too much and we are become catatonic. This grant will explore the mechanisms by which two families of potassium channels are regulated (a voltage-gated family called Kv3 of which there are four members (Kv3.1-3.4) and a family of leak (or flux-gated) potassium channels called two-pore or K2P channels.

Most studies of potassium channels are done in cell lines, but to understand their function, studies must be conducted in real neurons within an actual brain; hence we work in vitro, on tissue from the brains of humanely killed mice. These ion channels are nearly identical to those of humans. We can measure the brain activity and manipulate the potassium channels to test their contribution to specific tasks.

Our model system for this study is hearing and the brain. This is because listening requires fast processing and extreme precision in integrating information from both ears, so as to map sound objects and identify external threats (the sound of a car) or extract information from noisy environments (listening to a conversation in a bar). My laboratory has extensive experience of channel and auditory science.

There are over 80 genes for potassium channel subunits, so we work on a subset of 4 genes in a family known as Kv3 (potassium channel family three). Crucially, only two of these genes are expressed in the auditory brainstem, and we have transgenic knockout mice for both genes. We are most interested in the third gene of the Kv3 family (Kv3.3) as mutations of this gene are linked to hearing disorders.

We aim to discover why these channel subunits are so crucial for sound processing and to understand how mutations can produce disease. A mutation in Kv3.3 also causes a form of neurodegeneration in the cerebellum called spinocerebellar ataxia 13 (SCA13), so we anticipate that our basic science results will help understand mechanisms of hearing and also be important for understanding age-related hearing loss, which may in turn be relevant to understanding why neurons die in dementia.

This project will elucidate mechanisms of neuronal excitability and synaptic integration in the auditory brainstem. The objectives are: to understand the complimentary roles of 'voltage-gated' and 'leak' potassium channels in setting excitability, and then; to determine how changes in intrinsic excitability mediated by Kv3 channels contribute to auditory processing, hearing loss and mechanisms of degeneration.

Kv3 channels are expressed across the auditory brainstem in somata, axons and terminals. Channels are composed only of Kv3.1 and Kv3.3 subunits but functionally differs in adjacent nuclei (MNTB vs LSO). Spinocerebellar ataxia 13 (SCA13) is a Kv3.3 mutation damaging the cerebellum and interferes with sound localization in the brainstem, aspects of which are mimicked in a Kv3.3 knockout.

Preliminary data supports the hypothesis that Kv3 and K2P channels co-operate in setting intrinsic excitability and action potential (AP) repolarization in the LSO, and that Kv3.3 is a key subunit in determining Kv3 channel expression, location and function in auditory processing. We will:
1. Identify the channel subunits setting the resting membrane potential and AP repolarization in the LSO.
2. Identify the subunits and roles of K2P channels in AP repolarization.
3. Determine the specific role(s) of Kv3.1 vs Kv3.3 in AP repolarization at synaptic terminals (transmitter release) and neuronal soma in the MNTB and LSO (AP waveform and firing).
4. Determine the mechanisms by which Kv3.3 mutations undermine LSO auditory processing and contribute to hearing loss.

We can demonstrate differential expression of Kv3 and K2P subunits, and my lab has experience in all of the techniques to be used in this project; we have transgenic mice and validated CRISPR/cas9 gene editing. We will focus on the auditory brainstem but the results have broad implications for understanding neuronal processing, and mechanisms of hearing loss, neurodegeneration and ageing.

The immediate beneficiaries of this research will be other researchers into ion channel biophysics and physiology. Our experimental approach will provide data that computational neuroscientists in general and our collaborators in particular, will benefit in refining their models.

The first part of the study will benefit other auditory neuroscientists in defining the ionic mechanisms of the brain circuit (which determine sound localization based on interaural volume differences, by comparison between both ears); the same mechanisms also contribute to detection of information in noise (and hence intelligibility of vocal communication).

The second part of the project will demonstrate how the K2P class of potassium channels contribute to neuronal excitability and in particular will define the extent to which these channels act in concert with voltage-gated channels to determine action potential waveform and neuronal firing properties. This too has a predominantly academic impact, and is important as this potential role for K2P has never been tested in native cells under physiological conditions.

The linkage of our studies (on hearing and auditory processing) with ion channel (Kv3.3) mutations known to cause ataxia and neurodegeneration is an important long term impact issue. We will elucidate ionic mechanisms and use our existing transgenic models to define the role of Kv3.1 versus Kv3.3 subunits and determine how they impair neuronal processing. There is potential for this data to have significant impact on understanding the ionic basis of neurodegeneration in the cerebellum, where the same mutation is known to cause a human disease. So this result would provide mechanistic background for other investigators that are involved in clinical studies of neurodegeneration.

Our results have implications for protection of hearing after exposure to loud sounds and the for the risk of hearing-loss associated with ageing. The mechanisms in the brain which pre-dispose an individual to age-related hearing loss are poorly understood; we have established that our Kv3.3 knockout mouse shows an age-related phenotype. This project will also provide insight into how sound over-exposure may be studied in the brain of an animal model. Generation of a mouse model of spinocerebellar ataxia and hearing loss will give insights into mechanisms of deafness and have implications beyond hearing for the control of neuronal excitability.

We meet and discuss auditory and biophysical questions at many international conferences; and we have local networks of researchers with mutual interests in auditory processing, neuronal excitability, ion channel function and disease. We regularly meet with members of
Institute for Hearing Research (IHR, Nottingham) Sheffield and the UCL Ear Institute. So my laboratory has many opportunities to discuss data and inform others of the basic science and potential clinical implications of our research. Studies of neuronal excitability in the auditory pathway may in the long-term benefit tinnitus sufferers and provide insights into potential treatments. Our work will also benefit researchers studying other mechanisms of hearing loss associated with genetic mutations in the brain.