Modulation of hERG K+ Channel Deactivation by Voltage Sensor Relaxation

The hERG (human-ether-à-go-go-related gene) channel underlies the rapid delayed rectifier current, Ikr, in the heart, which is essential for normal cardiac electrical activity and rhythm. Slow deactivation is one of the hallmark features of the unusual gating characteristics of hERG channels, and plays a crucial role in providing a robust current that aids repolarization of the cardiac action potential. As such, there is significant interest in elucidating the underlying mechanistic determinants of slow hERG channel deactivation. Recent work has shown that the hERG channel S4 voltage sensor is stabilized following activation in a process termed relaxation. Voltage sensor relaxation results in energetic separation of the activation and deactivation pathways, producing a hysteresis, which modulates the kinetics of deactivation gating. Despite widespread observation of relaxation behaviour in other voltage-gated K+ channels, such as Shaker, Kv1.2 and Kv3.1, as well as the voltage-sensing phosphatase Ci-VSP, the relationship between stabilization of the activated voltage sensor by the open pore and voltage sensor relaxation in the control of deactivation has only recently begun to be explored. In this review, we discuss present knowledge and questions raised related to the voltage sensor relaxation mechanism in hERG channels and compare structure-function aspects of relaxation with those observed in related ion channels. We focus discussion, in particular, on the mechanism of coupling between voltage sensor relaxation and deactivation gating to highlight the insight that these studies provide into the control of hERG channel deactivation gating during their physiological functioning

Mechanistic Insight into Human ether-a-go-go-related Gene (hERG) K+ Channel Activation and Deactivation gating

hERG encodes the pore-forming α-subunit of the voltage-gated potassium channel that underlies the rapid delayed rectifier current, IKr, in the heart, which is essential for normal cardiac electrical activity and rhythm. Inherited mutations in, or pharmacological blockade of, hERG channels deplete the cardiac repolarization reserve, increasing the risk of life-threatening arrhythmias. The molecular bases of hERG gating events and drug binding are poorly understood. hERG channels display unique gating characteristics critical for their physiological function. They activate and deactivate slowly, yet inactivate and recover from inactivation rapidly. In addition, the promiscuous nature of drug interactions with hERG channels presents a therapeutic challenge for drug design and development. My thesis provides novel mechanistic and structural characterization of the unusual activation and deactivation gating processes of hERG. In my first study, I used a proline scan approach to define the activation gate region in hERG channels. Proximal substitutions (I655P-Q664P) impeded gate closure, trapping channels in the open state, while distal substitutions (R665P-Y667P) preserved normal gating, suggesting that Q664 marks the position of the activation gate in hERG. This is more than one helical turn lower than in related channels, which may allow for drug docking. Using two different approaches to measure voltage sensor gating in trapped open channels, I then demonstrated that slow activation is an intrinsic property of the voltage-sensing unit of hERG. In my second study, I showed that voltage-sensor stabilization slows hERG channel deactivation gating. I characterized the temporal sequence of events leading to voltage-sensor stabilization upon membrane depolarization. I showed that this occurs via two separable mechanisms, one derived from pore-gate-opening and the other from the voltage-sensing unit itself. In addition, I show that voltage sensor return in hERG channels is less energetically favourable than pore closure during repolarization and thus is what limits deactivation. Finally, I characterize the use of voltage clamp fluorimetry as a technique to track conformational rearrangements of the hERG voltage sensor associated with gating. These findings provide novel and in depth understanding regarding how hERG channels function and foundational knowledge relevant to finding targets for the treatment and management of cardiac arrhythmias

Loss-of-Function and Gain-of-Function Mutations in KCNQ5 Cause Intellectual Disability or Epileptic Encephalopathy

KCNQ5 is a highly conserved gene encoding an important channel for neuronal function; it is widely expressed in the brain and generates M-type current. Exome sequencing identified de novo heterozygous missense mutations in four probands with intellectual disability, abnormal neurological findings, and treatment-resistant epilepsy (in two of four). Comprehensive analysis of this potassium channel for the four variants expressed in frog oocytes revealed shifts in the voltage dependence of activation, including altered activation and deactivation kinetics. Specifically, both loss-of-function and gain-of-function KCNQ5 mutations, associated with increased excitability and decreased repolarization reserve, lead to pathophysiology