A study of the role of Kv8.2 a novel potassium channel gene in retinal physiology and phototransduction

The first step in vision is the capture of photons of light by the photoreceptors,t he rod and cones, that lie at the back of the retina. The human visual system operates over a very wide range of light levels from bright sunlight to almost complete darkness. This requires a corresponding wide range of sensitivity and the rod and cone photoreceptors are physiologiclly adapted to fulfill this need. Cones are relatively insensitive to light; they provide therefore vision in daylight the different classes also give colour vision. Rods on the other hand are very sensitive to light and operate at very low light levels. They do not provide for any perception of colour, hence objects appear monochromatic in dim light. The different sensitivities of cones and rods is determined by a range of processes that operate within the cells. We have recently identified the gene which encodes a potassium channel protein Kv8.2 that underlies a blinding disorder that is described as 'cone dystrophy with supernormal rod electroretinogram (ERG)'. As the named implies, the disorder affects the sensitivity of photoreceptors to light which is fundamental to the ability to see under different levels of illumination. Potassium (K+) channel proteins are central to the electrical activity of photoreceptors and are responsible in part for setting the current flow of the photoreceptors in the dark. This is directly related to the subseqent sensitivity of the photoreceptor to light. The objective of the project is to obtain a detailed understanding of the role of Kv8.2 in phototransduction. The approaches that will be used will include the study of an animal model with a knock-out mutation and in vitro studies of channel proteins. In this way, we will be able to define the role of relevant K+ channel in the intact retina from observations on the electrical activity of photoreceptors in mutant and normal mice. In addition, studies on the in vitro activity of the channel will define its pharmacology and interaction with other proteins.

The study of the regulation of phototransduction has been greatly facilitated by the development and application of techniques such as electroretinography and electrical recordings from single cells and this has been complemented by the study of inherited retinal diseases that arise from mutations in the component processes of phototransduction. We have recently defined the genetic basis for a blinding disorder that is described as 'cone dystrophy with supernormal rod electroretinogram (ERG)'. As the named implies, the disorder affects the sensitivity of photoreceptors to light and the time course of a response to a light flash, two critically important components of phototransduction and integral to the setting of dark background currents. The disease gene encodes a K+ channel protein Kv8.2 that acts as a modulator of activity when combined with other such proteins to form heteromultimeric channels but has no activity by itself. Our initial mutation detection screen identifed mutations in 10 different families that included nonsense, missense and deletion changes. The gene mutations show a recessive mode of inheritance which indicates that they are null mutations, consistent with the loss of channel modulation. The objective of the project is to obtain a detailed understanding of the role of Kv8.2 in phototransduction. The nature of the disorder indicates a fundamental role in setting the sensitivity to light and in the rate of response and the planned work will address these issues using a range of approaches that will include the study of a mouse gene knock-out line, in vitro electrophysiology using other K+ channels as partners, and in vitro expression to determine trafficking and stoichiometry.