hERG Channel Physiology and Biophysics
The hERG channel is a voltage-gated potassium (Kv) channel that is expressed in the heart. In contrast to other Kv channels, the gating properties of hERG channels are unique and poorly understood. hERG activates (open) and deactivates (close) slowly, yet inactivates and recovers from inactivation rapidly. These unusual gating properties afford hERG channels a critical role in the repolarization of the cardiac action potential and termination of excitability. We are using a range of electrophysiological techniques to understand the biophysical mechanisms underlying gating in hERG channels. We use patch clamp of mammalian cells and two electorde voltage clamp of Xenopus oocytes, as well as gating current recordings using COVG, and measurement of voltage sensor domain dyanmics using voltage clamp fluoirmetry and LRET spectroscopy.
Drug Interactions With hERG
hERG channels are the main cause of acquired LQTS. A diverse and extensive range of drugs block hERG channels resulting in life-threatening arrhythmias and sudden death. Interestingly, other potassium channels are not nearly so well blocked by these drugs. The promiscuous nature of drug interactions with hERG provides a significant challenge to drug design and development and toxicological screening against hERG has become routine practise. Interestingly, this process has revealed new compounds that activate hERG channels and may have therapeutic value in the management of LQTS. We use conventional and fluorescence-based electrophysiological techniques to investigate the mechanisms underlying drug inhibition and activation of hERG channels.
CRISPR-Cas9 editing of iPSC-derived cardiomyocytes
Cardiomyocytes derived from human iPSCs form monolayers of cells beating rhythmically as a functional syncytium. Optical mapping combined with programmed electrical stimulation enables the study of action potentials and calcium dynamics in these beating human cardiomyocytes with varying stimulation rates and patterns and in the presence or absence of adrenergic stimulation. In addition, gene-editing of isogenic control or patient-derived iPSCs allows us to study the effects of hERG channel variants on cardiac electrophysiology and to derive mutation-specific pharmacological strategies for rescuing function.
Gene-edited Zebrafish hearts as a model of cardiac repolarization disturbances
Zebrafish hearts are increasingly used as a model of human cardiac electrophysiology. Unlike small mammals, zebrafish hearts have an intrinsic beating rate and expression of ion currents that are similar to that observed in human hearts. We are using optical mapping of intact excised whole hearts to measure voltage and calcium dynamics during cardiac function as a relatively inexpensive and high throughput model for pharmacological screening of hERG channel function. We are also establishing and maintaining gene-edited (using CRISPR-Cas9) zebrafish lines to study inherited mutations associated with LQTS in humans in this readily translatable whole organ model.
Acidosis can occur as a result of myocardial ischemia in response to coronary artery occlusion and is associated with arrhythmia and sudden death. Acidosis inhibits hERG channel function and this may contribute to QT prolongation in early ischemia. We are investigating the molecular mechanisms by which protons inhibit hERG channel function.
KCNQ5 and epilepsy and intellectual disability
Intellectual disability and epilepsy are often caused by inherited gene mutations. Many different genes have been implicated, but one small group encodes for potassium channels. Gating (opening and closing) of these channels controls the flow of charged potassium ions out of brain cells, which in turn controls their excitability and nerve firing. Without coordinated gating of potassium channels, brain cells cannot form the new networks necessary for learning. Uncoordinated brain cell firing can also cause seizures. We are studying the impact of inherited mutations in KCNQ5 channels identified in individuals with intellectual disability and epilepsy and how dysfunction might be rescued pharmacologically with anti-epileptic therapies.